Science.gov

Sample records for archean spherule beds

  1. Early Archean Spherule Beds-Confirmation of Impact Origin

    NASA Technical Reports Server (NTRS)

    Shukolyukov, A.; Kyte, F. T.; Lugmair, G. W.; Lowe, D. R.; Byerly, G. R.

    2000-01-01

    The oldest record of major impact events on Earth may be a number of early Archean (3.5 to 3.2 Ga) spherule beds that have been identified in the Barberton Greenstone Belt, South Africa. Several field, petrographic, and geochemical criteria distinguish these beds from typical volcanic and clastic sediments. These criteria include the wide geographic distribution of two beds in a variety of depositional environments, the presence of relict quench textures, absence of juvenile volcaniclastic debris within the beds, and extreme enrichment of Ir and other platinum group elements (PGE) as compared to surrounding sediments. Some researchers, however, argued for a terrestrial origin for spherule bed formation, possibly related to volcanism and gold mineralization.

  2. Early Archean spherule beds of possible impact origin from Barberton, South Africa: A detailed mineralogical and geochemical study

    NASA Technical Reports Server (NTRS)

    Koeberl, Christian; Reimold, Wolf Uwe; Boer, Rudolf H.

    1992-01-01

    The Barberton Greenstone belt is a 3.5- to 3.2-Ga-old formation situated in the Swaziland Supergroup near Barberton, northeast Transvaal, South Africa. The belt includes a lower, predominantly volcanic sequence, and an upper sedimentary sequence (e.g., the Fig Tree Group). Within this upper sedimentary sequence, Lowe and Byerly identified a series of different beds of spherules with diameters of around 0.5-2 mm. Lowe and Byerly and Lowe et al. have interpreted these spherules to be condensates of rock vapor produced by large meteorite impacts in the early Archean. We have collected a series of samples from drill cores from the Mt. Morgan and Princeton sections near Barberton, as well as samples taken from underground exposures in the Sheba and Agnes mines. These samples seem much better preserved than the surface samples described by Lowe and Byerly and Lowe et al. Over a scale of just under 30 cm, several well-defined spherule beds are visible, interspaced with shales and/or layers of banded iron formation. Some spherules have clearly been deposited on top of a sedimentary unit because the shale layer shows indentions from the overlying spherules. Although fresher than the surface samples (e.g., spherule bed S-2), there is abundant evidence for extensive alteration, presumably by hydrothermal processes. In some sections of the cores sulfide mineralization is common. For our mineralogical and petrographical studies we have prepared detailed thin sections of all core and underground samples (as well as some surface samples from the S-2 layer for comparison). For geochemical work, layers with thicknesses in the order of 1-5 mm were separated from selected core and underground samples. The chemical analyses are being performed using neutron activation analysis in order to obtain data for about 35 trace elements in each sample. Major elements are being determined by XRF and plasma spectrometry. To clarify the history of the sulfide mineralization, sulfur isotopic

  3. Early Archean Spherule Beds: Chromium Isotopes Confirm Origin Through Multiple Impacts of Projectiles of Carbonaceous Chondrite Type

    NASA Technical Reports Server (NTRS)

    Kyte, Frank T.; Shukolyukov, Alex; Lugmair, Guenter W.; Lowe, Donald R.; Byerly, Gary R.

    2003-01-01

    Three Early Archean spherule beds from Barberton, South Africa, have anomalous Cr isotope compositions in addition to large Ir anomalies, confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites. The extra-terrestrial components in beds S2, S3, and S4 are estimated to be approx. l%, 50% - 60%, and 15% - 30%, respectively. These beds are probably the distal, and possibly global, ejecta from major large-body impacts. These impacts were probably much larger than the Cretaceous-Tertiary event, and all occurred over an interval of approx. 20 m.y., implying an impactor flux at 3.2 Ga that was more than an order of magnitude greater than the present flux.

  4. Early archean spherule beds in the Barberton mountain land, South Africa: Impact or terrestrial origin?

    NASA Astrophysics Data System (ADS)

    Reimold, Wolf Uwe; Koeberl, Christian; Johnson, Steven; McDonald, Iain

    The origin of multiple spherule-rich layers of millimeter to meter width, all occurring within the transition from the Fig Tree to the Onverwacht Group of the Barberton Greenstone Belt in South Africa, has been strongly debated during the last decade. One school subscribes to an origin by large meteorite impact, whereas others have preferred terrestrial processes. In particular, strong enrichments in siderophile elements, especially Ir, and chondrite-like PGE patterns for spherule layer samples have been cited as evidence favoring an impact origin. Recently, Cr isotopic signatures obtained for samples from two spherule layers have provided further support for this hypothesis. In contrast, our group has emphasized that secondary hydrothermal processes have pervasively overprinted the whole stratigraphy at this transition. Ir concentrations up to 5 times chondritic are suspect as primary impact-produced signatures. Here, we report new petrographic and geochemical data for samples from spherulitic horizons marking the S2 layer and from interlayered BIF, chert, and mudstone strata. In contrast to earlier work, the new samples were obtained from outside of the gold-sulfide mineralized ore zone on Agnes Mine. Both spherule and country rock samples are enriched in siderophile elements, with up to >1500 ppb Ir. Some of the highest values are related to clearly secondary fault and shear zone deposits. Chrome-spinel in spherule layers is often zoned. A proton microprobe study identified in one case the mineral gersdorffite, of likely secondary origin, as a carrier phase for Ir, whereas in other samples Ir must be contained in matrix silicates. New PGE analyses for more or less sulfidemineralized samples yielded uniformly flat, near-chondritic patterns.

  5. Early Archean Spherule Beds: Chromium Isotopes Confirm Origin through Multiple Impacts of Projectiles of Carbonaceous Chondrite Type: Comment and Reply

    NASA Technical Reports Server (NTRS)

    Kyte, Frank T.

    2003-01-01

    This is a exchange in the form of a comment and a reply in regards to an earlier article. The authors of the original article, consider it likely that virtually all of the projectile will condense with the silicate fraction, resulting in very little platinum group element fractionation in the final ejecta deposit. Further, we find no evidence in the commentator's, (i.e., Glikson), comment to support vapor fractionation. We note that the Pd/Ir ratios of published data on 2.56 Ga Hamersley Basin spherules are all greater than in chondrites, contrary to the assertion by Glikson. This is consistent with relatively high Pd concentrations (and Pd/Ir ratios) in crustal rocks.

  6. Early terrestrial impact events: Archean spherule layers in the Barberton Greenstone Belt, South Africa

    NASA Astrophysics Data System (ADS)

    Ozdemir, Seda; Koeberl, Christian; Schulz, Toni; Reimold, W. Uwe; Hofmann, Axel

    2015-04-01

    In addition to the oldest known impact structure on Earth, the 2.02-billion-year-old Vredefort Structure in South Africa, the evidence of Early Earth impact events are Archean spherule beds in South Africa and Australia. These spherules have been interpreted as condensation products from impact plumes and molten impact ejecta or/and impact ejecta that were melted during atmospheric re-entry [e.g., 1,2]. The 3.2-3.5 Ga spherule layers in the Barberton Greenstone Belt in South Africa currently represent the oldest known remnants of impact deposits on Earth. Aiming at identification of extraterrestrial components and to determine the diagenetic and metamorphic history of spherule layer intersections recently recovered in the CT3 drill core from the northeastern part of the Barberton Greenstone Belt, we have studied samples from these layers in terms of petrography and geochemistry. All samples, including spherule layer intersections and intercalating country rocks, were studied for mineral identification by optical and electron microscopy, as well as electron microprobe analysis (EPMA) at Natural History Museum Vienna and Museum für Naturkunde Berlin (MfN). Major and trace element compositions were determined via X-ray fluorescence spectrometry at MfN and instrumental neutron activation analysis (INAA) at University of Vienna. Os isotopes were measured by thermal ionization mass spectrometry (N-TIMS) at University of Vienna. Eighteen spherule beds are distributed over 150 meter drill core in CT3. Spherules are variably, deformed or undeformed. The high number of these layers may have been caused by tectonic duplication. Spherule beds are intercalated with shale, chert, carbonate, and/or sulfide deposits (country rocks). The size range of spherules is 0.5 to 2 mm, and some layers exhibit gradation. Shapes of spherules differ from spherical to ovoid, as well as teardrops, and spherules commonly show off-center vesicles, which have been interpreted as a primary

  7. PGE Chemistry and Systematics of Some Archean Spherule Layers in the Barberton Mountain Land

    NASA Astrophysics Data System (ADS)

    Mohr-Westheide, T.; Reimold, W. U.; Greshake, A.; Hoehnel, D.; Fritz, J.; Schmitt, R. T.; Salge, T.; Hofmann, A.; Oezdemir, S.; Schulz, T.; Koeberl, C.

    2015-07-01

    Comprehensive study of petrographic, mineralogical, and geochemical characteristics from a set of new samples of Archean spherule layers in the ICDP drill core BARB5 and drill core CT3 from the Barberton Greenstone Belt (BGB), South Africa.

  8. Discovering the Carrier Phase of the Extraterrestrial Component in Archean Spherule Layers, Barberton Greenstone Belt, South Africa

    NASA Astrophysics Data System (ADS)

    Mohr-Westheide, T.; Fritz, J.; Reimold, W. U.; Schmitt, R. T.; Hofmann, A.; Koeberl, C.; Luais, B.; Tagle, R.; Salge, T.; Hoehnel, D.

    2014-09-01

    Comprehensive study of sedimentary, petrographic, mineralogical, and geochemical characteristics from a set of new samples of Archean spherule layers in the ICDP Drill Core BARB5 from the Barite Valley.

  9. Archean spherule classification of CT3 drill core, Barberton Greenstone Belt (South Africa) based on petrography and mineral chemistry

    NASA Astrophysics Data System (ADS)

    Ozdemir, Seda; Koeberl, Christian; Mohr-Westheide, Tanja; Reimold, W. Uwe; Hofmann, Axel

    2016-04-01

    The impact history of the Early Archean Earth is not well documented. The oldest known impact structure is about 2 Ga years old; impact-related signatures in Precambrian rocks are scarce. The possible impact signature might be the Archean spherule layers that occur in the Barberton Greenstone Belt (BGB), Kaapvaal Craton, South Africa, and in the Pilbara Craton Western Australia, with ages of 3.2-3.4 and around 2.5 Ga [1]. These spherules were interpreted as impact-generated and ballistically emplaced silicate melt droplets [2]. This study is focused on petrographic and mineralogical characteristics from a set of newly drilled Archean spherule layers in drill core CT3 from the northeastern part of the BGB. The investigation of the three main intervals (A, B, and C, which include 2, 13, and 2 individual spherule layers, respectively) within CT3, contains the classification of spherules based on their shapes, textural features, deformation types, and mineral content. All of the intervals show spherule variation in those features. Therefore, the classification helps to understand if the spherules underwent processes such as tectonic deformation or if multiple impact events occurred in the area, which both might a reason of spherule layer duplications. The aim of the work is to differentiate various spherule types and the groundmasses in which they are embedded. The spherules within 17 identified spherule layers have been examined by optical microscopy (polarized and reflected) and secondary electron microscopy and were classified by shape and textural features. Subsequently, mineral phases and the chemical composition of the spherules and their matrices were investigated by using electron microprobe analysis. Regarding the shapes of the spherules they were divided into two main groups: undeformed and deformed. Undeformed spherules have spherical to ovoid as well as tear-drop shapes; deformed spherules were further subdivided into three main groups; flattened, crushed

  10. Compositional Grading in an Impact-produced Spherule Bed, Barberton Greenstone Belt, South Africa: A Key to Condensation History of Rock Vapor Clouds

    NASA Technical Reports Server (NTRS)

    Krull, A. E.; Lowe, D. R.; Byerly, G. R.

    2003-01-01

    The chemical and physical processes by which spherules form during the condensation of impact-produced rock vapor clouds are poorly understood. Although efforts have been made to model the processes of spherule formation, there is presently a paucity of field data to constrain the resulting theoretical models. The present study examines the vertical compositional variability in a single early Archean spherule bed in the Barberton Greenstone Belt (BGB), South Africa, in order to better identify the process by which impact vapor clouds condense and spherules form and accumulate. The BGB spherule beds are suitable for this type of study because of their great thickness, often exceeding 25cm of pure spherules, due to the massive sizes of the impactors. Two main problems complicate analysis of vertical compositional variability of graded spherule beds: (1) differential settling of particles in both the vapor and water column due to density and size differences and (2) turbulence within the vapor cloud. The present study compares sections of spherule bed S3 from four different depositional environments in the Barberton Greenstone Belt: (1) The Sheba Mine section (SAF-381) was deposited under fairly low energy conditions in deep water, providing a nice fallout sequence, and also has abundant Ni-rich spinels; (2) Jay's Chert section (SAF-380) was deposited in subaerial to shallow-water conditions with extensive post-depositional reworking by currents. The spherules also have preserved spinels; (3) the Loop Road section (loc. SAF-295; samp. KSA-7) was moderately reworked and has only rare preservation of spinels; and (4) the shallow-water Barite Syncline section (loc. SAF-206; samp KSA-1) has few to no spinels preserved and is not reworked. Although all of the spherule beds have been altered by silica diagenesis and K-metasomatism, most of the compositional differences between these sections appear to reflect their diagenetic histories, possibly related to their differing

  11. Inferred Primary Compositions of Archean Spherules Formed by the Condensation of an Impact-produced Rock Vapor Cloud, Barberton Greenstone Belt, South Africa

    NASA Technical Reports Server (NTRS)

    Krull, A. E.; Lowe, D. R.; Byerly, G. R.

    2003-01-01

    Based on the lunar cratering record, impacts were larger and more frequent on the early Earth than they are today. There is no persevered record of these early terrestrial impacts because rocks of this age have been obliterated by tectonism and erosion. The oldest known evidence of impacts on Earth lies in four beds (S1, S2, S3 and S4) in the Barberton Greenstone Belt (BGB), South Africa, ranging in age from about 3.24 to 3.47 Ga. These beds are composed in large part of sand-sized spherical particles, termed spherules, that are thought to have formed by the condensation of rock vapor clouds ejected above the atmosphere as a result of large impacts. Spherule beds S2 and S3 are both about 20 cm thick where composed entirely of fall-deposited spherules and up to a meter thick where spherules are mixed with locally derived debris. The diameters the bolides have been estimated to be between 20 and 50 km, based on bed thickness, size of the largest spherules, Ir fluence and extraterrestrial Cr.

  12. Impact glass spherules in the Chicxulub K-Pg event bed at Beloc, Haiti: Alteration patterns

    NASA Astrophysics Data System (ADS)

    Ritter, Xenia; Deutsch, Alexander; Berndt, Jasper; Robin, Eric

    2015-03-01

    We have investigated six impact glass spherules from the K-Pg event bed at Beloc, Haiti, using optical and electron microscopy, electron microprobe and in situ laser ablation-mass spectrometry (LA-ICP-MS; 37 trace elements, spot size 90-35 μm), in order to understand geochemical changes during alteration. The mm-sized glass spherules are partly or totally altered to smectite, but original textural features are preserved. The average trace-element composition of glass matches that one of the upper continental crust. Hints for a "meteoritic component" are lacking (Ni/Cr < 1.3 Pt below detection limit). Compared to this fresh glass, smectites are strongly depleted in trace elements, except for Li, Sc, V, Ni, Ga, Ge, and Ba. The chondrite-normalized REE distribution patterns are flat with subchondritic abundances, related to their very low degree of crystallinity. We observe a positive Eu and a strong negative Ce anomaly; the latter is explained by formation of an organic Ce4+-complex, soluble under reducing conditions. Zr/Hf of glasses and smectites is chondritic to superchondritic (35-40), whereas Nb/Ta in smectite is subchondritic (5-12) compared to Nb/Ta in the glass (~14-18). The low Nb/Ta is due to the low Nb concentrations in the smectite. Using in situ techniques with high spatial resolution, we have documented for the first time the significant changes in diagnostic elemental ratios during alteration of glass spherules. This has to be taken into account in the interpretation of geochemical data of not only impact materials but also volcanic glass, especially if bulk rock methods are used.

  13. A Good Impact Is Hard To Find (and Interpret): Lessons Learned From Studying Multiple Late Archean/Early Proterozoic Spherule Layers

    NASA Astrophysics Data System (ADS)

    Simonson, B. M.; Hassler, S. W.

    2009-12-01

    Assessing impact hazards can be approached by looking up (via real-time observations of potential threat objects) or down (finding the products and tabulating the effects of large impacts through Earth history). For the latter, craters and related structures receive the most attention, but even the largest are restricted to a very small fraction of the Earth’s surface. In contrast, impactors roughly commensurate with the end-K object generate ejecta layers that are global in extent. Although thin, distal layers can be preserved over vast areas. Moreover, preserved layers are always found in stratigraphic context, so they provide more information about planetary-scale environmental effects than do impact structures. Therefore distal ejecta layers are in many ways better “search targets” for assessing the prehistoric impact record. Since 1985, we have studied ejecta layers deposited ca. 2.63-2.49 Ga in exceptionally well preserved strata in the Hamersley Basin (Western Australia) and Griqualand West Basin (South Africa). To date we have recognized discrete layers in 8 different late Archean to earliest Proterozoic stratigraphic units, 5 in Australia and 3 in South Africa; via correlations they appear to be products of 4 large impacts. The one feature all the layers have in common is an abundance of sand-size, formerly molten silicate spherules that change little in size or composition over distances of hundreds of kilometers. This suggests the impacts responsible took place thousands of kilometers from the preserved layers. Textural and geochemical evidence suggests most or all of these impacts involved oceanic crust. The impact structures themselves have probably long since been subducted or otherwise altered beyond detection. With rare exceptions, the layers display sedimentary structures that indicate the impacts that formed them had strong environmental effects, including unusually high-energy waves and currents, the latter probably being large rip current

  14. Spherule beds 3.47-3.24 billion years old in the Barberton Greenstone Belt, South Africa: a record of large meteorite impacts and their influence on early crustal and biological evolution.

    PubMed

    Lowe, Donald R; Byerly, Gary R; Kyte, Frank T; Shukolyukov, Alexander; Asaro, Frank; Krull, Alexandra

    2003-01-01

    Four layers, S1-S4, containing sand-sized spherical particles formed as a result of large meteorite impacts, occur in 3.47-3.24 Ga rocks of the Barberton Greenstone Belt, South Africa. Ir levels in S3 and S4 locally equal or exceed chondritic values but in other sections are at or only slightly above background. Most spherules are inferred to have formed by condensation of impact-produced rock vapor clouds, although some may represent ballistically ejected liquid droplets. Extreme Ir abundances and heterogeneity may reflect element fractionation during spherule formation, hydraulic fractionation during deposition, and/or diagenetic and metasomatic processes. Deposition of S1, S2, and S3 was widely influenced by waves and/or currents interpreted to represent impact-generated tsunamis, and S1 and S2 show multiple graded layers indicating the passage of two or more wave trains. These tsunamis may have promoted mixing within a globally stratified ocean, enriching surface waters in nutrients for biological communities. S2 and S3 mark the transition from the 300-million-year-long Onverwacht stage of predominantly basaltic and komatiitic volcanism to the late orogenic stage of greenstone belt evolution, suggesting that regional and possibly global tectonic reorganization resulted from these large impacts. These beds provide the oldest known direct record of terrestrial impacts and an opportunity to explore their influence on early life, crust, ocean, and atmosphere. The apparent presence of impact clusters at 3.26-3.24 Ga and approximately 2.65-2.5 Ga suggests either spikes in impact rates during the Archean or that the entire Archean was characterized by terrestrial impact rates above those currently estimated from the lunar cratering record. PMID:12804363

  15. Spherule Beds 3.47-3.24 Billion Years Old in the Barberton Greenstone Belt, South Africa: A Record of Large Meteorite Impacts and Their Influence on Early Crustal and Biological Evolution

    NASA Technical Reports Server (NTRS)

    Lowe, Donald R.; Byerly, Gary R.; Kyte, Frank T.; Shukolyukov, Alexander; Asaro, Frank; Krull, Alexander

    2003-01-01

    Four layers, S1-S4, containing sand-sized spherical particles formed as a result of large meteorite impacts, occur in 3.47-3.24 Ga rocks of the Barberton Greenstone Belt, South Africa. Ir levels in S3 and S4 locally equal or exceed chondritic values but in other sections are at or only slightly above background. Most spherules are inferred to have formed by condensation of impact-produced rock vapor clouds, although some may represent ballistically ejected liquid droplets. Extreme Ir abundances and heterogeneity may reflect element fractionation during spherule formation, hydraulic fractionation during deposition, and/or diagenetic and metasomatic processes. Deposition of S1, S2, and S3 was widely influenced by waves and/or currents interpreted to represent impact-generated tsunamis, and S1 and S2 show multiple graded layers indicating the passage of two or more wave trains. These tsunamis may have promoted mixing within a globally stratified ocean, enriching surface waters in nutrients for biological communities. S2 and S3 mark the transition from the 300-million-year-long Onverwacht stage of predominantly basaltic and komatiitic volcanism to the late orogenic stage of greenstone belt evolution, suggesting that regional and possibly global tectonic reorganization resulted from these large impacts. These beds provide the oldest known direct record of terrestrial impacts and an opportunity to explore their influence on early life, crust, ocean, and atmosphere. The apparent presence of impact clusters at 3.26-3.24 Ga and approx. 2.65-2.5 Ga suggests either spikes in impact rates during the Archean or that the entire Archean was characterized by terrestrial impact rates above those currently estimated from the lunar cratering record.

  16. Plasma Generated Spherules

    NASA Astrophysics Data System (ADS)

    Ransom, C. J.

    2005-04-01

    Z-pinch plasma simulations have been performed that indicate the production of spherules under certain experimental parameters. (A. L. Peratt, private communication) While performing experiments dealing with the impact of plasma discharges on various materials, we observed that spherules were created at the surface of some of the materials. For specific materials and conditions, spherules were always produced. Both individual spherules and joined spherules were created. The size and shapes were nearly identical to items found by the Mars rover, Opportunity, and called ``blueberries.'' Sky & Telescope, June 2004, p. 20, among other sources indicated the blueberries were gray spherules composed of hematite. The experiments produced hematite spherules identical in appearance to those found on Mars. These experiments suggest how the newly discovered blueberries were formed on Mars while providing an explanation that does not depend on the presence of water.

  17. Noble metal abundances in an Early Archean impact deposit.

    PubMed

    Kyte, F T; Zhou, L; Lowe, D R

    1992-01-01

    We report detailed analyses on the concentrations of the noble metals Pd, Os, Ir, Pt, and Au in an early Archean spherule bed (S4) of probable impact origin from the lower Fig Tree Group, Barberton Greenstone Belt, South Africa. Compared to other sedimentary deposits of known or suspected impact origin, some noble metals are present in exceptionally high concentrations. Noble metal abundances are fractionated relative to abundances in chondrites with ratios of Os/Ir, Pt/Ir, Pd/Ir, and Au/Ir at only 80, 80, 41, and 2% of these values in CI chondrites. Although an extraterrestrial source is favored for the noble metal enrichment, the most plausible cause of the fractionation is by regional hydrothermal/metasomatic alteration. PMID:11537203

  18. Noble metal abundances in an early Archean impact deposit

    NASA Technical Reports Server (NTRS)

    Kyte, Frank T.; Zhou, Lei; Lowe, Donald R.

    1992-01-01

    Detailed analyses are reported on the concentrations of the noble metals Pd, Os, Ir, Pt, and Au in an early Archean spherule bed (S4) of probably impact origin from the lower Fig Tree Group, Barberton Greenstone Belt, South Africa. Compared to other sedimentary deposits of known or suspected impact origin, some noble metals are present in exceptionally high concentrations. Noble metal abundances are fractionated relative to abundances in chondrites with ratios of Os/Ir, Pt/Ir, Pd/Ir, and Au/Ir at only 80, 80, 41, and 2 percent of these values on CI chondrites. Although an extraterrestrial source is favored for the noble metal enrichment, the most plausible cause of the fractionation is by regional hydrothermal/metasomatic alteration.

  19. Evidence for and implications of an Early Archean terrestrial impact record

    NASA Technical Reports Server (NTRS)

    Lowe, Donald R.; Byerly, Gary R.

    1988-01-01

    Early Archean, 3.5 to 3.2 Ga, greenstone sequences in South Africa and Western Australia contain a well-preserved record of early terrestrial meteorite impacts. The main impact-produced deposits are layers, 10 cm to over 1 m thick, composed largely of sand-sized spherules, 0.1 to 4 mm in diameter. The beds studied to date show an assemblage of features indicating formation by the fall of debris from impact-generated ejecta clouds. Some presented data effectively rule out normal magmatic or sedimentary processes in the origin of these units and provide substantial support for an origin by large impacts on the early earth. The presence of at least four, remarkably thick, nearly pure spherule layers suggests that smaller-scale impact deposits may be even more abundant in these sequences. The existence of a well-preserved Archean terrestrial impact record suggests that a direct source of evidence is available regarding a number of important aspects of early earth history.

  20. Search for shock-metamorphosed grains in Precambrian spherule layers

    NASA Astrophysics Data System (ADS)

    Smith, Frank C.

    2014-10-01

    . This study has documented, apparently for the first time, a shock-induced, high-pressure polymorph (TiO2 II) from Archean terrestrial rocks. The Graenseso spherule layer (˜145 cm thick) is a dolomixtite that predominantly contains spherules and mm-to-dm-sized chert and carbonate intraclasts in a dolomitic matrix, and it records a singular debris flow within a low-energy marine environment.

  1. Diagenetic alteration of impact spherules in the Neoarchean Monteville layer, South Africa

    USGS Publications Warehouse

    Kohl, I.; Simonson, B.M.; Berke, M.

    2006-01-01

    Intercontinental correlation of distal Archean impact ejecta layers can be used to help create a global time-stratigraphic framework for early Earth events. For example, an impact spherule layer in the Neoarchean Monteville Formation (Griqualand West Basin, South Africa) may be correlated with layers in one or more formations in Western Australia. To help assess the degree to which diagenetic alteration would hinder such correlations, we performed a petrographic study of spherules in the Monteville layer. Most of the spherules in the Monteville layer have botryoidal rims composed of radial-fibrous K-feldspar, but compaction and replacement have greatly altered their appearance and mineralogy. Moreover, the Monteville spherule layer consists of three main subunits, and spherule compaction varies between subunits as well as across the Griqualand West region. Compaction is about three times greater in a medial spherulerich subunit as compared to a basal subunit rich in large intraclasts, resulting in better preservation of the shapes of melt particles in the latter. However, spherule rims have omparable numbers of fractures in both subunits, indicating the melt particles were fractured prior to compaction. Some spherules contain mica ribbons with a septarian geometry. Fracturing via rapid thermal quenching could help explain all of these features. f hot spherules possessing crystalline rims were thermally shocked when they hit the ocean, fractures would have the observed geometries and provide pathways for fluid infiltration and local replacement of glass by mica. Although heavily distorted, impact spherules in the Monteville layer are very similar to those in the Hesta occurrence of the Neoarchean Jeerinah spherule layer of the Hamersley Basin, even showing similar diagenetic histories. In this instance, diagenetic alteration may actually help rather than hinder intercontinental correlation of impact spherule layers. ??2006 Geological Society of America.

  2. Big Spherules near 'Victoria'

    NASA Technical Reports Server (NTRS)

    2006-01-01

    This frame from the microscopic imager on NASA's Mars Exploration Rover Opportunity shows spherules up to about 5 millimeters (one-fifth of an inch) in diameter. The camera took this image during the 924th Martian day, or sol, of Opportunity's Mars-surface mission (Aug. 30, 2006), when the rover was about 200 meters (650 feet) north of 'Victoria Crater.'

    Opportunity discovered spherules like these, nicknamed 'blueberries,' at its landing site in 'Eagle Crater,' and investigations determined them to be iron-rich concretions that formed inside deposits soaked with groundwater. However, such concretions were much smaller or absent at the ground surface along much of the rover's trek of more than 5 kilometers (3 miles) southward to Victoria. The big ones showed up again when Opportunity got to the ring, or annulus, of material excavated and thrown outward by the impact that created Victoria Crater. Researchers hypothesize that some layer beneath the surface in Victoria's vicinity was once soaked with water long enough to form the concretions, that the crater-forming impact dispersed some material from that layer, and that Opportunity might encounter that layer in place if the rover drives down into the crater.

  3. Spherule Size Distribution in the BARB5 ICDP Drill Core from the Barberton Greenstone Belt, South Africa

    NASA Astrophysics Data System (ADS)

    Hoehnel, D.; Mohr-Westheide, T.; Fritz, J.; Reimold, W. U.

    2014-09-01

    On the four 4 cm thick spherule layers of the ICDP BARB5 drill core grain size analysis has been performed. The grain size statistics do not indicate regular decrease of spherule sizes, so do not represent a single impact bed.

  4. Geological evidence for a 2.6-Ga strewn field of impact spherules in the Hamersley Basin of Western Australia

    NASA Technical Reports Server (NTRS)

    Simonson, Bruce M.

    1992-01-01

    Sand-sized spherules up to 1.7 mm across with spherulitic, vesicular, and other crystalline textures that consist mainly of K-feldspar help define a unique horizon in the well-preserved 2.6-Ga Wittenoom Formation in the Hamersley Basin of Western Australia. This layer is informally known as the spherule marker bed. In the northeastern part of the Hamersley Basin, similar spherules again occur at only one horizon, but here they are a minor constituent of a dolomitic debris-flow deposit known as the dolomixtite layer. The dolomixtite layer occurs in the Carawine Dolomite, which is stratigraphically equivalent to the Wittenoom Formation. Moreover, paleocurrent data from closely associated carbonate and volcaniclastic turbidites indicate the spherule marker bed was deposited in deeper-water paleoenvironments than the dolomixtite layer. Therefore, the dolomixtite layer is believed to be a proximal equivalent of the spherule marker bed. The layers that host the spherules are interpreted to be the deposits of a major sediment gravity flow that exhumed and redeposited most of the spherules after shallow burial, although the flow is not believed to have been a direct result of the proposed impact. The most likely site for the proposed impact would have been in the early Precambrian ocean close to the northeastern edge of the Pilbara Craton. Another thin horizon in the overlying Brockman Iron Formation contains spherules that again consist largely of K-feldspar and have internal textures strikingly similar to those of the Wittenoom Formation and Carawine Dolomite. The close resemblance of these spherules to those of the Wittenoom Formation and Carawine Dolomite suggests they also originated as impact melt droplets, even though they are admixed with volcaniclastic detritus. The stratigraphic separation between the two suggests that a second major impact occurred near the Hamersley Basin after a time interval of about 75 m.y. elapsed. This suggests the record of impacts in

  5. Spherule layers, crater scaling laws, and the population of ancient terrestrial impactors

    NASA Astrophysics Data System (ADS)

    Johnson, Brandon C.; Collins, Gareth S.; Minton, David A.; Bowling, Timothy J.; Simonson, Bruce M.; Zuber, Maria T.

    2016-06-01

    Ancient layers of impact spherules provide a record of Earth's early bombardment history. Here, we compare different bombardment histories to the spherule layer record and show that 3.2-3.5 Ga the flux of large impactors (10-100 km in diameter) was likely 20-40 times higher than today. The E-belt model of early Solar System dynamics suggests that an increased impactor flux during the Archean is the result of the destabilization of an inward extension of the main asteroid belt (Bottke et al., 2012). Here, we find that the nominal flux predicted by the E-belt model is 7-19 times too low to explain the spherule layer record. Moreover, rather than making most lunar basins younger than 4.1 Gyr old, the nominal E-belt model, coupled with a corrected crater diameter scaling law, only produces two lunar basins larger than 300 km in diameter. We also show that the spherule layer record when coupled with the lunar cratering record and careful consideration of crater scaling laws can constrain the size distribution of ancient terrestrial impactors. The preferred population is main-belt-like up to ∼50 km in diameter transitioning to a steep distribution going to larger sizes.

  6. Compositional Constraints on Hematite-Rich Spherule (Blueberry) Formation at Meridiani Planum, Mars

    NASA Technical Reports Server (NTRS)

    Schneider, A. L.; Mittlefehldt, D. W.; Gellert, R.; Jolliff, B.

    2007-01-01

    Meridiani Planum was chosen as the landing site for the Mars Exploration Rover Opportunity partially based on Mars Global Surveyor Thermal Emission Spectrometer data indicating an abundance of hematite. Hematite often forms through processes that involve water, so the site was a promising one to determine whether conditions on Mars were ever suitable for life. Opportunity struck pay dirt; it s Miniature Thermal Emission Spectrometer (Mini-TES) and Mossbauer Spectrometer (MB) confirmed the presence of hematite in sulfate-rich sedimentary beds and in lag deposits. Meridiani Planum rocks contain three main components: silicate phases, sulfate and possibly chloride salts, and ferric oxide phases such as hematite. Primary igneous phases are at low abundance despite the basaltic origin of the protoliths. Jarosite, an alkali ferric sulfate, was identified by Mossbauer. Some of the hematite is contained in the spherules, and some resides in finer grains in outcrops. Mossbauer and Mini-TES data indicate that hematite is a dominant constituent of the spherules. Panoramic Camera (Pancam) and Microscopic Imager (MI) images of spherule interiors show that hematite is present throughout. The exact composition of the spherules is unknown. Mini-TES only identifies a hematite signature in the spherules; any other constituents have an upper limit of 5-10% .The MB data are consistent with the spherules being composed of only hematite.

  7. Archean Microbial Mat Communities

    NASA Astrophysics Data System (ADS)

    Tice, Michael M.; Thornton, Daniel C. O.; Pope, Michael C.; Olszewski, Thomas D.; Gong, Jian

    2011-05-01

    Much of the Archean record of microbial communities consists of fossil mats and stromatolites. Critical physical emergent properties governing the evolution of large-scale (centimeters to meters) topographic relief on the mat landscape are (a) mat surface roughness relative to the laminar sublayer and (b) cohesion. These properties can be estimated for fossil samples under many circumstances. A preliminary analysis of Archean mat cohesion suggests that mats growing in shallow marine environments from throughout this time had cohesions similar to those of modern shallow marine mats. There may have been a significant increase in mat strength at the end of the Archean.

  8. Micro-spherules near the Kamil crater .

    NASA Astrophysics Data System (ADS)

    Bignami, L.; Guaita, C.; Pezzotta, F.; Zilioli, M.

    A large number of metallic micro-spheres has been found near the 45 m Kamil crater (22o 01'06''N, 26o05'15''E), produced about 5000 years ago by an extraordinary metallic meteorite (20% Nickel, 0.5% P as shreibersite). A complex SEM inspection inside and outside these micro-spherules provided some good indication about the source and the mechanism of formation of the spherules.

  9. ODP Hole 689B spherules and upper Eocene microtektite and clinopyroxene-bearing spherule strewn fields

    NASA Astrophysics Data System (ADS)

    Glass, B. P.; Koeberl, C.

    1999-03-01

    Montanari et al. (1993) reported a positive Ir anomaly in the upper Eocene sediments from Ocean Drilling Program Hole 689B on the Maud Rise, Southern Ocean. Vonhof (1998) described microtektites and clinopyroxene-bearing (cpx) spherules associated with the Ir anomaly in Hole 689B and suggested that they belong to the North American and equatorial Pacific cpx strewn fields, respectively. We searched a suite of 27 samples taken through the spherule layer from Hole 689B and we recovered 386 microtektites and 667 cpx spherules. We studied the petrography of the microtektites and cpx spherules and determined the major element compositions of 31 microtektites and 14 cpx spherules using energy dispersive x-ray analysis. We also determined the minor element compositions of eight microtektites using instrumental neutron activation analysis. We found that the peak abundance of cpx spherules is ~2 cm below the peak abundance of the microtektites (~128.7 m below sea floor), suggesting that the cpx spherule layer may be slightly older (~3-5 ka). The microtektites are mostly spherical and are generally transparent and colorless. They are similar to the North American microtektites in composition; the biggest differences being their generally lower Na2O and generally higher Zr, Ba, and Ir (up to 0.3 ppb) contents. We agree with Vonhof (1998) that the Hole 689B microtektites probably belong to the North American tektite strewn field. We calculate that the number of microtektites (>125 microns)/cm2 at Hole 689B is 52. This number is close to the concentration predicted by extrapolation of the trend of concentration versus distance from the Chesapeake Bay structure based on data from other North American microtektite-bearing sites. Thus, the North American strewn field may be at least four times larger than previously mapped. The Hole 689B cpx spherules range from translucent yellow to opaque black, but most are opaque tan to dark brown. They are generally spherical in shape and all

  10. Sr and Nd Data for Upper Eocene Spherule Layers

    NASA Technical Reports Server (NTRS)

    Liu, Shaobin; Glass, B. P.; Ngo, H. H.; Papanastassiou, D. A.; Wasserburg, G. J.

    2001-01-01

    Sr and Nd data for clinopyroxene-bearing (cpx) spherules from three sites support conclusions that there is only one cpx spherule layer and that the source crater may be Popigai. Additional information is contained in the original extended abstract.

  11. Metallic spherules in tektites from Isabela, Philippine Islands

    USGS Publications Warehouse

    Chao, E.C.T.; Adler, I.; Dwornik, E.J.; Littler, J.

    1962-01-01

    Iron-nickel spherules, as much as 0.5 mm in diameter, have been found completely embedded in some philippinites. The spherules consist mainly of kamacite with unidentified pink inclusions. The meteoritic origin of these spherules seems reasonable, suggesting that the tektites containing them were formed by asteroidal or meteoritic impact.

  12. Hematite spherules at Meridiani: Results from MI, Mini-TES, and Pancam

    NASA Astrophysics Data System (ADS)

    Calvin, W. M.; Shoffner, J. D.; Johnson, J. R.; Knoll, A. H.; Pocock, J. M.; Squyres, S. W.; Weitz, C. M.; Arvidson, R. E.; Bell, J. F.; Christensen, P. R.; de Souza, P. A.; Farrand, W. H.; Glotch, T. D.; Herkenhoff, K. E.; Jolliff, B. L.; Knudson, A. T.; McLennan, S. M.; Rogers, A. D.; Thompson, S. D.

    2008-12-01

    We report on observations of hematite-bearing spherules at Meridiani Planum made using the Microscopic Imager (MI), Mini-Thermal Emission Spectrometer (Mini-TES), and Panoramic Camera (Pancam) instruments on the Mars Exploration Rover Opportunity. Spherules were observed on soil surfaces and in outcrop rocks, both on undisturbed surfaces and in abraded surfaces ground using the Rock Abrasion Tool (RAT). Spherule size and shape change little along the 850 m eastward traverse from Eagle Crater to Endurance Crater, but spherules decrease and then slightly increase in size along the 6 km traverse from Endurance south to Victoria Crater. Local populations range from submillimeters to several millimeters in diameter. An additional small diameter (100 μm) size population is possible. An increase in irregular shapes is found near Victoria Crater. This, combined with the size decrease south of Endurance, suggests either a changing depositional environment, or variation in the duration and timing of diagenetic events. The dominant smaller size population observed early in the mission in aeolian areas and ripple crests is observed as the primary size population in abraded outcrop farther south. This suggests that successively younger beds are exposed at the surface along the southward traverse. Stratigraphically higher units removed by erosion could be recorded by the present surface lag deposit. Coordinated systematic observations are used to determine optical and infrared hematite indices of the surface soils in Pancam and Mini-TES. In spite of the systematic variation seen in MI, both Pancam and Mini-TES indices are highly variable based on the local surface, and neither show systematic trends south of Endurance. The lack of a 390 cm-1 feature in Mini-TES spectra suggests concentric or radial interior structure within the spherules at scales too fine for MI to observe. Mini-TES does not detect any silicate component in the spherules. A bound water component in soils or in

  13. Hematite spherules at Meridiani: results from MI, Mini-TES, and Pancam

    USGS Publications Warehouse

    Calvin, W.M.; Shoffner, J.D.; Johnson, J. R.; Knoll, A.H.; Pocock, J.M.; Squyres, S. W.; Weitz, C.M.; Arvidson, R. E.; Bell, J.F., III; Christensen, P.R.; de Souza, P. A., Jr.; Farrand, W. H.; Glotch, T.D.; Herkenhoff, K. E.; Jolliff, B.L.; Knudson, A.T.; McLennan, S.M.; Rogers, A.D.; Thompson, S.D.

