Mafic Oceanic Crust Research Articles

Hotter mantle but colder subduction in the Precambrian: What are the implications?

On contemporary Earth, subduction recycles mafic oceanic crust and associated volatile elements, creating new silicic continental crust in volcanic arcs. However, if the mantle was hotter in the Precambrian, the style of subduction, the depth of devolatilization and the formation of silicic continental crust may have been different. Consequently, the generation of the tonalite–trondhjemite–granodiorite (TTG) suite, which is characteristic of Archean crust, may not have been related to subduction. Here, we use a two-dimensional numerical magmatic-thermomechanical model to investigate intraoceanic subduction for contemporary mantle conditions and at higher mantle temperatures, as appropriate to the Precambrian. In each case, we characterize the thermal structure of the subducting plate and investigate magma compositions and production rates. We use these results to assess the potential growth of silicic continental crust associated with intraoceanic subduction at different mantle temperatures. For the Precambrian, in a set of experiments with ΔT = 150 K and decreasing subducting plate velocity, we find that the contemporary style of subduction was preceded by an arc-free regime dominated by rapid trench rollback and vigorous upwelling of asthenospheric mantle into the space created above the retreating slab. In this regime, formation of magmas by fluid-fluxed melting of the mantle is suppressed. Instead, decompression melting of upwelling asthenospheric mantle results in the widespread development of voluminous plateau-like basaltic magmas. In addition, retreating subduction at higher mantle temperature causes faster descent of the downgoing slab, leads to colder thermal gradients, similar to those associated with active subduction in the western Pacific today, and suppresses melting of the basaltic crust, limiting production of silicic (adakite-like) magmas. With increasing maturity of the subduction system, retreat of the subducting plate ceases, the role of decompression melting strongly decreases and fluid-fluxed melting of the mantle coupled with melting of the hydrated slab begins to produce basaltic and felsic arc volcanic rocks similar to those formed during contemporary subduction. In an additional series of individual experiments at various ΔT, an increase of the mantle temperature above ΔT = 150 K leads to episodic and short-lived subduction accompanied by limited production of silicic continental crust. The results of our experimental study demonstrate that a hotter mantle in the Precambrian changes dramatically both the slab dynamics and the processes of magma generation and crustal growth associated with intraoceanic subduction zones. These changes may preclude growth of the early Precambrian silicic continental crust by processes that were dominantly similar to those associated with contemporary subduction.

Geology and geochemistry of the Yuanjiacun banded iron formation in Shanxi Province, China: constraints on the genesis

AbstractThe Yuanjiacun banded iron formation (BIF) is hosted in lower Proterozoic metamorphic strata, and its structures are dominated by bands or streaks. Based on their differences in mineral compositions, the iron ores can be subdivided into haematite quartzite, magnetite quartzite, stilpnomelane magnetite quartzite and stilpnomelane haematite quartzite. The geochemical characteristics of the surrounding rocks show that the protoliths consisted of argillaceous and arenaceous sedimentary rocks. The predominant provenance was a high-maturity felsic sedimentary terrane. The absence of syn-depositional igneous rocks and the tectonic setting discrimination diagrams indicate that the Yuanjiacun BIF formed in a passive continental margin setting. Negligible terrigenous materials were involved in the precipitation of the Yuanjiacun BIF. The precipitation of the Yuanjiacun BIF was predominantly controlled by the mixing of seawater and hydrothermal fluids. Its metallogenic material originated from the leaching of mafic oceanic crust by hydrothermal fluids. The observed Ce anomaly deficiency and heavy Fe isotope enrichment indicate that the Yuanjiacun BIF formed in an anoxic marine environment. In a redox-stratified palaeo-ocean, the Yuanjiacun BIF formed in reducing seawater below the oxidation–reduction transition zone. The Si and O isotope compositions of quartz suggest that the formation of the Yuanjiacun BIF was closely related to submarine hydrothermal activity. The Si and Fe erupted from the seafloor and precipitated by supersaturation and biological oxidation under anoxic conditions, respectively.

