Jianghua Feng

Publication Date


Document Type


First Advisor

Perry, Eugene C., 1933-

Degree Name

M.S. (Master of Science)

Legacy Department

Department of Geology


Geochemistry; Geology; Stratigraphic--Precambrian; Submarine geology


The sulfur isotope age curve for the Phanerozoic ocean has been well established by studies of evaporite deposits, but only a few Precambrian evaporites persist. In this study, concentrations of sulfate in rocks, sulfur and oxygen isotope composition of sulfate, and oxygen isotope composition of chert and carbonates have been obtained world wide from samples ranging in age from 600 Ma to 3500 Ma. The original sulfur isotope composition of sulfate can be preserved in quartz and chert pseudomorphs after sulfate. Studies of remnant sulfate inclusions in chert and quartz nodules derived through dissolution of evaporites in Phanerozoic carbonate units reinforce this conclusion. In many cases, there are obvious differences in the oxygen isotope data between primary and secondary sulfates. Therefore, oxygen isotope composition of sulfate may be a good discriminator between primary and secondary sulfate sources. The sulfur data of this study show that the sulfur isotope age curve was close to 0%₀ (CDT standard) before 3.0 Ga, and somewhat before 2.0 Ga the range of variation of sulfur isotopes in sulfate appears to have increased to close to 15%₀. This change has been controlled mainly by marine hydrothermal reactions in mid-ocean ridges and bacterial sulfate reduction. The later shift (within the range 10 to 30%₀) observed for Phanerozoic evaporites may result mainly from bacterial reduction of marine sulfate. Samples of presumably marine sulfate preserved in early Archean rocks are unusually depleted in ¹⁸O compared to Phanerozoic marine sulfate. The δ¹⁸O range of variation for the Precambrian is about 4-18%₀. The similarity between δ¹⁸O variations of marine sulfate and chert may be explained by later metamorphism. However, the δ¹⁸O of Precambrian marine sulfate may have been dominated more by contemporaneous circulation of seawater through hot basalt along ocean ridges. Examination of δ¹⁸O for sixty-four of the Precambrian chert samples show that they formed under warm water conditions and most likely involved contributions of meteoric water, possibly in a sabkha environment. These data basically parallel the curves from previous studies of δ¹⁸O of chert formed in a pure marine environment. Thus differences in chert formation temperature and oxygen isotope composition of diagenetic solutions between deep marine and coastal facies may be essentially similar for Precambrian and Phanerozoic samples. Five samples of microfossiliferous chert have δ¹⁸O values consistent with δ¹⁸O values of pure marine original chert samples from previous studies.


Bibliography: pages [104]-110.


v, 110 pages




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