    2008-01-01

    We report on observations of hematite-bearing spherules at Meridiani Planum made using the Microscopic Imager (MI), Mini-Thermal Emission Spectrometer (Mini-TES), and Panoramic Camera (Pancam) instruments on the Mars Exploration Rover Opportunity. Spherules were observed on soil surfaces and in outcrop rocks, both on undisturbed surfaces and in abraded surfaces ground using the Rock Abrasion Tool (RAT). Spherule size and shape change little along the 850 m eastward traverse from Eagle Crater to Endurance Crater, but spherules decrease and then slightly increase in size along the 6 km traverse from Endurance south to Victoria Crater. Local populations range from submillimeters to several millimeters in diameter. An additional small diameter (100 μm) size population is possible. An increase in irregular shapes is found near Victoria Crater. This, combined with the size decrease south of Endurance, suggests either a changing depositional environment, or variation in the duration and timing of diagenetic events. The dominant smaller size population observed early in the mission in aeolian areas and ripple crests is observed as the primary size population in abraded outcrop farther south. This suggests that successively younger beds are exposed at the surface along the southward traverse. Stratigraphically higher units removed by erosion could be recorded by the present surface lag deposit. Coordinated systematic observations are used to determine optical and infrared hematite indices of the surface soils in Pancam and Mini-TES. In spite of the systematic variation seen in MI, both Pancam and Mini-TES indices are highly variable based on the local surface, and neither show systematic trends south of Endurance. The lack of a 390 cm?1 feature in Mini-TES spectra suggests concentric or radial interior structure within the spherules at scales too fine for MI to observe. Mini-TES does not detect any silicate component in the spherules. A bound water component in soils or in

  14. Nickel-iron spherules from aouelloul glass

    USGS Publications Warehouse

    Chao, E.C.T.; Dwornik, E.J.; Merrill, C.W.

    1966-01-01

    Nickel-iron spherules, ranging from less than 0.2 to 50 microns in diameter and containing 1.7 to 9.0 percent Ni by weight, occur in glass associated with the Aouelloul crater. They occur in discrete bands of siliceous glass enriched in dissolved iron. Their discovery is significant tangible evidence that both crater and glass originated from terrestrial impact.

  15. Oxygen isotopes in deep sea spherules

    NASA Technical Reports Server (NTRS)

    Mayeda, T. K.; Clayton, R. N.; Brownlee, D. E.

    1984-01-01

    The determination of the genetic relationships between the dust and small particles in the solar system, and the meteorites and larger bodies are examined. Oxygen isotopes proved useful in the identification of such relationships between one meteorite group and another. Of the various samples of submillimeter extraterrestrial particles available for laboratory study, only the deep sea spherules are abundant enough for precise oxygen isotope analysis using existing techniques. Complications arise in interpretation of the isotopic data, since these particles were melted during passage through the Earth's atmosphere, and have been in contact with seawater for prolonged periods. Spherules that were originally silicates are considered with the originally metallic ones to deduce their preterrestrial isotopic compositions. The type 1 spherules which enter the atmosphere as metallic particles, contain only atmospheric oxygen. The type S spherules contain a mixture of atmospheric oxygen and their original extraterrestrial oxygen. It is suggested that the Earth's mesosphere is strongly enriched in heavy isotopes of oxygen at altitudes near 90 km at which the iron particles are oxidized. Fractionation due to the combined diffusion of O atoms and O2 molecules may be responsible.

  16. Method for preparing hydrous zirconium oxide gels and spherules

    DOEpatents

    Collins, Jack L.

    2003-08-05

    Methods for preparing hydrous zirconium oxide spherules, hydrous zirconium oxide gels such as gel slabs, films, capillary and electrophoresis gels, zirconium monohydrogen phosphate spherules, hydrous zirconium oxide spherules having suspendable particles homogeneously embedded within to form a composite sorbent, zirconium monohydrogen phosphate spherules having suspendable particles of at least one different sorbent homogeneously embedded within to form a composite sorbent having a desired crystallinity, zirconium oxide spherules having suspendable particles homogeneously embedded within to form a composite, hydrous zirconium oxide fiber materials, zirconium oxide fiber materials, hydrous zirconium oxide fiber materials having suspendable particles homogeneously embedded within to form a composite, zirconium oxide fiber materials having suspendable particles homogeneously embedded within to form a composite and spherules of barium zirconate. The hydrous zirconium oxide spherules and gel forms prepared by the gel-sphere, internal gelation process are useful as inorganic ion exchangers, catalysts, getters and ceramics.

  17. Origin and diagenesis of K/T impact spherules -- From Haiti to Wyoming and beyond

    NASA Astrophysics Data System (ADS)

    Bohor, B. F.; Glass, B. P.

    1995-03-01

    altered Type 1 melt-droplet bodies. The spherules at both locations were deposited in a melt-ejecta layer as part of the KIT impact event. Previously, two types of relict impact glasses had been identified in the Haitian spherule beds: black glass of andesitic composition and high-Ca yellow glass with an unusually high S content. Most workers agree that the latter probably formed by impact melting and mixing of surficial carbonate (and minor anhydrite) rocks with the more deeply-buried crystalline parent rocks of the black glasses. However, some workers have suggested that an intermediate compositional gap exists between the two groups of glasses, implying a different origin than simple mixing of end members during impact. We report glass compositional analyses with values extending throughout this intermediate range, lending support to the impact-mixing model. Inclusions of CaSO4 found by us in relict yellow glasses further support this model.

  18. The Archean kerogen paradox

    NASA Astrophysics Data System (ADS)

    Ohmoto, H.

    2012-12-01

    The constituents of organic matter buried in sediments (kerogen) are classified into three types: (1) liable components that may be decomposed by aerobic and anaerobic microbes during the early diagenesis of sediments, and by thermal decomposition during the burial at ~3-5 km depths (T = 100 ~150°C) to generate bitumen (oil); (2) refractory components that may decompose into gaseous components at greater depths (T = 150-230°C); and (iii) inert components that may be converted to graphite during high temperature metamorphism. Laboratory experiments and field observations indicate that matured kerogen (i.e., refractory and inert components) is decomposed during the weathering under an oxygenated atmosphere by reaction C + O2 → CO2, promoted by aerobic organisms. This has resulted in the general absence of old detrital kerogen (except debris of vascular plants) in Phanerozoic-aged sedimentary rocks. In contrast, matured kerogen would not be decomposed during the weathering under a reducing atmosphere, because the reactions C + 2H2 → CH4, C + 2H2O → CO2 + 2H2 and 2C + 2H2O → CO2 + CH4 would not proceed at low temperatures even with the aid of anaerobic organisms. If such reactions could occur at low temperatures, sedimentary rocks, regardless of their age, would have lost all their kerogen before being buried to depths ~3 km. If the Archean atmosphere had been reducing, as postulated by the dominant paradigm of the early Earth, detrital kerogen should be ubiquitously present in Archean-aged sedimentary rocks. We should also find general increasing trends in both the ratio of detrital/syngenetic kerogens and the total amount of reduced C (syngenetic and detrital kerogens) in sedimentary rocks from ~3.8 Ga to ~2.5 Ga in age. Because the detrital kerogen had been subjected to metamorphism and weathering before being transported to the oceans, detrital kerogen would have different structures, textures, and elemental and isotopic ratios compared to the syngenetic

  19. Submicroscopic spherules and color of tektites

    USGS Publications Warehouse

    Thorpe, A.N.; Senftle, F.E.

    1964-01-01

    Magnetic susceptibility measurements of 18 tektites from various strewn fields have been made as a function of temperature from 77??K to room temperature. A relatively large temperature-independent component of the magnetic susceptibility was observed in all cases, and an analysis of the data shows that this component is the result of submicroscopic iron spherules in the tektites. An analysis of the color of tektites in terms of the magnetic measurements and also of the optical absorption spectra suggests that the basic color of all tektites is green or greenish-blue, and that the brown to black coloration in some tektites is due to finely dispersed Fe2O3 and/or many metallic spherules, both probably of colloidal size. ?? 1964.

  20. Examining Archean methanotrophy

    NASA Astrophysics Data System (ADS)

    Slotznick, Sarah P.; Fischer, Woodward W.

    2016-05-01

    The carbon isotope ratios preserved in sedimentary rocks can be used to fingerprint ancient metabolisms. Organic carbon in Late Archean samples stands out from that of other intervals with unusually low δ13C values (∼-45 to -60‰). It was hypothesized that these light compositions record ecosystem-wide methane cycling and methanotrophy, either of the aerobic or anaerobic variety. To test this idea, we studied the petrography and carbon and oxygen isotope systematics of well-known and spectacular occurrences of shallow water stromatolites from the 2.72 Ga Tumbiana Formation of Western Australia. We examined the carbonate cements and kerogen produced within the stromatolites, because methanotrophy is expected to leave an isotopic fingerprint in these carbon reservoirs. Mathematical modeling of Archean carbonate chemistry further reveals that methanotrophy should still have a discernible signature preserved in the isotopic record, somewhat diminished from those observed in Phanerozoic sedimentary basins due to higher dissolved inorganic carbon concentrations. These stromatolites contain kerogen with δ13Corg values of ∼ - 50 ‰. By microsampling different regions and textures within the stromatolites, we determined that the isotopic compositions of the authigenic calcite cements show a low degree of variation and are nearly identical to values estimated for seawater at this time; the lack of low and variable δ13Ccarb values implies that methanotrophy does not explain the low δ13Corg seen in the coeval kerogen. These observations do not support a methanotrophy hypothesis, but instead hint that the Late Archean may constitute an interval wherein autotrophs employed markedly different biochemical processes of energy conservation and carbon fixation.

  1. Analysis of Cosmic Spherule Candidates from the Kwajalein Micrometeorite Collection

    NASA Technical Reports Server (NTRS)

    Wozniakiewicz, P. J.; Price, M. C.; Bradley, J. P.; Ishii, H. A.; Russell, S. S.; Zolensky, M. E.; Brownlee, D. E.

    2015-01-01

    The Kwajalein micrometeorite collection utilised high volume air samplers fitted with 5 micrometer laser-etched polycarbonate membrane filters to capture particles directly from the atmosphere. The filters were changed weekly over several months throughout 2011/12, providing the opportunity to investigate the contemporary flux of micrometeorites. We recently reported the results of our initial survey of cosmic spherule-like particles on several of these filters. We identified three main groups of particle based on bulk compositions: 1. Silicate spherules rich in Mg, Ca and Fe, 2. Silicate spherules rich in Al, Ca, K and/or Na and 3. Fe-rich spherules. Abundances appeared to change over time suggesting links with celestial activity (e.g. meteor showers), however, spherules similar to groups 2 and 3 can be produced by terrestrial and anthropogenic activity (e.g. volcanic microspherules exhibit similar compositions to group 2 spherules and metallic spherules similar to those of group 3 can be formed during fuel combustion). We are now studying the internal structures and chemistries of these spherules and comparing against cosmic spherules identified in other collections to confrim their origins and further contrain the contemporary micrometeorite flux. Particles are being picked, embedded in resin and polished through to reveal their interiors. Here we will describe our ongoing analyses of these particles via SEM. We will also introduce our new collection using this method that is currently being performed in the Antarctic.

  2. Archean geotherms and supracrustal assemblages

    NASA Astrophysics Data System (ADS)

    Condie, Kent C.

    1984-06-01

    Metamorphic mineral assemblages suggest the existence of variable geotherms and lithospheric thicknesses beneath late Archean continental crust. Archean granite-greenstone terranes reflect steep geotherms (50-70°C/km) while high-grade terranes reflect moderate geotherms similar to present continental crust with high heat flow (25-40°C/km). Corresponding lithosphere thicknesses for each terrane during the late Archean are 35-50 km and 50-75 km, respectively. Early Archean (⩾ 3.0 b.y.) greenstones differ from late Archean (˜ 2.7 b.y.) greenstones by the rarity or absence of andesite and graywacke and the relative abundance of pelite, quartzite, and komatiite. Mature clastic sediments in early greenstones reflect shallow-water, stable-basin deposition. Such rocks, together with granite-bearing conglomerate and felsic volcanics imply the existence of still older granitic source terranes. The absence or rarity of andesite in early greenstones reflects the absence of tectonic conditions in which basaltic and tonalitic magmas are modified to produce andesite. A model is presented in which early Archean greenstones form at the interface between tonalite islands and oceanic lithosphere, over convective downcurrents; high-grade supracrustals form on stable continental edges or interiors; and late Archean greenstones form in intracontinental rifts over mantle plumes.

  3. Lunar and Planetary Science XXXV: Antarctic and Spherule Studies

    NASA Technical Reports Server (NTRS)

    2004-01-01

    The session "Antarctic and Spherule Studies" included the following reports:In Situ Magnetic Identification and Classification of Meteorites in Antarctica; Pb, Nd and Sr Isotopes in Aerosols Extracted from Snow, Berkner Island, Antarctica; Olivine Textures and Compositions in BIT-58 Ablation Debris; and Extraterrestrial Spherules with Fe-Ni Core and Pt Group Nuggets in Pleistocene Sediment from Hungary.

  4. Metal spherules in Wabar, Monturaqui, and Henbury impactites. [iron meteorites

    NASA Technical Reports Server (NTRS)

    Gibbons, R. V.; Horz, F.; Thompson, T. D.; Brownlee, D. E.

    1976-01-01

    The Fe, Ni, and Co contents of 450 spherules with diameters of 3 to 100 microns in impact glasses from three terrestrial meteorite craters are determined by electron microprobe analysis. The objects investigated include 250 spherules from Wabar (Saudi Arabia) impactite and 100 each from Monturaqui (Chile) and Henbury (Australia) impactites. The specimens and spherules are described, noting that the Wabar and Henbury craters were produced in sandstone while the Monturaqui crater was formed in siliceous igneous rocks and has an enigmatic sulfide abundance. It is found that the spherules are enriched in Ni and Co relative to the original meteorite Fe content and are enriched in Ni relative to Co for Ni contents greater than 50%. The results indicate that, on the average, Henbury spherules are more Ni-enriched than Monturaqui spherules, and Wabar spherules exhibit the least Ni-enrichment. It is suggested that the spherules could have formed without experiencing free flight and that instantaneous dissemination of meteoritic material at the meteorite-target contact was the major fractionation process.

  5. Method for preparing hydrous iron oxide gels and spherules

    DOEpatents

    Collins, Jack L.; Lauf, Robert J.; Anderson, Kimberly K.

    2003-07-29

    The present invention is directed to methods for preparing hydrous iron oxide spherules, hydrous iron oxide gels such as gel slabs, films, capillary and electrophoresis gels, iron monohydrogen phosphate spherules, hydrous iron oxide spherules having suspendable particles homogeneously embedded within to form composite sorbents and catalysts, iron monohydrogen phosphate spherules having suspendable particles of at least one different sorbent homogeneously embedded within to form a composite sorbent, iron oxide spherules having suspendable particles homogeneously embedded within to form a composite of hydrous iron oxide fiber materials, iron oxide fiber materials, hydrous iron oxide fiber materials having suspendable particles homogeneously embedded within to form a composite, iron oxide fiber materials having suspendable particles homogeneously embedded within to form a composite, dielectric spherules of barium, strontium, and lead ferrites and mixtures thereof, and composite catalytic spherules of barium or strontium ferrite embedded with oxides of Mg, Zn, Pb, Ce and mixtures thereof. These variations of hydrous iron oxide spherules and gel forms prepared by the gel-sphere, internal gelation process offer more useful forms of inorganic ion exchangers, catalysts, getters, dielectrics, and ceramics.

  6. Hydrothermal Synthesis of Hematite-Rich Spherules: Implications for Diagenesis and Hematite Spherule Formation in Outcrops at Meridiani Planum, Mars

    NASA Technical Reports Server (NTRS)

    Golden, D. C.; Ming, D. W.; Morris, R. V.; Graff, T. G.

    2007-01-01

    The Athena science payload onboard the Opportunity rover identified hematite-rich spherules (mean diameter of 4.2 +/- 0.8 mm) embedded in outcrops and occurring as lag deposits at Meridiani Planum. They have formed as diagenetic concretions from the rapid breakdown of pre-existing jarosite and other iron sulfates when chemically distinct groundwater passed through the sediments. Diagenetic, Fe-cemented concretions found in the Jurassic Navajo Formation, Utah and hematite-rich spherules found within sulfate-rich volcanic breccia on Mauna Kea volcano, Hawaii are possible terrestrial analogues for Meridiani spherules. The Navajo Formation concretions form in porous quartz arenite from the dissolution of iron oxides by reducing fluids and subsequent Fe precipitation to form spherical Fe- and Si-rich concretions. The Mauna Kea spherules form by hydrothermal, acid-sulfate alteration of basaltic tephra. The formation of hematite-rich spherules with similar chemical, mineralogical, and morphological properties to the Meridiani spherules is rare on Earth, so little is known about their formation conditions. In this study, we have synthesized in the laboratory hematite-rich spherules that are analogous in nearly all respects to the Meridiani spherules.

  7. The Bombardment of the Earth During the Hadean and Early Archean Eras

    NASA Astrophysics Data System (ADS)

    Marchi, S.; Bottke, W. F.; Elkins-Tanton, L. T.; Morbidelli, A.; Wuennemann, K.; Kring, D. A.; Bierhaus, M.

    2013-12-01

    Our knowledge of the Earth during the Hadean and early Archean eons (ca 4.5-3.5 Ga) is very limited, mainly because few rocks older than 3.8 Ga have been found (e.g. Harrison 2009). Hadean-era zircons have allowed us to glean important insights into this era, but their data has led to considerably different evolution models for the evolution of the early Earth; some predict a hellish world dominated by a molten surface with a sporadic steam atmosphere (e.g. Pollack 1997), while others have predicted a tranquil, cool surface with stable oceans (e.g. Wilde et al 2001; Valley et al 2002). To understand whether either model (or both) could be right, we believe it is useful to quantitatively examine the post Moon-forming impact bombardment of the early Earth. Over the last several years, through a combination of observations (e.g., Marchi et al 2012), theoretical models (e.g., Bottke et al 2012), and geochemical constraints from lunar rock (e.g. highly siderophile elements -HSE- abundances delivered to the Moon by impactors; the global number of lunar basins; the record of Archean-era impact spherule beds on Earth; Walker 2009; Neumann et al 2012), we have constructed a calibrated model of the early lunar impactor flux (Morbidelli et al 2012). Our results have now been extrapolated to the Earth, where they can make predictions about its early bombardment. Using a Monte Carlo code to account for the stochastic nature of major impacts, and constraining our results by the estimated HSE abundances of Earth's mantle (that were presumably delivered by impactors; Walker 2009; Bottke et al. 2010), we find the following trends. In the first ~100-200 Myr after the formation of the Moon, which we assume was created ~4.5 Ga, the Earth was almost entirely resurfaced by impacts. This bombardment, which included numerous D > 1000 km diameter impactors, should have vigorously mixed the crust and upper mantle. Between ~4.1-4.3 Ga, the impactor flux steadily decreased; though an uptick

  8. An Archean Biosphere Initiative

    NASA Technical Reports Server (NTRS)

    Anbar, A. D.; Boyd, E. S.; Buick, R.; Claire, M.; DesMarais, D.; Domagal-Goldman, D.; Eigenbrode, J.; Erwin, D.; Freeman, K.; Hazen, R.; Johnson, C.; Lyons, T.; Meadows, V.; Ohmoto, H.; Ono, S.; Peters, J. W.; Shapiro, B.; Summons, R.; Walter, M.

    2011-01-01

    The search for life on extrasolar planets will necessarily focus on the imprints of biolgy on the composition of planetary atmospheres. The most notable biological imprint on the modern terrestrial atmosphere is the presence of 21 % O2, However, during most of the past 4 billion years, life and the surface environments on Earth were profoundly different than they are today. It is therefore a major goal of the astrobiology community to ascertain how the O2 content of the atmosphere has varied with time. and to understand the causes of these variations. The NAI and NASA Exobiology program have played critical roles in developing our current understanding of the ancient Earth's atmosphere, supporting diverse observational, analytical, and computational research in geoscience, life science, and related fields. In the present incarnation of the NAI, ongoing work is investigating (i) variations in atmospheric O2 in the Archean to the Cambrian, (ii) characterization of the redox state of the oceans shortly before, during and after the Great Oxidation Event (GOE), and (iii) unraveling the complex connections between environmental oxygenation, global climate, and the evolution of life.

  9. Numbers, types, and compositions of an unbiased collection of cosmic spherules

    NASA Astrophysics Data System (ADS)

    Taylor, Susan; Lever, James H.; Harvey, Ralph P.

    2000-07-01

    Micrometeorites collected from the bottom of the South Pole Water Well (SPWW) may represent a complete, well-preserved sample of the cosmic dust that accreted on Earth from 1100-1500 AD. We classified 1588 cosmic spherules in the size range 50-800 μm. The collection has 41% barred olivine spherules, 17% glass spheres, 12% cryptocrystalline spherules, 11% porphyritic olivine spherules, 12% relic grain-bearing spherules, 3% scoriaceous spherules, 2% I-type spherules, 1% Ca-Al-Ti-rich (CAT) spherules and 1% G-type spherules. We also found bubbly glass spherules, spherules with glass caps, and ones with sulfide coatings, particles that are absent from other collections. A classification sequence of the stony spherules (scoriaceous, relic grain bearing, porphyritic, barred olivine, cryptocrystalline, glass, and CAT) is consistent with progressive heating and evaporation of Fe from chondritic materials. The modern-day accretion rate and size distribution measured at the SPWW can account for the stony spherules present in deep-sea collection through preferential dissolution of glass and small stony spherules. However, weathering alone cannot account for the high accretion rate of I-type spherules determined for two deep-sea collections. The SPWW collection provides data to constrain models of atmospheric-entry heating and to assess the effects of terrestrial weathering.

  10. Earth's Archean Impact Record In The ICDP Drilling "Barberton Mountain Land".

    NASA Astrophysics Data System (ADS)

    Fritz, Jörg; Schmitt, Ralf-Thomas; Reimold, Uwe; Koeberl, Christian; Mc Donald, Ian; Hofmann, Axel; Luais, Beatrice

    2013-04-01

    The marine meta-sedimentary successions in the "Barberton Mountain Land" are formed by Archean volcanic and sedimentary rocks including the oldest known impact ejecta layers on Earth. The chemical signature (high iridium concentrations, chromium isotopic ratios) of some of these up to tens of cm thick Archean spherule layers advocate that these ejecta deposits represent mainly extraterrestrial material [1]. These ejecta layers contain millimetre sized spherules that are larger and accumulated thicker layers compared to any impact ejecta layer known from Phanerozoic sediments, including the global ejecta layer of the Chicxulub impact catering event terminating the Mesozoic era of Earth's history [2]. The Archean spherule layers are interpreted as products of large impacts by 20 to >100 km diameter objects [3, 4]. Identifying traces of mega-impacts in Earth's ancient history could be of relevance for the evolution of atmosphere, biosphere, and parts of the Earth's crust during that time. In addition, recognizing global stratigraphic marker horizons is highly valuable for inter-correlating sedimentary successions between Archean cratons [5]. However estimates regarding size of the impact event and correlations between the different outcrops in the Barberton mountain land are complicated by post depositional alterations of the tectonically deformed sediments [6, 7]. The relatively fresh samples recovered from below the water table during the 2011-2012 ICDP drilling "Barberton Mountain Land" are promising samples to investigate and to discriminate primary and secondary features of these rare rocks. We plan to conduct 1) petrographic, micro-chemical and mineralogical characterization of the impact ejecta layers, 2) bulk chemical analyses of major and trace elements, and 3) LAICP- MS elemental mapping of platinum group element (PGE) distributions. and elemental analyses of moderately siderophile elements. This aims at 1) characterization of the ejecta layers, 2

  11. Proximal ejecta deposits of the K-Pg Chixulub impact: The case for carbonate impact melt spherules

    NASA Astrophysics Data System (ADS)

    Deutsch, Alex; Schulte, Peter

    2010-05-01

    When 65.5 million yrs. ago an about ~10 km-sized asteroid hit Earth (Chicxulub impact event), ejecta was distributed world-wide to form the Cretaceous-Paleogene (K-Pg) event bed. Continuous sections across this K-Pg boundary document unambiguously that the 'K-T' mass extinction was triggered by the Chicxulub event, not only because of the projectile's size but particularly by the specific composition of the target, namely a 3-km- (in the West) to about 4.5-km-thick (in the Gulf area) layer of volatile-rich carbonate and sulfate platform sediments on top of the crystalline basement (Schulte et al., 2010). Modeling, petrographic and geochemical studies on natural samples as well as experimental results show that shock pressure and high post-shock temperatures yield irreversible deformations and transformations on carbonate and sulfate target lithologies which are also expected to occur in the context of the Chicxulub event. Twofold devastating effects on life are predicted (i) dissociation of carbonates and sulfates with nearly instantaneous release of vast quantities of CO2, and of about 100 to 500 Gt sulfur triggering severe climate effects, and (ii) deposition of carbonate and sulfate melts together with silicate melts, causing short-term disruption of the thermal conditions proximal to the crater. Silicate impact glasses with high CaO contents occur, for example, as spherules in the K-Pg event bed at Haiti and in melt lithologies from drill cores (e.g., Yucatan-6, Chicxulub-1, Yaxcopoil-1). Carbonate melt glasses have not been reported so far in K-Pg event beds. Hence, the following question was answered so far: Where are the huge amounts of carbonates that suffered impact metamorphism and were ejected in the Chicxulub event? The Chicxulub ejecta deposits in the Gulf of Mexico area contain up to 80 wt% carbonates which, however, have been interpreted as precipitation product during diagenesis. In consequence, the abundant mm-sized ejecta spherules consisting of a

  12. Archean sedimentary systems and crustal evolution

    NASA Technical Reports Server (NTRS)

    Lowe, D. R.

    1985-01-01

    Current knowledge of preserved Archean sedimentary rocks suggests that they accumulated in at least three major depositional settings. These are represented generally by sedimentary units: (1) in early Archean, pre-3.0 Ga old greenstone belts, (2) on late Archean sialic cratons, and (3) in late Archean, post-3.0 Ga old greenstone belts. Research suggests that the Archean was characterized by at least two distinctive and largely diachronous styles of crustal evolution. Thick, stable early Archean simatic platforms, perhaps analogous to modern oceanic islands formed over hot spots, underwent a single cycle of cratonization to form stable continental blocks in the early Archean. Later formed Archean continents show a two stage evolution. The initial stage is reflected in the existence of older sialic material, perhaps representing incompletely cratonized areas or microcontinents of as yet unknown origin. During the second stage, late Archean greenstone belts, perhaps analogous to modern magmatic arcs or back arc basins, developed upon or adjacent to these older sialic blocks. The formation of this generation of Archean continents was largely complete by the end of the Archean. These results suggest that Archean greenstone belts may represent a considerable range of sedimentological and tectonic settings.

  13. Iridium and Spherules in Late Eocene Impact Deposits

    NASA Technical Reports Server (NTRS)

    Kyte, F. T.; Liu, S.

    2002-01-01

    We have been independently examining the Ir (FTK) and spherule (SL) contents of recently discovered late Eocene impact deposits from the south Atlantic and western Indian oceans. These include ODP Sites 1090 [14,15], 709 [lo], and 699 [Liu in prep.]. Iridium abundances at these sites are within the typical range reported for late Eocene deposits, with peak concentrations between 100 and 1000 pg/g. In Table 1 we present estimated net Ir fluences (in ng Ir/cm ) for these and nine other sites. Although there are fewer sites than the K/T boundary, the average of 9 ng Ir/cm2 is probably a good estimate of the late Eocene global flux. This is enough Ir for a 6 km comet (assuming 250 ng/g Ir, p=1.5), is sufficient to produce the Popigai or Chesapeake Bay structures, and is 16% of the flux estimated for the K/T boundary (55 ng/cm2 [ 161). Figure 1 shows the relative abundances of Ir, glassy microtektites and cpx-bearing spherules in sediments from Sites 699 and 1090, which are separated by only 3100 km. Although these two sites have similar Ir anomalies, the abundances of spherules are quite different. Site 1090 has well-defined peaks for both types of spherules, with a peak of 562 cpx spheruledg, while Site 699 contains only a few glassy microtektites and no cpx spherules. While the different abundances of spherules may reflect a heterogeneous distribution of spherules on the Earth s surface, an equally likely cause of this difference may be differential preservation of spherules in the sediment. recovered are only a trace residue of the initial impact deposit. Earlier work found 0.22 ng/g Ir in glassy microtektites from Site 689 [17], an insufficient concentration to support 0.16 ng/g in the bulk sediment at this site. We measured 15 ng/g Ir in a group of 95 cpx spherules from Site 1090 with sizes from 63 to -200 pm, a set typical of the size distribution at this site. Although this is a significant concentration it also cannot support the Ir peak. We presently lack

  14. K/T spherules from Haiti and Wyoming: Origin, diagenesis, and similarity to some microtektites

    NASA Technical Reports Server (NTRS)

    Bohor, B. F.; Glass, B. P.; Betterton, W. J.

    1993-01-01

    Spherules with relict glass cores in the K/T boundary bed of Haiti allow for a comparison of these bodies with hollow goyazite shells in the K/T boundary claystone of Wyoming and with younger microtektites of the Ivory Coast strewn field. Samples of the Haitian beds from undisturbed sections at Beloc, as determined by Jehanno et al., contain both hollow shells and relict glass cores rimmed by palagonite that has been partially converted to smectite. These palagonite rims developed from hydration zones formed when hot, splash-form droplets of andesitic impact glass were deposited into water. Mutual collisions between these droplets in the ejecta curtain may have formed point-source stresses on their surfaces. Initiation of hydration would be facilitated at these surface stress points and propagated radially into the glass. The inner surface of these merged hemispherical fronts appears mammillary, which is reflected as scalloping in Haitian relict glass cores.

  15. Upper Eocene Spherules at ODP Site 1090B

    NASA Technical Reports Server (NTRS)

    Liu, S.; Kyte, F. T.; Glass, B. P.; Gersonde, R.

    2000-01-01

    Our two labs independently discovered upper Eocene microtektites and microkrystites at ODP Site 1090, a new South Atlantic locality near the Agulhus Ridge. This is a significant new data point for the strewn fields of these spherules, which were recently extended into the Atlantic sector of the Southern Ocean when they were reported at ODP Site 689 on the Maude Rise. The microtektites have been regarded as related to North American tektites and the microkrystites as belonging to the clinopyroxene-bearing (cpx) spherule strewn field. Initial reports indicate that Site 1090 contains a complete sequence of upper Eocene sediments composed of diatom and nannofossil oozes. The magneto- and bio-stratigraphy indicate that impact-age sediments should occur in core 30X of Hole 1090B. One of us (FTK) took 2 cc samples at 10 cm intervals over 600 cm of core for Ir analyses and the senior author (SL) took 3 cc samples at 20 cm intervals to search for spherules. Both studies proved successful and additional samples were obtained to confirm initial results and better define the Ir anomaly and spherule abundances. Peak Ir concentrations of 0.97 ng/g were found at 1090B-30X-5, 105-106cm and 0.78 ng/g at 115-116 cm. Anomalous Ir concentrations (greater than 0.1 ng/g) extend over about 100 cm of core. Preliminary results indicate that the excess Ir at this site is about 25 ng per sq cm. About 380 microtektites (>63 pm) and 2492 microkrystites (>63 pm) were recovered over a 1.8 m interval with a peak abundance of microtektites (106/gram) and microkrystites (562/gram) at 1090B-30X- 5, 114-115 cm. The largest microtektite is approximately 960 x 1140 micron in size. About 55 % are spherical, and the rest are disc, cylinder, dumbbell, teardrop, or fragments. Most of the microtektites are transparent colorless, but a few are transparent pale brown or green. Preliminary data indicate that the microtektites at Site 1090 have similar major oxide compositions to those at Site 689. About 50% of

  16. Morphological Diversity of the Rod Spherule: A Study of Serially Reconstructed Electron Micrographs

    PubMed Central

    Li, Shuai; Mitchell, Joe; Briggs, Deidrie J.; Young, Jaime K.; Long, Samuel S.; Fuerst, Peter G.

    2016-01-01

    Purpose Rod spherules are the site of the first synaptic contact in the retina’s rod pathway, linking rods to horizontal and bipolar cells. Rod spherules have been described and characterized through electron micrograph (EM) and other studies, but their morphological diversity related to retinal circuitry and their intracellular structures have not been quantified. Most rod spherules are connected to their soma by an axon, but spherules of rods on the surface of the Mus musculus outer plexiform layer often lack an axon and have a spherule structure that is morphologically distinct from rod spherules connected to their soma by an axon. Retraction of the rod axon and spherule is often observed in disease processes and aging, and the retracted rod spherule superficially resembles rod spherules lacking an axon. We hypothesized that retracted spherules take on an axonless spherule morphology, which may be easier to maintain in a diseased state. To test our hypothesis, we quantified the spatial organization and subcellular structures of rod spherules with and without axons. We then compared them to the retracted spherules in a disease model, mice that overexpress Dscam (Down syndrome cell adhesion molecule), to gain a better understanding of the rod synapse in health and disease. Methods We reconstructed serial EM images of wild type and DscamGoF (gain of function) rod spherules at a resolution of 7 nm in the X-Y axis and 60 nm in the Z axis. Rod spherules with and without axons, and retracted spherules in the DscamGoF retina, were reconstructed. The rod spherule intracellular organelles, the invaginating dendrites of rod bipolar cells and horizontal cell axon tips were also reconstructed for statistical analysis. Results Stereotypical rod (R1) spherules occupy the outer two-thirds of the outer plexiform layer (OPL), where they present as spherical terminals with large mitochondria. This spherule group is highly uniform and composed more than 90% of the rod spherule

  17. Dynamic Mechanochemistry of Seismic Slip -Nano Spherules Lubrication

    NASA Astrophysics Data System (ADS)

    Tanaka, H.; Chen, W.; Chen, Y.; Song, Y.; Ma, K.

    2007-12-01

    The Chelungpu fault, which was activated during 1999Chi-Chi Earthquake, had been drilled (Hole A, B and C) to recover the earthquake slip zone materials. We present here the results of nano-scale observations for identified slip zone materials (Ma, Tanaka et al., 2006) by using HR-TEM and TXM technique. Minimum size of grains observed under HR-TEM is 3 nm. The grain size distribution for grains larger than 100 nm in diameter follows the fractal law and grain shape is highly irregular. Grains smaller than100 nm show some specific characteristics, that is, smaller the grains, more the spherical shapes and more equi-granular. Thus, the grains smaller than 100 nm are no longer described by fractal distribution model. By SAD and EDX analysis under HR-TEM, the nano spherules are mainly composed of crystallized quartz associated with minor amounts of carbonates and amorphous materials. Results of observations lead following three conclusions, (1) nano spherules are not generated just by fracturing based on their shapes and grain size distributions. (2) nano spherules would compose viscous materials enveloping larger fractured grains from SEM observations. (3) Mica clay minerals and feldspars are disappeared in ultra-fine grained layer. This implies that chemical process of dissolution - elements dissipation - SiO2 precipitation occurred associated with mechanical fracturing. Therefore nano spherules would be generated through mechano-chemical process during co-seismic slip. Dynamic shear strength drop by rapid slip experimentsare and formation of gelled materials are recently reported. Large differences of ultra-fine products between previous reports and our observations are existence of nano spherules and their crystallinity. If the nano- spherules are generated during seismic slip, dynamic weakening would be expected because mode of friction turns into rolling friction by huge amounts of equigranular and spherical grains. This may be alternative explanations for dynamic

  18. Extraterrestrial spherules in glacial sediment from the Transantarctic Mountains, Antarctica - Structure, mineralogy, and chemical composition

    NASA Technical Reports Server (NTRS)

    Koeberl, Christian; Hagen, Erik H.

    1989-01-01

    The morphological, mineralogical, and chemical characteristics of silicate and glassy spherules from a number of new localities in the Transantarctic Mountains were determined. The results indicate that these Antarctic spherules are of extraterrestrial origin. It is concluded that they, most likely, represent individual micrometeorites that melted during passage through the atmosphere; alternatively, they may be ablation spherules.

  19. Synthetic (Hydrothermal) Hematite-Rich Mars-Analog Spherules from Acid-Sulfate Brines: Implications for Formation and Diagenesis of Hematite Spherules in Outcrops at Meridiani Planum, Mars

    NASA Technical Reports Server (NTRS)

    Golden, D. C.; Ming, D. W.; Morris, R. V.; Graff, T. G.

    2008-01-01

    The Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) orbiter discovered a large area at Meridiani Planum (MP) covered with the Fe-oxide hematite (alpha-Fe2O3) [1,2]. This discovery and favorable landing site characteristics led to selection of MP as the landing site for the Opportunity Mars Exploration Rover (MER) [3]. The Athena science payload onboard the Opportunity rover identified hematite-rich spherules (mean spherule diameter approx.4.2+/-0.8 mm) embedded in S-rich outcrop rock and also as lag deposits of whole and broken spherules [4,5,6,7,8,9]. Although the chemical and mineralogical compositions of spherules are not fully constrained, Moessbauer spectrometer (MB) Miniature Thermal Emission Spectrometer (Mini-TES) and chemical analyses from the Alpha Particle X-Ray Spectrometer (APXS) are consistent with a hematite mineralogical composition and an oxide bulk chemical composition consisting of Fe2O3. MGS-TES, also provides an important constraint that emission from the hematite-rich spherules is dominated by emission along the crystallographic c-axis [1,2,10,11]. The formation of hematite-rich spherules with similar chemical, mineralogical, morphological, and crystallographic properties to the MP spherules is rare on Earth, to date, only two natural analogs have been proposed; one from Utah (Navaho Concretions) and the other from Mauna Kea, Hawaii [12,13]. In this study, we synthesized in the laboratory hematite-rich spherules using conditions that may have existed on Early Mars [14] and compared their properties to those for MP hematite spherules of Mars and the analog spherules from Utah and Mauna Kea in order to assess their relative merit as MP hematite spherule analogs. Such comparisons yield clues to the formation pathway for MP spherules.

  20. Extraterrestrial spherules in glacial sediment from the Transantarctic Mountains, Antarctica: Structure, mineralogy, and chemical composition

    SciTech Connect

    Koeberl, C. Lunar and Planetary Institute, Houston, TX ); Hagen, E.H. )

    1989-04-01

    Silicate and glassy spherules of probably extraterrestrial origin have been discovered in glacial till from localities in the Beardmore glacier region, Transantarctic Mountains, Antarctica. These spherules are similar in structure, appearance, and composition to spherules found in deep-sea sediments and in Greenland. Most spherules have a shiny and smooth dark surface, while some show a characteristic brickwork pattern composed of intergrown olivine crystals and magnetite dendrites. Several spherules have been sectioned after trace element analysis by INAA. The interiors of the spherules contain olivine crystals of varying size (predominantly of forsteritic composition) set in a glassy Fe-rich and Mg-poor matrix. The matrix contains very fine-grained dendritic magnetite crystals and rare accessory wustite. SEM and microprobe studies show that most of the spherules have not experienced significant weathering. Trace element analyses for individual particles confirm the extraterrestrial nature of the spherules. They have undifferentiated chondritic rare earth element (REE) patterns and are enriched in Ir and other siderophile elements (with Ir contents between 10 and > 1,000 ppb). Some spherules have experienced loss of volatile elements and some differentiation of siderophile elements (e.g., Ni). The Antarctic spherules--which might be micrometeorites that melted during passage through the atmosphere, or ablation spherules--are a significant addition to the collection of extraterrestrial microparticles. Of key importance is their high abundance in the sediment and the fact that they have been concentrated by glacial action.

  1. Are Archean lithospheric keels inverted?

    NASA Astrophysics Data System (ADS)

    Percival, J. A.; Pysklywec, R. N.

    2007-02-01

    Post-tectonic granite "blooms" in many Archean cratons manifest widespread crustal melting that cannot be explained by lithosphere removal, considering the long-term buoyancy of Archean cratonic keels and low mantle geotherms indicated by Archean diamondiferous lithosphere. Rather, heat may have been transferred from the mantle through a process of lithosphere inversion, driven by basal tractive forces acting on gravitationally unstable lithosphere. The top-heavy cell comprises eclogitic lower crust ( ρ = 3500 kg/m 3) lying above depleted mantle lithosphere ( ρ = 3300 kg/m 3), whose aggregate density remained less than that of surrounding asthenosphere ( ρ = 3340 kg/m 3). Parameterized numerical models of the inversion process show a > 40 m.y. pulse of maximum 1060 °C temperatures in the lower crust, sufficient to drive anhydrous melting, when overturned basal lithosphere reaches the Moho. In equilibrating to lower lithosphere conditions, the eclogitic cap may have yielded siliceous melts that infiltrated overlying, previously depleted mantle, producing the high Si/Mg characteristic of some cratonic peridotites. P-T paths calculated for the central part of the inverting cell criss-cross the graphite-diamond boundary, explaining development of large diamond crystals through numerous growth increments. The craton stabilization process follows terminal tectonism by about 50 m.y. as a result of initial cooling, eclogite formation and lithosphere stiffening. Related consequences in the crust include low-pressure regional metamorphism and widespread hydrothermal effects including some gold mineralization. Craton stability is attributed to the high strength of the depleted mantle lithosphere, its rectified density profile, and the presence of refractory compositions at the lithosphere-asthenosphere boundary.

