Hadean Crustal Processes Revealed from Oxygen Isotopes and U-Th-Pb Depth Profiling of Pre-4.0 Ga Detrital Zircons from Western Australia

NASA Technical Reports Server (NTRS)

Trail, D.; Mojzsis, S. J.; Harrison, T. M.

2005-01-01

Because physical and chemical processes of the past are determined from analysis of a preserved geologic record, little is known about terrestrial crustal processes of the first 500 Ma during the so-called Hadean Eon. What is known from direct measurements has been derived almost exclusively from the study of greater than 4.0 Ga detrital zircons from the Jack Hills, Western Australia. The geochemistry of these zircons has direct application to understanding the origin and evolution of the rocks during the Hadean because: (i) U-Th-Pb age determinations by ion microprobe suggests the presence of crust as early as 4.37 Ga, or shortly after lunar formation; (ii) high-resolution U-Th-Pb zircon depth profiles reported here reveal several episodes of zircon growth in the Hadean previously unrecognized; (iii) core regions of pre-4.0 Ga zircons with igneous compositions are enriched in O-18 and contain metaluminous and peraluminous mineral inclusions, both features indicative of S-type grainitod protoliths. Study of these ancient zircons provides a unique window into the first half billion years that permits assessment of the potential of the Hadean Earth to host an emergent biosphere.

Characterization of inclusions in terrestrial impact formed zircon: Constraining the formation conditions of Hadean zircon from Jack Hills, Western Australia

NASA Astrophysics Data System (ADS)

Faltys, J. P.; Wielicki, M. M.; Sizemore, T. M.

2017-12-01

Due to the discovery and subsequent geochemical analysis of Hadean terrestrial material (e.g. detrital zircon from Jack Hills, Western Australia), a dramatic paradigm shift has occurred in the hypothesized near surface conditions of the first 500 million years of Earth's evolution. From a hellish setting riddled with impactors and not fit for life to a much milder environment that may have been uniquely suitable for the origin of life. Geochemical analyses of these ancient materials have been used to suggest the presence of water at or near the surface as well as the existence of continental crust during the Hadean, both of which have been suggested as necessary for the origin of life. However, the intensity of extraterrestrial bombardment during the Hadean and the effects of such events on the origin of life remains poorly understood. Clearly, as evidenced by Phanerozoic impact events, extraterrestrial impactors have the potential to dramatically effect the environment, particularly the biosphere. Early Earth likely experienced multiple large impact events, as evidenced by the lunar record, however whether those impacts were sufficient to frustrate the origin of life remains an open question. Although multiple lines of evidence, including the inclusion population, suggest the formation of Hadean zircon from Jack Hills as crystallizing in an under-thrust environment from S-type magmas, a recent study has suggested their formation in an impact melt environment analogous to a portion of the Sudbury Igneous Complex at the Sudbury impact structure. To determine between these two formation scenarios we have under-taken an inclusion study of terrestrial impact formed zircon from four of the largest terrestrial impact structures (Sudbury, Canada; Manicouagan, Canada; Vredefort, South Africa; Morokweng, South Africa), to compare to the vast inclusion dataset that exists for Jack Hills zircon. Preliminary data suggests a different inclusion population, from Hadean zircon

What do we really know about Earth's early crust?

NASA Astrophysics Data System (ADS)

Rudnick, R. L.; Tang, M.

2016-12-01

The oldest minerals on Earth, the detrital Hadean Jack Hills zircons from western Australia, show evidence for their crystallization from hydrous, low temperature, granitic magmas. However, considerable debate centers on whether the parental melts are minimum-melt granites formed in subduction zone settings and implying widespread, evolved continental crust (e.g., Harrison, 2009, AREPS), or crystallized from the last differentiates of mafic magmas (Darling et al., 2009, Geology), or even late differentiates of impact melt sheets on a largely water-covered Earth (Kenny et al., 2016, Geology). Another means by which to interrogate the nature of Earth's early crust is through analyses of ancient fine-grained terrigenous sedimentary rocks such as shales or glacial diamictites, which provide averages of the surface of the Earth that is exposed to chemical weathering and erosion. From these studies it has long been known that Archean crust contained a higher proportion of mafic rocks. However, only recently has that proportion been constrained based on a change in the average MgO content of the upper continental crust from 15 wt.% at 3.2 Ga, to 4 wt.% at 2.6 Ga (Tang et al., 2016, Science). These data for terrigeneous sediments require the pre 3.2 Ga crust to be dominated by mafic rocks (only 10-40% `granite' s.l.) and to be high-standing and susceptible to subareal weathering and erosion, implying the mafic crust was thick (see Tang and Rudnick, this meeting). The dramatic transition that occurred in upper crustal composition between 3.2 and 2.6 Ga likely marks the onset of widespread subduction as a means of generating voluminous granite.

Multiple Hadean crystallization and reworking events preserved in individual Jack Hills zircon grains

NASA Astrophysics Data System (ADS)

Bellucci, Jeremy; Nemchin, Alexander; Whitehouse, Martin; Snape, Joshua

2017-04-01

Five Hadean (>3.9 Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by an improved secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale <5 μm, as well as document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of <10 μm, smaller than traditional ion microprobe spot analyses. These new scanning ion images and age determinations suggest that roughly half, if not all, previous analyses, including those of trace elements and various isotope systems, could have intersected several domains of unfractured zircon, thus making the interpretation of any trace element, Hf, or O isotopic data tenuous. Lastly, all of the grains analyzed here preserve at least two distinguishable 207Pb/206Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships. These secondary events took place during at ca. 4.3, 4.2, 4.1, 4.0, and 3.7 Ga potentially indicating a sequence of magmatic and/or metamorphic events that recycled some volume of early crust during the Hadean and into Paleo- to Mesoarchean several times with an apparent periodicity of ca. 100 Ma.

Cubic zirconia in >2370 °C impact melt records Earth's hottest crust

NASA Astrophysics Data System (ADS)

Timms, Nicholas E.; Erickson, Timmons M.; Zanetti, Michael R.; Pearce, Mark A.; Cayron, Cyril; Cavosie, Aaron J.; Reddy, Steven M.; Wittmann, Axel; Carpenter, Paul K.

2017-11-01

Bolide impacts influence primordial evolution of planetary bodies because they can cause instantaneous melting and vaporization of both crust and impactors. Temperatures reached by impact-generated silicate melts are unknown because meteorite impacts are ephemeral, and established mineral and rock thermometers have limited temperature ranges. Consequently, impact melt temperatures in global bombardment models of the early Earth and Moon are poorly constrained, and may not accurately predict the survival, stabilization, geochemical evolution and cooling of early crustal materials. Here we show geological evidence for the transformation of zircon to cubic zirconia plus silica in impact melt from the 28 km diameter Mistastin Lake crater, Canada, which requires super-heating in excess of 2370 °C. This new temperature determination is the highest recorded from any crustal rock. Our phase heritage approach extends the thermometry range for impact melts by several hundred degrees, more closely bridging the gap between nature and theory. Profusion of >2370 °C superheated impact melt during high intensity bombardment of Hadean Earth likely facilitated consumption of early-formed crustal rocks and minerals, widespread volatilization of various species, including hydrates, and formation of dry, rigid, refractory crust.

Are there impact-formed zircons in the Hadean record?

NASA Astrophysics Data System (ADS)

Wielicki, M. M.; Lu, X.; Bell, E. A.; Schmitt, A. K.; Harrison, T. M.

2008-12-01

Detrital Hadean zircons from the Jack Hills, Western Australia, show a remarkable cluster of crystallization temperatures at 680±25°C. This is particularly surprising as a simple model relating rock composition and Zr concentration predicts that a very broad spectrum of crystallization temperatures (ca. 650°C to 1000°C) with a median value of 780°C, would result from impact melting of the Earth's surface. Magmatic fractionation would tend to increase the aforementioned values. Given the predicted high rate of impacts during the Hadean, the absence of such a population in the Jack Hills zircons could signal a profound sampling problem, a hint of a history much different than previously supposed, or our lack of understanding of zircon formation due to impact related processes. We have begun to examine the latter issue by investigating the crystallization temperatures of zircons formed in melt sheets preserved in the geologic record. The Sudbury Igneous Complex, formed at 1850±3 Ma within the second largest impact crater on Earth, includes two igneous units termed the Black and Felsic Norites. Examination of zircons from each by SIMS confirms their crystallization age at 1847.3±2.2 Ma and yields Ti-in-zircon temperatures of 720°C and 750°C, respectively. This is consistent with that predicted from zircon saturation systematics. A statistical test indicates that the combined norite population is distinct from the Hadean temperature distribution. Thus the question arises: where are the Hadean zircons expected to have formed at >780°C via impact processes? Similar analysis is being pursued for zircons from the Vredefort Impact Structure, South Africa, which should provide further information on impact-formed zircon temperature spectra.

What Hf isotopes in zircon tell us about crust-mantle evolution

NASA Astrophysics Data System (ADS)

Iizuka, Tsuyoshi; Yamaguchi, Takao; Itano, Keita; Hibiya, Yuki; Suzuki, Kazue

2017-03-01

The 176Lu-176Hf radioactive decay system has been widely used to study planetary crust-mantle differentiation. Of considerable utility in this regard is zircon, a resistant mineral that can be precisely dated by the U-Pb chronometer and record its initial Hf isotope composition due to having low Lu/Hf. Here we review zircon U-Pb age and Hf isotopic data mainly obtained over the last two decades and discuss their contributions to our current understanding of crust-mantle evolution, with emphasis on the Lu-Hf isotope composition of the bulk silicate Earth (BSE), early differentiation of the silicate Earth, and the evolution of the continental crust over geologic history. Meteorite zircon encapsulates the most primitive Hf isotope composition of our solar system, which was used to identify chondritic meteorites best representative of the BSE (176Hf/177Hf = 0.282793 ± 0.000011; 176Lu/177Hf = 0.0338 ± 0.0001). Hadean-Eoarchean detrital zircons yield highly unradiogenic Hf isotope compositions relative to the BSE, providing evidence for the development of a geochemically enriched silicate reservoir as early as 4.5 Ga. By combining the Hf and O isotope systematics, we propose that the early enriched silicate reservoir has resided at depth within the Earth rather than near the surface and may represent a fractionated residuum of a magma ocean underlying the proto-crust, like urKREEP beneath the anorthositic crust on the Moon. Detrital zircons from world major rivers potentially provide the most robust Hf isotope record of the preserved granitoid crust on a continental scale, whereas mafic rocks with various emplacement ages offer an opportunity to trace the Hf isotope evolution of juvenile continental crust (from εHf[4.5 Ga] = 0 to εHf[present] = + 13). The river zircon data as compared to the juvenile crust composition highlight that the supercontinent cycle has controlled the evolution of the continental crust by regulating the rates of crustal generation and intra

The chemical evolution of Earth's emerged crust inferred from titanium isotopes

NASA Astrophysics Data System (ADS)

Greber, N. D.; Dauphas, N.; Bekker, A.; Ptáček, M. P.; Bindeman, I. N.; Hofmann, A.

2017-12-01

Earth's earliest crust was ultramafic/mafic in composition. In contrast, modern Earth consists of a mafic oceanic crust and a continental crust dominated by felsic rocks. The Hadean zircon record suggests that at around 4.0 Ga, Earth's crust included some felsic rocks but their proportion relative to mafic rocks has been the subject of much discussion [1]. Several studies have shown evidence that the early Archean continental crust was mostly mafic and transitioned from 3.0 to 2.0 Ga to a modern-like felsic crust. This change in the nature of continental crust was tied to the onset of plate tectonics, arguing that it is difficult to make a large proportion of felsic rocks in a non-subduction setting [2]. Understanding the nature of Earth's early continental crust is also critical as it controls the bio-nutrient supply to the oceans and influences Earth's climate. Most reconstructions of the composition of Earth's emerged crust rely on terrigenous sediments whose composition can be altered relative to source rocks by weathering, sediment transport and metasomatism. We present a novel approach based on the Ti isotopic composition (δ49Ti) of shales to reconstruct the chemical composition of emerged continental crust through time. This proxy is based on the observation that the δ49Ti value of igneous rocks increases with increasing SiO2 concentration. Komatiites and basalts have an identical δ49Ti value to the bulk silicate Earth (around +0.005‰). Rocks with a granitic composition can reach up to a δ49Ti value of +0.55‰ [3]. Therefore, by measuring the δ49Ti values of shales with continental provenance, the SiO2 content of the emerged continental crust can be estimated, providing constraints on the proportion of mafic to felsic rocks. We measured δ49Ti values of shales ranging in age from 3.5 Ga to present. The average δ49Ti value of shales is almost constant over the last 3.5 Ga and always heavier than that of mafic rocks. We applied a three

Geochemical constraints on the Hadean environment from mineral fingerprints of prokaryotes.

PubMed

Novoselov, Alexey A; Silva, Dailto; Schneider, Jerusa; Abrevaya, Ximena Celeste; Chaffin, Michael S; Serrano, Paloma; Navarro, Margareth Sugano; Conti, Maria Josiane; Souza Filho, Carlos Roberto de

2017-06-21

The environmental conditions on the Earth before 4 billion years ago are highly uncertain, largely because of the lack of a substantial rock record from this period. During this time interval, known as the Hadean, the young planet transformed from an uninhabited world to the one capable of supporting, and inhabited by the first living cells. These cells formed in a fluid environment they could not at first control, with homeostatic mechanisms developing only later. It is therefore possible that present-day organisms retain some record of the primordial fluid in which the first cells formed. Here we present new data on the elemental compositions and mineral fingerprints of both Bacteria and Archaea, using these data to constrain the environment in which life formed. The cradle solution that produced this elemental signature was saturated in barite, sphene, chalcedony, apatite, and clay minerals. The presence of these minerals, as well as other chemical features, suggests that the cradle environment of life may have been a weathering fluid interacting with dry-land silicate rocks. The specific mineral assemblage provides evidence for a moderate Hadean climate with dry and wet seasons and a lower atmospheric abundance of CO 2 than is present today.

Evolution of the earth's crust: Evidence from comparative planetology

NASA Technical Reports Server (NTRS)

Lowman, P. D., Jr.

1973-01-01

Geochemical data and orbital photography from Apollo, Mariner, and Venera missions were combined with terrestrial geologic evidence to study the problem of why the earth has two contrasting types of crust (oceanic and continental). The following outline of terrestrial crustal evolution is proposed. A global crust of intermediate to acidic composition, high in aluminum, was formed by igneous processes early in the earth's history; portions survive in some shield areas as granitic and anorthositic gneisses. This crust was fractured by major impacts and tectonic processes, followed by basaltic eruptions analogous to the lunar maria and the smooth plains of the north hemisphere of Mars. Seafloor spreading and subduction ensued, during which portions of the early continental crust and sediments derived therefrom were thrust under the remaining continental crust. The process is exemplified today in regions such as the Andes/Peru-Chile trench system. Underplating may have been roughly concentric, and the higher radioactive element content of the underplated sialic material could thus eventually cause concentric zones of regional metamorphism and magmatism.

Rapid change of atmosphere on the Hadean Earth: Beyond Habitable Trinity on a tightrope

NASA Astrophysics Data System (ADS)

Arai, T.; Maruyama, S.

2014-12-01

Surface environment of Hadean Earth is a key to bear life on the Earth. All of previous works assumed that high pCO2 has been decreased to a few bars in the first a few hundreds millions of years (e.g., Zhanle et al., 2011). However, this process is not easy because of material and process barriers as shown below. Four barriers are present. First, the ultra-acidic pH (<0.1) of 4.4Ga ocean prevented the precipitation of carbonates at mid-oceanic ridge through water-rock interaction after the birth of primordial ocean driven by plate tectonics or pseudo-plate tectonics system. To overcome this barrier, primordial (anorthosite + KREEP) continents must have been above sea-level to increase pH rapidly through hydrological process. Second, major cap rocks on the Hadean oceanic crust must have been komatiite with minor basaltic rocks to precipitate carbonates through water-rock interaction and transport them into mantle through subduction at higher than the intermediate P/T geotherm on the Benioff plane. If not, carbonate minerals are all decarbonated at shallower depths than the Moho plane. Komatiite production depends on mantle potential temperature which must have been rapidly decreased to yield only Fe-enriched MORB by 3.8Ga. Third, the primordial continents composed of anorthosite with subordinate amounts of KREEP basalts must have been annihilated by 4.0Ga to alter pH to be possible to precipitate carbonates by hydrothermal process. The value of pCO2 must have been decreased down to a few bars from c.a. 50 bars at TSI (total surface irradiance) = 75% under the restricted time limit. If failed, the Earth must have been Venus state which is impossible to bear life on the planet. Fourth is the role of tectonic erosion to destroy and transport the primordial continent of anorthosite into deep mantle by subduction. Anorthosite + KREEP was the mother's milk grow life on the Earth, but disappeared by 4.0Ga or even earlier, but alternatively granites were formed and

The Hadean upper mantle conundrum: evidence for source depletion and enrichment from Sm-Nd, Re-Os, and Pb isotopic compositions in 3.71 Gy boninite-like metabasalts from the Isua Supracrustal Belt, Greenland

NASA Astrophysics Data System (ADS)

Frei, Robert; Polat, Ali; Meibom, Anders

2004-04-01

radiogenic Os isotopic signature is not known. Compared with the Sm-Nd and Re-Os isotope systems, the Pb isotope systematics show evidence for substantial perturbation by postformational hydrothermal-metasomatic alteration processes accompanying an early Archean metamorphic event at 3510 ± 65 Ma and indicate that the U-Th-Pb system was partially opened to Pb-loss on a whole rock scale. Single stage mantle evolution models fail to provide a solution to the Pb isotopic data, which requires that a high-μ component was mixed with the depleted mantle component before or during the extrusion of the basalts. Relatively high 207Pb/204Pb ratios (compared to contemporaneous mantle), support the hypothesis that erosion products of the ancient terrestrial protocrust existed for several hundred My before recycling into the mantle before ∼3.7 Ga. Our results are broadly consistent with models favoring a time-integrated Hadean history of mantle depletion and with the existence of an early Hadean protocrust, the complement to the Hadean depleted mantle, which after establishment of subduction-like processes was, at least locally, recycled into the upper mantle before 3.7 Ga. Thus, already in the Hadean, the upper mantle seems to be characterized by geochemical heterogeneity on a range of length scales; one property that is shared with the modern upper mantle. However, a simple two component mixing scenario between depleted mantle and an enriched-, crustal component with a modern analogue can not account for the complicated and contradictory geochemical properties of this particular Hadean upper mantle source.

Exploring for early bombardments on Earth from pre-3.85 Fa thermal effects recorded in Hadean zircons - a status report

NASA Astrophysics Data System (ADS)

Mojzsis, S. J.; Abramov, O.; Harrison, T. M.; Kring, D. A.; Levison, H. F.; Trail, D.; Watson, E. B.

2008-12-01

We report on our progress with high-resolution ion microprobe U-Th-Pb depth profiles and Ti+REEs spot analysis which show that subsequent to their crystallization in melts under typical crustal conditions on Earth, some Hadean (pre-3.85 Ga) zircons record common age domains with unusual chemical and isotopic characteristics consistent with a high-temperature (possibly impact) origin. We have found evidence for later overprints caused by intense thermal alteration between 3.94-3.97 Ga in six of eight studied grains but no evidence for older events. These findings alert us to two fundamental things we did not know before about the probiotic potential of the Earth in the earliest solar system: (i) that the bombardment epoch did not result in complete 'Doomsday' scale destruction of the Earth's crust since the Moon-forming event at ca. 4.5 Ga; and (ii) age constraints on both sides of the ther-mally altered 3.94-3.97 Ga zircon domains are very good and so far our data show that no detectable thermal events are recorded by the zircons before ~3.97 Ga up to about 4.3 Ga. This observation is consistent with the output of new classes of dynamical models that successfully re-create the decay of impactor populations in the early solar system as recorded on the Moon and in meteorites.

What can zircon ages from the Jack Hills detrital zircon suite really tell us about Hadean geodynamics?

NASA Astrophysics Data System (ADS)

Whitehouse, Martin; Nemchin, Alexander

2015-04-01

As the only direct sample of the Hadean Earth, detrital zircon grains from the Jack Hills, Western Australia, have been the subject of intense investigation over the almost three decades since their discovery. A wide variety of geochemical and isotopic analyses of these grains, as well as their mineral inclusions, have been used variously to support two fundamentally different models for Hadean geodynamics: (i) Some form of (not necessarily modern-style) plate recycling generating felsic (continental-type?) crust at the boundaries [1, 2], or conversely (ii) the persistence of a long-lived, stagnant basaltic lid within which magmatism occurred as a result of internal temperature perturbations and/or impacts [3, 4], a model also generally consistent with a wide range of observations from post-Hadean geochemical reservoirs. Despite the considerable time and resources expended, the majority of these studies uncritically accept the individual U-Pb zircon ages, even though their veracity is key to many of the interpretations [5, 6]. We report here the results of an in-depth evaluation of all published (and new) U-Pb ages from the Jack Hills zircon suite in order to define age populations that can be used with a high degree of confidence in geodynamic interpretations. A notable problem in the interpretation of U-Pb data from ancient zircon grains (including those as young as the Neoarchean) is that disturbance of the systematics even several 100 Ma after crystallization causes data to spread along the concordia curve without becoming discernably discordant within the relatively large error bounds associated with U/Pb ages from in situ dating methods (e.g. SIMS). While 207Pb/206Pb ages are typically more precise, individually they provide no means to detect Pb-loss-induced younging. However, if two or preferably more analyses have been made in the same zircon growth zone, a reasonable evaluation of the possibility of Pb-loss can be made. In the available Jack Hills zircon

On the origin of cratonic `high-mu' isotopic signatures

NASA Astrophysics Data System (ADS)

Reimink, J. R.; Carlson, R.; Shirey, S. B.; Pearson, D. G.; Kamber, B. S.

2017-12-01

Some Archean cratons (i.e. Slave, Wyoming) contain Neoarchean granitoids with initial Pb isotopic compositions indicative of derivation from sources characterized by high time-integrated U/Pb ratios (high-mu [1]). Single-stage high-m precursor source reservoir separation from the depleted mantle occurred no later than 3.9 Ga [2]. However, multi-stage separation could have occurred in the Hadean, suggesting that recycling or reworking of Eoarchean/Hadean crust played a significant role in the generation of Neoarchean granitic crust in many cratons. The Sm-Nd system is similar to the U-Pb system in that it has a short-lived parent-daughter pair (146Sm-142Nd) that is sensitive to very early differentiation events, as well as a long-lived parent-daughter pair (147Sm-143Nd) that is sensitive to differentiation throughout all of Earth history. The 103 Ma half-life of 146Sm makes it sensitive only to Sm/Nd fractionation that occurred in the Hadean, providing a useful tracker for very early differentiation events. Indeed, evidence for Neoarchean remelting of ancient crust in another craton has come from analyses of the paired Sm-Nd isotope systems from the Hudson Bay terrane of the northeastern Superior Province. These results indicate that the source of 2.7 Ga Hudson Bay terrane granitoids was Hadean mafic crust, and not Eoarchean felsic crust [3]. Here, we present new data from Neoarchean granites located in the Slave and Wyoming cratons, along with modeling of the dual paired-isotope systems of U-Pb and Sm-Nd to achieve a tighter constraint on the composition of the precursors and the timing of their melting. Combining our newly collected 142Nd data with the high-m signature of these Neoarchean rocks, we evaluate precursor source separation ages along with the source Sm/Nd and U/Pb compositions. In the simplest end-member scenarios, use of the 142Nd system allows us to test whether the cratonic high-mu signature was created by melting of Hadean mafic crust or Eoarchean

The 3.66 Ga Nuvvuagittuq Tonalite-Trondhjemite-Granodiorite Suite: A Case of Hadean Anatexis

NASA Astrophysics Data System (ADS)

Stevenson, R. K.; Bizzarro, M.

2011-12-01

. Neodymium-142 Evidence Hadean Mafic Crust. Science 321 (Sept. 26), 1828-1831. Stevenson, R.K. and Patchett, P.J. (1990): Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons. Geochimica et Cosmochimica Acta 54, 1683-1697.

The impact environment of the Hadean Earth

USGS Publications Warehouse

Abramov, Oleg; Kring, David A. Kring; Mojzsis, Stephen J.

2013-01-01

Impact bombardment in the first billion years of solar system history determined in large part the initial physical and chemical states of the inner planets and their potential to host biospheres. The range of physical states and thermal consequences of the impact epoch, however, are not well quantified. Here, we assess these effects on the young Earth's crust as well as the likelihood that a record of such effects could be preserved in the oldest terrestrial minerals and rocks. We place special emphasis on modeling the thermal effects of the late heavy bombardment (LHB) – a putative spike in the number of impacts at about 3.9 Gyr ago – using several different numerical modeling and analytical techniques. A comprehensive array of impact-produced heat sources was evaluated which includes shock heating, impact melt generation, uplift, and ejecta heating. Results indicate that ∼1.5–2.5 vol.% of the upper 20 km of Earth's crust was melted in the LHB, with only ∼0.3–1.5 vol.% in a molten state at any given time. The model predicts that approximately 5–10% of the planet's surface area was covered by >1 km deep impact melt sheets. A global average of ∼600–800 m of ejecta and ∼800–1000 m of condensed rock vapor is predicted to have been deposited in the LHB, with most of the condensed rock vapor produced by the largest (>100-km) projectiles. To explore for a record of such catastrophic events, we created two- and three-dimensional models of post-impact cooling of ejecta and craters, coupled to diffusion models of radiogenic Pb*-loss in zircons. We used this to estimate what the cumulative effects of putative LHB-induced age resetting would be of Hadean zircons on a global scale. Zircons entrained in ejecta are projected to have the following average global distribution after the end of the LHB: ∼59% with no impact-induced Pb*-loss, ∼26% with partial Pb*-loss and ∼15% with complete Pb*-loss or destruction of the grain. In addition to the

The Athabasca Granulite Terrane and Evidence for Dynamic Behavior of Lower Continental Crust

NASA Astrophysics Data System (ADS)

Dumond, Gregory; Williams, Michael L.; Regan, Sean P.

2018-05-01

Deeply exhumed granulite terranes have long been considered nonrepresentative of lower continental crust largely because their bulk compositions do not match the lower crustal xenolith record. A paradigm shift in our understanding of deep crust has since occurred with new evidence for a more felsic and compositionally heterogeneous lower crust than previously recognized. The >20,000-km2 Athabasca granulite terrane locally provides a >700-Myr-old window into this type of lower crust, prior to being exhumed and uplifted to the surface between 1.9 and 1.7 Ga. We review over 20 years of research on this terrane with an emphasis on what these findings may tell us about the origin and behavior of lower continental crust, in general, in addition to placing constraints on the tectonic evolution of the western Canadian Shield between 2.6 and 1.7 Ga. The results reveal a dynamic lower continental crust that evolved compositionally and rheologically with time.

The Hadean, Through a Glass Telescopically: Observations of Young Solar Analogs

NASA Technical Reports Server (NTRS)

Gaidos, E. J.

1998-01-01

Investigations into the Earth's surface environment during the Hadean eon (prior to 3.8 Ga) are hampered by the paucity of the geological and geochemical record and the relative inaccessibility of better-preserved surfaces with possibly similar early histories (i.e., Mars). One approach is to observe nearby, young solar-mass stars as analogs to the Hadean Sun and its environment. A catalog of 38 G and early K stars within 25 pc was constructed based on main-sequence status, bolometric luminosity, lack of known stellar companions within 800 AU, and coronal X-ray luminosities commensurate with the higher activity of solar-mass stars <0.8 b.y. old. Spectroscopic data support the assignment of ages of 0.2 - 0.8 Ga for most of these stars. Observations of these objects will provide insight into external forces that influenced Hadean atmosphere, ocean, and surface evolution (and potential ecosystems), including solar luminosity evolution, the flux and spectrum of solar ultraviolet radiation, the intensity of the solar wind, and the intensity and duration of a late period of heavy bombardment. The standard model of solar evolution predicts a luminosity of 0.75 solar luminosity at the end of the Hadean, implying a terrestrial surface temperature inconsistent with the presence of liquid water and motivating atmospheric greenhouse models. An alternative model fo solar evolution that invokes mass loss, constructed to explain solar Li depletion, attenuates or reverses this luminosity evolution of the atmospheres of Earth and the other terrestrial planets. This model can be tested by Li abundance measurements. The continuum emission from stellar wind plasma during significant mass loss may be detectable at millimeter and radio wavelengths. The Earth (and Moon) experienced a period of intense bombardment prior to 3.8 Ga, long after accretion was completed in the inner solar system and possibly associated with the clearing of residual planetesimals in the outer solar system. Such

The Fe-Rich Clay Microsystems in Basalt-Komatiite Lavas: Importance of Fe-Smectites for Pre-Biotic Molecule Catalysis During the Hadean Eon

NASA Astrophysics Data System (ADS)

Meunier, Alain; Petit, Sabine; Cockell, Charles S.; El Albani, Abderrazzak; Beaufort, Daniel

2010-06-01

During the Hadean to early Archean period (4.5-3.5 Ga), the surface of the Earth’s crust was predominantly composed of basalt and komatiite lavas. The conditions imposed by the chemical composition of these rocks favoured the crystallization of Fe-Mg clays rather than that of Al-rich ones (montmorillonite). Fe-Mg clays were formed inside chemical microsystems through sea weathering or hydrothermal alteration, and for the most part, through post-magmatic processes. Indeed, at the end of the cooling stage, Fe-Mg clays precipitated directly from the residual liquid which concentrated in the voids remaining in the crystal framework of the mafic-ultramafic lavas. Nontronite-celadonite and chlorite-saponite covered all the solid surfaces (crystals, glass) and are associated with tiny pyroxene and apatite crystals forming the so-called “mesostasis”. The mesostasis was scattered in the lava body as micro-settings tens of micrometres wide. Thus, every square metre of basalt or komatiite rocks was punctuated by myriads of clay-rich patches, each of them potentially behaving as a single chemical reactor which could concentrate the organics diluted in the ocean water. Considering the high catalytic potentiality of clays, and particularly those of the Fe-rich ones (electron exchangers), it is probable that large parts of the surface of the young Earth participated in the synthesis of prebiotic molecules during the Hadean to early Archean period through innumerable clay-rich micro-settings in the massive parts and the altered surfaces of komatiite and basaltic lavas. This leads us to suggest that Fe,Mg-clays should be preferred to Al-rich ones (montmorillonite) to conduct experiments for the synthesis and the polymerisation of prebiotic molecules.

The oxidation state of Hadean magmas and implications for early Earth's atmosphere.

PubMed

Trail, Dustin; Watson, E Bruce; Tailby, Nicholas D

2011-11-30

Magmatic outgassing of volatiles from Earth's interior probably played a critical part in determining the composition of the earliest atmosphere, more than 4,000 million years (Myr) ago. Given an elemental inventory of hydrogen, carbon, nitrogen, oxygen and sulphur, the identity of molecular species in gaseous volcanic emanations depends critically on the pressure (fugacity) of oxygen. Reduced melts having oxygen fugacities close to that defined by the iron-wüstite buffer would yield volatile species such as CH(4), H(2), H(2)S, NH(3) and CO, whereas melts close to the fayalite-magnetite-quartz buffer would be similar to present-day conditions and would be dominated by H(2)O, CO(2), SO(2) and N(2) (refs 1-4). Direct constraints on the oxidation state of terrestrial magmas before 3,850 Myr before present (that is, the Hadean eon) are tenuous because the rock record is sparse or absent. Samples from this earliest period of Earth's history are limited to igneous detrital zircons that pre-date the known rock record, with ages approaching ∼4,400 Myr (refs 5-8). Here we report a redox-sensitive calibration to determine the oxidation state of Hadean magmatic melts that is based on the incorporation of cerium into zircon crystals. We find that the melts have average oxygen fugacities that are consistent with an oxidation state defined by the fayalite-magnetite-quartz buffer, similar to present-day conditions. Moreover, selected Hadean zircons (having chemical characteristics consistent with crystallization specifically from mantle-derived melts) suggest oxygen fugacities similar to those of Archaean and present-day mantle-derived lavas as early as ∼4,350 Myr before present. These results suggest that outgassing of Earth's interior later than ∼200 Myr into the history of Solar System formation would not have resulted in a reducing atmosphere.

Thermal events documented in Hadean zircons by ion microprobe depth profiles

NASA Astrophysics Data System (ADS)

Trail, Dustin; Mojzsis, Stephen J.; Harrison, T. Mark

2007-08-01

We report the first U-Th-Pb ion microprobe depth profiles of four Hadean zircons from the Jack Hills and Mount Narryer supracrustal belts of the Narryer Gneiss Complex (NGC), Western Australia. This ultra-high spatial resolution technique probes the age and origin of sub-micron features in individual crystals that can record episodes of zircon growth. Near-surface grain dates of 2700 Ma or older are coincident with post-depositional growth/modification. Some ages may coincide with documented pre-deposition metamorphic events for the NGC and igneous emplacement at ca. 3700 Ma. Separate events that do not correlate in time with known geologic episodes prior to the preserved rock record are also present on pre-4000 Ma zircons. We find evidence for a ˜3.9 Ga event, which is coterminous within age uncertainty with one or several large basin-forming impacts (e.g. Nectaris) on the Moon attributed to the late heavy bombardment of the inner solar system.

Using the magmatic record to constrain the growth of continental crust-The Eoarchean zircon Hf record of Greenland

NASA Astrophysics Data System (ADS)

Fisher, Christopher M.; Vervoort, Jeffrey D.

2018-04-01

Southern West Greenland contains some of the best-studied and best-preserved magmatic Eoarchean rocks on Earth, and these provide an excellent vantage point from which to view long-standing questions regarding the growth of the earliest continental crust. In order to address the questions surrounding early crustal growth and complementary mantle depletion, we present Laser Ablation Split Stream (LASS) analyses of the U-Pb and Hf isotope compositions of zircon from eleven samples of the least-altered meta-igneous rocks from the Itsaq (Amîtsoq) Gneisses of the Isukasia and Nuuk regions of southern West Greenland. This analytical technique allows a less ambiguous approach to determining the age and Hf isotope composition of complicated zircon. Results corroborate previous findings that Eoarchean zircon from the Itsaq Gneiss (∼3.85 Ga to ∼3.63 Ga) were derived from a broadly chondritic source. In contrast to the Sm-Nd whole rock isotope record for southern West Greenland, the zircon Lu-Hf isotope record provides no evidence for early mantle depletion, nor does it suggest the presence of crust older than ∼3.85 Ga in Greenland. Utilizing LASS U-Pb and Hf data from the Greenland zircons studied here, we demonstrate the importance of focusing on the magmatic (rather than detrital) zircon record to more confidently understand early crustal growth and mantle depletion. We compare the Greenland Hf isotope data with other Eoarchean magmatic complexes such as the Acasta Gneiss Complex, Nuvvuagittuq greenstone belt, and the gneissic complexes of southern Africa, and all lack zircons with suprachondritic Hf isotope compositions. In total, these data suggest only a very modest volume of crust was produced during (or survived from) the Hadean and earliest Eoarchean. There remains no record of planet-scale early Earth mantle depletion in the Hf isotope record prior to 3.8 Ga.

Paleomagnetism of Hadean and Archean Detrital Zircons from the Jack Hills, Western Australia

NASA Astrophysics Data System (ADS)

Weiss, B. P.; Lima, E. A.; Alexander, E.; Bell, E. A.; Boehnke, P.; Wielicki, M. M.; Harrison, M.; Fu, R. R.; Kehayias, P.; Glenn, D. R.; Walsworth, R. L.; Araujo, J. F. D.; Einsle, J. F.; Harrison, R.; Trail, D.; Watson, E. B.

2016-12-01

Determining the history of Earth's dynamo prior to the oldest known well-preserved rock record is one of the ultimate challenges in the field of paleomagnetism. The dynamo's early history has major implications for the evolution of the core, the initiation of plate tectonics, the physics of magnetic field generation, and the habitability of the early Earth. The only known minerals that might retain paleomagnetic records from well before 3.5 billion years ago (Ga) are detrital zircon crystals found in sedimentary rocks in Western Australia. Ranging up to 4.38 Ga in age, they are the oldest known terrestrial minerals. Tarduno et al. (2015) argued that detrital zircons contain records of an active dynamo dating back to 4.2 Ga. However, it has not been demonstrated that the zircons have escaped remagnetization during the intervening time since their formation (Weiss et al. 2016). Therefore, the age of magnetization in the Jack Hills zircons and the existence of a dynamo prior to 3.5 Ga have yet to be established. To address this issue, we have been studying the magnetism and thermal and aqueous alteration histories of single Archean and Hadean Jack Hills zircon crystals. Peak unblocking temperatures combined with electron backscatter diffraction indicate that the zircons contain inclusions of magnetite and hematite. Electron microscopy, X-ray tomography, and quantum diamond magnetometry indicate that much of the iron oxides in the zircons are associated with cracks and are therefore likely secondary. However, our newly developed Li-in-zircon geospeedometry technique shows for the first time that a small fraction of Hadean zircons retain sharp gradients in Li concentration (see figure), indicating they likely have never heated above the magnetite Curie temperature since their formation at >4 Ga. We describe thermal demagnetization and Thellier-Thellier paleointensity studies of these zircons and implications for the existence of a Hadean dynamo.

Hadean silicate differentiation revealed by anomalous 142Nd in the Réunion hotspot source

NASA Astrophysics Data System (ADS)

Peters, B. J.; Carlson, R.; Day, J. M.; Horan, M.

2017-12-01

Geochemical and geophysical data show that volcanic hotspots can tap ancient domains sequestered in Earth's deep mantle. Evidence from stable and long-lived radiogenic isotope systems has demonstrated that many of these domains result from tectonic and differentiation processes that occurred more than two billion years ago. Recent advances in the analysis of short-lived radiogenic isotopes have further shown that some hotspot sources preserve evidence for metal-silicate differentiation occurring within the first one percent of Earth's history. Despite these discoveries, efforts to detect variability in the lithophile 146Sm-142Nd (t1/2 = 103 Ma) system in Phanerozoic hotspot lavas have not yet detected significant global variation. We report 142Nd/144Nd ratios in Réunion Island basalts that are statistically distinct from the terrestrial Nd standard ranging to both higher and lower 142Nd/144Nd. Variations in 142Nd/144Nd, which total nearly 15 ppm on Réunion, are correlated with 3He/4He among both anomalous and non-anomalous samples. Such behavior implies that there were analogous changes in Sm/Nd and (U+Th)/3He that occurred during a Hadean silicate differentiation event and were not completely overprinted by the depleted mantle. Variations in the 142Nd-143Nd compositions of Réunion basalts can be explained by a single Hadean melting event producing enriched and depleted domains that partially re-mixed after 146Sm was no longer extant. Assuming differentiation occurred at pressures where perovskite is stable, anomalies of the magnitude observed in Réunion basalts require melting of at least 50% across a wide depth range, and up to 90% for melting at pressures near those of the deepest mantle. Models with best fits to Nd isotope data suggest this differentiation occurred around 4.40 Ga and mixing occurred after 4 Ga. This two-stage differentiation process nearly erased the ancient, anomalous 142Nd composition of the Réunion source and produced the relatively

Tracking Hadean processes in modern basalts with 142-Neodymium

NASA Astrophysics Data System (ADS)

Horan, M. F.; Carlson, R. W.; Walker, R. J.; Jackson, M.; Garçon, M.; Norman, M.

2018-02-01

The short-lived 146Sm→142 Nd isotope system (t1/2 = 103 Ma) provides constraints on the timing and processes of terrestrial silicate fractionation during the early Hadean. Although some Archean terranes preserve variability in 142Nd/144Nd, no anomalies have been resolved previously in young rocks. This study provides high precision 142Nd/144Nd data on a suite of ocean island basalts from Samoa and Hawaii previously shown to have variable depletions in 182W/184W that are inversely correlated with 3He/4He ratios. Improved analytical techniques and multiple replicate analyses of Nd show a variation in μ142 Nd values between -1.3 and +2.7 in the suite, relative to the JNdi standard. Given the reproducibility of the standard (±2.9 ppm, 2 SD), two Samoan samples exhibit resolved variability in their 142Nd/144Nd ratios outside of their 95% confidence intervals, suggesting minor variability in the Samoan hotspot. One sample from Samoa has a higher μ142 Nd of +2.7, outside the 95% confidence interval (±1.0 ppm) of the average of the JNdi standard. Limited, but resolved, variation in 142Nd/144Nd within the suite suggests the preservation of early Hadean silicate differentiation in the sources of at least some basalts from Samoa. Larger variations of 182W/184W and 3He/4He ratios in the same samples suggest that metal-silicate separation and mantle outgassing left a more persistent imprint on the accessible mantle compared to 142Nd/144Nd ratios which are impacted by early silicate differentiation.

Evidence for Buried "Pre-Noachian" Crust Pre-Dating the Oldest Observed Surface Units on Mars

NASA Technical Reports Server (NTRS)

Frey, H. V.; Frey, E. L.; Hartmann, W. K.; Tanaka, K. L. T.

2003-01-01

Even though the Early Noachian (EN) used in geologic mapping is undefined at the early end, it is often assumed in absolute chronologies to extend back to 4.6 BYA. We explored this assumption by searching for evidence of buried impact basins, in the largest occurrences of Early Noachian terrain. The hypothesis is that if such basins exist, they indicate crust which must predate the surface units mapped as the oldest on Mars, and those units must then be less than 4.6 BY old. Alternatively, if no such buried features are seen, then the surface units may represent crust of the same age below, which could in principle be as old as Mars. Here we show the former alternative is true. There must be crust older than the oldest mapped surface units. We also show that a number of Noachian terrains on Mars appear to have a common total (visible + buried) crater retention age. This might be either the age of the original (planet-wide?) crust of Mars, or may indicate crater saturation.

Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions.

PubMed

Walter, M J; Kohn, S C; Araujo, D; Bulanova, G P; Smith, C B; Gaillou, E; Wang, J; Steele, A; Shirey, S B

2011-10-07

A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

USGS Publications Warehouse

Jonathan, Patchett P.; Kouvo, O.; Hedge, C.E.; Tatsumoto, M.

1982-01-01

We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from ??Hf=0 to +14, or about 60% of the variability of the present mantle. The approximate ??Hf=2??Nd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf=??? 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships. The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive ??Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle. ?? 1981 Springer-Verlag.

Seawater osmium isotope evidence for a middle Miocene flood basalt event in ferromanganese crust records

USGS Publications Warehouse

Klemm, V.; Frank, M.; Levasseur, S.; Halliday, A.N.; Hein, J.R.

2008-01-01

Three ferromanganese crusts from the northeast, northwest and central Atlantic were re-dated using osmium (Os) isotope stratigraphy and yield ages from middle Miocene to the present. The three Os isotope records do not show evidence for growth hiatuses. The reconstructed Os isotope-based growth rates for the sections older than 10??Ma are higher than those determined previously by the combined beryllium isotope (10Be/9Be) and cobalt (Co) constant-flux methods, which results in a decrease in the maximum age of each crust. This re-dating does not lead to significant changes to the interpretation of previously determined radiogenic isotope neodymium, lead (Nd, Pb) time series because the variability of these isotopes was very small in the records of the three crusts prior to 10??Ma. The Os isotope record of the central Atlantic crust shows a pronounced minimum during the middle Miocene between 15 and 12??Ma, similar to a minimum previously observed in two ferromanganese crusts from the central Pacific. For the other two Atlantic crusts, the Os isotope records and their calibration to the global seawater curve for the middle Miocene are either more uncertain or too short and thus do not allow for a reliable identification of an isotopic minimum. Similar to pronounced minima reported previously for the Cretaceous/Tertiary and Eocene/Oligocene boundaries, possible interpretations for the newly identified middle Miocene Os isotope minimum include changes in weathering intensity and/or a meteorite impact coinciding with the formation of the No??rdlinger Ries Crater. It is suggested that the eruption and weathering of the Columbia River flood basalts provided a significant amount of the unradiogenic Os required to produce the middle Miocene minimum. ?? 2008 Elsevier B.V.

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.

PubMed

Audet, Pascal; Bostock, Michael G; Christensen, Nikolas I; Peacock, Simon M

2009-01-01

Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction-and indeed plate tectonics-to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson's ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson's ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

Icelandic-type crust

USGS Publications Warehouse

Foulger, G.R.; Du, Z.; Julian, B.R.

2003-01-01

Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 ?? 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15-30 ?? 5 km thick and begins where the velocity gradient decreases radically. This generally occurs at the V p ??? 6.5 km s-1 level. A low-velocity zone ??? 10 000 km2 in area and up to ??? 15 km thick occupies the lower crust beneath central Iceland, and may represent a submerged, trapped oceanic microplate. The crust-mantle boundary is a transition zone ???5 ?? 3 km thick throughout which V p increases progressively from ???7.2 to ???8.0 km s-1. It may be gradational or a zone of alternating high- and low-velocity layers. There is no seismic evidence for melt or exceptionally high temperatures in or near this zone. Isostasy indicates that the density contrast between the lower crust and the mantle is only ???90 kg m-3 compared with ???300 kg m-3 for normal oceanic crust, indicating compositional anomalies that are as yet not understood. The seismological crust is ???30 km thick beneath the Greenland-Iceland and Iceland-Faeroe ridges, and eastern Iceland, ???20 km beneath western Iceland, and ???40 km thick beneath central Iceland. This pattern is not what is predicted for an eastward-migrating plume. Low attenuation and normal V p/V s ratios in the lower crust beneath central and southwestern Iceland, and normal uppermost mantle velocities in general, suggest that the crust and uppermost mantle are subsolidus and cooler than at equivalent depths beneath the East Pacific Rise. Seismic data from Iceland have historically been interpreted both in terms of thin-hot and thick-cold crust models, both of which have been cited as supporting the plume

Evidence of synsedimentary microbial activity and iron deposition in ferruginous crusts of the Late Cenomanian Utrillas Formation (Iberian Basin, central Spain)

NASA Astrophysics Data System (ADS)

García-Hidalgo, José F.; Elorza, Javier; Gil-Gil, Javier; Herrero, José M.; Segura, Manuel

2018-02-01

Ferruginous sandstones and crusts are prominent sedimentary features throughout the continental (braided)-coastal siliciclastic (estuarine-tidal) wedges of the Late Cenomanian Utrillas Formation in the Iberian Basin. Crust types recognized are: Ferruginous sandy crusts (Fsc) with oxides-oxyhydroxides (hematite and goethite) concentrated on sandstone tops presenting a fibro-radial internal structure reminding organic structures that penetrate different mineral phases, suggesting the existence of bacterial activity in crust development; Ferruginous muddy crusts (Fmc) consisting of wavy, laminated, microbial mats, being composed mainly of hematite. On the other hand, a more dispersed and broader mineralization included as Ferruginous sandstones with iron oxides and oxyhydroxides (hematite and goethite) representing a limited cement phase on these sediments. The presence of microbial remains, ferruginous minerals, Microbially-induced sedimentary structures, microbial laminites and vertebrate tracks preserved due to the presence of biofilms suggest firstly a direct evidence of syn-depositional microbial activity in these sediments; and, secondly, that iron accumulation and ferruginous crusts development occurred immediately after deposition of the host, still soft sediments. Ferruginous crusts cap sedimentary cycles and they represent the gradual development of hard substrate conditions, and the development of a discontinuity surface at the top of the parasequence sets, related to very low sedimentary rates; the overlying sediments record subsequent flooding of underlying shallower environments; crusts are, consequently, interpreted as boundaries for these higher-order cycles in the Iberian Basin.

Magmatic intrusions in the lunar crust

NASA Astrophysics Data System (ADS)

Michaut, C.; Thorey, C.

2015-10-01

The lunar highlands are very old, with ages covering a timespan between 4.5 to 4.2 Gyr, and probably formed by flotation of light plagioclase minerals on top of the lunar magma ocean. The lunar crust provides thus an invaluable evidence of the geological and magmatic processes occurring in the first times of the terrestrial planets history. According to the last estimates from the GRAIL mission, the lunar primary crust is particularly light and relatively thick [1] This low-density crust acted as a barrier for the dense primary mantle melts. This is particularly evident in the fact that subsequent mare basalts erupted primarily within large impact basin: at least part of the crust must have been removed for the magma to reach the surface. However, the trajectory of the magma from the mantle to the surface is unknown. Using a model of magma emplacement below an elastic overlying layer with a flexural wavelength Λ, we characterize the surface deformations induced by the presence of shallow magmatic intrusions. We demonstrate that, depending on its size, the intrusion can show two different shapes: a bell shape when its radius is smaller than 4 times Λ or a flat top with small bended edges if its radius is larger than 4 times Λ[2]. These characteristic shapes for the intrusion result in characteristic deformations at the surface that also depend on the topography of the layer overlying the intrusion [3].Using this model we provide evidence of the presence of intrusions within the crust of the Moon as surface deformations in the form of low-slope lunar domes and floor-fractured craters. All these geological features have morphologies consistent with models of magma spreading at depth and deforming an overlying elastic layer. Further more,at floor-fractured craters, the deformation is contained within the crater interior, suggesting that the overpressure at the origin of magma ascent and intrusion was less than the pressure due to the weight of the crust removed by

Mass-spectrometric mining of Hadean zircons by automated SHRIMP multi-collector and single-collector U/Pb zircon age dating: The first 100,000 grains

NASA Astrophysics Data System (ADS)

Holden, Peter; Lanc, Peter; Ireland, Trevor R.; Harrison, T. Mark; Foster, John J.; Bruce, Zane

2009-09-01

The identification and retrieval of a large population of ancient zircons (>4 Ga; Hadean) is of utmost priority if models of the early evolution of Earth are to be rigorously tested. We have developed a rapid and accurate U-Pb zircon age determination protocol utilizing a fully automated multi-collector ion microprobe, the ANU SHRIMP II, to screen and date these zircons. Unattended data acquisition relies on the calibration of a digitized sample map to the Sensitive High Resolution Ion MicroProbe (SHRIMP) sample-stage co-ordinate system. High precision positioning of individual grains can be produced through optical image processing of a specified mount location. The focal position of the mount can be optimized through a correlation between secondary-ion steering and the spot position on the target. For the Hadean zircon project, sample mounts are photographed and sample locations (normally grain centers) are determined off-line. The sample is loaded, reference points calibrated, and the target positions are then visited sequentially. In SHRIMP II multiple-collector mode, zircons are initially screened (ca. 5 s data acquisition) through their 204Pb corrected 207Pb/206Pb ratio; suitable candidates are then analyzed in a longer routine to obtain better measurement statistics, U/Pb, and concentration data. In SHRIMP I and SHRIMP RG, we have incorporated the automated analysis protocol to single-collector measurements. These routines have been used to analyze over 100,000 zircons from the Jack Hills quartzite. Of these, ca. 7%, have an age greater than 3.8 Ga, the oldest grain being 4372 +/- 6 Ma (2[sigma]), and this age is part of a group of analyses around 4350 Ma which we interpret as the age when continental crust first began to coalesce in this region. In multi-collector mode, the analytical time taken for a single mount with 400 zircons is approximately 6 h; whereas in single-collector mode, the analytical time is ca. 17 h. With this productivity, we can produce

The Mafic Lower Crust of Neoproterozoic age beneath Western Arabia: Implications for Understanding African Lower Crust

NASA Astrophysics Data System (ADS)

Stern, R. J.; Mooney, W. D.

2011-12-01

We review evidence that the lower crust of Arabia - and by implication, that beneath much of Africa was formed at the same time as the upper crust, rather than being a product of Cenozoic magmatic underplating. Arabia is a recent orphan of Africa, separated by opening of the Red Sea ~20 Ma, so our understanding of its lower crust provides insights into that of Africa. Arabian Shield (exposed in W. Arabia) is mostly Neoproterozoic (880-540 Ma) reflecting a 300-million year process of continental crustal growth due to amalgamated juvenile magmatic arcs welded together by granitoid intrusions that make up as much as 50% of the Shield's surface. Seismic refraction studies of SW Arabia (Mooney et al., 1985) reveal two layers, each ~20 km thick, separated by a well-defined Conrad discontinuity. The upper crust has average Vp ~6.3 km/sec whereas the lower crust has average Vp ~7.0 km/sec, corresponding to a granitic upper crust and gabbroic lower crust. Neogene (<30 ma) lava fields in Arabia (harrats) extend over 2500 km, from Yemen to Syria. Many of these lavas contain xenoliths, providing a remarkable glimpse of the lower-crustal and upper-mantle lithosphere beneath W. Arabia. Lower crustal xenoliths brought up in 8 harrats in Saudi Arabia, Jordan, and Syria are mostly 2-pyroxene granulites of igneous (gabbroic, anorthositic, and dioritic) origin. They contain plagioclase, orthopyroxene, and clinopyroxene, and a few contain garnet and rare amphibole and yield mineral-equilibrium temperatures of 700-900°C. Pyroxene-rich and plagioclase-rich suites have mean Al2O3 contents of 13% and 19%, respectively: otherwise the two groups have similar elemental compositions, with ~50% SiO2 and ~1% TiO2, with low K2O (<0.5%) and Na2O (1-3%). Both groups show tholeiitic affinities, unrelated to their alkali basalt hosts. Mean pyroxene-rich and plagioclase-rich suites show distinct mean MgO contents (11% vs. 7%), Mg# (67 vs. 55), and contents of compatible elements Ni (169 vs. 66 ppm

Modeling crust-mantle evolution using radiogenic Sr, Nd, and Pb isotope systematics

NASA Astrophysics Data System (ADS)

Kumari, Seema; Paul, Debajyoti

2015-04-01

The present-day elemental and isotopic composition of Earth's terrestrial reservoirs can be used as geochemical constraints to study evolution of the crust-mantle system. A flexible open system evolutionary model of the Earth, comprising continental crust (CC), upper depleted mantle (UM) -source of mid-ocean ridge basalts (MORB), and lower mantle (LM) reservoir with a D" layer -source of ocean island basalts (OIB), and incorporating key radioactive isotope systematics (Rb-Sr, Sm-Nd, and U-Th-Pb), is solved numerically at 1 Ma time step for 4.55 Ga, the age of the Earth. The best possible solution is the one that produces the present-day concentrations as well as isotopic ratios in terrestrial reservoirs, compiled from published data. Different crustal growth scenarios (exponential, episodic, early and late growth), proposed in earlier studies, and its effect on the evolution of isotope systematics of terrestrial reservoirs is studied. Model simulations strongly favor a layered mantle structure satisfying majority of the isotopic constraints. In the successful model, which is similar to that proposed by Kellogg et al. (1999), the present-day UM comprises of 60% of mantle mass and extends to a depth 1600 km, whereas the LM becomes non-primitive and more enriched than the bulk silicate Earth, mainly due to addition of recycled crustal material. Modeling suggest that isotopic evolution of reservoirs is affected by the mode of crustal growth. Only two scenarios satisfied majority of the Rb-Sr and Sm-Nd isotopic constraints but failed to reproduce the present-day Pb-isotope systematics; exponential growth of crust (mean age, tc=2.3 Ga) and delayed and episodic growth (no growth for initial 900 Ma, tc=2.05 Ga) proposed by Patchett and Arndt (1986). However, assuming a slightly young Earth (4.45 Ga) better satisfies the Pb-isotope systematics. Although, the delayed crustal growth model satisfied Sr-Nd isotopic constraints, presence of early Hadean crust (4.03 and 4.4 Ga

A Hydrothermal-Sedimentary Context for the Origin of Life

PubMed Central

Hickman-Lewis, K.; Hinman, N.; Gautret, P.; Campbell, K.A.; Bréhéret, J.G.; Foucher, F.; Hubert, A.; Sorieul, S.; Dass, A.V.; Kee, T.P.; Georgelin, T.; Brack, A.

2018-01-01

Abstract Critical to the origin of life are the ingredients of life, of course, but also the physical and chemical conditions in which prebiotic chemical reactions can take place. These factors place constraints on the types of Hadean environment in which life could have emerged. Many locations, ranging from hydrothermal vents and pumice rafts, through volcanic-hosted splash pools to continental springs and rivers, have been proposed for the emergence of life on Earth, each with respective advantages and certain disadvantages. However, there is another, hitherto unrecognized environment that, on the Hadean Earth (4.5–4.0 Ga), would have been more important than any other in terms of spatial and temporal scale: the sedimentary layer between oceanic crust and seawater. Using as an example sediments from the 3.5–3.33 Ga Barberton Greenstone Belt, South Africa, analogous at least on a local scale to those of the Hadean eon, we document constant permeation of the porous, carbonaceous, and reactive sedimentary layer by hydrothermal fluids emanating from the crust. This partially UV-protected, subaqueous sedimentary environment, characterized by physical and chemical gradients, represented a widespread system of miniature chemical reactors in which the production and complexification of prebiotic molecules could have led to the origin of life. Key Words: Origin of life—Hadean environment—Mineral surface reactions—Hydrothermal fluids—Archean volcanic sediments. Astrobiology 18, 259–293. PMID:29489386

Evidence for biogenic processes during formation of ferromanganese crusts from the Pacific Ocean: implications of biologically induced mineralization.

PubMed

Wang, Xiao-Hong; Schlossmacher, Ute; Natalio, Filipe; Schröder, Heinz C; Wolf, Stephan E; Tremel, Wolfgang; Müller, Werner E G

2009-01-01

Ferromanganese [Fe/Mn] crusts formed on basaltic seamounts, gain considerable economic importance due to their high content of Co, Ni, Cu, Zn and Pt. The deposits are predominantly found in the Pacific Ocean in depths of over 1000m. They are formed in the mixing layer between the upper oxygen-minimum zone and the lower oxygen-rich bottom zone. At present an almost exclusive abiogenic origin of crust formation is considered. We present evidence that the upper layers of the crusts from the Magellan Seamount cluster are very rich in coccoliths/coccolithophores (calcareous phytoplankton) belonging to different taxa. Rarely intact skeletons of these unicellular algae are found, while most of them are disintegrated into their composing prisms or crystals. Studies on the chemical composition of crust samples by high resolution SEM combined with an electron probe microanalyzer (EPMA) revealed that they are built of distinct stacked piles of individual compartments. In the center of such piles Mn is the dominant element, while the rims of the piles are rich in Fe (mineralization aspect). The compartments contain coccospheres usually at the basal part. Energy dispersive X-ray spectroscopy (EDX) analyses showed that those coccospheres contain, as expected, CaCO3 but also Mn-oxide. Detailed analysis displayed on the surface of the coccolithophores a high level of CaCO3 while the concentration of Mn-oxide is relatively small. With increasing distance from the coccolithophores the concentration of Mn-oxide increases on the expense of residual CaCO3. We conclude that coccoliths/coccolithophores are crucial for the seed/nucleation phase of crust formation (biomineralization aspect). Subsequently, after the biologically induced mineralization phase Mn-oxide deposition proceeds "auto"catalytically.

Old Continental Crust Underlying Juvenile Oceanic Arc: Evidence From Northern Arabian-Nubian Shield, Egypt

NASA Astrophysics Data System (ADS)

Li, Xian-Hua; Abd El-Rahman, Yasser; Abu Anbar, Mohamed; Li, Jiao; Ling, Xiao-Xiao; Wu, Li-Guang; Masoud, Ahmed E.

2018-04-01

The Neoproterozoic Arabian-Nubian Shield (ANS) is the best preserved and the largest exposed Neoproterozoic juvenile crust on Earth. While the lithology and early Sr and Nd isotopic data demonstrate that the ANS crust is overwhelmingly juvenile, pre-ANS old zircon crystals have been increasingly recognized in the ANS igneous and sedimentary rocks, casting doubt on the "juvenility" of the ANS crust. In order to understand the origin of the old continental materials in the ANS and its roles in generation of juvenile oceanic arcs, we carry out for the first time an integrated in situ analysis of zircon U-Pb age and Hf-O isotopes for greywacke and felsic volcanic cobble samples from the Atud Formation in the Eastern Desert of northwestern part of the ANS. Our data indicate that the Atud Formation was deposited between ca. 720 and 700 Ma, concurrent with the production of oceanic arcs in the ANS. The Atud greywacke was derived from the erosion of a proximal arc terrane that contains numerous old continental crust materials. We identify for the first time a 755-Ma felsic volcanic cobble from the Atud Formation that is derived from old continental materials during juvenile crust production, suggesting presence of an old continental crust substrate that underlies the ANS. Our work demonstrates that reworking of old continental crust played important roles in generation of oceanic arcs in the northwestern ANS that is likely much less juvenile than previously thought. Thus, the crustal growth rates calculated based on estimates of temporal island arc development need to be revised.

Evidence for Buried "Pre-Noachian" Crust Pre-Dating the Oldest Observed Surface Units on Mars

NASA Technical Reports Server (NTRS)

Frey, H. V.; Frey, E. L.; Hartmann, W. K.; Tanaka, K. L. T.

2004-01-01

MOLA gridded data shows clear evidence for Quasi-Circular Depressions not visible on images in Early Noachian (EN) terrain units on Mars. We suggest these are buried impact basins that pre-date the superimposed craters whose high density makes these EN units the oldest visible at the surface of Mars. There is crust older than the oldest visible terrain units on Mars, and these EN units cannot date from 4.6 BYA. These and other Noachian units have similar total (visible + buried) crater retention ages, suggesting a common "pre-Noachian" crustal age OR crater saturation beyond which we cannot see.

Pervasive Layering in the Lunar Highland Crust: Evidence from Apollos 15, 16,and 17

NASA Technical Reports Server (NTRS)

Lowman, Paul D., Jr.; Yang, Tiffany

2005-01-01

This paper presents results of a photogeologic reconnaissance of 70 mm photographs taken on the lunar surface during the Apollo 15, 16, and 17 missions, whose primary objective was to investigate the lunar highland crust. Photographs at all three sites, notably the Apennine Front, show pervasive layered structure. These layers are easily distinguished from lighting artifacts, and are considered genuine crustal structures. Their number, thickness, and extent implies that they are lava flows, not ejecta blankets or intrusive features. They appear to be the upper part of the earliest lunar crust, possibly forming a layer tens of kilometers thick. Remote sensing studies (X-ray fluorescence and reflectance spectroscopy), indicate that the highland crust is dominantly a feldspathic basalt. It is concluded that the highland layers represent a global crust formed by eruptions of high-alumina basalt in the first few hundred million years of the Moon's history.

Mineralogical Evidence for the Bulk Transformation of Continental Crust to Ultrahigh-Pressure Conditions in Subduction Zones

NASA Astrophysics Data System (ADS)

Peterman, E. M.; Hacker, B. R.; Kylander-Clark, A. R.

2005-12-01

Evidence for (ultra)high-pressure --(U)HP-- metamorphism in modern orogenic belts and the preservation of exhumed (U)HP terranes around the world suggest that subduction and exhumation of continental crust plays an important role in Phanerozoic plate tectonics. The Western Gneiss region (WGR) of Norway, a major (U)HP province extending over 60,000 km2, provides an excellent opportunity to study how subduction to depths >100 km affects continental crust. By studying a ~60 km wide transect bounded to the north by Vartdalsfjorden and Rovdefjorden and the south by the Möre og Romsdal county boundary, we are able to examine mineralogical changes that occurred during subduction and exhumation within a rock composed predominantly of orthogneiss and variably transformed mafic bodies, which indicate the depths to which these rocks were subducted. Previous studies (e.g. Hacker et al., 2005) have suggested that Caledonian deformation in WGR host gneisses is primarily limited to brittle-ductile fabrics characterized by greenschist to lower-amphibolite facies metamorphism; the majority of the deformation in the rocks, including the pervasive foliation and foliation-parallel isoclinal folds, occurred between 1200 and 900 Ma. On the northern half of our study area, however, locally occurring neoblastic garnet crosscuts the foliation in the gneiss. The boundary of this garnet zone coincides with the local HP-UHP boundary, as determined by the presence of coesite in eclogite. Because garnet can retain information about changes in pressure and temperature, as well as the availability of water within the crust to catalyze chemical reactions, our findings suggest that 1) portions of the orthogneiss did transform at high pressures, 2) the presence of garnet within the orthogneiss may indicate conditions that approximate UHP and can therefore be useful in defining the boundaries between UHP and HP conditions, and 3) the growth of garnet during (U)HP metamorphism may be controlled by

Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

PubMed Central

Smirnov, Alexander; Hausner, Douglas; Laffers, Richard; Strongin, Daniel R; Schoonen, Martin AA

2008-01-01

Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200°C), borosilicate septum bottles (70°C) and HDPE tubes (22°C) in the presence of Fe and Ni metal, awaruite (Ni80Fe20) and tetrataenite (Ni50Fe50). In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites) were found to contain up to 22 ppm Ntot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N3-). This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic) ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts. PMID:18489746

Crusted scabies in an immunocompetent child: treatment with ivermectin.

PubMed

Gladstone, H B; Darmstadt, G L

2000-01-01

An 11-year-old girl presented to our clinic with recalcitrant crusted scabies despite repeated applications of topical scabicides. She had no history of corticosteroid use prior to onset of the eruption and no evidence of immunodeficiency. A combination of oral ivermectin, topical lindane, and keratolytics cleared the infestation. Our patient is exceptional in that she had no risk factors commonly associated with a propensity to develop crusted scabies. While topical therapy remains the first-line treatment for children with classic scabies, in the unusual instance of a child with recalcitrant, crusted scabies, ivermectin may offer an efficacious alternative, although it should be used with caution. We discuss the use of oral ivermectin for treatment of crusted scabies and the challenging comprehensive management needed for this socially stigmatizing condition.

Evidence for the Buried "Pre-Noachian" Crust Pre-Dating the Oldest Observed Surface Units on Mars

NASA Technical Reports Server (NTRS)

Frey, H. V.; Frey, E. L.; Hartmann, W. K.; Tanaka, K. L. T.

2003-01-01

MOLA gridded data shows clear evidence for Quasi-Circular Depressions not visible on images in Early Noachian (EN) terrain units on Mars. We suggest these are buried impact basins that pre-date the superimposed craters whose high density makes these EN units the oldest visible at the surface of Mars. There is crust older than the oldest visible terrain units on Mars, and these EN units cannot date from 4.6 BYA. These and other Noa-chian units have similar total (visible + buried) crater retention ages, suggesting a common "pre-Noachian" crustal age OR crater saturation beyond which we cannot see.

Planetary size critical to the preservation of primordial anorthosite-enriched continental crust and life potential

NASA Astrophysics Data System (ADS)

Dohm, J. M.; Maruyama, S.

2016-12-01

Primordial continental crust (PCC) of the Moon consists of anorthosite. Anorthosite has been discovered on the Martian surface as well, possibly of significant extent as on the Moon [1]. In the case of the Earth, the occurrence of anorthosite is observed to be limited in the geological record; however, lunar and Martian surface geology indicate that anorthosite may have been more universal on the Earth as PCC during the Hadean. We propose that differences in the presence of anorthosite-enriched PCC are due to planetary size. The reason why the PCC of Earth disappeared is explained by the strength and duration of mantle convection and plate tectonics, compared to the Moon and its relatively rapid cooling following the development of a magma ocean. We also theorize that Mars also retains its anorthosite-enriched PCC, which includes andesite and granite due to an early phase of plate tectonics that shut down prior to its complete destruction (roughly 3.93 Ga) as a result of its relatively small mass and rapid cooling [2]. Growing evidence of this includes anorthosite identified in Hellas rim materials [1], exposures of possible granite in more ancient terrain of Noachis Terra [3], and alluvial-fan materials of Peace Vallis in Gale crater, which have been interpreted to be representative of an ancient felsic crust [4], and in particular > 4.0 Ga Terra Cimmeria crustal basement exposed by the Gale impact [5]. Nutrient-enriched PCC is essential in determining the fate of the planet to be habitable or not. Mars has elevated habitability potential because of its PCC that was once exposed above an ocean that interacted with a relatively thick atmosphere through Sun-driven hydrological circulation, known as Habitable-Trinity conditions [6]. At this conference, we will discuss the significance of planetary size on both the preservation of anorthosite-enriched PCC on rocky planets and habitability potential, and why the Martian PCC will be a key target of exploration. [1

Scales of Heterogeneities in the Continental Crust and Upper Mantle

NASA Astrophysics Data System (ADS)

Tittgemeyer, M.; Wenzel, F.; Ryberg, T.; Fuchs, K.

1999-09-01

A seismological characterization of crust and upper mantle can refer to large-scale averages of seismic velocities or to fluctuations of elastic parameters. Large is understood here relative to the wavelength used to probe the earth.¶In this paper we try to characterize crust and upper mantle by the fluctuations in media properties rather than by their average velocities. As such it becomes evident that different scales of heterogeneities prevail in different layers of crust and mantle. Although we cannot provide final models and an explanation of why these different scales exist, we believe that scales of inhomogeneities carry significant information regarding the tectonic processes that have affected the lower crust, the lithospheric and the sublithospheric upper mantle.¶We focus on four different types of small-scale inhomogeneities (1) the characteristics of the lower crust, (2) velocity fluctuations in the uppermost mantle, (3) scattering in the lowermost lithosphere and on (4) heterogeneities in the mantle transition zone.

CRUST1.0: An Updated Global Model of Earth's Crust

NASA Astrophysics Data System (ADS)

Laske, G.; Masters, G.; Ma, Z.; Pasyanos, M. E.

2012-04-01

We present an updated global model of Earth's crustal structure. The new model, CRUST1.0, serves as starting model in a more comprehensive effort to compile a global model of Earth's crust and lithosphere, LITHO1.0. CRUST1.0 is defined on a 1-degree grid and is based on a new database of crustal thickness data from active source seismic studies as well as from receiver function studies. In areas where such constraints are still missing, for example in Antarctica, crustal thicknesses are estimated using gravity constraints. The compilation of the new crustal model initially follows the philosophy of the widely used crustal model CRUST2.0 (Bassin et al., 2000; http://igppweb.ucsd.edu/~gabi/crust2.html). Crustal types representing properties in the crystalline crust are assigned according to basement age or tectonic setting. The classification of the latter loosely follows that of an updated map by Artemieva and Mooney (2001) (http://www.lithosphere.info). Statistical averages of crustal properties in each of these crustal types are extrapolated to areas with no local seismic or gravity constraint. In each 1-degree cell, boundary depth, compressional and shear velocity as well as density is given for 8 layers: water, ice, 3-layer sediment cover and upper, middle and lower crystalline crust. Topography, bathymetry and ice cover are taken from ETOPO1. The sediment cover is essentially that of our sediment model (Laske and Masters, 1997; http://igppweb.ucsd.edu/~sediment.html), with several near-coastal updates. In the sediment cover and the crystalline crust, updated scaling relationships are used to assign compressional and shear velocity as well as density. In an initial step toward LITHO1.0, the model is then validated against our new global group velocity maps for Rayleigh and Love waves, particularly at frequencies between 30 and 40 mHz. CRUST1.0 is then adjusted in areas of extreme misfit where we suspect deficiencies in the crustal model. These currently include

Deep Crustal Melting and the Survival of Continental Crust

NASA Astrophysics Data System (ADS)

Whitney, D.; Teyssier, C. P.; Rey, P. F.; Korchinski, M.

