Composition Of Continental Crust Research Articles

Disparities in oxygen isotopes of detrital and igneous zircon identify erosional bias in crustal rock record

The detrital zircon oxygen isotope record has been used to describe 4.4 billion years of crustal evolution and maturation. Implicit in the application and use of large databases of oxygen isotope ratio (δ18O) from detrital zircon is that such data are more representative of the history of the continental crust than are exposed rocks. Here we show that the δ18O of igneous zircon in preserved rocks varies greatly from the detrital zircon record over time, both in terms of the magnitude of δ18O and the secular variation. Starting from 4.0 Ga and lasting throughout the Archean, the detrital zircon record shows consistently higher δ18O values than the rock record. In post-Archean times, detrital zircon δ18O is conspicuously higher during periods associated with supercontinent amalgamation. Differences between detrital and igneous zircon δ18O records imply that the igneous source rocks from which high δ18O detrital zircon are derived have been entirely eroded and are not represented in the known geological record. We postulate that the ‘missing’ record of high δ18O magmatic rocks consists of upper crustal sedimentary-derived melts from collisional belts, such as that found in the Himalayan orogen. The observation of higher δ18O in Archean detrital zircon further requires a revision of the current paradigm for Archean magmatism. More broadly, recognition of a ‘missing’ component of the upper crustal rock record calls into question the degree to which the detrital zircon record accurately reflects the composition of the bulk continental crust.

Decoupling of dissolved and particulate Li isotopes during estuarine processes

Lithium isotopes in marine authigenic or detrital sedimentary archives have been recently used to trace continental weathering over geologic timescales. However, interpretations are predominantly based on the assumption that riverine Li isotopic signals can be propagated through estuaries without modification. Here, we verify this hypothesis by investigating the behaviour of Li isotopes in the Changjiang (Yangtze) River estuary. We observe a conservative mixing of dissolved Li and its isotopes between the Changjiang River water and seawater. The dissolved δ 7 Li yields a non-linear increase with salinity, and a significant increase occurs during the initial water mixing. Through the studied transect, estuarine flocculation and resuspension processes cause the homogenisation of offshore particulate δ 7 Li values, which are close to the average composition of upper continental crust. This study provides clear and direct evidence that the riverine dissolved Li isotopic signal is not modified during estuarine processes in large rivers, but caution should be exercised when using detrital δ 7 Li in marginal seas to investigate past continental weathering.

Crustal reworking and growth during India–Asia continental collision: Insights from early Cenozoic granitoids in the central Lhasa Terrane, Tibet

Studying granitoids can provide important knowledge on the composition of continental crust and the geodynamic processes of continental growth. Nevertheless, the mechanisms of granitoids formation during continental collision remain uncertain. In this paper, we present new zircon U–Pb–Hf isotope and whole‐rock Sr–Nd isotope compositions, and major‐ and trace‐element data for granitoids from the Chacangka area of the central Lhasa Terrane, southern Tibet. The Chacangka quartz monzonites crystallized at ca. 61 Ma, whereas the monzonites and granites were emplaced at ca. 51 Ma. The quartz monzonites are peraluminous and have high 87Sr/86Sri ratios (0.7129–0.7131), and their low εNd(t) (−7.65 to −7.10) and zircon εHf(t) (−9.3 to −7.5) values are more depleted than those of ancient crustal basement beneath the central Lhasa Terrane, which suggests that they originated from a mixed source of juvenile and ancient lower crust. Compared with the quartz monzonites, the monzonites have much higher contents of MgO (2.34–3.62 wt%) and values of Mg# (42.3–46.3) and zircon εHf(t) (−6.9 to 1.9) and show more depleted Sr–Nd isotopes, indicating that more juvenile materials were involved in their source. The granites have Sr–Nd isotope compositions that are similar to those of the coeval monzonites but have higher SiO2 contents, lower CaO and MgO contents, and lower values of Mg#, suggesting that they were differentiates of the monzonites. The elemental and isotopic compositions of the Chacangka granitoids exhibit a marked transformation from 61 to 51 Ma, probably indicating breakoff of the Neo‐Tethyan oceanic slab. The Neo‐Tethyan slab breakoff not only induced the upwelling of deep material and eruption of magma but also caused re‐melting and destruction of ancient crustal basement of the central Lhasa Terrane.

К СТРАТИГРАФИИ И ГЕОДИНАМИКЕ ЗИЛАИРСКИХ ОТЛОЖЕНИЙ НА ЮЖНОМ УРАЛЕ

It is known that the upper shell of our planet is the earth's crust, which is different in composition on the continents and in the oceans. The composition of the continental crust is predominantly sialic, and the oceanic one is simatic. The capacity of the first is within 35-70 km, the second is close to 5-12. The formation of any type of the earth's crust is determined by the participation of specific geological processes, causal relationships of matter and geodynamics, which is justified by researchers on the basis of well-proven facts and judgments. The concepts used must be specific, in accordance with well-known definitions, such as: stratigraphy is a branch of geology that studies «the formation of rocks in their primary spatial relationships», and geodynamics is «a branch of geology that studies the forces and processes in the crust, mantle and core of the Earth, deep and surface movements of masses in time and space» [1]. The use of the conceptual framework and the study of numerous facts allow us to confidently identify the sequence of connections and the reasons for the relationship.

