Compositional differences between basaltic and gabbroic clasts in mesosiderites

Abstract

Two gabbroic pebbles and one pebble with basaltic textural affinities from the Vaca Muerta mesosiderite were analyzed by instrumental neutron activation analysis and electron microprobe. These analyses plus literature data reveal remarkable compositional differences between basaltic and gabbroic clasts in mesosiderites. On average, basaltic clasts have small (15%) depletions in light relative to heavy rare earth elements (REEs) and moderate positive Eu anomalies (i.e., H-chondrite-normalized La/Lu= 0.85; Eu/Sm= 1.30). The mean La/Lu and Eu/Sm abundance ratios of gabbroic clasts are at least 3.3 × smaller and 28 × greater, respectively. Intense meteoroid bombardment probably persisted until after the mesosiderite parent body had differentiated. Near-surface mixtures of major amounts of basalt and lesser amounts of cumulate eucritic rocks were essentially totally melted and rapidly cooled, accounting for the moderately fractionated REE patterns of most basaltic clasts. At greater depths, impact-heating partially melted cumulate gabbros; incompatible-element-rich eutectic liquids drained away, creating large REE fractionations in the residual rocks. These rocks cooled slowly and developed gabbroic textures.