PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF DEL PUERTO CANYON CARBONATE: BIOPRECIPITATION IN ULTRAMAFIC-HOSTED ALKALINE CREEKS (CALIFORNIA COAST RANGE, USA)

We studied the petrography and geochemistry of Ca-Mg carbonate cements from creek beds and drainage areas within the Del Puerto Ophiolite, situated roughly 100 km SE of San Francisco in the California Coast Range. The creeks are fed by rainwater and seeping spring water that has interacted with serpentinized ultramafic rocks; they are moderately alkaline (pH>8.3), with high Mg (~120 mg/L) and bicarbonate (>400 mg/L).

The carbonate cements line these creek beds, forming a conglomerate with creek sediments and clasts ranging in size from submillimeter-sized grains to pebbles several inches in diameter. Thermodynamic constraints preclude the precipitation of Mg-rich carbonate in such a setting, and therefore we assume that the cements formed by bioprecipitation. We examined ~20 petrographic thin sections from three creek sites, in order to characterize the cements in detail. In addition, we performed EMP and SEM-EDX analyses of selected areas. We observed four different cement textures: turbid cement with micritic regions, cement lamellae consisting of tens of bands growing parallel to grain boundaries, blocky laminar cement ranging in thickness from 5-30 microns and blanketing grain boundaries and these first two cement types, and a dentoidal carbonate lining void spaces. EMP analyses revealed a range in composition from approximately stoichiometric dolomite to calcite. Other interstitial materials included silts and alteration minerals typical of serpentinized host rocks. SEM-EDX analyses revealed relations between cement textures and compositional variations. Our geochemical data will be discussed in the context of biomineralization and preservation of biosignatures in the rock record. This work was supported in part by NSF’s REU program, AST-0552751.