V13B-2109
Hydrogen Peroxide Formation and pH Changes at Rock-Water Interface during Stressing
Common igneous and high-grade metamorphic rocks contain dormant defects, which become activated when stressed. They release electronic charge carriers, in particular defect electrons associated with O- states in a matrix of O2-. Known as 'positive holes' or pholes for short, the O- states can spread out of the stressed rock volume, travel along stress gradients over distances on the order of meters in the lab and probably over kilometers in the field. They carry a current, which can flow through meters of rock in the laboratory, probably tens of kilometers in the field. At rock-water interfaces the O- states turn into •O radicals, which subtract H from H2O, forming OH- in the rock surface and •OH radicals in the water. Two •OH combine to H2O2. In the process the pH becomes more acidic. The discovery of H2O2 formation at rock-water interfaces as part of stress- activated currents on the tectonically active Earth may help us better understand the oxidation of the early Earth and the evolution of early Life.
V13B-2110
Numerical Modeling Of Plate Tectonics Initiation And Its Style In The Early Earth
Plate tectonics is an outstanding example of a self organizing complex system, driven by the negative buoyancy of the thermal boundary layer resulting in subduction. Although the signature of plate tectonics is recognized with some confidence in the Phanerozoic geologic record of the continents, its action becomes less certain further back in time. The best way to improve our understanding of the early Earth is to combine our knowledge from petrological data and facilities of numerical modeling. Based on 2D petrological-thermomechanical numerical model of oceanic-continental subduction (using the I2VIS code (Gerya and Yuen, 2003)) we determined sharp first order transition from modern style of subduction to transitional "pre-subduction" tectonic regime at upper mantle temperature rising by around 160°C above the present one. This condition roughly corresponds to upper bound for Neoarchean (about 3Ga) mantle temperature (Davies, 1993; Komiya et al., 1999). In the "pre-subduction" tectonic regime plates are strongly internally deformable by intense percolation of melts continuously generated from underlying partially molten mantle. Their convergence results in shallow underthrusting of continental crust by oceanic plate. Oceanic plate goes quite far (to 200 km) under continental crust moving away and folding continental lithospheric mantle. In some experiments after the underthrusting oceanic plate starts to sink into the mantle forming at first stages two-sided subduction. In contrast to present time in the hotter "pre-subduction" regime there is no formation of backarc basin and new spreading center (the same was assumed by de Wit, 1998 for Archean time). Relatively shallow and hot tectonic style dominates. Oceanic plates weakened by sub- lithospheric melts are subjected to buckling and shallow undethrusting (rather then subduction) associated by local melting. This triggers formation of characteristic metamorphic and magmatic rocks found in Archean (such as, ultra-high-temperature and eclogite-high-pressure granulites, adakites and granitoids). Further increase in the mantle temperature (by around 250°C above present) causes transition from "pre- subduction regime" to "horizontal-tectonics" regime. At this stage horizontal movements of small deformable plate fragments are predominant and even shallow underthrusts do not form under imposed convergence. Our experiments also show that apart of mantle temperature another crucial parameter controlling the tectonic regime is degree of lithospheric weakening induced by upward movement of extracted sub- lithospheric melts. At the high Archean mantle temperatures when melts are always present in the upper mantle lithospheric weakening by melts should be low to preserve coherency of the plates and to allow stable subduction. Neither increasing radiogenic heat production nor lithospheric weakening by slab derived fluids has notable effects for the identified transitions in the early Earth.
V13B-2111
The Chemical Interplay of Serpentinization, Methanogenesis, and Carbonate Formation at 200C and 300 bar
Serpentinization (olivine hydrolysis) is the primary process contributing to abiotic generation of elemental hydrogen (H2) in active mid-ocean ridge hydrothermal systems and forearc environments. Carbon dioxide in these types of systems reacts with serpentinization-derived H2 to produce methane (CH4) and with cations in seawater (Ca2+, Mg2+, Fe2+) to precipitate carbonate minerals. Despite a growing body of field and experimental evidence, the reaction pathways, kinetics, and geochemical interplay of serpentinization, methanogenesis and carbonate formation are not fully understood. Here, we evaluated the reaction kinetics of CH4 versus carbonate genesis during serpentinization of average Earth olivines (Fo~90) under pressure-temperature (PT) conditions similar to mid-ocean ridge and forearc environments. Laboratory experiments were preformed at identical T and P conditions (200°C and 300 bar) using flexible gold reaction-cells in hydrothermal autoclaves located in the Water- Rock Interaction Laboratory at the U.S. Geological Survey in Menlo Park, CA. The experiments demonstrate that CH4 production was minimal in CO2-rich fluids and that CH4 generation was enhanced in a CO2-limited experiment producing an end-member CH4 concentration of 44 μM after 860 hours. Equilibrium thermodynamics and reaction rates observed in these experiments demonstrate that carbonate precipitation is favored in CO2-saturated systems and methanogenesis is hindered. These results highlight the competition between methanogenesis and carbonate production in hydrothermal environments leading to the conclusion that the production of CH4 is greater when the concentration of CO2 is undersaturated with respect to carbonate equilibrium.
