THE MILLBRIG ASH IN CONTEXT OF PRECEDING ERUPTIONS: PRELIMINARY U-PB AND TRACE-ELEMENT ANALYSIS OF ZIRCON IN ORDOVICIAN PENNSYLVANIA BENTONITES

Explosive Ordovician eruptions off the modern east coast of North America deposited extensive ash beds, now bentonite/meta-bentonite, across the continent. These represent a complex chapter of tectono-magmatic history. We focus on the Millbrig, locally known as B14, and three preceding ashes (B11, B12-the Deicke, and B13) from a 17.5 m thick stratigraphic succession in State College, PA. By placing the Millbrig in the context of these older ashes, we aim to contribute new insight into temporal relations, magmatic conditions, and source material for Ordovician volcanism. We focus on zircon, as post-depositional alteration to bentonites overprinted most of the other original material. We analyzed zircon with laser-ablation split-stream ICPMS (n=222), guided by CL imagery. Median U-Pb ages (<3% error) of B11 (457 Ma), B12 (481 Ma), B13 (453 Ma), and B14 (455 Ma) indicate that the magmas experienced closely timed crystallization. There are broad geochemical similarities between the deposits: on discrimination diagrams (U vs Yb, U/Yb vs Hf, and U/Yb vs Y) zircons from all four ashes cluster in the continental realm (Grimes et al., 2007). Model melt REE compositions indicate that all four ashes are enriched in LREE relative to chondrite (Nd-Gd ~250-35) and depleted in HREE (Sano et al., 2002, Claiborne et al., 2020). These similarities suggest zircon crystallization in a continental arc setting. Our preliminary data reveal intriguing differences between the ashes. On REE/CI plots, model melts for B11 show greater differences between MREE and HREE (Gd/Tm: ~3.5) compared to B12, B13, and B14 (Gd/Tm: ~2.3). B13 is more enriched in HREE (Dy-Lu: ~50) relative to B11, B12, and B14 (Dy-Lu: ~10). Hf-in-zircon reflects the extent of magma evolution: B11 is more primitive (8795 ppm) and B12 is more evolved (10410 ppm) than B13 and B14 (~9500 ppm). Using Ti-in-zircon as a proxy for temperature, B13 (13 ppm) experienced warmer crystallization conditions than B11, B12, and B14 (~7 ppm). These trace element signatures suggest unique magmatic conditions and extents of evolution leading up to each eruption. Our subsequent work will focus on the influence of the depositional environment on the accumulation of eruption products and investigate source materials contributing to these magmatic systems (as recorded by rare xenocrystic cores).