Altered Oceanic Crust Research Articles

Reply to comment on “How diverse is the source? Age, provenance, reworking, and overprint of Precambrian meta-sedimentary rocks of West Gondwana, Cameroon, from zircon U-Pb geochronology’’ by Hubert Mvondo and Thierry Bineli Betsi

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

A New Insight Into Seawater-Basalt Exchange Reactions Based on Combined δ18O—Δ′17O—87Sr/86Sr Values of Hydrothermal Fluids From the Axial Seamount Volcano, Pacific Ocean

The δ18O values of submarine vent fluids are controlled by seawater-basalt exchange reactions, temperature of exchange, and to a lesser extent, by phase separation. These variations are translated into the δ18O values of submarine hydrothermal fluids between ca. 0 and + 4‰, a range defined by pristine seawater and equilibrium with basalt. Triple oxygen isotope systematics of submarine fluids remains underexplored. Knowing how δ17O and δ18O change simultaneously during seawater-basalt reaction has a potential to improve i) our understanding of sub-seafloor processes and ii) the rock-based reconstructions of ancient seawater. In this paper, we introduce the first combined δ17O-δ18O-87Sr/86Sr dataset measured in fluids collected from several high-temperature smoker- and anhydrite-type vent sites at the Axial Seamount volcano in the eastern Pacific Ocean. This dataset is supplemented by measurements of major, trace element concentrations and pH indicating that the fluids have reacted extensively with basalt. The salinities of these fluids range between 30 and 110% of seawater indicating that phase separation is an important process, potentially affecting their δ18O. The 87Sr/86Sr endmember values range between 0.7033 and 0.7039. The zero-Mg endmember δ18O values span from -0.9 to + 0.8‰, accompanied by the Δ′17O0.528 values ranging from around 0 to −0.04‰. However, the trajectory at individual site varies. The endmember values of fluids from focused vents exhibit moderate isotope shifts in δ′18O up to +0.8‰, and the shifts in Δ′17O are small, about −0.01‰. The diffuse anhydrite-type vent sites produce fluids that are significantly more scattered in δ′18O—Δ′17O space and cannot be explained by simple isothermal seawater-basalt reactions. To explain the observed variations and to provide constraints on more evolved fluids, we compute triple O isotope compositions of fluids using equilibrium calculations of seawater-basalt reaction, including a non-isothermal reaction that exemplifies complex alteration of oceanic crust. Using a Monte-Carlo simulation of the dual-porosity model, we show a range of possible simultaneous triple O and Sr isotope shifts experienced by seawater upon reaction with basalt. We show the possible variability of fluid values, and the causal effects that would normally be undetected with conventional δ18O measurements.

Three-stage modification of lithospheric mantle: Evidence from petrology, in-situ trace elements, and Sr isotopes of mantle xenoliths in the Cenozoic basalts, northeastern North China Craton

Abstract We present mineralogical and geochemical compositions of mantle xenoliths from two Cenozoic basalt localities of the northeastern North China Craton. These xenoliths include lherzolite, harzburgite, and websterite. They are generally fertile in major elements and different from the typical cratonic lithosphere, which is consistent with previous hypotheses regarding craton destruction. The ratios of 87Sr/86Sr and (La/Yb)N of clinopyroxenes (Cpx) in one lherzolite are relatively low in the core but high in the rim. The center of the Cpx grain has a high U concentration. Changes in trace elements and Sr isotopes indicate that later stage high 87Sr/86Sr melt metasomatism superimposed on the early hydrous melt/fluid. The Cpxs in some xenoliths are low in Ti/Eu but high in Ca/Al and light rare earth elements, which indicates carbonate melt metasomatism. 87Sr/86Sr is increased in the core and decreased in the rim of most Cpx grains, which reflects the superposition of two-stage metasomatism. The early agent should be high in 87Sr/86Sr, and the recent agent should be low in 87Sr/86Sr. The Cpxs in olivine websterite are low in 87Sr/86Sr (0.70220–0.70320), which reflects the recent metasomatism of asthenosphere-derived melt. Collectively, these observations reflect a three-stage modification of the lithospheric mantle. First-stage hydrous melt/fluid could come from the dehydration of young subducted plates. Second-stage melt/fluid of high 87Sr/86Sr could derive from the partial melting of the subducted altered oceanic crust, and the recent melt/fluid of low 87Sr/86Sr should be from the asthenosphere.