Nitrogen loss and isotopic fractionation during granulite-facies metamorphism in the lower crust (Ivrea Zone, NW Italy)

The Ivrea Zone, exposed in the Southern Alps, affords an examination of nitrogen mobilization and storage in deep continental crust, in rocks spanning the amphibolite to granulite facies transition and that experienced multiple devolatilization reactions and partial melting. Such information is important for considerations of whole-Earth N cycling, as continental crust is thought to contain up to ~15% of global N, much of it organic in origin and thus tied to ancient-Earth biogeochemical processes.In these rocks, N is redistributed among silicate phases during devolatilization and both H2O-saturated and dehydration melting, showing particular affinity for potassic phases such as biotite, muscovite and alkali feldspar. In metasedimentary rocks, N concentrations decrease strongly with increasing grade, from the Kinzigite Zone (amphibolite facies) with 29 to 181 μg/g to the Stronalite Zone (granulite facies) containing only 10 to 15 μg/g. The decrease in N is intermediate in magnitude to that of Cs, showing somewhat greater loss, and that of Rb that shows smaller degrees of loss. The loss of N with increasing grade is due to breakdown of biotite, which is the main mineral host of N. It also reflects a strong preference of N for the fluid during dehydration and a moderate preference of N for the melt during partial melting. Accompanying this loss of N is a shift by up to 9‰ in δ15N towards lower values. The direction of this shift contrasts with what would be expected for loss of N into fluids speciated as either N2 or NH3 and likely reflects complex isotope fractionation effects during equilibration between melt, fluid, and rock, perhaps during evolving redox conditions. The lowest δ15N values for metabasaltic rocks in this suite are near that of the mantle (−5‰) and the array of values across the traverse could represent initial protolith heterogeneities and superimposed effects of magmatic degassing of the protolith. The leucosomes broadly overlap in N concentrations and δ15N values with their host rocks and lack any systematic relationships with metamorphic grade or type of host lithology.As has been noted in previous work, estimation of the size and isotopic composition of the N reservoir in continental crust is complicated its great lithologic heterogeneity and by the paucity of data for metamorphic rocks representing greater depths. Superimposed on the lithologic heterogeneity associated with varying protoliths is the ability of highly devolatilized and partially melted deep crustal rocks to store N over long time periods. We used our new data of deep crustal rocks from the Ivrea Zone, combined with published N data and based on lithological proportions in the crust, to provide a first estimate for the N content in the middle continental crust of 56 ± 4 μg/g. For the lower continental crust, we estimate a N content of 26 ± 4 μg/g N but note that the data base of granulite-facies samples is still small. Calculation of the total continental crust N content yields 74 ± 4 μg/g, which is in between previous estimates of 56, 60 and 88 μg/g N. Finally, we estimate the N isotopic composition of the bulk continental crust as δ15N = 4.3 ± 0.5.

Zirconium isotopic composition of the upper continental crust through time

The stable isotopic composition of insoluble, refractory elements such as titanium (Ti) or zirconium (Zr), which are modified by magmatic differentiation but, a priori, are poorly affected by weathering or diagenesis, serve as powerful potential proxies to reconstruct the compositional evolution of the continental crust. Here we present the evolution of the Zr stable isotopic compositions (δ94/90ZrIPGP-Zr, per mille deviation of 94Zr/90Zr from IPGP-Zr standard) of the continental crust through time, using 38 sedimentary samples from the upper continental crust (UCC), including 12 Holocene loesses from the Chinese Loess Plateau and Xinjiang, three oceanic sediments from the sea floor outboard of the Lesser Antilles island arc and 23 glacial diamictite composites with depositional ages ranging from ∼ 2.9 Ga to 0.3 Ga from South Africa, South America, Canada, USA and China. The samples show limited Zr isotopic variations with δ94/90ZrIPGP-Zr values of 0.043‰ - 0.109‰ for loess; 0.069‰ - 0.083‰ for oceanic sediments and 0.031‰ - 0.118‰ for glacial diamictites; their Zr-weighted average values are, 0.081 ± 0.044‰ (2SD, n = 12), 0.073 ± 0.015‰ (2SD, n = 3) and 0.078 ± 0.047‰ (2SD, n = 23), respectively. The isotopic similarity among loess, oceanic sediments and glacial diamictites, suggests that zircon enrichment effects previously documented in some sedimentary samples have not biased the Zr isotope compositions of these sedimentary rocks from their source rocks. Two groups with or without Zr enrichment have similar average δ94/90ZrIPGP-Zr values (0.075 ± 0.040‰ and 0.080 ± 0.046‰). There is no correlation between Zr isotope compositions and any proxy of chemical weathering (e.g., Al2O3/SiO2, Fe2O3/SiO2, CIA, K2O/Al2O3 and δ7Li). The δ94/90ZrIPGP-Zr values are quite constant for these sedimentary samples regardless of their depositional ages and locations. Therefore, the UCC appears to have had a constant Zr isotopic composition between 3 Ga and present, and is homogeneous at a large scale. Combining data for sedimentary reference materials from the literature and the sedimentary rocks in this study, we suggest a Zr-weighted δ94/90ZrIPGP-Zr value of 0.077 ± 0.058‰ (2SD, n = 44) for the UCC, which is statistically distinct (t test, p value = 2.88 × 10−10) and higher than that of the mantle (0.040 ± 0.044‰, n = 72). Combining the δ94/90ZrIPGP-Zr values of different terrestrial reservoirs, the δ94/90ZrIPGP-Zr of the BSE and bulk Earth is constrained to be 0.041 ± 0.041‰.

