Palagonite growth rates and O isotopes in basaltic hyaloclastites from natural samples and experiments
Abstract
Hyaloclastites form when an erupting lava interacts with an overlying glacier or body of water. Shards of quenched glass hydrate and rapidly devitrify at these conditions to form palagonite, a metastable phase transitional between basaltic glass and smectite clays. The extent of palagonite formation correlates with both the timescales of emplacement and the thermal history of the hyaloclastite. Oxygen isotope exchange between palagonite and the hydration waters, recorded as the δ18O value of palagonite, is a promising tool to help disentangle the roles of time and temperature, providing constraints on emplacement timescales.
Partial fluorination (reaction with the BrF5 reagent without a laser) of hyaloclastite strips away palagonite without affecting the anhydrous basaltic glass. This allows for the palagonite and unaltered glass to be analyzed separately. We apply this technique to basaltic hyaloclastites from the Reykjanes Peninsula in Iceland. Residual glasses after partial fluorination yield δ18O values consistent with regional whole rock values for basaltic lavas (+4.6 to +5.0‰), which confirms that all palagonite is converted during partial fluorination. In contrast, the palagonite values are extremely high with δ18O values of up to +16.5‰. The highest δ18O palagonite value comes from a sample that lacks any peaks in XRD spectra to suggest the growth of any secondary crystalline phases and has 10-20μm thick palagonite rims. Both extent of palagonitization and δ18O exchange will be calibrated by basaltic glass hydration and alteration experiments. These use an EMORB glass with 0.4 wt.% H2O as a starting material and tap water from Fairbanks, Alaska (δ18O = -19.19‰) at 89°, 130°, and 175°. The pressure at the highest T is ~1 MPa, which roughly correlates to an overpressure of 100-150 meters of ice (depending on the extent of snow and firn densification). The experimental run products will be analyzed for δ18O by the partial fluorination method and analyzed by electron microprobe to determine rates of palagonite formation (or alteration) and δ18O exchange rates at each temperature. The experimental results will then be compared to bulk and microanalytical geochemical and isotopic data from natural samples to constrain temperatures and timescales of hyaloclastite emplacement.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43G0163H
- Keywords:
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- 8408 Volcano/climate interactions;
- VOLCANOLOGY;
- 8414 Eruption mechanisms and flow emplacement;
- VOLCANOLOGY;
- 8424 Hydrothermal systems;
- VOLCANOLOGY;
- 8427 Subaqueous volcanism;
- VOLCANOLOGY