2017-12-01

Plate convergence involving continental lithosphere leads to crustal melting, which ultimately stabilizes the crust because it drives rapid upward flow of hot deep crust, followed by rapid cooling at shallow levels. Collision drives partial melting during crustal thickening (at 40-75 km) and/or continental subduction (at 75-100 km). These depths are not typically exceeded by crustal rocks that are exhumed in each setting because partial melting significantly decreases viscosity, facilitating upward flow of deep crust. Results from numerical models and nature indicate that deep crust moves laterally and then vertically, crystallizing at depths as shallow as 2 km. Deep crust flows en masse, without significant segregation of melt into magmatic bodies, over 10s of kms of vertical transport. This is a major mechanism by which deep crust is exhumed and is therefore a significant process of heat and mass transfer in continental evolution. The result of vertical flow of deep, partially molten crust is a migmatite dome. When lithosphere is under extension or transtension, the deep crust is solicited by faulting of the brittle upper crust, and the flow of deep crust in migmatite domes traverses nearly the entire thickness of orogenic crust in <10 million years. This cycle of burial, partial melting, rapid ascent, and crystallization/cooling preserves the continents from being recycled into the mantle by convergent tectonic processes over geologic time. Migmatite domes commonly preserve a record of high-T - low-P metamorphism. Domes may also contain rocks or minerals that record high-T - high-P conditions, including high-P metamorphism broadly coeval with host migmatite, evidence for the deep crustal origin of migmatite. There exists a spectrum of domes, from entirely deep-sourced to mixtures of deep and shallow sources. Controlling factors in deep vs. shallow sources are relative densities of crustal layers and rate of extension: fast extension (cm/yr) promotes efficient

The evolution of Mercury's crust: a global perspective from MESSENGER.

PubMed

Denevi, Brett W; Robinson, Mark S; Solomon, Sean C; Murchie, Scott L; Blewett, David T; Domingue, Deborah L; McCoy, Timothy J; Ernst, Carolyn M; Head, James W; Watters, Thomas R; Chabot, Nancy L

2009-05-01

Mapping the distribution and extent of major terrain types on a planet's surface helps to constrain the origin and evolution of its crust. Together, MESSENGER and Mariner 10 observations of Mercury now provide a near-global look at the planet, revealing lateral and vertical heterogeneities in the color and thus composition of Mercury's crust. Smooth plains cover approximately 40% of the surface, and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically. A low-reflectance, relatively blue component affects at least 15% of the surface and is concentrated in crater and basin ejecta. Its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron- and titanium-bearing oxides.

New evidence for long-distance fluid migration within the Earth's crust

NASA Astrophysics Data System (ADS)

Person, M.; Baumgartner, L.

1995-07-01

During the past decade, geologists have come to appreciate the interconnectedness of hydrologic, tectonic, thermal, and geochemical processes operating within the Earth's continental crust [Oliver, 1992]. This has led to a new geologically-based conceptual model of hydrology which is crustal-scale and is centered in plate tectonics theory (Fig. 1). From a geological perspective, the tectonic and thermal processes which drive plate motion are also responsible, either directly or indirectly, for inducing fluid motion across and through the continents. Supporting evidence for this emerging paradigm is based on observations of pervasive rock-water interactions associated with geologic processes as diverse as the chemical alteration of crustal rocks [Shelton et al, 1992; Elliott and Aronson, 1993; McManus and Hanor, 1993; Ague, 1991, 1994], devolatilization of minerals during burial and consequent metamorphism [Cox and Etheridge, 1989], the formation of energy and mineral deposits [Garven et al, 1993; and Cathles et al, 1993], remagnitization of ancient sedimentary rocks [McCabe and Elmore, 1989], the tectonic deformation of sedimentary basins [Oliver 1992, Ge and Garven, 1992], and the regulation of global climate [Caldeira et al, 1993, Kerrick and Caldeira, 1993, 1994]. This paper summarizes the many recent lines of theoretical, laboratory, and field evidence from diverse disciplines within the Earth Sciences supporting this emerging view of crustal-scale hydrology. Evidence for two types of long-distance fluid migration are highlighted: vertical pore water movement through crystalline rocks to depths greater than six km and lateral groundwater movement through sedimentary basins over hundereds of km. Also emphasized are the many driving mechanisms on fluid motion which are not typically considered in water quality and water supply investigations. Some geologic terms used in this paper, which may be unfamiliar to the reader, are defined in geologic dictionaries

Exploring the Hydrothermal System in the Chicxulub Crater and Implications for the Early Evolution of Life on Earth

NASA Astrophysics Data System (ADS)

Kring, D. A.; Schmieder, M.; Tikoo, S.; Riller, U. P.; Simpson, S. L.; Osinski, G.; Cockell, C. S.; Coolen, M.; Gulick, S. P. S.; Morgan, J. V.

2017-12-01

Impact cratering, particularly large basin-size craters with diameters >100 km, have the potential to generate vast subsurface hydrothermal systems. There were dozens of such impacts during the Hadean and early Archean, some of which vaporized seas for brief periods of time, during which the safest niches for early life may have been in those subsurface hydrothermal systems. The Chicxulub crater can serve as a proxy for those events. New IODP-ICDP core recovered by Expedition 364 reveals a high-temperature (>300 degree C) system that may have persisted for more than 100,000 years. Of order 105 to 106 km3 of crust was structurally deformed, melted, and vaporized within about 10 minutes of the impact. The crust had to endure immense strain rates of 104/s to 106/s, up to 12 orders of magnitude greater than those associated with igneous and metamorphic processes. The outcome is a porous, permeable region that is a perfect host for hydrothermal circulation across the entire diameter of the crater to depths up to 5 or 6 km. The target rocks at Chicxulub are composed of an 3 km-thick carbonate platform sequence over a crystalline basement composed of igneous granite, granodiorite, and a few other intrusive components, such as dolerite, and metamorphic assemblages composed, in part, of gneiss and mica schist. Post-impact hydrothermal alteration includes Ca-Na- and K-metasomatism, pervasive hydration to produce layered silicates, and lower-temperature vug-filling zeolites as the system cycled from high temperatures to low temperatures. While the extent of granitic crust on early Earth is still debated and, thus, the direct application of those mineral reactions to the Hadean and early Archean can be debated, the thermal evolution of the system should be applicable to diverse crustal compositions. It is important to point out that pre-impact thermal conditions of Hadean and early Archean crust can affect the size of an impact basin and, in turn, the proportion of that basin

Sequestration of volatiles in the martian crust through hydrated minerals

NASA Astrophysics Data System (ADS)

Mustard, J. F.; Ehlmann, B. L.; Poulet, F.; Fraeman, A. A.; Carter, J.

2011-12-01

The magnitude and history of volatile reservoirs is a key question in understanding Mars' evolution. The volumes of reservoirs for water through time have been estimated on the basis of morphology (e.g. Carr 1996) and modeling while the volume of active identifiable modern reservoirs such as the polar caps, the near-surface cryosphere, and the atmosphere are reasonably well known. One reservoir that has been hypothesized but not examined is the crust where water would be in the form of hydrous minerals. The OMEGA and CRISM experiments on Mars Express and Mars Reconnaissance Orbiter respectively, have shown that phyllosilicate minerals are commonly observed in the Noachian crust of Mars. Modeling has shown that depending on the location the abundance of clays and phyllosilicates can exceed 50% but more typically is less or absent, particularly in the Hesperian and younger terrains (Poulet 2007). Phyllosilicate-bearing outcrops have been observed in the deepest wall exposures of Valles Marineris (8 km below the rim) and in the central peaks of impact craters as large of 100 km. Modeling suggests that the porosity of the crust in maintained to approximate 8-10 km depth permitting the circulation of water to this depth and formation of phyllosilicate and other hydrated minerals. Based on these and other observations it is evident that at least the top 10 km of the crust can be considered to contain hydrated silicate minerals. These observations also show that phyllosilicates are globally present in Noachian crust. We use altered oceanic crust as an analog for the amount of alteration on Mars. Analyses show that the average volume fraction of hydrous phases in the lower oceanic crust is 10%. Simple calculations show this results in a water content of between 1 - 3%. If the upper 10 km of the martian crust is altered to this extent then a global equivalent thickness (GET) of water of 0.3 to 0.9 km is stored in the crust due to alteration minerals. This is comparable to

Hadean silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the Réunion hotspot source

NASA Astrophysics Data System (ADS)

Peters, Bradley J.; Carlson, Richard W.; Day, James M. D.; Horan, Mary F.

2018-03-01

Active volcanic hotspots can tap into domains in Earth’s deep interior that were formed more than two billion years ago. High-precision data on variability in tungsten isotopes have shown that some of these domains resulted from differentiation events that occurred within the first fifty million years of Earth history. However, it has not proved easy to resolve analogous variability in neodymium isotope compositions that would track regions of Earth’s interior whose composition was established by events occurring within roughly the first five hundred million years of Earth history. Here we report 142Nd/144Nd ratios for Réunion Island igneous rocks, some of which are resolvably either higher or lower than the ratios in modern upper-mantle domains. We also find that Réunion 142Nd/144Nd ratios correlate with helium-isotope ratios (3He/4He), suggesting parallel behaviour of these isotopic systems during very early silicate differentiation, perhaps as early as 4.39 billion years ago. The range of 142Nd/144Nd ratios in Réunion basalts is inconsistent with a single-stage differentiation process, and instead requires mixing of a conjugate melt and residue formed in at least one melting event during the Hadean eon, 4.56 billion to 4 billion years ago. Efficient post-Hadean mixing nearly erased the ancient, anomalous 142Nd/144Nd signatures, and produced the relatively homogeneous 143Nd/144Nd composition that is characteristic of Réunion basalts. Our results show that Réunion magmas tap into a particularly ancient, primitive source compared with other volcanic hotspots, offering insight into the formation and preservation of ancient heterogeneities in Earth’s interior.

Secular variations of iron isotopes in ferromanganese crusts: evidences for deeply sourced iron in the Pacific Ocean?

NASA Astrophysics Data System (ADS)

Rouxel, O. J.; Gueguen, B.

2016-12-01

Ferromanganese (Fe-Mn) crusts are potential archive of the Fe isotope composition of deep seawater through time. Here, we report Fe isotope composition of two pairs of Fe-Mn crusts collected on two volcanic seamounts from the Northern Pacific Ocean (Apuupuu Seamount, Hawaii) and the Southern Pacific Ocean (near Rurutu Island, Austral archipelago of French Polynesia). This approach allows (a) a direct comparison of the Fe isotope record in Fe-Mn crusts from the same seamount in order to address local effects, and (b) a comparison of geochemical composition of crusts between North and South Pacific in order to address the effect of more global geochemical processes. The results show that, despite different growth rates, diagenetic history, textures and geochemical patterns, Fe-Mn crusts from both North and South Pacific Oceans have fairly homogenous Fe isotope compositions over the last 17 Ma, yielding average δ56Fe values of -0.22 ± 0.20‰ (1sd, n = 54). The results also show striking correlations between Fe and Pb isotope ratios, indicating that local mixing between water masses is the main factor controlling Fe isotope composition in FeMn crusts. Recently, Horner et al. (2015) reported a range of δ56Fe values from -1.12‰ to 1.54‰ along a 76 Ma-old FeMn crust from the central pacific. However, secular variations of Fe isotopes inferred from other FeMn crusts in the Central North Pacific and Western Pacific (Yang and Rouxel, unpublished) show different patterns over the last 40 Ma, with δ56Fe ranging from -0.07 to -0.61‰ (n=81). Hence, the application of Fe isotopes as paleoceanographic proxies to trace deeply sourced iron at the scale of oceanic basins should be used with caution, prompting for an integrative approach combining diverse yet complimentary geochemical proxies.

Petrogenesis and tectonics of the Acasta Gneiss Complex derived from integrated petrology and 142Nd and 182W extinct nuclide-geochemistry

NASA Astrophysics Data System (ADS)

Reimink, Jesse R.; Chacko, Thomas; Carlson, Richard W.; Shirey, Steven B.; Liu, Jingao; Stern, Richard A.; Bauer, Ann M.; Pearson, D. Graham; Heaman, Larry M.

2018-07-01

The timing and mechanisms of continental crust formation represent major outstanding questions in the Earth sciences. Extinct-nuclide radioactive systems offer the potential to evaluate the temporal relations of a variety of differentiation processes on the early Earth, including crust formation. Here, we investigate the whole-rock 182W/184W and 142Nd/144Nd ratios and zircon Δ17O values of a suite of well-studied and lithologically-homogeneous meta-igneous rocks from the Acasta Gneiss Complex, Northwest Territories, Canada, including the oldest-known zircon-bearing rocks on Earth. In the context of previously published geochemical data and petrogenetic models, the new 142Nd/144Nd data indicate that formation of the Hadean-Eoarchean Acasta crust was ultimately derived from variable sources, both in age and composition. Although 4.02 Ga crust was extracted from a nearly bulk-Earth source, heterogeneous μ142Nd signatures indicate that Eoarchean rocks of the Acasta Gneiss Complex were formed by partial melting of hydrated, Hadean-age mafic crust at depths shallower than the garnet stability field. By ∼3.6 Ga, granodioritic-granitic rocks were formed by partial melting of Archean hydrated mafic crust that was melted at greater depth, well into the garnet stability field. Our 182W results indicate that the sources to the Acasta Gneiss Complex had homogeneous, high-μ182W on the order of +10 ppm-a signature ubiquitous in other Eoarchean terranes. No significant deviation from the terrestrial mass fractionation line was found in the triple oxygen isotope (16O-17O-18O) compositions of Acasta zircons, confirming homogeneous oxygen isotope compositions in Earth's mantle by 4.02 Ga.

Evidence from xenoliths for a dynamic lower crust, eastern Mojave Desert, California

USGS Publications Warehouse

Hanchar, John M.; Miller, Calvin F.; Wooden, Joseph L.; Bennett, Victoria C.; Staude, John-Mark G.

1994-01-01

Garnet-rich xenoliths in a Tertiary dike in the eastern Mojave Desert, California, preserve information about the nature and history of the lower crust. These xenoliths record pressures of ∼ 10–12 kbar and temperatures of ∼ 750–800°C. Approximately 25% have mafic compositions and bear hornblende + plagioclase + clinopyroxene + quartz in addition to garnet. The remainder, all of which contain quartz, include quartzose, quartzofeldspathic, and aluminous (kyanite±sillimanite-bearing) varieties. Most xenoliths have identifiable protoliths—mafic from intermediate or mafic igneous rocks, quartzose from quartz-rich sedimentary rocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathic from feldspathic sediments and/or intermediate to felsic igneous rocks. However, many have unusual chemical compositions characterized by high FeO(t), FeO(t)/MgO, Al2O3, and Al2O3/CaO, which correspond to high garnet abundance. The mineralogy and major-and trace-element compositions are consistent with the interpretation that the xenoliths are the garnet-rich residues of high-pressure crustal melting, from which granitic melt was extracted. High 87Sr/86Sr and low 143Nd/144Nd, together with highly discordant zircons from a single sample with Pb/Pb ages of ∼ 1.7 Ga, demonstrate that the crustal material represented by the xenoliths is at least as old as Early Proterozoic. This supracrustal-bearing lithologic assemblage may have been emplaced in the lower crust during either Proterozoic or Mesozoic orogenesis, but Sr and Nd model ages> 4 Ga require late Phanerozoic modification of parent/daughter ratios, presumably during the anatectic event. Pressures of equilibration indicate that peak metamorphism and melting occurred before the Mojave crust had thinned to its current thickness of <30 km. The compositions of the xenoliths suggest that the lower crust here is grossly similar to estimated world-wide lower-crustal compositions in terms of silica and mafic content

Continental crust

USGS Publications Warehouse

Pakiser, L.C.

1964-01-01

The structure of the Earth’s crust (the outer shell of the earth above the M-discontinuity) has been intensively studied in many places by use of geophysical methods. The velocity of seismic compressional waves in the crust and in the upper mantle varies from place to place in the conterminous United States. The average crust is thick in the eastern two-thirds of the United States, in which the crustal and upper-mantle velocities tend to be high. The average crust is thinner in the western one-third of the United States, in which these velocities tend to be low. The concept of eastern and western superprovinces can be used to classify these differences. Crustal and upper-mantle densities probably vary directly with compressional-wave velocity, leading to the conclusion that isostasy is accomplished by the variation in densities of crustal and upper-mantle rocks as well as in crustal thickness, and that there is no single, generally valid isostatic model. The nature of the M-discontinuity is still speculative.

Steady State Growth of Continental Crust?

NASA Astrophysics Data System (ADS)

Bowring, S. A.; Bauer, A.; Dudas, F. O.; Schoene, B.; McLean, N. M.

2012-12-01

any age. If one accepts that the probability of preserving old crust decreases with increasing age, the few exposures of rocks older than 3.5 Ga should not be surprising. The thickness and compositional differences between Archean and younger lithospheric mantle are not fully understood nor is the role of thicker buoyant mantle in preserving continental crust; these lead to the question of whether the preserved rock record is representative of what formed. It is notable that the oldest known rocks, the ca. 4.0 Ga Acasta Gneisses, are tonalities-granodiorites-granites with evidence for the involvement of even older crust and that the oldest detrital zircons from Australia (ca. 4.0-4.4 Ga) are thought to have been derived from granitoid sources. The global Hf and Nd isotope databases are compatible with both depleted and enriched sources being present from at least 4.0 Ga to the present and that the lack of evolution of the MORB source or depleted mantle is due to recycling of continental crust throughout earth history. Using examples from the Slave Province and southern Africa, we argue that Armstrong's concept of steady state crustal growth and recycling via plate tectonics still best explains the modern geological and geochemical data.

Formation of Fe-Mn crusts within a continental margin environment

USGS Publications Warehouse

Conrad, Tracey A.; Hein, James R.; Paytan, Adina; Clague, David A.

2017-01-01

This study examines Fe-Mn crusts that form on seamounts along the California continental-margin (CCM), within the United States 200 nautical mile exclusive economic zone. The study area extends from approximately 30° to 38° North latitudes and from 117° to 126° West longitudes. The area of study is a tectonically active northeast Pacific plate boundary region and is also part of the North Pacific Subtropical Gyre with currents dominated by the California Current System. Upwelling of nutrient-rich water results in high primary productivity that produces a pronounced oxygen minimum zone. Hydrogenetic Fe-Mn crusts forming along the CCM show distinct chemical and mineral compositions compared to open-ocean crusts. On average, CCM crusts contain more Fe relative to Mn than open-ocean Pacific crusts. The continental shelf and slope release both Fe and Mn under low-oxygen conditions. Silica is also enriched relative to Al compared to open-ocean crusts. This is due to the North Pacific silica plume and enrichment of Si along the path of deep-water circulation, resulting in Si enrichment in bottom and intermediate waters of the eastern Pacific.The CCM Fe-Mn crusts have a higher percentage of birnessite than open-ocean crusts, reflecting lower dissolved seawater oxygen that results from the intense coastal upwelling and proximity to zones of continental slope pore-water anoxia. Carbonate fluorapatite (CFA) is not present and CCM crusts do not show evidence of phosphatization, even in the older sections. The mineralogy indicates a suboxic environment under which birnessite forms, but in which pH is not high enough to facilitate CFA deposition. Growth rates of CCM crusts generally increase with increasing water depth, likely due to deep-water Fe sources mobilized from reduced shelf and slope sediments.Many elements of economic interest including Mn, Co, Ni, Cu, W, and Te have slightly or significantly lower concentrations in CCM crusts relative to crusts from the Pacific

Consequences of the low density of the lunar primary crust on its magmatic history (Invited)

NASA Astrophysics Data System (ADS)

Michaut, C.; Thorey, C.

2013-12-01

The lunar highlands are very old, with ages covering a timespan between 4.5 to 4.2 Gyr, and probably formed by flotation of light plagioclase minerals on top of the lunar magma ocean. The lunar crust provides thus an invaluable evidence of the geological and magmatic processes occurring in the first times of the terrestrial planets history. According to the last estimates from the GRAIL mission, the lunar primary crust is particularly light and relatively thick. This low-density crust acted as a barrier for the dense primary mantle melts. This is particularly evident in the fact that subsequent mare basalts erupted primarily within large impact basins: at least part of the crust must have been removed for the magma to reach the surface. However, the trajectory of the magma from the mantle to the surface is unknown. Here, we provide evidence of intrusions within the crust of the Moon as surface deformations in the form of low-slope lunar domes and floor-fractured craters. All these geological features have morphologies consistent with models of magma spreading at depth and deforming an overlying elastic layer. Furthermore, at floor-fractured craters, the deformation is contained within the crater interior, suggesting that the overpressure at the origin of magma ascent and intrusion was less than the pressure due to the weight of the crust removed by impact. The pressure release due to material removal by impact is significant over a depth equivalent to the crater radius. Because many of these floor-fractured craters are relatively small, i.e. less than 20 to 30 km in radius, this observation suggests that the magma at the origin of the intrusion was already stored within or just below the crust, in deeper intrusions. Thus, a large fraction of the mantle melt might have stored at depth below or within the light primary crust before reaching shallower layers. And hence, magma intrusions must have had a large influence on the thermal and geological evolution of the

Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.

PubMed

Wilde, S A; Valley, J W; Peck, W H; Graham, C M

2001-01-11

No crustal rocks are known to have survived since the time of the intense meteor bombardment that affected Earth between its formation about 4,550 Myr ago and 4,030 Myr, the age of the oldest known components in the Acasta Gneiss of northwestern Canada. But evidence of an even older crust is provided by detrital zircons in metamorphosed sediments at Mt Narryer and Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, where grains as old as approximately 4,276 Myr have been found. Here we report, based on a detailed micro-analytical study of Jack Hills zircons, the discovery of a detrital zircon with an age as old as 4,404+/-8 Myr--about 130 million years older than any previously identified on Earth. We found that the zircon is zoned with respect to rare earth elements and oxygen isotope ratios (delta18O values from 7.4 to 5.0%), indicating that it formed from an evolving magmatic source. The evolved chemistry, high delta18O value and micro-inclusions of SiO2 are consistent with growth from a granitic melt with a delta18O value from 8.5 to 9.5%. Magmatic oxygen isotope ratios in this range point toward the involvement of supracrustal material that has undergone low-temperature interaction with a liquid hydrosphere. This zircon thus represents the earliest evidence for continental crust and oceans on the Earth.

Biological Soil Crust Web Site

Science.gov Websites

www.soilcrust.org Crust 101 Advanced Gallery References CCERS site Links Biological Soil Crusts Textbook Corrections Level of Development Index Biological soil crusts are the community of organisms , mosses, liverworts and lichens. A Field Guide to Biological Soil Crusts of Western U.S. Drylands: Common

Biological Soil Crust Technical Reference

Science.gov Websites

Technical Reference Our understanding of the biology, ecology and physiology of biological soil crusts has published studies on soil crusts. The attached PDF file is a 90-page report that summarizes our current state of knowledge about biological soil crusts, with emphasis on crusts found in the western United

Genomic evidence for the Wood-Ljungdahl pathway for carbon fixation in warm basaltic ocean crust

NASA Astrophysics Data System (ADS)

Smith, A. R.; Fisk, M. R.; Mueller, R.; Colwell, F. S.; Mason, O. U.; Popa, R.

2016-12-01

Microbial life in the deep suboceanic aquifer can harness geochemical energy resulting from water-rock reactions and contribute to carbon cycling in the ocean via primary production, or chemosynthesis. Iron-bearing minerals such as olivine in oceanic crust can produce molecular hydrogen, small molecular weight hydrocarbons, and hydrogen sulfide as they react with seawater. Although this generally occurs in serpentinizing systems at very high temperatures deep in the subsurface, it has also been hypothesized to drive the subseafloor microbial ecosystems present in shallower basaltic aquifers. We present genome-based evidence for chemolithoautotrophic microbes present on the surface of olivine incubated in Juan de Fuca Ridge basaltic ocean crust for a 4-year period. These metagenome-derived genomes show dominant taxa capable of using both branches of the Wood-Ljungdahl pathway for carbon fixation and energy generation. This pathway uses molecular hydrogen potentially derived from the olivine surface as it reacts with seawater and CO2 which is inherent to seawater. These taxa were not reported from aquifer fluid samples, but have been found only in association with mineral surfaces in this study location. Most taxa in this simple community are distant relatives of cultured taxa; therefore this genome information is crucial to understanding how the subseafloor aquifer community is structured, how it obtains energy, how it cycles carbon, and gives us keys to help cultivate these organisms in the laboratory. Our findings also support the Subsurface Lithoautotrophic Microbial Ecosystem (SLiME) hypothesis and have implications for understanding life on early Earth and the potential for life in the Martian subsurface.

Anatexis, hybridization and the modification of ancient crust: Mesozoic plutonism in the Old Woman Mountains area, California

USGS Publications Warehouse

Miller, C.F.; Wooden, J.L.

1994-01-01

A compositionally expanded array of granitic (s.l.) magmas intruded the > 2 Ga crust of the Old Woman Mountains area between 160 and 70 Ma. These magmas were emplaced near the eastern (inland) edge of the Jurassic/Cretaceous arcs of western North America, in an area where magma flux, especially during the Jurassic, was considerably lower than to the west. The Jurassic intrusives and over half of the Cretaceous intrusives are predominantly metaluminous and variable in composition; a major Cretaceous suite comprises only peraluminous monzogranite. Only the Jurassic intrusions show clear evidence for the presence of mafic liquids. All units, including the most mafic rocks, reveal isotopic evidence for a significant crustal component. However, none of the Mesozoic intrusives matches in isotopic composition either average pre-intrusion crust or any major unit of the exposed crust. Elemental inconsistencies also preclude closed system derivation from exposed crust. Emplacement of these magmas, which doubled the volume of the mid- to upper crust, did not dramatically change its elemental composition. It did, however, affect its Nd and especially Sr isotopic composition and modify some of the distinctive aspects of the elemental chemistry. We propose that Jurassic magmatism was open-system, with a major influx of mantle-derived mafic magma interacting strongly with the ancient crust. Mesozoic crustal thickening may have led to closed-system crustal melting by the Late Cretaceous, but the deep crust had been profoundly modified by earlier Mesozoic hybridization so that crustal melts did not simply reflect the original crustal composition. The clear evidence for a crustal component in magmas of the Old Woman Mountains area may not indicate any fundamental differences from the processes at work elsewhere in this or other magmatic arcs where the role of pre-existing crust is less certain. Rather, a compositionally distinctive, very old crust may simply have yielded a more

Lithium in Jack Hills zircons: Evidence for extensive weathering of Earth's earliest crust

NASA Astrophysics Data System (ADS)

Ushikubo, Takayuki; Kita, Noriko T.; Cavosie, Aaron J.; Wilde, Simon A.; Rudnick, Roberta L.; Valley, John W.

2008-08-01

In situ Li analyses of 4348 to 3362 Ma detrital zircons from the Jack Hills, Western Australia by SIMS reveal that the Li abundances (typically 10 to 60 ppm) are commonly over 10,000 times higher than in zircons crystallized from mantle-derived magmas and in mantle-derived zircon megacrysts (typically < 2 ppb). High Li concentrations in zircons (10 to 250 ppm) have also been found in igneous zircons from three continental parent rocks: granites, Li-rich pegmatites, and migmatites in pelitic metasediment. The substitution of trivalent cations (REEs and Y) in zircon correlates with Li + 1 and P + 5 , suggesting that an interstitial site for Li, as well as the xenotime substitution for P, provides charge balance for REEs. Li is thus fixed in the zircon structure by coupled substitutions, and diffusive changes in [Li] composition are rate-limited by slow diffusion of REEs. The Jack Hills zircons also have fractionated lithium isotope ratios ( δ7Li = - 19 to + 13‰) about five times more variable than those recorded in primitive ocean floor basalts (2 to 8‰), but similar to continental crust and its weathering products. Values of δ7Li below - 10‰ are found in zircons that formed as early as 4300 Ma. The high Li compositions indicate that primitive magmas were not the source of Jack Hills zircons and the fractionated values of δ7Li suggest that highly weathered regolith was sampled by these early Archean magmas. These new Li data provide evidence that the parent magmas of ancient zircons from Jack Hills incorporated materials from the surface of the Earth that interacted at low temperature with liquid water. These data support the hypothesis that continental-type crust and oceans existed by 4300 Ma, within 250 million years of the formation of Earth and the low values of δ7Li suggest that weathering was extensive in the early Archean.

Raising the continental crust

NASA Astrophysics Data System (ADS)

Campbell, Ian H.; Davies, D. Rhodri

2017-02-01

The changes that occur at the boundary between the Archean and Proterozoic eons are arguably the most fundamental to affect the evolution of Earth's continental crust. The principal component of Archean continental crust is Granite-Greenstone Terranes (GGTs), with granites always dominant. The greenstones consist of a lower sequence of submarine komatiites and basalts, which erupted onto a pre-existing Tonalite-Trondhjemite-Granodiorite (TTG) crust. These basaltic rocks pass upwards initially into evolved volcanic rocks, such as andesites and dacites and, subsequently, into reworked felsic pyroclastic material and immature sediments. This transition coincides with widespread emplacement of granitoids, which stabilised (cratonised) the continental crust. Proterozoic supra-crustal rocks, on the other hand, are dominated by extensive flat-lying platform sequences of mature sediments, which were deposited on stable cratonic basements, with basaltic rocks appreciably less abundant. The siliceous TTGs cannot be produced by direct melting of the mantle, with most hypotheses for their origin requiring them to be underlain by a complimentary dense amphibole-garnet-pyroxenite root, which we suggest acted as ballast to the early continents. Ubiquitous continental pillow basalts in Archean lower greenstone sequences require the early continental crust to have been sub-marine, whereas the appearance of abundant clastic sediments, at higher stratigraphic levels, shows that it had emerged above sea level by the time of sedimentation. We hypothesise that the production of komatiites and associated basalts, the rise of the continental crust, widespread melting of the continental crust, the onset of sedimentation and subsequent cratonisation form a continuum that is the direct result of removal of the continent's dense amphibole-garnet-pyroxenite roots, triggered at a regional scale by the arrival of a mantle plume at the base of the lithosphere. Our idealised calculations suggest

The Oceanic Crust.

ERIC Educational Resources Information Center

Francheteau, Jean

1983-01-01

The earth's oceanic crust is created and destroyed in a flow outward from midocean ridges to subduction zones, where it plunges back into the mantle. The nature and dynamics of the crust, instrumentation used in investigations of this earth feature, and research efforts/findings are discussed. (JN)

Mesozoic invasion of crust by MORB-source asthenospheric magmas, U.S. Cordilleran interior

NASA Astrophysics Data System (ADS)

Leventhal, Janet A.; Reid, Mary R.; Montana, Art; Holden, Peter

1995-05-01

Mafic and ultramafic xenoliths entrained in lavas of the Cima volcanic field have Nd and Sr isotopic ratios indicative of a source similar to that of mid-ocean ridge basalt (MORB). Nd and Sr internal isochrons demonstrate a Late Cretaceous intrusion age. These results, combined with evidence for emplacement in the lower crust and upper mantle, indicate invasion of the lower crust by asthenospheric magmas in the Late Cretaceous. Constituting the first prima facie evidence for depleted-mantle magmatism in the Basin and Range province prior to late Cenozoic volcanism, these results lend key support to models suggesting crustal heating by ascent of asthenosphere in the Mesozoic Cordilleran interior.

Basin Excavation, Lower Crust, Composition, and Bulk Moon Mass balance in Light of a Thin Crust

NASA Technical Reports Server (NTRS)

Jolliff, B. L.; Korotev, R. L.; Ziegler, R. A.

2013-01-01

New lunar gravity results from GRAIL have been interpreted to reflect an overall thin and low-density lunar crust. Accordingly, crustal thickness has been modeled as ranging from 0 to 60 km, with thinnest crust at the locations of Crisium and Moscoviense basins and thickest crust in the central farside highlands. The thin crust has cosmochemical significance, namely in terms of implications for the Moon s bulk composition, especially refractory lithophile elements that are strongly concentrated in the crust. Wieczorek et al. concluded that the bulk Moon need not be enriched compared to Earth in refractory lithophile elements such as Al. Less Al in the crust means less Al has been extracted from the mantle, permitting relatively low bulk lunar mantle Al contents and low pre- and post-crust-extraction values for the mantle (or the upper mantle if only the upper mantle underwent LMO melting). Simple mass-balance calculations using the method of [4] suggests that the same conclusion might hold for Th and the entire suite of refractory lithophile elements that are incompatible in olivine and pyroxene, including the KREEP elements, that are likewise concentrated in the crust.

Introduction to Biological Soil Crusts

Science.gov Websites

Introduction to Biological Soil Crusts In more arid regions, vegetative cover is generally sparse. Open spaces are usually covered by biological soil crusts, a highly specialized community of cyanobacteria, mosses , and lichens (Figure 1). Biological soil crusts are commonly found in semiarid and arid environments

Collisional stripping of planetary crusts

NASA Astrophysics Data System (ADS)

Carter, Philip J.; Leinhardt, Zoë M.; Elliott, Tim; Stewart, Sarah T.; Walter, Michael J.

2018-02-01

Geochemical studies of planetary accretion and evolution have invoked various degrees of collisional erosion to explain differences in bulk composition between planets and chondrites. Here we undertake a full, dynamical evaluation of 'crustal stripping' during accretion and its key geochemical consequences. Crusts are expected to contain a significant fraction of planetary budgets of incompatible elements, which include the major heat producing nuclides. We present smoothed particle hydrodynamics simulations of collisions between differentiated rocky planetesimals and planetary embryos. We find that the crust is preferentially lost relative to the mantle during impacts, and we have developed a scaling law based on these simulations that approximates the mass of crust that remains in the largest remnant. Using this scaling law and a recent set of N-body simulations of terrestrial planet formation, we have estimated the maximum effect of crustal stripping on incompatible element abundances during the accretion of planetary embryos. We find that on average approximately one third of the initial crust is stripped from embryos as they accrete, which leads to a reduction of ∼20% in the budgets of the heat producing elements if the stripped crust does not reaccrete. Erosion of crusts can lead to non-chondritic ratios of incompatible elements, but the magnitude of this effect depends sensitively on the details of the crust-forming melting process on the planetesimals. The Lu/Hf system is fractionated for a wide range of crustal formation scenarios. Using eucrites (the products of planetesimal silicate melting, thought to represent the crust of Vesta) as a guide to the Lu/Hf of planetesimal crust partially lost during accretion, we predict the Earth could evolve to a superchondritic 176Hf/177Hf (3-5 parts per ten thousand) at present day. Such values are in keeping with compositional estimates of the bulk Earth. Stripping of planetary crusts during accretion can lead to

Experimental evidence for use of aluminum in zircon as a new tracer to distinguish peraluminous and metaluminous melts

NASA Astrophysics Data System (ADS)

Wang, Y.; Trail, D.

2017-12-01

Aluminum (Al) is the third most abundant rock-forming element in Earth's crust. It also substitutes into the zircon lattice as trace impurity. For zircons that lack parent rock information, e.g. Hadean zircon (>4.0 Ga), Al in zircon could be a potentially useful tool to investigate the formation and evolution of the parental magma. Aluminum saturation index (ASI; molar ratio of Al2O3/(CaO+Na2O+K2O)) is widely used to distinguish peraluminous rocks (ASI>1) from metaluminous rocks (ASI<1). We grew zircons in piston cylinder from granitic melts, with different ASI values but same temperature, pressure and water content. Synthetic zircons with variable sizes, as large as 20 µm, were observed under SEM and analyzed using LA-ICP-MS. Our analyses reveal that the Al content of zircons that crystalized from a peraluminous melt (ASI=1.25) is, on average, one magnitude higher than crystals from peralkaline melt (ASI=0.5), e.g., 100s of ppm, vs. 10s of ppm. This indicates that aluminum solubility in zircon is related to the ASI value of parental melt. This experimental result agrees with studies of natural zircons, where Al contents of peraluminous zircons ( 10 ppm) are an order of magnitude higher than zircons from metaluminous rocks ( 1.3 ppm) (Trail et al., 2017). We suggest that melts with ASI>1 can result in an elevated alumina activity, which leads to zircons with elevated Al contents. The notably higher Al concentration in experimental zircons (10s to 100s ppm), compared with natural zircons (1 to 10 ppm), could be explained by the much higher crystallization temperature, e.g., >1100°C vs. <900°C. Both field and experimental studies support the notion that a relationship exists between the Al content in zircon and the melt ASI value. This conclusion has several meaningful applications: 1) it may be used to infer source rock characteristics of Hadean zircons, and to shed light on the formation and evolution of the earliest crust; 2) it may provide additional

Weathering crusts on peridotite

NASA Astrophysics Data System (ADS)

Bucher, Kurt; Stober, Ingrid; Müller-Sigmund, Hiltrud

2015-05-01

Chemical weathering of dark-green massive peridotite, including partly serpentinized peridotite, produces a distinct and remarkable brown weathering rind when exposed to the atmosphere long enough. The structure and mineral composition of crusts on rocks from the Ronda peridotite, Spain, have been studied in some detail. The generic overall weathering reaction serpentinized peridotite + rainwater = weathering rind + runoff water describes the crust-forming process. This hydration reaction depends on water supply from the outcrop surface to the reaction front separating green peridotite from the brown crust. The reaction pauses after drying and resumes at the front after wetting. The overall net reaction transforms olivine to serpentine in a volume-conserving replacement reaction. The crust formation can be viewed as secondary serpentinization of peridotite that has been strongly altered by primary hydrothermal serpentinization. The reaction stoichiometry of the crust-related serpentinization is preserved and reflected by the composition of runoff waters in the peridotite massif. The brown color of the rind is caused by amorphous Fe(III) hydroxide, a side product from the oxidation of Fe(II) released by the dissolution of fayalite component in olivine.

Timing the oxidation of Earth's crust: Evidence from big data records of manganese mineralization

NASA Astrophysics Data System (ADS)

Hummer, D. R.; Golden, J. J.; Hystad, G.; Downs, R. T.; Eleish, A.; Liu, C.; Ralph, J.; Morrison, S.; Meyer, M.; Hazen, R. M.

2017-12-01

A great deal of work has focused on unravelling the oxygenation of Earth's early atmosphere and oceans, which took place during and after the Great Oxidation Event (1). Recently, field and experimental methods have also been used to examine the timing of mantle oxidation, especially near subduction zones (2). However, very little information is available on the timing of crustal oxidation. To examine the oxidation of Earth's shallow crust, we analyzed records of manganese (Mn) mineral occurrences across geologic time from a database of 2666 mineral-locality data pairs (mindat.org as of 20 Nov. 2015) that had associated geologic ages in the literature. Manganese is a redox-sensitive transition element with oxidation states of +2, +3, and +4, whose average oxidation state in the geologic record can be used as a proxy for the oxygenation of the shallow crust, where Mn mineralization typically occurs. Analysis revealed that Mn mineralization older than 600 Ma contained mostly Mn2+ mineral species, with isolated localities containing Mn3+ and Mn4+ species. During the Phanerozoic, the average oxidation state of Mn follows the same trend as reconstructions of atmospheric oxygen (3), but on a 66+1 Myr delay (as calculated using a least squares fitting procedure). This contrasts with a delay of hundreds of millions of years for the oxidation of molybdenum, which forms much deeper in the crust (4). We interpret these time lags as the time necessary to equilibrate various crustal depths to atmospheric oxygen fugacity through infiltration of oxidizing fluids and tectonic mixing processes. Analysis of other redox-sensitive transition metals (such as Cr, V, and Fe) using big data techniques may reveal a strategy for timing the oxidation of different portions of Earth's crust. (1) T.W. Lyons, C.T. Reinhard, N.J. Planavsky, Nature 506, 307-315 (2014). (2) M. Brounce, et al. Geology 43, 775-778 (2015). (3) N.M. Bergman, T.M. Lenton, A.J. Watson, Am. J. Sci. 304, 397-437 (2004). (4

Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc

NASA Astrophysics Data System (ADS)

Freymuth, Heye; Ivko, Ben; Gill, James B.; Tamura, Yoshihiko; Elliott, Tim

2016-08-01

We address the question of whether melting of the mafic oceanic crust occurs beneath ordinary volcanic arcs using constraints from U-Series (238U/232Th, 230Th/232Th and 226Ra/230Th) measurements. Alteration of the top few hundred meters of the mafic crust leads to strong U enrichment. Via decay of 238U to 230Th, this results in elevated (230Th/232Th) (where brackets indicate activity ratios) over time-scales of ∼350 ka. This process leads to the high (230Th/232Th), between 2.6 and 11.0 in the mafic altered oceanic crust (AOC) sampled at ODP Sites 801 and 1149 near the Izu-Bonin-Mariana arc. Th activity ratios in the Izu arc lavas range from (230Th/232Th) = 1.2-2.0. These values are substantially higher than those in bulk sediment subducting at the Izu trench and also extend to higher values than in mid-ocean ridge basalts and the Mariana arc. We show that the range in Th isotope ratios in the Izu arc lavas is consistent with the presence of a slab melt from a mixed source consisting of AOC and subducted sediments with an AOC mass fraction of up to approximately 80 wt.% in the component added to the arc lava source. The oceanic plate subducting at the Izu arc is comparatively cold which therefore indicates that temperatures high enough for fluid-saturated melting of the AOC are commonly achieved beneath volcanic arcs. The high ratio of AOC/sediments of the slab melt component suggested for the Izu arc lavas requires preferential melting of the AOC. This can be achieved when fluid-saturated melting of the slab is triggered by fluids derived from underlying subducted serpentinites. Dehydration of serpentinites and migration of the fluid into the overlying crust causes melting to start within the AOC. The absence of a significant sediment melt component suggests there was insufficient water to flux both AOC and overlying sediments.

The extent of continental crust beneath the Seychelles

NASA Astrophysics Data System (ADS)

Hammond, J. O. S.; Kendall, J.-M.; Collier, J. S.; Rümpker, G.

2013-11-01

The granitic islands of the Seychelles Plateau have long been recognised to overlie continental crust, isolated from Madagascar and India during the formation of the Indian Ocean. However, to date the extent of continental crust beneath the Seychelles region remains unknown. This is particularly true beneath the Mascarene Basin between the Seychelles Plateau and Madagascar and beneath the Amirante Arc. Constraining the size and shape of the Seychelles continental fragment is needed for accurate plate reconstructions of the breakup of Gondwana and has implications for the processes of continental breakup in general. Here we present new estimates of crustal thickness and VP/VS from H-κ stacking of receiver functions from a year long deployment of seismic stations across the Seychelles covering the topographic plateau, the Amirante Ridge and the northern Mascarene Basin. These results, combined with gravity modelling of historical ship track data, confirm that continental crust is present beneath the Seychelles Plateau. This is ˜30-33 km thick, but with a relatively high velocity lower crustal layer. This layer thins southwards from ˜10 km to ˜1 km over a distance of ˜50 km, which is consistent with the Seychelles being at the edge of the Deccan plume prior to its separation from India. In contrast, the majority of the Seychelles Islands away from the topographic plateau show no direct evidence for continental crust. The exception to this is the island of Desroche on the northern Amirante Ridge, where thicker low density crust, consistent with a block of continental material is present. We suggest that the northern Amirantes are likely continental in nature and that small fragments of continental material are a common feature of plume affected continental breakup.

Mobility of partially molten crust, heat and mass transfer, and the stabilization of continents

NASA Astrophysics Data System (ADS)

Teyssier, Christian; Whitney, Donna L.; Rey, Patrice F.

2017-04-01

The core of orogens typically consists of migmatite terrains and associated crustal-derived granite bodies (typically leucogranite) that represent former partially molten crust. Metamorphic investigations indicate that migmatites crystallize at low pressure (cordierite stability) but also contain inclusions of refractory material (mafic, aluminous) that preserve evidence of crystallization at high pressure (HP), including HP granulite and eclogite (1.0-1.5 GPa), and in some cases ultrahigh pressure (2.5-3.0 GPa) when the continental crust was subducted (i.e. Norwegian Caledonides). These observations indicate that the partially molten crust originates in the deep crust or at mantle depths, traverses the entire orogenic crust, and crystallizes at shallow depth, in some cases at the near-surface ( 2 km depth) based on low-T thermochronology. Metamorphic assemblages generally show that this nearly isothermal decompression is rapid based on disequilibrium textures (symplectites). Therefore, the mobility of partially molten crust results in one of the most significant heat and mass transfer mechanisms in orogens. Field relations also indicate that emplacement of partially molten crust is the youngest major event in orogeny, and tectonic activity essentially ceases after the partially molten crust is exhumed. This suggests that flow and emplacement of partially molten crust stabilize the orogenic crust and signal the end of orogeny. Numerical modeling (open source software Underworld; Moresi et al., 2007, PEPI 163) provides useful insight into the mechanisms of exhumation of partially molten crust. For example, extension of thickened crust with T-dependent viscosity shows that extension of the shallow crust initially drives the mobility of the lowest viscosity crust (T>700°C), which begins to flow in a channel toward the zone of extension. This convergent flow generates channel collision and the formation of a double-dome of foliation (two subdomes separated by a steep

Recycling lower continental crust in the North China craton.

PubMed

Gao, Shan; Rudnick, Roberta L; Yuan, Hong-Ling; Liu, Xiao-Ming; Liu, Yong-Sheng; Xu, Wen-Liang; Ling, Wen-Li; Ayers, John; Wang, Xuan-Che; Wang, Qing-Hai

2004-12-16

Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

N-MORB crust beneath Fuerteventura in the easternmost part of the Canary Islands: evidence from gabbroic xenoliths

NASA Astrophysics Data System (ADS)

Neumann, Else-Ragnhild; Vannucci, Riccardo; Tiepolo, Massimo

2005-09-01

Gabbro xenoliths reported in this paper were collected in northern Fuerteventura, the Canary Island located closest to the coast of Africa. The xenoliths are very fresh and consist of Ti-Al-poor clinopyroxene + plagioclase (An87-67) + olivine (Fo72-86) ± orthopyroxene. Clinopyroxene and orthopyroxene are constantly and markedly depleted in light rare earth elements (LREE) relative to heavy REE (HREE), as expected for cumulus minerals formed from highly refractory N-MORB-type melts. In contrast, whole-rock Primordial Mantle-normalized trace element patterns range from mildly S-shaped (mildly depleted in Pr-Sm relative to both the strongly incompatible elements Rb-La and the HREE) to enriched. Estimates show that the trace element compositions of the rocks and their minerals are compatible with formation as N-MORB gabbro cumulates, which have been infiltrated at various extents (≤1% to >5%) by enriched alkali basaltic melts. The enriched material is mainly concentrated along grain boundaries and cracks through mineral grains, suggesting that the infiltration is relatively recent, and is thus associated with the Canary Islands magmatism. Our data contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean. No evidence of continental material that might reflect attenuated continental crust in the area has been found. Gabbro xenoliths with REE and trace element compositions similar to those exhibited by the Fuerteventura gabbros are also found among gabbro xenoliths from the islands of La Palma (western Canary Islands) and Lanzarote. The compositions of the most depleted samples from these islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. The presence

Density Sorting During the Evolution of Continental Crust

NASA Astrophysics Data System (ADS)

Kelemen, P. B.; Behn, M. D.; Hacker, B. R.

2015-12-01

We consider two settings - in addition to "delamination" of arc lower crust - in which dense, mafic eclogites founder into the convecting mantle while buoyant, felsic lithologies accumulate at the base of evolving continental crust. Arc processes play a central role in generating continental crust, but it remains uncertain how basaltic arc crust is transformed to andesitic continental crust. Dense, SiO2-poor products of fractionation may founder from the base of arc crust by "delamination", but lower arc crust after delamination has significantly different trace elements compared to lower continental crust (LCC). In an alternative model, buoyant magmatic rocks generated at arcs are first subducted, mainly via subduction erosion. Upon heating, these buoyant lithologies ascend through the mantle wedge or along a subduction channel, and are "relaminated" at
the base of overlying crust (e.g., Hacker et al EPSL 11, AREPS 15). Average buoyant lavas and plutons
for the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs fall within the range of estimated LCC major and trace elements. Relamination is more efficient in generating continental crust than delamination. Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle. There is a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across the region where Tibetan crust overlies Indian crust, into thickened Tibetan crust. About half the subducted Indian crust is present, whereas the other half is missing. Data (Vp/Vs; Miocene lavas formed by interaction of continental crust with mantle; xenolith thermometry) indicate 1000°C or more from ca 50 km depth to the Moho since the Miocene. We build on earlier studies (LePichon et al Tectonics 92, T'phys 97; Schulte-Pelkum et al Nature 05; Monsalve et al JGR 08) to advance the hypothesis that rapid growth of garnet occurs at 70-80 km and 1000°C within subducting Indian crust. Dense eclogites founder

Granitoid formation is ineffective in isotopically homogenizing continental crust: Evidence from archean rocks of the Wind River Mountains, Wyoming

DOE Office of Scientific and Technical Information (OSTI.GOV)

Frost, C.D.; Hulsebosch, T.P.; Chamberlain, K.R.

1992-01-01

The Archean core of the Laramide Wind River uplift records evidence of at least three major granitoid-forming episodes. The oldest, the Dry Creek gneiss (DCG), was emplaced by 2.8 Ga and occupies the northeastern part of the range. Mafic, pelitic and ultramafic inclusions occur in the DCG. Elsewhere in the Wind River Mountains there is evidence for crustal components as old as 3.8 Ga. The Bridger batholith (BB), intruded at 2.67 Ga, is found in the west-central Wind River Mountains. The Wind River batholith (WRB) refers to the youngest Late Archean granodiorites and granites which are found throughout the rangemore » and includes granitoids previously name the Louis Lake, Bears Ears, Popo Agie, and Middle Mountain intrusions. Although granitoids of the Wind River batholith have been dated at 2.63 and 2.55 Ga, they are considered together here because there is a complete gradation in rock type and because definite intrusive contacts are scarce. The DCG, BB, and WRB each span the metaluminous/peraluminous boundary and are indistinguishable on Harker diagrams. Each has variable trace element and isotopic characteristics which do not correlate with silica content. Although the isotopic characteristics of these granitoids may be explained by mixing of variable amounts of preexisting continental crust and contemporary depleted mantle, this hypothesis is difficult to reconcile with the evolved nature of even those samples with the most mantle-like isotopic signatures. The authors suggest that each of these granitoid batholiths was formed primarily by remelting of pre-existing heterogeneous continental crust, and that the granite-forming process was not effective in obliterating these trace element and isotopic heterogeneities. Isotopic homogeneity in granitoid batholiths may reflect the isotopic homogeneity of their sources rather than an effective magmatic mixing process.« less

The Deep Crust Magmatic Refinery, Part 2 : The Magmatic Output of Numerical Models.

NASA Astrophysics Data System (ADS)

Bouilhol, P.; Riel, N., Jr.; Van Hunen, J.

2016-12-01

Metamorphic and magmatic processes occurring in the deep crust ultimately control the chemical and physical characteristic of the continental crust. A complex interplay between magma intrusion, crystallization, and reaction with the pre-existing crust provide a wide range of differentiated magma and cumulates (and / or restites) that will feed the upper crustal levels with evolved melt while constructing the lower crust. With growing evidence from field and experimental studies, it becomes clearer that crystallization and melting processes are non-exclusive but should be considered together. Incoming H2O bearing mantle melts will start to fractionate to a certain extent, forming cumulates but also releasing heat and H2O to the intruded host-rock allowing it to melt in saturated conditions. The end-result of such dynamic system is a function of the amount and composition of melt input, and extent of reaction with the host which is itself dependent on the migration mode of the melts. To better constrain lower crust processes, we have built up a numerical model [see Riel et al. associated abstract for methods] to explore different parameters, unravelling the complex interplay between melt percolation / crystallization and degassing / re-melting in a so called "hot zone" model. We simulated the intrusion of water bearing mantle melts at the base of an amphibolitized lower crust during a magmatic event that lasts 5 Ma. We varied several parameters such as Moho depth and melt rock ratio to better constrain what controls the final melt / lower crust composition.. We show the evolution of the chemical characteristics of the melt that escape the system during this magmatic event, as well as the resulting lower crust characteristics. We illustrate how the evolution of melt major elements composition reflects the progressive replacement of the crust towards compositions that are dominated by the mantle melt input. The resulting magmas cover a wide range of composition from

Simulations of the Neutron Star Crust

NASA Astrophysics Data System (ADS)

Schramm, Stefan; Nandi, Rana

The properties of the neutron star crust are crucially important for many physical processes occurring in the star. For instance, the crustal transport coefficients define the temperature evolution of accreting stars after bursts, which can be compared to observation. Furthermore, the structure of the inner crust can modify the neutrino transport through the matter considerably, significantly impacting the dynamics of supernova explosions. Therefore, we perform numerical studies of the inner crust, and among other aspects, investigate the dependence of the pasta phase on the isospin properties of the nuclear interactions. To this end we developed an efficient computer code to simulate the inner and outer crust using molecular dynamics techniques. First results of the simulations and insights into the crust-core transition are presented.

Evolution of the Archean Mohorovičić discontinuity from a synaccretionary 4.5 Ga protocrust

NASA Astrophysics Data System (ADS)

Hamilton, Warren B.

2013-12-01

This review evaluates and rejects the currently dominant dogmas of geodynamics and geochemistry, which are based on 1950s-1970s assumptions of a slowly differentiating Earth. Evidence is presented for evolution of mantle, crust, and early Moho that began with fractionation of most crustal components, synchronously with planetary accretion, into mafic protocrust by ~ 4.5 Ga. We know little about Hadean crustal geology (> 3.9 Ga) except that felsic rocks were then forming, but analogy with Venus, and dating from the Moon, indicate great shallow disruption by large and small impact structures, including huge fractionated impact-melt constructs, throughout that era. The mantle sample and Archean (< 3.9 Ga) crustal geology integrate well. The shallow mantle was extremely depleted by early removal of thick mafic protocrust, which was the primary source of the tonalite, trondhjemite, and granodiorite (TTG) that dominate preserved Archean crust to its base, and of the thick mafic volcanic rocks erupted on that crust. Lower TTG crust, kept mobile by its high radioactivity and by insulating upper crust, rose diapirically into the upper crust as dense volcanic rocks sagged synformally. The mobile lower crust simultaneously flowed laterally to maintain subhorizontal base and surface, and dragged overlying brittler granite-and-greenstone upper crust. Petrologically required garnet-rich residual protocrust incrementally delaminated, sank through low-density high-mantle magnesian dunite, and progressively re-enriched upper mantle, mostly metasomatically. Archean and earliest Proterozoic craton stabilization and development of final Mohos followed regionally complete early delamination of residual protocrust, variously between ~ 2.9 and 2.2 Ga. Where some protocrust remained, Proterozoic basins, filled thickly by sedimentary and volcanic rocks, developed on Archean crust, beneath which delamination of later residual protocrust continued top-down enrichment of upper mantle. That

Numerical investigation of deep-crust behavior under lithospheric extension

NASA Astrophysics Data System (ADS)

Korchinski, Megan; Rey, Patrice F.; Mondy, Luke; Teyssier, Christian; Whitney, Donna L.

2018-02-01

What are the conditions under which lithospheric extension drives exhumation of the deep orogenic crust during the formation of gneiss domes? The mechanical link between extension of shallow crust and flow of deep crust is investigated using two-dimensional numerical experiments of lithospheric extension in which the crust is 60 km thick and the deep-crust viscosity and density parameter space is explored. Results indicate that the style of extension of the shallow crust and the path, magnitude, and rate of flow of deep crust are dynamically linked through the deep-crust viscosity, with density playing an important role in experiments with a high-viscosity deep crust. Three main groups of domes are defined based on their mechanisms of exhumation across the viscosity-density parameter space. In the first group (low-viscosity, low-density deep crust), domes develop by lateral and upward flow of the deep crust at km m.y-1 velocity rates (i.e. rate of experiment boundary extension). In this case, extension in the shallow crust is localized on a single interface, and the deep crust traverses the entire thickness of the crust to the Earth's near-surface in 5 m.y. This high exhuming power relies on the dynamic feedback between the flow of deep crust and the localization of extension in the shallow crust. The second group (intermediate-viscosity, low-density deep crust) has less exhuming power because the stronger deep crust flows less readily and instead accommodates more uniform extension, which imparts distributed extension to the shallow crust. The third group represents the upper limits of viscosity and density for the deep crust; in this case the low buoyancy of the deep crust results in localized thinning of the crust with large upward motion of the Moho and lithosphere-asthenosphere boundary. These numerical experiments test the exhuming power of the deep crust in the formation of extensional gneiss domes.

Microbial habitability of the Hadean Earth during the late heavy bombardment.

PubMed

Abramov, Oleg; Mojzsis, Stephen J

2009-05-21

Lunar rocks and impact melts, lunar and asteroidal meteorites, and an ancient martian meteorite record thermal metamorphic events with ages that group around and/or do not exceed 3.9 Gyr. That such a diverse suite of solar system materials share this feature is interpreted to be the result of a post-primary-accretion cataclysmic spike in the number of impacts commonly referred to as the late heavy bombardment (LHB). Despite its obvious significance to the preservation of crust and the survivability of an emergent biosphere, the thermal effects of this bombardment on the young Earth remain poorly constrained. Here we report numerical models constructed to probe the degree of thermal metamorphism in the crust in the effort to recreate the effect of the LHB on the Earth as a whole; outputs were used to assess habitable volumes of crust for a possible near-surface and subsurface primordial microbial biosphere. Our analysis shows that there is no plausible situation in which the habitable zone was fully sterilized on Earth, at least since the termination of primary accretion of the planets and the postulated impact origin of the Moon. Our results explain the root location of hyperthermophilic bacteria in the phylogenetic tree for 16S small-subunit ribosomal RNA, and bode well for the persistence of microbial biospheres even on planetary bodies strongly reworked by impacts.

Generation of felsic crust in the Archean: a geodynamic modeling perspective

NASA Astrophysics Data System (ADS)

Sizova, Elena; Gerya, Taras; Stüwe, Kurt; Brown, Michael

2015-04-01

The relevance of contemporary tectonics to the formation of the Archean terrains is a matter of vigorous debate. Higher mantle temperatures and higher radiogenic heat production in the past would have impacted on the thickness and composition of the oceanic and continental crust. As a consequence of secular cooling, there is generally no modern analog to assist in understanding the tectonic style that may have operated in the Archean. For this reason, well-constrained numerical modeling, based on the fragmentary evidence preserved in the geological record, is the most appropriate tool to evaluate hypotheses of Archean crust formation. The main lithology of Archean terrains is the sodic tonalite-trondhjemite-granodiorite (TTG) suite. Melting of hydrated basalt at garnet-amphibolite to eclogite facies conditions is considered to be the dominant process for the generation of the Archean TTG crust. Taking into account geochemical signatures of possible mantle contributions to some TTGs, models proposed for the formation of Archean crust include subduction, melting at the bottom of thickened continental crust and fractional crystallization of mantle-derived melts under water-saturated conditions. We evaluated these hypotheses using a 2D coupled petrological-thermomechanical numerical model with initial conditions appropriate to the Eoarchean-Mesoarchean. As a result, we identified three tectonic settings in which intermediate to felsic melts are generated by melting of hydrated primitive basaltic crust: 1) delamination and dripping of the lower primitive basaltic crust into the mantle; 2) local thickening of the primitive basaltic crust; and, 3) small-scale crustal overturns. In addition, we consider remelting of the fractionated products derived from underplated dry basalts as an alternative mechanism for the formation of some Archean granitoids. In the context of a stagnant lid tectonic regime which is intermittently terminated by short-lived subduction, we identified

A search for thermal excursions from ancient extraterrestrial impacts using Hadean zircon Ti-U-Th-Pb depth profiles.

PubMed

Abbott, Sunshine S; Harrison, T Mark; Schmitt, Axel K; Mojzsis, Stephen J

2012-08-21

Few terrestrial localities preserve more than a trace lithic record prior to ca. 3.8 Ga greatly limiting our understanding of the first 700 Ma of Earth history, a period inferred to have included a spike in the bolide flux to the inner solar system at ca. 3.85-3.95 Ga (the Late Heavy Bombardment, LHB). An accessible record of this era may be found in Hadean detrital zircons from the Jack Hills, Western Australia, in the form of μm-scale epitaxial overgrowths. By comparing crystallization temperatures of pre-3.8 Ga zircon overgrowths to the archive of zircon temperature spectra, it should, in principle, be possible to identify a distinctive impact signature. We have developed Ti-U-Th-Pb ion microprobe depth profiling to obtain age and temperature information within these zircon overgrowths and undertaken a feasibility study of its possible use in identifying impact events. Of eight grains profiled in this fashion, four have overgrowths of LHB-era age. Age vs. temperature profiles reveal a period between ca. 3.85-3.95 Ga (i.e., LHB era) characterized by significantly higher temperatures (approximately 840-875 °C) than do older or younger zircons or zircon domains (approximately 630-750 °C). However, temperatures approaching 900 °C can result in Pb isotopic exchange rendering interpretation of these profiles nonunique. Coupled age-temperature depth profiling shows promise in this role, and the preliminary data we report could represent the first terrestrial evidence for impact-related heating during the LHB.

Titanium isotopic evidence for felsic crust and plate tectonics 3.5 billion years ago.

PubMed

Greber, Nicolas D; Dauphas, Nicolas; Bekker, Andrey; Ptáček, Matouš P; Bindeman, Ilya N; Hofmann, Axel

2017-09-22

Earth exhibits a dichotomy in elevation and chemical composition between the continents and ocean floor. Reconstructing when this dichotomy arose is important for understanding when plate tectonics started and how the supply of nutrients to the oceans changed through time. We measured the titanium isotopic composition of shales to constrain the chemical composition of the continental crust exposed to weathering and found that shales of all ages have a uniform isotopic composition. This can only be explained if the emerged crust was predominantly felsic (silica-rich) since 3.5 billion years ago, requiring an early initiation of plate tectonics. We also observed a change in the abundance of biologically important nutrients phosphorus and nickel across the Archean-Proterozoic boundary, which might have helped trigger the rise in atmospheric oxygen. Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Earthquakes in Stable Continental Crust.

ERIC Educational Resources Information Center

Johnston, Arch C.; Kanter, Lisa R.

1990-01-01

Discussed are some of the reasons for earthquakes which occur in stable crust away from familiar zones at the ends of tectonic plates. Crust stability and the reactivation of old faults are described using examples from India and Australia. (CW)

Experimental and geochemical evidence for derivation of the El Capitan Granite, California, by partial melting of hydrous gabbroic lower crust

USGS Publications Warehouse

Ratajeski, K.; Sisson, T.W.; Glazner, A.F.

2005-01-01

Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850??C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635-661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486-1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening. ?? Springer-Verlag 2005.

Hydrogenetic Ferromanganese Crusts of the California Continental Margin

NASA Astrophysics Data System (ADS)

Conrad, Tracey A.

Hydrogenetic Ferromanganese (Fe-Mn) crusts grow from seawater and in doing so sequester elements of economic interest and serve as archives of past seawater chemistry. Ferromanganese crusts have been extensively studied in open-ocean environments. However, few studies have examined continent-proximal Fe-Mn crusts especially from the northeast Pacific. This thesis addresses Fe-Mn crusts within the northeast Pacific California continental margin (CCM), which is a dynamic geological and oceanographic environment. In the first of three studies, I analyzed the chemical and mineralogical composition of Fe-Mn crusts and show that continental-proximal processes greatly influence the chemistry and mineralogy of CCM Fe-Mn crusts. When compared to global open-ocean Fe-Mn crusts, CCM crusts have higher concentrations of iron, silica, and thorium with lower concentrations of many elements of economic interest including manganese, cobalt, and tellurium, among other elements. The mineralogy of CCM Fe-Mn crusts is also unique with more birnessite and todorokite present than found in open-ocean samples. Unlike open-ocean Fe-Mn crusts, carbonate-fluorapatite is not present in CCM crusts. This lack of phosphatization makes CCM Fe-Mn crusts excellent candidates for robust paleoceanography records. The second and third studies in this thesis use isotope geochemistry on select CCM Fe-Mn crusts from four seamounts in the CCM to study past terrestrial inputs into the CCM and sources and behavior of Pb and Nd isotopes over the past 7 million years along the northeast Pacific margin. The second study focuses on riverine inputs into the Monterey Submarine Canyon System and sources of the continental material. Osmium isotopes in the crusts are compared to the Cenozoic Os seawater curve to develop an age model for the samples that show the crusts range in age of initiation of crust growth from approximately 20 to 6 Myr. Lead and neodymium isotopes measured in select Fe-Mn crusts show that

Evidence for strong lateral seismic velocity variation in the lower crust and upper mantle beneath the California margin

USGS Publications Warehouse

Lai, Voon; Graves, Robert; Wei, Shengji; Helmberger, Don

2017-01-01

Regional seismograms from earthquakes in Northern California show a systematic difference in arrival times across Southern California where long period (30–50 seconds) SH waves arrive up to 15 seconds earlier at stations near the coast compared with sites towards the east at similar epicentral distances. We attribute this time difference to heterogeneity of the velocity structure at the crust-mantle interface beneath the California margin. To model these observations, we propose a fast seismic layer, with thickness growing westward from the San Andreas along with a thicker and slower continental crust to the east. Synthetics generated from such a model are able to match the observed timing of SH waveforms better than existing 3D models. The presence of a strong upper mantle buttressed against a weaker crust has a major influence in how the boundary between the Pacific plate and North American plate deforms and may explain the observed asymmetric strain rate across the boundary.

Mechanical and statistical evidence of the causality of human-made mass shifts on the Earth's upper crust and the occurrence of earthquakes

NASA Astrophysics Data System (ADS)

Klose, Christian D.

2013-01-01

A global catalog of small- to large-sized earthquakes was systematically analyzed to identify causality and correlatives between human-made mass shifts in the upper Earth's crust and the occurrence of earthquakes. The mass shifts, ranging between 1 kt and 1 Tt, result from large-scale geoengineering operations, including mining, water reservoirs, hydrocarbon production, fluid injection/extractions, deep geothermal energy production and coastal management. This article shows evidence that geomechanical relationships exist with statistical significance between (a) seismic moment magnitudes M of observed earthquakes, (b) lateral distances of the earthquake hypocenters to the geoengineering "operation points" and (c) mass removals or accumulations on the Earth's crust. Statistical findings depend on uncertainties, in particular, of source parameter estimations of seismic events before instrumental recoding. Statistical observations, however, indicate that every second, seismic event tends to occur after a decade. The chance of an earthquake to nucleate after 2 or 20 years near an area with a significant mass shift is 25 or 75 %, respectively. Moreover, causative effects of seismic activities highly depend on the tectonic stress regime in which the operations take place (i.e., extensive, transverse or compressive). Results are summarized as follows: First, seismic moment magnitudes increase the more mass is locally shifted on the Earth's crust. Second, seismic moment magnitudes increase the larger the area in the crust is geomechanically polluted. Third, reverse faults tend to be more trigger-sensitive than normal faults due to a stronger alteration of the minimum vertical principal stress component. Pure strike-slip faults seem to rupture randomly and independently from the magnitude of the mass changes. Finally, mainly due to high estimation uncertainties of source parameters and, in particular, of shallow seismic events (<10 km), it remains still very difficult to

Growth of the lower continental crust via the relamination of arc magma

NASA Astrophysics Data System (ADS)

He, Yumei; Zheng, Tianyu; Ai, Yinshuang; Hou, Guangbing; Chen, Qi-Fu

2018-01-01

How does continental crust transition from basaltic mantle-derived magmas into an andesitic composition? The relamination hypothesis has been presented as an alternative dynamical mechanism to classical delamination theory to explain new crust generation and has been supported by petrological and geochemical studies as well as by thermomechanical numerical modeling. However, direct evidence of this process from detailed seismic velocity structures is lacking. Here, we imaged the three-dimensional (3D) velocity structures of the crust and uppermost mantle beneath the geologically stable Ordos terrane of the North China Craton (NCC). We identify a region of continental crust that exhibits extreme growth using teleseismic data and an imaging technique that models the Common Conversion Point (CCP) stacking profiles. Our results show an approximately 400 × 400 km2 wide growth zone that underlies the primitive crust at depths of 30-50 km and exhibits a gradual increase of velocity with depth. The upper layer of the growth zone has a shear wave velocity of 3.6-3.9 km/s (Vp = 6.2-6.8 km/s), indicating felsic material, and the lower layer has a shear wave velocity of 4.1-4.3 km/s (Vp = 7.2-7.5 km/s), which corresponds to mafic material. We suggest that this vertical evolution of the layered structure could be created by relamination and that the keel structure formed by relamination may be the root of the supernormal stability of the ancient Ordos terrane.

Carbon fixation in oceanic crust: Does it happen, and is it important?

NASA Astrophysics Data System (ADS)

Orcutt, B.; Sylvan, J. B.; Rogers, D.; Lee, R.; Girguis, P. R.; Carr, S. A.; Jungbluth, S.; Rappe, M. S.

2014-12-01

The carbon sources supporting a deep biosphere in igneous oceanic crust, and furthermore the balance of heterotrophy and autotrophy, are poorly understood. When the large reservoir size of oceanic crust is considered, carbon transformations in this environment have the potential to significantly impact the global carbon cycle. Furthermore, igneous oceanic crust is the most massive potential habitat for life on Earth, so understanding the carbon sources for this potential biosphere are important for understanding life on Earth. Geochemical evidence suggests that warm and anoxic upper basement is net heterotrophic, but the balance of these processes in cooler and potentially oxic oceanic crust are poorly known. Here, we present data from stable carbon isotope tracer incubations to examine carbon fixation in basalts collected from the Loihi Seamount, the Juan de Fuca Ridge, and the western flank of the Mid-Atlantic Ridge, to provide a first order constraint on the rates of carbon fixation on basalts. These data will be compared to recently available assessments of carbon cycling rates in fluids from upper basement to synthesize our current state of understanding of the potential for carbon fixation and respiration in oceanic crust. Moreover, we will present new genomic data of carbon fixation genes observed in the basalt enrichments as well as from the subsurface of the Juan de Fuca Ridge flank, enabling identification of the microbes and metabolic pathways involved in carbon fixation in these systems.

Millennial-scale ocean acidification and late Quaternary decline of cryptic bacterial crusts in tropical reefs.

PubMed

Riding, R; Liang, L; Braga, J C

2014-09-01

Ocean acidification by atmospheric carbon dioxide has increased almost continuously since the last glacial maximum (LGM), 21,000 years ago. It is expected to impair tropical reef development, but effects on reefs at the present day and in the recent past have proved difficult to evaluate. We present evidence that acidification has already significantly reduced the formation of calcified bacterial crusts in tropical reefs. Unlike major reef builders such as coralline algae and corals that more closely control their calcification, bacterial calcification is very sensitive to ambient changes in carbonate chemistry. Bacterial crusts in reef cavities have declined in thickness over the past 14,000 years with largest reduction occurring 12,000-10,000 years ago. We interpret this as an early effect of deglacial ocean acidification on reef calcification and infer that similar crusts were likely to have been thicker when seawater carbonate saturation was increased during earlier glacial intervals, and thinner during interglacials. These changes in crust thickness could have substantially affected reef development over glacial cycles, as rigid crusts significantly strengthen framework and their reduction would have increased the susceptibility of reefs to biological and physical erosion. Bacterial crust decline reveals previously unrecognized millennial-scale acidification effects on tropical reefs. This directs attention to the role of crusts in reef formation and the ability of bioinduced calcification to reflect changes in seawater chemistry. It also provides a long-term context for assessing anticipated anthropogenic effects. © 2014 John Wiley & Sons Ltd.

