Hydrothermal calcite in the Elephant Moraine · rock exposures of the Mawson diamictite in the...
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Hydrothermal calcite in the Elephant Moraine C. FAURE and K.S. TAYLOR Department of Geology and Mineralogy and Institute of Polar Studies Oliw State University Columbus, Ohio 43210 L.M. JONES Conoco, Inc. Ponca City, Oklahoma 74603 In the course of geologic mapping of the Elephant Moraine on the east antarctic ice sheet (76°17'34.9"S 157°20'04.9"E) during austral summer 1984-1985, Faure and Taylor (1985) collected several specimens of "black botryoidal calcite, composed of radiating acicular crystals that resemble stromatolites." Sipiera and Landis (1985) suggested that these rocks were deposited in part by algae with interlayered calcrete. However, Faure and Taylor (1985) excluded a marine origin for this material because the isotopic composition of strontium differs from that of sea- water and suggested a hydrothermal origin instead. Faure and Taylor considered that the calcite may have formed by volcanic activity during the Jurassic period. Therefore, they searched rock exposures of the Mawson diamictite in the Allan Hills but did not find samples of the black calcite in that formation. Additional studies by Montello (1986) and Taylor (1986) also did not provide any new evidence bearing on the origin of the black calcite clasts. The specimen pictured in the figure was collected by W.A. Cassidy of the University of Pittsburgh. Calcite from this and other specimens is significantly enriched in strontium-87 (the strontium-87/strontium-86 ratio equals 0.71417 ± 0.00002), car- bon-12 (delta carbon-13 equals -22.9 parts per thousand, PDB standard) and oxygen-16 (delta oxygen-18 equals -21.1 parts per thousand, standard mean ocean water) compared with calcite of marine origin. The enrichment in carbon-12 is similar to that of calcite associated with coal in th Allan Hills for which Faure and Botoman (1984) reported delta carbon-13 equals -16.9 parts per thousand and the strontium-87/strontium-86 ratio equals 0.7138. The enrichment in oxygen-16 indicates that the calcite from the Elephant Moraine could only have precipitated in isotopic equilibrium with glacial meltwater. Ice at the base of the east antarctic ice sheet has an average isotopic oxygen value of about -40 parts per thousand (Epstein, Sharp, and Cow 1970). There- fore, the temperature at which the black calcite precipitated from water of that isotope composition was about 85°C, based on the fractionation equation of O'Neil, Clayton, and Mayeda (1969). A temperature of this magnitude implies that the black calcite formed as a result of volcanic activity under the east antarctic ice sheet. The enrichment of the black calcite in carbon-12 suggests that it formed in part from carbon dioxide derived from the coal seams of the Weller Formation in the Beacon Supergroup. The isotopic composition of strontium in the black calcite is similar, to that of carbonate beds and concretions in the Beacons of southern Victoria Land (Faure and Barrett 1973). A ic- Photograph of a specimen of black calcite from the Elephant Mor- aine. (This specimen was collected by W.A. Cassidy.) hydrothermal origin is also consistent with the very low total organic carbon content of 0.15 percent in the calcite. One of the calcite samples included in the study was contrib- uted by John Schutt. Michael L. Strobel determined con- centrations of rubidium and strontium. We thank Conoco, Inc., for the use of their facilities. This project is supported by National Science Foundation grant DPP 83-14136. References Epstein, S., R.P. Sharp, and A.J. Cow. 1970. Antarctic ice sheet: Stable isotope analyses of Byrd Station cores and interhemispheric climatic implications. Science, 168, 1570-1572. Faure, C. and P.J. Barrett. 1973. Strontium isotope composition of non- marine carbonate rocks from the Beacon Supergroup of the Transan- tarctic Mountains. Journal of Sedimentary Petrology, 43, 447-457. Faure, C., and C. Botoman. 1984. Origin of epigenetic calcite in coal from Antarctica and Ohio based on isotope compositions of oxygen, carbon and strontium. Isotope Geoscience (Chemical Geology), 2, 313-324. Faure, C., and K.S. Taylor. 1985. The geology and origin of the Elephant Moraine on the east antarctic ice sheet. Antarctic Journal of the U.S., 20(5), 11-12. Montello, J.M. 1986. The provenance of till at the Elephant Moraine on the East Antarctic ice sheet. (Masters thesis, Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio.) O'Neil, JR., R.N. Clayton, and T. Mayeda, 1969. Oxygen isotope frac- tionation in divalent metal carbonates. Journal Chemical Physics, 51, 5547-5558. Sipiera, PP., and C.A. Landis. 1985. A preliminary report on a possible stromatolite find from the Elephant Moraine, Antarctica: A potential directional indicator for ice movement. (Abstract.) Workshop oil Meteorites,, Mainz, Federal Republic of Germany 10-12 July 1985. Taylor, K.S. 1986. The lithologic composition of clasts oft/ic Elephant Moraine, Allan Hills area, South Victoria Land, Antarctica. (Masters thesis, De- partment of Geology, Kent State University, Kent, Ohio.) 1986 REVIEW 21
Transcript of Hydrothermal calcite in the Elephant Moraine · rock exposures of the Mawson diamictite in the...
