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Journal of Petrology Volume 42 Number 11 Pages 2173-2174 2001
© Oxford University Press 2001
Reply to Comment by Kerr and Arndt
SCHOOL OF OCEAN AND EARTH SCIENCE, UNIVERSITY OF SOUTHAMPTON, SOUTHAMPTON SO14 3ZH, UK
Received June 27, 2001; Revised typescript accepted June 27, 2001
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INTRODUCTION |
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 TOP INTRODUCTION REFERENCES |
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The discussion by Kerr & Arndt (2001)
on the problem of producing a workable classification for the high-Mg rocks is most welcome, and I am glad of the opportunity of debate, as there most certainly are two sides to the argument. They advocate a path of classification that emphasizes textural features, here the spinifex texture. The majority of the Subcommission took the view that for volcanic rocks, in which the mineralogical mode could not be estimated, chemical composition was the next best property to consider, with textural features a further consideration. The Subcommission also took the view that one aspect of the high-Mg rocks was the distinction of komatiite from picrite (the meimechites and picrobasalts need not concern us here). The 1989 IUGS classification had both komatiite and picrite with MgO >18 wt %, leaving a field picrobasalt for most of the ultrabasic rocks with <18 wt % MgO. This proved unsatisfactory and the proposed contrast in alkali contents unhelpful, so the Subcommission felt obliged to reconsider its earlier conclusions. Kerr & Arndt focus their discussion concerning komatiite on the spinifex texture, and the Gorgona example usefully makes evident the problems faced by the Subcommission. Which is more important, a texture that indicates the conditions of crystallization, or the chemical composition, which relates to the chemistry of the parental magma? This begs the more fundamental question: should two rocks produced from two chemically identical magmas be considered as different rocks if they crystallized under different conditions? Surely the same initial magma should be treated as of more significance than any difference in the subsequent textures produced. Also, why should the spinifex texture be solely restricted to komatiites? May not the conditions necessary to produce the spinifex texture occur in other magmas, and is not that useful information? Perhaps it is significant that at Gorgona many of the spinifex-textured rocks are picrites whereas in Precambrian cases they are komatiites.
Although the Subcommission considers that the classification of komatiite is best based on its chemistry (Le Bas, 2000), in the second edition of Le Maitre et al. (in press), as in the 1989 first edition, the common occurrence of the spinifex texture is acknowledged in the ‘Glossary of Terms’. It says that komatiites ‘usually display well-developed spinifex textures’, thus recognizing that the spinifex texture is highly rated in the identification of komatiite. It is unfortunate that Kerr & Arndt turned to Le Bas & Streckeisen (1991) for information on the classification of the high-Mg rocks. That paper was on the principles of classification and gave only a summary of the classifications and not the detail, which in this case was significant.
The example of the Curaçao flow brings in another problem, that of cumulates. Although rather hidden in the IUGS classification (Le Maitre et al., 1989, p. 29), it did include the significant statement that in considering the TAS classification ‘rocks that ... have undergone crystal accumulation should be used with caution’, acknowledging the difficulty these rocks impose. But it is assumed that the classification can contend with such rocks within reason; that disclaimer is regrettably necessary. However, most cumulate rocks are coarse grained, for which the mineral-based plutonic QAPF classification is appropriate. It should be pointed out here that in the second edition of Le Maitre et al. (in press), coarse grained is now defined as having >3 mm average grain size. Identifying a coarse-grained volcanic rock, formerly a contradiction in terms, should be welcomed as extending our understanding of volcanic processes.
This reply is clearly not going to satisfy Kerr & Arndt. But trying to improve the definition of one particular rock without considering the adjacent rocks in a classification scheme is not a good solution. It is not acceptable that it could be ‘sufficient simply to describe such rocks as "the basal olivine-rich parts of differentiated flows" or some such similar phrase’. And no solution is offered for the picrites. It is perhaps unfortunate that Kerr & Arndt refer to Thompson & Gibson (2000) as causing confusion when they classified some non-spinifex Gorgona rocks as komatiites, when all they did was to follow the latest IUGS recommendations. Is following recommendations confusing? Without nomenclature commissions operating through consultation and major consensus, the nomenclature in many branches of science would become a cacophony of individual views.
It could be that a lesson might be learnt here: that chemical analyses of igneous rocks should be accompanied by a petrographic description that allows the reader to assess the processes that contributed to its formation.
Twenty-five years ago, Streckeisen wrote a paper entitled ‘To each plutonic rock its proper name’ in Earth-Science Reviews. That philosophy should be applicable to all igneous rocks. If the profession can cope with ‘granites and granites’, it should be able to cope with ‘komatiites and komatiites’.
I am grateful to several colleagues in the Subcommission’s Working Group who assisted me in preparing this reply.
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FOOTNOTES |
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*E-mail: mjlb{at}soc.soton.ac.uk.
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REFERENCES |
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TOPINTRODUCTIONREFERENCES |
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Kerr, A. C. & Arndt, N. T. (2001). A note on the IUGS reclassification of the high-Mg and picritic volcanic rocks. Journal of Petrology 42, 2169–2171.
Le Bas, M. J. (2000). IUGS reclassification of the high-Mg and picritic volcanic rocks. Journal of Petrology 41, 1467–1470.
Le Bas, M. J. & Streckeisen, A. L. (1991). The IUGS systematics of igneous rocks. Journal of the Geological Society, London 148, 825–833.
Le Maitre et al. (1989). A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Oxford: Blackwell Scientific.
Streckeisen, A. L. (1976). To each plutonic rock its proper name. Earth-Science Reviews 12, 1–33.
Thompson, R. N. & Gibson, S.A. (2000). Transient high temperatures in mantle plume heads inferred from magnesian olivines in Phanerozoic picrites. Nature 407, 502–506.
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