Mathematics – Logic
Scientific paper
Oct 1964
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1964gecoa..28.1631e&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 28, Issue 10-11, pp.1631-1668
Mathematics
Logic
17
Scientific paper
Ten isofacial blueschist facies rocks were investigated from the K tu-Bizan District, eastern Shikoku, Japan, previously mapped by (1963). Twenty of the constituent minerals have been newly analysed, 17 by conventional chemical methods, and 3 by electron microprobe techniques. Combined with 15 previously determined wet chemical analyses of minerals from the same rocks, mineralogical data include a total of: 8 sodic amphiboles; 2 sodic pyroxenes; 9 white micas; 4 chlorites; 6 garnets; and 6 epidotes. Unit cell dimensions and optical properties for the newly analysed minerals have been determined, and available constants for all 35 minerals are tabulated. Chemographic phase relations indicate that the investigated rocks crystallized under physical conditions transitional between greenschist and blueschist facies, but within the latter facies. Intercalations of actinolite-bearing and sodic amphibole-bearing rocks probably result from differences in oxidation state, as demonstrated by graphic analysis; this conclusion agrees with an hypothesis previously proposed by . Element partitioning among coexisting phases has been investigated; these are the lowest-grade metamorphic rocks for which such a study has been undertaken up to the present time. Aluminum, iron, magnesium and manganese appear to show systematic distributions between the mineral pairs sodie amphibole-white mica, sodic amphibole-epidote, white mica-chlorite and white mica-epidote. Non-systematic partitioning of these elements between garnets and the other minerals may be explained in any of three ways: 1. (1) at least some of the garnets are metastable in the present phase assemblages 2. (2) manganese in garnet exerts a profound influence on the partitioning of ferrous iron and magnesium 3. (3) some of the garnets have reached their Mg solubility limit. Petrographic relations support but do not prove the hypothesis of garnet metastability. Mn +++ appears to be diadochic with aluminum and ferric iron, Mn ++ with ferrous iron and magnesium in garnets and epidotes, but not in the other minerals studied. Distributions of manganese, calcium and alkalis show apparently systematic equilibrium relations, and pronounced fractionation. Chemical distributions of the elements are explicable on the basis of valencies, ionic radii and sizes of the various structural sites. The observed systematic partitioning of elements among coexisting minerals implies that K tu-Bizan blueschists achieved a close approach to chemical equilibrium during the metamorphism, at least in hand specimen sized volumes of rock.
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