24. PRE-ERUPTIVE H₂O AND CO₂ CONTENTS OF MAFIC MAGMAS FROM THE SUBMARINE TO EMERGENT SHIELD STAGES OF GRAN CANARIA ¹

Paul J. Wallace ²

ABSTRACT

Volcaniclastic sediments recovered from the clastic apron of Gran Canaria during Leg 157 provide a rare opportunity to study basaltic volcanism from the early shield-building phase of an oceanic island. Infrared spectroscopic analyses of trapped melt (glass) inclusions from Miocene hyaloclastites, many of which are picritic, generally show very low pre-eruptive dissolved H₂O concentrations (0.08 to ~0.2 wt% H₂O). Two glass inclusions from a differentiated basaltic hyaloclastite have significantly higher water concentrations (1.2–1.4 wt%). The speciation of water in these two inclusions is consistent with the high-temperature equilibrium for water in basaltic melts, indicating that the high total dissolved H₂O concentrations are not caused by secondary hydration. The low dissolved H₂O concentrations of most of the glass inclusions are comparable to the lowest values found in mid-ocean-ridge basalts, whereas the H₂O-rich glass inclusions are similar to the highest values reported for submarine alkalic glasses from Hawaii. Most inclusions have dissolved carbonate contents between 100 and 350 ppm CO₂ . A single inclusion has a much higher CO₂ content (1426 ppm) that indicates pyroxene crystallization and inclusion entrapment at a pressure of ~2800 bars. All other inclusions must have formed by pyroxene crystallization at an average pressure of 500 ± 100 bars, equivalent to ~1-3 km depth beneath the subaerial Miocene volcanic edifice of Gran Canaria. Such low pressures of crystallization for picritic magmas suggest that during the shield-building phase of Gran Canaria, ascending batches of picritic magma were sufficiently CO₂ -rich to be positively buoyant relative to shallow-stored magma. Comparison of degassing models for basaltic melts with measured vesicularities of altered glass shards from the hyaloclastites indicates that much of the glass shard and crystal debris, from which the hyaloclastites were deposited, originally formed in submarine eruptions at water depths <~500 m.

¹ Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998. Proc. ODP, Sci. Results, 157: College Station, TX (Ocean Drilling Program).
² Ocean Drilling Program and Dept. of Geology and Geophysics, Texas A&M Uni-versity, College Station, TX 77845, U.S.A. paul_wallace@odp.tamu.edu