The Mercury Isotopic Composition of Earth's Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust

AbstractThe element mercury (Hg) can develop large mass‐independent fractionation (MIF) (Δ199Hg) due to photo‐chemical reactions at Earth's surface. This results in globally negative Δ199Hg for terrestrial sub‐aerially‐derived materials and positive Δ199Hg for sub‐aqueously‐derived marine sediments. The mantle composition least affected by crustal recycling is estimated from high‐3He/4He lavas from Samoa and Iceland, providing an average of Δ199Hg = 0.00 ± 0.10, Δ201Hg = −0.02 ± 0.0.09, δ202Hg = −1.7 ± 1.2; 2SD, N = 11. By comparison, a HIMU‐type lava from Tubuai exhibits positive Δ199Hg, consistent with altered oceanic crust in its mantle source. A Samoan (EM2) lava has negative Δ199Hg reflecting incorporation of continental crust materials into its source. Three Pitcairn lavas exhibit positive Δ199Hg which correlate with 87Sr/86Sr, consistent with variable proportions of continental (low Δ199Hg and high 87Sr/86Sr) and oceanic (high Δ199Hg and low 87Sr/86Sr) crustal material in their mantle sources. These observations indicate that MIF signatures offer a powerful tool for examining atmosphere‐deep Earth interactions.

Cycling of CO2 and N2 Along the Hikurangi Subduction Margin, New Zealand: An Integrated Geological, Theoretical, and Isotopic Approach

AbstractWe present a quantitative assessment of the input and output of CO2 and N2 along the Hikurangi margin based on the chemical and stable isotope composition of sediments and basalts (from IODP 375), previously accreted metasedimentary rocks, and volcanic/hydrothermal gases (together with noble gas data for the latter). We compare these results with 3‐D thermo‐petrologic models for four lithologic structures, representing different plateau inputs. The model results indicate that 59%–85% of initially subducted C and 5%–12% of N is lost from the slab during metamorphism, with both volatiles being dominantly sourced from altered oceanic crust with some contribution from subducted sediment at the forearc‐arc transition (75–90 km depth). The δ13CVPDB and CO2/3He values for the arc gases range from −8.3 to −1.4‰ and 2 × 109 to 2.7 × 1011, indicating contributions from slab carbonate, organic C, and mantle C of 67%, 30%, and 3%, respectively. The δ15Nair and N2/36Ar values of arc gases are −1.0 to +2.3‰ and 1.54 × 104 to 1.9 × 105, indicating slab and mantle contributions of 74% and 26%. The δ13C signature of gases requires addition of organic C by tectonic erosion and/or shallow crustal assimilation. These calculations yield whole‐margin fluxes of 5.4–7.0 Tg/yr for CO2 and 0.0022–0.0057 Tg/yr for N2, corresponding to ∼2.2% and 1%–30% of the global CO2 and N2 flux from subaerial volcanoes worldwide (assuming no loss during transit). This unique assessment of volatile cycling could prove useful in refining regional and global estimates of volatile recycling efficiency.

Evidence for tungsten mobility during oceanic crust alteration

Being a mobile element, the budget of W in the oceanic crust, an important geochemical reservoir, may be severely affected by hydrothermal alteration, but W data for altered oceanic crust (AOC) have not been available so far. In this study, we present the first high precision W concentration data obtained by isotope dilution and MC-ICPMS, along with high precision Th, U, Ta, Zr, Nb, Lu, and Hf data, for a complete section of altered upper oceanic crust (IODP-Borehole 1256D) down to the basal gabbros. As W, Th, U and Ta are similar melt incompatible, the latter were selected as framework to study W enrichment.In the studied section of altered oceanic crust, preserved alteration styles range from low temperature (< 150 °C) seawater altered lavas to high temperature (> 400 °C) altered granoblastic dikes. Tungsten is systematically enriched over Th, U, and Ta in the whole depth succession. Uranium is also hydrothermally enriched locally, whereas Th and Ta both behaved entirely immobile over the entire depth succession. Based on U-series data, most alteration activity can be confined in age to >350 kyrs. The strongest W enrichments were found in contact zones between the different lithological units, at the sites of extreme fluid alteration.Generally, the elemental ratios of W/Th, Ta/W and W/U show only minor overlap with pristine MORB ratios and may even exceed the values reported for global arc lavas with respect to W enrichment. The selective enrichment of W in many arc lavas may therefore be partially inherited from altered oceanic crust. Nevertheless, there are examples of arcs where subducted oceanic crust dominates the trace element budget, but the lavas do not show pronounced W enrichment. We propose three models to explain this conundrum, (a) retention of W in accessory phases, (b) selective enrichment of subduction zone fluids in U and Th relative to W, and (c) incomplete sampling of AOC by hole 1256D, leaving W depleted portions in the oceanic crust unsampled.