Metal sources in the Proterozoic Vazante-Paracatu sediment-hosted Zn District, Brazil: Constraints from Pb isotope compositions of meta-siliciclastic units

ABSTRACT Different types of sediment-hosted whole-rock Pb isotope (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) compositions were determined from phyllites, carbonaceous phyllites (>1% TOC), and meta-litharenites belonging to the Serra do Garrote Formation, which is part of the Proterozoic Vazante Group, Brazil. Results were integrated with lithogeochemistry in order to identify the Pb isotopic signature of Zn enrichment (up to 0.24 wt.% Zn) associated with meta-siliciclastic-hosted sulfide mineralization that formed prior to the Brasiliano Orogeny (850 to 550 Ma) in order to (1) understand the nature of siliciclastic sediment sources, (2) identify possible metal sources in pre-orogenic meta-siliciclastic-hosted Zn mineralization, and (3) evaluate the genetic links between the Zn enrichment in the relatively reduced phyllite package, and different styles of syn-orogenic Zn ± Pb mineralization (hypogene Zn-silicate and Zn-Pb sulfide) in overlying dolomitic carbonates throughout the Vazante-Paracatu Zn District, Brazil. The whole-rock 206Pb/204Pb and 207Pb/204Pb isotope ratios of meta-siliciclastic rocks plot as positively sloping, sub-parallel arrays with radiogenic, upper continental crust compositions, which could represent a detrital contribution from at least two upper continental crust sources. However, the 206Pb/204Pb versus207Pb/204Pb isotope system does not distinguish between Zn-enriched samples and un-mineralized samples. In the whole-rock 206Pb/204Pb–208Pb/204Pb plot, Zn-enriched samples form a flat trend of lower 208Pb/204Pb values (38.3 to 39.5) compared to the Zn-poor ones that follow common upper crustal trends. Zinc-enriched samples have low whole-rock Th/U values (<4) and higher whole-rock U concentrations compared to unmineralized samples. These support the hypothesis that U (± Pb) was added by pre-orogenic metalliferous fluids, which were in turn derived from underlying Paleoproterozoic and Archean basement rocks. Due to U addition, the original whole-rock thorogenic and uranogenic Pb isotope systems were decoupled in mineralized samples. Pre-orogenic metalliferous fluids have similar present-day first-order characteristics, including: (1) relatively high U/Pb and (2) low Th/U values, when compared to galena in the major carbonate-hosted Zn ± Pb deposits (Vazante, Morro Agudo, Ambrosia, Fagundes) in the Vazante Group. These results support the hypothesis that Zn-rich layers and veins in mineralized carbonaceous phyllites could be linked to the same origins as carbonate-hosted mineral deposits throughout the Vazante Basin, but further data are warranted. We suggest that the tectonic evolution of the Vazante Basin saw multiple phases of Zn-rich mineralization over protracted time periods from around 1200 to 550 Ma.

High-pressure syntheses and crystal structure analyses of a new low-density CaFe2O4-related and CaTi2O4-type MgAl2O4 phases

