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Akira Ishikawa - One of the best experts on this subject based on the ideXlab platform.
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rhenium osmium isotopes and highly siderophile elements in Ultramafic Rocks from the eoarchean saglek block northern labrador canada implications for archean mantle evolution
Geochimica et Cosmochimica Acta, 2017Co-Authors: Graham D Pearson, Kenneth D Collerson, Katsuhiko Suzuki, Akira Ishikawa, Tsuyoshi KomiyaAbstract:Abstract We determined highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) concentrations and 187Os/188Os ratios for Ultramafic Rocks distributed over the Eoarchean gneiss complex of the Saglek-Hebron area in northern Labrador, Canada in order to constrain to what extent variations in HSE abundances are recorded in Early Archean mantle that have well-resolved 182W isotope anomalies relative to the present-day mantle (∼+11 ppm: Liu et al., 2016). The samples analysed here have been previously classified into two suites: mantle-derived peridotites occurring as tectonically-emplaced slivers of lithospheric mantle, and metakomatiites comprising mostly pyroxenitic layers in supracrustal units dominated by amphibolites. Although previous Sm-Nd and Pb-Pb isotope studies provided whole-rock isochrons indicative of ∼3.8 Ga protolith formation for both suites, our whole-rock Re-Os isotope data on a similar set of samples yield considerably younger errorchrons with ages of 3612 ± 130 Ma (MSWD = 40) and 3096 ± 170 Ma (MSWD = 10.2) for the metakomatiite and lithospheric mantle suites, respectively. The respective initial 187Os/188Os = 0.10200 ± 18 for metakomatiites and 0.1041 ± 18 for lithospheric mantle Rocks are within the range of chondrites. Re-depletion Os model ages for unradiogenic samples from the two suites are consistent with the respective Re-Os errorchrons (metakomatiite TRD = 3.4–3.6 Ga; lithospheric mantle TRD = 2.8–3.3 Ga). These observations suggest that the two Ultramafic suites are not coeval. However, the estimated mantle sources for the two Ultramafics suites are similar in terms of their broadly chondritic evolution of 187Os/188Os and their relative HSE patterns. In detail, both mantle sources show a small excess of Ru/Ir similar to that in modern primitive mantle, but a ∼20% deficit in absolute HSE abundances relative to that in modern primitive mantle (metakomatiite 74 ± 18% of PUM; lithospheric mantle 82 ± 10% of PUM), consistent with the ∼3.8 Ga Isua mantle source and Neoarchean komatiite sources around the world (∼70–86% of PUM). This demonstrates that the lower HSE abundances are not unique to the sources of komatiites, but rather might be a ubiquitous feature of Archean convecting mantle. This tentatively suggests that chondritic late accretion components boosted the convecting mantle HSE inventory after core separation in the Hadean, and that the Eoarchean to Neoarchean convecting mantle was depleted in its HSE content relative to that of today. Further investigation of Archean mantle-derived Rocks is required to explore this hypothesis.
