Marine geology and geophysics
Marine geology and geophysics Dating of detrital feldspar in sediment from RISP site J-9 GUNTER FAURE
and KAREN S. TAYLOR
Department of Geology and Mineralogy and Institute of Polar Studies The Ohio State University Columbus, Ohio 43210
Detrital feldspar in the glacial-marine sediment at Ross Ice Shelf Project (RIsP) site J-9 originated from West Antarctica and provides information about the age of the bedrock under the ice sheet. This study is based on analyses of bulk sample PNw-23 recovered during the 1977-78 field season at site J-9 (dough and Hansen, 1979; Webb, 1978, 1979; Brady and Martin, 1979). A sample weighing 64.43 grams (dry) was sieved into eight specific size fractions (figure 1). The presence of feldspar was confirmed by X-ray diffraction, and concentrates of this mineral were obtained by use of a mag-
60 50 4-)
40 4.)
30 20 10
Quartz
20 U 10 C to C
20U S.-
10 0 20
Fel
A' •
r
Muscovite + Illite
ci . CC 4.)
C
U
0
20 Chlorite 10 I I I I I____
I I I I 0 15.6 62.5 250 1000 4000
Grain size in Micrometers Figure 1. Grain-size distribution and abundances of minerals in sample ppiw-23, RISP site J-9 (1977-78). Key: A and B = feldspar fractions dated by rubidium-strontium method.
124
netic separator. After leaching with dilute hydrochloric acid to dissolve calcium carbonate, and after removal of residual clay coatings by ultrasonic treatment, the concentrates were combined into two samples labeled A (-18+ 120 mesh) and B (-120+230 mesh). These were ground to -200 mesh and homogenized for dating by the rubidium-strontium method. Feldspar is known to have excellent retentivity for radiogenic strontium-87 and is therefore well suited for dating by the rubidium-strontium method (Faure, 1977). Concentrations of rubidium and strontium were determined by X-ray fluorescence. The isotopic composition of strontium was measured by mass spectrometry. The results can be interpreted in terms of dates that have magnitudes dependent on the assumed value of the initial strontium-87/strontium-86 ("'Sr/"'Sr) ratio. A series of discordant dates was calculated for each sample for initial 87 Sr/ 86 Sr ratios, ranging from 0.704 to 0.716 (figure 2). A concordant date of 174 million years for both samples was obtained with an initial 87SrI86Sr ratio of 0.71552. This calculation is based on the reasonable assumption that the feldspar grains in different size fractions originated from the same bedrock sources in West Antarctica and therefore should yield the same date. The date of 174 million years may be a result of the mixing of feldspar of different ages. In this case, additional assumptions about the ages of contributing bedrock sources are necessary before this date can be eval uated. If the feldspar originated primarily from rocks having the same age, then the date obtained for the feldspar is a valid estimate of the age of the bedrock. This interpretation is supported by the fact that the date and initial 87 SrI86Sr ratio of the feldspar are similar to those of granites at Mount Chapman, Whitmore Moun tains, for which Kovach and Faure (1978) reported a rubidium-strontium isochron date of 173 million years and an initial 87 Sr/86 Sr ratio of 0.7148. This date conFirmed a potassium-argon date of 176±5 million years For biotite from the Linck Nunataks granite of the Whitmore Mountains published earlier by Craddock (1970) md by Webers et al. (1979). The above comparisons lead to two tentative conclu;ions. First, feldspar in sediment sample PNw-23 probmbly originated from igneous and/or metamorphic rocks f Early Jurassic age that crystallized about 174 million years ago. Second, no feldspar derived from older rocks, such as those of the Transantarctic Mountains, has been detected. The sediment sample from RISP site J-9 (1977-78) was made available by Dennis S. Cassidy, Antarctic Ma-
0.
0.
0 100 200 300 400 500 600 Date in Millions of Years
Figure 2. Variation of rubidium-strontium dates with assumed values of the initial strontium-87/strontium-86 ratio.
rifle Geology Research Facility, Florida State University. This research has been supported by the Division of Polar Programs of the National Science Foundation through grant DPP 77-21505. References Brady, H., and H. Martin. 1979. Ross Sea region in the Middle Miocene: A glimpse into the past. Science, 203: 437-38. Clough, J . W., and B. L. Hansen. 1979. The Ross Ice Shelf Project. Science, 203: 433-34. Craddock, C. 1970. Radiometric age map of Antarctica. Antarctic Map Folio Series, folio 12—Geology, plate 19. New York: American Geographic Society. Faure, G. 1977. Principles of isotope Geology. New York: John Wiley and Sons. Kovach, J . , and G. Faure. 1978. Rubidium-strontium geochronology of granitic rocks from Mt. Chapman, Whitmore Mountains, West Antarctica. Antarctic Journal of the United States, 13(4): 17-18. Webb, P. N., 1978. Initial report on geological materials collected at RISP Site J9, 1977-78. RISP Technical Report 781. Lincoln: University of Nebraska. Webb, P. N. 1979. The glaciomarine sediments from beneath the northern Ross Ice Shelf, Antarctica. Science, 203, 43537. Webers, G. F., C. Craddock, M. A. Rogers, and J.J. Anderson. 1979. Geology of the Whitmore Mountains. In Antarctic Geosciences, ed. C. Craddock. Madison: University of Wisconsin Press.
