Physics
Scientific paper
Jan 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50...59e&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 1, pp.59-68
Physics
2
Scientific paper
The mineralogy, O 18 , and D of the <0.1 m fraction of 22 Cretaceous bentonites and the mineralogy and O 18 of the < 0.1 m fraction of 14 adjacent shales collected from outcrops in the Sweetgrass Arch and Disturbed Belt, Montana, have been determined. Mixed-layer illite/smectite (I/S) is the dominant mineral in the bulk bentonite and usually the only mineral in the < 0.1 m fraction. I/S is also the major clay mineral in the shales. The diagenetic grade in bentonite is qualitatively given by the percentage of illite layers in I/S, which varies from 2 to 25 (Sweetgrass Arch) to as high as 95 (Disturbed Belt). O 18 of < 0.1 m bentonite generally decreases from about +20%. to about +13%. with increasing diagenetic grade. On a plot of D versus O 18 , data for the < 0.1 m bentonite define a field that generally parallels, but falls on the meteoric water line side of the smectite-water line (Savin and Epstein, 1970). O 18 of bulk bentonite is 1 to 3%. more negative than the O 18 of the < 0.1 m fraction, due to the presence of volcanic quartz and feldspar. O 18 of several size fractions of clay-sized quartz separated from the bentonite varies from +11%. to +24%., and, in a given bentonite, generally increases with decreasing grain size. Among the different bentonites, the O 18 range of the different grain sizes decreases as the percentage of illite layers in the coexisting I/S increases. The O 18 of 0.1-0.5 m shale quartz is generally 1 to 4%. more positive than clay-sized quartz from an adjacent bentonite, and the O 18 of < 0.1 m I/S concentrate of shales is generally < 1 to 4%. more negative than the < 0.1 m I/S from an adjacent bentonite. Isotopic temperatures, interpreted to be maximum burial temperatures, range between about 160°C (shale), to about 250°C (bentonite). The isotopic data can be interpreted using the stages: 1. 1) deposition of volcanic glassy ash containing some quartz and feldspar; 2. 2) devitrification into mostly 100% expandable smectite with a O 18 of about 20%. and D of -60 to -80%.; 3. 3) tectonic burial beneath thrust sheets in the Disturbed Belt region which induced elevated temperatures that allowed oxygen and hydrogen isotope reequilibration as the percentage of illite layers in I/S increased and diagenetic quartz formed; and 4. 4) exhumation of the strata with preservation of O 18 / O 16 and D/H attained during burial.
Eslinger Eric V.
Yeh Hsueh-Wen
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