230Th- 238U disequilibrium systematics in oceanic tholeiites from 21°N on the East Pacific Rise

Abstract

Significant disequilibrium occurs between ^(230)Th and its parent, ^(238)U, in a suite of fresh basalt glasses from the RISE Project study area at 21°N on the East Pacific Rise. The (^(230)Th/^(232)Th) activity ratios observed for eight of nine samples from the crest of the axis at this site are constant within analytical uncertainty, with a value of 1.22. This observed homogeneity of (^(230)Th/^(232)Th) has two possible interpretations. First, the measured (^(230)Th/^(232)Th) can be considered to indicate a mantle-source for the RISE basalts with Th/U of 2.5. This interpretation, however, conflicts with the proposed correlation between (^(230)Th/^(232)Th) and ^(87)Sr/^(86)Sr [1] which predicts that (^(230)Th/^(232)Th) should equal 1.33 at the RISE site. A second possible interpretation is that radioactive decay of ^(230)Th, in the basalts themselves or in a magma chamber, has decreased (^(230)Th/^(232)Th) from 1.33 to the observed values. The required time span is 11,000 to > 100,000 years. However, petrologic arguments rule against long residence time in a magma chamber, and the spreading rate of this section of the East Pacific Rise (6 cm/yr) predicts that the maximum age for axis basalts is 27,000 years. Both interpretations of the measured (^(230)Th/^(232)Th) imply a low Th/U ratio for the RISE basalt source and suggest that the MORB source at this location is depleted in Th with respect to U relative to primitive mantle or bulk earth. In spite of their constant (^(230)Th/^(232)Th), the basalts from 21°N have wide ranges of measured Th/U and thorium and uranium concentrations, in apparent conflict with the common assumption that these two elements have very small, similar crystal-liquid partition coefficients. Participation of an accessory phase with high Th and U concentrations during partial melting or fractional crystallization appears to be required to explain this anomaly. Major and other trace element compositions provide more information about petrogenetic processes. Although fractional crystallization can explain the variations in major element composition, a more complex process such as continuous melting is required to account for the observed trends in trace element compositions. In addition to its use as a tracer, the ^(230)Th-^(238)U disequilibrium technique is capable of putting some constraints on the timing of fractionation events, and it may eventually lead to a method for dating very young mid-ocean ridge basalts.