Textural properties of iron-rich phases in H ordinary chondrites and quantitative links to the degree of thermal metamorphism

Abstract

The textural characteristics of opaque iron-rich phases (kamacite–taenite and troilite) have been quantified in the eight H-chondrites (two H4, three H5 and three H6) that have been the subject of previous thermo-chronological studies. These samples are of interest as they have temperature–time paths during cooling that have been shown to be consistent with radiogenic heating by 26Al on a single parent-body, thus offering the possibility to quantitatively link textural characteristics to thermal history. In addition to these eight samples, two other H5 samples (Forest City & Misshof) and two primitive achondrites (Acapulco & Lodran) were studied for comparison. The textural characteristics measured include: (i) phase proportions, (ii) the length of metal-sulphide contacts, (iii) dihedral angle at contacts with silicate grains, (iv) grain shape and circularity, (v) grain size and size distributions. The absolute and relative proportions of metals and sulphides are found to be approximately constant in all studied H chondrites, consistent with evolution in a chemically closed system. With increasing degree of thermal metamorphism, H-chondrites are found to show evidence for separation of metal and sulphide phases, increasing grain circularity, increasing grain size, and modification of size distributions characterized by the elimination of small grains. Variations of these parameters are found to be almost identical for sulphides and metals suggesting similar growth mechanisms for these two phases. Furthermore, trends between samples place them consistently in the same order: Sainte Marguerite (H4), Forest Vale (H4), Nadiabondi (H5), Richardton (H5), Forest City (H5), Misshof (H5), Allegan (H5), Kernouvé (H6), Guareña (H6) and Estacado (H6). In all cases Acapulco and Lodran extend the trends observed among the H-chondrites. In general, it is found that characteristics requiring material transport over shorter length scales (i.e. within grains) show greater variation for low petrographic type (H4/H5) and reach textural equilibrium earlier in the sequence than characteristics which require transport over larger distances (i.e. between grains). In the latter case (e.g. slopes of crystal size distribution), variations are most marked for H6 samples, trends that are significantly extended by Acapulco and Lodran, highlighting the role of silicate melt on variations in textural properties. Crystal size distributions imply normal grain growth (NGG) for both metals and sulphides, possibly controlled by grain boundary migration of olivine and/or pyroxene. Comparison of these results with geochemically constrained thermal models of the H-chondrite parent body shows an excellent correlation between average crystal sizes, and inferred depths in the original parent body, consistent with expectations based on thermal modelling. This study highlights the potential of grain-size as a quantitative marker of the degree of thermal metamorphism, although further work on a wider set of samples will be required to explore the limits of this approach.