Influence of substrate rocks on Fe Mn crust composition

NASA Astrophysics Data System (ADS)

Hein, James R.; Morgan, Charles L.

1999-05-01

Principal Component and other statistical analyses of chemical and mineralogical data of Fe-Mn oxyhydroxide crusts and their underlying rock substrates in the central Pacific indicate that substrate rocks do not influence crust composition. Two ridges near Johnston Atoll were dredged repetitively and up to seven substrate rock types were recovered from small areas of similar water depths. Crusts were analyzed mineralogically and chemically for 24 elements, and substrates were analyzed mineralogically and chemically for the 10 major oxides. Compositions of crusts on phosphatized substrates are distinctly different from crusts on substrates containing no phosphorite. However, that relationship only indicates that the episodes of phosphatization that mineralized the substrate rocks also mineralized the crusts that grew on them. A two-fold increase in copper contents in crusts that grew on phosphatized clastic substrate rocks, relative to crusts on other substrate rock types, is also associated with phosphatization and must have resulted from chemical reorganization during diagenesis. Phosphatized crusts show increases in Sr, Zn, Ca, Ba, Cu, Ce, V, and Mo contents and decreases in Fe, Si, and As contents relative to non-phosphatized crusts. Our statistical results support previous studies which show that crust compositions reflect predominantly direct precipitation from seawater (hydrogenetic), and to lesser extents reflect detrital input and diagenetic replacement of parts of the older crust generation by carbonate fluorapatite.

Influence of substrate rocks on Fe-Mn crust composition

USGS Publications Warehouse

Hein, J.R.; Morgan, C.L.

1999-01-01

Principal Component and other statistical analyses of chemical and mineralogical data of Fe-Mn oxyhydroxide crusts and their underlying rock substrates in the central Pacific indicate that substrate rocks do not influence crust composition. Two ridges near Johnston Atoll were dredged repetitively and up to seven substrate rock types were recovered from small areas of similar water depths. Crusts were analyzed mineralogically and chemically for 24 elements, and substrates were analyzed mineralogically and chemically for the 10 major oxides. Compositions of crusts on phosphatized substrates are distinctly different from crusts on substrates containing no phosphorite. However, that relationship only indicates that the episodes of phosphatization that mineralized the substrate rocks also mineralized the crusts that grew on them. A two-fold increase in copper contents in crusts that grew on phosphatized clastic substrate rocks, relative to crusts on other substrate rock types, is also associated with phosphatization and must have resulted from chemical reorganization during diagenesis. Phosphatized crusts show increases in Sr, Zn, Ca, Ba, Cu, Ce, V, and Mo contents and decreases in Fe, Si, and As contents relative to non-phosphatized crusts. Our statistical results support previous studies which show that crust compositions reflect predominantly direct precipitation from seawater (hydrogenetic), and to lesser extents reflect detrital input and diagenetic replacement of parts of the older crust generation by carbonate fluorapatite.

Crusted scabies is associated with increased IL-17 secretion by skin T cells.

PubMed

Liu, X; Walton, S F; Murray, H C; King, M; Kelly, A; Holt, D C; Currie, B J; McCarthy, J S; Mounsey, K E

2014-11-01

Scabies is an ectoparasitic infestation by the mite Sarcoptes scabiei. Although commonly self-limiting, a fraction of patients develop severely debilitating crusted scabies. The immune mechanisms underlying the development of crusted scabies are unclear, and undertaking longitudinal infection studies in humans is difficult. We utilized a porcine model to compare cellular immune responses in peripheral blood and skin of pigs with different clinical manifestations of scabies (n = 12), and in uninfected controls (n = 6). Although clinical symptoms were not evident until at least 4 weeks post-infestation, the numbers of peripheral IFNγ-secreting CD4(+) T cells and γδ T cells increased in infected pigs from week 1 post-infestation. γδ T cells remained increased in the blood at week 15 post-infestation. At week 15, skin cell infiltrates from pigs with crusted scabies had significantly higher CD8(+) T cell, γδ T cell and IL-17(+) cell numbers than those with ordinary scabies. Peripheral IL-17 levels were not increased, suggesting that localized skin IL-17-secreting T cells may play a critical role in the pathogenesis of crusted scabies development. Given the potential of anti-IL-17 immunotherapy demonstrated for other inflammatory skin diseases, this study may provide a novel therapeutic avenue for patients with recurrent crusted scabies. © 2014 John Wiley & Sons Ltd.

A case report of crusted scabies in an adult patient with Down syndrome.

PubMed

Nagsuk, Phillips; Moore, Rachel; Lopez, Lisa

2015-08-15

Crusted (Norwegian) scabies is a severe manifestation of the contagious skin infection caused by Sarcoptes scabiei. Crusted scabies has been well described in patients with known immunocompromised states. Treatment may be complicated by delayed diagnosis and/or inadequate treatment. This infection may not rank highly on one's differential diagnosis in the absence of an immunocompromised state, highlighting the uniqueness of the case being presented. Several papers describe immunocompromised children with Down syndrome who are infected with crusted scabies. We present a case of infection in an adult with Down syndrome without evidence of an immunocompromised state. Our patient came to us with a 13-month history of progressively worsening symptoms, the last 4-6 weeks of that time period being most dramatic, despite various treatments. We performed tissue biopsy, culture, and laboratory evaluations, which revealed numerous mites and bacterial superinfection. Crusted scabies infection may occur in adult age individuals with Down syndrome regardless of immune status, leading us to encourage practitioners to consider this condition when presented with patients of this population. We also highlight the need for further exploration of disease prevalence in this patient population.

A search for thermal excursions from ancient extraterrestrial impacts using Hadean zircon Ti-U-Th-Pb depth profiles

PubMed Central

Abbott, Sunshine S.; Harrison, T. Mark; Schmitt, Axel K.; Mojzsis, Stephen J.

2012-01-01

Few terrestrial localities preserve more than a trace lithic record prior to ca. 3.8 Ga greatly limiting our understanding of the first 700 Ma of Earth history, a period inferred to have included a spike in the bolide flux to the inner solar system at ca. 3.85–3.95 Ga (the Late Heavy Bombardment, LHB). An accessible record of this era may be found in Hadean detrital zircons from the Jack Hills, Western Australia, in the form of μm-scale epitaxial overgrowths. By comparing crystallization temperatures of pre-3.8 Ga zircon overgrowths to the archive of zircon temperature spectra, it should, in principle, be possible to identify a distinctive impact signature. We have developed Ti-U-Th-Pb ion microprobe depth profiling to obtain age and temperature information within these zircon overgrowths and undertaken a feasibility study of its possible use in identifying impact events. Of eight grains profiled in this fashion, four have overgrowths of LHB-era age. Age vs. temperature profiles reveal a period between ca. 3.85–3.95 Ga (i.e., LHB era) characterized by significantly higher temperatures (approximately 840–875 °C) than do older or younger zircons or zircon domains (approximately 630–750 °C). However, temperatures approaching 900 °C can result in Pb isotopic exchange rendering interpretation of these profiles nonunique. Coupled age-temperature depth profiling shows promise in this role, and the preliminary data we report could represent the first terrestrial evidence for impact-related heating during the LHB. PMID:22869711

Xe incorporation in crust and upper mantle minerals: new experimental and theoretical evidences

NASA Astrophysics Data System (ADS)

Celine, C.; Sanloup, C.; Blanchard, M.; Lazzeri, M.; Balan, E.; Hudspeth, J.

2017-12-01

Storage of Xe in silicate minerals has been proposed to explain the `Missing Xenon' issue, i.e. the low Xe abundance in the Earth's and Mars' atmospheres compared to other noble gases [1]. However, data about Xe incorporation in minerals remain scarce due to high Xe volatility preventing studies at ambient conditions. Xe incorporations in olivine [2] and quartz [3] have been proposed based on experimental evidences at high pressures and temperatures. Nevertheless, Xe incorporation mechanisms remained so far only hypothetical. We present here new in situ X-ray diffraction data in diamond-anvil cell showing Xe incorporation in widespread silicate minerals of the continental crust (feldspars and quartz) and of the upper mantle (olivine) at relevant pressure and temperature conditions. Significant variations in cell parameters are retrieved in Xe-rich minerals as well as new peaks, implying change in the crystal structures linked to Xe incorporation. Theoretical calculations have been performed (using the density functional theory) to propose Xe incorporation sites able to reproduce experimental observations. In olivine, a Xe for Si substitutions is proposed, with up to 0.4 at% Xe potentially stored in olivine at depth. These new constraints on Xe incorporation at depths in silicate minerals, often neglected, could be crucial in the `Missing Xenon' issue. [1] Anders and Owen (1977) Science 198, 453-465, [2] Sanloup et al. (2011) Geochim. Cosmochim. Acta 75, 6271-6284, [3] Sanloup et al., (2005) Science, 310, 1174-1177

Reactions between komatiite and CO2-rich seawater at 250 and 350 °C, 500 bars: implications for hydrogen generation in the Hadean seafloor hydrothermal system

NASA Astrophysics Data System (ADS)

Ueda, Hisahiro; Shibuya, Takazo; Sawaki, Yusuke; Saitoh, Masafumi; Takai, Ken; Maruyama, Shigenori

2016-12-01

To understand the chemical nature of hydrothermal fluids in the komatiite-hosted seafloor hydrothermal system in the Hadean, we conducted two hydrothermal serpentinization experiments involving synthetic komatiite and a CO2-rich acidic NaCl fluid at 250 and 350 °C, 500 bars. During the experiments, the komatiites were strongly carbonated to yield iron-rich dolomite (3-9 wt.% FeO) at 250 °C and calcite (<0.8 wt.% FeO) at 350 °C, respectively. The carbonation of komatiites suppressed H2 generation in the fluids. The steady-state H2 concentrations in the fluid were approximately 0.024 and 2.9 mmol/kg at 250 and 350 °C, respectively. This correlation between the Fe content in carbonate mineral and the H2 concentration in the fluid suggests that the incorporation of ferrous iron into the carbonate mineral probably limited magnetite formation and consequent generation of hydrogen during the serpentinization of komatiites. The H2 concentration of the fluid at 350 °C corresponds to that of modern H2-rich seafloor hydrothermal systems, such as the Kairei hydrothermal field, where hydrogenotrophic methanogens dominate in the prosperous microbial ecosystem. Accordingly, the high-temperature serpentinization of komatiite would provide the H2-rich hydrothermal environments that were necessary for the emergence and early evolution of life in the Hadean ocean. In contrast, H2-rich fluids may not have been generated by serpentinization at temperatures below 250 °C because carbonate minerals become more stable with decreasing temperature in the komatiite-H2O-CO2 system.

Melting of subducted continental crust: Geochemical evidence from Mesozoic granitoids in the Dabie-Sulu orogenic belt, east-central China

NASA Astrophysics Data System (ADS)

Zhao, Zi-Fu; Liu, Zhi-Bin; Chen, Qi

2017-09-01

Syn-collisional and postcollisional granitoids are common in collisional orogens, and they were primarily produced by partial melting of subducted continental crust. This is exemplified by Mesozoic granitoids from the Dabie-Sulu orogenic belt in east-central China. These granitoids were emplaced in small volumes in the Late Triassic (200-206 Ma) and the Late Jurassic (146-167 Ma) but massively in the Early Cretaceous (111-143 Ma). Nevertheless, all of them exhibit arc-like trace element distribution patterns and are enriched in Sr-Nd-Hf isotope compositions, indicating their origination from the ancient continental crust. They commonly contain relict zircons with Neoproterozoic and Triassic U-Pb ages, respectively, consistent with the protolith and metamorphic ages for ultrahigh-pressure (UHP) metaigneous rocks in the Dabie-Sulu orogenic belt. Some granitoids show low zircon δ18O values, and SIMS in-situ O isotope analysis reveals that the relict zircons with Neoproterozoic and Triassic U-Pb ages also commonly exhibit low δ18O values. Neoproterozoic U-Pb ages and low δ18O values are the two diagnostic features that distinguish the subducted South China Block from the obducted North China Block. Thus, the magma source of these Mesozoic granitoids has a genetic link to the subducted continental crust of the South China Block. On the other hand, these granitoids contain relict zircons with Paleoproterozoic and Archean U-Pb ages, which are present in both the South and North China Blocks. Taken together, the Mesozoic granitoids in the Dabie-Sulu orogenic belt and its hanging wall have their magma sources that are predominated by the continental crust of the South China Block with minor contributions from the continental crust of the North China Block. The Triassic continental collision between the South and North China Blocks brought the continental crust into the thickened orogen, where they underwent the three episodes of partial melting in the Late Triassic, Late

Osmium isotope and highly siderophile element systematics of the lunar crust

NASA Astrophysics Data System (ADS)

Day, James M. D.; Walker, Richard J.; James, Odette B.; Puchtel, Igor S.

2010-01-01

Coupled 187Os/ 188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 ± 0.3 pg g - 1 Os, 1.5 ± 0.6 pg g - 1 Ir, 6.8 ± 2.7 pg g - 1 Ru, 16 ± 15 pg g - 1 Pt, 33 ± 30 pg g - 1 Pd and 0.29 ± 0.10 pg g - 1 Re (˜ 0.00002 × CI) and Re/Os ratios that were modestly elevated ( 187Re/ 188Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (˜ 0.00007 × CI) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle-crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments. If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust-mantle concentration ratios ( D-values) must be ≤ 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust-mantle HSE partitioning for the Earth and Moon are

Osmium isotope and highly siderophile element systematics of the lunar crust

USGS Publications Warehouse

Day, J.M.D.; Walker, R.J.; James, O.B.; Puchtel, I.S.

2010-01-01

Coupled 187Os/188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 ?? 0.3 pg g- 1 Os, 1.5 ?? 0.6 pg g- 1 Ir, 6.8 ?? 2.7 pg g- 1 Ru, 16 ?? 15 pg g- 1 Pt, 33 ?? 30 pg g- 1 Pd and 0.29 ?? 0.10 pg g- 1 Re (??? 0.00002 ?? CI) and Re/Os ratios that were modestly elevated (187Re/188Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (??? 0.00007 ?? CI) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle-crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments. If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust-mantle concentration ratios (D-values) must be ??? 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust-mantle HSE partitioning for the Earth and Moon are somewhat

Transdomes sampling of lower and middle crust

NASA Astrophysics Data System (ADS)

Teyssier, C. P.; Whitney, D. L.; Roger, F.; Rey, P. F.

2015-12-01

Migmatite transdomes are formed by lateral and upward flow of partially molten crust in transtension zones (pull-apart structures). In order to understand the flow leading to this type of domes, 3D numerical models were set-up to simulate the general case of an extensional domain located between two strike-slip faults (pull-apart or dilational bridge). Results show that upper crust extension induces flow of the deep, low-viscosity crust, with rapid upward movement of transdome material when extension becomes localized. At this point a rolling hinge detachment allows rapid removal of upper crust. The internal structure of transdomes includes a subvertical high strain zone located beneath the zone of localized upper crust extension; this shear zone separates two elongate subdomes of foliation that show refolded/sheath folds. Lineation tends to be oriented dominantly subhorizontal when the amount of strike-slip motion is greater than the amount of upward flow of dome rocks. Models also predict nearly isothermal decompression of transdome material and rapid transfer of ~50 km deep rocks to the near surface. These model results are compared to the structural and metamorphic history of several transdomes, and in particular the Variscan Montagne Noire dome (French Massif Central) that consists of two domes separated by a complex high strain zone. The Montagne Noire dome contains ~315 Ma eclogite bodies (U-Pb zircon age) that record 1.4 GPa peak pressure. The eclogite bodies are wrapped in highly sheared migmatite that yield 314-310 Ma monazite ages interpreted as the metamorphism and deformation age. Based on these relations we conclude that the Montagne Noire transdome developed a channel of partially molten crust that likely entrained eclogite bodies from the deep crust (~50 km) before ascending to the near-surface. One implication of this work is that the flowing crust was deeply seated in the orogen although it remained a poor recorder of peak pressure of metamorphism

Physics of Neutron Star Crusts.

PubMed

Chamel, Nicolas; Haensel, Pawel

2008-01-01

The physics of neutron star crusts is vast, involving many different research fields, from nuclear and condensed matter physics to general relativity. This review summarizes the progress, which has been achieved over the last few years, in modeling neutron star crusts, both at the microscopic and macroscopic levels. The confrontation of these theoretical models with observations is also briefly discussed.

Biological Soil Crusts: Webs of Life in the Desert

USGS Publications Warehouse

Belnap, Jayne

2001-01-01

Although the soil surface may look like dirt to you, it is full of living organisms that are a vital part of desert ecosystems. This veneer of life is called a biological soil crust. These crusts are found throughout the world, from hot deserts to polar regions. Crusts generally cover all soil spaces not occupied by green plants. In many areas, they comprise over 70% of the living ground cover and are key in reducing erosion, increasing water retention, and increasing soil fertility. In most dry regions, these crusts are dominated by cyanobacteria (previously called blue-green algae), which are one of the oldest known life forms. Communities of soil crusts also include lichens, mosses, microfungi, bacteria, and green algae. These living organisms and their by-products create a continuous crust on the soil surface. The general color, surface appearance, and amount of coverage of these crusts vary depending on climate and disturbance patterns. Immature crusts are generally flat and the color of the soil, which makes them difficult to distinguish from bare ground. Mature crusts, in contrast, are usually bumpy and dark-colored due to the presence of lichens, mosses, and high densities of cyanobacteria and other organisms.

The Birth of a Cratonic Core: Petrologic Evolution of the Hadean-Eoarchean Acasta Gneiss Complex

NASA Astrophysics Data System (ADS)

Reimink, J. R.; Chacko, T.; Davies, J.; Pearson, D. G.; Stern, R. A.; Heaman, L. M.; Carlson, R.; Shirey, S. B.

2016-12-01

Granitoid magmatism within the 4.02-3.6 Ga Acasta Gneiss Complex records distinct whole-rock compositional changes during the building the Slave Craton. Previously1,2 we suggested that these signatures implied petrologic changes from initiation of evolved crust formation in an Iceland-like setting to partial melting of hydrated mafic crust at increasing depth through time, culminating in relatively voluminous magmatism at 3.6 Ga. Increasing La/Yb in these rocks suggest increasing depth of melting (and increasing residual garnet content) with time, ending in emplacement of rocks comparable to other Archean TTG suites3, with both high pressure (high La/Yb) and low pressure (low La/Yb) rocks represented at 3.6 Ga. Data from rocks with variable La/Yb that crystallized 3.6 Ga allow us to evaluate potential mechanisms for formation of rocks of this age such as subduction/accretion or intracrustal melting/delamination. Despite major and trace element compositional and age variability, zircon oxygen isotope compositions from a wide variety of rocks are extremely consistent (+6.0-6.5 ‰ from 3.9-2.9 Ga), implying a similar source, one that had been altered by surface waters1. Potential source rocks include the upper portion of oceanic crust, which contains a large portion of mafic crust that had been altered at low temperatures (e.g., 4). Paired whole rock and zircon radiogenic isotopic data are especially sensitive to the extent of pre-existing felsic material in the region, as well as the longevity of primary, basaltic rocks prior to their reworking into more evolved crust. New paired zircon Hf and whole rock Nd isotope data collected from these samples show variably unradiogenic signatures and allow an exploration of similarities and disparities between crust formation in the Acasta Gneiss Complex and other Paleoarchean-Mesoarchean crustal blocks. [1] Reimink et al., 2016. Precambrian Research 281, 453-472. [2] Reimink et al., 2014 Nature Geoscience 7, 529-533. [3

Phosphatization Associated Features of Ferromanganese Crusts at Lemkein Seamount, Marshall Islands

NASA Astrophysics Data System (ADS)

Choi, J.; Lee, I.; Park, B. K.; Kim, J.

2014-12-01

Old layers of ferromanganese crusts, especially in the Pacific Ocean, have been affected by phosphatization. Ferromanganese crusts on Lemkein seamount in Marshall Islands also are phosphatized (3.3 to 4.2 wt % of P concentration). Furthermore, they have characteristic features that are different from other ferromanganese crusts. These features occur near the phosphorite, which were thought to fill the pore spaces of ferromanganese crusts. Inside the features, ferromanganese crusts are botryoidally precipitated from the round-boundary. The features of the phosphatized lower crusts of Lemkein seamount are observed using microscope and SEM. Elemental compositions of the selected samples were analyzed by SEM-EDS. Based on the observation and analysis of samples, three characteristic structures are identified: (1) phosphate-filled circles, (2) tongue-shaped framboidal crust, and (3) massive framboidal crust. The phosphate-filled circles are mostly composed of phosphorite, and they include trace fossils such as foraminifera. Phosphatized ferromanganese crusts exist at the boundary of this structure. The tongue-shaped crust is connected with the lips downward, and ferromanganese crusts inside the tongue show distinct growth rim. The massive framboidal crust is located below the tongue. Ferromanganese crusts in the massive framboidal crust are enveloped by phosphate, and some of the crusts are phosphatized. Around the structures, Mn oxide phase is concentrated as a shape of corona on BSE image. All of the structures are in the phosphatized crusts that show columnar growth of ferromanganese crusts and have sub-parallel lamination. These observation and chemical analysis of the ferromanganese crusts can provide a clue of diagenetic processes during the formation of ferromanganese crusts.

USArray Imaging of North American Continental Crust

NASA Astrophysics Data System (ADS)

Ma, Xiaofei

The layered structure and bulk composition of continental crust contains important clues about its history of mountain-building, about its magmatic evolution, and about dynamical processes that continue to happen now. Geophysical and geological features such as gravity anomalies, surface topography, lithospheric strength and the deformation that drives the earthquake cycle are all directly related to deep crustal chemistry and the movement of materials through the crust that alter that chemistry. The North American continental crust records billions of years of history of tectonic and dynamical changes. The western U.S. is currently experiencing a diverse array of dynamical processes including modification by the Yellowstone hotspot, shortening and extension related to Pacific coast subduction and transform boundary shear, and plate interior seismicity driven by flow of the lower crust and upper mantle. The midcontinent and eastern U.S. is mostly stable but records a history of ancient continental collision and rifting. EarthScope's USArray seismic deployment has collected massive amounts of data across the entire United States that illuminates the deep continental crust, lithosphere and deeper mantle. This study uses EarthScope data to investigate the thickness and composition of the continental crust, including properties of its upper and lower layers. One-layer and two-layer models of crustal properties exhibit interesting relationships to the history of North American continental formation and recent tectonic activities that promise to significantly improve our understanding of the deep processes that shape the Earth's surface. Model results show that seismic velocity ratios are unusually low in the lower crust under the western U.S. Cordillera. Further modeling of how chemistry affects the seismic velocity ratio at temperatures and pressures found in the lower crust suggests that low seismic velocity ratios occur when water is mixed into the mineral matrix

Composition of the crust beneath the Kenya rift

USGS Publications Warehouse

Mooney, W.D.; Christensen, N.I.

1994-01-01

We infer the composition of the crust beneath and on the flanks of the Kenya rift based on a comparison of the KRISP-90 crustal velocity structure with laboratory measurements of compressional-wave velocities of rock samples from Kenya. The rock samples studied, which are representative of the major lithologies exposed in Kenya, include volcanic tuffs and flows (primarily basalts and phonolites), and felsic to intermediate composition gneisses. This comparison indicates that the upper crust (5-12 km depth) consists primarily of quartzo-feldspathic gneisses and schists similar to rocks exposed on the flanks of the rift, whereas the middle crust (12-22 km depth) consists of more mafic, hornblende-rich metamorphic rocks, probably intruded by mafic rocks beneath the rift axis. The lower crust on the flanks of the rift may consist of mafic granulite facies rocks. Along the rift axis, the lower crust varies in thickness from 9 km in the southern rift to only 2-3 km in the north, and has a seismic velocity substantially higher than the samples investigated in this study. The lower crust of the rift probably consists of a crust/mantle mix of high-grade metamorphic rocks, mafic intrusives, and an igneous mafic residuum accreted to the base of the crust during differentiation of a melt derived from the upper mantle. ?? 1994.

Nuclear Reactions in the Crusts of Accreting Neutron Stars

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lau, Rita; Beard, Mary; Gupta, Sanjib S.

X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density ismore » $$\\rho =2\\times {10}^{12}\\,{\\rm{g}}\\,{\\mathrm{cm}}^{-3}$$ using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond $$\\rho =2\\times {10}^{12}\\,{\\rm{g}}\\,{\\mathrm{cm}}^{-3}$$. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 owing to the N = 82 shell closure. In agreement with previous work, we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. As a result, this work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.« less

Nuclear Reactions in the Crusts of Accreting Neutron Stars

NASA Astrophysics Data System (ADS)

Lau, R.; Beard, M.; Gupta, S. S.; Schatz, H.; Afanasjev, A. V.; Brown, E. F.; Deibel, A.; Gasques, L. R.; Hitt, G. W.; Hix, W. R.; Keek, L.; Möller, P.; Shternin, P. S.; Steiner, A. W.; Wiescher, M.; Xu, Y.

2018-05-01

X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density is ρ =2× {10}12 {{g}} {cm}}-3 using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond ρ =2× {10}12 {{g}} {cm}}-3. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 owing to the N = 82 shell closure. In agreement with previous work, we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. This work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.

Nuclear Reactions in the Crusts of Accreting Neutron Stars

DOE PAGES

Lau, Rita; Beard, Mary; Gupta, Sanjib S.; ...

2018-05-24

X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density ismore » $$\\rho =2\\times {10}^{12}\\,{\\rm{g}}\\,{\\mathrm{cm}}^{-3}$$ using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond $$\\rho =2\\times {10}^{12}\\,{\\rm{g}}\\,{\\mathrm{cm}}^{-3}$$. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 owing to the N = 82 shell closure. In agreement with previous work, we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. As a result, this work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.« less

Detrital Zircon Geochronology of Sedimentary Rocks of the 3.6 - 3.2 Ga Barberton Greenstone Belt: No Evidence for Older Continental Crust

NASA Astrophysics Data System (ADS)

Drabon, N.; Lowe, D. R.; Byerly, G. R.; Harrington, J.

2017-12-01

The crustal setting of early Archean greenstone belts and whether they formed on or associated with blocks of older continental crust or in more oceanic settings remains a major issue in Archean geology. We report detrital zircon U-Pb age data from sandstones of the 3.26-3.20 Ga Fig Tree and Moodies Groups and from 3.47 to 3.23 Ga meteorite impact-related deposits in the 3.55-3.20 Ga Barberton greenstone belt (BGB), South Africa. The provenance signatures of these sediments are characterized by zircon age peaks at 3.54, 3.46, 3.40, 3.30, and 3.25 Ga. These clusters are coincident either with the ages of major episodes of felsic to intermediate igneous activity within and around the belt or with the ages of thin felsic tuffs reflecting distant volcanic activity. Only 15 of the reported 3410 grains (<0.5%) pre-date the age of the oldest rocks in the BGB. The extreme rarity of zircons older than the felsic components of the BGB itself, even after widespread deformation, uplift, and deep erosion of the BGB, implies that an older continental substrate is unlikely to have existed beneath or adjacent to the BGB. Ten of the 15 pre -BGB zircons were recovered from a single meteorite impact-related layer and may have been derived from far beyond the BGB by impact-related processes. The remaining old zircons could represent felsic rocks in older, unexposed parts of the BGB sequence, but are too few to provide evidence for a continental source. This finding offers further evidence that the large, thick, high-standing, highly evolved blocks of continental crust with an andesitic bulk composition that characterize the Earth during younger geologic times were scarce in the early Archean.

Clay minerals trap hydrogen in the Earth's crust: Evidence from the Cigar Lake uranium deposit, Athabasca

NASA Astrophysics Data System (ADS)

Truche, Laurent; Joubert, Gilles; Dargent, Maxime; Martz, Pierre; Cathelineau, Michel; Rigaudier, Thomas; Quirt, David

2018-07-01

Hydrogen (H2)-rich fluids are observed in a wide variety of geologic settings including gas seeps in serpentinized ultramafic rocks, sub-seafloor hydrothermal vents, fracture networks in crystalline rocks from continental and oceanic crust, and volcanic gases. Natural hydrogen sources can sustain deep microbial ecosystems, induce abiotic hydrocarbons synthesis and trigger the formation of prebiotic organic compounds. However, due to its extreme mobility and small size, hydrogen is not easily trapped in the crust. If not rapidly consumed by redox reactions mediated by bacteria or suitable mineral catalysts it diffuses through the rocks and migrates toward the surface. Therefore, H2 is not supposed to accumulate in the crust. We challenge this view by demonstrating that significant amount of H2 may be adsorbed by clay minerals and remain trapped beneath the surface. Here, we report for the first time H2 content in clay-rich rocks, mainly composed of illite, chlorite, and kaolinite from the Cigar Lake uranium ore deposit (northern Saskatchewan, Canada). Thermal desorption measurements reveal that H2 is enriched up to 500 ppm (i.e. 0.25 mol kg-1 of rock) in these water-saturated rocks having a very low total organic content (<0.5 wt%). Such hydrogen uptake is comparable and even exceeds adsorbed methane capacities reported elsewhere for pure clay minerals or shales. Sudoite (Al-Mg di-trioctahedral chlorite) is probably the main mineral responsible for H2 adsorption in the present case. The presence of multiple binding sites in interlinked nanopores between crystal layers of illite-chlorite particles offers the ideal conditions for hydrogen sorption. We demonstrate that 4 to 17% of H2 produced by water radiolysis over the 1.4-Ga-lifetime of the Cigar Lake uranium ore deposit has been trapped in the surrounding clay alteration haloes. As a result, sorption processes on layered silicates must not be overlooked as they may exert an important control on the fate and

Evidence for mafic lower crust in Tanzania, East Africa, from joint inversion of receiver functions and Rayleigh wave dispersion velocities

NASA Astrophysics Data System (ADS)

Julià, Jordi; Ammon, Charles J.; Nyblade, Andrew A.

2005-08-01

The S-wave velocity structure of Precambrian terranes in Tanzania, East Africa is modelled by jointly inverting receiver functions and surface wave dispersion velocities from the 1994-1995 Tanzania broad-band seismic experiment. The study region, which consists of an Archean craton surrounded by Proterozoic mobile belts, forms a unique setting for evaluating Precambrian crustal evolution. Our results show a uniform crustal structure across the region, with a 10-15 km thick upper crust with VS= 3.4-3.5 km s-1, overlying a gradational lower crust with S-wave velocities up to 4.1 km s-1 at 38-42 km depth. The upper-mantle lid displays uniform S-wave velocities of 4.5-4.7 km s-1 to depths of 100-150 km and overlays a prominent low-velocity zone. This low-velocity zone is required by the dispersion and receiver function data, but its depth interval is uncertain. The high crustal velocities within the lowermost crust characterize the entire region and suggest that mafic lithologies are present in both Archean and Proterozoic terranes. The ubiquitous mafic lower crust can be attributed to underplating associated with mafic dyke emplacement. This finding suggests that in East Africa there has been little secular variation in Precambrian crustal development.

Petrology and Wavespeeds in Central Tibet Indicate a Partially Melted Mica-Bearing Crust

NASA Astrophysics Data System (ADS)

Hacker, B. R.; Ritzwoller, M. H.; Xie, J.

2013-12-01

S-wave speeds and Vp/Vs ratios in the middle to deep crust of Tibet are best explained by a partially melted, mica-bearing middle to lower crust with a subhorizontal to gently dipping foliation. Surface-wave tomography [e.g., Yang et al., 2012; Xie et al., 2013] shows that the central Tibetan Plateau (the Qiangtang block) is characterized by i) slow S-wave speeds of 3.3-3.5 km/s at depths from 20-25 km to 45-50 km, ii) S-wave radial anisotropy of at least 4% (Vsh > Vsv) with stronger anisotropy in the west than the east [Duret et al., 2010], and iii) whole-crust Vp/Vs ratios in the range of 1.73-1.78 [Xu et al., 2013]. The depth of the Curie temperature for magnetite inferred from satellite magnetic measurements [Alsdorf and Nelson, 1999], the depth of the α-β quartz transition inferred from Vp/Vs ratios [Mechie et al., 2004], and the equilibration pressures and temperatures of xenoliths erupted from the mid-deep crust [Hacker et al., 2000] indicate that the thermal gradient in Qiangtang is steep, reaching 1000°C at 30-40 km depth. This thermal gradient crosses the dehydration-melting solidi for crustal rocks at 20-30 km depth, implying the presence or former presence of melt in the mid-deep crust. These temperatures do not require the wholesale breakdown of mica at these depths, because F and Ti can stabilize mica to at least 1300°C [Dooley and Patino Douce, 1996]. Petrology suggests, then, that the Qiangtang middle to deep crust consists of a mica-bearing residue from which melt has been extracted or is being extracted. Wavespeeds calculated for mica-bearing rocks with a subhorizontal to gently dipping foliation and minor silicate melt are the best match to the wavespeeds and anisotropy observed by seismology. Alsdorf, D., and D. Nelson, The Tibetan satellite magnetic low: Evidence for widespread melt in the Tibetan crust?, Geology, 27, 943-946, 1999. Dooley, D.F., and A.F. Patino Douce, Fluid-absent melting of F-rich phlogopite + rutile +quartz, American

Microbial community structure in three deep-sea carbonate crusts.

PubMed

Heijs, S K; Aloisi, G; Bouloubassi, I; Pancost, R D; Pierre, C; Sinninghe Damsté, J S; Gottschal, J C; van Elsas, J D; Forney, L J

2006-10-01

Carbonate crusts in marine environments can act as sinks for carbon dioxide. Therefore, understanding carbonate crust formation could be important for understanding global warming. In the present study, the microbial communities of three carbonate crust samples from deep-sea mud volcanoes in the eastern Mediterranean were characterized by sequencing 16S ribosomal RNA (rRNA) genes amplified from DNA directly retrieved from the samples. In combination with the mineralogical composition of the crusts and lipid analyses, sequence data were used to assess the possible role of prokaryotes in crust formation. Collectively, the obtained data showed the presence of highly diverse communities, which were distinct in each of the carbonate crusts studied. Bacterial 16S rRNA gene sequences were found in all crusts and the majority was classified as alpha-, gamma-, and delta- Proteobacteria. Interestingly, sequences of Proteobacteria related to Halomonas and Halovibrio sp., which can play an active role in carbonate mineral formation, were present in all crusts. Archaeal 16S rRNA gene sequences were retrieved from two of the crusts studied. Several of those were closely related to archaeal sequences of organisms that have previously been linked to the anaerobic oxidation of methane (AOM). However, the majority of archaeal sequences were not related to sequences of organisms known to be involved in AOM. In combination with the strongly negative delta 13C values of archaeal lipids, these results open the possibility that organisms with a role in AOM may be more diverse within the Archaea than previously suggested. Different communities found in the crusts could carry out similar processes that might play a role in carbonate crust formation.

Evidence of Tectonic Rotations and Magmatic Flow Within the Sheeted Dike Complex of Super-Fast Spread Crust Exposed at the Pito Deep Rift

NASA Astrophysics Data System (ADS)

Horst, A. J.; Varga, R. J.; Gee, J. S.; Karson, J. A.

2008-12-01

Escarpments bounding the Pito Deep Rift expose cross-sections into ~3 Ma oceanic crust accreted at a super-fast spreading (>140 mm/yr) segment of the East Pacific Rise (EPR). Dikes within the sheeted dike complex persistently strike NE, parallel to local abyssal hill lineaments and magnetic anomaly stripes, and dip SE, outward and away from the EPR. During the Pito Deep 2005 Cruise, both ALVIN and JASON II used the Geocompass to fully orient a total of 69 samples [63 basaltic dikes, 6 massive gabbros] collected in situ. Paleomagnetic analyses of these oriented samples provide a quantitative constraint of kinematics of structural rotations of dikes. Magnetic remanence of dike samples indicates a dominant normal polarity with almost all directions rotated clockwise from the expected direction. The most geologically plausible model to account for these dispersions using these data coupled with the general orientation of the dikes incorporates two different structural rotations: 1) A horizontal-axis rotation that occurred near the EPR axis, related to sub-axial subsidence, and 2) A clockwise vertical-axis rotation, associated with the rotation of the Easter microplate consistent with current models. Additionally, the anisotropy of magnetic susceptibility (AMS) of dike samples indicates rock fabric and magmatic flow direction within dikes. In most samples, two of three AMS eigenvectors lie near the dike plane orientations. Generally, Kmin lies perpendicular to dike planes, while Kmax is often shallow within the dike planes, indicating dominantly subhorizontal magma flow. Steep Kmax in a few samples indicates vertical flow directions that suggest either primary flow or gravitational back-flow during waning stages of dike intrusion. These results provide the first direct evidence for primarily horizontal magma flow in sheeted dikes of super-fast spread oceanic crust. Results for Pito Deep Rift and previous results for Hess Deep Rift reveal outward dipping dikes that are

Lunar and terrestrial crust formation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Walker, D.

1983-11-15

Planetary crusts may be accreted, produced in primordial differentiation, or built up piecemeal by serial magmatism. The existence of old, polygenetic, laterally heterogeneous, partial melt rocks in the lunar highlands suggests that the moon produced its early crust by serial magmatism. This view can be reconciled with lunar Eu anomalies, previously thought to support the magma ocean model of crust formation, if complications in the fractionation of mare basalts are reconized. Phase equilibrium and magmatic density information for mare basalts suggest a model in which plagioclase fractionation can occur even though plagioclase is not a near-liquidus phase. The crytic fractionationmore » of clinopryoxene in MORB provides a precedent for this model. The necessity for a lunar magma ocean is questioned, but a role for a terrestrial magma ocean of sorts at depth is suggested.« less

Nitrogen fixation in biological soil crusts from southeast Utah, USA

USGS Publications Warehouse

Belnap, Jayne

2002-01-01

Biological soil crusts can be the dominant source of N for arid land ecosystems. We measured potential N fixation rates biweekly for 2 years, using three types of soil crusts: (1) crusts whose directly counted cells were >98% Microcoleus vaginatus (light crusts); (2) crusts dominated by M. vaginatus, but with 20% or more of the directly counted cells represented by Nostoc commune and Scytonema myochrous (dark crusts); and (3) the soil lichen Collema sp. At all observation times, Collema had higher nitrogenase activity (NA) than dark crusts, which had higher NA than light crusts, indicating that species composition is critical when estimating N inputs. In addition, all three types of crusts generally responded in a similar fashion to climate conditions. Without precipitation within a week of collection, no NA was recorded, regardless of other conditions being favorable. Low (<1°C) and high (>26°C) temperatures precluded NA, even if soils were moist. If rain or snow melt had occurred 3 or less days before collection, NA levels were highly correlated with daily average temperatures of the previous 3 days (r2=0.93 for Collema crusts; r2=0.86 for dark crusts and r2=0.83 for light crusts) for temperatures between 1°C and 26°C. If a precipitation event followed a long dry period, NA levels were lower than if collection followed a time when soils were wet for extended periods (e.g., winter). Using a combination of data from a recording weather datalogger, time-domain reflectometry, manual dry-down curves, and N fixation rates at different temperatures, annual N input from the different crust types was estimated. Annual N input from dark crusts found at relatively undisturbed sites was estimated at 9 kg ha–1 year–1. With 20% cover of the N-fixing soil lichen Collema, inputs are estimated at 13 kg ha–1 year–1. N input from light crusts, generally indicating soil surface disturbance, was estimated at 1.4 kg ha–1 year–1. The rates in light crusts are

Black manganese-rich crusts on a Gothic cathedral

NASA Astrophysics Data System (ADS)

Macholdt, Dorothea S.; Herrmann, Siegfried; Jochum, Klaus Peter; Kilcoyne, A. L. David; Laubscher, Thomas; Pfisterer, Jonas H. K.; Pöhlker, Christopher; Schwager, Beate; Weber, Bettina; Weigand, Markus; Domke, Katrin F.; Andreae, Meinrat O.

2017-12-01

Black manganese-rich crusts are found worldwide on the façades of historical buildings. In this study, they were studied exemplarily on the façade of the Freiburger Münster (Freiburg Minster), Germany, and measured in-situ by portable X-ray fluorescence (XRF). The XRF was calibrated to allow the conversion from apparent mass fractions to Mn surface density (Mn mass per area), to compensate for the fact that portable XRF mass fraction measurements from thin layers violate the assumption of a homogeneous measurement volume. Additionally, 200-nm femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs LA-ICP-MS) measurements, scanning transmission X-ray microscopy-near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS), Raman spectroscopy, and imaging by light microscopy were conducted to obtain further insight into the crust material, such as potential biogenic contributions, element distributions, trace element compositions, and organic functional groups. While black crusts of various types are present at many places on the minster's facade, crusts rich in Mn (with a Mn surface density >150 μg cm-2) are restricted to a maximum height of about 7 m. The only exceptions are those developed on the Renaissance-Vorhalle (Renaissance Portico) at a height of about 8 m. This part of the façade had been cleaned and treated with a silicon resin as recently as 2003. These crusts thus accumulated over a period of only 12 years. Yet, they are exceptionally Mn-rich with a surface density of 1200 μg cm-2, and therefore require an accumulation rate of about 100 μg cm-2 Mn per year. Trace element analyses support the theory that vehicle emissions are responsible for most of the Mn supply. Lead, barium, and zinc correlate with manganese, indicating that tire material, brake pads, and resuspended road dust are likely to be the element sources. Microscopic investigations show no organisms on or in the Mn-rich crusts. In contrast, Mn-free black

Investigating the Early Carbon Cycle Using Carbonaceous Inclusions and Dissolved Carbon in Detrital Zircon

NASA Astrophysics Data System (ADS)

Bell, E. A.; Boehnke, P.; Harrison, M.; Mao, W. L.

2015-12-01

Because the terrestrial rock record extends only to ~4 Ga and older materials thus far identified are limited to detrital zircons, information about volatile abundances and cycles on early Earth is limited. Carbon, for instance, plays an important role not only in the modern biosphere but also in deep recycling of materials between the crust and mantle. We are investigating the record of carbon abundance and origin in Hadean zircons from Jack Hills (W. Australia) using two main approaches. First, carbon may partition into the zircon structure at trace levels during crystallization from a magma, and better understanding of this partitioning behavior will allow for zircon's use as a monitor of magmatic carbon contents. We have measured carbon abundances in zircon from a variety of igneous rocks (gabbro; I-, A-, and S-type granitoids) via SIMS and found that although abundances are typically low (average raw 12C/30Si ~ 1x10-6), S-type granite zircons can reach a factor of 1000 over this background. Around 10% of Hadean zircons investigated show similar enrichments, consistent with other evidence for the derivation of many Jack Hills zircons from S-type granitoids and with the establishment of modern-level carbon abundances in the crust by ca. 4.2 Ga. Diamond and graphite inclusions reported in the Jack Hills zircons by previous studies proved to be contamination by polishing debris, leaving the true abundance of these materials in the population uncertain. On a second front, we have identified and investigated primary carbonaceous inclusions in these zircons. From a population of over 10,000 Jack Hills zircons, we identified one concordant 4.10±0.01 Ga zircon that contains primary graphite inclusions (so interpreted due to their enclosure in a crack-free zircon host as shown by transmission X-ray microscopy and their crystal habit). Their δ13CPDB of -24±5‰ is consistent with a biogenic origin and, in the absence of a likely inorganic mechanism to produce such a

What Do We Really Know About Early Earth? Less Than We Claim.

NASA Astrophysics Data System (ADS)

Harrison, M.; Bell, E. A.; Boehnke, P.

2016-12-01

The ubiquity of origin myths suggests that our species has an innate need to explain how Earth formed and evolved. Myth fabrication is in part controlled by limitations of the available historical record. When our community encountered its limit - there are no known rocks older than 4.02 Ga - it chose the paradigm of a desiccated, molten, continent-free wasteland and called it the Hadean. Over the past 15 years, motivated largely by study of >4 Ga zircons, aspects of this story have been displaced to include granite weathering and sediment cycling in the presence of H2O. While encouraging that observational data now informs at least part of our early Earth paradigm, other elements appear unchanged. For example, the view that significant continental crust or plate interactions didn't emerge until 3 Ga are argued on the basis of changes at that time in diamond inclusions, shale composition, zircon age spectra, and arc rock associations. However, they share 3 flawed, interrelated assumptions (lithospheric thermal structure and zircon productivity are time independent and the Archean rock record is unbiased) that greatly weaken their evidentiary value. It is axiomatic that we cannot know if earliest Earth was similar to present day or more akin to our longstanding myth from rocks given their >4.02 Ga absence. However, we are not without a lithic record and data from zircons as old as 4.38 Ga are decidedly more consistent with the former view than the latter. What compelled us to create an origin myth in the absence of empirical evidence? While science is distinguished from mythology by its emphasis on verification, its practitioners may be as subject to the same existential needs as any primitive society. Given high expected early radioactivity and impact flux, it was irresistible to explain the lack of Hadean continental crust by its non-existence rather than the equally plausible notion that it was consumed by the same processes operating on the planet today. If

Hawaiian submarine manganese-iron oxide crusts - A dating tool?

USGS Publications Warehouse

Moore, J.G.; Clague, D.A.

2004-01-01

Black manganese-iron oxide crusts form on most exposed rock on the ocean floor. Such crusts are well developed on the steep lava slopes of the Hawaiian Ridge and have been sampled during dredging and submersible dives. The crusts also occur on fragments detached from bedrock by mass wasting, on submerged coral reefs, and on poorly lithified sedimentary rocks. The thickness of the crusts was measured on samples collected since 1965 on the Hawaiian Ridge from 140 dive or dredge localities. Fifty-nine (42%) of the sites were collected in 2001 by remotely operated vehicles (ROVs). The thinner crusts on many samples apparently result from post-depositional breakage, landsliding, and intermittent burial of outcrops by sediment. The maximum crust thickness was selected from each dredge or dive site to best represent crusts on the original rock surface at that site. The measurements show an irregular progressive thickening of the crusts toward the northwest-i.e., progressive thickening toward the older volcanic features with increasing distance from the Hawaiian hotspot. Comparison of the maximum crust thickness with radiometric ages of related subaerial features supports previous studies that indicate a crust-growth rate of about 2.5 mm/m.y. The thickness information not only allows a comparison of the relative exposure ages of two or more features offshore from different volcanoes, but also provides specific age estimates of volcanic and landslide deposits. The data indicate that some of the landslide blocks within the south Kona landslide are the oldest exposed rock on Mauna Loa, Kilauea, or Loihi volcanoes. Crusts on the floors of submarine canyons off Kohala and East Molokai volcanoes indicate that these canyons are no longer serving as channelways for downslope, sediment-laden currents. Mahukona volcano was approximately synchronous with Hilo Ridge, both being younger than Hana Ridge. The Nuuanu landslide is considerably older than the Wailau landslide. The Waianae

Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in California

USGS Publications Warehouse

Coleman, D.S.; Frost, T.P.; Glazner, A.F.

1992-01-01

Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.

Crusted scabies-associated immune reconstitution inflammatory syndrome

PubMed Central

2012-01-01

Background Despite the widely accepted association between crusted scabies and human immunodeficiency virus (HIV)-infection, crusted scabies has not been included in the spectrum of infections associated with immune reconstitution inflammatory syndrome in HIV-infected patients initiating antiretroviral therapy. Case presentation We report a case of a 28-year-old Mexican individual with late HIV-infection, who had no apparent skin lesions but soon after initiation of antiretroviral therapy, he developed an aggressive form of crusted scabies with rapid progression of lesions. Severe infestation by Sarcoptes scabiei was confirmed by microscopic examination of the scale and skin biopsy. Due to the atypical presentation of scabies in a patient responding to antiretroviral therapy, preceded by no apparent skin lesions at initiation of antiretroviral therapy, the episode was interpreted for the first time as “unmasking crusted scabies-associated immune reconstitution inflammatory syndrome”. Conclusion This case illustrates that when crusted scabies is observed in HIV-infected patients responding to antiretroviral therapy, it might as well be considered as a possible manifestation of immune reconstitution inflammatory syndrome. Patient context should be considered for adequate diagnosis and treatment of conditions exacerbated by antiretroviral therapy-induced immune reconstitution. PMID:23181485

Scaly scalp associated with crusted scabies: case series.

PubMed

Anbar, T S; El-Domyati, M B; Mansour, H A; Ahmad, H M

2007-07-13

The diagnosis of crusted scabies is becoming more relevant due to the increase in number of immunocompromised patients. To date, more than 200 cases have been reported in the literature. However, crusted scabies seems to be under-diagnosed because of its unusual presentations. In this case series we present history, clinical manifestations, KOH smear, and histopathological findings of a series of four patients with crusted scabies. Scaly scalp was a prominent feature of the disease in all cases. Examination of and treatment of the scalp of patients with suspected crusted scabies should not be neglected. A KOH smear from the scalp offers a simple and reliable technique for diagnosis.

Ferromanganese crusts as indicators for paleoceanographic events in the NE Atlantic

NASA Astrophysics Data System (ADS)

Koschinsky, A.; Halbach, P.; Hein, J. R.; Mangini, A.

Hydrogenetic ferromanganese crusts reflect the chemical conditions of the seawater from which they formed. Fine-scale geochemical analysis of crust layers in combination with age determinations can therefore be used to investigate paleoceanographic changes which are recorded in geochemical gradients in the crusts. At Tropic seamount (off northwest Africa), uniform crust growth influenced by terrigenous input from the African continent occurred during approximately the past 12Ma. Phosphatization of these crusts is minor. In contrast, crusts from Lion seamount, located between Madeira and the Portuguese coast, display a much more variable growth history. A pronounced increase in Ni, Cu, and Zn is observed in some intervals of the crusts, which probably reflects increased surface productivity. A thick older phosphatized generation occurs in many samples. Hydrographic profiles indicate that Mediterranean outflow water (MOW) may play an important role in the composition of these crusts. 10Be dating of one sample confirms that the interruption of the MOW during the Messinian salinity crisis (6.2-5Ma ago) resulted in changes in element composition. Sr-isotope dating of the apatite phase of the old crust generation has been carried out to obtain a minimum age for the older generation of Atlantic crusts and to determine whether crust phosphatization in the Atlantic can be related to phosphatization episodes recorded in Pacific crusts. The preliminary data show that the old phosphatized crust generation might be as old as approximately 30-40Ma.

Ferromanganese crusts as indicators for paleoceanographic events in the NE Atlantic

USGS Publications Warehouse

Koschinsky, A.; Halbach, P.; Hein, J.R.; Mangini, A.

1996-01-01

Hydrogenetic ferromanganese crusts reflect the chemical conditions of the seawater from which they formed. Fine-scale geochemical analysis of crust layers in combination with age determinations can therefore be used to investigate paleoceanographic changes which are recorded in geochemical gradients in the crusts. At Tropic seamount (off northwest Africa), uniform crust growth influenced by terrigenous input from the African continent occurred during approximately the past 12 Ma. Phosphatization of these crusts is minor. In contrast, crusts from Lion seamount, located between Madeira and the Portuguese coast, display a much more variable growth history. A pronounced increase in Ni, Cu, and Zn is observed in some intervals of the crusts, which probably reflects increased surface productivity. A thick older phosphatized generation occurs in many samples. Hydrographic profiles indicate that Mediterranean outflow water (MOW) may play an important role in the composition of these crusts. 10Be dating of one sample confirms that the interruption of the MOW during the Messinian salinity crisis (6.2-5 Ma ago) resulted in changes in element composition. Sr-isotope dating of the apatite phase of the old crust generation has been carried out to obtain a minimum age for the older generation of Atlantic crusts and to determine whether crust phosphatization in the Atlantic can be related to phosphatization episodes recorded in Pacific crusts. The preliminary data show that the old phosphatized crust generation might be as old as approximately 30-40 Ma.

Crusted ("Norwegian") scabies in a specialist HIV unit: successful use of ivermectin and failure to prevent nosocomial transmission.

PubMed Central

Corbett, E L; Crossley, I; Holton, J; Levell, N; Miller, R; De Cock, K M

1996-01-01

A nosocomial outbreak of scabies in a specialist inpatient HIV unit resulted from a patient admitted with crusted scabies. Treatment of his infestation with topical scabicides alone failed and he remained infectious for several weeks. His infestation was then eradicated with combined topical treatment and oral ivermectin. In total, 14 (88%) out of 19 ward staff became symptomatic, and 4 (21%) had evidence of scabies on potassium hydroxide examination of skin scrapings. The ward infection control policy was changed to distinguish patients with crusted scabies from those with ordinary scabies. A second patient with crusted scabies was treated with combined oral and topical therapy early in his admission and nursed with more stringent isolation procedures. No nosocomial transmission occurred and his infestation responded rapidly to treatment. Patients with crusted scabies require strict barrier nursing if nosocomial transmission is to be avoided. Ivermectin combined with topical scabicides may be a more efficacious treatment than topical scabicides alone in such patients. Images PMID:8698358

Magnetic field effects on the crust structure of neutron stars

NASA Astrophysics Data System (ADS)

Franzon, B.; Negreiros, R.; Schramm, S.

2017-12-01

We study the effects of high magnetic fields on the structure and on the geometry of the crust in neutron stars. We find that the crust geometry is substantially modified by the magnetic field inside the star. We build stationary and axis-symmetric magnetized stellar models by using well-known equations of state to describe the neutron star crust, namely, the Skyrme model for the inner crust and the Baym-Pethick-Sutherland equation of state for the outer crust. We show that the magnetic field has a dual role, contributing to the crust deformation via the electromagnetic interaction (manifested in this case as the Lorentz force) and by contributing to curvature due to the energy stored in it. We also study a direct consequence of the crust deformation due to the magnetic field: the thermal relaxation time. This quantity, which is of great importance to the thermal evolution of neutron stars, is sensitive to the crust properties, and, as such, we show that it may be strongly affected by the magnetic field.

The potential roles of biological soil crusts in dryland hydrologic cycles

USGS Publications Warehouse

Belnap, J.

2006-01-01

Biological soil crusts (BSCs) are the dominant living cover in many drylands of the world. They possess many features that can influence different aspects of local hydrologic cycles, including soil porosity, absorptivity, roughness, aggregate stability, texture, pore formation, and water retention. The influence of biological soil crusts on these factors depends on their internal and external structure, which varies with climate, soil, and disturbance history. This paper presents the different types of biological soil crusts, discusses how crust type likely influences various aspects of the hydrologic cycle, and reviews what is known and not known about the influence of biological crusts on sediment production and water infiltration versus runoff in various drylands around the world. Most studies examining the effect of biological soil crusts on local hydrology are done by comparing undisturbed sites with those recently disturbed by the researchers. Unfortunately, this greatly complicates interpretation of the results. Applied disturbances alter many soil features such as soil texture, roughness, aggregate stability, physical crusting, porosity, and bulk density in ways that would not necessarily be the same if crusts were not naturally present. Combined, these studies show little agreement on how biological crusts affect water infiltration or runoff. However, when studies are separated by biological crust type and utilize naturally occurring differences among these types, results indicate that biological crusts in hyperarid regions reduce infiltration and increase runoff, have mixed effects in and regions, and increase infiltration and reduce runoff in semiarid cool and cold drylands. However, more studies are needed before broad generalizations can be made on how biological crusts affect infiltration and runoff. We especially need studies that control for sub-surface soil features such as bulk density, micro- and macropores, and biological crust structure. Unlike

The potential roles of biological soil crusts in dryland hydrologic cycles

NASA Astrophysics Data System (ADS)

Belnap, Jayne

2006-10-01

Biological soil crusts (BSCs) are the dominant living cover in many drylands of the world. They possess many features that can influence different aspects of local hydrologic cycles, including soil porosity, absorptivity, roughness, aggregate stability, texture, pore formation, and water retention. The influence of biological soil crusts on these factors depends on their internal and external structure, which varies with climate, soil, and disturbance history. This paper presents the different types of biological soil crusts, discusses how crust type likely influences various aspects of the hydrologic cycle, and reviews what is known and not known about the influence of biological crusts on sediment production and water infiltration versus runoff in various drylands around the world. Most studies examining the effect of biological soil crusts on local hydrology are done by comparing undisturbed sites with those recently disturbed by the researchers. Unfortunately, this greatly complicates interpretation of the results. Applied disturbances alter many soil features such as soil texture, roughness, aggregate stability, physical crusting, porosity, and bulk density in ways that would not necessarily be the same if crusts were not naturally present. Combined, these studies show little agreement on how biological crusts affect water infiltration or runoff. However, when studies are separated by biological crust type and utilize naturally occurring differences among these types, results indicate that biological crusts in hyperarid regions reduce infiltration and increase runoff, have mixed effects in arid regions, and increase infiltration and reduce runoff in semiarid cool and cold drylands. However, more studies are needed before broad generalizations can be made on how biological crusts affect infiltration and runoff. We especially need studies that control for sub-surface soil features such as bulk density, micro- and macropores, and biological crust structure

Quantifying glassy and crystalline basalt partitioning in the oceanic crust

NASA Astrophysics Data System (ADS)

Moore, Rachael; Ménez, Bénédicte

2016-04-01

The upper layers of the oceanic crust are predominately basaltic rock, some of which hosts microbial life. Current studies of microbial life within the ocean crust mainly focus on the sedimentary rock fraction, or those organisms found within glassy basalts while the potential habitability of crystalline basalts are poorly explored. Recently, there has been recognition that microbial life develops within fractures and grain boundaries of crystalline basalts, therefore estimations of total biomass within the oceanic crust may be largely under evaluated. A deeper understanding of the bulk composition and fractionation of rocks within the oceanic crust is required before more accurate estimations of biomass can be made. To augment our understanding of glassy and crystalline basalts within the oceanic crust we created two end-member models describing basalt fractionation: a pillow basalt with massive, or sheet, flows crust and a pillow basalt with sheeted dike crust. Using known measurements of massive flow thickness, dike thickness, chilled margin thickness, pillow lava size, and pillow lava glass thickness, we have calculated the percentage of glassy versus crystalline basalts within the oceanic crust for each model. These models aid our understanding of textural fractionation within the oceanic crust, and can be applied with bioenergetics models to better constrain deep biomass estimates.

FERROMANGANESE CRUST RESOURCES IN THE PACIFIC AND ATLANTIC OCEANS.

USGS Publications Warehouse

Commeau, R.F.; Clark, A.; Johnson, Chad; Manheim, F.T.; Aruscavage, P. J.; Lane, C.M.

1984-01-01

Ferromanganese crusts on raised areas of the ocean floor have joined abyssal manganese nodules and hydrothermal sulfides as potential marine resources. Significant volumes of cobalt-rich (about 1% Co) crusts have been identified to date within the US Exclusive Economic Zone (EEZ) in the Central Pacific: in the NW Hawaiian Ridge and Seamount region and in the seamounts in the Johnston Island and Palmyra Island regions. Large volumes of lower grade crusts, slabs, and nodules are also present in shallow ( greater than 1000 m) waters on the Blake plateau, off Florida-South Carolina in the Atlantic Ocean. Data on ferromanganese crusts have been increased by recent German and USGS cruises, but are still sparse, and other regions having crust potential are under current investigation. The authors discuss economic potentials for cobalt-rich crusts in the Central Pacific and Western North Atlantic oceans, with special reference to US EEZ areas. Additional research is needed before more quantitative resource estimates can be made.

Elemental composition of the Martian crust.

PubMed

McSween, Harry Y; Taylor, G Jeffrey; Wyatt, Michael B

2009-05-08

The composition of Mars' crust records the planet's integrated geologic history and provides clues to its differentiation. Spacecraft and meteorite data now provide a global view of the chemistry of the igneous crust that can be used to assess this history. Surface rocks on Mars are dominantly tholeiitic basalts formed by extensive partial melting and are not highly weathered. Siliceous or calc-alkaline rocks produced by melting and/or fractional crystallization of hydrated, recycled mantle sources, and silica-poor rocks produced by limited melting of alkali-rich mantle sources, are uncommon or absent. Spacecraft data suggest that martian meteorites are not representative of older, more voluminous crust and prompt questions about their use in defining diagnostic geochemical characteristics and in constraining mantle compositional models for Mars.

Deep-ocean ferromanganese crusts and nodules

USGS Publications Warehouse

Hein, James R.; Koschinsky, Andrea

2014-01-01

Ferromanganese crusts and nodules may provide a future resource for a large variety of metals, including many that are essential for emerging high- and green-technology applications. A brief review of nodules and crusts provides a setting for a discussion on the latest (past 10 years) research related to the geochemistry of sequestration of metals from seawater. Special attention is given to cobalt, nickel, titanium, rare earth elements and yttrium, bismuth, platinum, tungsten, tantalum, hafnium, tellurium, molybdenum, niobium, zirconium, and lithium. Sequestration from seawater by sorption, surface oxidation, substitution, and precipitation of discrete phases is discussed. Mechanisms of metal enrichment reflect modes of formation of the crusts and nodules, such as hydrogenetic (from seawater), diagenetic (from porewaters), and mixed diagenetic–hydrogenetic processes.

Rocks of the early lunar crust

NASA Technical Reports Server (NTRS)

James, O. B.

1980-01-01

Data are summarized which suggest a model for the early evolution of the lunar crust. According to the model, during the final stages of accretion, the outer part of the moon melted to form a magma ocean approximately 300 km deep. This ocean fractionated to form mafic and ultramafic cumulates at depth and an overlying anorthositic crust made up of ferroan anorthosites. Subsequent partial melting in the primitive mantle underlying the crystallized magma ocean produced melts which segregated, moved upward, intruded the primordial crust, and crystallized to form layered plutons consisting of Mg-rich plutonic rocks. Intense impact bombardment at the lunar surface mixed and melted the rocks of the two suites to form a thick layer of granulated debris, granulitic breccias, and impact-melt rocks.

A Lower-Crust or Mantle Source for Mineralizing Fluids Beneath the Olympic Dam IOCG Deposit, Australia: New Evidence From Magnetotelluric Sounding

NASA Astrophysics Data System (ADS)

Heinson, G.

2005-12-01

The iron-oxide-copper-gold (IOCG) Olympic Dam (OD) deposit, situated along the margin of the Proterozoic Gawler Craton, South Australia, is the world's largest uranium deposit, and sixth largest copper deposit; it also contains significant reserves of gold, silver and rare-earth elements (REE). Gaining a better understanding of the mechanisms for genesis of the economic mineralisation is fundamental for defining exploration models in similar crustal-settings. To delineate crustal structures that may constrain mineral system fluid pathways, coincident deep crustal seismic and magnetotelluric (MT) transects were obtained along a 220 km section that crosses OD and the major crustal boundaries. We present results from 58 long-period (10-104 s) MT sites, with site spacing of 5 to 10 km. A 2D inversion of all MT data to a depth of 100 km shows four notable features: (a) sedimentary cover sequences with low resistivity (<20 Ω.m) thicken to 10 km towards the northern cover sequences of the Adelaide Rift Complex; (b) a northeast-dipping crustal boundary separates a highly resistive (>1000 Ω.m) Archaean crustal core, from a more conductive crust to the north (typically <500 Ω.m); (c) to the north of OD, the crust to about 20 km is quite resistive (~1000 Ω.m), but the lower crust is much more conductive (<100 Ω.m); and (d) beneath OD, we image a low-resistivity region (<100 Ω.m) throughout the crust, coincident with a seismically transparent region. We argue that the cause of the low-resistivity and low-reflectivity region beneath OD may be due to the upward movement of crustal-volatiles that have deposited conductive graphite mineralisation along grain boundaries, simultaneously annihilating acoustic impedance boundaries. The source of the volatiles may be from the mantle-degassing or retrograde metamorphism of the lower crust associated with Proterozoic crustal deformation.

Biogeochemical Signals from Deep Microbial Life in Terrestrial Crust

PubMed Central

Fukuda, Akari; Komatsu, Daisuke D.; Hirota, Akinari; Watanabe, Katsuaki; Togo, Yoko; Morikawa, Noritoshi; Hagiwara, Hiroki; Aosai, Daisuke; Iwatsuki, Teruki; Tsunogai, Urumu; Nagao, Seiya; Ito, Kazumasa; Mizuno, Takashi

2014-01-01

In contrast to the deep subseafloor biosphere, a volumetrically vast and stable habitat for microbial life in the terrestrial crust remains poorly explored. For the long-term sustainability of a crustal biome, high-energy fluxes derived from hydrothermal circulation and water radiolysis in uranium-enriched rocks are seemingly essential. However, the crustal habitability depending on a low supply of energy is unknown. We present multi-isotopic evidence of microbially mediated sulfate reduction in a granitic aquifer, a representative of the terrestrial crust habitat. Deep meteoric groundwater was collected from underground boreholes drilled into Cretaceous Toki granite (central Japan). A large sulfur isotopic fractionation of 20–60‰ diagnostic to microbial sulfate reduction is associated with the investigated groundwater containing sulfate below 0.2 mM. In contrast, a small carbon isotopic fractionation (<30‰) is not indicative of methanogenesis. Except for 2011, the concentrations of H2 ranged mostly from 1 to 5 nM, which is also consistent with an aquifer where a terminal electron accepting process is dominantly controlled by ongoing sulfate reduction. High isotopic ratios of mantle-derived 3He relative to radiogenic 4He in groundwater and the flux of H2 along adjacent faults suggest that, in addition to low concentrations of organic matter (<70 µM), H2 from deeper sources might partly fuel metabolic activities. Our results demonstrate that the deep biosphere in the terrestrial crust is metabolically active and playing a crucial role in the formation of reducing groundwater even under low-energy fluxes. PMID:25517230

Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust

USGS Publications Warehouse

Akinin, V.V.; Miller, E.L.; Wooden, J.L.

2009-01-01

Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep

Evidence for partial melt in the crust beneath Mt. Paektu (Changbaishan), Democratic People’s Republic of Korea and China

USGS Publications Warehouse

Kyong-Song, Ri; Hammond, James O. S.; Chol-Nam, Ko; Hyok, Kim; Yong-Gun, Yun; Gil-Jong, Pak; Chong-Song, Ri; Oppenheimer, Clive; Liu, Kosima W.; Iacovino, Kayla D.; Kum-Ran, Ryu

2016-01-01

Mt. Paektu (also known as Changbaishan) is an enigmatic volcano on the border between the Democratic People’s Republic of Korea (DPRK) and China. Despite being responsible for one of the largest eruptions in history, comparatively little is known about its magmatic evolution, geochronology, or underlying structure. We present receiver function results from an unprecedented seismic deployment in the DPRK. These are the first estimates of the crustal structure on the DPRK side of the volcano and, indeed, for anywhere beneath the DPRK. The crust 60 km from the volcano has a thickness of 35 km and a bulk VP/VS of 1.76, similar to that of the Sino-Korean craton. The VP/VS ratio increases ~20 km from the volcano, rising to >1.87 directly beneath the volcano. This shows that a large region of the crust has been modified by magmatism associated with the volcanism. Such high values of VP/VS suggest that partial melt is present in the crust beneath Mt. Paektu. This region of melt represents a potential source for magmas erupted in the last few thousand years and may be associated with an episode of volcanic unrest observed between 2002 and 2005.

Evidence for micronutrient limitation of biological soil crusts: Importance to arid-lands restoration

USGS Publications Warehouse

Bowker, M.A.; Belnap, J.; Davidson, D.W.; Phillips, S.L.

2005-01-01

Desertification is a global problem, costly to national economies and human societies. Restoration of biological soil crusts (BSCs) may have an important role to play in the reversal of desertification due to their ability to decrease erosion and enhance soil fertility. To determine if there is evidence that lower fertility may hinder BSC recolonization, we investigated the hypothesis that BSC abundance is driven by soil nutrient concentrations. At a regional scale (north and central Colorado Plateau, USA), moss and lichen cover and richness are correlated with a complex water-nutrient availability gradient and have approximately six-fold higher cover and approximately two-fold higher species richness on sandy soils than on shale-derived soils. At a microscale, mosses and lichens are overrepresented in microhabitats under the north sides of shrub canopies, where water and nutrients are more available. At two spatial scales, and at the individual species and community levels, our data are consistent with the hypothesis that distributions of BSC organisms are determined largely by soil fertility. The micronutrients Mn and Zn figured prominently and consistently in the various analyses, strongly suggesting that these elements are previously unstudied limiting factors in BSC development. Structural-equation modeling of our data is most consistent with the hypothesis of causal relationships between the availability of micronutrients and the abundance of the two major nitrogen (N) fixers of BSCs. Specifically, higher Mn availability may determine greater Collema tenax abundance, and both Mn and Zn may limit Collema coccophorum; alternative causal hypotheses were less consistent with the data. We propose experimental trials of micronutrient addition to promote the restoration of BSC function on disturbed lands. Arid lands, where BSCs are most prevalent, cover ???40% of the terrestrial surface of the earth; thus the information gathered in this study is potentially useful

Micron-Scale Correlations Among Ti, P, Ce, and Y in Hadean Jack Hills Zircons

NASA Astrophysics Data System (ADS)

Hofmann, A. E.; Cavosie, A. J.; Valley, J. W.; Eiler, J. M.

2007-12-01

Detrital zircons and the inclusions found therein are our only mineralogical constraints on geologic events that occurred on the Hadean Earth. These zircons are commonly small (ca. <100 μm in the longest dimension) and preserve micron to sub-micron chemical zonations indicative of a dynamic petrological history. Trace elements within zircon are of particular interest because concentrations and ratios of these elements can provide information regarding chemical and physical conditions during zircon growth. In this study, we analyzed Hadean-age detrital zircons from Archean metasediment in the Jack Hills (Australia) using the Caltech Microanalysis Center Cameca NanoSIMS 50L. Trace elements analyzed included Ti, P, Ce, and Y. Ti- thermometry [1,2,3] can potentially constrain growth and/or re-equilibration temperatures of zircons; P, Ce, and Y are known to enter the zircon lattice by the coupled xenotime-type substitution mechanism: (Y, REE)3+ + P5+ = Zr4+ + Si4+ [5]. The 89Y/28Si ratio was observed to correlate with, and was used as a proxy for, cathodoluminescence (CL) banding. Growth features manifested in CL (e.g., sector, oscillatory zoning) were observed in all zircons analyzed. CL zones vary from <1 μm to several microns in width; therefore, the NanoSIMS---with a beam diameter resolved to ca. 250 nm on the sample surface when operating with an O- primary beam---is uniquely suited for this scale of analysis. Regions displaying CL banding were imaged as 20 x 20 μm areas. All elements were normalized to 28Si; 49Ti/28Si ratios were converted to [Ti] via calibration based on analyses of synthetic, high-Ti zircons (provided by B. Watson) that were independently analyzed on Caltech's JEOL JXA-8200 electron microprobe. We observe three types of relationships between trace element distribution and CL banding in the zircons imaged: 1) strong positive correlations between CL banding, P, Ce, and Ti; 2) subtle positive correlations between CL banding, P, Ce, and Ti; 3) no

Fluids of the Lower Crust: Deep Is Different

NASA Astrophysics Data System (ADS)

Manning, Craig E.

