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

An investigation of the boron isotopic composition of hydrated berates in the Kramer lacustrine deposit of southern California was undertaken in an effort to better understand 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 delta(11)B for borax through the 24.9-m-thick Na-borate core facies is +0.1 to +1.7 parts per thousand except for samples in and just below a clay-rich 6-m interval where delta(11)B varies from -5.1 to +2.3 parts per thousand. The delta(11)B 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 parts per thousand, whereas two samples from a 6.1-m-thick basal Na-Ca-borate facies both yield -2.1 parts per thousand. The delta(11)B of colemanite from near the top of the upper Na-Ca-borate facies is -8.6 parts per thousand. The small range of delta(11)B through much of the Na-borate facies indicates that the source waters of the borax-precipitating lake varied little in delta(11)B for a time interval of (5-7). 10(4) yr. The delta(11)B 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 delta(11)B 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.