Meta-peridotites outcropping at different structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to re-examine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre-Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism of Alpine age, comprising an Eocene (M1) blueschist event strongly overprinted by a Miocene Barrovian-type event (M2). Structural concordance with the country rocks and metasomatic zonation at the contact with the felsic host rocks indicate that the meta-peridotites have experienced the M2 metamorphism. This conclusion is supported by the similarity between metamorphic temperatures of the ultrabasic rocks and those of the host rocks. Maximum temperatures of 730– 760 °C were calculated for the upper-amphibolite facies meta-peridotites (Fo–En–Hbl–Chl–Spl), associated with sillimanite gneisses and migmatites. Relict phases in ultrabasics of different structural levels indicate two distinct pre-M2 histories: whereas the cover-associated horizons have been affected by lowgrade serpentinization prior to metamorphism, the basementassociated meta-peridotites show no signs of serpentinization and instead preserve some of their original mantle assemblage. The geochemical affinities of the two groups are also different. The basement-associated meta-peridotites retain their original composition indicating derivation by fractional partial melting of primitive lherzolite, whereas serpentinization has led to almost complete Ca-loss in the second group. The cover-associated ultrabasics are interpreted as remnants of an ophiolite sequence obducted on the adjacent continental shelf early in the Alpine orogenesis. In contrast, the basement-associated meta-peridotites were tectonically interleaved with the Naxos section at great depth during the Alpine collision and high P/T metamorphism. Their emplacement at the base of the orogenic wedge is inferred to have involved isobaric cooling from temperatures of c. 1050 °C within the spinel lherzolite field to eclogite facies temperatures of c. 600 °C.
The /sup 18/O//sup 16/O fractionation accompanying the hydrothermal crystallization of quartz from silicic acid at 265/sup 0/ to 465/sup 0/C has been studied in order to assess the influences of (a) rate of quartz formation, (b) the nature of the reaction mechanism, and (c) temperature. At 360/sup 0/ and 465/sup 0/C, there are no indications of rate effects influencing isotope partitioning, but at 265/sup 0/C (where fractionation factors show an unusally large scatter) the evidence is inconclusive in this respect. All reaction appears to occur through solution-precipitation processes, with the intermediate phases, cristobalite and silica K, giving identical fractionation factors (within experimental errors) to quartz samples formed at the same temperature. The temperature-dependence of fractionation in the range 265/sup 0/ to 465/sup 0/C is given by the equation 10/sup 3/ ln ..cap alpha..(SiO/sub 2/--H/sub 2/O) = 3.05 x 10/sup 6/T/sup -2/ - 2.09. The data are in good agreement with other experimental calibrations of the quartz-water fractionation.
Research Article| March 01, 1983 On the mechanisms and kinetics of oxygen isotope exchange in quartz and feldspars at elevated temperatures and pressures ALAN MATTHEWS; ALAN MATTHEWS 1Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637 Search for other works by this author on: GSW Google Scholar JULIAN R. GOLDSMITH; JULIAN R. GOLDSMITH 1Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637 Search for other works by this author on: GSW Google Scholar ROBERT N. CLAYTON ROBERT N. CLAYTON 1Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 606372Present address: (Matthews) Department of Geology, Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel. Clayton is also with the Enrico Fermi Institute and Department of Chemistry. Search for other works by this author on: GSW Google Scholar Author and Article Information ALAN MATTHEWS 1Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637 JULIAN R. GOLDSMITH 1Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637 ROBERT N. CLAYTON 1Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 606372Present address: (Matthews) Department of Geology, Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel. Clayton is also with the Enrico Fermi Institute and Department of Chemistry. Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1983) 94 (3): 396–412. https://doi.org/10.1130/0016-7606(1983)94<396:OTMAKO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation ALAN MATTHEWS, JULIAN R. GOLDSMITH, ROBERT N. CLAYTON; On the mechanisms and kinetics of oxygen isotope exchange in quartz and feldspars at elevated temperatures and pressures. GSA Bulletin 1983;; 94 (3): 396–412. doi: https://doi.org/10.1130/0016-7606(1983)94<396:OTMAKO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Stereological and scanning electron microscope (SEM) analyses of quartz-water and plagioclase feldspar–water interactions have been combined with 18O/17O and 17O/16O measurements of the associated isotope exchange, in an attempt to develop an understanding of mineral-water interactions at elevated temperatures and pressures. The quartz-water exchange (T = 250 to 600 °C; Ph2o = 1 to 22 kbar) is dominated by an "Ostwald ripening" recrystallization mechanism, in which reprecipitated quartz develops as faceted overgrowths on original grains that have been rounded by initial solution processes. Correspondingly, there is a linear decrease in specific surface area (S̅) as a function of fractional extent of exchange (f). The unrecrystallized quartz cores exchange through diffusional mechanisms that also become dominant in later stages when recrystallization becomes retarded and increasingly ineffective as a mechanism of isotopic exchange. This transition from solution-precipitation to diffusional mechanisms accounts for a "plateau" pressure effect on exchange rate, observed in later stages of exchange. Substantial recrystallization does not occur in feldspar-water exchange at 500 and 600 °C, Ph2o = 2 to 15 kbar; the feldspar grains rapidly degrade into smaller fragments and become incoherently coated by a small amount of fine precipitated crystals. Subsequently, isotopic exchange occurs without further textural change and at constant S̅. These observations, the preservation of Al/Si order in albite throughout exchange, the high calculated values of oxygen-diffusion coefficients and possible dependence of these coefficients on (Ph2o)½ indicate inhibition of feldspar recrystallization and are consistent with diffusional mechanisms for penetration of water into the feldspar and exchange with shared oxygen atoms in alumino-silica tetrahedra. Equilibrium 18O/16O fractionation factors for both quartz-water and albite-water agree well with those measured previously in this laboratory. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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