2018-05-01

Deep fluids are important for the evolution and properties of the lower continental and arc crust in tectonically active settings. They comprise four components: H2O, nonpolar gases, salts, and rock-derived solutes. Contrasting behavior of H2O-gas and H2O-salt mixtures yields immiscibility and potential separation of phases with different chemical properties. Equilibrium thermodynamic modeling of fluid-rock interaction using simple ionic species known from shallow-crustal systems yields solutions too dilute to be consistent with experiments and resistivity surveys, especially if CO2 is added. Therefore, additional species must be present, and H2O-salt solutions likely explain much of the evidence for fluid action in high-pressure settings. At low salinity, H2O-rich fluids are powerful solvents for aluminosilicate rock components that are dissolved as polymerized clusters. Addition of salts changes solubility patterns, but aluminosilicate contents may remain high. Fluids with Xsalt = 0.05 to 0.4 in equilibrium with model crustal rocks have bulk conductivities of 10‑1.5 to 100 S/m at porosity of 0.001. Such fluids are consistent with observed conductivity anomalies and are capable of the mass transfer seen in metamorphic rocks exhumed from the lower crust.

Psoriasis or crusted scabies.

PubMed

Goyal, N N; Wong, G A

2008-03-01

We describe a case of a 67-year-old woman with a 1-year history of nail thickening and a non-itchy erythematous scaly eruption on the fingertips. She was diagnosed with psoriasis and started on methotrexate after having had no response to topical calcipotriol. The diagnosis was reviewed after it was revealed by another consultant that the patient's husband had been attending dermatology clinics for several years with chronic pruritus, which had been repeatedly thought to be due to scabies. Our patient was found to have crusted scabies after a positive skin scraping showed numerous mites. She was treated with topical permethrin, keratolytics and oral ivermectin. We also review the literature on crusted scabies and its management, with recommendations.

Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187Os and 142Nd isotopic domains and the preservation of Hadean 129Xe in the modern convecting mantle

NASA Astrophysics Data System (ADS)

Jackson, M. G.; Shirey, S. B.; Hauri, E. H.; Kurz, M. D.; Rizo, H.

2016-07-01

The Re-Os systematics in 13 peridotite xenoliths hosted in young (<0.39 myr) rejuvenated lavas from the Samoan island of Savai'i and 8 peridotite xenoliths from 6 to 10 myr old lavas from the Austral island of Tubuai have been examined to evaluate the history of the oceanic mantle in this region. Modal mineralogy, trace element compositions and 187Os/188Os ratios suggest that these peridotites are not cognate or residual to mantle plumes but rather samples of Pacific oceanic lithosphere created at the ridge. Savai'i and Tubuai islands lie along a flow line in the Pacific plate, and provide two snapshots (separated by over 40 Ma in time) of Pacific mantle that originated in the same region of the East Pacific rise. Tubuai xenoliths exhibit 187Os/188Os from 0.1163 to 0.1304, and Savai'i (Samoa) xenoliths span a smaller range from 0.1173 to 0.1284. The 187Os/188Os ratios measured in Tubuai xenoliths are lower than (and show no overlap with) basalts from Tubuai. The 187Os/188Os of the Savai'i xenoliths overlap the isotopic compositions of lavas from the island of Savai'i, but also extend to lower 187Os/188Os than the lavas. 3He/4He measurements of a subset of the xenoliths range from 2.5 to 6.4 Ra for Tubuai and 10.8 to 12.4 Ra for Savai'i. Like abyssal peridotites and xenoliths from oceanic hotspots that sample the convecting mantle, Os isotopes from the Savai'i and Tubuai xenolith suites are relatively unradiogenic, but do not preserve a record of depleted early-formed (Hadean and Archean) mantle domains expected from earlier cycles of ridge-related depletion, continent extraction, or subcontinental lithospheric mantle erosion. The lack of preservation of early-formed, geochemically-depleted Os-isotopic and 142Nd/144Nd domains in the modern convecting mantle contrasts with the preservation of early-formed (early-Hadean) 129Xe/130Xe isotopic heterogeneities in the convecting mantle. This can be explained if the initial isotopic signatures in Re-Os and Sm-Nd systems

Kinetics of the crust thickness development of bread during baking.

PubMed

Soleimani Pour-Damanab, Alireza; Jafary, A; Rafiee, Sh

2014-11-01

The development of crust thickness of bread during baking is an important aspect of bread quality and shelf-life. Computer vision system was used for measuring the crust thickness via colorimetric properties of bread surface during baking process. Crust thickness had a negative and positive relationship with Lightness (L (*) ) and total color change (E (*) ) of bread surface, respectively. A linear negative trend was found between crust thickness and moisture ratio of bread samples. A simple mathematical model was proposed to predict the development of crust thickness of bread during baking, where the crust thickness was depended on moisture ratio that was described by the Page moisture losing model. The independent variables of the model were baking conditions, i.e. oven temperature and air velocity, and baking time. Consequently, the proposed model had well prediction ability, as the mean absolute estimation error of the model was 7.93 %.

Structure of the lower crust beneath the Carolina Trough, U.S. Atlantic continental margin

USGS Publications Warehouse

Tréhu, Anne M.; Ballard, A.; Dorman, L.M.; Gettrust, J.F.; Klitgord, Kim D.; Schreiner, A.

1989-01-01

Data from three large-offset seismic profiles provide information on the crustal structure beneath the Carolina trough. The profiles, obtained by the U.S. Geological Survey, the Naval Oceanographic Research Development Agency, and the Scripps Institution of Oceanography in 1985, were oriented parallel to the trough and were located (1) seaward of the East Coast Magnetic Anomaly (ECMA), which is generally thought to represent the boundary between oceanic and continental crust; (2) along the axis of the trough between the ECMA and the hinge zone, which is thought to reflect the landward limit of highly stretched and altered transitional crust; and (3) along the Carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting. These data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high-velocity (>7.1 km/s) lower crustal material that extends beneath the Carolina trough and the adjacent ocean basin. This lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough, thins to about 5 km seaward of the ECMA, and is either very thin or absent landward of the hinge zone. It is interpreted to represent material that was underplated beneath and/or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase. These data thus support the recent conclusions of White et al. (1987b) and Mutter et al. (1988) that the initiation of seafloor spreading is attended in many, if not most, cases by the generation of an anomalously large volume of melt.

Nonradial oscillation modes of compact stars with a crust

NASA Astrophysics Data System (ADS)

Flores, Cesar Vásquez; Hall, Zack B.; Jaikumar, Prashanth

2017-12-01

Oscillation modes of isolated compact stars can, in principle, be a fingerprint of the equation of state (EoS) of dense matter. We study the non-radial high-frequency l =2 spheroidal modes of neutron stars and strange quark stars, adopting a two-component model (core and crust) for these two types of stars. Using perturbed fluid equations in the relativistic Cowling approximation, we explore the effect of a strangelet or hadronic crust on the oscillation modes of strange stars. The results differ from the case of neutron stars with a crust. In comparison to fluid-only configurations, we find that a solid crust on top of a neutron star increases the p -mode frequency slightly with little effect on the f -mode frequency, whereas for strange stars, a strangelet crust on top of a quark core significantly increases the f -mode frequency with little effect on the p -mode frequency.

The crust structure of the Morasko meteorite - a preliminary hypothesis

NASA Astrophysics Data System (ADS)

Stankowski, Wojciech T. J.

2017-03-01

A small piece of the Morasko meteorite, weighing 970 g, yields traces of its journey through the Earth's atmosphere and of its impact into a mineral substrate, such as reflected in the meteorite's crust. This is seen in the crust structure in the form of sintered as well as fusion and semi-fusion layers for which ablative niches are optimum sites. Subsequent weathering processes have resulted in significant mineralogical changes in the crusts. The meteorite crusts originated during polygenetic processes.

Recycled oceanic crust in the source of 90-40 Ma basalts in North and Northeast China: Evidence, provenance and significance

NASA Astrophysics Data System (ADS)

Xu, Yi-Gang

2014-10-01

Major, trace element and Sr-Nd-Pb isotopic data of basalts emplaced during 90-40 Ma in the North and Northeast China are compiled in this review, with aims of constraining their petrogenesis, and by inference the evolution of the North China Craton during the late Cretaceous and early Cenozoic. Three major components are identified in magma source, including depleted component I and II, and an enriched component. The depleted component I, which is characterized by relatively low 87Sr/86Sr (<0.7030), moderate 206Pb/204Pb (18.2), moderately high εNd (∼4), high Eu/Eu∗ (>1.1) and HIMU-like trace element characteristics, is most likely derived from gabbroic cumulate of the oceanic crust. The depleted component II, which distinguishes itself by its high εNd (∼8) and moderate 87Sr/86Sr (∼0.7038), is probably derived from a sub-lithospheric ambient mantle. The enriched component has low εNd (2-3), high 87Sr/86Sr (>0.7065), low 206Pb/204Pb (17), excess Sr, Rb, Ba and a deficiency of Zr and Hf relative to the REE. This component is likely from the basaltic portion of the oceanic crust, which is variably altered by seawater and contains minor sediments. Comparison with experimental melts and trace element modeling suggest that these recycled oceanic components may be in form of garnet pyroxenite/eclogite. These components are young (<0.5 Ga) and show an Indian-MORB isotopic character. Given the share of this isotopic affinity by the extinct Izanaghi-Pacific plate, currently stagnated within the mantle transition zone, we propose that it ultimately comes from the subducted Pacific slab. Eu/Eu∗ and 87Sr/86Sr of the 90-40 Ma magmas increases and decreases, respectively, with decreasing emplacement age, mirroring a change in magma source from upper to lower parts of subducted oceanic crust. Such secular trends are created by dynamic melting of a heterogeneous mantle containing recycled oceanic crust. Due to different melting temperature of the upper and lower ocean

Continued Evidence for Input of Chlorine into the Martian Crust from Degassing of Chlorine-Rich Martian Magmas with Implications for Potential Habitability

NASA Astrophysics Data System (ADS)

Filiberto, J.; Gross, J.

2014-12-01

The chlorine-concentration (or salinity) of a fluid affects the potential for that fluid to be a habitable environment, with most known terrestrial organisms preferring low salinity fluids [1, 2]. The Martian crust (as analyzed by the Gamma Ray Spectrometer) is chlorine-rich with up to 0.8 wt% Cl; while the MER rovers Spirit and Opportunity as well as MSL Curiosity have analyzed rocks with even higher chlorine concentrations [e.g., 3]. This suggests that any potential fluid flowing through the crust would have high chlorine concentrations and therefore high salinity. Here we investigate the bulk and mineral chemistry of the SNC meteorites to constrain the pre-eruptive chlorine concentrations of Martian magmas as the potential source of chlorine in the Martian crust. Bulk SNC meteorites have Cl concentrations similar to terrestrial Mid Ocean Ridge Basalts which would suggest a Cl content of the Martian interior similar to that of the Earth [4]. However, based on Cl/La ratios, the Martian interior actually has 2-3 times more Cl than the Earth [5]. This is also reflected in the composition of Cl-rich minerals within the SNC meteorites [5, 6] and suggests that the pre-eruptive parental magmas to the SNC meteorites were Cl-rich. Eruption and degassing of such Cl-rich magmas would have delivered Cl to the Martian crust, thereby increasing the salinity of any fluids within the crust. [1] Rothschild L.J. and R.L. Mancinelli (2001) Nature. 409: 1092-1101. [2] Sharp Z.D. and D.S. Draper (2013) EPSL. 369-370: 71-77. [3] Taylor G.J. et al. (2010) GRL. 37: L12204. [4]. Burgess R. et al (2013) GCA 77: 793. [5] Filiberto J. and A.H. Treiman (2009) Geology. 37: 1087-1090. [6] McCubbin F.M. et al. (2013) MaPS. 48: 819-853.

Evidence for Thin Oceanic Crust on the Extinct Aegir Ridge, Norwegian Basin, N.E. Atlantic Derived from Satellite Gravity Inversion

NASA Astrophysics Data System (ADS)

Greenhalgh, E. E.; Kusznir, N. J.

2006-12-01

Satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the N.E. Atlantic. The inversion of gravity data to determine crustal thickness incorporates a lithosphere thermal gravity anomaly correction for both oceanic and continental margin lithosphere. Predicted crustal thicknesses in the Norwegian Basin are between 7 and 4 km on the extinct Aegir oceanic ridge which ceased sea-floor spreading in the Oligocene. Crustal thickness estimates do not include a correction for sediment thickness and are upper bounds. Crustal thicknesses determined by gravity inversion for the Aegir Ridge are consistent with recent estimates derived using refraction seismology by Breivik et al. (2006). Failure to incorporate a lithosphere thermal gravity anomaly correction produces an over-estimate of crustal thickness. Oceanic crustal thicknesses within the Norwegian Basin are predicted by the gravity inversion to increase to 9-10 km eastwards towards the Norwegian (Moere) and westwards towards the Jan Mayen micro-continent, consistent with volcanic margin continental breakup at the end of the Palaeocene. The observation (from gravity inversion and seismic refraction studies) of thin oceanic crust produced by the Aegir ocean ridge in the Oligocene has implications for the temporal evolution of asthenosphere temperature under the N.E. Atlantic during the Tertiary. Thin Oligocene oceanic crust may imply cool (normal) asthenosphere temperatures during the Oligocene in contrast to elevated asthenosphere temperatures in the Palaeocene and Miocene-Recent as indicated by volcanic margin formation and the formation of Iceland respectively. Gravity inversion also predicts a region of thin oceanic crust to the west of the northern part of the Jan Mayen micro-continent and to the east of the thicker oceanic crust currently being formed at the Kolbeinsey Ridge. Thicker crust (c.f. ocean basins) is

[Effects of soil crusts on surface hydrology in the semiarid Loess hilly area].

PubMed

Wei, Wei; Wen, Zhi; Chen, Li-Ding; Chen, Jin; Wu, Dong-Ping

2012-11-01

Soil crusts are distributed extensively in the Chinese Loess Plateau and play key roles in surface hydrological processes. In this study, a typical loess hilly region in Anjiagou catchment, Dingxi city, Gansu province was selected as the study region, and soil crusts in the catchment were investigated. Then, the hydrological effect of soil crusts was studied by using multi-sampling and hydrological monitoring experiments. Several key results were shown as follows. Firstly, compared with bared soil without crust cover, soil crusts can greatly reduce the bulk density, improve the porosity of soil, and raise the holding capacity of soil moisture which ranges from 1.4 to 1.9 times of that of bared soil. Secondly, the role of soil crust on rainfall interception was very significant. Moss crust was found to be strongest on rainfall interception, followed by synantectic crusts and lichen crusts. Bared soil without covering crusts was poorest in resisting rainfall splash. Thirdly, hydrological simulation experiments indicate that soil crusts play a certain positive role in promoting the water infiltration capacity, and the mean infiltration rate of the crusted soil was 2 times higher than that of the no-crust covered soils. While the accumulated infiltrated water amounts was also far higher than that of the bared soil.

Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth

NASA Astrophysics Data System (ADS)

Chowdhury, Priyadarshi; Gerya, Taras; Chakraborty, Sumit

2017-09-01

The rock record and geochemical evidence indicate that continental recycling has been occurring since the early history of the Earth. The stabilization of felsic continents in place of Earth's early mafic crust about 3.0 to 2.0 billion years ago, perhaps due to the initiation of plate tectonics, implies widespread destruction of mafic crust during this time interval. However, the physical mechanisms of such intense recycling on a hotter, (late) Archaean and presumably plate-tectonic Earth remain largely unknown. Here we use thermomechanical modelling to show that extensive recycling via lower crustal peeling-off (delamination but not eclogitic dripping) during continent-continent convergence was near ubiquitous during the late Archaean to early Proterozoic. We propose that such destruction of the early mafic crust, together with felsic magmatism, may have caused both the emergence of silicic continents and their subsequent isostatic rise, possibly above the sea level. Such changes in the continental character have been proposed to influence the Great Oxidation Event and, therefore, peeling-off plate tectonics could be the geodynamic trigger for this event. A transition to the slab break-off controlled syn-orogenic recycling occurred as the Earth aged and cooled, leading to reduced recycling and enhanced preservation of the continental crust of present-day composition.

1.8 billion years of fluid-crust interaction: A zircon oxygen isotope record for the lower crust, western Churchill Province, Canadian Shield

NASA Astrophysics Data System (ADS)

Petts, Duane C.; Moser, Desmond E.; Longstaffe, Frederick J.; Davis, William J.; Stern, Richard A.

2014-04-01

The western Churchill Province of the Canadian Shield experienced a prolonged and complex formation history (ca. 4.04 to 1.70 Ga), with evidence for multiple episodes of orogenesis and regional magmatic activity. Here we report on the oxygen isotopic compositions of garnet and zircon recovered from lower crustal xenoliths, which have U-Pb ages between ca. 3.5 and 1.7 Ga. Overall, zircon from four metabasite xenoliths from the Rankin Inlet sample suite have δ18O values ranging from + 5.5 to + 8.6‰. Zircon from three metatonalite/anorthosite xenoliths and five metabasite xenoliths from the Repulse Bay sample suite have δ18O values of + 5.6 to + 8.3‰. High δ18O values (> + 6.0‰) for the oldest igneous zircon cores (ca. 3.5 Ga and 3.0-2.6 Ga) indicate that their metatonalite/anorthosite protolith magmas were generated from, or had assimilated, supracrustal rocks that interacted previously with surface-derived fluids. Igneous zircon cores (ca. 2.9-2.6 Ga) from one metabasite xenolith have δ18O values of + 5.6 to + 6.4‰, which suggests a formation from a mantle-derived basaltic/gabbroic magma. Metamorphic zircon cores (ca. 2.0-1.9 Ga) from one metabasite xenolith commonly have δ18O values between + 6.0 and + 6.3‰, which is indicative of a basalt/gabbro protolith and localized reworking of the lower crust caused by regional-scale plate convergence. The wide range of δ18O values (+ 5.5 to + 8.3‰) for ca. 1.75-1.70 Ga metamorphic zircon rims (identified in all xenoliths) indicates regional transient heating and reworking of mantle- and supracrustal-derived crust, induced by magmatic underplating along the crust-mantle boundary.

Fiskenaesset Anorthosite Complex: Stable isotope evidence for shallow emplacement into Archean ocean crust

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peck, W.H.; Valley, J.W.

1996-06-01

Oxygen and hydrogen isotope ratios indicate that unusual rocks at the upper contact of the Archean Fiskenaesset Anorthosite Complex at Fiskenaesset Harbor (southwest Greenland) are the products of hydrothermal alteration by seawater at the time of anorthosite intrusion. Subsequent granulite-facies metamorphism of these Ca-poor and Al- and Mg-rich rocks produced sapphirine- and kornerupine-bearing assemblages. Because large amounts of surface waters cannot penetrate to depths of 30 km during granulite-facies metamorphism, the isotopic signature of the contact rocks must have been obtained prior to regional metamorphism. The stable isotope and geochemical characteristics of the contact rocks support a model of shallowmore » emplacement into Archean ocean crust for the Fiskenaesset Anorthosite Complex. 45 refs., 3 figs., 2 tabs.« less

The nature of orogenic crust in the central Andes

NASA Astrophysics Data System (ADS)

Beck, Susan L.; Zandt, George

2002-10-01

The central Andes (16°-22°S) are part of an active continental margin mountain belt and the result of shortening of the weak western edge of South America between the strong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield. We have combined receiver function and surface wave dispersion results from the BANJO-SEDA project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure. The major results are as follows: (1) The crust supporting the high elevations is thick and has a felsic to intermediate bulk composition. (2) The relatively strong Brazilian lithosphere is underthrusting as far west (65.5°W) as the high elevations of the western part of the Eastern Cordillera (EC) but does not underthrust the entire Altiplano. (3) The subcrustal lithosphere is delaminating piecemeal under the Altiplano-EC boundary but is not completely removed beneath the central Altiplano. The Altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation, leading to lower crustal flow and flat topography. In contrast, in the high-relief, inland-sloping regions of the EC and sub-Andean zone, the upper crust is still strongly coupled across the basal thrust of the fold-thrust belt to the underthrusting Brazilian Shield lithosphere. Subcrustal shortening between the Altiplano and Brazilian lithosphere appears to be accommodated by delamination near the Altiplano-EC boundary. Our study suggests that orogenic reworking may be an important part of the "felsification" of continental crust.

Mid-to-Lower-level Plutonic Rocks From Crust of the Southern Mariana Forearc: Implications for Growth of Continental Crust

NASA Astrophysics Data System (ADS)

Fryer, P.; Reagan, M.

2006-12-01

Tonalitic plutonic rocks dredged from the southern Mariana forearc are similar in terms of major element composition to tonalitic plutonic rocks of the Tanzawa Mountains on the Izu Peninsula of Japan. The tonalites of the Tanzawa Mountains have been interpreted to represent mid-lower crustal plutonic rocks that make up the 6.0 to 6.3 km/s layer identified in seismic velocity profiles of the Izu arc at 32°N. The tonalities of the southern Mariana forearc may be analogous to the Tanzawa tonalities in terms of lithology and presumably seismic velocities, but have distinctive trace element and isotopic compositions. The exposure of these rocks on the southern Mariana forearc in a location where it is narrower by up to 80 km than elsewhere along its strike indicates a truncation of the arc lithosphere by tectonic erosion in the southern Mariana forearc. If tectonic processes in the forearc have exposed silicic plutonic rock of the arc lithosphere within 150 km of the volcanic front, then the structure of the Mariana arc and forearc is likely similar to that of the Izu arc, where seismic velocity structure suggests 25% of the arc/forearc lithosphere is comprised of a mid-crustal level tonalitic plutonic complex. The trace element and Sr isotopic compositions of the tonalities dredged from the Mariana forearc links them to a suprasubduction-zone environment. The Pb isotopic compositions, however, are consistent with crystallization ages that may be as old as Cretaceous. The compositions of these tonalites differ markedly from those of silicic volcanic rocks that have erupted throughout the history of the IBM arc and suggest that they represent a minor component of the arc. Nevertheless, the presence of Cretaceous tonalites in the Mariana forearc suggests that a portion of its crust may predate subduction initiation. The presence of silicic mid-to-lower crustal level plutonics beneath the Mariana arc as well as Eocene rhyolites on Saipan indicate that average major

Snow and Ice Crust Changes over Northern Eurasia since 1966

NASA Astrophysics Data System (ADS)

Bulygina, O.; Groisman, P. Y.; Razuvaev, V.; Radionov, V.

2009-12-01

When temperature of snow cover reaches zero Celsius first time since its establishment, snowmelt starts. In many parts of the world this process can be lengthy. The initial amount of heat that “arrives” to the snowpack might be insufficient for complete snowmelt, during the colder nights re-freeze of the melted snow may occur (thus creating the ice crust layers), and a new cold front (or the departure of the warm front that initiated melt) can decrease temperatures below the freezing point again and stop the snowmelt completely. It well can be that first such snowmelt occurs in winter (thaw day) and for several months thereafter snowpack stays on the ground. However, even the first such melt initiates a process of snow metamorphosis on its surface changing snow albedo and generating snow crust as well as on its bottom generating ice crust. Once emerged, the crusts will not disappear until the complete snowmelt. Furthermore, these crusts have numerous pathways of impact on the wild birds and animals in the Arctic environment as well as on domesticated reindeers. In extreme cases, the crusts may kill some wild species and prevent reindeers’ migration and feeding. Ongoing warming in high latitudes created situations when in the western half of Eurasian continent days with thaw became more frequent. Keeping in mind potential detrimental impacts of winter thaws and associated with them snow/ice crust development, it is worthwhile to study directly what are the major features of snow and ice crust over Eurasia and what is their dynamics. For the purpose of this study, we employed the national snow survey data set archived at the Russian Institute for Hydrometeorological Information. The dataset has routine snow surveys run throughout the cold season each decade (during the intense snowmelt, each 5 days) at all meteorological stations of the former USSR, thereafter, in Russia since 1966. Prior to 1966 snow surveys are also available but the methodology of

Under the sea: microbial life in volcanic oceanic crust.

PubMed

Edwards, Katrina J; Wheat, C Geoffrey; Sylvan, Jason B

2011-09-06

Exploration of the microbiology in igneous, 'hard rock' oceanic crust represents a major scientific frontier. The igneous crust harbours the largest aquifer system on Earth, most of which is hydrologically active, resulting in a substantial exchange of fluids, chemicals and microorganisms between oceanic basins and crustal reservoirs. Study of the deep-subsurface biosphere in the igneous crust is technically challenging. However, technologies have improved over the past decade, providing exciting new opportunities for the study of deep-seated marine life, including in situ and cross-disciplinary experimentation in microbiology, geochemistry and hydrogeology. In this Progress article, we describe the recent advances, available technology and remaining challenges in the study of the marine intraterrestrial microbial life that is harboured in igneous oceanic crust.

Crusts: biological

USGS Publications Warehouse

Belnap, Jayne; Elias, Scott A.

2013-01-01

Biological soil crusts, a community of cyanobacteria, lichens, mosses, and fungi, are an essential part of dryland ecosystems. They are critical in the stabilization of soils, protecting them from wind and water erosion. Similarly, these soil surface communities also stabilized soils on early Earth, allowing vascular plants to establish. They contribute nitrogen and carbon to otherwise relatively infertile dryland soils, and have a strong influence on hydrologic cycles. Their presence can also influence vascular plant establishment and nutrition.

Galactic-cosmic-ray-produced 3He in a ferromanganese crust: any supernova 60Fe excess on earth?

PubMed

Basu, S; Stuart, F M; Schnabel, C; Klemm, V

2007-04-06

An excess of 60Fe in 2.4-3.2 x 10(6) year old ferromanganese crust (237 KD) from the deep Pacific Ocean has been considered as evidence for the delivery of debris from a nearby supernova explosion to Earth. Extremely high ;{3}He/;{4}He (up to 6.12 x 10(-3)) and 3He concentrations (up to 8 x 10(9) atoms/g) measured in 237 KD cannot be supernova-derived. The helium is produced by galactic cosmic rays (GCR) and delivered in micrometeorites that have survived atmospheric entry to be trapped by the crust. 60Fe is produced by GCR reactions on Ni in extraterrestrial material. The maximum (3)He/(60)Fe of 237 KD (80-850) is comparable to the GCR (3)He/(60)Fe production ratio (400-500) predicted for Ni-bearing minerals in iron meteorites. The excess 60Fe can be plausibly explained by the presence of micrometeorites trapped by the crust, rather than injection from a supernova source.

Cyanobacterial crust induction using two non-previously tested cyanobacterial inoculants: crusting capability and role of EPSs

NASA Astrophysics Data System (ADS)

Mugnai, Gianmarco; Rossi, Federico; De Philippis, Roberto

2017-04-01

The use of cyanobacteria as soil improvers and bio-conditioners (a technique often referred to as algalization) has been studied for decades. Several studies proved that cyanobacteria are feasible eco-friendly candidates to trigger soil fertilization and enrichment from agricultural to arid and hyper-arid systems. This approach can be successful to achieve stabilization and rehabilitation of degraded environments. Much of the effectiveness of algalization is due to the productivity and the characteristics of extracellular polysaccharides (EPSs) which, among their features, embed soil particles and promote the development of a first stable organo-mineral layer (cyanobacterial crusts). In natural settings, cyanobacterial crust induction represents a first step of a succession that may lead to the formation of mature biological soil crusts (Lan et al., 2014). The aim of this research was to investigate the crusting capabilities, and the characteristics of excreted EPSs by two newly tested non-heterocystous cyanobacterial inoculants, in microcosm experiments carried out using oligothrophic sand collected from sand dunes in Negev Desert, Israel. The cyanobacteria tested were Schizothrix AMPL1601, originally isolated from biocrusts collected in Hobq Desert, Inner Mongolia (China) and Leptolyngbia ohadii, originally isolated from biocrusts collected in Negev Desert, Israel. Inoculated microcosms were maintained at 30 °C in a growth chamber under continuous illumination and minimal water availability. Under such stressing conditions, and for a three-months incubation time, the growth and the colonization of the strains in the microcosms were monitored. At the same time, EPSs production and their chemical and macromolecular characteristics were determined by applying a methodology optimized for the purpose. Notably, EPSs were analyzed in two operationally-defined fractions, one more dispersed in the crust matrix (loosely bound EPSs, LB-EPSs) and one more condensed and

The Location and most Viable Magnetic Mineral of the Magnetic Layer of Mars Crust

NASA Astrophysics Data System (ADS)

Boutin, D.; Arkani-Hamed, J.

2010-12-01

The discovery of strong magnetic anomalies of remanant origin over the southern hemisphere of Mars [1] has provided the challenge to estimate the thickness of the magnetic crust and identify magnetic minerals capable of producing the anomalies. The power spectral analysis of the magnetic anomalies suggests a magnetic crust of 46 km thickness [2]. Estimates of depth to Curie temperature of viable magnetic mineral at about 4 Ga imply that the potentially magnetic layer must have been in the upper 70 km of the crust [3], and that the lower ~10 km must have been effectively demagnetized since by viscous decay [4]. The rock magnetic measurements show appreciable demagnetization at hydrostatic pressures up to 1.2 GPa [5], consistent with the above estimate of the magnetic layer thickness. The distinct lack of magnetic signature of many giant impact basins indicates that the impacts have demagnetized the crust. Detailed study of the magnetic anomalies surrounding Hellas, Isidis, and Argyre suggests that the area inside ~80% of the basin radius is almost completely demagnetized [6], as is confirmed by recent investigations [7,8]. First we use the evidence from these giant basins and show that Pierazzo et al. [1997] shock pressure distribution model with maximum decay exponent is most viable for Martian crust among the 6 models proposed. Using this model, we then determine the demagnetization of the crust by impacts that can create 10-500 km diameter craters. The surface of Mars is saturated by craters of diameters <100 km, which have completely demagnetized the upper ~10 km of Mars. The impacts that create 200-500 km diameter craters are capable of demagnetizing the entire crust beneath the craters. Second, we model topography, gravity, and magnetic data over all craters of diameters 300-600 km located in the southern hemisphere of Mars. The topography and gravity data suggest that majority of the craters are isostatically compensated and have distinct mantle plugs

Effects of Canary hotspot volcanism on structure of oceanic crust off Morocco

NASA Astrophysics Data System (ADS)

Holik, James S.; Rabinowitz, Philip D.; Austin, James A., Jr.

1991-07-01

Analysis of over 6400 km of multichannel seismics (MCS) and 50 sonobuoy reflection and refraction experiments reduced both in the domain of X-T and tau-p shows that a region within the Jurassic Quiet Zone off Morocco underwent dramatic changes as a result of the passage of the lithosphere over the Canary hotspot commencing approximately 60 Ma. A seismic unit (UCF), interpreted as volcanic in origin, is observed within the sediments in a region characterized by a broad bathymetric swell. It shows diffractions from its upper surface and an internally chaotic seismic facies and pinches out between sedimentary units of continuous, subparallel facies. A velocity inversion is noted between the UCF (4.7km/s) and the underlying sediment (3.1 km/s). The UCF is time transgressive; it lies near the Cretaceous/s Tertiary boundary in the northern portion of the study area and is younger to the south. Kinematic studies of the movement of the Canary hotspot relative to Africa show that the hotspot first appeared off NW Africa about 60 Ma and was located beneath oceanic crust in the region where the UCF is observed. Depth-to-basement measurements in areas not effected by the hotspot show a consistent linear trend of increased depth with age. In areas effected by the hotspot the thermal rejuvenation is evident as basement depths shoal with increased proximity to the present hotspot. The reheating of the crust resets the thermal age of the lithosphere with many of the properties of crust of a younger age. Subsidence curves of the reheated crust off Morocco show good correlation to subsidence curves of other reheated crust on a global basis. A zone characterized by high crustal velocities, (7.1-7.4 km/s) and greater crustal thicknesses (by ˜1-2 km) is observed in an area that corresponds to the bathymetric swell, the region of the UCF, and the reelevated basement. The high velocities and increased crustal thickness are interpreted to be the result of underplating and assimilation of

Eleven years of itching: a case report of crusted scabies.

PubMed

Kutlu, Nurdan S; Turan, Enver; Erdemir, Asli; Gürel, Mehmet S; Bozkurt, Erol

2014-08-01

Crusted scabies is a rare and highly contagious form of scabies that is characterized by uncontrolled proliferation of mites in the skin, extensive hyperkeratotic scaling, crusted lesions, and variable pruritus. We report the case of a 48-year-old man with an 11-year history of pruritic, hyperkeratotic, psoriasiform plaques and widespread erythematous papules that was diagnosed as crusted scabies.

[Crusted scabies induced by topical corticosteroids: A case report].

PubMed

Bilan, P; Colin-Gorski, A-M; Chapelon, E; Sigal, M-L; Mahé, E

2015-12-01

The frequency of scabies is increasing in France. Crusted (or Norwegian) scabies is a very contagious form of scabies because of the huge number of mites in the skin. It is observed in patients suffering from immunodepression, motor or sensory deficiency, or mental retardation. The clinical presentation, except for the classic manifestation of scabies, is characterized by crusted lesions. Treatment is not easy and requires hospitalization. Topical corticosteroids are frequently used for children's dermatological diseases. Their long-term and inappropriate application in an infested scabies child can induce crusted scabies. We report on a case of an 8-year-old boy who developed crusted scabies induced by topical corticosteroid application. We discuss the therapeutic aspects of this severe form of scabies. Copyright © 2015 Elsevier Masson SAS. All rights reserved.

Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust

USGS Publications Warehouse

McCubbin, Francis M.; Boyce, Jeremy W.; Novak-Szabo, Timea; Santos, Alison; Tartese, Romain; Muttik, Nele; Domokos, Gabor; Vazquez, Jorge A.; Keller, Lindsay P.; Moser, Desmond E.; Jerolmack, Douglas J.; Shearer, Charles K.; Steele, Andrew; Elardo, Stephen M.; Rahman, Zia; Anand, Mahesh; Delhaye, Thomas; Agee, Carl B.

2016-01-01

The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali-rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K-Ar, U-Pb in apatite, and U-Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre-Amazonian high-temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification.

Reduction of acrylamide content in bread crust by starch coating.

PubMed

Liu, Jie; Liu, Xiaojie; Man, Yong; Liu, Yawei

2018-01-01

A technique of starch coating to reduce acrylamide content in bread crust was proposed. Bread was prepared in accordance with a conventional procedure and corn or potato starch coating was brushed on the surface of the fermented dough prior to baking. Corn starch coating caused a decrease in acrylamide of 66.7% and 77.1% for the outer and inner crust, respectively. The decrease caused by the potato starch coating was 68.4% and 77.4%, respectively. Starch coating reduced asparagine content significantly (43.4-82.9%; P < 0.01)in both the outer and inner crust. A lower temperature (difference of 10-20 °C) in combination with a higher moisture content (maximum difference of 8%) of bread crust were a result of starch coating, which effectively shortened the time span (4-8 min) over which acrylamide could form and accumulate. The present study demonstrates that starch coating could be a simple, effective and practical application for reducing acrylamide levels in bread crust without changing the texture and crust color of bread. © 2017 Society of Chemical Industry. © 2017 Society of Chemical Industry.

The Deep Crust Magmatic Refinery, Part 1: A Coupled Thermodynamic and Two-phase Flow Model

NASA Astrophysics Data System (ADS)

Riel, N., Jr.; Bouilhol, P.; Van Hunen, J.; Velic, M.; Magni, V.

2016-12-01

Metamorphic and magmatic processes occurring in the deep crust ultimately control the chemical and physical characteristic of the continental crust. A complex interplay between magma intrusion, crystallization, and reaction with the pre-existing crust provide a wide range of differentiated magma and cumulates (and / or restites) that will feed the upper crustal levels with evolved melt while constructing the lower crust. With growing evidence from field and experimental studies, it becomes clearer that crystallization and melting processes are non-exclusive but should be considered together. Incoming H2O bearing mantle melts will start to fractionate to a certain extent, forming cumulates but also releasing heat and H2O to the intruded host-rock allowing it to melt in saturated conditions. The end-result of such dynamic system is a function of the amount and composition of melt input, and extent of reaction with the host which is itself dependent on the migration mode of the melts. To assess the dynamics of this deep magmatic system we developed a new 2-D two-phase flow code using finite volume method. Our formulation takes into account: (i) melt flow through a viscous porous matrix with temperature- and melt-content dependent host-rock viscosity, (ii) heat transfer, assuming local thermal equilibrium between solid and liquid, (iii) thermodynamic modelling of stable phases, (iv) injection of fractionated melt from crystallizing basalt at the Moho and (v) chemical advection of both the solid and liquid compositions. Here we present the core of our modelling approach, especially the petrological implementation. We show in details that our thermodynamic model can reproduce well both the sub- and supra solidus phase relationship and composition of the host-rock. We apply our method to an idealized amphibolite lower crust that is affected by a magmatic event represented by the intrusion of a wet mantle melt into the crust at Moho depth. The models [see Bouilhol et al

NO gas loss from biologically crusted soils in Canyonlands National Park, Utah

USGS Publications Warehouse

Barger, N.N.; Belnap, J.; Ojima, D.S.; Mosier, A.

2005-01-01

In this study, we examined N gas loss as nitric oxide (NO) from N-fixing biologically crusted soils in Canyonlands National Park, Utah. We hypothesized that NO gas loss would increase with increasing N fixation potential of the biologically crusted soil. NO fluxes were measured from biologically crusted soils with three levels of N fixation potential (Scytonema-Nostoc-Collema spp. (dark)>Scytonema-Nostoc-Microcoleus spp. (medium)>Microcoleus spp. (light)) from soil cores and field chambers. In both cores and field chambers there was a significant effect of crust type on NO fluxes, but this was highly dependent on season. NO fluxes from field chambers increased with increasing N fixation potential of the biologically crusted soils (dark>medium>light) in the summer months, with no differences in the spring and autumn. Soil chlorophyllasis Type a content (an index of N fixation potential), percent N, and temperature explained 40% of the variability in NO fluxes from our field sites. Estimates of annual NO loss from dark and light crusts was 0.04-0.16 and 0.02-0.11-N/ha/year. Overall, NO gas loss accounts for approximately 3-7% of the N inputs via N fixation in dark and light biologically crusted soils. Land use practices have drastically altered biological soil crusts communities over the past century. Livestock grazing and intensive recreational use of public lands has resulted in a large scale conversion of dark cyanolichen crusts to light cyanobacterial crusts. As a result, changes in biologically crusted soils in arid and semi-arid regions of the western US may subsequently impact regional NO loss. ?? Springer 2005.

Phantom Archean crust in Mangaia hotspot lavas and the meaning of heterogeneous mantle

NASA Astrophysics Data System (ADS)

Herzberg, C.; Cabral, R. A.; Jackson, M. G.; Vidito, C.; Day, J. M. D.; Hauri, E. H.

2014-06-01

Lavas from Mangaia in the Cook-Austral island chain, Polynesia, define an HIMU (or high μ, where μ=U238/Pb204) global isotopic end-member among ocean island basalts (OIB) with the highest 206,207,208Pb/204Pb. This geochemical signature is interpreted to reflect a recycled oceanic crust component in the mantle source. Mass independently fractionated (MIF) sulfur isotopes indicate that Mangaia lavas sampled recycled Archean material that was once at the Earth's surface, likely hydrothermally-modified oceanic crust. Recent models have proposed that crust that is subducted and then returned to the surface in a mantle plume is expected to transform to pyroxenite/eclogite during transit through the mantle. Here we examine this hypothesis for Mangaia using high-precision electron microprobe analysis on olivine phenocrysts. Contrary to expectations of a crustal component and, hence pyroxenite, results show a mixed peridotite and pyroxenite source, with peridotite dominating. If the isotopic compositions were inherited from subduction of recycled oceanic crust, our work shows that this source has phantom-like properties in that it can have its lithological identity destroyed while its isotope ratios are preserved. This may occur by partial melting of the pyroxenite and injection of its silicic melts into the surrounding mantle peridotite, yielding a refertilized peridotite. Evidence from one sample reveals that not all pyroxenite in the melting region was destroyed. Identification of source lithology using olivine phenocryst chemistry can be further compromised by magma chamber fractional crystallization, recharge, and mixing. We conclude that the commonly used terms mantle “heterogeneities” and “streaks” are ambiguous, and distinction should be made of its lithological and isotopic properties.

Evaporative losses from soils covered by physical and different types of biological soil crusts

USGS Publications Warehouse

Chamizo, S.; Cantón, Y.; Domingo, F.; Belnap, J.

2013-01-01

Evaporation of soil moisture is one of the most important processes affecting water availability in semiarid ecosystems. Biological soil crusts, which are widely distributed ground cover in these ecosystems, play a recognized role on water processes. Where they roughen surfaces, water residence time and thus infiltration can be greatly enhanced, whereas their ability to clog soil pores or cap the soil surface when wetted can greatly decrease infiltration rate, thus affecting evaporative losses. In this work, we compared evaporation in soils covered by physical crusts, biological crusts in different developmental stages and in the soils underlying the different biological crust types. Our results show that during the time of the highest evaporation (Day 1), there was no difference among any of the crust types or the soils underlying them. On Day 2, when soil moisture was moderately low (11%), evaporation was slightly higher in well-developed biological soil crusts than in physical or poorly developed biological soil crusts. However, crust removal did not cause significant changes in evaporation compared with the respective soil crust type. These results suggest that the small differences we observed in evaporation among crust types could be caused by differences in the properties of the soil underneath the biological crusts. At low soil moisture (<6%), there was no difference in evaporation among crust types or the underlying soils. Water loss for the complete evaporative cycle (from saturation to dry soil) was similar in both crusted and scraped soils. Therefore, we conclude that for the specific crust and soil types tested, the presence or the type of biological soil crust did not greatly modify evaporation with respect to physical crusts or scraped soils.

Aleutian basin oceanic crust

USGS Publications Warehouse

Christeson, Gail L.; Barth, Ginger A.

2015-01-01

We present two-dimensional P-wave velocity structure along two wide-angle ocean bottom seismometer profiles from the Aleutian basin in the Bering Sea. The basement here is commonly considered to be trapped oceanic crust, yet there is a change in orientation of magnetic lineations and gravity features within the basin that might reflect later processes. Line 1 extends ∼225 km from southwest to northeast, while Line 2 extends ∼225 km from northwest to southeast and crosses the observed change in magnetic lineation orientation. Velocities of the sediment layer increase from 2.0 km/s at the seafloor to 3.0–3.4 km/s just above basement, crustal velocities increase from 5.1–5.6 km/s at the top of basement to 7.0–7.1 km/s at the base of the crust, and upper mantle velocities are 8.1–8.2 km/s. Average sediment thickness is 3.8–3.9 km for both profiles. Crustal thickness varies from 6.2 to 9.6 km, with average thickness of 7.2 km on Line 1 and 8.8 km on Line 2. There is no clear change in crustal structure associated with a change in orientation of magnetic lineations and gravity features. The velocity structure is consistent with that of normal or thickened oceanic crust. The observed increase in crustal thickness from west to east is interpreted as reflecting an increase in melt supply during crustal formation.

Workshop on the Growth of Continental Crust

NASA Technical Reports Server (NTRS)

Ashwal, Lewis D. (Editor)

1988-01-01

Constraints and observations were discussed on a fundamental unsolved problem of global scale relating to the growth of planetary crusts. All of the terrestrial planets were considered, but emphasis was placed on the Earth's continental crust. The title of each session is: (1) Extraterrestrial crustal growth and destruction; (2) Constraints for observations and measurements of terrestrial rocks; (3) Models of crustal growth and destruction; and (4) Process of crustal growth and destruction.

Crustal Stretching Style and Lower Crust Flow of the South China Sea Northern Margin

NASA Astrophysics Data System (ADS)

Bai, Y.; Dong, D.; Runlin, D.

2017-12-01

There is a controversy about crustal stretching style of the South China Sea (SCS) northern margin mainly due to considerable uncertainty of stretching factor estimation, for example, as much as 40% of upper crust extension (Walsh et al., 1991) would be lost by seismic profiles due to poor resolution. To discover and understand crustal stretching style and lower crustal flow on the whole, we map the Moho and Conrad geometries based on gravity inversion constrained by deep seismic profiles, then according to the assumption of upper and lower crust initial thickness, upper and lower crust stretching factors are estimated. According to the comparison between upper and lower crust stretching factors, the SCS northern margin could be segmented into three parts, (1) sediment basins where upper crust is stretched more than lower crust, (2) COT regions where lower crust is stretched more than upper crust, (3) other regions where the two layers have similar stretching factors. Stretching factor map shows that lower crust flow happened in both of COT and sediment basin regions where upper crust decouples with lower crust due to high temperature. Pressure contrast by sediment loading in basins and erosion in sediment-source regions will lead to lower crust flow away from sediment sink to source. Decoupled and fractured upper crust is stretched further by sediment loading and the following compensation would result in relatively thick lower crust than upper crust. In COT regions with thin sediment coverage, low-viscosity lower crust is easier to thin in extensional environment, also the lower crust tends to flow away induced by magma upwelling. Therefore, continental crust on the margin is not stretching in a constant way but varies with the tectonic setting changes. This work is supported by National Natural Science Foundation of China (Grant No. 41506055, 41476042) and Fundamental Research Funds for the Central Universities China (No.17CX02003A).

Magnetization of the oceanic crust: TRM or CRM?

NASA Technical Reports Server (NTRS)

Raymond, C. A.; Labrecque, J. L.

1987-01-01

A model was proposed in which chemical remanent magnetization (CRM) acquired within the first 20 Ma of crustal evolution may account for 80% of the bulk natural remanent magnetization (NRM) of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low temperature alteration. The CRM intensity and direction are controlled by the post-emplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis, and the enhanced magnetization of the Cretaceous Quiet Zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further studies indicate varying rates of TRM decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer.

High-temperature hydrothermal circulation in the lower oceanic crust at fast spreading ridges: Reconciling geophysical and geochemical constraints

NASA Astrophysics Data System (ADS)

Wilcock, W.

2003-04-01

) inferred from oxygen isotope studies are often cited as evidence of limited circulation but when interpreted physically they are actually sufficient to transport a substantial proportion of the heat required to solidify and cool the lower crust. Nevertheless the geophysical constraints are also compatible with circulation in a two-layer double diffusive system favored by many researchers, in which the lower crust is cooled by a recirculating brine cell.

Osmium isotope variations in the oceans recorded by Fe-Mn crusts

USGS Publications Warehouse

Burton, K.W.; Bourdon, B.; Birck, J.-L.; Allegre, C.J.; Hein, J.R.

1999-01-01

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe-Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re/Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya-Tibet region is not a major source for Os. Recently formed Fe-Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (???4 km) possess a remarkably unradiogenic Os isotope composition (187Os/186Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input

Seismic wide-angle constraints on the crust of the southern Urals

NASA Astrophysics Data System (ADS)

Carbonell, R.; Gallart, J.; PéRez-Estaún, A.; Diaz, J.; Kashubin, S.; Mechie, J.; Wenzel, F.; Knapp, J.

2000-06-01

A wide-angle seismic reflection/refraction data set was acquired during spring 1995 across the southern Urals to characterize the lithosphere beneath this Paleozoic orogen. The wide-angle reflectivity features a strong frequency dependence. While the lower crustal reflectivity is in the range of 6-15 Hz, the PmP is characterized by frequencies below 6 Hz. After detailed frequency filtering, the seismic phases constrain a new average P wave velocity crustal model that consists of an upper layer of 5.0-6.0 km/s, which correlates with the surface geology; 5-7 km depths at which the velocities increase to 6.2-6.3 km/s; 10-30 km depths at which, on average, the crust is characterized by velocities of 6.6 km/s; and finally, the lower crust, from 30-35 km down to the Moho, which has velocities ranging from 6.8 to 7.4 km/s. Two different S wave velocity models, one for the N-S and one for the E-W, were derived from the analysis of the horizontal component recordings. Crustal sections of Poisson's ratio and anisotropy were calculated from the velocity models. The Poisson's ratio increases in the lower crust at both sides of the root zone. A localized 2-3% anisotropy zone is imaged within the lower crust beneath the terranes east of the root. This feature is supported by time differences in the SmS phase and by the particle motion diagrams, which reveal two polarized directions of motion. Velocities are higher in the central part of the orogen than for the Siberian and eastern plates. These seismic recordings support a 50-56 km crustal thickness beneath the central part of the orogen in contrast to Moho depths of ≈ 45 km documented at the edges of the transect. The lateral variation of the PmP phase in frequency content and in waveform can be taken as evidence of different genetic origins of the Moho in the southern Urals.

Greenhouse gas microbiology in wet and dry straw crust covering pig slurry.

PubMed

Hansen, Rikke R; Nielsen, Daniel Aa; Schramm, Andreas; Nielsen, Lars P; Revsbech, Niels P; Hansen, Martin N

2009-01-01

Liquid manure (slurry) storages are sources of gases such as ammonia (NH(3)) and methane (CH(4)). Danish slurry storages are required to be covered to reduce NH(3) emissions and often a floating crust of straw is applied. This study investigated whether physical properties of the crust or crust microbiology had an effect on the emission of the potent greenhouse gases CH(4) and nitrous oxide (N(2)O) when crust moisture was manipulated ("dry", "moderate", and "wet"). The dry crust had the deepest oxygen penetration (45 mm as compared to 20 mm in the wet treatment) as measured with microsensors, the highest amounts of nitrogen oxides (NO(2)(-) and NO(3)(-)) (up to 36 mumol g(-1) wet weight) and the highest emissions of N(2)O and CH(4). Fluorescent in situ hybridization and gene-specific polymerase chain reaction (PCR) were used to detect occurrence of bacterial groups. Ammonia-oxidizing bacteria (AOB) were abundant in all three crust types, whereas nitrite-oxidizing bacteria (NOB) were undetectable and methane-oxidizing bacteria (MOB) were only sparsely present in the wet treatment. A change to anoxia did not affect the CH(4) emission indicating the virtual absence of aerobic methane oxidation in the investigated 2-mo old crusts. However, an increase in N(2)O emission was observed in all crusted treatments exposed to anoxia, and this was probably a result of denitrification based on NO(x)(-) that had accumulated in the crust during oxic conditions. To reduce overall greenhouse gas emissions, floating crust should be managed to optimize conditions for methanotrophs.

Rheology and Thermal State of Titan's Crust: Potential Role of Methane Clathrates

NASA Astrophysics Data System (ADS)

Basu Sarkar, D.; Elwood Madden, M.

2014-12-01

Gravity and topography data including new results obtained from recent Cassini RADAR and Visible and Infrared Mapping Spectrometer (VIMS) observations suggest that Titan has a rigid and conductive crust, greater than 40 km in thickness (Beghin et al., 2012; Hemingway et al., 2013; Lopes et al., 2013; Mitri et al., 2014; Lefevre et al., 2014; Baland et al., 2014). In this work we employed rheological models based on clathrate hydrate stability fields and modeled geothermal gradients to investigate how clathrate hydrates may influence the rheology of Titan's crust. Our findings suggest that a thick, rigid, and conductive crust composed of pure water ice is unlikely. Instead, a mixed phase crust comprised of water ice and clathrate hydrates, with up to 40 to 50% methane clathrates, results in thermal conductivity, viscosity, and density values consistent with Cassini observations. We modeled variations in Rayleigh number with crustal thickness for different crustal compositions assuming constant viscosity. Addition of methane clathrates makes the mixed ice-clathrate crust more viscous resulting in smaller Rayleigh numbers with depth compared to ice-only models. This slower rate of increase in Rayleigh number is also associated with increasing critical Rayleigh numbers, and hence, the potential thickness of a rigid, conductive crust. Modeling basal viscosity for different crustal compositions, following McKinnon (2006), we also determined that a methane clathrate-rich conductive crust would likely be much thicker (~60 km) than a non-convecting pure water-ice crust (~12 km). Titan's carbon content constrained by different formation models (Tobie et al., 2012) shows that even a pure methane clathrate crust is possible. However, a pure methane clathrate crust is unlikely because it would be relatively thin, less than 20 km due to clathrate's low thermal conductivity. Therefore, a mixed phase crust may explain both the geophysical observations and significant methane

Hafnium isotope stratigraphy of ferromanganese crusts

PubMed

Lee; Halliday; Hein; Burton; Christensen; Gunther

1999-08-13

A Cenozoic record of hafnium isotopic compositions of central Pacific deep water has been obtained from two ferromanganese crusts. The crusts are separated by more than 3000 kilometers but display similar secular variations. Significant fluctuations in hafnium isotopic composition occurred in the Eocene and Oligocene, possibly related to direct advection from the Indian and Atlantic oceans. Hafnium isotopic compositions have remained approximately uniform for the past 20 million years, probably reflecting increased isolation of the central Pacific. The mechanisms responsible for the increase in (87)Sr/(86)Sr in seawater through the Cenozoic apparently had no effect on central Pacific deep-water hafnium.

Evolution of the lunar highland crust

NASA Technical Reports Server (NTRS)

Taylor, S. R.; Bence, A. E.

1975-01-01

The evolution of three distinct element associations in the lunar highland crust is discussed in terms of the Taylor-Jakes model which involves melting of most of the moon during accretion. Sources for (1) high Ca, Al, Sr, Eu, (2) high Mg and Cr, and (3) high K, REE, Zr, Hf, Nb are suggested. Bombardment by large projectiles during the differentiation process causes melting and mixing, which produces a wide range of compositions in the crust. The formation of dunite, troctolite, high-, medium-, and low-K Fra Mauro basalts, and rocks close to the olivine-spinel-plagioclase peritectic point is considered.

Supply-limited horizontal sand drift at an ephemerally crusted, unvegetated saline playa

USGS Publications Warehouse

Gillette, Dale A.; Niemeyer, T.C.; Helm, P.J.

2001-01-01

A site at Owens Dry Lake was observed for more than 4 years. The site was a vegetation-free saline playa where the surface formed "ephemeral crusts," crusts that form after rainfall. Sometimes these crusts were destroyed and often a layer of particles on the crust would engage in vigorous aeolian activity. Three "phases" of active sand drifting are defined as almost no movement (extreme supply limitation), loose particles on crust with some degree of sand drift (moderate supply limitation), and unlimited source movement corresponding to a destroyed surface crust (unlimited supply). These "phases" occurred 45, 49, and 6% of the time, respectively. The accumulation of loose particles on the crust was mostly the result of in situ formation. Crusted sediments with loose particles on top can exhibit mass flux rates about the same as for noncrusted sediments. Crusted sediments limit or eliminate sand drift in two conditions: for rough crusts that effect a sufficiently high threshold friction velocity (above the wind friction velocity) and for limited amounts of loose particles on the crust where particle supply is less than would be transported in normal saltation for a thick sandy surface. These "supply-limited" cases are similar to wind erosion of limited spilled material on a hard concrete surface. We quantified "supply limitation" by defining a "potential" or "supply unlimited" sand drift function Q = AG where A represents supply limitation that decreases as the particle source is depleted. Here Q is the mass of sand transported through a surface perpendicular to the ground and to the wind and having unit width during time period t, and G = ??? u*(u*2 - u*t2) dt for u* > u*t. G is integrated for the same time period t as for Q, u* is the friction velocity of the wind, and u*t is the threshold friction velocity of the wind. Hard crusts (usually formed in the summer) tended to show almost no change of threshold friction velocity with time and often gave total

Diffuse degassing through magmatic arc crust (Invited)

NASA Astrophysics Data System (ADS)

Manning, C. E.; Ingebritsen, S.

2013-12-01

The crust of magmatic arcs plays an important role in the volatile cycle at convergent margins. The fluxes of subduction- and arc-related volatiles such as H2O, C, Cl, S are poorly known. It is commonly believed that gases emitted from volcanoes account nearly quantitatively for the volatiles that cross the Moho beneath the volcanic front. This volcanic degassing may occur during eruption, emission from summit fumaroles and hot springs, or more 'diffuse' delivery to volcano flanks. However, several observations suggest that volatiles also transit arc crust by even more diffuse pathways, which could account for significant volatile loss on long time and length scales. Active metamorphism of arc crust produces crustal-scale permeability that is sufficient to transport a large volume of subducted volatiles (Ingebritsen and Manning, 2002, PNAS, 99, 9113). Arc magmas may reach volatile saturation deeper than the maximum depths recorded by melt inclusions (e.g., Blundy et al., 2010, EPSL, 290, 289), and exhumed sections of magmatic arc crust typically record voluminous plutons reflecting magma crystallization and volatile loss at depths well below the volcanic edifice. At shallower depths, topographically driven meteoric groundwater systems can absorb magmatic volatiles and transport them laterally by tens of km (e.g., James et al., 1999, Geology, 27, 823; Evans et al., 2002, JVGR, 114, 291). Hydrothermal ore deposits formed at subvolcanic depths sequester vast amounts of volatiles, especially sulfur, that are only returned to the surface on the time scale of exhumation and/or erosion. Water-rich metamorphic fluids throughout the crust can readily carry exsolved volcanic gases because the solubilities of volatile bearing minerals such as calcite, anhydrite, and fluorite are quite high at elevated pressure and temperature (e.g., Newton and Manning, 2002, Am Min, 87, 1401; 2005, J Pet, 46, 701; Tropper and Manning, 2007, Chem Geol, 242, 299). Taken together, these

Constraints on cooling of the lower ocean crust from epidote veins in the Wadi Gideah section, Oman Ophiolite

NASA Astrophysics Data System (ADS)

Diehl, Alexander; Bieseler, Bastian; Bach, Wolfgang

2017-04-01

Determining the depth, extent, and timing of high-temperature hydrothermal alteration in the ocean crust is key to understanding how the lower oceanic crust is cooled. We report data from 18 epidote veins from the Wadi Gideah section in the Wadi Tayin block, which is a reference section for alteration of the lower crust formed at a fast oceanic spreading center. 87Sr/86Sr ratios feature a narrow range from 0.70429 to 0.70512, while O isotope compositions vary between - 0.7 and +4.9‰ in δ18OSMOW. These compositions indicate uniform water-rock ratios between 1 and 2 and formation temperatures in the range of 300 to 450˚ C. There is no systematic trend in Sr and O isotope compositions down section. Fluid inclusion entrapment temperatures for a subset of four samples linearly increase from 338˚ C to 465˚ C in lowermost 3 km of crust of the Wadi Gideah section. Salinities are uniform throughout and scatter closely around seawater values. We developed a numerical cooling model to assign possible crustal ages to the thermal gradients observed. For pure conductive cooling, these ages range between 4 and 20 Ma. Our thermal model runs with a high Nusselt number (Nu) of 20 down to the base of the crust indicate that the epidote veins may record this near-axial deep circulation in crust of only 0.1 Ma (5-7 km off axis). When off-axis circulation is shut off in the more distal flanks, however, massive conductive reheating of the lower crust by as much as 200˚ C is predicted to take place. But there is no evidence for prograde metamorphic reactions in the samples we studied (or other hydrothermally altered oceanic gabbros). An intermediate model, in which Nu is 20 down to 2 km for the first 0.1 Ma and Nu is then 4 down to 6.5 km depth off axis to 1 Ma, is consistent with the permeability distribution within the ocean crust and predicts a thermal gradient for the lower crust that matches the observed one for ages between 1 and 3 Ma. The most plausible explanation for the

Microenvironments and microscale productivity of cyanobacterial desert crusts

USGS Publications Warehouse

Garcia-Pichel, F.; Belnap, Jayne

1996-01-01

We used microsensors to characterize physicochemical microenvironments and photosynthesis occurring immediately after water saturation in two desert soil crusts from southeastern Utah, which were formed by the cyanobacteria Microcoleus vaginatus Gomont, Nostoc spp., and Scytonema sp. The light fields within the crusts presented steep vertical gradients in magnitude and spectral composition. Near-surface light-trapping zones were formed due to the scattering nature of the sand particles, but strong light attenuation resulted in euphotic zones only ca. 1 mm deep, which were progressively enriched in longer wavelengths with depth. Rates of gross photosynthesis (3.4a??9.4 mmol O2A?ma??2A?ha??1) and dark respiration (0.81a??3.1 mmol Oa??2A?ma??2A?ha??1) occurring within 1 to several mm from the surface were high enough to drive the formation of marked oxygen microenvironments that ranged from oxygen supersaturation to anoxia. The photosynthetic activity also resulted in localized pH values in excess of 10, 2a??3 units above the soil pH. Differences in metabolic parameters and community structure between two types of crusts were consistent with a successional pattern, which could be partially explained on the basis of the microenvironments. We discuss the significance of high metabolic rates and the formation of microenvironments for the ecology of desert crusts, as well as the advantages and limitations of microsensor-based methods for crust investigation.

Linking biological soil crust diversity to ecological functions

NASA Astrophysics Data System (ADS)

Glaser, Karin; Borchhardt, Nadine; Schulz, Karoline; Mikhailyuk, Tatiana; Baumann, Karen; Leinweber, Peter; Ulf, Karsten

2016-04-01

Biological soil crusts (BSCs) are an association of different microorganisms and soil particles in the top millimeters of the soil. They are formed by algae, cyanobacteria, microfungi, bacteria, bryophytes and lichens in various compositions. Our aim was to determine and compare the biodiversity of all occurring organisms in biogeographically different habitats, ranging from polar (both Arctic and Antarctic), subpolar (Scandinavia), temperate (Germany) to dry regions (Chile). The combination of microscopy and molecular techniques (next-generation sequencing) revealed highly diverse crust communities, whose composition clustered by region and correlates with habitat characteristics such as water content. The BSC biodiversity was then linked to the ecological function of the crusts. The functional role of the BSCs in the biogeochemical cycles of carbon, nitrogen and phosphorous is evaluated using an array of state of the art soil chemistry methods including Py-FIMS (pyrolysis field ionization mass spectrometry) and XANES (x-ray absorbance near edge structure). Total P as well as P fractions were quantified in all BSCs, adjacent soil underneath and comparable nearby soil of BSC-free areas revealing a remarkable accumulation of total phosphorous and a distinct pattern of P fractions in the crust. Further, we observed an indication of a different P-speciation composition in the crust compared with BSC-free soil. The data allow answering the question whether BSCs act as sink or source for these compounds, and how biodiversity controls the biogeochemical function of BSCs.

Production and recycling of oceanic crust in the early Earth

NASA Astrophysics Data System (ADS)

van Thienen, P.; van den Berg, A. P.; Vlaar, N. J.

2004-08-01

Because of the strongly different conditions in the mantle of the early Earth regarding temperature and viscosity, present-day geodynamics cannot simply be extrapolated back to the early history of the Earth. We use numerical thermochemical convection models including partial melting and a simple mechanism for melt segregation and oceanic crust production to investigate an alternative suite of dynamics which may have been in operation in the early Earth. Our modelling results show three processes that may have played an important role in the production and recycling of oceanic crust: (1) Small-scale ( x×100 km) convection involving the lower crust and shallow upper mantle. Partial melting and thus crustal production takes place in the upwelling limb and delamination of the eclogitic lower crust in the downwelling limb. (2) Large-scale resurfacing events in which (nearly) the complete crust sinks into the (eventually lower) mantle, thereby forming a stable reservoir enriched in incompatible elements in the deep mantle. New crust is simultaneously formed at the surface from segregating melt. (3) Intrusion of lower mantle diapirs with a high excess temperature (about 250 K) into the upper mantle, causing massive melting and crustal growth. This allows for plumes in the Archean upper mantle with a much higher excess temperature than previously expected from theoretical considerations.

Formation of ferric iron crusts in Quaternary sediments of Lake Baikal, Russia, and implications for paleoclimate

USGS Publications Warehouse

Deike, R.G.; Granina, L.; Callender, E.; McGee, J.J.

1997-01-01

be explained by: (1) coupled adsorption of aqueous Ca and P by a colloidal ferric hydrous oxide; (2) loss of Fe from a Ca-P-Fe phase; or (3) oxidation of vivianite to a metastable mineral phase that gradually loses Ca and gains Fe. The first explanation is favored, because there is no petrographic evidence for either the existence of an originating Ca-P-Fe phase, or, for the oxidation of vivianite. Further, it is suggested that by continually equalizing surface charge, Ca allows more phosphate to be adsorbed leading to thicker crusts and longer preservation after burial.

Seismic structure of the Slave craton crust

NASA Astrophysics Data System (ADS)

Barantseva, O.; Vinnik, L. P.; Farra, V.; van der Hilst, R. D.; Artemieva, I. M.; Montagner, J. P.

2017-12-01

We present P- and S-receiver functions for 20 stations along a 200-km-long NNW-SSE seismological profile across the Slave craton, and estimate the average crustal Vp/Vs ratio which is indicative of rock composition. We observe high Vp/Vs ratio ( 1.85-2.00) for the bulk crust and elevated Vp values at a depth range from 20-30 km to 40 km. High Vp values (>7.0 km/s) suggest mafic composition of the lower crust. In case of dry lower crustal rocks, the Vp/Vs ratio is expected to range from 1.6 to 1.8, which is lower than the observed values of 1.9-2.0. Laboratory studies show that Vp/Vs 1.9-2.0 can be explained by the presence of numerous cracks saturated with an incompressible fluid. Our results are at odds with the structure of the cratonic crust in many regions worldwide, and may suggest a unique geodynamic evolution of the Slave crust. Possible explanations for the observed crustal structure include the presence of an underplated mafic material, possibly related to intraplate magmatism or paleosubduction. Receiver functions are highly sensitive to the change of acoustic impedance and S-wave velocities, but do not resolve the internal seismic structure with a high precision. We extend our study of the crustal structure by using ambient noise tomography (ANT). We measure Rayleigh wave dispersion from Green's functions that are estimated from one-year noise cross-correlation (NCF). The phase velocity maps are inverted for 1D wave speed profiles which are then combined to form 2D and 3D models of the crust of the Slave Province. The combined results of RF analyses and ANT are interpreted in terms of crustal structure, composition, and evolution.

Reformulation of pizza crust in restaurants may increase whole-grain intake among children.

PubMed

Tritt, Aimee; Reicks, Marla; Marquart, Len

2015-06-01

Whole-grain intake among children is well below recommendations. The purpose of the present study was to test the acceptability and liking of pizza made with whole-grain crust compared with refined-grain crust among children in restaurant and school settings. Plate waste data were collected via observation from child restaurant patrons consuming pizza made with either whole-grain or refined-grain crust. Waste was estimated by trained observers over eight months (August 2012-March 2013). Percentage waste was calculated and compared by crust type. A taste test was conducted with school children who tasted pizza made with whole-grain crust alongside pizza made with refined-grain crust and rated their liking of each product. Liking ratings were compared by crust type. Five Green Mill restaurant (a Midwest US chain) locations and one elementary school in the Minneapolis/St. Paul metropolitan area, Minnesota, USA. Child restaurant patrons (n 394) and school children (n 120, grades 3-5). Children consumed as much of the pizza made with whole-grain crust (42·1 %) as the pizza made with refined-grain crust (44·6 %; P=0·55), based on an average serving size of 350-400 g. Liking ratings for both types of pizza were high (>4·5 of 5) and did not differ by crust type (P=0·47). These positive consumption and liking outcomes indicate that whole-grain pizza crust is well accepted among children in a restaurant setting. The impact on whole-grain intake could be substantial if large, national restaurant chains served pizza made with whole-grain crust.

Contrasting effects of microbiotic crusts on runoff in desert surfaces

NASA Astrophysics Data System (ADS)

Kidron, Giora J.; Monger, H. Curtis; Vonshak, Ahuva; Conrod, William

2012-02-01

Microbiotic crusts (MCs) play an important role in surface hydrology by altering runoff yield. In order to study the crust's role on water redistribution, rainfall and runoff were measured during 1998-2000 at three sites within the northern Chihuahuan Desert, New Mexico, USA: the Sevilleta National Wildlife Refuge (SEV), the White Sands National Monument (WS), and the Jornada Experimental Range (JER). Whereas quartz and gypsum sand characterize the SEV and WS sites, respectively, both of which have high infiltration rates, silty alluvial deposits characterize the JER site. Runoff was measured in four pairs of 1.8-6.4 m 2 plots having MCs, one of which was scalped in each pair. No runoff was generated at WS, whether on the crusted or the scalped plots. Runoff was however generated at SEV and JER, being higher on the crusted plots at SEV and lower on the JER plots. The results were explained by the combined effect of (a) parent material and (b) the crust properties, such as species composition, microrelief (surface roughness) and exopolysaccharide (EPS) content (reflected in the ratio of carbohydrates to chlorophyll). Whereas the effective rainfall, the fines and the EPS content were found to explain runoff initiation, the effective rainfall and the crust microrelief were found to explain the amount of runoff at SEV and JER where runoff generation took place. The findings attest to the fundamental role of the parent material and the crust's species composition and properties on runoff and hence to the complex interactions and the variable effects that MCs have on dryland hydrology.

Continental crust beneath southeast Iceland.

PubMed

Torsvik, Trond H; Amundsen, Hans E F; Trønnes, Reidar G; Doubrovine, Pavel V; Gaina, Carmen; Kusznir, Nick J; Steinberger, Bernhard; Corfu, Fernando; Ashwal, Lewis D; Griffin, William L; Werner, Stephanie C; Jamtveit, Bjørn

2015-04-14

The magmatic activity (0-16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland--and especially the Öræfajökull volcano--is characterized by a unique enriched-mantle component (EM2-like) with elevated (87)Sr/(86)Sr and (207)Pb/(204)Pb. Here, we demonstrate through modeling of Sr-Nd-Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2-6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume.

Continental crust beneath southeast Iceland

PubMed Central

Torsvik, Trond H.; Amundsen, Hans E. F.; Trønnes, Reidar G.; Doubrovine, Pavel V.; Gaina, Carmen; Kusznir, Nick J.; Steinberger, Bernhard; Corfu, Fernando; Ashwal, Lewis D.; Griffin, William L.; Werner, Stephanie C.; Jamtveit, Bjørn

2015-01-01

The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland—and especially the Öræfajökull volcano—is characterized by a unique enriched-mantle component (EM2-like) with elevated 87Sr/86Sr and 207Pb/204Pb. Here, we demonstrate through modeling of Sr–Nd–Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2–6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume. PMID:25825769

Seismic imaging of extended crust with emphasis on the western United States

USGS Publications Warehouse

McCarthy, J.; Thompson, G.A.

1988-01-01

Understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust. This is particularly true in areas where the last tectonic event was extensional, such as the Basin and Range province of the western United States and much of western Europe. In these regions, a characteristic reflective pattern has emerged, whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective. In the metamorphic-core-complex belt in the western United States, where extension can be as much as an order of magnitude greater than in the more classic continental rift zones, the lower crustal reflectivity thickens and rises, yielding a picture of a crust that is reflective throughout. If metamorphic core complexes are representative of extended continental crust world-wide, then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world. -from Authors

Copper-nickel-rich, amalgamated ferromanganese crust-nodule deposits from Shatsky Rise, NW Pacific

USGS Publications Warehouse

Hein, J.R.; Conrad, T.A.; Frank, M.; Christl, M.; Sager, W.W.

