The composition of subduction zone fluids and the origin of the trace element enrichment in arc magmas

Greta Rustioni,Andreas Audetat,Hans Keppler

doi:10.1007/s00410-021-01810-8

Greta Rustioni, Andreas Audetat + Show 1 more

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https://doi.org/10.1007/s00410-021-01810-8

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Abstract

The partitioning of major and trace elements between eclogite and aqueous fluids with variable salinity was studied at 700–800 °C and 4–6 GPa in piston cylinder and multi anvil experiments. Fluid compositions were determined using the diamond trap technique combined with laser ablation ICP-MS measurements in the frozen state. In addition to NaCl, SiO2 is the main solute in the fluids. The fluid/eclogite partition coefficients of the large ion lithophile elements (LILE), such as Rb, Cs, Sr, and Ba as well as those of the light rare earths (LREE), of Pb, and of U increase by up to three orders of magnitude with salinity. These elements will therefore be efficiently transported by saline fluids. On the other hand, typical high field strength elements, such as Ti, Nb, and Ta, are not mobilized even at high salinities. Increasing temperature and pressure gradually increases the partitioning into the fluid. In particular, Th is mobilized by silica-rich fluids at 6 GPa already at low salinities. We show that we can fully reproduce the trace element enrichment pattern of primitive arc basalts by adding a few percent of saline fluid (with 5–10 wt% Cl) released from the basaltic slab to the zone of melting in the mantle wedge. Assuming 2 wt% of rutile in the eclogite equilibrated with the saline fluid produces a negative Nb Ta anomaly that is larger than in most primitive arc basalts. Therefore, we conclude that the rutile fraction in the subducted eclogite below most arcs is likely < 1 wt%. In fact, saline fluids would even produce a noticeable negative Nb Ta anomaly without any rutile in the eclogite residue. Metasomatism by sediment melts alone, on the other hand, is unable to produce the enrichment pattern seen in arc basalts. We, therefore, conclude that at least for primitive arc basalts, the release of hydrous fluids from the basaltic part of the subducted slab is the trigger for melting and the main agent of trace element enrichment. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible. In the supplementary material, we provide a “Subduction Calculator” in Excel format, which allows the calculation of the trace element abundance pattern in primitive arc basalts as function of fluid salinity, the amount of fluid released from the basaltic part of the subducted slab, the fluid fraction added to the source, and the degree of melting.

Highlights

Magma generation in subduction zones is likely the main mechanism for the growth of the continental crust since the onset of plate tectonics (e.g. Hawkesworth et al 2019)
This idea would be generally consistent with the typical trace element enrichment pattern seen in arc magmas (e.g. Kelemen et al 2005), which features strong enrichments in large ion lithophile (LILE) elements, such as Rb, Cs, Sr, and Ba, which are usually considered to be soluble in aqueous fluids, while the poorly soluble high field strength elements (HFSE), such as Ti, Zr, Hf, Nb, and Ta are depleted
A common notion in the recent literature is that aqueous fluids are “too dilute” to cause the trace element enrichment pattern seen in typical arc magmas (e.g. Hermann et al 2006; Spandler and Pirard 2013)

Summary

Introduction

Magma generation in subduction zones is likely the main mechanism for the growth of the continental crust since the onset of plate tectonics (e.g. Hawkesworth et al 2019). Gill 1981; Arculus and Powell 1986; Tatsumi 1989; Peacock 1990) This idea would be generally consistent with the typical trace element enrichment pattern seen in arc magmas Many experimental studies have been carried out in the last few decades to constrain the composition of aqueous fluids in equilibrium with minerals of the subducted slab at high pressures and temperatures (Brenan et al 1994, 1995; Keppler 1996; Stalder et al 1998; Johnson and Plank 1999; Kessel et al 2005; Bali et al 2011, 2012; Tsay et al 2014, 2017). A common notion in the recent literature is that aqueous fluids are “too dilute” to cause the trace element enrichment pattern seen in typical arc magmas (e.g. Hermann et al 2006; Spandler and Pirard 2013)

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