Melting of subducted slab dictates trace element recycling in global arcs

Temperature-dependent trace element fractionation during melting of subducted slab can explain the composition of arc magmas.

High Sulfur in Primitive Arc Magmas, Its Origin and Implications

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.

Metals in subduction related magmatism: Insights from melt inclusions and associated glassy groundmass from the Southern Kermadec Arc, New Zealand

<p>The southern Kermadec Arc – Havre Trough (SKAHT) is an intra-oceanic arc – back-arc system where the Pacific plate is subducting beneath the Australian plate. The Kermadec volcanic arc front consists of 33 volcanic centres, four of which host hydrothermal mineralization (Brothers, Haungaroa, Rumble II West, and Clark) such as volcanogenic massive sulfide (VMS) deposits, which are characterised by high concentrations of base and precious metals (e.g., Au, Cu, Zn, Pb). The sources of these metals are strongly tied to the metal contents within underlying magmatic rocks and associated magmatic systems with which the hydrothermal fluids interact. Understanding the sources, movements, and accumulation of metals associated with porphyry copper and exhalative base metal deposits within a subduction – arc setting remains limited. This study reports major, trace, and volatile element contents in basaltic groundmass glasses and olivine-hosted melt inclusions from lavas from four locations within the arc – back-arc setting of the SKAHT. The focus is on understanding the controls on base metal (Pb, Cu, Zn, Mo, V) contents in the magmas. The sample locations, Rumble III and Rumble II West volcanoes, and back-arc Basins D and I, form an arc-perpendicular transect extending from arc front into the back-arc. The analysed melt inclusion and groundmass glasses are all basalt to basaltic andesite in composition, with back-arc basin samples more mafic than arc front volcano samples. The magmatic evolution of the melts is primarily controlled by crystal fractionation of olivine + pyroxene + plagioclase. All glasses have undergone variable degassing, indicated by an absence of detectable CO₂ and curvilinear decreases in S contents with increasing SiO₂. Of the volatile phases analysed, only Cl appears unaffected by degassing. Distinct compositional differences are apparent between arc front and back-arc melts. The arc front magmas formed from higher degrees of melting of a less fertile mantle source and are more enriched in trace elements then the back-arc magmas due to greater additions of slab-derived aqueous fluids to their source. Magmas from a single arc front volcano (Rumble II West) incorporate melts that have tapped variably enriched sources, indicating heterogeneity of the mantle at small scales. Significant variation in mantle composition, however, is also apparent laterally along strike of the arc. Rumble III volcano and Basin I lie on an arc-perpendicular transect south of Rumble II West volcano and Basin D. Their greater enrichment in trace elements and higher concentrations of base metals than Rumble II West and Basin D lavas can be attributed to higher fluxes of subduction derived components. Base metals (Cu, Zn, Pb, Mo, and V) are variably enriched in the SKAHT melts compared with typical mid-ocean ridge basalts with relative enrichments in the order Pb >> Cu > Mo, V > Zn. All metals appear to be affected by mantle metasomatism related to slab-derived fluids, either directly from slab components introduced to the mantle source (e.g., Pb) or through mobilisation of metals within the ambient mantle wedge. The apparently compatible behaviour of Zn, Cu, and V in the mantle means that these elements may be enriched in arc front magmas relative to back-arc magmas by higher degrees of partial melting and/or melting of more depleted sources. All base metals behave incompatibly in the magma during crystal fractionation between 48 – 56 wt.% SiO₂. Lead and Cu concentrations, however, begin to level out from ~ 52 wt.% SiO₂ suggesting some subsequent loss to fractionating volatile phases as metal sulfide complexes. Rumble III samples show a decrease in metal concentration (Pb, Cu, V), from melt inclusions to groundmass glasses, suggestive of more significant loss associated with sulfur degassing. Although other factors such as heat generation, hydrothermal flow, fault systems, and magma venting are key in the development of VMS deposits, this study shows that variations in subduction parameters can significantly affect metal concentrations in arc magmas that may host hydrothermal systems, and hence the amount of metals available to be scavenged into the deposits.</p>

Metals in subduction related magmatism: Insights from melt inclusions and associated glassy groundmass from the Southern Kermadec Arc, New Zealand