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widespread tungsten isotope anomalies and w mobility in crustal and mantle Rocks of the eoarchean saglek block northern labrador canada implications for early earth processes and w recycling
Earth and Planetary Science Letters, 2016Co-Authors: Jingao Liu, Akira Ishikawa, Mathieu Touboul, Richard J Walker, Graham D PearsonAbstract:Abstract Well-resolved 182W isotope anomalies, relative to the present mantle, in Hadean–Archean terrestrial Rocks have been interpreted to reflect the effects of variable late accretion and early mantle differentiation processes. To further explore these early Earth processes, we have carried out W concentration and isotopic measurements of Eoarchean Ultramafic Rocks, including lithospheric mantle Rocks, meta-komatiites, a layered Ultramafic body and associated crustal gneisses and amphibolites from the Uivak gneiss terrane of the Saglek Block, northern Labrador, Canada. These analyses are augmented by in situ W concentration measurements of individual phases in order to examine the major hosts of W in these Rocks. Although the W budget in some Rocks can be largely explained by a combination of their major phases, W in other Rocks is hosted mainly in secondary grain-boundary assemblages, as well as in cryptic, unidentified W-bearing ‘nugget’ minerals. Whole rock W concentrations in the Ultramafic Rocks show unexpected enrichments relative, to elements with similar incompatibilities. By contrast, W concentrations are low in the Uivak gneisses. These data, along with the in situ W concentration data, suggest metamorphic transport/re-distribution of W from the regional felsic Rocks, the Uivak gneiss precursors, to the spatially associated Ultramafic Rocks. All but one sample from the lithologically varied Eoarchean Saglek suite is characterized by generally uniform ∼ + 11 ppm enrichments in 182W relative to Earth's modern mantle. Modeling shows that the W isotopic enrichments in the Ultramafic Rocks were primarily inherited from the surrounding 182W-rich felsic precursor Rocks, and that the W isotopic composition of the original Ultramafic Rocks cannot be determined. The observed W isotopic composition of mafic to Ultramafic Rocks in intimate contact with ancient crust should be viewed with caution in order to plate constraints on the early Hf–W isotopic evolution of the Earth's mantle with regard to late accretionary processes. Although 182W anomalies can be erased via mixing in the convective mantle, recycling of 182W-rich crustal Rocks into the mantle can produce new mantle sources with anomalous W isotopic compositions that can be tapped at much later times and, hence, this process should be considered as a mechanism for the generation of 182W-rich Rocks at any subsequent time in Earth history.
Graham D Pearson - One of the best experts on this subject based on the ideXlab platform.
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rhenium osmium isotopes and highly siderophile elements in Ultramafic Rocks from the eoarchean saglek block northern labrador canada implications for archean mantle evolution
Geochimica et Cosmochimica Acta, 2017Co-Authors: Graham D Pearson, Kenneth D Collerson, Katsuhiko Suzuki, Akira Ishikawa, Tsuyoshi KomiyaAbstract:Abstract We determined highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) concentrations and 187Os/188Os ratios for Ultramafic Rocks distributed over the Eoarchean gneiss complex of the Saglek-Hebron area in northern Labrador, Canada in order to constrain to what extent variations in HSE abundances are recorded in Early Archean mantle that have well-resolved 182W isotope anomalies relative to the present-day mantle (∼+11 ppm: Liu et al., 2016). The samples analysed here have been previously classified into two suites: mantle-derived peridotites occurring as tectonically-emplaced slivers of lithospheric mantle, and metakomatiites comprising mostly pyroxenitic layers in supracrustal units dominated by amphibolites. Although previous Sm-Nd and Pb-Pb isotope studies provided whole-rock isochrons indicative of ∼3.8 Ga protolith formation for both suites, our whole-rock Re-Os isotope data on a similar set of samples yield considerably younger errorchrons with ages of 3612 ± 130 Ma (MSWD = 40) and 3096 ± 170 Ma (MSWD = 10.2) for the metakomatiite and lithospheric mantle suites, respectively. The respective initial 187Os/188Os = 0.10200 ± 18 for metakomatiites and 0.1041 ± 18 for lithospheric mantle Rocks are within the range of chondrites. Re-depletion Os model ages for unradiogenic samples from the two suites are consistent with the respective Re-Os errorchrons (metakomatiite TRD = 3.4–3.6 Ga; lithospheric mantle TRD = 2.8–3.3 Ga). These observations suggest that the two Ultramafic suites are not coeval. However, the estimated mantle sources for the two Ultramafics suites are similar in terms of their broadly chondritic evolution of 187Os/188Os and their relative HSE patterns. In detail, both mantle sources show a small excess of Ru/Ir similar to that in modern primitive mantle, but a ∼20% deficit in absolute HSE abundances relative to that in modern primitive mantle (metakomatiite 74 ± 18% of PUM; lithospheric mantle 82 ± 10% of PUM), consistent with the ∼3.8 Ga Isua mantle source and Neoarchean komatiite sources around the world (∼70–86% of PUM). This demonstrates that the lower HSE abundances are not unique to the sources of komatiites, but rather might be a ubiquitous feature of Archean convecting mantle. This tentatively suggests that chondritic late accretion components boosted the convecting mantle HSE inventory after core separation in the Hadean, and that the Eoarchean to Neoarchean convecting mantle was depleted in its HSE content relative to that of today. Further investigation of Archean mantle-derived Rocks is required to explore this hypothesis.