V13B-2112
Importance of micro-scale oxygen isotopic study in Gunflint cherts (1.9 Ga): new constraints on paleo-temperatures reconstructions
Isotopic composition of sedimentary Precambrian cherts contains a record of the Precambrian environment through their oxygen isotopes (δ18O). Indeed, their δ18O values may reflect their temperature of formation, hence the temperature of the ocean provided the seawater δ18O remained constant around 0 ± 3 permil (Holmden and Muehlenbachs, 1993) during these last 3.5 Ga (Knauth and Lowe, 1978; Robert and Chaussidon , 2006). However, this temperature record can be modified by isotopic exchange with hydrothermal or metamorphic fluids. For a given age, the δ18O values of cherts show large variations (Knauth and Lowe, 2003; Perry and Lefticariu, 2003; Robert and Chaussidon, 2006) which complicates the paleo-temperatures reconstructions. To better understand the origin of these local variations, we have analyzed with SIMS, µm-scale δ18O variations in five cherts from the 1.9 Ga old Gunflint iron-formation (Canada) that stands among the least metamorphosed Precambrian cherts. Five chert samples, containing different types of silica (microcrystalline quartz, drusy quartz, megaquartz, and quartz veins), minor carbonates (siderite and ankerite), hematite and pyrite, were analysed (> 100 spots per sample) by multicollector CAMECA ims 1270 ion microprobe (CRPG-CNRS in Nancy). The external reproducibility on quartz standards was of ± 0.17 permil (1 σ, n=65). The Gunflint cherts show (i) detrital quartz with δ18O 10-12 permil lower than microcrystalline quartz, (ii) a typical 3-6 permil, δ18O range for microcrystalline quartz and (iii) quartz veins with δ18O 5-7 permil, lower than microcrystalline quartz. Variations in the proportion of the different types of silica appear to explain the isotopic heterogeneity observed the whole rock sample scale (i.e. sample 4 of 06/30/84; bulk = 22.75 ± 0.3 permil as compared with a mean δ18O calculated from the measured δ18O values of the different phases of silica = 23.18 ± 0.8 permil). Isotopic mapping of quartz veins shows that the oxygen isotopic composition of microquartz was preserved during hydrothermal circulation. Considering microcrystalline quartz only, a δ18O range of 3-6 permil is found; such a variation is likely to reflect diagenetic processes (Kolodny and Epstein, 1976). We develop an isotopic model of diagenetic evolution of quartz and fluid with different steps of precipiation-recrystallization. This model can give access to the temperature of equilibrium with seawater of the precursors of cherts, which may allow to better constrain oceanic paleo-temperatures in the Precambrian. Holmden C. and Muehlenbachs K., (1993) Science 259, 1733-1736. Knauth L.P. and Lowe R.D., (1978) Earth and Planetary Science Letters. 41, 209-222. Knauth L.P. and Lowe D.R., (2003) Geological Society of America Bulletin 115, 566-580. Kolodny Y. and Epstein S., (1976) Geochim. Cosmochim. Acta 40, 1195-1209. Perry E. C. and Lefticariu L., (2003) In: Treatise on Geochemistry, vol 7, pp. 99-113. Robert F. and Chaussidon M., (2006) Nature. 443, 969-971.