125
and KAREN S. TAYLOR
Department of Geology and Mineralogy and Institute of Polar Studies The Ohio State University Columbus, Ohio 43210
Detrital feldspar in the glacial-marine sediment at Ross Ice Shelf Project (RIsP) site J-9 originated from West Antarctica and provides information about the age of the bedrock under the ice sheet. This study is based on analyses of bulk sample PNw-23 recovered during the 1977-78 field season at site J-9 (dough and Hansen, 1979; Webb, 1978, 1979; Brady and Martin, 1979). A sample weighing 64.43 grams (dry) was sieved into eight specific size fractions (figure 1). The presence of feldspar was confirmed by X-ray diffraction, and concentrates of this mineral were obtained by use of a mag-
60 50 4-)
40 4.)
30 20 10
Quartz
20 U 10 C to C
20U S.-
10 0 20
Fel
A' •
r
Muscovite + Illite
ci . CC 4.)
C
U
0
20 Chlorite 10 I I I I I____
I I I I 0 15.6 62.5 250 1000 4000
Grain size in Micrometers Figure 1. Grain-size distribution and abundances of minerals in sample ppiw-23, RISP site J-9 (1977-78). Key: A and B = feldspar fractions dated by rubidium-strontium method.
124
netic separator. After leaching with dilute hydrochloric acid to dissolve calcium carbonate, and after removal of residual clay coatings by ultrasonic treatment, the concentrates were combined into two samples labeled A (-18+ 120 mesh) and B (-120+230 mesh). These were ground to -200 mesh and homogenized for dating by the rubidium-strontium method. Feldspar is known to have excellent retentivity for radiogenic strontium-87 and is therefore well suited for dating by the rubidium-strontium method (Faure, 1977). Concentrations of rubidium and strontium were determined by X-ray fluorescence. The isotopic composition of strontium was measured by mass spectrometry. The results can be interpreted in terms of dates that have magnitudes dependent on the assumed value of the initial strontium-87/strontium-86 ("'Sr/"'Sr) ratio. A series of discordant dates was calculated for each sample for initial 87 Sr/ 86 Sr ratios, ranging from 0.704 to 0.716 (figure 2). A concordant date of 174 million years for both samples was obtained with an initial 87SrI86Sr ratio of 0.71552. This calculation is based on the reasonable assumption that the feldspar grains in different size fractions originated from the same bedrock sources in West Antarctica and therefore should yield the same date. The date of 174 million years may be a result of the mixing of feldspar of different ages. In this case, additional assumptions about the ages of contributing bedrock sources are necessary before this date can be eval uated. If the feldspar originated primarily from rocks having the same age, then the date obtained for the feldspar is a valid estimate of the age of the bedrock. This interpretation is supported by the fact that the date and initial 87 SrI86Sr ratio of the feldspar are similar to those of granites at Mount Chapman, Whitmore Moun tains, for which Kovach and Faure (1978) reported a rubidium-strontium isochron date of 173 million years and an initial 87 Sr/86 Sr ratio of 0.7148. This date conFirmed a potassium-argon date of 176±5 million years For biotite from the Linck Nunataks granite of the Whitmore Mountains published earlier by Craddock (1970) md by Webers et al. (1979). The above comparisons lead to two tentative conclu;ions. First, feldspar in sediment sample PNw-23 probmbly originated from igneous and/or metamorphic rocks f Early Jurassic age that crystallized about 174 million years ago. Second, no feldspar derived from older rocks, such as those of the Transantarctic Mountains, has been detected. The sediment sample from RISP site J-9 (1977-78) was made available by Dennis S. Cassidy, Antarctic Ma-
0.
0.
0 100 200 300 400 500 600 Date in Millions of Years
Figure 2. Variation of rubidium-strontium dates with assumed values of the initial strontium-87/strontium-86 ratio.
rifle Geology Research Facility, Florida State University. This research has been supported by the Division of Polar Programs of the National Science Foundation through grant DPP 77-21505. References Brady, H., and H. Martin. 1979. Ross Sea region in the Middle Miocene: A glimpse into the past. Science, 203: 437-38. Clough, J . W., and B. L. Hansen. 1979. The Ross Ice Shelf Project. Science, 203: 433-34. Craddock, C. 1970. Radiometric age map of Antarctica. Antarctic Map Folio Series, folio 12—Geology, plate 19. New York: American Geographic Society. Faure, G. 1977. Principles of isotope Geology. New York: John Wiley and Sons. Kovach, J . , and G. Faure. 1978. Rubidium-strontium geochronology of granitic rocks from Mt. Chapman, Whitmore Mountains, West Antarctica. Antarctic Journal of the United States, 13(4): 17-18. Webb, P. N., 1978. Initial report on geological materials collected at RISP Site J9, 1977-78. RISP Technical Report 781. Lincoln: University of Nebraska. Webb, P. N. 1979. The glaciomarine sediments from beneath the northern Ross Ice Shelf, Antarctica. Science, 203, 43537. Webers, G. F., C. Craddock, M. A. Rogers, and J.J. Anderson. 1979. Geology of the Whitmore Mountains. In Antarctic Geosciences, ed. C. Craddock. Madison: University of Wisconsin Press.
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