The geochemical characteristics of mantle xenoliths (spinel lherzolites) in Quaternary alkaline basalts from the northwestern Ethiopian plateau provide new insight into the conditions of melting associated with the Afar plume. The major element compositions of the spinel lherzolites are in the range of those of the primitive mantle. The high modal content of the clinopyroxene (14–23%) indicate that the mantle xenoliths are fertile spinel lherzolites that have experienced insignificant partial melting. However, enrichment in highly incompatible elements (LREE, Ba, Pb, Th, and U) and the absence of any typical metasomatic minerals indicate that the spinel lherzolites underwent an event of later cryptic metasomatic enrichment induced by plume–related, hydrous fluid–rich silicate melts. The 87Sr/86Sr (0.70184–0.70324), 143Nd/144Nd (0.51313–0.51338), and 206Pb/204Pb (17.17–19.06) values of the clinopyroxene separate from the spinel lherzolites display systematic variations between those of depleted MORB mantle (DMM) and enriched compositions interpreted as related to the Afar mantle plume. The mineral compositions yield an estimated temperature and pressure in the range of 921–973 °C and 13–17 kbar, respectively, indicating that the mantle xenoliths originated at shallow depths of 40 to 55 km above the Afar mantle plume.
White mica K–Ar age dating was carried out for metapelites in the Hakoishi sub–unit and a meta–pillow basalt in the Tobiishi sub–unit of the Kurosegawa belt (Kamimura et al., 2012), Yatsushiro district, Kyushu, Japan. The Hakoishi sub–unit suffered the lawsonite–blueschist facies metamorphism with peak conditions of 0.7–0.9 GPa and <300 °C. Phengite separate with K content of 5.323 wt% from OD 28 gives 299.0 ± 6.1 Ma and those with K content of 1.222 wt% and 2.142 from OD 113 give 280.2 ± 5.8 Ma and 245.3 ± 5.1 Ma, respectively. The Tobiishi sub–unit suffered the prehnite–pumpellyite facies metamorphism with the peak conditions round <0.4 GPa and <350 °C. White mica named as alumino–celadonite with a composition range of Si from 3.79 to 3.94 and Fe3+ from 0.30 to 0.48 for Oxygen = 11 basis is developed in amygdules of meta–pillow basalts, closely associated with pumpellyite and chlorite. Alumino–celadonite separates obtained from pillow basalt, SOD01–B, vary K content from 5.601 to 2.343 wt% and give K–Ar ages from 181 to 173 Ma. The peak P–T conditions of the all studied samples were much lower than the closure temperature of Ar in white micas, so those data can be interpreted as the peak metamorphic timing of each sub–unit, i.e., the existence of both late Paleozoic (299–245 Ma) lawsonite blueschist facies metamorphic rocks and Mesozoic (181–173 Ma) prehnite–pumpellyite facies metamorphic rocks are newly identified from the Kurosegawa belt in Kyushu, although Mesozoic epidote–blueschist facies metamorphic rocks have been reported in the relevant area. Obtained metamorphic ages and grade from the Hakoishi and Tobiishi sub–units are almost similar with those of the Osayama area in the Renge belt (Tsujimori and Itaya, 1999) and of the Katsuyama area in the Suo belt of the Inner Zone of Southwest Japan, respectively (Hashimoto, 1968; Nishimura, 1998). These data support the Inner Zone origin of the Kurosegawa belt proposed by Isozaki and Itaya (1991).
A new method for quantitatively joining compositional maps measured by a scanning X–ray analytical microscope (SXAM) to visualize a larger scale element distribution (i.e., a joined element map) is proposed, and applied to the analysis of a 25–cm–long sample across a reaction zone from high–grade metamorphic rock. The method involves the in situ measurement of a standard material during a sample scan, which enables correction of the different sensitivities of multiple maps. The appropriate background intensity correction, spectrum processing, and X–ray intensity correction proposed in this study enable the production of a semiquantitative element map at a decimeter scale with relatively high resolution (~ 0.1 mm). The one–dimensional quantitative transect across the reaction zone has high resolution as well as high precision (e.g., relative standard deviation of <2% for Fe). The transect shows both a sharp boundary controlled by phase stability (as well as a millimeter–scale gradual reaction boundary) and a decimeter–scale gradual compositional gradient simultaneously, and these features are difficult to identify using conventional methods (i.e., electron probe microanalyzer, X–ray fluorescence analysis, or SXAM with prior data processing). These compositional gradients, which range from submillimeter to decimeters in length, provide a key to understanding the formation mechanisms of rock and/or mineral reaction zones.
A Raman geothermometer, which utilizes the degree of graphitization of carbonaceous material, has been widely applied to estimate the recrystallization temperature of metapelite. This study evaluates the degree of graphitization of carbonaceous material, which is affected by several factors, and tests the robustness of the Raman carbonaceous material geothermometer defined by the R2 [= D1/(G + D1 + D2) area ratio] value. The main results are as follows. (1) Laser radiation over 6 mW at the sample surface caused a significant decrease in the R2 value, owing to the local increase in surface temperature of the carbonaceous material, and thus gave an overestimation of the recrystallization temperature. On the contrary, laser irradiation of 2 mW showed no distinct alteration of the spectrum during continuous analyses up to 120 s. (2) Carbonaceous materials occurring as matrix and inclusion phases in silicate minerals in a thin section showed no significant difference in R2 value. (3) The average R2 value of 10 samples collected from an outcrop at a scale of 2–3 m was 0.483 ± 0.012, corresponding to a temperature of 416 ± 5 °C. This result implies that an arbitrary sample can likely represent the R2 value of the entire outcrop from which the sample was collected. (4) No distinct alteration of the R2 value around a shear zone width of 1–1.5 m was measured in the metapelites. The degree of graphitization of carbonaceous material was not noticeably altered by deformation during exhumation and local fracturing. Moreover, the Raman analysis of the carbonaceous material under the appropriate laser power condition can estimate the peak metamorphic temperature of rocks regardless of scale from thin section to outcrop.
Purple–cathodoluminescent calcite associated with periclase and survival dolomite has been found from the skarn minerals in the Kanehira mine located in the eastern part of Hiroshima Prefecture. Paragenesis of these minerals suggests that the decomposition of dolomite at 700–750 °C under dry condition during skarnitization could promote the production of characteristic calcite. Cathodoluminescence (CL) spectroscopy of the calcite reveals its blue emissions related to defect centers possibly derived from its origin, also similar CL features in the dolomite. Spectral deconvolution of the CL in the calcite and dolomite clarified two emission components at 2.67 and 3.30 eV, of which former corresponds to structural defect by thermally–influenced stress and the latter to intrinsic defect comparable to ‘back ground blue’ previously reported in calcite. The facts imply that the calcite with purple CL might leave the defects in its structure during thermal decomposition of dolomite at relatively high–temperature skarnization.