Paper No. 95-15
Presentation Time: 11:30 AM
MINERAL ECOLOGY AND EVOLUTION OF MANGANESE: USING REDOX SENSITIVE MINERALS TO PROBE EARTH’S HISTORY
Manganese is the 12th most abundant element in the Earth’s crust (~1000 ppm), has three geologically relevant oxidation states (+2, +3, +4), and 543 out of 5027 (~11%) of known mineral species contain Mn as a major element. By taking advantage of large digital databases, mineralogy can be used in new ways to probe Earth’s unique composition and history. We have used the IMA and Mindat.org mineralogical databases to characterize the frequencies and redox states of all 543 Mn mineral species, in order to gain insight into Mn mineral diversity and the evolving redox state of Earth’s crust. Our analysis, using 19,114 mineral-locality data pairs, reveals that Mn mineral species are very well described by a large number of rare event (LNRE) statistical model, which predicts that ~732 Mn minerals exist on Earth but ~189 are undiscovered. Assignment of oxidation states indicates that a majority of Mn-bearing minerals (433) contain Mn2+, while 89 contain Mn3+ and 38 contain Mn4+, with some overlap due to mixed valence species. These subsets (containing Mn in different oxidation states) also conform reasonably well to a LNRE distribution, and occur in proportions consistent with very low oxygen fugacity in the crust (near the hematite-magnetite buffer, at which only Mn2+ is stable) for the first 2.3 Ga of Earth history. Systematics of the appearance of Mn minerals in the geologic record and implications for the redox history of the crust, including the Great Oxidation Event, will be discussed.