Trace elements in ilmenite: A way to discriminate provenance or age in coastal sands

Abstract

Trace elements and Ti percentage in ilmenite grains magnetically separated from modern and late Pleistocene coastal sands of southeastern Virginia and northwestern North Carolina were used to distinguish different deposits. Multivariate analysis of ilmenite composition (Ti, Mn, Mg, Cr, V, Ni, and Cu) from coastal deposits and potential source rivers enabled the identification of dominant source rivers. Using the trace-element content of one mineral instead of heavy-mineral suites eliminated most of the hydraulic sorting, selective weathering, and intrastratal solution problems that often obscure heavy-mineral provenance determinations. Most ilmenite grains lacked exsolution or twinning, which are common to ilmenite; however, there were no significant optical differences between river and coastal deposits, and thus weathering effects were considered to be negligible in provenance determinations based on ilmenite composition. Owing to the dynamic mixing of beach sands during deposition, they contained more homogeneous ilmenite trace-element values than did river or bay sands. Late Pleistocene and modern beach deposits were compositionally similar, but different from associated bay sands. Bay sands were more similar to different source river deposits than were beach sands. Despite a similar primary or distal provenance, subtle differences in the mixture of proximal sources were revealed between the ilmenite composition of samples from a modern and a late Pleistocene beach deposit. Besides aiding in provenance determination, ilmenite trace-element content thus might be used for distinguishing beach deposits of different ages and for subsurface correlation of discontinuous segments from a barrier-island chain or other similarly well-mixed sand deposit.