Lithium isotopes in kimberlites, lamproites and lamprophyres as tracers of source components and processes related to supercontinent cycles

Abstract Our pilot study reveals potential Li isotope fingerprints recorded in the Mesoproterozoic ( c. 1.4–1.1 Ga) kimberlites, lamproites and lamprophyres from the Eastern Dharwar Craton and Paleocene (62 Ma) orangeite from the Bastar Craton in India. The new data are interpreted in the context of available Li isotope composition of lamproitic to lamprophyric rocks occurring in Variscan (Bohemian Massif) and Alpine–Himalayan (SW Tibet) orogenic belts formed in response to Gondwana–Pangea amalgamation and break-up. As a result of the development of supercontinents, kimberlites from the Eastern Dharwar Craton and ‘orangeite’ from the Bastar Craton show clear presence of a component with a heavy Li isotope signature (δ 7 Li up to 9.7‰) similar to ancient altered oceanic crust, whereas the Eastern Dharwar Craton lamproites (2.3–6.3‰) and lamprophyres (3.3–6.7‰) show Li isotope signatures indicative of a dominant contribution from heterogeneous lithospheric mantle. Variscan lamprophyric to lamproitic rocks and post-collisional mantle-derived (ultra)potassic volcanic rocks from SW Tibet, i.e. rocks from the orogenic belts outside the cratonic areas, are characterized by a clear Li isotope shift towards an isotopically lighter component (δ 7 Li as low as –9.5‰) comparable with the involvement of evolved continental crust and high-pressure metamorphic rocks in their orogenic mantle source. Such components with isotopically light Li are strikingly missing in the source of cratonic kimberlites, lamproites and lamprophyres.

Sulphur isotopes (δ34S and Δ33S) in sulphides from cratonic mantle eclogites: A glimpse of volatile cycling in ancient subduction zones

Multiple sulphur isotopic compositions of sulphides from Kaapvaal craton mantle eclogites allow to elucidate the recycling of sulphur into the deep Earth and to differentiate between recycled crust and mantle origins of eclogite-hosted sulphides, including the precious metals that they capture. We present multiple sulphur isotope ratio measurements by secondary ion mass spectrometry for sulphides from a collection of mantle-derived eclogite xenoliths from Proterozoic and Mesozoic kimberlite occurrences in South Africa (Premier, Roberts Victor, Jagersfontein). Previous work established that the host eclogites have elemental and oxygen isotopic compositions in support of seawater-altered oceanic lithosphere protoliths, and for many of these xenolith suites Archean ages have been suggested.The eclogite-hosted sulphides have δ34S values from −5.7 to +29‰, with the upper end of this wide range representing the highest-ever recorded δ34S composition of material derived from the Earth's mantle. The Δ33S values range from −0.29 to +0.18‰ and do not record significant mass-independent sulphur isotope fractionation, i.e., there is no compelling S-MIF signature. Most of the sulphide grains have δ34S values that fall within a range between −6 and +4‰, and they probably retain an isotopic record of sulphides that formed originally within altered oceanic crust. In contrast, the highly positive δ34S values from +13 to +29‰ detected in sulphide grains from a single eclogite xenolith are similar to those of marine sulphates, which were probably a minor sulphur component of the oceanic crustal protolith. The lack of a significant S-MIF signature in the eclogitic sulphides that show δ34S evidence for a recycled crust origin implies that this sulphur component stems from a <2.4Ga post-Archean surficial reservoir. This finding suggests that the cratonic mantle eclogites may have formed from post-Archean oceanic crust (e.g., Paleoproterozoic eclogite protoliths), or – as is preferred here – the ‘surficial’ sulphur was introduced into the cratonic root during relatively young metasomatic events and is thus unrelated to eclogite petrogenesis and Archean continent formation.

Intraslab remobilization of nitrogen during early subduction facilitates deep nitrogen recycling: Insights from the blueschists in the Heilongjiang Complex in NE China

