Coupled dichotomies of apatite and fluid composition during contact metamorphism of siliceous carbonate rocks
-
Sarah C. Penniston-Dorland
and
John M. Ferry
Abstract
The XCO2 recorded by mineral-fluid equilibria in contact metamorphosed siliceous carbonates commonly defines two groups of rocks in the same aureole. One group records relatively low XCO2 that results from infiltration of chemically reactive H2O-rich fluid. The other records relatively high XCO2, up to 0.99, that results from decarbonation reactions with little or no infiltration. A complementary dichotomy in apatite compositions exists in five contact aureoles in Italy, Scotland, and the U.S.A. Apatite in the low-XCO2 group is close to an F-OH solid solution. Apatite in the high-XCO2 group is a relatively Cl-rich Cl-F-OH solution. The halogen content of fluid coexisting with analyzed apatite was characterized in two aureoles to determine the origin and significance of the dichotomy in apatite composition. Calculated αHF/aH2O, αHF/αHCl, αHF, and mFT (the total F molality of fluid) are systematically higher in fluid coexisting with the low-XCO2 group. In contrast, αHCl/αH2O in the high-XCO2 group may be higher than or overlap with αHCl/αH2O in the low-XCO2 group. Calculated αHCl and mClT in the high-XCO2 group are lower than or overlap with aHCl and mClT in the low-XCO2 group. The Cl-rich apatites in the high-XCO2 group are explained by crystallization at relatively low αH2O, αHF, and mFT rather than at high αHCl or mClT. In comparison, the F-OH apatites in the low-XCO2 group formed by infiltration of rock by and equilibration with relatively H2O-rich, high mFT/mClT fluid, reflecting the same metasomatic process responsible for F-rich humite-group minerals and skarns in many contact aureoles. Calculated halogen contents indicate that the non-CO2 fraction of fluid in equilibrium with both groups had modest, seawater-like salinity, and that the reactive H2O- and F-rich fluid that infiltrated the low-XCO2 group had a plutonic source.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Presidential Address To The Mineralogical Society Of Amrica, Boston, November 6, 2001: Some Precambrian Banded Iron-Formations (BiIFs) From Around The World: Their Age, Geologic Setting, Mineralogy, Metamorphism, Geochemistry, And Origin
- Effects of high pressure and high temperature on cation ordering in magnesioferrite, MgFe2O4, using in situ synchrotron X-ray powder diffraction up to 1430 K and 6 GPa
- Raman spectroscopy and vibrational analyses of albite: From 25 °C through the melting temperature
- A new occurrence of xitieshanite [Fe3+(SO4)Cl⋅6H2O] crystals in acid-mine seepways, Green Valley, Vigo County, Indiana, U.S.A.
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- Crystal chemistry of NaMgF3 perovskite at high pressure and temperature
- Hyperfine electric field gradient tensors at Fe2+ sites in octahedral layers: Toward understanding oriented single-crystal Mössbauer spectroscopy measurements of micas
- Does the bazhenovite structure really contain a thiosulfate group? A structural and spectroscopic study of a sample from the type locality
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- Absolute quantification by powder X-ray diffraction of complex mixtures of crystalline and amorphous phases for applications in the Earth sciences
- Detrital illite crystals identified from crystallite thickness measurements in siliciclastic sediments
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- Low-pressure and low-temperature K-bearing kosmochloric diopside from the Osayama serpentinite mélange, SW Japan
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- Structural changes accompanying the phase transformation between leadhillite and susannite: A structural study by means of in situ high-temperature single-crystal X-ray diffraction
- Optic properties of centimeter-sized crystals determined in air with the spindle stage using EXCALIBRW
- Zoltaiite, a new barium-vanadium nesosubsilicate mineral from British Columbia: Description and crystal structure
- Mn-tourmaline from island of Elba (Italy): Crystal chemistry
- Letter: An infrared study of carbon-oxygen bonding in magnesite to 60 GPa
- Letter: Metals in quartz-hosted melt inclusions: Natural facts and experimental artifacts
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