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Kent C Condie - One of the best experts on this subject based on the ideXlab platform.
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rapid mantle convection drove massive crustal thickening in the late Archean
Geochimica et Cosmochimica Acta, 2020Co-Authors: Roberta L Rudnick, Ming Tang, Cinty A Lee, Kent C CondieAbstract:Abstract The lithospheric mantle beneath Archean cratons is conspicuously refractory and thick compared to younger continental lithosphere ( Jordan, 1988 , Boyd, 1989 ; Lee and Chin, 2014), but how such thick lithospheres formed is unclear. Using a large global geochemical database of Archean igneous crustal rocks overlying these thick cratonic roots, we show from Gd/Yb– and MnO/FeOT–SiO2 trends that crustal differentiation required continuous garnet fractionation. Today, these signatures are only found where crust is anomalously thick (60–70 km), as in the Northern and Central Andes and Southern Tibet. The widespread garnet signature in Archean igneous suites suggests that thickening occurred not only in the lithospheric mantle but also in the crust during continent formation in the late Archean. Building thick crust requires tectonic thickening or magmatic inflation rates that can compete against gravitational collapse through lower crustal flow, which would have been enhanced in the Archean when geotherms were hotter and crustal rocks weaker. We propose that Archean crust and mantle lithosphere formed by thickening over mantle downwelling sites with minimum strain rates on the order of 10−13–10−12 s−1, requiring mantle flow rates associated with late Archean crust formation to be 10–100 times faster than today.
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A great thermal divergence in the mantle beginning 2.5 Ga: Geochemical constraints from greenstone basalts and komatiites
Geoscience Frontiers, 2016Co-Authors: Kent C Condie, Richard C. Aster, Jeroen Van HunenAbstract:Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time. Four types of mantle are characterized from incompatible element distributions in basalts and komatiites: depleted, hydrated, enriched and mantle from which komatiites are derived. Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean, which we ascribe to thermal and convective evolution. Prior to 2.5 Ga, depleted and enriched mantle have indistinguishable thermal histories, whereas at 2.5–2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle. Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes, but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle. During this time, however, high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts. Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment of Ti, Al, Ca and Na in basalts derived from enriched mantle sources. After 2.5 Ga, increases in Mg# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time. A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.
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the building blocks of continental crust evidence for a major change in the tectonic setting of continental growth at the end of the Archean
Gondwana Research, 2013Co-Authors: Kent C Condie, Alfred KronerAbstract:Abstract Oceanic arcs are commonly cited as primary building blocks of continents, yet modern oceanic arcs are mostly subducted. Also, lithosphere buoyancy considerations show that oceanic arcs (even those with a felsic component) should readily subduct. With the exception of the Arabian–Nubian orogen, terranes in post-Archean accretionary orogens comprise 50%) produced in continental arcs. Felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs. They have lower Th/Yb, Nb/Yb, Sr/Y and La/Yb ratios, reflecting shallow mantle sources in which garnet did not exist in the restite during melting. The bottom line of these geochemical differences is that post-Archean continental crust does not begin life in oceanic arcs. On the other hand, the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs. During the Archean, however, oceanic arcs may have been thicker due to higher degrees of melting in the mantle, and oceanic lithosphere would be more buoyant. These arcs may have accreted to each other and to oceanic plateaus, a process that eventually led to the production of Archean continental crust. After the Archean, oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges, hence, oceanic lithosphere is more subductable. Widespread propagation of plate tectonics in the late Archean may have led not only to rapid production of continental crust, but to a change in the primary site of production of continental crust, from accreted oceanic arcs and oceanic plateaus in the Archean to primarily continental arcs thereafter.
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Did the character of subduction change at the end of the Archean? Constraints from convergent-margin granitoids
Geology, 2008Co-Authors: Kent C CondieAbstract:Large ion lithophile and high field strength element distributions in juvenile upper continental crust are controlled chiefly by the abundance of tonalite-trondhjemite-granodiorite (TTG) in the Archean shifting to a combination of TTG, calc-alkaline granitoid, and graywacke control thereafter. Geochemical differences between TTG and high-silica adakites do not require production of most TTG magmas in descending slabs. Changes in the ratio of TTG to calc-alkaline granitoids after 2.5 Ga indicate that Archean subduction zones must have differed from younger subduction zones in two very important ways: (1) a deep mafic crust served as a TTG magma source (either as thickened crust or in descending slabs), and (2) they did not give rise to significant volumes of calc-alkaline magma. Thickened mafic crust in the Late Archean may have resulted from plate jams in subduction zones caused by thicker oceanic crust and oceanic plateaus produced during Late Archean mantle thermal events.
