Application of Olivine-Spinel Eqilibria to Extraterrestrial Igneous Systems

where YCr = atomic proportion of Cr/(Cr+Al) in spinel and KD =Al2O3/Al2O3 in wt. %. The Al-in-olivine geothermometer is calibrated at 1 bar, 1250 – 1450oC, FMQ + 1.3 to FMQ – 1.5, and predicts experimental temperatures within ± 20oC. Furthermore, experiments conducted at IW – 1 from [3] and 1GPa data from [4] do not display systematic deviation from the current calibrations (Fig. 1), indicating the Al-in-olivine geothermometer is applicable to planetary redox states and interior pressures. In similar fashion to many terrestrial ultramafic rock types (e.g., large igneous provinces, peridotites, primitive basalts), olivine and spinel ([Mg,Fe][Al,Cr]2O4) coexist in several extraterrestrial samples including: pallasites (interpreted to be samples from the core-mantle boundary of differentiated asteroids), martian olivine-phyric shergottites, and primitive lunar dunites and troctolites. Thus, the ubiquity of olivine-spinel-bearing igneous lithologies indicates the Alin-olivine geothermometer can be broadly applied to constrain early thermal evolution of igneous provinces and differentiated bodies throughout the solar system. In this abstract, we summarize the general application of olivine-spinel equilibria and Al-in-olivine geothermometry to extraterrestrial igneous systems and present preliminary Al-in-olivine data of main group pallasite Brahin 4859-1 as a case study. Al-in-Olivine Geothermometry & Olivine-Spinel Equilibria: Perhaps the most common basis for determining the early thermal history of igneous samples relies on the temperature dependent partitioning of Mg between olivine and melt since olivine is the primary crystallizing silicate phase of most mantle-derived melts at low pressure [e.g., 5-8]. Olivine-melt geothermometry therefore has the advantage of estimating the crystallization temperatures of olivine-bearing samples, which in turn can be used to infer mantle potential temperatures of the terrestrial planets and Moon [8]. However, olivine-melt geothermometry can yield conflicting temperatures depending on estimates and assumptions of melt fraction, pressure of crystallization, and choice of the olivine-melt Fe-Mg equilibrium exchange coefficient, or KD = [XFe/XMg] x [XMg/XFe] [e.g., 2,7,9-11]. For instance, olivine-melt geothermometry has been used both to argue for [10,11] and against [7,9] similar mantle potential temperatures below regions of intraplate volcanism and mid ocean ridges. Moreover, many natural olivine-bearing samples contain trace amounts of spinel, complicating estimates of liquidus temperatures obtained via olivine-melt equilibria alone [e.g., 1,2,12]. Spinel is commonly interpreted to be a near-liquidus phase in natural rocks (supported by several experimental studies), occurring as euhedral to anhedral phenocrysts and crystalline inclusions within host-olivine [e.g.,13,12-14]. Therefore, olivine-spinel geothermometry estimates a near-liquidus co-saturation temperature (commonly within ~10oC of the liquidus) [2], providing minimum constraints on the liquidus temperatures of mantle derived melts. Similar to olivine-melt equilibria however, early olivine-spinel geothermometers relied on the temperature dependent partitioning of Fe-Mg, and could easily be reset during sub-solidus re-equilibration due to the rapid inter-diffusion of Fe-Mg between olivine and spinel [1,15]. Al-in-olivine geothermometry is less susceptible to short-lived changes in temperature and sub-solidus reequilibration because the diffusion rates of Al are << FeMg [1,2]. T(C) = 10,000 0.512 + 0.873YCr − 0.91ln(KD ) − 273