Two undeformed metagranite samples were collected from the UHP Brossasco–Isasca Unit (BIU) of Dora–Maira Massif, Italy to carry out the laser step–heating 40Ar/39Ar analyses of individual biotite crystals. The metagranites occur as undeformed domains (m– to ten of meters in size) within strongly deformed augen–gneiss. They still preserve their medium– to coarse– grained igneous texture and are composed mainly of K–feldspar, plagioclase pseudomorph, quartz, and biotite that preserve their original igneous shape but are either re–equilibrated or replaced by new phases.
Three biotite crystals from the first sample have similar age spectra showing 400 to 300 Ma, except for the first fraction (500–1500 Ma). On the contrary, the age spectra of five biotite crystals from the second sample are significantly different; these biotites show saddle shape 40Ar/39Ar age spectra, except for one crystal. The oldest fractions have ages (800 to 1300 Ma) three to four times older than that of the granite protolith (which is late Permian). This extremely high intensity of excess argon could be due to an ‘Excess–Argon Wave’ (EAW) phenomenon, occurred during the quick exhumation of the BIU, combined with the extremely short ductile deformation history. The observed variation of the biotite age spectra may reflect the different trapping processes of EAW and/or localized source of EAW.
Traditional metamorphic geology admitted that the appearance of specific index mineral, such as garnet, indicated the rocks experienced higher peak metamorphic condition. It is largely true, but it is still unclear what is going on in the rocks at the very start of the garnet formation. One of the key must lie in the area where apparent isograd is outcropped. Nagatoro area is located in the Kanto Mountains, and it is where the low grade metamorphic rocks of the Sanbagawa metamorphic belt are exposed. Appearance of garnet is the index of the higher metamorphic grade in this area. Several outcrops in Nagatoro area are known to contain garnet, thus being good samples of the outcropped isograd. In this study, spatial distribution of garnet within such an outcrop several tens of meters long was investigated. The samples are pelitic schists, and the mineral assemblage was basically quartz, plagioclase, muscovite, and chlorite. 36 of the 55 samples contained garnet. Garnet grains were small, most of them with diameters less than 100 µm. Most of the garnet grains were euhedral to subhedral and were found within micaceous lamellae which form the foliations of the pelitic metamorphic rocks. The micaceous lamellae were constituted mainly by muscovite with lesser amount of chlorite. Occurrence of garnet–bearing rocks within the outcrop seemed to be restricted in certain structural layers. The structural layers are known to be nearly parallel to the lithologic boundary in this area. Mapped chemical profiles of garnet revealed that the garnet grains exhibited euhedral growth of the crystal. The core part was relatively large and homogeneous, with quite a high Mn end–member (spessartine) content (XSps > 50%). Irregular shaped inner core was preserved. These features indicate that the texture preserves the beginning stage of garnet growth. The trend of chemical composition of garnet rim and adjacent chlorite is consistent with the bulk rock chemistry control. Spatial distribution of garnet, at the start of its growth, was probably controlled by the bulk rock chemistry.
The Mineoka Belt in central Japan is a Paleogene accretionary complex with the various kinds of volcanic rocks and plutonic rocks formed in multiple ages. A basaltic lapilli tuff derived from the Hota Group was collected from the Mineoka Belt, Boso Peninsula to reveal the genetic relationship of the Mineoka Belt with the Izu–Bonin–Mariana (IBM) arc. The whole–rock composition of the basaltic lapilli tuff shows island arc tholeiite affinity, and the zircon U–Pb dating yields about 18 Ma. The geochemical signatures of the basaltic lapilli tuff are similar to those of the Eocene plutonic rocks in the Mineoka Belt, but the zircon U–Pb age is much younger than those of the plutonic rocks. The idea that the arc–related rocks in the Belt are fragments of the IBM arc can explain the two different ages of the Mineoka arc–related rocks. The island arc plutonic rocks are derived from IBM middle to lower crust formed during Eocene to Oligocene, whereas the Early Miocene basaltic rock is most likely to be a fragment of arc products formed in the IBM volcanism at the end of the Miocene back–arc spreadings.
We report the discovery of mullite coexisting with sillimanite in a fused pelitic fragment, so–called buchite from Asama volcano, Japan. TEM observation revealed a core–rim texture in the examined fibrous minerals: cores are mullite with characteristic glass inclusions, and rims are sillimanite with abundant anti–phase boundaries. These two phases have common crystal axis directions and coherent boundaries, and thus have eluded accurate identification by previous workers using other analytical methods, e.g., optical microscopy, electron microprobe analysis, or XRD experiments. This sub–micrometric core–rim texture can explain inconsistencies among previous analytical results on the same fibrous mineral from Asama. We show that mullite formed from sillimanite with incongruent melting at high temperature, and, upon slight cooling, the outer parts of mullite grains reacted with the surrounding melt to retrogressively form sillimanite rims with abundant anti–phase boundaries. This texture indicates a compositional gap between sillimanite (Al2SiO5) and 3:2 mullite (3Al2O3∙2SiO2) and is evidence against a low–pressure complete solid solution between the two phases.
Organic acids are biological molecules that present abundantly in the earth’s surface environments. Low–molecular–weight dicarboxylic acids are reactive organic acids containing two carboxyl groups in the molecular structure. To evaluate the effect of these dicarboxylic acids on the formation of CaCO3 minerals, formation experiments were performed by the batch method using 100 ml solutions containing 5.0 mM Ca2+ and Mg2+, 20.0 mM HCO3−, and 0.0, 0.1, 0.5, 1.0, 2.0, or 5.0 mM oxalic acid, malonic acid, or glutaric acid at 25 °C for 10 days. In addition, adsorption experiments with the dicarboxylic acids on the surfaces of calcite and aragonite were conducted to reveal the adsorption affinity for the surfaces of CaCO3 minerals. The results confirmed that the dicarboxylic acids inhibited significantly the formation of aragonite and favored the formation of calcite depending on the molecular structure in the following order: oxalic acid > malonic acid > glutaric acid. Notably, oxalic acid had much greater effect on the CaCO3 formation at lower concentrations, while glutaric acid showed no clear effect even at higher concentrations. The adsorption experiments revealed that the dicarboxylic acids exhibited much higher adsorption affinity for the surface of aragonite than for the calcite surface and showed following order of adsorption affinity for both aragonite and calcite surfaces: oxalic acid > malonic acid > glutaric acid. This is consistent with their effectiveness in the CaCO3 formation. Therefore, the adsorption affinity of the dicarboxylic acids for the surface of CaCO3 minerals contributed to the inhibition of aragonite formation and also the resultant formation of calcite by favorable adsorption of the molecules on the surface of aragonite over that of calcite.