He and Ne isotopes in oceanic crust: implications for noble gas recycling in the mantle
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In an attempt to determine the helium and neon isotopic composition of the lower oceanic crust, we report new noble gas measurements on 11 million year old gabbros from Ocean Drilling Program site 735B in the Indian Ocean. The nine whole rock samples analyzed came from 20 to 500 in depth below the seafloor. Helium contents vary from 3.3 x 10(-10) to 2.5 x 10(-7) ccSTP/g by crushing and from 5.4 X 10(-8) to 2.4 X 10(-7) ccSTP/g by melting. He-3/He-4 ratios vary between 2.2 and 8.6 Ra by crushing and between 2.9 and 8.2 by melting. The highest R/Ra ratios are similar to the mean mid-ocean ridge basalt (MORB) ratio of 8 +/- 1. The lower values are attributed to radiogenic helium from in situ a-particle production during uranium and thorium decay. Neon isotopic ratios are similar to atmospheric ratios, reflecting a significant seawater circulation in the upper 500 in of exposed crust at this site. MORB-like neon, with elevated Ne-20/Ne-22 and Ne-21/Ne-22 ratios, was found in some high temperature steps of heating experiments, but with very small anomalies compared to air. These first results from the lower oceanic crust indicate that subducted lower oceanic crust has an atmospheric Ne-20/Ne-22 ratio. Most of this neon must be removed during the subduction process, if the ocean crust is to be recirculated in the upper mantle, otherwise this atmospheric neon will overwhelm the upper mantle neon budget. Similarly, the high (U+Th)/He-3 ratio of these crustal gabbros will generate very radiogenic He-4/He-3 ratios on a 100 Ma time scale, so lower oceanic crust cannot be recycled into either MORB or oceanic island basalt without some form of processing. (C) 2003 Elsevier B.V. All rights reserved.