Strontium isotopic study of sediment from the Ross Sea
Strontium isotopic study of sediment from the Ross Sea
Sea at 78 0 31.0'S. 164'24.7'W. during USNS Eltanin cruise 32. The strontium-87/strontium-86 ratios of the samples analyzed range from 0.7119 to 0.7220. Rubidium and strontium concentrations range, respectively, from 126 to 164 parts per million and from 113 to 174 parts per million.
JACK KOVACH
Department of Geology and Geography Muskingum College New Concord, Ohio 43762 and GUNTER FAURE
Institute of Polar Studies The Ohio State University Columbus, Ohio 43210 As part of a continuing study of the geochemistry and strontium isotopic composition of sediment deposited in the Ross Sea (Faure and Bannigan, 1975; Faure and Boger, 1976; Shaffer and Faure, 1976; Kovach and Faure, 1976; and Kovach and Faure, in preparation), we present here a preliminary report summarizing the results of a study of the strontium-87/strontium-86 ratios and of the rubidium and strontium concentrations of the fine-grained (less than 150 microns) noncarbonate fractions of sediment samples from core E32-25 raised from a depth of 327 fathoms in the Ross
In earlier studies, Shaffer and Faure (1976) and Faure and Boger (1976) showed that the detrital silicate fractions of sediment deposited in the Ross Sea in late Tertiary to Recent time are mixtures of two components having contrasting strontium -87/strontium -86 ratios and strontium concentrations. One of these components consists of materials derived from the weathering and erosion of igneous, sedimentary, and metamorphic rocks of Precambrian to Cretaceous age that form the bedrock of East and West Antarctica (Craddock, 1970). The other component originates from Late Cenozoic volcanic rocks of the McMurdo Volcanic Group of Ross Island and Victoria Land (Kyle and Cox, 1974) and/or from the Mesozoic and Cenozoic volcanics of Marie Byrd Land in West Antarctica (Le Masurier, 1972, figure 1). The observed strontium-87/strontium-86 ratios and strontium concentrations in sediment samples from core E32-25 fit a hyperbolic curve. This result supports the hypothesis that the detrital silicate fraction of the Late Cenozoic sediment in the Ross Sea is dominantly a twocomponent mixture of material derived from the weathering and erosion of old sialic rocks (high strontium-87/ strontium-86 ratios, low strontium concentrations) and
87 Sr 86
20
Sr
0.728 60
0.724 100
0.720 140
K
U
0.716
180 0.712
220 0.708
A. Mixing line for detrital non-carbonate fraction of core E32-25 from the Ross Sea. B: Concentration of volcanogenic detritus as a function of depth in the core. October 1977
260 0.704 0.002
0.004
0.006
0.008 1/Sr,pp.
0.010 1
S
0.012
0
10 20 30
300
Basalt
77
young volcanic rocks of basaltic composition (low strontium87/strontium-86 ratios, high strontium concentrations). The measured strontium-87/strontium-86 ratios and strontium concentrations can be used to estimate the concentrations of volcanogenic detritus in the sediment samples by means of a two-component mixing model developed by Boger and Faure (1974) and Shaffer and Faure (1976). The mixing equation was derived by fitting a straight line to the data points in coordinates of strontium-87/strontium-86 and the reciprocals of the strontium concentrations (figure A). Figure A demonstrates that the data points form a straight line (correlation coefficient = 0.95) whose equation is given in the figure. The strontium concentrations of the basaltic (B) and sialic (S) components can be calculated from this mixing equation by substituting assumed values of the strontium87/strontium-86 ratio (0.704 ± 0.001 for the basaltic component and 0.730 ± 0.005 for the sialic component). After the strontium concentrations of the basaltic and sialic components have been determined in this way, they can be used to calculate the concentration of the basaltic component of any given sample from the strontium concentration of that sample. Estimates of the concentration of volcanogenic detritus in sediment samples from core E32-25 calculated in that fashion are given in figure B. The concentration of volcanogenic detritus varies with depth in the core and may be used to define stratigraphic layers which may be useful in correlating sediment layers in piston cores collected elsewhere in the Ross Sea. The results of this study provide additional baseline data for anticipated future studies of sediment cores to be recovered from beneath the Ross Ice Shelf by the Ross Ice Shelf Project. The samples for this study (owned by the National Science Foundation) were made available by D.S. Cassidy, Antarctic Marine Geology Research Facility, Florida State University, Tallahassee. This research was supported by National Science Foundation grant OPP 72-00459.
