Contact metamorphism on 4 Vesta and the Petersburg polymict eucrite

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Using equations for thermal diffusion, we model the temperature profiles in metamorphic aureoles surrounding 1km and 3km diameter spherical intrusions containing eucrite and diogenite magmas in the crust of 4 Vesta. The aureoles created by thermal diffusion experience temperatures significantly elevated above the regional thermal gradient for 10^3 to 10^4years, which is comparable to the period of time required for the intrusions to crystallize. Temperatures of wall rock adjacent to a magma body intruded at mid-crustal levels can approach those in the lower part of the crust. In contrast, it has previously been suggested that thermal processes and, by implication, regional metamorphism could have affected 4 Vesta for a period of 106-108years. Heating of underlying material by a lava flow or layer of impact melt of moderate thickness would have lasted for a period of 1-10years. In an effort to obtain approximate cooling rates, we numerically fit theoretical diffusion profiles to the observed compositional profile of a 30-μm-wide Fe-rich alteration rim at the edge of a fragment of diogenitic orthopyroxene in the Petersburg polymict eucrite, which was affected by late-stage metamorphism. Using two sets of diffusion coefficients that are similar at 800°-1000°C , but very different at lower temperatures (e.g., 500°C ), the calculated diffusion profiles that most closely fit the compositional profile suggest cooling rates ranging from 0.12°C /yr (a period of 4000years) to 0.25°C /yr (2000years). These cooling rates are very similar to those expected for contact metamorphism, and, hence, the Petersburg meteorite may represent a polymict eucrite breccia derived from near an intrusive magma body. However, this interpretation is not unique as this alteration profile might also have been formed by regional metamorphism at low temperatures.

Contact metamorphism on 4 Vesta and the Petersburg polymict eucrite

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