Olivine-Hosted Melt Inclusions From Fernandina Lavas: the Volatile Content of the Galapagos Plume and Evidence for Melt Interaction With Plagioclase-rich Cumulate
Abstract
Fernandina volcano is considered the center of the Galapagos hotspot for three reasons: it is located at the leading (western) edge of the Galapagos Archipelago with respect to plate motion, its lavas are characterized by extremely high 3He/4He, and it has historically been the most active volcano in the archipelago. This study examines olivine (Fo81-87)-hosted melt inclusions from lavas on the submarine flanks of Fernandina, and focuses on the trace element and volatile contents of pre-aggregated melts. The inclusions are geochemically divided into two groups: the first group is characterized by trace element enrichment and primitive-mantle (PM) normalized patterns similar to whole rock data for lavas from Fernandina. The second group is characterized by significant depletion in incompatible trace elements with unusual (Sr/Nd)PM and (Ba/Th)PM ratios greater than unity. This geochemical signature is typical of plagioclase-rich cumulates; thus, melt inclusion compositions suggest interaction between plagioclase-undersaturated melts and plagioclase-rich cumulate during melt percolation within the oceanic lithosphere. Although this "plagioclase-signature" has been previously observed in the whole rock composition of picritic lavas from central volcanoes in the archipelago, it has not been observed in primitive lavas from Fernandina and Isabela volcanoes. Melt inclusions form prior to eruption, are commonly trapped at pressures exceeding eruption pressure, and are protected by their crystal host from eruptive degassing. Therefore the analysis of melt inclusions is a unique and more constructive approach to understanding the volatile (CO2, H2O, F, Cl, S) content of the Galapagos plume than erupted magmas. Melt inclusions that appear unaffected by the interaction of plume-derived melts with plagioclase-rich cumulate suggest a significantly higher volatile content than MORB (H2O ~0.63-1.10 wt %, CO2 ~80-1800 ppm, S ~300-1500 ppm, Cl ~100-220 ppm, F ~260-470 ppm). Average H2O/Ce (~250), Cl/Ba (~1.4), and F/Nd (~21) ratios are consistent with previously reported values for other high 3He/4He OIB. The CO2- H2O solubility model of Dixon et al (1995) gives an average minimum degassing pressure of ~1 kbar, corresponding to a depth of ~3 km. This is not inconsistent with the work of Geist et al (2006) suggesting a minimum depth of 1-2 km for the magma chamber located beneath Fernandina.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V23C0642K
- Keywords:
-
- 1043 Fluid and melt inclusion geochemistry;
- 1065 Major and trace element geochemistry;
- 3037 Oceanic hotspots and intraplate volcanism;
- 3615 Intra-plate processes (1033;
- 8415);
- 8410 Geochemical modeling (1009;
- 3610)