Abstract Understanding the behavior of chalcophile elements during the evolution of arc magmas is critical to refining models for the formation and distribution of porphyry copper deposits used in mineral exploration. Because magmas in continental arcs undergo copper depletion during their early differentiation, a widely held hypothesis posits that the removed copper is locked at the base of the crust in copper-rich cumulates that form due to early sulfide saturation. Testing this hypothesis requires direct evidence for such copper-rich reservoirs and a comprehensive understanding of the mechanisms driving sulfide saturation. Interaction between oxidized magmas and reducing crustal material in island arcs has been shown to be an efficient process causing sulfide saturation. However, the extent to which crustal assimilation impacts the flux of chalcophile elements during magmatism in thick continental arcs remains to be established. Here, we provide a deep perspective into these problems by studying a suite of subarc cumulate rocks from the Acadian orogen, New England (USA). These cumulates record the imprint of subduction zone magmatism and represent the residues left behind during the genesis of intermediate to evolved Acadian magmas (ca. 410 Ma). We find that the most primitive Acadian cumulates are enriched in copper (up to ~730 µg g–1) hosted by sulfide phases, providing direct evidence for the formation of lower crustal copper-rich reservoirs. The Acadian cumulates reveal a wide range of δ34S values, from –4.9‰ in the ultramafic rocks to 8‰ in the most evolved mafic rocks. The negative δ34S values observed in the most primitive and copper-rich cumulates (avg –3‰) reflect the assimilation of isotopically light sulfur from surrounding sulfidic and graphite-bearing metasedimentary rocks (δ34S of –19 to –12‰), whereas the more evolved cumulates with positive δ34S signatures may have formed from different magma batches that experienced less sediment assimilation. The assimilation of these reducing metasedimentary rocks caused a critical drop in oxygen fugacity (~DFMQ –2.5 to –1.9; FMQ = fayalite-quartz-magnetite buffer) in the evolving magmas, ultimately leading to extensive sulfide saturation and the consequent formation of copper-rich subarc cumulates. Assimilation-driven sulfide saturation may be a common process at the root of thickened arc crusts that triggers the formation of lower crustal copper-rich reservoirs, which play a pivotal role in the fate of copper during arc magmatism. Thus, deeply buried reducing metasedimentary crustal material at the base of continental arcs can act as a barrier to the magmatic flux of chalcophile elements and may play a crucial role in the genesis and distribution of porphyry copper deposits.
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Sulfide geochemistry of cumulates from the Lesser Antilles arc
The sulfur isotope composition of volcanic rocks in arcs can be difficult to constrain because significant fractionation can occur during degassing. Mafic and ultramafic cumulates represent the least degassed part of the magmatic arc system, thereby offering an opportunity to investigate undegassed sulfur in arcs. Recent work on high pressure metamorphic rocks has suggested that subducted materials can retain their original isotopic composition to sub-arc depths. In particular, extreme negative δ34S values can be retained in subducted sediments. The purpose of this project is to investigate to what extent these deep subduction zone processes are reflected in the sulfur isotope signature of arc magmas.
In the Lesser Antilles arc, there is a gradual decrease in terrigenous sediment being subducted from south to north. An estimated ~15% subducted sediment in the south and ~2% in the north is reflected in the chemical and isotopic composition of the Lesser Antilles arc magmas. Sulfides in these magma- derived cumulates record the earliest stages of magma evolution and are a more faithful monitor of the sulfur isotopic composition of the magma source region in the mantle than erupting lavas. We hypothesize that the decrease in terrigenous sediment being subducted from the south to north will be reflected in the S isotopes in cumulate samples.
Samples of mafic and ultramafic cumulates have been collected from fourteen islands across the Lesser Antilles arc. Primary rock types are olivine gabbro, amphibole gabbro, plagioclase gabbro, and olivine gabbronorite. Sulfide minerals include pyrite, chalcopyrite, and pyrrhotite, and typically occur as spherical blebs. Sulfides are found primarily as inclusions in clinopyroxene, amphibole, olivine, and plagioclase. Sulfides occur less frequently as inclusions in magnetite and within the matrix. Analyses of sulfur isotopes in cumulate sulfides are currently underway. The decrease in the amount sediment being subducted from south to north in the Lesser Antilles arc should result in δ34S values that increase from south to north (more sediment subducted = more negative δ34S values).
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- Award ID(s):
- 1725301
- NSF-PAR ID:
- 10108912
- Date Published:
- Journal Name:
- Goldschmidt Abstracts
- Page Range / eLocation ID:
- 2045
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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