Stability of the hydrous phases of Al-rich phase D and Al-rich phase H in deep subducted oceanic crust

To understand the stability of hydrous phases in mafic oceanic crust under deep subduction conditions, high-pressure and high-temperature experiments were conducted on two hydrous basalts using a Kawai-type multi-anvil apparatus at 17–26 GPa and 800–1200 °C. In contrast to previous studies on hydrous basalt that reported no hydrous phases in this pressure range, we found one or two hydrous phases in all run products at or below 1000 °C. Three hydrous phases, including Fe-Ti oxyhydroxide, Al-rich phase D and Al-rich phase H, were present at the investigated P-T conditions. At T ≤ 1000 °C, Fe-Ti oxyhydroxide is stable at 17 GPa, Al-rich phase D is stable at 18–23 GPa, and Al-rich phase H is stable at 25–26 GPa. Our results, in combination with published data on the stability of hydrous phases at lower pressures, suggest that a continuous chain of hydrous phases may exist in subducting cold oceanic crust (≤1000 °C): lawsonite (0–8 GPa), Fe-Ti oxyhydroxide (8–17 GPa), Al-rich phase D (18–23 GPa), and Al-rich phase H (>23 GPa). Therefore, in cold subduction zones, mafic oceanic crust, in addition to peridotite, may also carry a substantial amount of water into the mantle transition zone and the lower mantle.

Experimental Phase Relations in Altered Oceanic Crust: Implications for Carbon Recycling at Subduction Zones

The phase relationships in altered mafic oceanic crust (K2O, CO2 and H2O bearing) have been investigated to constrain and quantify the processes of carbon transfer from the slab to the mantle wedge at subduction zones. We report experiments at 2·5–4·5 GPa and 700–900°C in which gas in the experimental charges is analysed by gas chromatography to constrain the volatile composition of the aqueous fluid or hydrous melt. A phengite-bearing epidote-eclogite with dolomite and/or magnesite is stable at subsolidus conditions. The wet solidus at fO2 of NNO (nickel–nickel oxide) was found between 700 and 750°C at 2·5–3·5 GPa and 800–850°C at 4·5 GPa, similar to the CO2-free systems. This observation indicates a low amount of CO2 in the aqueous fluid phase at the wet solidus, in agreement with a measured X(CO2) of 0·059 ± 0·003 at 3 GPa, 700°C and 0·038 ± 0·003 at 3·5 GPa, 700°C. Experiments performed at higher fO2, using either an oxidized starting material or a Re–ReO2 buffer, resulted in a shift of the solidus to higher temperatures. Higher fO2 results in a higher X(CO2) in the aqueous fluid and the reduced water activity leads to a shift in the solidus to higher temperatures. Above the solidus, both textural observations and analyses of the gas enclosed in the capsule suggest that CO2 solubility in the silicate melt increases with increasing P–T conditions. At 4·5 GPa, more than 70% of carbonate at 850°C and 100% at 900°C was dissolved in the hydrous silicate melt. From textural observation it is not clear whether the high carbon content of the melt is related to an increased solubility of carbonate in the hydrous silicate melt or reflects an immiscible carbonatite melt. In any case, partial melting of altered oceanic crust at moderately oxidizing conditions (NNO) and pressures >4 GPa provides an efficient means for the transfer of carbon from the slab to the mantle wedge in intermediate to hot subduction zones. Significant amounts of subducted carbon can thus be brought back to the atmosphere via arc magmatism on relatively short time scales of less than 10 Myr.

Archean crustal compositions promote full mantle convection

Higher mantle potential temperatures characterized the early Earth, resulting in thicker, more mafic oceanic crust entering subduction systems. This change in the composition of subducted slabs, combined with the enhanced temperature contrast between the slab and ambient mantle, would have altered the buoyancy forces driving subduction in the early Earth. Here we investigate this “compositional effect” through a combination of petrologic and thermal modeling. Specifically, we construct density profiles for sinking slabs under modern and early Earth conditions based on a range of mafic crust and mantle compositions. Slab and mantle densities are then determined from mineral assemblages calculated using the thermodynamic modeling program Perple_X along slab geotherms estimated from an analytic thermal model. Consistent with previous studies, we find that modern MORB compositions are typically less dense than the ambient mantle in the basalt barrier zone, located immediately beneath the mantle transition zone. By contrast, possible early Earth oceanic crust compositions are denser than ambient mantle at all depths down to 1000 km. This compositional effect results in slabs that would have more readily penetrated the transition zone, promoting single-layered convection and effective mantle mixing in the early Earth.

The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine ophiolites

AbstractArc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidized or oxidizing components from the subducting slab to the mantle wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride, and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex, and the Zermatt‐Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P‐T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S‐bearing and C‐bearing fluids, this suggests that relatively small amounts of Zn are mobilized from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction.

Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc

We address the question of whether melting of the mafic oceanic crust occurs beneath ordinary volcanic arcs using constraints from U-Series (238U/232Th, 230Th/232Th and 226Ra/230Th) measurements. Alteration of the top few hundred meters of the mafic crust leads to strong U enrichment. Via decay of 238U to 230Th, this results in elevated (230Th/232Th) (where brackets indicate activity ratios) over time-scales of ∼350ka. This process leads to the high (230Th/232Th), between 2.6 and 11.0 in the mafic altered oceanic crust (AOC) sampled at ODP Sites 801 and 1149 near the Izu-Bonin-Mariana arc. Th activity ratios in the Izu arc lavas range from (230Th/232Th)=1.2–2.0. These values are substantially higher than those in bulk sediment subducting at the Izu trench and also extend to higher values than in mid-ocean ridge basalts and the Mariana arc. We show that the range in Th isotope ratios in the Izu arc lavas is consistent with the presence of a slab melt from a mixed source consisting of AOC and subducted sediments with an AOC mass fraction of up to approximately 80wt.% in the component added to the arc lava source. The oceanic plate subducting at the Izu arc is comparatively cold which therefore indicates that temperatures high enough for fluid-saturated melting of the AOC are commonly achieved beneath volcanic arcs. The high ratio of AOC/sediments of the slab melt component suggested for the Izu arc lavas requires preferential melting of the AOC. This can be achieved when fluid-saturated melting of the slab is triggered by fluids derived from underlying subducted serpentinites. Dehydration of serpentinites and migration of the fluid into the overlying crust causes melting to start within the AOC. The absence of a significant sediment melt component suggests there was insufficient water to flux both AOC and overlying sediments.

Chromium isotope fractionation during subduction-related metamorphism, black shale weathering, and hydrothermal alteration

Chromium (Cr) isotopes are an emerging proxy for redox processes at Earth's surface. However, many geological reservoirs and isotope fractionation processes are still not well understood. The purpose of this contribution is to move forward our understanding of (1) the Earth's high temperature Cr isotope inventory and (2) Cr isotope fractionations during subduction-related metamorphism, black shale weathering and hydrothermal alteration. The examined basalts and their metamorphosed equivalents yielded δ53Cr values falling within a narrow range of −0.12±0.13‰ (2SD, n=30), consistent with the previously reported range for the bulk silicate Earth (BSE). Compilations of currently available data for fresh silicate rocks (43 samples), metamorphosed silicate rocks (50 samples), and mantle chromites (39 samples) give δ53Cr values of −0.13±0.13‰, −0.11±0.13‰, and −0.07±0.13‰, respectively. Although the number of high-temperature samples analyzed has tripled, the originally proposed BSE range appears robust. This suggests very limited Cr isotope fractionation under high temperature conditions. Additionally, in a highly altered metacarbonate transect that is representative of fluid-rich regional metamorphism, we did not find resolvable variations in δ53Cr, despite significant loss of Cr. This work suggests that primary Cr isotope signatures may be preserved even in instances of intense metamorphic alteration at relatively high fluid–rock ratios. Oxidative weathering of black shale at low pH creates isotopically heavy mobile Cr(VI). However, a significant proportion of the Cr(VI) is apparently immobilized near the weathering surface, leading to local enrichment of isotopically heavy Cr (δ53Cr values up to ~0.5‰). The observed large Cr isotope variation in the black shale weathering profile provides indirect evidence for active manganese oxide formation, which is primarily controlled by microbial activity. Lastly, we found widely variable δ53Cr (−0.2‰ to 0.6‰) values in highly serpentinized peridotites from ocean drilling program drill cores and outcropping ophiolite sequences. The isotopically heavy serpentinites are most easily explained through a multi-stage alteration processes: Cr loss from the host rock under oxidizing conditions, followed by Cr enrichment under sulfate reducing conditions. In contrast, Cr isotope variability is limited in mildly altered mafic oceanic crust.

Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc

The fate of crustal material recycled into the convecting mantle by plate tectonics is important for understanding the chemical and physical evolution of the planet. Marked isotopic variability of Mo at the Earth's surface offers the promise of providing distinctive signatures of such recycled material. However, characterisation of the behaviour of Mo during subduction is needed to assess the potential of Mo isotope ratios as tracers for global geochemical cycles. Here we present Mo isotope data for input and output components of the archetypical Mariana arc: Mariana arc lavas, sediments from ODP Sites 800, 801 and 802 near the Mariana trench and the altered mafic, oceanic crust (AOC), from ODP Site 801, together with samples of the deeper oceanic crust from ODP Site 1256. We also report new high precision Pb isotope data for the Mariana arc lavas and a dataset of Pb isotope ratios from sediments from ODP Sites 800, 801 and 802. The Mariana arc lavas are enriched in Mo compared to elements of similar incompatibility during upper mantle melting, and have distinct, isotopically heavy Mo (high 98Mo/95Mo) relative to the upper mantle, by up to 0.3 parts per thousand. In contrast, the various subducting sediment lithologies dominantly host isotopically light Mo. Coupled Pb and Mo enrichment in the Mariana arc lavas suggests a common source for these elements and we further use Pb isotopes to identify the origin of the isotopically heavy Mo. We infer that an aqueous fluid component with elevated [Mo], [Pb], high 98Mo/95Mo and unradiogenic Pb is derived from the subducting, mafic oceanic crust. Although the top few hundred metres of the subducting, mafic crust have a high 98Mo/95Mo, as a result of seawater alteration, tightly defined Pb isotope arrays of the Mariana arc lavas extrapolate to a fluid component akin to fresh Pacific mid-ocean ridge basalts. This argues against a flux dominantly derived from the highly altered, uppermost mafic crust or indeed from an Indian-like mantle wedge. Thus we infer that the Pb and Mo budgets of the fluid component are dominated by contributions from the deeper, less altered (cooler) portion of the subducting Pacific crust. The high 98Mo/95Mo of this flux is likely caused by isotopic fractionation during dehydration and fluid flow in the slab. As a result, the residual mafic crust becomes isotopically lighter than the upper mantle from which it was derived. Our results suggest that the continental crust produced by arc magmatism should have an isotopically heavy Mo composition compared to the mantle, whilst a contribution of deep recycled oceanic crust to the sources of some ocean island basalts might be evident from an isotopically light Mo signature.

Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

Liberation of fluids during subduction of oceanic crust is thought to transfer sulfur into the overlying sub-arc mantle. However, despite the importance of sulfur cycling through magmatic arcs to climate change, magma oxidation and ore formation, there has been little investigation of the metamorphic reactions responsible for sulfur release from subducting slabs. Here, we investigate the relative stability of anhydrite (CaSO4) and pyrite (FeS2) in subducted basaltic oceanic crust, the largest contributor to the subducted sulfur budget, to place constraints on the processes controlling sulfur release. Our analysis of anhydrite stability at high pressures suggests that this mineral should dominantly dissolve into metamorphic fluids released across the transition from blueschist to eclogite facies (∼450–650 °C), disappearing at lower temperatures on colder geothermal trajectories. In contrast, we suggest that sulfur release via conversion of pyrite to pyrrhotite occurs at temperatures above 750 °C. This higher temperature stability is indicated by the preservation of pyrite–bornite inclusions in coesite-bearing eclogites from the Sulu Belt in China, which reached temperatures of at least 750 °C. Thus, sulfur may be released from subducting slabs in two separate pulses; (1) varying proportions of SO2, HSO4− and H2S are released via anhydrite breakdown at the blueschist–eclogite transition, promoting oxidation of remaining silicates in some domains, and (2) H2S is released via pyrite breakdown well into the eclogite facies, which may in some circumstances coincide with slab melting or supercritical liquid generation driven by influx of serpentinite-derived fluids. These results imply that the metallogenic potential in the sub-arc mantle above the subducting slab varies as a function of subduction depth, having the greatest potential above the blueschist–eclogite transition given the association between oxidised magmas and porphyry Cu(–Au–Mo) deposits. We speculate that this zoned sulfur liberation might be one of the factors that lead to the apparently redox-influenced zoned distribution of ore deposit types in the Andean arc. Furthermore, given the lack of sulfate-associated sea floor oxidation prior to the second great oxidation event, the pattern of sulfur transfer from the slab to the sub-arc mantle likely changed over time, becoming shallower and more oxidised from the Neoproterozoic onwards.

Evolution of the Early to Middle Ordovician Popelogan arc in New Brunswick, Canada, and adjacent Maine, USA: Record of arc-trench migration and multiple phases of rifting

A detailed tectonic analysis of the magmatic evolution of the Early to Middle Ordovician west-facing Popelogan arc in New Brunswick and adjacent Maine is presented based on combining new U-Pb zircon radiometric age dates (thermal ionization mass spectrometry and sensitive high-resolution ion microprobe) with existing age constraints on the various magmatic events recognized in this arc system. The Popelogan arc had a life span of nearly 20 m.y. (ca. 476–457 Ma) and becomes progressively younger toward the northwest. Existing lithogeochemical and isotope data combined with field relationships and new zircon inheritance data indicate that the Popelogan arc was built on the leading edge of Ganderia and represents a predominantly continental calc-alkaline arc. Incursions of arc to within-plate–like tholeiitic magmatism correspond to rifting events temporally and spatially linked to trenchward migration of the arc magmatic axis, isolation of arc ribbons, and formation of mafic oceanic and transitional crust in the associated Tetagouche backarc basin. Arc-trench migration and backarc basin opening were caused by a retreating subduction zone. Slab retreat prior to 467 Ma was at least partially accommodated by arc migration, but thereafter, it was mainly accommodated by extension and spreading in the associated Tetagouche backarc basin. New U-Pb zircon radiometric ages and lithogeochemistry of calc-alkaline felsic tuff beds interlayered with the oceanic backarc rocks of the Fournier Supergroup provide a direct link between the Tetagouche backarc rocks and coeval calc-alkaline arc activity in the Popelogan arc. The oldest incursion of arc tholeiite (470–467 Ma) corresponds to cessation of the ca. 476–470 Ma calc-alkaline Meductic phase of the Popelogan arc, the onset of the calc-alkaline Balmoral phase (467–457 Ma) farther to the west, and formation of oceanic and mafic-dominated transitional crust in the Canoe Landing Lake Formation of the Tetagouche backarc basin. A less-well-constrained second incursion of tholeiitic magmatism between 465 and 459 Ma corresponds to rifting of the Tetagouche arc ribbon and formation of the ophiolitic Devereaux complex in the Fournier Supergroup. Cessation of calc-alkaline arc magmatism and eruption of transitional and alkalic mafic volcanic rocks between 459 and 455 Ma may represent slab breakoff following accretion of the Popelogan arc to composite Laurentia, and/or ridge subduction immediately prior to arc-continent collision.

