A new method of reconstituting the P–T conditions of fluid circulation in an accretionary prism (Shimanto, Japan) from microthermometry of methane-bearing aqueous inclusions

Abstract

In paleo-accretionary prisms and the shallow metamorphic domains of orogens, circulating fluids trapped in inclusions are commonly composed of a mixture of salt water and methane, producing two types of fluid inclusions: methane-bearing aqueous and methane-rich gaseous fluid inclusions. In such geological settings, where multiple stages of deformation, veining and fluid influx are prevalent, textural relationships between aqueous and gaseous inclusions are often ambiguous, preventing the microthermometric

Fluid circulation in the depths of accretionary prisms: an example of the Shimanto Belt, Kyushu, Japan

“…Further details on apparatus and measurement conditions can be found in Raimbourg et al (2014b). The temperature accuracy for these measurements is around ± 1°C over the whole temperature range of investigation, from -120 to +300°C.…”

“…Shapes range from elliptical to very irregular. They present two phases at ambient temperatures and correspond to a mixture of salt water and methane (Raimbourg et al, 2014b). Due to the generally high density of inclusions, it is most often difficult to distinguish the nature (i.e.…”

“…Rather than modern seawater, Early Miocene seawater, with a salinity in the range 37-39 wt.‰ (Hay et al, 2006), provides a better estimate of seawater contemporaneous with vein formation, if one considers that the veins are associated with the main tectonic stage of deformation (Raimbourg et al, 2014a). The addition of CH 4 to salt water also depresses T mi , with a dependence we estimated with the software Fitane (Raimbourg et al, 2014b), using the model for density of aqueous solutions by Mao and Duan (2008) and the CH 4 solubility model by Duan et al (1992). To estimate the CH 4 concentration in aqueous fluid inclusions from the Hyuga mélange, one can note that another kind of inclusions, rich in methane, was described in the same quartz veins (Kondo et al, 2005), implying that at some stage the aqueous fluid was saturated in methane and CH 4 was present as a free phase.…”

“…beyond~1 km below seafloor, circulating fluids are only constrained by fluid inclusions trapped in quartz or carbonate veins in fossil prisms. In most accretionary prisms, including the Shimanto Belt in Japan (Kondo et al, 2005;Raimbourg et al, 2014b;Sakaguchi, 1999), Kodiak Island in Alaska (Vrolijk et al, 1988) or the Franciscan Complex in California (Dalla Torre et al, 1996;Sadofsky and Bebout, 2004), fluid inclusions contain a mixture of lowsalinity water and methane, for temperatures up to~300°C. Fluid inclusions from higher-grade rocks are markedly different as methane is replaced by carbon dioxide, as was shown on a transect through the Central Alps by Mullis et al (1994).…”

“…These studies, if they give a relatively consistent picture of the nature of the fluid, yields limited insights into the patterns of their circulation. Variations in pressure or redox state, possibly related to earthquakes (Vrojlik, 1987;Yamaguchi et al, 2011) or mixture of disequilibrium fluids (Raimbourg et al, 2014b) point to complex flow geometry.…”