Partial Melting and Explosive Volcanism on the Acapulcoite-Lodranite Parent Body

Details Partial Melting and Explosive Volcanism on the Acapulcoite-Lodranite Parent Body Partial Melting and Explosive Volcanism on the Acapulcoite-Lodranite Parent Body

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..393m&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 393

Mathematics

Logic

1

Acapulco, Acapulcoites, Krymka, Lodran, Lodranites, Monument Draw, Partial Melting, Volcanism

Scientific paper

Acapulcoites and lodranites represent a wide range of partial melt residues from a common parent body [1-3]. Acapulcoites experienced Fe,Ni-FeS eutectic melting, with no silicate partial melting. Fe,Ni-FeS melt formed veins hundreds of micrometers in length in Acapulco, and formed centimeter-sized veins in Monument Draw, suggesting greater melt mobility in the latter. In lodranites, both Fe,Ni-FeS and plagioclase-pyroxene (basaltic) partial melting occurred. Fe,Ni-FeS melt is absent in EET 84302, but plagioclase partial melt remains. Basaltic partial melt has migrated short distances in Y 8002, as indicated by the occurrence of interstitial plagioclase, and Fe,Ni-FeS and basaltic partial melts were removed from Lodran. The heat source for melting remains uncertain. Partial melting did not result from local (micrometer- scale) shock melting, and all minerals in acapulcoites and lodranites were heated to >=950 degrees C [1,2]. Indeed, nonimpact heating of Krymka to 900 degrees C produced similar Fe,Ni-FeS veins as observed in Acapulco [4]. We cannot unequivocally eliminate impact heating and subsequent annealing of the entire parent body, but we consider this unlikely: Impact heats and melts only small portions of target rocks, while most of the energy is dissipated through brecciation and ejection of rock, yet only a few lodranites and no acapulcoites show shock effects (including silicate melt veins, which would not be readily annealed), and none are brecciated. There are no meteorites in the world's collections that formed by crystallization of the removed partial melts. It is possible that this results from poor sampling. However, wide ranges of Fa contents, oxygen isotopic compositions, and peak temperatures among acapulcoites-lodranites suggest that we have samples from widely separated areas in this body, and that poor sampling is not likely to be responsible for the lack of these rocks. Instead, we argue that partial melts were removed from the acapulcoite-lodranite body and lost into space 4.5 b.y. ago by explosive volcanism, which can occur on bodies <100 km in radius if a few hundred parts per million of volatiles are present in the melt [5-7]. Our measurements of Acapulco and Lodran confirm that Lodran is more highly depleted in volatiles relative to most unequilibrated ordinary chondrites than is Acapulco, consistent with the suggestion that partial melt-gas mixtures were removed from Lodran by explosive volcanism. Furthermore, although the size of the acapulcoite- lodranite parent body is poorly constrained, cooling rates for these meteorites of tens of degrees C/m.y. [8] are consistent with a relatively small body, allowing for removal of partial melts by explosive volcanism. Petrologic evidence is consistent with models of melt migration in asteroidal- sized parent bodies [6]. In Acapulco, <1% partial melting of the Fe,Ni-FeS eutectic created pressures of ~10 MPa in liquid pockets, forming veins comparable in length to the grain size (~200 micrometers). In Monument Draw, somewhat higher degrees of Fe,Ni-FeS melting (~3%) resulted in higher pressures (~40 MPa) and longer dikes (~25 cm). In neither case would these melts reach the parent body surface. In EET 84302, total Fe,Ni-FeS eutectic melting (~4-5%) caused melt to migrate along cracks and escape. With ~8% melting in Yamato 8002, melt pressures of ~100 MPa caused melt-pocket growth, creating dike pathways. In Lodran, complete melting of the Fe,Ni-FeS eutectic and plagioclase (~15-20 %) formed pathways that allowed melt migration to the parent body surface and removal by explosive volcanism. Melt migration is complex due to the presence of two immiscible liquids (Fe,Ni-FeS and basaltic) and exsolved gas. We are continuing efforts to model this process. References: [1] McCoy et al. (1992) LPS XXIII, 871. [2] McCoy et al. (1992) Meteoritics, 27, 258. [3] McCoy et al. (1993) LPS XXIV, 945. [4] McSween et al. (1978) Proc. LPSC 9th, 1437. [5] Wilson and Keil (1991) EPSL, 104, 505. [6] Keil and Wilson (1993) EPSL, in press. [7] Muenow et al. (1992) GCA, 56, 4267. [8] Bogard et al. (1993) LPS XXIV, 141.

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