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high field strength element ratios in Archean basalts a window to evolving sources of mantle plumes
Lithos, 2005Co-Authors: Kent C CondieAbstract:Abstract In terms of high field strength element ratios Nb/Th, Zr/Nb, Nb/Y and Zr/Y, most basalts from non-arc type Archean greenstones are similar to oceanic plateau basalts, suggestive of mantle plume sources. A large number of these basalts have ratios similar to primitive mantle composition. Perhaps the Archean mantle was less fractionated than at present and “primitive mantle” comprised much of the deep mantle and made a significant contribution to mantle plumes. The near absence of Archean greenstone basalts similar to NMORB in composition is also consistent with a relatively unfractionated mantle in which a shallow depleted source (DM) was volumetrically insignificant. The element ratios in basalts also indicate the existence of recycled components (HIMU, EM1, EM2) in the mantle by the Late Archean. This suggests that oceanic lithosphere was recycled into the deep mantle and became incorporated in some mantle plumes by the Late Archean. High field strength element ratios also indicate an important contribution of continental crust or/and subcontinental lithosphere to some non-arc Archean greenstone basalts. This implies that at least thin continental lithosphere was relatively widespread in the Archean.
Justin L Payne - One of the best experts on this subject based on the ideXlab platform.
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crustal thickening and clay controls on o isotope variation in global magmatism and siliciclastic sedimentary rocks
Earth and Planetary Science Letters, 2015Co-Authors: Justin L Payne, Karin M Barovich, Norman J Pearson, Martin Hand, David McinerneyAbstract:New compilations of global O isotope data from zircon and siliciclastic sedimentary rocks highlight an increasing range in δ18O values in both systems since the late Archean. This is consistent with an increased clay component in sedimentary rocks and subsequent incorporation into igneous rocks. Each of these factors can arguably be achieved by increased crustal thickening in the late Archean resulting in greater burial and melting of supracrustal rocks and increased chemical weathering and recycling of upper crustal rocks. Despite the suggested change in tectonic regimes in the late Archean, stochastic modelling in this study demonstrates that δ18O data do not provide evidence for a secular decrease in the proportion of mantle-derived magmas in granitoid rocks. Instead, best-fit models indicate that juvenile input and reworking of supracrustal material vary with respect to the short term (100–200 Myr) tectonic cycles preserved in the continental crust. Hence, major step changes in global tectonic regimes in the post-Hadean, such as the initiation of subduction in the mid- to late Archean, are not supported by global zircon O isotope datasets and instead minor, progressive changes are indicated for Earth's tectonic regimes.
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crustal thickening and clay controls on o isotope variation in global magmatism and siliciclastic sedimentary rocks
Earth and Planetary Science Letters, 2015Co-Authors: Justin L Payne, Karin M Barovich, Norman J Pearson, Martin Hand, David McinerneyAbstract:New compilations of global O isotope data from zircon and siliciclastic sedimentary rocks highlight an increasing range in δ18O values in both systems since the late Archean. This is consistent with an increased clay component in sedimentary rocks and subsequent incorporation into igneous rocks. Each of these factors can arguably be achieved by increased crustal thickening in the late Archean resulting in greater burial and melting of supracrustal rocks and increased chemical weathering and recycling of upper crustal rocks. Despite the suggested change in tectonic regimes in the late Archean, stochastic modelling in this study demonstrates that δ18O data do not provide evidence for a secular decrease in the proportion of mantle-derived magmas in granitoid rocks. Instead, best-fit models indicate that juvenile input and reworking of supracrustal material vary with respect to the short term (100–200 Myr) tectonic cycles preserved in the continental crust. Hence, major step changes in global tectonic regimes in the post-Hadean, such as the initiation of subduction in the mid- to late Archean, are not supported by global zircon O isotope datasets and instead minor, progressive changes are indicated for Earth's tectonic regimes.
Clark M Johnson - One of the best experts on this subject based on the ideXlab platform.
