The Relative Rates of Secondary Hydration in Basalt and Rhyolite, and the use of δD as a Paleoclimate Indicator: Implications for Paleoenvironmental and Volcanic Degassing Studies

Abstract:

The δD-H₂O correlation is important for volcanic degassing and secondary hydration trends. We utilize the caibration of the TC/EA – MAT 253 continuous flow system, which permits us to analyze wt.% H₂O and its δD extracted from 1–8 mg of glass with as little as 0.1 wt% H₂O. Tephra that has been secondarily hydrated with meteoric water is widely used as a paleoenvironmental tool, but the rate of secondary hydration, the relative amounts of primary magmatic (degassed) and secondary meteoric water, and the retention of primary and secondary δD values are not well understood. To quantify these processes, we use a natural experiment involving dated Holocene tepha in Kamchatka and Oregon. Our research illustrates the drastic difference in hydration rates between silicic (hydrated after ~1.5 ka) and mafic tephra, which is not hydrated in the Holocene (similar to results for submarine volcanic glasses), and andesitic tephra with intermediate degrees of hydration. The 0.05-7.3 ka basaltic scoria from Klyuchevskoy volcano retains ≤0.45 wt.% primary magmatic H₂O, with δD values from -99 to -121 ‰. Four other 0.05–7.6 ka basaltic tephra units from Kamchatka with <57 wt.% SiO₂ all have wt.% H₂O 0.21–0.84 and δD values ranging from -90 – -145 ‰. The 1.0–7.6 ka andesitic tephra have slightly higher water contents (0.9–3.0 %) and slightly lower δD values (-113 – -146 ‰). Seven 0.3–7.9 ka silicic samples with SiO₂ >65 wt.% have higher (1.5 -3.4) wt.% H₂O and δD values between -115 – -160 ‰. We interpret the lower δD values and higher water contents (opposite of the magmatic degassing trend) to be a characteristic of secondary hydration in regions of higher latitude such as Kamchatka and Oregon. We are also investigating 7.7 ka Mt. Mazama tephra in Oregon that are known to be fully hydrated and cover nearly 5000 km² northeast of Crater Lake and range in elevation from ~1.3–1.9 km to understand the δD and δ¹⁸O details of the hydrated water’s correspondence with local Holocene meteoric waters. In the future, we plan to use a combination of δD in mid-high latitude precipitation to delineate δD-H₂O hydration trends to better understand the distinction between primary magmatic and secondary meteoric water in volcanic glass, and the exchange of hydrogen isotopes between OH^- and H₂O_mol sites in volcanic glass.