Soichi Omori

A fluid factory in solid Earth

Global material circulation in our planet from the surface to the bottom of the mantle is controlled by a combination of plate, plume, and “anti–plate” tectonics, where fluids and melts play an active role. Whereas crustal fluids are dominated by CO 2 and H 2 O, with subordinate CH 4 and N 2 , the volatiles in the lower mantle are speculated to be dominantly CO 2 . The process of Archean subduction aided in the sequestration of CO 2 from the ocean-atmosphere system, with the ultimate probable destination being the mantle. When a rising “superplume” hits the tectosphere, the predicted carbonated continental keel would become enriched in CO 2 . During the Phanerozoic, the carbonated upper mantle was drastically reduced in size, as speculated from the scarcity of dry, “ultrahot” orogens. Free fluid circulation within Earth is present only in restricted zones, mostly along the plate boundaries and intracontinental rifts, and particularly along subduction zones, where the fluid is mostly water dominated. The propagating water front in the deep mantle in modern Earth may correspond to the apparent increase in pressure through geologic time, which might be one of the reasons for a general lack of ultrahigh-pressure metamorphic belts in the Archean and their prevalence in the Phanerozoic orogenic belts.

Ultrahigh-pressure (UHP) low-Al titanites from carbonate-bearing rocks in Dabieshan-Sulu UHP terrane, eastern China

Abstract Titanites previously reported from high-pressure rocks commonly contain high Al and F contents. This finding has led previous workers to conclude that high Al and F in titanite can expand its stability field to high-pressure and ultrahigh-pressure (UHP) conditions. In this study, a coesite inclusion was identified in titanite of aragonite- and jadeite-bearing gneiss from the Dabieshan UHP metamorphic terrane of eastern China. We also found UHP titanite in carbonate-bearing garnet clinopyroxenite from the Sulu UHP terrane. Interestingly, both types of the UHP titanite mentioned above contain low Al and F contents. Therefore, our data indicate that the high-Al and F contents are not necessary criteria for stabilizing titanite to UHP conditions. It is inferred that the UHP titanite formed via the reaction TiO2 + CaCO3 + SiO2 = CaTiSiO5 + CO2, and that the presence of the UHP assemblage aragonite + rutile + clinopyroxene + coesite in the host rock and a very low XCO₂ value in the coexisting fluid are crucial conditions for the formation of these UHP titanites

Imbricated ocean-plate stratigraphy and U-Pb zircon ages from tuff beds in cherts in the Ballantrae complex, SW Scotland

Ocean-plate stratigraphy (basalt, chert, clastics) of the Balcreuchan Group in the Ballantrae complex is repeated by layer-parallel thrusts to form duplex structures south of Bennane Head, ~5 km north of Ballantrae. Five tuff beds, all in chert, have similar U-Pb zircon ages of ca. 470 ± 10 Ma. The geometrical polarity of the duplexes and the zircon ages provide new constraints on the tectonic evolution of the accretionary wedge of the Ballantrae complex. Northwestward thrusting of the duplexes suggests that subduction was from the northwest to the southeast, and this polarity at the leading edge of the main ophiolite body is consistent with the northward younging of the volcanic arc rocks of the Ballantrae complex.