AbstractThree single crystals of CaTi2O4 (CT)-type, CaFe2O4 (CF)-type, and new low-density CaFe2O4 (LD-CF) related MgAl2O4 were synthesized at 27 GPa and 2500 °C and also CT-type MgAl2O4 at 45 GPa and 1727 °C using conventional and advanced multi-anvil technologies, respectively. The structures of CT-type and LD-CF related MgAl2O4 were analyzed by single-crystal X-ray diffraction. The lattice parameters of the CT-type phases synthesized at 27 and 45 GPa were a = 2.7903(4), b = 9.2132(10), and c = 9.3968(12) Å, and a = 2.7982(6), b = 9.2532(15), and c = 9.4461(16) Å, respectively, (Z = 4, space group: Cmcm) at ambient conditions. This phase has an AlO6 octahedral site and an MgO8 bicapped trigonal prism with two longer cation-oxygen bonds. The LD-CF related phase has a novel structure with orthorhombic symmetry (space group: Pnma), and lattice parameters of a = 9.207(2), b = 3.0118(6), and c = 9.739(2) Å (Z = 4). The structural framework comprises tunnel-shaped spaces constructed by edge- and corner-sharing of AlO6 and a 4+1 AlO5 trigonal bipyramid, in which MgO5 trigonal bipyramids are accommodated. The CF-type MgAl2O4 also has the same space group of Pnma but a slightly different atomic arrangement, with Mg and Al coordination numbers of 8 and 6, respectively. The LD-CF related phase has the lowest density of 3.50 g/cm3 among MgAl2O4 polymorphs, despite its high-pressure synthesis from the spinel-type phase (3.58 g/cm3), indicating that the LD-CF related phase formed via back-transformation from a high-pressure phase during the recovery. Combined with the previously determined phase relations, the phase transition between CF-and CT-type MgAl2O4 is expected to have a steep Clapeyron slope. Therefore, CT-type phase may be stable in basaltic- and continental-crust compositions at higher temperatures than the average mantle geotherm in the wide pressure range of the lower mantle. The LD-CF related phase could be found in shocked meteorites and used for estimating shock conditions.

Trace elements in the Athabasca Bituminous Sands: A geochemical explanation for the paucity of environmental contamination by chalcophile elements

The Athabasca Bituminous Sands (ABS) in northern Alberta, Canada, represent one of the largest reserves of hydrocarbons on the planet, yet there is remarkably little published data on the abundance of potentially toxic trace elements (TEs) in this resource. Here, we present the concentrations and review the relevance of 30 TEs in bulk samples of ABS as well as the organic and mineral fraction of representative samples. The distribution of TEs is dichotomous: they occur primarily in the organic fraction (Mo, Ni, Re, V and Se) or almost exclusively in the mineral fraction (virtually all of the other TEs). Except for Mo and Re, TEs in the ABS are depleted relative to the composition of the Upper Continental Crust (UCC), a reference level commonly used in quantifying the extent of contamination by TEs in the environment. Based on the published data available for comparison, TE concentrations in ABS are similar to those of sandstones, well below the average value for shale, and far below the values reported for organic-rich, black shales. The data presented here explains why recent studies of contamination of air, water, soil, plants in this region of northern Alberta, when viewed critically, reveal limited enrichments of chalcophile TEs, relative to crustal abundance. The abundance and distribution of TEs in the ABS also explains why atmospheric transport of TEs is largely restricted to the immediate region (< 50 km) surrounding the open pit bitumen mines, and dispersion patterns resemble those of dust deposition. Based on these findings, most of the chalcophile TEs mobilized by mining are expected to have limited bioaccessibility and bioavailability, simply because they are mainly hosted by silicate minerals with limited solubility at ambient pH.

Evidence for highly refractory, heat producing element-depleted lower continental crust: Some implications for the formation and evolution of the continents

The composition of the lower continental crust is estimated via the analysis of granulite xenoliths, granulite terrains and geophysical properties. All three proxies generally agree on the lower crust's refractory nature, dominated by mafic granulites. Estimates weighted using seismic velocity reference models yield lower Th and U concentrations and higher K/U ratios than granulite xenolith averages, while terrain granulites are often much less refractory than xenoliths.Here we present new data for lower crustal xenoliths from central Queensland, an understudied part of the xenolith-bearing eastern Australian basalt provinces. The granulite chemistry was estimated using the reconstitution approach in which the modal mineralogy and in situ chemical analyses are combined. High-resolution energy-dispersive X-ray spectroscopy mapping revealed that K is enriched in anastomosing grain boundary networks and fractures in granulite and co-occurring mantle peridotite xenoliths. Laser-ablation inductively-coupled-plasma mass-spectrometry 2-D traverses show that the same networks are also significantly enriched in many highly incompatible elements. There are sharp concentration contrasts with neighbouring phases for elements with very different diffusivities (Li and U), suggesting that the networks formed during entrainment, decompression and heating within the host basalt. We propose that undetected inclusion of such late enrichment skews xenolith chemistry estimates to non-representative, overly fertile compositions. In the case of the studied xenoliths, the carrier basalts are not very strongly enriched in highly incompatible elements, and even when the K-rich networks are included in the reconstitution, the resulting granulite chemistry is very refractory with 0.52 wt% K2O, 0.07 ppm Th and 0.03 ppm U.The locally dominant lithology of lower crustal xenoliths is simple two-pyroxene, plagioclase, ilmenite granulite with few accessory phases. The granulite mineralogy and chemistry were compared with results from thermodynamic models of prograde anatexis of different metabasites, variably hydrated. The comparison shows that the granulites could be restitic calc-alkaline basalts or diorites that experienced episodic melt extraction accumulating up to 50–60% total melt loss at very high temperatures (950–1050 °C), implying that the temperatures recorded by two-pyroxene thermometry (750–830 °C) do not capture the thermal maximum. The corresponding upper crustal section of the northern New England Orogen exposes a range of Devonian to Cretaceous granitoids, some of which have complementary features to the granulites, including the low modal abundance of plagioclase; low relative abundance of Ti, very high Rb/Ba ratios, and high Th/U ratios. Together, the data suggest extensive and protracted melting of the original lower crust upon lithospheric thinning and concomitant magmatic underplating. The required high temperatures favour picritic over basaltic underplates. In such a setting, the gravity-driven delamination of more mafic garnet-rich restites and olivine-rich mafic-ultramafic underplating material is physically plausible. This delamination could help explain the long-established mass balance issue for the formation of continental crust in general.

