Major Volatiles and Helium Isotopes in Site 864 Basalt Glasses

Abstract

The abundance of major volatiles and helium isotopes was measured in three basalt samples from lithological Unit 1 in Ocean Drilling Program Hole 142-864A on "zero-age" oceanic crust. Total volatile content varies from 2.71 to 3.50 cm 3 STP/g in the samples, with H 2 O (0.16 to 0.21 wt% for the rock as a whole) and CO 2 (0.04 wt%) as the main components. Concentration ranges of other gases are as follows (in I0" 3 wt%): N 2 = 9-27; CO = 3-8; O 2 = \.l-\2\ H 2 O = 1.7-1.9, CH 4 = 0.45-0.92. The release behavior of different volatiles during their extraction from the rock by heating within the temperature interval 130°-1230°C was also investigated. From one-third to one-half of the total volatile content was released at 900°-1050°C, whereas the main part of He was released at <750°C. The content of He, I0" 6 cm 3 STP/g, varies from 37-39 in the gross powder fractions (0.5 to 1.0 mm) to 21-22 in the fine ones (<0.05 mm). The 3 He/ 4 He values in the same fractions are 1.18 × I0" 5 and 1.04 × I0" 5 , respectively. The C/ 3 He value in the total mass of volatiles is-1.9X10 9. The obtained results are within the range of data available for normal MORB tholeiite glass.