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widespread tungsten isotope anomalies and w mobility in crustal and mantle Rocks of the eoarchean saglek block northern labrador canada implications for early earth processes and w recycling
Earth and Planetary Science Letters, 2016Co-Authors: Jingao Liu, Akira Ishikawa, Mathieu Touboul, Richard J Walker, Graham D PearsonAbstract:Abstract Well-resolved 182W isotope anomalies, relative to the present mantle, in Hadean–Archean terrestrial Rocks have been interpreted to reflect the effects of variable late accretion and early mantle differentiation processes. To further explore these early Earth processes, we have carried out W concentration and isotopic measurements of Eoarchean Ultramafic Rocks, including lithospheric mantle Rocks, meta-komatiites, a layered Ultramafic body and associated crustal gneisses and amphibolites from the Uivak gneiss terrane of the Saglek Block, northern Labrador, Canada. These analyses are augmented by in situ W concentration measurements of individual phases in order to examine the major hosts of W in these Rocks. Although the W budget in some Rocks can be largely explained by a combination of their major phases, W in other Rocks is hosted mainly in secondary grain-boundary assemblages, as well as in cryptic, unidentified W-bearing ‘nugget’ minerals. Whole rock W concentrations in the Ultramafic Rocks show unexpected enrichments relative, to elements with similar incompatibilities. By contrast, W concentrations are low in the Uivak gneisses. These data, along with the in situ W concentration data, suggest metamorphic transport/re-distribution of W from the regional felsic Rocks, the Uivak gneiss precursors, to the spatially associated Ultramafic Rocks. All but one sample from the lithologically varied Eoarchean Saglek suite is characterized by generally uniform ∼ + 11 ppm enrichments in 182W relative to Earth's modern mantle. Modeling shows that the W isotopic enrichments in the Ultramafic Rocks were primarily inherited from the surrounding 182W-rich felsic precursor Rocks, and that the W isotopic composition of the original Ultramafic Rocks cannot be determined. The observed W isotopic composition of mafic to Ultramafic Rocks in intimate contact with ancient crust should be viewed with caution in order to plate constraints on the early Hf–W isotopic evolution of the Earth's mantle with regard to late accretionary processes. Although 182W anomalies can be erased via mixing in the convective mantle, recycling of 182W-rich crustal Rocks into the mantle can produce new mantle sources with anomalous W isotopic compositions that can be tapped at much later times and, hence, this process should be considered as a mechanism for the generation of 182W-rich Rocks at any subsequent time in Earth history.
Tsuyoshi Komiya - One of the best experts on this subject based on the ideXlab platform.