Evolution of seawater continentally-sourced Nd isotopic composition prior to and during the Great Oxidation Event

An ongoing debate concerns what initiated the Great Oxidation Event (GOE) and associated glaciation between ~ 2.45 and 2.2 Ga. One possibility is the emergence of continental landmasses and the increase of subaerial igneous province weathering during the Late Archean. We test this hypothesis in the Hamersley Basin by reporting Nd-isotope data from a succession of iron formations (IFs), mudstone/siltstones and glacial diamictites from the Boolgeeda Iron Formation and overlying Turee Creek Group deposited during the GOE. In a 147Sm/144Nd - ɛNd(t) diagram, the data define a negative trend indicating the contribution of a high ɛNd(t) ~ +3 hydrothermal component and a strongly negative ɛNd(t) ~ -9 crustal component, which is compatible with the Nd-isotope composition of the upper continental crust but also of the underlying felsic volcanics of the Woongarra Rhyolite and crustally-contaminated mafic volcanics of the Fortescue Group. A less pronounced negative trend originating from the same hydrothermal source but correlated with non-contaminated ultramafic Fortescue volcanics (ɛNd(t) ~ -2) is observed for the older Joffre, Dales Gorge and Marra Mamba IFs. As Nd-isotopes are not sensitive to redox conditions, the major shift of Nd-isotopic compositions at ~ 2.45 Ga cannot be linked to a change in the weathering regime, rather to a change in the nature of the continental surface exposed to weathering. One explanation is that the Sm-Nd sources for sediments deposited before and during the GOE were locally derived from the underlying subaerial LIPs, reflecting a change in the geodynamic context of deposition and/or hydrographic network and catchment areas. Another explanation could be a significant change in the nature of fluid–rock interactions due to the increase role of weathering processes associated with the emergence of continental landmasses. Additional Nd isotope data from different cratons worldwide are needed, however, to infer as to whether or not the marked shift in Nd isotope compositions recorded in the Turee Creek Group reflect a change in the global hydrological cycle. Our data support the role of large subaerial magmatic provinces as triggers of the rise of atmospheric oxygen and the onset of glaciations at the beginning of the Proterozoic.

Reliability of detrital marine sediments as proxy for continental crust composition: The effects of hydrodynamic sorting on Ti and Zr isotope systematics

The isotopic composition of the detrital sediment record harbours a valuable proxy for estimating the composition of the erodible upper crust since the Archaean. Refractory elements such as titanium (Ti) and zirconium (Zr) can display systematic variations in their isotopic composition as a result of magmatic differentiation. Hence, the isotopic composition of such elements in detrital sediments could potentially be used to infer the average composition (e.g., SiO2 content) of their source region, even when elemental systematics are obfuscated by weathering and diagenetic processes. A key premise of this approach is that the isotopic composition of sediments remains unbiased relative to their protolith. To what extent isotopic fractionation can occur during sedimentary processes, notably the hydrodynamic sorting of heavy mineral assemblages with contrasting isotopic compositions, remains poorly understood. We investigate the effects of such processes on the Ti and Zr isotope composition of a suite of detrital sediments from the Eastern Mediterranean Sea (EMS). These sediments are binary mixtures of two main provenance components, Saharan dust and Nile sediment, with strongly contrasting mineralogical and geochemical signatures.The EMS sediments display clear evidence for hydrodynamic sorting of zircon, expressed as a large variation in Zr/Al2O3 and deviation of εHf relative to the terrestrial εNd-εHf array. Our new data, however, do not show pronounced Zr isotope variation resulting from either hydrodynamic sorting of zircon or sediment provenance. Although this agrees with theoretical models that predict negligible equilibrium zircon-melt Zr isotope fractionation, it contrasts with recent observations suggesting that kinetic Zr isotope fractionation might be a common feature in igneous rocks. For the EMS sediments, the negligible shift in Zr isotope composition through hydrodynamic sorting means that fine-grained samples accurately reflect the composition of their source. The nearly overlapping Zr isotope compositions of Sahara- and Nile-derived sediment, however, mean that Zr isotopes, in this case, have insufficient resolution to be a useful provenance proxy.Titanium behaves differently. A small but resolvable, systematic difference in Ti isotope composition is observed between the Sahara and Nile provenance components. Samples with a strong Saharan dust signature show some Ti isotope evidence for hydrodynamic sorting of oxides in tandem with zircon, but a much stronger effect is inferred for Nile sediment. Regression of the EMS sediment samples shows that the Ti isotope composition of the Nile-derived component is strongly fractionated compared to its protolith, the Ethiopian flood basalts. Whereas Ti in Nile sediment is carried in essentially unmodified concentration, and by inference isotope composition, from its sources to the delta, large-scale hydrodynamic sorting of Fe-Ti oxides occurs in the littoral cell. This process causes a decrease in TiO2/Al2O3 of the residual fine-grained sediment fraction and shifts its Ti isotope composition to heavier compositions. The potential of such an “oxide effect” in detrital sediments has implications for crustal evolution models that use Ti isotopes as a proxy for the proportion of felsic crust and can account for the observed scatter in the shale record.