Actinolitic rinds on low-T mafic blueschist blocks in the Franciscan shale-matrix mélange near San Simeon: Implications for metasomatism and tectonic history

In the mélange terrane of the Franciscan accretionary prism, Mg-rich actinolite/tremolite “rinds” have long been known to be present on many high-T, garnet ± epidote-bearing, mafic blueschist and eclogite blocks, but they were thought to be absent on low-T lawsonite + chlorite blueschist blocks. By analogy with rinds on high-T blocks, these rinds on low-T blocks must have been produced by metasomatic reaction with ultramafic rock. The rinds on high-T blocks have led to the speculation that the high-T Franciscan blueschists were uplifted to the surface in serpentinite diapirs, while other processes exhumed the volumetrically more numerous low-T blocks. This paper reports the discovery of actinolitic rinds on the margins of nine low-T, lawsonite ± epidote Franciscan blueschists that are, or were, clearly encased in the shale-matrix mélange exposed in seacliffs and wavecut benches near San Simeon, California. Petrographic and geochemical analyses show these Mg-rich rinds are similar to those found on high-T garnet-bearing blocks.The bulk composition indicates the protolith of the blueschist blocks was mafic oceanic crust. The high Fe3+/∑Fe contents indicate it was probably the uppermost hydrated volcanic part of the crust. The block/ultramafic rock association was created by faulting during subduction initiation as fragments from the top of the subducting ocean crust were imbricated with mantle materials at the base of the ophiolitic leading edge of the North American plate. The chemistry of a block margin altered to actinolitic rind indicates Si, Na, Pb, Cu, and Sr were lost from the blocks while K, Rb, Cs, and Ba were added. Most pieces of actinolitic rind are Mg- and Ni-rich because they are metasomatized ultramafic material that was enriched in Si, K, Rb, Cs, and Ba. These metasomatic changes indicate the fluids that hydrated the mantle and altered the mafic blocks ascended from subducting and/or underplated sediments. Both high-T and low-T blueschists that have actinolitic rinds were uplifted to near the surface after they sank or were plucked from the base of the overriding plate by subduction-generated mélange that upwelled from depths of >15 km.

Temporal source evolution and crustal contamination at Lopevi Volcano, Vanuatu Island Arc

Here we present a new geochemical study of Lopevi volcano, one the most active volcanoes in the Vanuatu island arc. We focus on the temporally well-defined sequence of lava flows emitted since 1960, and for the first time, on pre-1960 volcanic products, including high-MgO basalts and felsic andesites, the most evolved lavas sampled so far on this island. This work reports the first Pb and Hf isotopic study of lavas from Lopevi island. These lavas display correlations between differentiation indexes such as SiO2 content and isotopic ratios. The felsic andesites extend the known correlations with both the least (Sr–Pb) and the most (Nd–Hf) radiogenic isotopic compositions on the island. Our results confirm that the rising magma interacted with the sub-arc crust. Assimilation–Fractional Crystallization (AFC) quantitative modeling of trace element ratios and isotopic compositions requires 1% and 10% of assimilated partial melts of a mafic oceanic crust to account for the pre- and post-1960 lavas, respectively. The post-1960 lavas differ from the former lavas emitted ~20years earlier by enrichments in fluid mobile elements (K, Ba, Rb…), Th, and Light Rare Earth Elements (LREE). We ascribe these features to slight variations in the metasomatic agent added to the sub-arc mantle and ultimately derived from the subducted lithosphere. However, the contrasting time scales involved in subducted lithosphere dehydration and magma genesis, relative to the time elapsed between eruptions of the two lava series, suggest that two different portions of mantle which have undergone slightly different metasomatism, gave birth to the Lopevi lavas. These distinct magmas are still present beneath the volcano.

