HEMMING, N. G., G. N. HANSON, and W. J. MEYERS, State University of New York at Stony Brook, Stony Brook, NY

ABSTRACT: Host Rock Control on the Boron Concentration and Isotopic Composition of Ground Water from the Edwards Aquifer of Central Texas

The isotopic composition of boron in ground water sampled from carbonate

host rocks (Edwards Group and Georgetown Limestone) from three locations in central Texas is +25 to +32 per mil (relative to National Bureau of Standards SRM 951 boric acid). This is similar to modern marine carbonates which are +22 +/- 3 per mil. These data are consistent with a model where the Cretaceous carbonate host rocks and associated evaporites are the dominant source of boron in the water samples. This assumes Cretaceous seas had a similar isotopic composition to modern oceans, and consequently Cretaceous carbonates have a boron isotopic composition similar to modern carbonates. Modern marine carbonates have boron isotopic compositions from +19 to +25 per mil and boron concentrations from 11 to 75 ppm, while modern sea water has a boron isotopic composition of about +40 per m l and boron concentration of 4.5 ppm. We have interpreted that the boron incorporation into carbonates is strongly controlled by pH. The isotopic offset of boron in carbonates from boron in modern sea water is the result of incorporation of the isotopically lighter B(OH)<4>{-} species in carbonates. This species comprises about 20% of aqueous boron in sea water and has the same calculated isotopic composition as modem marine carbonates at pH=8.

Water samples were collected from Natural Bridge Caverns, near San Antonio, Texas (pH=7.3), Aquarena Springs, San Marcos, Texas (pH=7.4), and Inner Space Caverns, Georgetown, Texas (pH=8.1) The boron concentrations range from 0.034 to 0.050 ppm. Dilute rain water interacting with carbonate host rocks releases boron into the ground water system. Meteoric cements and cave deposits may precipitate from these ground waters. These secondary precipitates are predicted to have lower concentrations than the original rock because of: (1) the generally low pH of ground water and (2) the low concentration in the ground water. These precipitates are also predicted to have isotopic compositions offset by approximately -18 per mil from the ground water. Therefore, we predict that meteoric alteratio of carbonate rocks will result in lower boron concentrations and isotopic compositions.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.