Mafic Archean continental crust prohibited exhumation of orogenic UHP eclogite

The absence of ultrahigh pressure (UHP) orogenic eclogite in the geological record older than c. 0.6 Ga is problematic for evidence of subduction having begun on Earth during the Archean (4.0–2.5 Ga). Many eclogites in Phanerozoic and Proterozoic terranes occur as mafic boudins encased within low-density felsic crust, which provides positive buoyancy during subduction; however, recent geochemical proxy analysis shows that Archean continental crust was more mafic than previously thought, having greater proportions of basalt and komatiite than modern-day continents. Here, we show via petrological modelling that secular change in the petrology and bulk composition of upper continental crust would make Archean continental terranes negatively buoyant in the mantle before reaching UHP conditions. Subducted or delaminated Archean continental crust passes a point of no return during metamorphism in the mantle prior to the stabilization of coesite, while Proterozoic and Phanerozoic terranes remain positively buoyant at these depths. UHP orogenic eclogite may thus readily have formed on the Archean Earth, but could not have been exhumed, weakening arguments for a Neoproterozoic onset of subduction and plate tectonics. Further, isostatic balance calculations for more mafic Archean continents indicate that the early Earth was covered by a global ocean over 1 km deep, corroborating independent isotopic evidence for large-scale emergence of the continents no earlier than c. 3 Ga. Our findings thus weaken arguments that early life on Earth likely emerged in shallow subaerial ponds, and instead support hypotheses involving development at hydrothermal vents in the deep ocean.

Ore formation of the Dayakou emerald deposit (Southwest China) constrained by chemical and boron isotopic composition of tourmaline

The Cretaceous Dayakou emerald deposit (southwest China) hosted in the Neoproterozoic schists and granofels is a magmatic-hydrothermal ore system. In this paper, we present the petrological and geochemical study of tourmaline from the Dayakou deposit, aiming to constrain the source of ore-forming fluid and emerald genesis. Two types of tourmaline, namely type-I formed in emerald-bearing quartz veins and type-II in non-mineralized quartz veins, are distinguished and analyzed in situ for their chemical and boron isotopic compositions. All tourmalines are close to the dravite endmember and belong to the alkali group. The type-I tourmaline has a higher Fe/(Fe + Mg) ratio (0.28–0.47) with Li enrichment (21–227 ppm). The type-II tourmaline, however, shows a lower Fe/(Fe + Mg) ratio (0.14–0.34) with relatively higher Co (2–3 ppm) and REEs (2–21 ppm) content. Boron isotopic composition in tourmaline (δ11B = −15.8‰ to −10.4‰) is closed to the average boron composition of continental crust. The boron isotope variations could be the result of rapid precipitation of tourmaline from a magmatic-hydrothermal fluid or the mixing with a metamorphic fluid. Based on the chemical composition and enrichment in granitophile elements (e.g., Li, Be, Sn) and transition metals (e.g., Ti, V, Cr) in tourmaline, we propose that the Dayakou deposit is dominantly controlled by the fluid-rock interaction between the metamorphic basement and possibly reduced magmatic-hydrothermal fluids. Beryllium is transported as fluoride complex in fluids. Hydrothermal fluids continuously interacted with surrounding rocks to extract Cr, V, and Ca into fluids, resulting in extensive crystallization of fluorine minerals (e.g., fluorite, fluorapatite, etc.) and subsequent destabilization of Be-F complexes. The large amounts of Be, Cr and V enriched in the fluid then combine to form emeralds. The Li, Co, and REEs contents of tourmaline can be used as the exploration indicator to target emerald mineralization in this region.

