Clinopyroxene/Melt Trace Element Partitioning in Sodic Alkaline Magmas

Charles D Beard,Vincent J Van Hinsberg,Max Wilke,John Stix

doi:10.1093/petrology/egz052

Abstract

Abstract Clinopyroxene is a key fractionating phase in alkaline magmatic systems, but its impact on metal enrichment processes, and the formation of REE + HFSE mineralisation in particular, is not well understood. To constrain the control of clinopyroxene on REE + HFSE behaviour in sodic (per)alkaline magmas, a series of internally heated pressure vessel experiments was performed to determine clinopyroxene–melt element partitioning systematics. Synthetic tephriphonolite to phonolite compositions were run H2O-saturated at 200 MPa, 650–825°C with oxygen fugacity buffered to log f O2 ≈ ΔFMQ + 1 or log f O2 ≈ ΔFMQ +5. Clinopyroxene–glass pairs from basanitic to phonolitic fall deposits from Tenerife, Canary Islands, were also measured to complement our experimentally-derived data set. The REE partition coefficients are 0·3–53, typically 2–6, with minima for high-aegirine clinopyroxene. Diopside-rich clinopyroxene (Aeg5–25) prefer the MREE and have high REE partition coefficients (DEu up to 53, DSm up to 47). As clinopyroxene becomes more Na- and less Ca-rich (Aeg25–50), REE incorporation becomes less favourable, and both the VIM1 and VIIIM2 sites expand (to 0·79 Å and 1·12 Å), increasing DLREE/DMREE. Above Aeg50 both M sites shrink slightly and HREE (VIri ≤ 0·9 Å ≈ Y) partition strongly onto the VIM1 site, consistent with a reduced charge penalty for REE3+ ↔ Fe3+ substitution. Our data, complemented with an extensive literature database, constrain an empirical model that predicts trace element partition coefficients between clinopyroxene and silicate melt using only mineral major element compositions, temperature and pressure as input. The model is calibrated for use over a wide compositional range and can be used to interrogate clinopyroxene from a variety of natural systems to determine the trace element concentrations in their source melts, or to forward model the trace element evolution of tholeiitic mafic to evolved peralkaline magmatic systems.

Highlights

Trace element systematics provide a powerful methodology to investigate and model magmatic processes (Spera & Bohrson, 2001; Troll & Schmincke, 2002; Boudreau, 2004; Xu et al, 2010; Girnis et al, 2013; Mungall & Brenan, 2014), but their interpretation requires precise knowledge of mineral/liquid element partition coefficients
Trace element data are reported for eleven experimental charges. 25 additional experiments were rejected as their run temperatures were super-liquidus or subsolidus, or because their growth textures were indicative of disequilibrium (e.g. Supplementary Data Fig. S3)
We focused on 3þ ions that cover a wide range of radii and fitted lattice-strain parameters for both the VIM1 and VIIIM2 sites of clinopyroxene (Fig. 9): Dicpx=melt 1⁄4 D0M2 exp Parabolae for 3þ ions were fitted for the VIM1 and VIIIM2 sites using the REE, Ga and Al assigned to the VIM1 site of clinopyroxene (Fig. 9a)

Summary

Introduction

Trace element systematics provide a powerful methodology to investigate and model magmatic processes (Spera & Bohrson, 2001; Troll & Schmincke, 2002; Boudreau, 2004; Xu et al, 2010; Girnis et al, 2013; Mungall & Brenan, 2014), but their interpretation requires precise knowledge of mineral/liquid element partition coefficients. The approach has been applied to studies of mafic systems and mantle melting processes (Niu, 2004; Workman & Hart, 2005; Foley et al, 2013; Coumans et al, 2016; Peters et al, 2017).

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Discussion

Conclusion