Sm-Nd chronology and petrogenesis of mesosiderites

Abstract

We have obtained Sm-Nd data from four mesosiderite silicate clasts, including three clasts with a variety of textures from the Vaca Muerta type 1A mesosiderite and one gabbroic clast from the Mt. Padbury mesosiderite. The gabbroic Vaca Muerta Pebble 12 and basaltic Pebble 16 yield identical ^(147)Sm- ^(143)Nd ages of 4.48 ± 0.19 AE and 4.48 ± 0.09 AE, 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. All clasts show a correlation of ^(142)Nd /^(144)Nd with ^(147)Sm /^(144)Nd , and provide unequivocal evidence for the presence of live ^(146)Sm at the time of their formation. Calculated initial ^(146)Sm /^(144)Sm values range from 0.004 (Pebble 5) to 0.006 (Pebbles 12, 16, and Mt. Padbury) and are generally consistent with the ^(147)Sm- ^(143)Nd ages. However, discordance of whole-rock leach and residue data and some disagreement between ^(146)Sm- ^(142)Nd relative ages and ^(147)Sm- ^(143)Nd absolute ages indicate small but significant disturbances to the Sm-Nd systematics. The ranges of ages and initial ^(146)Sm /^(144)Sm and ^(143)Nd /^(144)Nd values suggest that each of these silicate clasts underwent a separate, protracted evolution on its parent body prior to mixing with metal. Textural and trace element criteria indicate that individual clasts from the same mesosiderite 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, whereas Pebbles 5 and 16 and Mt. Padbury have nearly chondritic Sm/Nd with bulk REE concentrations higher than chondrites by factors of 5 to 15. In general, the Sm-Nd systematics of mesosiderite silicates require formation of the silicates on a parent planet which underwent relatively early and extreme differentiation. Preservation of diverse, old ages and the presence of ^(146)Sm imply that metal-silicate mixing did not seriously alter the Sm-Nd isotopic memories of these clasts. We present a model for metal-silicate mixing which combines the cooling history with isotopic reequilibration for the case of thermal blanketing. We show that the total amount of isotopic reequilibration in a sample can be related to the initial temperature, depth of burial, grain size, and diffusion parameters. Application of this model to the silicate clasts 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 in 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 temperatures determined from studies of exsolution in the metal phase, we infer that heat was transferred rapidly from hot metal to cold silicate material during initial metal-silicate mixing, and that the deeply buried portions of the mixture then cooled slowly after reaching thermal equilibrium at ~600-700°C. The data from this study point to the following history for the mesosiderite parent body: (1) differentiation of a silicate parent body within the first ~50 m.y. of solar system history to form diverse parent magmas; (2) emplacement of primitive and differentiated mafic magmas near the planetary surface, and extensive differentiation in the near surface environment; (3) formation and reworking of regolith breccias through impact 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.y. after the origin of the solar system; (5) slow cooling from temperatures of ~700°C to produce the observed nickel diffusion profiles in the iron phase; (6) mild impact metamorphism and brecciation to obtain the much younger K-Ar ages; and (7) recent collisions or perturbations sending mesosiderite fragments into Earth-crossing trajectories.

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