V13B-2113
Characterization and component tracing of Banded Iron Formations deposited during the ~2.7 Ga superplume event: An example from the Tati Greenstone Belt, Northeastern Botswana
Major and trace element, samarium (Sm)–neodymium (Nd) and lead (Pb) isotopic analyses of individual mesobands of five Banded Iron Formations (BIFs) and associated metavolcanic and metasedimentary rocks from the Neoarchean Tati Greenstone Belt (TGB, Northeastern Botswana) were conducted in order to characterize and determine the source(s) and depositional environment(s) of these chemical sediments and to compare their features with other Archean BIFs. Rare earth element (REE)-yttrium (Y) patterns of individual BIF mesobands from the TGB show features characteristic of other Archean BIFs; these are (relative to Post Archean Australian Shale (PAAS) normalization), LREE depletions relative to MREE and HREE, positive lanthanum La/La*PAAS, europium Eu/Eu*PAAS, and Y/holmium (Ho) ratios. The TGB BIFs also lack defined cerium Ce/Ce*PAAS anomalies. Generally, the REY patterns are similar to modern seawater and together with low concentrations of high-field strength elements these features are indicative of an essentially detritus-free precipitation. Uranogenic Pb isotope data for the BIFs define individual correlation lines with slopes corresponding to apparent ages of ~2.7 Ga, interpreted as closely reflecting the depositional time frames. An exception to this are the BIFs at Mupane mine in the central part of the TGB which exhibit disturbed U-Pb isotope systematics. They indicate a pronounced resetting around 2.0 Ga. This is also reflected by Pb isotope data of gold-bearing sulfides and Pb-stepwise leaching (PbSL) data of garnets from associated metasedimentary rocks at this location. The garnet PbSL data define a correlation line in uranogenic Pb isotope space with a slope corresponding to an apparent age of 1976±88 Ma. This age is interpreted as a metamorphic age that can be correlated with ~2.0 Ga tectono- metamorphic events within the adjacent Limpopo orogenic belt, and which apparently have, also affected parts of the adjacent Zimbabwe craton to the north. Elevated, but variable 207Pb/204Pb relative to 206Pb/204Pb ratios of BIFs are indicative of a high-μ (238U/204Pb) prehistory of their source materials which, in accordance with Nd model ages defined by the BIFs, can best be modeled by a 3.0 – 3.2 Ga extraction of these sources from a depleted mantle reservoir. Sm–Nd isotopic relationships between silica (SiO2)- and iron (Fe2O3)-rich mesobands of one of the studied BIFs point to essentially two REY sources controlling the depositional environment. The first is characterized by elevated Sm-Nd ratios and a negative inferred εNd(2.7 Ga) value of -2.5 and is associated with high Fe fluxes. The second source, associated with high Si fluxes, is instead characterized by lower Sm-Nd ratios and a slightly positive inferred εNd(2.7 Ga) value of +0.4. While the association of high Fe concentrations and elevated Sm-Nd in BIF mesobands is characteristic of a hydrothermal seawater input, the Sm-Nd isotopic characterization of this source, unlike other Archean BIFs, points to a significantly LREE enriched (rather than depleted) mantle source. This finding is compatible with the potential existence of a subcontinental lithospheric mantle reservoir beneath the Zimbabwe and Kaapvaal craton.
V13B-2114
Unravelling the Maggie Hays and Emily Ann nickel sulphide deposits via a multidisciplinary study of the Archaean Lake Johnston Greenstone Belt, Yilgarn Craton, Western Australia.
In order to better understand the architecture and structural evolution of the Archaean Lake Johnston Greenstone Belt and predict, the distribution of remobilized magmatic ultramafic nickel-sulphide mineralization in the belt, a multidisciplinary study has been carried out on the central area of the greenstone belt around the Maggie Hays and Emily Ann nickel-sulphide deposits. The nickel-sulphide deposits are hosted in a sequence of amphibolite facies rocks, severely affected by metamorphic and magmatic events. Geochronology, structural and metamorphic petrology and 3D geological modelling have been undertaken. Through a common georeferencing using Geomodeller 3D modelling software, all available data (geological map, interpretative cross-sections, drill hole and structural information) have been integrated into a 3D model to constrain the thickness of the lithologies and their geometry. Applying the THERMOCALC software coupled to the analysis of textural relationships between the main metamorphic phases, the peak P-T conditions were estimated from garnet rims and matrix minerals. The P-T evolution was deduced from garnet core compositions. The U-Pb SHRIMP method on monazite was utilized to constrain the time parameter of the P-T-t path. Two major thermal events have been inferred from the field relationships. The D1 event was part of regional metamorphism associated with a strong NNE-SSW shortening. The D1 event is associated with the development of kilometre-scale recumbent folds currently preserved at the Emily Ann Ni-deposit during top-to-the-NNE transport. A 2634 +/- 4.6 Ma SHRIMP age of folded dykes in the deposit provides an upper age limit on the D1 event. During this first event, massive nickel sulphides were mechanically remobilized into major fold hinges. The D2 event was associated with peak metamorphic temperatures, and was linked to the emplacement of the granite intrusions into the belt. The D2 event is characterized in the field by an intense top-to-the-SSE shearing. This deformation has substantially remobilized the massive nickel sulphide ore. Garnet-bearing pegmatitic intrusions that cross cut the ductile fabrics associated with peak metamorphism are common at the Maggie Hays deposit, and provide a lower U-Pb SHRIMP age constraint on the D2 event of 2631 +/- 2.9 Ma. The intense shearing observed occurred at upper amphibolite facies, and is inferred to be a key component of massive sulphide remobilization. The studied samples reached metamorphic peak conditions of 596-678 +/- 65 °C and 5-7 +/-2.1 kbars during D2. The combination of different applied approaches is helping to build a better understanding of stratigraphic and structural relationships in the greenstone belt and is helping to develop new exploration targets. Key words: Yilgarn craton, SHRIMP U-Pb dating, metamorphic petrology, 3D modelling.