References
Boger, P.D., and G. Faure. 1974. Strontium-isotope stratigraphy of a Red Sea core. Geology, 2(4): 181-183. Craddock, C. 1970. Geologic Map of Antarctica. (scale 1:5,000,000). New York, American Geographical Society. Faure, G., andJ.L. Bannigan. 1975. Geochemistry and mineralogy of DSDP core 270, Ross Sea. Antarctic Journal of the U.S., X(5): 256-257. Faure, G., and J.L. Boger. 1976. Geochemical and isotopic study of sediment from unit I, DSDP site 270, Ross Sea. Antarctic Journal of the U.S., XI(3): 163.165. Kovach, J . , and G. Faure. 1976. Strontium isotope studies of Late Cenozoic sediments from the Ross Sea, Antarctica. Geological Society ofAmerican Abstracts with Programs, 8(6): 962.963. Kovach, J . , and G. Faure. In preparation. The sources and abundance of volcanogenic sediment in piston cores from the Ross Sea, Antarctica. Kyle, P.R., andJ.W. Cole, 1974. Structural control of volcanism in the McMurdo Volcanic Group, Antarctica. Bulletin of Volcanology, 38: 16.25. 78
Le Masurier, W.E. 1972. Volcanic record of Cenozoic glacial history of Marie Byrd Land. In: Antarctic Geology and Geophysics (R.J. Adie, ed.). Univertetsforlaget, Oslo. 251-258. Shaffer, N.R., and G. Faure. 1976. Regional variation of 87Sr/86Sr ratios and mineral compositions of sediment from the Ross Sea, Antarctica. Geological Society of America. Bulletin, 87(11): 1491-1500.
Sedimentary geochemical processes near the PacificAntarctic Ridge IVAR ZEMMELS
Antarctic Research Facility Department of Geology The Florida State University Tallahassee, Florida 32306 Metalliferous sediments have been reported from several locations on the Pacific Antarctic Ridge (PAR) by Nayudu (1971) and Piper (1973). The origin of metalliferous sediments on the East Pacific Rise to the north has been attributed to hydrothermal processes associated with volcanism and sea-floor spreading (Bostrom and Peterson, 1966, 1969; Bender etal., 1971). The purpose of this study was to establish geochemical trends in the vicinity of the PAR, to ascertain the presence of hydrothermal metallogenic activity on the PAR, and to interpret the distribution of hydrothermal deposits in the light of two current theories of metallogensis on active midoceanic ridges. These are: (a) mineralized, juvenile, thermal fluids emanate from the axial zone of active faulting and volcanism and precipitate the metals in the vicinity of the axial zone (Bostrom, 1973) and (b) isolated, open, hydrothermal convective cells that circulate sea water are operative in the oceanic crust even at some distance from the axial zone and may be a source of mineralized fluids (Lister, 1972; Williams et al., 1974; Sclater et al., 1974; Anderson and Halunen, 1974). One hundred and twenty surface samples from the core collection at the Antarctic Research Facility, raised from the PAR, Southeast, and Southwest Pacific Basins south of 40°S. between 80° and 160 0W. were analyzed for Si, Al, Ti, Ca, Mg, Na, K, Fe, Mn, Cu, Zn, Ni, Co, Pb, and Sr using atomic absorption spectrophotometry, and for P (as PO43) using colorimetry. Sediments on the PAR are enriched in Ti, Fe, Mn, Cu, Zn, Ni, Co, Pb, and P. Volcaniclastic materials invariably constitute less than 1 percent of the sediment and cannot account for the element enrichment on the PAR. Only 8 of 50 PAR samples were classed as Al-poor sediments (i.e., Al/[Al + Fe + Mn] less than 0.20). A group of calcareous. Al-poor sediments in the vicinity of 40°S showed a Srdepletion that is suggestive of calcite recrystallization at elevated temperatures or in the presence of Sr-poor pore fluids (Kinsman, 1969). ANTARCTIC JOURNAL
Sea at 78 0 31.0'S. 164'24.7'W. during USNS Eltanin cruise 32. The strontium-87/strontium-86 ratios of the samples analyzed range from 0.7119 to 0.7220. Rubidium and strontium concentrations range, respectively, from 126 to 164 parts per million and from 113 to 174 parts per million.