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rhenium osmium isotopes and highly siderophile elements in Ultramafic Rocks from the eoarchean saglek block northern labrador canada implications for archean mantle evolution
Geochimica et Cosmochimica Acta, 2017Co-Authors: Graham D Pearson, Kenneth D Collerson, Katsuhiko Suzuki, Akira Ishikawa, Tsuyoshi KomiyaAbstract:Abstract We determined highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) concentrations and 187Os/188Os ratios for Ultramafic Rocks distributed over the Eoarchean gneiss complex of the Saglek-Hebron area in northern Labrador, Canada in order to constrain to what extent variations in HSE abundances are recorded in Early Archean mantle that have well-resolved 182W isotope anomalies relative to the present-day mantle (∼+11 ppm: Liu et al., 2016). The samples analysed here have been previously classified into two suites: mantle-derived peridotites occurring as tectonically-emplaced slivers of lithospheric mantle, and metakomatiites comprising mostly pyroxenitic layers in supracrustal units dominated by amphibolites. Although previous Sm-Nd and Pb-Pb isotope studies provided whole-rock isochrons indicative of ∼3.8 Ga protolith formation for both suites, our whole-rock Re-Os isotope data on a similar set of samples yield considerably younger errorchrons with ages of 3612 ± 130 Ma (MSWD = 40) and 3096 ± 170 Ma (MSWD = 10.2) for the metakomatiite and lithospheric mantle suites, respectively. The respective initial 187Os/188Os = 0.10200 ± 18 for metakomatiites and 0.1041 ± 18 for lithospheric mantle Rocks are within the range of chondrites. Re-depletion Os model ages for unradiogenic samples from the two suites are consistent with the respective Re-Os errorchrons (metakomatiite TRD = 3.4–3.6 Ga; lithospheric mantle TRD = 2.8–3.3 Ga). These observations suggest that the two Ultramafic suites are not coeval. However, the estimated mantle sources for the two Ultramafics suites are similar in terms of their broadly chondritic evolution of 187Os/188Os and their relative HSE patterns. In detail, both mantle sources show a small excess of Ru/Ir similar to that in modern primitive mantle, but a ∼20% deficit in absolute HSE abundances relative to that in modern primitive mantle (metakomatiite 74 ± 18% of PUM; lithospheric mantle 82 ± 10% of PUM), consistent with the ∼3.8 Ga Isua mantle source and Neoarchean komatiite sources around the world (∼70–86% of PUM). This demonstrates that the lower HSE abundances are not unique to the sources of komatiites, but rather might be a ubiquitous feature of Archean convecting mantle. This tentatively suggests that chondritic late accretion components boosted the convecting mantle HSE inventory after core separation in the Hadean, and that the Eoarchean to Neoarchean convecting mantle was depleted in its HSE content relative to that of today. Further investigation of Archean mantle-derived Rocks is required to explore this hypothesis.
Peter B Kelemen - One of the best experts on this subject based on the ideXlab platform.
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lipid biomarker record of the serpentinite hosted ecosystem of the samail ophiolite oman and implications for the search for biosignatures on mars
Astrobiology, 2020Co-Authors: S A Newman, Sara A Lincoln, Shane S Oreilly, Xiaolei Liu, Everett L Shock, Peter B KelemenAbstract:Serpentinization is a weathering process in which Ultramafic Rocks react with water, generating a range of products, including serpentine and other minerals, in addition to H2 and low-molecular-wei...
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In situ carbon mineralization in Ultramafic Rocks: Natural processes and possible engineered methods
Energy Procedia, 2018Co-Authors: Peter B Kelemen, Greeshma Gadikota, Roger D. Aines, E. Bennett, Sally M. Benson, E. Carter, J. A. Coggon, J. C. De Obeso, Owen Evans, G. M. DippleAbstract:Abstract In this invited review, we summarize the main results of ongoing research on “in situ” carbon mineralization in Ultramafic Rocks, including outcrop studies in Oman (e.g., [1, 2]), investigation of carbon mass transfer in subduction zones from the Oman Drilling Project (e.g., [3-7]), laboratory investigations (e.g., [8-12]) and numerical modeling (e.g., [13-17]) of the pressure of crystallization and reaction-driven cracking, and assessment of the rate, cost and capacity of various proposed methods for engineered carbon mineralization [18, 19].
