The composition of mineral phases and their modal proportions have been determined for three representative Eoalpine eclogites from the Saualpe type locality/Eastern Austria (sample SKP31) and the Pohorje Massif/Slovenia (CM31/03 and CM15/01) using electron microprobe, laser ICP-MS, IR spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole+quartz inclusions (COIs) in omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as H2O carriers in these eclogites. For omphacites in CM31/03 with a zonal distribution of COIs, a comparison of water and trace element concentrations of areas containing COIs and those free of COIs and a comparison with the trace element concentration of calcic amphibole indicate that COIs have formed through an open-system alteration of clinopyroxene and not through a closed system exsolution process. In sample SKP31, both textural and mineral chemical evidence suggests that COIs did not form by exsolution involving a Ca-Eskola component in clinopyroxene but formed by progressive growth under eclogite-facies P-T conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz-calcic amphibole grains in the matrix. Bulk H2O contents of the eclogites are between ca. 750 and 2150 ppm with 6-25% of the total water contributed by nominally anhydrous minerals (NAMs). Because of high modal amounts of 37-65%, omphacite is the major nominally anhydrous water carrier, containing 145-580 ppm H2O with significant concentration variations on a thin section scale. Due to their very low H2O concentrations of b5-10 ppm (garnet, kyanite) or insignificant modal amounts <=3% (rutile) the remaining NAMs contribute less than 1.5% to the bulk eclogite H2O content. Calcic amphibole forming part of COIs may be a major carrier of H2O as evidenced by CM31/03 containing both COIs and texturally primary calcic amphibole. In this sample calcic amphibole of the COIs contributes 63% whereas primary calcic amphibole only accounts for 13% of the bulk water. The relative order of H2O concentrations in NAMs is H2Orutile>H2Oomphacite>>H2Ogarnet for CM31/03 and CM15/01 and H2Orutile?H2Oomphacite>>H2Ogarnet in SKP31.
Oriented quartz + calcic amphibole inclusions in omphacite from the Saualpe and Pohorje Mountain eclogites, Eastern Alps an assessment of possible formation mechanisms based on IR- and mineral chemical data and water storage in Eastern Alpine eclogites.
Zanetti A
2008
Abstract
The composition of mineral phases and their modal proportions have been determined for three representative Eoalpine eclogites from the Saualpe type locality/Eastern Austria (sample SKP31) and the Pohorje Massif/Slovenia (CM31/03 and CM15/01) using electron microprobe, laser ICP-MS, IR spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole+quartz inclusions (COIs) in omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as H2O carriers in these eclogites. For omphacites in CM31/03 with a zonal distribution of COIs, a comparison of water and trace element concentrations of areas containing COIs and those free of COIs and a comparison with the trace element concentration of calcic amphibole indicate that COIs have formed through an open-system alteration of clinopyroxene and not through a closed system exsolution process. In sample SKP31, both textural and mineral chemical evidence suggests that COIs did not form by exsolution involving a Ca-Eskola component in clinopyroxene but formed by progressive growth under eclogite-facies P-T conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz-calcic amphibole grains in the matrix. Bulk H2O contents of the eclogites are between ca. 750 and 2150 ppm with 6-25% of the total water contributed by nominally anhydrous minerals (NAMs). Because of high modal amounts of 37-65%, omphacite is the major nominally anhydrous water carrier, containing 145-580 ppm H2O with significant concentration variations on a thin section scale. Due to their very low H2O concentrations of b5-10 ppm (garnet, kyanite) or insignificant modal amounts <=3% (rutile) the remaining NAMs contribute less than 1.5% to the bulk eclogite H2O content. Calcic amphibole forming part of COIs may be a major carrier of H2O as evidenced by CM31/03 containing both COIs and texturally primary calcic amphibole. In this sample calcic amphibole of the COIs contributes 63% whereas primary calcic amphibole only accounts for 13% of the bulk water. The relative order of H2O concentrations in NAMs is H2Orutile>H2Oomphacite>>H2Ogarnet for CM31/03 and CM15/01 and H2Orutile?H2Oomphacite>>H2Ogarnet in SKP31.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.