JACK KOVACH
Department of Geology and Geography Muskingum College New Concord, Ohio 43762 and GUNTER FAURE
Institute of Polar Studies The Ohio State University Columbus, Ohio 43210 As part of a continuing study of the geochemistry and strontium isotopic composition of sediment deposited in the Ross Sea (Faure and Bannigan, 1975; Faure and Boger, 1976; Shaffer and Faure, 1976; Kovach and Faure, 1976; and Kovach and Faure, in preparation), we present here a preliminary report summarizing the results of a study of the strontium-87/strontium-86 ratios and of the rubidium and strontium concentrations of the fine-grained (less than 150 microns) noncarbonate fractions of sediment samples from core E32-25 raised from a depth of 327 fathoms in the Ross
In earlier studies, Shaffer and Faure (1976) and Faure and Boger (1976) showed that the detrital silicate fractions of sediment deposited in the Ross Sea in late Tertiary to Recent time are mixtures of two components having contrasting strontium -87/strontium -86 ratios and strontium concentrations. One of these components consists of materials derived from the weathering and erosion of igneous, sedimentary, and metamorphic rocks of Precambrian to Cretaceous age that form the bedrock of East and West Antarctica (Craddock, 1970). The other component originates from Late Cenozoic volcanic rocks of the McMurdo Volcanic Group of Ross Island and Victoria Land (Kyle and Cox, 1974) and/or from the Mesozoic and Cenozoic volcanics of Marie Byrd Land in West Antarctica (Le Masurier, 1972, figure 1). The observed strontium-87/strontium-86 ratios and strontium concentrations in sediment samples from core E32-25 fit a hyperbolic curve. This result supports the hypothesis that the detrital silicate fraction of the Late Cenozoic sediment in the Ross Sea is dominantly a twocomponent mixture of material derived from the weathering and erosion of old sialic rocks (high strontium-87/ strontium-86 ratios, low strontium concentrations) and
87 Sr 86
20
Sr
0.728 60
0.724 100
0.720 140
K
U
0.716
180 0.712
220 0.708
A. Mixing line for detrital non-carbonate fraction of core E32-25 from the Ross Sea. B: Concentration of volcanogenic detritus as a function of depth in the core. October 1977
260 0.704 0.002
0.004
0.006
0.008 1/Sr,pp.
0.010 1
S
0.012
0
10 20 30
300
Basalt
77
young volcanic rocks of basaltic composition (low strontium87/strontium-86 ratios, high strontium concentrations). The measured strontium-87/strontium-86 ratios and strontium concentrations can be used to estimate the concentrations of volcanogenic detritus in the sediment samples by means of a two-component mixing model developed by Boger and Faure (1974) and Shaffer and Faure (1976). The mixing equation was derived by fitting a straight line to the data points in coordinates of strontium-87/strontium-86 and the reciprocals of the strontium concentrations (figure A). Figure A demonstrates that the data points form a straight line (correlation coefficient = 0.95) whose equation is given in the figure. The strontium concentrations of the basaltic (B) and sialic (S) components can be calculated from this mixing equation by substituting assumed values of the strontium87/strontium-86 ratio (0.704 ± 0.001 for the basaltic component and 0.730 ± 0.005 for the sialic component). After the strontium concentrations of the basaltic and sialic components have been determined in this way, they can be used to calculate the concentration of the basaltic component of any given sample from the strontium concentration of that sample. Estimates of the concentration of volcanogenic detritus in sediment samples from core E32-25 calculated in that fashion are given in figure B. The concentration of volcanogenic detritus varies with depth in the core and may be used to define stratigraphic layers which may be useful in correlating sediment layers in piston cores collected elsewhere in the Ross Sea. The results of this study provide additional baseline data for anticipated future studies of sediment cores to be recovered from beneath the Ross Ice Shelf by the Ross Ice Shelf Project. The samples for this study (owned by the National Science Foundation) were made available by D.S. Cassidy, Antarctic Marine Geology Research Facility, Florida State University, Tallahassee. This research was supported by National Science Foundation grant OPP 72-00459.