V13B-2115
Composition and Origin of Archean Migmatites from Iisalmi, Central Finland
Besides greenstone belts, the most known prominent rock type of the Archean is tonalite-trondhjemite- granodiorite (TTG) representing continental crust. Changes in the geochemical behavior of TTG during the Archean are explained by increasingly deep melting of the subducted slab towards the end of the Archean, resulting in increasing interaction of slab melts with the overlying mantle wedge [1]. We present major and trace element data for migmatized, late-Archean TTGs and associated amphibolites from three different locations in Central Finland. We investigated in particular the content of the transition trace elements Cr, Ni, Co and Zn within mesosome and neosome (leucosome + melanosome) of metatexitic TTGs to ascertain whether they developed by metamorphic segregation and partial melting or by another process. Our data show that some melanosomes have elevated Cr (> 70 ppm) and Ni (> 40 ppm) contents, and cannot be produced by simple partial melting of unmodified TTG with subsequent metamorphic segregation and fractional crystallization of the neosome. Rather, these 'melanosomes' represent basaltic melts, now amphibolites, which intruded syn- to early post-tectonically into the TTGs, and were subsequently deformed, giving the appearance of strongly banded metatexitic migmatites. In contrast to these 'contaminated' TTGs, most of the investigated samples show typical TTG characteristics, but no particular geochemical evidence for an increasing interaction with an overlying mantle wedge. Contamination by basaltic dyking may be an alternative explanation for the increase in MgO and decrease in SiO2 of migmatitic TTG with time during the Archean. [1] Martin, H. and Moyen, J.-F., 2002: Geology 30: 319-322.
V13B-2116
300 million years of komatiite evolution of the Barberton Greenstone Belt
The 3.5-3.2 Ga Barberton greenstone belt in South Africa is the type locality of komatiite and ultramafic lavas erupted throughout most of its 300 m.y. history. Despite 30 years of investigation, there are very few complete geochemical analyses of these rocks and we are undertaking a detailed analytical study, including major and trace elements, Nd, Hf, Os, O isotopes and melt inclusion studies, of samples from four stratigraphic formations that span the complete history of the belt. The oldest well preserved komatiites are the in 3.5 Ga Komati Formation near the base of the sequence. These have sub-chondritic Al/Ti and depleted HREE and are thought to have formed during deep (>300km) partial melting in a hot mantle plume. The majority are non-vesicular and show no evidence of having erupted as hydrous lavas, but some uncommon examples contain large amygdales and are coarse-grained, and they may have contained some water. The 3.3 Ga Weltevreden komatiites have slightly super-chondritic Al/Ti and a positively sloping REE profiles, characteristics consistent with shallower melting of a source that either was enriched in garnet or had previously undergone extensive partial melting. Some Weltevreden komatiites contain olivine with the composition Fo96. The liquid composition inferred using this composition and whole-rock compositions contains 33% MgO. If anhydrous, as supported by analysis of melt inclusions, this magma erupted at about 1660° C making it the hottest magma on earth. Two types of komatiites occur in the 3.3 Ga Mendon Formation. The first has Al/Ti and HREE similar to Komati formation; the second has highly super-chondritic Al/Ti which may indicate a garnet enriched source.
