Geochemistry of herringbone calcite from an ancient Egyptian quarry

by
Sarah A. Tourre and Dawn Y. Sumner

2000, GSA Annual Meeting, Abstracts


Well preserved herringbone calcite and associated fibrous calcite from ancient Egyptian to modern quarries precipitated in fractures and caves in the Middle Eocene Mokattam Formation and equivalent units, Eastern Desert, Egypt. Ancient Egyptians used it in monuments and sculptures, and archeologists have studied herringbone calcite to determine its geographic provenance. The origin of herringbone calcite is also geologically interesting because of its unique macroscopic banding and optical properties. It also has been proposed as an indicator of anoxia, which is inconsistent with new results from the Egyptian herringbone calcite.

The geochemistry of herringbone calcite and associated fibrous calcite e indicates that they precipitated from warm, meteoric water. Na concentrations less than the detection limit of 100 ppm suggest that herringbone and fibrous calcite did not precipitated from seawater or a brine. Delta 18O values range from -10.38 to -12.47 per mil +/- 0.06 VPDB and are consistent with precipitation at temperatures of 46-86 degrees Celsius if the initial delta 18O of the water was 0 to -4 per mil. Delta 13C values range from -0.74 to -7.39 per mil +/- 0.04 VPDB, indicating that some CO2 was derived from organic decay.

Elemental analyses suggest that herringbone and fibrous calcite precipitated from oxidized, Sr-rich waters. Mn and Fe concentrations in calcite are less than or equal to 20 ppm and 125 +/- 4 ppm, respectively, despite the presence of abundant iron oxides, demonstrating oxidizing precipitation conditions. Sr concentrations range from 200-5,130 +/- 30 ppm, and Mg content ranges from 0.5-6.5 +/- 0.2 mol% MgCO3. In general, Sr and Mg concentrations are lowest in less well preserved opaque herringbone and fibrous calcite and highest in well preserved translucent and creamy calcite. Translucent fibrous calcite and translucent and creamy herringbone calcite show positive correlations among Mg concentrations, Sr concentrations, and delta 13C. Two models may explain the correlations: 1) variable mixing of a low Mg, Sr, delta 13C water with a high Mg, Sr, delta 13C water and 2) variable influxes of organically derived CO2 causing changes in precipitation rate and resulting crystallographic influences on elemental incorporation.



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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616
sumner@geology.ucdavis.edu

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