Uptake of carbon and sulfur during seafloor serpentinization and the effects of subduction metamorphism in Ligurian peridotites

The concentrations and isotope compositions of carbon and sulfur in Ligurian serpentinites having different metamorphic histories were analyzed in order to investigate the behavior of sulfur and carbon during seafloor serpentinization and during subduction metamorphism. Serpentinites associated with gabbros and with seafloor hydrothermal sulfide deposits in northern Apennine ophiolites that exhibit only slight effects of low-grade metamorphism have high sulfur contents (190–1440ppm total sulfur) and δ34STotal S values of up to 9.8‰ through interaction with hydrothermal fluids in high-temperature ultramafic-hosted hydrothermal systems on the seafloor. Fluid circulation was driven by mafic intrusions and the hydrothermal fluids contained a mixture of basaltic and seawater-derived sulfur. The serpentinites contain 250–480ppm total carbon having δ13CTotal C values of −9.4 to −6.0‰, reflecting mixtures of seawater carbonate (~0‰) and reduced (organic) carbon having δ13C≈−25‰ and derived from multiple sources. Entrainment of cold seawater into the subsurface beneath seafloor hydrothermal sulfide deposits maintained locally low temperatures and enabled microbial reduction of seawater sulfate. Combined with previous work documenting low-temperature serpentinization on the seafloor away from such gabbro-driven high-temperature hydrothermal activity, these results provide an integrated view of serpentinization processes in tectonically extended oceanic basement exposed in oceanic core complexes.High-P–T antigorite serpentinites in the Voltri massif (Ligurian Alps) have sulfur and carbon contents and isotope compositions similar to those of the Apennine ophiolites, indicating that the protoliths underwent similar hydrothermal processes on the seafloor. Recrystallization of chrysotile and lizardite to antigorite and minor partial dehydration to olivine during subduction metamorphism result in no significant changes in the contents or isotope compositions of sulfur or carbon in the serpentinites. We estimate that the uptake of carbon and sulfur by serpentinites in oceanic basement is comparable to or greater than that per unit volume of mafic oceanic crust. The high P–T stability of serpentine can, however, make serpentinites important for transport of carbon and sulfur to greater depths in subduction zones.

Combined 238U–230Th and 235U–231Pa constraints on the transport of slab-derived material beneath the Mariana Islands

We present 235U–231Pa measurements for a suite of well-characterised lavas from the Mariana Islands. There is limited variability in (231Pa/235U), 1.18–1.55, with the lowest values typically found in the incompatible element depleted samples. We also report the complementary 238U–234U–230Th–232Th analyses, which are in good agreement previous measurements on these same samples. In keeping with this earlier work, we interpret the range of (230Th/238U) from 0.65 to 1.06 as the result of variable contributions of an enriched, sediment melt component coupled with a near constant flux of a relatively U-rich aqueous fluid, derived from the subducted mafic oceanic crust to the sub-arc mantle. We reassess the composition of the subducting sedimentary assemblage and find that its (230Th/232Th) was previously under-estimated.We performed quantitative models to reproduce the coupled 238U–230Th–232Th and 235U–231Pa data starting from a mantle source modified by subduction-related components according to traditional assumptions of a Pa (and Th)-free fluid and a sedimentary component in 231Pa–235U and 238U–230Th equilibrium. This scenario does not allow the U-series isotope compositions of the Mariana lavas to be reproduced. Successful solutions are instead possible with a sediment melt component with relatively high (230Th/232Th) ∼0.9 and a modest 230Th- and 231Pa excess, (∼30%). The need for a 230Th- and 231Pa-excess in the sedimentary component implies that the sediment melted <150ka before eruption of the lavas that carry this signature and that it melted to sufficiently high degree to exhaust monazite. In contrast, the low (230Th/238U) of the aqueous fluid component is explained by its derivation from lower temperature, mafic crust that contained residual allanite. The maximum (230Th/232Th) of this fluid is set by the well-defined array of lavas on an equiline plot and this argues for a fresh, rather than significantly altered MORB protolith. Moreover, in-growth is insufficient to account for the 231Pa excesses observed in the Marianas lavas given a Pa-free fluid. Thus we infer that the combined U–Th–Pa signatures of the Marianas are more strongly controlled by the initial compositions of the slab components than by mantle melting. Creation of U-series disequilibria is strongly favoured by the presence of key accessory phases, such as allanite, monazite and zircon, that are likely to be stable in the slab and we suggest that the variable U–Th–Pa systematics of arc lavas globally may well reflect the contrasting stabilities of accessory phases in subduction zones with different thermal and compositional controls.

