A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites

Eric Quirico,Michèle Bourot-Denise,Christophe Robin,Gilles Montagnac,Pierre Beck

doi:10.1016/j.gca.2011.03.009

Abstract

The thermal history of a series of EH3 and EL3 chondrites has been investigated by studying the degree of structural order of the organic matter (OM) located and characterized in matrix areas by Raman micro-spectroscopy. By comparison with unequilibrated ordinary chondrites (UOCs) and CO and CV carbonaceous chondrites, the following petrologic types have been assigned to various E chondrites: Sahara 97096 and Allan Hills 84206: 3.1–3.4; Allan Hills 85170 and Parsa: 3.5; Allan Hills 85119: 3.7; Qingzhen, MacAlpine Hills 88136 and MacAlpine Hills 88184: 3.6–3.7. The petrologic type of Qingzhen is consistent with the abundance of the P3 noble gas component, a sensitive tracer of the grade of thermal metamorphism. The petrologic types are qualitatively consistent with the abundance of fine-grained matrix for the whole series. No significant effects of shock processes on the structure of OM were observed. However such processes certainly compete with thermal metamorphism and the possibility of an effect cannot be fully discarded, in particular in the less metamorphosed objects. The OM precursors accreted by the EH3 and EL3 parent bodies appear to be fairly similar to those of UOCs and CO and CV carbonaceous chondrites. Raman data however show some slight structural differences that could be partly accounted for by shock processes. The metamorphic history of EH3 and EL3 chondrites has often been described as complex, in particular regarding the combined action of shock and thermal metamorphism. Because OM maturity is mostly controlled by the temperature of peak metamorphism, it is possible to distinguish between the contributions of long duration thermal processes and that of shock processes. Comparison of the petrologic types with the closure temperatures previously derived from opaque mineral assemblages has revealed that the thermal history of EH3 and EL3 chondrites is consistent with a simple asteroidal onion shell model. Thermal metamorphism in enstatite chondrites appears to be fairly similar to that which takes place in other chondrite classes. The complex features recorded by mineralogy and petrology and widely reported in the literature appear to be mostly controlled by shock processes.

Highlights

Angrites are a rare group of achondrites notable for their critical silica undersaturation, lack of shock metamorphic features and ancient crystallization ages [1,2,3]
The grain has a d30Si-value of 20 ± 50& with an Al abundance of approximately 1.5% relative to Si and other light elements well below 1%
We present new highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance and 187Os/188Os data for low-Ti mare basalt meteorites, Apollo 12 samples, and diogenite meteorites to explore the late accretion histories of the Moon and asteroid (4) Vesta, the probable diogenite parent body

Summary

Introduction

Angrites are a rare group of achondrites notable for their critical silica undersaturation, lack of shock metamorphic features and ancient crystallization ages [1,2,3]. Along with reporting mineralogical diversity, we examine the scale of textural diversity using a new method where 8 x-ray maps are combined into a single image, allowing us to characterize distributions of specific subtypes of HEDs. Chondritic organic matter might have evolved on its parent body where subjected to shock metamorphism [1,2]. Bulk major and trace element compositions were measured on a very small fragment of the stone; subsequent analyses on more representative powders of paired stones lead us to present revised compositional data, which are much more consistent with the observed mineralogy and have significant implications. It has been suggested [2] that the name diogenite be expanded to include rocks containing >40 vol% olivine (previously termed olivine diogenites by us [3]), and even to related rocks composed of more than 90vol% olivine (of which there are at least five known examples [4]).

Objectives

Methods

Results

Discussion

Conclusion