The Nishisonogi jadeitite is a peculiar example among Japanese jadeitites in terms of the age of formation and its genesis as metasomatic replacement (R–type). The Nishisonogi jadeitite is characterized by jadeite with a quartz–inclusion–rich core and quartz–free rim. The pressure–temperature (P–T) conditions of the Nishisonogi jadeitite is constrained by the assemblage of jadeite + quartz and of clinozoisite + paragonite + quartz. The jadeitite occurs as tectonic blocks in a sepentinite mélange together with metasomatic rocks such as omphacitite, albitite and zoisite rock. The protolith of the jadeitite is likely plagiogranite or adakitic granite as suggested by the rare earth element (REE) pattern of zircon. The zircon has a euhedral core with a U–Pb age of 126 ± 6 Ma with exhibiting sharp and flat boundary with metamorphic rim, which has a U–Pb age of 84 ± 6 Ma. The rim age is almost consistent with the 95–90 Ma age of the peak metamorphism of the coherent schists, as a host of the serpentinite mélange, as determined by 40Ar/39Ar method applied to glaucophane and phengite. Some jadeitite blocks show lithological zonations with albitite and other rocks in scale of several tens of centimeters. These zonations likely formed in the stability field of albite during exhumation of the serpentinite mélange. The isocon analysis of whole rock compositions and the singular value decomposition analysis of mineral reactions show that SiO2, Na2O, and Fe2O3 evolved from the jadeitite towards serpentinite according to the retrograde reactions.
Paleozoic jadeitite–bearing serpentinite–matrix mélange represents the oldest mantle wedge record of a Pacific–type subduction zone of proto–Japan. Most jadeitites are fluid precipitates (P–type), but some jadeitites are metasomatic replacement (R–type) which preserve relict minerals and protolith textures. The beauty and preciousness of some gem–quality, semi–translucent varieties of jadeitites in the Itoigawa–Omi area led to the designation of jadeitite as the national stone of Japan by the Japan Association of Mineralogical Sciences. Zircon geochronology indicates jadeitite formed prior to Late Paleozoic Renge metamorphism that formed blueschist and rare eclogite. For example, in the Itoigawa–Omi and Osayama localities, older jadeitites and younger high–pressure/ low–temperature metamorphic rocks in a single mélange complex imply different histories for the subduction channel and jadeite–bearing serpentinite–matrix mélange. This suggests that the jadeitite–hosted mélange (or serpentinized peridotite) can stay within the mantle wedge for a considerable time; thus recrystallization, resorption, and re–precipitation of jadeitite can continue in the mantle wedge environment. Therefore, studies of Paleozoic jadeitites in Japan have great potential to elucidate the earliest stages of orogenic growth (oceanward–accretion and landward–erosion) associated with the subduction of the paleo–Pacific oceanic plates, and to test geophysical observations of modern analogues from a mixture of fossilized mantle wedges and subduction channels.
The mode of occurrence of jadeitic pyroxenes and their origins were reviewed using literatures published in the Sanbagawa belt (sensu lato), SW Japan, and the Kamuikotan belt, Hokkaido. Jadeite + quartz assemblage is found from tectonic blocks both in the Yorii area of the Kanto Mountains in the Sanbagawa belt and the Kamuikotan Gorge area, and its formation timing is predated to the main metamorphism in each area, where albite is stable along with metamorphic pyroxenes with lower jadeite contents (namely Jd<50). The jadeite + quartz assemblage is recently found as inclusions in garnet grains in some rocks with peculiarly lower CaO and/or MgO bulk compositions, which suffered the eclogite facies metamorphism in the Sanbagawa belt of Shikoku area, although it is not detected from the matrix of the eclogite facies rocks. Recent zirconology applied to the jadeite + quartz rock in the Yorii area causes a hot argument on its origin, i.e., the wholesale metasomatic replacement origin or the vein precipitation origin. In either case, this methodology gave new insight on fluid activity recorded in Jurassic or post–Jurassic subduction zone.
Jadeitite from the New Idria serpentinite body of California is a fluid precipitation–to–metasomatic product. Optical cathodoluminescence (CL) microscopy of the jadeitite revealed that vein–filling ‘pure’ jadeites (mostly 97–99.9 mol% jadeite) exhibit bright luminescence, whereas ‘impure’ jadeites (mostly 75–95 mol% jadeite) in pale–greenish matrix show dark luminescence. The ‘pure’ jadeites in the veins are composed of mixtures of red, blue and dull blue CL–colored domains, showing growth textures (oscillatory bands). The ‘impure’ jadeites in the pale–greenish matrix with dark luminescence have a higher augite component (up to 5.37 wt% FeO), implying that the CL property is due to significant amount of Fe2+ to act as a quencher. CL spectra of the blue CL–colored domains of the vein–filling ‘pure’ jadeite have a doublet broad emission peak centered at ∼ 320 and ∼ 360 nm in the ultraviolet (UV) to blue region. In the red CL–colored domains, a broad asymmetric emission peak at ∼ 700 nm is also recognized together with the doublet UV–blue emission peak. Comparing monochromatic CL images in the UV–blue (300–400 nm) and red (650–750 nm) emission regions with X–ray elemental maps, luminescence centers contributing the UV–blue and red CL emission peaks were assigned. The red emission peak of the ‘pure’ jadeite with subtle augite component would be attributed to lattice defects related to Ca2+, Fe2+ (or Fe3+) and Mg2+ deficiency and/or excess centers in M1 or M2 sites. Alternatively, transition metal ions (Mn2+ and Fe3+) or rare earth elements in the M1 and M2 sites as impurity centers, might contribute to the red emission peak. As the UV–blue emissions correlate with Al3+ content, i.e. purity of jadeite component, they might be related to Na+ and/or Al3+ defect centers.
The density of the jadeite (NaAlSi2O6) melt has been measured up to 6.5 GPa and 2273 K using the X–ray absorption technique at beamline 13–BM–D of the Advanced Photon Source. A fit of the pressure–density–temperature data to the high temperature Birch–Murnaghan equation of state yielded the following thermoelastic parameters: density, ρ0 = 2.36 g/cm3, isothermal bulk modulus, KT0 = 21.5 ± 0.8 GPa, its pressure derivative, K0′ = 8.9 ± 1.2, and the temperature derivative (∂KT/∂T)P = −0.0021 ± 0.0011 GPa/K at a reference temperature T0 = 1473 K. The densification of jadeite melt at low pressures is primarily dominated by topological changes in the structure, including a decrease in T–O–T angle and breaking and reforming of the T–O bond (T = Si4+, Al3+). Compressibilities of jadeite, albite, diopside, phonolite and peridotite melts display a systematic trend: the K0–K0′ plot of these silicate melts exhibits an inverse linear relation.
Jadeite occurs as the shocked product of albite feldspar in shocked meteorites, and is one of the most common high–pressure polymorphs in shock–melt veins of meteorites. The characteristic textures of jadeite in shocked ordinary chondrites show that some of jadeite crystals were formed from originally albite feldspar by a solid–state transformation and some were crystallized from a shock–induced albite melt. Based on these textures of jadeite together with the other high–pressure mineral assemblages and their crystallization kinetics, we can estimate the impact conditions such as impact velocity and parent–body size.
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