Inorganic Geochemistry of Carbonate Shelf Rocks: ABSTRACT

Abstract

Carbonate rocks constitute approximately 20% of the sedimentary record; their economic value is even more important than this percentage would indicate. For example, carbonate rocks contain about 50% of the world's known petroleum reserves; they serve as important host material for base-metal deposits; and they are important industrial minerals. At the time of formation, by organic or inorganic processes, carbonate rocks consist primarily of the two calcium carbonate minerals, aragonite and calcite. Aragonite is metastable with respect to normal, low-magnesium calcite. In addition to calcium, these minerals commonly contain varying amounts of other divalent cations, especially magnesium, strontium, manganese, iron, and barium. The ecosystem of the depositional environment is reflected by the trace-element composition of the carbonates. For example, strontium and magnesium content of carbonates increases near a reef complex and reflects the aragonitic and high-magnesian calcitic carbonates of organic origin. The inorganic precipitation of aragonite rather than calcite is favored by the presence of strontium ion, warm water, hi h pH, high ionic strength, and pronounced supersaturation of the water with respect to calcite. The precipitation of calcite is inhibited by a high magnesium content of the solution. Calcite may contain several mole percent magnesium which substitutes for calcium in the lattice; some organisms contain as much as 30 mole percent magnesium. High-magnesian calcite is even more metastable than aragonite and generally inverts to low-magnesian or relatively pure calcite. Aragonite, with time, generally inverts to calcite although it is known to occur in shells from rocks at least as old as early Paleozoic. Indeed, one of the enigmas of carbonate geochemistry is that normal modern marine deposits are composed predominantly of the metastable phases, aragonite and high-magnesian calcite, whereas ncient rocks are chiefly low-magnesian calcite and dolomite. End_Page 720------------------------------ Recent dolomite has now been found in several geologic environments--in restricted marine environments, closed basis, and reefs. Other than reefs, these occurrences cannot account for the enormous amount of dolomitic rock in the record. One might say that the dolomite problem is in reality a magnesium problem for the main difficulty seems to be to get sufficient magnesium in solution to dolomitize calcite or aragonite. It is probable that many regional dolomites are formed by the action of interstitial waters, of sufficient magnesium activity, on a calcitic or aragonitic precursor. The isotopes of carbon and oxygen have played a significant role in unraveling prior history of carbonate deposits. Oxygen isotopes have been extremely useful in developing a paleotemperature scale for determining the ecological environment of ancient marine organisms. Carbon-isotope composition has been used to differentiate between marine and nonmarine limestone deposits. However, these data must be interpreted with care; recent work has indicated that, above the water table and in a tropical environment, extensive carbon-isotope alteration may occur which can greatly change the isotopic composition of any or all carbonate minerals above the regional water table. Also, evaporative processes may alter the isotopic composition of a solution and of the minerals precipitating from it; t ese effects can lead to erroneous interpretations of ancient geologic environments. Trace element and isotopic studies of the carbonate rock minerals and similar studies of the coexisting aqueous phase can aid greatly in understanding the environment of deposition, diagenetic changes which have occurred, in predicting regions of porosity development and dolomitization, and in unraveling conditions in the geologic past. End_of_Article - Last_Page 721------------