A boron isotopic study of a mineralogically zoned lacustrine borate deposit: The Kramer deposit, California, U.S.A

An investigation of the boron isotopic composition of hydrated borates in the Kramer lacustrine deposit of southern California was undertaken in an effort to better inderstand the origins of mineralogically zoned deposits of this type. Twenty-one samples from fifteen depths along a drill core through the deposit reveal that isotopic zoning accompanies mineral zoning. The range of δ 11B for borax through the 24.9-m-thick Na-borate core facies is + 0.1 to + 1.7%. except for samples inand just below a clay-rich 6-m interval where δ 11B varies from −5.1 to + 2.3%. The δ 11B of three cottonball ulexite samples in a 3.0-m-thick NaCa-borate facies above the Na-borate facies ranges from −5.5 to −4.6%., whereas two samples from a 6.1-m-thick basal NaCa-borate facies both yield −2.1%. The δ 11B of colemanite from near the top of the upper NaCa-borate facies is −8.6%. The small range of δ 11B through much of the Na-borate facies indicates that the source waters of the borax-precipitating lake varied little in δ 11B for a time interval of (5−7) · 10 4 yr. The δ 11B variations within and among borax crystals in the 6-m clay-rich interval, some of which are estimated to have occurred over a period of weeks, were probably produced by a combination of pH change and Rayleigh effect during partial desiccation cycles. The distinctly different ranges of δ 11B exhibited in the upper and lower NaCa-borate facies and the distinctive cottonball habit of the crystal aggregates indicate that ulexite originated through growth in the lake-margin muds.