V13B-2117
Palaeoarchean Barite Deposits in the Barberton Greenstone Belt: Origin and Links to Early Microbial Life
Barite deposits are considered important for identifying microbial S cycling in Archean rocks since they can provide information about S isotopes in coexisting sulfate and sulfide minerals. However the degree to which barite and pyrite in metasedimentary rocks are related remains unclear. In this study we have investigated the origin of barite and pyrite in four main horizons seen in both outcrop and fresh drill core material from the Lower Mapepe formation (3.26 to 3.23 Ga), Barberton Greenstone Belt, South Africa. Host rocks include shales, cherts, tuffs and conglomerates that are variably silicified and/or affected by carbonate alteration. The high-energy depositional environment of the host rocks, mineralogical textures, barite chemistry and the occurrence of feldspars from the rarely-found celsian-hyalophane-orthoclase series suggest a seafloor exhalative origin for the barite. In contrast pyrite is closely associated with cherts and dolomitic units where rare earth element and Y data support a marine influence. Pyrite chemistry (Co/Ni= 0.1-1, Se/S <5 x 10- 5) also indicates a low temperature sedimentary origin. Multiple S isotope data (32S, 33S, 34S, determined by SIMS) for pyrite indicates a number of arrays with limited δ34S fractionation at constant Δ33S associated with individual syn-sedimentary microcrystalline pyrite layers. Isolated euhedral pyrites in massive chert and barite rich units show much more scatter and larger degrees of Δ33S variation (-1 to +4 ‰). Our results are consistent with models invoking microbial mass dependent fractionation of a heterogeneous elemental sulfur source derived from atmospheric photolysis. The sulfate reservoir can also be linked to photolysis but there is no clear relationship between the barite and pyrite S isotope data, suggesting that microbial (or abiotic) sulfate reduction was absent at this time or that the basinal sulfate concentration must have remained significantly lower than the mM level prior to barite precipitation.
V13B-2118
Deep-water environments for the putative Early Archean life: Chert units depositional facies, Warrawoona Group, Western Australia
The principal objective of this work is to find and interpret evidence for the depositional conditions of the greenstone-belt facies that possibly hosted primitive life forms. That information is critical to any attempt to assess the habitability of Early Earth's environments, as well as their potential for fossil preservation. Chert members of the Duffer Fm and the Apex chert, which bears fossil-like filaments (3.465 Ga), are exposed in the vicinity of Marble Bar. The Marble Bar Chert is made up of dm-scale sequences of interbedded red, white and black finely crystalline silica. Sticky beds consist of flow-oriented clasts (i.e., large, angular, elongated, light-colored clasts showing load-structures, and regularly-inclined and vertically- oriented imbrications) interpreted here as gravity flows of early-lithified sediments. The Chinaman Pool Chert Mb is incised by a deep scour upon which sandstone beds with chert interlayers onlap. This unit includes coarse mafic sands, and conglomerates with chert lithoclasts and felsic pebbles. Locally, m-scale growth faults localize sand-filled channels. The depositional architectures and facies of this upper unit are indicative of a deep-water fan depositional system. This interpretation conflicts with the current view that chert units derived from hydrothermalism overprint of original shallow deposits, and that hydrothermalism hampered microfossil preservation. Alternatively, we suggest that deep-water environments and early lithification of siliceous sediments may have favored life preservation. Earth's oldest stromatolites (3.49-3.47 Ga) of the Dresser Fm are exposed in the North Pole Dome area. Autochtonous stromatolites consist of banded microstructures with iron-rich, wavy and wrinkle laminae, and form regular domes. The stromatolite layers cap a sandstone bed at the top of which asymmetric ripples show steeply-inclined sets of laminae that typically represent overturned ripples at uppermost turbiditic flow deposits. The stromatolitic domes are sealed by finely-banded cherts. A large block of rocks yields a variety of stromatolitic morphotypes including cuspate swales, domes and flat-topped individual columns that cap large intraclasts. Stromatolite occurrences are here interbedded with sea-floor and early diagenetic barite precipitates. Tilted beds and short disharmonic folds are indicative of slumping and suggest the block was transported down the depositional slope. Stromatolite forms of the North Pole Dome share much analogy with the pelagic stromatolites of condensed sequences of the Phanerozoic (i.e., omission surfaces and hard substrates related to minimal regular sediment input and/or irregular massive inputs in distal and deep marine environments). It seems that life could have originated in fairly distal and deep subaqueous environments. How far and how deep cannot be specified at this stage. The potential of such depositional environments to support and preserve microbial life will be discussed.