<p>The southern Kermadec Arc – Havre Trough (SKAHT) is an intra-oceanic arc – back-arc system where the Pacific plate is subducting beneath the Australian plate. The Kermadec volcanic arc front consists of 33 volcanic centres, four of which host hydrothermal mineralization (Brothers, Haungaroa, Rumble II West, and Clark) such as volcanogenic massive sulfide (VMS) deposits, which are characterised by high concentrations of base and precious metals (e.g., Au, Cu, Zn, Pb). The sources of these metals are strongly tied to the metal contents within underlying magmatic rocks and associated magmatic systems with which the hydrothermal fluids interact. Understanding the sources, movements, and accumulation of metals associated with porphyry copper and exhalative base metal deposits within a subduction – arc setting remains limited. This study reports major, trace, and volatile element contents in basaltic groundmass glasses and olivine-hosted melt inclusions from lavas from four locations within the arc – back-arc setting of the SKAHT. The focus is on understanding the controls on base metal (Pb, Cu, Zn, Mo, V) contents in the magmas. The sample locations, Rumble III and Rumble II West volcanoes, and back-arc Basins D and I, form an arc-perpendicular transect extending from arc front into the back-arc. The analysed melt inclusion and groundmass glasses are all basalt to basaltic andesite in composition, with back-arc basin samples more mafic than arc front volcano samples. The magmatic evolution of the melts is primarily controlled by crystal fractionation of olivine + pyroxene + plagioclase. All glasses have undergone variable degassing, indicated by an absence of detectable CO₂ and curvilinear decreases in S contents with increasing SiO₂. Of the volatile phases analysed, only Cl appears unaffected by degassing. Distinct compositional differences are apparent between arc front and back-arc melts. The arc front magmas formed from higher degrees of melting of a less fertile mantle source and are more enriched in trace elements then the back-arc magmas due to greater additions of slab-derived aqueous fluids to their source. Magmas from a single arc front volcano (Rumble II West) incorporate melts that have tapped variably enriched sources, indicating heterogeneity of the mantle at small scales. Significant variation in mantle composition, however, is also apparent laterally along strike of the arc. Rumble III volcano and Basin I lie on an arc-perpendicular transect south of Rumble II West volcano and Basin D. Their greater enrichment in trace elements and higher concentrations of base metals than Rumble II West and Basin D lavas can be attributed to higher fluxes of subduction derived components. Base metals (Cu, Zn, Pb, Mo, and V) are variably enriched in the SKAHT melts compared with typical mid-ocean ridge basalts with relative enrichments in the order Pb >> Cu > Mo, V > Zn. All metals appear to be affected by mantle metasomatism related to slab-derived fluids, either directly from slab components introduced to the mantle source (e.g., Pb) or through mobilisation of metals within the ambient mantle wedge. The apparently compatible behaviour of Zn, Cu, and V in the mantle means that these elements may be enriched in arc front magmas relative to back-arc magmas by higher degrees of partial melting and/or melting of more depleted sources. All base metals behave incompatibly in the magma during crystal fractionation between 48 – 56 wt.% SiO₂. Lead and Cu concentrations, however, begin to level out from ~ 52 wt.% SiO₂ suggesting some subsequent loss to fractionating volatile phases as metal sulfide complexes. Rumble III samples show a decrease in metal concentration (Pb, Cu, V), from melt inclusions to groundmass glasses, suggestive of more significant loss associated with sulfur degassing. Although other factors such as heat generation, hydrothermal flow, fault systems, and magma venting are key in the development of VMS deposits, this study shows that variations in subduction parameters can significantly affect metal concentrations in arc magmas that may host hydrothermal systems, and hence the amount of metals available to be scavenged into the deposits.</p>

Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems

Crystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation of mantle-derived arc magmas. Various pressure–temperature trajectories were explored to cover a range of potential magma ascent paths and to investigate the role of decompression on phase equilibria and liquid lines of descent (LLD). Fractional crystallisation was approached in a step-wise manner by repetitively synthesising new starting materials chemically corresponding to liquids formed in previous runs. Experiments were performed at temperatures ranging from 1140 to 870 °C with 30 °C steps, and pressure was varied between 0.8 and 0.2 GPa with 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO2) was buffered close to the Ni-NiO equilibrium (NNO). An additional fractionation series was conducted at fO2 corresponding to the Re-ReO2 buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) were run in piston cylinder apparatus while 0.2 GPa runs were conducted in externally heated pressure vessels. Resulting liquid lines of descent follow calc-alkaline differentiation trends where the onset of pronounced silica enrichment coincides with the saturation of amphibole and/or Fe–Ti–oxide. Both pressure and fO2 exert crucial control on the stability fields of olivine, pyroxene, amphibole, plagioclase, and Fe–Ti–oxide phases and on the differentiation behaviour of arc magmas. Key observations are a shift of the olivine–clinopyroxene cotectic towards more clinopyroxene-rich liquid composition, an expansion of the plagioclase stability field and a decrease of amphibole stability with decreasing pressure. Decompression-dominated ascent trajectories result in liquid lines of descent approaching the metaluminous compositional range observed for typical arc volcanic rocks, while differentiation trends obtained for cooling-dominated trajectories evolve to peraluminous compositions, similar to isobaric liquid lines of descent at elevated pressures. Experiments buffered at RRO provide a closer match with natural calc-alkaline differentiation trends compared to fO2 conditions close to NNO. We conclude that decompression-dominated fractionation at oxidising conditions represents one possible scenario for arc magma differentiation.

