H2O and CO2 contents of basaltic glasses from Mauna Kea volcano, Hawaii

In 1999, the Hawaii Scientific Drilling Project recovered 3098 meters of drill core from Mauna Loa and Mauna Kea volcanoes on the island of Hawaii. Using FTIR spectroscopy, we measured H₂O and CO₂ concentrations in 145 basaltic glasses from the Mauna Kea section of the drill core. All glass samples are from submarine units (hyaloclastites, pillow basalts, intrusives, and massive basalts) at depths of 1103-3096 mbsl (meters below sea level). The analyzed samples contain 0.06-0.84 wt% H₂O and <20-106 ppm CO₂. With one exception, hyaloclastites (1103-2889 mbsl) are degassed (0.06-0.16 wt% H₂O, <20 ppm CO₂); one deep sample (2860 mbsl) contains 0.67 wt% H₂O and 43 ppm CO₂. Pillow basalts define two groups: degassed pillows (0.09-0.15 wt% H₂O, <20 ppm CO₂) shallower than 2132 mbsl, and undegassed pillows (0.16-0.75 wt% H₂O, <20-85 ppm CO₂) deeper than ~2236 mbsl. Intrusive units contain 0.57-0.79 wt% H₂O and 53-106 ppm CO₂ and generally have volatile contents equal to or greater than the volcanic units into which they have intruded. The 11 massive basalts (1126-2502 mbsl) measured have a range of H₂O contents (0.09-0.84 wt%), but only one sample had a CO₂ content (27 ppm) above background. Comparing measured OH and H₂O in all of the samples to experimentally determined water speciation in rapidly quenched basaltic glasses (Dixon 1995) suggests that several pillow basalts have elevated molecular H₂O contents. However, many of the samples with high molecular H₂O are near intrusive units, suggesting that the molecular H₂O content was elevated during interaction with fluids heated by the nearby intrusions. If eruption depths correspond to the pressures at which liquids with the volatile contents of the pillows would be vapor-saturated, then the deepest pillow basalts were erupted at ~1600 mbsl. Combining the present depths of these pillows ( ~3100 mbsl) with a subsidence rate of 2.5 mm/year (Moore 1996), leads to an estimated eruption age of ~600,000 years for the deepest recovered pillow basalts. Although CO₂ concentrations in our samples were modified on degassing during ascent and eruption (and thus reflect eruption pressures), the measured range of H₂O contents over limited depth intervals cannot be explained by degassing. In the section from 2236-2424 where the glasses trend toward "transitional" basaltic compositions, H₂O correlates positively with K₂O, but in strongly tholeiitic sections H₂O is decoupled from K₂O. These results suggest that the sources of the basalts are heterogeneous with respect to H₂O and that these heterogeneities are decoupled from variations in incompatible elements such as K₂O. Moore, J.G., B.L. Ingram, K.R. Ludwig, and D.A. Clague, Coral ages and island subsidence, Hilo drill hole, J. Geophys Res., 101, 11599-11605, 1996. Dixon, J.E., E.M. Stolper, and J.R. Holloway, An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part I: Calibration and Solubility Models, J. Petrol., 36, 1607-1631, 1995.