Arc magmas sourced from mélange diapirs in subduction zones

Horst R. Marschall; John C. Schumacher

doi:10.1038/ngeo1634

The role of sulphur on the melting of Ca-poor sediment and on trace element transfer in subduction zones: an experimental investigation

Journal of Petrology ◽

10.1093/petrology/egab005 ◽

2021 ◽

Author(s):

Anne-Aziliz Pelleter ◽

Gaëlle Prouteau ◽

Bruno Scaillet

Keyword(s):

Trace Element ◽

Partial Melting ◽

Subduction Zones ◽

Sediment Flux ◽

Arc Magmas ◽

Trace Element Contents ◽

The Stability ◽

Element Transfer ◽

High Sediment

Abstract We performed phase equilibrium experiments on a natural Ca-poor pelite at 3 GPa, 750-1000 °C, under moderately oxidizing conditions, simulating the partial melting of such lithologies in subduction zones. Experiments investigated the effect of sulphur addition on phase equilibria and compositions, with S contents of up to ∼ 2.2 wt. %. Run products were characterized for their major and trace element contents, in order to shed light on the role of sulphur on the trace element patterns of melts produced by partial melting of oceanic Ca-poor sediments. Results show that sulphur addition leads to the replacement of phengite by biotite along with the progressive consumption of garnet, which is replaced by an orthopyroxene-kyanite assemblage at the highest sulphur content investigated. All Fe-Mg silicate phases produced with sulphur, including melt, have higher MgO/(MgO+FeO) ratios (relative to S-free/poor conditions), owing to Fe being primarily locked up by sulphide in the investigated redox range. Secular infiltration of the mantle wedge by such MgO and K2O-rich melts may have contributed to the Mg and K-rich character of the modern continental crust. Addition of sulphur does not affect significantly the stability of the main accessory phases controlling the behaviour of trace elements (monazite, rutile and zircon), although our results suggest that monazite solubility is sensitive to S content at the conditions investigated. The low temperature (∼ 800 °C) S-bearing and Ca-poor sediment sourced slab melts show Th and La abundances, Th/La systematics and HFSE signatures in agreement with the characteristics of sediment-rich arc magmas. Because high S contents diminish phengite and garnet stabilities, S-rich and Ca-poor sediment sourced slab melts have higher contents of Rb, B, Li (to a lesser extent), and HREE. The highest ratios of La/Yb are observed in sulphur-poor runs (with a high proportion of garnet, which retains HREE) and beyond the monazite out curve (which retains LREE). Sulphides appear to be relatively Pb-poor and impart high Pb/Ce ratio to coexisting melts, even at high S content. Overall, our results show that Phanerozoic arc magmas from high sediment flux margins owe their geochemical signature to the subduction of terrigenous, sometimes S-rich, sediments. In contrast, subduction of such lithologies during Archean appears unlikely or unrecorded.

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The Fluid Mobilities of K and Zr in Subduction Zones: Thermodynamic Constraints

Minerals ◽

10.3390/min11040394 ◽

2021 ◽

Vol 11 (4) ◽

pp. 394

Author(s):

Richen Zhong ◽

Min Zhang ◽

Chang Yu ◽

Hao Cui

Keyword(s):

Trace Element ◽

Subduction Zones ◽

Critical Role ◽

High Field Strength ◽

Trace Element Pattern ◽

Arc Magmas ◽

Thermodynamic Simulations ◽

Element Pattern ◽

Element Transfer ◽

Complex Ion

A subduction zone plays a critical role in forging continental crust via formation of arc magmas, which are characteristically enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs). This trace element pattern results from the different mobilities of LILEs and HFSEs during slab-to-wedge mass transfer, but the mechanisms of trace element transfer from subducting crusts are not fully understood. In this study, thermodynamic simulations are carried out to evaluate the mobilities of K and Zr, as representative cases of LILE and HFSE, respectively, in slab fluids. The fluids buffered by basaltic eclogite can dissolve > 0.1 molal of K at sub-arc depths (~3 to 5.5 GPa). However, only minor amounts of K can be liberated by direct devolatilization of altered oceanic basalt, because sub-arc dehydration mainly takes place at temperatures < 600 °C (talc-out), wherein the fluid solubility of K is very limited (<0.01 molal). Therefore, serpentinite-derived fluids are required to flush K from the eclogite. The solubility of K can be enhanced by the addition of NaCl to the fluid, because fluid Na+ can unlock phengite-bonded K via a complex ion exchange. Finally, it is further confirmed that Zr and other HFSEs are immobile in slab fluids.