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the petrogenesis of Ultramafic Rocks in the 3 7ga isua supracrustal belt southern west greenland geochemical evidence for two distinct magmatic cumulate trends
Gondwana Research, 2015Co-Authors: Peter B Kelemen, Kristoffer Szilas, Minik T RosingAbstract:Ultramafic Rocks with MgO > 25 wt.% are found as large slivers within the Eoarchaean Isua supracrustal belt. Some of these Ultramafic bodies have previously been proposed to represent relicts of residual mantle. In this study we present new bulk-rock major, trace and platinum-group element data, as well as mineral chemistry for these Rocks in order to constrain their origin. The Ultramafic Rocks form two distinct geochemical trends, which project away from local tholeiitic basalts and boninite-like volcanic Rocks, respectively. The Ultramafic Rocks have FeOt contents of up to 16 wt.% and Al2O3 up to 11 wt.%, with abundant normative orthopyroxene. Their trace element patterns are broadly parallel with the two types of volcanic Rocks with which they are associated. All analysed Rocks have fractionated chondrite-normalised platinum-group element patterns with relatively low Os and Ir abundances. The few spinel grains that were found to be potentially primary (Fe3 +# < 10) have Cr# of around 73 and most have Mg# of about 23. None of the above geochemical features are compatible with a residual mantle origin of the Ultramafic Rocks in the Isua supracrustal belt. Instead, these data suggest an origin of the Ultramafic Rocks by accumulation of mainly olivine + spinel during fractional crystallisation of the tholeiitic basalts and the boninite-like volcanic series, and possibly continued crystallisation of plagioclase ± orthopyroxene to form the more Al-rich cumulates. This interpretation is supported by modelling of the liquid evolution and the corresponding bulk-cumulates, for the two volcanic sequences. We find that the observed and calculated liquid evolutions match reasonably well, and can be explained mainly by olivine and spinel crystallisation. However, depending on the amount of accumulated plagioclase and orthopyroxene, the observed bulk-cumulate requires additional input from the evolving liquids to account for the elevated SiO2, Al2O3, TiO2, normative orthopyroxene and the trace element abundances of the Ultramafic Rocks. We conclude that igneous crystal fractionation, in combination with cumulate–liquid interaction is capable of explaining all of the geochemical variation observed for these Ultramafic Rocks, and that there is no evidence for residual mantle Rocks in the Isua supracrustal belt.
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geochemistry and magmatic history of eclogites and Ultramafic Rocks from the chinese continental scientific drill hole subduction and ultrahigh pressure metamorphism of lower crustal cumulates
Chemical Geology, 2008Co-Authors: Keqing Zong, Peter B KelemenAbstract:Abstract Three distinct groups of eclogites (low-Mg–Ti eclogites, high-Ti eclogites and Mg-rich eclogites) and Ultramafic Rocks from the depth interval of 100–680 m of the Chinese Continental Scientific Drill Hole were studied. The low Mg#s (= 100 ⁎ molar Mg/(Mg + Fe)) (81–84%) and low Ni (1150–1220 ppm) and high Fe 2 O 3 total (13–15 wt.%) contents of Ultramafic Rocks suggest a cumulate origin. Mg-rich eclogites show middle and heavy REE enrichments, which could not be produced by metamorphic growth of garnet. Instead, if the Rocks formed from a light REE enriched magma, there may be an igneous precursor for some garnets in their protolith. Alternatively, perhaps they formed from a light REE depleted magma without garnet. The high-Ti eclogites are characterized by unusually high Fe 2 O 3 total contents (up to 24.5 wt.%) and decoupling of high TiO 2 from low Nb and Ta contents. These features cannot be produced by concentration of rutile during UHP metamorphism (even for samples with TiO 2 > 4 wt.%) of high-Ti basalts, but could be attributed to crystal fractionation of titanomagnetite (for those with TiO 2 2 > ∼ 4 wt.%). Thus, we suggest that protoliths of the high-Ti eclogites were titanomagnetite/ilmenite-rich gabbroic cumulates. As a whole, the low-Mg–Ti eclogites are geochemically complementary to the high-Ti eclogites, Mg-rich eclogites and Ultramafic Rocks, and could be metamorphic products of gabbroic/dioritic cumulates formed by high degree crystal fractionation. All these observations suggest that parental materials of the Ultramafic rock-eclogite assemblage could represent a complete sequence of fractional crystallization of tholeiitic or picritic magmas at intermediate to high pressure, which were later carried to ultrahigh-pressure conditions during a continental collision event.
