ISOTOPIC-THERMOCHRONOLOGIC CHARACTERIZATION OF APATITE AS A NEW PROXY FOR PROVENANCE ANALYSIS

Many studies of sedimentary provenance have used tools such as major and trace element geochemistry of clastic sediment, heavy mineral analysis, and U-Pb dating of detrital zircon to characterize sediment sources. In previous studies of modern sediment from the Stepladder Mountains, SE California, neither trace element geochemistry nor detrital zircon ages properly correlated source to sink. This is unexpected as these first-cycle sediments were collected within 7 km of the Stepladder pluton, a 74 Ma granodioritic body that is a point source for the sediment. Despite these near ideal conditions, for most sediment samples, of a variety of grain sizes, the K-S test (i.e. the null hypothesis that two zircon subpopulations were derived from the same source) is rejected. In order to overcome this lack of correlation of source to sink, called the “Stepladder Effect”, we are developing a novel dual technique to characterize apatite from both the pluton and the sediment. By jointly obtaining (U-Th)/He dates and Sr isotopic compositions of the same apatite from the pluton and detrital samples, our initial goals are to constrain the recent thermal history of the pluton, determine the original magma sources, and evaluate if the approach is effective as a new dual provenance proxy.

In order to test the viability of the new isotopic-thermochronologic method, the Durango apatite standard was analyzed, yielding (U-Th)/He dates of 31.5 ± 1.1 Ma which is within previously published values. Subsequently, apatite from the Stepladder pluton were analyzed, yielding a (U-Th)/He apparent age of 20.7 ± 1.90 Ma, consistent with a reported Stepladder Mountain apatite fission track age of ~24 Ma (Foster et al., 1990). Apatite from the pluton yielded ⁸⁷Sr/⁸⁶Sr of 0.71056, consistent with an origin of the granitic magma by anatexis of Precambrian crust. Detrital apatite from the alluvium will next be isotopically and thermochronologically characterized in order to test the fidelity of this new provenance tool, which seems to have a very promising outcome.