The Cretaceous to Paleogene within-plate magmatism of Pachino-Capo Passero (southeastern Sicily) and Adria (La Queglia and Pietre Nere, southern Italy): geochemical and isotopic evidence against a plume-related origin of circum-Mediterranean magmas

The Pachino-Capo Passero centre, located at the south-easternmost portion of Sicily, represents an Upper Cretaceous volcano within the Pelagian Block, a fragment in lithospheric continuity with the African Plate. The volcanic rocks of the Pachino-Capo Passero centre show fairly homogeneous mafic characteristics with a typical within-plate signature. Two groups with different levels of alkali and incompatible trace element enrichment have been recognised. Trace element modelling indicates that the two groups might be originated at different degrees of partial melting. The geochemical data suggest that Cretaceous rocks of Pachino-Capo Passero originated at greater depth than the neighbouring Neogene volcanic products of the Hyblean Plateau, likely from an asthenospheric mantle source. Sr-Nd-Pb isotope compositions of Pachino-Capo Passero rocks are presented along with those of Paleocene and Eocene lamprophyric dikes and small intrusive bodies from the adjacent Adria block, now representing the Apulian foreland ( i.e ., Pietre Nere and La Queglia dykes) in the context of investigating the presence and origin of a Common Mantle Reservoir beneath the Central Mediterranean area. The Sr-Nd-Pb isotopes of the Pachino-Capo Passero rocks are extremely similar to the recently redefined FOZO component whilst other suites ( i.e ., Pietre Nere) might require additional enriched components. Highly radiogenic Pb isotopes, along with high TiO 2 contents and fractionated MREE/HREE argue for the presence of significant amount of recycled mafic oceanic crust in the mantle sources of these magmas as eclogitic lithologies dispersed within a depleted peridotitic mantle. The Sr-Nd-Pb isotope compositions of the studied rocks are also compared with literature data from the Canary Islands, as representative of a putative plume component contaminating the mantle region. The data show a clear distinction, especially in Δ7/4 vs . Δ8/4, indicating that the age and composition of the mafic crust recycled within the source of Pachino-Capo Passero rocks must be older and bear higher time-integrated U/Th ratios. The geochemical and isotope composition of Pachino-Capo Passero magmas, Pietre Nere and La Queglia is interpreted as a feature of the ambient asthenospheric mantle, due to the extremely widespread process of prolonged subduction and recycling of mafic oceanic crust of variable age and composition. Therefore no need for a unique reservoir related to a mantle plume is envisaged.

Another sea area separated from the Panthalassic Ocean in the Norian, the Late Triassic: The lowest Sr isotopic composition of the Ishimaki limestone in central Japan

Mt. Ishimaki is the Jurassic accretionary complex of the Chichibu Belt in Toyohashi City, near Nagoya in central Japan. The Ishimaki limestone is thought to be seamount-type limestone. The P1 elements of the conodonts Norigondolella navicula and Ancyrogondolella quadrata found in the limestone indicate it is of Norian age. The Sr isotopic compositions of 45 Ishimaki limestone samples ranged from 0.7055 to 0.7077. Eighteen of these samples had lower Sr isotopic compositions than the lowest Sr isotopic composition (0.7068) of seawater throughout the Phanerozoic. The Sr isotopic compositions in the limestone block are generally lower at the base of the block and higher at the top. The present Sr isotopic compositions of the Ishimaki limestone are unlikely to have been reduced by post-depositional alteration because most of the limestone samples had a low amount of Mn (<300ppm) or high Sr/Mn ratios (>2) and the conodont elements had low (1–2) CAI (conodont alteration index) values. Additionally, there was little acid-insoluble residue. Thus, the low Sr isotopic compositions are thought to represent the strontium of the past ambient seawater. The low Sr isotopic compositions are in complete disagreement with the generally recognised range of seawater Sr isotopic compositions in the Norian stage of the Late Triassic (0.7075–0.7078) because the depositional environment of the Ishimaki limestone was closed or semi-closed from the Panthalassic Ocean. Therefore, the Sr isotopic composition of the limestone differs from that of the Panthalassic seawater. The low Sr isotopic compositions were probably affected by Sr inflows from mafic oceanic crust by hydrothermal fluid circulation or from hinterlands surrounded by mafic rocks by river water circulation.