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a high continental weathering flux into paleoArchean seawater revealed by strontium isotope analysis of 3 26 ga barite
Earth and Planetary Science Letters, 2016Co-Authors: Donald R Lowe, Aaron M Satkoski, Brian L Beard, Max Coleman, Clark M JohnsonAbstract:Controls on Archean seawater chemistry remain controversial. Many studies have suggested that it was largely controlled by oceanic hydrothermal fluid circulation. Recent work, however, from clastic sequences, Hf–O isotope data from detrital zircons, and models for the Rb/Sr evolution of the continental crust suggest that intense continental weathering and low-temperature surface alteration were more important than previously thought during the early Archean. This is consistent with biogeochemical studies that suggest the Archean had a diverse microbial ecology, which would, in part, need to be sustained by nutrients (e.g., phosphorus) that were derived from continental weathering. To further quantify continental weathering during the early Archean, we analyzed 3.26 Ga barite from the Fig Tree Group, South Africa for strontium, oxygen, and sulfur isotope compositions. We propose that the seawater component of the barite is characterized by 87Sr/86Sr ratios >0.701, which is significantly more radiogenic than contemporaneous mantle (∼0.7007–0.7008). The radiogenic nature of seawater at this time suggests that the continental weathering flux at 3.26 Ga had a large impact on ocean chemistry 400 million years earlier than previously suggested.
W L Griffin - One of the best experts on this subject based on the ideXlab platform.
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high mg carbonatitic melts in diamonds kimberlites and the sub continental lithosphere
Earth and Planetary Science Letters, 2011Co-Authors: Y Weiss, W L Griffin, D R Bell, Oded NavonAbstract:Abstract The trace elements of high-Mg carbonatitic high-density fluids (HDFs) trapped in six fibrous diamonds from Siberia exhibit patterns that are highly similar to those of Group I kimberlites, but are slightly more fractionated. The patterns of both are similar to the average pattern of post-Archaean xenoliths from the sub-continental lithospheric mantle (SCLM). The Siberian high-Mg carbonatitic HDFs are highly enriched in incompatible elements and have compositions comparable to those of high-Mg HDFs from Kankan, Guinea. However, in detail the latter show depletion of K, Rb, Cs, Nb and Ta and enrichment in Ba, Th, U and LREE relative to the Siberian HDFs. These differences correspond closely to those between the patterns of Group II and Group I kimberlites, respectively. Mixing, fractionation and melting were explored as possible scenarios to explain these similarities and to constrain the possible genetic relationships between HDFs, kimberlites and the SCLM. Addition of 2.5% of Group I kimberlitic magma or 0.5% of the Udachnaya high-Mg HDFs to a depleted peridotite closely reproduces the post-Archaean SCLM pattern. The formation of high-Mg HDFs through fractionation of kimberlitic magma calls for 80% crystallization of olivine, clinopyroxene, garnet, carbonate and ilmenite. However, mismatches in K, Rb, Y and Ho abundances, and absence of the postulated fractionating minerals as inclusions suggest other petrogenetic scenarios are more likely. High-Mg HDFs and kimberlites can be produced by melting of a common source. The pattern of the calculated source for Siberian HDF and Group I kimberlites resembles that of average post-Archean, rather than Archean, SCLM. Batch melting of such a source can produce high-Mg HDFs at 0.5% partial melting and Group I kimberlites at ~ 2%. Kankan HDFs and Group II kimberlites can be produced by 0.1 and 0.8% melting of average Archaean SCLM that carries phlogopite ± Fe–Ti oxides. The close correspondence between the trace-element composition of surface kimberlites and HDFs that were trapped at depth indicates that kimberlitic melts do not change their incompatible trace element contents much on their way to the surface (except for a possible loss of alkalis). The new data on the HDFs suggest a close genetic relation between high-Mg carbonatitic HDFs and kimberlites and reveals the similarity of the trace element of both to that of the post-Archaean SCLM. This similarity may reflect the interaction of such melts with the lithospheric keel, its melting to produce HDF and/or kimberlites or melting of deeper sources that led to formation of HDFs and kimberlite and to widespread metasomatism of the SCLM.
