マントル捕獲岩の希ガス研究：希ガスで探るマントルメタソマティズム

Abstract

Because of chemical inertness of noble gases, their isotopic compositions trapped in mantle-derived xenoliths provide valuable information about mantle processes. Here we present a review of noble gas studies of mantle xenoliths from several tectonic settings with specific attention to mantle metasomatism. Numerous metasomatic traces have been identified as noble gas isotopic anomalies found in fluid or melt inclusions or in minerals of metasomatic origin in the suboceanic and subcontinental lithosphere. The noble gas isotopic ratio of MORB source, which is generally regarded as representing the suboceanic upper mantle, is characterized by a quite uniform 3He/4He ratio and a high 40Ar/36Ar ratio of up to 40000. On the other hand, low 3He/4He and 40Ar/36Ar ratios compared to those of MORBs have been reported from some subcontinental ultramafic xenoliths. This phenomenon is explainable in terms of metasomatism by a slab-derived component at the continental/convergent plate margin causing enrichment of U and Th, parent nuclides of 4He, and of atmospheric Ar in the mantle wedge. Metasomatic signatures attributable to deep mantle plume are observable as a higher 3He/4He ratio than the MORB value and a distinct trend in Ne three-isotope plot from that of MORBs, both in oceanic and continental areas. In addition, noble gas isotope exchange between the mantle xenolith and its host magma are often observed. By applying several methods for extraction of noble gases and careful selection of samples, noble gases can serve as a powerful tool to distinguish these metasomatic agents. Furthermore, noble gas analysis of small pieces of mantle xenoliths or individual fluid/melt inclusion using a laser microprobe in combination with other analytical techniques for detection of major volatile components, such as micro-Raman spectroscopy, will clarify the origin of volatiles in mantle xenoliths.