Clinopyroxene/Melt Trace Element Partitioning in Sodic Alkaline Magmas. (9th November 2019)

• Record Type: Journal Article
• Authors: Beard, Charles D; van Hinsberg, Vincent J; Stix, John; Wilke, Max
• Is Part Of: Journal of petrology. Volume 60:Number 9(2019:Sep.)
• Page Start: 1797
• Page End: 1823
• Publication Date: 2019-11-09
• Subjects: aegirine -- experimental petrology -- Canary Islands -- peralkaline -- rare earth elements; Petrology -- Periodicals; 552
• DOI: 10.1093/petrology/egz052
• Languages: English
• ISSNs: 0022-3530

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 H2 O-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 ( D Eu up to 53, D Sm up to 47). As clinopyroxene becomes more Na- and less Ca-rich (Aeg25–50 ), REE incorporation becomes less favourable, and both the VI M1 and VIII M2 sites expand (to 0·79 Å and 1·12 Å), increasing D LREE /D MREE . Above Aeg50 both M sites shrink slightly and HREE ( VI r i ≤ 0·9 Å ≈ Y) partition strongly onto the VI M1 site, consistent with a reduced charge penalty for REE 3+ ↔ Fe 3+ 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.