The initial review of twenty randomly selected IPG Group I axe thin-sections revealed small differences in mineralogy that eventually led to the formation of the sub-groups described above. During the examination of these axe thin-sections it was felt a more systematic and quantitative approach to gauging the slight differences seen in thin-sections may prove beneficial, especially to less skilled petrologists. Therefore, an attempt to measure differences in thin-sections using an empirically based quantitative 'scoring' system was devised. This system is based upon observations of thin-section colour, grain size, degree of alteration, size and shape of opaque minerals and the presence of a number of readily identifiable minerals, and is described in detail below.
All IPG Group I thin-sections are various shades of green to the naked eye. The scores 1-9 indicate the darkness of the thin-section, with 1 representing a transparent, colourless thin-section, and 9 representing a near opaque, very dark green thin-section. A score of 10 indicates another predominant, non-green colour (encountered mainly in rock rather than axe thin-sections). Initially, a small number of Group I axe thin-sections (20) were arranged in a row on a white background and divided into groups of visually equal colour, with the largest population frequency (mode) given a score of 5. All remaining thin-sections were then assigned a colour score using the initial sorting as a colour template.
Noting that the original mineralogy had been altered to some degree and that the original grains may be obscured or altered, a simple visual approach to assessing grain size was adopted. Using a grain size card (supplied by GEO Supplies Ltd, Chapeltown, Sheffield), each thin-section was viewed with a x10 hand lens, and the best match between grain size displayed on the grain size card and the thin-section established. This measurement was then complemented by re-estimating the grain size using the polarising microscope at x40 and x100 magnification (4.0mm and 1.6mm field of view (fov) respectively). In reality, the average of the longest dimension of pyroxene grains, with their ubiquitous amphibolitic alteration, determined the thin-section grain size recorded since these grains (pseudomorphs) were the most visible evidence of original rock texture. Table 7 details the scores assigned to the grain sizes displayed on the grain size chart values. In general, the observed score for axes was around 6, i.e. an average grain size of 1mm.
A scoring system relating to the amount of alteration of the pyroxenes contained within the thin-section is detailed in Table 7. The concept behind this subjective scoring is that pyroxene, altered to some amount, is very common in IPG Group I axe thin-sections and in many greenstone rocks. The scoring is based on visually estimating the percentage of alteration of the pyroxene grains seen in the thin-section and the number of grains altered. The type of alteration (usually to amphibole/chlorite) is not recorded, nor is the grain size or nature of the grains (e.g. ophitic). The range of scores in Table 7 anticipates the potential use of this procedure on basic igneous rocks with little pyroxene alteration. Pyroxenes within most greenstone axe thin-sections were altered to such an extent that the norm is scored at 6-7; that is, generally less than 25% of the primary pyroxene remains unaltered in the thin-sections.
The scoring regime based on the progressive alteration of feldspar is detailed in Table 7. Feldspars found in IPG Group I and IPG Group III thin-sections were usually heavily altered to a very fine-grained mineral, probably sericite, and often had radiating bundles of bladed actinolite within the original crystal perimeter. The difficulties with identifying the alteration product(s) means that feldspar scores are related to the evidence for the original feldspar remaining and not the type of alteration. Further, it was often not possible to determine the actual variety of feldspar as the alteration clouded the assessment of twinning and other evidence (relief, twinning angles, etc.) so the table of scores relates to a visual estimate of the amount of unaltered feldspar found in the thin-section. It should be noted that secondary albite may have been incorrectly diagnosed as primary feldspar. The considerable alteration to feldspars within axe thin-sections means that the thin-section norm is 7-9; that is very little primary feldspar remains.
Accessory opaque minerals are present in every Group I and III thin-section. The scoring system, detailed in Table 7, is based upon the shape and size of the opaque grains and not the quantity or type. In practice the score was based on the larger grains seen under the microscope, as most thin-sections contained fine, <0.1mm, rounded grains associated with the alteration of pyroxene to chlorite/amphibole. The scores represent the wide range of opaque grain size and shape encountered. The most common scores for axes are 6 and 8; that is fine-medium grained, larger grains being elongate/ladder-like in appearance.
The fine-grained nature of the alteration of the Group I thin-sections made it very difficult to determine what type of amphibole or mica was present. Hence the simple scoring system is based upon the presence of primary (Score – 1) or secondary (Score – 2) amphibole/mica. Lack of amphibole/mica was scored as 0. In practice only one thin-section was found to contain primary magmatic amphibole (e.g. CUD003) and all thin-sections contained some form of secondary mica (usually chlorite and biotite), hence the value of this particular form of scoring in this situation is minimal. However, presence/absence of amphibole/mica is a good discriminatory factor and may prove useful in future research, hence its inclusion in the quantitative system.
A simple scoring system based on the lack of, probable, and certain evidence for, epidote has been used. The absence of epidote in the thin-section was scored as 0. Presence of very fine-grained, high-relief, cloudy masses seen within feldspar alteration were taken as probable evidence for the presence of epidote and scored 1. In some cases it is possible that sphene has been incorrectly identified as epidote (and vice versa), with the very fine grains making it difficult for positive identification. Distinct, often small (<0.2mm), grains clearly of epidote or clinozoisite scored 2 on the empirical 0, 1, 2 scale.
Apatite is an easily recognised accessory mineral, with distinctive hexagonal basal sections often clearly visible when surrounded by alteration products. The lack of apatite in thin-section was scored as 0, with the presence of small grains (<0.1mm) scoring 1 and larger grains (>0.1mm) scoring 2. Plentiful (visible throughout the thin-section), but fine-grained apatite also scored 2 on the empirical 0, 1, 2 scale.
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Last updated: Wed Jul 29 2009