10-14 Comparative Geochemical Analysis of Water–Rock Interactions in a Subterranean Lava Tube, Hawaiʻi

Session: Undergraduate and Graduate Geoscience Student Showcase (Posters)

Poster Booth No.: 71

Presenting Author:

Harper Buchwald

Authors:

Buchwald, Harper O.¹, Rotz, Rachel R.², Craddock, Robert A.³, MacDonald, James H.⁴, Sanderlin, Stacy L.⁵, Brigham, Cassandra⁶

(1) Environmental Geology Program, Florida Gulf Coast University, Ft. Myers, FL, USA, (2) Florida Gulf Coast University, Fort Myers, FL, , (3) Smithsonian Institution, Washington, DC, , (4) Florida Gulf Coast University, Fort Myers, FL, , (5) Florida Gulf Coast University, Ft. Myers, FL, USA, (6) Arizona State University, Tempe, Arizona, USA,

Abstract:

This study examines water–rock interactions within a lava tube on the southeastern tip of the Island of Hawaiʻi to improve understanding of aqueous geochemistry in basaltic environments, including the Moon and Mars. Three sediment samples were collected from the wet lava tube floor, three rock samples from the east and west walls, and three mineral precipitates from wall surfaces. Five water samples were collected from the front, middle, and back of the tube. Temperature, pH, and salinity were measured in situ using a YSI, and samples were retained for ionic, isotopic, and metal analyses. Sediment and rock samples were crushed in a ball mill, pressed into pellets, and analyzed using portable X-ray fluorescence (pXRF) to determine elemental composition. Powder X-ray diffraction (PXRD) identified kaolinite, halloysite, calcite, thenardite, and gypsum. All rocks are classified as porphyritic alkali basalt and plot within the ocean island basalt (OIB) field on the Ti–V diagram. Rock samples are enriched in Mg, Cr, and Ni, with higher Ni in rock than sediment reflecting weathering and geochemical differentiation. Average field measurements indicate cool water (18.9 °C), near-neutral pH (7.68), and slightly brackish conditions (6.6 psu); isotopic compositions cluster between δ¹⁸O = −4.26 to −4.50‰ and δD = −25.8 to −26.9‰ (VSMOW), and major ions include Ca²⁺, Mg²⁺, and Cl⁻. These data indicate meteoric groundwater weakly mixed with seawater. Aqueous samples for metal analysis are still under analysis. Comparisons of rock, sediment, and groundwater chemistry clarify water–rock exchange processes relevant to planetary basaltic systems.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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