AN EMPIRICAL-MODEL FOR THE CALCULATION OF SPINEL MELT EQUILIBRIA IN MAFIC IGNEOUS SYSTEMS AT ATMOSPHERIC-PRESSURE .1. CHROMIAN SPINELS

In order to develop a model for simulating naturally occurring chromia n spinel compositions, we have processed published experimental data o n chromian spinel-melt equilibrium. Out of 259 co-existing spinel-melt experiments reported in the literature, we have selected 118 composit ions on the basis of run time, melt composition and experimental techn ique. These data cover a range of temperatures 1150-1500 degrees C, ox ygen fugacities of -13<log f(O2)<-0.7, and bulk compositions ranging f rom basalt and norite, to komatiite. Six major spinel components with Cr3+, Al3+, Ti4+, Mg2+, Fe3+ and Fe2+-bearing end-members were conside red for the purpose of describing chromite saturation as a function of melt composition, temperature and oxygen fugacity at 1 atmosphere pre ssure (0.101 MPa). The empirically calibrated mineral-melt expression based on multiple linear regressions is: In K-i(Sp)=A/T(K)+B log f(O2) +C ln (Fe3+/Fe2+)(L)+D ln R(L) + E, where K-i(Sp) is an equilibrium co nstant and R(L) is a melt structure-chemical parameter (MSCP). Twenty- eight forms of equilibrium constants were considered, including single distribution coefficients, exchange equilibrium constants, formation constants for AB(2)O(4) components, as well as simple ''spinel cation ratios'' For each form of the equilibrium constants, a set of 16 combi nations of the MSCPs have been investigated. The MSCP is present in th e form of composite ratios [e.g., Si/O, NBO/T,(Al+Si)/Si, or (Na+K)/Al ] or as simple cation ratios (e.g., Mg/Fe2+). For the calculation of F e3+ and Fe2+ species in silicate melts, we used existing equations, wh ereas the Fe3+/Fe2+ ratio of spinels was calculated from the spinel st oichiometry. The regression parameters that best repoduce the experime ntal data were for the following constants: (Fe3+/Fe2+)(Sp), (Mg/Fe2+) (Sp)/(Mg/Fe2+)(L), (Cr/Al)(Sp)/(Cr/Al)(L), K-FeCr2O4, and Ti-Sp/Ti-L. These expressions have been combined into a single program called SPIN MELT, which calculates chromite crystallization temperature and compos ition at a given f(O2) with an average accuracy of similar to 10 degre es C and 1-2 mol%. An example of the use of SPINMELT is presented for a magma parental to the Bushveld Complex.