SM-ND CHRONOLOGY AND PETROGENESIS OF MESOSIDERITES

We have obtained Sm-Nd data from four mesosiderite silicate clasts, in cluding three clasts with a variety of textures from the Vaca Muerta t ype 1A mesosiderite and one gabbroic clast from the Mt. Padbury mesosi derite. The gabbroic Vaca Muerta Pebble 12 and basaltic Pebble 16 yiel d identical Sm-147-Nd-143 ages of 4.48 +/- 0.19 AE and 4.48 +/- 0.09 A E, respectively, while the highly recrystallized Pebble 5 gives an age of 4.42 +/- 0.02 AE; Mt. Padbury yields an age of 4.52 +/- 0.04 AE. A ll clasts show a correlation of Nd-142/Nd-144 with Sm-147/Nd-144, and provide unequivocal evidence for the presence of live Sm-146 at the ti me of their formation. Calculated initial Sm-146/Sm-144 values range f rom 0.004 (Pebble 5) to 0.006 (Pebbles 12, 16, and Mt. Padbury) and ar e generally consistent with the Sm-147-Nd-143 ages. However, discordan ce of whole-rock leach and residue data and some disagreement between Sm-146-Nd-142 relative ages and Sm-147-Nd-143 absolute ages indicate s mall but significant disturbances to the Sm-Nd systematics. The ranges of ages and initial Sm-146/Sm-144 and Nd-143/Nd-144 values suggest th at each of these silicate clasts underwent a separate, protracted evol ution on its parent body prior to mixing with metal. Textural and trac e element criteria indicate that individual clasts from the same mesos iderite often had very different igneous sources and thermal histories prior to their incorporation in the meteorite. Pebble 12 is extremely LREE depleted, probably a result of several melt extraction events, w hereas Pebbles 5 and 16 and Mt. Padbury have nearly chondritic Sm/Nd w ith bulk REE concentrations higher than chondrites by factors of 5 to 15. In general, the Sm-Nd systematics of mesosiderite silicates requir e formation of the silicates on a parent planet which underwent relati vely early and extreme differentiation. Preservation of diverse, old a ges and the presence of Sm-146 imply that metal-silicate mixing did no t seriously alter the Sm-Nd isotopic memories of these clasts. We pres ent a model for metal-silicate mixing which combines the cooling histo ry with isotopic reequilibration for the case of thermal blanketing. W e show that the total amount of isotopic reequilibration in a sample c an be related to the initial temperature, depth of burial, grain size. and diffusion parameters. Application of this model to the silicate c lasts measured in this study indicates that if the metal and silicate were thermally equilibrated above the metal solidus temperature during mixing, then the clasts must have been buried no deeper than 1-10 m i n regolith during the initial high-temperature cooling phase in order to prevent the Sm-Nd systems from being extensively reset. In order to reconcile these results with the slow cooling rates at lower temperat ures determined from studies of exsolution in the metal phase, we infe r that heat was transfer-red rapidly from hot metal to cold silicate m aterial during initial metal-silicate mixing, and that the deeply buri ed portions of the mixture then cooled slowly after reaching thermal e quilibrium at approximately 600-700-degrees-C. The data from this stud y point to the following history for the mesosiderite parent body: (1) differentiation of a silicate parent body within the first approximat ely 50 m.y. of solar system history to form diverse parent magmas; (2) emplacement of primitive and differentiated mafic magmas near the pla netary surface, and extensive differentiation in the near surface envi ronment; (3) formation and reworking of regolith breccias through impa ct gardening at the near surface of the body; (4) mixing of molten Fe- Ni metal with the regolith followed by rapid cooling 100-150 m.v. afte r the origin of the solar system; (5) slow cooling from temperatures o f approximately 700-degrees-C to produce the observed nickel diffusion profiles in the iron phase; (6) mild impact metamorphism and brecciat ion to obtain the much younger K-Ar ages; and (7) recent collisions or perturbations sending mesosiderite fragments into Earth-crossing traj ectories.