  2. Geochemistry of Cenozoic microtektites and clinopyroxene-bearing spherules

    NASA Astrophysics Data System (ADS)

    Glass, Billy P.; Huber, Heinz; Koeberl, Christian

    2004-10-01

    We have determined the major and trace element compositions of 176 individual microtektites/spherules from the Australasian, Ivory Coast, and North American microtektite and clinopyroxene-bearing (cpx) spherule layers. Trace element contents for up to 30 trace elements were determined by instrumental neutron activation analysis (INAA), and major element compositions were determined using energy dispersive X-ray (EDX) analysis in combination with a scanning electron microscope (SEM). In addition, petrographic data were obtained for the cpx spherules using the SEM and EDX. This is the first trace element study of individual Australasian microtektites, and the data revealed the presence of a previously unrecognized group of Australasian microtektites with high contents of Ni (up to 471 ppm). In previous studies the high-Mg (HMg) Australasian microtektites were thought to be related to the HMg Australasian tektites, but our trace element data suggest that the high-Ni (HNi) Australasian microtektites, rather than the high-Mg microtektites, are related to the high-Mg Australasian tektites. We find that Cenozoic microtektites/spherules from a given layer can be distinguished from microtektites/spherules from other layers as a group, but it is not always possible to determine which layer an individual microtektite/spherule came from based only on trace element compositions. The cpx spherules and most of the microtektites have Cr, Co, and Ni contents that are higher than the average contents of these elements in the upper continental crust, suggesting the presence of a meteoritic component. The highest Cr, Co, and Ni contents are found in the cpx spherules (and low-Si cpx-related microtektites). Unetched to slightly etched cpx spherules have Ni/Cr and Ni/Co ratios that generally lie along mixing curves between the average upper continental crust and chondrites. The best fit appears to be with an LL chondrite. The moderately to heavily etched cpx spherules have values that

  3. Cretaceous-tertiary boundary spherules and Cenozoic microtektites: Similarities and differences

    NASA Technical Reports Server (NTRS)

    Glass, B. P.; Bohor, Bruce F.; Betterton, William J.

    1993-01-01

    Bohor and Betterton pointed out that the K-T spherules can be divided into three groups. Their Type 1 spherules appear to be found in or adjacent to North America, particularly the Western Interior and in Haiti and Mexico. The Type 1 spherules occur in the lower part of the K-T boundary clay below an Ir anomaly. It is the Type 1 spherules which are most similar to microtektites. The discovery of K-T boundary spherules in Beloc, Haiti, and Mimbral, Mexico, with residual tektite-like glass cores supports the hypothesis that the Type 1 spherules are diagenetically altered microtektites. The similarities and differences of the Type 1 K-T boundary spherules to previously described Cenozoic microtektites are discussed.

  4. 10Be and 26Al in Individual Cosmic Spherules

    NASA Astrophysics Data System (ADS)

    Nishiizumi, K.; Arnold, J. R.; Chaffee, M. W.; Finkel, R. C.; Southon, J.; Brownlee, D. E.; Harvey, R. P.

    1992-07-01

    Cosmic spherules and fragments in the size range 0.1-1 mm, originally extracted magnetically from deep-sea deposits, have now been isolated from ice in Greenland and Antarctica (Maurette et al., 1986; Koeberl and Hagen, 1989; Harvey and Maurette, 1990). Studies of cosmogenic radionuclides in individual spherules made possible by accelerator mass spectrometry (Raisbeck et al., 1985; Nishiizumi et al., 1991, 1992), have verified the model calculations that suggested that the spherules are mainly fused micrometeoroids, rather than spall droplets from larger objects (Dohnanyi, 1978; Grun et al., 1985; Olinger et al., 1990). However, the exposure lifetimes observed are much longer than those deduced from models (10^5- 10^7 years as against a few times 10^4) (Dohnanyi, 1978; Grun et al., 1985). As new sources of spherules become available, it is important to verify their extraterrestrial origin, and to see whether they confirm or modify earlier findings. It is also interesting to continue to search for spall droplets. In this paper we report on a group of large spherules collected from glacial till near Lewis Cliff, Antarctica (84.3 degrees S, 161.6 degrees E) (Harvey and Maurette, 1990). The concentrations of cosmogenic ^10Be and ^26Al were measured in 10 such spherules (LC-7 to LC-16). Typically these spherules have a lower Fe content, and are more weakly magnetic, than those studied earlier. In addition to these spherules, we analyzed 8 individual deep sea spherules (90-250 micrograms) (Murrell et al., 1980). All particles were individually mounted in acrylic resin and a small surface was polished flat with aluminum oxide. The quantitative elemental analysis of these polished surfaces was performed using an electron microprobe. After electron microprobe analysis, each particle was dissolved and Be and Al were separated for AMS. The ^10Be and ^26Al concentrations were determined at LLNL. In this and a preceding group of LC and deep-sea particles, three (LC-6, reported

  5. New insights from old spherules: Os-W isotope and HSE evidence for Paleoarchean meteorite bombardment of the Earth

    NASA Astrophysics Data System (ADS)

    Schulz, T.; Luguet, A. A.; Koeberl, C.

    2014-12-01

    Introduction: Although still debated, spherule beds in the Barberton Mountain Land (~3.4 Ga) are suspected to represent remnants of impact-generated and ballistically emplaced silicate melt droplets [e.g. 1]. Such deposits provide the only window into the late stages of the heavy meteorite bombardment on Earth as their source craters have long since been obliterated. In order to identify a possible meteoritic component and, if successful, to discuss potential projectile materials, we are performing a detailed Os-W isotope as well as HSE abundance study on spherule layers from the recently drilled ICDP BARB5 core (grid location 25°30`50.76``S, 31°33`10.08``E). Samples and Methods: Samples were taken from a spherule-containing meta-sedimentary core section discovered between 510 and 512 m depth. About 100 mg of homogenized sample powders were spiked with a mixed 190Os, 185Re, 191Ir and 194Pt tracer and treated in a high pressure asher using inverse aqua regia, followed by conventional extraction schemes for Os and the other HSEs [4]. Chemical and Os isotope measurements (via N-TIMS) were performed in Vienna, whereas HSE measurements were undertaken via ICP MS in Bonn. Results and Discussion: Our preliminary Os isotope data reveal a trend between samples exhibiting high spherule to matrix ratios (187Os/188Os ~0.106 and Os ~0.4 ppm) and samples with lower ones (187Os/188Os up to ~0.304 and Os ~0.008 ppm). Notably, the most unradiogenic samples exhibit carbonaceous-chondrite-like initial 187Os/188Os and HSE ratios, whereas all other samples are clear non-chondritic. These findings support an extraterrestrial contribution in the spherules and can be interpreted compared to conclusions drawn from a Cr isotope study performed on similar samples [3], possibly representing a different impact event and favouring a chondritic projectile. However, further considerations based on precise Os/W ratio determinations and high-precision 182W isotope data, will be presented at the

  6. Primary magmatic carbon in ureilites - Evidence from cohenite-bearing metallic spherules

    NASA Technical Reports Server (NTRS)

    Goodrich, C. A.; Berkley, J. L.

    1986-01-01

    The compositions and mineralogy of metal-sulfide spherules trapped in ureilites are studied. Electron microprobe analyses of metallic phases and sulfides in spherules were conducted and backscattered electron images of metallic spherules in olivine and in ureilites are presented. It is observed that the spherules are composed of cohenite, Fe-Ni metal, troilite, and rare phosphorus-bearing minerals. The intergranular metal and graphite in the ureilites are analyzed. The phase relations of cohenite-bearing spherules are considered. Spherules represent Fe-Ni-C-S alloys that exist as immiscible metallic liquid droplets in the silicate magma from which ureilite olivine and pigeonite crystallized. The data reveal that graphite crystals occur as inclusions in olivine and ureilites and therefore graphite in ureilites is a primary magnetic component. It is hypothesized that graphite is a residue from the solid source materials that melted to produce the ureilite parent magma or crystallized from the magma.

  7. Gene expression in human fungal pathogen Coccidioides immitis changes as arthroconidia differentiate into spherules and mature

    PubMed Central

    2013-01-01

    Background Coccidioides immitis is a dimorphic fungus that causes disease in mammals, including human beings. It grows as a mycelium containing arthroconidia in the soil and in the host arthroconidia differentiates into a unique structure called a spherule. We used a custom open reading frame oligonucleotide microarray to compare the transcriptome of C. immitis mycelia with early (day 2) and late stage (day 8) spherules grown in vitro. All hybridizations were done in quadruplicate and stringent criteria were used to identify significantly differentially expressed genes. Results 22% of C. immitis genes were differentially expressed in either day 2 or day 8 spherules compared to mycelia, and about 12% of genes were differentially expressed comparing the two spherule time points. Oxireductases, including an extracellular superoxide dismutase, were upregulated in spherules and they may be important for defense against oxidative stress. Many signal transduction molecules, including pleckstrin domain proteins, protein kinases and transcription factors were downregulated in day 2 spherules. Several genes involved in sulfur metabolism were downregulated in day 8 spherules compared to day 2 spherules. Transcription of amylase and α (1,3) glucan synthase was upregulated in spherules; these genes have been found to be important for differentiation to yeast in Histoplasma. There were two homologs of 4-hydroxyphenylpyruvate dioxygenase (4-HPPD); transcription of one was up- and the other downregulated. We tested the effect of a 4-HPPD inhibitor, nitisinone, on mycelial and spherule growth and found that it inhibited mycelial but not spherule growth. Conclusions Transcription of many genes was differentially expressed in the process of arthroconidia to spherule conversion and spherule maturation, as would be expected given the magnitude of the morphologic change. The transcription profile of early stage (day 2) spherules was different than late stage (day 8) endosporulating

  8. The Archean Nickel Famine Revisited.

    PubMed

    Konhauser, Kurt O; Robbins, Leslie J; Pecoits, Ernesto; Peacock, Caroline; Kappler, Andreas; Lalonde, Stefan V

    2015-10-01

    Iron formations (IF) preserve a history of Precambrian oceanic elemental abundance that can be exploited to examine nutrient limitations on early biological productivity. However, in order for IF to be employed as paleomarine proxies, lumped-process distribution coefficients for the element of interest must be experimentally determined or assumed. This necessitates consideration of bulk ocean chemistry and which authigenic ferric iron minerals controlled the sorption reactions. It also requires an assessment of metal mobilization reactions that might have occurred in the water column during particle descent and during post-depositional burial. Here, we summarize recent developments pertaining to the interpretation and fidelity of the IF record in reconstructions of oceanic trace element evolution. Using an updated compilation, we reexamine and validate temporal trends previously reported for the nickel content in IF (see Konhauser et al., 2009 ). Finally, we reevaluate the consequences of methanogen Ni starvation in the context of evolving views of the Archean ocean-climate system and how the Ni famine may have ultimately facilitated the rise in atmospheric oxygen. PMID:26426143

  9. Were kinetics of Archean calcium carbonate precipitation related to oxygen concentration?

    NASA Technical Reports Server (NTRS)

    Sumner, D. Y.; Grotzinger, J. P.

    1996-01-01

    Archean carbonates commonly contain decimetre- to metre-thick beds consisting entirely of fibrous calcite and neomorphosed fibrous aragonite that precipitated in situ on the sea floor. The fact that such thick accumulations of precipitated carbonate are rare in younger marine carbonates suggests an important change in the modes of calcium carbonate precipitation through time. Kinetics of carbonate precipitation depend on the concentration of inhibitors to precipitation that reduce crystallization rates and crystal nuclei formation, leading to kinetic maintenance of supersaturated solutions. Inhibitors also affect carbonate textures by limiting micrite precipitation and promoting growth of older carbonate crystals on the sea floor. Fe2+, a strong calcite-precipitation inhibitor, is thought to have been present at relatively high concentrations in Archean seawater because oxygen concentrations were low. The rise in oxygen concentration at 2.2-1.9 Ga led to the removal of Fe2+ from seawater and resulted in a shift from Archean facies, which commonly include precipitated beds, to Proterozoic facies, which contain more micritic sediment and only rare precipitated beds.

  10. Mechanisms for Planetary Spherules Formation and Alteration: Salar Grande, Chile -- An Example of Volcanic/Aqueous Processes Interactions

    NASA Astrophysics Data System (ADS)

    Ukstins Peate, I.; Cabrol, N. A.; Grin, E. A.; French, R.; Dressing, C.; Franklin, T.; Parsons, K.; Piatek, J. L.; Chong, G.

    2009-03-01

    Silica nodules and hematite spherules are observed at Salar Grande and Monturaqui, Atacama Desert, Chile. The Planetary Spherules Project investigates formation, deposition and alteration processes as analogs to Gusev Crater and Meridiani, Mars.

  11. Method for preparing hydrous titanium oxide spherules and other gel forms thereof

    DOEpatents

    Collins, J.L.

    1998-10-13

    The present invention are methods for preparing hydrous titanium oxide spherules, hydrous titanium oxide gels such as gel slabs, films, capillary and electrophoresis gels, titanium monohydrogen phosphate spherules, hydrous titanium oxide spherules having suspendible particles homogeneously embedded within to form a composite sorbent, titanium monohydrogen phosphate spherules having suspendible particles of at least one different sorbent homogeneously embedded within to form a composite sorbent having a desired crystallinity, titanium oxide spherules in the form of anatase, brookite or rutile, titanium oxide spherules having suspendible particles homogeneously embedded within to form a composite, hydrous titanium oxide fiber materials, titanium oxide fiber materials, hydrous titanium oxide fiber materials having suspendible particles homogeneously embedded within to form a composite, titanium oxide fiber materials having suspendible particles homogeneously embedded within to form a composite and spherules of barium titanate. These variations of hydrous titanium oxide spherules and gel forms prepared by the gel-sphere, internal gelation process offer more useful forms of inorganic ion exchangers, catalysts, getters and ceramics. 6 figs.

  12. Method for preparing hydrous titanium oxide spherules and other gel forms thereof

    DOEpatents

    Collins, Jack L.

    1998-01-01

    The present invention are methods for preparing hydrous titanium oxide spherules, hydrous titanium oxide gels such as gel slabs, films, capillary and electrophoresis gels, titanium monohydrogen phosphate spherules, hydrous titanium oxide spherules having suspendible particles homogeneously embedded within to form a composite sorbent, titanium monohydrogen phosphate spherules having suspendible particles of at least one different sorbent homogeneously embedded within to form a composite sorbent having a desired crystallinity, titanium oxide spherules in the form of anatase, brookite or rutile, titanium oxide spherules having suspendible particles homogeneously embedded within to form a composite, hydrous titanium oxide fiber materials, titanium oxide fiber materials, hydrous titanium oxide fiber materials having suspendible particles homogeneously embedded within to form a composite, titanium oxide fiber materials having suspendible particles homogeneously embedded within to form a composite and spherules of barium titanate. These variations of hydrous titanium oxide spherules and gel forms prepared by the gel-sphere, internal gelation process offer more useful forms of inorganic ion exchangers, catalysts, getters and ceramics.

  13. Archean sedimentary styles and early crustal evolution

    NASA Technical Reports Server (NTRS)

    Lowe, D. R.

    1986-01-01

    The distinctions between and implications of early and late Archean sedimentary styles are presented. Early Archean greenstone belts, such as the Barberton of South Africa and those in the eastern Pilbar Block of Australia are characterized by fresh or slightly reworked pyroclastic debris, orthochemical sediments such as carbonates, evaporites, and silica, and biogenic deposits including cherts and stromatolitic units. Terrigenous deposits are rare, and it is suggested that early Archean sediments were deposited on shallow simatic platforms, with little or no components derived from sialic sources. In contrast, late Archean greenstone belts in the Canadian Shield and the Yilgarn Block of Australia contain coarse terrigenous clastic rocks including conglomerate, sandstone, and shale derived largely from sialic basement. Deposition appears to have taken place in deepwater, tectonically unstable environments. These observations are interpreted to indicate that the early Archean greenstone belts formed as anorogenic, shallow water, simatic platforms, with little or no underlying or adjacent continental crust, an environment similar to modern oceanic islands formed over hot spots.

  14. Green spherules from Apollo 15 - Inferences about their origin from inert gas measurements.

    NASA Technical Reports Server (NTRS)

    Lakatos, S.; Yaniv, A.; Heymann, D.

    1973-01-01

    Green spherules from the 'clod' 15426 and from fines 15421 contain about 100 times less trapped inert gases than normal bulk fines from Apollo 15. These spherules have apparently never been directly exposed to the solar wind. Spherules from other fines contain about 10 times more trapped gas than those from the 'clod.' The gas in the former is surface correlated. However, spherules from fines 15401 are exceptionally gas-poor. The trapped gases can be of solar-wind origin, but this origin requires a two-stage model for the spherules from the clods. Another possibility is that the gases were absorbed from an ambient gas phase. The trapped gases may also be assumed to represent primordial lunar gas. The composition of this gas is then similar to the 'solar' or 'unfractionated' component of gas-rich meteorites, but unlike that in most of the carbonaceous chondrites.

  15. Iron and nickel isotopic mass fractionation in deep-sea spherules

    NASA Technical Reports Server (NTRS)

    Davis, Andrew M.; Brownlee, Donald E.

    1993-01-01

    Magnetite-wuestite spherules collected from deep-sea sediments are thought to have originally been Fe-Ni metal particles at the top of the atmosphere that were oxidized and melted during entry into the earth's atmosphere. Some likely sources for the metal particles are Fe-Ni interplanetary dust particles (IDP's) and metal or sulfide from stony IDP's that separated after melting. Davis et al. reported that four of these spherules are enriched in the heavy isotopes of iron, with enrichments of 8-23%/amu. We have developed a technique for analysis of both iron and nickel isotopes on the same ion microprobe spot and have applied this technique to a number of deep-sea spherules in order to better understand the processes leading to isotopic mass fractionation. Eight spherules show iron and nickel isotopic mass fractionation, with iron and nickel enriched in the heavy isotopes by 10-19%/amu and 4-32%/amu, respectively. If the mass fractionations are due to Rayleigh fractionation during evaporation, these spherules lost 76-94% of their original mass. We have analyzed the four magnetite-wuestite spherules for which iron isotopic data were reported by Davis et al. as well as four new spherules.

  16. Template RNA Length Determines the Size of Replication Complex Spherules for Semliki Forest Virus

    PubMed Central

    Kallio, Katri; Hellström, Kirsi; Balistreri, Giuseppe; Spuul, Pirjo; Jokitalo, Eija

    2013-01-01

    The replication complexes of positive-strand RNA viruses are always associated with cellular membranes. The morphology of the replication-associated membranes is altered in different ways in different viral systems, but many viruses induce small membrane invaginations known as spherules as their replication sites. We show here that for Semliki Forest virus (SFV), an alphavirus, the size of the spherules is tightly connected with the length of the replicating RNA template. Cells with different model templates, expressed in trans and copied by the viral replicase, were analyzed with correlative light and electron microscopy. It was demonstrated that the viral-genome-sized template of 11.5 kb induced spherules that were ∼58 nm in diameter, whereas a template of 6 kb yielded ∼39-nm spherules. Different sizes of viral templates were replicated efficiently in trans, as assessed by radioactive labeling and Northern blotting. The replication of two different templates, in cis and trans, yielded two size classes of spherules in the same cell. These results indicate that RNA plays a crucial determining role in spherule assembly for SFV, in direct contrast with results from other positive-strand RNA viruses, in which either the presence of viral RNA or the RNA size do not contribute to spherule formation. PMID:23760239

  17. Major and trace element geochemistry of S-type cosmic spherules

    NASA Astrophysics Data System (ADS)

    Rudraswami, N. G.; Shyam Prasad, M.; Babu, E. V. S. S. K.; Vijaya Kumar, T.

    2016-04-01

    Micrometeorites that pass through the Earth's atmosphere undergo changes in their chemical compositions, thereby making it difficult to understand if they are sourced from the matrix, chondrules, or calcium-aluminum-rich inclusions (CAIs). These components have the potential to provide evidence toward the understanding of the early solar nebular evolution. The variations in the major element and trace element compositions of 155 different type (scoriaceous, relict bearing, porphyritic, barred, cryptocrystalline, and glass) of S-type cosmic spherules are investigated with the intent to decipher the parent sources using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry. The S-type cosmic spherules appear to show a systematic depletion in volatile element contents, but have preserved their refractory trace elements. The trends in their chemical compositions suggest that the S-type spherules comprise of components from similar parent bodies, that is, carbonaceous chondrites. Large fosteritic relict grains observed in this investigation appear to be related to the fragments of chondrules from carbonaceous chondrites. Furthermore, four spherules (two of these spherules enclose spinels and one comprised entirely of a Ca-Al-rich plagioclase) show enhanced trace element enrichment patterns that are drastically different from all the other 151 cosmic spherules. The information on the chemical composition and rare earth elements (REEs) on cosmic spherules suggest that the partially to fully melted ones can preserve evidences related to their parent bodies. The Ce, Eu, and Tm anomalies found in the cosmic spherules have similar behavior as that of chondrites. Distinct correlations observed between different REEs and types of cosmic spherules reflect the inherited properties of the precursors.

  18. Ridge-trench collision in Archean and Post-Archean crustal growth: Evidence from southern Chile

    NASA Technical Reports Server (NTRS)

    Nelson, E. P.; Forsythe, R. D.

    1988-01-01

    The growth of continental crust at convergent plate margins involves both continuous and episodic processes. Ridge-trench collision is one episodic process that can cause significant magmatic and tectonic effects on convergent plate margins. Because the sites of ridge collision (ridge-trench triple junctions) generally migrate along convergent plate boundaries, the effects of ridge collision will be highly diachronous in Andean-type orogenic belts and may not be adequately recognized in the geologic record. The Chile margin triple junction (CMTJ, 46 deg S), where the actively spreading Chile rise is colliding with the sediment-filled Peru-Chile trench, is geometrically and kinematically the simplest modern example of ridge collision. The south Chile margin illustrates the importance of the ridge-collision tectonic setting in crustal evolution at convergent margins. Similarities between ridge-collision features in southern Chile and features of Archean greenstone belts raise the question of the importance of ridge collision in Archean crustal growth. Archean plate tectonic processes were probably different than today; these differences may have affected the nature and importance of ridge collision during Archean crustal growth. In conclusion, it is suggested that smaller plates, greater ridge length, and/or faster spreading all point to the likelihood that ridge collision played a greater role in crustal growth and development of the greenstone-granite terranes during the Archean. However, the effects of modern ridge collision, and the processes involved, are not well enough known to develop specific models for the Archean ridge collison.

  19. Siderophile element fractionation in meteor crater impact glasses and metallic spherules

    NASA Technical Reports Server (NTRS)

    Mittlefehldt, David W.; See, T. H.; Scott, E. R. D.

    1993-01-01

    Meteor Crater, Arizona provides an opportunity to study, in detail, elemental fractionation processes occurring during impacts through the study of target rocks, meteorite projectile and several types of impact products. We have performed EMPA and INAA on target rocks, two types of impact glass and metallic spherules from Meteor Crater. Using literature data for the well studied Canyon Diablo iron we can show that different siderophite element fractionations affected the impact glasses than affected the metallic spherules. The impact glasses primarily lost Au, while the metallic spherules lost Fe relative to other siderophile elements.

  20. Discovery of coesite and shocked quartz associated with the upper Eocene cpx spherule layer

    NASA Technical Reports Server (NTRS)

    Liu, S.; Kyte, T.; Glass, B. P.

    2002-01-01

    At least two major impact ejecta layers have been discovered in upper Eocene strata. The upper layer is the North American microtektite layer. lt consists tektite fragments, microtektites, and shocked mineral grains (e.g., quartz and feldspar with multiple sets of PDFs, coesite and reidite (a high-pressure polymorph of zircon)). The slightly older layer contains clinopyroxene-bearing (cpx) spherules and microtektites associated with an Ir anomaly. The North American tektite layer may be derived from the Chesapeake Bay impact structure, and the cpx spherule layer may from the Popigai impact crater. A cpx spherule layer associated with a positive Ir anomaly was recently found at ODP Site 709, western Indian Ocean. A large sample (Hole 709C, core 31, section 4, 145-150 cm), originally used for a study of interstitial water by shipboard scientists, was acquired for the purpose of recovering a large number of spherules for various petrographic and geochemical studies. A split of the sample (50.35 g) was disaggregated and wet-sieved. More than 17,000 cpx spherules and several hundred microtektites (larger than 125 microns) were recovered from the sample. Rare white opaque grains were observed in the 125-250 micron size fraction after removal of the carbonate component using dilute HCI. Seven of the white opaque grains were X-rayed using a Gandolfi camera and six were found to be coesite (probably mixed with lechatelierite). Eighty translucent colorless grains from the 63-125 micron size fraction were studied with a petrographic microscope. Four of the grains exhibit one to two sets of planar deformation features (PDFs). The only other possible known occurrence of shocked minerals associated with the cpx spherule layer is at Massignano, Italy, where pancake-shaped clay spherules (thought to be diagenetically altered cpx spherules are associated with a positive Ir anomaly and Ni- rich spinel crystals. Shocked quartz grains with multiple sets of PDFs also occur at this site

  1. Venus and the Archean Earth: Thermal considerations

    NASA Technical Reports Server (NTRS)

    Sleep, N. H.

    1989-01-01

    The Archean Era of the Earth is not a direct analog of the present tectonics of Venus. In this regard, it is useful to review the state of the Archean Earth. Most significantly, the temperature of the adiabatic interior of the Earth was 200 to 300 C hotter than the current temperature. Preservation biases limit what can be learned from the Archean record. Archean oceanic crust, most of the planetary surface at any one time, has been nearly all subducted. More speculatively, the core of the Earth has probably cooled more slowly than the mantle. Thus the temperature contrast above the core-mantle boundary and the vigor of mantle plumes has increased with time on the Earth. The most obvious difference between Venus and the present Earth is the high surface temperature and hence a low effective viscosity of the lithosphere. In addition, the temperature contrast between the adiabatic interior and the surface, which drives convection, is less on Venus than on the Earth. It appears that the hot lithosphere enhanced tectonics on the early Venus significantly enough that its interior cooled faster than the Earth's. The best evidence for a cool interior of Venus comes from long wavelength gravity anomalies. The low interior temperatures retard seafloor spreading on Venus. The high surface temperatures on Venus enhance crustal deformation. That is, the lower crust may become ductile enough to permit significant flow between the upper crust and the mantle. There is thus some analogy to modern and ancient areas of high heat flow on the Earth. Archean crustal blocks typically remained stable for long intervals and thus overall are not good analogies to the deformation style on Venus.

  2. Carbon and Oxygen Isotopic Measurements of K/T Boundary Spherules from Haiti

    NASA Astrophysics Data System (ADS)

    Hough, R. M.; Sigurdsson, H.; Franchi, I. A.; Wright, I. P.; Pillinger, C. T.; Gilmour, I.

    1993-07-01

    Glass spherules thought to be tektites from Haiti have previously been analyzed for their mineralogy and chemical composition to identify their origin and mode of formation [1]. They contain bubbles and occur in various colors dependent upon the original target rock. To investigate these spherules and the nature of any gas phase, several dark brown glasses have been analyzed for their carbon content and isotope composition, using stepped combustion analysis and static mass spectrometry. Both brown and yellow spherules were analyzed for oxygen isotope composition using laser fluorination and conventional dynamic gas-source mass spectrometry. Some spherules were analyzed whole for carbon but one was broken into fragments for the purpose of replication. Individual fragments were initially analyzed and found to yield a total of 0.2 wt% carbon in two components of different isotopic composition. The first, released between 350-400 degrees C had a delta^13C of -22 per mil whereas the second, between 500-600 degrees C had a delta^13C of -6.3 per mil. As the lower temperature release was presumed to be contaminated, other spherule fragments were pre- treated with 0.1M chromic acid to remove organic and carbonate components. Analyses of cleaned fragments indicated a variable carbon content from 0.005 to 2.6 wt% carbon but still with two isotopically different components. The first with a delta^13C of -0.8 per mil and the second, a delta^13C of -19.0 per mil. The spherules are both variable and heterogeneous. The -19 per mil component is apparently present in most of the spherules and released by 600 degrees C. A component with a similar combustion temperature and delta^13C has been encountered in K/T residues containing nanodiamonds [2]. There is currently no information available confirming its identity, but it does not appear to be surficial or an oxidizable organic. Identification of these carbon components by future work may reveal a possible source and mode of

  3. Monturaqui Crater (Atacama Desert, Chile) as a Mars Analog: Exploring the Impact Spherule Hypothesis for Meridiani

    NASA Astrophysics Data System (ADS)

    Piatek, J. L.; Ukstins Peate, I.; Cabrol, N. A.; Grin, E. A.; Chong, G.

    2010-03-01

    Monturaqui is a young terrestrial impact in the Atacama Desert. The impact affected volcanic rocks and created a vesicular impactite that contains iron oxide spherules, representing a potential analog for "blueberries" in Meridiani Planum, Mars.

  4. Composition of Meridiani Hematite-rich Spherules: A Mass-Balance Mixing-Model Approach

    NASA Astrophysics Data System (ADS)

    Jolliff, B. L.; Athena Science Team

    2005-03-01

    A mass-balance model using APXS data and microscopic images indicates that the composition of spherules ("blueberries"), found at the Meridiani site by the Mars Exploration Rover Opportunity and thought to be concretions, contain ~45-60 wt% hematite.

  5. Petrography, PGE Contents and Os Isotope Systematics of Barberton CT3 Spherule Layers

    NASA Astrophysics Data System (ADS)

    Ozdemir, S.; Schulz, T.; Koeberl, C.; Reimold, W. U.; Mohr-Westheide, T.

    2014-09-01

    We focus on some spherule layers from the northeastern part of the Barberton Greenstone Belt, South Africa and aim to determine their diagenetic-metamorphic history and to assess their possible impact origin by petrographic and geochemical analyses.

  6. Spherulitic Growth of Hematite Under Hydrothermal Conditions: Insights into the Growth Mechanism of Hematite Spherules at Meridiani Planum Mars.

    NASA Technical Reports Server (NTRS)

    Ming, Douglas W.; Golden, D. C.; Morris, R. V.

    2010-01-01

    Hematite-rich spherules were discovered embedded in sulfate-rich outcrop rock and as lag deposits of whole and broken spherules by the Opportunity rover at Meridiani Planem [1-6]. The Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), which has a wider spectral range compared to the Mars Exploration Rover Mini-TES, provided an important constraint that hematite-rich spherules are dominated by emission along the crystallographic c-axis [7-10]. We have previously synthesized hematite spherules whose mineralogic, chemical, and crystallographic properties are strikingly similar to those for the hematite-rich spherules at Meridiani Planum [11]. The spherules were synthesized in the laboratory along with hydronium jarosite and minor hydronium alunite from Fe-Al-Mg-S-Cl acid sulfate solutions under hydrothermal conditions. The reaction sequence was (1) precipitation of hydronium jarosite, (2) jarosite dissolution and precipitation of hematite spherules, and (3) precipitation of hydronium alunite upon depletion of hydronium jarosite. The spherules exhibit a radial growth texture with the crystallographic c-axis aligned along the radial direction, so that thermal emission spectra have no hematite emissivity minimum at approx.390/cm similar to the emission spectra returned by MGS TES. The objective of this paper is to expand on our initial studies [11] to examine the morphological evolution during growth of spherules starting from sub-micrometer crystals to spherules many orders of magnitude in size.

  7. Nickel-iron spherules in tektites - Non-meteoritic in origin

    NASA Technical Reports Server (NTRS)

    Ganapathy, R.; Larimer, J. W.

    1983-01-01

    The concentrations of several diagnostic trace elements were determined in two comparatively large NiFe spherules extracted from tektites. The purpose of the study was to obtain some clues about the chemistry of the projectile that is presumed responsible for the formation of these tektites. However, the trace element pattern is distinctly terrestrial implying that the spherules are the result of in-situ reduction of the host rock and are not meteoritic in origin.

  8. Neutron activation analysis of stoney spherules from a marine sediment sample

    NASA Technical Reports Server (NTRS)

    Millar, H. T., Jr.; Englert, P.

    1984-01-01

    The identification of extraterrestrial material in samples collected at the surface of the Earth is discussed. Criteria were established for black magnetic spherules which involve the presence of: Fe, Ni, and Co in iron meteoritic ratios, wustite, and Fe-Ni metal while reliable criteria for stoney spherules are not well established. Neutron activation analysis was performed on eight stony spherules separated from the same marine sediment used by Millard and Finkelman. The 22 elements were determined by Compton suppression and triple coincidence gamma counting. It is found that Fe, Mg, Al, Ni, Cr, Co, Ir, and Sc are the best discriminators between chondritic and terrestrial compositions. Three of the spherules have compositions very close to chondrites and of these, two contain 0.5 and 0.25 ppm Ir. The other five spherules contain much less than chondritic concentrations of Ni but this element may be segregated and lost during ablation of the parent meteorite. One of these five low Ni spherules contains 2.9 ppm Ir while the other four contain less than 0.05 ppm Ir.

  9. Oxygen isotopic composition of relict olivine grains in cosmic spherules: Links to chondrules from carbonaceous chondrites

    NASA Astrophysics Data System (ADS)

    Rudraswami, N. G.; Prasad, M. Shyam; Nagashima, K.; Jones, R. H.

    2015-09-01

    Most olivine relict grains in cosmic spherules selected for the present study are pristine and have not been disturbed during their atmospheric entry, thereby preserving their chemical, mineralogical and isotopic compositions. In order to understand the origin of the particles, oxygen isotope compositions of relict olivine grains in twelve cosmic spherules collected from deep sea sediments of the Indian Ocean were studied using secondary ion mass spectrometry. Most of the data lie close to the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line, with Δ17O ranging from -5‰ to 0‰. The data overlap oxygen isotopic compositions of chondrules from carbonaceous chondrites such as CV, CK, CR and CM, which suggests that chondrules from carbonaceous chondrites are the source of relict grains in cosmic spherules. Chemical compositions of olivine in cosmic spherules are also very similar to chondrule olivine from carbonaceous chondrites. Several olivine relict grains in three cosmic spherules are 16O-rich (Δ17O -21.9‰ to -18.7‰), similar to oxygen isotopic compositions observed in calcium aluminum rich inclusions (CAIs), amoeboid olivine aggregates (AOAs), and some porphyritic chondrules from carbonaceous chondrites. These grains appear to have recorded the initial oxygen isotopic composition of the inner solar nebula. Three olivine grains from two cosmic spherules have δ18O values >+20‰, which could be interpreted as mixing with stratospheric oxygen during atmospheric entry.

  10. Possible mechanism for explaining the origin and size distribution of Martian hematite spherules

    NASA Astrophysics Data System (ADS)

    Misra, Anupam K.; Acosta-Maeda, Tayro E.; Scott, Edward R. D.; Sharma, Shiv K.

    2014-03-01

    Mysterious hematite spherules, also known as “blueberries”, observed at Meridiani Planum on Mars have been widely accepted as concretions which are formed by precipitation of aqueous fluids. One of the biggest mysteries is that all observed Martian blueberries are limited in size with maximum diameter of 6.2 mm. In contrast, terrestrial concretions are not size limited. In this article, we discuss significant differences between Martian blueberries and Earth concretion analogs. Puzzling observations from Mars Exploration Rovers Opportunity and Spirit suggest that the spherules may not be concretions but are cosmic spherules formed by ablation of meteorites. The perfect spherical shape of spherules, their observed size limit, and all other physical properties are easily explained by a meteorite ablation model. Evidence that some of these spherules are only few years old strongly constrains concretion and other growth mechanisms related to aqueous processes that require the existence of water on Mars in its recent history. The large number of hematite spherules in Meridiani Planum may be due to a big rare iron meteorite impact event in this region sometime in the past.

  11. Reappraisal of hydrocarbon biomarkers in Archean rocks.

    PubMed

    French, Katherine L; Hallmann, Christian; Hope, Janet M; Schoon, Petra L; Zumberge, J Alex; Hoshino, Yosuke; Peters, Carl A; George, Simon C; Love, Gordon D; Brocks, Jochen J; Buick, Roger; Summons, Roger E

    2015-05-12

    Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼ 2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories. PMID:25918387

  12. Reappraisal of hydrocarbon biomarkers in Archean rocks

    NASA Astrophysics Data System (ADS)

    French, Katherine L.; Hallmann, Christian; Hope, Janet M.; Schoon, Petra L.; Zumberge, J. Alex; Hoshino, Yosuke; Peters, Carl A.; George, Simon C.; Love, Gordon D.; Brocks, Jochen J.; Buick, Roger; Summons, Roger E.

    2015-05-01

    Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories.

  13. Reappraisal of hydrocarbon biomarkers in Archean rocks

    PubMed Central

    French, Katherine L.; Hallmann, Christian; Hope, Janet M.; Schoon, Petra L.; Zumberge, J. Alex; Hoshino, Yosuke; Peters, Carl A.; George, Simon C.; Love, Gordon D.; Brocks, Jochen J.; Buick, Roger; Summons, Roger E.

    2015-01-01

    Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories. PMID:25918387

  14. Identification of an Archean marine oxygen oasis

    SciTech Connect

    Riding, Dr Robert E; Fralick, Dr Philip; Liang, Liyuan

    2014-01-01

    The early Earth was essentially anoxic. A number of indicators suggest the presence of oxygenic photosynthesis 2700 3000 million years (Ma) ago, but direct evidence for molecular oxygen (O2) in seawater has remained elusive. Here we report rare earth element (REE) analyses of 2800 million year old shallowmarine limestones and deep-water iron-rich sediments at Steep Rock Lake, Canada. These show that the seawater from which extensive shallow-water limestones precipitated was oxygenated, whereas the adjacent deeper waters where iron-rich sediments formed were not. We propose that oxygen promoted limestone precipitation by oxidative removal of dissolved ferrous iron species, Fe(II), to insoluble Fe(III) oxyhydroxide, and estimate that at least 10.25 M oxygen concentration in seawater was required to accomplish this at Steep Rock. This agrees with the hypothesis that an ample supply of dissolved Fe(II) in Archean oceans would have hindered limestone formation. There is no direct evidence for the oxygen source at Steep Rock, but organic carbon isotope values and diverse stromatolites in the limestones suggest the presence of cyanobacteria. Our findings support the view that during the Archean significant oxygen levels first developed in protected nutrient-rich shallow marine habitats. They indicate that these environments were spatially restricted, transient, and promoted limestone precipitation. If Archean marine limestones in general reflect localized oxygenic removal of dissolved iron at the margins of otherwise anoxic iron-rich seas, then early oxygen oases are less elusive than has been assumed.

  15. Accretion of the Archean Slave province

    NASA Astrophysics Data System (ADS)

    Kusky, T. M.

    1989-01-01

    Continental rift models have long been applied to the Archean Slave province of northwestern Canada. A reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones. Regions composed of different rock suites (e.g., orthogneisses vs. mafic volcanics) are separated by high-strain zones recording large displacements. It is proposed that the high-strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis. From west to east, these include the Anton terrane, interpreted as an Archean microcontinent; the Sleepy Dragon terrane, possibly an exhumed more eastern part of the Anton terrane; the Contwoyto terrane, a westward-verging fold and thrust belt containing tectonic slivers of greenstone volcanics; and the Hackett River volcanic terrane, interpreted as an Archean island arc. The Contwoyto and Hackett River terranes represent a paired accretionary prism and island-arc system that formed above an east-dipping subduction zone. These collided with the Anton microcontinent, producing a basement nappe, expressed as the Sleepy Dragon terrane, during the main accretion event within the Slave province. The whole tectonic assemblage was intruded by late-kinematic to postkinematic granitoids.

  16. LA-ICP-MS and Textural Analyses of Impact Spherules from the 2.54 Ga Bee Gorge Spherule Layer, Western Australia

    NASA Astrophysics Data System (ADS)

    Deutsch, A.; Metzler, K.; Berndt, J.; Langenhorst, F.

    2013-08-01

    We have analyzed spherules in thin sections from the 2.54 Bee Gorge ejecta layer with special emphasize on the origin of textural features. We try to constrain the geochemical composition of the target using trace element analysis with LA-ICP-MS.

  17. Type I Cosmic Spherules: Key to a Major, But Poorly Sampled, Asteroid Population?

    NASA Technical Reports Server (NTRS)

    Nyquist, L. E.

    1999-01-01

    Herzog et al. have determined Fe, Ni, and Cr abundances in Type I cosmic spherules recovered from the deep sea, and also the isotopic fractionation of these elements during passage of the spherules through the terrestrial atmosphere. Isotopic fractionation for all three elements is typically large, approx. 16%/amu, corresponding to evaporative mass losses of approx. 80 - 85%, assuming Rayleigh distillation from an open system. The corrected, pre-atmospheric, Cr/Ni and Fe/Ni ratios, are compared to ratios in bulk chondrites and chondritic metal. Although the calculated pre-atmospheric Fe/Ni ratio for the spherules is relatively constant at 19 +/- 4 (sigma(sub mean)) the calculated pre-atmospheric Cr/Ni ratios vary by about two orders of magnitude. The Cr/Ni ratios are thus powerful discriminators for possible modes of origin of the spherules. For example, iron meteorites typically have low Cr contents and low Cr/Ni ratios, less or = 3 x 10(exp -4). Thus, Type I spherules do not appear to be ablation products of iron meteorites, in contrast to an earlier suggestion.