V13B-2119
3.2 Ga hydrothermal sedimentary sequence: DXCL drilling Project, West Pilbara, Australia
"Dixon Island" Cleaverville (DXCL) Drilling Project" was successfully performed during August 3 through 9, 2007, to obtain modern-weathering free fresh geologic samples of the 3.2 Ga Cleaverville Group at the Cleaverville Beach, west Pilbara, Australia. There are three cycles of volcanic and chemical sedimentary sequences; Lagoon, Dixon Island, Snapper Beach formations to the top. Each formation consist mainly of hydrothermal volcanic rocks (komatiite, rhyolite tuff and pillow basalt), organic-rich black chert, and iron-rich chert. We drilled three holes (CL1, CL2, and DX) that cover two formations (Snapper Beach and Dixon Island Formations of the Cleaverville Group). The drillcores CL1 (66.1m long) and CL2 (44.4m long) belong to the lower part of the Snapper Beach Formation, and the drillcore DX (100.15m long) belongs to the upper part of the Dixon Island Formation and Dixon Pillow Basalt. The total length of the drillcores is approximately 200m. We observed at least 40~50m deep uppermost sections that are visibly affected by modern weathering; however, the remaining portions are indeed "fresh," evidenced by preservation of fine grains/laminations and nodules of pyrite. All sequences of CL 1, CL 2, and DX are younging to southwest, which is identified by cross laminations and graded bed. The core of the DX consist very fine laminated black shale/gray chert/pyrite alteration. Well cyclic sedimentation is identified as annual event or hydrothermal activity cycles. The CL 1 and CL 2 cores are identified bedded black shale and fine grain volcanic sand layer. They contain finely laminated volcanic sandstone and very finely laminated black shale (mostly organic-rich and/or pyrite- rich) and gray-white chert. Total organic carbon (TOC) of the black shales are about 2-3%, and 13C is average 28°. TOC content is higher than that of average of content of black chert bed (0.1-0.2 %; Kiyokawa et al., 2006) in the Dixon Island Formation. These high organic content is higher than that of our expected. Coarsening upward sequence and non silicified sequence of the Snapper Beach Formation is shallower condition than that of the Dixon Island Formation. Sedimentary environments of the Cleaverville Group contain the burial history of oceanic volcanic caldera or base of the volcano of the oceanic island arc condition. Kiyokawa et al. 2006. GSA Bulletin, Jan/Feb. 1-22.
V13B-2120
Simultaneous generation of Hadean and Archean crust and buoyant cratonic roots by vertical tectonics
Archean cratons are characterized by granite-greenstone belts (GSB), by the predominance of tonalite- trondhjemite-granodiorite (TTG) type granites, and by the strong, depleted, and unusually buoyant sub- cratonic lithospheric mantle (SCLM). Individually, these features remain enigmatic, and together they provide no clear picture of the style of tectonics operating in the Hadean and early Archean. Starting with the observation that GSBs strongly resemble natural salt diapirs, we present models of crustal diapirism which show that under appropriate thermal and rheological conditions, dense low-viscosity volcanics overlying a felsic basement will overturn diapirically in as little as 10 Ma, displacing as much as 60% of the volcanics to the lower crust. This surprisingly fast overturn rate suggests that diapiric overturn dominated crustal tectonics in the Early Earth. Moreover, the return of mafic and ultramafic basalts to the lower crust provides a mechanism for the formation of TTGs, and the restite of melting of metabasalts at this depth strongly resembles the harzburgitic mantle. Hence, we suggest that repeated cycles of ultramafic volcanism and rapid crustal recycling by diapirism may provide a tectonic setting for the formation of both GSBs and TTGs, as well as a top-down accumulation mechanism for the formation of the Neo-Hadean and Paleo-Archean SCLM. The Archean crust we see today represents the last stages of this predominantly vertical system of tectonics, as the earth cooled and diapirism became less efficient. This is consistent with field evidence showing a transition through the Archean from a vertical to a horizontal tectonic regime.
