Towards characterising rhyolitic tephra layers from New Zealand with rapid, non-destructive μ-XRF core scanning

Tephra layers are of importance for the construction of reliable age control in late Quaternary paleoenvironmental and volcanic hazard studies, especially in volcanically-active settings such as the North Island of New Zealand. However, their identification involves time-consuming and destructive processing steps, making the application of non-destructive μ-XRF core scanners potentially advantageous for tephra identification. Here, we investigate the potential of the Itrax μ-XRF core scanner to differentiate between rhyolitic tephra layers sourced from various northern New Zealand rhyolitic volcanic centres deposited in maar lakes of the Auckland Volcanic Field. In their macroscopic form these tephra layers are usually visibly distinct when surrounded by a dark, organic-rich sediment matrix, although their attribution to source volcanic centre and eruption typically requires examination of their mineral assemblages, combined with chemical fingerprinting of the rhyolite glass shards. We demonstrate that μ-XRF core scanning of rhyolitic tephra layers from the Taupo Volcanic Zone and Tuhua Volcanic Centre can also allow identification, and sometimes differentiation, of the tephra using μ-XRF-derived elemental counts, especially high Si, K, Ca and very low Br and Ti. Different rhyolite tephra layers vary in their relative abundances of major, minor and trace elements as is evident from electron microprobe and LA-ICP-MS analyses of their glass shards. Mo-tube based μ-XRF cannot detect Na nor Mg and is of lower reliability for the lighter elements (Ca, Al) which play an important role in traditional tephra fingerprinting. Nevertheless, we are able to demonstrate that μ-XRF core scanning data can distinguish between previously identified tephra layers using multivariate statistics. Furthermore, the study emphasises the need for a standard protocol for μ-XRF core scanning of tephra layers for this approach to be more widely applicable, especially to aid or be a substitute for conventional geochemical approaches used for tephra fingerprinting