Nitrogen (N) in subducting slab resides in both sediments and altered oceanic crust (AOC). The extent of subduction-zone metamorphic devolatilization of N in these reservoirs determines the amount of remaining N for deep recycling. However, N behavior inside the subduction zone, particularly below the forearc region, remains poorly understood so far. Here, we studied the pillow-shaped epidote-blueschist facies meta-basalts (hereafter refer to as blueschists) in the Heilongjiang Complex in Northeast China, aiming to understand the N behavior during the early subduction stage. Results show that these blueschists contain N (13.9 to 122.6 ppm; average: 51.0 ppm) up to seven times higher than the AOC (including pillow lavas) entering global trenches (N < 19 ppm; average 7 ppm). The N concentrations of the blueschists correlate with the Ba and Th concentrations, indicating that significant N enrichment of the blueschists took place inside the subduction zone in addition to small N enrichment on seafloor. The N concentrations and δ15N values of the blueschists show two distinct mixing trends, indicating that two types of ammonium (NH4+) were added into the blueschists. One is characterized by relatively high abundance and positive δ15N value (~ +3‰), which can be attributed to the direct assimilation of NH4+ in metamorphic fluids derived from the sedimentary components in the mélange. The other is characterized by relatively low abundance and extremely low δ15N values (~ −16‰). This NH4+ is consistent with a product of abiotic reduction of sediment-derived N2. This study demonstrates that the labile N in sediments released during early subduction can be at least partially (if not all) re-fixed into the more refractory meta-igneous part of the subducting slab, which facilitates the retention of N for deep subduction and recycling.

Potassium isotopic composition of low-temperature altered oceanic crust and its impact on the global K cycle

To constrain the behavior of K isotopes during the low-temperature alteration of oceanic crust and reveal its impact on the global K cycle, we measured the K isotopic compositions of 49 fresh and altered basalts recovered from the Integrated Ocean Drilling Program (IODP) Sites U1365 (~100 Ma) and U1368 (~13.5 Ma) in the South Pacific Ocean. The measured basalts representing the uppermost oceanic crust have been subjected to low-temperature (<150 °C) alteration of different types and intensities as reflected by different secondary mineral modal abundances and element mobility. Both Sites have remarkably low sedimentation rates (0.71 and 1.1 m/Myr, respectively). The altered basalts from both Sites show higher K2O contents and K/Nb ratios than fresh basalts, suggesting the addition of K during low-temperature alteration. The measured δ41K values of the altered basalts vary over a large range (−0.76‰ to −0.17‰) compared to unaltered basalts, indicating significant K isotopic fractionation during low-temperature alteration of the oceanic crust. We propose that the K isotopic characteristics of the altered basalts were caused by the combined effects of equilibrium and kinetic isotopic fractionations between seawater and alteration minerals during the incorporation and adsorption of K into alteration minerals. A weighted average δ41K value of −0.40 ± 0.33‰ (2sd) is given for the low-temperature altered oceanic crust (AOC) at Sites U1365 and U1368, which is indistinguishable from the fresh N-MORB value (−0.44 ± 0.17‰), but significantly lower than its K source, i.e., seawater (δ41K = +0.12 ± 0.07‰, 2sd). Therefore, low-temperature AOC serves as an important sink for K and is a cause for the heavy K isotopic composition of seawater. In addition, the heterogeneous K isotopic compositions of the low-temperature AOC, together with large scale fractionation during various crustal processes, further highlights the utility of the K isotope system to trace subducted crustal materials.

Mantle source of tephritic porphyry in the Tarim Large Igneous Province constrained from Mg, Zn, Sr, and Nd isotope systematics: Implications for deep carbon cycling

AbstractThe nature and source of magmatism associated with large igneous provinces (LIPs) remain disputed. Here we investigate the role of recycled materials that contributed to mantle heterogeneity in the Tarim Large Igneous Province (TLIP) in China through integrated Zn–Mg–Sr–Nd isotopes of a rare tephritic rock suite. The Sr–Nd isotopes [(87Sr/86Sr)i = 0.70368–0.70629; εNd(t) = −0.25–4.64] and δ26Mg values (−0.23&amp;#x89; to −0.34&amp;#x89;) of the tephritic porphyries are consistent with a normal mantle origin. In contrast, the whole rock and pyroxene phenocrysts yield δ66Zn values of +0.28&amp;#x89; to +0.46&amp;#x89; and +0.30&amp;#x89; to +0.39&amp;#x89;, which are slightly heavier than those of the terrestrial mantle (+0.16 ± 0.06&amp;#x89;) and mid-oceanic-ridge basalts (MORBs) (+0.27 ± 0.05&amp;#x89;). We exclude the possibility that the heavy Zn isotopes of the Wajilitag tephritic porphyries are caused by magmatic processes such as fractional crystallization and partial melting and correlate the isotopic features to the role of altered oceanic crust along with magnesite in the mantle source. The Wajilitag tephritic porphyry displays trace-element patterns similar to those of the melts from natural hornblendite, especially those of hornblendite + peridotite. Additionally, the geochemical characteristics such as enrichment in Nb and Ta, depletion in K, high TiO2, and constant Na2O/K2O ratios also suggest that the tephritic porphyries were derived from an amphibole-bearing source contributed by altered oceanic crust along with sedimentary carbonates. Our study provides insight into the contribution of subducted materials to the mantle heterogeneity beneath the TLIP and highlights the role of subduction in the deep carbon cycle and subducted slab-lithosphere-plume interaction in the generation of LIPs.

