Cooling Rates of Y980459 and Dag476 Shergottites on the Basis of Fe-mg Zoning of Olivine

Introduction: We have developed a model to calculate the cooling rate (or burial depth) by using the Fe-Mg chemical zoning profile of olivine, considering diffusional modification of zoning profiles as crystals grow by fractional crystallization from a melt [e.g., 1]. We applied the model to calculating cooling rates of olivines in the Semarkona (LL3.00) chondrite [2]. In this abstract, we applied the model to calculating the cooling rate of martian meteorites. Yamato (Y) 980459 is an olivine-phyric shergottite that consists of olivine megacryst with groundmass composed of olivine, pyroxene and glassy mesostasis, and no plagioclase [e.g., 3]. The core compositions of olivine and pyroxene are the most mafic among martian meteorites found so far. Dar al Gani (DaG) 476 is an olivine-phyric shergottite that consists of olivine megacrysts with the groundmass composed of pyrox-ene and maskelynite [e.g., 4]. Koizumi [5] pointed out that both shergottites might have been derived from the same melt. We assumed that a single and linear cooling for these shergottites and calculated their cooling rates by employing Fe-Mg zoning profiles of olivine megacrysts. Model: Crystallization of olivine starts at temperature T S and ends at temperature T E as temperature decreases (Fig. 1). We assume that the olivine crystal is a sphere, and that olivine crystal growth is parabolic [e.g., 6], that is, dR/dt is proportional to 1/R, where R is the radius of crystal and t is time. Fe-Mg zoning in olivine basically formed by fractional crystallization. We calculated an Fe-Mg zoning profile (primary zoning profile) by using the Rayleigh equation for closed-system fractional crystallization. The equation is C L /C 0 = F KD-1 , where C 0 and C L are the initial concentration in the bulk liquid and the concentration in the observed liquid, respectively. K D is the distribution coefficient, and F is the fraction of liquid remaining. We used the distribution coefficient for Fe/Mg of 0.30 [e.g., 7]. For the cooling rate calculation, we assume that cooling is linear (continuous) from starting temperature (T S) to closure temperature (T F) at which Fe-Mg interdiffusion essentially ceases considering the cooling rate. T F is lower than ending temperature (T E). Diffusional modification also starts at T S , which is starting time of crystallization, and ends at closure temperature (T F). As the crystal grows, Fe-Mg zoning forms in accordance with the Rayleigh equation for closed-system fractional crystallization, and diffusional