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the origin and evolution of Archean lithospheric mantle
Precambrian Research, 2003Co-Authors: Norman J Pearson, W L Griffin, Suzanne Y Oreilly, Natsue Abe, Sonja Aulbach, R M Davies, B J Doyle, K KiviAbstract:Abstract The composition of the subcontinental lithospheric mantle (SCLM) varies in a systematic way with the age of the last major tectonothermal event in the overlying crust. This secular evolution in SCLM composition implies quasi-contemporaneous formation (or modification) of the crust and its underlying mantle root, and indicates that crust and mantle in many cases have remained linked through their subsequent history. Archean SCLM is distinctively different from younger mantle; it is highly depleted, commonly is strongly stratified, and contains rock types (especially subcalcic harzburgites) that are essentially absent in younger SCLM. Some, but not all, Archean SCLM also has higher Si/Mg than younger SCLM. Attempts to explain the formation of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic mantle (“lithospheric stacking”), founder on the marked differences in geochemical trends between Archean xenolith suites and Phanerozoic examples of highly depleted mantle, such as abyssal peridotites, island-arc xenolith suites and ophiolites. In Archean xenolith suites, positive correlations between Fe, Cr and Al imply that no Cr–Al phase (i.e. spinel or garnet) was present on the liquidus during the melting. This situation is in direct contrast to the geochemical patterns observed in highly depleted peridotites from modern environments, which are controlled by the presence of spinel during melting. It is more likely that Archean SCLM represents residues and/or cumulates from high-degree melting at significant depths, related to specifically Archean processes involving major mantle overturns or megaplumes. The preservation of island-arc like SCLM at shallow levels in some sections (e.g. Slave Craton, E. Greenland) suggests that this specifically Archean tectonic regime may have coexisted with a shallow regime more similar to modern plate tectonics. Preliminary data from in situ Re–Os dating of sulfide minerals in mantle-derived peridotites suggest that much Archean SCLM may have formed in a small number of such major events >3.0 Ga ago. The survival of Archean crust may have been critically determined by the availability of large plugs of very buoyant SCLM (a “life-raft model” of craton formation). Many Archean SCLM sections have been strongly affected by Proterozoic and Phanerozoic metasomatism, and much of the observed secular evolution in SCLM composition, at least through Proterozoic time, may reflect the progressive modification of relict, buoyant Archean lithosphere.
David Mcinerney - One of the best experts on this subject based on the ideXlab platform.
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crustal thickening and clay controls on o isotope variation in global magmatism and siliciclastic sedimentary rocks
Earth and Planetary Science Letters, 2015Co-Authors: Justin L Payne, Karin M Barovich, Norman J Pearson, Martin Hand, David McinerneyAbstract:New compilations of global O isotope data from zircon and siliciclastic sedimentary rocks highlight an increasing range in δ18O values in both systems since the late Archean. This is consistent with an increased clay component in sedimentary rocks and subsequent incorporation into igneous rocks. Each of these factors can arguably be achieved by increased crustal thickening in the late Archean resulting in greater burial and melting of supracrustal rocks and increased chemical weathering and recycling of upper crustal rocks. Despite the suggested change in tectonic regimes in the late Archean, stochastic modelling in this study demonstrates that δ18O data do not provide evidence for a secular decrease in the proportion of mantle-derived magmas in granitoid rocks. Instead, best-fit models indicate that juvenile input and reworking of supracrustal material vary with respect to the short term (100–200 Myr) tectonic cycles preserved in the continental crust. Hence, major step changes in global tectonic regimes in the post-Hadean, such as the initiation of subduction in the mid- to late Archean, are not supported by global zircon O isotope datasets and instead minor, progressive changes are indicated for Earth's tectonic regimes.
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crustal thickening and clay controls on o isotope variation in global magmatism and siliciclastic sedimentary rocks
Earth and Planetary Science Letters, 2015Co-Authors: Justin L Payne, Karin M Barovich, Norman J Pearson, Martin Hand, David McinerneyAbstract:New compilations of global O isotope data from zircon and siliciclastic sedimentary rocks highlight an increasing range in δ18O values in both systems since the late Archean. This is consistent with an increased clay component in sedimentary rocks and subsequent incorporation into igneous rocks. Each of these factors can arguably be achieved by increased crustal thickening in the late Archean resulting in greater burial and melting of supracrustal rocks and increased chemical weathering and recycling of upper crustal rocks. Despite the suggested change in tectonic regimes in the late Archean, stochastic modelling in this study demonstrates that δ18O data do not provide evidence for a secular decrease in the proportion of mantle-derived magmas in granitoid rocks. Instead, best-fit models indicate that juvenile input and reworking of supracrustal material vary with respect to the short term (100–200 Myr) tectonic cycles preserved in the continental crust. Hence, major step changes in global tectonic regimes in the post-Hadean, such as the initiation of subduction in the mid- to late Archean, are not supported by global zircon O isotope datasets and instead minor, progressive changes are indicated for Earth's tectonic regimes.