Locally Derived Sediment, Yet Chemically and Chronologically Unique – Overcoming the "Stepladder Effect" in Provenance Studies

Abstract:

The trace element composition of clastic sedimentary rocks has frequently been used to characterize sediment provenance. To test the efficacy of whole-rock chemical composition as a provenance indicator, researchers examined alluvium solely derived from the 74 Ma Stepladder pluton (SP), in the Mojave Desert of SE California. Despite the low degree of chemical alteration and minimal transport distance neither trace element concentration or immobile elements ratios accurately reflected that of the source pluton. Furthermore, there is a considerable discrepancy between the frequency of zircon U-Pb ages from the SP and its derived alluvium. The majority (~ 75%) of the zircon grains from the SP are ~ 74 Ma and the remaining 25% are ~ 1.65 Ga. In contrast, ~ 50 to 85% of the detrital zircon, of varying grain sizes, yielded Proterozoic ages. Both the Kolmogorov-Smirnov (KS) and Likeness tests were used to compare all 120 possible samples pairs; the KS test rejected the null hypothesis that the zircon are derived from the same population, and the likeness test showed that most sediments are not like the source. These results, which we refer to as the “Stepladder effect”, led us to consider Sr isotopic composition as a potentially more robust metric of provenance. Several of the sediment samples have similar present-day ⁸⁷Sr/⁸⁶Sr to that determined for four SP samples which range from 0.71055 to 0.71129. However, three sediment samples are distinctly different, suggesting a sedimentary sorting effect (e.g. concentration of biotite in finer sediments). To minimize this confounding effect we will determine the Sr isotopic composition of detrital apatite and biotite to compare with the same minerals of SP source. Using these specific minerals, which represent end-members of Rb-Sr spectrum, may help to constrain the degree of mixing with an externally sourced material (e.g. aeolian-derived grains) that have been added to the alluvium shedding further light on the Stepladder Effect.