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

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.

Miocene and Pliocene Silicic Coromandel Volcanic Zone Tephras from ODP Site 1124-C: Petrogenetic Applications and Temporal Evolution

<p>The Coromandel Volcanic Zone (CVZ) was the longest-lived area of volcanism in New Zealand hosting the commencement of large explosive rhyolitic and ignimbrite forming eruptions. The NW trending Coromandel Peninsula is the subaerial remnant of the Miocene-Pliocene CVZ, which is regarded as a tectonic precursor to the Taupo Volcanic Zone (TVZ), currently the most dynamic and voluminous rhyolitic volcanic centre on Earth. This study presents new single glass shard major and trace element geochemical analyses for 72 high-silica volcanic tephra layers recovered from well-dated deep-sea sediments of the SW Pacific Ocean by the Ocean Drilling Program (ODP) Leg 181. ODP Site 1124, ~720 km south and east from the CVZ, penetrated sediments of the Rekohu Drift yielding an unprecedented record of major explosive volcanic eruptions owing to the favourable location and preservation characteristics at this site. This record extends onshore eruptive sequences of CVZ explosive volcanism that are obscured by poor exposure, alteration, and erosion and burial by younger volcanic deposits. Tephra layers recovered from Site 1124 are well-dated through a combination of biostratigraphic and palaeomagnetic methods allowing the temporal geochemical evolution of the CVZ to be reconstructed in relation to changes in the petrogenesis of CVZ arc magmas from ~ 10 to 2 Ma. This thesis establishes major and trace element geochemical "fingerprints" for all Site 1124-C tephras using well-established (wavelength dispersive electron probe microanalysis) and new (laser ablation inductively coupled plasma mass spectrometry) in situ single glass shard microanalytical techniques. Trace element analysis of Site 1124-C glass shards (as small as 20 um) demonstrate that trace element signatures offer a more specific, unequivocal characterisation for distinguishing (and potentially correlating) between tephras with nearly identical major element compositions. The Site 1124-C core contains 72 unaltered Miocene-Pliocene volcanic glass-shard-bearing laminae > 1 cm thick that correspond to 83 or 84 geochemical eruptive units. Revised eruptive frequencies based on the number of geochemical eruptive units identified represent at least one eruption every 99 kyr for the late Miocene and one per 74 kyr for the Pliocene. The frequency of tephra deposition throughout the history of the CVZ has not been constant, rather reflecting pulses of major explosive eruptions resulting in closely clustered groups of tephra separated by periods of reduced activity, relative volcanic quiescence or non-tephra deposition. As more regular activity became prevalent in the Pliocene, it was accompanied by more silicic magma compositions. Rhyolitic volcanic glass shards are characterised by predominantly calc-alkaline and minor high-K enriched major element compositions. Major element compositional variability of the tephras deposited between 10 Ma and 2 Ma reveals magma batches with pre-eruptive compositional gradients implying a broad control by fractional crystallisation. Trace element characterisation of glass shards reveals the role of magmatic processes that are not readily apparent in the relatively homogeneous major element compositions. Multi-element diagrams show prominent negative Sr and Ti anomalies against primitive mantle likely caused by various degrees of plagioclase and titanomagnetite fractional crystallisation in shallow magma chambers. Relative Nb depletion, characteristic of arc volcanism, is moderate in CVZ tephras. HFSEs (e.g. Nb, Zr, Ti) and HREEs (e.g. Yb, Lu) remain immobile during slab fluid flux suggesting they are derived from the mantle wedge. LILE (e.g. Rb, Cs, Ba, Sr) and LREE (e.g. La, Ce) enrichments are consistent with slab fluid contribution. B/La and Li/Y ratios can be used as a proxy for the flux of subducting material to the mantle wedge, they suggest there is a strong influence from this component in the generation of CVZ arc magmas, potentially inducing melting. CVZ tephra show long-term coherent variability in trace element geochemistry. Post ~ 4 Ma tephras display a more consistent, less variable, chemical fingerprint that persists up to and across the CVZ/TVZ transition at ~ 2 Ma. Initiation of TVZ volcanism may have occurred earlier than is presently considered, or CVZ to TVZ volcanism may have occurred without significant changes in magma generation processes.</p>