  18. Isolation and Characterization of a Galactosamine Wall from Spores and Spherules of Physarum polycephalum

    PubMed Central

    McCormick, J. Justin; Blomquist, Judith C.; Rusch, Harold P.

    1970-01-01

    The myxomycete, Physarum polycephalum, can be induced under laboratory conditions to form two different hard-walled forms, spores and spherules. Characterization of both types of walls revealed only a single sugar, galactosamine. It was identified after acid hydrolysis of the isolated walls by chromatography in three solvent systems, by its positive reaction with ammoniacal silver nitrate, ninhydrin, Galactostat, and the Elson-Morgan test, and by ninhydrin degradation to lyxose. Galactosamine was present as a polymer with solubility characteristics the same as the β1-4–linked glucosamine polymer (chitosan). The walls were also found to contain about 2% protein. Spherule walls revealed a single glycoprotein on gel electrophoresis. Spore walls contained a similar protein component. The phosphate content of isolated spherule walls was 9.8%, and that of spore walls was 1.4%. Spore walls also contained about 15% melanin which was shown to be similar to fungal melanin. A novel method was used to measure the rate of mature spherule formation based on the loss of extractability of P. polycephalum natural pigment. The presence of a rare galactosamine polymer in P. polycephalum spore and spherule walls as the only carbohydrate suggests that the myxomycetes are not closely related to the fungi or the protozoa. PMID:16559084

  19. Sulphur tales from the early Archean world

    NASA Astrophysics Data System (ADS)

    Montinaro, A.; Strauss, H.

    2016-07-01

    Sedimentary and magmatic rocks and their distinct sulphur isotopic signatures indicate the sources and processes of sulphur cycling, in particular through the analysis of all four stable sulphur isotopes (32S, 33S, 34S and 36S). Research over the past 15 years has substantially advanced our understanding of sulphur cycling on the early Earth, most notably through the discovery of mass-independently fractionated sulphur isotopic signatures. A strong atmospheric influence on the early Archean global sulphur cycle is apparent, much in contrast to the modern world. Diverse microbially driven sulphur cycling is clearly discernible, but its importance for Earth surface environments remains to be quantified.

  20. Iron isotopes in an Archean ocean analogue

    NASA Astrophysics Data System (ADS)

    Busigny, Vincent; Planavsky, Noah J.; Jézéquel, Didier; Crowe, Sean; Louvat, Pascale; Moureau, Julien; Viollier, Eric; Lyons, Timothy W.

    2014-05-01

    Iron isotopes have been extensively used to trace the history of microbial metabolisms and the redox evolution of the oceans. Archean sedimentary rocks display greater variability in iron isotope ratios and more markedly negative values than those deposited in the Proterozoic and Phanerozoic. This increased variability has been linked to changes in either water column iron cycling or the extent of benthic microbial iron reduction through time. We tested these contrasting scenarios through a detailed study of anoxic and ferruginous Lac Pavin (France), which can serve as a modern analogue of the Archean ocean. A depth-profile in the water column of Lac Pavin shows a remarkable increase in dissolved Fe concentration (0.1-1200 μM) and δ56Fe values (-2.14‰ to +0.31‰) across the oxic-anoxic boundary to the lake bottom. The largest Fe isotope variability is found at the redox boundary and is related to partial oxidation of dissolved ferrous iron, leaving the residual Fe enriched in light isotopes. The analysis of four sediment cores collected along a lateral profile (one in the oxic layer, one at the redox boundary, one in the anoxic zone, and one at the bottom of the lake) indicates that bulk sediments, porewaters, and reactive Fe mostly have δ56Fe values near 0.0 ± 0.2‰, similar to detrital iron. In contrast, pyrite δ56Fe values in sub-chemocline cores (60, 65, and 92 m) are highly variable and show significant deviations from the detrital iron isotope composition (δ56Fepyrite between -1.51‰ and +0.09‰; average -0.93‰). Importantly, the pyrite δ56Fe values mirror the δ56Fe of dissolved iron at the redox boundary—where near quantitative sulfate and sulfide drawdown occurs—suggesting limited iron isotope fractionation during iron sulfide formation. This finding has important implications for the Archean environment. Specifically, this work suggests that in a ferruginous system, most of the Fe isotope variability observed in sedimentary pyrites can

  1. Sulfate was a trace constituent of Archean seawater

    NASA Astrophysics Data System (ADS)

    Crowe, Sean A.; Paris, Guillaume; Katsev, Sergei; Jones, CarriAyne; Kim, Sang-Tae; Zerkle, Aubrey L.; Nomosatryo, Sulung; Fowle, David A.; Adkins, Jess F.; Sessions, Alex L.; Farquhar, James; Canfield, Donald E.

    2014-11-01

    In the low-oxygen Archean world (>2400 million years ago), seawater sulfate concentrations were much lower than today, yet open questions frustrate the translation of modern measurements of sulfur isotope fractionations into estimates of Archean seawater sulfate concentrations. In the water column of Lake Matano, Indonesia, a low-sulfate analog for the Archean ocean, we find large (>20 per mil) sulfur isotope fractionations between sulfate and sulfide, but the underlying sediment sulfides preserve a muted range of δ34S values. Using models informed by sulfur cycling in Lake Matano, we infer Archean seawater sulfate concentrations of less than 2.5 micromolar. At these low concentrations, marine sulfate residence times were likely 103 to 104 years, and sulfate scarcity would have shaped early global biogeochemical cycles, possibly restricting biological productivity in Archean oceans.

  2. Sulfate was a trace constituent of Archean seawater.

    PubMed

    Crowe, Sean A; Paris, Guillaume; Katsev, Sergei; Jones, CarriAyne; Kim, Sang-Tae; Zerkle, Aubrey L; Nomosatryo, Sulung; Fowle, David A; Adkins, Jess F; Sessions, Alex L; Farquhar, James; Canfield, Donald E

    2014-11-01

    In the low-oxygen Archean world (>2400 million years ago), seawater sulfate concentrations were much lower than today, yet open questions frustrate the translation of modern measurements of sulfur isotope fractionations into estimates of Archean seawater sulfate concentrations. In the water column of Lake Matano, Indonesia, a low-sulfate analog for the Archean ocean, we find large (>20 per mil) sulfur isotope fractionations between sulfate and sulfide, but the underlying sediment sulfides preserve a muted range of δ(34)S values. Using models informed by sulfur cycling in Lake Matano, we infer Archean seawater sulfate concentrations of less than 2.5 micromolar. At these low concentrations, marine sulfate residence times were likely 10(3) to 10(4) years, and sulfate scarcity would have shaped early global biogeochemical cycles, possibly restricting biological productivity in Archean oceans. PMID:25378621

  3. First discovery of the organic materials in deep-sea iron cosmic spherule

    NASA Technical Reports Server (NTRS)

    Hanchang, Peng; Peicang, XU

    1993-01-01

    The dust impact mass analyzer (PUMA) carried by the spacecraft Vega 1, Vega 2 and Giotto has provided the first direct measurements of the physical and chemical properties of cometary dust. The results indicate that most of the cometary dust particles are rich in light elements such as H, C, N, and O, suggesting the validity of models that describe the cometary dust as including organic material. Up to now, there were none found with the organic material from the deep-sea cosmic spherules. We have determined this from the deep-sea iron cosmic spherules collected from the North Pacific. An iron cosmic spherule (382 microns in diameter) was determined by the Laser Raman Microprobe.

  4. Archean collisional tectonics in SW Montana

    SciTech Connect

    Mogk, D.; Rickmond, D.; Salt, K.; Clark, M.; Mueller, P.; Lafrenze, D.; Wooden, J.; Henry, D.

    1985-01-01

    The Archean continental crust of SW Montana evolved through alternating cycles of stable platform sedimentation followed by crustal thickening through collisional tectonics. The ancient sialic crust in the Beartooth Mountains served as the nucleus for accretion of younger terranes to the west. The oldest orogenic cycle recognized in the Beartooth Mountains involves a 3.4 Ga old supracrustal sequence which was metamorphosed in the granulite facies (T=700-800/sup 0/C, P=6Kb, 35/sup 0/C/Km); deep burial is interpreted as the result of collisional tectonic thickening. The second orogenic cycle is subduction related and has produced 2.8 Ga old andesites, 2.75 Ga old calc-alkaline intrusives, upper amphibolite grade metamorphism, transcurrent faulting (in the North Snowy Block and Yankee Jim canyon at 2.8 Ga) and nappe emplacement. In the central Beartooths post-orogenic granites intrude pelitic schists (T=600/sup 0/C, P=8Kb, 25/sup 0/C/Km). West of the Beartooths the basement consists of 2.75-2.70 Ga old, tectonically telescoped coarse clastics (Gallatin, Madison Ranges) and stable platform sequences (Gravelly, Tobacco Root, Ruby Ranges). Nappe formation and granulite-migmatite (700-750/sup 0/C) associations are common, suggesting deep burial through tectonic thickening. A later-kinematic mesozonal (8Kb) qtz diorite-granodiorite batholithic complex is present in the northern Madison Range. Quartzofeldspathic paragneisses in the westernmost Archean basement are derived from either a continental or island arc source.

  5. Late Archean rise of aerobic microbial ecosystems

    PubMed Central

    Eigenbrode, Jennifer L.; Freeman, Katherine H.

    2006-01-01

    We report the 13C content of preserved organic carbon for a 150 million-year section of late Archean shallow and deepwater sediments of the Hamersley Province in Western Australia. We find a 13C enrichment of ≈10‰ in organic carbon of post-2.7-billion-year-old shallow-water carbonate rocks relative to deepwater sediments. The shallow-water organic-carbon 13C content has a 29‰ range in values (−57 to −28‰), and it contrasts with the less variable but strongly 13C-depleted (−40 to −45‰) organic carbon in deepwater sediments. The 13C enrichment likely represents microbial habitats not as strongly influenced by assimilation of methane or other 13C-depleted substrates. We propose that continued oxidation of shallow settings favored the expansion of aerobic ecosystems and respiring organisms, and, as a result, isotopic signatures of preserved organic carbon in shallow settings approached that of photosynthetic biomass. Facies analysis of published carbon-isotopic records indicates that the Hamersley shallow-water signal may be representative of a late Archean global signature and that it preceded a similar, but delayed, 13C enrichment of deepwater deposits. The data suggest that a global-scale expansion of oxygenated habitats accompanied the progression away from anaerobic ecosystems toward respiring microbial communities fueled by oxygenic photosynthesis before the oxygenation of the atmosphere after 2.45 billion years ago. PMID:17043234

  6. Analysis of glassy spherules extracted from Carpathian Mesozoic lime stone by μPIXE method

    NASA Astrophysics Data System (ADS)

    Uzonyi, I.; Kiss, Á. Z.; Solt, P.; Dosztály, L.; Kákay Szabó, O.; Detre, Cs. H.

    1998-04-01

    Twenty-four glassy spherules collected from three Mesozoic sites of the Carpathian Basin have been analysed for major, minor and trace element composition using the scanning proton microprobe facility of the Institute of Nuclear Research (ATOMKI) in Debrecen. μPIXE analyses revealed surprisingly high Ca and Ba concentrations (CaO ˜40%, BaO ˜2.5%) which have never been reported before. Samples proved to be homogeneous for the major elements (Si, Ca). From geological considerations it may be inferred that these spherules are terrestrial impact products, though, the details of the impact events have not been fully cleared yet.

  7. Size distribution of interplanetary iron and stony particles related with deep-sea spherules

    NASA Technical Reports Server (NTRS)

    Matsuzaki, H.; Yamakoshi, K.

    1993-01-01

    To study origin and evolution of the interplanetary dust, it is very important to investigate the size distribution. Here the changes of the size distributions of meteoroid particles due to the ablative effects during atmospheric entry were investigated by numerical computer simulation. Using the results, the pre-atmospheric size distributions of the interplanetary dust particles could be estimated from that of ablated spherules taken from deep-sea sediments. We are now analyzing deep-sea spherules from some aspects and examining if we could get any information about the interplanetary dust.

  8. Accretionary lapilli, tektites, or concretions: the ubiquitous spherules of Meridiani Planum

    NASA Astrophysics Data System (ADS)

    DiGregorio, Barry E.

    2004-11-01

    One of the most enigmatic discoveries made by the NASA Mars Exploration Rover Opportunity (MER-B) at the Meridiani Planum landing site are the ubiquitous spherules referred to as "blueberries" by the science team. They cover the entire landing area and can be seen in every direction within view of the rover cameras. Subsequent analysis of a small grouping of the spherules laying on top of a rock outcrop by Mossbauer spectroscopy showed an intense hematite signature not found on the rock or in the surrounding basaltic soils. Spherules were also found attached to and embedded within sedimentary sulfate rock outcrops found at the landing area that have been determined by the MER science team as having been formed in an acidic liquid water environment. The appearance of most of the Meridiani spherules is strikingly similar to the morphology and size of terrestrial accretionary lapilli and show similarities to terrestrial tektites. Accretionary lapilli are spherical balls and fragments with a concentric layered structure that are formed by a variety of mechanisms including hydrovolcanic eruptions, geysers and large meteorite impacts in water. Tektites are glassy impact spherules that form as a result of large meteorite impacts and also seem apparent in some of the rover images. Tektites can be perfectly spherical or have teardrop and dumbbell shapes. A lack of a visible volcanic source capable of producing high volumes of accretionary lapilli as seen in the MER-B images, in combination with the strong spectral signature of hematite, that some of the spherules display, led the MER science team to favor a concretion hypothesis thus far. All of these types of spherules involve interaction of with surface water or ice to form. Problems exist in explaining how the Martian "concretions", if that is indeed what they are, are of such uniform size and have such a wide distribution. Evidence from Martian orbit and on the surface indicate that the Meridiani Planum landing ellipse

  9. Contrasting Aerodynamic Morphology and Geochemistry of Impact Spherules from Lonar Crater, India: Some Insights into Their Cooling History

    NASA Astrophysics Data System (ADS)

    Ray, D.; Misra, S.

    2014-11-01

    The ~50 or 570 ka old Lonar crater, India, was excavated in the Deccan Trap flood basalt of Cretaceous age by the impact of a chondritic asteroid. The impact-spherules known from within the ejecta around this crater are of three types namely aerodynamically shaped sub-mm and mm size spherules, and a sub-mm sized variety of spherule, described as mantled lapilli, having a core consisting of ash-sized grains, shocked basalt and solidified melts surrounded by a rim of ash-sized materials. Although, information is now available on the bulk composition of the sub-mm sized spherules (Misra et al. in Meteorit Planet Sci 7:1001-1018, 2009), almost no idea exists on the latter two varieties. Here, we presented the microprobe data on major oxides and a few trace elements (e.g. Cr, Ni, Cu, Zn) of mm-sized impact spherules in unravelling their petrogenetic evolution. The mm-sized spherules are characterised by homogeneous glassy interior with vesicular margin in contrast to an overall smooth and glassy-texture of the sub-mm sized spherules. Undigested micro-xenocrysts of mainly plagioclase, magnetite and rare clinopyroxene of the target basalt are present only at the marginal parts of the mm-sized spherules. The minor relative enrichment of SiO2 (~3.5 wt% in average) and absence of schlieren structure in these spherules suggest relatively high viscosity of the parent melt droplets of these spherules in comparison to their sub-mm sized counterpart. Chemically homogeneous mm-sized spherule and impact-melt bomb share similar bulk chemical and trace element compositions and show no enrichment in impactor components. The general depletion of Na2O within all the Lonar impactites was resulted due to impact-induced volatilisation effect, and it indicates the solidification temperature of the Lonar impactites close to 1,100 °C. The systematic geochemical variation within the mm-sized spherules (Mg# ~0.38-0.43) could be attributed to various level of mixing between plagioclase

  10. Alteration of submarine volcanic rocks in oxygenated Archean oceans

    NASA Astrophysics Data System (ADS)

    Ohmoto, H.; Bevacqua, D.; Watanabe, Y.

    2009-12-01

    Most submarine volcanic rocks, including basalts in diverging plate boundaries and andesites/dacites in converging plate boundaries, have been altered by low-temperature seawater and/or hydrothermal fluids (up to ~400°C) under deep oceans; the hydrothermal fluids evolved from shallow/deep circulations of seawater through the underlying hot igneous rocks. Volcanogenic massive sulfide deposits (VMSDs) and banded iron formations (BIFs) were formed by mixing of submarine hydrothermal fluids with local seawater. Therefore, the behaviors of various elements, especially of redox-sensitive elements, in altered submarine volcanic rocks, VMSDs and BIFs can be used to decipher the chemical evolution of the oceans and atmosphere. We have investigated the mineralogy and geochemistry of >500 samples of basalts from a 260m-long drill core section of Hole #1 of the Archean Biosphere Drilling Project (ABDP #1) in the Pilbara Craton, Western Australia. The core section is comprised of ~160 m thick Marble Bar Chert/Jasper Unit (3.46 Ga) and underlying, inter-bedded, and overlying submarine basalts. Losses/gains of 65 elements were quantitatively evaluated on the basis of their concentration ratios against the least mobile elements (Ti, Zr and Nb). We have recognized that mineralogical and geochemical characteristics of many of these samples are essentially the same as those of hydrothermally-altered modern submarine basalts and also those of altered volcanic rocks that underlie Phanerozoic VMSDs. The similarities include, but are not restricted to: (1) the alteration mineralogy (chlorite ± sericite ± pyrophyllite ± carbonates ± hematite ± pyrite ± rutile); (2) the characteristics of whole-rock δ18O and δ34S values; (3) the ranges of depletion and enrichment of Si, Al, Mg, Ca, K, Na, Fe, Mn, and P; (4) the enrichment of Ba (as sulfate); (5) the increases in Fe3+/Fe2+ ratios; (6) the enrichment of U; (7) the depletion of Cr; and (8) the negative Ce anomalies. Literature data

  11. Zircon Archean of the Transuralian megazone

    NASA Astrophysics Data System (ADS)

    Krasnobaev, A. A.; Puchkov, V. N.; Puzhakov, B. A.; Busharina, S. V.; Sergeeva, N. D.

    2015-12-01

    The Il'inka metamorphic complex (IC) is located in the Transuralian megazone at the latitude of the Chelyabinsk granite pluton, east of the Chelyabinsk graben. The petrological, mineralogical, and age data on the IC indicate the presence of Archean complexes during its formation. Taking into account the importance of the age data on IC, zircons were additionally analyzed using a SHRIMP. For the Transuralian megazone, the analytical data allowed us for the first time to establish the presence of the Neoarchean (2715 ± 15 Ma) substance and two stages of metamorphism of gneisses. The early stage was in the Paleoproterozoic (1970-2130 Ma). The metamorphism of 648 ± 18 Ma ends the evolution of IC.

  12. Middle Archean continent formation by crustal delamination

    NASA Astrophysics Data System (ADS)

    Zegers, Tanja E.; van Keken, Peter E.

    2001-12-01

    The processes that created the first large cratonic areas such as the Pilbara and the Kaapvaal remain poorly understood. Models based on the uniformitarian extrapolation of present-day arc volcanic processes to a hotter early Earth have not adequately explained the observations in these terranes. Here we propose an alternative mechanism for the formation of the earliest continental crust. The formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust. Such delamination results in uplift, extension, and the production of tonalite, trondhjemite, and granodiorite (TTG) suites as recorded in Middle Archean cratons. The available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction-based hypotheses.

  13. Phosphate microaggregates in Archean sediments. [Abstract only

    NASA Technical Reports Server (NTRS)

    Mojzsis, S.; Fan, G. Y.; Arrhenius, G.

    1994-01-01

    Light microscopy conducted on samples of Archean sediments reveals phosphate microaggregates which are suggestive of a biotic origin (Arrhenius et al., 1993). These aggregates, typically 15 micrometers wide and 50 micrometers long, are thought to be the mineral remains of colonies of microorganisms that lived during the late Archean Eon (greater than or equal to 2.5 Ga). Confocal microscopy was used to study the structures of these microaggregates in three dimensions. Samples used in this study are from the lowermost section of drill core taken from the Dales Gorge Member of the Brockman Iron-Formation (Hamersley Basin) in Western Australia. These sediments are well-preserved and escaped extensive metamorphism typically experienced by older rocks of this type. Two types of samples were prepared for study under the microscope: thin sections (30 micrometers) for transmitted light microscopy to study the general rock texture and to locate the grains of interest, and thick sections (3mm) for confocal microscopy to determine the 3-D structure of the aggregates in situ. The samples have been carefully polished so that they may be directly placed on the oil-immersion lens without the use of a cover slip. No chemical treatments of the surfaces have been performed. The aggregates often form clusters, although isolated aggregates have also been found. The clusters tend to distribute along microbands in the rocks. Electron microprobe analyses show that the phosphate grains and their inclusions, besides calcium and phosphorus, contain no major elements heavier than sodium. The proportions of calcium to phosphorus, the absence of stoichiometric amounts of other cations such as magnesium and iron, as well as optical properties suggest apatite as the mineral form.

  14. The origins of I-type spherules and the atmospheric entry of iron micrometeoroids

    NASA Astrophysics Data System (ADS)

    Genge, Matthew J.

    2016-06-01

    The Earth's extraterrestrial dust flux includes a wide variety of dust particles that include FeNi metallic grains. During their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed I-type spherules. These particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments. Understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 AU. This article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials. The results indicate that iron micrometeoroids experience peak temperatures 300-800 K higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of <50 μm. The lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates, which enhances their abundance among micrometeorites by a factor of 2. The abundance of I-types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite-derived MMs arriving at Earth. Furthermore, carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the I-type spherule flux. Events involving such objects, therefore, cannot be recognized from I-type spherule abundances in the geological record.

  15. Composition of Meridiani Hematite-rich Spherules: A Mass-Balance Mixing-Model Approach

    NASA Technical Reports Server (NTRS)

    jOLLIFF, b. l.

    2005-01-01

    One of the great surprises of the Mars Exploration Rovers (MER) mission is the discovery at Meridiani Planum that the surface hematite signature observed from orbit is attributable largely to a surface enrichment of hematite-rich spherules, thought to be concretions, that have weathered out of rocks similar to the underlying sulfate-rich rock formation [1]. A strong hematite signature has been observed by the Mini-TES [2] and by in-situ measurements of spherule-rich targets by the Mossbauer spectrometer (MB) [3] and the alpha-particle X-ray spectrometer (APXS) [4]. The Mini-TES derived spectrum of spherule-rich targets on the plains is consistent with nearly pure coarse-grained hematite, with perhaps as little as 5-10 areal % of other components [2]. The occurrence and abundance of the spherules as the bearer of the widespread hematite signature observed by MGS TES over much of Meridiani Planum is significant for global remote sensing, and their occurrence as concretions in the outcrop lithology is significant for the diagenetic history and role of water in the formation of the sedimentary rock formation [5].

  16. The origins of I-type spherules and the atmospheric entry of iron micrometeoroids

    NASA Astrophysics Data System (ADS)

    Genge, Matthew J.

    2016-04-01

    The Earth's extraterrestrial dust flux includes a wide variety of dust particles that include FeNi metallic grains. During their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed I-type spherules. These particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments. Understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 AU. This article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials. The results indicate that iron micrometeoroids experience peak temperatures 300-800 K higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of <50 μm. The lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates, which enhances their abundance among micrometeorites by a factor of 2. The abundance of I-types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite-derived MMs arriving at Earth. Furthermore, carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the I-type spherule flux. Events involving such objects, therefore, cannot be recognized from I-type spherule abundances in the geological record.

  17. Beryllium-10 and Aluminum-26 in Individual Cosmic Spherules from Antarctica

    NASA Technical Reports Server (NTRS)

    Nishiizumi, K.; Arnold, J. R.; Brownlee, D. E.; Caffee, M. W.; Finkel, R. C.; Harvey, R. P.

    1995-01-01

    We present data for the cosmogenic nuclides Be-10 and A-26 in a suite of 24 extraterrestrial spherules, collected from Antarctic moraines and deep sea sediments. All of the 10 large spherules collected in glacial till at Lewis Cliff are extraterrestrial. As in earlier work, the great majority of particles show prominent solar cosmic-ray (SCR) production of Al-26, indicating bombardment ages on the order of 106 years or even longer. These long ages are in direct contradiction to model ages for small particles in the inner Solar System and may require reconsideration of models of small particle lifetimes. A small fraction of the particles so far measured (6/42) possess cosmogenic radionuclide patterns consistent with predictions for meteoroid spall droplets. We believe that most of the spherules were bombarded in space primarily as bodies not much larger than their present size. The content of in situ produced Be-10 and Al-26 in quartz pebbles in the same moraine suggests that these spherules may have on average a significant terrestrial age.

  18. Hematite Spherules in Basaltic Tephra Altered Under Aqueous, Acid-Sulfate Conditions on Mauna Kea Volcano, Hawaii: Possible Clues for the Occurrence of Hematite-Rich Spherules in the Burns Formation at Meridiani Planum, Mars

    NASA Technical Reports Server (NTRS)

    Morris, R. V.; Ming, D. W.; Graff, T. G.; Arvidson, R. E.; Bell, J. F., III; Squyres, S. W.; Mertzman, S. A.; Gruener, J. E.; Golden, D. C.; Robinson, G. A.

    2005-01-01

    Iron-rich spherules (>90% Fe2O3 from electron microprobe analyses) approx.10-100 microns in diameter are found within sulfate-rich rocks formed by aqueous, acid-sulfate alteration of basaltic tephra on Mauna Kea volcano, Hawaii. Although some spherules are nearly pure Fe, most have two concentric compositional zones, with the core having a higher Fe/Al ratio than the rim. Oxide totals less than 100% (93-99%) suggest structural H2O and/or /OH. The transmission Moessbauer spectrum of a spherule-rich separate is dominated by a hematite (alpha-Fe2O3) sextet whose peaks are skewed toward zero velocity. Skewing is consistent with Al(3+) for Fe(3+) substitution and structural H2O and/or /OH. The grey color of the spherules implies specular hematite. Whole-rock powder X-ray diffraction spectra are dominated by peaks from smectite and the hydroxy sulfate mineral natroalunite as alteration products and plagioclase feldspar that was present in the precursor basaltic tephra. Whether spherule formation proceeded directly from basaltic material in one event (dissolution of basaltic material and precipitation of hematite spherules) or whether spherule formation required more than one event (formation of Fe-bearing sulfate rock and subsequent hydrolysis to hematite) is not currently constrained. By analogy, a formation pathway for the hematite spherules in sulfate-rich outcrops at Meridiani Planum on Mars (the Burns formation) is aqueous alteration of basaltic precursor material under acid-sulfate conditions. Although hydrothermal conditions are present on Mauna Kea, such conditions may not be required for spherule formation on Mars if the time interval for hydrolysis at lower temperatures is sufficiently long.

  19. Large Quantities of Melt-Quenched Impact Spherules in Late Pleistocene Alaskan and Yukon "Muck" Deposits

    NASA Astrophysics Data System (ADS)

    Hagstrum, J. T.; Firestone, R. B.; West, A.; Weaver, J. C.; Bunch, T. E.; Kimbel, D. R.

    2012-12-01

    The so-called "muck" deposits of Alaska and the upper Yukon are wind-transported, silt-dominated, organic-rich sediments, including paleosols, erosional unconformities, and buried forests with in situ stumps, that have accumulated in creek valleys over at least the last 2 million years. Underlying the frozen muck are gold-bearing gravels, and removal of Late Pleistocene muck layers in mining operations has uncovered a remarkable collection of usually broken and disarticulated megafaunal bones and carcasses. Previously, Fe-rich particles have been found embedded in a number of mammoth tusks and a bison skull, and those particles have high-Ni and low-Ti compositions indicative of an extraterrestrial origin. These fossils range in age from ~21 to 37 ka B.P. Additional fossil skulls (bison, rangifer, mammoth) and a mammoth tusk from Alaska and the Yukon Territory have been found in museum and government collections with embedded Fe-rich particles as well, and nine skulls also contain significant quantities of original host sediment within them. This associated sediment was removed and examined for the presence of spherules and other cosmic impact proxies. The additional megafaunal bones are estimated to date from between 13 to 40 ka, and radiocarbon dating of samples from these specimens is currently underway. Magnetic grains were extracted from aliquots of bulk sediment from each of the fossil skulls. The magnetic fractions ranged from ~5 to 44 g/kg, averaging 23.6 g/kg. We then examined each sample fraction for magnetic spherules. Two samples contained rounded detrital magnetite and no spherules, while the other seven samples contained numerous melt-quenched magnetic spherules ranging in abundance from ~1000 to 18,000/kg, averaging ~8000/kg. We performed SEM-EDS analyses on 49 selected spherules and identified two distinct compositional populations. One group from a mammoth skull is predominately aluminosilicate (Al2O3 = 30.7 wt.%, SiO2 = 34.4 wt.%, FeO = 23.4 wt

  20. Geology and geochronology of granitoid and metamorphic rocks of late Archean age in northwestern Wisconsin

    USGS Publications Warehouse

    Sims, P.K.; Peterman, Z.E.; Zartman, R.E.; Benedict, F.C.

    1985-01-01

    Granitoid rocks of the Puritan Quartz Monzonite and associated biotite gneiss and amphibolite in northwestern Wisconsin compose the southwestern part of the Puritan batholith of Late Archean age. They differ from rocks in the Michigan segment of the batholith in having been deformed by brittle-ductile deformation and partly recrystallized during shearing accompanying development of the midcontinent rift system of Keweenawan (Middle Proterozoic) age. Granitoid rocks ranging in composition from granite to tonalite are dominant in the Wisconsin part of the batholith. To the north of the Mineral Lake fault zone, they are massive to weakly foliated and dominantly of granite composition, whereas south of the fault zone they are more strongly foliated and mainly of tonalite composition. Massive granite, leucogranite, and granite pegmatite cut the dominant granitoid rocks. Intercalated with the granitoid rocks in small to large conformable bodies are biotite gneiss, amphibolite, and local tonalite gneiss. Metagabbro dikes of probable Early Proterozoic age as much as 15 m thick cut the Archean rocks. Rubidium-strontium whole-rock data indicate a Late Archean age for the granitoids and gneisses, but data points are scattered and do not define a single isochron. Zircon from two samples of tonalitic gneiss for uranium-thorium-Iead dating define a single chord on a concordia diagram, establishing an age of 2,735?16 m.y. The lower intercept age of 1,052?70 m.y. is in close agreement with rubidium-strontium and potassium-argon biotite ages from the gneisses. Two episodes of deformation and metamorphism are recorded in the Archean rocks. Deformation during the Late Archean produced a steep west-northwest-oriented foliation and gently plunging fold axes and was accompanied by low amphibolite-facies metamorphism of the bedded rocks. A younger deformation resulting from largely brittle fracture was accompanied by retrogressive metamorphism; this deformation is most evident adjacent

  1. An archean suture zone in the Tobacco Root Mountains? (1984) Evolution of Archean Continental Crust, SW Montana (1985)

    NASA Technical Reports Server (NTRS)

    Mogk, D. W.; Kain, L.

    1985-01-01

    The Lake Plateau area of the Beartooth Mountains, Montana were mapped and geochemically sampled. The allochthonous nature of the Stillwater Complex was interpreted as a Cordilleran-style continental margin. The metamorphic and tectonic history of the Beartooth Mountains was addressed. The Archean geology of the Spanish Peaks area, northern Madison Range was addressed. A voluminous granulite terrain of supracrustal origin was identified, as well as a heretofore unknown Archean batholithic complex. Mapping, petrologic, and geochemical investigations of the Blacktail Mountains, on the western margin of the Wyoming Province, are completed. Mapping at a scale of 1:24000 in the Archean rocks of the Gravelly Range is near completion. This sequence is dominantly of stable-platform origin. Samples were collected for geothermometric/barometric analysis and for U-Pb zircon age dating. The analyses provide the basis for additional geochemical and geochronologic studies. A model for the tectonic and geochemical evolution of the Archean basement of SW Montana is presented.

  2. An archean suture zone in the Tobacco Root Mountains? (1984) Evolution of Archean Continental Crust, SW Montana (1985)

    NASA Astrophysics Data System (ADS)

    Mogk, D. W.; Kain, L.

    The Lake Plateau area of the Beartooth Mountains, Montana were mapped and geochemically sampled. The allochthonous nature of the Stillwater Complex was interpreted as a Cordilleran-style continental margin. The metamorphic and tectonic history of the Beartooth Mountains was addressed. The Archean geology of the Spanish Peaks area, northern Madison Range was addressed. A voluminous granulite terrain of supracrustal origin was identified, as well as a heretofore unknown Archean batholithic complex. Mapping, petrologic, and geochemical investigations of the Blacktail Mountains, on the western margin of the Wyoming Province, are completed. Mapping at a scale of 1:24000 in the Archean rocks of the Gravelly Range is near completion. This sequence is dominantly of stable-platform origin. Samples were collected for geothermometric/barometric analysis and for U-Pb zircon age dating. The analyses provide the basis for additional geochemical and geochronologic studies. A model for the tectonic and geochemical evolution of the Archean basement of SW Montana is presented.

  3. Geochemistry of precambrian carbonates. II. Archean greenstone belts and Archean sea water

    SciTech Connect

    Veizer, J. ); Hoefs, J. ); Lowe, D.R. ); Thurston, P.C. )

    1989-04-01

    Carbonate rocks with geological attributes of marine sediments are a minor component of the Archean greenstone belts. Despite their relative scarcity, these rocks are important because they record chemical and isotopic properties of coeval oceans. The greenstones containing such carbonates appear to cluster at {approximately}2.8 {plus minus} 0.2 and {approximately}3.5 {plus minus} 0.1 Ga ago. The samples for the younger group are from the Abitibi, Yellowknife, Wabigoon, Michipicoten and Uchi greenstone belts of Canada and the Upper Greenstones of Zimbabwe. The older group includes the Swaziland Supergroup of South Africa, Warrawoona Group of Australia and the Sargur marbles of India. Mineralogically, the carbonates of the younger greenstones are mostly limestones and of the older ones, ferroan dolomites (ankerites); the latter with some affinities to hydrothermal carbonates. In mineralized areas with iron ores, the carbonate minerals are siderite {plus minus} ankerite, irrespective of the age of the greenstones. Iron-poor dolomites represent a later phase of carbonate generation, related to post-depositional tectonic faulting. The original mineralogy of limestone sequences appears to have been an Sr-rich aragonite. The Archean carbonates yield near-mantle Sr isotopic values, with ({sup 87}Sr/{sup 86}Sr){sub o} of 0.7025 {plus minus} 0.0015 and 0.7031 {plus minus} 0.0008 for younger and older greenstones, respectively. The mineralogical and chemical attributes of Archean carbonates are consistent with the proposition that the composition of the coeval oceans may have been buffered by a pervasive interaction with the mantle, that is, with the oceanic crust and the coeval ubiquitous volcanosedimentary piles derived from mantle sources.

  4. Tomographic location of potential melt-bearing phenocrysts in lunar glass spherules

    SciTech Connect

    Ebel, D.S.; Fogel, R.A.; Rivers, M.L.

    2005-02-04

    Apollo 17 orange glass spherules contain olivine phenocrysts with melt inclusions from depth. Tomography (<2micron/pxl) of >200 spherules located 1 phenocryst. We will try to find melt inclusions and obtain original magma volatiles and compositions. In 1971, Apollo 17 astronauts collected a 10 cm soil sample (74220) comprised almost entirely of orange glass spherules. Below this, a double drive-tube core sampled a 68 cm thick horizon comprised of orange glass and black beads (crystallized equivalents of orange glass). Primitive lunar glass spherules (e.g.-A17 orange glasses) are thought to represent ejecta from lunar mare fire fountains. The fire-fountains were apparently driven by a combination of C-O gas exsolution from orange glass melt and the oxidation of graphite. Upon eruption, magmas lost their volatiles (e.g., S, CO, CO{sub 2}) to space. Evidence for volatile escape remains as volatile-rich coatings on the exteriors of many spherules. Moreover, it showed that Type I and II Fe-Ni-rich metal particles found within orange glass olivine phenocrysts, or free-floating in the glass itself, are powerful evidence for the volatile driving force for lunar fire fountains. More direct evidence for the volatile mechanism has yet to be uncovered. Issues remaining include: the exact composition of magmatic volatiles; the hypothesized existence of graphite in the magma; the oxygen fugacity of the magma and of the lunar interior. In 1996 reported a single {approx}450 micron, equant olivine phenocryst, containing four glassy melt inclusions (or inclusion cores), the largest {approx}30micron in size, in a thin section of the 74001/2 drill core. The melt is assumed to sample the parent magma of the lunar basalts at depth, evidenced by the S content of the inclusion (600 ppm) which is 400 ppm greater than that of the orange glass host. Such melts potentially contain a full complement of the volatile components of the parent magma, which can be analyzed by infrared spectroscopy

  5. Early Archean (approximately 3.4 Ga) prokaryotic filaments from cherts of the apex basalt, Western Australia: The oldest cellularly preserved microfossils now known

    NASA Technical Reports Server (NTRS)

    Schopf, J. W.

    1991-01-01

    In comparison with that known from later geologic time, the Archean fossil record is miniscule: although literally hundreds of Proterozoic formations, containing more that 2800 occurrences of bona fide microfossils are now known, fewer than 30 units containing some 43 categories of putative microfossils (the vast majority of which are of questionable authenticity) have been reported from the Archean. Among the oldest known fossils are Early Archean filaments reported from cherts of the Towers Formation and the Apex Basalt of the 3.3-3.6 Ga-old Warrawoona Group of Western Australia. The paleobiologic significance of the Towers Formation microstructures is open to question: thin aggregated filaments are properly regarded as dubiomicrofossils (perhaps biogenic, but perhaps not); therefore, they cannot be regarded as firm evidence of Archean life. Although authentic, filamentous microfossiles were reported from a second Towers Formation locality, because the precise layer containing the fossiliferous cherts was not relocated, this discovery can neither be reconfirmed by the original collector nor confirmed independently by other investigators. Discovery of microfossils in bedded cherts of the Apex Basalt, the stratigraphic unit immediately overlying the Towers Formation, obviates the difficulties stored above. The cellularly preserved filaments of the Apex Basalt meet all of the criteria required of a bona fide Archean microfossils. Recent studies indicate that the Apex assemblage includes at least six morphotypes of uniseriate filaments, composed of barrel-shaped, discoidal, or quadrate cells and exhibiting rounded or conical terminal cells and medial bifurcated and paired half-cells that reflect the occurrence of prokaryotic binary cell division. Interestingly, the majority of these morphotypes are morphologically more similar to extant cyanobacteria than to modern filamentous bacteria. Prokaryotes seem clearly to have been hypobradytelic, and the evidence suggests

  6. Evidence for a complex archean deformational history; southwestern Michipicoten Greenstone Belt, Ontario

    NASA Technical Reports Server (NTRS)

    Mcgill, George E.; Shrady, Catherine H.

    1986-01-01

    The Michipicoten Greenstone Belt extends for about 150 km ENE from the northeastern angle of Lake Superior. In common with many other Archean greenstone belts, it is characterized by generally steep bedding dips and a distribution of major lithologic types suggesting a crudely synclinal structure for the belt as a whole. Detailed mapping and determination of structural sequence demonstrates that the structure is much more complex. The Archean history of the belt includes formation of at least three regionally significant cleavages, kilometer-scale overturning, extensive shearing, and diabase intrusion. Most well defined, mappable 'packages' of sedimentary rocks appear to be bounded by faults. These faults were active relatively early in the structural history of the belt, when extensive overturning also occurred. Steepening of dips, NW-SE shortening, development of steep NE cleavage, and pervasive shearing all postdate the early faulting and the regional overturning, obscuring much of the detail needed to define the geometry of the earlier structures. The results obtained so far suggest, however, that the Michipicoten Greenstone Belt underwent an early stage of thrusting and associated isoclinal folding, probably in a convergent tectonic environment.

  7. Tectonic implications of Archean anorthosite occurrences

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.; Morrison, D. A.; Maczuga, D. E.

    1988-01-01

    The occurrences of megacrystic anorthosite and basalt in a variety of geologic settings were reviewed and it was found that these rock types occur in a variety of tectonic settings. Anorthosites and megacrystic basalts are petrogenetically related and are found in oceanic volcanic crust, cratons, and shelf environments. Although megacrystic basalts are most common in Archean terranes, similar occurrences are observed in rocks of early Proterozoic age, and even in young terranes such as the Galapagos hotspot. Based on inferences from experimental petrology, all of the occurrences are apparently associated with similar parental melts that are relatively Fe-rich tholeiites. The megacrystic rocks exhibit a two- (or more)-stage development of plagioclase, with the megacrysts having relatively uniform composition produced under nearly isothermal and isochemical conditions over substantial periods of time. The anorthosites appear to have intruded various crustal levels from very deep to very shallow. The petrogenetic indicators, however, suggest that conditions of formation of the Precambrian examples were different from Phanerozoic occurrences.