V13B-2121
The End of Flat Earth and the Coupling of the Earth's Geochemical Reservoirs
Because of a warmer and more buoyant geotherm continents were unable to sustain topography larger than 2500 m until the late Archaean. Our numerical experiments suggest that during the Neoarchean (2.8-2.5 Ga) the continental lithosphere evolved through a rheological threshold allowing for the development of significant topography and erosion. This rather rapid strengthening was directly linked to the cooling and strengthening of the upper mantle and had profound consequences for the coupling of the Earth's geochemical reservoirs. During the Neoarchean, the Earth went through a period of global changes during which exogenic envelopes recorded major shifts in composition toward modern values. We propose that during the Neoarchean the exogenic envelopes, that were until then coupled to the mantle through submarine hydrothermal processes and volcanism also became coupled to the continental crust through relief-generating tectonics processes and erosion, hence changing the balance between mantle versus crustal interaction with the exogenic Earth. Silicate weathering and erosion being a very efficient sink for atmospheric CO2, we propose that the strengthening and emergence of the continental lithosphere in the late Archean was a very important factor not only contributing to the cooling of the Earth' surface but also the oxygenation of the Earth's atmosphere by providing large amount of key nutrient to the biosphere, in particular phosphorus a biomass limiting element almost exclusively stored in the felsic continental crust.
V13B-2122
Plate boundary processes at the Hadean-Eoarchean transition
The time of the onset of plate interactions on Earth is unknown. Have plate tectonics-like processes been a general feature of Earth since its earliest history? It is important to know this at least when considering planetary heat-loss, element recycling and habitats for the origin of life. We report direct evidence for plate boundary processes before 3.7 Ga from the geochemistry of the oldest supracrustal rocks, which include paragneisses, orthogneisses and mafic schists, from Eoarchean terranes in West Greenland (>3.83 Ga), northern Quebec (>3.75 Ga) and in a Hadean rock from northwestern Canada (ca. 4.03 Ga). Compositions for these lithologies strongly resemble the products of contemporary arc and back-arc plutonic and volcanosedimentary systems. Metamorphic equivalents of tonalite-trondhjemite suites captured in the oldest terranes show that continental precursors were widespread on Earth since at least ca. 3.83 Ga. Yet earlier geochemical indicators from oxygen- and hafnium isotopes, and mineral inclusions in Hadean detrital zircons (3.83 – 4.38 Ga), reinforce the view that subduction-like settings where partial melting of a subduction slab (or thickened mafic lower crust?) have operated on Earth since ~150 Myr after the formation of the Moon. Apparently, heterogeneities induced by earlier Hadean crust-forming events were almost entirely lost by vigorous early mantle mixing by the time of formation of the oldest known rocks.
V13B-2123
Shocked Minerals in Siliciclastic Sediments From the Vredefort Dome: A New Approach Towards Searching for Impact Evidence From the Early Earth
Understanding the intense meteorite bombardment hypothesized to have dominated the early evolution of the Earth is a fundamental goal of planetary geology. While the surface of the Moon preserves a record of early impacts, terrestrial evidence remains elusive; no Hadean impact structures have been identified on Earth. The vast majority of terrestrial impact structures are removed by erosion shortly after crater formation, however corresponding sedimentary records have not been identified. In this study we report the occurrence of detrital grains of shocked metamorphosed quartz and zircon in siliciclastic sediments from the channel of the Vaal River, a large meandering river actively eroding the 2023 Ma Vredefort Dome. The sediment samples analyzed all contain numerous detrital grains of shocked quartz which preserve a single orientation of decorated planar deformation features (PDFs), and also detrital shocked zircon grains that contain one to three sets of parallel planar fractures (PF). The recognition that impact evidence in the form of shocked minerals can persist in siliciclastic sediments up to 2 billion years after an impact provides a new investigative tool for identifying ancient impact-related detritus from structures that are long since eroded. The Hadean rock record is fragmentary, however the existence of Hadean detritus has been demonstrated by the documentation of >4000 Ma zircons in Australia, China, USA, and Canada. The geochemistry of the Hadean zircons strongly suggests they originated in quartz-saturated granitoids that likely contained abundant quartz. We hypothesize that large volumes of shocked quartz and zircon were produced during Hadean impact events. While the Hadean impact structures have long since eroded, shocked detritus may be preserved in Archean sediments. The results from the Vredefort Dome demonstrate the viability of this potential record, and in particular offers promise that traces of the lost record of impacts on the Early Earth may be preserved in siliciclastic rocks.