2012-01-01

A unique set of ferromanganese crusts and nodules collected from Shatsky Rise (SR), NW Pacific, were analyzed for mineralogical and chemical compositions, and dated using Be isotopes and cobalt chronometry. The composition of these midlatitude, deep-water deposits is markedly different from northwest-equatorial Pacific (PCZ) crusts, where most studies have been conducted. Crusts and nodules on SR formed in close proximity and some nodule deposits were cemented and overgrown by crusts, forming amalgamated deposits. The deep-water SR crusts are high in Cu, Li, and Th and low in Co, Te, and Tl concentrations compared to PCZ crusts. Thorium concentrations (ppm) are especially striking with a high of 152 (mean 56), compared to PCZ crusts (mean 11). The deep-water SR crusts show a diagenetic chemical signal, but not a diagenetic mineralogy, which together constrain the redox conditions to early oxic diagenesis. Diagenetic input to crusts is rare, but unequivocal in these deep-water crusts. Copper, Ni, and Li are strongly enriched in SR deep-water deposits, but only in layers older than about 3.4 Ma. Diagenetic reactions in the sediment and dissolution of biogenic calcite in the water column are the likely sources of these metals. The highest concentrations of Li are in crust layers that formed near the calcite compensation depth. The onset of Ni, Cu, and Li enrichment in the middle Miocene and cessation at about 3.4 Ma were accompanied by changes in the deep-water environment, especially composition and flow rates of water masses, and location of the carbonate compensation depth.

Wüstite in the fusion crust of Almahata Sitta sulfide-metal assemblage MS-166: Evidence for oxygen in metallic melts

NASA Astrophysics Data System (ADS)

Horstmann, Marian; Humayun, Munir; Harries, Dennis; Langenhorst, Falko; Chabot, Nancy L.; Bischoff, Addi; Zolensky, Michael E.

2013-05-01

Meteorite fusion crusts form during the passage of a meteoroid through the Earth's atmosphere and are highly oxidized intergrowths as documented by the presence of e.g., oxides. The porous and irregular fusion crust surrounding the Almahata Sitta sulfide-metal assemblage MS-166 was found highly enriched in wüstite (Fe1-xO). Frictional heating of the outer portions of the assemblage caused partial melting of predominantly the Fe-sulfide and minor amounts of the outer Ni-rich portions of the originally zoned metal in MS-166. Along with melting significant amounts of oxygen were incorporated into the molten fusion crust and mainly FeS was oxidized and desulfurized to form wüstite. Considerable amounts of FeS were lost due to ablation, whereas the cores of the large metal grains appear largely unmelted leaving behind metal grains and surrounding wüstite-rich material (matte). Metal grains along with the surrounding matte typically form an often highly porous framework of globules interconnected with the matte. Although textures and chemical composition suggest that melting of Fe,Ni metal occurred only partially (Ni-rich rims), there is a trace elemental imprint of siderophile element partitioning influenced by oxygen in the metallic melt as indicated by the behavior of W and Ga, the two elements significantly affected by oxygen in a metallic melt. It is remarkable that MS-166 survived the atmospheric passage as troilite inclusions in iron meteorites are preferentially destroyed.

Osmium isotope stratigraphy of a marine ferromanganese crust

USGS Publications Warehouse

Klemm, V.; Levasseur, S.; Frank, M.; Hein, J.R.; Halliday, A.N.

2005-01-01

Ferromanganese crusts provide records of long term change in ocean circulation and continental weathering. However, calibrating their age prior to 10 Ma has been entirely based on empirical growth rate models using Co concentrations, which have inherently large uncertainties and fail to detect hiatuses and erosional events. We present a new method for dating these crusts by measuring their osmium (Os) isotope record and matching it to the well-known marine Os isotope evolution of the past 80 Ma. The well-characterised crust CD29-2 from the central Pacific, was believed to define a record of paleooceanographic change from 50 Ma. Previous growth rate estimates based on the Co method are consistent with the new Os isotope stratigraphy but the dating was grossly inaccurate due to long hiatuses that are now detectable. The new chronology shows that it in fact started growing prior to 70 Ma in the late Cretaceous and stopped growing or was eroded between 13.5 and 47 Ma. With this new technique it is now possible to exploit the full potential of the oceanographic and climatic records stored in Fe-Mn crusts. ?? 2005 Elsevier B.V. All rights reserved.

Self-Consistent Generation of Continental Crust in Global Mantle Convection Models

NASA Astrophysics Data System (ADS)

Jain, C.; Rozel, A. B.; Tackley, P.

2016-12-01

Numerical modelling commonly shows that mantle convection and continents have strong feedbacks on each other (Philips and Coltice, JGR 2010; Heron and Lowman, JGR 2014), but the continents are always inserted a priori while basaltic (oceanic) crust is generated self-consistently in such models (Rolf et al., EPSL 2012). We aim to implement self-consistent generation of continental crust in global models of mantle convection using StagYY (Tackley, PEPI 2008). The silica-rich continental crust appears to have been formed by fractional melting and crystallization in episodes of relatively rapid growth from late Archaean to late Proterozoic eras (3-1 Ga) (Hawkesworth & Kemp, Nature 2006). It takes several stages of differentiation to generate continental crust. First, the basaltic magma is extracted from the pyrolitic mantle. Second, it goes through eclogitic transformation and then partially melts to form Na-rich Tonalite-Trondhjemite-Granodiorite (TTG) which rise to form proto-continents (Rudnick, Nature 1995; Herzberg & Rudnick, Lithos 2012). TTGs dominate the grey gneiss complexes which make up most of the continental crust. Based on the melting conditions proposed by Moyen (Lithos, 2011), we parameterize TTG formation and henceforth, the continental crust. Continental crust can also be destroyed by subduction or delamination. We will investigate continental growth and destruction history in the models spanning the age of the Earth.

Self-consistent generation of continental crust in global mantle convection models

NASA Astrophysics Data System (ADS)

Jain, Charitra; Rozel, Antoine; Tackley, Paul

2017-04-01

Numerical modeling commonly shows that mantle convection and continents have strong feedbacks on each other (Philips and Coltice, JGR 2010; Heron and Lowman, JGR 2014), but the continents are always inserted a priori while basaltic (oceanic) crust is generated self-consistently in such models (Rolf et al., EPSL 2012). We aim to implement self-consistent generation of continental crust in global models of mantle convection using StagYY (Tackley, PEPI 2008). The silica-rich continental crust appears to have been formed by fractional melting and crystallization in episodes of relatively rapid growth from late Archean to late Proterozoic eras (3-1 Ga) (Hawkesworth & Kemp, Nature 2006). It takes several stages of differentiation to generate continental crust. First, the basaltic magma is extracted from the pyrolitic mantle. Second, it goes through eclogitic transformation and then partially melts to form Na-rich Tonalite-Trondhjemite-Granodiorite (TTG) which rise to form proto-continents (Rudnick, Nature 1995; Herzberg & Rudnick, Lithos 2012). TTGs dominate the grey gneiss complexes which make up most of the continental crust. Based on the melting conditions proposed by Moyen (Lithos, 2011), we parameterize TTG formation and henceforth, the continental crust. Continental crust can also be destroyed by subduction or delamination. We will investigate continental growth and destruction history in the models spanning the age of the Earth.

Permeability of continental crust influenced by internal and external forcing

USGS Publications Warehouse

Rojstaczer, S.A.; Ingebritsen, S.E.; Hayba, D.O.

2008-01-01

The permeability of continental crust is so highly variable that it is often considered to defy systematic characterization. However, despite this variability, some order has been gleaned from globally compiled data. What accounts for the apparent coherence of mean permeability in the continental crust (and permeability-depth relations) on a very large scale? Here we argue that large-scale crustal permeability adjusts to accommodate rates of internal and external forcing. In the deeper crust, internal forcing - fluxes induced by metamorphism, magmatism, and mantle degassing - is dominant, whereas in the shallow crust, external forcing - the vigor of the hydrologic cycle - is a primary control. Crustal petrologists have long recognized the likelihood of a causal relation between fluid flux and permeability in the deep, ductile crust, where fluid pressures are typically near-lithostatic. It is less obvious that such a relation should pertain in the relatively cool, brittle upper crust, where near-hydrostatic fluid pressures are the norm. We use first-order calculations and numerical modeling to explore the hypothesis that upper-crustal permeability is influenced by the magnitude of external fluid sources, much as lower-crustal permeability is influenced by the magnitude of internal fluid sources. We compare model-generated permeability structures with various observations of crustal permeability. ?? 2008 The Authors Journal compilation ?? 2008 Blackwell Publishing Ltd.

Wind erodibility response of physical and biological crusts to rain and flooding

NASA Astrophysics Data System (ADS)

Aubault, H.; Bullard, J. E.; Strong, C. L.; Ghadiri, H.; McTainsh, G. H.

2015-12-01

Soil surface crusts are important controllers of the small-scale wind entrainment processes that occur across all dust source regions globally. The crust type influences water and wind erosion by impacting infiltration, runoff, threshold wind velocity and surface storage capacity of both water and loose erodible material. The spatial and temporal patterning of both physical and biological crusts is known to change with rainfall and flooding. However, little is known about the impact of differing water quantity (from light rainfall through to flooding) on soil crusting characteristics (strength, roughness, sediment loss). This study compares the response of two soil types (loamy sand - LS, sandy loam - SL) with and without BSCs to three different rainfall events (2mm, 8mm, 15mm). Two BSC treatments were used one that simulated a young cyanobacteria dominated crust and an older flood induced multi species biological crust. For both soil types, soil surface strength increased with increasing rainfall amount with LS having consistently higher resistance to rupture than SL. Regardless of texture, soils with BSCs were more resistant and strength did not change in response to rainfall impact. Soil loss due to wind erosion was substantially higher on bare LS (4 times higher) and SL (3 times higher) soils compared with those with BSCs. Our results also show that young biological crust (formed by the rainfall event) have reduced soil erodibility with notably greater strength, roughness and reduced sediment losses when compared to soils with physical crust. Interestingly though, the erodibility of the old BSC did not differ greatly from that of the young BSC with respect to strength, roughness and sediment loss. This raises questions regarding the rapid soil surface protection offered by young colonising cyanobacteria crusts. Further analyses exploring the role of biological soil crusts on surface response to rainfall and wind saltation impact are ongoing.

Physical properties and seismic structure of Izu-Bonin-Mariana fore-arc crust: Results from IODP Expedition 352 and comparison with oceanic crust

NASA Astrophysics Data System (ADS)

Christeson, G. L.; Morgan, S.; Kodaira, S.; Yamashita, M.; Almeev, R. R.; Michibayashi, K.; Sakuyama, T.; Ferré, E. C.; Kurz, W.

2016-12-01

Most of the well-preserved ophiolite complexes are believed to form in suprasubduction zone (SSZ) settings. We compare physical properties and seismic structure of SSZ crust at the Izu-Bonin-Mariana (IBM) fore arc with oceanic crust drilled at Holes 504B and 1256D to evaluate the similarities of SSZ and oceanic crust. Expedition 352 basement consists of fore-arc basalt (FAB) and boninite lavas and dikes. P-wave sonic log velocities are substantially lower for the IBM fore arc (mean values 3.1-3.4 km/s) compared to Holes 504B and 1256D (mean values 5.0-5.2 km/s) at depths of 0-300 m below the sediment-basement interface. For similar porosities, lower P-wave sonic log velocities are observed at the IBM fore arc than at Holes 504B and 1256D. We use a theoretical asperity compression model to calculate the fractional area of asperity contact Af across cracks. Af values are 0.021-0.025 at the IBM fore arc and 0.074-0.080 at Holes 504B and 1256D for similar depth intervals (0-300 m within basement). The Af values indicate more open (but not necessarily wider) cracks in the IBM fore arc than for the oceanic crust at Holes 504B and 1256D, which is consistent with observations of fracturing and alteration at the Expedition 352 sites. Seismic refraction data constrain a crustal thickness of 10-15 km along the IBM fore arc. Implications and inferences are that crust-composing ophiolites formed at SSZ settings could be thick and modified after accretion, and these processes should be considered when using ophiolites as an analog for oceanic crust.

Formation of ferromanganese crusts on northwest intertropical Pacific seamounts: Electron photomicrography and microprobe chemistry

USGS Publications Warehouse

Jeong, K.S.; Jung, H.-S.; Kang, J.-K.; Morgan, C.L.; Hein, J.R.

2000-01-01

Seven ferromanganese crusts from the northwest intertropical Pacific seamounts were analyzed for photomicroscopic growth structures, microprobe chemistry, and ages based on Co-chronometer growth rate. The crusts on the Marshall Islands seamounts are thick and ale divided into phosphatized lower older and nonphosphatized upper younger growth generations: the older crust consists of compact laminations and columns impregnated with carbonate fluoapatite (CFA), whereas the younger crust is characterized by porous botryoids and columns of ??-MnO2 and Fe oxyhydroxide. The crusts on the Federated States of Micronesia (FSM) and Palau Islands seamounts are thin and are often incorporated with inorganic opal-A in the uppermost part, comprising the younger generation. Some crusts show scours and fractures. Although the growth of crusts has been often interrupted by mass failure of slope sediments, the crusts on the Marshall Islands seamounts are estimated to have grown at rate of about 3 mm/Ma since the middle Eocene and to have been phosphatized in the late Oligocene during the host seamounts were located beneath the equatorial zone of high productivity. Prolonged infiltration of the oxygen minimum zone (OMZ) water into shallower water older crusts redistributed crust composition by precipitating CFA, enriching subsequent amounts of Mn and Ni, and removing some Co. The younger crust has formed at slower rate (about 2 mm/Ma) under the stronger influence of bottom-water circulation in the north of the equatorial zone, concentrating abundant Co. In the uppermost part of some crusts, siliceous skeletons transform with burial to inorganic opal-A and Si-rich Fe oxyhydroxide, suggesting that biosilica diagenesis can enhance crust growth. (C) 2000 Elsevier Science B.V.

Soil stabilization by biological soil crusts in arid Tunisia

NASA Astrophysics Data System (ADS)

Guidez, Sabine; Couté, Alain; Bardat, Jacques

2015-04-01

As part of the fight against desertification (LCD) in arid Tunisia, we have been able to highlight the important role played by biological soil crusts (BSC) in soil stabilization. The identification of the major species of cyanobacteria, lichens and bryophytes, their adaptation and terrestrial colonization strategies in this high climatic constraints area through their morpho-anatomical criteria have been set. In addition to their biological composition, their internal arrangement (i.e. texture and microstructure) reflects the structural stability of BSC against erosion. Precisely, the aggregative power of cyanobacteria and their ways of moving inside a soil, the capacity of mosses to grow through the sediments and lichens ability to bind at particles on surface, thus stabilizing the substrate have been demonstrated. Then, the three biological components ability to capture soil particles has been widely illustrated, proving the major environmental contribution of BSC in arid areas biological crusts formation, providing that soils will experience an increase of organic matter and fine particles rates subsequently gaining faster and better stability. Although the thickness and the morphology of crusts are related to the cover rates of these different biological components, the water properties of the latter, studied at the environmental SEM, illustrate their important role in altering the water cycle. Thus, the mixed crusts, i.e. with good presence of three biological components, cause the highest runoff rates by their ability to retain the water and spread on the surface. In spite of a swelling coefficient in presence of water higher than cryptogams, the cyanobacterial crusts located in newly stabilized areas of our studied region, remain finally insufficiently dense to impact surface hydrology. But, we showed after all that the cyanobacteria, pioneer species, have a certain environmental role. The lichen crusts cause a increased runoff because the lichens have a

Origin of silicic crust by rifting and bimodal plume volcanism in the Afar Depression

NASA Astrophysics Data System (ADS)

Ghatak, A.; Basu, A. R.; Ebinger, C. J.

2010-12-01

the lower mantle, and this source zone showed little variation over the past 30 Ma. Some of the silicic lavas fall distinctly outside the plume field toward more radiogenic 87Sr/86Sr at relatively restricted Nd and Pb isotopic compositions. This excursion in Sr-isotopic ratios of the silicic lavas, in concert with their positive Ce-anomaly, is interpreted to be due to mixing of the Afar plume derived basaltic magma with fluids from saline aquifers. We conclude that the bimodal lavas are consanguineous, the silicic lavas being generated by fractional crystallization of plagioclase in a lower mantle plume-derived basaltic magma-chamber, caused by the interaction with saline aquifers. The generation of bimodal volcanism from parental primitive basalts without any contribution from pre-existing continental crust in Dabbahu may explain other similar intraplate magmatism including early Archean-Hadean continental crust formation prior to onset of arc-volcanism.

[Development and succession of artificial biological soil crusts and water holding characteristics of topsoil].

PubMed

Wu, Li; Chen, Xiao-Guo; Zhang, Gao-Ke; Lan, Shu-Bin; Zhang, De-Lu; Hu, Chun-Xiang

2014-03-01

In order to understand the improving effects of cyanobacterial inoculation on water retention of topsoil in desert regions, this work focused on the development and succession of biological soil crusts and water holding characteristics of topsoil after cyanobacterial inoculation in Qubqi Desert. The results showed that after the artificial inoculation of desert cyanobacteria, algal crusts were quickly formed, and in some microenvironments direct succession of the algal crusts to moss crusts occurred after 2-3 years. With the development and succession of biological soil crusts, the topsoil biomass, polysaccharides content, crust thickness and porosity increased, while the soil bulk density decreased. At the same time, with crust development and succession, the topsoil texture became finer and the percents of fine soil particles including silt and clay contents increased, while the percents of coarse soil particles (sand content) decreased proportionately. In addition, it was found that with crust development and succession, the water holding capacity and water content of topsoil showed an increasing trend, namely: moss crust > algal crusts > shifting sand. The water content (or water holding capacity) in algal and moss crusts were 1.1-1.3 and 1.8-2.2 times of those in shifting sand, respectively. Correlation analysis showed that the water holding capacity and water content of topsoil were positively correlated with the crust biomass, polysaccharides content, thickness, bulk density, silt and clay content; while negatively correlated with the porosity and sand content. Furthermore, stepwise regression analysis showed that the main factor affecting water content was the clay content, while that affecting water holding capacity was the porosity.

Late-time Cooling of Neutron Star Transients and the Physics of the Inner Crust

NASA Astrophysics Data System (ADS)

Deibel, Alex; Cumming, Andrew; Brown, Edward F.; Reddy, Sanjay

2017-04-01

An accretion outburst onto a neutron star transient heats the neutron star’s crust out of thermal equilibrium with the core. After the outburst, the crust thermally relaxes toward equilibrium with the neutron star core, and the surface thermal emission powers the quiescent X-ray light curve. Crust cooling models predict that thermal equilibrium of the crust will be established ≈ 1000 {days} into quiescence. Recent observations of the cooling neutron star transient MXB 1659-29, however, suggest that the crust did not reach thermal equilibrium with the core on the predicted timescale and continued to cool after ≈ 2500 {days} into quiescence. Because the quiescent light curve reveals successively deeper layers of the crust, the observed late-time cooling of MXB 1659-29 depends on the thermal transport in the inner crust. In particular, the observed late-time cooling is consistent with a low thermal conductivity layer near the depth predicted for nuclear pasta that maintains a temperature gradient between the neutron star’s inner crust and core for thousands of days into quiescence. As a result, the temperature near the crust-core boundary remains above the critical temperature for neutron superfluidity, and a layer of normal neutrons forms in the inner crust. We find that the late-time cooling of MXB 1659-29 is consistent with heat release from a normal neutron layer near the crust-core boundary with a long thermal time. We also investigate the effect of inner crust physics on the predicted cooling curves of the accreting transient KS 1731-260 and the magnetar SGR 1627-41.

New Synthesis of Ocean Crust Velocity Structure From Two-Dimensional Profiles

NASA Astrophysics Data System (ADS)

Christeson, G. L.; Goff, J.; Carlson, R. L.; Reece, R.

2017-12-01

The velocity structure of typical oceanic crust consists of Layer 2, where velocities increase rapidly with depth from seafloor, and Layer 3, which is thicker and has a lower velocity gradient. Previous syntheses have found no correlation of velocity structure with spreading rate, even though we know that magmatic processes differ between slow-spreading and fast-spreading crust. We present a new synthesis of ocean crust velocity structure, compiling observations from two-dimensional studies in the Atlantic, Pacific, and Indian ocean basins. The Layer 2/3 boundary was picked from each publication at a change in gradient either on velocity-depth functions or contour plots (with at least 0.5 km/s contour interval), or from the appropriate layer boundary for layered models. We picked multiple locations at each seismic refraction profile if warranted by model variability. Preliminary results show statistically significant differences in average Layer 2 and Layer 3 thicknesses between slow-spreading and superfast-spreading crust, with Layer 2 thinner and Layer 3 thicker for the higher spreading rate crust. The thickness changes are about equivalent, resulting in no change in mean crustal thickness. The Layer 2/3 boundary is often interpreted as the top of the gabbros; however, a comparison with mapped magma lens depths at the ridge axis shows that the boundary is typically deeper than average axial melt lens depth at superfast-spreading crust, and shallower at intermediate-spreading crust.

Response of desert biological soil crusts to alterations in precipitation frequency

USGS Publications Warehouse

Belnap, J.; Phillips, S.L.; Miller, M.E.

2004-01-01

Biological soil crusts, a community of cyanobacteria, lichens, and mosses that live on the soil surface, occur in deserts throughout the world. They are a critical component of desert ecosystems, as they are important contributors to soil fertility and stability. Future climate scenarios predict alteration of the timing and amount of precipitation in desert environments. Because biological soil crust organisms are only metabolically active when wet, and as soil surfaces dry quickly in deserts during late spring, summer, and early fall, the amount and timing of precipitation is likely to have significant impacts on the physiological functioning of these communities. Using the three dominant soil crust types found in the western United States, we applied three levels of precipitation frequency (50% below-average, average, and 50% above-average) while maintaining average precipitation amount (therefore changing both timing and size of applied events). We measured the impact of these treatments on photosynthetic performance (as indicated by dark-adapted quantum yield and chlorophyll a concentrations), nitrogenase activity, and the ability of these organisms to maintain concentrations of radiation-protective pigments (scytonemin, beta-carotene, echinenone, xanthophylls, and canthaxanthin). Increased precipitation frequency produced little response after 2.5 months exposure during spring (1 April-15 June) or summer (15 June-31 August). In contrast, most of the above variables had a large, negative response after exposure to increased precipitation frequency for 6 months spring-fall (1 April-31 October) treatment. The crusts dominated by the soil lichen Collema, being dark and protruding above the surface, dried the most rapidly, followed by the dark surface cyanobacterial crusts (Nostoc-Scytonema-Microcoleus), and then by the light cyanobacterial crusts (Microcoleus). This order reflected the magnitude of the observed response: crusts dominated by the lichen Collema

Lead isotope evolution across the Neoproterozoic boundary between craton and juvenile crust, Bayuda Desert, Sudan

NASA Astrophysics Data System (ADS)

Evuk, David; Lucassen, Friedrich; Franz, Gerhard

2017-11-01

Metaigneous mafic and ultramafic rocks from the juvenile Neoproterozoic Arabian Nubian Shield (ANS) and the Proterozoic, reworked Saharan Metacraton (SMC) have been analysed for major- and trace elements and Sr, Nd, and Pb isotopes. Most of the rocks are amphibolites metamorphosed at amphibolite facies conditions, some with relicts of a granulite facies stage. The other rocks are metapyroxenites, metagabbros, and some ultramafic rocks. Trace element compositions of the metabasaltic (dominantly tholeiitic) rocks resemble the patterns of island arcs and primitive lavas from continental arcs. Variable Sr and Nd isotope ratios indicate depleted mantle dominance for most of the samples. 207Pb/204Pb signatures distinguish between the influence of high 207Pb/204Pb old SMC crust and depleted mantle signatures of the juvenile ANS crust. The Pb isotope signatures for most metabasaltic rocks, metapyroxenites and metagabbros from SMC indicate an autochthonous formation. The interpretation of the new data together with published evidence from mafic xenoliths on SMC and ophiolite from ANS allows an extrapolation of mantle evolution in time. There are two lines of evolution in the regional mantle, one, which incorporates potential upper crust material during Neoproterozoic, and a second one with a depleted mantle signature since pre-Neoproterozoic that still is present in the Red Sea and Gulf of Aden spreading centres.

Dynamics of Crust Dissolution and Gas Release in Tank 241-SY-101

DOE Office of Scientific and Technical Information (OSTI.GOV)

SD Rassat; CW Stewart; BE Wells

2000-01-26

Due primarily to an increase in floating crust layer thickness, the waste level in Hanford Tank 241-SY-101 (SY-101) has grown appreciably, and the flammable gas volume stored in the crust has become a potential hazard. To remediate gas retention in the crust and the potential for buoyant displacement gas releases from the nonconnective layer at the bottom of the tank, SY-101 will be diluted to dissolve a large fraction of the solids that allow the waste to retain gas. In this work we develop understanding of the state of the tank waste and some of its physical properties, investigate howmore » added water will be distributed in the tank and affect the waste, and use the information to evaluate mechanisms and rates of waste solids dissolution and gas release. This work was completed to address these questions and in support of planning and development of controls for the SY-101 Surface Level Rise Remediation Project. Particular emphasis is given to dissolution of and gas release from the crust, although the effects of back-dilution on all waste layers are addressed. The magnitude and rates of plausible gas release scenarios are investigated, and it is demonstrated that none of the identified mechanisms of continuous (dissolution-driven) or sudden gas release, even with conservative assumptions, lead to domespace hydrogen concentrations exceeding the lower flammability limit. This report documents the results of studies performed in 1999 to address the issues of the dynamics, of crust dissolution and gas release in SY-101. It contains a brief introduction to the issues at hand; a summary of our knowledge of the SY-101 crust and other waste properties, including gas fractions, strength and volubility; a description of the buoyancy and dissolution models that are applied to predict the crust response to waste transfers and back dilution; and a discussion of the effectiveness of mixing for water added below the crust and the limited potential for significant

Hydromechanical Modeling of Fluid Flow in the Lower Crust

NASA Astrophysics Data System (ADS)

Connolly, J.

2011-12-01

The lower crust lies within an ambiguous rheological regime between the brittle upper crust and ductile sub-lithospheric mantle. This ambiguity has allowed two schools of thought to develop concerning the nature of fluid flow in the lower crust. The classical school holds that lower crustal rocks are inviscid and that any fluid generated by metamorphic devolatilization is squeezed out of rocks as rapidly as it is produced. According to this school, permeability is a dynamic property and fluid flow is upward. In contrast, the modern school uses concepts from upper crustal hydrology that presume implicitly, if not explicitly, that rocks are rigid or, at most, brittle. For the modern school, the details of crustal permeability determine fluid flow and as these details are poorly known almost anything is possible. Reality, to the extent that it is reflected by inference from field studies, offers some support to both schools. In particular, evidence of significant lateral and channelized fluid flow are consistent with flow in rigid media, while evidence for short (104 - 105 y) grain-scale fluid-rock interaction during much longer metamorphic events, suggests that reaction-generated grain-scale permeability is sealed rapidly by compaction; a phenomenon that is also essential to prevent extensive retrograde metamorphism. These observations provide a compelling argument for recognizing in conceptual models of lower crustal fluid flow that rocks are neither inviscid nor rigid, but compact by viscous mechanisms on a finite time-scale. This presentation will review the principle consequences of, and obstacles to, incorporating compaction in such models. The role of viscous compaction in the lower crust is extraordinarily uncertain, but ignoring this uncertainty in models of lower crustal fluid flow does not make the models any more certain. Models inevitably invoke an initial steady state hydraulic regime. This initial steady state is critical to model outcomes because it

Molybdenum Cycling During Crust Formation and Destruction

NASA Astrophysics Data System (ADS)

Greaney, A. T.; Rudnick, R. L.

2016-12-01

Molybdenum geochemistry has become an important tool for tracking the redox state of the early atmosphere and oceans as well as the emergence and sustainability of Mo-cofactored enzymes. However, in order for Mo to be enriched in the oceans, it must first be weathered out of the crust. Sulfides that weather in the presence of atmospheric O2have historically been deemed the predominant crustal source of Mo. Here, we test this assumption by determining the mineralogical hosts of Mo in Archean, Proterozoic, and Phanerozoic upper crustal rocks, using LA-ICP-MS. We also investigate Mo behavior during igneous differentiation and continental crust formation. We find that molybdenite, MoS2, is a weatherable sulfide source of Mo, but common igneous sulfides are not because their Mo concentrations are too low. However, molybdenite is uncommon in the upper continental crust. By contrast, volcanic glass is much more abundant and is a significant weatherable source of Mo that readily breaks down to release oxidized, soluble Mo whether or not atmospheric O2is present. Other common crustal mineral hosts of Mo are Ti-bearing phases like titanite, ilmenite, magnetite, and rutile that are resistant to weathering. Significant Mo depletion (relative to Ce and Pr) is observed in nearly every granitic rock analyzed in our study, but is not observed in OIB or MORB (Jenner and O'Neill, 2012). There are two possible reasons for this: 1) Mo is removed from cooling plutons during fluid expulsion, or 2) Mo is fractionated during igneous differentiation. The first scenario is a likely explanation given the solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest a fractionating phase like rutile may sequester Mo in the lower crust. Additionally, a correlation between Mo/Ce and inferred tectonic setting (enrichments observed in rift-related plutons) suggest an overall tectonic influence on the availability of Mo in the upper crust.

The inverse microconglomerate test: Definition and application to the preservation of Paleoarchean to Hadean magnetizations in metasediments of the Jack Hills, Western Australia

NASA Astrophysics Data System (ADS)

Cottrell, Rory; Tarduno, John; Bono, Richard; Dare, Matthew

2016-04-01

We introduce a new paleomagnetic field test, the inverse microconglomerate test. In contrast with traditional conglomerate tests, which target specimens that might preserve primary magnetizations, the inverse microconglomerate test focuses on magnetic carriers having unblocking temperatures less than peak metamorphic temperatures. These mineral carriers are expected to carry a consistent direction of remagnetization. Hence, the inverse microconglomerate test evaluates whether coherent magnetizations are retained on a grain/mineral scale in a given sedimentary rock sample. By defining the remagnetization direction, it also serves as a benchmark for comparison of magnetizations from other grains/minerals having unblocking temperatures higher than peak metamorphic conditions (i.e., potential primary magnetizations). We apply this new test to sediments of the Jack Hills (JH), Yilgarn craton, Western Australia. For the JH sediments we focus on fuchsite, a secondary Cr-mica that contains relict Cr-Fe spinels capable of recording remanent magnetizations. We find that JH fuchsite grains retain consistent magnetic directions at unblocking temperatures between ˜270 and 340 oC, which defines a positive test. This direction does not reproduce a nominal 1078-1070 Ma remagnetization reported by Weiss et al. (EPSL, 2015) that we interpret as an artifact of inappropriate use of averaging and statistics. The thermochemical remanent magnetization recorded by the fuchsite was most likely imparted during peak JH metamorphic conditions at ˜2650 Ma. Our inverse microconglomerate test complements a positive microconglomerate test and large scale positive conglomerate test conducted on JH cobbles (Tarduno and Cottrell, EPSL, 2013), further supporting evidence that JH zircons record Paleoarchean to Hadean primary magnetizations at high (greater than 550 oC) unblocking temperatures (Tarduno et al., Science, 2015). More generally, the new inverse microconglomerate test may aid in

The role of non-rainfall water on physiological activation in desert biological soil crusts

NASA Astrophysics Data System (ADS)

Zheng, Jiaoli; Peng, Chengrong; Li, Hua; Li, Shuangshuang; Huang, Shun; Hu, Yao; Zhang, Jinli; Li, Dunhai

2018-01-01

Non-rainfall water (NRW, e.g. fog and dew), in addition to rainfall and snowfall, are considered important water inputs to drylands. At the same time, biological soil crusts (BSCs) are important components of drylands. However, little information is available regarding the effect of NRW inputs on BSC activation. In this study, the effects of NRW on physiological activation in three BSC successional stages, including the cyanobacteria crust stage (Crust-C), moss colonization stage (Crust-CM), and moss crust stage (Crust-M), were studied in situ. Results suggest NRW inputs hydrated and activated physiological activity (Fv/Fm, carbon exchange, and nitrogen fixation) in BSCs but led to a negative carbon balance and low rates of nitrogen fixation in BSCs. One effective NRW event could hydrate BSCs for 7 h. Following simulated rainfall, the physiological activities recovered within 3 h, and net carbon gain occurred until 3 h after hydration, whereas NRW-induced physiological recovery processes were slower and exhibited lower activities, leading to a negative carbon balance. There were significant positive correlations between NRW amounts and the recovered values of Fv/Fm in all the three BSC stages (p < .001). The thresholds for Fv/Fm activation decreased with BSC succession, and the annual effective NRW events increased with BSC succession, with values of 29.8, 89.2, and 110.7 in Crust-C, Crust-CM and Crust-M, respectively. The results suggest that moss crust and moss-cyanobacteria crust use NRW to prolong metabolic activity and reduce drought stress more efficiently than cyanobacteria crusts. Therefore, these results suggest that BSCs utilize NRW to sustain life while growth and biomass accumulation require precipitation (rainfall) events over a certain threshold.

Magnetoemission of magnetar crust

NASA Astrophysics Data System (ADS)

Kondratyev, V. N.; Korovina, Yu. V.

2017-05-01

The magnetoemission of crusts of magnetars (ultramagnetized neutron stars) is considered as an origin of repeated soft gamma-ray bursts. It is shown that all observations of such bursts can be described and systematized on the basis of amodel of randomly jumping interacting moments that includes quantum fluctuations and internuclear magnetic interaction in inhomogeneous crusty nuclear matter.

Microbial Response to UV Exposure and Nitrogen Limitation in Desert Soil Crusts

NASA Astrophysics Data System (ADS)

Fulton, J. M.; Van Mooy, B. A.

2016-12-01

Microbiotic soil crusts have diverse biomarker distributions and C and N stable isotopic compositions that covary with soil type. Sparse plant cover and the relative lack of soil disturbance in arid/semi-arid landscapes allows populations of soil cyanobacteria to develop along with fungi and heterotrophic bacteria. Microbial communities in this extreme environment depend in part on the production of scytonemin, a UV protective pigment, by cyanobacteria near the top of the crust. N limitation of microbial growth also affects soil crust population dynamics, increasing the requirement of N2fixation by diazotrophic cyanobacteria. We collected 56 soil crust samples from 27 locations throughout the Great Salt Lake Desert, including four transects spanning high-elevation, erosion-dominated soils to lower elevation soils dominated by silt-accumulation. Erosion-dominated soil surfaces included rounded gravel and cobbles; in the interstices there were poorly-developed microbiotic crusts on sandy loam with low δ15N values near 0‰ that point toward microbial growth dependent on cyanobacterial N2 fixation. Nutrients regenerated by heterotrophic bacteria may have been eroded from the system, providing a positive feedback for N2 fixation. High scytonemin:chlorophyll a ratios suggest that cyanobacteria required enhanced protection from UV damage in these crusts. A similar increase in scytonemin:chlorophyll a ratio during soil crust rehydration experiments also points toward the importance of UV protection. Glycolipid:phospholipid ratios were lowest where N2 fixation was favored, however, suggesting that the cyanobacterial population was relatively small, possibly because of the metabolic cost of N2fixation. Microbiotic crusts on silt loam soils, on the other hand, had higher δ15N values between 3.5 and 7.8‰, consistent with heterotrophic growth and nutrient recycling. Lower scytonemin:chlorophyll a ratios suggest that relatively high photosynthetic activity was supported in

Chronology and complexity of early lunar crust

NASA Technical Reports Server (NTRS)

Dasch, E. J.; Ryder, G.; Nyquist, L. E.

1989-01-01

The petrology and chronology of early lunar crust is examined using the least equivocal of the available petrographic and age data on lunar rock samples, and the possible processes which produced the lunar crust are discussed. The results suggest that the lunar anorthositic crust was formed by about 120 Ma after the primary accretion of the moon at 4.56 Ga. At least some members of the diverse Mg-suites of rocks, such as norites, troctolites, and dunites, crystallized within a very few 100s of Ma after 4.56 Ga. A trace-element-rich material (KREEP) was formed by about 4.3 Ga ago, and this residue was subsequently reworked in melting and impact processes such that most samples which contain it have ages around 3.9-4.0 Ga. The findings also suggest that the onset of ferrous mare basalt volcanism began about 4.33 Ga, much earlier than was once assumed, and was still in process before the end of the most intense period of bombardment (3.9-4.0 Ga ago).

Physical Properties and Seismic Structure of Izu-Bonin-Mariana Fore Arc crust: Results From IODP Expedition 352 and Comparison with Oceanic Crust

NASA Astrophysics Data System (ADS)

Christeson, G. L.; Morgan, S.; Kodaira, S.; Yamashita, M.

2015-12-01

Most of the well-preserved ophiolite complexes are believed to form in supra-subduction zone settings. One of the goals of IODP Expedition 352 was to test the supra-subduction zone ophiolite model by drilling forearc crust at the northern Izu-Bonin-Mariana (IBM) system. IBM forearc drilling successfully cored 1.22 km of volcanic lavas and underlying dikes at four sites. A surprising observation is that basement compressional velocities measured from downhole logging average ~3.0 km/s, compared to values of 5 km/s at similar basement depths at oceanic crust sites 504B and 1256D. Typically there is an inverse relationship in extrusive lavas between velocity and porosity, but downhole logging shows similar porosities for the IBM and oceanic crust sites, despite the large difference in measured compressional velocities. These observations can be explained by a difference in crack morphologies between IBM forearc and oceanic crust, with a smaller fractional area of asperity contact across cracks at EXP 352 sites than at sites 504B and 1256D. Seismic profiles at the IBM forearc image many faults, which may be related to the crack population.

[Effects of bio-crust on soil microbial biomass and enzyme activities in copper mine tailings].

PubMed

Chen, Zheng; Yang, Gui-de; Sun, Qing-ye

2009-09-01

Bio-crust is the initial stage of natural primary succession in copper mine tailings. With the Yangshanchong and Tongguanshan copper mine tailings in Tongling City of Anhui Province as test objects, this paper studied the soil microbial biomass C and N and the activities of dehydrogenase, catalase, alkaline phosphatase, and urease under different types of bio-crust. The bio-crusts improved the soil microbial biomass and enzyme activities in the upper layer of the tailings markedly. Algal crust had the best effect in improving soil microbial biomass C and N, followed by moss-algal crust, and moss crust. Soil microflora also varied with the type of bio-crust. No'significant difference was observed in the soil enzyme activities under the three types of bio-crust. Soil alkaline phosphatase activity was significantly positively correlated with soil microbial biomass and dehydrogenase and urease activities, but negatively correlated with soil pH. In addition, moss rhizoid could markedly enhance the soil microbial biomass and enzyme activities in moss crust rhizoid.

Norwegian crusted scabies: an unusual case presentation.

PubMed

Maghrabi, Michael M; Lum, Shireen; Joba, Ameha T; Meier, Molly J; Holmbeck, Ryan J; Kennedy, Kate

2014-01-01

Scabies is a contagious condition that is transmitted through direct contact with an infected person and has been frequently associated with institutional and healthcare-facility outbreaks. The subtype Norwegian crusted scabies can masquerade as other dermatologic diseases owing to the heavy plaque formation. Successful treatment has been documented in published reports, including oral ivermectin and topical permethrin. Few case studies documenting the treatment of Norwegian crusted scabies have reported the use of surgical debridement as an aid to topical and/or oral treatment when severe plaque formation has been noted. A nursing home patient was admitted to the hospital for severe plaque formation of both feet. A superficial biopsy was negative for both fungus and scabies because of the severity of the plaque formation on both feet. The patient underwent a surgical, diagnostic biopsy of both feet, leading to the diagnosis of Norwegian crusted scabies. A second surgical debridement was then performed to remove the extensive plaque formation and aid the oral ivermectin and topical permethrin treatment. The patient subsequently made a full recovery and was discharged back to the nursing home. At 2 and 6 months after treatment, the patient remained free of scabies infestation, and the surgical wound had healed uneventfully. The present case presentation has demonstrated that surgical debridement can be complementary to the standard topical and oral medications in the treatment of those with Norwegian crusted scabies infestation. Copyright © 2014 American College of Foot and Ankle Surgeons. Published by Elsevier Inc. All rights reserved.

Observational constraints on neutron star crust-core coupling during glitches

NASA Astrophysics Data System (ADS)

Newton, W. G.; Berger, S.; Haskell, B.

2015-12-01

We demonstrate that observations of glitches in the Vela pulsar can be used to investigate the strength of the crust-core coupling in a neutron star and provide a powerful probe of the internal structure of neutron stars. We assume that glitch recovery is dominated by the torque exerted by the mutual friction-mediated recoupling of superfluid components of the core that were decoupled from the crust during the glitch. Then we use the observations of the recoveries from two recent glitches in the Vela pulsar to infer the fraction of the core that is coupled to the crust during the glitch. We then analyse whether crustal neutrons alone are sufficient to drive glitches in the Vela pulsar, taking into account crustal entrainment. We use two sets of neutron star equations of state (EOSs) which span crust and core consistently and cover a conservative range of the slope of the symmetry energy at saturation density 30 < L < 120 MeV. The two sets differ in the stiffness of the high density EOS. We find that for medium to stiff EOSs, observations imply >70 per cent of the moment of inertia of the core is coupled to the crust during the glitch, though for softer EOSs L ≈ 30 MeV as little as 5 per cent could be coupled. We find that only by extending the region where superfluid vortices are strongly pinned into the core by densities at least 0.016 fm-3 above the crust-core transition density does any EOS reproduce the observed glitch activity.

Magnetization of the Lunar Crust

NASA Technical Reports Server (NTRS)

Carley, R. A.; Whaler, K. A.; Purucker, M. E.; Halekas, J. S.

2012-01-01

Magnetic fields measured by the satellite Lunar Prospector show large scale features resulting from remanently magnetized crust. Vector data synthesized at satellite altitude from a spherical harmonic model of the lunar crustal field, and the radial component of the magnetometer data, have been used to produce spatially continuous global magnetization models for the lunar crust. The magnetization is expressed in terms of localized basis functions, with a magnetization solution selected having the smallest root-mean square magnetization for a given fit to the data, controlled by a damping parameter. Suites of magnetization models for layers with thicknesses between 10 and 50 km are able to reproduce much of the input data, with global misfits of less than 0.5 nT (within the uncertainties of the data), and some surface field estimates. The magnetization distributions show robust magnitudes for a range of model thicknesses and damping parameters, however the magnetization direction is unconstrained. These global models suggest that magnetized sources of the lunar crust can be represented by a 30 km thick magnetized layer. Average magnetization values in magnetized regions are 30-40 mA/m, similar to the measured magnetizations of the Apollo samples and significantly weaker than crustal magnetizations for Mars and the Earth. These are the first global magnetization models for the Moon, providing lower bounds on the magnitude of lunar crustal magnetization in the absence of multiple sample returns, and can be used to predict the crustal contribution to the lunar magnetic field at a particular location.

Cobalt- and platinum-rich ferromanganese crusts and associated substrate rocks from the Marshall Islands

USGS Publications Warehouse

Hein, J.R.; Schwab, W.C.; Davis, A.

1988-01-01

Ferromanganese crusts cover most hard substrates on seafloor edifices in the central Pacific basin. Crust samples and their associated substrates from seven volcanic edifices of Cretaceous age along the Ratak chain of the Marshall Islands are discussed. The two most abundant substrate lithologies recovered were limestone, dominantly fore-reef slope deposits, and volcanic breccia composed primarily of differentiated alkalic basalt and hawaiite clasts in a phosphatized carbonate matrix. The degree of mass wasting on the slopes of these seamounts is inversely correlated with the thickness of crusts. Crusts are generally thin on limestone substrate. Away from areas of active mass-wasting processes, and large atolls, crusts may be as thick as 10 cm maximum. The dominant crystalline phase in the Marshall Islands crusts is ??-MnO2 (vernadite). High concentrations of cobalt, platinum and rhodium strongly suggest that the Marshall Islands crusts are a viable source for these important metals. Many metals and the rare earth elements vary significantly on a fine scale through most crusts, thus reflecting the abundances of different host mineral phases in the crusts and changes in seawater composition with time. High concentrations of cobalt, nickel, titanium, zinc, lead, cerium and platinum result from a combination of their substitution in the iron and manganese phases and their oxidation potential. ?? 1988.

Crusted scabies due to indiscriminate use of glucocorticoid therapy in infant.

PubMed

Lima, Fernanda Carvalho da Rocha; Cerqueira, Ana Maria Mósca; Guimarães, Manuela Boleira Sieiro; Padilha, Carolina Barbosa de Sousa; Craide, Fernanda Helena; Bombardelli, Marina

2017-01-01

Crusted or Norwegian scabies is a parasitic infectious disease caused by Sarcoptes scabiei var. hominis that mainly affects immunocompromised individuals and those with neurological patients. We report a case of crusted scabies in a 4-month-old infant who had been treated erroneously for atopic dermatitis with high doses of corticosteroids. This initial misdiagnosis associated with the abusive use of corticosteroid facilitated the evolution of scabies to crusted scabies and its main complications of secondary infection and sepsis.

Rayleigh-wave dispersion reveals crust-mantle decoupling beneath eastern Tibet.

PubMed

Legendre, Cédric P; Deschamps, Frédéric; Zhao, Li; Chen, Qi-Fu

2015-11-09

The Tibetan Plateau results from the collision of the Indian and Eurasian Plates during the Cenozoic, which produced at least 2,000 km of convergence. Its tectonics is dominated by an eastward extrusion of crustal material that has been explained by models implying either a mechanical decoupling between the crust and the lithosphere, or lithospheric deformation. Discriminating between these end-member models requires constraints on crustal and lithospheric mantle deformations. Distribution of seismic anisotropy may be inferred from the mapping of azimuthal anisotropy of surface waves. Here, we use data from the CNSN to map Rayleigh-wave azimuthal anisotropy in the crust and lithospheric mantle beneath eastern Tibet. Beneath Tibet, the anisotropic patterns at periods sampling the crust support an eastward flow up to 100°E in longitude, and a southward bend between 100°E and 104°E. At longer periods, sampling the lithospheric mantle, the anisotropic structures are consistent with the absolute plate motion. By contrast, in the Sino-Korean and Yangtze cratons, the direction of fast propagation remains unchanged throughout the period range sampling the crust and lithospheric mantle. These observations suggest that the crust and lithospheric mantle are mechanically decoupled beneath eastern Tibet, and coupled beneath the Sino-Korean and Yangtze cratons.

Comets, Carbonaceous Meteorites, and the Origin of the Biosphere

NASA Technical Reports Server (NTRS)

Hoover, Richard B.

2005-01-01

The biosphere comprises the Earth s crust, atmosphere, oceans, and ice caps and the living organisms that survive within this habitat. The discoveries of barophilic chemolithoautotrophic thermophiles living deep within the crust and in deep-sea hydrothermal vents, and psychrophiles in permafrost and deep within the Antarctic Ice Sheet indicate the Earth s biosphere is far more extensive than previously recognized. Molecular biomarkers and Bacterial Paleontology provide evidence that life appeared very early on the primitive Earth and the origin of the biosphere is closely linked with the emergence of life. The role of comets, meteorites, and interstellar dust in the delivery of water, organics and prebiotic chemicals has long been recognized. Deuterium enrichment of seawater and comets indicates that comets delivered oceans to the early Earth. Furthermore, the similarity of the D/H ratios and the chemical compositions of CI carbonaceous meteorites and comets indicate that the CI meteorites may be remnants of cometary nuclei with most volatiles removed. Comets, meteorites, and interstellar dust also contain complex organic chemicals, amino acids, macromolecules, and kerogen-like biopolymers and may have played a crucial role in the delivery of complex organics and prebiotic chemicals during the Hadean (4.5-3.8 Gyr) period of heavy bombardment. The existence of indigenous microfossils of morphotypes of cyanobacteria in the CI and CM carbonaceous meteorites suggests that the paradigm that life originated endogenously in the primitive oceans of early Earth may require re-consideration. Recent data on the hot (300-400 K) black crust on comet P/Halley and Stardust images of P/Wild 2 showing depressions, tall cliffs, and pinnacles, indicate the presence of thick, durable, dark crusts on comets. If cavities within the ice and crust sustain vapor pressures in excess of 10 millibar, then localized pools of liquid water and brines could exist within the comet. Since life

Growth of continental crust and its episodic reworking over >800 Ma: evidence from Hf-Nd isotope data on the Pietersburg block (South Africa)

NASA Astrophysics Data System (ADS)

Laurent, Oscar; Zeh, Armin; Moyen, Jean-François; Doucelance, Régis; Martin, Hervé

2014-05-01

The formation and evolution of the continental crust during the Precambrian, and in particular during the Archaean eon (4.0-2.5 Ga), is still a matter of debate. In particular, it is not yet clear in which tectonic environment the genesis of crust took place and how the large volume of granitoid rocks that form ~70% of the Archaean crust were extracted from the mantle. Many studies highlighted that radiogenic isotope systems, especially Lu-Hf and Sm-Nd, are powerful tools to unravel the respective extent of crustal growth and recycling in Archaean terranes. This work presents coupled Hf and Nd isotope data (analyzed both in situ in accessory minerals and in whole rock samples) of Meso- to Neoarchaean granitoids, applied to unravel the processes of crust formation and evolution of the Pietersburg crustal block in South Africa. This crustal segment, the northermost one of the Archaean Kaapvaal Craton, is separated from older crust (3.65-3.10 Ga) by a large-scale suture zone, and the processes related to amalgamation of both blocks and their subsequent evolution are still unclear. The Pietersburg block is made up of a wide range of Archaean granitoid rocks, including tonalite-trondhjemite-granodiorite (TTG) series, high-K monzogranites as well as (grano)diorites belonging to the so-called "sanukitoid" group [1], all intruded by late Paleoproterozoic alkaline complexes. Age determinations highlighted two stages of granitoid formation: (1) TTG magmatism took place episodically over >400 Ma between 3.34 and 2.89 Ga, with a major pulse at 2.97-2.90 Ga; while (2) all the other (high-K) granitoid types emplaced subsequently between 2.84 and 2.69 Ga before a long magmatic shutdown until the intrusion of alkaline complexes at ~2.00 Ga [2-3]. Isotope systematics reveal that these two stages are related to juvenile crust formation and crust reworking, respectively. Indeed, all Hf-Nd isotope data from TTG gneisses are suprachondritic, pointing to a juvenile origin and precluding

A new model of lunar crust: asymmetry in crustal composition and evolution

NASA Astrophysics Data System (ADS)

Arai, Tomoko; Takeda, Hiroshi; Yamaguchi, Akira; Ohtake, Makiko

2008-04-01

Earlier models of lunar crustal formation as a simple flotation of ferroan anorthosites (FAN) do not account for the diverse crustal composition revealed by feldspathic lunar meteorites and granulites in the Apollo samples. Based on the integrated results of recent studies of lunar meteorites and global chemical and mineralogical maps, we propose a novel asymmetric crust model with a ferroan, noritic, nearside crust and a magnesian, troctolitic farside crust. Asymmetric crystallization of a primordial magma ocean can be one possibility to produce a crust with an asymmetric composition. A post-magma-ocean origin for a portion of the lunar crust is also possible and would account for the positive eNd value for FAN and phase equilibria. The formation of giant basins, such as the South Pole-Aitken (SPA) basin may have significant effects on resurfacing of the early lunar crust. Thus, the observed surface composition of the feldspathic highland terrane (FHT) represents the combined results of magma ocean crystallization, post-magma-ocean magmatism and resurfacing by basin formation. The Mg/(Mg+Fe) ratios, rock types, and mineral compositions of the FHT and the South Pole-Aitken basin Terrane (SPAT) obtained from the KAGUYA mission, coupled with further mineralogical and isotopic studies of lunar meteorites, will facilitate an assessment of the feasibility of the proposed crust model and improve understanding of lunar crustal genesis and evolution.

Intrusion of Magmatic Bodies Into the Continental Crust: 3-D Numerical Models

NASA Astrophysics Data System (ADS)

Gorczyk, Weronika; Vogt, Katharina

2018-03-01

Magma intrusion is a major material transfer process in the Earth's continental crust. Yet the mechanical behavior of the intruding magma and its host are a matter of debate. In this study we present a series of numerical thermomechanical simulations on magma emplacement in 3-D. Our results demonstrate the response of the continental crust to magma intrusion. We observe change in intrusion geometries between dikes, cone sheets, sills, plutons, ponds, funnels, finger-shaped and stock-like intrusions, and injection time. The rheology and temperature of the host are the main controlling factors in the transition between these different modes of intrusion. Viscous deformation in the warm and deep crust favors host rock displacement and plutons at the crust-mantle boundary forming deep-seated plutons or magma ponds in the lower to middle crust. Brittle deformation in the cool and shallow crust induces cone-shaped fractures in the host rock and enables emplacement of finger- or stock-like intrusions at shallow or intermediate depth. Here the passage of magmatic and hydrothermal fluids from the intrusion through the fracture pattern may result in the formation of ore deposits. A combination of viscous and brittle deformation forms funnel-shaped intrusions in the middle crust. Intrusion of low-density magma may more over result in T-shaped intrusions in cross section with magma sheets at the surface.

Flow of material under compression in weak lower continental crust can cause post-rift uplift of passive continental margins

NASA Astrophysics Data System (ADS)

Chalmers, James

2014-05-01

There are mountain ranges up to more than 2 km high along many passive continental margins (e.g. Norway, eastern Australia, eastern Brazil, SE and SW Africa, east and west Greenland etc.), dubbed Elevated Passive Continental Margins (EPCMs). EPCMs contain several features in common and observations indicate that uplift of these margins took place after continental break-up. There are many explanations for their formation but none that satisfy all the observations. Lack of a geodynamical mechanism has meant that there has been difficulty in getting the community to accept the observational evidence. Formation of a passive continental margin must take place under conditions of tension. After rifting ceases, however, the margin can come under compression from forces originating elsewhere on or below its plate, e.g. orogeny elsewhere in the plate or sub-lithospheric drag. The World Stress Map (www.world-stress-mp.org) shows that, where data exists, all EPCMs are currently under compression. Under sufficient compression, crust and/or lithosphere can fold, and Cloetingh & Burov (2010) showed that many continental areas may have folded in this way. The wavelengths of folding observed by Cloetingh & Burov (2010) imply that the lower crust is likely to be of intermediate composition; granitic lower crust would fold with a shorter wavelength and basic lower crust would mean that the whole lithosphere would have to fold as a unit resulting in a much longer wavelength. Continental crust more than 20 km thick would be separated from the mantle by a weak layer. However, crust less thick than that would contain no weak layers would become effectively annealed to the underlying strong mantle. Under sufficient horizontal compression stress, material can flow in the lower weak layer towards a continental margin from the continental side. The annealed extended crust and mantle under the rift means, however, that flow cannot continue towards the ocean. Mid- and lower crustal material

Crust-mantle coupling mechanism in Cameroon, West Africa, revealed by 3D S-wave velocity and azimuthal anisotropy

NASA Astrophysics Data System (ADS)

Ojo, Adebayo Oluwaseun; Ni, Sidao; Chen, Haopeng; Xie, Jun

2018-01-01

To understand the depth variation of deformation beneath Cameroon, West Africa, we developed a new 3D model of S-wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion. We found that the Cameroon Volcanic Line (CVL) is well delineated by slow phase velocities in contrast with the neighboring Congo Craton, in agreement with previous studies. Apart from the Congo Craton and the Oubanguides Belt, the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere. The direction of fast axis in the upper crust is mostly NE-SW, but trending approximately N-S around Mt. Oku and the southern CVL. The observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities. A widespread zone of weak-to-zero azimuthal anisotropy in the mid-lower crust shows evidence for vertical mantle flow or isotropic mid-lower crust. In the uppermost mantle, the fast axis direction changed from NE-SW to NW-SE around Mt. Oku and northern Cameroon. This suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed. NE-SW fast azimuths are observed beneath the Congo Craton and are consistent with the absolute motion of the African plate, suggesting a mantle origin for the observed azimuthal anisotropy. Our tomographically derived fast directions are consistent with the local SKS splitting results in some locations and depths, enabling us to constrain the origin of the observed splitting. The different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in Cameroon.

The origin of continental crust: Outlines of a general theory

NASA Technical Reports Server (NTRS)

Lowman, P. D., Jr.

1985-01-01

The lower continental crust, formerly very poorly understood, has recently been investigated by various geological and geophysical techniques that are beginning to yield a generally agreed on though still vague model (Lowman, 1984). As typified by at least some exposed high grade terranes, such as the Scottish Scourian complex, the lower crust in areas not affected by Phanerozoic orogeny or crustal extension appears to consist of gently dipping granulite gneisses of intermediate bulk composition, formed from partly or largely supracrustal precursors. This model, to the degree that it is correct, has important implications for early crustal genesis and the origin of continental crust in general. Most important, it implies that except for areas of major overthrusting (which may of course be considerable) normal superposition relations prevail, and that since even the oldest exposed rocks are underlain by tens of kilometers of sial, true primordial crust may still survive in the lower crustal levels (of. Phinney, 1981).

Crusted scabies due to indiscriminate use of glucocorticoid therapy in infant*

PubMed Central

Lima, Fernanda Carvalho da Rocha; Cerqueira, Ana Maria Mósca; Guimarães, Manuela Boleira Sieiro; Padilha, Carolina Barbosa de Sousa; Craide, Fernanda Helena; Bombardelli, Marina

2017-01-01

Crusted or Norwegian scabies is a parasitic infectious disease caused by Sarcoptes scabiei var. hominis that mainly affects immunocompromised individuals and those with neurological patients. We report a case of crusted scabies in a 4-month-old infant who had been treated erroneously for atopic dermatitis with high doses of corticosteroids. This initial misdiagnosis associated with the abusive use of corticosteroid facilitated the evolution of scabies to crusted scabies and its main complications of secondary infection and sepsis. PMID:29186253

Crustal Structure and Evidence for a Hales Discontinuity Beneath the Seychelles Microcontinent

NASA Astrophysics Data System (ADS)

Hammond, J.; Kendall, J.; Collier, J.; Rumpker, G.; Pilidou, S.; Stuart, G.

2005-12-01

It is well known that the Seychelles Plateau consists of a sliver of continental crust cast adrift during the formation of the Indian ocean. However the extent of the continental crust beneath the microcontinent and the cause of its isolation is poorly understood. Here we use receiver functions, interstation phase velocities obtained from surface waves, and wide angle reflections from controlled-source seismic data to investigate the lithospheric structure of the region. The H-κ method is used to calculate depths and Poison's ratio at 26 temporary stations distributed across the plateau and Mascarene basin. The Vp/V_s ratios and depths at stations on the plateau are typical of continental crust. To explain the major features of the RFs a simple two layer crust is proposed for the island of Mahé. The islands of Silhouette and Nord display a more complex crust consistent with the islands volcanic history. Praslin and its satellite islands display a simpler crust but display signs of a deeper discontinuity (~40 km) beneath the Moho which is possible evidence for underplating associated with Deccan age volcanism. Bird Island (Moho~18 km) and Desroche (Moho~23 km) show signs of being situated on islands above the transition from continental to oceanic crust. Alphonse, Coetivy and Platte all show receiver functions expected for oceanic crust, with Moho depths ~10 km. Inter-station phase velocity inversions from surface waves support these results with paths sampling the plateau region showing dispersion curves expected for continental crust, and those travelling between stations off the plateau showing evidence for oceanic crust. A deeper arrival is observed on the plateau stations at ~7 s or ~65 km. This feature is also seen in wide-angle controlled source work and the inter-station phase velocity inversions. Candidate interpretion for this Hales discontinuity include a Precambrian suture assoicated with shallow subduction or a shear-zone assoicated with deformation

Growth of early continental crust by partial melting of eclogite.

PubMed

Rapp, Robert P; Shimizu, Nobumichi; Norman, Marc D

2003-10-09

The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic ('grey gneiss') and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of 'average' Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, 'arc-like' crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.

Magnetic structure of the crust

NASA Technical Reports Server (NTRS)

Wasilewski, P.

1985-01-01

The bibuniqueness aspect of geophysical interpretation must be constrained by geological insight to limit the range of theoretically possible models. An additional step in depth understanding of the relationship between rock magnetization and geological circumstances on a grand scale is required. Views about crustal structure and the distribution of lithologies suggests a complex situation with lateral and vertical variability at all levels in the crust. Volcanic, plutonic, and metamorphic processes together with each of the observed anomalies. Important questions are addressed: (1) the location of the magnetic bottom; (2) whether the source is a discrete one or are certain parts of the crust cumulatively contributing to the overall magnetization; (3) if the anomaly to some recognizable surface expression is localized, how to arrive at a geologically realistic model incorporating magnetization contrasts which are realistic; (3) in the way the primary mineralogies are altered by metamorphism and the resulting magnetic contracts; (4) the effects of temperature and pressure on magnetization.

Formation of continental crust by intrusive magmatism

NASA Astrophysics Data System (ADS)

Rozel, A. B.; Golabek, G. J.; Jain, C.; Tackley, P. J.; Gerya, T.

2017-09-01

How were the continents formed in the Earth? No global numerical simulation of our planet ever managed to generate continental material self-consistently. In the present study, we show that the latest developments of the convection code StagYY enable to estimate how to produce the early continents, more than 3 billion years ago. In our models, melting of pyrolitic rocks generates a basaltic melt and leaves behind a depleted solid residue (a harzburgite). The melt generated in the mantle is transported to the surface. Only basaltic rocks melting again can generate continental crust. Should the basaltic melt always reach the open air and cool down? Should the melt be intruded warm in the pre-existing crust? The present study shows that both processes have to be considered to produce continents. Indeed, granitoids can only be created in a tight window of pressure-temperature. If all basalt is quickly cooled by surface volcanism, the lithosphere will be too cold. If all basalt is intruded warm below the crust then the lithosphere will be too warm. The key is to have both volcanism and plutonism (intrusive magmatism) to reach the optimal temperature and form massive volumes of continental material.

Increasing cotton stand establishment in soils prone to soil crusting

USDA-ARS?s Scientific Manuscript database

Many factors can contribute to poor cotton stand establishment, and cotton is notorious for its weak seedling vigor. Soil crusting can be a major factor hindering cotton seedling emergence in many of the cotton production regions of the US and the world. Crusting is mainly an issue in silty soils ...

Seismic observation of a sharp post-garnet phase transition within the Farallon crust: Evidence for oceanic plateau subduction

NASA Astrophysics Data System (ADS)

Maguire, R.; Ritsema, J.

2017-12-01

The tectonic evolution of North America over the past 150 million years was heavily influenced by the complex subduction history of the Farallon plate. In particular, Laramide mountain building may have been triggered by the initiation of flat slab subduction in the late Cretaceous. While it has been proposed that the cause of slab flattening is related to the subduction of an oceanic plateau[1], direct geophysical evidence of a subducted oceanic plateau is lacking. Here, using P-to-S receiver functions, we detect a sharp seismic discontinuity at 720-km depth beneath the southeastern United States and Gulf of Mexico. We interpret this discontinuity as a garnet-to-bridgmanite phase transition occurring within a thickened Farallon crust. Our results are consistent with a subducted oceanic plateau (likely the conjugate half of the Hess rise) which is foundering below the base of the mantle transition zone. Additionally, we find a strong 520-km discontinuity beneath the southeastern United States which may indicate a hydrous transition zone due to the release of H2O from the Farallon slab. These results provide insight into the dynamics of flat slab subduction as well as the tectonic history of North America. [1] Livaccari, R. F., Burke, K., & Şengör, A. M. C. (1981). Was the Laramide orogeny related to subduction of an oceanic plateau? Nature, v. 289, p. 276-278, doi: 10.1038/289276a0

MARSIS radar sounder evidence of buried basins in the northern lowlands of Mars.

PubMed

Watters, Thomas R; Leuschen, Carl J; Plaut, Jeffrey J; Picardi, Giovanni; Safaeinili, Ali; Clifford, Stephen M; Farrell, William M; Ivanov, Anton B; Phillips, Roger J; Stofan, Ellen R

2006-12-14

A hemispheric dichotomy on Mars is marked by the sharp contrast between the sparsely cratered northern lowland plains and the heavily cratered southern highlands. Mechanisms proposed to remove ancient crust or form younger lowland crust include one or more giant impacts, subcrustal transport by mantle convection, the generation of thinner crust by plate tectonics, and mantle overturn following solidification of an early magma ocean. The age of the northern lowland crust is a significant constraint on these models. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on the European Space Agency's Mars Express spacecraft is providing new constraints on the martian subsurface. Here we show evidence of buried impact basins ranging in diameter from about 130 km to 470 km found over approximately 14 per cent of the northern lowlands. The number of detected buried basins >200 km in diameter indicates that the lowland crust is ancient, dating back to the Early Noachian epoch. This crater density is a lower limit because of the likelihood that not all buried basins in the area surveyed by MARSIS have been detected. An Early Noachian age for the lowland crust has been previously suggested on the basis of a large number of quasi-circular topographic depressions interpreted to be evidence of buried basins. Only a few of these depressions in the area surveyed by MARSIS, however, correlate with the detected subsurface echoes. On the basis of the MARSIS data, we conclude that the northern lowland crust is at least as old as the oldest exposed highland crust. This suggests that the crustal dichotomy formed early in the geologic evolution of Mars.

Comparison of characteristic of soils with and without salt crust soils in a hyper-arid floodplain

NASA Astrophysics Data System (ADS)

LI, X.; Feng, G.

2017-12-01

Soil salt crusts have been shown to restrict soil erosion, and influence the water and salt movement in soil, was great concern in the world. However, there is little information for the comparison of characteristic of soil with and without salt crust in a hyper-arid flood plains. The objective of this study was to investigate paired samples of salt crusts and the surface soil without a salt curst in the flood plain of Tarim River in China. The results revealed that the salt crust soils most distributed in shrubland dominated by Tarimax, in which account for 73%, the wetland was followed, with shallower groundwater table (<2.4 m). The salt crust was comprised of salt greater than 109 g·kg-1, crust was not found on the soils as its salt content less than 89 g·kg-1. The salt content of soils either with crust or without crust ranged from 89 to 109 g·kg-1. The salt crust thickness had positive correlation with salt content (R2=0.61), and also with crust strength (R2=0.64). Compared with soils without salt crust, the salt crust soils had more clay, silt and soil organic matter content. It was found that those soils were located in low-lying area, experience relatively higher frequent overflowing flood. This study revealed that the flood did not reduce salt content in top soils. Salt crust was probably formed due to salt accumulation from shallow groundwater (e.g. <2.4 m) in this region.

Accessory Mineral Records of Early Earth Crust-Mantle Systematics: an Example From West Greenland

NASA Astrophysics Data System (ADS)

Storey, C. D.; Hawkesworth, C. J.

2008-12-01

Conditions for the formation and the nature of Earth's early crust are enigmatic due to poor preservation. Before c.4 Ga the only archives are detrital minerals eroded from earlier crust, such as the Jack Hills zircons in western Australia, or extinct isotope systematics. Zircons are particularly powerful since they retain precise records of their ages of crystallisation, and the Lu-Hf radiogenic isotope and O stable isotope systematics of the reservoir from which they crystallised. In principle, this allows insight into the nature of the crust, the mantle reservoir from which the melt was extracted and any reworked material incorporated into that melt. We have used in situ methods to measure U-Pb, O and Lu-Hf within single zircon crystals from tonalitic gneisses from West Greenland in the vicinity of the Isua Supracrustal Belt. They have little disturbed ages of c.3.8 Ga, mantle-like O isotope signatures and Lu-Hf isotope signatures that lie on the CHUR evolution line at 3.8 Ga. These samples have previously been subjected to Pb isotope feldspar and 142Nd whole rock analysis and have helped constrain models in which early differentiation of a proto-crust must have occurred. The CHUR-like Lu-Hf signature, along with mantle-like O signature from these zircons suggests juvenile melt production at 3.8 Ga from undifferentiated mantle, yet the other isotope systems preclude this possibility. Alternatively, this is further strong evidence for a heterogeneous mantle in the early Earth. Whilst zircons afford insight into the nature of the early crust and mantle, it is through the Sm-Nd system that the mantle has traditionally been viewed. Titanite often contains several thousand ppm Nd, making it amenable to precise analysis, and is a common accessory phase. It has a reasonably high closure temperature for Pb and O, and it can retain cores with older ages and distinct REE chemistry. It is often the main accessory phase alongside zircon, and it is the main carrier of Nd

Transition from continental to oceanic crust on the Wilkes-Adelie margin of Antarctica

NASA Astrophysics Data System (ADS)

Eittreim, Stephen L.

1994-12-01

The Wilkes-Adelie margin of East Antarctica, a passive margin rifted in the Early Cretaceous, has an unusually reflective Moho which can be traced seismically across the continent-ocean transition. Velocity models and depth sections were constructed from a combined set of U.S. and French multichannel seismic reflection lines to investigate the transition from continental to oceanic crust. These data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and, in places, by a shoaling of Moho. The Moho reflection is continuous across the edge of oceanic crust, and gradually deepens landward under the continental edge. A marginal rift basin, some tens of kilometers in width, lies in the transition between continental and oceanic crust, contains an average of about 4 km of synrift sediment that is prograded in places, and has characteristics of a former rift valley, now subsided to about 10 km. Three types of reflections in the seismic data are interpreted as volcanic deposits: (1) high-amplitude reflections that floor the marginal rift basin, (2) irregularly seaward dipping sequences that comprise an anomalously thick edge of oceanic crust, and (3) highly irregular and diffractive reflections from oceanic crustal basins that cap a normal-thickness ocean crust. The present depth to the prefit surface of continental crust is compatible with passive margin subsidence since 95 Ma, corrected for its load of synrift and postrift sediment and mechanically stretched by factors of beta = 1.8 or higher. Comparison of seismic crustal thickness measurements with inferred crustal thinning from subsidence analysis shows agreement for areas where beta less than 4. In areas where beta greater than 4, measured thickness is greater than that inferred from subsidence analysis, a result that could be explained by underplating the crust beneath the marginal rift basin.

Calorimetric studies of cryptogamic crust metabolism in response to temperature, water vapor, and liquid water

Treesearch

Dorothy A. Stradling; Tonya Thygerson; Bruce N. Smith; Lee D. Hansen; Richard S. Criddle; Rosemary L. Pendleton

2001-01-01

Cryptogamic crusts are communities composed of lichens, cyanobacteria, algae, mosses, and fungi. These integrated soil crusts are susceptible to disturbance, but if intact, appear to play a role in providing nutrients, especially nitrogen, to higher plants. It is not known how or under what conditions desert crusts can grow. Crust samples from localities on the...

USArray Imaging of Continental Crust in the Conterminous United States

NASA Astrophysics Data System (ADS)

Ma, Xiaofei; Lowry, Anthony R.