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Isotopic Compositions of Plagioclase From Plutonic Xenoliths Reveal Crustal Assimilation Below Martinique, Lesser Antilles Arc

Frontiers in Earth Science ◽

10.3389/feart.2021.682583 ◽

2021 ◽

Vol 9 ◽

Author(s):

J. R. Brown ◽

G. F. Cooper ◽

G. M. Nowell ◽

C. G. Macpherson ◽

I. Neill ◽

...

Keyword(s):

Subduction Zones ◽

Lesser Antilles ◽

Sr Isotope ◽

Crustal Assimilation ◽

Arc Magmas ◽

Isotopic Compositions ◽

Mantle Sources ◽

Show Evidence ◽

Sediment Addition ◽

Lesser Antilles Arc

The chemical and isotopic compositions of volcanic arc lavas often show evidence for involvement of a sedimentary component during magma genesis. Determining where this sedimentary component is added to arc magmas is of vital importance for constraining the extent to which sediments and volatiles are recycled at subduction zones. Lavas from Martinique in the Lesser Antilles arc have wide ranging isotopic compositions extending to highly radiogenic values (e.g. 87/Sr/86Sr up to ∼0.710) that could, in principle, be explained by sediment addition to the mantle source or by crustal assimilation in the upper plate. We use Sr isotopic compositions of plagioclase from Martinique plutonic xenoliths to provide evidence supporting the crustal assimilation hypothesis. Plagioclase from plutonic xenoliths formed in the mid-crust (∼12 km) show a restricted range of unradiogenic Sr isotope ratios (87Sr/86Sr = 0.7041–0.7042) whereas plagioclase from upper crustal plutonic xenoliths (∼6 km) show greater intra-sample variation and more radiogenic Sr isotopic compositions up to 87Sr/86Sr = 0.7047. This trend is also observed in plutonic xenolith whole rock 87Sr/86Sr. Combined, these results indicate that the range of Sr isotope compositions becomes larger and more radiogenic in Martinique magmas as a result of sediment assimilation at shallow crustal levels. This is supported by Assimilation-Fractional Crystallization modeling, which shows that assimilation of chemically and isotopically heterogenous crustal sediments can produce the isotopic variation in Martinique plutonic xenoliths and lavas. Our results highlight the importance of constraining crustal contributions from the upper plate before using arc lava geochemistry to quantify sediment and volatile recycling at subduction zones and assessing potential heterogeneity of arc mantle sources.

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Deserpentinization in Subduction Zones as a Source of Oxidation in Arcs: A Reality Check

Journal of Petrology ◽

10.1093/petrology/egab016 ◽

2021 ◽

Author(s):

Katy A Evans ◽

B Ronald Frost

Keyword(s):

Subduction Zones ◽

Ultramafic Rocks ◽

Thermodynamic Models ◽

Arc Magmas ◽

Reality Check ◽

Deep Earth ◽

Tectonic Settings ◽

Iron Bearing ◽

Arc Magma

Abstract Previous studies have concluded that dehydration of serpentinites in subduction zones produces oxidizing fluids that are the cause of oxidized arc magmas. Here, observations of natural samples and settings are combined with thermodynamic models to explore some of the factors that complicate interpretation of the observations that form the basis of this conclusion. These factors include: the variability of serpentinite protoliths; the roles of carbon and sulfur in serpentinite evolution; variability in serpentinization in different tectonic settings; changes in the bulk compositions of ultramafic rocks during serpentinization; fundamental differences between serpentinization and deserpentinization; and the absence of precise geothermobarometers for ultramafic rocks. The capacity of serpentinite-derived fluids to oxidize sub-arc magma is also examined. These fluids can transport redox budget as carbon-, sulfur-, and iron-bearing species. Iron- and carbon-bearing species might be present in sufficient concentrations to transport redox budget deep within subduction zones, but are not viable transporters of redox budget at the temperatures of antigorite breakdown, which produces the largest proportion of fluid released by serpentinite dehydration. Sulfur-bearing species can carry significant redox budget, and calculations using the Deep Earth Water (DEW) model show that these species might be stable during antigorite breakdown. However, oxygen fugacities of ∼ΔFMQ +3 (where FMQ refers to the fayalite–magnetite–quartz buffer, and ΔFMQ is Log fO2 – Log fO2,FMQ), which is close to, or above, the hematite–magnetite buffer at the conditions of interest, are required to stabilize oxidized sulfur-bearing species. Pseudosection calculations indicate that these conditions might be attained at the conditions of antigorite breakdown if the starting serpentinites are sufficiently oxidized, but further work is required to assess the variability of serpentinite protoliths, metamorphic pressures and temperatures, and to confirm the relative positions of the mineral buffers with relation to changes in fluid speciation.

