Mantle vs. Crust: Untangling influences on ocean island basalt stable O and H isotopic composition via tandem olivine-glass analyses at Kamaʻehuakanaloa Volcano, Hawaiʻi

Cunningham, Molly Jean

Mantle vs. Crust: Untangling influences on ocean island basalt stable O and H isotopic composition via tandem olivine-glass analyses at Kamaʻehuakanaloa Volcano, Hawaiʻi

Files

Cunningham_hawii_0085O_11559.pdf (1.71 MB)

Date

2022

Authors

Cunningham, Molly Jean

Advisor

Pietruszka, Aaron J.

Department

Earth and Planetary Sciences

Abstract

The stable O and H isotopic composition of ocean island basalts reflect the interplay of deep and shallow magmatic processes such as melting of a heterogeneous mantle source and crustal contamination. For Hawaiian volcanoes, it has been debated whether lavas retain their source-derived isotopic compositions, or if these signatures are overprinted by assimilation of hydrothermally altered materials. New δD and δ18O analyses of glass and olivine from the youngest Hawaiian volcano, Kamaʻehuakanaloa (Kamaʻehu; formerly Lōʻihi), clarify the extent to which magmatic contamination influences O and H isotope ratios of erupted lavas. We find that in most samples, assimilation of seawater-derived fluids is minor: this process has elevated δD, Cl/K2O, and H2O in some glasses, but most samples retain mantle-like δD (-60 to -90‰) until eruption. In contrast, Kamaʻehu lavas demonstrate variation in olivine δ18O (δ18Ool = ~4.5 to 5.4‰) and glass δ18O (δ18Ogl = ~5.0 to 6.2‰) that is greater than that expected from melting of simple peridotitic mantle. We find that different regions of the volcanic edifice erupt lavas that are compositionally distinct in their δ18O: North Rift Zone lavas are relatively 18O-enriched (δ18Ogl = ~5.6‰); South Rift Zone lavas are relatively 18O-depleted (δ18Ogl = ~5.3‰); and lavas from the summit region have intermediate δ18O values (δ18Ogl = ~5.4‰). We resolve these observations into an isotopically consistent model of the Kamaʻehu magmatic plumbing system. Over time, differences in the temperature of circulating hydrothermal fluids may have altered basalt in the volcanic edifice to high δ18O in the NRZ and low δ18O in the SRZ. Magmas with initial mantle-derived δ18O (δ18Oliq = ~5.4‰) ascend into the shallow volcanic plumbing system and assimilate this hydrothermally altered rock, causing individual magmas to shift toward the δ18O value of local assimilants. The degree of this contamination may be greatest in the rift zones and least in the summit due to a higher rate of magma supply to the summit reservoir system. δ18Ool tracks with regional differences in δ18Ogl, indicating that the assimilation process begins before or during olivine fractionation. Finally, olivine entrainment and lack of homogenization prior to eruption preserve isotopic heterogeneity on small spatial scales in erupted lavas. Variations in δ18O for Hawaiian lavas thus may be controlled by processes operating within the shallow volcanic plumbing system, overprinting variation derived from melting of a heterogeneous mantle source. To determine the true, mantle-derived O isotopic signature of a volcano, systematic analyses of many samples are required.

Keywords

Geochemistry, Hawaiian volcanoes, hydrogen isotopes, magmatic assimilation, mantle, oxygen isotopes, seamount

URI

https://hdl.handle.net/10125/104620

Extent

41 pages

Rights

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Collections

M.S. - Earth and Planetary Sciences

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