2017-12-01

The thickness and bulk composition of continental crust provide important constraints on the evolution and dynamics of continents. Crustal mineralogy and thickness both may influence gravity anomalies, topographic elevation, and lithospheric strength, but prior to the inception of EarthScope's USArray, seismic measurements of crustal thickness and properties useful for inferring lithology are sparse. Here we improve upon a previously published methodology for joint inversion of Bouguer gravity anomalies and seismic receiver functions by using parameter space stacking of cross correlations of modeled synthetic and observed receiver functions instead of standard H-κ amplitude stacking. The new method is applied to estimation of thickness and bulk seismic velocity ratio, vP/vS, of continental crust in the conterminous United States using USArray and other broadband network data. Crustal thickness variations are reasonably consistent with those found in other studies and show interesting relationships to the history of North American continental formation. Seismic velocity ratios derived in this study are more robust than in other analyses and hint at large-scale variations in composition of continental crust. To interpret the results, we model the pressure-/temperature-dependent thermodynamics of mineral formation for various crustal chemistries, with and without volatile constituents. Our results suggest that hydration lowers bulk crustal vP/vS and density and releases heat in the shallow crust but absorbs heat in the lowermost crust (where plagioclase breaks down to pyroxene and garnet resulting in higher seismic velocity). Hence, vP/vS variations may provide a useful proxy for hydration state in the crust.

Thickness of Mercury's crust from MESSENGER gravity and altimetry data

NASA Astrophysics Data System (ADS)

Padovan, S.; Wieczorek, M. A.; Margot, J. L.; Tosi, N.; Solomon, S. C.

2014-12-01

The major igneous events that form and shape the crust of a rocky body, such as magma ocean solidification and volcanism, affect the interior thermo-chemical evolution through control on the bulk volatile content, partitioning of heat-producing elements, and heat loss. Therefore, characterizing the crust of a body provides information on that object's origin, differentiation, and subsequent geologic evolution. For Mercury, the crust may hold clues in particular to the still poorly understood processes of formation of this planet. Analysis of geoid-to-topography ratios (GTRs) has been previously applied to infer the thickness of the crust of the Moon, Mars, and Venus. We perform a similar analysis for Mercury with the gravity and altimetry data acquired by the MESSENGER spacecraft. We consider only the northern hemisphere, where the gravity field and topography are well constrained. We assume that Airy isostasy is the principal mechanism of support of variations in topography, and we therefore exclude from the analysis regions that might not be compatible with this assumption, such as large expanses of smooth plains and large impact basins. For a conservative range of densities of the crust, we infer a crustal thickness of 35±18 km (one standard deviation). This new mean value is substantially less than earlier estimates that were based on viscous relaxation of topography, on the relation between the low-degree gravity field and equatorial ellipticity, and on the depth of the brittle-ductile transition as constrained by models of thrust faulting and thermal evolution. This relatively thin crust allows for the possibility of excavation of mantle material during the formation of large impact basins (such as Caloris). Such material might be observed with instruments on MESSENGER and the BepiColombo spacecraft now in development.

Influence of surface crusting on infiltration of a loess plateau soil

USDA-ARS?s Scientific Manuscript database

Surface sealing and crusting are common widespread processes that occur in many cultivated soils worldwide, especially in arid and semiarid regions. Soil crusting negatively affects water infiltration, increases surface runoff, reduces seedling emergence, restricts air exchange between the soil and ...

SEM observation on microbial like structure in the ferromanganese crust from the western Pacific Magellan Seamount

NASA Astrophysics Data System (ADS)

Yang, K.; Park, H.; Baik, H.; Kim, J.; Park, K. R.; Yoon, J.; Kim, J. W.

2016-12-01

Understanding the biogeochemical process in the Fe-Mn crust layer is important to reconstruct the paleo-environment when the Fe-Mn crust layer forms. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Electron Energy Loss Spectroscopy (EELS), and Polymerase Chain Reaction (PCR) were utilized to determine the redox states of Fe/Mn and microbial diversity at each layer. Samples were dredged from the western Pacific Magellan Seamount (OSM11) that consists of five well-defined layers from the rim (L1) to the core (L5). Some microbial like structures of sheath-like with filaments (L1 - L3), capsule-shaped (L2), fossilized coccolith mounds with phosphatized globules (L4), and bean-shaped (L4) were detected in entire layers. The cross sectional observation of bean-shaped microbe like structures encrusted with Fe-vernadite (L3) by Scanning Transmission Electron Microscopy (STEM) and Focused Ion Beam (FIB) technique revealed 1-μm diameter cavity in the center and porous structures of encrusting Fe-vernadite in periphery. Moreover, the organic carbon in the center cavity compared with inorganic C (from carbonate) in periphery was differentiated by C-K edge EELS spectra, suggesting that the microbe used to occupy. Indeed, the PCR analysis indicated the presence of functional gene (cumA; 1056bp & coxC; 810bp) association with Mn & Fe oxidizer that promote the formation of the crust. The cloning and sequencing of DNA PCR fragments revealed the appearance of geobacter species in L3 (G. sulfurreducens and G. lovleyi). The DNA molecular biological analysis and SEM direct observations suggest the evidence of biotic process in the formation of Fe-Mn crust.

Magma Transport from Deep to Shallow Crust and Eruption

NASA Astrophysics Data System (ADS)

White, R. S.; Greenfield, T. S.; Green, R. G.; Brandsdottir, B.; Hudson, T.; Woods, J.; Donaldson, C.; Ágústsdóttir, T.

2016-12-01

We have mapped magma transport paths from the deep (20 km) to the shallow (6 km) crust and in two cases to eventual surface eruption under several Icelandic volcanoes (Askja, Bardarbunga, Eyjafjallajokull, Upptyppingar). We use microearthquakes caused by brittle fracture to map magma on the move and tomographic seismic studies of velocity perturbations beneath volcanoes to map the magma storage regions. High-frequency brittle failure earthquakes with magnitudes of typically 0-2 occur where melt is forcing its way through the country rock, or where previously frozen melt is repeatedly re-broken in conduits and dykes. The Icelandic crust on the rift zones where these earthquakes occur is ductile at depths greater than 7 km beneath the surface, so the occurrence of brittle failure seismicity at depths as great as 20 km is indicative of high strain rates, for which magma movement is the most likely explanation. We suggest that high volatile pressures caused by the exsolution of carbon dioxide in the deep crust is driving the magma movement and seismicity at depths of 15-20 km. Eruptions from shallow crustal storage areas are likewise driven by volatile exsolution, though additional volatiles, and in particular water are also involved in the shallow crust.

Structure of the crust and upper mantle in the western United States

USGS Publications Warehouse

Pakiser, L.C.

1963-01-01

Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2,000 stations in the western conterminous United States as a part of the VELA UNIFORM program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The speed of compressional waves in the upper-mantle rocks ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province. In general, the speed of compressional waves in the upper-mantle rocks tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The crust tends to be thick in regions where the speed of compressional waves in the upper-mantle rocks (and presumably the density) is high, and tends to be relatively thin where the speed of compressional waves in the upper-mantle rocks (and density) is lower. With in the Basin and Range province, crustal thickness seems to vary directly with regional altitude above sea level. Evidence that a layer of intermediate compressional-wave speed exists in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper- mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic

Microstructure, texture and colour development during crust formation on whole muscle chicken fillets.

PubMed

Barbut, S

2013-01-01

1. The development of crust during a 22-min period was evaluated in an oven, and in previously cooked-in-bag products (no crust) placed in an oven for 10 min. The oven-roasted products started to develop a thin (2-4 μm) crust layer after 4 min. At that point, the colour of the fillets turned white but no browning was observed. As roasting time increased, crust thickness and shear force increased, the product turned brown and eventually black at certain spots. 2. Light microscopy revealed the shrinking of muscle fibres close to the surface, as they also lost water. At a certain point, tears between the different layers started to appear. The inner muscle fibres also progressively shrank and the spaces between them increased. Microscopy of cook-in-bag products revealed no crust formation during heating. Upon moving to the oven, crust started to form but was much faster compared with the other products. 3. Cook-in-the-bag samples showed a higher rate of cook loss during the first 12 min (to internal 70°C) compared with oven heating. This could have been due to the fast heating rate in water and/or no crust formation. 4. White colour was fully formed on water-cooked fillets within 2 min (L* = 83), while it was gradually forming on oven-roasted samples (max L* of 79 after 12 min). 5. Shear force measurements showed an increase in both treatments up to 18 min, with a decrease thereafter (when dry crust started to crack).

Terrestrial aftermath of the Moon-forming impact.

PubMed

Sleep, Norman H; Zahnle, Kevin J; Lupu, Roxana E

2014-09-13

Much of the Earth's mantle was melted in the Moon-forming impact. Gases that were not partially soluble in the melt, such as water and CO2, formed a thick, deep atmosphere surrounding the post-impact Earth. This atmosphere was opaque to thermal radiation, allowing heat to escape to space only at the runaway greenhouse threshold of approximately 100 W m(-2). The duration of this runaway greenhouse stage was limited to approximately 10 Myr by the internal energy and tidal heating, ending with a partially crystalline uppermost mantle and a solid deep mantle. At this point, the crust was able to cool efficiently and solidified at the surface. After the condensation of the water ocean, approximately 100 bar of CO2 remained in the atmosphere, creating a solar-heated greenhouse, while the surface cooled to approximately 500 K. Almost all this CO2 had to be sequestered by subduction into the mantle by 3.8 Ga, when the geological record indicates the presence of life and hence a habitable environment. The deep CO2 sequestration into the mantle could be explained by a rapid subduction of the old oceanic crust, such that the top of the crust would remain cold and retain its CO2. Kinematically, these episodes would be required to have both fast subduction (and hence seafloor spreading) and old crust. Hadean oceanic crust that formed from hot mantle would have been thicker than modern crust, and therefore only old crust underlain by cool mantle lithosphere could subduct. Once subduction started, the basaltic crust would turn into dense eclogite, increasing the rate of subduction. The rapid subduction would stop when the young partially frozen crust from the rapidly spreading ridge entered the subduction zone. © 2014 The Author(s) Published by the Royal Society. All rights reserved.

The lunar crust - A product of heterogeneous accretion or differentiation of a homogeneous moon

NASA Technical Reports Server (NTRS)

Brett, R.

1973-01-01

The outer portion of the moon (including the aluminum-rich crust and the source regions of mare basalts) was either accreted heterogeneously or was the product of widespread differentiation of an originally homogeneous source. Existing evidence for and against each of these two models is reviewed. It is concluded that the accretionary model presents more problems than it solves, and the model involving differentiation of an originally homogeneous moon is considered to be more plausible. A hypothesis for the formation of mare basalts is advanced.

Mechanical impedance of soil crusts and water content in loamy soils

NASA Astrophysics Data System (ADS)

Josa March, Ramon; Verdú, Antoni M. C.; Mas, Maria Teresa

2013-04-01

Soil crust development affects soil water dynamics and soil aeration. Soil crusts act as mechanical barriers to fluid flow and, as their mechanical impedance increases with drying, they also become obstacles to seedling emergence. As a consequence, the emergence of seedling cohorts (sensitive seeds) might be reduced. However, this may be of interest to be used as an effective system of weed control. Soil crusting is determined by several factors: soil texture, rain intensity, sedimentation processes, etc. There are different ways to characterize the crusts. One of them is to measure their mechanical impedance (MI), which is linked to their moisture level. In this study, we measured the evolution of the mechanical impedance of crusts formed by three loamy soil types (clay loam, loam and sandy clay loam, USDA) with different soil water contents. The aim of this communication was to establish a mathematical relationship between the crust water content and its MI. A saturated soil paste was prepared and placed in PVC cylinders (50 mm diameter and 10 mm height) arranged on a plastic tray. Previously the plastic tray was sprayed with a hydrophobic liquid to prevent the adherence of samples. The samples on the plastic tray were left to air-dry under laboratory conditions until their IM was measured. To measure IM, a food texture analyzer was used. The equipment incorporates a mobile arm, a load cell to apply force and a probe. The arm moves down vertically at a constant rate and the cylindrical steel probe (4 mm diameter) penetrates the soil sample vertically at a constant rate. The equipment is provided with software to store data (time, vertical distance and force values) at a rate of up to 500 points per second. Water content in crust soil samples was determined as the loss of weight after oven-drying (105°C). From the results, an exponential regression between MI and the water content was obtained (determination coefficient very close to 1). This methodology allows

Closer Look at Lunar Highland Crust

NASA Image and Video Library

2012-12-05

This image depicting the porosity of the lunar highland crust was derived using bulk density data from NASA GRAIL mission and independent grain density measurements from NASA Apollo moon mission samples as well as orbital remote-sensing data.

The Origin of Silicic Arc Crust - Insights from the Northern Pacific Volcanic Arcs through Space and Time

NASA Astrophysics Data System (ADS)

Straub, S. M.; Kelemen, P. B.

2016-12-01

The remarkable compositional similarities of andesitic crust at modern convergent margins and the continental crust has long evoked the hypothesis of similar origins. Key to understanding either genesis is understanding the mode of silica enrichment. Silicic crust cannot be directly extracted from the upper mantle. Hence, in modern arcs, numerous studies - observant of the pervasive and irrefutable evidence of melt mixing - proposed that arc andesites formed by mixing of mantle-derived basaltic melts and fusible silicic material from the overlying crust. Mass balance requires the amount of silicic crust in such hybrid andesites to be on the order to tens of percent, implying that their composition to be perceptibly influenced by the various crustal basements. In order to test this hypothesis, major and trace element compositions of mafic and silicic arc magmas with arc-typical low Ce/Pb< 10 of Northern Pacific arcs (Marianas through Mexico) were combined with Pb isotope ratios. Pb isotope ratios are considered highly sensitive to crustal contamination, and hence should reflect the variable composition of the oceanic and continental basement on which these arcs are constructed. In particular, in thick-crust continental arcs where the basement is isotopically different from the mantle and crustal assimilation thought to be most prevalent, silicic magmas must be expected to be distinct from those of the associated mafic melts. However, in a given arc, the Pb isotope ratios are constant with increasing melt silica regardless of the nature of the basement. This observation argues against a melt origin of silicic melts from the crustal basement and suggest them to be controlled by the same slab flux as their co-eval mafic counterparts. This inference is validated by the spatial and temporal pattern of arc Pb isotope ratios along the Northern Pacific margins and throughout the 50 million years of Cenozoic evolution of the Izu Bonin Mariana arc/trench system that are can

Coulomb crystals in neutron star crust

NASA Astrophysics Data System (ADS)

Baiko, D. A.

2014-03-01

It is well known that neutron star crust in a wide range of mass densities and temperatures is in a crystal state. At a given density, the crystal is made of fully ionized atomic nuclei of a single species immersed in a nearly incompressible (i.e., constant and uniform) charge compensating background of electrons. This model is known as the Coulomb crystal model. In this talk we analyze thermodynamic and elastic properties of the Coulomb crystals and discuss various deviations from the ideal model. In particular, we study the Coulomb crystal behavior in the presence of a strong magnetic field, consider the effect of the electron gas polarizability, outline the main properties of binary Coulomb crystals, and touch the subject of quasi-free neutrons permeating the Coulomb crystal of ions in deeper layers of neutron star crust.

The ancient lunar crust, Apollo 17 region

NASA Technical Reports Server (NTRS)

James, O. B.

1992-01-01

The Apollo 17 highland collection is dominated by fragment-laden melt rocks, generally thought to represent impact melt from the Serenitatis basin-forming impact. Fortunately for our understanding of the lunar crust, the melt rocks contain unmelted clasts of preexisting rocks. Similar ancient rocks are also found in the regolith; most are probably clasts eroded out of melt rocks. The ancient rocks can be divided into groups by age, composition, and history. Oldest are plutonic igneous rocks, representing the magmatic components of the ancient crust. The younger are granulitic breccias, which are thoroughly recrystallized rocks of diverse parentages. The youngest are KREEPy basalts and felsites, products of relatively evolved magmas. Some characteristics of each group are given.

Near-isothermal conditions in the middle and lower crust induced by melt migration.

PubMed

Depine, Gabriela V; Andronicos, Christopher L; Phipps-Morgan, Jason

2008-03-06

The thermal structure of the crust strongly influences deformation, metamorphism and plutonism. Models for the geothermal gradient in stable crust predict a steady increase of temperature with depth. This thermal structure, however, is incompatible with observations from high-temperature metamorphic terranes exhumed in orogens. Global compilations of peak conditions in high-temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure, for both isothermal decompression and isobaric cooling paths. Here we develop simple one-dimensional thermal models that include the effects of melt migration. These models show that long-lived plutonism results in a quasi-steady-state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust. The models also predict that the upward advection of heat by melt generates granulite facies metamorphism, and widespread andalusite-sillimanite metamorphism in the upper crust. Once the quasi-steady-state thermal profile is reached, the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions, thus setting the stage for gravitational collapse, exhumation and isothermal decompression after the onset of plutonism. Near-isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that, in part, control the shape of the geotherm.

Evaporative sodium salt crust development and its wind tunnel derived transport dynamics under variable climatic conditions

NASA Astrophysics Data System (ADS)

Nield, Joanna M.; McKenna Neuman, Cheryl; O'Brien, Patrick; Bryant, Robert G.; Wiggs, Giles F. S.

2016-12-01

Playas (or ephemeral lakes) can be significant sources of dust, but they are typically covered by salt crusts of variable mineralogy and these introduce uncertainty into dust emission predictions. Despite the importance of crust mineralogy to emission potential, little is known about (i) the effect of short-term changes in temperature and relative humidity on the erodibility of these crusts, and (ii) the influence of crust degradation and mineralogy on wind speed threshold for dust emission. Our understanding of systems where emission is not driven by impacts from saltators is particularly poor. This paper describes a wind tunnel study in which dust emission in the absence of saltating particles was measured for a suite of climatic conditions and salt crust types commonly found on Sua Pan, Botswana. The crusts were found to be non-emissive under climate conditions characteristic of dawn and early morning, as compared to hot and dry daytime conditions when the wind speed threshold for dust emission appears to be highly variable, depending upon salt crust physicochemistry. Significantly, sodium sulphate rich crusts were found to be more emissive than crusts formed from sodium chloride, while degraded versions of both crusts had a lower emission threshold than fresh, continuous crusts. The results from this study are in agreement with in-situ field measurements and confirm that dust emission from salt crusted surfaces can occur without saltation, although the vertical fluxes are orders of magnitude lower (∼10 μg/m/s) than for aeolian systems where entrainment is driven by particle impact.

Numerical Experiments on the Role of the Lower Crust in the Development of Extension-driven Gneiss Domes

NASA Astrophysics Data System (ADS)

Korchinski, M.; Rey, P. F.; Teyssier, C. P.; Mondy, L. S.; Whitney, D.

2016-12-01

Flow of orogenic crust is a critical geodynamic process in the chemical and physical evolution of continents. Deeply sourced rocks are transported to the near surface within gneiss domes, which are ubiquitous features in orogens and extensional regions. Exhumation of material within a gneiss dome can occur as the result of tectonic stresses, where material moves into space previously occupied by the shallow crust as the result of extension localized along a detachment system. Gravitationally driven flow may also contribute to exhumation. This research addresses how physical parameters (density, viscosity) of the deep crust (base of brittle crust to Moho) impact (1) the localization of extension in the shallow crust, and (2) the flow of deep crust by tectonic and non-tectonic stresses. We present 2D numerical experiments in which the density (2900-3100 kg m-3) and viscosity (1e19-1e21 Pa s) of the deep crust are systematically varied. Lateral and vertical transport of deep crustal rocks toward the gneiss dome occurs across the entire parameter space. A low viscosity deep crust yields localized extension in the upper crust and crustal-scale upward flow; this case produces the highest exhumation. A high viscosity deep crust results in distributed thinning of the upper crust, which suppresses upward mass transport. The density of the deep crust has only a second-order effect on the shallow crust extension regime. We capture the flow field generated after the cessation of extension to evaluate mass transport that is not driven by tectonic stresses. Upward transport of material within the gneiss dome is present across the entire parameter space. In the case of a low-viscosity deep crust, horizontal flow occurs adjacent to the dome above the Moho; this flow is an order of magnitude higher than that within the dome. Density variations do not drastically alter the flow field in the low viscosity lower crust. However, a high density and high viscosity deep crust results in

Continental crust formation: Numerical modelling of chemical evolution and geological implications

NASA Astrophysics Data System (ADS)

Walzer, U.; Hendel, R.

2017-05-01

Oceanic plateaus develop by decompression melting of mantle plumes and have contributed to the growth of the continental crust throughout Earth's evolution. Occasional large-scale partial melting events of parts of the asthenosphere during the Archean produced large domains of precursor crustal material. The fractionation of arc-related crust during the Proterozoic and Phanerozoic contributed to the growth of continental crust. However, it remains unclear whether the continents or their precursors formed during episodic events or whether the gaps in zircon age records are a function of varying preservation potential. This study demonstrates that the formation of the continental crust was intrinsically tied to the thermoconvective evolution of the Earth's mantle. Our numerical solutions for the full set of physical balance equations of convection in a spherical shell mantle, combined with simplified equations of chemical continent-mantle differentiation, demonstrate that the actual rate of continental growth is not uniform through time. The kinetic energy of solid-state mantle creep (Ekin) slowly decreases with superposed episodic but not periodic maxima. In addition, laterally averaged surface heat flow (qob) behaves similarly but shows peaks that lag by 15-30 Ma compared with the Ekin peaks. Peak values of continental growth are delayed by 75-100 Ma relative to the qob maxima. The calculated present-day qob and total continental mass values agree well with observed values. Each episode of continental growth is separated from the next by an interval of quiescence that is not the result of variations in mantle creep velocity but instead reflects the fact that the peridotite solidus is not only a function of pressure but also of local water abundance. A period of differentiation results in a reduction in regional water concentrations, thereby increasing the temperature of the peridotite solidus and the regional viscosity of the mantle. By plausibly varying the

Sequestration of Tellurium From Seawater by Ferromanganese Crusts: A XANES/EXAFS Perspective

NASA Astrophysics Data System (ADS)

Hein, J. R.; Bargar, J.; Koschinsky, A.; Dunham, R.; Halliday, A. N.

2007-12-01

Marine iron-oxyhydroxide/manganese-oxide crusts (Fe-Mn crusts) provide the richest known source of tellurium (Te). Te averages about 50 ppm in Fe-Mn crusts distributed globally, with concentrations locally up to 210 ppm. The sorption of Te onto Fe-Mn crusts likely controls the dominant redox species and concentration of Te in the global ocean (Hein et al., 2003). However, little is known about the mechanisms by which Te is sequestered by Fe-Mn crusts and Fe-Mn colloids in the water column, and then stabilized in the Fe/Mn oxyhydroxide/oxide framework. Two primary hypotheses are being tested: (a) Te(IV) is initially the predominant adsorbed species, which is subsequently oxidized on the Fe-oxyhydroxide and/or Mn oxide phases in natural systems and in sorption experiments. (b) Once oxidized, Te(VI) remains tightly bound to the Fe phase in Fe-Mn crusts as adsorbed surface complexes. These hypotheses are being examined by using the Stanford Synchrotron Radiation Laboratory's (SSRL) synchrotron-based XANES (x-ray absorption near-edge structure) spectroscopy to assess Te oxidation state in natural samples and samples in which Te(IV) and Te(VI) were sorbed onto synthetic and natural FeOOH and Mn oxides. EXAFS (extended x-ray absorption fine structure) spectroscopy is being used to resolve the local molecular-scale structure around Te in these same samples. Data have thus far been obtained for six Fe-Mn crusts from a variety of geographic locations and water depths of occurrence, with differing chemical compositions; and two model compounds, Te(IV) sorbed on FeOOH and Te(IV) sorbed on MnO2. XANES data show that for all six Fe-Mn crust samples, 85 to 100 percent of the Te occurs as Te(VI). For the model compounds, about 65 percent of the Te(IV) sorbed onto the MnO2 had oxidized to Te(VI) by the time (one week) the sample was analyzed, whereas Te sorbed onto FeOOH remained at about 100 percent Te(IV). The most striking result from the EXAFS data is that all spectra for the

The global Moho depth map for continental crust

NASA Astrophysics Data System (ADS)

Baranov, Alexey; Morelli, Andrea

2014-05-01

Different tectonic units cover the continents: platform, orogens and depression structures. This structural variability is reflected both in thickness and physical properties of the crust. We present a new global Moho map for the continental crust, derived from geophysical data selected from the literature and regional crustal models. The Moho depth is represented with a resolution of 1x1 on a Cartesian grid. A large volume of new data has been analyzed: mostly active seismic experiments, as well as receiver functions and geological studies. We have used the following regional studies: for Europe and Greenland, models EPcrust [Molinari and Morelli, 2011]and EUNAseis [Artemieva and Thybo, 2103]; for North Asia, Moho models from [Cherepanova et al., 2013; Iwasaki et al., 2013; Pavlenkova, 1996]; for Central and Southern Asia, model AsCrust [Baranov, 2010] with updates for India [Reddy and Rao, 2013]; China [Teng et al., 2013];Arabian [Mechie et al., 2013]; for Africa, the model by [Pasyanos and Nyblade, 2007] as a framework and added many others regional studies; for South America, models by [Assumpção et al.,2013; Chulick et al.,2013; Lloyd et al., 2010]; for North America, the model by [Keller, 2013]; for Australia, the model by [Salmon et al., 2013]; for Antarctica, model ANTMoho [Baranov and Morelli, 2013] with update for West Antarctica (POLENET project, [Chaput et al., 2013]). For two orogens we have found the maximum depth at - 75 km (Tibet and Andes). In our model the average thickness of the continental crust is about 34 km (st. deviation 9 km) whereas in CRUST 2.0 model the average Moho for continental areas is about 38 km. The new Moho model for continents exhibits some remarkable disagreement at places with respect to global model CRUST 2.0. The difference in crustal thickness between these two models may amount up to 30 km, mainly due to improved resolution of our model's Moho boundary. There are significant changes in several regions: among them

The Stirring of Oceanic Crust in the Mantle: How it Changes with Time?

NASA Astrophysics Data System (ADS)

McNamara, A. K.; Li, M.

2017-12-01

The Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific are considerably-sized seismic anomalies in the lower mantle that likely play a key role in global mantle convection. Unfortunately, we do not know what they are, and hypotheses include thermal megaplumes, plume clusters, primordial piles, thermochemical superplumes, and large accumulations of ancient, subducted oceanic crust. Discovering which of these are the cause of LLSVPs will provide fundamental understanding toward the nature of global-scale mantle convection. Here, we focus on two of the possibilities: primordial piles and accumulations of subducted oceanic crust. In previous work, it seemed clear that each provide a distinguishably-different morphology: primordial piles are clearly defined entities with sharp edges and tops, whereas accumulations of oceanic crust appear quite messy and have fuzzy, advective boundaries, particularly at their tops. Therefore, it was thought that by performing seismic studies that define the tops of LLSVPs, we could distinguish between these possibilities. Here, we ask the following question: Can piles formed by ancient oceanic crust eventually "clean themselves up" and evolve into structures that more-resemble what we think primordial piles should look like at the present day? Here, we present geodynamics work that demonstrates that this is indeed the case. The driving mechanism is a thinning of oceanic crust through time (as the mantle cools, there is less melt at ridges, and therefore, crust is thinner). We find that in the early, hotter Earth, if crust is on the order of 20-30 km thick, it will accumulate into messy piles at the base of the mantle. As crust thins beyond a critical thinness, it will stop accumulating and be stirred into the background mantle instead. Once crust stops accumulating in the lower mantle, the pre-existing messy piles begin to sharpen into well-defined piles with sharp edges and tops. Furthermore, we find that this

Oceanic crust in the mid-mantle beneath Central-West Pacific subduction zones: Evidence from S-to-P converted waveforms

NASA Astrophysics Data System (ADS)

He, X.

2015-12-01

The fate of subducted slabs is enigmatic, yet intriguing. We analyze seismic arrivals at ~20-50 s after the direct P wave in an array in northeast China (NECESSArray) recordings of four deep earthquakes occurring beneath the west-central Pacific subduction zones (from the eastern Indonesia to Tonga region). We employ the array analyzing techniques of 4th root vespagram and beam-form analysis to constrain the slowness and back azimuth of later arrivals. Our analyses reveal that these arrivals have a slightly lower slowness value than the direct P wave and the back azimuth deviates slightly from the great-circle direction. Along with calculation of one-dimensional synthetic seismograms, we conclude that the later arrival is corresponding to an energy of S-to-P converted at a scatterer below the sources. Total five scatterers are detected at depths varying from ~700 to 1110 km in the study region. The past subducted oceanic crust most likely accounts for the seismic scatterers trapped in the mid-mantle beneath the west-central subduction zones. Our observation in turn reflects that oceanic crust at least partly separated from subducted oceanic lithosphere and may be trapped substantially in the mid-mantle surrounding subduction zones, in particular in the western Pacific subduction zones.

Eastern Indian 3800-million-year-old crust and early mantle differentiation

USGS Publications Warehouse

Basu, A.R.; Ray, S.L.; Saha, A.K.; Sarkar, S.N.

1981-01-01

Samarium-neodymium data for nine granitic and tonalite gneisses occurring as remnants within the Singhbhum granite batholith in eastern India define an isochron of age 3775 ?? 89 ?? 106 years with an initial 143Nd/144Nd ratio of 0.50798 ?? 0.00007. This age contrasts with the rubidium-strontium age of 3200 ?? 106 years for the same suite of rocks. On the basis of the new samarium-neodynium data, field data, and petrologic data, a scheme of evolution is proposed for the Archean crust in eastern India. The isotopic data provide evidence that parts of the earth's mantle were already differentiated with respect to the chondritic samarium-neodymium ratio 3800 ?? 106 years ago.

Misdiagnosed crusted scabies in an AIDS patient leads to hyperinfestation.

PubMed

Yari, Niloofar; Malone, C Helen; Rivas, Antonio

2017-03-01

Crusted scabies is a severe, highly contagious form of classic scabies caused by the mite Sarcoptes scabiei var hominis . Crusted scabies is more common in immunosuppressed populations and overcrowded environments. In this condition, the host's immune system is overwhelmed and unable to defend against the mites on the skin, resulting in hyperinfestation of the host. Diagnosis can be challenging because the condition resembles other common skin conditions, such as plaque psoriasis. Furthermore, delayed diagnosis and inappropriate treatment can lead to worsening of the condition. We report a case of crusted scabies that was initially misdiagnosed in a 34-year-old incarcerated man with multidrug-resistant human immunodeficiency virus/AIDS. The patient had a complicated but complete recovery after treatment with permethrin and ivermectin was instituted.

Global lunar crust - Electrical conductivity and thermoelectric origin of remanent magnetism

NASA Technical Reports Server (NTRS)

Dyal, P.; Parkin, C. W.; Daily, W. D.

1977-01-01

An upper limit is placed on the average crustal conductivity from an investigation of toroidal (V x B) induction in the moon, using ten-minute data intervals of simultaneous lunar orbiting and surface magnetometer data. Crustal conductivity is determined as a function of crust thickness. For an average global crust thickness of about 80 km, the crust surface electrical conductivity is of the order of 1 hundred millionth mho/m. The toroidal-induction results lower the surface-conductivity limit obtained from poloidal-induction results by approximately four orders of magnitude. In addition, a thermoelectric (Seebeck effect) generator model is presented as a magnetic-field source for thermoremanent magnetization of the lunar crust during its solidification and cooling. Magnetic fields from 1000 to 10,000 gammas are calculated for various crater and crustal geometries. Solidified crustal material cooling through the iron Curie temperature in the presence of such ancient lunar fields could have received thermoremanent magnetization consistent with that measured in most returned lunar samples.

Crust and mantle of the gulf of Mexico

USGS Publications Warehouse

Moore, G.W.

1972-01-01

A SEEMING paradox has puzzled investigators of the crustal structure of the Gulf of Mexico since Ewing et al.1 calculated that a unit area of the rather thick crust in the gulf contains less mass than does a combination of the crust and enough of the upper mantle to make a comparable thickness in the Atlantic Ocean. They also noted that the free-air gravity of the gulf is essentially normal and fails by a large factor to be low enough to reflect the mass difference that they calculated. We propose a solution to this problem. ?? 1972 Nature Publishing Group.

Impacts of biological soil crust disturbance and composition on C and N loss from water erosion

USGS Publications Warehouse

Barger, N.N.; Herrick, J.E.; Van Zee, J.; Belnap, J.

2006-01-01

In this study, we conducted rainfall simulation experiments in a cool desert ecosystem to examine the role of biological soil crust disturbance and composition on dissolved and sediment C and N losses. We compared runoff and sediment C and N losses from intact late-successional dark cyanolichen crusts (intact) to both trampled dark crusts (trampled) and dark crusts where the top 1 cm of the soil surface was removed (scraped). In a second experiment, we compared C and N losses in runoff and sediments in early-successional light cyanobacterial crusts (light) to that of intact late-successional dark cyanolichen crusts (dark). A relatively high rainfall intensity of approximately 38 mm per 10-min period was used to ensure that at least some runoff was generated from all plots. Losses of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and ammonium (NH 4+ ) were significantly higher from trampled plots as compared to scraped and intact plots. Sediment C and N losses, which made up more than 98% of total nutrient losses in all treatments, were more than 4-fold higher from trampled plots relative to intact plots (sediment C g/m2, intact = 0.74, trampled = 3.47; sediment N g/m2, intact = 0.06, trampled = 0.28). In light crusts, DOC loss was higher relative to dark crusts, but no differences were observed in dissolved N. Higher sediment loss in light crusts relative to dark crusts resulted in 5-fold higher loss of sediment-bound C and N. Total C flux (sediment + dissolved) was on the order of 0.9 and 7.9 g/m2 for dark and light crusts, respectively. Sediment N concentration in the first minutes after runoff from light crusts was 3-fold higher than the percent N of the top 1 cm of soil, suggesting that even short-term runoff events may have a high potential for N loss due to the movement of sediments highly enriched in N. Total N loss from dark crusts was an order of magnitude lower than light crusts (dark = 0.06 g N/m2, light = 0.63 g/m2). Overall, our

Oxygen isotopes in garnet and accessory minerals to constrain fluids in subducted crust

NASA Astrophysics Data System (ADS)

Rubatto, Daniela; Gauthiez-Putallaz, Laure; Regis, Daniele; Rosa Scicchitano, Maria; Vho, Alice; Williams, Morgan

2017-04-01

Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. Garnet and U-Pb accessory minerals constitute some of the most robust and ubiquitous minerals in subducted crust and can preserve multiple growth zones that track the metamorphic evolution of the sample they are hosted in. Microbeam investigation of the chemical (major and trace elements) and isotopic composition (oxygen and U-Pb) of garnet and accessory minerals is used to track significant fluid-rock interaction at different stages of the subduction system. This approach requires consideration of the diffusivity of oxygen isotopes particularly in garnet, which has been investigated experimentally. The nature of the protolith and ocean floor alteration is preserved in relict accessory phases within eclogites that have been fully modified at HP conditions (e.g. Monviso and Dora Maira units in the Western Alps). Minerals in the lawsonite-blueschists of the Tavsanli zone in Turkey record pervasive fluid exchange between mafic and sedimentary blocks at the early stage of subduction. High pressure shear zones and lithological boundaries show evidence of intense fluid metasomatism at depth along discontinuities in Monviso and Corsica. In the UHP oceanic crust of the Zermatt-Saas Zone, garnet oxygen isotopes and tourmaline boron isotopes indicate multistage fluid infiltration during prograde metamorphism. Localized exchanges of aqueous fluids are also observed in the subducted continental crust of the Sesia-Lanzo Zone. In most cases analyses of distinct mineral zones enable identification of multiple pulses of fluids during the rock evolution.

Outer crust of nonaccreting cold neutron stars

NASA Astrophysics Data System (ADS)

Rüster, Stefan B.; Hempel, Matthias; Schaffner-Bielich, Jürgen

2006-03-01

The properties of the outer crust of nonaccreting cold neutron stars are studied by using modern nuclear data and theoretical mass tables, updating in particular the classic work of Baym, Pethick, and Sutherland. Experimental data from the atomic mass table from Audi, Wapstra, and Thibault of 2003 are used and a thorough comparison of many modern theoretical nuclear models, both relativistic and nonrelativistic, is performed for the first time. In addition, the influences of pairing and deformation are investigated. State-of-the-art theoretical nuclear mass tables are compared to check their differences concerning the neutron drip line, magic neutron numbers, the equation of state, and the sequence of neutron-rich nuclei up to the drip line in the outer crust of nonaccreting cold neutron stars.

Geochemical and Nd-Sr-Pb isotope characteristics of synorogenic lower crust-derived granodiorites (Central Damara orogen, Namibia)

NASA Astrophysics Data System (ADS)

Simon, I.; Jung, S.; Romer, R. L.; Garbe-Schönberg, D.; Berndt, J.

2017-03-01

The 547 ± 7 Ma old Achas intrusion (Damara orogen, Namibia) includes magnesian, metaluminous to slightly peraluminous, calcic to calc-alkalic granodiorites and ferroan, metaluminous to slightly peraluminous, calc-alkalic to alkali-calcic leucogranites. For the granodiorites, major and trace element variations show weak if any evidence for fractional crystallization whereas some leucogranites are highly fractionated. Both, granodiorites and leucogranites are isotopically evolved (granodiorites: εNdinit: - 12.4 to - 20.5; TDM: 2.4-1.9; leucogranites: εNdinit: - 12.1 to - 20.6, TDM: 2.5-2.0), show similar Pb isotopic compositions, and may be derived from late Archean to Paleoproterozoic crustal source rocks. Comparison with melting experiments and simple partial melting modeling indicate that the granodiorites may be derived by extensive melting (> 40%) at 900-950 °C under water-undersaturated conditions (< 5 wt.% H2O) of felsic gneisses. Al-Ti and zircon saturation thermometry of the most primitive granodiorite sample yielded temperatures of ca. 930 °C and ca. 800 °C. In contrast to other lower crust-derived granodiorites and granites of the Central Damara orogen, the composition of the magma source is considered the first-order cause of the compositional diversity of the Achas granite. Second-order processes such as fractional crystallization at least for the granodiorites were minor and evidence for coupled assimilation-fractional crystallization processes is lacking. The most likely petrogenetic model involves high temperature partial melting of a Paleoproterozoic felsic source in the lower crust ca. 10-20 Ma before the first peak of regional high-temperature metamorphism. Underplating of the lower crust by magmas derived from the lithospheric mantle may have provided the heat for melting of the basement to produce anhydrous granodioritic melts.

Helium isotopes in ferromanganese crusts from the central Pacific Ocean

USGS Publications Warehouse

Basu, S.; Stuart, F.M.; Klemm, V.; Korschinek, G.; Knie, K.; Hein, J.R.

2006-01-01

Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High 4He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4He. 3He/4He of VA13/2 samples varies from 18.5 to 1852 Ra due almost entirely to variation in the extraterrestrial He contribution. The highest 3He/4He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3He concentration of the crusts rules out whole, undegassed 4–40 μm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.

Generation of mantle heterogeneity by oceanic crust recycling: how well can we match geochemical and geophysical observations? (Invited)

NASA Astrophysics Data System (ADS)

van Keken, P. E.; Brandenburg, J. P.; Hauri, E. H.; Ballentine, C. J.

2009-12-01

The heterogeneity of the Earth's mantle is expressed in complementary geochemical and geophysical signatures, where the geochemistry provides a time-integrated signal and the geophysics tends to see a recent snapshot of the Earth's interior. While the geophysical evidence tends to support a form of whole mantle convection that is moderated by rheological and phase changes below the transition zone, the geochemical observations have been generally used to support the presence of long-lived and isolated reservoirs. Recent dynamical modeling (Brandenburg et al., EPSL, 2008) employed high resolution finite modeling of mantle convection using an energetically consistent simulation of tectonic plates. A suite of models was developed with a dynamic vigor similar to that of the present day earth. The recycling of oceanic crust combined with a two-stage formation of the continental crust leads to a satisfactory match to the observed spread between HIMU-DMM-EM1 in multiple isotope systems without invoking recycling of continental crust. Due to the rheological contrast between upper and lower mantle there is a natural occurrence of a well-mixed upper mantle overlaying a chemically more heterogeneous lower mantle. The pooling of dense oceanic crust provides the formation of dense piles at the base of the mantle. Together with the occurrence of slabs that thicken and/or stagnate at the 670 discontinuity we find reasonable correspondance with the present day tomographic signatures. At present the models fail to explain noble gas systematics, even when taking the suggested high compatibility of helium into account.

Sub-millimeter scale magnetostratigraphy and environmental magnetism of ferromanganese crusts using a scanning SQUID microscope

NASA Astrophysics Data System (ADS)

Oda, H.; Noguchi, A.; Yamamoto, Y.; Usui, A.; Ito, T.; Kawai, J.; Takahashi, H.

2017-12-01

Ferromanganese crusts are chemical sedimentary rock composed mainly of iron-manganese oxide. Because the ferromanganese crusts grow very slowly on the sea floor at rates of 3-10 mm/Ma, long-term deep-sea environmental changes can be reconstructed from the ferromanganese crusts. Thus, it is important to provide reliable age model for the crusts. For the past decades 10Be/9Be dating method has been used extensively to give age models for crusts younger than 15 Ma. Alternatively, sub-millimeter scale magnetostratigraphic study on a ferromanganese crust sample using a scanning SQUID (superconducting quantum interference device) microscope (Kawai et al., 2016; Oda et al., 2016) has been applied successfully (e.g. Oda et al., 2011; Noguchi et al. 2017). Also, environmental magnetic mapping was successful for the ferromanganese crust from the Takuyo Daigo Seamount (Noguchi et al., 2017). The ferromanganese crust used in this study was sampled from the Hanzawa Seamount, Ryukyu trench and the Shotoku Seamount. The vertical component of the magnetic field above thin section samples of the ferromanganese crust was measured using the scanning SQUID microscope on 100 μm grids. Magnetic mapping of the Hanzawa Seamount shows sub-millimeter scale magnetic stripes parallel to lamina. By correlating the boundaries of magnetic stripes with known geomagnetic reversals, we estimated that average growth rate of the Hanzawa Seamount is 2.67 +/- 0.04 mm/Ma , which is consistent with that deduced from the 10Be/9Be dating method (2.56 +/- 0.04 mm/Ma). The crust sample from the Shotoku Seamount used by Oda et al. (2011) shows prominent periodical lamination. Further details are going to be discussed together with the environmental magnetic mapping.

A review of shear wave splitting in the crack-critical crust

NASA Astrophysics Data System (ADS)

Crampin, Stuart; Chastin, Sebastien

2003-10-01

Over the last 15 years, it has become established that crack-induced stress-aligned shear wave splitting, with azimuthal anisotropy, is an inherent characteristic of almost all rocks in the crust. This means that most in situ rocks are pervaded by fluid-saturated microcracks and consequently are highly compliant. The evolution of such stress-aligned fluid-saturated grain-boundary cracks and pore throats in response to changing conditions can be calculated, in some cases with great accuracy, using anisotropic poro-elasticity (APE). APE is tightly constrained with no free parameters, yet dynamic modelling with APE currently matches a wide range of phenomena concerning anisotropy, stress, shear waves and cracks. In particular, APE has allowed the anisotropic response of a reservoir to injection to be calculated (predicted with hindsight), and the time and magnitude of an earthquake to be correctly stress-forecast. The reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems. This crack-critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre-fracturing deformation of the crust and for solid-earth geophysics and geology. We review past, present and speculate about the future of shear wave splitting in the crack-critical crust. Shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass. There is some good news and some bad news for conventional geophysics. Many accepted phenomena are no longer valid at high spatial and temporal resolution. A major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes. However, at least in some circumstances, the behaviour of the rock in the highly complex inhomogeneous Earth may be calculated and the response predicted, opening the way to possible control by feedback. The need is

Clinopyroxene precursors to amphibole sponge in arc crust

PubMed Central

Smith, Daniel J.

2014-01-01

The formation of amphibole cumulates beneath arc volcanoes is a key control on magma geochemistry, and generates a hydrous lower crust. Despite being widely inferred from trace element geochemistry as a major lower crustal phase, amphibole is neither abundant nor common as a phenocryst phase in arc lavas and erupted pyroclasts, prompting some authors to refer to it as a ‘cryptic’ fractionating phase. This study provides evidence that amphibole develops by evolved melts overprinting earlier clinopyroxene—a near-ubiquitous mineral in arc magmas. Reaction-replacement of clinopyroxene ultimately forms granoblastic amphibole lithologies. Reaction-replacement amphiboles have more primitive trace element chemistry (for example, lower concentrations of incompatible Pb) than amphibole phenocrysts, but still have chemistries suitable for producing La/Yb and Dy/Yb ‘amphibole sponge’ signatures. Amphibole can fractionate cryptically as reactions between melt and mush in lower crustal ‘hot zones’ produce amphibole-rich assemblages, without significant nucleation and growth of amphibole phenocrysts. PMID:25002269

Deep-sea mud volcanoes - a window to alteration processes in old oceanic crust?

NASA Astrophysics Data System (ADS)

Hensen, Christian; Scholz, Florian; Nuzzo, Marianne; Valadares, Vasco; Terrinha, Pedro; Liebetrau, Volker; Kaul, Norbert; Manzoni, Sonia; Schmidt, Mark; Gràcia, Eulàlia

2013-04-01

A number of deep sea mud volcanoes (>4700 m water depth) were discovered during a recent expedition with the German research vessel Meteor along a prominent WSW-ENE trending strike-slip fault (SWIM 1; Zitellini et al., 2009) in the western extension of the Gulf of Cadiz (NE Atlantic). Mud volcanism was unambiguously related to tectonic activity along the fault and fluids expelled at these sites show a very distinct geochemical composition that has not been reported from any other mud volcano to date. In previous studies on deep-water mud volcanoes in the Western Gulf of Cadiz accretionary wedge it was hypothesized that the discharge fluids were affected by alteration processes occurring in the old (>140 Ma) and deeply buried (>4 km) oceanic crust (Scholz et al., 2009; Sallarès et al, 2011). This hypothesis is supported by recent findings at the mud volcanoes located to the west of the realm of tectonic deformation driven by the accretionary wedge of the Gulf of Cadiz. Pore water geochemical analyses revealed fluid sources from oceanic crust and oldest sedimentary strata. Regardless of the ultimate source, these findings suggest that large strike-slip faults may play a significant, yet unrecognized role in terms of fluid circulation and element redistribution. To date, hot vents and cold seeps occurring at active spreading centers and forearcs of subduction zones have been pinpointed as hotspots of fluid activity. However, bearing in mind that transform-type plate boundaries are equal in length compared to other types of plate boundaries, fluid exchange at this type of plate boundary may provide a similarly important pathway for water and element exchange between the lithosphere and ocean. Sallarès V., Gailler A., Gutscher M.-A., Graindorge D., Bartolomé R., Gràcia E., Díaz J., Dañobeitia J.J. and Zitellini N. (2011) Seismic evidence for the presence of Jurassic oceanic crust in the central Gulf of Cadiz (SW Iberian margin), Earth and Planetary Science Letters

Seismic character of the crust and upper mantle beneath the Sierra Nevada

NASA Astrophysics Data System (ADS)

Frassetto, A.; Gilbert, H.; Zandt, G.; Owens, T. J.; Jones, C.

2008-12-01

Recent geophysical studies of the Southern Sierra Nevada suggest that the removal of a gravitationally unstable, eclogitic residue links to recent volcanism and uplift in the Eastern Sierra. The Sierra Nevada EarthScope Project (SNEP) investigates the extent of this process beneath Central and Northern Sierra Nevada. We present receiver functions, which provide estimates of crustal thickness and Vp/Vs and image the response of the crust and upper mantle to lithospheric removal. For completeness this study combines data from the 2005-2007 SNEP broadband experiment, EarthScope's BigFoot Array, regional backbone stations, and earlier PASSCAL deployments. We analyze transects of teleseismic receiver functions generated using a common-conversion-point stacking algorithm. These identify a narrow, "bright" conversion from the Moho at depths of ~25-35 km along the crest of the Eastern Sierra and adjacent Basin and Range northward to the Cascade Arc. Trade-off analysis using the primary conversion and reverberations shows a high Vp/Vs (~1.9) throughout the Eastern Sierra, which may relate to partial melt present in the lower crust. To the west the crust-mantle boundary vanishes beneath the western foothills. However, low frequency receiver functions do image the crust-mantle boundary exceeding 50 km depth along the foothills to the west and south of Yosemite National Park. Unusually deep, intraplate earthquakes (Ryan et al., this session) occur in the center of this region. The frequency dependence of the Moho conversion implies a gradational increase from crust to mantle wavespeeds over a significant depth interval. The transition from a sharp to gradational Moho probably relates to the change from a delaminated granitic crust to crust with an intact, dense, eclogitic residue. The spatial correlation and focal mechanisms of the deep earthquakes suggest that a segment of this still intact residue is currently delaminating.

Spatial modeling of biological soil crusts to support rangeland assessment and monitoring

USGS Publications Warehouse

Bowker, M.A.; Belnap, J.; Miller, M.E.

2006-01-01

Biological soil crusts are a diverse soil surface community, prevalent in semiarid regions, which function as ecosystem engineers and perform numerous important ecosystem services. Loss of crusts has been implicated as a factor leading to accelerated soil erosion and other forms of land degradation. To support assessment and monitoring efforts aimed at ensuring the sustainability of rangeland ecosystems, managers require spatially explicit information concerning potential cover and composition of biological soil crusts. We sampled low disturbance sites in Grand Staircase-Escalante National Monument (Utah, USA) to determine the feasibility of modeling the potential cover and composition of biological soil crusts in a large area. We used classification and regression trees to model cover of four crust types (light cyanobacterial, dark cyanobacterial, moss, lichen) and 1 cyanobacterial biomass proxy (chlorophyll a), based upon a parsimonious set of GIS (Geographic Information Systems) data layers (soil types, precipitation, and elevation). Soil type was consistently the best predictor, although elevation and precipitation were both invoked in the various models. Predicted and observed values for the dark cyanobacterial, moss, and lichen models corresponded moderately well (R 2 = 0.49, 0.64, 0.55, respectively). Cover of late successional crust elements (moss + lichen + dark cyanobacterial) was also successfully modeled (R2 = 0.64). We were less successful with models of light cyanobacterial cover (R2 = 0.22) and chlorophyll a (R2 = 0.09). We believe that our difficulty modeling chlorophyll a concentration is related to a severe drought and subsequent cyanobacterial mortality during the course of the study. These models provide the necessary reference conditions to facilitate the comparison between the actual cover and composition of biological soil crusts at a given site and their potential cover and composition condition so that sites in poor condition can be

Composition of the lower crust of the Arabian Plate: a xenolith perspective

NASA Astrophysics Data System (ADS)

Al-Mishwat, Ali T.; Nasir, Sobhi J.

2004-01-01

Petrological and geochemical data for a suite of mafic granulite xenoliths in Cenozoic alkali basalts from Saudi Arabia, Jordan and Syria provide a unique opportunity to explore the composition and nature of the lower crust beneath the Arabian Plate. Two mineralogically and chemically distinct groups of xenoliths occur. Group I is composed of two pyroxenes and plagioclase approximately in equal amounts. Group II is plagioclase rich and has variable proportions of orthopyroxene and clinopyroxene. The xenolith mineral assemblages and geothermobarometry of coexisting minerals suggest that these xenoliths represent basaltic cumulates that crystallized under high-pressure conditions in the lower crust. The xenoliths possibly form a part of a lower crustal gabbroic intrusive complex that underlies the Arabian Plate and may represent mafic roots of an arc complex of Pan-African age beneath Arabia. The xenolith data are compatible with available geophysical models on crust thickness and layering. The crust is between 20 and 40 km thick, and its lower part consists of mafic meta-igneous granulites. The chemical averages of xenoliths from different parts of the Arabian Plate are more mafic than the estimated present-day average of model lower crust.

Global occurrence of tellurium-rich ferromanganese crusts and a model for the enrichment of tellurium

USGS Publications Warehouse

Hein, J.R.; Koschinsky, A.; Halliday, A.N.

2003-01-01

Hydrogenetic ferromanganese oxyhydroxide crusts (Fe-Mn crusts) precipitate out of cold ambient ocean water onto hard-rock surfaces (seamounts, plateaus, ridges) at water depths of about 400 to 4000 m throughout the ocean basins. The slow-growing (mm/Ma) Fe-Mn crusts concentrate most elements above their mean concentration in the Earth's crust. Tellurium is enriched more than any other element (up to about 50,000 times) relative to its Earth's crustal mean of about 1 ppb, compared with 250 times for the next most enriched element. We analyzed the Te contents for a suite of 105 bulk hydrogenetic crusts and 140 individual crust layers from the global ocean. For comparison, we analyzed 10 hydrothermal stratabound Mn-oxide samples collected from a variety of tectonic environments in the Pacific. In the Fe-Mn crust samples, Te varies from 3 to 205 ppm, with mean contents for Pacific and Atlantic samples of about 50 ppm and a mean of 39 ppm for Indian crust samples. Hydrothermal Mn samples have Te contents that range from 0.06 to 1 ppm. Continental margin Fe-Mn crusts have lower Te contents than open-ocean crusts, which is the result of dilution by detrital phases and differences in growth rates of the hydrogenetic phases. Correlation coefficient matrices show that for hydrothermal deposits, Te has positive correlations with elements characteristic of detrital minerals. In contrast, Te in open-ocean Fe-Mn crusts usually correlates with elements characteristic of the MnO2, carbonate fluorapatite, and residual biogenic phases. In continental margin crusts, Te also correlates with FeOOH associated elements. In addition, Te is negatively correlated with water depth of occurrence and positively correlated with crust thickness. Q-mode factor analyses support these relationships. However, sequential leaching results show that most of the Te is associated with FeOOH in Fe-Mn crusts and ???10% is leached with the MnO2. Thermodynamic calculations indicate that Te occurs

Processes of lithosphere evolution: New evidence on the structure of the continental crust and uppermost mantle

USGS Publications Warehouse

Artemieva, I.M.; Mooney, W.D.; Perchuc, E.; Thybo, H.

2002-01-01

We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomography studies of Eurasia and from global and regional data on seismic anisotropy. These data as documented in the papers of this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passive and active), subduction, and lithospheric basal erosion due to a relative motion of cratonic keels and the convective mantle. ?? 2002 Elsevier Science B.V. All rights reserved.

Responses of photosynthetic properties and chloroplast ultrastructure of two moss crusts from a desert biological soil crust to supplementary UV-B radiation

NASA Astrophysics Data System (ADS)

Hui, Rong; Li, Xinrong; Zhao, Yang; Pan, Yanxia

2016-04-01

Our understanding of plant responses to supplementary ultraviolet-B (UV-B) radiation due to stratospheric ozone depletion has improved over recent decades. However, research on biological soil crusts (BSCs) is scarce and it remains controversial. Laboratory studies were conducted to investigate the influence of UV-B radiation on the Bryum argenteum and Didymodon vinealis isolated from BSCs, which are both dominant species in moss crusts found within patches of shrubs and herbs in the Tengger Desert of northern China. The aim of the current work was to evaluate whether supplementary UV-B radiation affected photosynthetic properties and chloroplast ultrastructure of two moss crusts and whether response differences were observed between the crusts. Four levels of UV-B radiation of 2.75 (control), 3.08, 3.25, and 3.41 W m-2 was achieved using fluorescence tube systems for 10 days, simulating 0, 6, 9, and 12% of stratospheric ozone at the latitude of Shapotou, respectively. We measured photosynthetic apparatus as assessed by chlorophyll a fluorescence parameters, photosynthetic pigment contents, and observations of chloroplast ultrastructure. Additionally, soluble proteins and UV-B absorbing compounds were simultaneously investigated. The results of this study showed that chlorophyll a fluorescence parameters (i.e., the maximal quantum yield of PSII photochemistry, the effective quantum yield of PSII photochemistry, and photochemical quenching coefficient), photosynthetic pigment contents, soluble protein contents, total flavonoid contents and the ultrastructure were negatively influenced by elevated UV-B radiation and the degree of detrimental effects significantly increased with the intensity of UV-B radiation. Moreover, results demonstrated that the negative effects on photosynthesis and chloroplast ultrastructure were more serious in B. argenteum than that in D. vinealis. These results may not only provide a potential mechanism for supplemental UV-B effects on

Impact of biological soil crusts and desert plants on soil microfaunal community composition

USGS Publications Warehouse

Darby, B.J.; Neher, D.A.; Belnap, J.

2010-01-01

Carbon and nitrogen are supplied by a variety of sources in the desert food web; both vascular and non-vascular plants and cyanobacteria supply carbon, and cyanobacteria and plant-associated rhizosphere bacteria are sources of biological nitrogen fixation. The objective of this study was to compare the relative influence of vascular plants and biological soil crusts on desert soil nematode and protozoan abundance and community composition. In the first experiment, biological soil crusts were removed by physical trampling. Treatments with crust removed had fewer nematodes and a greater relative ratio of bacterivores to microphytophages than treatments with intact crust. However, protozoa composition was similar with or without the presence of crusts. In a second experiment, nematode community composition was characterized along a spatial gradient away from stems of grasses or shrubs. Although nematodes generally occurred in increasing abundance nearer to plant stems, some genera (such as the enrichment-type Panagrolaimus) increased disproportionately more than others (such as the stress-tolerant Acromoldavicus). We propose that the impact of biological soil crusts and desert plants on soil microfauna, as reflected in the community composition of microbivorous nematodes, is a combination of carbon input, microclimate amelioration, and altered soil hydrology. ?? Springer Science + Business Media B.V. 2009.

Large-scale subduction of continental crust implied by India-Asia mass-balance calculation

NASA Astrophysics Data System (ADS)

Ingalls, Miquela; Rowley, David B.; Currie, Brian; Colman, Albert S.

2016-11-01

Continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle. When two continents collide, the mass balance for the continental crust is therefore assumed to be maintained. Here we use estimates of pre-collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the India-Asia collisional system. Using the current best estimates for the timing of the diachronous onset of collision between India and Eurasia, we find that about 50% of the pre-collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at Earth's surface today--represented by the mass preserved in the thickened crust that makes up the Himalaya, Tibet and much of adjacent Asia, as well as southeast Asian tectonic escape and exported eroded sediments. This implies large-scale subduction of continental crust during the collision, with a mass equivalent to about 15% of the total oceanic crustal subduction flux since 56 million years ago. We suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent-continent collisions is reflected in some ocean crust and ocean island basalt geochemistry. The subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry.

Earth's oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns

NASA Astrophysics Data System (ADS)

Wiemer, Daniel; Schrank, Christoph E.; Murphy, David T.; Wenham, Lana; Allen, Charlotte M.

2018-05-01

During the early Archaean, the Earth was too hot to sustain rigid lithospheric plates subject to Wilson Cycle-style plate tectonics. Yet by that time, up to 50% of the present-day continental crust was generated. Preserved continental fragments from the early Archaean have distinct granite-dome/greenstone-keel crust that is interpreted to be the result of a gravitationally unstable stratification of felsic proto-crust overlain by denser mafic volcanic rocks, subject to reorganization by Rayleigh-Taylor flow. Here we provide age constraints on the duration of gravitational overturn in the East Pilbara Terrane. Our U-Pb ages indicate the emplacement of 3,600-3,460-million-year-old granitoid rocks, and their uplift during an overturn event ceasing about 3,413 million years ago. Exhumation and erosion of this felsic proto-crust accompanied crustal reorganization. Petrology and thermodynamic modelling suggest that the early felsic magmas were derived from the base of thick ( 43 km) basaltic proto-crust. Combining our data with regional geochronological studies unveils characteristic growth cycles on the order of 100 million years. We propose that maturation of the early crust over three of these cycles was required before a stable, differentiated continent emerged with sufficient rigidity for plate-like behaviour.

[Nitrogen fixation potential of biological soil crusts in Heidaigou open coal mine, Inner Mongolia, China].

PubMed

Zhang, Peng; Huang, Lei; Hu, Yi-gang; Zhao, Yang; Wu, Yong-chen

2016-02-01

Nitrogen limitation is common in terrestrial ecosystems, and it is particularly severe in damaged ecosystems in arid regions. Biological soil crusts (BSCs) , as a crucial component of recovered vegetation, play a vital role in nitrogen fixation during the ecological restoration processes of damaged ecosystems in arid and semi-arid regions. In this study, two dominant types of BSCs (i.e., cyanobacterial-algal crusts and moss crusts) that are widely distributed in the re-vegetated area of Heidaigou open pit coal mine were investigated. Samples were collected in the field and their nitrogenase activities (NA) were measured in the laboratory. The responses of NA to different hydro-thermal factors and the relationships between NA and herbs in addition to crust coverage were analyzed. The results indicated that BSCs under reconstructed vegetation at different succession stages, abandoned land and natural vegetation showed values of NA ranging from 9 to 150 µmol C2H4 . m-2 . h-1, and the NA value of algae crust (77 µmol C2H4 . m-2 . h-1) was markedly higher than that of moss crust (17 µmol C2H4 . m-2 . h-1). In the re-vegetated area, cyanobacterial-algal crust and moss crust under shrub-herb had higher NA values than those of crusts under arbor-shrnb and arbor-shrub-herb. The relationship between NA of the two BSCs and soil relative water content (10% - 100%) as well as culture temperature (5-45 °C) were of quadratic function. With elevated water content and cultural temperature, the NA values increased at the initial stage and then decreased, and reached the maximum value at 25 °C of cultural temperature and 60% or 80% of relative water content. The NA of cyanobacterial-algal crust had a significant quadratic function with herb coverage, as NA declined when herb coverage was higher than 20%. A significant negative correlation was observed between the NA of moss crusts and herb coverage. The NA values of the two types of BSCs had a significant positive correlation

Soil nematode communities are ecologically more mature beneath late- than early-successional stage biological soil crusts

USGS Publications Warehouse

Darby, B.J.; Neher, D.A.; Belnap, J.

2007-01-01

Biological soil crusts are key mediators of carbon and nitrogen inputs for arid land soils and often represent a dominant portion of the soil surface cover in arid lands. Free-living soil nematode communities reflect their environment and have been used as biological indicators of soil condition. In this study, we test the hypothesis that nematode communities are successionally more mature beneath well-developed, late-successional stage crusts than immature, early-successional stage crusts. We identified and enumerated nematodes by genus from beneath early- and late-stage crusts from both the Colorado Plateau, Utah (cool, winter rain desert) and Chihuahuan Desert, New Mexico (hot, summer rain desert) at 0-10 and 10-30 cm depths. As hypothesized, nematode abundance, richness, diversity, and successional maturity were greater beneath well-developed crusts than immature crusts. The mechanism of this aboveground-belowground link between biological soil crusts and nematode community composition is likely the increased food, habitat, nutrient inputs, moisture retention, and/or environmental stability provided by late-successional crusts. Canonical correspondence analysis of nematode genera demonstrated that nematode community composition differed greatly between geographic locations that contrast in temperature, precipitation, and soil texture. We found unique assemblages of genera among combinations of location and crust type that reveal a gap in scientific knowledge regarding empirically derived characterization of dominant nematode genera in deserts soils and their functional role in a crust-associated food web. ?? 2006 Elsevier B.V. All rights reserved.

CRUST 5.1: A global crustal model at 5° x 5°

USGS Publications Warehouse

Mooney, Walter D.; Laske, Gabi; Masters, T. Guy

1998-01-01

We present a new global model for the Earth's crust based on seismic refraction data published in the period 1948–1995 and a detailed compilation of ice and sediment thickness. An extensive compilation of seismic refraction measurements has been used to determine the crustal structure on continents and their margins. Oceanic crust is modeled with both a standard model for normal oceanic crust, and variants for nonstandard regions, such as oceanic plateaus. Our model (CRUST 5.1) consists of 2592 5° × 5° tiles in which the crust and uppermost mantle are described by eight layers: (1) ice, (2) water, (3) soft sediments, (4) hard sediments, (5) crystalline upper, (6) middle, (7) lower crust, and (8) uppermost mantle. Topography and bathymetry are adopted from a standard database (ETOPO-5). Compressional wave velocity in each layer is based on field measurements, and shear wave velocity and density are estimated using recently published empirical Vp- Vs and Vp-density relationships. The crustal model differs from previous models in that (1) the thickness and seismic/density structure of sedimentary basins is accounted for more completely, (2) the velocity structure of unmeasured regions is estimated using statistical averages that are based on a significantly larger database of crustal structure, (3) the compressional wave, shear wave, and density structure have been explicitly specified using newly available constraints from field and laboratory studies. Thus this global crustal model is based on substantially more data than previous models and differs from them in many important respects. A new map of the thickness of the Earth's crust is presented, and we illustrate the application of this model by using it to provide the crustal correction for surface wave phase velocity maps. Love waves at 40 s are dominantly sensitive to crustal structure, and there is a very close correspondence between observed phase velocities at this period and those predicted by CRUST 5

Physiological variation among native and exotic winter annual plants associated with microbiotic crusts in the Mojave Desert

USGS Publications Warehouse

DeFalco, L.A.; Detling, J.K.; Tracy, C.R.; Warren, S.D.

2001-01-01

Microbiotic crusts are important components of many aridland soils. Research on crusts typically focuses on the increase in soil fertility due to N-fixing micro-organisms, the stabilization of soils against water and wind erosion and the impact of disturbance on N-cycling. The effect of microbiotic crusts on the associated plant community has received little attention. We quantified the influence of crusts on the production, species diversity, nutrient content and water relations of winter annual plant species associated with microbiotic soil crusts in the northeast Mojave Desert. Shoot biomass of winter annuals was 37% greater and plant density was 77% greater on crusts than were biomass and density on soils lacking crust cover (=bare soils). This greater production of annuals on crusts was likely due to enhanced soil conditions including an almost two-fold increase in soil organic matter and inorganic N compared to bare soils. Crusted soils also had 53% greater volumetric water content than bare soils during November and December, the time when winter annuals become established. As plant development progressed into spring, however, soil water availability decreased: More negative plant xylem water potentials were associated with greater plant biomass on crusted soils. Plants associated with microbiotic soil crusts had lower concentrations of N in shoots (mg N g-1 dry mass). However, total shoot N (mg N m-2) was the same in plants growing on the different soil types when biomass production peaked in April. Shoots had similar patterns in their concentration and content of P. Species diversity of annuals was not statistically different between the two soil types. Yet, while native annuals comprised the greatest proportion of shoot biomass on bare soils, exotic forbs and grasses produced more biomass on crusts. Total shoot nutrient content (biomass x concentration) of the two exotic annual species examined was dramatically greater on crusts than bare soils; only one

Physiological variation among native and exotic winter annuals associated with microphytic soil crusts in the Mojave Desert

USGS Publications Warehouse

DeFalco, Lesley; Detling, James K.; Tracy, C. Richard; Warren, Steven D.

2001-01-01

Microbiotic crusts are important components of many aridland soils. Research on crusts typically focuses on the increase in soil fertility due to N-fixing micro-organisms, the stabilization of soils against water and wind erosion and the impact of disturbance on N-cycling. The effect of microbiotic crusts on the associated plant community has received little attention. We quantified the influence of crusts on the production, species diversity, nutrient content and water relations of winter annual plant species associated with microbiotic soil crusts in the northeast Mojave Desert. Shoot biomass of winter annuals was 37% greater and plant density was 77% greater on crusts than were biomass and density on soils lacking crust cover (=bare soils). This greater production of annuals on crusts was likely due to enhanced soil conditions including an almost two-fold increase in soil organic matter and inorganic N compared to bare soils. Crusted soils also had 53% greater volumetric water content than bare soils during November and December, the time when winter annuals become established. As plant development progressed into spring, however, soil water availability decreased: More negative plant xylem water potentials were associated with greater plant biomass on crusted soils. Plants associated with microbiotic soil crusts had lower concentrations of N in shoots (mg N g−1 dry mass). However, total shoot N (mg N m−2) was the same in plants growing on the different soil types when biomass production peaked in April. Shoots had similar patterns in their concentration and content of P. Species diversity of annuals was not statistically different between the two soil types. Yet, while native annuals comprised the greatest proportion of shoot biomass on bare soils, exotic forbs and grasses produced more biomass on crusts. Total shoot nutrient content (biomass×concentration) of the two exotic annual species examined was dramatically greater on crusts than bare soils; only

Magnetization of the oceanic crust - Thermoremanent magnetization of chemical remanent magnetization?

NASA Technical Reports Server (NTRS)

Raymond, C. A.; Labrecque, J. L.

1987-01-01

A model was proposed in which chemical remanent magnetization (CRM) acquired within the first 20 Ma of crustal evolution may account for 80 percent of the bulk natural remanent magnetization (NRM) of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low temperature alteration. The CRM intensity and direction are controlled by the post-emplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness dicrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis, and the enhanced magnetization of the Cretaceous Quiet Zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further studies indicate varying rates of TRM decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer.

Construction of Continental Crust at the Central American and Philippines Arc Systems

NASA Astrophysics Data System (ADS)

Whattam, S. A.; Stern, R. J.

2016-12-01

Whether or not magmatic arcs evolve compositionally with time and the processes responsible remain controversial. Resolution of this question requires reconstructing arc geochemical evolution at the level of discrete arc systems, as has been done for IBM, Central America, and the Greater Antilles. Emphasis should be on arcs built on oceanic crust because interaction with continental crust complicates interpretations. The Philippines are a particularly attractive target because this may be the best example where proto-continental crust has been generated and processed in Cretaceous and younger time. Here, we show how this question could be addressed for the Philippines using the well-studied Central American Volcanic Arc System (CAVAS) as an example. For the CAVAS, we avoided the northern arc segment because these are (Guatemala) or maybe (El Salvador) sections built on continental crust. Geochemical and isotopic data were compiled for 1031 samples of lavas and intrusive rocks from the 1100 km-long segment built on thickened, initially plume-derived oceanic crust over its 75 million year lifespan (Panama, Costa Rica, Nicaragua) . The most striking observation is the overall evolution of the CAVAS to more incompatible element enriched and ultimately continental-like compositions with time. Models entailing progressive arc magmatic enrichment are generally supported by the CAVAS record. Progressive enrichment of the oceanic CAVAS with time reflects changes in mantle wedge composition and decreased melting due to arc crust thickening, which was kick-started by the involvement of enriched plume mantle. Progressive crustal thickening and associated changes in the sub-arc thermal regime resulted in decreasing degrees of partial melting over time, which allowed for progressive enrichment of the CAVAS and ultimately the production of continental-like crust in Panama and Costa Rica by 16-10 Ma. Our similar study of the Philippine Arc system is in its infancy but earlier

A Comparison of Microbial Communities from Deep Igneous Crust

NASA Astrophysics Data System (ADS)

Smith, A. R.; Flores, G. E.; Fisk, M. R.; Colwell, F. S.; Thurber, A. R.; Mason, O. U.; Popa, R.

2013-12-01

Recent investigations of life in Earth's crust have revealed common themes in organism function, taxonomy, and diversity. Capacities for hydrogen oxidation, carbon fixation, methanogenesis and methanotrophy, iron and sulfur metabolisms, and hydrocarbon degradation often predominate in deep life communities, and crustal mineralogy has been hypothesized as a driving force for determining deep life community assemblages. Recently, we found that minerals characteristic of the igneous crust harbored unique communities when incubated in the Juan de Fuca Ridge flank borehole IODP 1301A. Here we present attached mineral biofilm morphologies and a comparison of our mineral communities to those from a variety of locations, contamination states, and igneous crustal or mineralogical types. We found that differences in borehole mineral communities were reflected in biofilm morphologies. Olivine biofilms were thick, carbon-rich films with embedded cells of uniform size and shape and often contained secondary minerals. Encrusted cells, spherical and rod-shaped cells, and tubes were indicative of glass surfaces. We also found that the attached communities from incubated borehole minerals were taxonomically more similar to native, attached communities from marine and continental crust than to communities from the aquifer water that seeded it. Our findings further support the hypothesis that mineralogy selects for microbial communities that have distinct phylogenetic, morphological, and potentially functional, signatures. This has important implications for resolving ecosystem function and microbial distributions in igneous crust, the largest deep habitat on Earth.