Kristoffer Szilas - One of the best experts on this subject based on the ideXlab platform.
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Corundum formation by metasomatic reactions in Archean metapelite, SW Greenland: Exploration vectors for ruby deposits within high-grade greenstone belts
Elsevier, 2018Co-Authors: Chris Yakymchuk, Kristoffer SzilasAbstract:Corundum (ruby-sapphire) is known to have formed in situ within Archean metamorphic Rocks at several localities in the North Atlantic Craton of Greenland. Here we present two case studies for such occurrences: (1) Maniitsoq region (Kangerdluarssuk), where kyanite paragneiss hosts ruby corundum, and (2) Nuuk region (Storø), where sillimanite gneiss hosts ruby corundum. At both occurrences, Ultramafic Rocks (amphibole-peridotite) are in direct contact with the ruby-bearing zones, which have been transformed to mica schist by metasomatic reactions. The bulk-rock geochemistry of the ruby-bearing Rocks is consistent with significant depletion of SiO2 in combination with addition of Al2O3, MgO, K2O, Th and Sr relative to an assumed aluminous precursor metapelite. Phase equilibria modelling supports ruby genesis from the breakdown of sillimanite and kyanite at elevated temperatures due to the removal of SiO2. The juxtaposition of relatively silica- and aluminum-rich metasedimentary Rocks with low silica Ultramafic Rocks established a chemical potential gradient that leached/mobilized SiO2 allowing corundum to stabilize in the former Rocks. Furthermore, addition of Al2O3 via a metasomatic reaction is required, because Al/Ti is fractionated between the aluminous precursor metapelites and the resulting corundum-bearing mica schist. We propose that Al was mobilized either by complexation with hydroxide at alkaline conditions, or that Al was transported as K-Al-Si-O polymers at deep crustal levels. The three main exploration vectors for corundum within Archean greenstone belts are: (1) amphibolite- to granulite-facies metamorphic conditions, (2) the juxtaposition of Ultramafic Rocks and aluminous metapelite, and (3) mica-rich reactions zones at their interface. Keywords: Ultramafic Rocks, Kyanite, Sillimanite, Metasomatism, Pseudosection, Desilicificatio
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the petrogenesis of Ultramafic Rocks in the 3 7ga isua supracrustal belt southern west greenland geochemical evidence for two distinct magmatic cumulate trends
Gondwana Research, 2015Co-Authors: Peter B Kelemen, Kristoffer Szilas, Minik T RosingAbstract:Ultramafic Rocks with MgO > 25 wt.% are found as large slivers within the Eoarchaean Isua supracrustal belt. Some of these Ultramafic bodies have previously been proposed to represent relicts of residual mantle. In this study we present new bulk-rock major, trace and platinum-group element data, as well as mineral chemistry for these Rocks in order to constrain their origin. The Ultramafic Rocks form two distinct geochemical trends, which project away from local tholeiitic basalts and boninite-like volcanic Rocks, respectively. The Ultramafic Rocks have FeOt contents of up to 16 wt.% and Al2O3 up to 11 wt.%, with abundant normative orthopyroxene. Their trace element patterns are broadly parallel with the two types of volcanic Rocks with which they are associated. All analysed Rocks have fractionated chondrite-normalised platinum-group element patterns with relatively low Os and Ir abundances. The few spinel grains that were found to be potentially primary (Fe3 +# < 10) have Cr# of around 73 and most have Mg# of about 23. None of the above geochemical features are compatible with a residual mantle origin of the Ultramafic Rocks in the Isua supracrustal belt. Instead, these data suggest an origin of the Ultramafic Rocks by accumulation of mainly olivine + spinel during fractional crystallisation of the tholeiitic basalts and the boninite-like volcanic series, and possibly continued crystallisation of plagioclase ± orthopyroxene to form the more Al-rich cumulates. This interpretation is supported by modelling of the liquid evolution and the corresponding bulk-cumulates, for the two volcanic sequences. We find that the observed and calculated liquid evolutions match reasonably well, and can be explained mainly by olivine and spinel crystallisation. However, depending on the amount of accumulated plagioclase and orthopyroxene, the observed bulk-cumulate requires additional input from the evolving liquids to account for the elevated SiO2, Al2O3, TiO2, normative orthopyroxene and the trace element abundances of the Ultramafic Rocks. We conclude that igneous crystal fractionation, in combination with cumulate–liquid interaction is capable of explaining all of the geochemical variation observed for these Ultramafic Rocks, and that there is no evidence for residual mantle Rocks in the Isua supracrustal belt.