Early–Middle Paleozoic granitoids in Gorny Altai, Russia: Implications for continental crust history and magma sources

Major-element and trace-element compositions, as well as Nd isotope ratios and protolith model ages of Early to Middle Paleozoic granitoids from the Russian Gorny Altai are summarized and discussed in terms of the compositions and tectonic settings. The observed geographic variations of compositions in the granitoid assemblages correlate with three groups of late Neoproterozoic to Middle Paleozoic terranes corresponding to oceanic, island arc, and turbidite basin settings. Plutonic rocks in the oceanic and island arc terranes are commonly low-alkali metaluminous tonalite and trondhjemite that formed by melting of oceanic crust. The island arc terranes also contain tonalite–trondjemite–granodioritic and tonalite–granodiorite–granitic rocks, with higher alkali and potassium contents, which result from anatectic melting of volcanics and sediments. Granitoids in the turbidite basins are potassic peraluminous monzogranite produced by anatectic melting of upper crustal turbidites, as well as subalkaline tonalite, granodiorite, and granite derived from upper crustal metasediments and metamorphosed mafic oceanic crust underlying the turbidite basins. Alkaline granitoids emplaced during two major tectonomogamatic events (Early and Late Devonian) and are present in terranes of all three settings. Their source is of crustal origin, like in other groups of granitoids, but it was a mixture of crustal and mantle material. Thus the compositional trends of Early and Middle Paleozoic granitoids in the Gorny Altai record the history of continental crust.

Boninites as windows into trace element mobility in subduction zones

Boninites are subduction-related rocks originating from re-melting of highly depleted mantle sources left after extraction of tholeiitic melts. Due to their depleted nature, the incompatible trace element inventory of boninites is virtually entirely inherited from slab components without a significant contribution from the refractory mantle wedge. Thus, boninites constitute an excellent window into processes controlling trace element mobilization at the slab–mantle wedge interface. In order to constrain the behaviour of trace elements in subduction zones with a special emphasis on high field strength elements, we analyzed low-Ca boninites and associated tholeiitic basalts from Cape Vogel, Papua New Guinea (PNG) and compare them with compositions of high-Ca boninites and associated tholeiitic basalts from Cyprus. High-precision HFSE (Nb, Ta, Zr, Hf, W) concentration data of the boninites and associated tholeiitic basalts were obtained by isotope dilution. Major, trace element, and Sr–Nd–Hf–Pb isotope compositions clearly document a significant contribution of slab-derived melts involved in the petrogenesis of the PNG boninites, whereas only fluid-like subduction components were involved in the petrogenesis of the PNG basalts and the Cyprus suite. Low-Ca boninites from PNG are derived from a more refractory mantle source (∼21% depletion) than the high-Ca boninites from Cyprus (∼11% depletion) and their respective tholeiitic precursors (<10% depletion). In agreement with the more depleted nature of their mantle source, boninites exhibit a significantly stronger overprint by slab components. High-precision HFSE data indicate that, in comparison to LILE, a somewhat lower but measurable mobilization of all investigated HFSE in both slab-derived fluids and melts is evident. Modelling calculations demonstrate that the subduction components dominate the LILE budget and also largely control LREE and HFSE abundances in the boninite sources. Notably, the increasing influence of slab-derived fluids results in a decrease of the negative Nb–Ta anomaly, most likely reflecting a similar mobility of Nb–Ta and LREE at higher pressures near the critical point of fluid-melt miscibility. Ratios of Zr/Hf and Nb/Ta in the melt-like slab components dominating in the sources of the PNG boninites were probably fractionated in equilibrium with garnet–amphibolitic mafic oceanic crust. HFSE ratios in the Cyprus boninites are best explained by dehydration of subducted pelagic sediments in the absence of Ti-rich phases such as rutile. Our results also confirm previous assertions that the mobility of HFSE decreases in the order Sb > W–Mo > Nb–Ta > Zr–Hf. Furthermore, Mo–W systematics may provide a potential novel tracer for the amount, composition and redox state of subducted pelagic sediments that contribute to the geochemical budget of intra-oceanic arc systems.

Palaeozoic Lachlan orogen, Australia; accretion and construction of continental crust in a marginal ocean setting: isotopic evidence from Cambrian metavolcanic rocks

Abstract The Lachlan orogen developed as a classic accretionary orogen in an oceanic setting between the palaeo-Pacific subduction zone and the Australian craton. Direct evidence for the composition and age of the lower crust and the basement to the thick Palaeozoic turbidite fan of the Lachlan orogen is limited. Exposures of Cambrian metavolcanic rocks and geophysical data suggest that most of the basement is the mafic oceanic crust along with possible small fragments of older continental crust. The trace element compositions of Cambrian metavolcanic rocks in the western and central Lachlan orogen are similar to those of volcanic rocks formed in modern back-arc and forearc settings. Pb, Nd and Sr isotopic data from these Cambrian rocks suggest a supra-subduction zone setting with little or no influence of continental crust other than subducted sediment.