As a follow-up of a symposium on mantle xenoliths held at Kanazawa University in 2004, we here publish a special issue composed of seven review articles dealing with mantle xenoliths. Mantle-derived xenoliths of peridotite and related rocks play an important role as a direct insight into Earth’s interior, and have been examined in various ways. We would like to widely introduce the current situation of mantle xenolith studies to non-specialists as well as to enlighten young scientists as to the mantle materials and their importance.

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We discuss about geological aspects of deep-seated xenoliths in volcanics from the Japan arcs. We can compare xenoliths in magmas with gravels in rivers. Dams on the rivers are equivalent to magma reservoirs; the former hinder the gravels from downstream transportation as the latter may prevent the deep-seated xenoliths from uprise by accumulation. Rivers with protected banks where gravels are not caught are compared with fixed magmatic conduits where wall rocks are hardly sampled as xenoliths. The magmas that are originated as deep as possible, move upward as fast as possible, and do not form reservoirs are appropriate for providing deep-seated xenoliths as the rivers with deep source and without dams and protected banks are favorable for providing gravels for prospecting geology upstream.
    Modes of occurrence of deep-seated xenoliths on the Japan arcs suggest us the following. (1) Xenoliths are sorted in an uprising magmatic column; the magma catches deeper-seated, higher-density rocks earlier than shallower-seated, lower-density ones as xenoliths. The proportion of mantle xenoliths dramatically increases upward in stratigraphy of xenolith-producing monogenetic volcanoes. Inversely crustal xenoliths are predominant in the lower eruptives, especially in basal pyroclastics. (2) Rocks are selectively caught as xenoliths by magma. If young, fragile cumulative dikes formed by preceding magmas, subsequent magmas would trace them and do not catch wall rocks, which may be invisible as xenoliths. (3) The upper mantle to lower crust of the SW Japan arc is heterogeneous in thickness of cumulus mantle as well as in development of younger cumulative rocks. The mantle peridotite is cryptically or patently metasomatized by melts for the young cumulates beneath the monogenetic volcano clusters. (4) The upper mantle to lower crust of the NE Japan arc is heterogeneous in hydration. Aqueous fluids would move selectively along cracked or sheared parts to hydrate.

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Recent developments in instrumentation for in situ trace element analysis of peridotite minerals give us valuable data set of petrological and geochemical insights of the upper mantle. Here, we summarize petrographical, petrological and geochemical characteristics of mantle xenoliths from the Northeastern and Southwestern Japan arcs. They have clear correlations between microtexture and mineral compositions in terms of both major and trace elements. That suggests that influx-assisted melt extraction occurs simultaneously with deformation/recrystalization in the upper mantle. Their characteristics of trace-elements in clinopyroxene are distinguished from those of abyssal peridotite and peridotite xenoliths from continental regions. The geochemistry of Japan arcs’ clinopyroxenes have characteristics of arc-type mantle source; i.e., low light rare earth element (LREE) and high field strength element (HFSE; Ti, Zr and so on), rather high heavy rare earth (HREE) relative to LREE elements and Sr concentrations. They also have a rather constant Ti/Zr ratio, variable REE patterns, and relatively low LREE/HREE ratios. The simple melt extraction model can not explain these characteristics. These features are due to metasomatism, which is different from carbonatite metasomatism.

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Fluid inclusions in mantle-derived minerals can serve as a messenger from the deep Earth. If CO₂ is a dominant phase of the fluid, the relationship between intensity ratio and frequency separation of the Fermi diad bands in the Raman spectra of CO₂ can be used for determination of density of the inclusions. The intensity ratio and the frequency separation between the peaks thereby increase with density of CO₂. Kawakami et al . (2003) have established the relationship between density of CO₂ and the frequency separation of the Fermi diad bands using the Raman data on CO₂ fluid with densities from 0.1 to 1.21 g/cm³, including super critical fluids at 58-59°C. Thus, micro-Raman spectroscopic analysis allows us to reveal multiple densities of the small fluid inclusions by one-by-one density analysis.
    Generally, inclusions show CO₂ densities (pressure) specific to the individual host minerals in the order of spinel > orthopyroxene ∼ clinopyroxene » olivine. The density of CO₂ reflects how strong host minerals are to withstand the pressure differential between the inclusion’s internal pressure and the external environmental pressure during transport of xenoliths to near the Earth's surface. Olivine underwent considerable plastic deformation resulting in the density reduction of CO₂ fluid inclusions. On the other hand, the slightly higher density of CO₂ in spinel can be explained by elastic deformation of the minerals during ascent and cooling of the xenoliths. Conversely, the density of CO₂ inclusions in pyroxene will work as a useful geobarometer requisite for discussions on the origin of mantle-derived minerals.