  8. Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago.

    PubMed

    Wittke, James H; Weaver, James C; Bunch, Ted E; Kennett, James P; Kennett, Douglas J; Moore, Andrew M T; Hillman, Gordon C; Tankersley, Kenneth B; Goodyear, Albert C; Moore, Christopher R; Daniel, I Randolph; Ray, Jack H; Lopinot, Neal H; Ferraro, David; Israde-Alcántara, Isabel; Bischoff, James L; DeCarli, Paul S; Hermes, Robert E; Kloosterman, Johan B; Revay, Zsolt; Howard, George A; Kimbel, David R; Kletetschka, Gunther; Nabelek, Ladislav; Lipo, Carl P; Sakai, Sachiko; West, Allen; Firestone, Richard B

    2013-06-01

    Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world's premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event. PMID:23690611

  9. Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

    NASA Astrophysics Data System (ADS)

    Wittke, James H.; Weaver, James C.; Bunch, Ted E.; Kennett, James P.; Kennett, Douglas J.; Moore, Andrew M. T.; Hillman, Gordon C.; Tankersley, Kenneth B.; Goodyear, Albert C.; Moore, Christopher R.; Daniel, I. Randolph, Jr.; Ray, Jack H.; Lopinot, Neal H.; Ferraro, David; Israde-Alcántara, Isabel; Bischoff, James L.; DeCarli, Paul S.; Hermes, Robert E.; Kloosterman, Johan B.; Revay, Zsolt; Howard, George A.; Kimbel, David R.; Kletetschka, Gunther; Nabelek, Ladislav; Lipo, Carl P.; Sakai, Sachiko; West, Allen; Firestone, Richard B.

    2013-06-01

    Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world's premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.

  10. Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

    PubMed Central

    Wittke, James H.; Weaver, James C.; Bunch, Ted E.; Kennett, James P.; Kennett, Douglas J.; Moore, Andrew M. T.; Hillman, Gordon C.; Tankersley, Kenneth B.; Goodyear, Albert C.; Moore, Christopher R.; Daniel, I. Randolph; Ray, Jack H.; Lopinot, Neal H.; Ferraro, David; Israde-Alcántara, Isabel; Bischoff, James L.; DeCarli, Paul S.; Hermes, Robert E.; Kloosterman, Johan B.; Revay, Zsolt; Howard, George A.; Kimbel, David R.; Kletetschka, Gunther; Nabelek, Ladislav; Lipo, Carl P.; Sakai, Sachiko; West, Allen; Firestone, Richard B.

    2013-01-01

    Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world’s premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event. PMID:23690611

  11. Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

    USGS Publications Warehouse

    Wittke, James H.; Weaver, James C.; Bunch, Ted E.; Kennett, James P.; Kennett, Douglas J.; Moore, Andrew M.T.; Hillman, Gordon C.; Tankersly, Kenneth B.; Goodyear, Albert C.; Moore, Christopher R.; Daniel, I. Randolph, Jr.; Ray, Jack H.; Lopinot, Neal H.; Ferraro, David; Israde-Alcántara, Isabel; Bischoff, James L.; DeCarli, Paul S.; Hermes, Robert E.; Kloosterman, Johan B.; Revay, Zsolt; Howard, George A.; Kimbel, David R.; Kletetschka, Gunther; Nabelek, Ladislav; Lipo, Carl P.; Sakai, Sachiko; West, Allen; Firestone, Richard B.

    2013-01-01

    Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world’s premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.

  12. Compositional Models of Hematite-Rich Spherules (Blueberries) at Meridiani Planum, Mars and Constraints on Their Formation

    NASA Astrophysics Data System (ADS)

    Schneider, A.; Mittlefehldt, D.

    2006-10-01

    The Mars Exploration Rover Opportunity discovered hematite-rich spherules (``blueberries'') believed to be diagenetic concretions formed in the bedrock in stagnant or slow-moving groundwater. These spherules likely precipitated from solution, but their origins are poorly understood. Three formation mechanisms are possible: inclusive, replacive and displacive. The first would result in a distinct spherule composition compared to the other two. We propose that chemical clues may help to constrain the nature of blueberry formation. We used Alpha Particle X-ray Spectrometer data for undisturbed soils that were blueberry-free and with visible blueberries at the surface in Microscopic Imager images. We made plots of the elements versus iron for the spherule-rich soils and compared them to a mixing line representative of a pure hematite end member spherule (called ``the zero model''). This modeled the replacive formation mechanism, in which pure hematite would replace all of the original material. If the spherules grew inclusively, chemical data should reflect a compositional component of the rock grains included during formation. Four models were developed to test for possible compositions of a rock component. These models could not easily explain the APXS data and thus demonstrate that the most plausible rock compositions are not components of blueberries.

  13. Archean Subduction or Not? The Archean Volcanic Record Re-assessed.

    NASA Astrophysics Data System (ADS)

    Pearce, Julian; Peate, David; Smithies, Hugh

    2013-04-01

    Methods of identification of volcanic arc lavas may utilize: (1) the selective enrichment of the mantle wedge by 'subduction-mobile' elements; (2) the distinctive preconditioning of mantle along its flow path to the arc front; (3) the distinctive combination of fluid-flux and decompression melting; and (4) the effects of fluids on crystallization of the resulting magma. It should then be a simple matter uniquely to recognise volcanic arc lavas in the Geological Record and so document past subduction zones. Essentially, this is generally true in the oceans, but generally not on the continents. Even in recent, fresh lavas and with a full battery of element and isotope tools at our disposal, there can be debate over whether an arc-like geochemical signature results from active subduction, an older, inherited subduction component in the lithosphere, or crustal contamination. In the Archean, metamorphism, deformation, a different thermal regime and potential non-uniformitarian tectonic scenarios make the fingerprinting of arc lavas particularly problematic. Not least, the complicating factor of crustal contamination is likely to be much greater given the higher magma and crustal temperatures and higher magma fluxes prevailing. Here, we apply new, high-resolution immobile element fingerprinting methods, based primarily on Th-Nb fractionation, to Archean lavas. In the Pilbara, for example, where there is a volcanic record extending for over >500 m.y., we note that lavas with high Th/Nb (negative Nb anomalies) are common throughout the lava sequence. Many older formations also follow a basalt-andesite-dacite-rhyolite (BADR) sequence resembling present-day arcs. However, back-extrapolation of their compositions to their primitive magmas demonstrates that these were almost certainly crustally-contaminated plume-derived lavas. By contrast, this is not the case in the uppermst part of the sequence where even the most primitive magmas have significant Nb anomalies. The

  14. Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary.

    PubMed

    Wu, Yingzhe; Sharma, Mukul; LeCompte, Malcolm A; Demitroff, Mark N; Landis, Joshua D

    2013-09-17

    One or more bolide impacts are hypothesized to have triggered the Younger Dryas cooling at ∼12.9 ka. In support of this hypothesis, varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the Younger Dryas boundary (YDB). We show that bulk sediment and/or magnetic grains/microspherules collected from the YDB sites in Arizona, Michigan, New Mexico, New Jersey, and Ohio have (187)Os/(188)Os ratios ≥1.0, similar to average upper continental crust (= 1.3), indicating a terrestrial origin of osmium (Os) in these samples. In contrast, bulk sediments from YDB sites in Belgium and Pennsylvania exhibit (187)Os/(188)Os ratios <1.0 and at face value suggest mixing with extraterrestrial Os with (187)Os/(188)Os of ∼0.13. However, the Os concentration in bulk sample and magnetic grains from Belgium is 2.8 pg/g and 15 pg/g, respectively, much lower than that in average upper continental crust (=31 pg/g), indicating no meteoritic contribution. The YDB site in Pennsylvania is remarkable in yielding 2- to 5-mm diameter spherules containing minerals such as suessite (Fe-Ni silicide) that form at temperatures in excess of 2000 °C. Gross texture, mineralogy, and age of the spherules appear consistent with their formation as ejecta from an impact 12.9 ka ago. The (187)Os/(188)Os ratios of the spherules and their leachates are often low, but Os in these objects is likely terrestrially derived. The rare earth element patterns and Sr and Nd isotopes of the spherules indicate that their source lies in 1.5-Ga Quebecia terrain in the Grenville Province of northeastern North America. PMID:24009337

  15. Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary

    PubMed Central

    Wu, Yingzhe; Sharma, Mukul; LeCompte, Malcolm A.; Demitroff, Mark N.; Landis, Joshua D.

    2013-01-01

    One or more bolide impacts are hypothesized to have triggered the Younger Dryas cooling at ∼12.9 ka. In support of this hypothesis, varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the Younger Dryas boundary (YDB). We show that bulk sediment and/or magnetic grains/microspherules collected from the YDB sites in Arizona, Michigan, New Mexico, New Jersey, and Ohio have 187Os/188Os ratios ≥1.0, similar to average upper continental crust (= 1.3), indicating a terrestrial origin of osmium (Os) in these samples. In contrast, bulk sediments from YDB sites in Belgium and Pennsylvania exhibit 187Os/188Os ratios <<1.0 and at face value suggest mixing with extraterrestrial Os with 187Os/188Os of ∼0.13. However, the Os concentration in bulk sample and magnetic grains from Belgium is 2.8 pg/g and 15 pg/g, respectively, much lower than that in average upper continental crust (=31 pg/g), indicating no meteoritic contribution. The YDB site in Pennsylvania is remarkable in yielding 2- to 5-mm diameter spherules containing minerals such as suessite (Fe-Ni silicide) that form at temperatures in excess of 2000 °C. Gross texture, mineralogy, and age of the spherules appear consistent with their formation as ejecta from an impact 12.9 ka ago. The 187Os/188Os ratios of the spherules and their leachates are often low, but Os in these objects is likely terrestrially derived. The rare earth element patterns and Sr and Nd isotopes of the spherules indicate that their source lies in 1.5-Ga Quebecia terrain in the Grenville Province of northeastern North America. PMID:24009337

  16. Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary

    NASA Astrophysics Data System (ADS)

    Wu, Yingzhe; Sharma, Mukul; LeCompte, Malcolm A.; Demitroff, Mark N.; Landis, Joshua D.

    2013-09-01

    One or more bolide impacts are hypothesized to have triggered the Younger Dryas cooling at ∼12.9 ka. In support of this hypothesis, varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the Younger Dryas boundary (YDB). We show that bulk sediment and/or magnetic grains/microspherules collected from the YDB sites in Arizona, Michigan, New Mexico, New Jersey, and Ohio have 187Os/188Os ratios ≥1.0, similar to average upper continental crust (= 1.3), indicating a terrestrial origin of osmium (Os) in these samples. In contrast, bulk sediments from YDB sites in Belgium and Pennsylvania exhibit 187Os/188Os ratios <<1.0 and at face value suggest mixing with extraterrestrial Os with 187Os/188Os of ∼0.13. However, the Os concentration in bulk sample and magnetic grains from Belgium is 2.8 pg/g and 15 pg/g, respectively, much lower than that in average upper continental crust (=31 pg/g), indicating no meteoritic contribution. The YDB site in Pennsylvania is remarkable in yielding 2- to 5-mm diameter spherules containing minerals such as suessite (Fe-Ni silicide) that form at temperatures in excess of 2000 °C. Gross texture, mineralogy, and age of the spherules appear consistent with their formation as ejecta from an impact 12.9 ka ago. The 187Os/188Os ratios of the spherules and their leachates are often low, but Os in these objects is likely terrestrially derived. The rare earth element patterns and Sr and Nd isotopes of the spherules indicate that their source lies in 1.5-Ga Quebecia terrain in the Grenville Province of northeastern North America.

  17. Magnetic Properties through the Archean/Paleoproterozoic Transition from the Pilbara Craton, Western Australia: Bio-environmental Implications

    NASA Astrophysics Data System (ADS)

    Isambert, A.; Carlut, J. H.; Bouquerel, H.; Pecoits, E.; Philippot, P.; Vennin, E.; Ader, M.; Thomazo, C.; Buoncristiani, J. F.; Baton, F.; Le Huen, A. L.; Muller, E.; Deldicque, D.; Sforna, M. C.

    2015-12-01

    The origin of iron oxides in Archean and Paleoproterozoic Banded Iron Formations is still a matter of debate. We report here low and high temperature magnetic properties, susceptibility and saturation magnetization results coupled with scanning microscope, transmission electron microscopy, Raman observations and microprobe analyses along a 60 meters section, which encompasses the uppermost Archean Boolgeeda Iron Formation and its transition into the lower Paleoproterozoic Kungarra Formation in the Pilbara Craton, Western Australia. With the exception of two volcanoclastic intervals characterized by low susceptibility and magnetization, nearly pure magnetite is identified as the main magnetic carrier in all iron-rich layers including hematite-bearing jasper beds. The relative magnetic contribution of magnetite and hematite throughout the section is evidenced by IRM acquisition curves. We observed a sharp decrease in magnetization at the Archean-Proterozoic transition and a general trend in the Verwey temperature. Two populations of magnetically distinct magnetites are reported from a 2 meter-thick interval lying within the late Archean section of the core. Each population shows a specific Verwey transition temperature: one around 120-124K and the other in the range of 105-110K. The two Verwey transitions are interpreted to reflect two distinct stoichiometry and likely two stages of magnetite crystallization. The 120-124K transition is attributed to nearly pure stoichiometric magnetite, whereas SEM, TEM and microprobe observations suggest that the lower temperature transition is related to chemically impure silician magnetite. Microbial-induced partial substitution of iron by silicon is suggested here. This is supported by an increase in Total Organic Carbon (TOC) in the same interval and Raman spectroscopy data showing a close association of organic carbon with magnetite.

  18. Tomographic Location of Potential Melt-Bearing Phenocrysts in Lunar Glass Spherules

    NASA Technical Reports Server (NTRS)

    Ebel, D. S.; Fogel, R. A.; Rivers, M. L.

    2005-01-01

    In 1971, Apollo 17 astronauts collected a 10 cm soil sample (74220) comprised almost entirely of orange glass spherules. Below this, a double drive-tube core sampled a 68 cm thick horizon comprised of orange glass and black beads (crystallized equivalents of orange glass). Primitive lunar glass spherules (e.g.-A17 orange glasses) are thought to represent ejecta from lunar mare fire fountains [1, 2]. The fire-fountains were apparently driven by a combination of C-O gas ex-solution from orange glass melt and the oxidation of graphite [3, 4]. Upon eruption, magmas lost their volatiles (e.g., S, CO, CO2) to space. Evidence for volatile escape remains as volatile-rich coatings on the exteriors of many spherules [e.g., 5,6]. Moreover, [7] showed that Type I and II Fe-Ni-rich metal particles found within orange glass olivine phenocrysts, or free-floating in the glass itself, are powerful evidence for the volatile driving force for lunar fire fountains.

  19. Studies on coccidioides immitis. II. Physiological studies on in vitro spherulation.

    PubMed

    BROOKS, L D; NORTHEY, W T

    1963-01-01

    Brooks, Lula D. (Arizona State University, Tempe) and William T. Northey. Studies on Coccidioides immitis. II. Physiological studies on in vitro spherulation. J. Bacteriol. 85:12-15. 1963.-Studies on the amino acids assayed in Edamine revealed that the amino acids phenylalanine, tyrosine, and tryptophan, in addition to the amino acid derivatives dihydroxyphenylalanine (DOPA), epinephrine, tyramine monohydrochloride (tyramine HCl), and pyrocatechol play an important role in the Coccidioides immitis conversion process. In the presence of phenylalanine, tyrosine, tryptophan, DOPA, pyrocatechol, dextrose, and biotin, conversion as well as maturation was demonstrated. However, in epinephrine and tyramine HCl combinations, conversion occurred without maturation of the spherules. The availability of increased amounts of melanin precursors is suggested as a possible explanation for the increased susceptibility of the dark-skinned races to disseminated coccidioidomycosis. Conversion and maturation were also noted when lactic and fumaric acids served as carbon sources. The conversion of a limited number of arthrospores to spherules and their subsequent maturation in the presence of only phenol and biotin suggest the possibility that the key to the C. immitis conversion process lies in the availability of certain ring structures. PMID:14015812

  20. Convergent Plate Boundary Processes in the Archean: Evidence from Greenland

    NASA Astrophysics Data System (ADS)

    Polat, A.

    2014-12-01

    The structural, magmatic and metamorphic characteristics of Archean greenstone belts and associated TTG (tonalite, trondhjemite and granodiorite) gneisses in southern West Greenland are comparable to those of Phanerozoic convergent plate margins, suggesting that Archean continents grew mainly at subduction zones. These greenstone belts are composed mainly of tectonically juxtaposed fragments of oceanic crust including mafic to ultramafic rocks, with minor sedimentary rocks. Volcanic rocks in the greenstone belts are characterized mainly by island arc tholeiitic basalts, picrites, and boninites. The style of deformation and geometry of folds in 10 cm to 5 m wide shear zones are comparable to those occur on 1 to 50 km scale in the greenstone belts and TTG gneisses, suggesting that compressional tectonic processes operating at convergent plate boundaries were the driving force of Archean crustal accretion and growth. Field observations and trace element data suggest that Archean continental crust grew through accretion of mainly island arcs and melting of metamorphosed mafic rocks (amphibolites) in thickened arcs during multiple tectonothermal events. Fold patterns on cm to km scale are consistent with at least three phases of deformation and multiple melting events generating TTG melts that intruded mainly along shear zones in accretionary prism and magmatic arcs. It is suggested that Archean TTGs were produced by three main processes: (1) melting of thickened oceanic island arcs; (2) melting of subducted oceanic crust; and (3) differentiation of basaltic melts originating from metasomatized sub-arc mantle wedge peridotites.

  1. Some examples of deep structure of the Archean from geophysics

    NASA Technical Reports Server (NTRS)

    Smithson, S. B.; Johnson, R. A.; Pierson, W. R.

    1986-01-01

    The development of Archean crust remains as one of the significant problems in earth science, and a major unknown concerning Archean terrains is the nature of the deep crust. The character of crust beneath granulite terrains is especially fascinating because granulites are generally interpreted to represent a deep crustal section. Magnetic data from this area can be best modeled with a magnetized wedge of older Archean rocks (granulitic gneisses) underlying the younger Archean greenstone terrain. The dip of the boundary based on magnetic modeling is the same as the dip of the postulated thrust-fault reflection. Thus several lines of evidence indicate that the younger Archean greenstone belt terrain is thrust above the ancient Minnesota Valley gneiss terrain, presumably as the greenstone belt was accreted to the gneiss terrain, so that the dipping reflection represents a suture zone. Seismic data from underneath the granulite-facies Minnesota gneiss terrain shows abundant reflections between 3 and 6 s, or about 9 to 20 km. These are arcuate or dipping multicyclic events indicative of layering.

  2. Geology and tectonics of the Archean Superior Province, Canadian Shield

    NASA Technical Reports Server (NTRS)

    Card, K. D.

    1986-01-01

    Superior Province consists mainly of Late Archean rocks with Middle Archean gneisses in the south, and possibly in the north. The Late Archean supracrustal sequences are of island arc and interarc affinity and are cut by abundant plutonic rocks, including early arc-related intrusions, late synorogenic intrusions, and post-orogenic plutons that are possibly the product of crustal melting caused by thermal blanketing of newly-thickened continental crust combined with high mantle heat flux. The contemporaneity of magmatic and deformational events along the lengths of the belts is consistent with a subduction-dominated tectonic regime for assembly of the Kenoran Orogen. Successive addition of volcanic arcs accompanied and followed by voluminous plutonism resulted in crustal thickening and stabilization of the Superior craton prior to uplift of Kapuskasing granulites, emplacement of the Matachewan diabase dykes, and Early Proterozoic marginal rifting.

  3. The early Earth -- A perspective on the Archean

    SciTech Connect

    Hamilton, W.B. )

    1993-04-01

    Dominant models of Archean tectonics and magmatism involve plate-tectonic mechanisms. Common tenets of geochemistry (e.g., model ages) and petrology visualize a cold-accreted Earth in which primitive mantle gradually fractionated to produce crust during and since Archean time. These popular assumptions appear to be incompatible with cosmologic and planetologic evidence and with Archean geology. All current quantitative and semiquantitative theories agree that the Earth was largely or entirely melted (likely superheated) by giant impacts, including the Mars-size impact which splashed out the Moon, and by separation of the core. The Earth at [approximately]4.5 Ga was a violently convecting anhydrous molten ball. Both this history and solar-system position indicate the bulk Earth to be more refractory than chondrite. The outer part of whatever sold shell developed was repeatedly recycled by impacts before 3.9 Ga. Water and CO[sub 2] were added by impactors after the Moon-forming event; the mantle is not a source of primordial volatiles, but rather is a sink that has depleted the hydrosphere. Voluminous liquidus ultramafic lava (komatiite) indicates that much Archean upper mantle was above its solidus. Only komatiitic and basaltic magma entered Archean crust from the mantle. Variably hydrous contamination, secondary melting, and fractionation in the crust produced intermediate and felsic melts. Magmatism was concurrent over vast tracts. Within at least the small sample of Archean crust that has not been recycled into the mantle, heat loss was primarily by voluminous, dispersed magmatism, not, as in the modern Earth, primarily through spreading windows through the crust. Only in Proterozoic time did plate-tectonic mechanisms become prevalent.

  4. The character of the Moho and lower crust within Archean cratons and the tectonic implications

    NASA Astrophysics Data System (ADS)

    Abbott, Dallas H.; Mooney, Walter D.; VanTongeren, Jill A.

    2013-12-01

    Undisturbed mid Archean crust (stabilized by 3.0-2.9 Ga) has several characteristics that distinguish it from post Archean crust. Undisturbed mid-Archean crust has a low proportion of internal seismic boundaries (as evidenced by converted phases in seismic receiver functions), lacks high seismic velocities in the lower crust and has a sharp, flat Moho. Most of the seismic data on mid-Archean crust comes from the undisturbed portions of the Kaapvaal and Zimbabwe (Tokwe segment) cratons. Around 67-74% of younger Archean crust (stabilized by 2.8-2.5 Ga) has a sharp, flat Moho. Much of the crust with a sharp, flat Moho also lacks strong internal seismic boundaries, but there is not a one to one correspondence. In cases where its age is known, basaltic lower crust in Archean terranes is often but not always the result of post Archean underplating. Undisturbed mid-Archean cratons are also characterized by lower crustal thicknesses (Archean median range = 32-39 km vs. post-Archean average = 41 km) and lower crustal seismic velocities. These observations are shown to be distinct from those observed in any modern-day tectonic environment. The data presented here are most consistent with a model in which Archean crust undergoes delamination of dense lithologies at the garnet-in isograd resulting in a flat, sharp Moho reflector and a thinner and more felsic-intermediate crust. We discuss the implications of this model for several outstanding paradoxes of Archean geology.

  5. Archean crustal evolution of the northern North China Craton

    NASA Technical Reports Server (NTRS)

    Qian, Xianglin; Chen, Yaping; Liu, Jinzhong

    1988-01-01

    The Archean granultie facies rocks of the North China (Sino-Korean) Craton mostly occur inside the northern boundary forming a unique and spectacular granulite belt trending roughly E-W from eastern Hebei, North China in the east to Mt. Daqinchan, western Inner Mongolia in the west, ranging about 1,000 km long. Over the years in the middle portion of this Archean high-grade metamorphic belt a stratigraphic unconformity between the khondalite rock assemblage and the medium in composition granulite assemblage in Datong-Xinghe area is determined. The geological structural properties of the North China Craton are discussed.

  6. Chemistry and petrology of Fe-Ni beads from different types of cosmic spherules: Implication for precursors

    NASA Astrophysics Data System (ADS)

    Rudraswami, N. G.; Prasad, M. Shyam; Babu, E. V. S. S. K.; Kumar, T. Vijaya

    2014-11-01

    Fe-Ni beads are observed to occur in all three (Stony, Glass, Iron) types of cosmic spherules collected from deep sea sediments of the Indian Ocean. Fe-Ni beads in cosmic spherules can provide insights for understanding metal segregation mechanisms and their refractory metal element (RME: Re, Os, W, Ir, Ru, Mo, Pt, Rh including Pd) compositions can help ascertain their precursor meteorites. We measured RME compositions of 55 Fe-Ni beads using LA-ICP-MS in all three basic types of cosmic spherules selected after examining ∼2000 cosmic spherules. The RMEs of Fe-Ni beads provide unique information on formation and differentiation during atmospheric entry. The variability in the concentration of the RMEs depends on the initial mass of the cosmic spherules, volatility, temperature attained and efficiency in metal segregation during entry. The CI chondrite and Os normalized RME compositions of the beads display a pattern that is close to CI chondritic composition. The presence of Pd, a non-refractory metal having condensation temperature similar to Fe, in Fe-Ni beads of all types of cosmic spherules indicates that the heating undergone was below its vaporization temperature. Not all parent bodies lead to the formation of beads, the precursor needs to exceed a certain minimum size and temperature to facilitate the metal to get segregated into beads. The minimum size of a parent particle that could enclose a Fe-Ni bead is estimated to have a size ∼1 mm. This places constraints on the sizes of materials that are ablated during entry, and the accompanying mass loss during entry. Our study further points out that all the three basic types of cosmic spherules have a chondritic origin based on their RME distribution patterns. Only metal-rich carbonaceous chondrites contain the required quantities of metal for the formation of Fe-Ni beads during atmospheric entry and during this process the RMEs are also efficiently segregated into these beads.

  7. Origin of iron oxide spherules in the banded iron formation of the Bababudan Group, Dharwar Craton, Southern India

    NASA Astrophysics Data System (ADS)

    Orberger, Beate; Wagner, Christiane; Wirth, Richard; Quirico, Eric; Gallien, Jean Paul; Derré, Colette; Montagnac, Gilles; Noret, Aurélie; Jayananda, Mudlappa; Massault, Marc; Rouchon, Virgile

    2012-06-01

    The banded iron formation of the Bababudan Group (Western Dharwar Craton, India) is composed of millimetric to centimetric alternating quartz and grey to red Fe-oxide bands. Major phases are quartz and martite (hematized magnetite) with minor Fe-sulfides and Ca-Mg-Fe-carbonates. Micrometric Fe-oxide spherules fill cavities in discontinuous micrometric layers of Fe-oxides that occur in the massive quartz layers and at the interface of massive Fe-oxide and quartz layers. The spherules are composed of micrometric radial plates of hematite intergrown with nanometric magnetite. These spherules contain carbonaceous matter (CM) with nanometric Fe-particles and have low N contents (˜900 ppm; CM1). The spherule formation is attributed to a low temperature hydrothermal process (150-200 °C) at around 2.52 Ga, possibly favored by the presence of CM. These hydrothermal fluids dissolved diagenetic interstitial sulfides or carbonates creating cavities which, provided space for the spherule precipitation. Carbonaceous matter of semi-anthracite maturity is encapsulated in quartz grains adjacent to the Fe-oxide spherules (CM2) and it is thus concluded that CM1 and CM2 are most likely contemporaneous and of the same origin, either incorporated at the time of BIF formation or during the hydrothermal event at 2.52 Ga from the underlying phyllitised black shales. Carbonaceous matter (CM3) was also found around the Fe-oxide spherules and the martite grains. CM3 has much higher N contents (>5000 ppm), is of a lower maturity than CM1 and CM2, and is related to weathering, which is also indicated by the presence of goethite and kaolinite. The δ13C of all CMs varies from -19.4 to -24.7‰, similar to values measured in the underlying phyllitised black shales and likely reflect denitrifying microbial activity.

  8. A revised, hazy methane greenhouse for the Archean Earth.

    PubMed

    Haqq-Misra, Jacob D; Domagal-Goldman, Shawn D; Kasting, Patrick J; Kasting, James F

    2008-12-01

    Geological and biological evidence suggests that Earth was warm during most of its early history, despite the fainter young Sun. Upper bounds on the atmospheric CO2 concentration in the Late Archean/Paleoproterozoic (2.8-2.2 Ga) from paleosol data suggest that additional greenhouse gases must have been present. Methanogenic bacteria, which were arguably extant at that time, may have contributed to a high concentration of atmospheric CH4, and previous calculations had indicated that a CH4-CO2-H2O greenhouse could have produced warm Late Archean surface temperatures while still satisfying the paleosol constraints on pCO2. Here, we revisit this conclusion. Correction of an error in the CH4 absorption coefficients, combined with the predicted early onset of climatically cooling organic haze, suggest that the amount of greenhouse warming by CH4 was more limited and that pCO2 must therefore have been 0.03 bar, at or above the upper bound of the value obtained from paleosols. Enough warming from CH4 remained in the Archean, however, to explain why Earth's climate cooled and became glacial when atmospheric O2 levels rose in the Paleoproterozoic. Our new model also shows that greenhouse warming by higher hydrocarbon gases, especially ethane (C2H6), may have helped to keep the Late Archean Earth warm. PMID:19093801

  9. Petrogenesis of calcic plagioclase megacrysts in Archean rocks

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.; Morrison, D. A.

    1986-01-01

    Anorthositic complexes with large equidimensional plagioclase grains of highly calcic composition occur in nearly all Archean cratons. Similar plagioclase occur as megacrysts in many Archean sills, dikes, and volcanic flows. In the Canadian Shield these units occur throughout the Archean portions of the entire shield and are particularly common as dikes over an area of a few 100,000 sq km in Ontario and Manitoba during a period of at least 100 m.y. in many different rock types and metamorphic grades. The plagioclase generally occurs in three modes: as inclusions in mafic intrusions at various stages of fractionation, as crystal segregations in anorthosite complexes, or as megacrysts in fractionated sills, dikes, and flows. Most occurrences suggest that the plagioclase was formed elsewhere before being transported to its present location. The evidence seems to be quite clear that occurrences of these types of calcic plagioclase require: (1) ponding of a relatively undifferentiated Archean tholeiitic melt at some depth; (2) isothermal crystallization of large, equidimensional homogeneous plagioclase crystals; (3) separation of the plagioclase crystals from any other crystalline phases; (4) further fractionation of melt; (5)transport of various combinations of individual plagioclase crystals and clusters of crystals by variously fractionated melts; and (6) emplacement as various types of igneous intrusions or flows.

  10. Archean metamorphic sequence and surfaces, Kangerdlugssuaq Fjord, East Greenland

    NASA Technical Reports Server (NTRS)

    Kays, M. A.

    1986-01-01

    The characteristics of Archean metamorphic surfaces and fabrics of a mapped sequence of rocks older than about 3000 Ma provide information basic to an understanding of the structural evolution and metamorphic history in Kangerdlugssuaq Fjord, east Greenland. This information and the additional results of petrologic and geochemical studies have culminated in an extended chronology of Archean plutonic, metamorphic, and tectonic events. The basis for the chronology is considered, especially the nature of the metamorphic fabrics and surfaces in the Archean sequence. The surfaces, which are planar mineral parageneses, may prove to be mappable outside Kangerdlugssuaq Fjord, and if so, will be helpful in extending the events that they represent to other Archean sequences in east Greenland. The surfaces will become especially important reference planes if the absolute ages of their metamorphic assemblages can be determined in at least one location where strain was low subsequent to their recrystallization. Once an isochron is obtained, the dynamothermal age of the regionally identifiable metamorphic surface is determined everywhere it can be mapped.

  11. Archean komatiite volcanism controlled by the evolution of early continents.

    PubMed

    Mole, David R; Fiorentini, Marco L; Thebaud, Nicolas; Cassidy, Kevin F; McCuaig, T Campbell; Kirkland, Christopher L; Romano, Sandra S; Doublier, Michael P; Belousova, Elena A; Barnes, Stephen J; Miller, John

    2014-07-15

    The generation and evolution of Earth's continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that in the Archean, the formation and stabilization of continents also controlled the location, geochemistry, and volcanology of the hottest preserved lavas on Earth: komatiites. These magmas typically represent 50-30% partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the Archean-Proterozoic Earth. As a result, it is vital to constrain and understand the processes that govern their localization and emplacement. Here, we combined Lu-Hf isotopes and U-Pb geochronology to map the four-dimensional evolution of the Yilgarn Craton, Western Australia, and reveal the progressive development of an Archean microcontinent. Our results show that in the early Earth, relatively small crustal blocks, analogous to modern microplates, progressively amalgamated to form larger continental masses, and eventually the first cratons. This cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks. The dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmosphere, while also providing a fundamental control on the distribution of major magmatic ore deposits. PMID:24958873

  12. Major, trace element and oxygen isotope study of glass cosmic spherules of chondritic composition: The record of their source material and atmospheric entry heating

    NASA Astrophysics Data System (ADS)

    Cordier, Carole; Folco, Luigi; Suavet, Clément; Sonzogni, Corinne; Rochette, Pierre

    2011-09-01

    New geochemical data on cosmic spherules (187 major element, 76 trace element, and 10 oxygen isotope compositions) and 273 analyses from the literature were used to assess the chemical diversity observed among glass cosmic spherules with chondritic composition. Three chemical groups of glass spherules are identified: normal chondritic spherules, CAT-like spherules (where CAT refers to Ca-Al-Ti-rich spherules), and high Ca-Al spherules. The transition from normal to high Ca-Al spherules occurs through a progressive enrichment in refractory major elements (on average from 2.3 wt.% to 7.0 wt.% for CaO, 2.8 wt.% to 7.2 wt.% for Al 2O 3, and 0.14 wt.% to 0.31 wt.% for TiO 2) and refractory trace elements (from 6.2 μg/g to 19.3 μg/g for Zr and 1.6CI-4.3CI for Rare Earth Elements-REEs) relative to moderately refractory elements (Mg, Si) and volatile elements (Rb, Na, Zn, Pb). Based on a comparison with experimental works from the literature, these chemical groups are thought to record progressive heating and evaporation during atmospheric entry. The evaporative mass losses evaluated for the high Ca-Al group (80-90%) supersede those of the CAT spherules which up to now have been considered as the most heated class of stony cosmic spherules. However, glass cosmic spherules still retain isotopic and elemental evidence of their source and precursor mineralogy. Four out of the 10 normal and high Ca-Al spherules analysed for oxygen isotopes are related to ordinary chondrites ( δ18O = 13.2-17.3‰ and δ17O = 7.6-9.2‰). They are systematically enriched in Ni and Co (Ni = 24-500 μg/g) with respect to spherules related to carbonaceous chondrites (Ni < 1.2 μg/g, δ18O = 13.1-28.0‰ and δ17O = 5.1-14.0‰). REE abundances in cosmic spherules, which are not fractionated according to parent body or atmospheric entry heating, can then be used to unravel the precursor mineralogy. Spherules with flat REE pattern close to unity when normalized to CI are the most abundant in our

  13. Chromium-Isotope and iridium-Abundance Measurements for Late Eocene Impact-Derived Spherule Deposits

    NASA Astrophysics Data System (ADS)

    Kyte, F. T.; Shukolyukov, A.; Hildebrand, A. R.; Lugmair, G. W.; Hanova, J.

    2004-05-01

    The late Eocene (approx. 35 Ma) was a time of multiple large body impacts superimposed within an interval of dust accretion. At least two spherule layers are preserved in deep sea sediments: North American microtektites and the slightly older cpx spherules. The two largest impact structures in the Cenozoic, the 45 km Chesapeake Bay structure and the 100 km Popigai structure, are indicated as the respective spherule sources. Enhanced 3He concentrations extending across a 3 m.y. duration in upper Eocene sediments from the Massignano quarry in Italy indicate accretion of <50 micron dust over this time interval. To characterize one of the impactors we have analyzed splits from the 125-250 micron cpx spherules from ODP 709C, and two fractions from the Massignano layer. Splits of each sample were analyzed for minor and trace elements by instrumental activation analysis (INAA) including Ir, Cr, Fe, Ni, and Co. Additional splits were analyzed for their Cr-isotopic composition, using thermal ionization mass spectrometry. Significant concentrations of Ir were found in all samples, with the highest levels in the Massignano coarse sample and the lowest in the 709C sample. In all cases, element/Ir ratios are much higher than in chondritic meteorites; this may reflect elemental fractionation due to preferential concentration of Cr in spinel growing in the impact fireball. The Cr-isotopic compositions of the 709C and Massignano coarse samples are both non-terrestrial with a positive epsilon 53, indicating a 53Cr/52Cr ratio higher than in terrestrial materials. Microprobe surveys showed that the Massignano samples had significant fine grained oxide grains (mixed with the spherules in the coarse sample) that were not Ni- or Cr-rich, including Ti-rich spinels (likely terrestrial contaminants). In contrast, the ODP 709C sample is a pure extract of generally well-preserved cpx spherules composed of clinopyroxene in a glass matrix. Both the Massignano coarse and the 709C cpx samples

  14. Dating Carbonaceous Matter in Archean Cherts by Electron Paramagnetic Resonance

    PubMed Central

    Bourbin, M.; Derenne, S.; Binet, L.; Le Du, Y.; Westall, F.; Kremer, B.; Gautret, P.

    2013-01-01

    Abstract Ancient geological materials are likely to be contaminated through geological times. Thus, establishing the syngeneity of the organic matter embedded in a mineral matrix is a crucial step in the study of very ancient rocks. This is particularly the case for Archean siliceous sedimentary rocks (cherts), which record the earliest traces of life. We used electron paramagnetic resonance (EPR) for assessing the syngeneity of organic matter in cherts that have a metamorphic grade no higher than greenschist. A correlation between the age of Precambrian samples and the shape of their EPR signal was established and statistically tested. As thermal treatments impact organic matter maturity, the effect of temperature on this syngeneity proxy was studied; cyanobacteria were submitted to cumulative short thermal treatment at high temperatures followed by an analysis of their EPR parameters. The resulting carbonaceous matter showed an evolution similar to that of a thermally treated young chert. Furthermore, the possible effect of metamorphism, which is a longer thermal event at lower temperatures, was ruled out for cherts older than 2 Gyr, based on the study of Silurian cherts of the same age and same precursors but various metamorphic grades. We determined that even the most metamorphosed sample did not exhibit the lineshape of an Archean sample. In the hope of detecting organic contamination in Archean cherts, a “contamination-like” mixture was prepared and studied by EPR. It resulted that the lineshape analysis alone does not allow contamination detection and that it must be performed along with cumulative thermal treatments. Such treatments were applied to three Archean chert samples, making dating of their carbonaceous matter possible. We concluded that EPR is a powerful tool to study primitive organic matter and could be used in further exobiology studies on low-metamorphic grade samples (from Mars for example). Key Words: Kerogen—Sedimentary rocks—Contamination—Spectroscopy—Archean

  15. Elemental depletions in Antarctic micrometeorites and Arctic cosmic spherules: Comparison and relationships

    NASA Technical Reports Server (NTRS)

    Presper, T.; Kurat, G.; Koeberl, C.; Palme, H.; Maurette, Michel

    1993-01-01

    Antarctic micrometeorites (MM's) and Arctic cosmic spherules (CS's) have bulk compositions comparable to those of chondritic meteorites. However, abundance of Na, Ca, Mn, Ni, Co, and S are commonly lower in MM's and CS's as compared to chondrites. Our SEM, EMP, and INAA studies suggest that these elemental depletions in unmelted MM's are likely to be due to leaching of soluble components from the MM's in the upper atmosphere and the melt ice water. Depletions in CS's appear to be mainly due to volatilization during melting in the atmosphere or to sampling bias during aggregate formation or parent rock break-up.

  16. Deep-sea spherules from Pacific clay - Mass distribution and influx rate. [extraterrestrial origins from optical and electron microscopy

    NASA Technical Reports Server (NTRS)

    Murrell, M. T.; Davis, P. A., Jr.; Nishiizumi, K.; Millard, H. T., Jr.

    1980-01-01

    From 411 kg of Pacific clay, 22 mg of stony spherules and 50 mg of iron spherules larger than 150 microns were concentrated. The extraterrestrial origin of these particles was evaluated with the aid of optical and electron microscopy and atomic absorption elemental analysis. An expression for the integral number of stony particles from this sediment in the mass range 20-300 micrograms was derived. The world-wide influx rate of stony particles in the mass range which survive atmospheric heating and ocean sediment storage is calculated to be 90 tons/yr. The relative contributions of ablation debris vs fused interplanetary dust to the influx of stony spherules is discussed, but no conclusions could be made.

  17. Identification of hydroxyapatite spherules provides new insight into subretinal pigment epithelial deposit formation in the aging eye.

