Abstract

The partitioning behaviors of zinc and manganese can serve as important tracers of diageneticprocesses in limestones. A review of the literature indicates k calcite (super Zn (super 2+) ) , the trace element partition coefficient of zinc into calcite, equals 5.5 and k calcite (super Mn (super 2+) ) nearly equal e 15, at the probable temperatures and solution compositions under which most limestone lithification is achieved. Since k is greater than unity for both these elements, the trace element/calcium ratio in a calcite being precipitated is always greater than the ratio in the solution. Thus in aragonite/calcite transformation by dissolution-reprecipitation in a closed system, the initial calcite is greatly enriched in either zinc or manganese relative to the parent aragonite. As closed system, homogeneous dissolution-reprecipitation progresses, the enrichment of the calcite falls exponentially until the ratio in the calcite being precipitated equals the ratio in the aragonite being dissolved. In an open system, dissolution-reprecipitation usually results in a calcite that is enriched in zinc or manganese. The more pure water that enters and leaves the diagenetic site, the greater excess loss of calcium occurs, since calcium preferentially remains in the liquid phase during calcite precipitation. In order to integrate the information provided by the distributions of manganese and zinc in calcites with that of strontium and magnesium, a general model of trace element behavior is proposed. Using the parameters of distribution coefficient, water flow relative to reaction rate (open or closed), and chemistry of the water before it enters the diagenetic site, it is possible to determine the enrichment or depletion of a calcite relative to its parent aragonite. Four distinct situations are noted. Autodepletion refers to the preferential loss of trace element from the diagenetic site due to a high water flow rate relative to reaction rate, where the trace element has a partition coefficient less than unity. A calcite depleted in trace element relative to the parent aragonite (or calcite) results. Autoenrichment is the preferential removal of calcium from the diagenetic site, with consequent enrichment of the calcite in trace element. This may occur for trace elements with k greater than unity. Alloenrichment is the result of calcite precipitation from a liquid that was enriched in trace element (a trace element/calcium ratio greater than that of the parent aragonite or calcite) before it entered the diagenetic site. It may occur for any value of k, but the trace element/calcium ratio of the liquid required for such enrichment increases with decreasing k. Allodepletion results when a liquid rich in calcium and poor in trace element flushes through the diagenetic site. For values of k close to one, each cycle of liquid may introduce so much calcium that after the liquid dissolves aragonite, its trace element/calcium ratio is still less than that of the aragonite. A depleted calcite is precipitated. Examination of the Zn (super 2+) , Mn (super 2+) , Mg (super 2+) , and Sr (super 2+) content of Pleistocene corals from Barbados, West Indies, reveals an increase in Mn (super 2+) and Mg (super 2+) as a result of aragonite/calcite transformation. Sr (super 2+) was lost in this process and Zn (super 2+) remained approximately the same. Note that unlike k calcite (super Zn (super 2+) ) and k calcite (super Mn (super 2+) ) , k calcite (super Sr (super 2+) ) and k calcite (super Mg (super 2+) ) are both < 1. Such changes denote an open diagenetic water system. Strong intercorrelations among these elements were found in corals altered under late vadose conditions. These suggest that liquid flow rate controlled the trace element chemistry of these corals. Corals altered under phreatic conditions did not exhibit such systematic behavior; their chemistry was likely influenced by neighboring diagenetic events. The general trace element model gives a quantitative indication of the "openness" of these diagenetic systems. On average each liter of diagenetic liquid dissolved and reprecipitated between 10 and 20 millimoles of calcium carbonate. The trace element model can be applied to calcite/calcite recrystallization and cement precipitation later in the evolution of a limestone. Where there is no significant external source of these trace cations, later calcites should be depleted in strontium and magnesium, and enriched in zinc and manganese.

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