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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.

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This study evaluates the validity of several pyroxene geothermometers by comparing calculations with recent experimental studies that cover wide range of pressure and pyroxene composition. The most accurate thermometer for the multicomponent system among previously proposed models is that of Taylor (1998). This model yields temperatures ±32°C (1σ) for the multicomponent system, but provides temperatures lower than the experimental temperatures at low pressures in the CaO-MgO-SiO₂ (CMS) system.
    We propose a revised model of Wells (1977)'s pyroxene thermometer as,
    lnK = {3867 − 9.686(P − 20)}/T + 5.518 × 10⁻³T + 2.44X_Fe^Opx − 12.48
    , where the equilibrium constant K is the same as that of Wood and Banno (1973). This yields temperatures ± 35°C (1σ) for the multicomponent system and ±21°C for the CMS system.

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Platinum group elements (PGE), gold, and rhenium are the siderophiles which show contrasting geochemical distribution with lithophile elements such as rare earth elements. The PGE abundances are generally very low on the order of ppb or less in felsic rocks, and are relatively high in ultramafic rocks. The PGE pattern is the plot of abundances of PGE and Au, normalized by CI-chondrite value or primitive mantle value, in descending order of melting point, i.e., Os, Ir, Ru, Rh, Pt, Pd, and Au. The pattern shows characteristic varieties on their rock species or tectonic settings. The PGE and Au can be classified into two sub-groups on the basis of their geochemical behavior: the Ir group (I-PGE; Os, Ir, and Ru) and the Pd group (P-PGE; Rh, Pt, Pd, and Au). Compared with the I-PGE, the P-PGEs tend to be disturbed during metasomatism or hydrothermal events. Investigation of the PGE behavior in ultramafic xenoliths and their host rocks would clarify the interactions which xenoliths have experienced in upper mantle materials or during the uplift.

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We review petrological and chemical characteristics of silicic glasses and associated crystalline phases in mantle derived peridotite xenoliths reported from various volcanic rocks at the oceanic island, continental rift, and island arc settings.
    Most of silicic glasses in peridotite xenoliths reported from oceanic islands and continental rifts are rich in alkalis and silica-undersaturated (SiO₂ = 57.4 wt%, Qz-(Ne + Lu) = − 4.45 wt% on average) and accompanied with secondary clinopyroxene and olivine. In the oceanic island and continental rift settings, almost all silicic glasses reported were primarily silica-undersaturated but these compositions were converted into silica-oversaturated by reacting with orthopyroxene. On the other hand, almost all glasses in the sub-arc peridotite xenoliths examined are silica-oversaturated (SiO₂ = 63.0 wt%, Qz-(Ne + Lu) =16.6 wt% on average) and accompanied with secondary orthopyroxene, indicating their primary silica-oversaturated characters.
    Silica-oversaturated melts such as silicic glasses in the sub-arc peridotites can be produced either by hydrous partial melting of mantle peridotite (e.g., Hirose, 1997) or by melting or dehydration of basaltic slab materials (e.g., Prouteau et al., 2001). Silicic but silica-undersaturated melts such as silicic glasses, which are included in peridotite xenoliths from oceanic islands and continental rifts, can be produced either by small-degree partial melting of mantle peridotite (e.g., Falloon et al., 1997) or by reaction between carbonatitic melt and mantle peridotite (e.g., Green and Wallace, 1988).

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