Evidence for transitional and mildly alkalic eruptions during Hawai'i's dominantly tholeiitic shield-building stage: Insights from the Kulanaokuaiki Tephra (≥1.0 ka) at Kīlauea Volcano, HI

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Vitric clasts from a marker horizon in the Kulanaokuaiki Tephra, deposited on the summit and flanks of Kīlauea Volcano, HI, during a prolonged period of explosive eruptions and low magma supply >1.0 ka, show unusual enrichments in alkalis relative to silica and in incompatible elements, in contrast with the volcano's dominantly tholeiitic shield-building lavas. The clasts are transitional basalts, with compositions near the tholeiitic-alkalic boundary (Macdonald and Katsura, 1964). Nearly uniform in composition across ∼200 km2, the clasts are the most proximal to the summit among rare occurrences of transitional and mildly alkalic shield-stage eruptions at Kīlauea. In contrast with the volcano's effusive shield-building style, stratigraphic evidence suggests the clasts were deposited in one of Kīlauea's most vigorous explosive eruptions in the past 2.5 ka—an episode punctuated by high fountaining along with an ∼12 km-tall ash plume. The tephra's composition is consistent with those of five unusual shield-stage Kīlauea lavas. All contain <50.0 wt% SiO2 and 3.0–4.0 wt% Na2O + K2O. All but one contain ≥0.70 wt% K2O, ≥3.0 wt% TiO2, and > 0.30 wt% P2O5. The transitional Kulanaokuaiki clasts also display elevated abundances and ratios of incompatible trace elements (e.g., ∼19 ppm La, La/Yb ∼ 8.6). Compositionally, these early shield-stage clasts and lavas resemble those erupted at the end of shield building at older Hawaiian volcanoes, attributed to lower degrees of partial mantle melting. Chemical constraints and extrinsic circumstances suggest that similar episodes of transitional and mildly alkalic volcanism will likely recur throughout shield building.

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Journal of Volcanology and Geothermal Research