Controls on palagonitization versus pedogenic weathering of basaltic tephra: Evidence from the consolidation and geochemistry of the Keanakako'i Ash Member, Kilauea Volcano

On the summit of Kilauea Volcano, incipient palagonitization and consolidation of vitric tephras from the <500‐year‐old Keanakako'i Ash Member of the Puna Basalt have occurred only adjacent to caldera‐bounding faults. Calcite intergrown with palagonitized glass has δ18O values indicative of a hydrothermal origin. Whereas the δ13C of the calcite suggests that it has sequestered carbon from atmospheric as well as mantle sources, radiocarbon dates from the calcites, in addition to calcites from modern steam vents at Kilauea, indicate that >90% of the carbon is mantle derived. We hypothesize that the palagonitized zones may denote the location of fossil steam vents, similar to active vents exposed along circumferential faults on both the north and the south sides of the Kilauea Caldera. Elsewhere, Keanakako'i tephras are weathering into pedogenic products that closely reflect modern environmental conditions. Under acidic soil conditions (pH <6.0), tephra is undergoing dissolution, with the development of opaline crusts on outcrop faces. Under more neutral soil pH (6.5–7.8), the pedogenic products reflect the degree of desilication of tephra mitigated by leaching: dominantly smectite where mean annual rainfall is <50 cm/yr and dominantly kaolinite, allophane, and/or imogolite where rainfall exceeds 250 cm/yr.

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