The abundance of major volatiles and helium isotopes were measured in three samples of basaltic glasses from East-Pacific Rise (EPR). These samples are represent the lithological Unit 1 in Ocean Drilling Program Hole
142-864A on "zero-age" oceanic crust.
Total volatile contents vary from 2.71 to 3.5 cm3STP/g in the samples, with H2O (0.16 to 0.21 wt% for the rock as whole) and CO2 (0.04 wt%) as the main components. Concentration ranges of other gases are as follows (in 10-3 wt%): N2 = 9-27; CO = 3-8; O2 = 1.7-4.2; H2 = 1.7-1.9;
CH4 = 0.45-0.92. The release behavior of different volatiles during their thermal extraction from the rock within the temperature interval 130°-1230°C was also investigated. The sum of H2O and CO2 contents is noticeably less than the total volatile contents from EPR, Mid-Atlantic Ridge (MAR) and Galapagos Spreading Center (GSC) (Byers et al., 1986).
The H2O/CO2 ratio in the Site 864 basalts is the inverse of that observed at 21°N on the EPR and is the similar to that in the MAR rocks. The Site 864 basalts contain 2 times as much H2O as GPS rocks and 1.5 times as much as those from the 21°N area. In contrast, the CO2 and CO contents in the studied samples are the least values among those observed from other spreading centers, whereas the CH4 content is the highest. Partial contents of volatile components in the studied samples vary significantly, except for CO2. The differences in contents of some components are interrelated: the increase of H2 and CO is accompanied by a proportional decrease of CH4, as well as of N2. This may possibly result from chemical reactions in which H2, CO2 and CH4 participate. A significant part of the volatile components is released from the rocks under heating at >750°C, during which the vesicles decrepitate and the specimen melts, whereas almost all the helium is released at <750°C (i.e., before the separation of volatiles enclosed within the vesicles).
Hence, the release of helium from vesicles begin before their destruction, presumably as a result of preferential termal diffusion of helium through the basalt glass. Helium abundance in the Site 864 rocks, (37 to 40)¥10-6 cm3STP/g, is similar to the highest He concentrations found in some glasses from 21°N at the EPR (Ozima and Zashu, 1982). The values of the 3He/4He ratio in the Site 864 basalts vary slightly according to the size of the studied fragments. The ratio is a little lower in helium extracted from the fine powder (<0.05mm) where the most vesicles were destroyed. This suggest that the glass matrix is contaminated by seawater containing dissolved atmospheric helium. The gross powder (0.5-1.0 mm) contains helium with a 3He/4He ratio of 1.18¥10-5 inherent to MORB. Calculated ratio of the isotopic composition in vesicles is (1.32-1.38)¥10-5. This is one of the highest known 3He/4He values ever measured in MORB basalts.
The C/3He ratio in the Site 864 rocks is 1.9¥109; it is a typical value for MORB glasses. Thus, the rocks recovered from Hole 864A do not differ in helium isotope composition, as well as in the abundance of helium and carbon, from normal mid-oceanic ridge basalts. There is no evidence for the contamination of their parental melts by either the recicling crust (lithosphere), or the undepleted mantle (i.e., by material from any reservoirs distinguishable from the depleted mantle).
Byers, C.D., Garcia, M.O. & Muenow, D.W., Earth Planet. Sci. Lett. 79, 9-20 (1986).
Ozima, M. & Zashu, S., Earth Planet. Sci. Lett. 62, 24-40 (1982).