Ringwoodite Microstructures in L-chondrites Rc106 and Acfer040: Implicatoins for Transformation Mechanisms

1. Introduction Ringwooodite, produced by shock metamorphism, is common in and adjacent to melt veins in highly shocked chondrites. Although ringwoodite can crystallize from the silicate melt in the shock-vein or pockets, most of the easily observed ringwoodite in shock veins is formed by the transformation of olivine in host-rock fragments entrained in the melt or olivine along shock-vein margins. Ringwoodite is an important indicator of shock stage S6 [1] because it is commonly deep blue in color and therefore easily observed in thin section. The transformation of olivine to ringwoodite is important because the transformation kinetics can be used to constrain shock duration if one knows P-T conditions and transformation mechanisms. Here we examine the mi-crostructures in ringwoodites from L chondrites RC106 and Acfer040 to better understand transformation mechanisms of ringwoodite. Ringwoodite has been reported in many shocked chondrites since its discovery in the L chondrite Ten-ham nearly 40 years ago [2]. Observations of ring-woodite lamellae in partially transformed olivine in S6 chondrites, Sixiangkou, Yamato791384, and Tenham [3-6] illustrate that the transformation mechanisms commonly include heterogeneous nucleation of ring-woodite along defects in olivine. The ringwoodite la-mellae in Sixiangkou were interpreted [3] to have formed by the same coherent intracrystalline transformation mechanism observed in experimentally transformed samples by Kerschhofer et al. [7-9]. The kinetic data of Kerschhofer et al. [7-9] were used to infer a shock duration from about several seconds (at 1500°C-1700°C). In 2006, Chen et al. [10] reinter-preted the mechanism as incoherent growth, and used Fe-Mg inter-diffusion data to infer an unrealistic shock duration of several minutes (at 1100 °C). A recent report of ringwoodite rims on wadsleyite cores in Peace River [11] have been used to argue that olivine actually melts during its transformation to ringwoodite. With so many possible transformation mechanisms , it is important to examine the ringwoodite mi-crostructures in a variety of samples to better understand the range of transformation mechanism. Here we report new polarizing light and electron microscopy as well as Raman spectroscopy results on ringwoodite in RC106 and Acfer 040.