An experimental study of partial melting and fractional crystallization on the HED parent body.
Ashcroft, H. O. and Wood, B. J.
Meteoritics & Planetary Science. doi: 10.1111/maps.12556
We have performed an experimental and modeling study of the partial melting behavior of the HED parent body and of the fractional crystallization of liquids derived from its mantle. We estimated the mantle composition by assuming chondritic ratios of refractory lithophile elements, adjusting the Mg# and core size to match the density and moment of inertia of Vesta, and the compositions of Mg-rich olivines found in diogenites. The liquidus of a mantle with Mg# (=100*[Mg/(Mg+Fe)]) 80 is ~1625 °C and, under equilibrium conditions, the melt crystallizes olivine alone until it is joined by orthopyroxene at 1350 °C. We synthesized the melt from our 1350 °C experiment and simulated its fractional crystallization path. Orthopyroxene crystallizes until it is replaced by pigeonite at 1200 °C. Liquids become eucritic and crystal assemblages resemble diogenites below 1250 °C. MELTS correctly predicts the olivine liquidus but overestimates the orthopyroxene liquidus by ~70 °C. Predicted melt compositions are in reasonable agreement with those generated experimentally. We used MELTS to determine that the range of mantle compositions that can produce eucritic liquids and diogenitic solids in a magma ocean model is Mg# 75–80 (with chondritic ratios of refractory elements). A mantle with Mg# ~ 70 can produce eucrites and diogenites through sequential partial melting.”
We find that a HED parent body with refractory elements (including Fe, Mg, and Si) in chondritic proportions results in a model consistent with the density and moment of inertia of Vesta if the mantle has an Mg# of about 80. In this case, the core would be 15–20% of the mass of Vesta and the mantle would contain olivine of approximately the same Mg# as the most Mg-rich diogenites. The liquidus temperature of the mantle is ~1625 °C with olivine being the sole precipitating phase in the interval 1625–1350 °C. Separation of the mantle melt at 1350 °C, the point at which orthopyroxene appears, simulates equilibrium partial melting of the mantle and ensures that the liquid line of descent is dominated by pyroxene fractionation. Under these conditions, 45% equilibrium partial melting of the mantle leads to a melt which, during fractional crystallization, produces eucritic liquids and diogenitic solid assemblages at temperatures below ~1300 °C. We thus find that our mantle composition is in the acceptable range for the generation of the HED meteorites by equilibrium partial melting followed by fractional crystallization of the segregated melt (Righter and Drake 1997).
We found that the MELTS program performs well at predicting the olivine liquidus temperature, overestimates the orthopyroxene liquidus by ~70 °C, and predicts melt compositions in good agreement with those observed at any given MgO content. Given its utility, we used the MELTS program to investigate the range of mantle compositions which can lead to eucritic melt compositions and which precipitate pyroxenes of diogenitic composition. We find that, given the assumption of chondritic ratios of refractory lithophile elements, the range of mantle Mg#s which generate eucrites and diogenites by the two-stage process is 75–80. A Monte Carlo approach suggests that small variations (±10% relative to their major element abundance) in CaO, MgO, and Al2O3 content of the HED parent body mantle remain consistent with generation of eucrites (liquids) and diogenites but that the SiO2 content of the mantle must be greater than 43 wt% in order to generate orthopyroxene during fractional crystallization of the segregated partial melts. Although the magma ocean model generates a good approximation to the compositions of eucrites and diogenites by equilibrium partial melting followed by fractional crystallization, it is apparent that the range of solid compositions requires more complexity of process. Incomplete re-equilibration, re-melting of cumulates, and differential melt segregation during cooling are probably required to match the observed compositional ranges in terms of incompatible trace elements.”