SPINEL LHERZOLITE XENOLITHS, QUANTITATIVE CHARACTERIZATION OF TEXTURES

Abstract

Peridotite xenoliths brought to the surface by alkali basalt volcanism are thought to originate in and represent the Earth’s upper mantle. Both between various sites of occurrence and also within a given site, they can show a range of mineralogical grain size, shape, degree of crystallisation and arrangement that can be broadly described as “texture”. These textures have been the subject of interpretation based mainly on qualitative description. Whilst this is undoubtedly useful, the descriptions tend to reflect the experience and understanding of the words used by various workers. As a result a number of authors have proposed systems with increasingly complex nomenclature that, however, in the end cannot avoid the inherent uncertainty of the subjective nature of qualitative criteria and assessment. To address this problem methods for the physical measurement of textural features are being explored so that hypotheses can be stated and tested in terms of numbers. Rock thin-sections, whilst only providing areas and outline boundaries of the mineral grains in the plane cut through the rock by the section, offer the most immediately accessible objects for measurement that may be regarded as related to the overall texture. An outline structure of mineral identities (skeletonisation) provides a basis for these measurements. These measurements must be of sufficient number for derived parameters to have statistical significance. To this end a method has been devised for the scanning of whole or at least a large proportion of standard thin sections of mantle xenoliths, from different sites, to produce calibrated colour (RGB) white light and crossed polar images. These images were achieved using a NIKON Coolscan IV scanner together with a NIKON Medical Slide Holder FHG1 modified to take Polariod inserts. An electron microscope slot specimen support grid was attached to each area to be scanned to provide a physical dimension calibration reference. These images were then combined with the use of an illuminated panel to produce a traced outline of the constituent mineral identities according to a set of recognition rules. The traced outlines were then scanned to produce images that could be loaded into computer programs for the measurement of physical parameters. To date skeletal images have been prepared from sixteen xenolith examples from two sites. From some samples more than one skeletal image was prepared, to examine different areas of the sample and assess the effects of different preparation procedures. More images for xenolith samples from different site are in the process of preparation. The number of individual constituent grain areas assessed from the skeletal images prepared so far ranges from 218 for the coarsest example to a little over two and a half thousand with one of the smallest grained examples. For the examples examined so far, a plot of the mean grain area, as seen in the thin section, against the standard deviation of the areas measured, gives a linear relationship, with ranges for the coarse-grained protogranular, through porphyroclastic down to the small-grained equigranular examples. This then provides a numerical assessment by means of which samples from different studies may be compared. Histograms of the grain measurement distributions are also being examined, together with other parameters, which are being considered in relation to crystal size distribution and particulate process theories.