References
Boger, P.D., and G. Faure. 1974. Strontium-isotope stratigraphy of a Red Sea core. Geology, 2(4): 181-183. Craddock, C. 1970. Geologic Map of Antarctica. (scale 1:5,000,000). New York, American Geographical Society. Faure, G., andJ.L. Bannigan. 1975. Geochemistry and mineralogy of DSDP core 270, Ross Sea. Antarctic Journal of the U.S., X(5): 256-257. Faure, G., and J.L. Boger. 1976. Geochemical and isotopic study of sediment from unit I, DSDP site 270, Ross Sea. Antarctic Journal of the U.S., XI(3): 163.165. Kovach, J . , and G. Faure. 1976. Strontium isotope studies of Late Cenozoic sediments from the Ross Sea, Antarctica. Geological Society ofAmerican Abstracts with Programs, 8(6): 962.963. Kovach, J . , and G. Faure. In preparation. The sources and abundance of volcanogenic sediment in piston cores from the Ross Sea, Antarctica. Kyle, P.R., andJ.W. Cole, 1974. Structural control of volcanism in the McMurdo Volcanic Group, Antarctica. Bulletin of Volcanology, 38: 16.25. 78
Le Masurier, W.E. 1972. Volcanic record of Cenozoic glacial history of Marie Byrd Land. In: Antarctic Geology and Geophysics (R.J. Adie, ed.). Univertetsforlaget, Oslo. 251-258. Shaffer, N.R., and G. Faure. 1976. Regional variation of 87Sr/86Sr ratios and mineral compositions of sediment from the Ross Sea, Antarctica. Geological Society of America. Bulletin, 87(11): 1491-1500.
Sedimentary geochemical processes near the PacificAntarctic Ridge IVAR ZEMMELS
Antarctic Research Facility Department of Geology The Florida State University Tallahassee, Florida 32306 Metalliferous sediments have been reported from several locations on the Pacific Antarctic Ridge (PAR) by Nayudu (1971) and Piper (1973). The origin of metalliferous sediments on the East Pacific Rise to the north has been attributed to hydrothermal processes associated with volcanism and sea-floor spreading (Bostrom and Peterson, 1966, 1969; Bender etal., 1971). The purpose of this study was to establish geochemical trends in the vicinity of the PAR, to ascertain the presence of hydrothermal metallogenic activity on the PAR, and to interpret the distribution of hydrothermal deposits in the light of two current theories of metallogensis on active midoceanic ridges. These are: (a) mineralized, juvenile, thermal fluids emanate from the axial zone of active faulting and volcanism and precipitate the metals in the vicinity of the axial zone (Bostrom, 1973) and (b) isolated, open, hydrothermal convective cells that circulate sea water are operative in the oceanic crust even at some distance from the axial zone and may be a source of mineralized fluids (Lister, 1972; Williams et al., 1974; Sclater et al., 1974; Anderson and Halunen, 1974). One hundred and twenty surface samples from the core collection at the Antarctic Research Facility, raised from the PAR, Southeast, and Southwest Pacific Basins south of 40°S. between 80° and 160 0W. were analyzed for Si, Al, Ti, Ca, Mg, Na, K, Fe, Mn, Cu, Zn, Ni, Co, Pb, and Sr using atomic absorption spectrophotometry, and for P (as PO43) using colorimetry. Sediments on the PAR are enriched in Ti, Fe, Mn, Cu, Zn, Ni, Co, Pb, and P. Volcaniclastic materials invariably constitute less than 1 percent of the sediment and cannot account for the element enrichment on the PAR. Only 8 of 50 PAR samples were classed as Al-poor sediments (i.e., Al/[Al + Fe + Mn] less than 0.20). A group of calcareous. Al-poor sediments in the vicinity of 40°S showed a Srdepletion that is suggestive of calcite recrystallization at elevated temperatures or in the presence of Sr-poor pore fluids (Kinsman, 1969). ANTARCTIC JOURNAL
