Compositional zoning in sodic amphiboles from the blueschist facies

Abstract

SUMMARY. The sodic amphiboles possess two independent chemical substitution series (Fe 3 +-A1 and Fe z + Mg) that combine to provide a 'plane' of compositions. Yet at no single T and P are compositions covering the whole plane stable: (i) pure riebeckite exists under low-P conditions but breaks down in normal blueschists to give deerite; (ii) ferro-glaucophane is in competition at all except the lowest blneschist temperatures with almandine garnet; (iii) magnesio-riebeckite is stable at high-T and low-P but within the blueschist facies is replaced by the alternative higher density aegirine-talc assemblage; and (iv) glaucophane is stable only at high-P.At higher T and P than those of the blueschists, competition from NaCa pyroxenes, garnets, and deerite first erodes, and then removes, nearly all sodic amphibole compositions. At low-P the normal sodic amphiboleforming reactions from stilpnomelane and chlorite (in the presence of iron oxides, albite, etc.) produce an initial 'riebeckitic' amphibole that subsequently becomes more glaucophanitic with increasing P. Under certain conditions, perhaps connected with hydrous fluid overpressures, these reactions become transposed such that crossitic compositions become replaced whilst ferroglaucophane to glancophane compositions remain stable.

Subduction initiation and early evolution of the Easton metamorphic suite, northwest Cascades, Washington

“…DR5 [see footnote 1]). All metamorphic temperatures from the greenschist/blueschist are similar within uncertainty, although the highest average temperature was obtained from the S 1 G fabric, consistent with the presence of riebeckite cores in glaucophane, which are interpreted to result from the growth of riebeckite early at higher-temperature and higher-pressure conditions, and glaucophane later at lower temperature and pressure (Brown, 1974;Wood, 1980).…”

Geochemistry and Origin of Banded Ore of Manganese Belt, Eastern Carpathians, Romania.

“…The minerals of this manganiferrous lithology are: quartz, brown Mn-aegirine-augite, blue alkali Mn-Mg-riebeckite, yellow alkali Mg-Liamphibole (ferrighoseite), microcline, albite, spessartine-almandine, magnetite, and hematite. The forming conditions of this rock, P>9Kb and T <450 0 C, has been evaluated from experimental Jadeite /Glaucophane-Aegirine/Mg-riebeckite diagram (Wood, 1980) and from the Jd-Ae-Q diagram (Brown & Ghent, 1983), based on the jadeite component of aegirine, and that of glaucophane component of Mg-riebeckite. The paragenesis aegirine-magnesioriebeckite is newly formed on older one.…”