Mathematics – Logic
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996lpi....27..839m&link_type=abstract
Lunar and Planetary Science, volume 27, page 839
Mathematics
Logic
2
Cooling History, Crystallization, Experiments, Meteorites: Snc
Scientific paper
The SNC meteorites have been the subject of intense interest owing to their likely Martian origin. While numerous petrologic studies have been conducted, experimental studies of their crystallization are sparse. Petrographic studies and partial-melting experiments were conducted using Zagami and Shergotty. The partial-melting experiments produced pyroxenes of compositions comparable to the Mg-rich cores of natural shergottite pyroxenes ~1140degrees C, crystallizing ~45% pyroxene, suggesting that shergottites were cumulates. Recent petrologic studies of Zagami measured Mg-rich core abundances of 20 +/- 5%, far below the inferred 45%. These studies inferred widely different crystallization histories. Atwo-stage magmatic history with Mg-rich pyroxene crystallization in a deep-seated (1-2 kb) magma chamber followed by eruption of a thick, phenocryst-bearing lava flow and cooling at ~0.1-0.01 degrees C/hr was favored by McCoy and Brearley, while a single-stage history cooling at 5-20 degrees C/hr and undercooling was supported by Treiman and Sutton. The difference in Mg-rich pyroxene abundances, the possibility of a complex pressure-cooling history, and the lack of dynamic crystallization experiments to constrain the cooling history of Zagami all suggested that a new experimental study was needed. Our equilibrium crystallization experiments are similar, producing ~38% core-composition pyroxene at 1150 degrees C. Differences in pyroxene abundances are not due to experimental error (e.g., fO2), but reflect differences in pressure and/or composition between the experiments and natural samples. Our dynamic crystallization experiments do not produce Zagami-like textures at cooling rates => 5 degrees C/hr. Textures are dependent on both nuclei density and cooling rate. Slower cooling rates (<= 2 degrees C/hr) and relatively few nuclei produce textures more Zagami-like, but pyroxene zoning and mesostases texture are unlike Zagami. Zagami probably formed during slower cooling (< 0.5 degree C/hr), and perhaps with a multi-stage history.
Lofgren Gary E.
McCoy Timothy James
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