MICROBIALLY MEDIATED DISSOLUTION OF DETRITAL SANIDINE AND ITS REPRECIPITATION AS AUTHIGENIC ALBITE AND QUARTZ WITHIN EDIACARAN STROMATOLITES—UNIFYING MICROBIAL DISSOLUTION AND PRECIPITATION PROCESSES

Detrital and authigenic feldspars have been found throughout the carbonate matrix of Ediacaran stromatolites in the Deep Spring Formation, Nevada. Mineral phases include endmember sanidine and albite, as well as andesine and abundant quartz. Morphological characteristics of authigenic feldspars here are disparate from those previously reported from carbonates, instead enigmatically displaying wormy morphologies like those found in igneous rocks such as granophyric or melt textures. We propose that the same textures can form from the co-precipitation of mineral phases from heterogeneous solutions within the mineralizing lower portions of an actively growing stromatolite. This model requires the dissolution of detrital sanidine grains by microbial metabolites to utilize K⁺ for osmotic balance, the capture of dissolved Si and Al ions by high-surface-area organic molecules such as exopolymers, and the eventual release of these ions after degradation of organics leading to precipitation of authigenic albite, quartz, and an unidentified K⁺-rich clay within the forming calcite matrix.

Several lines of evidence suggest wormy textures in this environment imply coprecipitation; 1) etch pit formation on large detrital sanidine grains similar to previously reported microbially mediated dissolution features, 2) large, smooth grains of sanidine and andesine with albite rims suggesting partial dissolution of detrital grains and templated authigenesis, 3) mixed sanidine- and andesine-cored grains suggest albite rim grew authigenically, 4) highly abundant albite and quartz grains on the scale of a few microns to submicron dispersed throughout the carbonate matrix, 5) textures differ between the microbialite laminae and those found in the interstitial matrix between stromatolites, as well as between microbialite reef horizons. All grains display granophyric-like wormy textures but differ slightly in phase abundances and texture, suggesting intrinsic properties or microbial mat regions are responsible for authigenic mineralization, as opposed to later-stage diagenesis of the rock through hydrothermal alteration. Together, these lines of evidence suggest that early, authigenic feldspar and quartz is possible within an actively growing stromatolite allowing for syndepositional Ar/Ar dating.