V13B-2124
Onset of convection in a basally heated spherical shell application to planets
Convective instabilities related to the early dynamics of planetary mantles just after core formation play an important role in the subsequent evolution. Although these early stages of planetary dynamics are likely to imply more complex phenomena such as global melting and fractional solidification, little is known about the onset of solid-state convection in a fluid with temperature-dependent viscosity heated from below. Here, we investigate onset times of convection in a spherical shell in order to obtain scaling laws with Rayleigh number, viscosity parameter describing the dependency on the temperature and geometry of the shell. The influence of the mechanical boundary condition is also studied: free-slip is relevant for planetary mantles overlying a fluid core while no-slip may better approximate the boundary condition between two solid layers (e.g. between an icy layer and a silicate core in some of the icy satellites). We performed three dimensional numerical experiments in a spherical shell using the OEDIPUS program (Choblet, 2005; Choblet et al., 2007). The fluid is incompressible, its viscosity is temperature dependent and the Boussinesq approximation is used. We systematically investigate the onset time and wavelength of the first instabilities. Furthermore, in order to better understand the processes associated to the birth of convection, 3D results are compared to onset times obtained with two simple methods: the linear stability (LS) analysis and the growth of the Rayleigh- Taylor (R-T) instabilities. For the LS analysis, the values of the onset time are much smaller due to the "frozen time" approach. Moreover, the dependency of the onset time on the Rayleigh number is overestimated, especially for the free-slip conditions, where the effect of the frozen time is even more significant due to kinematic effects. For the R-T instability analysis, however, the onset times are also slightly underestimated, the agreement with 3D numerical simulations is good in terms of efficient scaling relationships. For both free-slip and no-slip boundary condition, the scaling may be written in the form t~(Ra*)a, where a is approximately -2/3 and Ra*=Ra(μ(θ*)) is A Rayleigh number specifically associated to a relevant viscosity (temperature) value. Planetary implications are envisioned.
V13B-2125
Inter-mineral Magnesium Isotope Fractionation in the Mantle and Implications for the Supra-chondritic 25Mg/24Mg of Earth
The magnesium isotopic composition of Earth is poorly constrained despite significant advances in methods for measuring Mg isotope ratios in rocks. One impediment to establishing 25Mg/24Mg and 26Mg/24Mg of Earth is the lack of constraints on inter-mineral Mg isotope fractionations at high temperatures. Advances in computational chemistry afford the capacity to predict quantitatively Mg isotope fractionations among high-temperature minerals. High-precision MC-ICPMS measurements in turn provide the opportunity to test these predictions in well-characterized samples. Toward this end, we present new high-precision measurements of 25Mg/24Mg and 26Mg/24Mg in mantle minerals and meteorites. Our results for minerals from the San Carlos volcanic field xenoliths show that there is measurable and systematic fractionation in Mg isotope ratios between constituent minerals. The observed order from highest to lowest 25Mg/24Mg is spinel > clinopyroxene > orthopyroxene > olivine. The fractionation between spinel and olivine suggests an equilibration temperature of 810° ± 30° C based on the temperature dependence obtained from ab initio calculations. This temperature is consistent with independent T indicators, suggesting that spinel and olivine are in Mg isotopic equilibrium in these mantle rocks. Clinopyroxene, on the other hand, is not in Mg isotopic equilibrium with spinel and olivine if the predicted temperature-dependent fractionations are correct. Whole-rock samples of the carbonaceous chondrites Allende (CV3) and Orgueil (CI1) have the lowest δ25Mg values (relative to DSM3) measured as part of this study with δ25Mg of -0.21 and -0.19‰, respectively. We conclude that chondrite meteorites are not 0 on the DSM3 scale. The fact that primitive solar system materials are lower in δ25Mg than DSM3 is supported by our pallasite olivine (n=2) δ25Mg values of -0.16‰. Olivines from the lunar highlands and from Kilauea basalt are similar to pallasite olivine with δ 25Mg values of -0.15‰ with a range of ± 0.02. All of the olivines of igneous origin that we have measured have a narrow range of δ 25Mg values between about -0.18 and - 0.13‰. Olivine has the lowest 25Mg/24Mg values in mafic and ultramafic rocks. Olivine δ 25Mg ≥ chondrite therefore requires that Earth is supra-chondritic in 25Mg/24Mg (e.g., Wiechert and Halliday 2007, EPSL 256, 360).