Evidence for post-eruptive modification of Li-isotopes and water content in olivine and clinopyroxene phenocrysts from Austral Islands basalts during cooling of lava flows
Abstract
Compositional variations in phenocrysts and melt inclusions are increasingly used to examine magma mixing, degassing, and small-scale source heterogeneities. Water concentrations in pyroxene phenocrysts and olivine-hosted melt inclusions from ocean island basalts have been used to constrain pre-eruptive magma water contents. Both bulk and in situ Li-isotope analyses of phenocrysts and melt inclusions have also been used to constrain Li-isotope variations in mantle source reservoirs (Kobayashi et al., 2004; Chan et al., 2009). However, slow cooling of thick flows allows diffusive exchange between phenocrysts (and inclusions) and basalt matrix. The effects of such exchange must be quantified for phenocryst-based geochemical studies to provide accurate information on magmatic processes and source composition.
We have examined Li-isotope variations in olivine and clinopyroxene phenocrysts in basalts from Raivavae and Rapa, Austral Islands, that were previously characterized for pyroxene H2O content. Pyroxene δ7Li spans a much greater range (-10 to +3.2 ‰) than bulk rocks (mostly +2.8 to +5.9 ‰), and is positively correlated with both average phenocryst size (0.8-6.3 mm) and average pyroxene H2O/Ce (3-63). Olivine δ7Li spans a similar range (-6.9 to +5.4 ‰), and is negatively correlated with measured [Li]ol/[Li]WR. High [Li] and low δ7Li in mineral phenocrysts likely results from diffusive influx of Li into phenocryst phases as magma cooling and crystallization increases the [Li] in the residual liquid and increases the mineral/matrix KD. The correlation with phenocryst [H2O] indicates volatile loss from phenocrysts during cooling. Rim-core transect measurements of [Li] in olivine from one sample with anomalously low olivine δ7Li (-6.9 ‰) indicates a Li-rich rim of ~100 μm thickness. Although the bulk δ7Li values of both pyroxene and (to a lesser extent) olivine have been modified by diffusive exchange, the cores of large (>4 mm) phenocrysts appear to record near-primary δ7Li and H2O/Ce values. Combination of in situ Li-isotope and [H2O] measurements thus may allow extraction of useful information regarding initial magmatic volatile contents even in samples that have experienced variable volatile loss during cooling. Kobayashi et al., Chem. Geol., 2004; Chan, L-H et al., Chem. Geol., 2009- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51I0176L
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 8430 Volcanic gases;
- VOLCANOLOGY;
- 8450 Planetary volcanism;
- VOLCANOLOGY