Solubility controlled noble gas fractionation during magmatic degassing: Implications for noble gas compositions of primary melts of OIB and MORB

摘要

Noble gas abundances in basaltic glasses from ocean islands (OIBs) are generally lower than those of mid-oceanic ridge basalts (MORBs), contrary to most geodynamic models which usually require that the source of OIBs is less degassed (resulting in higher primordial noble gas abundances) and more trace element enriched (resulting in higher radiogenic noble gas abundances) than the MORB source. Therefore, noble gas abundances in OIBs are often thought to have been reduced by extensive gas loss from the magma before eruption.The extent of magmatic degassing can be tested as it will cause characteristic changes in the composition of the volatiles; notably the He-4/Ar-40* ratio (where Ar-40* is Ar-40 corrected for atmospheric contamination) will increase in residual volatiles due to the higher solubility of He relative to Ar. The degree of He-As fractionation for a given fraction of gas loss depends on the ratio of the solubilities, S-He/S-Ar, which is sensitive to (among other things) the CO2 and H2O content of the basalt at the time of degassing.From a global database of OIB and MORB glasses, we show that He-4/Ar-40* ratios of MORB glasses are broadly consistent with degassing of a magma with an initial Ar-40 of approximate to1.5 x 10(-5) ccSTP/g, i.e., similar to that of the "popping rock." However, OIB glasses generally have lower Ar-40* concentration for a given He-4/Ar-40*. While this would appear to require lower Ar-40* abundances in the undegassed OIB magmas, the higher volatile contents of OIBs will reduce S-He/S-Ar (relative to MORBs) during degassing. By modeling S-He/S-Ar in OIBs, it is possible to show that extensive degassing of OIBs can occur without dramatically increasing the He-4/Ar-40* ratio. We show that undegassed Ar-40 concentrations of OEB magmas were probahl. similar to those of MORBs. Copyright (C) 2005 Elsevier Ltd.