    PubMed

    Thompson, Richard B; Reffatto, Valentina; Bundy, Jacob G; Kortvely, Elod; Flinn, Jane M; Lanzirotti, Antonio; Jones, Emrys A; McPhail, David S; Fearn, Sarah; Boldt, Karsten; Ueffing, Marius; Ratu, Savanjeet Guy Singh; Pauleikhoff, Laurenz; Bird, Alan C; Lengyel, Imre

    2015-02-01

    Accumulation of protein- and lipid-containing deposits external to the retinal pigment epithelium (RPE) is common in the aging eye, and has long been viewed as the hallmark of age-related macular degeneration (AMD). The cause for the accumulation and retention of molecules in the sub-RPE space, however, remains an enigma. Here, we present fluorescence microscopy and X-ray diffraction evidence for the formation of small (0.5-20 μm in diameter), hollow, hydroxyapatite (HAP) spherules in Bruch's membrane in human eyes. These spherules are distinct in form, placement, and staining from the well-known calcification of the elastin layer of the aging Bruch's membrane. Secondary ion mass spectrometry (SIMS) imaging confirmed the presence of calcium phosphate in the spherules and identified cholesterol enrichment in their core. Using HAP-selective fluorescent dyes, we show that all types of sub-RPE deposits in the macula, as well as in the periphery, contain numerous HAP spherules. Immunohistochemical labeling for proteins characteristic of sub-RPE deposits, such as complement factor H, vitronectin, and amyloid beta, revealed that HAP spherules were coated with these proteins. HAP spherules were also found outside the sub-RPE deposits, ready to bind proteins at the RPE/choroid interface. Based on these results, we propose a novel mechanism for the growth, and possibly even the formation, of sub-RPE deposits, namely, that the deposit growth and formation begin with the deposition of insoluble HAP shells around naturally occurring, cholesterol-containing extracellular lipid droplets at the RPE/choroid interface; proteins and lipids then attach to these shells, initiating or supporting the growth of sub-RPE deposits. PMID:25605911

  18. Bed bugs.

    PubMed

    Foulke, Galen T; Anderson, Bryan E

    2014-09-01

    The term bed bug is applied to 2 species of genus Cimex: lectularius describes the common or temperate bed bug, and hemipterus its tropical cousin. Cimex lectularius is aptly named; its genus and species derive from the Latin words for bug and bed, respectively. Though the tiny pest is receiving increased public attention and scrutiny, the bed bug is hardly a new problem. PMID:25577850

  19. Spherules from the Cretaceous/Tertiary boundary clay at Gubbio, Italy: the problem of outcrop contamination

    SciTech Connect

    Montanari, A.

    1986-12-01

    Surficial outcrop contamination has occurred in some well-known stratigraphic sections of carbonate rocks in the northern Apennines. A critical case involves several contaminated clay partings, including the Cretaceous/Tertiary boundary clay in the classic Bottaccione section near Gubbio, Italy. These clay layers contain shiny spherules which, in several recent studies, have been said to consist of volcanic glass and have been used to support the hypothesis that the terminal Cretaceous mass extinction was caused by widespread volcanism. Laboratory tests, however, indicate that these shiny spherules are made of HF-insoluble and combustible material and are therefore of recent biological origin. These objects were introduced into the Cretaceous/Tertiary boundary clay and other clay layers from the surrounding soil along with abundant detrital contaminants derived from erosion of the middle Miocene flysch exposed at the head of the Bottaccione Gorge. They are completely different from the altered and flattened microtektitelike spheroids that are found only in the iridium-rich Cretaceous/Tertiary boundary clay and that provide strong evidence for a large impact.

  20. Spherulization as a process for the exudation of chemical cues by the encrusting sponge C. crambe

    PubMed Central

    Ternon, Eva; Zarate, Lina; Chenesseau, Sandrine; Croué, Julie; Dumollard, Rémi; Suzuki, Marcelino T.; Thomas, Olivier P.

    2016-01-01

    Ecological interactions in the marine environment are now recognized to be partly held by chemical cues produced by marine organisms. In particular, sponges are sessile animals thought to rely on the bioactive substances they synthesize to ensure their development and defense. However, the mechanisms leading the sponges to use their specialized metabolites as chemical cues remain unknown. Here we report the constant release of bioactive polycyclic guanidinic alkaloids by the Mediterranean sponge Crambe crambe into the dissolved and the particulate phases using a targeted metabolomics study. These compounds were proven to be stored into already described specialized (spherulous) sponge cells and dispersed into the water column after release through the sponge exhaling channels (oscula), leading to a chemical shield surrounding the sponge. Low concentrations of these compounds were demonstrated to have teratogenic effects on embryos of a common sea squirt (ascidian). This mechanism of action called spherulization may therefore contribute to the ecological success of encrusting sponges that need to extend their substrate cover to expand. PMID:27381941

  1. Spherulization as a process for the exudation of chemical cues by the encrusting sponge C. crambe.

    PubMed

    Ternon, Eva; Zarate, Lina; Chenesseau, Sandrine; Croué, Julie; Dumollard, Rémi; Suzuki, Marcelino T; Thomas, Olivier P

    2016-01-01

    Ecological interactions in the marine environment are now recognized to be partly held by chemical cues produced by marine organisms. In particular, sponges are sessile animals thought to rely on the bioactive substances they synthesize to ensure their development and defense. However, the mechanisms leading the sponges to use their specialized metabolites as chemical cues remain unknown. Here we report the constant release of bioactive polycyclic guanidinic alkaloids by the Mediterranean sponge Crambe crambe into the dissolved and the particulate phases using a targeted metabolomics study. These compounds were proven to be stored into already described specialized (spherulous) sponge cells and dispersed into the water column after release through the sponge exhaling channels (oscula), leading to a chemical shield surrounding the sponge. Low concentrations of these compounds were demonstrated to have teratogenic effects on embryos of a common sea squirt (ascidian). This mechanism of action called spherulization may therefore contribute to the ecological success of encrusting sponges that need to extend their substrate cover to expand. PMID:27381941

  2. Deformation history of Archean metasedimentary rocks of the Beartooth mountains in the vicinity of the Mineral Hill mine, Jardine, Montana

    SciTech Connect

    Jablinski, J.D.; Holst, T.B. )

    1992-09-01

    Archean metasedimentary rocks of the South Snowy Block of the Beartooth Mountains, in the vicinity of Jardine, Montana, consist predominantly of schistose rocks with rare iron formation. These rocks are intruded by Precambrian granitic stocks and minor mafic dikes and sills. Evidence for three phases of folding and late-stage kinking is found within the metasedimentary rocks, whereas rocks of the Crevice Mountain stock (2,700 Ma) are unaffected by any of these events. The first folding event involved the development of isoclinal, recumbent folds of varying scale. F[sub 1] fold hinges are rare, most commonly observed underground in Mineral Hill. An S[sub 1] schistosity has developed axial planar to these folds. This schistosity, which is subparallel to bedding, is very well developed and ubiquitous in the metasedimentary rocks of the Jardine region. Two later phases of folding are also recognized. F[sub 2] folds are nearly upright with gently to moderately plunging fold hinges. Temperature and pressure conditions during deformation, as revealed by calculations from microprobe analyses, suggest that the peak of metamorphism occurred at a temperature of about 560 C and a pressure of 2.9 kb. Thin section observations indicate that the metamorphic peak accompanied the formation of S[sub 1] schistosity. Structural, metamorphic, and geochemical data are consistent with the hypothesis that the metasedimentary rock of the Jardine region are allochthonous and constitute one of a number of tectonostratigrphic terranes in the western Beartooth Mountains that were juxtaposed tectonically against the western margin of an Archean continent during a Late Archean collisional event.

  3. Development of Archean crust in the Wind River Mountains, Wyoming

    NASA Technical Reports Server (NTRS)

    Frost, C. D.; Koesterer, M. E.; Koesterer, M. E.; Koesterer, M. E.; Koesterer, M. E.

    1986-01-01

    The Wind River Mountains are a NW-SE trending range composed almost entirely of high-grade Archean gneiss and granites which were thrust to the west over Phanerozoic sediments during the Laramide orogeny. Late Archean granites make up over 50% of the exposed crust and dominates the southern half of the range, while older orthogneisses and magnatites form most of the northen half of the range. Locally these gneisses contain enclaves of supracrustal rocks, which appear to be the oldest preserved rocks in the range. Detailed work in the Medina Mountain area of the central Wind River Mountains and reconnaissance work throughout much of the northern part of the range has allowed definition of the sequence of events which marked crustal development in this area. The sequence of events are described.

  4. Archean microfossils: a reappraisal of early life on Earth.

    PubMed

    Altermann, Wladyslaw; Kazmierczak, Józef

    2003-11-01

    The oldest fossils found thus far on Earth are c. 3.49- and 3.46-billion-year-old filamentous and coccoidal microbial remains in rocks of the Pilbara craton, Western Australia, and c. 3.4-billion-year-old rocks from the Barberton region, South Africa. Their biogenicity was recently questioned and they were reinterpreted as contaminants, mineral artefacts or inorganic carbon aggregates. Morphological, geochemical and isotopic data imply, however, that life was relatively widespread and advanced in the Archean, between 3.5 and 2.5 billion years ago, with metabolic pathways analogous to those of recent prokaryotic organisms, including cyanobacteria, and probably even eukaryotes at the terminal Archean. PMID:14596897

  5. Hospitable archean climates simulated by a general circulation model.

    PubMed

    Wolf, E T; Toon, O B

    2013-07-01

    Evidence from ancient sediments indicates that liquid water and primitive life were present during the Archean despite the faint young Sun. To date, studies of Archean climate typically utilize simplified one-dimensional models that ignore clouds and ice. Here, we use an atmospheric general circulation model coupled to a mixed-layer ocean model to simulate the climate circa 2.8 billion years ago when the Sun was 20% dimmer than it is today. Surface properties are assumed to be equal to those of the present day, while ocean heat transport varies as a function of sea ice extent. Present climate is duplicated with 0.06 bar of CO2 or alternatively with 0.02 bar of CO2 and 0.001 bar of CH4. Hot Archean climates, as implied by some isotopic reconstructions of ancient marine cherts, are unattainable even in our warmest simulation having 0.2 bar of CO2 and 0.001 bar of CH4. However, cooler climates with significant polar ice, but still dominated by open ocean, can be maintained with modest greenhouse gas amounts, posing no contradiction with CO2 constraints deduced from paleosols or with practical limitations on CH4 due to the formation of optically thick organic hazes. Our results indicate that a weak version of the faint young Sun paradox, requiring only that some portion of the planet's surface maintain liquid water, may be resolved with moderate greenhouse gas inventories. Thus, hospitable late Archean climates are easily obtained in our climate model. PMID:23808659

  6. Dating carbonaceous matter in archean cherts by electron paramagnetic resonance.

    PubMed

    Bourbin, M; Gourier, D; Derenne, S; Binet, L; Le Du, Y; Westall, F; Kremer, B; Gautret, P

    2013-02-01

    Ancient geological materials are likely to be contaminated through geological times. Thus, establishing the syngeneity of the organic matter embedded in a mineral matrix is a crucial step in the study of very ancient rocks. This is particularly the case for Archean siliceous sedimentary rocks (cherts), which record the earliest traces of life. We used electron paramagnetic resonance (EPR) for assessing the syngeneity of organic matter in cherts that have a metamorphic grade no higher than greenschist. A correlation between the age of Precambrian samples and the shape of their EPR signal was established and statistically tested. As thermal treatments impact organic matter maturity, the effect of temperature on this syngeneity proxy was studied; cyanobacteria were submitted to cumulative short thermal treatment at high temperatures followed by an analysis of their EPR parameters. The resulting carbonaceous matter showed an evolution similar to that of a thermally treated young chert. Furthermore, the possible effect of metamorphism, which is a longer thermal event at lower temperatures, was ruled out for cherts older than 2 Gyr, based on the study of Silurian cherts of the same age and same precursors but various metamorphic grades. We determined that even the most metamorphosed sample did not exhibit the lineshape of an Archean sample. In the hope of detecting organic contamination in Archean cherts, a "contamination-like" mixture was prepared and studied by EPR. It resulted that the lineshape analysis alone does not allow contamination detection and that it must be performed along with cumulative thermal treatments. Such treatments were applied to three Archean chert samples, making dating of their carbonaceous matter possible. We concluded that EPR is a powerful tool to study primitive organic matter and could be used in further exobiology studies on low-metamorphic grade samples (from Mars for example). PMID:23397956

  7. Workshop on a Cross Section of Archean Crust

    NASA Technical Reports Server (NTRS)

    Ashwal, L. D. (Editor); Card, K. D. (Editor)

    1983-01-01

    Various topics relevant to crustal genesis, especially the relationship between Archean low - and high-grade terrains, were discussed. The central Superior Province of the Canadian Shield was studied. Here a 120 km-wide transition from subgreenschist facies rocks of the Michipicoten greenstone belt to granulite facies rocks of the Kapuskasing structural zone represents an oblique cross section through some 20 km of crust, uplifted along a northwest-dipping thrust fault.

  8. Crust formation and plate motion in the early archean.

    PubMed

    Kröner, A; Layer, P W

    1992-06-01

    Mounting evidence for voluminous continental crust formation in the early Archean involving intracrustal melting and selective preservation of granitoid rocks suggests that initial crust formation crust formation and growth were predominantly by magmatic underplating in plumegenerated Iceland-type settings. Collision of these early islands to give rise to larger blocks is suggested by extensive horizontal shortening in both supracrustal and granitoid assemblages. Preservation of early Archean high-grade gneisses that were once at depths of 20 to 30 kilometers implies that these blocks developed thick, subcrustal roots despite high mantle heat flow. Rigid continental plates must have existed since at least 3.5 billion years ago, and greenstone belts (composed of mixed metavolcanic and metasedimentary sequences intruded by granitoid plutons) probably developed on or near these microcontinents. Paleomagnetic data with good age control from at least one ancient craton suggest that plate motion was at normal minimum average velocities of about 17 millimeters per year with respect to the poles during the period 3.5 billion to 2.4 billion years ago. If this is true on a global scale, Archean plate motion was not faster than in later geologic times. PMID:17791608

  9. Chemical constraints on the evolution of Archean continental crust

    NASA Technical Reports Server (NTRS)

    Weaver, B. L.; Tarney, J.

    1985-01-01

    One of the challenges of Archean geochronology is to find isotopic systems that preserve an indication of a rock's primary age in spite of the effects of later metamorphism. Zircon dating has been used widely with considerable success but not without difficulty, especially in polymetamorphic terrains. Zircons in such cases commonly are found to have lost radiogenic Pb, and despite fractionizing the zircons or abrading them to remove disturbed portions; it often is not possible to define a pattern of Pb loss from which the original age can confidently be inferred. The refinement of techniques to enable extremely small samples, or even single crystals, to be analyzed has contributed greatly to solving the problem but even those techniques cannot resolve the micron scale isotopic heterogeneities within single zircons in which much of their history is recorded. That can only be done by ion microprobe. Progress reports on studies of four Archean rocks, each of which illustrates the power and potential of ion microprobe analysis in solving problems of Archean geochronology are discussed.

  10. Archean komatiite volcanism controlled by the evolution of early continents

    PubMed Central

    Mole, David R.; Fiorentini, Marco L.; Thebaud, Nicolas; Cassidy, Kevin F.; McCuaig, T. Campbell; Kirkland, Christopher L.; Romano, Sandra S.; Doublier, Michael P.; Belousova, Elena A.; Barnes, Stephen J.; Miller, John

    2014-01-01

    The generation and evolution of Earth’s continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that in the Archean, the formation and stabilization of continents also controlled the location, geochemistry, and volcanology of the hottest preserved lavas on Earth: komatiites. These magmas typically represent 50–30% partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the Archean–Proterozoic Earth. As a result, it is vital to constrain and understand the processes that govern their localization and emplacement. Here, we combined Lu-Hf isotopes and U-Pb geochronology to map the four-dimensional evolution of the Yilgarn Craton, Western Australia, and reveal the progressive development of an Archean microcontinent. Our results show that in the early Earth, relatively small crustal blocks, analogous to modern microplates, progressively amalgamated to form larger continental masses, and eventually the first cratons. This cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks. The dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmosphere, while also providing a fundamental control on the distribution of major magmatic ore deposits. PMID:24958873

  11. Bridging Two Worlds: From the Archean to the Proterozoic

    NASA Technical Reports Server (NTRS)

    Schopf, J. William

    2000-01-01

    As now known, the Archean and Proterozoic appear to have been different worlds: the geology (tectonic style, basinal distribution, dominant rock types), atmospheric composition (O2, CO21, CH4), and surface environment (day-length, solar luminosity, ambient temperature) all appear to have changed over time. And virtually all paleobiologic indicators can be interpreted as suggesting there were significant biotic differences as well: (1) Stromatolites older than 2.5 Ga are rare relative to those of the Proterozoic; their biotic components are largely unknown; and the biogenicity of those older than approx. 3.2 Ga has been questioned. (2) Bona fide microfossils older than approx. 2.4 Ga are rare, poorly preserved, and of uncertain biological relations. Gaps of hundreds of millions of years in the known record make it impossible to show that Archean microorganisms are definitely part of the 2.4 Ga-to-present evolutionary continuum. and (3) In rocks older than approx. 2.2 Ga, the sulfur isotopic record is subject to controversy; phylogenetically distinctive bio-markers are unknown; and nearly a score of geologic units contain organic carbon anomalously light isotopically (relative to that of the Proterozoic and Phanerozoic) that may reflect the presence of Archaeans ("Archaebacteria of earlier classifications) but may not (since cellularly preserved Archean-age Archaeans have never been identified).

  12. How widely is the Andean type of continental margin represented in the Archean

    NASA Technical Reports Server (NTRS)

    Burke, Kevin

    1988-01-01

    Application of the principle of uniformitarianism to the Archean was discussed in a search for evidence of Archean-type continental margins in Archean rocks. The author cautioned that Archean rocks represent only 2 percent of the current exposure of the continents, half of which is in the North American Superior Province. Care must be taken in interpreting the global tectonic significance of relatively small exposures of Archean rocks, such as South India. Andean margins were characterized by their elongate shape, magmatic associations, and isotopic signatures. Although the compositional evidence alone will always be ambiguous, it was suggested that supporting structural evidence may aid in the identification of Archean Andean margins. Andean margin remains have been recognized in the Superior Province of Canada by these criteria, and the author suggested that the Closepet granite of South India may represent another example.

  13. How sharp is the sharp Archean Moho? Example from eastern Superior Province

    NASA Astrophysics Data System (ADS)

    Levin, Vadim; VanTongeren, Jill A.; Servali, Andrea

    2016-03-01

    The Superior Province of North America has not experienced major internal deformation for nearly 2.8 Gyr, preserving the Archean crust in its likely original state. We present seismological evidence for a sharp (less than 1 km) crust-mantle boundary beneath three distinct Archean terranes and for a more vertically extensive boundary at sites likely affected by the 1.2-0.9 Ga Grenville orogeny. At all sites crustal thickness is smaller than expected for the primary crust produced by melting under higher mantle potential temperature conditions of Archean time. Reduced thickness and an abrupt contrast in seismic properties at the base of the undisturbed Archean crust are consistent with density sorting and loss of the residues through gravitational instability facilitated by higher temperatures in the upper mantle at the time of formation. Similar sharpness of crust-mantle boundary in disparate Archean terranes suggests that it is a universal feature of the Archean crustal evolution.

  14. Low Temperature Magnetic Properties of the Late Archean Boolgeeda Iron Formation (Hamersley Group, Western Australia): Environmental Implications

    NASA Astrophysics Data System (ADS)

    Carlut, Julie; Isambert, Aude; Bouquerel, Hélène; Pecoits, Ernesto; Philippot, Pascal; Vennin, Emmanuelle; Ader, Magali; Thomazo, Christophe; Buoncristiani, Jean-François; Baton, Franck; Muller, Elodie; Deldicque, Damien

    2015-05-01

    The origin of the iron oxides in Archean and Paleoproterozoic Banded Iron Formations is still a debated question. We report low and high temperature magnetic properties, susceptibility and saturation magnetization results joined with scanning microscope observations within a 35 meters section of the Late Archean Boolgeeda Iron Formation of the Hamersley Group, Western Australia. With the exception of two volcanoclastic intervals characterized by low susceptibility and magnetization, nearly pure magnetite is identified as the main magnetic carrier in all iron-rich layers including hematite-rich jasper beds. Two populations of magnetically distinct magnetites are reported from a 2 meter-thick interval within the section. Each population shows a specific Verwey transition temperature: one around 120-124 K and the other in the range of 105-110 K. This temperature difference is interpreted to reflect two distinct stoichiometry and likely two episodes of crystallization. The 120-124K transition is attributed to nearly pure stoichiometric magnetite, SEM and microprobe observations suggest that the lower temperature transition is related to chemically impure silician magnetite. Microbial-induced partial substitution of iron by silicon is suggested here. This is supported by an increase in Total Organic Carbon (TOC) in the same interval.

  15. Composition and origin of Archean lower crust, Northern Tanzania

    NASA Astrophysics Data System (ADS)

    Mansur, A. T.; Manya, S.; Rudnick, R.

    2008-12-01

    Granulite-facies xenoliths from tuff cones erupted on the margin of the Tanzanian craton and within the adjacent Mozambique belt in northern Tanzania offer an opportunity to assess the role of lower crustal processes in the tectonic evolution of these two terranes. Both terranes are Archean, but record very different histories, starting in the Proterozoic and continuing today. Whereas the craton experienced little metamorphism or igneous activity following its stabilization around 2.8 Ga, Archean rocks of the Mozambique belt in the study area experienced at least one episode of high-grade metamorphism during the East African orogeny (ca. 640 Ma). Today, the East African rift exists at the contact between the Mozambique belt and the craton, implying a fundamental lithospheric weakness at this boundary. Granulite xenoliths come from Labait, on the craton margin, and Lashaine and Naibor Soito in the metamorphic belt. Most xenoliths are mafic and all are igneous in origin. Cratonic xenoliths (pl- opx±cpx±gt±hbl) are primarily anhydrous two-pyroxene granulites that likely originated as crystallized high-Ni, Archean basaltic melts. Xenoliths from the Mozambique belt are dominated by mafic granulites (pl-cpx-gt±opx) at Lashaine and banded, mafic to intermediate granulites at Naibor Soito. Positive Sr and Eu anomalies imply that the Lashaine granulites originated as plagioclase cumulates. The wide range in SiO2 (47-65 wt%) and correlation of Ni-MgO in the Naibor Soito xenoliths suggests they may have originated as igneous rocks that subsequently underwent partial melting to form the mafic (pl- opx±cpx±gt±hbl±bt) and felsic bands (pl-qtz-opx±kfs). U-Pb zircon ages for xenoliths from both terranes are Archean, as are most TDM ages, though younger TDM ages are seen in some Lashaine samples that were contaminated by rift magma. High pressures (up to 2.7GPa) are recorded by the Mozambique belt xenoliths, suggesting equilibration in thickened crust during the East

  16. Constraining Archean Earth's Atmosphere with the Geological Record

    NASA Astrophysics Data System (ADS)

    Horan, A. M.; Domagal-Goldman, S. D.; Claire, M.

    2014-12-01

    A warm, water-bearing Archean Earth, when the Sun was young and faint, remains a paradox to the scientific world. Abundant geological data suggests that Archean Earth had standing water at the surface, despite the fainter Sun. An explanation of this paradox is vital to the understanding of Earth's history and coevolution with life. If the surface of the planet was not being kept warm by the Sun, which was 25% less luminous than now, it must have been kept warm a different way—by an atmospheric composition high in greenhouse gases. Constraints on these gases come from the geological record, which have provided proxies for the redox state of the atmosphere (limiting H2 and O2), the total atmospheric pressure, and the partial pressure of certain gases such as carbon dioxide (CO2) and methane (CH4). Previous attempts at solutions to the paradox are consistent with some, but not all, of the geological proxies. The constraints are used as inputs for a 1-D photochemical code, which calculates atmospheric composition and predicts the abundances of atmospheric gases that affect climate, particularly methane (CH4) and gaseous hydrogen (H2). A coupled 1-D radiative-convective climate code is then used to calculate the corresponding surface temperature. Critically, the improved photochemical code maintains strict redox boundary conditions, and is being further updated to ensure that the redox fluxes from volcanoes and mid-ocean ridge vents are consistent with both each other and the redox state of the mantle. These code improvements will lead to changes in both the inputs to the atmosphere from volcanoes and the sink for oxidants at mid-ocean ridges, in turn affecting the abundance of redox-sensitive greenhouse gases such as CH4 and H2. The main purpose of this project is to extend simulations of the Archean surface environment down into the mantle, and to search for a solution to the faint young sun paradox that is consistent with the geological proxies. Beyond having

  17. Hazy Archean Earth as an Analog for Hazy Earthlike Exoplanets

    NASA Astrophysics Data System (ADS)

    Arney, Giada; Meadows, Victoria; Domagal-Goldman, Shawn; Claire, Mark; Schwieterman, Edward

    2015-01-01

    Hazy exoplanets may be common (Bean et al. 2010, Sing et al. 2011, Kreidberg et al 2014), and in our solar system, Venus and Titan have photochemically-produced hazes. There is evidence that Earth itself had a hydrocarbon haze in the Archean (Zerkle et al. 2012, Domagal-Goldman et al. 2008) with important climatic effects (Pavlov et al. 2001, Trainer et al. 2006, Haqq-Misra et al. 2008, Wolf and Toon 2012). We use a 1D coupled photochemical-climate model and a line-by-line radiative transfer model to investigate the climactic and spectral impacts of a fractal hydrocarbon haze on Archean Earth. The haze absorbs significantly at shorter wavelengths and can strongly suppress the Rayleigh scattering tail, a broadband effect that would be remotely detectable at low spectral resolution at wavelengths less than 0.5 μm. Hazes may have a more significant impact on transit transmission spectra. Using the transit transmission radiative transfer model developed by Misra et al. (2014) to generate hazy Archean spectra, we find that even a thin hydrocarbon haze masks the lower atmosphere from the visible into the near infrared where the haze optical depth exceeds unity. The transit transmission spectra we generate for hazy Archean Earth are steeply sloped like the Titan solar occultation spectrum observed by Robinson et al. (2014). Thick hazes can also cool the planet significantly: for example, the thick fractal haze generated around Archean Earth with 0.3% CH4, 1% CO2 and 1 ppm C2H6 cools the planet from roughly 290 K without the haze to below freezing with the haze. Finally, we investigate the impact of host star spectral type on haze formation, comparing the hazes generated around a solar-type star to those generated at an Earth analog planet around the M dwarf AD Leo. Our results indicate hazes around M dwarfs for the same initial atmospheric composition may be thinner due to decreased UV photolysis of methane and other hydrocarbons needed for haze formation. Earthlike

  18. Spinel-bearing spherules condensed from the Chicxulub impact-vapor plume

    NASA Astrophysics Data System (ADS)

    Ebel, Denton S.; Grossman, Lawrence

    2005-04-01

    Formation of the giant Chicxulub crater off Mexico's Yucatan Peninsula coincided with deposition of the global Ir-rich Cretaceous-Tertiary (K-T) stratigraphic boundary layer ca. 65 Ma. The boundary is marked most sharply by abundant spherules containing unaltered grains of magnesioferrite spinel. Here we predict for the first time the sequential condensation of solids and liquids from the plume of vaporized rock expected from oblique K-T impacts. We predict highly oxidizing plumes that condense silicate liquid droplets bearing spinel grains whose compositions closely match those marking the actual boundary. Systematic global variations in spinel composition are consistent with higher condensation temperatures for spinels found at Atlantic and European sites than for those in the Pacific.

  19. Boron isotopes in Archean cherts: investigating early Earth marine conditions

    NASA Astrophysics Data System (ADS)

    Lemarchand, D.; Jeambrun, M.; van Bergen, M. J.; van Cappellen, P.

    2009-12-01

    The Archean Eon was a period of intense modifications of the Earth surface environment that led to the emergence of life in the early ocean. Therefore, the chemistry of Archean seawater and its relationship to that of the solid Earth, through oceanic and continental crust alteration, remain a matter of great interest and debate. Here, we present new boron (B) isotopes data in well-characterized cherts (ca. 3.5 Ga) from the Pilbara Craton, in order to provide new constraints on their depositional environments. Boron isotopes offer a potentially powerful tool for investigating seawater/rock interactions. The modern differences between B isotopes in seawater, continental crust and oceanic crust greatly facilitate the identification of B sources, while mineral precipitation processes are also accompanied by large isotopic fractionations. We analyzed B concentrations and δ11B in 12 subsamples from two types of stratiform chert; one type (C-chert) is interpreted as resulting from direct precipitation of a seawater-hydrothermal fluid mixture, the second type (S-chert) as resulting from silicification of detritic sediment precursors (Van den Boorn et al., 2007). The C-cherts subsamples show relative large ranges of δ11B values (from -3 to +16 ‰) and B concentrations (0.4-20 ppm). The cm scale variations argue against significant post depositional alteration and thus support the hypothesis that the primary signature is preserved. The similarity of our results with B isotopes in modern cherts suggests that Archean seawater conditions during chert deposition were rather close to those in the modern ocean, at least with respect to B isotopes. In particular, the marine B isotopic budget was likely comparable to the present one. If this is correct, the B isotopes data imply that Archean seawater pH was neutral to slightly basic (pH ≈ 7-9). The S-chert subsamples show remarkably constant δ11B values (≈ -20 ‰) despite a wide range of B concentrations (3-120 ppm). These

  20. Triple sulfur isotope composition of Late Archean seawater sulfate

    NASA Astrophysics Data System (ADS)

    Paris, G.; Fischer, W. W.; Sessions, A. L.; Adkins, J. F.

    2013-12-01

    Multiple sulfur isotope ratios in Archean sedimentary rocks have provided powerful insights into the behavior of the ancient sulfur cycle, the redox state of fluid Earth, and the timing of the rise of atmospheric oxygen [1]. Most processes fractionate sulfur isotopes in proportion to their mass differences, but the Archean sulfur isotope record is marked by pronounced mass-independent fractionation (MIF, Δ33S≠0). The origin of these signatures has been traditionally interpreted as the result of photolysis of SO2 from short wavelength UV light, with positive Δ33S values recorded in pyrite and negative Δ33S values in sulfate-bearing phases [2]. This long-held hypothesis rests on observations of negative Δ33S from enigmatic barite occurrences from mixed volcanic sedimentary strata in Mesoarchean greenstone terrains. Despite forming the framework for understanding Archean sulfur cycle processes [3], it is largely untested [3]. It is largely untested. Consequently, the biggest challenge to our current understanding of the early sulfur cycle is a poor understanding of the isotopic composition of seawater sulfate. Sulfate evaporite minerals are absent from Archean strata and the sulfur isotope record is written entirely by measurements of pyrite. Carbonate associated sulfate (CAS) provides an important archive for assaying the isotopic composition of ancient seawater sulfate It has been exploited in many studies of Phanerozoic and Proterozoic sulfate but have been only marginally used thus far for Archean samples because of the extremely low concentration of CAS in limestones and dolomites from this era. We have developed a novel MC-ICP-MS approach to solve this problem [4]. This new method lowers the detection limit by up to three orders of magnitude for δ34S and Δ33S measurements, enabling to work on a few nmols of sulfate which represent only tens of mg of sample powders micromilled from specific carbonate textures. Two stratigraphic sections from the 2

  1. Relationship between high- and low-grade Archean terranes: Implications for early Earth paleogeography

    NASA Technical Reports Server (NTRS)

    Eriksson, K. A.

    1986-01-01

    The Western Gneiss Terrain (WGT) of the Yilgarn Block, Western Australia was studied. The WGT forms an arcuate belt of Archean gneisses that flank the western margin of the Yilgarn Block. In general the WGT is composed of high-grade orthogneisses and paragneisses which contain supracrustal belts composed largely of siliciclastic metasediments and subordinate iron formation. The platformal nature of the metasedimentary belts and lack of obvious metavolcanic lithologies contrasts with the composition of typical Yilgarn greenstones to the east. Radiometric data from WGT rocks indicates that these rocks are significantly older than Yilgarn rocks to the east (less than 3.3 Ga) and this has led to the suggestion that the WGT represents sialic basement to Yilgarn granite-greenstone belts. The Mount Narryer region exposes the northernmost occurrence of high-grade metasediments within the WGT and consists of quartz-rich clastic metasediments at upper amphibolite to granulite grade. Most occurrences of supracrustal rocks in this region comprise isolated lenses within the gneissic basement. However, at Mount Narryer a unique sequence of metaclastics with preserved bedding provide an unusual window into the parentage of similar supracrustal bodies in this region.

  2. Accretionary origin for the late Archean Ashuanipi Complex of Canada

    NASA Technical Reports Server (NTRS)

    Percival, J. A.

    1988-01-01

    The Ashuanipi complex is one of the largest massif granulite terrains of the Canadian Shield. It makes up the eastern end of the 2000 km long, lower-grade, east-west belts of the Archean Superior Province, permitting lithological, age and tectonic correlation. Numerous lithological, geochemical and metamorphic similarities to south Indian granulites suggest common processes and invite comparison of tectonic evolution. The Ashuanipi granulite terrain of the Cannadian Superior Province was studied in detail, and an origin through self-melting of a 55 km thick accretionary wedge seems possible.

  3. Geological Mapping of Fortuna Tessera (V-2): Venus and Earth's Archean Process Comparisons

    NASA Technical Reports Server (NTRS)

    Head, James W.; Hurwitz,D. M.; Ivanov, M. A.; Basilevsky, A. T.; Kumar, P. Senthil

    2008-01-01

    The geological features, structures, thermal conditions, interpreted processes, and outstanding questions related to both the Earth's Archean and Venus share many similarities and we are using a problem-oriented approach to Venus mapping, guided by insight from the Archean record of the Earth, to gain new insight into the evolution of Venus and Earth's Archean. The Earth's preserved and well-documented Archean record provides important insight into high heat-flux tectonic and magmatic environments and structures and the surface of Venus reveals the current configuration and recent geological record of analogous high-temperature environments unmodified by subsequent several billion years of segmentation and overprinting, as on Earth. Elsewhere we have addressed the nature of the Earth's Archean, the similarities to and differences from Venus, and the specific Venus and Earth-Archean problems on which progress might be made through comparison. Here we present the major goals of the Venus-Archean comparison and show how preliminary mapping of the geology of the V-2 Fortuna Tessera quadrangle is providing insight on these problems. We have identified five key themes and questions common to both the Archean and Venus, the assessment of which could provide important new insights into the history and processes of both planets.

  4. Significance of an in vitro phenomenon in which murine erythrocytes are lysed by autologous spleen cells and spherules of Coccidioides immitis.

    PubMed Central

    Danley, D L; Hilger, A E; Boonlayangoor, P; Millhouse, E W

    1979-01-01

    Spleen cells from mice immunized with a variety of antigens and incubated in vitro with killed spherules of Coccidioides immitis lyse six to eight times more autologous murine erythrocytes than normal spleen cells and spherules. Cellular and biochemical events in this phenomenon were investigated to ascertain its significance. Kinetic studies suggested that hemolysis results from the activation of some immune cells by spherules. The capacity of spherules to activate these cells is rather unusual because, of the inert particles tested, only zymosan A and crude chitin demonstrated comparable activity. Furthermore, although the hemolytic phenomenon occurred in serum-free medium, more lysis was effected by immune cells and opsonized spherules or zymosan A than by immune cells and untreated fungal particles. Sheep, chicken, and human erythrocytes were not lysed in the hemolytic phenomenon; however, hemoglobin in chicken and sheep erythrocytes was oxidized. Both the murine erythrocyte lysis and oxidation of ovine hemoglobin correlated with the reduction of Nitro Blue Tetrazolium by immune cells adherent to spherules, and both phenomena appeared to be mediated by H2O2 released into the medium by activated cells. Spleen cells reactive with spherules could not be depleted by treatment with iron carbonyl, antiimmunoglobulin plus complement, or anti-brain-associated theta plus complement, but they were partially or completely depleted after rosette formation with erythrocytes coated with antibody or murine complement. Using light and electron microscopy, we noted that immune spleens contained more neutrophils than normal spleens, that these neutrophils reduced Nitro Blue Tetrazolium after stimulation with phorbol myristate acetate, and that they were the most prevalent cell type adherent to spherules after incubation in vitro. Images Fig. 4 Fig. 5 PMID:370005

  5. Altered spherules of impact melt and associated relic glass from the K/T boundary sediments in Haiti

    NASA Technical Reports Server (NTRS)

    Kring, David A.; Boynton, William V.

    1991-01-01

    Partially to wholly altered glass spherules produced by impact-induced shock melting have been found in the K/T boundary sediments of Haiti which also contain grains of shocked quartz. The relic glass has an approximately dacitic composition, and although grossly similar in composition to most previously described tektite glasses, it is slightly enriched in Ca and slightly depleted in Si, suggesting the Haitian glass was produced either from a target with a greater fraction of carbonate and anhydrite lithologies and fewer silicate units than the targets from which most other tektites were produced, and/or from one with a significant mafic component. The composition of the glass can best be reconciled with a continental margin terrane, consistent with studies of shocked mineral phases reported elsewhere. The thickness of the deposit in which the impact spherules occur indicates the source of the ejecta was in the proto-Caribbean region.

  6. Altered spherules of impact melt and associated relic glass from the K/T boundary sediments in Haiti

    SciTech Connect

    Kring, D.A.; Boynton, W.V. )

    1991-06-01

    Partially to wholly altered glass spherules produced by impact-induced shock melting have been found in the K/T boundary sediments of Haiti which also contain grains of shocked quartz. The relic glass has an approximately dacitic composition, and although grossly similar in composition to most previously described tektite glasses, it is slightly enriched in Ca and slightly depleted in Si, suggesting the Haitian glass was produced either from a target with a greater fraction of carbonate and anhydrite lithologies and fewer silicate units than the targets from which most other tektites were produced, and/or from one with a significant mafic component. The composition of the glass can best be reconciled with a continental margin terrane, consistent with studies of shocked mineral phases reported elsewhere. The thickness of the deposit in which the impact spherules occur indicates the source of the ejecta was in the proto-Caribbean region.

  7. Stored mafic/ultramafic crust and early Archean mantle depletion

    NASA Technical Reports Server (NTRS)

    Chase, Clement G.; Patchett, P. J.

    1990-01-01

    Both early and late Archean rocks from greenstone belts and felsic gneiss complexes exhibit positive epsilon(Nd) values of +1 to +5 by 3.5 Ga, demonstrating that a depleted mantle reservoir existed very early. The amount of preserved pre-3.0 Ga continental crust cannot explain such high epsilon values in the depleted residue unless the volume of residual mantle was very small: a layer less than 70 km thick by 3.0 Ga. Repeated and exclusive sampling of such a thin layer, especially in forming the felsic gneiss complexes, is implausible. Extraction of enough continental crust to deplete the early mantle and its destructive recycling before 3.0 Ga ago requires another implausibility, that the sites of crustal generation of recycling were substantially distinct. In contrast, formation of mafic or ultramafic crust analogous to present-day oceanic crust was continuous from very early times. Recycled subducted oceanic lithosphere is a likely contributor to present-day hotspot magmas, and forms a reservoir at least comparable in volume to continental crust. Subduction of an early mafic/ultramafic oceanic crust and temporary storage rather than immediate mixing back into undifferentiated mantle may be responsible for the depletion and high epsilon(Nd) values of the Archean upper mantle.

  8. THE COSMIC-RAY INTENSITY NEAR THE ARCHEAN EARTH

    SciTech Connect

    Cohen, O.; Drake, J. J.; Kota, J.

    2012-11-20

    We employ three-dimensional state-of-the-art magnetohydrodynamic models of the early solar wind and heliosphere and a two-dimensional model for cosmic-ray transport to investigate the cosmic-ray spectrum and flux near the Archean Earth. We assess how sensitive the cosmic-ray spectrum is to changes in the sunspot placement and magnetic field strength, the large-scale dipole magnetic field strength, the wind ram pressure, and the Sun's rotation period. Overall, our results confirm earlier work that suggested the Archean Earth would have experienced a greatly reduced cosmic-ray flux than is the case today. The cosmic-ray reduction for the early Sun is mainly due to the shorter solar rotation period and tighter winding of the Parker spiral, and to the different surface distribution of the more active solar magnetic field. These effects lead to a global reduction of the cosmic-ray flux at 1 AU by up to two orders of magnitude or more. Variations in the sunspot magnetic field have more effect on the flux than variations in the dipole field component. The wind ram pressure affects the cosmic-ray flux through its influence on the size of the heliosphere via the pressure balance with the ambient interstellar medium. Variations in the interstellar medium pressure experienced by the solar system in orbit through the Galaxy could lead to order of magnitude changes in the cosmic-ray flux at Earth on timescales of a few million years.

  9. Post-orogenic thermal evolution of newborn Archean continents

    NASA Astrophysics Data System (ADS)

    Jaupart, C.; Mareschal, J.-C.

    2015-12-01

    The post-orogenic thermal evolution of newborn cratons in the Archean is marked by high-temperature metamorphism and plutonic activity that lag accretion by several tens of million years. The source of the heat that is required remains controversial. Here, we show that such late activity is consistent with the thermal evolution of new continental crust that adjusts to heat released by radioactive decay. Quantitative results depend on the total amount of radioactive elements in the newborn crust. Using heat flow and heat production data from the Archean Superior Province of the Canadian Shield, we show that temperatures ≈800-900 °C were reached in the lower crust a few tens of million years after the final accretion event. The timing of post-orogenic metamorphism is sensitive to the thermal structure acquired at the end of accretion. For the Superior Province, the relatively short time-lag between the end of accretion and metamorphism suggests that the lithosphere was thin or had been heated up by sustained magma percolation.