Microtektites and glassy cosmic spherules from new sites in the Transantarctic Mountains, Antarctica

AbstractWe report on the geochemical analyses of glassy spherules from sediments at three Transantarctic Mountain locations and the discovery of Australasian microtektites at two of these sites. Australasian microtektites are present at Mt. Raymond (RY) in the Grosvenor Mountains and Meteorite Moraine (MM) at Walcott Névé, in the Beardmore Glacier region of Antarctica. The microtektites were identified based on their pale yellow appearance, the high concentrations of silica (SiO2 = 60 ± 7 wt%) and alumina (Al2O3 = 23 ± 4 wt%), and a K2O/Na2O &gt; 1, which are all characteristics of microtektites and distinct from spherules of meteoritic origin. Additionally, trace element patterns for these microtektites match the upper continental crust compositions with enrichments in refractory elements and depletions in volatile elements, most likely as a result of melting and vaporization of source material. The presence of Australasian microtektites in RY sediment confirms the recent Australasian strewn field extension to Antarctica and the presence of highly volatile depleted microtektites. In addition to microtektites, thousands of chondritic spherules and a few unique differentiated cosmic spherules were identified in RY, MM, and Jacobs Nunatak sediments. Two unique spherules were calculated to have Fe/Mn ratios similar to micrometeorites assumed to be derived from Vesta (Fe/Mn 33.2 ± 0.5 atom%) and two other unique spherules are extremely rich in refractory components (Al2O3 ~ 30% and TiO2 = ~2%). The three sites examined are evidently successful cosmic dust and impact debris collectors, and thus are useful traps for recording and examining the nature of influx events.

Petrogenesis of highly fractionated leucogranite in the Himalayas: The Early Miocene Cuonadong example

There has been considerable debate concerning the petrogenesis of Cenozoic highly felsic, strongly peraluminous, leucogranites within the Himalayan–Tibetan orogen, with two contrasting viewpoints regarding their origin, either as primary melts from parting melting or magmatic differentiation. This debate hampers our understanding of the tectono‐magmatic evolution and associated uplift and exhumation processes during Himalayan orogenesis. This study attempts to resolve this debate by means of an integrated petrographic, geochronological, and geochemical study on the Cenozoic Cuonadong leucogranites (including two‐mica granites, garnet‐bearing granites, and tourmaline‐bearing granites) in the eastern part of the south Himalayan–Tibetan orogen. One SHIRIMP zircon U–Pb dating of two‐mica granites and four LA–ICP–MS monazite U–Th–Pb dating of all three types of granites yield ages of ca. 21.9–19.7 Ma. Combined with previous published continuous U–Th–Pb zircon/monazite ages varying from 16.5 to 21.8 Ma, indicating that Cuonadong leucogranites have experienced continuously multiple episodes of intrusion and long‐term crystallization from ca.16 to 22 Ma, and lasted for ca. 6 Myrs. Major and trace element geochemistry combined with whole‐rock Sr–Nd isotope data suggest that the Cuonadong's three types of leucogranites are highly fractionated and garnet‐ and tourmaline‐bearing granites are more evolved. The Cuonadong leucogranite melts are mainly derived from metapelites within the High Himalayan Crystalline Sequence, and underwent fractional crystallization of biotite, monazite, and plagioclase during melt migration. This study indicates Cuonadong leucogranites are Cenozoic highly fractionated strongly peraluminous leucogranites, which, combined with previous intermittent documentations of fractionated leucogranites, point to an increasing maturation of the continental crust composition during continental orogenesis.

The composition and weathering of the continents over geologic time

The composition of continental crust records a history of construction by tectonics and destruction by physical and chemical erosion. Quantitative constraints on how both igneous addition and chemical weathering have modified the continents bulk composition are essential for understanding the evolution of geodynamics and climate. We have extracted temporal trends in sediments protolith composition and weathering intensity from the largest available compilation of sedimentary major-element compositions, of ~ 15,000 samples from 4.0 Ga to the present. To do this we used a new analytical method which inverts whole sedimentary compositions for protolith composition and weathering intensity simultaneously. We find that the average Archean upper continental crust was silica rich and had a similar compositional diversity to modern continents. This is consistent with an early-Archean, or earlier, onset of plate tectonics. In the Archean, chemical weathering was ~ 25 % more efficient at sequestering CO2 than in subsequent time periods. Since 2.0 Ga, over long (> 0.5 Ga) timescales, the crustal weathering intensity has remained largely constant. On shorter timescales over the Phanerozoic, the intensity of weathering is correlated to global climate state, consistent with silicate weathering feedback acting in response to changes in CO2 outgassing.

Anoxic continental surface weathering recorded by the 2.95 Ga Denny Dalton Paleosol (Pongola Supergroup, South Africa)