Mars Crust: Made of Basalt

NASA Astrophysics Data System (ADS)

Taylor, G. J.

2009-05-01

By combining data from several sources, Harry Y. (Hap) McSween (University of Tennessee), G. Jeffrey Taylor (University of Hawaii) and Michael B. Wyatt (Brown University) show that the surface of Mars is composed mostly of basalt not unlike those that make up the Earth's oceanic crust. McSween and his colleagues used data from Martian meteorites, analyses of soils and rocks at robotic landing sites, and chemical and mineralogical information from orbiting spacecraft. The data show that Mars is composed mostly of rocks similar to terrestrial basalts called tholeiites, which make up most oceanic islands, mid-ocean ridges, and the seafloor beneath sediments. The Martian samples differ in some respects that reflect differences in the compositions of the Martian and terrestrial interiors, but in general are a lot like Earth basalts. Cosmochemistst have used the compositions of Martian meteorites to discriminate bulk properties of Mars and Earth, but McSween and coworkers' synthesis shows that the meteorites differ from most of the Martian crust (the meteorites have lower aluminum, for example), calling into question how diagnostic the meteorites are for understanding the Martian interior.

Outer crust of nonaccreting cold neutron stars

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ruester, Stefan B.; Hempel, Matthias; Schaffner-Bielich, Juergen

The properties of the outer crust of nonaccreting cold neutron stars are studied by using modern nuclear data and theoretical mass tables, updating in particular the classic work of Baym, Pethick, and Sutherland. Experimental data from the atomic mass table from Audi, Wapstra, and Thibault of 2003 are used and a thorough comparison of many modern theoretical nuclear models, both relativistic and nonrelativistic, is performed for the first time. In addition, the influences of pairing and deformation are investigated. State-of-the-art theoretical nuclear mass tables are compared to check their differences concerning the neutron drip line, magic neutron numbers, the equationmore » of state, and the sequence of neutron-rich nuclei up to the drip line in the outer crust of nonaccreting cold neutron stars.« less

Temperature distribution in the crust and mantle

NASA Technical Reports Server (NTRS)

Jeanloz, R.; Morris, S.

1986-01-01

In an attempt to understand the temperature distribution in the earth, experimental constraints on the geotherm in the crust and mantle are considered. The basic form of the geotherm is interpreted on the basis of two dominant mechanisms by which heat is transported in the earth: (1) conduction through the rock, and (2) advection by thermal flow. Data reveal that: (1) the temperature distributions through continental lithosphere and through oceanic lithosphere more than 60 million years old are practically indistinguishable, (2) crustal uplift is instrumental in modifying continental geotherms, and (3) the average temperature through the Archean crust and mantle was similar to that at present. It is noted that current limitations in understanding the constitution of the lower mantle can lead to significant uncertainties in the thermal response time of the planetary interior.

The nature of the crust under Cayman Trough from gravity

USGS Publications Warehouse

ten Brink, Uri S.; Coleman, D.F.; Dillon, William P.

2002-01-01

Considerable crustal thickness variations are inferred along Cayman Trough, a slow-spreading ocean basin in the Caribbean Sea, from modeling of the gravity field. The crust to a distance of 50 km from the spreading center is only 2–3 km thick in agreement with dredge and dive results. Crustal thickness increases to ∼5.5 km at distances between 100 and 430 km west of the spreading center and to 3.5–6 km at distances between 60 and 370 km east of the spreading center. The increase in thickness is interpreted to represent serpentinization of the uppermost mantle lithosphere, rather than a true increase in the volume of accreted ocean crust. Serpentinized peridotite rocks have indeed been dredged from the base of escarpments of oceanic crust rocks in Cayman Trough. Laboratory-measured density and P-wave speed of peridotite with 40–50% serpentine are similar to the observed speed in published refraction results and to the inferred density from the model. Crustal thickness gradually increases to 7–8 km at the far ends of the trough partially in areas where sea floor magnetic anomalies were identified. Basement depth becomes gradually shallower starting 250 km west of the rise and 340 km east of the rise, in contrast to the predicted trend of increasing depth to basement from cooling models of the oceanic lithosphere. The gradual increase in apparent crustal thickness and the shallowing trend of basement depth are interpreted to indicate that the deep distal parts of Cayman Trough are underlain by highly attenuated crust, not by a continuously accreted oceanic crust.

Oceanic crust recycling and the formation of lower mantle heterogeneity

NASA Astrophysics Data System (ADS)

van Keken, Peter E.; Ritsema, Jeroen; Haugland, Sam; Goes, Saskia; Kaneshima, Satoshi

2016-04-01

The Earth's lower mantle is heterogeneous at multiple scales as demonstrated for example by the degree-2 distribution of LLSVPs seen in global tomography and widespread distribution of small scale heterogeneity as seen in seismic scattering. The origin of this heterogeneity is generally attributed to leftovers from Earth's formation, the recycling of oceanic crust, or a combination thereof. Here we will explore the consequences of long-term oceanic crust extraction and recycling by plate tectonics. We use geodynamical models of mantle convection that simulate plates in an energetically consistent manner. The recycling of oceanic crust over the age of the Earth produces persistent lower mantle heterogeneity while the upper mantle tends to be significantly more homogeneous. We quantitatively compare the predicted heterogeneity to that of the present day Earth by tomographic filtering of the geodynamical models and comparison with S40RTS. We also predict the scattering characteristics from S-P conversions and compare these to global scattering observations. The geophysical comparison shows that lower mantle heterogeneity is likely dominated by long-term oceanic crust recycling. The models also demonstrate reasonable agreement with the geochemically observed spread between HIMU-EM1-DMM in ocean island basalts as well as the long-term gradual depletion of the upper mantle as observed in Lu-Hf systematics.

Don’t bust the biological soil crust: Preserving and restoring an important desert resource

Treesearch

Sue Miller; Steve Warren; Larry St. Clair

2017-01-01

Biological soil crusts are a complex of microscopic organisms growing on the soil surface in many arid and semi-arid ecosystems. These crusts perform the important role of stabilizing soil and reducing or eliminating water and wind erosion. One of the largest threats to biological soil crusts in the arid and semi-arid areas of the western United States is mechanical...

Mars primordial crust: unique sites for investigating proto-biologic properties.

PubMed

Perry, Randall S; Hartmann, William K

2006-12-01

The Martian meteorite collection suggests that intact outcrops or boulder-scale fragments of the 4.5 Ga Martian crust exist within tens of meters of the present day surface of Mars. Mars may be the only planet where such primordial crust samples, representing the first 100 Ma of a planet's environment, are available. The primordial crust has been destroyed on Earth by plate tectonics and other geological phenomena and is buried on the Moon under hundreds or thousands of meters of megaregoltih. Early Mars appears to have been remarkably similar to early Earth, and samples of rock from the first few Ma or first 100 Ma may reveal "missing link" proto-biological forms that could shed light on the transition from abiotic organic chemistry to living cells. Such organic snapshots of nascent life are unlikely to be found on Earth.

Laboratory experiments duplicate conditions in the Earth’s crust

USGS Publications Warehouse

Peselnick, L.; Dieterich, J.H.; Stewart, R.M.

1974-01-01

An experimental device that simulates conditions in the Earth's crust at depths of up to 30 kilometers has been constructed by geophysicists working at the U.S Geological Survey laboratories in Menlo Park, California. A high pressure "bomb" is being used to experimentally measure the velocity of seismic waves in different types of rock at various confining pressures and temperatures. The principal purpose of these measurements is to determine the elastic and non-elastic properties of rocks and minerals under conditions of high-pressure such as exist deep in the Earth's crust. 

First Investigation of the Microbiology of the Deepest Layer of Ocean Crust

PubMed Central

Mason, Olivia U.; Nakagawa, Tatsunori; Rosner, Martin; Van Nostrand, Joy D.; Zhou, Jizhong; Maruyama, Akihiko; Fisk, Martin R.; Giovannoni, Stephen J.

2010-01-01

The gabbroic layer comprises the majority of ocean crust. Opportunities to sample this expansive crustal environment are rare because of the technological demands of deep ocean drilling; thus, gabbroic microbial communities have not yet been studied. During the Integrated Ocean Drilling Program Expeditions 304 and 305, igneous rock samples were collected from 0.45-1391.01 meters below seafloor at Hole 1309D, located on the Atlantis Massif (30 °N, 42 °W). Microbial diversity in the rocks was analyzed by denaturing gradient gel electrophoresis and sequencing (Expedition 304), and terminal restriction fragment length polymorphism, cloning and sequencing, and functional gene microarray analysis (Expedition 305). The gabbroic microbial community was relatively depauperate, consisting of a low diversity of proteobacterial lineages closely related to Bacteria from hydrocarbon-dominated environments and to known hydrocarbon degraders, and there was little evidence of Archaea. Functional gene diversity in the gabbroic samples was analyzed with a microarray for metabolic genes (“GeoChip”), producing further evidence of genomic potential for hydrocarbon degradation - genes for aerobic methane and toluene oxidation. Genes coding for anaerobic respirations, such as nitrate reduction, sulfate reduction, and metal reduction, as well as genes for carbon fixation, nitrogen fixation, and ammonium-oxidation, were also present. Our results suggest that the gabbroic layer hosts a microbial community that can degrade hydrocarbons and fix carbon and nitrogen, and has the potential to employ a diversity of non-oxygen electron acceptors. This rare glimpse of the gabbroic ecosystem provides further support for the recent finding of hydrocarbons in deep ocean gabbro from Hole 1309D. It has been hypothesized that these hydrocarbons might originate abiotically from serpentinization reactions that are occurring deep in the Earth's crust, raising the possibility that the lithic microbial

First investigation of the microbiology of the deepest layer of ocean crust.

PubMed

Mason, Olivia U; Nakagawa, Tatsunori; Rosner, Martin; Van Nostrand, Joy D; Zhou, Jizhong; Maruyama, Akihiko; Fisk, Martin R; Giovannoni, Stephen J

2010-11-05

The gabbroic layer comprises the majority of ocean crust. Opportunities to sample this expansive crustal environment are rare because of the technological demands of deep ocean drilling; thus, gabbroic microbial communities have not yet been studied. During the Integrated Ocean Drilling Program Expeditions 304 and 305, igneous rock samples were collected from 0.45-1391.01 meters below seafloor at Hole 1309D, located on the Atlantis Massif (30 °N, 42 °W). Microbial diversity in the rocks was analyzed by denaturing gradient gel electrophoresis and sequencing (Expedition 304), and terminal restriction fragment length polymorphism, cloning and sequencing, and functional gene microarray analysis (Expedition 305). The gabbroic microbial community was relatively depauperate, consisting of a low diversity of proteobacterial lineages closely related to Bacteria from hydrocarbon-dominated environments and to known hydrocarbon degraders, and there was little evidence of Archaea. Functional gene diversity in the gabbroic samples was analyzed with a microarray for metabolic genes ("GeoChip"), producing further evidence of genomic potential for hydrocarbon degradation--genes for aerobic methane and toluene oxidation. Genes coding for anaerobic respirations, such as nitrate reduction, sulfate reduction, and metal reduction, as well as genes for carbon fixation, nitrogen fixation, and ammonium-oxidation, were also present. Our results suggest that the gabbroic layer hosts a microbial community that can degrade hydrocarbons and fix carbon and nitrogen, and has the potential to employ a diversity of non-oxygen electron acceptors. This rare glimpse of the gabbroic ecosystem provides further support for the recent finding of hydrocarbons in deep ocean gabbro from Hole 1309D. It has been hypothesized that these hydrocarbons might originate abiotically from serpentinization reactions that are occurring deep in the Earth's crust, raising the possibility that the lithic microbial

Noble metals in ferromanganese crusts from marginal seas of the Northwest Pacific

NASA Astrophysics Data System (ADS)

Astakhova, N. V.

2017-07-01

Based on data on the concentration of noble metals (Au, Ag, Pt, Os, Ir, and Ru) in bulk samples of ferromanganese crusts, the presence of inclusions of micro- and nanosized grains of Ag, Au, Pd, and Pt, often with impurities of other elements, as well as their chaotic distribution, three sources of incorporation of these metals into ore crusts of Far Eastern seas are suggested: seawater, postvolcanic gas-hydrothermal fluids, and hydrothermal plumes. The presence of grains of platinoids and gold in ferromanganese crusts on only some mounts may result from peculiarities in the formation of volcanic rocks on the ancient continental basement.

40K-(40)Ar constraints on recycling continental crust into the mantle

PubMed

Coltice; Albarede; Gillet

2000-05-05

Extraction of potassium into magmas and outgassing of argon during melting constrain the relative amounts of potassium in the crust with respect to those of argon in the atmosphere. No more than 30% of the modern mass of the continents was subducted back into the mantle during Earth's history. It is estimated that 50 to 70% of the subducted sediments are reincorporated into the deep continental crust. A consequence of the limited exchange between the continental crust and the upper mantle is that the chemistry of the upper mantle is driven by exchange of material with the deep mantle.

A Discussion of SY-101 Crust Gas Retention and Release Mechanisms

DOE Office of Scientific and Technical Information (OSTI.GOV)

SD Rassat; PA Gauglitz; SM Caley

1999-02-23

The flammable gas hazard in Hanford waste tanks was made an issue by the behavior of double-shell Tank (DST) 241-SY-101 (SY-101). Shortly after SY-101 was filled in 1980, the waste level began rising periodically, due to the generation and retention of gases within the slurry, and then suddenly dropping as the gases were released. An intensive study of the tank's behavior revealed that these episodic releases posed a safety hazard because the released gas was flammable, and, in some cases, the volume of gas released was sufficient to exceed the lower flammability limit (LFL) in the tank headspace (Allemann etmore » al. 1993). A mixer pump was installed in SY-101 in late 1993 to prevent gases from building up in the settled solids layer, and the large episodic gas releases have since ceased (Allemann et al. 1994; Stewart et al. 1994; Brewster et al. 1995). However, the surface level of SY-101 has been increasing since at least 1995, and in recent months the level growth has shown significant and unexpected acceleration. Based on a number of observations and measurements, including data from the void fraction instrument (VFI), we have concluded that the level growth is caused largely by increased gas retention in the floating crust. In September 1998, the crust contained between about 21 and 43% void based on VFI measurements (Stewart et al. 1998). Accordingly, it is important to understand the dominant mechanisms of gas retention, why the gas retention is increasing, and whether the accelerating level increase will continue, diminish or even reverse. It is expected that the retained gas in the crust is flammable, with hydrogen as a major constituent. This gas inventory would pose a flammable gas hazard if it were to release suddenly. In May 1997, the mechanisms of bubble retention and release from crust material were the subject of a workshop. The evaluation of the crust and potential hazards assumed a more typical void of roughly 15% gas. It could be similar to

Influence of development stage and disturbance of physical and biological soil crusts on soil water erosion

NASA Astrophysics Data System (ADS)

Chamizo, S.; Cantón, Y.; Lázaro, R.; Solé-Benet, A.; Calvo-Cases, A.; Miralles, I.; Domingo, F.

2009-04-01

Most soils exposed to rainfall are prone to sealing and crusting processes causing physical soil crusts (PSCs). When climate and soil stability conditions are suitable, PSCs can be consolidated by a complex community consisting of cyanobacteria, bacteria, green algae, microfungi, lichens and bryophytes, which are collectively known as biological soil crust (BSC). The influence of soil crusts on erosion processes is complex: crusts may reduce detachment, increasing soil stability and protecting soil against raindrop impact, although that protection will depend on the type of soil crust and the stage of development; they can also build up runoff, suggesting that downstream erosion may actually be increased or favoured water harvesting to vegetated areas. On the other hand, BSCs have been demonstrated to be very vulnerable to disturbance which in turn can lead to accelerate soil erosion and other forms of land degradation. Incorporation of the response of different type of soil crusts and the effects of their disturbance is highly likely to improve the prediction of runoff and water erosion models in arid and semi-arid catchments. The objective of this work is to analyse the erosional response of PSCs and BSCs in different stages of their development and subject to distinct disturbances when extreme rainfalls intensities are applied at plot scale in semiarid environments. Small plots on the most representative crust types, corresponding to different stages of crust development, in two semiarid ecosystems in SE Spain, El Cautivo (in the Tabernas Desert) and Amoladeras (in the Natural Park Cabo de Gata-Níjar), were selected and three disturbance treatments were applied on each crust type: a) no disturbance (control), b) trampling, stepping 100 times over the crust and c) scraping. Two consecutive rainfall simulation experiments (50 mm/h rainfall intensity) were carried out on each plot: the first on dry soil and the second, 30 minutes later, on wet soil conditions

Apulian crust: Top to bottom

NASA Astrophysics Data System (ADS)

Amato, Alessandro; Bianchi, Irene; Agostinetti, Nicola Piana

2014-12-01

We investigate the crustal seismic structure of the Adria plate using teleseismic receiver functions (RF) recorded at 12 broadband seismic stations in the Apulia region. Detailed models of the Apulian crust, e.g. the structure of the Apulian Multi-layer Platform (AMP), are crucial for assessing the presence of potential décollements at different depth levels that may play a role in the evolution of the Apenninic orogen. We reconstruct S-wave velocity profiles applying a trans-dimensional Monte Carlo method for the inversion of RF data. Using this method, the resolution at the different depth level is completely dictated by the data and we avoid introducing artifacts in the crustal structure. We focus our study on three different key-elements: the Moho depth, the lower crust S-velocity, and the fine-structure of the AMP. We find a well defined and relatively flat Moho discontinuity below the region at 28-32 km depth, possibly indicating that the original Moho is still preserved in the area. The lower crust appears as a generally low velocity layer (average Vs = 3.7 km/s in the 15-26 km depth interval), likely suggestive of a felsic composition, with no significant velocity discontinuities except for its upper and lower boundaries where we find layering. Finally, for the shallow structure, the comparison of RF results with deep well stratigraphic and sonic log data allowed us to constrain the structure of the AMP and the presence of underlying Permo-Triassic (P-T) sediments. We find that the AMP structure displays small-scale heterogeneities in the region, with a thickness of the carbonates layers varying between 4 and 12 km, and is underlain by a thin, discontinuous layer of P-T terrigenous sediments, that are lacking in some areas. This fact may be due to the roughness in the original topography of the continental margins or to heterogeneities in its shallow structure due to the rifting process.

Soil surface disturbances in cold deserts: Effects on nitrogenase activity in cyanobacterial-lichen soil crusts

USGS Publications Warehouse

Belnap, Jayne

1996-01-01

CyanobacteriaMichen soil crusts can be a dominant source of nitrogen for cold-desert ecosystems. Effects of surface disturbance from footprints, bike and vehicle tracks on the nitrogenase activity in these crusts was investigated. Surface disturbances reduced nitrogenase activity by 30-100%. Crusts dominated by the cyanobacterium Microcoleus vaginatus on sandy soils were the most susceptible to disruption; crusts on gypsiferous soils were the least susceptible. Crusts where the soil lichen Collema tenax was present showed less immediate effects; however, nitrogenase activity still declined over time. Levels of nitrogenase activity reduction were affected by the degree of soil disruption and whether sites were dominated by cyanobacteria with or without heterocysts. Consequently, anthropogenic surface disturbances may have serious implications for nitrogen budgets in these ecosystems.

The significance of slab-crusted lava flows for understanding controls on flow emplacement at Mount Etna, Sicily

NASA Astrophysics Data System (ADS)

Guest, John E.; Stofan, Ellen R.

2005-04-01

Slab-crusted flows on Mount Etna, Sicily are defined here as those whose crust has ridden on the flow core without significant disruption or deformation and have a high length to width ratio. They typically erupt from ephemeral boccas as late-stage products on dominantly aa flow fields, such as that of the 1983 eruption on Mount Etna. Slab-crusted flows tend to inflate mainly as they approach and after they reach the maximum length of slab-crust formation, the flow interior acting as a preferential pathway for injecting lava under a stable crust. Coalescence of vesicles under successive crusts causes separation between core and crust giving a new cooling surface within the flow, on which ropy surfaces (and occasionally aa textures) of limited areal extent may develop. Slab-crusted flows tend to form at ephemeral boccas together with other surface textural types including toes, ropy pahoehoe sheets and aa flows. This suggests that, on Etna, slab-crusted flows form from lava of the same rheological properties as both aa and pahoehoe textured flows. They do not represent a transition between aa and pahoehoe as argued for toothpaste flows in Hawaii. We conclude that slab-crusted flows on Etna owe their morphology to a relatively high critical ratio of effusion rate to advance rate, related to vent cross-sectional area and the slope over which the flow forms.

Evolution of Nd and Pb isotopes in Central Pacific seawater from ferromanganese crusts

USGS Publications Warehouse

Ling, H.F.; Burton, K.W.; O'Nions, R. K.; Kamber, B.S.; Von Blanckenburg, F.; Gibb, A.J.; Hein, J.R.

1997-01-01

Hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts. By analysing Nd, Pb and Be isotope profiles within crusts it is possible to reconstruct seawater tracer histories. Depth profiles of 10Be/9Be ratios in three Pacific ferromanganese crusts have been used to obtain growth rates which are between 1.4 and 3.8 mm/Ma. Nd and Pb isotopes provide intact records of isotopic variations in Pacific seawater over the last 20 Ma or more. There were only small changes in Pb isotope composition in the last 20 Ma. This indicates a constant Pb composition for the erosional sources and suggests further that erosional Nd inputs may have been uniform too. ??ND values vary considerably with time and most probably reflect changes in ocean circulation. The ??ND values of the crusts not only vary as a function of age but also as a function of water depth. From 25 to 0 Ma, crust VA13/2 from 4.8 km water depth has a similar pattern of ??ND variation to the two shallower crusts from 1.8 and 2.3 km, but about 1.0 to 1.5 units more negative. This suggests that ??ND stratification in Pacific seawater, as demonstrated for the present day, has been maintained for at least 20 Ma. Each crust shows a decrease in ??ND from 3-5 Ma to the present, which is interpreted in terms of an increase in the NADW component present in the Pacific. From 10 to 3-5 Ma ago the crusts show an increase in ??ND. This suggests a decreasing role for a deep water source with ??ND less than circum-Pacific sources. In this regard the Panamanian gateway restriction from ???10 Ma with final closure at 3-5 Ma may have played an important role in reducing access of Atlantic-derived Nd to the Pacific.

Shear-velocity structure, radial anisotropy and dynamics of the Tibetan crust

NASA Astrophysics Data System (ADS)

Agius, Matthew R.; Lebedev, Sergei

2014-12-01

Geophysical and geological data suggest that Tibetan middle crust is a partially molten, mechanically weak layer, but it is debated whether this low-viscosity layer is present beneath the entire plateau, what its properties are, how it deforms, and what role it has played in the plateau's evolution. Broad-band seismic surface waves yield resolution in the entire depth range of the Tibetan crust and can be used to constrain its shear-wave velocity structure (indicative of crustal composition, temperature and partial melting) and radial anisotropy (indicative of the patterns of deformation). We measured Love- and Rayleigh-wave phase-velocity curves in broad period ranges (up to 7-200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds of interstation measurements, made with cross-correlation and waveform-inversion methods. Shear-velocity profiles were then determined by extensive series of non-linear inversions of the data, designed to constrain the depth-dependent ranges of isotropic-average shear speeds and radial anisotropy. Shear wave speeds within the Tibetan middle crust are anomalously low and, also, show strong lateral variations across the plateau. The lowest mid-crustal shear speeds are found in the north and west of the plateau (˜3.1-3.2 km s-1), within a pronounced low-velocity zone. In southeastern Tibet, crustal shear wave speeds increase gradually towards southeast, whereas in the north, the change across the Kunlun Fault is relatively sharp. The lateral variations of shear speeds within the crust are indicative of those in temperature. A mid-crustal temperature of 800 °C, reported previously, can account for the low shear velocities across Lhasa. In the north, the temperature is higher and exceeds the solidus, resulting in partial melting that we estimate at 3-6 per cent. Strong radial anisotropy is required by the data in western-central Tibet (>5 per cent) but not in northeastern Tibet. The amplitude

Evolution of Fractal Parameters through Development Stage of Soil Crust

NASA Astrophysics Data System (ADS)

Ospina, Abelardo; Florentino, Adriana; Tarquis, Ana Maria

2016-04-01

Soil surface characteristics are subjected to changes driven by several interactions between water, air, biotic and abiotic components. One of the examples of such interactions is provided through biological soil crusts (BSC) in arid and semi-arid environments. BSC are communities composed of cyanobacteria, fungi, mosses, lichens, algae and liverworts covering the soil surface and play an important role in ecosystem functioning. The characteristics and formation of these BSC influence the soil hydrological balance, control the mass of eroded sediment, increase stability of soil surface, and influence plant productivity through the modification of nitrogen and carbon cycle. The site of this work is located at Quibor and Ojo de Agua (Lara state, Venezuela). The Quibor Depression in Venezuela is a major agricultural area being at semi-arid conditions and limited drainage favor the natural process of salinization. Additionally, the extension and intensification of agriculture has led to over-exploitation of groundwater in the past 30 years (Méndoza et al., 2013). The soil microbial crust develops initially on physical crusts which are mainly generated since wetting and drying, being a recurrent feature in the Quíbor arid zone. The microbiotic crust is organic, composed of macro organisms (bryophytes and lichens) and microorganisms (cyanobacteria, fungi algae, etc.); growing on the ground, forming a thickness no greater than 3 mm. For further details see Toledo and Florentino (2009). This study focus on characterize the development stage of the BSC based on image analysis. To this end, grayscale images of different types of biological soil crust at different stages where taken, each image corresponding to an area of 12.96 cm2 with a resolution of 1024x1024 pixels (Ospina et al., 2015). For each image lacunarity and fractal dimension through the differential box counting method were calculated. These were made with the software ImageJ/Fraclac (Karperien, 2013

Seismic characteristics of central Brazil crust and upper mantle: A deep seismic refraction study

USGS Publications Warehouse

Soares, J.E.; Berrocal, J.; Fuck, R.A.; Mooney, W.D.; Ventura, D.B.R.

2006-01-01

A two-dimensional model of the Brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the Porangatu and Cavalcante lines, each approximately 300 km long. When the lines were deployed, they overlapped by 50 km, forming an E-W transect approximately 530 km long across the Tocantins Province and western Sa??o Francisco Craton. The Tocantins Province formed during the Neoproterozoic when the Sa??o Francisco, the Paranapanema, and the Amazon cratons collided, following the subduction of the former Goia??s ocean basin. Average crustal VP and VP/VS ratios, Moho topography, and lateral discontinuities within crustal layers suggest that the crust beneath central Brazil can be associated with major geological domains recognized at the surface. The Moho is an irregular interface, between 36 and 44 km deep, that shows evidences of first-order tectonic structures. The 8.05 and 8.23 km s-1 P wave velocities identify the upper mantle beneath the Porangatu and Cavalcante lines, respectively. The observed seismic features allow for the identification of (1) the crust has largely felsic composition in the studied region, (2) the absence of the mafic-ultramafic root beneath the Goia??s magmatic arc, and (3) block tectonics in the foreland fold-and-thrust belt of the northern Brasi??lia Belt during the Neoproterozoic. Seismic data also suggested that the Bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle. Finally, the Goia??s-Tocantins seismic belt can be interpreted as a natural seismic alignment related to the Neoproterozoic mantle domain. Copyright 2006 by the American Geophysical Union.

Coesite Assemblages in Deep Continental Lithosphere: Additional Evidence for a Protolith from Subduction of Oceanic Crust

NASA Astrophysics Data System (ADS)

Sobolev, N.

2005-12-01

Inclusions in diamonds (DIs) represent an important source of information about the composition of continental lithospheric mantle. The isolated coesite inclusions in two diamonds (Harris, 1968) and a full set of eclogitic minerals (coesite (Cs), garnet (Ga), omphacite Cpx)) in two Yakutian diamonds (Sobolev et al., 1976), followed by finds of Cs-eclogite xenoliths (Smyth and Hatton, 1977; Ponomarenko et al., 1977) testify to the importance of coesite as a constituent of eclogitic rocks in deep lithospheric environment. Since these earlier times, coesite has been documented in more than 250 natural diamonds from 25 localities worldwide. Some 40 xenoliths of Cs-eclogites were found both in South African and Yakutian kimberlites. However, >50% of DIs of coesite are related to only four (4) diamond localities, including Guaniamo, Venezuela (Sobolev et al., 1998, 2003), Argyle (Jaques et al., 1989; Sobolev et al., 1989), New South Wales, all Australia (Sobolev et al., 1984; Meyer et al., 1997), and North Yakutian alluvials (Sobolev et al., 1999). All described DIs with coesite are from a wide range of assemblages: websterites to kyanite eclogites; grospydites and calcsilicate assemblages, with a large range in Gt [3.7-28.7 wt.% CaO] and Cpx [ 0.9-8.8 wt.% Na2O] compositions. In spite of these occurrences in diamonds, to the present, no coesite has been detected within the assemblage of minerals making up some 400 diamondiferous-eclogite xenoliths; similarly, no diamonds have been found in any Cs-eclogite xenoliths. This apparent paradox may be caused by coesite alteration in the diamondiferous eclogites, whereas coesite eclogites may have formed only outside of the diamond stability field. Indeed, coesite eclogites (without diamonds) may occupy a shallower position within continental lithosphere compared with the normal E-type diamond source. This indicates a broadly basaltic chemistry of the deep eclogitic environment, additional evidence for a protolith from the

The South Scandinavian crust: Structural complexities from seismic reflection and refraction profiling

NASA Astrophysics Data System (ADS)

Kinck, J. J.; Husebye, E. S.; Lund, C.-E.

1991-04-01

Pioneering work on mapping the Scandinavian crust commenced in the early 1960s and since then numerous profiling surveys have been undertaken, particularly as part of the on-going EUGENO-S project. However, the most significant contribution to mapping crustal structural details came from the M.V. Mobil Search cruises in the Skagerrak and off the West coast of Norway (16 s TWT reflection profiling). All past and present crustal profiling results have been integrated to produce detailed maps of Moho depths and crustal thicknesses for South Scandinavia. The thinnest crust is found in the North Sea and Skagerrak (approximately 20 km), while East-central Sweden features very thick crust (approximately 50 km). Other interesting features are the apparent correlation between crustal thinning and sedimentation/subsidence, magmatic activity, earthquake occurrences and the tectonic age of the crust. Moho depths and the crustal thicknesses clearly reflect the tectonic evolution and the present structural features of the region investigated.

Mars: The initial emplacement of ground ice in response to the thermal evolution of its early crust

NASA Technical Reports Server (NTRS)

Clifford, Stephen M.

1993-01-01

Given the geomorphic evidence for the widespread occurrence of water and ice in the early martian crust, and the difficulty involved in accounting for this distribution given the present climate, it has been suggested that the planet's early climate was originally more Earth-like, permitting the global emplacement of crustal H2O by direct precipitation as snow or rain. The resemblance of the martian valley networks to terrestrial runoff channels, and their almost exclusive occurrence in the planet's ancient (approximately 4 billion year old) heavily cratered terrain, is often cited as evidence of just such a period. An alternative school of thought suggests that the early climate did not differ substantially from that of today. Advocates of this view find no compelling reason to invoke a warmer, wetter period to explain the origin of the valley networks. Rather, they cite evidence that the primary mechanism of valley formation was ground water sapping, a process that does not require that surface water exist in equilibrium with the atmosphere. However, while sapping may successfully explain the origin of the small valleys, it fails to address how the crust was initially charged with ice as the climate evolved towards its present state. Therefore, given the uncertainty regarding the environmental conditions that prevailed on early Mars, the initial emplacement of ground ice is considered from two perspectives: (1) that the early climate started warm and wet, but gradually cooled with time; and (2) that it never differed substantially from that of today.

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

NASA Astrophysics Data System (ADS)

van der Werf, Thomas; Chatzaras, Vasileios; Marcel Kriegsman, Leo; Kronenberg, Andreas; Tikoff, Basil; Drury, Martyn R.

2017-12-01

The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene-Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750-890 °C and pressures of 400-560 MPa, corresponding to 15-22 km depth. A hot crustal geotherm of 40 ° C km-1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12-33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12-17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1-1.3×1020 Pa ṡ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016-1019 Pa ṡ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ṡ s

Remnants of Eoarchean continental crust derived from a subducted proto-arc.

PubMed

Ge, Rongfeng; Zhu, Wenbin; Wilde, Simon A; Wu, Hailin

2018-02-01

Eoarchean [3.6 to 4.0 billion years ago (Ga)] tonalite-trondhjemite-granodiorite (TTG) is the major component of Earth's oldest remnant continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths, but whether this requires subduction remains enigmatic. Recent studies show that most Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction. We report a suite of newly discovered Eoarchean tonalitic gneisses dated at ~3.7 Ga from the Tarim Craton, northwestern China. These rocks are probably the oldest high-pressure TTGs so far documented worldwide. Thermodynamic and trace element modeling demonstrates that the parent magma may have been generated by water-fluxed partial melting of moderately enriched arc-like basalts at 1.8 to 1.9 GPa and 800° to 830°C, indicating an apparent geothermal gradient (400° to 450°C GPa -1 ) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high-pressure TTGs were generated by partial melting of a subducted proto-arc during arc accretion. Our model implies that modern-style plate tectonics was operative, at least locally, at ~3.7 Ga and was responsible for generating some of the oldest continental nuclei.

Remnants of Eoarchean continental crust derived from a subducted proto-arc

PubMed Central

Ge, Rongfeng; Zhu, Wenbin; Wilde, Simon A.; Wu, Hailin

2018-01-01

Eoarchean [3.6 to 4.0 billion years ago (Ga)] tonalite-trondhjemite-granodiorite (TTG) is the major component of Earth’s oldest remnant continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths, but whether this requires subduction remains enigmatic. Recent studies show that most Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction. We report a suite of newly discovered Eoarchean tonalitic gneisses dated at ~3.7 Ga from the Tarim Craton, northwestern China. These rocks are probably the oldest high-pressure TTGs so far documented worldwide. Thermodynamic and trace element modeling demonstrates that the parent magma may have been generated by water-fluxed partial melting of moderately enriched arc-like basalts at 1.8 to 1.9 GPa and 800° to 830°C, indicating an apparent geothermal gradient (400° to 450°C GPa−1) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high-pressure TTGs were generated by partial melting of a subducted proto-arc during arc accretion. Our model implies that modern-style plate tectonics was operative, at least locally, at ~3.7 Ga and was responsible for generating some of the oldest continental nuclei. PMID:29487901

Continental crust formation on early Earth controlled by intrusive magmatism

NASA Astrophysics Data System (ADS)

Rozel, A. B.; Golabek, G. J.; Jain, C.; Tackley, P. J.; Gerya, T.

2017-05-01

The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present-day Earth, two crust-mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism and the “Plutonic squishy lid” tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite-trondhjemite-granodiorite (TTG) continental crust but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.

Continental crust formation on early Earth controlled by intrusive magmatism.

PubMed

Rozel, A B; Golabek, G J; Jain, C; Tackley, P J; Gerya, T

2017-05-18

The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present-day Earth, two crust-mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism and the "Plutonic squishy lid" tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite-trondhjemite-granodiorite (TTG) continental crust but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.

Archean upper crust transition from mafic to felsic marks the onset of plate tectonics.

PubMed

Tang, Ming; Chen, Kang; Rudnick, Roberta L

2016-01-22

The Archean Eon witnessed the production of early continental crust, the emergence of life, and fundamental changes to the atmosphere. The nature of the first continental crust, which was the interface between the surface and deep Earth, has been obscured by the weathering, erosion, and tectonism that followed its formation. We used Ni/Co and Cr/Zn ratios in Archean terrigenous sedimentary rocks and Archean igneous/metaigneous rocks to track the bulk MgO composition of the Archean upper continental crust. This crust evolved from a highly mafic bulk composition before 3.0 billion years ago to a felsic bulk composition by 2.5 billion years ago. This compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks, suggesting the onset of global plate tectonics at ~3.0 billion years ago. Copyright © 2016, American Association for the Advancement of Science.

Imaging exhumed lower continental crust in the distal Jequitinhonha basin, Brazil

NASA Astrophysics Data System (ADS)

Loureiro, A.; Schnürle, P.; Klingelhöfer, F.; Afilhado, A.; Pinheiro, J.; Evain, M.; Gallais, F.; Dias, N. A.; Rabineau, M.; Baltzer, A.; Benabdellouahed, M.; Soares, J.; Fuck, R.; Cupertino, J. A.; Viana, A.; Matias, L.; Moulin, M.; Aslanian, D.; Vinicius Aparecido Gomes de Lima, M.; Morvan, L.; Mazé, J. P.; Pierre, D.; Roudaut-Pitel, M.; Rio, I.; Alves, D.; Barros Junior, P.; Biari, Y.; Corela, C.; Crozon, J.; Duarte, J. L.; Ducatel, C.; Falcão, C.; Fernagu, P.; Le Piver, D.; Mokeddem, Z.; Pelleau, P.; Rigoti, C.; Roest, W.; Roudaut, M.; Salsa Team

2018-07-01

Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust.

EAG Eminent Speaker: Two types of Archean continental crust: plume and plate tectonics on early Earth

NASA Astrophysics Data System (ADS)

Van Kranendonk, M. J.

2012-04-01

Over 4.5 billion years, Earth has evolved from a molten ball to a cooler planet with large continental plates, but how and when continents grew and plate tectonics started remain poorly understood. In this paper, I review the evidence that 3.5-3.2 Ga continental nuclei of the Pilbara (Australia) and Kaapvaal (southern Africa) cratons formed as thick volcanic plateaux over hot, upwelling mantle and survived due to contemporaneous development of highly depleted, buoyant, unsubductable mantle roots. This type of crust is distinct from, but complimentary to, high-grade gneiss terranes, as exemplified by the North Atlantic Craton of West Greenland, which formed through subduction-accretion tectonics on what is envisaged as a vigorously convecting early Earth with small plates. Thus, it is proposed that two types of crust formed on early Earth, in much the same way as in modern Earth, but with distinct differences resulting from a hotter Archean mantle. Volcanic plateaux provided a variety of stable habitats for early life, including chemical nutrient rich, shallow-water hydrothermal systems and shallow marine carbonate platforms.

Growth of continental crust: Clues from Nd isotopes and Nb-Th relationships in mantle-derived magmas

NASA Technical Reports Server (NTRS)

Arndt, N. T.; Chauvel, C.; Jochum, K.-P.; Gruau, G.; Hofmann, A. W.

1988-01-01

Isotope and trace element geochemistry of Precambrian mantle derived rocks and implications for the formation of the continental crust is discussed. Epsilon Nd values of Archean komatiites are variable, but range up to at least +5, suggesting that the Archean mantle was heterogeneous and, in part, very depleted as far back as 3.4 to 3.5 Ga. This may be taken as evidence for separation of continental crust very early in Earth history. If these komatiite sources were allowed to evolve in a closed system, they would produce modern day reservoirs with much higher epsilon Nd values than is observed. This implies recycling of some sort of enriched material, perhaps subducted sediments, although other possibilities exist. Archean volcanics show lower Nb/Th than modern volcanics, suggesting a more primitive mantle source than that observed nowadays. However, Cretaceous komatiites from Gorgona island have similar Nb/Th to Archean volcanics, indicating either the Archean mantle source was indeed more primitive, or Archean magmas were derived from a deep ocean island source like that proposed for Gorgona.

Growth of continental crust: Clues from Nd isotopes and Nb-Th relationships in mantle-derived magmas

NASA Astrophysics Data System (ADS)

Arndt, N. T.; Chauvel, C.; Jochum, K.-P.; Gruau, G.; Hofmann, A. W.

Isotope and trace element geochemistry of Precambrian mantle derived rocks and implications for the formation of the continental crust is discussed. Epsilon Nd values of Archean komatiites are variable, but range up to at least +5, suggesting that the Archean mantle was heterogeneous and, in part, very depleted as far back as 3.4 to 3.5 Ga. This may be taken as evidence for separation of continental crust very early in Earth history. If these komatiite sources were allowed to evolve in a closed system, they would produce modern day reservoirs with much higher epsilon Nd values than is observed. This implies recycling of some sort of enriched material, perhaps subducted sediments, although other possibilities exist. Archean volcanics show lower Nb/Th than modern volcanics, suggesting a more primitive mantle source than that observed nowadays. However, Cretaceous komatiites from Gorgona island have similar Nb/Th to Archean volcanics, indicating either the Archean mantle source was indeed more primitive, or Archean magmas were derived from a deep ocean island source like that proposed for Gorgona.

Biological soil crust succession impact on soil moisture and temperature in the sub-surface along a rainfall gradient

NASA Astrophysics Data System (ADS)

Zaady, E.; Yizhaq, H.; Ashkenazy, Y.

2012-04-01

Biological soil crusts produce mucilage sheets of polysaccharides that cover the soil surface. This hydrophobic coating can seal the soil micro-pores and thus cause reduction of water permeability and may influence soil temperature. This study evaluates the impact of crust composition on sub-surface water and temperature over time. We hypothesized that the successional stages of biological soil crusts, affect soil moisture and temperature differently along a rainfall gradient throughout the year. Four experimental sites were established along a rainfall gradient in the western Negev Desert. At each site three treatments; crust removal, pure sand (moving dune) and natural crusted were monitored. Crust successional stage was measured by biophysiological and physical measurements, soil water permeability by field mini-Infiltrometer, soil moisture by neutron scattering probe and temperature by sensors, at different depths. Our main interim conclusions from the ongoing study along the rainfall gradient are: 1. the biogenic crust controls water infiltration into the soil in sand dunes, 2. infiltration was dependent on the composition of the biogenic crust. It was low for higher successional stage crusts composed of lichens and mosses and high with cyanobacterial crust. Thus, infiltration rate controlled by the crust is inverse to the rainfall gradient. Continuous disturbances to the crust increase infiltration rates, 3. despite the different rainfall amounts at the sites, soil moisture content below 50 cm is almost the same. We therefore predict that climate change in areas that are becoming dryer (desertification) will have a positive effect on soil water content and vice versa.

Plutonic rocks in the Mineoka-Setogawa ophiolitic mélange, central Japan: Fragments of middle to lower crust of the Izu-Bonin-Mariana Arc?

NASA Astrophysics Data System (ADS)

Ichiyama, Yuji; Ito, Hisatoshi; Hokanishi, Natsumi; Tamura, Akihiro; Arai, Shoji

2017-06-01

A Paleogene accretionary complex, the Mineoka-Setogawa Belt, is distributed around the Izu Collision Zone, central Japan. Plutonic rocks of gabbro, diorite and tonalite compositions are included as fragments and dykes in an ophiolitic mélange in this belt. Zircon U-Pb dating of the plutonic rocks indicates that they were formed at ca. 35 Ma simultaneously. These ages are consistent with Eocene-Oligocene tholeiite and calc-alkaline arc magmatism in the Izu-Bonin-Mariana (IBM) Arc and exclude several previous models for the origin of the Mineoka-Setogawa ophiolitic rocks. The geochemical characteristics of these plutonic rocks are similar to those of the Eocene-Oligocene IBM tholeiite and calc-alkaline volcanic rocks as well as to the accreted middle crust of the IBM Arc, the Tanzawa Plutonic Complex. Moreover, their lithology is consistent with those of the middle and lower crust of the IBM Arc estimated from the seismic velocity structure. These lines of evidence strongly indicate that the plutonic rocks in the Mineoka-Setogawa ophiolitic mélange are fragments of the middle to lower crust of the IBM Arc. Additionally, the presence of the Mineoka-Setogawa intermediate to felsic plutonic rocks supports the hypothesis that intermediate magma can form continental crust in intra-oceanic arcs.

Small-scale density variations in the lunar crust revealed by GRAIL

NASA Astrophysics Data System (ADS)

Jansen, J. C.; Andrews-Hanna, J. C.; Li, Y.; Lucey, P. G.; Taylor, G. J.; Goossens, S.; Lemoine, F. G.; Mazarico, E.; Head, J. W.; Milbury, C.; Kiefer, W. S.; Soderblom, J. M.; Zuber, M. T.

2017-07-01

Data from the Gravity Recovery and Interior Laboratory (GRAIL) mission have revealed that ∼98% of the power of the gravity signal of the Moon at high spherical harmonic degrees correlates with the topography. The remaining 2% of the signal, which cannot be explained by topography, contains information about density variations within the crust. These high-degree Bouguer gravity anomalies are likely caused by small-scale (10‧s of km) shallow density variations. Here we use gravity inversions to model the small-scale three-dimensional variations in the density of the lunar crust. Inversion results from three non-descript areas yield shallow density variations in the range of 100-200 kg/m3. Three end-member scenarios of variations in porosity, intrusions into the crust, and variations in bulk crustal composition were tested as possible sources of the density variations. We find that the density anomalies can be caused entirely by changes in porosity. Characteristics of density anomalies in the South Pole-Aitken basin also support porosity as a primary source of these variations. Mafic intrusions into the crust could explain many, but not all of the anomalies. Additionally, variations in crustal composition revealed by spectral data could only explain a small fraction of the density anomalies. Nevertheless, all three sources of density variations likely contribute. Collectively, results from this study of GRAIL gravity data, combined with other studies of remote sensing data and lunar samples, show that the lunar crust exhibits variations in density by ± 10% over scales ranging from centimeters to 100‧s of kilometers.

Response of Surface Soil Hydrology to the Micro-Pattern of Bio-Crust in a Dry-Land Loess Environment, China

PubMed Central

Wei, Wei; Yu, Yun; Chen, Liding

2015-01-01

The specific bio-species and their spatial patterns play crucial roles in regulating eco-hydrologic process, which is significant for large-scale habitat promotion and vegetation restoration in many dry-land ecosystems. Such effects, however, are not yet fully studied. In this study, 12 micro-plots, each with size of 0.5 m in depth and 1 m in length, were constructed on a gentle grassy hill-slope with a mean gradient of 8° in a semiarid loess hilly area of China. Two major bio-crusts, including mosses and lichens, had been cultivated for two years prior to the field simulation experiments, while physical crusts and non-crusted bare soils were used for comparison. By using rainfall simulation method, four designed micro-patterns (i.e., upper bio-crust and lower bare soil, scattered bio-crust, upper bare soil and lower bio-crust, fully-covered bio-crust) to the soil hydrological response were analyzed. We found that soil surface bio-crusts were more efficient in improving soil structure, water holding capacity and runoff retention particularly at surface 10 cm layers, compared with physical soil crusts and non-crusted bare soils. We re-confirmed that mosses functioned better than lichens, partly due to their higher successional stage and deeper biomass accumulation. Physical crusts were least efficient in water conservation and erosion control, followed by non-crusted bare soils. More importantly, there were marked differences in the efficiency of the different spatial arrangements of bio-crusts in controlling runoff and sediment generation. Fully-covered bio-crust pattern provides the best option for soil loss reduction and runoff retention, while a combination of upper bio-crust and lower bare soil pattern is the least one. These findings are suggested to be significant for surface-cover protection, rainwater infiltration, runoff retention, and erosion control in water-restricted and degraded natural slopes. PMID:26207757

Response of Surface Soil Hydrology to the Micro-Pattern of Bio-Crust in a Dry-Land Loess Environment, China.

PubMed

Wei, Wei; Yu, Yun; Chen, Liding

2015-01-01

The specific bio-species and their spatial patterns play crucial roles in regulating eco-hydrologic process, which is significant for large-scale habitat promotion and vegetation restoration in many dry-land ecosystems. Such effects, however, are not yet fully studied. In this study, 12 micro-plots, each with size of 0.5 m in depth and 1 m in length, were constructed on a gentle grassy hill-slope with a mean gradient of 8° in a semiarid loess hilly area of China. Two major bio-crusts, including mosses and lichens, had been cultivated for two years prior to the field simulation experiments, while physical crusts and non-crusted bare soils were used for comparison. By using rainfall simulation method, four designed micro-patterns (i.e., upper bio-crust and lower bare soil, scattered bio-crust, upper bare soil and lower bio-crust, fully-covered bio-crust) to the soil hydrological response were analyzed. We found that soil surface bio-crusts were more efficient in improving soil structure, water holding capacity and runoff retention particularly at surface 10 cm layers, compared with physical soil crusts and non-crusted bare soils. We re-confirmed that mosses functioned better than lichens, partly due to their higher successional stage and deeper biomass accumulation. Physical crusts were least efficient in water conservation and erosion control, followed by non-crusted bare soils. More importantly, there were marked differences in the efficiency of the different spatial arrangements of bio-crusts in controlling runoff and sediment generation. Fully-covered bio-crust pattern provides the best option for soil loss reduction and runoff retention, while a combination of upper bio-crust and lower bare soil pattern is the least one. These findings are suggested to be significant for surface-cover protection, rainwater infiltration, runoff retention, and erosion control in water-restricted and degraded natural slopes.

Can weak crust explain the correlation of geoid and topography on Venus?

NASA Technical Reports Server (NTRS)

Buck, W. Roger

1993-01-01

The effect on geoid and topography of low viscosity crust overlying a steady-state convecting mantle is estimated under the assumption that the shear between crust and mantle does not alter the mantle flow. The weak crustal layer can change the sign of the geoid to topography ratio (admittance). The positive long wavelength admittance for Venus is consistent with a weak crust overlying a mantle with a viscosity that increases strongly with depth. The accepted interpretation of the strong positive correlation of geoid and topography on Venus, is that the convecting mantle of Venus has a constant viscosity with depth. Topography results from vertical normal stresses caused by mantle convection and highlands occur where mantle upwells. For topography to be supported by normal stress, the time scale for crustal flow must be long compared to the time scale for changes in the pattern of mantle flow. Because the high surface temperature of Venus may cause the crust to have a low viscosity, this assumption may be false. Topography should then be dominated by shear coupling between the crust and mantle. In the absence of a crustal layer, convection in a constant viscosity layer gives rise to a geoid anomaly that correlates positively with surface topography. When the viscosity in the layer increases with depth by several orders of magnitude, the surface topography and geoid anomaly become anti-correlated.

A Geochemical View on the Interplay Between Earth's Mantle and Crust

NASA Astrophysics Data System (ADS)

Chauvel, C.

2017-12-01

Over most of Earth history, oceanic and continental crust was created and destroyed. The formation of both types of crust involves the crystallization and differentiation of magmas producing by mantle melting. Their destruction proceeds by mechanical erosion and weathering above sea level, chemical alteration on the seafloor, and bulk recycling in subduction zones. All these processes enrich of some chemical element and deplete others but each process has its own effect on chemical elements. While the flux of material from mantle to crust is well understood, the return flux is much more complex. In contrast to mantle processes, erosion, weathering, chemical alteration and sedimentary processes strongly decouple elements such as the rare earths and high-field strength elements due to their different solubilities in surface fluids and mineralogical sorting during transport. Soluble elements such as strontium or uranium are quantitatively transported to the ocean by rivers and decoupled from less soluble elements. Over geological time, such decoupling significantly influences the extent to which chemical elements remain at the Earth's surface or find their way back to the mantle through subduction zones. For example, elements like Hf or Nd are retained in heavy minerals on continents whereas U and Sr are transported to the oceans and then in subduction zones to the mantle. The consequence is that different radiogenic isotopic systems give disparate age estimates for the continental crust; e.g, Hf ages could be too old. In subduction zones, chemical elements are also decoupled, due to contrasting behavior during dehydration or melting in subducting slabs. The material sent back into the mantle is generally enriched in non-soluble elements while most fluid-mobile elements return to the crust. This, in turn, affects the relationship between the Rb-Sr, Sm-Nd, Lu-Hf and U-Th-Pb isotopic systems and creates correlations unlike those based on magmatic processes. By

Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life

USGS Publications Warehouse

Kargel, J.S.; Kaye, J.Z.; Head, J. W.; Marion, G.M.; Sassen, R.; Crowley, J.K.; Ballesteros, O.P.; Grant, S.A.; Hogenboom, D.L.

2000-01-01

We have considered a wide array of scenarios for Europa's chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below. We postulate that, following formation of the jovian system, the europan evolutionary sequence has as its major links: (a) initial carbonaceous chondrite rock, (b) global primordial aqueous differentiation and formation of an impure primordial hydrous crust, (c) brine evolution and intracrustal differentiation, (d) degassing of Europa's mantle and gas venting, (e) hydrothermal processes, and (f) chemical surface alteration. Our models were developed in the context of constraints provided by Galileo imaging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous differentiation from a carbonaceous CI or CM chondrite precursor, without further chemical processing, would result in a crust/ocean enriched in magnesium sulfate and sodium sulfate, consistent with Galileo spectroscopy. Within the bounds of this simple model, a wide range of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean/crust compositions, e.g., an ocean/crust of sodium carbonate or sulfuric acid, or a crust containing abundant clathrate hydrates. Realistic chemical-physical evolution scenarios differ greatly in detailed predictions, but they generally call for a highly impure and chemically layered crust. Some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geological consequences of the physical-chemical structures predicted from these scenarios. These predicted consequences and observed aspects of Europa's geology may serve as a basis for further analys is

Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu-Bonin-Mariana Arc: Syntectonic granitic crust formation during back-arc rifting

NASA Astrophysics Data System (ADS)

Tani, Kenichiro; Dunkley, Daniel J.; Chang, Qing; Nichols, Alexander R. L.; Shukuno, Hiroshi; Hirahara, Yuka; Ishizuka, Osamu; Arima, Makoto; Tatsumi, Yoshiyuki

2015-08-01

A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan-West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu-Bonin-Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.

Natural constraints on the rheology of the lower continental crust (Musgrave Ranges, Central Australia)

NASA Astrophysics Data System (ADS)

Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Camacho, Alfredo; Pennacchioni, Giorgio

2015-04-01

on a statistical basis, and an estimate of the bulk pseudotachylyte composition for small areas, avoiding clasts and heterogeneous composition of the former melt. The combination with thermodynamic modelling using PerpleX is used to test the results from geothermobarometry. The estimated conditions are similar to the ductile shear zones and support evidence for synchronous action of brittle faulting and viscous shearing in the lower crust.

Open system models of isotopic evolution in Earth's silicate reservoirs: Implications for crustal growth and mantle heterogeneity

NASA Astrophysics Data System (ADS)

Kumari, Seema; Paul, Debajyoti; Stracke, Andreas

2016-12-01

the UM within the initial 500 Myr, as a result of early formation of CC, which supports other evidence in favor of the presence of Hadean continental crust. Therefore, a chondritic Th/U ratio (4 ± 0.2) in the UM until 2 Gyr appears rather unlikely. We find that the κ conundrum - the observation that measured Th/U ratios and those deduced from 208Pb-206Pb isotope systematics differ - is a natural outcome of an open system evolution in which preferential recycling of U for the past 2 Gyr has played a dominant role. Overall, our simulations strongly favor exponential crustal growth, starting in the early Hadean, the transient preservation of compositionally distinct mantle reservoirs over billion year time periods, and a generally less incompatible element depleted, but non-primitive composition of the lower mantle.

Contraction or expansion of the Moon's crust during magma ocean freezing?

PubMed Central

Elkins-Tanton, Linda T.; Bercovici, David

2014-01-01

The lack of contraction features on the Moon has been used to argue that the Moon underwent limited secular cooling, and thus had a relatively cool initial state. A cool early state in turn limits the depth of the lunar magma ocean. Recent GRAIL gravity measurements, however, suggest that dikes were emplaced in the lower crust, requiring global lunar expansion. Starting from the magma ocean state, we show that solidification of the lunar magma ocean would most likely result in expansion of the young lunar crust, and that viscous relaxation of the crust would prevent early tectonic features of contraction or expansion from being recorded permanently. The most likely process for creating the expansion recorded by the dikes is melting during cumulate overturn of the newly solidified lunar mantle. PMID:25114310

Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

NASA Astrophysics Data System (ADS)

Barker, A. K.; Coogan, L. A.; Gillis, K. M.; Weis, D.

2008-06-01

Fluid flow through the axial hydrothermal system at fast spreading ridges is investigated using the Sr-isotopic composition of upper crustal samples recovered from a tectonic window at Pito Deep (NE Easter microplate). Samples from the sheeted dike complex collected away from macroscopic evidence of channelized fluid flow, such as faults and centimeter-scale hydrothermal veins, show a range of 87Sr/86Sr from 0.7025 to 0.7030 averaging 0.70276 relative to a protolith with 87Sr/86Sr of ˜0.7024. There is no systematic variation in 87Sr/86Sr with depth in the sheeted dike complex. Comparison of these new data with the two other localities that similar data sets exist for (ODP Hole 504B and the Hess Deep tectonic window) reveals that the extent of Sr-isotope exchange is similar in all of these locations. Models that assume that fluid-rock reaction occurs during one-dimensional (recharge) flow lead to significant decreases in the predicted extent of isotopic modification of the rock with depth in the crust. These model results show systematic misfits when compared with the data that can only be avoided if the fluid flow is assumed to be focused in isolated channels with very slow fluid-rock exchange. In this scenario the fluid at the base of the crust is little modified in 87Sr/86Sr from seawater and thus unlike vent fluids. Additionally, this model predicts that some rocks should show no change from the fresh-rock 87Sr/86Sr, but this is not observed. Alternatively, models in which fluid-rock reaction occurs during upflow (discharge) as well as downflow, or in which fluids are recirculated within the hydrothermal system, can reproduce the observed lack of variation in 87Sr/86Sr with depth in the crust. Minimum time-integrated fluid fluxes, calculated from mass balance, are between 1.5 and 2.6 × 106 kg m-2 for all areas studied to date. However, new evidence from both the rocks and a compilation of vent fluid compositions demonstrates that some Sr is leached from the

Fast Radio Bursts from the Collapse of Strange Star Crusts

NASA Astrophysics Data System (ADS)

Zhang, Yue; Geng, Jin-Jun; Huang, Yong-Feng

2018-05-01

Fast radio bursts (FRBs) are transient radio sources at cosmological distances. No counterparts in other bands have been observed for non-repeating FRBs. Here we suggest the collapse of strange star (SS) crusts as a possible origin for FRBs. SSs, which are composed of almost equal numbers of u, d, and s quarks, may be encapsulated by a thin crust of normal hadronic matter. When a SS accretes matter from its environment, the crust becomes heavier and heavier. It may finally collapse, leading to the release of a large amount of magnetic energy and plenty of electron/positron pairs on a very short timescale. Electron/positron pairs in the polar cap region of the SS can be accelerated to relativistic velocities, streaming along the magnetic field lines to form a thin shell. FRBs are produced by coherent emission from these electrons when the shell is expanding. Basic characteristics of observed FRBs can be explained in our model.

Syn-collisional felsic magmatism and continental crust growth: A case study from the North Qilian Orogenic Belt at the northern margin of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Chen, Shuo; Niu, Yaoling; Xue, Qiqi

2018-05-01

The abundant syn-collisional granitoids produced and preserved at the northern Tibetan Plateau margin provide a prime case for studying the felsic magmatism as well as continental crust growth in response to continental collision. Here we present the results from a systematic study of the syn-collisional granitoids and their mafic magmatic enclaves (MMEs) in the Laohushan (LHS) and Machangshan (MCS) plutons from the North Qilian Orogenic Belt (NQOB). Two types of MMEs from the LHS pluton exhibit identical crystallization age ( 430 Ma) and bulk-rock isotopic compositions to their host granitoids, indicating their genetic link. The phase equilibrium constraints and pressure estimates for amphiboles from the LHS pluton together with the whole rock data suggest that the two types of MMEs represent two evolution products of the same hydrous andesitic magmas. In combination with the data on NQOB syn-collisional granitoids elsewhere, we suggest that the syn-collisional granitoids in the NQOB are material evidence of melting of ocean crust and sediment. The remarkable compositional similarity between the LHS granitoids and the model bulk continental crust in terms of major elements, trace elements, and some key element ratios indicates that the syn-collisional magmatism in the NQOB contributes to net continental crust growth, and that the way of continental crust growth in the Phanerozoic through syn-collisional felsic magmatism (production and preservation) is a straightforward process without the need of petrologically and physically complex processes.

Analysis of volatile compounds in gluten-free bread crusts with an optimised and validated SPME-GC/QTOF methodology.

PubMed

Pico, Joana; Antolín, Beatriz; Román, Laura; Gómez, Manuel; Bernal, José

2018-04-01

The aroma of bread crust, as one of the first characteristics perceived, is essential for bread acceptance. However, gluten-free bread crusts exhibit weak aroma. A SPME-GC/QTOF methodology was optimised with PCA and RSM and validated for the quantification of 44 volatile compounds in bread crust, extracting 0.75 g of crust at 60 °C for 51 min. LODs ranged between 3.60 and 1760 μg Kg -1 , all the R 2 were higher than 0.99 and %RSD for precision and %Er for accuracy were lower than 9% and 12%, respectively. A commercial wheat bread crust was quantified, and furfural was the most abundant compound. Bread crusts of wheat starch and of japonica rice, basmati rice and teff flours were also quantified. Teff flour and wheat starch crusts were very suitable for improving gluten-free bread crust aroma, due to their similar content in 2-acetyl-1-pyrroline and 4-hydroxy-2,5-dimethyl-3(2H)-furanone compared to wheat flour crust and also for their high content in pyrazines. Copyright © 2018 Elsevier Ltd. All rights reserved.

Self-Consistent Generation of Primordial Continental Crust in Global Mantle Convection Models

NASA Astrophysics Data System (ADS)

Jain, C.; Rozel, A.; Tackley, P. J.

2017-12-01

We present the generation of primordial continental crust (TTG rocks) using self-consistent and evolutionary thermochemical mantle convection models (Tackley, PEPI 2008). Numerical modelling commonly shows that mantle convection and continents have strong feedbacks on each other. However in most studies, continents are inserted a priori while basaltic (oceanic) crust is generated self-consistently in some models (Lourenco et al., EPSL 2016). Formation of primordial continental crust happened by fractional melting and crystallisation in episodes of relatively rapid growth from late Archean to late Proterozoic eras (3-1 Ga) (Hawkesworth & Kemp, Nature 2006) and it has also been linked to the onset of plate tectonics around 3 Ga. It takes several stages of differentiation to generate Tonalite-Trondhjemite-Granodiorite (TTG) rocks or proto-continents. First, the basaltic magma is extracted from the pyrolitic mantle which is both erupted at the surface and intruded at the base of the crust. Second, it goes through eclogitic transformation and then partially melts to form TTGs (Rudnick, Nature 1995; Herzberg & Rudnick, Lithos 2012). TTGs account for the majority of the Archean continental crust. Based on the melting conditions proposed by Moyen (Lithos 2011), the feasibility of generating TTG rocks in numerical simulations has already been demonstrated by Rozel et al. (Nature, 2017). Here, we have developed the code further by parameterising TTG formation. We vary the ratio of intrusive (plutonic) and extrusive (volcanic) magmatism (Crisp, Volcanol. Geotherm. 1984) to study the relative volumes of three petrological TTG compositions as reported from field data (Moyen, Lithos 2011). Furthermore, we systematically vary parameters such as friction coefficient, initial core temperature and composition-dependent viscosity to investigate the global tectonic regime of early Earth. Continental crust can also be destroyed by subduction or delamination. We will investigate

Key Factors Influencing Rapid Development of Potentially Dune-Stabilizing Moss-Dominated Crusts

PubMed Central

Bu, Chongfeng; Zhang, Kankan; Zhang, Chunyun; Wu, Shufang

2015-01-01

Biological soil crusts (BSCs) are a widespread photosynthetic ground cover in arid and semiarid areas. They have many positive ecological functions, such as increasing soil stability, and reducing water and wind erosion. Using artificial technology to achieve the rapid development of BSCs is expected to become a low-cost and highly beneficial ecological restoration measure. In the present study, typical moss-dominated crusts in a region characterized by mobile dunes (Mu Us Sandland, China) were collected, and a 40-day cultivation experiment was performed to investigate key factors, including watering frequency, light intensity and a nutrient addition, which affect the rapid development of moss crusts and their optimal combination. The results demonstrated that watering frequency and illumination had a significant positive effect (P=0.049, three-factor ANOVA) and a highly significant, complicated effect (P=0.000, three-factor ANOVA), respectively, on the plant density of bryophytes, and a highly significant positive effect on the chlorophyll a and exopolysaccharide contents (P=0.000, P=0.000; P=0.000, P=0.000; one-way ANOVA). Knop nutrient solution did not have a significant positive but rather negative effect on the promotion of moss-dominated crust development (P=0.270, three-factor ANOVA). Moss-dominated crusts treated with the combination of moderate-intensity light (6,000 lx) + high watering frequency (1 watering/2 days) - Knop had the highest moss plant densities, while the treatment with high-intensity light (12,000 lx) + high watering frequency (1 watering/2 days) + Knop nutrient solution had higher chlorophyll a contents than that under other treatments. It is entirely feasible to achieve the rapid development of moss crusts under laboratory conditions by regulating key factors and creating the right environment. Future applications may seek to use cultured bryophytes to control erosion in vulnerable areas with urgent needs. PMID:26230324

A novel clinical grading scale to guide the management of crusted scabies.

PubMed

Davis, Joshua S; McGloughlin, Steven; Tong, Steven Y C; Walton, Shelley F; Currie, Bart J

2013-01-01

Crusted scabies, or hyperinfestation with Sarcoptes scabiei, occurs in people with an inadequate immune response to the mite. In recent decades, data have emerged suggesting that treatment of crusted scabies with oral ivermectin combined with topical agents leads to lower mortality, but there are no generally accepted tools for describing disease severity. Here, we describe a clinical grading scale for crusted scabies and its utility in real world practice. In 2002, Royal Darwin Hospital (RDH), a hospital in tropical Australia developed and began using a clinical grading scale to guide the treatment of crusted scabies. We conducted a retrospective observational study including all episodes of admission to RDH for crusted scabies during the period October 2002-December 2010 inclusive. Patients who were managed according to the grading scale were compared with those in whom the scale was not used at the time of admission but was calculated retrospectively. There were 49 admissions in 30 patients during the study period, of which 49 (100%) were in Indigenous Australians, 29 (59%) were male and the median age was 44.1 years. According to the grading scale, 8 (16%) episodes were mild, 24 (49%) were moderate, and 17 (35%) were severe. Readmission within the study period was significantly more likely with increasing disease severity, with an odds ratio (95% CI) of 12.8 (1.3-130) for severe disease compared with mild. The patients managed according to the grading scale (29 episodes) did not differ from those who were not (20 episodes), but they received fewer doses of ivermectin and had a shorter length of stay (11 vs. 16 days, p = 0.02). Despite this the outcomes were no different, with no deaths in either group and a similar readmission rate. Our grading scale is a useful tool for the assessment and management of crusted scabies.

The effect of syntectonic hydration on rock strength, fabric evolution, and polycrystalline flow in mafic lower continental crust rocks

NASA Astrophysics Data System (ADS)

Getsinger, A.; Hirth, G.

2014-12-01

Strain localization is significantly enhanced by the influx of fluid; however, processes associated with deformation in polycrystalline material, fluid infiltration, and the evolution of creep processes and rock fabric with increasing strain localization are not well constrained for many lower crust lithologies. We combine field and experimental observations of mafic rocks deforming at lower crust pressure, temperature, and water conditions to examine strain localization processes associated with the influx of fluid, strength dependence of fabric evolution, and flow law parameters for amphibolite. General shear experiments were conducted in a Griggs rig on powdered basalt (≤5 µm starting grain size) with up to 1 wt% water at lower continental crust conditions (750˚ to 850˚C, 1GPa). Amphibole formed during deformation exhibits both a strong shape preferred orientation (SPO) and lattice preferred orientation (LPO). With increasing strain, the amphibole (and clinopyroxene) LPO strengthens and rotates to [001] maximum aligned sub-parallel to the flow direction and SPO, which indicates grain rotation during deformation. Plagioclase LPO increases from random to very weak in samples deformed to high strain. As the amphibole LPO rotates and strengthens, the mechanical strength decreases. The correlation of the SPO and LPO coupled with the rheological evidence for diffusion creep (n ≈ 1.5) indicates that the amphibole fabric results from grain growth and rigid grain rotation during deformation. The coevolution of LPO (and grain rotation) and mechanical weakening coupled with the absence of grain size reduction in our samples suggests that strength depends on the formation of a strong mineral LPO. Both our field and experimental data demonstrate that fluid intrusion into the mafic lower crust initiates syn-deformational, water-consuming reactions, creating a rheological contrast between wet and dry lithologies that promotes strain localization. Additionally, the

Estimating the formation age distribution of continental crust by unmixing zircon ages

NASA Astrophysics Data System (ADS)

Korenaga, Jun

2018-01-01

Continental crust provides first-order control on Earth's surface environment, enabling the presence of stable dry landmasses surrounded by deep oceans. The evolution of continental crust is important for atmospheric evolution, because continental crust is an essential component of deep carbon cycle and is likely to have played a critical role in the oxygenation of the atmosphere. Geochemical information stored in the mineral zircon, known for its resilience to diagenesis and metamorphism, has been central to ongoing debates on the genesis and evolution of continental crust. However, correction for crustal reworking, which is the most critical step when estimating original formation ages, has been incorrectly formulated, undermining the significance of previous estimates. Here I suggest a simple yet promising approach for reworking correction using the global compilation of zircon data. The present-day distribution of crustal formation age estimated by the new "unmixing" method serves as the lower bound to the true crustal growth, and large deviations from growth models based on mantle depletion imply the important role of crustal recycling through the Earth history.

Impact of Environmental Factors and Biological Soil Crust Types on Soil Respiration in a Desert Ecosystem

PubMed Central

Feng, Wei; Zhang, Yuqing; Jia, Xin; Wu, Bin; Zha, Tianshan; Qin, Shugao; Wang, Ben; Shao, Chenxi; Liu, Jiabin; Fa, Keyu

2014-01-01

The responses of soil respiration to environmental conditions have been studied extensively in various ecosystems. However, little is known about the impacts of temperature and moisture on soils respiration under biological soil crusts. In this study, CO2 efflux from biologically-crusted soils was measured continuously with an automated chamber system in Ningxia, northwest China, from June to October 2012. The highest soil respiration was observed in lichen-crusted soil (0.93±0.43 µmol m−2 s−1) and the lowest values in algae-crusted soil (0.73±0.31 µmol m−2 s−1). Over the diurnal scale, soil respiration was highest in the morning whereas soil temperature was highest in the midday, which resulted in diurnal hysteresis between the two variables. In addition, the lag time between soil respiration and soil temperature was negatively correlated with the soil volumetric water content and was reduced as soil water content increased. Over the seasonal scale, daily mean nighttime soil respiration was positively correlated with soil temperature when moisture exceeded 0.075 and 0.085 m3 m−3 in lichen- and moss-crusted soil, respectively. However, moisture did not affect on soil respiration in algae-crusted soil during the study period. Daily mean nighttime soil respiration normalized by soil temperature increased with water content in lichen- and moss-crusted soil. Our results indicated that different types of biological soil crusts could affect response of soil respiration to environmental factors. There is a need to consider the spatial distribution of different types of biological soil crusts and their relative contributions to the total C budgets at the ecosystem or landscape level. PMID:25050837

Small-Scale Density Variations in the Lunar Crust Revealed by GRAIL

NASA Technical Reports Server (NTRS)

Jansen, J. C.; Andrews-Hanna, J. C.; Li, Y.; Lucey, P. G.; Taylor, G. J.; Goossens, S.; Lemoine, F. G.; Mazarico, E.; Head, J. W., III; Milbury, C.;

Models of a partially hydrated Titan interior with clathrate crust

NASA Astrophysics Data System (ADS)

Lunine, J. I.; Castillo-Rogez, J.