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origin of mesoarchaean arc related Rocks with boninite komatiite affinities from southern west greenland
Lithos, 2012Co-Authors: Kristoffer Szilas, Robert Frei, Tomas Naeraa, Anders Schersten, Henrik Stendal, Vincent J Van Hinsberg, Thomas F Kokfelt, Minik T RosingAbstract:We report whole-rock elemental and Sm-Nd isotope geochemical data from mafic-Ultramafic supracrustal Rocks from the Nunatak 1390 area in southern West Greenland. Additionally, we report the metamorphic temperature history for these Rocks as derived from tourmaline thermometry on a tourmalinite inlier, as well as in situ U-Pb, Hf and O isotopic data from zircons extracted from tonalite-trondhjemite-granodiorite (TTG) gneisses that intruded the mafic-Ultramafic sequence. The supracrustal Rocks from the Nunatak 1390 area have a minimum age of c. 2900 Ma defined by U-Pb zircon ages of cross-cutting aplite sheets of TTG composition. The supracrustal sequence comprises mafic Rocks with pillow structures and Ultramafic Rocks with no evidence of their protolith. They all have amphibolite-facies mineral assemblages and a peak metamorphic temperature of approximately 550 degrees C. The mafic sequence has relatively flat trace element patterns (La-N/Sm-N of 0.70-2.4) and mostly negative Nb-anomalies (Nb/Nb* of 0.30-1.0) and resembles modern island arc tholeiites. The mafic sequence can be divided into a high- and low-Ti group, where the former group has lower MgO, and significantly higher contents of incompatible elements such as TiO2, P2O5, Zr, Nb and Th. The Ultramafic Rocks have major and trace element compositions similar to Ti-enriched/Karasjok-type komatiites described in the literature. However, there are no textural indications that the Ultramafic Rocks from Nunatak 1390 are komatiites sensu stricto. The low-Ti group of the mafic sequence appears to have been derived from a N-MORB source, whereas the high-Ti group and the Ultramafic Rocks appear to have been derived from a mantle source that is more enriched than the N-MORB source. However, there is no difference in the initial epsilon Nd of the mafic and Ultramafic Rocks. Additionally, assimilation-fractional-crystallisation (AFC) modelling is consistent with this enrichment being caused by introduction of juvenile low-silica adalcite (slab-melt) into the mantle source region. Accordingly, we propose that the mafic and Ultramafic Rocks were derived from a similar type of mantle source, but that the Ultramafic Rocks were derived from a previously depleted mantle source that was refertilised by slab melts in a subduction zone setting. The high MgO contents of the Ultramafic Rocks could thus reflect a second stage of partial melting of a refractory mantle in a process similar to that which is suggested for the formation of modern boninites. We propose that the mafic-Ultramafic sequence represents an island arc that evolved initially as a juvenile complex (c. 3000 Ma). However, inherited zircon grains in aplites and Hf isotope data recorded by the second intrusive TTG phase (c. 2850-2870 Ma), show that mixing with older pre-existing crust occurred during this event. Because the regional crust is dominated by TTGs of this younger age, our data suggests that it likely formed by accretion and melting of arcs of different ages and/or contamination of juvenile arcs by pre-existing continental crust rather than entirely by juvenile arc differentiation or melting. Our data thus supports melting of thickened mafic crust in an accretionary setting, rather than direct slab melting, as a mechanism for Archaean crust formation. (C) 2012 Elsevier B.V. All rights reserved.
