Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.282r&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 282
Mathematics
Logic
Scientific paper
Recent petrologic, chemical, and isotopic analyses of mesosiderite clasts and whole rocks allow construction of the evolutionary history of the mesosiderite parent body (MPB) as shown in the figure. I. Initial Melting ~4.56 Ga Ago. Numerous chronologic measurements indicate that initial melting of the MPB occurred shortly after accretion, at about the same time that the eucrites and angrites formed. For example, a zircon grain from a basaltic clast in Vaca Muerta yielded a Pb-Pb age of 4.563 Ga (Ireland and Wlotzka, 1992). The 4.56-Ga event on the MPB is probably responsible for forming the monogenic basaltic clasts that compositionally resemble typical eucrites (Rubin and Mittlefehldt, 1992). II. Crustal Remelting ~4.47 Ga Ago. Internal Sm-Nd isochron ages of 4.47 Ga have been determined for gabbroic pebble 12 from Vaca Muerta and a basaltic clast from Mt. Padbury (Stewart et al., 1991). Pebble 12 is one of many clasts in mesosiderites that are highly depleted in incompatible elements and probably represent remelted crustal rocks (Rubin and Mittlefehldt, 1992). Because variation of Fe/Mn with Fe/Mg in pyroxenes from these clasts follows a trend defined by FeO reduction from a melt (Mittlefehldt, 1990), and because the reducing agents (e.g., P) were likely to have been introduced during metal-silicate mixing, the age of pebble 12 indicates that metal-silicate mixing had to have occurred >=4.47 Ga ago. III. Localized Impact Melting 4.5-3.9 Ga Ago. Localized impact melting and metamorphism were widespread on the eucrite parent body as indicated by many ages that were partially or totally reset <4.56 Ga ago. Impact melt material is common in mesosiderites of all petrologic types, and ages between 4.5-3.9 Ga are preserved. Quench-textured pebble 5 from Vaca Muerta, which is likely to have formed from a localized impact melt of material resembling a cumulate eucrite (Rubin and Jerde, 1987), has an internal isochron age of 4.42 Ga (Stewart et al., 1992). The high temperature of these localized events places them before the 3.9-Ga event (below); however, because no extensive crustal remelting is evident, the metamorphism must have occurred after 4.47 Ga ago. The mesosiderite breccias were assembled during this phase of MPB history. IV. Collisional Disruption and Reassembly ~3.9 Ga Ago. Mesosiderites were degassed by a major thermal event ~3.9 Ga ago that heated the rocks to ~500 degrees C; this event may mark collisional disruption and gravitational reassembly of the MPB (Bogard et al., 1990). The surface breccias were deeply buried at this time and heated somewhat above the Fe-Ni solvus, resetting the metallographic cooling rates to ~1 degree C/Ma. This event may also account for some of the reported disturbances in the Rb- Sr, Sm-Nd, and Pb-Pb ages of a few mesosiderites. V. Impact Excavation and Ejection <<3.9 Ga Ago. Mesosiderites have cosmic ray exposure ages ranging from ~10-150 Ma, which represent the epoch when individual mesosiderites were excavated from the MPB or from a >=10-m-size MPB fragment. References: Bogard D.D., Garrison D.H., Jordan J.L. and Mittlefehldt D. (1990) Geochim. Cosmochim. Acta 54, 2549-2564. Ireland T.R. and Wlotzka F. (1992) Earth Planet. Sci. Lett. 109, 1-10. Mittlefehldt D.W. (1990) Geochim. Cosmochim. Acta 54, 1165-1173. Rubin A.E. and Jerde E.A. (1987) Earth Planet. Sci. Lett. 84, 1-14. Rubin A.E. and Mittlefehldt D.W. (1992) Geochim. Cosmochim. Acta 56, 827-840. Stewart B.W., Cheng Q.C., Papanastassiou D.A. and Wasserburg G.J. (1991) Lunar Planet. Sci. (abstract) 22, 1333. Stewart B.W., Papanastassiou D.A. and Wasserburg G.J. (1992) Lunar Planet. Sci. (abstract) 23, 1365.
Mittlefehldt Dave W.
Rubin Alan E.
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