Journal of Conference Abstracts

Replacement of Aragonite by Dolomite in a 12.4 ka Tufa Mound, Great Salt Lake, Utah

Vicki Pedone (vicki.pedone@csun.edu)¹ & J. A. D. Dickson (jadd1@esc.cam.ac.uk)²

¹ Dept Geological Sciences, California State University, Northridge, CA 91330-8266, USA

² Dept. Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK

On the west side of the Great Salt Lake, a series of large, >1.5-m-thick, crudely layered stromatolitic tufa mounds are partly exposed at the present-day lake elevation of ~1280 m. With the exception of rare calcitic tufa (AMS radiocarbon age of 12.4 ka), the mounds consist of extremely porous dolostone, with about 10% relict aragonite. In thin section, dolomite preserves fine details of the structure grumeleuse fabric of the porous microbial framework of the aragonitic tufa. The most striking textural difference is that in aragonitic parts of the tufa, pores are lined by fibrous aragonite cement; and in the dolomitized parts, pores are lined by clear, nearly spheroidal dolomite. SEM images show that at the dolomite-aragonite interface, aragonite matrix and cement are partly dissolved. Scattered among 1- to 3-micron, slightly bladed aragonite micrite and 10- to 50-micron-long aragonite cement needles are 4- to 10-micron, spheroidal dolomite crystals with stepped faces and rounded edges and corners. In general, the contact between the two minerals is sharp. Dolomite spheroids grow over and around aragonite needles. In rare cases, the contact between dolomite and aragonitic micrite is ragged; and the dolomite spheroids are only partly formed. This suggests that dolomitization can only proceed by dissolution of the aragonite. This close association could explain the retention of original micritic fabric by the dolomite, despite the increase in crystal size. However, dolomite is not mimetic because it preserves neither the original size nor shape of aragonite micrite and cement. In pervasively dolomitized areas, crystals vary in size from 4 to 20 microns and retain the rounded shapes. Dolomite in the tufa is more soluble in cold HCl than standard dolomite and stains a faint pink with alizarin red-S. Microprobe analyses of the dolomite (n=94) yield an average 54.1 mole% CaCO₃, in good agreement with the 44 to 45 mole% MgCO₃ content determined from the position of the 10.4 reflection in the XRD pattern. XRD data also show that the dolomite is poorly ordered. Using 01.5/11.0 as a measure of order in the crystal, standard dolomite has a ratio of 0.52. Dolomite from the Great Salt Lake has a ratio of 0.18. The water of the Great Salt Lake has likely been an excellent dolomitizing fluid since the end of pluvial conditions ~12,500 years ago. For example, during the last highstand in 1986 when the mounds were fully submerged, the water had a Mg/Ca ratio of 11 and a Mg abundance of 1.7 g/L. During the Holocene, the mounds have been emergent for longer periods than they have been submergent. Hence, dolomitization probably has occurred in discreet pulses of during episodes of submergence. Owing to the pervasive replacement, the process must occur rapidly during each pulse. Expansion of the lake during episodes of submergence might have resulted in decreasing the saturation of the water with respect to aragonite to the point where aragonite could dissolve and dolomite precipitate.

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