Hydrothermal calcite in the ElephantMoraine
C. FAURE and K.S. TAYLOR
Department of Geology and Mineralogyand
Institute of Polar StudiesOliw State University
Columbus, Ohio 43210
L.M. JONES
Conoco, Inc.Ponca City, Oklahoma 74603
In the course of geologic mapping of the Elephant Moraine onthe east antarctic ice sheet (76°17'34.9"S 157°20'04.9"E) duringaustral summer 1984-1985, Faure and Taylor (1985) collectedseveral specimens of "black botryoidal calcite, composed ofradiating acicular crystals that resemble stromatolites." Sipieraand Landis (1985) suggested that these rocks were deposited inpart by algae with interlayered calcrete. However, Faure andTaylor (1985) excluded a marine origin for this material becausethe isotopic composition of strontium differs from that of sea-water and suggested a hydrothermal origin instead. Faure andTaylor considered that the calcite may have formed by volcanicactivity during the Jurassic period. Therefore, they searchedrock exposures of the Mawson diamictite in the Allan Hills butdid not find samples of the black calcite in that formation.Additional studies by Montello (1986) and Taylor (1986) also didnot provide any new evidence bearing on the origin of the blackcalcite clasts.
The specimen pictured in the figure was collected by W.A.Cassidy of the University of Pittsburgh. Calcite from this andother specimens is significantly enriched in strontium-87 (thestrontium-87/strontium-86 ratio equals 0.71417 ± 0.00002), car-bon-12 (delta carbon-13 equals -22.9 parts per thousand, PDBstandard) and oxygen-16 (delta oxygen-18 equals -21.1 parts perthousand, standard mean ocean water) compared with calciteof marine origin. The enrichment in carbon-12 is similar to thatof calcite associated with coal in th Allan Hills for which Faureand Botoman (1984) reported delta carbon-13 equals -16.9 partsper thousand and the strontium-87/strontium-86 ratio equals0.7138.
The enrichment in oxygen-16 indicates that the calcite fromthe Elephant Moraine could only have precipitated in isotopicequilibrium with glacial meltwater. Ice at the base of the eastantarctic ice sheet has an average isotopic oxygen value of about-40 parts per thousand (Epstein, Sharp, and Cow 1970). There-fore, the temperature at which the black calcite precipitatedfrom water of that isotope composition was about 85°C, basedon the fractionation equation of O'Neil, Clayton, and Mayeda(1969). A temperature of this magnitude implies that the blackcalcite formed as a result of volcanic activity under the eastantarctic ice sheet.
The enrichment of the black calcite in carbon-12 suggests thatit formed in part from carbon dioxide derived from the coalseams of the Weller Formation in the Beacon Supergroup. Theisotopic composition of strontium in the black calcite is similar,to that of carbonate beds and concretions in the Beacons ofsouthern Victoria Land (Faure and Barrett 1973). Aic-
Photograph of a specimen of black calcite from the Elephant Mor-aine. (This specimen was collected by W.A. Cassidy.)
hydrothermal origin is also consistent with the very low totalorganic carbon content of 0.15 percent in the calcite.
One of the calcite samples included in the study was contrib-uted by John Schutt. Michael L. Strobel determined con-centrations of rubidium and strontium. We thank Conoco, Inc.,for the use of their facilities.
This project is supported by National Science Foundationgrant DPP 83-14136.
References
Epstein, S., R.P. Sharp, and A.J. Cow. 1970. Antarctic ice sheet: Stableisotope analyses of Byrd Station cores and interhemispheric climaticimplications. Science, 168, 1570-1572.
Faure, C. and P.J. Barrett. 1973. Strontium isotope composition of non-marine carbonate rocks from the Beacon Supergroup of the Transan-tarctic Mountains. Journal of Sedimentary Petrology, 43, 447-457.
Faure, C., and C. Botoman. 1984. Origin of epigenetic calcite in coalfrom Antarctica and Ohio based on isotope compositions of oxygen,carbon and strontium. Isotope Geoscience (Chemical Geology), 2,313-324.
Faure, C., and K.S. Taylor. 1985. The geology and origin of the ElephantMoraine on the east antarctic ice sheet. Antarctic Journal of the U.S.,20(5), 11-12.
Montello, J.M. 1986. The provenance of till at the Elephant Moraine on theEast Antarctic ice sheet. (Masters thesis, Department of Geology andMineralogy, Ohio State University, Columbus, Ohio.)
O'Neil, JR., R.N. Clayton, and T. Mayeda, 1969. Oxygen isotope frac-tionation in divalent metal carbonates. Journal Chemical Physics, 51,5547-5558.
Sipiera, PP., and C.A. Landis. 1985. A preliminary report on a possiblestromatolite find from the Elephant Moraine, Antarctica: A potentialdirectional indicator for ice movement. (Abstract.) Workshop oil
Meteorites,, Mainz, Federal Republic of Germany 10-12 July1985.
Taylor, K.S. 1986. The lithologic composition of clasts oft/ic Elephant Moraine,Allan Hills area, South Victoria Land, Antarctica. (Masters thesis, De-partment of Geology, Kent State University, Kent, Ohio.)
1986 REVIEW 21