Calcium isotope compositions of arc magmas: Implications for Ca and carbonate recycling in subduction zones

Calcium (Ca) is a fluid-mobile element and plays a pivotal role in regulating carbon cycling through the precipitation of calcium carbonate. Calcium isotopes have been proposed as a promising tracer for deep carbon recycling because of the higher Ca content and distinctive Ca isotope composition in carbonate relative to the mantle. To test the effect of carbonate recycling on Ca isotope composition in arc magma and determine the fate of carbonate in subduction zones, we analyzed a suite of magmatic rocks from Central America, Southern Lesser Antilles, and Central-eastern Aleutian arcs. These three island arcs have variable thermal regimes from intermediate to cool and sediment types from carbonate-rich to carbonate-poor. Thirty-six arc magmas display δ44/40Ca values from 0.70‰ to 1.00‰ which are similar to the range of fresh mid-ocean ridge basalts (MORBs). Their average δ44/40Ca (0.82 ± 0.12‰, 2SD, n = 36) is lower than the estimates of Bulk Silicate Earth (~0.94‰, BSE) and can be ascribed to the effect of mantle partial melting. Despite strong evidence of C and Sr isotopes that various degrees of carbonate metasomatism occurred in the source of these rocks, they do not show Ca isotope variation. Based on the flux model, the buffering effect of the mantle wedge can homogenize the different carbonate Ca isotope signals at subduction zones where carbonate recycling efficiency is lower than 30% or the carbonates are mainly brought by carbonate-bearing altered oceanic crust. For subduction zones with carbonate recycling efficiency higher than 30%, the MORB-like δ44/40Ca in arc magmas may also reflect the storage of Ca in the mantle wedge and the retention of Ca in the descending slab beyond sub-arc depth. Combined with the fact that carbonate Ca isotope signals have been observed in OIBs and intra-plate igneous rocks, our new data indicates that most subducted Ca may finally enter the deep mantle via corner flow or within the descending slab.

Fluid pathway evolution and mass transfer during Mg-dominated mineral transformations

Understanding the alteration of oceanic crust is of fundamental importance to unraveling geochemical and physicochemical processes on Earth. Mineral transformation during geological processes can result in mass transfer, changes in solid mass and volume while generating pathways that enhance fluid access to more remote areas of the crust and mantle. In this study, two hydrothermal experiments were conducted at different conditions, one at 160 °C and ~ 6 bar, and the other at 500 °C and 5 kbar. The reaction products were characterized using XRD and HRTEM. Saponite and talc were identified as products by XRD patterns in low- and high-temperature systems, respectively. This is consistent with HRTEM observations, in which layers with a thickness of ~1.2 and 1.0 nm were observed in products. These results confirm the successful transformation of brucite to saponite at 160 °C, and saponite to talc at 500 °C. HRTEM images clearly show the direct connection between brucite layers and saponite layers and between saponite layers and talc layers, suggesting that solid-state transformation is the main mechanism in both reaction processes. Based on the structural differences, the transformation from brucite to saponite undergoes as much as a 1.6-fold increase in volume and created expandable interlayer spacings of ~0.55 nm, which allows for fluid transport. Further conversion of saponite to talc at 500 °C results in a volume decrease. Inter-particle porosities are created during the progressive reduction of the layer spacing commencing from the outer edges of saponite crystals, potentially enhancing fluid flow. Therefore, the observation that mineral transformations proceed from the edges to interiors of the minerals suggests that fluid pathways at the nanoscale were generated during mineral transformations. The thermostability of the structural water was enhanced through mineral transformations, although water content decreased from ~30.9 wt% to ~4.7 wt%, with implications for water transfer processes in a down-going plate from the surface into Earth's interior. Also, ~2.0 wt% exchangeable cations were sequestered and preserved in the interlayer spaces during the transformation of brucite to saponite. These exchangeable cations, however, were ultimately expelled with the reduction of interlayer spacing during the conversion of saponite to talc. Thus, mineral transformations may have significant effects on mass transfer in various geological processes.