  10. Dismembered Archean ophiolite in the SE. Wind River Mountains, Wyoming

    SciTech Connect

    Harper, G.D.

    1985-01-01

    Ophiolitic rocks occur as wall rocks of the 2.7 Ga Louis Lake batholith near Atlantic City, Wyoming. All of the Archean rocks are strongly deformed and metamorphosed to a greenschist and amphibolite facies, but relict structures and textures are commonly preserved. These include the following, from west to east: (1) metadiabase with rare coarse-grained metagabbro; (2) ultramafic rocks and metagabbro; (3) amphibolite, locally pillowed, overlain(.) by pelitic schist, banded iron formation, and quartzite; and (4) pillow lavas, massive sills or flows, and minor metasedimentary rocks. Slice 1 locally contains parallel dike margins and rare metagabbro screens; these features suggest that it may represent a sheeted dike complex. Slice 2 locally contains ultramafic rocks having relict cumulus textures and igneous layering, corresponding to the cumulus portion of an ophiolite. The pillow lavas of slice 4 and possibly slice 3 are interpreted as comprising the extrusive portion of the ophiolite. The immobile trace element chemistry (Ti, V, Zr, Y, Cr, Ni) of slice 1 and 4 is very similar and supports a cogenetic origin, whereas pillow lavas of slice 3 are somewhat distinct. The metadiabases and lavas of slices 1 and 4 are similar to modern mid-ocean ridge basalt, whereas lavas of slice 3 are more similar to island-arc tholeiites. Rare high-Ti basaltic komatiites occur in slices 1 and 4, but have very distinct trace element chemistry and probably represent later off-axis dikes. The ophiolitic rocks are interpreted to represent the remains of Archean oceanic crust.

  11. Manganese carbonates as possible biogenic relics in Archean settings

    NASA Astrophysics Data System (ADS)

    Rincón-Tomás, Blanca; Khonsari, Bahar; Mühlen, Dominik; Wickbold, Christian; Schäfer, Nadine; Hause-Reitner, Dorothea; Hoppert, Michael; Reitner, Joachim

    2016-07-01

    Carbonate minerals such as dolomite, kutnahorite or rhodochrosite are frequently, but not exclusively generated by microbial processes. In recent anoxic sediments, Mn(II)carbonate minerals (e.g. rhodochrosite, kutnahorite) derive mainly from the reduction of Mn(IV) compounds by anaerobic respiration. The formation of huge manganese-rich (carbonate) deposits requires effective manganese redox cycling in an oxygenated atmosphere. However, putative anaerobic pathways such as microbial nitrate-dependent manganese oxidation, anoxygenic photosynthesis and oxidation in ultraviolet light may facilitate manganese cycling even in an early Archean environment, without the availability of oxygen. In addition, manganese carbonates precipitate by microbially induced processes without change of the oxidation state, e.g. by pH shift. Hence, there are several ways how these minerals could have been formed biogenically and deposited in Precambrian sediments. We will summarize microbially induced manganese carbonate deposition in the presence and absence of atmospheric oxygen and we will make some considerations about the biogenic deposition of manganese carbonates in early Archean settings.

  12. Early Archean tonalite gneiss in the upper peninsula of Michigan

    NASA Technical Reports Server (NTRS)

    Peterman, Z. E.; Zartman, R. E.; Sims, P. K.

    1986-01-01

    Geochronological results on tonalite gneiss of northern Michigan that is 3.56 Ga or slightly older is presented. Tonalitic augen gneiss and structurally overlying biotite gneiss and schist are exposed in a dome near Watersmeet. They are part of an extensive gneiss terrane of southern Minnesota, Wisconsin and Michigan that includes rocks of early to late Archean ages and lies south of the Wawa volcanic subprovince. Two samples of the augen gneiss and one of the biotite gneiss show zircon grains of similar shape, zoning, color, and development of crystal faces. These zircons give Pb/U isotopic ratios that plot on a chord of 3,560 + or - 40 m.y. upper intersect and of 1,250 + or m.y. lower intersect. The 3,560 m.y. number is believed to be a minimum age because analysis of one of the least discordant zircon fractions by ion microprobe that gave a nearly concordant age of 3,650 m.y. The 1,250 m.y. lower intersect is without geological significance: it is interpreted to be a result of multiple lead loss at 2.7, 1.8, and 0.5 Ga by U/Pb in zircon. Archean rocks 10 to 25 km northwest of the Watersmeet dome give a 2.75 Ga age on zircons. Quartz monzonite here is dated at 2.65 Ga.

  13. Early Archean tonalite gneiss in the upper peninsula of Michigan

    NASA Astrophysics Data System (ADS)

    Peterman, Z. E.; Zartman, R. E.; Sims, P. K.

    Geochronological results on tonalite gneiss of northern Michigan that is 3.56 Ga or slightly older is presented. Tonalitic augen gneiss and structurally overlying biotite gneiss and schist are exposed in a dome near Watersmeet. They are part of an extensive gneiss terrane of southern Minnesota, Wisconsin and Michigan that includes rocks of early to late Archean ages and lies south of the Wawa volcanic subprovince. Two samples of the augen gneiss and one of the biotite gneiss show zircon grains of similar shape, zoning, color, and development of crystal faces. These zircons give Pb/U isotopic ratios that plot on a chord of 3,560 + or - 40 m.y. upper intersect and of 1,250 + or m.y. lower intersect. The 3,560 m.y. number is believed to be a minimum age because analysis of one of the least discordant zircon fractions by ion microprobe that gave a nearly concordant age of 3,650 m.y. The 1,250 m.y. lower intersect is without geological significance: it is interpreted to be a result of multiple lead loss at 2.7, 1.8, and 0.5 Ga by U/Pb in zircon. Archean rocks 10 to 25 km northwest of the Watersmeet dome give a 2.75 Ga age on zircons. Quartz monzonite here is dated at 2.65 Ga.

  14. Mineralogy and composition of Archean Crust, Greenland: A pilot study

    NASA Technical Reports Server (NTRS)

    Goetz, Alexander F. H.; Curtiss, Brian

    1989-01-01

    The Portable Instant Display and Analysis Spectrometer (PIDAS) was taken to southwestern Greenland to investigate in situ the potential application of AVIRIS to estimate the mineralogy and composition of rocks exposed in Archean terranes. The goal was to determine the feasibility of using a high spectral resolution scanner to find and study pristine rocks, those that have not been altered by subsequent deformation and metamorphism. The application of AVIRIS data to the problems in Greenland is logical. However, before a costly deployment of the U-2 aircraft to Greenland is proposed, this study was undertaken to acquire the spectral data necessary to verify that mineralogical mapping in the environmental conditions found there is possible. Although field conditions were far from favorable, all of the major objectives of the study were addressed. One of the major concerns was that lichens would obscure the rock surfaces. It was found that the spectral signature of the lichens was distinct from the underlying rocks. Thus, a spectrum of a rock outcrop, with its partial cover of lichens, can be un-mixed into rock and lichen components. The data acquired during the course of this study supports the conclusion that areas of pristine Archean crust can be differentiated from that which has experienced low grade alteration associated with Proterizoic faulting.

  15. Hematite Spherules of Meridiani Planum: Implications for Aqueous History at the Site

    NASA Astrophysics Data System (ADS)

    Calvin, W. M.

    2004-12-01

    The thermal infrared spectral signature of bulk, grey hematite was the chemical "beacon" that focused the selection of Meridiani Planum as the landing site for the Mars Exploration Rover Opportunity, and aqueous processes were favored for its formation. Orbital data suggesting more bound water in accessory minerals at this location also supported this interpretation. After landing January 24, 2004, the Mini-TES instrument rapidly confirmed the thermal spectral signature of bulk hematite in soils on the plains surrounding Eagle crater and unevenly distributed within the crater. Observations within Eagle crater soon uncovered unusual spherical grains in abundance surrounding the outcrop and Microscopic Imager (MI) showed these grains eroding from within these rocks. They were dubbed "blueberries" by the team due to their spherical nature and their grey or blue appearance compared to their surroundings in various color composites of Pancam images. Extensive observations with the Mössbauer, Alpha-Particle X-ray Spectrometer (APXS) and Mini-TES instruments, especially of the "Berry Bowl" (i.e. with berries and adjacent berry-free rock), confirmed that these spherules are dominantly composed of hematite. Pancam spectra of individual spheres also match laboratory spectra of hematite. These spheres are found within and around outcrop rocks, across the plains of Meridiani, and rolling into the interior of Endurance crater. They are ubiquitous and remarkably even in size and tone. Grains are spherical to subspherical typically 2 to 6 mm in diameter. Opportunity has performed numerous operations with the Rock Abrasion Tool (RAT), and has sliced through individual spherules. The appearance after ratting shows limited or no interior structure and they remain uniformly grey in color. They can preserve scratches from the grinding wheel or become dislodged by ratting, indicating they are harder than the host rock. Several lines of evidence suggest the spheres are post

  16. Archean Pb Isotope Evolution: Implications for the Early Earth.

    NASA Astrophysics Data System (ADS)

    Vervoort, J. D.; Thorpe, R.; Albarede, F.; Blichert-Toft, J.

    2008-12-01

    The U-Pb isotope system provides us with a powerful tool for understanding the chemical evolution of the Earth. Pb isotopes in Archean rocks, however, have not been widely utilized because U mobility makes initial Pb isotope ratios from old silicate rocks difficult, if not impossible, to determine. Galenas in syngenetic volcanogenic massive sulfide (VMS) deposits, however, provide snapshots of initial Pb ratios because their Pb isotopic composition is time invariant at their formation (U/Pb=0). The Pb isotopic record from galenas from rocks of all age have been utilized for over 70 years to answer a wide range of scientific problems beginning with Al Nier's pioneering work analyzing Pb isotopes in the 1930's but are no longer widely used by the isotopic community because they have been produced by older TIMS techniques. We have begun a re-examination of Archean Pb by an extensive analysis of over 100 galena samples from Archean VMS deposits throughout the Superior and Slave Provinces in Canada as well as from other VMS deposits in Finland, South Africa and Western Australia. The goal of this work is to provide modern, high precision measurements and update an old, but venerable, Pb isotopic data set. We feel these data provide important constraints on not only the Pb isotopic evolution of the Earth, but planetary differentiation and recycling processes operating in the first 2 b.y. of Earth's history. Our analytical techniques include dissolving the Pb sulfide minerals, purifying them with ion chromatography, and analyzing them using MC-ICPMS at both Washington State University (Neptune) and Ecole Normale Superieure in Lyon, France (Nu). All Pb solutions are doped with Tl in order to correct for mass fractionation. In this abstract we report preliminary galena Pb isotope data from 6 VMS deposits in the Abitibi greenstone belt: Chibougamu, Matagami, Noranda, Normetal, Timmins, and Val d"Or. These deposits are all approximately 2.7 Ga in age but in detail vary from 2

  17. What is the Age and Origin of the Spherule Bearing Layer in some Ross Sea Cores?

    NASA Astrophysics Data System (ADS)

    Yeh, C.; Abbott, D. H.; Anders, M. H.; Breger, D.

    2012-12-01

    L.P. Khyranina (1985) was the first to suggest that there were two structures present on the Antarctic continental shelf beneath the Ross Sea. One was the ~100 km Bowers impact crater candidate. However, there was no solid evidence that proved an extraterrestrial impact produced the Bowers structure. Debate has ensued for decades on whether the Bowers structure was an impact crater. Now we have come close to finding an answer. We used core samples taken near the crater and sieved into four size fractions (>250,>125,>63,>38 μm). Impact ejecta candidates were then picked from cores ELT 32-08, ELT 32-03, ELT 32-43, ELT 32-06, DSDP 273, DSDP 274, and NPB 95-01 39KC and were analyzed for chemical composition and physical features. All but DSDP site 274 contain pure SiO2 glasses that resemble tektites. We also found candidates for flow textured impact glass and shocked quartz. This led us to conclude that the origin of the spherule-bearing layer was from an impact. With the use of dated core NBP 95-01 39KC, we have constrained the age to lie between 7,305±80 BP and 11,150±95 BP (corrected radiocarbon ages) (Cunningham et al., 1999).

  18. Late Archean Euxinia as a Window into Early Biogeochemical Cycles

    NASA Astrophysics Data System (ADS)

    Scott, C.; Bekker, A.; Reinhard, C.; Lyons, T. W.

    2009-12-01

    A number of transition metals present in seawater in trace amounts (10-10 to 10-7 moles/L) are nevertheless bioessential micronutrients, utilized in a wide range of cellular activities. Because their abundances in seawater are largely a reflection of redox-controlled sources and sinks, Precambrian biogeochemists increasingly focus on the interrelated nature of major redox transitions, the chemical composition of the oceans, and the evolution of life on Earth. Of particular interest are temporal trends in seawater inventories of elements utilized in the nitrogen cycle, both nitrogen fixation (Fe, V, Mo) and denitrification (Cu). Recent work on the link between trace metal abundance and the biologically mediated nitrogen cycle has focused on the Proterozoic Eon, when oxidative weathering was well established and sulfidic conditions were common in the deep ocean. However, we know little about trace metal availability during the Archean Eon, when oxygenic photosynthesis first appeared on Earth and began to alter the chemical composition of the oceans and atmosphere. The development of euxinic conditions, or anoxic and sulfidic bottom waters, provides important information regarding the cycling of major elements such as C, S and Fe. However, euxinic black shales can also provide a record of trace metal abundance. Mo is highly enriched in these shales and displays a conspicuous covariation with the concentration of total organic carbon (TOC). Furthermore, it has been demonstrated that the ratio Mo/TOC is proportional to the concentration of Mo in seawater. Cu and V are also enriched in euxinic black shales, and both correlate with TOC. By analogy with Mo, it is likely that the ratios Cu/TOC and V/TOC also contain information on the concentration of these transition metals in seawater. Here we present C-S-Fe systematics as well as trace metal concentrations from black shales of the Roy Hill Member of the late Archean Jeerinah Formation. Fe speciation indicates that the

  19. Air pollution history elucidated from anthropogenic spherules and their magnetic signatures in marine sediments offshore of Southwestern Taiwan

    NASA Astrophysics Data System (ADS)

    Horng, Chorng-Shern; Huh, Chih-An; Chen, Kuo-Hang; Huang, Pin-Ru; Hsiung, Kan-Hsi; Lin, Hui-Ling

    2009-03-01

    Kaohsiung City and its neighborhood in the southwestern coastal plain of Taiwan have suffered serious air pollution since the region became the largest center for heavy-industry on the island. In order to unravel the air pollution history of the region, four 210Pb- and 137Cs-dated sediment box cores recovered in 2006 from offshore of this area were chosen for magnetic and petrographic analyses. The data were used to distinguish changes in concentration, composition and grain size of magnetic particles in the sediments due to inputs of anthropogenic magnetic spherules. Sedimentation rates have been reasonably constant for the last one hundred years, except at the core tops which were affected by a turbidite layer induced by a typhoon in 2005. Down-core profiles of mass-specific magnetic susceptibility ( χ) and saturation isothermal remanent magnetization (SIRM) are similar among the cores, and reflect similar trends to magnetic spherule counts. This reveals that χ and SIRM of modern marine sediments can be used as air pollution indicators for nearby industrialized upwind areas. The studied record indicates that industrialization of the area was gradual during 1950-1980 and boomed afterward, resulting in a high production of airborne magnetic spherules, which is consistent with evidence for poor air quality at that time. Optical and scanning electron microscopic (SEM) surveys of magnetic extracts indicate that the magnetic spherules have grain sizes ranging from a few micrometers up to 50 μm and consist mainly of iron oxides with variable Si, Al, and Ca contents. X-ray diffraction analysis on magnetic extracts from different depths in the cores further indicates that magnetite and pyrrhotite, which are derived from terrigenous detritus, form the magnetic constituents of the sediments before the area was industrialized. In contrast, during the industrial boom, anthropogenic magnetite and hematite spherules became the dominant magnetic particles in the sediments

  20. Short-lived chemical heterogeneities in the archean mantle with implications for mantle convection.

    PubMed

    Blichert-Toft, J; Albarede, F

    1994-03-18

    The neodymium isotope and samarium-neodymium systematics of 2.7-billion-year-old mantle-derived magmas indicate that the lifetime of chemical heterogeneities was much shorter in the Archean mantle than in the modern mantle. Isotopic evidence is compatible with a Rayleigh number 100 times larger and convection 10 times faster in the Late Archean compared with the present-day mantle. Modern plate tectonics thus may be an improbable analog for the Archean. Chemical heterogeneities in the mantle may originate upon magma migration and mineralogical phase changes rather than by recycling of oceanic and continental crust. PMID:17744788

  1. Methane Greenhouses and Anti-Greenhouses During the Archean Era

    NASA Astrophysics Data System (ADS)

    Kasting, J. F.; Pavlov, A. A.

    2002-12-01

    Climate and life are coupled today through the biogeochemical carbon cycle, but they may have been even more tightly coupled in the distant past when atmospheric O2 levels were lower. The finding of mass-independently fractionated S isotopes in Archean rocks confirms that pO2 was very low, probably <10-13 times the present level, prior to 2.3 Ga (1). The Sun was also some 20 percent less luminous at this time (2). High CO2 levels were initially proposed to solve this `faint young Sun problem' (3); however, these levels are in conflict in data from paleosols (4). CH4 is an alternative greenhouse gas which could have kept the Archean climate warm if present at concentrations of 0.01-0.1 percent by volume (5). The primary source of methane is biological. CH4 is produced by methanogenic bacteria that today live in anaerobic environments such as the intestines of ruminants and the water-logged soils underlying rice paddies. During the Archean, however, methanogens should have been widespread, and the methane they produced would have had a long photochemical lifetimes, around 10,000 years (6). Most methanogens are thermophiles or hyperthermophiles, and those which are more thermophilic have shorter doubling times than those that prefer cooler temperatures. This suggests that a positive feedback loop may have existed, whereby methanogens warmed the climate by releasing CH4, which in turn promoted the proliferation of faster-growing methanogens. This positive feedback would have been halted, however, once the ratio of CH4 to CO2 in the atmosphere exceeded unity. At this point, polymerization of CH4 by solar UV radiation would have caused the formation of an organic haze layer similar to that observed today on Titan. Such a haze layer would have cooled the climate by creating an `anti-greenhouse effect.' This creates an overall negative feedback loop that may have been responsible for maintaining a stable Archean climate. The rise of O2 at 2.3 Ga disrupted this equilibrium

  2. The Petrogenetic significance of Plagioclase megacrysts in Archean rocks

    NASA Technical Reports Server (NTRS)

    Morrison, D. A.; Phinney, W. C.; Maczuga, D. E.

    1988-01-01

    The petrogenetic significance of plagioclase megacryst-bearing Archean rocks was considered. It was suggested that these developed in mid-to upper-crustal magma chambers that were repeatedly replenished. Crystallization of megacrysts from a primitive liquid that evolves to an Fe-rich tholeiite (with LREE enrichment) is nearly isothermal and is an equilibrium process. Cumulates probably form near the margins of the chambers and liquids with megacrysts are periodically extracted and can appear as volcanics. Some flows and intrusives are found in arc-like settings in greenstone belts. Megacrystic dikes represent large volumes of melt and dike swarms such as the Metachawan swarm of Ontario suggest multiple sources of similar compositions. A complex series of melt ponding and migration are probable and involve large amounts of liquid.

  3. Tracing fluid pathways in Archean hydrothermal systems with imaging spectroscopy

    NASA Astrophysics Data System (ADS)

    von Ruitenbeek, F. J. A.; Cudahy, T.; Hale, M.; van der Werff, H. M. A.; van der Meer, F. D.

    2008-09-01

    Abstract Fossil submarine hydrothermal systems in Archean greenstone belts and other geologic terranes are important because of their relationship with volcanic massive sulfide (VMS) mineral deposits and their association with environments that are favorable for early forms of life. Interpretation and reconstruction of these systems is difficult because of their geologic complexity. Airborne imaging spectroscopy provides information about the presence, abundance, and composition of near-infrared active minerals at continuous spatial coverage and high spatial resolution, and can therefore be used to obtain new geologic insights into of the Archean hydrothermal systems. It was applied to the Panorama VMS-district in the Soanesville greenstone belt, Western Australia. Results from the analyses of 189 hand specimen showed that the wavelength position of the main absorption feature of white micas, a proxy for their Al content, varied between 2195 nm and 2225 nm. These wavelength variations and the relative abundance of white micas were used to reconstruct fossil fluid pathways from low-temperature recharge to hightemperature discharge zones. Results also showed that the absorption-wavelength variations of white micas could be mapped from airborne imaging spectroscopy using a stochastic method where the presence of white mica minerals and their absorption wavelengths in field measurements were predicted from hyperspectral band ratios. Analysis of the spatial patterns in segmented images, covering 52 km2, of white mica probability and their absorption wavelengths and their comparison with field data resulted in the identification of regional scale hydrothermal fluid pathways, a regional-scale K alteration event, and differences in hydrothermal regime between the northern and southern parts of the test area.

  4. Late Archean mafic volcanism in the Rainy Lake area, Minnesota

    SciTech Connect

    Day, W.C.

    1985-01-01

    The Late Archean greenstone-granite terrane of the Rainy Lake area of Minnesota contains a bimodal suite of mafic and felsic volcanic and coeval intrusive rocks. New geochemical data show that the mafic rocks occur in three distinct suites: (1) low-Ti olivine- and quartz-tholeiite, (2) high-Ti quartz-tholeiite and basaltic andesite, and (3) calc-alkaline lamprophyric monzodiorite and quartz diorite. The low-Ti tholeiites have only slightly evolved Mg-numbers from 53-63, Ni=125-300 ppm, and MORB-like REE. In contrast, the high-Ti tholeiites are more evolved, with Mg*=26-48, Ni=43-135 ppm, and higher total REE. Compared to the tholeiitic suites, the monzodiorite suite has more primitive Mg-numbers, with Mg*=70-78, Ni<410 ppm, and anomalously high LREE. The two tholeiitic suites cannot be genetically related by simple fractionation from a single parent magma; however, lower degrees of partial melting (<8 percent) of a mantle source (spinel periodotite) with REE=2-4 times chondrites could have produced the high-Ti tholeiites, and higher degrees of melting (20-30 percent) of a similar source could have generated the low-Ti tholeiites. In contrast, the monzodiorite suite must have been generated from either a LREE-rich or (and) a garnet-bearing source (garnet periodotite). The authors conclude that shallow melting (<40-50 km) within the Archean mantle in the Rainy Lake area produced the tholeiitic rocks, and that deep melting (>40-50 km) generated the lamprophyric monzodiorites.

  5. Archean evolution of the Leo Rise and its Eburnean reworking

    NASA Astrophysics Data System (ADS)

    Thiéblemont, Denis; Goujou, Jean Christian; Egal, Emmanuel; Cocherie, Alain; Delor, Claude; Lafon, Jean Michel; Fanning, C. Mark

    2004-06-01

    Recent geological mapping in southeastern Guinea, supported by zircon dating, has called into question traditional understanding concerning the evolution of the Leo Rise. Gneiss dated at about 3540 Ma appears to constitute the earliest evidence for continental accretion within the Leo Rise. The existence of a Leonian depositional cycle at about 3000 Ma is confirmed, marked by volcanic and sedimentary rocks that can be correlated with the Loko Group in Sierra Leone. The span of ages (3244-3050 Ma) suggests that the Leonian cycle comprises different episodes whose respective chronology is as yet uncertain. Clearly distinct from the Leonian cycle, the Liberian cycle (˜2900-2800 Ma) is represented in Guinea by granite and migmatite (˜2910-2800 Ma), reflecting remobilization of the ancient Archean basement and deformation of the Leonian rocks; no deposition is associated with this cycle. After the Liberian, the Nimba and Simandou successions, containing Liberian detrital zircons, are assigned to the Birimian (˜2200-2000 Ma). Finally, Eburnean tectonism caused intense deformation of the Archean craton, accompanied by high-grade metamorphism and the intrusion of granite and syenite with ages between 2080 and 2020 Ma. The evolution of the Kénéma-Man domain, attributed to the cumulated effect of the Leonian and Liberian cycles, is thus in part Eburnean. We can suppose, therefore, that the NNE-SSW-trending structures attributed to the Liberian in Sierra Leone are, in fact, Eburnean. The Kambui Supergroup, also affected by this tectonism, should thus be assigned to the Birimian rather than the Liberian, which would explain its similarities with the Nimba and Simandou successions.

  6. Microtektite spherules from proximal K-Pg sections: alteration patterns and clues to precursor melt lithologies

    NASA Astrophysics Data System (ADS)

    Belza, J.; Goderis, S.; Smit, J.; Vanhaecke, F. F.; Baert, K.; Terryn, H.; Claeys, P. F.

    2013-05-01

    In the proximal K-Pg ejecta deposits all around the Gulf of Mexico, impact melt from the Chicxulub crater occurs predominantly in the form of glassy spherules or 'microtektites', which are considered droplets of melt that travelled ballistically from the point of impact and quenched rapidly during flight. In most localities, spherules are severely altered. Pristine, unaltered glass is very scarce and is rarely found in the center of a smectite/chlorite shell. Although the micro-tektite glass has been shown to be heterogeneous even on the μm-scale, geochemical data have solely been reported from bulk samples. Simple binary mixing between the sedimentary carbonate and the currently described underlying crystalline basement does not fully account for the variety of tektite impact melt products, which severely complicates their interpretation. Using LA-ICP-MS, we have conducted spatially resolved analysis of trace elements in fresh, unaltered microtektite glasses and their surrounding alteration phase. This approach is unique amongst previous studies, in that it offers the opportunity to study in situ and with high spatial resolution the mixing of different target lithologies and the variation of the trace element budget during alteration. We present data from three groups of microtektite glasses at the Mimbral (NE Mexico) and Beloc (Haiti) locality: high-SiO2 green and red glasses, high-Ca yellow glass, and black glasses. All green and red glasses exhibit low oxides totals (85-90 wt%) as determined by Electron Microprobe analyses, suggesting the presence of H2O or hydrous alteration products. Raman spectroscopy clearly shows the spectrum of a hydrated glass, not an alteration phase. Their relatively high Raman index of polymerization (compared to black glass) is consistent with a normalized (water-free basis) major element composition high in SiO2 and low in network modifiers (Mg, Ca and Fe). Black and yellow glasses are not hydrated but are surrounded by a double

  7. Coccidioides Endospores and Spherules Draw Strong Chemotactic, Adhesive, and Phagocytic Responses by Individual Human Neutrophils

    PubMed Central

    Lee, Cheng-Yuk; Thompson III, George R.; Hastey, Christine J.; Hodge, Gregory C.; Lunetta, Jennine M.; Pappagianis, Demosthenes; Heinrich, Volkmar

    2015-01-01

    Coccidioides spp. are dimorphic pathogenic fungi whose parasitic forms cause coccidioidomycosis (Valley fever) in mammalian hosts. We use an innovative interdisciplinary approach to analyze one-on-one encounters between human neutrophils and two forms of Coccidioides posadasii. To examine the mechanisms by which the innate immune system coordinates different stages of the host response to fungal pathogens, we dissect the immune-cell response into chemotaxis, adhesion, and phagocytosis. Our single-cell technique reveals a surprisingly strong response by initially quiescent neutrophils to close encounters with C. posadasii, both from a distance (by complement-mediated chemotaxis) as well as upon contact (by serum-dependent adhesion and phagocytosis). This response closely resembles neutrophil interactions with Candida albicans and zymosan particles, and is significantly stronger than the neutrophil responses to Cryptococcus neoformans, Aspergillus fumigatus, and Rhizopus oryzae under identical conditions. The vigorous in vitro neutrophil response suggests that C. posadasii evades in vivo recognition by neutrophils through suppression of long-range mobilization and recruitment of the immune cells. This observation elucidates an important paradigm of the recognition of microbes, i.e., that intact immunotaxis comprises an intricate spatiotemporal hierarchy of distinct chemotactic processes. Moreover, in contrast to earlier reports, human neutrophils exhibit vigorous chemotaxis toward, and frustrated phagocytosis of, the large spherules of C. posadasii under physiological-like conditions. Finally, neutrophils from healthy donors and patients with chronic coccidioidomycosis display subtle differences in their responses to antibody-coated beads, even though the patient cells appear to interact normally with C. posadasii endospores. PMID:26070210

  8. Rare earth element patterns in Archean high-grade metasediments and their tectonic significance

    NASA Technical Reports Server (NTRS)

    Taylor, Stuart Ross; Rudnick, Roberta L.; Mclennan, Scott M.; Eriksson, Kenneth A.

    1986-01-01

    REE data on metasedimentary rocks from two different types of high-grade Archean terrains are presented and analyzed. The value of REEs as indicators of crustal evolution is explained; the three geologic settings (in North America, Southern Africa, and Australia) from which the samples were obtained are described; and the data are presented in extensive tables and graphs and discussed in terms of metamorphic effects, the role of accessory phases, provenance, and tectonic implications (recycling, the previous extent of high-grade terrains, and a model of Archean crustal growth). The diversity of REE patterns in shallow-shelf metasediments is attributed to local provenance, while the Eu-depleted post-Archean patterns are associated with K-rich plutons from small, stable early Archean terrains.

  9. Dating Archean zircon by ion microprobe: New light on an old problem

    NASA Technical Reports Server (NTRS)

    Williams, I. S.; Kinny, P. D.; Black, L. P.; Compston, W.; Froude, D. O.; Ireland, T. R.

    1985-01-01

    Ion microprobe analysis of zircons from three sites (Watersmeet Dome in northern Michigan, Mount Sones in eastern Antarctica, and Mount Narryer in western Australia) is discussed. Implications of the results to Archean geochronology and early Earth crust composition are addressed.

  10. 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. PMID:26798012

  11. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics

    NASA Astrophysics Data System (ADS)

    Tang, Ming; Chen, Kang; Rudnick, Roberta L.

    2016-01-01

    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.

  12. Nature of the Coast Batholith, Southeastern Alaska: Are there Archean analogs

    NASA Technical Reports Server (NTRS)

    Barker, Fred; Arth, J. G.

    1988-01-01

    The comparison of Phanerozoic Andean margins and their possible Archean analogs was made. Geochemical and isotopic data was presented for the episodic intrusion of the elongate, continental margin Coast batholith of southeastern Alaska and British Columbia. The batholith was characterized as having been formed in direct response to subduction in accreted terranes of oceanic or slope origin. It was concluded that there were good analogs of the Coast batholith in Archean plutonic suites.

  13. Magnetic Fe, Si, Al-Rich Impact Spherules from the P-T Boundary Layer at Graphite Peak, Antarctica

    NASA Technical Reports Server (NTRS)

    Petaev, M. I.; Jacobsen, S. B.; Basu, A. R.; Becker, L.

    2004-01-01

    The geological boundary between Triassic and Permian strata coincides with the greatest life extinction in the Earth's history. Although the cause of the extinction is still the subject of intense debates, recent discoveries in the P-T boundary layer of shocked quartz grains, fullerenes with the extraterrestrial noble gases, Fe metal nuggets, and chondritic meteorite fragments all point to a powerful collision of Earth with a celestial body in the late Permian. Here we report the discovery of magnetic Fe, Si, Al-rich impact spherules which accompany the chondritic meteorite fragments in some samples from the P-T boundary layer at Graphite Peak, Antarctica.

  14. A Glass Spherule of Questionable Impact Origin from the Apollo 15 Landing Site: Unique Target Mare Basalt

    NASA Technical Reports Server (NTRS)

    Ryder, Graham; Delano, John W.; Warren, Paul H.; Kallemeyn, Gregory W.; Dalrymple, G. Brent

    1996-01-01

    A 6 mm-diameter dark spherule, 15434,28, from the regolith on the Apennine Front at the Apollo 15 landing site has a homogeneous glass interior with a 200 microns-thick rind of devitrified or crystallized melt. The rind contains abundant small fragments of Apollo 15 olivine-normative mare basalt and rare volcanic Apollo 15 green glass. The glass interior of the spherule has the chemical composition, including a high FeO content and high CaO/Al2O3, of a mare basalt. Whereas the major element and Sc, Ni, and Co abundances are similar to those of low-Ti mare basalts, the incompatible elements and Sr abundances are similar to those of high-Ti mare basaits. The relative abundance patterns of the incompatible trace elements are distinct from any other lunar mare basalts or KREEP; among these distinctions are a much steeper slope of the heavy rare earth elements. The 15434,28 glass has abundances of the volatile element Zn consistent with both impact glasses and crystalline mare basalts, but much lower than in glasses of mare volcanic origin. The glass contains siderophile elements such as Ir in abundances only slightly higher than accepted lunar indigenous levels, and some, such as Au, are just below such upper limits. The age of the glass, determined by the Ar-40/Ar-39 laser incremental heating technique, is 1647 +/- 11 Ma (2 sigma); it is expressed as an age spectrum of seventeen steps over 96% of the Ar-38 released, unusual for an impact glass. Trapped argon is negligible. The undamaged nature of the sphere demonstrates that it must have spent most of its life buried in regolith; Ar-38 cosmic ray exposure data suggest that it was buried at less than 2m but more than a few centimeters if a single depth is appropriate. That the spherule solidified to a glass is surprising; for such a mare composition, cooling at about 50 C/s is required to avoid crystallization, and barely attainable in such a large spherule. The low volatile abundances, slightly high siderophile

  15. Bed Bugs FAQs

    MedlinePlus

    ... Tropical Diseases Laboratory Diagnostic Assistance [DPDx] Parasites Home Bed Bugs FAQs Recommend on Facebook Tweet Share Compartir On ... are bed bugs treated and prevented? What are bed bugs? Bed bugs ( Cimex lectularius ) are small, flat, parasitic ...

  16. New Discoveries From The Archean Biosphere Drilling Project (ABDP)

    NASA Astrophysics Data System (ADS)

    Nedachi, M.

    2004-12-01

    The Archean Biosphere Drilling Project (ABDP), an international scientific drilling project involving scientists from the USA, Australia and Japan, was initiated in Pilbara Craton, Western Australia. The scientific objectives of the ABDP are the identification of microfossils and biomarkers, the clarification of geochemical environment of the early Earth, and the understanding of geophysical contribution to the co-evolution of life and environment. Through 2003 and 2004 activities, we have drilled 150 _| 300 m deep holes to recover _gfresh_h (modern weathering-free) geologic formations that range from 3.5 to 2.7 Ga in age. The drilling targets were: (1) 3.46 Ga Towers Formation, (2) mid-Archean Mosquito Formation, (3) 2.77 Ga Mt Roe Basalt, (4) 2.76 Ga Tumbiana Formation, (5) 2.74 Ga Hardey Formation. The initial investigations on the ABDP drill cores by Japanese members have already produced many exciting and interesting data and observations. 3.46 Ga Marble Bar Jasper could provide clues to the argument about the early photosynthetic cyanobacteria that have produced free oxygen and have evolved the oxygen level on the earth. There have been many ideas how the hematite in jasper was formed. Our most important discoveries are the confirmations that hematite, magnetite and siderite precipitated separately as primary minerals, and that there is a remaining texture which resembles microfossil using FE-SEM, ESCA, Laser-Raman and cathodoluminescence. Taking into account the carbon isotopic ratios of remains from _|25 to _|40 permil, these iron oxides might be biogenic. We need to identify the iron bacteria in detail to deduce the early earth_fs surface environment. In addition, the black shale of Apex Basalt overlying Marble Bar Jasper contains organic carbon from 0.7 to 5.2 percent, and the carbon isotopic ratio of which is from -26 to -30 per mil, suggesting that various microbes inhabited in the early Archean ocean. 2.77 Ga Mt Roe Basalt, which is composed of

  17. Geostable molecules and the Late Archean 'Whiff of Oxygen'

    NASA Astrophysics Data System (ADS)

    Summons, R. E.; Illing, C. J.; Oduro, H. D.; French, K. L.; Ono, S.; Hallmann, C.; Strauss, H.

    2012-12-01

    exhibits a 'MIF' signal that is significantly amplified compared to co-occurring pyrite sulfur. Limited isotopic exchange between the organic and inorganic sulfur pools suggests Archean origin of these organic sulfur compounds. We also report new results from the 2012 Agouron Pilbara drilling project. Anbar A.D. et al. A whiff of oxygen before the great oxidation event. Science 317, 1903-1906. (2007). Bosak T. et al., Morphological record of oxygenic photosynthesis in conical stromatolites. Proc. Natl. Acad. Sci. USA 106:10939-10943 (2009). Kopp, R.E. et al.,The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis. Proc. Natl. Acad. Sci. USA 102: 11131-11136 (2005). Waldbauer J.R. et al., Late Archean molecular fossils from the Transvaal Supergroup record the antiquity of microbial diversity and aerobiosis. Precambrian Research 169, 28-47 (2008). Waldbauer J.R. et al., 2011. Microaerobic steroid biosynthesis and the molecular fossil record of Archean life. Proceedings of the National Academy of Sciences (USA) 108, 13409-13414

  18. A CaAl4O7-bearing refractory spherule from Murchison - Evidence for very high-temperature melting in the solar nebula

    NASA Astrophysics Data System (ADS)

    Simon, S. B.; Yoneda, S.; Grossman, L.; Davis, A. M.

    1994-04-01

    Results are presented of analyses (by a SEM equipped with an EDXS system, INAA, and an electron microprobe) of a unique refractory spherule, B6, recovered from the Murchison C2 chondrite. The spinel-, hibonite-, CaAl4O7-, perovskite-bearing spherule appears to have been almost completely molten, requiring exposure to the highest temperature yet inferred for any refractory inclusion. Strong evidence is presented that the spherule crystallized from a liquid. The presence of measurable excess Al2O3 in B6 spinel despite the normal a(Al2O3)/a(MgO) ratio of the inclusion is further evidence of its unusually high temperature history. Despite the high peak temperature, no volatility-derived fractionation of REEs or isotopic fractionation of Mg occurred, probably because cooling was rapid enough for a very rapid formation of a spinel shell which sealed off the molten interior of the inclusion from the nebular gas.

  19. Heterogeneously hydrated mantle beneath the late Archean Yilgarn Craton

    NASA Astrophysics Data System (ADS)

    Ivanic, T. J.; Nebel, O.; Jourdan, F.; Faure, K.; Kirkland, C. L.; Belousova, E. A.

    2015-12-01

    Archean mafic-ultramafic melts, crystallized as layered intrusions in the upper crust and extruded as komatiitic flows, are primary probes of upper mantle chemistry. However, the message from their primary chemical composition can be compromised by different modes of contamination. Contaminants are typically cryptic in terms of their geochemical and isotopic signals but may be related to metasomatised mantle sources, ambient crustal assimilation or subduction-related inputs. In this work we present critical evidence for both dry and wet Archean mantle sources for two juxtaposed layered intrusions in the Australian Yilgarn Craton. The ca. 2813 Ma Windimurra and ca. 2800 Ma Narndee Igneous Complexes in Western Australia are two adjacent layered intrusions and would be expected to derive via similar mantle sections. A key difference in their chemistry is the presence of crystal-bound water in the Narndee Igneous Complex, represented primarily by abundant hornblende. Such a primary hydrous phase is notably absent in the Windimurra Igneous Complex. New 40Ar/39Ar plateau ages for fresh Narndee hornblende (weighted mean: 2805 ± 14 Ma, MSWD = 1.8, probability = 0.18) agrees with the published U-Pb age of 2800 ± 6 Ma for the complex and is consistent with a magmatic origin for this phase. Zircon Hf and whole-rock Hf and Nd isotopes for the Narndee Igneous Complex indicate only minor crustal contamination, in agreement with H and O isotope values in amphibole and O isotope values in rare zircon crystals, plagioclase and pyroxene within both complexes. These findings illustrate a fast temporal transition, in proximal bodies, from anhydrous to hydrous mantle sources with very minor crustal contamination. These large layered mafic-ultramafic intrusions are igneous bodies with a primitive chemical bulk composition that requires large degrees of mantle melting. This has been attributed by many workers to mantle plume activity, yet not without dispute, as subduction-related flux

  20. Geochemical evolution of magmatism in Archean granite-greenstone terrains

    NASA Astrophysics Data System (ADS)

    Samsonov, A. V.; Larionova, Yu. O.

