Publication Date

1984

Document Type

Dissertation/Thesis

First Advisor

Perry, Eugene C., 1933-

Degree Name

M.S. (Master of Science)

Legacy Department

Department of Geology

LCSH

Mineralogy--South Africa--Cape Province; Metamorphism (Geology)--South Africa--Cape Province

Abstract

Oxygen and carbon isotopic compositions of minerals were analyzed from cores in the Asbesheuwels Iron Formations and associated Makganyene Diamictite. Materials for this study came from the vicinity of Postmasburg, Cape Province, South Africa. Temperature of metamorphism in this region, estimated from oxygen isotope fractionation between pairs of the minerals quartz, carbonate and magnetite, is 250°C. Large variations in the ¹⁸O values of carbonates on the scale of centimeters indicate that the Kuruman Iron Formation acted as a series of closed subsystems during diagenesis and remained closed to post-metamorphic reactions for 2 billion years. Graded contacts between diamictite and lean iron formation units in the Makganyene Diamictite indicate that cool water conditions prevailed during iron formation deposition. These findings lend support to the hypothesis that the ocean temperature has remained relatively constant, and that the δ¹⁸O of the oceans has evolved from negative values in the Pre-Cambrian to near zero at present. Isotopic variations in core from the Makganyene Diamictite show definite trends that are a function of a diagenetic reaction such as 5Fe₂O₃ + (C)^organic = FeCO₃ + 3Fe₃O₄. By this reaction, a significant component of ¹³C-depleted organic carbon would be added to the carbonate fraction of the lean iron formation units within the Makganyene Diamictite. This carbon would presumably equilibrate in ¹⁸O with magnetite at the temperature of diagenesis, resulting in sympathetic δ¹⁸O[sub mag] and δ¹³C[sub carb] curves and inverse (18)^(O)^carb and δ¹³C[sub carb] curves. Due to its coarse grained texture, the siderite from the core C.S. 119 did not enter into substantial isotopic exchange with magnetite or quartz. Therefore, most of the oxygen isotopic exchange in this core occurred between quartz and magnetite, which consequently have negatively correlated ¹⁸O contents. Conversely, a section of core taken from Coretsie South 120 (C.S. 120) displays a wide range of (18)^(O)^carb values that closely correlates with percent magnetite. Furthermore, the lowest (18)^(O)^carb values in C.S. 120 are usually associated with very fine grained siderite. These trends indicate that siderite exchanged isotopes with quartz and magnetite while it was fine grained. During metamorphism, siderite that recrystallized to large crystals did not enter into isotopic reactions and became isolated from the system.

Comments

Bibliography : pages 68-74.

Extent

vi, 76 pages

Language

eng

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

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