The Molybdenum isotope subduction recycling conundrum: A case study from the Tongan subduction zone, Western Alps and Alpine Corsica

Molybdenum isotopes have emerged as novel tracers for high-temperature igneous and metamorphic processes. The debate remains to what extent different subducted slab lithologies, such as oceanic crust and marine sediments, contribute to the Mo isotope signature of arc magmas and, hence, exert different controls on the terrestrial Mo cycle. Here we investigate Mo isotope systematics from input to output at the Tongan subduction zone: Arc lavas from different Tongan islands, pelagic sediments and altered oceanic crust (AOC) samples from DSDP site 595/596 on the subducting Pacific plate. For complementary insights into the fate of Mo and its isotopic signatures during prograde subduction metamorphism, we also present data of metasediments and variably altered AOC-type eclogites from the Zermatt-Saas ophiolite, Switzerland and Italy, and the Schistes Lustrés Complex in Alpine Corsica.Manganese oxide-rich pelagic sediments from DSDP site 595/596 show variable, depth-dependent Mo/Mn ratios and Mo isotope compositions controlled by diagenetic reactions. As subducted equivalents, Mn-rich eclogitic metapelites display lower Mo contents and δ98/95Mo ratios compared to their non-subducted protolith. This indicates prominent loss of Mo along with isotope fractionation during early subduction metamorphism. In comparison to unaltered MORB, low temperature seafloor alteration has shifted Mo/Ce and δ98/95Mo in studied AOC samples towards lower ratios, in the range of most mafic eclogites published so far. However, some mafic eclogites show even lower Mo/Ce and δ98/95Mo ratios compared to AOC, likely due to fluid-related Mo loss upon early subduction and preferential incorporation of light Mo into residual rutile.Our data document a prominent loss of isotopically heavy Mo before and upon early subduction metamorphism at shallow depths in the forearc region. Moreover, when prograde rutile crystallizes at ~30 km depth, it fixes the largest fraction of Mo in the subducting material. This creates an “arc Mo-conundrum” as devolatization of the slab at subarc depths is not able to account for the fluid-mobile Mo source responsible for observed higher Mo/Ce and δ98/95Mo in Tongan arc lavas compared to the mantle. As an alternative scenario, Mo mobilization by slab-derived aqueous fluids during the early stages of subduction into the forearc mantle produces serpentinites enriched in Mo with a possible heavy Mo isotopic signature. Mechanical transport and devolatization of forearc serpentinites at subarc regions is a plausible alternative recycling process accounting for the observed Mo systematics in Tongan arc lavas. This is supported by positive covariations of Mo/Ce and δ98/95Mo with elements such as As, Sb, and Cs, which are thought to be mostly released from the subducting material during early stages of subduction. We propose that a multi-stage recycling of metasomatized forearc mantle can be an important process in recycling of Mo and possibly other elements.

Hydrothermal recycling of sedimentary ammonium into oceanic crust and the Archean ocean at 3.24 Ga

Abstract The Archean ocean supported a diverse microbial ecosystem, yet studies suggest that seawater was largely depleted in many essential nutrients, including fixed nitrogen. This depletion was in part a consequence of inefficient nutrient recycling under anoxic conditions. Here, we show how hydrothermal fluids acted as a recycling mechanism for ammonium (NH4+) in the Archean ocean. We present elemental and stable isotope data for carbon, nitrogen, and sulfur from shales and hydrothermally altered volcanic rocks from the 3.24 Ga Panorama district in Western Australia. This suite documents the transfer of NH4+ from organic-rich sedimentary rocks into underlying sericitized dacite, similar to what is seen in hydrothermal systems today. On modern Earth, hydrothermal fluids that circulate through sediment packages are enriched in NH4+ to millimolar concentrations because they efficiently recycle organic-bound N. Our data show that a similar hydrothermal recycling process dates back to at least 3.24 Ga, and it may have resulted in localized centers of enhanced biological productivity around hydrothermal vents. Last, our data provide evidence that altered oceanic crust at 3.24 Ga was enriched in nitrogen, and, when subducted, it satisfies the elemental and isotopic source requirements for a low-N, but 15N-enriched, deep mantle nitrogen reservoir as sampled by mantle plumes.

