Thursday, May 25, 2006
Surprisingly rapid magma ascent
Basaltic magmas from southern Patagonia have brought to the surface xenoliths containing olivine grains with rims that are depleted in hydrogen relative to the central core. The hydrogen profiles represent a dehydration process having occurred during ascent in the host magma. Demouchy et al (2006) have used experimental hydrogen diffusion data to fit the observed profiles, constraining the xenolith’s ascent rate. Surprisingly, some of these xenoliths appear to have reached the surface from 60-70 km depth within several hours.
Demouchy S, Jacobsen SD, Gaillard F, Stern CR, ‘Rapid magma ascent recorded by water diffusion profiles in mantle olivine’, Geology 2006;34(6):429-432.
Abstract: Mechanisms and rates of magma ascent play a critical role in eruption dynamics but remain poorly constrained phenomena. Water, dissolved in mantle minerals as hydrogen and partitioned into the magma during ascent, may provide clues to quantifying magma ascent rates prior to eruption. We determined the dehydration profiles in olivine crystals from peridotite mantle xenoliths within the Pali-Aike alkali basalt from Patagonia, Chile. The results demonstrate that the amount of water stored in the uppermost mantle has likely been underestimated due to water loss during transport. Using experimental diffusion data for hydrogen, we estimate that the xenoliths reached the surface from 60-70 km depth in several hours, a surprisingly rapid rise comparable to ascent rates for kimberlite magmas.
Demouchy S, Jacobsen SD, Gaillard F, Stern CR, ‘Rapid magma ascent recorded by water diffusion profiles in mantle olivine’, Geology 2006;34(6):429-432.
Abstract: Mechanisms and rates of magma ascent play a critical role in eruption dynamics but remain poorly constrained phenomena. Water, dissolved in mantle minerals as hydrogen and partitioned into the magma during ascent, may provide clues to quantifying magma ascent rates prior to eruption. We determined the dehydration profiles in olivine crystals from peridotite mantle xenoliths within the Pali-Aike alkali basalt from Patagonia, Chile. The results demonstrate that the amount of water stored in the uppermost mantle has likely been underestimated due to water loss during transport. Using experimental diffusion data for hydrogen, we estimate that the xenoliths reached the surface from 60-70 km depth in several hours, a surprisingly rapid rise comparable to ascent rates for kimberlite magmas.
