More on the cooling history of angrite LEW 86010

Details More on the cooling history of angrite LEW 86010 More on the cooling history of angrite LEW 86010

Mar 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993lpi....24..967m&link_type=abstract

In Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 2: G-M p 967-968 (SEE N94-16173 03-91)

Mathematics

Logic

2

Cooling, Meteorites, Meteoritic Composition, Mineralogy, Olivine, Calcium, Diffusion Coefficient, Pyroxenes, Trace Elements

Scientific paper

Antarctic angrite LEW 86010 has many chemical and mineralogical characteristics which suggest it is closely related to Angra dos Reis (ADOR). However, these meteorites have had very different thermal histories. Olivines and pyroxenes in ADOR are nearly homogeneous, suggesting very slow cooling or extensive subsolidus equilibration. In contrast, LEW 86010 pyroxenes are extensively zoned in both major and trace elements, while olivines are nearly homogeneous, suggesting that cooling of this sample was slow enough to homogenize olivines, but too fast to homogenize pyroxenes. In an earlier study, we used Ca diffusion gradients associated with exsolution lamellae of kirsh-steinite in LEW 86010 olivines to estimate the cooling rate of this sample. We obtained cooling rates of a few x.01 deg C/yr, corresponding to a burial depth of approximately 102 m under solid rock (thermal diffusivity = 0.004 sq cm/s), or approximately m under regolith material (10-5 sq cm/s). Unfortunately, this calculation was very sensitive to the assumed temperature of initial exsolution, which we approximated from the estimated initial olivine bulk Ca content, and also to the extrapolated values for the Ca diffusion coefficient (DCa). This abstract reports new cooling rate estimates obtained using a refined approach that includes the initial temperature of exsolution as one of the fitted parameters. The purpose of our new calculations is (1) to reduce uncertainties in cooling rate due to uncertainties in bulk olivine Ca content and initial exsolution temperature, (2) to investigate the effects of using different values for DCa, and (3) to determine how well constrained the cooling rate is for any particular Ca diffusion coefficient.

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