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Recycling of Marine Carbonate Induced Calcium Isotope Heterogeneity of Arc Magmas in Subduction Zones

10.20944/preprints202107.0523.v1 ◽

2021 ◽

Author(s):

Xue-Gang Chen ◽

Tao Wu ◽

Qin Gao ◽

Yu-Ming Lai

Keyword(s):

Subduction Zones ◽

Essential Element ◽

Chemical Compositions ◽

Carbon Cycles ◽

Arc Magmas ◽

Marine Carbonate ◽

Depleted Mantle ◽

Mid Ocean Ridge ◽

Sedimentary Carbonate ◽

Ocean Ridge

Calcium (Ca) is an essential element constituting sedimentary carbonate in subducting sediments. Ca isotopic characteristics of subduction-related rocks could provide insight into the behavior and budget of carbonate and carbon cycles in subduction zones, due to the distinctive δ44/40Ca ranges of sedimentary carbonate with respect to the mantle. Here, we studied the Ca isotopic compositions of arc magmas from the Northern Luzon arc (NLA), which are evolved from a depleted mantle metasomatized by slab-derived fluids and sediment melts. The δ44/40Ca values range from 0.76 ± 0.04&permil; to 1.01 ± 0.03&permil; and cover the typical ranges for bulk silica earth (BSE, ~ 0.94&permil;) and fresh mid-ocean ridge basalt (MORB, ~ 0.83&permil;). The Ca isotopes of NLA volcanics are not dominantly determined by the effects of mantle partial melting or fractional crystallization, nor significantly modified by secondary alteration. Instead, the δ44/40Ca values of NLA volcanics are controlled by the subduction-related metasomatism. The metasomatism by slab-derived fluids (mainly expelled from altered oceanic crust, AOC) dramatically elevated the contents of fluid-mobile elements (e.g., Ba and Pb) with respect to fluid-immobile elements (e.g., Ce). This process, however, rarely modified the Ca isotopes, possibly ascribed to the δ44/40Ca similarity between AOC and the depleted mantle. The δ44/40Ca values significantly correlated with subduction indicators (e.g., Sr-Nd isotopes, Ba/Nb, Ce/Pb, and Nb/La), demonstrating the Ca isotopes of NLA volcanics are mainly controlled by the metasomatism of sediment melts subducting from the South China Sea (SCS). Based on the thermal structures and chemical compositions of sediments subducting into global trenches, we propose that carbonate Ca isotopic signals can only be observed in the arcs with high sedimentary Ca fluxes and temperature-pressure conditions well beyond the solidus of H2O-saturated sediment melting, e.g., NLA, Nicaragua, Guatemala, Colombia, Peru, South Chile, North Vanuatu, New Zealand, and Kermadec. The absence of such signals in other arcs suggests either limited sedimentary fluxes or much of the subducting sedimentary carbonate has been survived during plate subduction to enter the deep mantle.

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High Sulfur in Primitive Arc Magmas, Its Origin and Implications

Minerals ◽

10.3390/min12010037 ◽

2021 ◽

Vol 12 (1) ◽

pp. 37

Author(s):

Michael Zelenski ◽

Vadim S. Kamenetsky ◽

Nikolai Nekrylov ◽

Alkiviadis Kontonikas-Charos

Keyword(s):