Iron mobilization during continental weathering was pervasive before the Great Oxidation Event (GOE) that started at around 2.43 billion years (Ga) ago, due to the soluble nature of reduced iron. However, various geochemical proxies indicate transient oxygenation during deposition of the Mesoarchean (∼2.95 Ga) Pongola Supergroup, South Africa, which suggests that continental weathering could have also occurred under transiently oxic conditions before the GOE. We analyzed trace elemental and Fe, Ti, and Cr isotopic compositions of the ca. 2.95 Ga Denny Dalton paleosol in the Pongola Supergroup to better understand continental weathering and redox conditions in the ancient critical zone, and the nature of geochemical fluxes from the continents to the oceans and marine sediments. Iron isotope systematics are consistent with a model where Fe was released during intense leaching from the paleosol to concentrate in Fe-rich groundwaters in the deeper part of the soil horizon. We show for the first time that Fe isotopic fractionation during Mesoarchean continental weathering was limited, and Fe enrichments and depletions are coupled with those of divalent transition metals, Co, Ni, and particularly Zn. This suggests that Fe redox cycling was not involved in paleosol formation, and Fe2+ was mobilized under anoxic conditions. Chromium isotopes are also unfractionated relative to the parent igneous rock in this paleosol, which precludes removal of isotopically heavy Cr6+ and thus supports anoxic continental weathering. We show that previously reported Cr isotopic fractionation in another Denny Dalton paleosol profile does not follow a Cr6+ leaching trend, but instead scales with Cr enrichment and may reveal Cr enrichment from post-burial fluids. Thus, there is no clear evidence for an oxidative continental weathering during deposition of the Pongola Supergroup. Titanium isotopes are not significantly fractionated in the paleosol, suggesting that continental weathering and erosion in the Archean did not fractionate Ti isotopes. Similarly, Ni/Co and Th/Sc ratios are reasonably conserved, which validates their use as a robust proxy for upper continental crust composition in shales, whereas La/Sc, Cr/Zn and Cr/U ratios are highly variable relative to the provenance composition, which suggests that caution should be used when applying these ratios in shale studies of the ancient upper continental crust composition.

Correlation of trace element composition of oils and other caustobiolites with chemical content of different types of biota and the Upper, Middle and the Lower Earth's Crust

The trace element (TE) content of oils is known to have a polygenetic character and to be inherited from an organic matter, surrounding rocks and from formation waters. The presence of some TEs indicates the involvement of the deep source, at least from the Lower Crust. However, these conclusions have a qualitative character. We present the results of the correlation analysis of TE content of oils with the chemical compositions of the Upper, Middle, and Lower Crust, with different types of organic matter, clays, and causetobiolites. A logarithmic instead of a linear scale for TE concentration values are used in calculations because the examining values change by several orders of magnitude. The TE compositions of clays, coals, and shales correlate better with the composition of the upper continental crust and an organic matter. In contrast, the TE contents in the majority of oils correlate stronger with the Lower crust, which indicates a significant contribution from the Lower continental crust. This finding points to the role that the uprising flow of fluid plays in the process of the formation of oil. Only young oils from Kamchatka and from White Tiger huge oil fields indicate a better correlation with the Upper crust. This finding is explained by the lower depth of formation of the uprising flow of deep waters because of higher deep temperatures in Kamchatka and in the White Tiger oil field area. The obtained trend of change of TE content in clays and different caustobiolites oils including is interpreted as a mixing line between the subsurface end member (which is characterized by high correlation with the chemical content of the Upper crust and biota) and the deep end member (high correlation with the chemical content of the Lower crust).

Late Summer Peak and Scavenging-Dominant Metal Fluxes in Particulate Export Near a Seamount in the Western North Pacific Subtropical Gyre

We measured sedimentation fluxes around JA06 Seamount (Xufu Guyot; 19°30′N, 158°00′E) as part of an environmental baseline survey in Japan’s exploration area for cobalt-rich crusts in the subtropical Northwest Pacific. Sinking particles were collected at the flat top (sediment trap depths: 900 and 1000 m) and northeastern base (sediment trap depths: 1000 and 4720 m) of the seamount from June 2016 to April 2017. Total mass fluxes were very low, with average values of 4.3–4.9 mg m–2 d–1 and 9.3 mg m–2 d–1 in the shallow traps at the flat top and base, respectively, which is consistent with an oligotrophic system. The lower fluxes at the flat top probably reflect lower productivity of siliceous microplankton, such as diatoms. However, we were unable to substantiate any potential mechanisms for this difference in productivity and cannot evaluate whether this is representative of typical conditions. When combined with previous observations at two adjacent seamounts, our results indicate widespread seasonality in sediment fluxes with a peak in late summer (August–September). However, satellite data indicate that summer is the season with the lowest primary production. This discrepancy could be explained either by phytoplankton blooms fueled by symbiotic nitrogen fixation that only cause minor increases in surface-ocean chlorophyll or short-lived blooms induced by passing typhoons under thick cloud cover. At the base site, we also analyzed material and element transport rates from shallow to deep waters. Half of the organic matter and >80% of the carbonate in sinking particles was not degraded in the water column, suggesting that most of the regeneration of these materials occurs near or on the sediment surface. Furthermore, four major processes appeared to control elemental fluxes in the area: lithogenic (Al, Ti, Fe), carbonate (Mg, Ca, Sr), biogenic (+scavenging) (Ni, Zn, Cd, Pb), and scavenging (V, Mn, Co, Cu, rare earth elements) processes. The estimation of excess flux based on the composition of upper continental crust demonstrated that >85% of total Mn, Co, Ni, Cu, Zn, Cd, and Pb fluxes were attributable to scavenging (+biogenic uptake). Scavenging-dominant metal fluxes are likely ubiquitous in the oligotrophic open ocean.