Chemical feedbacks during magma degassing control chlorine partitioning and metal extraction in volcanic arcs

B Tattitch,J Blundy,R R Loucks,C Chelle-Michou

doi:10.1038/s41467-021-21887-w

B Tattitch, J Blundy + Show 2 more

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https://doi.org/10.1038/s41467-021-21887-w

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Abstract

Hydrous fluids released from subducting oceanic lithosphere fuel arc magmatism and associated hydrothermal mineralization, including formation of porphyry copper deposits. Critical magma degassing parameters are the depth, chemistry and style of fluid release during magma ascent, notably the behaviour of chlorine, a key metal-transporting ligand. Currently, understanding is limited by restricted data on fluid-melt partitioning of chlorine as a function of pressure and magma chemistry, and the complex interplay between the two that occurs in polybaric magmatic systems. Here we present experimental determinations of chlorine partitioning as a function of fluid and melt composition at pressures from 50 to 800 MPa. We provide, for the first time, a quantitative understanding of chlorine and copper evolution that is valid for shallow, deep or transcrustal differentiation and degassing. Monte Carlo simulations using our new data reproduce the chemical evolution of melt inclusions from arc volcanoes and fluid inclusions from upper crustal intrusions and porphyry copper deposits. Our results not only provide a novel chemical framework for understanding magma degassing, but quantify the primacy of magmatic chlorine concentration at the point of fluid saturation in promoting efficient copper extraction from magmas.

Highlights

Hydrous fluids released from subducting oceanic lithosphere fuel arc magmatism and associated hydrothermal mineralization, including formation of porphyry copper deposits
To address the lack of appropriate fluid-melt partitioning data we performed fluid-saturated experiments at 50–800 MPa, 800–950 °C using as starting materials natural metaluminous calc-alkaline rhyolite, dacite and andesite to which were added NaCl–KCl–HCl solution and quartz cylinders
We tested our approach against melt inclusion (MI) data from two arc volcanoes, Mount St

Summary

Introduction

Hydrous fluids released from subducting oceanic lithosphere fuel arc magmatism and associated hydrothermal mineralization, including formation of porphyry copper deposits. Conventional concepts predicated on second-boiling of large, relatively volatile-poor (≤5 wt% H2O) melt-rich “magma chambers” in the shallow crust[5,6,7] are being challenged by models of vertically extensive, longlived, mid- to lower-crustal crystal mushes containing a volatilerich (8–15 wt% H2O) intergranular melt[1,8,9,10,11,12,13] Such high dissolved H2O concentrations create problems for standard approaches to tracking magma degassing using H2O and CO2 because the latter is extensively degassed by the time magmas reach shallow sub-volcanic or ore-forming domains. Predictions Subsequent ebxapseerdimoenntst2h0e–25ahvaigilhalbiglehteDdCflultihd=emaeldt didtiaotna1a9l influences of major element composition of melts and total chlorine content of the system, yet many of these studies used melt compositions distinct from typical subduction zone magmas, hampering their quantitative application to arc volcanism and mineralization These studies often relied on imprecise mass balance methods (especially those with two-phase fluid assemblages) to determine fluid composition. Previous attempts to model Cl and Cu behaviour during degassing[26,27,28] provided useful insights, but were unable to describe the complex chemical feedbacks inherent in the evolution of chemically diverse, polybaric magmatic systems

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