Melt Flux from the Mantle Regulates the Crustal Processing and δ18O Variations of Kamaʻehuakanaloa Magmas
Authors: Pietruszka Aaron J , Cunningham Molly J , Bindeman Ilya N , Garcia Michael O , Boro Joseph R , Burns Dale H , Jiang Peng 
Published: January 2025

Ocean-island basalts display a relatively large range in their oxygen isotopic composition (δ18O) compared to mid-ocean ridge basalts (MORB). The origin of these variations in δ18O—due to a combination of crustal contamination and/or mantle heterogeneity—is controversial. Kamaʻehuakanaloa (formerly Lōʻihi Seamount) is an active submarine Hawaiian pre-shield volcano that is last known to have erupted in 1996. Basalts from Kamaʻehu (for short) are derived from a distinctive 3He-rich deep mantle source within the Hawaiian plume, yet they commonly experience at least two types of shallow crustal contamination based on enrichments in seawater-derived Cl and 234U. Here, we present oxygen isotopic analyses of volcanic glasses (n = 102 from 53 samples) and single olivine crystals (n = 47 from eight samples) for tholeiitic (n = 28), transitional (n = 4), and alkalic (n = 18) basalts, and three hawaiites from Kamaʻehu. The average δ18O values of both glass and olivine from the North Rift Zone and NE summit platform (~5.6‰ and 5.1‰, respectively) are higher (at &gt;95% confidence) than those from the South Rift Zone and the SW summit platform (~5.3‰ and 4.9‰). The glass alkalic index (AI&lt;0 is the switch from alkalic to tholeiitic lavas as the degree of mantle melting increases) and incompatible element enrichment (K2O/TiO2) increases with decreasing MgO and CaO/Al2O3. The northern Kamaʻehu glasses are more frequently alkalic (~68%), more enriched, and more differentiated than those from southern Kamaʻehu (~81% tholeiitic or transitional). Model eruption ages from 226Ra–230Th disequilibria suggest that the transition from alkalic to tholeiitic volcanism at Kamaʻehu was nearly complete by ~2 ka. The predominantly alkalic northern lavas likely record an earlier phase of the volcano’s eruptive history that has since been covered by the more recent eruptions of tholeiitic basalts to the south. These observations suggest that melt flux from the mantle (recorded by the AI values and K2O/TiO2 ratios) regulates the crustal processing and δ18O variations of Kamaʻehu magmas. The mantle-controlled transition from alkalic to tholeiitic volcanism at Kamaʻehu led to the more frequent supply of larger magma batches produced by higher degrees of mantle melting, establishment of an active shallow hydrothermal system for high-temperature alteration of the volcanic edifice, and a decreasing extent of clinopyroxene fractionation. The average δ18O values of glass (~5.4‰) and olivine (~5.0‰) from Kamaʻehu—similar to the most depleted MORB—represent the best estimate for mantle-derived magma at this volcano. The higher average δ18O values of the glass and olivine from northern Kamaʻehu result from assimilation of volcanic edifice that was altered by seawater-rock interaction at low temperature, whereas the lower δ18O values from southern Kamaʻehu result from assimilation of such materials altered at high temperature.