Influence of high marine Ca/SO4 ratio on alteration of submarine basalts at 2.41 Ga documented by triple O and Sr isotopes of epidote

Over the course of Earth’s history, marine sulfate concentrations have been increasing in response to long-term atmospheric oxygenation. In contrast to modern oceans, where abundant sulfate precipitates in hot oceanic crust as anhydrite, Precambrian oceans contained much less (~0–10 mM) sulfate, so that submarine hydrothermal systems were comparatively poor in anhydrite. As a step towards exploring the role of chemical evolution of seawater solutes, we investigate the reaction between basalt and seawater that took place at the ca. 2.43–2.41 Ga Vetreny Belt (Karelia craton, NW Russia) using fluid inclusion and multi-isotope measurements complemented by reactive transport and static aqueous-mineral equilibrium calculations. Using fluid inclusion measurements by LA-ICP-MS, we constrain the Sr concentration in the least modified seawater-derived fluids and address the effect of phase separation. Then, we complement the previous δ18O – Δ′17O datasets with new 87Sr/86Sr measurements performed on 2.41 Ga epidote from the Vetreny Belt, and recent (0–6 Ma) oceanic epidote from Reykjanes, Iceland and the drilling site 504B in the eastern Pacific Ocean. The 2.41 Ga epidote with 87Sr/86Srinitial of 0.7029–0.7042 and Δ′17O of –0.06 to 0.00‰ is best explained by a relatively high fraction (~90%) of marine Sr that was delivered from contemporaneous seawater with 87Sr/86Sr ≈ 0.7045, and without significant removal by early anhydrite. Using Monte-Carlo simulation of a dual-porosity model, we constrain the range of possible exchange trajectories based on the variability of physical parameters (porosity, fluid flow velocity, fracture spacing, recrystallization rates). Further, we use a series of static equilibrium seawater-basalt reaction calculations with emphasis on the possible range of marine Ca/SO4 values at 2.41 Ga. Our calculations demonstrate that co-existing quartz and epidote in absence of feldspars represent equilibrium with less-evolved hydrothermal fluids. Consequently, equilibrium assemblage of quartz and epidote provide an insightful archive for marine Sr. Based on our modeling and assumptions about marine 87Sr/86Sr and Sr/Ca ratios, the 2.41 Ga epidotes document a seawater-basalt reaction where the initial fluid contained between 30 and 40 mM of Ca and 0–10 mM SO4, representing a high marine input and the possible effect of phase separation. Based on our data, we suggest that high Ca/SO4 ratio of seawater (≫1) and low concentration of anhydrite in submarine systems of the contemporaneous oceans promote a higher fraction of seawater Sr to be permanently stored in silicates of altered oceanic crust. In contrast, modern altered oceanic crust is depleted in radiogenic Sr due to partitioning into anhydrite, which partly returns into the ocean upon cooling.

Barium isotope evidence for recycled crustal materials in the mantle source of continental basalts

To investigate the potential of using Barium (Ba) isotopes to trace recycled crustal materials, we report high-precision Ba isotopic compositions for 22 well-characterized samples of Cenozoic basalts from the Taihang Mountains, North China Craton. The basalts have δ138/134Ba [= (138Ba/134Basample/138Ba/134BaSRM3104a – 1) × 1000] of −0.06‰ to +0.03‰(±0.05‰，2SD), lower than the depleted upper mantle (+0.03‰ to +0.15‰). Because Ba isotope fractionation during basaltic magmatic differentiation is negligible, the light Ba isotopic compositions likely reflect the involvement of isotopically light crustal materials in the mantle source. The Ba isotopic variations, combined with the data of elements and SrNd isotopes of the basalts, indicate that the light Ba isotopic compositions mainly resulted from marine sediments rather than altered oceanic crust. The quantitative modeling using Sr-Nd-Ba isotopic compositions suggests that about 0.5% to 3% of marine sediments were involved in generating the continental basalts. Our study here highlights the potential of using Ba isotopes to trace recycled materials in the mantle.

Thallium elemental and isotopic systematics in ocean island lavas

The Earth’s mantle exhibits marked chemical heterogeneity. We provide an examination of thallium systematics in ocean island basalts (OIB): new high-precision trace element analyses, including Tl, and Tl isotopic compositions for 48 OIB spanning the entire range of observed Sr-Nd-Hf-Pb isotope ratios. All investigated OIB are characterised by ubiquitous Tl depletion requiring OIB mantle sources to have Tl concentrations as low as 0.2 ng/g, which is an order of magnitude lower than estimates for the primitive mantle and similar to Tl concentrations inferred for the depleted mantle. The low Tl concentrations inferred for OIB mantle sources are interpreted to reflect near quantitative removal of Tl during subduction and inefficient Tl recycling into the deeper mantle. If true, the Tl isotopic composition of surface materials may not be readily translated to the mantle sources of OIB.The new OIB dataset shows a >10 ε-unit range in primary isotopic variation, from ε205Tl = −6.4 to +6.6. However, the majority of samples (32 of 48) are within uncertainty of mantle values (ε205Tl = −2 ± 1), and show no co-variation with radiogenic isotopic composition. Notably, OIB with only minor Tl depletion (11 samples) have Tl isotopic compositions outside the mantle range. The Tl concentration contrast between the mantle and inputs such as sediments and altered basalt is so great that minor additions (<1% by mass) of high-Tl material will dominate the isotopic budget of a lava, with decoupling of Tl and radiogenic isotopic compositions as an expected result. Thallium isotopic compositions of OIB are therefore difficult to link directly to radiogenic isotope variations and the mantle components they may reflect. Indeed, if isotopically distinct Tl from altered oceanic crust and/or sediments were efficiently recycled into the mantle and sampled via OIB, more variation in the Tl isotopic composition of OIB would be expected than is observed. The markedly unsystematic primary Tl isotopic variations in OIB therefore likely reflect the residual Tl isotopic composition of subducted material, and/or Tl acquired en route to the surface via shallow-level crustal assimilation.