2012-04-01

We present an updated model of the interior evolution of Titan over time, assuming the silicate core was hydrated early in Titan's history and is dehydrating over time. The original model presented in Castillo-Rogez and Lunine (2010) was motivated by a Cassini-derived moment of inertia (Iess et al., 2010) for Titan too large to be accommodated by classical fully differentiated models in which an anhydrous silicate core was overlain by a water ice (with possible perched ocean) mantle. Our model consisted of a silicate core still in the process of dehydrating today, a situation made possible by the leaching of radiogenic potassium from the silicates into the liquid water ocean. The crust of Titan was assumed to be pure water ice I. The model was consistent with the moment of inertia of Titan, but neglected the presence of large amounts of methane in the upper crust invoked to explain methane's persistence at present and through geologic time (Tobie et al. 2006). We have updated our model with such a feature. We have also improved our modeling with a better physical model for the dehydration of antigorite and other hydrated minerals. In particular our modeling now simulates heat advection resulting from water circulation (e.g., Seipold and Schilling 2003), rather than the purely conductive heat transfer regime assumed in the first version of our model. The modeling proceeds as in Castillo-Rogez and Lunine (2010), with the thermal conductivity of the methane clathrate crust rather than that of ice I. The former is several times lower than that of the latter, and the two have rather different temperature dependences (English and Tse, 2009). The crust turns out to have essentially no bearing on the temperature of the silicate core and hence the timing of dehydration, but it profoundly affects the thickness of the high-pressure ice layer beneath the ocean. Indeed, with the insulating methane clathrate crust, there must be a liquid water ocean beneath the methane clathrate

Tectonic slicing of subducting oceanic crust along plate interfaces: Numerical modeling

NASA Astrophysics Data System (ADS)

Ruh, J. B.; Le Pourhiet, L.; Agard, Ph.; Burov, E.; Gerya, T.

2015-10-01

Multikilometer-sized slivers of high-pressure low-temperature metamorphic oceanic crust and mantle are observed in many mountain belts. These blueschist and eclogite units were detached from the descending plate during subduction. Large-scale thermo-mechanical numerical models based on finite difference marker-in-cell staggered grid technique are implemented to investigate slicing processes that lead to the detachment of oceanic slivers and their exhumation before the onset of the continental collision phase. In particular, we investigate the role of the serpentinized subcrustal slab mantle in the mechanisms of shallow and deep crustal slicing. Results show that spatially homogeneous serpentinization of the sub-Moho slab mantle leads to complete accretion of oceanic crust within the accretionary wedge. Spatially discontinuous serpentinization of the slab mantle in form of unconnected patches can lead to shallow slicing of the oceanic crust below the accretionary wedge and to its deep slicing at mantle depths depending on the patch length, slab angle, convergence velocity and continental geothermal gradient. P-T paths obtained in this study are compared to natural examples of shallow slicing of the Crescent Terrane below Vancouver Island and deeply sliced crust of the Lago Superiore and Saas-Zermatt units in the Western Alps.

Contraction or expansion of the Moon's crust during magma ocean freezing?

PubMed

Elkins-Tanton, Linda T; Bercovici, David

2014-09-13

The lack of contraction features on the Moon has been used to argue that the Moon underwent limited secular cooling, and thus had a relatively cool initial state. A cool early state in turn limits the depth of the lunar magma ocean. Recent GRAIL gravity measurements, however, suggest that dikes were emplaced in the lower crust, requiring global lunar expansion. Starting from the magma ocean state, we show that solidification of the lunar magma ocean would most likely result in expansion of the young lunar crust, and that viscous relaxation of the crust would prevent early tectonic features of contraction or expansion from being recorded permanently. The most likely process for creating the expansion recorded by the dikes is melting during cumulate overturn of the newly solidified lunar mantle. © 2014 The Author(s) Published by the Royal Society. All rights reserved.

Seismic properties of the crust and uppermost mantle of North America

NASA Technical Reports Server (NTRS)

Braile, L. W.; Hinze, W. J.; Vonfrese, R. R. B.; Keller, G. R.

1983-01-01

Seismic refraction profiles for the North American continent were compiled. The crustal models compiled data on the upper mantle seismic velocity (P sub n), the crustal thickness (H sub c) and the average seismic velocity of the crystalline crust (V sub p). Compressional wave parameters were compared with shear wave data derived from surface wave dispersion models and indicate an average value for Poisson's ratio of 0.252 for the crust and of 0.273 for the uppermost mantle. Contour maps illustrate lateral variations in crustal thickness, upper mantle velocity and average seismic velocity of the crystalline crust. The distribution of seismic parameters are compared with a smoothed free air anomaly map of North America and indicate that a complidated mechanism of isostatic compensation exists for the North American continent. Several features on the seismic contour maps also correlate with regional magnetic anomalies.

[Crusted scabies in HIV/AIDS infected patients. Report of 15 cases].

PubMed

Tirado-Sánchez, Andrés; Bonifaz, Alexandro; Montes de Oca-Sánchez, Griselda; Araiza-Santibañez, Javier; Ponce-Olivera, Rosa María

2016-01-01

Crusted (Norwegian) scabies is a rare disease that occurs in patients with compromised immune system like patients with HIV/AIDS. We report 15 cases of crusted scabies in patients with HIV/AIDS successfully treated with oral ivermectin. The mean age of the patients was 43.7±8.06 and the diagnosis was made at a median of 5 months. All patients were diagnosed with HIV/AIDS treatment with antiretroviral therapy. Patients were treated with repeated doses of oral ivermectin with different schemes with good tolerance and efficacy with full resolution and without recurrence. Ivermectin is the treatment of choice for crusted scabies; it is tolerable and accessible to the patient. Immunosuppressed patients are those with the highest risk of acquiring that disease; we highlight the importance of lesion scraping to perform a correct and early diagnosis.

Seismic Signature of the Continental Crust: What Thermodynamics Says. An Example From the Italian Peninsula

NASA Astrophysics Data System (ADS)

Diaferia, G.; Cammarano, F.

2017-12-01

Unraveling the temperature distribution and composition of Earth's crust is key for understanding its origin, evolution, and mechanical behavior. Models of compressional (VP) and shear wave (VS) velocity are obtained from seismological studies and can be interpreted in terms of temperature and composition, using relationship defined through laboratory experiments. These empirical evidences often do not properly account for the effects driven by temperature, pressure, water content, and phase change of minerals. In this study, we use thermodynamic modeling to properly investigate the role of these variables in affecting seismic properties, as a tool to guide (joint) inversion and interpretation of geophysical data. We find that mineralogical phase transitions can be more seismically relevant than a change in chemical composition. In particular, the α-β quartz transition would cause a jump in acoustic impedance and VP/VS ratio >8%, occurring in the 15-25 km depth range, depending on the thermal gradient. Moreover, in the case of a cold lower crust, the consumption of plagioclase in favor of high-velocity minerals might represent another relevant seismic discontinuity. Different chemical compositions proposed for the Italian crust would be seismically indistinguishable, since they give overlapping seismic properties. Values of VS < 3.6 km s-1 would imply a strong contribution of sediments and/or partial melt. The VS/density ratio shows a narrow variability, suggesting that densities at depth can be directly derived in first approximation from VS.

Successional stage of biological soil crusts: an accurate indicator of ecohydrological condition

USGS Publications Warehouse

Belnap, Jayne; Wilcox, Bradford P.; Van Scoyoc, Matthew V.; Phillips, Susan L.

2013-01-01

Biological soil crusts are a key component of many dryland ecosystems. Following disturbance, biological soil crusts will recover in stages. Recently, a simple classification of these stages has been developed, largely on the basis of external features of the crusts, which reflects their level of development (LOD). The classification system has six LOD classes, from low (1) to high (6). To determine whether the LOD of a crust is related to its ecohydrological function, we used rainfall simulation to evaluate differences in infiltration, runoff, and erosion among crusts in the various LODs, across a range of soil depths and with different wetting pre-treatments. We found large differences between the lowest and highest LODs, with runoff and erosion being greatest from the lowest LOD. Under dry antecedent conditions, about 50% of the water applied ran off the lowest LOD plots, whereas less than 10% ran off the plots of the two highest LODs. Similarly, sediment loss was 400 g m-2 from the lowest LOD and almost zero from the higher LODs. We scaled up the results from these simulations using the Rangeland Hydrology and Erosion Model. Modelling results indicate that erosion increases dramatically as slope length and gradient increase, especially beyond the threshold values of 10 m for slope length and 10% for slope gradient. Our findings confirm that the LOD classification is a quick, easy, nondestructive, and accurate index of hydrological condition and should be incorporated in field and modelling assessments of ecosystem health.

Fluids of the lower crust and upper mantle: deep is different

NASA Astrophysics Data System (ADS)

Manning, C. E.

2017-12-01

Deep fluids are important for the evolution and properties of the lower crust and upper mantle in tectonically active settings. Uncertainty about their chemistry has led past workers to use upper crustal fluids as analogues. However, recent results show that fluids at >15 km differ fundamentally from shallow fluids and help explain high-pressure metasomatism and resistivity patterns. Deep fluids are comprised of four components: H2O, non-polar gases (chiefly CO2), salts (mostly alkali chlorides), and rock-derived solutes (dominated by aluminosilicates and related components). The first three generally define the solvent properties of the fluid, and models must account for observations that H2O activity may be quite low. The contrasting behavior of H2O-gas and H2O-salt mixtures yields immiscibility in the ternary system, which can lead to separation of two phases with fundamentally different chemical and transport properties. Thermodynamic modeling of equilibrium between rocks and H2O using simple ionic species known from shallow-crustal systems yields solutions possessing total dissolved solids and ionic strength that are too low to be consistent with experiments and resistivity surveys. Addition of CO2 further lowers bulk solubility and conductivity. Therefore, additional species must be present in H2O, and H2O-salt solutions likely explain much of the evidence for fluid action in high-P settings. At low salinity, H2O-rich fluids are powerful solvents for aluminosilicate rock components that are dissolved as previously unrecognized polymerized clusters. Experiments show that, near H2O-saturated melting, Al-Si polymers comprise >80% of solutes. The stability of these species facilitates critical critical mixing in rock-H2O systems. Addition of salt (e.g., NaCl) changes solubility patterns, but aluminosilicate contents remain high. Thermodynamic models indicate that the ionic strength of fluids with Xsalt = 0.05 to 0.4 and equilibrated with model crustal rocks have

Biotic soil crusts in relation to topography, cheatgrass, and fire in the Columbia Basin, Washington

USGS Publications Warehouse

Ponzetti, Jeanne; McCune, B.; Pyke, David A.

2007-01-01

We studied lichen and bryophyte soil crust communities in a large public grazing allotment within a sagebrush steppe ecosystem in which the biotic soil crusts are largely intact. The allotment had been rested from grazing for 12 years, but experienced an extensive series of wildfires. In the 350, 4 ?? 0.5 m plots, stratified by topographic position, we found 60 species or species groups that can be distinguished in the field with a hand lens, averaging 11.5 species groups per plot. Lichen and bryophyte soil crust communities differed among topographic positions. Draws were the most disturbed, apparently from water erosion in a narrow channel and mass wasting from the steepened sides. Presumably because of this disturbance, draws had the lowest average species richness of all the topographic strata we examined. Biotic crust species richness and cover were inversely related to cover of the invasive annual, cheatgrass (Bromus tectorum), and positively related to cover of native bunchgrasses. Integrity of the biotic crust was more strongly related to cheatgrass than to fire. In general, we observed good recovery of crusts following fire, but only in those areas dominated by perennial bunchgrasses. We interpret the resilience of the biotic crust, in this case, to the low abundance of cheatgrass, low amounts of soil disturbance and high moss cover. These fires have not resulted in an explosion of the cheatgrass population, perhaps because of the historically low levels of livestock grazing.

Insights into chemical weathering of the upper continental crust from the geochemistry of ancient glacial diamictites

NASA Astrophysics Data System (ADS)

Li, Su; Gaschnig, Richard M.; Rudnick, Roberta L.

2016-03-01

Glacial diamictites, with ages ranging from ∼2900 to 0.01 Ma, record the changing composition of the upper continental crust through time (Gaschnig et al., 2014). Li concentrations and isotopic compositions, combined with Pb isotopic compositions, chemical index of alteration (CIA) values and relative Sr concentrations are used here to assess the degree of chemical weathering recorded in these deposits and the origin of this signature. The δ7Li values of most of the diamictites (ranging from -3.9 to +3.5) are lower than those of mantle-derived basalts (+3.7 ± 2, 2σ), and the low δ7Li values are generally accompanied by high CIA and low Sr/Sr∗ values (or Sr depletion factor, Sr/Sr∗ = Sr/(Ce∗Nd)0.5), reflecting a weathering signature that may have derived from pre-depositional, syn-depositional, and/or post-depositional weathering processes. Profiles through three glacial diamictites with relatively high CIA (a fresh road cut of the Neoproterozoic Nantuo Formation (CIA = 62-69), and drill cores through the Paleoproterozoic Timeball Hill (CIA = 66-75) and Duitschland Formations (CIA = 84-91)) do not show evidence of significant post-depositional weathering. High Th/U, reflecting loss of uranium during oxidative weathering, is seen in all Paleozoic and Neoproterozoic diamictites and a few Paleoproterozoic deposits. Pb isotopic systematics suggest that this signature was largely inherited from preexisting crust, although a subset of samples (the Neoproterozoic Konnarock, Paleozoic Dwyka, and several of the Paleoproterozoic Duitschland samples) appears to have experienced post-depositional U loss. Modern glaciomarine sediments record little weathering (CIA = 47, Sr/Sr∗ = 0.7, δ7Li = +1.8), consistent with the cold temperatures accompanying glacial periods, and suggesting that limited syn-depositional weathering has occurred. Thus, the chemical weathering signature observed in ancient glacial diamictites appears to be largely inherited from the upper

[Crusted scabies: A review].

PubMed

Jouret, G; Bounemeur, R; Presle, A; Takin, R

2016-04-01

Crusted scabies is a rare and severe form of infestation by Sarcoptes scabies var. hominis. It is characterized by profuse hyperkeratosis containing over 4000 mites per gram of skin, with treatment being long and difficult. The condition is both direct and indirectly contagious. It has a central role in epidemic cycles of scabies, the incidence of which is on the rise in economically stable countries. Recent discoveries concerning the biology of mites, the pathophysiology of hyperkeratosis and the key role of IL-17 in this severe form open up new therapeutic perspectives. Copyright © 2016 Elsevier Masson SAS. All rights reserved.

Deformation and rupture of the oceanic crust may control growth of Hawaiian volcanoes

USGS Publications Warehouse

Got, J.-L.; Monteiller, V.; Monteux, J.; Hassani, R.; Okubo, P.

2008-01-01

Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate. Despite the apparent simplicity of the parent process - emission of magma onto the oceanic crust - the resulting edifices display some topographic complexity. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown. Here we use seismological investigations and finite-element mechanical modelling to show that the load exerted by large Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes. ??2008 Nature Publishing Group.

Deformation and rupture of the oceanic crust may control growth of Hawaiian volcanoes.

PubMed

Got, Jean-Luc; Monteiller, Vadim; Monteux, Julien; Hassani, Riad; Okubo, Paul

2008-01-24

Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate. Despite the apparent simplicity of the parent process--emission of magma onto the oceanic crust--the resulting edifices display some topographic complexity. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown. Here we use seismological investigations and finite-element mechanical modelling to show that the load exerted by large Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes.

Carbon and nitrogen fixation differ between successional stages of biological soil crusts in the Colorado Plateau and Chihuahuan Desert

USGS Publications Warehouse

Housman, D.C.; Powers, H.H.; Collins, A.D.; Belnap, J.

2006-01-01

Biological soil crusts (cyanobacteria, mosses and lichens collectively) perform essential ecosystem services, including carbon (C) and nitrogen (N) fixation. Climate and land-use change are converting later successional soil crusts to early successional soil crusts with lower C and N fixation rates. To quantify the effect of such conversions on C and N dynamics in desert ecosystems we seasonally measured diurnal fixation rates in different biological soil crusts. We classified plots on the Colorado Plateau (Canyonlands) and Chihuahuan Desert (Jornada) as early (Microcoleus) or later successional (Nostoc/Scytonema or Placidium/Collema) and measured photosynthesis (Pn), nitrogenase activity (NA), and chlorophyll fluorescence (Fv/Fm) on metabolically active (moist) soil crusts. Later successional crusts typically had greater Pn, averaging 1.2-1.3-fold higher daily C fixation in Canyonlands and 2.4-2.8-fold higher in the Jornada. Later successional crusts also had greater NA, averaging 1.3-7.5-fold higher daily N fixation in Canyonlands and 1.3-25.0-fold higher in the Jornada. Mean daily Fv/Fm was also greater in later successional Canyonlands crusts during winter, and Jornada crusts during all seasons except summer. Together these findings indicate conversion of soil crusts back to early successional stages results in large reductions of C and N inputs into these ecosystems.

Is Ishtar Terra a thickened basaltic crust?

NASA Technical Reports Server (NTRS)

Arkani-Hamed, Jafar

1992-01-01

The mountain belts of Ishtar Terra and the surrounding tesserae are interpreted as compressional regions. The gravity and surface topography of western Ishtar Terra suggest a thick crust of 60-110 km that results from crustal thickening through tectonic processes. Underthrusting was proposed for the regions along Danu Montes and Itzpapalotl Tessera. Crustal thickening was suggested for the entire Ishtar Terra. In this study, three lithospheric models with total thicknesses of 40.75 and 120 km and initial crustal thicknesses of 3.9 and 18 km are examined. These models could be produced by partial melting and chemical differentiation in the upper mantle of a colder, an Earth-like, and a hotter Venus having temperatures of respectively 1300 C, 1400 C, and 1500 C at the base of their thermal boundary layers associated with mantle convection. The effects of basalt-granulite-eclogite transformation (BGET) on the surface topography of a thickening basaltic crust is investigated adopting the experimental phase diagram and density variations through the phase transformation.

Ecological succession, hydrology and carbon acquisition of biological soil crusts measured at the micro-scale.

PubMed

Tighe, Matthew; Haling, Rebecca E; Flavel, Richard J; Young, Iain M

2012-01-01

The hydrological characteristics of biological soil crusts (BSCs) are not well understood. In particular the relationship between runoff and BSC surfaces at relatively large (>1 m(2)) scales is ambiguous. Further, there is a dearth of information on small scale (mm to cm) hydrological characterization of crust types which severely limits any interpretation of trends at larger scales. Site differences and broad classifications of BSCs as one soil surface type rather than into functional form exacerbate the problem. This study examines, for the first time, some hydrological characteristics and related surface variables of a range of crust types at one site and at a small scale (sub mm to mm). X-ray tomography and fine scale hydrological measurements were made on intact BSCs, followed by C and C isotopic analyses. A 'hump' shaped relationship was found between the successional stage/sensitivity to physical disturbance classification of BSCs and their hydrophobicity, and a similar but 'inverse hump' relationship exists with hydraulic conductivity. Several bivariate relationships were found between hydrological variables. Hydraulic conductivity and hydrophobicity of BSCs were closely related but this association was confounded by crust type. The surface coverage of crust and the microporosity 0.5 mm below the crust surface were closely associated irrespective of crust type. The δ (13)C signatures of the BSCs were also related to hydraulic conductivity, suggesting that the hydrological characteristics of BSCs alter the chemical processes of their immediate surroundings via the physiological response (C acquisition) of the crust itself. These small scale results illustrate the wide range of hydrological properties associated with BSCs, and suggest associations between the ecological successional stage/functional form of BSCs and their ecohydrological role that needs further examination.

Influence of mid-crustal rheology on the deformation behavior of continental crust in the continental subduction zone

NASA Astrophysics Data System (ADS)

Li, Fucheng; Sun, Zhen; Zhang, Jiangyang

2018-06-01

Although the presence of low-viscosity middle crustal layer in the continental crust has been detected by both geophysical and geochemical studies, its influence on the deformation behavior of continental crust during subduction remains poorly investigated. To illustrate the crustal deformation associated with layered crust during continental subduction, we conducted a suite of 2-D thermo-mechanical numerical studies with visco-brittle/plastic rheology based on finite-differences and marker-in-cell techniques. In the experiments, we established a three-layer crustal model with a quartz-rich middle crustal layer embedded between the upper and lower continental crust. Results show that the middle crustal layer determines the amount of the accreted upper crust, maximum subduction depth, and exhumation path of the subducted upper crust. By varying the initial effective viscosity and thickness of the middle crustal layer, the further effects can be summarized as: (1) a rheologically weaker and/or thicker middle crustal layer results in a larger percentage of the upper crust detaching from the underlying slab and accreting at the trench zone, thereby leading to more serious crustal deformation. The rest of the upper crust only subducts into the depths of high pressure (HP) conditions, causing the absence of ultra-high pressure (UHP) metamorphic rocks; (2) a rheologically stronger and/or thinner middle crustal layer favors the stable subduction of the continental crust, dragging the upper crust to a maximum depth of ∼100 km and forming UHP rocks; (3) the middle crustal layer flows in a ductile way and acts as an exhumation channel for the HP-UHP rocks in both situations. In addition, the higher convergence velocity decreases the amount of subducted upper crust. A detailed comparison of our modeling results with the Himalayan collisional belt are conducted. Our work suggests that the presence of low-viscosity middle crustal layer may be another possible mechanism for

Disordered Nuclear Pasta, Magnetic Field Decay, and Crust Cooling in Neutron Stars

NASA Astrophysics Data System (ADS)

Horowitz, C. J.; Berry, D. K.; Briggs, C. M.; Caplan, M. E.; Cumming, A.; Schneider, A. S.

2015-01-01

Nuclear pasta, with nonspherical shapes, is expected near the base of the crust in neutron stars. Large-scale molecular dynamics simulations of pasta show long lived topological defects that could increase electron scattering and reduce both the thermal and electrical conductivities. We model a possible low-conductivity pasta layer by increasing an impurity parameter Qimp . Predictions of light curves for the low-mass x-ray binary MXB 1659-29, assuming a large Qimp, find continued late time cooling that is consistent with Chandra observations. The electrical and thermal conductivities are likely related. Therefore, observations of late time crust cooling can provide insight on the electrical conductivity and the possible decay of neutron star magnetic fields (assuming these are supported by currents in the crust).

Disordered nuclear pasta, magnetic field decay, and crust cooling in neutron stars.

PubMed

Horowitz, C J; Berry, D K; Briggs, C M; Caplan, M E; Cumming, A; Schneider, A S

2015-01-23

Nuclear pasta, with nonspherical shapes, is expected near the base of the crust in neutron stars. Large-scale molecular dynamics simulations of pasta show long lived topological defects that could increase electron scattering and reduce both the thermal and electrical conductivities. We model a possible low-conductivity pasta layer by increasing an impurity parameter Q_{imp}. Predictions of light curves for the low-mass x-ray binary MXB 1659-29, assuming a large Q_{imp}, find continued late time cooling that is consistent with Chandra observations. The electrical and thermal conductivities are likely related. Therefore, observations of late time crust cooling can provide insight on the electrical conductivity and the possible decay of neutron star magnetic fields (assuming these are supported by currents in the crust).

A review of crust and upper mantle structure studies of the Snake River Plain-Yellowstone volcanic system: A major lithospheric anomaly in the western U.S.A.

USGS Publications Warehouse

Iyer, H.M.

1984-01-01

The Snake River Plain-Yellowstone volcanic system is one of the largest, basaltic, volcanic field in the world. Here, there is clear evidence for northeasterly progression of rhyolitic volcanism with its present position in Yellowstone. Many theories have been advanced for the origin of the Snake River Plain-Yellowstone system. Yellowstone and Eastern Snake River Plain have been studied intensively using various geophysical techniques. Some sparse geophysical data are available for the Western Snake River Plain as well. Teleseismic data show the presence of a large anomalous body with low P- and S-wave velocities in the crust and upper mantle under the Yellowstone caldera. A similar body in which compressional wave velocity is lower than in the surrounding rock is present under the Eastern Snake River Plain. No data on upper mantle anomalies are available for the Western Snake River Plain. Detailed seismic refraction data for the Eastern Snake River Plain show strong lateral heterogeneities and suggest thinning of the granitic crust from below by mafic intrusion. Available data for the Western Snake River Plain also show similar thinning of the upper crust and its replacement by mafic material. The seismic refraction results in Yellowstone show no evidence of the low-velocity anomalies in the lower crust suggested by teleseismic P-delay data and interpreted as due to extensive partial melting. However, the seismic refraction models indicate lower-than-normal velocities and strong lateral inhomogeneities in the upper crust. Particularly obvious in the refraction data are two regions of very low seismic velocities near the Mallard Eake and Sour Creek resurgent domes in the Yellowstone caldera. The low-velocity body near the Sour Creek resurgent dome is intepreted as partially molten rock. Together with other geophysical and thermal data, the seismic results indicate that a sub-lithospheric thermal anomaly is responsible for the time-progressive volcanism along the

The Role of Carbon in Exotic Crust Formation on Mercury

NASA Technical Reports Server (NTRS)

Vander Kaaden, Kathleen E.; McCubbin, Francis M.

2018-01-01

The terrestrial planets that comprise our inner Solar System, including the Moon, are all rocky bodies that have differentiated into a crust, mantle, and core. Furthermore, all of these bodies have undergone various igneous processes since their time of primary crust formation. These processes have resurfaced each of these bodies, at least in part, resulting in the production of a secondary crust, to which Mercury is no exception. From its first flyby encounter with Mercury on January 14, 2008, the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft collected data on the structure, chemical makeup, and density of the planet among other important characteristics. The X-Ray Spectrometer on board MESSENGER measured elevated abundances of sulfur and low abundances of iron, suggesting the planets oxygen fugacity (fO2) is several log10 units below the Iron-Wustite buffer. Similar to the role of other volatiles (e.g. sulfur) on highly reducing planetary bodies, carbon is expected to behave differently in an oxygen starved environment than it does in an oxygen enriched environment (e.g., Earth).

The Moho as a magnetic boundary. [Earth crust-mantle boundary

NASA Technical Reports Server (NTRS)

Wasilewski, P. J.; Thomas, H. H.; Mayhew, M. A.

1979-01-01

Magnetism in the crust and the upper mantle and magnetic results indicating that the seismic Moho is a magnetic boundary are considered. Mantle derived rocks - peridotites from St. Pauls rocks, dunite xenoliths from the Kaupulehu flow, and peridotite, dunite, and eclogite xenoliths from Roberts Victor and San Carlos diatremes - are weakly magnetic with saturation magnetization values from 0.013 emu/gm to less than 0.001 emu/gm which is equivalent to 0.01 to 0.001 wt% Fe304. Literature on the minerals in mantle xenoliths shows that metals and primary Fe304 are absent, and that complex Cr, Mg, Al, and Fe spinels are dominant. These spinels are non-magnetic at mantle temperatures, and the crust/mantle boundary can be specified as a magnetic mineralogy discontinuity. The new magnetic results indicate that the seismic Moho is a magnetic boundary, the source of magnetization is in the crust, and the maximum Curie isotherm depends on magnetic mineralogy and is located at depths which vary with the regional geothermal gradient.

Subduction of lower continental crust beneath the Pamir imaged by receiver functions from the seismological TIPAGE network

NASA Astrophysics Data System (ADS)

Schneider, F. M.; Yuan, X.; Schurr, B.; Mechie, J.; Sippl, C.; Kufner, S.; Haberland, C. A.; Minaev, V.; Oimahmadov, I.; Gadoev, M.; Abdybachaev, U.; Orunbaev, S.

2013-12-01

As the northwestern promontory of the Tibetan Plateau, the Pamir forms an outstanding part of the India-Asia convergence zone. The Pamir plateau has an average elevation of more than 4000 m surrounded by peaks exceeding 7000 m at its northern, eastern and southern borders. The Pamir is thought to consist of the same collage of continental terranes as Tibet. However, in this region the Indian-Asian continental collision presents an extreme situation since, compared to Tibet, in the Pamir a similar amount of north-south convergence has been accommodated within a much smaller distance. The Pamir hosts a zone of intermediate depth earthquakes being the seismic imprint of Earth's most spectacular active intra-continental subduction zone. We present receiver function (RF) images from the TIPAGE seismic profile giving evidence that the intermediate depth seismicity is situated within a subducted layer of lower continental crust: We observe a southerly dipping 10-15 km thick low-velocity zone (LVZ), that starts from the base of the crust and extends to a depth of more than 150 km enveloping the intermediate depth earthquakes that have been located with high precision from our local network records. In a second northwest to southeast cross section we observe that towards the western Pamir the dip direction of the LVZ bends to the southeast following the geometry of the intermediate depth seismic zone. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust. These observations provide important implications for the geodynamics of continental collision: First, it shows that under extreme conditions lower crust can be brought to mantle depths despite its buoyancy, a fact that is also testified by the exhumation of ultra-high pressure metamorphic rocks. Recent results from teleseismic tomography show a signal of Asian mantle lithosphere down to 600 km depth, implying a great

Higher-order symmetry energy and neutron star core-crust transition with Gogny forces

NASA Astrophysics Data System (ADS)

Gonzalez-Boquera, C.; Centelles, M.; Viñas, X.; Rios, A.

2017-12-01

Background: An accurate determination of the core-crust transition is necessary in the modeling of neutron stars for astrophysical purposes. The transition is intimately related to the isospin dependence of the nuclear force at low baryon densities. Purpose: To study the symmetry energy and the core-crust transition in neutron stars using the finite-range Gogny nuclear interaction and to examine the deduced crustal thickness and crustal moment of inertia. Methods: The second-, fourth-, and sixth-order coefficients of the Taylor expansion of the energy per particle in powers of the isospin asymmetry are analyzed for Gogny forces. These coefficients provide information about the departure of the symmetry energy from the widely used parabolic law. The neutron star core-crust transition is evaluated by looking at the onset of thermodynamical instability of the liquid core. The calculation is performed with the exact Gogny equation of state (EoS) (i.e., the Gogny EoS with the full isospin dependence) for the β -equilibrated matter of the core, and also with the Taylor expansion of the Gogny EoS in order to assess the influence of isospin expansions on locating the inner edge of neutron star crusts. Results: The properties of the core-crust transition derived from the exact EoS differ from the predictions of the Taylor expansion even when the expansion is carried through sixth order in the isospin asymmetry. Gogny forces, using the exact EoS, predict the ranges 0.094 fm-3≲ρt≲0.118 fm-3 for the transition density and 0.339 MeVfm-3≲Pt≲0.665 MeVfm-3 for the transition pressure. The transition densities show an anticorrelation with the slope parameter L of the symmetry energy. The transition pressures are not found to correlate with L . Neutron stars obtained with Gogny forces have maximum masses below 1.74 M⊙ and relatively small moments of inertia. The crustal mass and moment of inertia are evaluated and comparisons are made with the constraints from observed

Crusted scabies of the scalp in a patient with systemic lupus erythematosus.

PubMed

Yee, Brittany E; Carlos, Casey A; Hata, Tissa

2014-10-15

Crusted scabies is a severe, hyperkeratotic, psoriasiform disorder associated with immune suppression. Affected individuals typically present with crusted hyperkeratotic lesions in a variety of locations. This condition can lead to severe complications: institutional outbreaks and secondary bacterial infections associated with sepsis and high mortality. A 37-year-old woman with a 12-year history of systemic lupus erythematosus treated with prednisone, methotrexate, and plaquenil presented with a three-week history of a painful scalp rash with adherent yellow scale. Skin biopsy and tissue culture were consistent with a diagnosis of crusted scabies with superficial bacterial infection. The patient was treated with oral ivermectin and permethrin cream, as well as ciprofloxacin for the bacterial infection. At one-week follow-up, the scalp was no longer tender and hyperkeratotic plaques had significantly improved. At one-month follow-up, the affected scalp demonstrated further improvement with decreasing erythema and alopecia with follicular ostia. Our case highlights the atypical presentation of crusted scabies with primary scalp involvement and need for vigilance in recognizing and appropriately treating this condition to prevent the consequences of longstanding infection. Combination treatment with ivermectin and permethrin is appropriate management for this condition.

Dynamics of Crust Dissolution and Gas Release in Tank 241-SY-101

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rassat, Scot D.; Stewart, Charles W.; Wells, Beric E.

2000-01-24

Due primarily to an increase in floating crust thickness, the waste level in Tank 241-SY-101 has grown appreciably and the flammable gas volume stored in the crust has become a potential hazard. To remediate gas retention in the crust and the potential for buoyant displacement gas releases from the nonconvective layer at the bottom of the tank, SY-101 will be diluted to dissolve a large fraction of the solids that allow the waste to retain gas. The plan is to transfer some waste out and back-dilute with water in several steps. In this work, mechanisms and rates of waste solidsmore » dissolution and gas releases are evaluated theoretically and experimentally. Particular emphasis is given to crust dissolution processes and associated gas releases, although dissolution and gas release from the mixed-slurry and nonconvective layers are also considered. The release of hydrogen gas to the tank domespace is modeled for a number of scenarios. Under the tank conditions expected at the time of back-dilution, no plausible continuous or sudden gas release scenarios resulting in flammable hydrogen concentrations were identified.« less

Paleoceanographic conditions on the São Paulo Ridge, SW Atlantic Ocean, for the past 30 million years inferred from Os and Pb isotopes of a hydrogenous ferromanganese crust

NASA Astrophysics Data System (ADS)

Goto, Kosuke T.; Nozaki, Tatsuo; Toyofuku, Takashi; Augustin, Adolpho H.; Shimoda, Gen; Chang, Qing; Kimura, Jun-Ichi; Kameo, Koji; Kitazato, Hiroshi; Suzuki, Katsuhiko

2017-12-01

Hydrogenous ferromanganese (Fe-Mn) crusts can provide records of long-term environmental changes during the Cenozoic. To understand the paleoceanographic conditions in the southwestern Atlantic Ocean, we investigated depth profiles of major- and trace-element concentrations as well as Os and Pb isotopic compositions in a Fe-Mn crust collected from the southern flank of the São Paulo Ridge in the southwestern Atlantic. Major and trace element data plotted on ternary Mn-Fe-10×(Ni+Co+Cu) and rare-earth element plus yttrium (REY) discrimination diagrams indicate that the analyzed sample is a typical hydrogenous Fe-Mn crust. The obtained 187Os/188Os data were matched to the Cenozoic seawater Os isotope evolution curve reconstructed from pelagic sediments. The result suggests that the Fe-Mn crust has accreted over 30 Myr with growth rates of 0.5-3 mm/Myr, although the sample likely grew in two directions during the early stage of its growth. We found no evidence of growth hiatus in the sample, which may contrast with the growth histories of many Pacific Fe-Mn crusts. Hence, the conditions favorable for the accretion of hydrogenous Fe-Mn crusts were likely to have developed on the São Paulo Ridge over the past 30 Myr. The Pb isotopic compositions show very limited ranges (e.g., 206Pb/204Pb=18.80-18.85), and are similar to those of pre-anthropogenic seawater in the Southern Ocean. As the São Paulo Ridge is located near the Vema Channel, which is presently a major path of Antarctic Bottom Water, we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge, and played a vital role in the Fe-Mn crust formation since 30 Ma.

Geophysical Monitoring of Geodynamic Processes of Central Armenia Earth Crust

NASA Astrophysics Data System (ADS)

Avetyan, R.; Pashayan, R.

2016-12-01

The method of geophysical monitoring of earth crust was introduced. It allows by continuous supervision to track modern geodynamic processes of Armenia. Methodological practices of monitoring come down to allocation of a signal which reflects deformation of rocks. The indicators of deformations are not only deviations of geophysical indicators from certain background values, but also parameters of variations of these indicators. Data on changes of parameters of barometric efficiency and saw tooth oscillations of underground water level before seismic events were received. Low-amplitude periodic fluctuations of water level are the reflection of geodynamic processes taking place in upper levels of earth crust. There were recorded fluctuations of underground water level resulting from luni-solar tides and enabling to control the systems of borehole-bed in changes of voluminous deformations. The slow lowering (raising) of underground water level in the form of trend reflects long-period changes of stress-deformative state of environment. Application of method promotes identification of medium-term precursors on anomalous events of variations of geomagnetic field, change of content of subsoil radon, dynamics of level of underground water, geochemistry and water temperature. Increase of activity of geodynamic processes in Central Armenian tectonic complex is observed to change macro component Na+, Ca2+, Mg2-, CL-, SO42-, HCO3-, H4SiO4, pH and gas - CO2 structure of mineral water. Modern geodynamic movements of earth crust of Armenia are the result of seismic processes and active geodynamics of deep faults of longitudinal and transversal stretching. Key Words: monitoring, hydrogeodynamics, geomagnetic field, seismicity, deformation, earth crust

Transient Creep of a Composite Lower Crust. 1; Constitutive Theory

NASA Technical Reports Server (NTRS)

Ivins, Erik R.; Sammis, Charles G.

1996-01-01

A composite model is proposed to describe the time-dependent response of the Earth's lower crust. The motivation for such it model is twofold: First, new observations of widespread postseismic deformation indicate that the deep continental crust responds viscoelastically, having both long-and short-term decay times. Second, by any number of observationally based rationales, the lower crust is compositionally and structurally heterogeneous over many length scales. For heterogeneities that have much smaller characteristic lengths than the minimum deformation wavelength of interest, the aggregate rheology can be described by composite media theory. For wavelengths of the order of the thickness of the lower crust (approx. = 25-40 km) and larger, composite theory may be applied to heterogeneities that are smaller than about several hundred meters, or equivalent to the vertical extent of a thick lower crustal mylonitic shear zone. The composite media theory developed here is constructed using both Eshelhy-Mori-Tanaka theory for aligned generalized spheroidal inclusions and a generalized self-consistent method. The inclusions and matrix are considered to be Maxwellian viscoelastic: a rheology that is consistent with past homogeneous models of postseismic stress relaxation. The composite theory presented here introduces a transient response to a suddenly imposed stress field which does not appear in homogeneous Maxwell models. Analytic expressions for the amplitude and duration of the transient and for the effective long-and short-term viscosities of the composite are given which describe the sensitivity to inclusion concentration (phi), to shape, and to ratio of inclusion-to-matrix viscosity (R).

Evaluating the importance of metamorphism in the foundering of continental crust.

PubMed

Chapman, Timothy; Clarke, Geoffrey L; Piazolo, Sandra; Daczko, Nathan R

2017-10-12

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet-clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa.

A study regarding the stability of the primordial crust of asteroid Ceres

NASA Astrophysics Data System (ADS)

Formisano, Michelangelo; Federico, Costanzo; De Sanctis, Maria Cristina; De Angelis, Simone

2016-04-01

Ceres is a particular object of the solar system, since it is a "transition body" between the icy satellites of the outer solar system and the rocky bodies of the inner part. Probably it is differentiated [1,2], i.e. it has a core made of "rock" (silicates) with a weak presence of metals, a large icy mantle and a rocky crust. In particular, it has been proposed the existence on the surface of the ammoniated phyllosilicates, compatible with an outer solar system origin [3]. Also water in clay minerals, brucite, and iron-rich serpentine have been proposed to exist on the surface [4]. Ice directly on the surface regolith seems to be very unstable: numerical simulations of [5] indicate that it can last for very few orbits. A crust made of a mixture of ice and rock is potentially unstable. In the solar system, for example, Callisto has such a crust but its surface temperature is below the critical temperature for the Rayleigh-Taylor instability [6]: this seems not to be the case of Ceres. In this work, we verify the stability of the primordial crust, by assuming a certain initial composition (ice and rock) and thickness. We assume a post-differentiation Ceres, made of three layers (rocky core, icy mantle and crust). The key role is played by the viscosity of the layers, which influenced the survival or not of the primordial crust. We applied the method of the parametrized thermal convection widely diffused in literature. [1] McCord, T.B. and Sotin, C., 2005, JGR 110 [2] Castillo-Rogez, J.C., and McCord, T.B., 2010, Icarus 205, 443-459 [3] De Sanctis, M.C. et al., 2015, doi:10.1038/nature16172 [4] Rivkin, A.S., et al., 2014, Space Sci Rev, 95-116, 163, doi 10.1007/s11214-010-9677-4 [5] Formisano, M., et al., 2016, MRAS 455, 1892-1904 [6] Shoji, D. and Kurita, K., 2014, doi:10.1002/2014JE004695.

A Novel Clinical Grading Scale to Guide the Management of Crusted Scabies

PubMed Central

Tong, Steven Y. C.; Walton, Shelley F.; Currie, Bart J.

2013-01-01

Background Crusted scabies, or hyperinfestation with Sarcoptes scabiei, occurs in people with an inadequate immune response to the mite. In recent decades, data have emerged suggesting that treatment of crusted scabies with oral ivermectin combined with topical agents leads to lower mortality, but there are no generally accepted tools for describing disease severity. Here, we describe a clinical grading scale for crusted scabies and its utility in real world practice. Methodology/Principal Findings In 2002, Royal Darwin Hospital (RDH), a hospital in tropical Australia developed and began using a clinical grading scale to guide the treatment of crusted scabies. We conducted a retrospective observational study including all episodes of admission to RDH for crusted scabies during the period October 2002–December 2010 inclusive. Patients who were managed according to the grading scale were compared with those in whom the scale was not used at the time of admission but was calculated retrospectively. There were 49 admissions in 30 patients during the study period, of which 49 (100%) were in Indigenous Australians, 29 (59%) were male and the median age was 44.1 years. According to the grading scale, 8 (16%) episodes were mild, 24 (49%) were moderate, and 17 (35%) were severe. Readmission within the study period was significantly more likely with increasing disease severity, with an odds ratio (95% CI) of 12.8 (1.3–130) for severe disease compared with mild. The patients managed according to the grading scale (29 episodes) did not differ from those who were not (20 episodes), but they received fewer doses of ivermectin and had a shorter length of stay (11 vs. 16 days, p = 0.02). Despite this the outcomes were no different, with no deaths in either group and a similar readmission rate. Conclusions/Significance Our grading scale is a useful tool for the assessment and management of crusted scabies. PMID:24069468

Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data

NASA Astrophysics Data System (ADS)

Bécel, Anne; Shillington, Donna J.; Nedimović, Mladen R.; Webb, Spahr C.; Kuehn, Harold

2015-08-01

Multi-channel seismic (MCS) reflection profiles across the Pacific Plate south of the Alaska Peninsula reveal the internal structure of mature oceanic crust (48-56 Ma) formed at fast to intermediate spreading rates during and after a major plate re-organization. Oceanic crust formed at fast spreading rates (half spreading rate ∼ 74 mm /yr) has smoother basement topography, thinner sediment cover with less faulting, and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates (half spreading rate ∼ 28- 34 mm /yr). MCS data across fast-spreading oceanic crust formed during plate re-organization contain abundant bright reflections, mostly confined to the lower crust above a highly reflective Moho transition zone, which has a reflection coefficient (RC) of ∼0.1. The lower crustal events dip predominantly toward the paleo-ridge axis at ∼10-30°. Reflections are also imaged in the uppermost mantle, which primarily dip away from the ridge at ∼10-25°, the opposite direction to those observed in the lower crust. Dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re-organization, where a Moho reflection is weak or absent. Our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt. The abundance and reflection amplitude strength of these events (RC ∼ 0.15) can be explained by a combination of solidified melt that was segregated within the shear structures, mylonitization of the shear zones, and crystal alignment, all of which can result in anisotropy and constructive signal interference. Formation of shear zones with this geometry requires differential motion between the crust and upper mantle, where the upper mantle moves away from the ridge

Energy conservation in the earth's crust and climate change.

PubMed

Mu, Yao; Mu, Xinzhi

2013-02-01

Among various matters which make up the earth's crust, the thermal conductivity of coal, oil, and oil-gas, which are formed over a long period of geological time, is extremely low. This is significant to prevent transferring the internal heat of the earth to the thermal insulation of the surface, cooling the surface of the earth, stimulating biological evolution, and maintaining natural ecological balance as well. Fossil energy is thermal insulating layer in the earth's crust. Just like the function of the thermal isolation of subcutaneous fatty tissue under the dermis of human skin, it keeps the internal heat within the organism so it won't be transferred to the skin's surface and be lost maintaining body temperature at low temperatures. Coal, oil, oil-gas, and fat belong to the same hydrocarbons, and the functions of their thermal insulation are exactly the same. That is to say, coal, oil, and oil-gas are just like the earth's "subcutaneous fatty tissue" and objectively formed the insulation protection on earth's surface. This paper argues that the human large-scale extraction of fossil energy leads to damage of the earth's crust heat-resistant sealing, increasing terrestrial heat flow, or the heat flow as it is called, transferring the internal heat of the earth to Earth's surface excessively, and causing geotemperature and sea temperature to rise, thus giving rise to global warming. The reason for climate warming is not due to the expansion of greenhouse gases but to the wide exploitation of fossil energy, which destroyed the heat insulation of the earth's crust, making more heat from the interior of the earth be released to the atmosphere. Based on the energy conservation principle, the measurement of the increase of the average global temperature that was caused by the increase of terrestrial heat flow since the Industrial Revolution is consistent with practical data. This paper illustrates "pathogenesis" of climate change using medical knowledge. The

Composition of weathering crusts on sandstones from natural outcrops and architectonic elements in an urban environment.

PubMed

Marszałek, Mariola; Alexandrowicz, Zofia; Rzepa, Grzegorz

2014-12-01

This work presents mineralogical and chemical characteristics of weathering crusts developed on sandstones exposed to various air pollution conditions. The samples have been collected from sandstone tors in the Carpathian Foothill and from buildings in Kraków. It has been stated that these crusts differ in both fabric and composition. The sandstone black crust from tors is rich in organic matter and composed of amorphous silica. Sulphate incrustations accompanied by dust particles have been only sometimes observed. Beneath the black crust, a zone coloured by iron (oxyhydr)oxides occurs. The enrichment of the surface crust in silica and iron compounds protects the rock interior from atmospheric impact. The sandstones from architectonic details are also covered by a thin carbon-rich black crust, but they are visibly loosened. Numerous salts, mainly gypsum and halite, crystallise here, thus enhancing deterioration of the rock. Moreover, spherical particles originated from industrial emissions are much more common. Gypsum in natural outcrops, forms isolated and well-developed crystals, whilst these found on the architectonic details are finer and densely cover the surface. Such diversity reflects various concentrations of acid air pollutants in solutions.

Crusted scabies: clinical and immunological findings in seventy-eight patients and a review of the literature.

PubMed

Roberts, L J; Huffam, S E; Walton, S F; Currie, B J

2005-06-01

To describe the clinical and immunological features of crusted scabies in a prospectively ascertained cohort of 78 patients. All patients requiring inpatient treatment for crusted scabies in the 'top end' of the northern territory of Australia over a 10 year period were prospectively identified. Demographics, risk factors, and immunological parameters were retrospectively compiled from their medical records and pathology databases. More than half the patients with crusted scabies had identifiable immunosuppressive risk factors. Eosinophilia and elevated IgE levels occurred in 58% and 96% of patients, respectively, with median IgE levels 17 times the upper limit of normal. Seventeen percent had a history of leprosy but 42% had no identifiable risk factors. There was a decrease in mortality after the introduction of a treatment protocol consisting of multiple doses of ivermectin combined with topical scabicides and keratolytic therapy. Crusted scabies often occurs in patients with identifiable immunosuppressive risk factors. In patients without such risk factors, it is possible that the crusted response to infection results from a tendency to preferentially mount a Th2 response. The treatment regime described was associated with a reduction in mortality. This is the largest reported case series of crusted scabies.

Melting and differentiation in Venus with a cold start: A mechanism of the thin crust formation

NASA Technical Reports Server (NTRS)

Solomatov, Viatcheslav S.; Stevenson, David J.

1992-01-01

Recent works argue that the venusian crust is thin: less than 10-30 km. However, any convective model of Venus unavoidably predicts melting and a fast growth of the basaltic crust, up to its maximum thickness of about 70 km limited, by the gabbro-eclogite phase transition. The crust is highly buoyant due to both its composition and temperature and it is problematic to find a mechanism providing its effective recycling and thinning in the absence of plate tectonics. There are different ways to solve this contradiction. This study suggests that a thin crust can be produced during the entire evolution of Venus if Venus avoided giant impacts.

Anaerobic Fungi: A Potential Source of Biological H2 in the Oceanic Crust

PubMed Central

Ivarsson, Magnus; Schnürer, Anna; Bengtson, Stefan; Neubeck, Anna

2016-01-01

The recent recognition of fungi in the oceanic igneous crust challenges the understanding of this environment as being exclusively prokaryotic and forces reconsiderations of the ecology of the deep biosphere. Anoxic provinces in the igneous crust are abundant and increase with age and depth of the crust. The presence of anaerobic fungi in deep-sea sediments and on the seafloor introduces a type of organism with attributes of geobiological significance not previously accounted for. Anaerobic fungi are best known from the rumen of herbivores where they produce molecular hydrogen, which in turn stimulates the growth of methanogens. The symbiotic cooperation between anaerobic fungi and methanogens in the rumen enhance the metabolic rate and growth of both. Methanogens and other hydrogen-consuming anaerobic archaea are known from subseafloor basalt; however, the abiotic production of hydrogen is questioned to be sufficient to support such communities. Alternatively, biologically produced hydrogen could serve as a continuous source. Here, we propose anaerobic fungi as a source of bioavailable hydrogen in the oceanic crust, and a close interplay between anaerobic fungi and hydrogen-driven prokaryotes. PMID:27433154

Surface disturbance of cryptobiotic soil crusts: nitrogenase activity, chlorophyll content, and chlorophyll degradation

USGS Publications Warehouse

Belnap, Jayne; Harper, Kimball T.; Warren, Steven D.

1994-01-01

Cryptobiotic soil crusts are an important component of semiarid and arid ecosystems. An important role of these crusts is the contribution of fixed nitrogen to cold‐desert ecosystems. This study examines the residual effects of various intensities and combinations of different surface disturbances (raking, scalping, and tracked vehicles) on nitrogenase activity, chlorophyll content, and chlorophyll degradation in these soil crusts. Nine months after disturbance chlorophyll content of disturbed soils was not statistically different from undisturbed controls, except in the scalped treatments, indicating recovery of this characteristic is fairly quick unless surface material is removed. Differences in chlorophyll degradation among treatments were not statistically significant. However, nitrogenase activity in all treatments showed tremendous reductions, ranging from 77–97%, when compared to the control, indicating this characteristic is slow to recover. Consequently, assessment of crustal recovery from disturbance must include not only visual and biomass characteristics but other physiological measurements as well. Areas dominated by these crusts should be managed conservatively until the implications of crustal disturbance is better understood.

Drilling the Oceanic Lower Crust and Mantle

DTIC Science & Technology

1989-11-01

East Pacific Rise near 21 ...A. Bideau, R.D. and Hekinian, R. 1983, Ultramafics and mafic rocks from the Garret transform fault near 13󈧢’S on the East Pacific Rise : igneous...Science Foundation. older crust formed at the East Pacific Rise . The JOIDES Planning Committee should immediately constitute a Deep Crustal

Effects of dietary bread crust Maillard reaction products on calcium and bone metabolism in rats.

PubMed

Roncero-Ramos, Irene; Delgado-Andrade, Cristina; Haro, Ana; Ruiz-Roca, Beatriz; Morales, Francisco J; Navarro, María Pilar

2013-06-01

Maillard reaction products (MRP) consumption has been related with the development of bone degenerative disorders, probably linked to changes in calcium metabolism. We aimed to investigate the effects of MRP intake from bread crust on calcium balance and its distribution, and bone metabolism. During 88 days, rats were fed control diet or diets containing bread crust as source of MRP, or its soluble high molecular weight, soluble low molecular weight or insoluble fractions (bread crust, HMW, LMW and insoluble diets, respectively). In the final week, a calcium balance was performed, then animals were sacrified and some organs removed to analyse calcium levels. A second balance was carried out throughout the experimental period to calculate global calcium retention. Biochemical parameters and bone metabolism markers were measured in serum or urine. Global calcium bioavailability was unmodified by consumption of bread crust or its isolate fractions, corroborating the previously described low affinity of MRP to bind calcium. Despite this, a higher calcium concentration was found in femur due to smaller bones having a lower relative density. The isolate consumption of the fractions altered some bone markers, reflecting a situation of increased bone resorption or higher turnover; this did not take place in the animals fed the bread crust diet. Thus, the bread crust intake does not affect negatively calcium bioavailability and bone metabolism.

Temporal Dynamics of Sodic Playa Salt Crust Patterns: Implications for Aeolian Dust Emission Potential

NASA Astrophysics Data System (ADS)

Nield, J. M.; King, J.; Bryant, R. G.; Wiggs, G.; Eckardt, F. D.; Thomas, D. S.; Washington, R.

2013-12-01

Salt pans (or playas) are common in arid environments and can be major sources of windblown mineral dust, but there are uncertainties associated with their dust emission potential. These landforms typically form crusts which modify both their erosivity and erodibility by limiting sediment availability, modifying surface and aerodynamic roughness and limiting evaporation rates and sediment production. Here we show the relationship between seasonal surface moisture change and crust pattern development based on both remote-sensing and field surface and atmospheric measurements. We use high resolution (sub-cm) terrestrial laser scanning (TLS; ground-based lidar) surveys over weekly, monthly and annual timescales to accurately characterise crustal ridge thrusting and collapse. This can be as much as 2 mm/day on fresh pan areas that have recently been reset by flooding. Over a two month period, this ridge growth can change aerodynamic roughness length values by 6.5 mm. At the same time, crack densities across the surface increase and this raises the availability of erodible fluffy, low density dust source sediment stored below the crust layer. Ridge spaces are defined in the early stages of crust development, as identified by Fourier Transform analysis, but wider wavelengths become more pronounced over time. We present a conceptual model accounting for the driving forces (subsurface, surface and atmospheric moisture) and feedbacks between these and surface shape that lead to crust pattern trajectories between highly emissive degraded surfaces and less emissive ridged or continuous crusts. These findings improve our understanding of temporal changes in dust availability and supply from playa source regions.

Fractal Scaling of Particle Size Distribution and Relationships with Topsoil Properties Affected by Biological Soil Crusts

PubMed Central

Gao, Guang-Lei; Ding, Guo-Dong; Wu, Bin; Zhang, Yu-Qing; Qin, Shu-Gao; Zhao, Yuan-Yuan; Bao, Yan-Feng; Liu, Yun-Dong; Wan, Li; Deng, Ji-Feng

2014-01-01

Background Biological soil crusts are common components of desert ecosystem; they cover ground surface and interact with topsoil that contribute to desertification control and degraded land restoration in arid and semiarid regions. Methodology/Principal Findings To distinguish the changes in topsoil affected by biological soil crusts, we compared topsoil properties across three types of successional biological soil crusts (algae, lichens, and mosses crust), as well as the referenced sandland in the Mu Us Desert, Northern China. Relationships between fractal dimensions of soil particle size distribution and selected soil properties were discussed as well. The results indicated that biological soil crusts had significant positive effects on soil physical structure (P<0.05); and soil organic carbon and nutrients showed an upward trend across the successional stages of biological soil crusts. Fractal dimensions ranged from 2.1477 to 2.3032, and significantly linear correlated with selected soil properties (R2 = 0.494∼0.955, P<0.01). Conclusions/Significance Biological soil crusts cause an important increase in soil fertility, and are beneficial to sand fixation, although the process is rather slow. Fractal dimension proves to be a sensitive and useful index for quantifying changes in soil properties that additionally implies desertification. This study will be essential to provide a firm basis for future policy-making on optimal solutions regarding desertification control and assessment, as well as degraded ecosystem restoration in arid and semiarid regions. PMID:24516668

Seismic structure of the crust and uppermost mantle of South America and surrounding oceanic basins

USGS Publications Warehouse

Chulick, Gary S.; Detweiler, Shane; Mooney, Walter D.

2013-01-01

We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (Pn and Sn). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ∼1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9–7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average Pn velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.

Moment of inertia of neutron star crust in alternative and modified theories of gravity

NASA Astrophysics Data System (ADS)

Staykov, Kalin V.; Ekşi, K. Yavuz; Yazadjiev, Stoytcho S.; Türkoǧlu, M. Metehan; Arapoǧlu, A. Savaş

2016-07-01

The glitch activity of young pulsars arises from the exchange of angular momentum between the crust and the interior of the star. Recently, it was inferred that the moment of inertia of the crust of a neutron star is not sufficient to explain the observed glitches. Such estimates are presumed in Einstein's general relativity in describing the hydrostatic equilibrium of neutron stars. The crust of the neutron star has a spacetime curvature of 14 orders of magnitude larger than that probed in solar system tests. This makes gravity the weakest constrained physics input in the crust-related processes. We calculate the ratio of the crustal to the total moment of inertia of neutron stars in the scalar-tensor theory of gravity and the nonperturbative f (R )=R +a R2 gravity. We find for the former that the crust-to-core ratio of the moment of inertia does not change significantly from what is inferred in general relativity. For the latter, we find that the ratio increases significantly from what is inferred in general relativity in the case of high mass objects. Our results suggest that the glitch activity of pulsars may be used to probe gravity models, although the gravity models explored in this work are not appropriate candidates.

Development of a carbonate crust on alkaline nuclear waste sludge at the Hanford site.

PubMed

Page, Jason S; Reynolds, Jacob G; Ely, Tom M; Cooke, Gary A

2018-01-15

Hard crusts on aging plutonium production waste have hindered the remediation of the Hanford Site in southeastern Washington, USA. In this study, samples were analyzed to determine the cause of a hard crust that developed on the highly radioactive sludge during 20 years of inactivity in one of the underground tanks (tank 241-C-105). Samples recently taken from the crust were compared with those acquired before the crust appeared. X-ray diffraction and scanning electron microscopy (SEM) indicated that aluminum and uranium phases at the surface had converted from (hydr)oxides (gibbsite and clarkeite) into carbonates (dawsonite and cejkaite) and identified trona as the cementing phase, a bicarbonate that formed at the expense of thermonatrite. Since trona is more stable at lower pH values than thermonatrite, the pH of the surface decreased over time, suggesting that CO 2 from the atmosphere lowered the pH. Thus, a likely cause of crust formation was the absorption of CO 2 from the air, leading to a reduction of the pH and carbonation of the waste surface. The results presented here help establish a model for how nuclear process waste can age and can be used to aid future remediation and retrieval activities. Copyright © 2017 Elsevier B.V. All rights reserved.

Diazotrophic Community Structure and Function in Two Successional Stages of Biological Soil Crusts from the Colorado Plateau and Chihuahuan Desert

USGS Publications Warehouse

Yeager, C.M.; Kornosky, J.L.; Housman, D.C.; Grote, E.E.; Belnap, J.; Kuske, C.R.

2004-01-01

The objective of this study was to characterize the community structure and activity of N2-fixing microorganisms in mature and poorly developed biological soil crusts from both the Colorado Plateau and Chihuahuan Desert. Nitrogenase activity was approximately 10 and 2.5 times higher in mature crusts than in poorly developed crusts at the Colorado Plateau site and Chihuahuan Desert site, respectively. Analysis of nifH sequences by clone sequencing and the terminal restriction fragment length polymorphism technique indicated that the crust diazotrophic community was 80 to 90% heterocystous cyanobacteria most closely related to Nostoc spp. and that the composition of N2-fixing species did not vary significantly between the poorly developed and mature crusts at either site. In contrast, the abundance of nifH sequences was approximately 7.5 times greater (per microgram of total DNA) in mature crusts than in poorly developed crusts at a given site as measured by quantitative PCR. 16S rRNA gene clone sequencing and microscopic analysis of the cyanobacterial community within both crust types demonstrated a transition from a Microcoleus vaginatus-dominated, poorly developed crust to mature crusts harboring a greater percentage of Nostoc and Scytonema spp. We hypothesize that ecological factors, such as soil instability and water stress, may constrain the growth of N2-fixing microorganisms at our study sites and that the transition to a mature, nitrogen-producing crust initially requires bioengineering of the surface microenvironment by Microcoleus vaginatus.

Crust and uppermost mantle structure of the Kyushu-Palau Ridge, remnant arc on the Philippine Sea plate

NASA Astrophysics Data System (ADS)

Nishizawa, Azusa; Kaneda, Kentaro; Oikawa, Mitsuhiro

2016-02-01

We acquired 27 wide-angle seismic profiles to investigate variation in crustal structure along the Kyushu-Palau Ridge (KPR), a 2600-km-long remnant island arc in the center of the Philippine Sea plate; 26 lines were shot across the strike of the KPR at 13°-31°N, and one was shot along the northernmost KPR. The derived P-wave velocity (Vp) models show that the KPR has a crustal thickness of 8-23 km, which is thicker than the neighboring backarc basin oceanic crusts of the West Philippine Basin to the west and the Shikoku and Parece Vela Basins to the east. While the KPR crust consists mainly of lower crusts with a Vp of 6.8-7.2 km/s, the thicker crust contains a thick middle crust with Vp of 6.0-6.8 km/s. In general, the KPR crust is thicker in the north than in the south. The uppermost mantle velocities just below the KPR bathymetric highs are lower than 8.0 km/s and are commonly associated with a slightly high Vp of 7.2 km/s at the base of the crust. Large amplitude reflection signals are sometimes observed at far offsets on several lines suggesting the existence of several reflectors at depths of 23-40 km in the mantle beneath the KPR. The characteristics of these reflections are similar to these observed beneath the Izu-Ogasawara (Bonin) island arc, the tectonically conjugate arc of the KPR before backarc basin spreading. Very thin crust and high Pn velocities characterize the transition between the KPR and the eastern basins, which is probably a relic of the initial stage of the rifting. West of the KPR, the crust varies in structure from north to south as a result of the different tectonic settings in which it evolved.

The Ivrea zone as a model for the distribution of magnetization in the continental crust

NASA Technical Reports Server (NTRS)

Wasilewski, P.; Fountain, D. M.

1982-01-01

Units are identified within the Ivrea zone of northern Italy exhibiting magnetic susceptibilities greater than 0.0005 cgs, saturation magnetization values above 0.009 emu/cu cm, and Curie points as high as 570-580 C. Amphibolites from the granulite-amphibolite facies transition, and the mafic-ultramafic granulite facies lithologies exhibit high values of initial susceptibility and saturation remanence, are laterally continuous, and may be considered as a deep crustal source for long-wavelength anomalies in low-geothermal gradient areas. Evidence is presented which suggests that such mafic-ultramafic bodies as those exposed in the Toce valley were synmetamorphic additions to the base of the crust.

Magnetization of lower oceanic crust and upper mantle

NASA Astrophysics Data System (ADS)

Kikawa, E.

2004-05-01

The location of the magnetized rocks of the oceanic crust that are responsible for sea-floor spreading magnetic anomalies has been a long-standing problem in geophysics. The recognition of these anomalies was a key stone in the development of the theory of plate tectonics. Our present concept of oceanic crustal magnetization is much more complex than the original, uniformly magnetized model of Vine-Matthews-Morley Hypothesis. Magnetic inversion studies indicated that the upper oceanic extrusive layer (Layer 2A of 0.5km thick) was the only magnetic layer and that it was not necessary to postulate any contribution from deeper parts of oceanic crust. Direct measurements of the magnetic properties of the rocks recovered from the sea floor, however, have shown that the magnetization of Layer 2A, together with the observations that this layer could record geomagnetic field reversals within a vertical section, is insufficient to give the required size of observed magnetic anomalies and that some contribution from lower intrusive rocks is necessary. Magnetization of oceanic intrusive rocks were observed to be reasonably high enough to contribute to sea-floor spreading magnetic anomalies, but were considered somewhat equivocal until late 1980Os, in part because studies had been conducted on unoriented dredged and ophiolite samples and on intermittent DSDP/ODP cores. Since ODP Leg 118 that cored and recovered continuous 500m of oceanic intrusive layer at Site 735B, Southwest Indian Ridge with an extremely high recovery of 87 percent, there have been several ODP Legs (legs 147, 153, 176, 179 and 209) that were devoted to drilling gabbroic rocks and peridotites. In terms of the magnetization intensities, all of the results obtained from these ODP Legs were supportive of the model that a significant contribution must come from gabbros and peridotites and the source of the lineated magnetic anomalies must reside in most of the oceanic crust as well as crust-mantle boundary

CHAMP Magnetic Anomalies of the Antarctic Crust

NASA Technical Reports Server (NTRS)

Kim, Hyung Rae; Gaya-Pique, Luis R.; vonFrese, Ralph R. B.; Taylor, Patrick T.; Kim, Jeong Woo

2003-01-01

Regional magnetic signals of the crust are strongly masked by the core field and its secular variations components and hence difficult to isolate in the satellite measurements. In particular, the un-modeled effects of the strong auroral external fields and the complicated- behavior of the core field near the geomagnetic poles conspire to greatly reduce the crustal magnetic signal-to-noise ratio in the polar regions relative to the rest of the Earth. We can, however, use spectral correlation theory to filter the static lithospheric and core field components from the dynamic external field effects. To help isolate regional lithospheric from core field components, the correlations between CHAMP magnetic anomalies and the pseudo magnetic effects inferred from gravity-derived crustal thickness variations can also be exploited.. Employing these procedures, we processed the CHAMP magnetic observations for an improved magnetic anomaly map of the Antarctic crust. Relative to the much higher altitude Orsted and noisier Magsat observations, the CHAMP magnetic anomalies at 400 km altitude reveal new details on the effects of intracrustal magnetic features and crustal thickness variations of the Antarctic.

Why Does Some Subducted Continental Crust Escape Deformation and Transformation?

NASA Astrophysics Data System (ADS)

Garber, J. M.; Kylander-Clark, A. R.; Stearns, M.; Seward, G.; Hacker, B. R.

2016-12-01

Titanite geochemistry and geochronology from the Western Gneiss Region (WGR) of Norway shows that large portions of continental crust were deeply subducted and exhumed without significantly deforming or transforming, indicating heterogeneous mechanical behavior of crust at mantle depths. Titanite is stable in granitic rocks over a significant P-T range, and contains numerous major, minor, and trace elements that record 1) P-T conditions, 2) interactions with other major and accessory phases, and 3) information about the composition of coeval melts and fluids. A large titanite LA-ICP-MS dataset from WGR granitic gneisses and leucosomes yields a record that depends strongly on textural setting: some titanites formed after the decomposition of other phases at mantle depths, but other titanites preserve inherited isotopic dates and chemistry with variable recrystallization at grain rims. Differences in rock textures, as well as changes in volatile abundances (F and OH) among different titanite populations, suggest that the persistence of crustal minerals at mantle depths is coupled with limited prograde fluid infiltration, which restricted the efficacy of metamorphic reactions, likely increased rock strength, and caused heterogeneities in the density of the subducting slab. The observed expanses of untransformed, undeformed granitic crust may have been critical to coupling the continental crust to underlying mantle lithosphere, which prevented significant internal deformation of the slab (e.g,. Young et al., 2007; Butler et al., 2015). Finally, our statistical approach shows the utility of dimensional reduction in geochemical studies: rather than comparing individual element or isotopic abundances or ratios, principal components or discriminant analyses can condense variables and help efficiently distinguish between distinct geologic agents or geochemical reservoirs.

238U-234U-230Th disequilibrium in hydrogenous oceanic Fe-Mn crusts: Palaeoceanographic record or diagenetic alteration?

USGS Publications Warehouse

Chabaux, F.; O'Nions, R. K.; Cohen, A.S.; Hein, J.R.

1997-01-01

A detailed TIMS study of (234Uexc/238U), (230Th/232Th), and Th/U ratios have been performed on the outermost margin of ten hydrogenous Fe-Mn crusts from the equatorial Pacific Ocean and west-central Indian Ocean. Th/U concentration ratios generally decrease from the crust's surface down to 0.5-1 mm depth and growth rates estimated by uranium and thorium isotope ratios are significantly different in Fe-Mn crusts from the Peru Basin and the west-central Indian Ocean. Fe-Mn crusts from the same geographical area define a single trend in plots of Ln (234Uexc/238U) vs. Ln(230Th/232Th) and Th/U ratios vs. age of the analysed fractions. Results suggest that (1) hydrogenous Fe-Mn crusts remain closed-systems after formation, and consequently (2) the discrepancy observed between the 230Th and 234U chronometers in Fe-Mn crusts, and the variations of the Th/U ratios through the margin of Fe-Mn crusts, are not due to redistribution of uranium and thorium isotopes after oxyhydroxide precipitation, but rather to temporal variations of both Th/U and initial thorium activity ratios recorded by the Fe-Mn layers. Implications of these observations for determination of Fe-Mn crust growth-rates are discussed. Variations of both Th/U and initial Th activity ratios in Fe-Mn crusts might be related to changes in particle input to seawater and/or changes in ocean circulation during the last 150 ka. Copyright ?? 1997 Elsevier Science Ltd.

Hydrodynamic behaviour of crusted soils in the Sahel: a possible cause for runoff increase?

NASA Astrophysics Data System (ADS)

Malam Abdou, M.; Vandervaere, J.-P.; Bouzou Moussa, I.; Descroix, L.

2012-04-01

Crusted soils are in extension in the Sahel. As rainfall has decreased over the past decades (it is now increasing again in the central Sahel) and no significant change was observed in rainfall intensity and in its time and space distribution, it is supposed that land use management is the main cause for crusts cover increase. Fallow shortening, lack of manure, and land overexploitation (wood harvesting, overgrazing) are frequently cited as main factors of soil degradation. Based on field measurements in some small catchments of Western Niger, the hydrodynamics behaviour of the newly crusted soils of this area is described, mostly constituted by erosion crusts. A strong fall in soil saturated conductivity and in the active porosity as well as a rise in bulk density all lead to a quick onset of runoff production. Results are shown from field experiments in sedimentary and basement areas leading to similar conclusions. In both contexts, runoff plot production was measured at the rain event scale from 10-m2 parcels as well as at the catchment outlet. Soil saturated conductivity was reduced by one order of magnitude when crusting occurs, leading to a sharp runoff coefficient increase, from 4% in a weeded millet field and 10% in an old fallow to more than 60% in a erosion-crusted topsoil at the plot scale. At the experimental catchment scale, runoff coefficient has doubled in less than 20 years. In pure Sahelian basins, this resulted in endorheism breaching, and in a widespread river discharge increase. For some right bank tributaries of the Niger River, discharge is three times higher now than before the drought years, in spite of the remaining rainfall deficit. On the other hand, a general increase in flooding hazard frequency is observed in the whole Sahelian stripe. The role of surface crusts in the Sahel is discussed leading to the implementation of new experiments in the future.