    2006-05-01

    Evolution of Archean magmatism is one of the key problems concerning the early formation stages of the Earth crust and biosphere, because that evolution exactly controlled variable concentrations of chemical elements in the World Ocean, which are important for metabolism. Geochemical evolution of magmatism between 3.5 and 2.7 Ga is considered based on database characterizing volcanic and intrusive rock complexes of granite-greenstone terrains (GGT) studied most comprehensively in the Karelian (2.9-2.7 Ga) and Kaapvaal (3.5-2.9 Ga) cratons and in the Pilbara block (3.5-2.9 Ga). Trends of magmatic geochemical evolution in the mentioned GGTs were similar in general. At the early stage of their development, tholeiitic magmas were considerably enriched in chalcophile and siderophile elements Fe2O3, MgO, Cr, Ni, Co, V, Cu, and Zn. At the next stage, calc-alkaline volcanics of greenstone belts and syntectonic TTG granitoids were enriched in lithophile elements Rb, Cs, Ba, Th, U, Pb, Nb, La, Sr, Be and others. Elevated concentrations of both the “crustal” and “mantle-derived” elements represented a distinctive feature of predominantly intrusive rocks of granitoid composition, which were characteristic of the terminal stage of continental crust formation in the GGTs, because older silicic rocks and lithospheric mantle were jointly involved into processes of magma generation. On the other hand, the GGTs different in age reveal specific trends in geochemical evolution of rock associations close in composition and geological position. First, the geochemical cycle of GGT evolution was of a longer duration in the Paleoarchean than in the Meso-and Neoarchean. Second, the Paleoarche an tholeiitic associations had higher concentrations of LREE and HFSE (Zr, Ti, Th, Nb, Ta, Hf) than their Meso-and Neoarchean counterparts. Third, the Y and Yb concentrations in Paleoarchean calc-alkaline rock associations are systematically higher than in Neoarchean rocks of the same type

  1. Carbonaceous cherts in the Barberton greenstone belt and their significance for the study of early life in the Archean record.

    PubMed

    Hofmann, Axel; Bolhar, Robert

    2007-04-01

    The 3.5-3.2 Ga old volcano-sedimentary succession of the Barberton greenstone belt (South Africa) is characterized by lithological units that are repeated in a regular manner. Komatiitic, basaltic, and dacitic volcanic and volcaniclastic sequences are capped by zones of silica enrichment, followed by bedded carbonaceous cherts. Stratiform and crosscutting carbonaceous chert veins are common in silica alteration zones and bedded cherts. A detailed field study of several chert horizons and chert veins that range in age from 3.47 to 3.30 Ga revealed the importance of syndepositional hydrothermal activity for their origin. Bedded cherts consist of silicified detrital and tuffaceous sediments that were deposited on the seafloor. Silicification took place at the sediment-water interface as a result of diffuse upflow of low-temperature hydrothermal fluids, which gave rise to the formation of impermeable chert caps. Fluid overpressure resulted in the breaching of the cap rocks at times. Chert veins contain angular host rock fragments, replace wall rocks, and show evidence of multiple vein fillings and in situ brecciation of earlier generations of vein fillings. They represent hydraulic fractures that were initiated by overpressuring of the hydrothermal system. The vein networks were infilled, partly by hydrothermal chert precipitates, and partly by still unconsolidated (not yet silicified) sedimentary material derived from overlying sedimentary horizons. Field, petrographic, isotopic, and trace element evidence indicate that most carbonaceous matter represents sedimentary material that originated by biogenic processes in the Archean oceans and not by hydrothermal processes in the subsurface. PMID:17480166

  2. Archean foreland basin tectonics in the Witwatersrand, South Africa

    NASA Technical Reports Server (NTRS)

    Burke, K.; Kidd, W. S. F.; Kusky, T. M.

    1986-01-01

    The Witwatersrand Basin of South Africa is the best-known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch-type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These features indicate that the Witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an Andean-type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes above zones of shallow subduction. It is suggested that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision-related plateau, now represented by the Limpopo Province. Similarities are seen between this phase of Witwatersrand Basin evolution and that of active basins north of the Tibetan Plateau. The geologic evidence does not agree with earlier suggestions that the Witwatersrand strata were deposited in a rift or half-graben.

  3. Archean foreland basin tectonics in the Witwatersrand, South Africa

    SciTech Connect

    Burke, K.; Kidd, W.S.F.; Kusky, T.M.

    1986-06-01

    The Witwatersrand Basin of South Africa is the best-known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch-type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These features indicate that the Witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an Andean-type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes above zones of shallow subduction. It is suggested that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision-related plateau, now represented by the Limpopo Province. Similarities are seen between this Phase of Witywatersrand Basin evolution and that of active basins north of the Tibetan Plateau. The geologic evidence does not agree with earlier suggestions that the Witwatersrand strata were deposited in a rift or half-graben. 64 references.

  4. Archean foreland basin tectonics from the Witwatersrand, South Africa

    SciTech Connect

    Burke, K.; Kidd, W.S.F.; Kusky, T.M.

    1985-01-01

    The Witwatersrand Basin of South Africa is the best-known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch-type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These and other features indicate that the Witwatersrand strata were deposited in a foreland basin. A regional geologic synthesis suggests that his basin developed initially on the cratonward side of an Andean-type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes above zones of shallow subduction. We suggest that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision-related plateau, now represented by the Limpopo Province. Striking similarities are seen between this phase of Witwatersrand Basin evolution and active basins located north of the Tibetan Plateau. The geologic evidence is not so compatible with earlier suggestions that the Witwatersrand strata were deposited in a rift or half-graben.

  5. Archean foreland basin tectonics in the Witwatersrand, South Africa

    NASA Technical Reports Server (NTRS)

    Burke, K.; Kidd, W. S. F.; Kusky, T. M.

    1986-01-01

    The Witwatersrand Basin of South Africa is the best-known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch-type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These features indicate that the Witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an Andean-type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes above zones of shallow subduction. It is suggested that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision-related plateau, now represented by the Limpopo Province. Similarities are seen between this Phase of Witywatersrand Basin evolution and that of active basins north of the Tibetan Plateau. The geologic evidence does not agree with earlier suggestions that the Witwatersrand strata were deposited in a rift or half-graben.

  6. Dating of Archean basement in northeastern Wyoming and southern Montana.

    USGS Publications Warehouse

    Peterman, Z.E.

    1981-01-01

    Rb-Sr whole-rock and U-Pb zircon ages of granite and gneiss cores from three deep drill holes extend known occurrences of Archean rocks in the subsurface of NE Wyoming and S Montanta. Rb-Sr and K- Ar mineral ages are discordant and reflect early or middle Proterozoic disturbance. Highly altered rocks occur in a thin zone immediately below the sub-Cambrian unconformity. Samples from a few metres deeper in the basement are much fresher but show the effects of this alteration in filled fractures and thin adjacent alteration haloes. Whole-rock Rb-Sr systems have retaioned a fair degree of integrity in spite of increased susceptibility to modification because of the disturbed mineral systems. Interaction of the rocks with water a few metres below the sub-Cambrian unconformity probably occurred for only a relatively short time. Fractures filled rapidly with secondary minerals such as chlorite, anhydrite, and carbonate to maintain a relatively impermeable crystalline basement in which the silicates and their contained isotopic systems were preserved.- Author

  7. A Coupled General Circulation Model of the Archean Earth

    NASA Astrophysics Data System (ADS)

    Wolf, E. T.; Toon, O. B.

    2011-12-01

    We present results from a new coupled general circulation model suitable for deep paleoclimate studies. Particular interest is given to the faint young Sun paradox. The model is based on the Community Earth System Model maintained by the National Center for Atmospheric Research [1]. Prognostic atmosphere, ocean, land, ice, and hydrological cycle models are coupled. A new correlated-k radiative transfer model has been implemented allowing accurate flux calculations for anoxic atmospheres containing high concentrations of CO2 and CH4 [2, 3]. This model represents a significant improvement upon one-dimensional radiative-convective climate models used previously to study ancient climate [4]. Cloud and ice albedo feedbacks will be accurately quantified and new constraints on Archean surface temperatures will be revealed. References [1] Collins W.D. et al. "Description of the NCAR Community Atmosphere Model (CAM 3.0)." NCAR Technical Note, 2004. [2] Toon O.B., McKay, C.P., Ackerman, T.P. "Rapid Calculation of Radiative Heating Rates and Photodissociation Rates in Inhomogeneous Multiple Scattering Atmospheres." J. Geo. Res., 94(D13), 16287 - 16301, 1989. [3] Mlawer, E.J., et al. "Radiative transfer for inhomogeneous atmospheres: RRTM, a validated correlated-k model for the longwave." J. Geo. Res., 102(D14), 16663 - 16682, 1997. [4] Kasting J.F., Pollack, J.B., Crisp, D. "Effects of High CO2 Levels on Surface Temperature and Atmospheric Oxidation State of the Early Earth." J. Atm. Chem., 1, 403-428, 1984.

  8. Analysis of lipid biomarkers in rocks of Archean crystalline basement

    NASA Astrophysics Data System (ADS)

    Shekhovtsova, Nina V.; Osipov, George A.; Verkhovtseva, Nadejda V.; Pevzner, Lev A.

    2003-01-01

    The Earst-European platform Archean crystalline basement rocks opened by Vorotilov Deep Well (VDW) were studied within depths 2575 - 2805 m. VDW was drilled through Puchezh-Katunki impact structure and opened some rocks characterized by high magnetic saturation. Micro-dispersed structure of magnetite indicated on a possibility of its biogenic origin. Really some pure cultures of magnet-ordered compound producing bacilli were isolated. Thus, the identification of fatty acids and other lipid components of microbial cells inside rocks was made to establish the iron reduction role in common biochemical activity in deep subsurface. 34 microbial lipid markers were detected by gas chromatography -- mass-spectrometry and 22 species were identidied by private database. Bacteria of g. Bacillus reached 6.8% in subsurface communities. However, representatives of gg. Clostridium (37.1 - 33.2%) and Rhodococcus (27.6 - 33.7%) were absolute dominants within studied depth interval. Geochemical conditions in situ as well as physiological features of these micro-organisms allow to constitute a following trophic chain: subsurface fluid hyudrocarbons --> it oxidizing rhodococci --> free aminoacids and biomass proteins (products of rhodococci metabolism) --> it fermenting clostridia. This syntrophic association may be a new basement for subsurface ecosystem and can support the magnet-ordered compounds production.

  9. The Archean crust in the Wawa-Chapleau-Timmins region. A field guidebook prepared for the 1983 Archean Geochemistry-Early Crustal Genesis Field Conference

    NASA Technical Reports Server (NTRS)

    Percival, J. A.; Card, K. D.; Sage, R. P.; Jensen, L. S.; Luhta, L. E.

    1983-01-01

    This guidebook describes the characteristics and interrelationships of Archean greenstone-granite and high-grade gneiss terrains of the Superior Province. A 300-km long west to east transect between Wawa and Timmins, Ontario will be used to illustrate regional-scale relationships. The major geological features of the Superior Province are described.

  10. 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

  11. The Raman-Derived Carbonization Continuum: A Tool to Select the Best Preserved Molecular Structures in Archean Kerogens

    PubMed Central

    Rouzaud, Jean-Noël; Derenne, Sylvie; Bourbin, Mathilde; Westall, Frances; Kremer, Barbara; Sugitani, Kenichiro; Deldicque, Damien; Robert, François

    2016-01-01

    Abstract The search for indisputable traces of life in Archean cherts is of prime importance. However, their great age and metamorphic history pose constraints on the study of molecular biomarkers. We propose a quantitative criterion to document the thermal maturity of organic matter in rocks in general, and Archean rocks in particular. This is definitively required to select the best candidates for seeking non-altered sample remnants of life. Analysis of chemical (Raman spectroscopy, 13C NMR, elemental analysis) and structural (HRTEM) features of Archean and non-Archean carbonaceous matter (CM) that was submitted to metamorphic grades lower than, or equal to, that of greenschist facies showed that these features had all undergone carbonization but not graphitization. Raman-derived quantitative parameters from the present study and from literature spectra, namely, R1 ratio and FWHM-D1, were used to draw a carbonization continuum diagram showing two carbonization stages. While non-Archean samples can be seen to dominate the first stage, the second stage mostly consists of the Archean samples. In this diagram, some Archean samples fall at the boundary with non-Archean samples, which thus demonstrates a low degree of carbonization when compared to most Archean CM. As a result, these samples constitute candidates that may contain preserved molecular signatures of Archean CM. Therefore, with regard to the search for the oldest molecular traces of life on Earth, we propose the use of this carbonization continuum diagram to select the Archean CM samples. Key Words: Archean—Early life—Kerogen—Raman spectroscopy—Carbonization. Astrobiology 16, 407–417. PMID:27186810

  12. The Building of the Archean Superior Craton: Thermal Perspective

    NASA Astrophysics Data System (ADS)

    Jaupart, C. P.; Mareschal, J. C.

    2014-12-01

    The building of a craton involves the extraction of continental crust from the Earth's mantle and the lateral accretion of juvenile volcanic terranes. Ascertaining which conditions allow a newborn continental assemblage to survive requires information on its mechanical strength, which depends on the amount and vertical distribution of radioactive elements in the crust. There is thus a connection between crust formation mechanisms and a successful amalgamation process. To address outstanding questions concerning Archean cratons, the Superior province in Canada is the perfect region because it contains a well preserved geological record of accretion that provides compelling evidence for plate tectonic processes at 2.7 Ga. At almost the same time, the rate of continental growth decreased significantly, which may result from either slower crust formation or enhanced destruction through erosion and subduction. These issues are linked to the strength of the newborn continent. The extensive heat flow data set now available in the Superior Province reveals a clear demarcation between a chemically depleted and differentiated craton core and weakly differentiated enriched juvenile accreted terranes. The Superior craton was thus made of a strong core surrounded by weak terranes. This dichotomy implies that the accretion process could not involve complex imbrication of the accreted belts into the craton core. Subsequently, the craton may have been protected from convective disruption or delamination by its weak margins. Differences between the craton core and accreted terranes may be due to different crustal extraction processes, such as melting in a mantle plume or magmatism in a subduction zone. If subduction started at about 3 Ga, as advocated by several authors, the assembly and survival of large cratons may well be a consequence of this key shift in mantle activity. Alternatively, the chemical depletion of the craton core may be due to a prolonged history of internal

  13. Ferrous Silicate Spherules with Euhedral Fe,Ni-Metal Grains in CH Carbonaceous Chondrites: Evidence for Condensation Under Highly Oxidizing Conditions

    NASA Technical Reports Server (NTRS)

    Krot, A. N.; Meibom, A.; Petaev, M. I.; Keil, K.; Zolensky, M. E.; Saito, A.; Mukai, M.; Ohsumi, K.

    2000-01-01

    A population of ferrous silicate spherules composed of cryptocrystalline ol-px-normative material, +/-SiO2-rich glass and rounded-to-euhedral Fe,Ni-metal grains preserved a condensation signature of the precursors formed under oxidizing conditions.

  14. A comparison of the chemistry of pseudotachylyte breccias in the Archean Levack Gneisses of the Sudbury structure, Ontario

    NASA Technical Reports Server (NTRS)

    Thompson, Lucy M.; Spray, John G.

    1992-01-01

    The Archean Levack Gneisses of the North Range host millimeter-thick veins and centimeter-thick lenses of pseudotachylyte, as well as substantially larger meter-wide, dykelike bodies of pseudotachylytic 'breccia'. The 'breccia' occurs up to several tens of kilometers away from the Sudbury Igneous Complex and is commonly sited within or near joints and other natural weaknesses such as bedding, dyke contacts, and lithological boundaries. The larger 'breccia' dykes comprise a generally dark matrix containing rounded to subrounded and occasionally angular rock fragments derived predominantly from Levack Gneiss. Selected samples of bulk Sudbury Breccia and Sudbury Breccia matrices were chemically analyzed and compared to existing data on the Levack Gneisses and Sudbury Breccia. The matrices are apparently enriched in Fe and, to a lesser extent, Mg, Ti, and Ca compared to the wallrocks and the majority of clasts. This enrichment can be partly explained by the preferential cataclasis and/or frictional melting of hydrous ferromagnesian wallrock minerals, but also appear to require contamination by more basic exotic lithologies. This suggests that certain components of pseudotachylitic Sudbury Breccia have undergone significant transport during their formation.

  15. Packed Bed Reactor Experiment

    NASA Video Gallery

    The purpose of the Packed Bed Reactor Experiment in low gravity is to determine how a mixture of gas and liquid flows through a packed bed in reduced gravity. A packed bed consists of a metal pipe ...

  16. LA-ICP-MS analysis of trace elements in glass spherules of the El'gygytgyn impact structure, Siberia

    NASA Astrophysics Data System (ADS)

    Adolph, Leonie; Deutsch, Alex

    2010-05-01

    The 3.58±0.04 Ma old El'gygytgyn impact structure (Central Chukotka, NE Siberia) with a diameter of 18 km (Gurov and Gurova 1979, Layer 2000) is one of only two terrestrial craters with a volcanic target; therefore, analysis of its target and impact lithologies is of basic interest for comparative planetology. Lake El'gygytgyn is a very valuable climate archive in the Arctic as it was neither covered by glaciers (Melles et al. 2007) nor has the lake ever fallen dry. Climate and impact research were the rationale for the ICDP drilling project that finished successfully in spring 2009. Impactites like melt rocks and breccias are rarely found in outcrops yet are present in the 80 m terrace of Lake El'gygytgyn (Gurov and Gurova 1979). Numerous investigations on petrography, shock metamorphism, and geochemistry of impactites from El'gygytgyn have been published so far (e.g. Gurov et al. 2007). We report the first trace element data for seven 30- to 760-μm-sized impact glass spherules that have been collected about10 km off the crater center from a terrace deposit of the Enmyvaam River outside the crater rim. The spherules are translucent with colors ranging from amber, dark brown to nearly black; they contain a few circular bubbles, schlieren, and very rarely mineral clasts and breccia fragments. Major elements were measured with the JEOL JXA 8600 MX Superprobe, 31 trace elements were analyzed with the Finnigan Element2 LA-ICP-MS with 5 Hz, 8-9 J/cm2 at with Si as internal, and NIST612 as external standard (Institut f. Mineralogie, WWU Münster). The spot size was 60 μm. All spherules show a very homogeneous major and trace element distribution yet clear differences exist between the samples in the SiO2 content (in weight percent) 53-68: four of the glasses are dacitic, two andesitic, and one basaltic-andesitic in composition. In addition, MgO (2.1-9.2), K2O (0.6-3.3), and (in ppm) Ni (317-1096), Co (25-79), Zr (100-169), Rb (18-107), and Ba (459-1092) display wide

  17. A glass spherule of questionable impact origin from the Apollo 15 landing site: Unique target mare basalt

    USGS Publications Warehouse

    Ryder, G.; Delano, J.W.; Warren, P.H.; Kallemeyn, G.W.; Dalrymple, G.B.

    1996-01-01

    A 6 mm-diameter dark spherule, 15434,28, from the regolith on the Apennine Front at the Apollo 15 landing site has a homogeneous glass interior with a 200 ??m-thick rind of devitrified or crystallized melt. The rind contains abundant small fragments of Apollo 15 olivine-normative mare basalt and rare volcanic Apollo 15 green glass. The glass interior of the spherule has the chemical composition, including a high FeO content and high CaO/Al2O3, of a mare basalt. Whereas the major element and Sc, Ni, and Co abundances are similar to those of low-Ti mare basalts, the incompatible elements and Sr abundances are similar to those of high-Ti mare basalts. The relative abundance patterns of the incompatible trace elements are distinct from any other lunar mare basalts or KREEP; among these distinctions are a much steeper slope of the heavy rare earth elements. The 15434,28 glass has abundances of the volatile element Zn consistent with both impact glasses and crystalline mare basalts, but much lower than in glasses of mare volcanic origin. The glass contains siderophile elements such as Ir in abundances only slightly higher than accepted lunar indigenous levels, and some, such as Au, are just below such upper limits. The age of the glass, determined by the 40Ar/39Ar laser incremental heating technique, is 1647 ?? 11 Ma (2 ??); it is expressed as an age spectrum of seventeen steps over 96% of the 39Ar released, unusual for an impact glass. Trapped argon is negligible. The undamaged nature of the sphere demonstrates that it must have spent most of its life buried in regolith; 38Ar cosmic ray exposure data suggest that it was buried at less than 2m but more than a few centimeters if a single depth is appropriate. That the spherule solidified to a glass is surprising; for such a mare composition, cooling at about 50??C s-1 is required to avoid crystallization, and barely attainable in such a large spherule. The low volatile abundances, slightly high siderophile abundances, and

  18. Sm-Nd isotope study of early Archean rocks, Qianan, Hebei Province, China

    NASA Technical Reports Server (NTRS)

    Huang, X.; Bi, Z.; Depaolo, D. J.

    1986-01-01

    The geochronology of amphibolitic and gneiss rocks in the Qianan region in China is examined. Sm-Nd isotopic measurements were performed to determine if the rocks existed in the early Archean age. The average age for the amphibolite samples is 3.50 + or - 0.08 billion years with an initial epsilon(Nd) value of 3.3 + or - 0.3, and for the gneiss samples the average age is also 3.5 billion years. The high initial epsilon(Nd) value indicates that the mantle magma source is depleted of magmaphile elements and the geochemistry of the early Archean mantle is similar to the modern upper mantle.

  19. Sink or Swim? the Role of Intracrustal Differentiation in the Generation of Compositional Diversity and Crustal Delamination in the Archean

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Herzberg, C. T.; Kaus, B.; Johnson, T. E.; Brown, M.

    2014-12-01

    Significant debate exists regarding the processes of crustal formation and stabilization in the Archean, with some researchers advocating for continuous subduction-like processes throughout earth history, and others advocating crustal recycling by lithospheric delamination or 'drip tectonics'. Much of the debate hinges on whether Archean mantle potential temperatures (Tp) were significantly hotter than the present day. The rock record of non-arc Archean primary magma compositions (Herzberg et al., 2010) has been used to infer higher ambient Tp (Tp = 1500-1650C) during the Archean, causing high melt fractions during decompression melting, and leading to extreme primary (oceanic) crustal thicknesses of 30-40 km (Herzberg and Rudnick, 2012). Such crustal thicknesses might inhibit subduction, in which case an alternative mechanism of crustal recycling would be required. In their recent paper, Johnson et al. (2014) showed that at Tp > 1500C, the lower portions of a thick homogenous Archean primary crust generated would be density unstable with respect to the ambient mantle. Additionally, they showed that given realistic rheological constraints, large-scale lower crustal delamination is a very efficient crustal recycling mechanism at Tp >1600C. The Archean crust, however, is likely to be internally differentiated. Here we present pMELTS and Perple_X modeling results on the intracrustal differentiation of Archean primary crust, resulting in the formation of TTG-like granitoids in the upper crust and a lower crust dominated by clinopyroxenite. Using the composition and density profiles generated by intracrustal differentiation, our geodynamic modeling extends the Tp over which efficient crustal delamination will occur to lower values, consistent with those likely throughout the Archean. Efficient crustal differentiation and delamination of dense mafic residues throughout the Archean may explain the apparent paucity of mafic lithologies relative to TTGs that characterize

  20. An Archean Terrestrial Fractionation Line for Oxygen Isotopes

    NASA Astrophysics Data System (ADS)

    Rumble, D.; Blake, R. E.; Bao, H.; Bowring, S.; Komiya, T.; Rosing, M.; Ueno, Y.

    2008-12-01

    The Terrestrial Fractionation Line (TFL) for oxygen isotopes is defined by 17O/16O and 18O/16O analyses of meteoric waters, seawater, sedimentary, metamorphic, and igneous rocks and constituent minerals. Interlaboratory measurements of the slope of the TFL on a plot of d18O vs. d17O revealed eclogitic garnets with a slope of 0.526 and hydrothermal quartz of 0.524 from rocks younger than 0.8 Ga (Giga years before present). New measurements show Archean metamorphic rocks and minerals from Barberton, (3.2 Ga, S. Africa), Isua (3.8 Ga, Greenland), and Acasta (4.0 Ga, Canada) have a slope of 0.524 +/- 0.002 (95% confidence, MSWD = 0.66). Analysis of Ag3PO4 prepared from apatite mineral separates from Isua meta-sediments gives a slope of 0.509 +/- 0.022 (95% confidence, MSWD = 0.59). Taken at face value, steeper slopes on a d17O vs. d18O diagram indicate an approach towards isotope exchange equilibrium. Lower slopes are expected when isotope fractionation is kinetically controlled. The lower slope of 0.509 for Isua apatite suggests that the formation of orthophosphate was kinetically controlled. Kinetic fractionations are known to occur during catalysis of reactions by enzymes secreted by microbes. Enzymatic catalysis confers an advantage on organisms because energy-producing reactions may be induced to occur at lower temperature conditions more accessible to the organism. May it be definitively concluded that enzymatic catalysis was responsible for the measured 0.509 slope? No, abiotic kinetic fractionation cannot be disproven with existing data. The preparation of Ag3PO4 from apatite may have introduced kinetic fractionation as an analytical artifact. Conclusions fully supported by the data suggest: (1) Mixing accompanying the violent birth of the Earth- Moon system had already succeeded in establishing Earth's current oxygen isotope composition by 4.0 Ga; and (2) No trace of an episode of late heavy meteorite bombardment remains in the oxygen isotope compositions of

  1. Texture-specific Si isotope variations in Barberton Greenstone Belt cherts record low temperature fractionations in early Archean seawater

    NASA Astrophysics Data System (ADS)

    Stefurak, Elizabeth J. T.; Fischer, Woodward W.; Lowe, Donald R.

    2015-02-01

    Sedimentary cherts are unusually abundant in early Archean (pre-3.0 Ga) sequences, suggesting a silica cycle that was profoundly different than the modern system. Previously applied for the purpose of paleothermometry, Si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration, precipitation, diagenesis, and metamorphism. Early Archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition, cementation, silicification, and recrystallization. To add a new layer of insight into the chemistry of early cherts, we have used wavelength-dispersive spectroscopy and then secondary ion mass spectrometry (SIMS) to produce elemental and Si and O isotope ratio data from banded black-and-white cherts from the Onverwacht Group of the Barberton Greenstone Belt, South Africa. This geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations. SIMS allows the comparison of Si and O isotope ratios of distinct silica phases, including black carbonaceous chert beds and bands (many including well-defined sedimentary grains), white relatively pure chert bands including primary silica granules, early cavity-filling cements, and later quartz-filled veins. Including all chert types and textures analyzed, the δ30Si dataset spans a range from -4.78‰ to +3.74‰, with overall mean 0.20‰, median 0.51‰, and standard deviation 1.30‰ (n = 1087). Most samples have broadly similar δ30Si distributions, but systematic texture-specific δ30Si differences are observed between white chert bands (mean +0.60‰, n = 750), which contain textures that represent primary and earliest diagenetic silica phases, and later cavity-filling cements (mean -1.41‰, n = 198). We observed variations at a ∼100 μm scale indicating a lack of Si isotope homogenization at this scale during

  2. Molecular evidence of Late Archean archaea and the presence of a subsurface hydrothermal biosphere.

    PubMed

    Ventura, Gregory T; Kenig, Fabien; Reddy, Christopher M; Schieber, Juergen; Frysinger, Glenn S; Nelson, Robert K; Dinel, Etienne; Gaines, Richard B; Schaeffer, Philippe

    2007-09-01

    Highly cracked and isomerized archaeal lipids and bacterial lipids, structurally changed by thermal stress, are present in solvent extracts of 2,707- to 2,685-million-year-old (Ma) metasedimentary rocks from Timmins, ON, Canada. These lipids appear in conventional gas chromatograms as unresolved complex mixtures and include cyclic and acyclic biphytanes, C36-C39 derivatives of the biphytanes, and C31-C35 extended hopanes. Biphytane and extended hopanes are also found in high-pressure catalytic hydrogenation products released from solvent-extracted sediments, indicating that archaea and bacteria were present in Late Archean sedimentary environments. Postdepositional, hydrothermal gold mineralization and graphite precipitation occurred before metamorphism (approximately 2,665 Ma). Late Archean metamorphism significantly reduced the kerogen's adsorptive capacity and severely restricted sediment porosity, limiting the potential for post-Archean additions of organic matter to the samples. Argillites exposed to hydrothermal gold mineralization have disproportionately high concentrations of extractable archaeal and bacterial lipids relative to what is releasable from their respective high-pressure catalytic hydrogenation product and what is observed for argillites deposited away from these hydrothermal settings. The addition of these lipids to the sediments likely results from a Late Archean subsurface hydrothermal biosphere of archaea and bacteria. PMID:17726114

  3. Molecular evidence of Late Archean archaea and the presence of a subsurface hydrothermal biosphere

    PubMed Central

    Ventura, Gregory T.; Kenig, Fabien; Reddy, Christopher M.; Schieber, Juergen; Frysinger, Glenn S.; Nelson, Robert K.; Dinel, Etienne; Gaines, Richard B.; Schaeffer, Philippe

    2007-01-01

    Highly cracked and isomerized archaeal lipids and bacterial lipids, structurally changed by thermal stress, are present in solvent extracts of 2,707- to 2,685-million-year-old (Ma) metasedimentary rocks from Timmins, ON, Canada. These lipids appear in conventional gas chromatograms as unresolved complex mixtures and include cyclic and acyclic biphytanes, C36–C39 derivatives of the biphytanes, and C31–C35 extended hopanes. Biphytane and extended hopanes are also found in high-pressure catalytic hydrogenation products released from solvent-extracted sediments, indicating that archaea and bacteria were present in Late Archean sedimentary environments. Postdepositional, hydrothermal gold mineralization and graphite precipitation occurred before metamorphism (≈2,665 Ma). Late Archean metamorphism significantly reduced the kerogen's adsorptive capacity and severely restricted sediment porosity, limiting the potential for post-Archean additions of organic matter to the samples. Argillites exposed to hydrothermal gold mineralization have disproportionately high concentrations of extractable archaeal and bacterial lipids relative to what is releasable from their respective high-pressure catalytic hydrogenation product and what is observed for argillites deposited away from these hydrothermal settings. The addition of these lipids to the sediments likely results from a Late Archean subsurface hydrothermal biosphere of archaea and bacteria. PMID:17726114

  4. Constraining the location of the Archean--Proterozoic suture in the Great Basin based on magnetotelluric soundings

    USGS Publications Warehouse

    Rodriguez, Brian D.; Sampson, Jay A.

    2012-01-01

    It is important to understand whether major mining districts in north-central Nevada are underlain by Archean crust, known to contain major orogenic gold deposits, or, alternatively, by accreted crust of the Paleoproterozoic Mojave province. Determining the location and orientation of the Archean-Proterozoic suture zone between the Archean crust and Mojave province is also critical because it may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. In the Great Basin, the attitude of the suture zone is unknown because it is concealed below cover. A regional magnetotelluric sounding profile along the Utah-Nevada State line reveals a deeply penetrating, broad electrical conductor that may be the Archean-Proterozoic suture zone in the northwest corner of Utah. This major crustal conductor's strike direction is northwest, where it broadens to about 80 km wide below about 3-km depth. These results suggest that the southwestern limit of intact Archean crust in this part of the Great Basin is farther north than previously reported. These results also suggest that the major gold belts in north-central Nevada are located over the Paleoproterozoic Mojave province, and the Archean terrain lies northeast in the northwest corner of Utah. Rifted Archean crust segments south and west of the suture suggest that future mineral exploration northeast of current mineral trends may yield additional gold deposits.

  5. Diversity in the Archean biosphere: new insights from NanoSIMS.

    PubMed

    Oehler, Dorothy Z; Robert, François; Walter, Malcolm R; Sugitani, Kenichiro; Meibom, Anders; Mostefaoui, Smail; Gibson, Everett K

    2010-05-01

    The origin of organic microstructures in the approximately 3 Ga Farrel Quartzite is controversial due to their relatively poor state of preservation, the Archean age of the cherts in which they occur, and the unusual spindle-like morphology of some of the forms. To provide more insight into the significance of these microstructures, nano-scale secondary ion mass spectrometry (NanoSIMS) maps of carbon, nitrogen, sulfur, silicon, and oxygen were obtained for spheroidal and spindle-shaped constituents of the Farrel Quartzite assemblage. Results suggest that the structures are all bona fide approximately 3 Ga microfossils. The spindles demonstrate an architecture that is remarkable for 3 Ga organisms. They are relatively large, robust, and morphologically complex. The NanoSIMS element maps corroborate their complexity by demonstrating an intricate, internal network of organic material that fills many of the spindles and extends continuously from the body of these structures into their spearlike appendages. Results from this study combine with previous morphological and chemical analyses to argue that the microstructures in the Farrel Quartzite comprise a diverse assemblage of Archean microfossils. This conclusion adds to a growing body of geochemical, stromatolitic, and morphological evidence that indicates the Archean biosphere was varied and well established by at least approximately 3 Ga. Together, the data paint a picture of Archean evolution that is one of early development of morphological and chemical complexity. The evidence for Archean evolutionary innovation may augur well for the possibility that primitive life on other planets could adapt to adverse conditions by ready development of diversity in form and biochemistry. PMID:20528196

  6. Exploring Archean seawater sulfate via triple S isotopes in carbonate associated sulfate.

    NASA Astrophysics Data System (ADS)

    Paris, G.; Fischer, W. W.; Sessions, A. L.; Adkins, J. F.

    2015-12-01

    Multiple sulfur isotope ratios in Archean sedimentary rocks provide powerful insights into the behavior of the ancient sulfur cycle, the redox state of fluid Earth, and the timing of the rise of atmospheric oxygen [1]. The Archean sulfur isotope record is marked by pronounced mass-independent fractionation (Δ33S≠0)—signatures widely interpreted as the result of SO2 photolysis from "short-wavelength" UV light resulting in a reduced phase carrying positive Δ33S values (ultimately recorded in pyrite) and an oxidized phase carrying negative Δ33S values carried by sulfate [2]. Support for this hypothesis rests on early laboratory experiments and observations of negative Δ33S from barite occurrences in mixed volcanic sedimentary strata in Mesoarchean greenstone terrains. Despite forming the framework for understanding Archean sulfur cycle processes, this hypothesis is still largely untested, notably due to the lack of sulfate minerals in Archean strata. Using a new MC-ICP-MS approach combined with petrography and X-ray spectroscopy we have generated a growing S isotope dataset from CAS extracted from Archean carbonates from a range of sedimentary successions, including: the 2.6 to 2.521 Ga Campbellrand-Malmani carbonate platform (Transvaal Supergroup, South Africa), 2.7 Ga Cheshire Formation (Zimbabwe), and 2.9 Ga Steep Rock Formation (Canada). Importantly, we observe positive δ34S and Δ33S values across a range of different lithologies and depositional environments. These results demonstrate that dissolved sulfate in seawater was characterized by positive Δ33S values—a result that receives additional support from recent laboratory and theoretical experiments [e.g. 4, 5]. [1] Farquhar et al., 2000, Science [2] Farquhar et al., 2001, Journal of Geophysical Research: Planets [3] Paris et al., 2014, Science. [4] Whitehill et al., 2013, Proceedings of the National Academy of Sciences. [5] Claire et al., 2014 Geochimica et Cosmochimica Acta

  7. Tracking the Archean-Proterozoic suture zone in the northeastern Great Basin, Nevada and Utah

    USGS Publications Warehouse

    Rodriguez, B.D.; Williams, J.M.

    2008-01-01

    It is important to know whether major mining districts in north-central Nevada are underlain by crust of the Archean Wyoming craton, known to contain major orogenic gold deposits or, alternatively, by accreted crust of the Paleoproterozoic Mojave province. Determining the location and orientation of the Archean-Proterozoic suture zone between these provinces is also important because it may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. The suture zone is exposed in northeastern Utah and south-western Wyoming and exhibits a southwest strike. In the Great Basin, the suture zone strike is poorly constrained because it is largely concealed below a Neoproterozoic-Paleozoic miogeocline and Cenozoic basin fill. Two-dimensional resistivity modeling of three regional north-south magnetotelluric sounding profiles in western Utah, north-central Nevada, and northeastern Nevada, and one east-west profile in northeastern Nevada, reveals a deeply penetrating (>10 km depth), broad (tens of kilometers) conductor (1-20 ohm-meters) that may be the Archean-Proterozoic suture zone, which formed during Early Proterozoic rifting of the continent and subsequent Proterozoic accretion. This major crustal conductor changes strike direction from southwest in Utah to northwest in eastern Nevada, where it broadens to ???100 km width that correlates with early Paleozoic rifting of the continent. Our results suggest that the major gold belts may be over-isolated blocks of Archean crust, so Phanerozoic mineral deposits in this region may be produced, at least in part, from recycled Archean gold. Future mineral exploration to the east may yield large gold tonnages. ?? 2008 Geological Society of America.

  8. The origin or the Archean Jardine iron formation-hosted lode gold deposit. Montana

    SciTech Connect

    Ping, Liu.

    1992-06-09

    While there is considerable controversy concerning the origin of greenstone-hosted lode gold deposits of Archean age, there is a general consensus that these deposits are epigenetic. By contrast, iron formation-hosted lode gold deposits of Archean or Proterozoic age are considered either epigenetic or syngenetic. At least three genetic models have been proposed for these gold deposits: a syngenetic model involving simultaneous deposition of gold and the iron formation; an epigenetic model involving a later introduction of gold, arsenic, and sulfur into the iron formation; and a multistage model involving primary concentration of gold during deposition of iron formation followed by remobilization and reconcentration of gold during later events. The Jardine district is one of only three Archean lode gold districts in the United States that have reserves of greater than 300,000 ounces of gold. The other two are the South Pass-Atlantic City district, Wyoming, and the Ropes mine, Michigan. The fact that two of the three districts are in the Wyoming province suggests that the province might be an Archean gold province similar to Archean provinces in Canada. Placer gold was discovered near Jardine in 1866, and gold quartz veins were mined in the 1880's at Mineral Hill. Exploration by the Jardine Joint Venture has concentrated on the Jardine area, including Crevasse Mountain, where minor lode gold mineralization occurs in quartz-biotite schists. In order to complement previous geochemical, mineralogical, petrological and structural studies, the present study has concentrated on fluid inclusion, stable isotope, and electron microprobe studies with the intention of determining: (1) the source of the ore-forming fluids and gold, and (2) the genetic relationship between gold mineralization and iron formation, alteration and metamorphism.

  9. The origin or the Archean Jardine iron formation-hosted lode gold deposit. Montana

    SciTech Connect

    Ping, Liu

    1992-06-09

    While there is considerable controversy concerning the origin of greenstone-hosted lode gold deposits of Archean age, there is a general consensus that these deposits are epigenetic. By contrast, iron formation-hosted lode gold deposits of Archean or Proterozoic age are considered either epigenetic or syngenetic. At least three genetic models have been proposed for these gold deposits: a syngenetic model involving simultaneous deposition of gold and the iron formation; an epigenetic model involving a later introduction of gold, arsenic, and sulfur into the iron formation; and a multistage model involving primary concentration of gold during deposition of iron formation followed by remobilization and reconcentration of gold during later events. The Jardine district is one of only three Archean lode gold districts in the United States that have reserves of greater than 300,000 ounces of gold. The other two are the South Pass-Atlantic City district, Wyoming, and the Ropes mine, Michigan. The fact that two of the three districts are in the Wyoming province suggests that the province might be an Archean gold province similar to Archean provinces in Canada. Placer gold was discovered near Jardine in 1866, and gold quartz veins were mined in the 1880`s at Mineral Hill. Exploration by the Jardine Joint Venture has concentrated on the Jardine area, including Crevasse Mountain, where minor lode gold mineralization occurs in quartz-biotite schists. In order to complement previous geochemical, mineralogical, petrological and structural studies, the present study has concentrated on fluid inclusion, stable isotope, and electron microprobe studies with the intention of determining: (1) the source of the ore-forming fluids and gold, and (2) the genetic relationship between gold mineralization and iron formation, alteration and metamorphism.

  10. The Archean geology of the Godthabsfjord Region, southern west Greenland (includes excursion guide)

    NASA Technical Reports Server (NTRS)

    Mcgregor, V. R.; Nutman, A. P.; Friend, C. R. L.

    1986-01-01

    The part of the West Greenland Archean gneiss complex centered around Godthabsfjord and extending from Isukasia in the north to south Faeringehavn is studied. Extensive outcrops of 3800 to 3400 Ma rocks can provide some direct evidence of conditions and processes that operated on the Earth in the early Archean. However, the ways in which primary characteristics have been modified by later deformation, metamorphism, and chemical changes are first taken into account. The rocks exposed are the products of two major phases of accretion of continental crust, at 3800 to 3700 Ma and 3100 to 29 Ma. The main features of these two accretion phases are similar, but careful study of the least modified rocks may reveal differences related to changes in the Earth in the intervening period. The combination of excellent exposure over an extensive area, relatively detailed geological mapping of much of the region, and a considerable volume of isotopic and other geochemical data gives special insights into processes that operated at moderately deep levels of the crust in the Archean. Of particular interest is the effect of late Archean granulite facies metamorphism on early Archean rocks, especially the extent to which isotope systems were disturbed. Similar processes may well have partly or wholly destroyed evidence of more ancient components of other high grade terrains. This account does not attempt to be an exhaustive review of all work carried out on the geology of the region. Rather, it attempts to summarize aspects of the geology and some interest in the context of early crustal genesis.