Thermodynamic controls on element partitioning between titanomagnetite and andesitic\u2013dacitic silicate melts

R H Sievwright,J J Wilkinson,A J Berry,H St C O’Neill

doi:10.1007/s00410-017-1385-6

Abstract

Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO2) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg2+, Mn2+, Co2+, Ni2+, Zn2+) and Cu2+/Cu+ with increasing fO2 between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO2 implies a reciprocal relationship between a(Fe2+O) and a(Fe3+O1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO2 in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO2 in silicic, but less so in mafic bulk systems.

Highlights

Magnetite is a common liquidus phase in andesitic–dacitic magmas
Results in the dacitic bulk system focus on experiments using JA-1 with added Fe2O3, where titanomagnetite was present over a greater range of fO2 and temperature
We suggest that thermodynamic equilibria between mineral and melt and its associated changes in a(Fe2+O) and a(Fe3+O1.5) in the melt offer a more accurate explanation of partitioning

Summary

Introduction

Magnetite is a common liquidus phase in andesitic–dacitic magmas. We present new experimental data on the partition coefficients of key elements between magnetiterich spinel and andesitic–dacitic melts which will help in interpreting the petrogenesis of such magmas. The trace-element chemistry of magnetite and Fe–Ti oxides in general has been recognised as a useful tool for interpreting the formation environment of igneous rocks (Dare et al 2012). The use of magnetite as a petrogenetic indicator requires full understanding of the controls on element partitioning. To develop a better understanding of the partitioning behaviour of a wide range of elements into magnetite, a set of experiments were conducted at atmospheric pressure (0.1 MPa) as a function of oxygen fugacity (fO2 − FMQ + 0.2 to FMQ + 3.7) and temperature (1070–1120 °C)

Methods

Results

Discussion

Conclusion