Subduction Zones ◽

Melt Inclusions ◽

Solubility Limit ◽

Calc Alkaline ◽

Sulfur Solubility ◽

Arc Magmas ◽

Tolbachik Volcano ◽

Magmatic Sulfide ◽

Low Sulfur ◽

Basaltic Melts

Sulfur contents in 98.5% of melt inclusions (MI) from calc-alkaline subduction basalts do not exceed 4000 ppm, whereas experimentally established limits of sulfur solubility in basaltic melts with high fO2 (characteristic of subduction zones, e.g., QFM + 2) surpass 14,000 ppm. Here we show that primitive (Mg# 62-64) subduction melts may contain high sulfur, approaching the experimental limit of sulfur solubility. Up to 11,700 ppm S was measured in olivine-hosted MI from primitive arc basalt from the 1941 eruption of the Tolbachik volcano, Kamchatka. These MI often contain magmatic sulfide globules (occasionally enriched in Cu, Ni, and platinum-group elements) and anhydrite enclosed within a brown, oxidized glass. We conclude that the ubiquitous low sulfur contents in MI may originate either from insufficient availability of sulfur in the magma generation zone or early magma degassing prior to inclusion entrapment. Our findings extend the measured range of sulfur concentrations in primitive calc-alkaline basaltic melts and demonstrate that no fundamental limit of 4000 ppm S exists for relatively oxidized subduction basalts, where the maximum sulfur content may approach the solubility limit determined by crystallization of magmatic anhydrite.

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Continental Crust formation in the Archean vs modern times

10.5194/egusphere-egu2020-5997 ◽

2020 ◽

Author(s):

Oliver Jagoutz ◽

Benjamin Klein ◽

Max W Schmidt ◽

Nico Küter

Keyword(s):

Continental Crust ◽

Subduction Zones ◽

Major Elements ◽

Modern Times ◽

Arc Magmas ◽

Crucial Difference ◽

Dry Conditions ◽

Water Saturated ◽

Liquid Line Of Descent

<p>When subduction initiated and contributed to formation of Continental crust is uncertain. A crucial difference between subduction zones magma and e.g. plume related magmatism is the role of H2O in the magma formed. Subduction zones magma are frequently wet and follow a liquid line of descent (LLD) that differs from dry plume related magmas. We developed a qualitative hygrometer based on major elements that allow to distinguish between LLD formed at water saturated condition from those that formed at dry conditions. While arc magmas can by dry at times, plume related magmas are generally dry. So wet LLD are a hall mark of subduction. In this talk we will compare the modern arc record with the Archean rock record to investigate if Archean rocks formed due to a wet or dry LLD.&#160;</p>

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Noble Metals in Arc Basaltic Magmas Worldwide: A Case Study of Modern and Pre-Historic Lavas of the Tolbachik Volcano, Kamchatka

Frontiers in Earth Science ◽

10.3389/feart.2021.791465 ◽

2021 ◽

Vol 9 ◽

Author(s):

Anton Kutyrev ◽

Michael Zelenski ◽

Nikolai Nekrylov ◽

Dmitry Savelyev ◽

Alkiviadis Kontonikas-Charos ◽

...

Keyword(s):

Subduction Zones ◽

Noble Metals ◽

Mantle Wedge ◽

Mantle Melting ◽

Magmatic Differentiation ◽

Arc Magmas ◽

Promising Tool ◽

Tolbachik Volcano ◽

Single Arc

Platinum-group elements (PGE) and gold are a promising tool to assess the processes of mantle melting beneath the subduction zones. However, fractionation processes in magmas inevitably overwrite the initial metal budgets of magmas, making constraints on the melting processes inconclusive. Moreover, little is still known about the geochemical behavior of a particular metal in a single arc magmatic system, from mantle melting towards magma solidification. Here we compare noble metals in lavas from several eruptions of the Tolbachik volcano (Kamchatka arc) to better understand the effects of magma differentiation, estimate primary melt compositions and make constraints on the mantle melting. We show that Ir, Ru, Rh and, to a lesser extent, Pt are compatible during magmatic differentiation. The pronounced incompatible behavior of Cu and Pd, observed in Tolbachik magmas, rules out the significant influence of sulfide melts on the early magmatic evolution in this particular case. Gold is also incompatible during magmatic differentiation; however, its systematics can be affected by the inferred gold recycling in the plumbing system of Tolbachik. Although the Tolbachik lavas show only slightly higher PGE fractionation than in MORB, a notable negative Ru anomaly (higher Pt/Ru and Ir/Ru) is observed. We attribute this to be a result of greater oxidation in the subarc mantle (by 1–4 log units), which promotes crystallization of Ru-bearing phases such as Fe3+-rich Cr-spinel and laurite. The estimated Pd contents for the parental melt of the Tolbachik lavas approaches 6.5 ppb. This is several times higher than reported MORB values (1.5 ± 0.5 ppb), suggesting the enrichment of Pd in the mantle wedge. Our results highlight the influence of the subduction-related processes and mantle wedge refertilization on the noble metal budgets of arc magmas.

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