Progressive spatial and temporal evolution of tectonic triggers and metasomatized mantle lithosphere sources for orogenic gold mineralization in a Triassic convergent margin: Kunlun-Qinling Orogen, central China

Abstract Whether orogenic gold deposits formed from crustal or subcrustal sources is debated, and their link to orogenic processes is ambiguous. Gold mineralization in the Triassic East Kunlun–West Qinling Orogen, China, displays a spatial zonation in terms of its ages and stable isotope compositions. In the West Qinling segment, most gold deposits formed in a back-arc setting at 220∼210 Ma during a collisional episode within late slab rollback. These deposits have dominant δ34S of 5∼15‰ and δ18Ofluid of 10∼14‰, whereas those formed in the suture zone at 210∼170 Ma, during a post-collisional episode after slab break-off, have lower δ34S of −5∼+5‰ and δ18Ofluid of 6∼10‰. In the East Kunlun segment, those deposits that formed in a continental-arc setting and its related suture zone at 240∼200 Ma, in collisional to post-collisional episodes associated with slab break-off, have δ34S and δ18Ofluid values that are essentially similar to those in the West Qinling suture. δ34S values of ore sulfide separates and rims of zoned pyrites that have mantle-like signatures, in contrast with crustal signatures of host rocks, are indicative of subcrustal ore-fluid sources. The combined chronological and stable isotope shifts are consistent with a model in which ore fluids for gold mineralization in a back-arc setting were sourced from mantle lithosphere that was metasomatized by subducted oceanic sediment; whereas those in a continental-arc setting—including its suture zone—were sourced from fluid derived from altered oceanic crust. This study thus provides new insights into the complexity of orogenic gold systems in evolving orogens.

Halogen (F, Cl) Concentrations and Sr‐Nd‐Pb‐B Isotopes of the Basaltic Andesites From the Southern Okinawa Trough: Implications for the Recycling of Subducted Serpentinites

AbstractSerpentinites are increasingly recognized as playing an important role in the global geochemical cycle. However, discriminating the contributions of serpentinites to arc magmas from those of other subduction components is challenging. The Okinawa Trough is a back‐arc basin developed behind the Ryukyu subduction zone, where magmas are extensively affected by sediment subduction. In this study, we reported the F‐Cl concentrations and Sr‐Nd‐Pb‐B isotopes of basaltic andesites from the Yaeyama Graben, Yonaguni Graben, and Irabu Knoll in the southern Okinawa Trough. The Irabu Knoll lavas show the most enrichment of fluid‐mobile elements and F ± Cl, and have the heaviest B isotopes (δ11B: +6.6 ± 1.5‰). They also have decoupled Sr‐Nd isotopes: higher 87Sr/86Sr (∼0.7049) but have no obvious decrease of 143Nd/144Nd (∼0.5128). Results from slab dehydration modeling and mixing calculations suggest that the heavy δ11B in the Irabu Knoll lavas is not consistent with fluids derived from altered oceanic crust (AOC), sediments, or wedge serpentinites (formed in the mantle wedge), but rather from slab serpentinites (formed within the subducting plate); sediments control the subduction input of Nd, whereas the decoupled Sr‐Nd isotopes are most likely due to the excess radiogenic Sr carried by AOC fluids. Our results imply that recycling of serpentinite fluids and AOC fluids are usually coupled in subduction zones, as the arc lavas influenced by subducted serpentinite generally show Sr‐Nd isotopes decoupling. The large variation of Sr‐Nd‐B isotopes observed in a relatively localized area is consistent with a focused migration through the mantle wedge of components from multiple sources.