Presentation Information
[R8-05]Constraint on CO2 content in multiphase solid inclusions in a garnet pyroxene rock in southern Bohemia, Czech Republic
*Kosuke NAEMURA1, Rin Abe1, Yuki Hagiwara2 (1. Iwate University, 2. JAMSTEC)
Keywords:
Multiphase solid inclusion,ultrahigh pressure metamorphism,garnet pyroxene rock,carbon dioxide,heating stage
High-pressure to ultrahigh-pressure metamorphic rocks may contain melts and supercritical fluids generated in deep plate convergence zones - at depths of 50 to 200 km - that are trapped in high-pressure minerals such as garnet. Melts and supercritical fluids cooled and crystallized to form multiphase solid inclusions. Recently, several studies focused on their chemical composition and its relationship with post-orogenic ultrapotassic magmatism (e.g. Borghini et al., 2019). These studied demonstrated that parental melts or supercritical fluids contain substantial amounts of carbon dioxide up to 2-3 wt% (Borghini et al., 2023). In previous research, we conducted a homogenization experiment for multiphase solid inclusions from the Bohemian garnet pyroxene rock and obtained a parental melt composition: SiO2=70-74 wt%, Al2O3=12 wt%, MgO=0.3-2.0 wt%, CaO=1.1-1.8 wt%, K2O=6.0-6.5 wt%. Since the total weight mass of melts is about 87 wt%, the mass of fluid components in the melt is estimated to be more than 10 wt%. In this study, we conducted a heating experiment of multiphase solid inclusion using Linkam heating stage. After some trial and error, we found that when melt inclusions were held at 1,100 degrees for 10 minutes, most of the inclusions decrepitated, but when they survived, bubbles appeared in the homogenized melts. After quench, the bubbles were analyzed by use of Raman spectroscopy. All bubbles were identified as pure carbon dioxide whose maximum densities were determined at 0.23 g/cm3 by method of Hagiwara and Kuwatani (2025). Here we assume that the bubbles and glass are spheres with diameters of 5 and 20 μm, respectively. We assume that the amount of CO2 in the melt at atmospheric pressure is zero. Based on this assumption, we estimated the CO2 concentration in the parental melt of the MS inclusions to be 1,360 ppm at most This estimated value is equivalent to the CO2 concentration in homogenized glass of multiphase solid inclusions in UHP rocks, as determined by SIMS in previous studies. In the Bohemian Massif, carbonatite melts have been found within garnet peridotites, and different theories have been proposed regarding their origin: one theory (Kotkova et al., 2021) posits that carbonatite formed as a residual liquid when silicate melt crystallized garnet pyroxene rock, while another theory (Naemura et al., 2018) suggests that a small amount of carbonatite existed in the mantle as a primordial melt. Using the carbon dioxide concentration in the melt observed within garnet pyroxene rocks as a clue, I would like to discuss which of these theories is more plausible.