Photon Factory Activity Report 2014 #32 (2015) B

AR-NE5C / 2013G140

Decarbonation and melting of MgCO3 with SiO2
under high temperature and high pressure
Sho KAKIZAWA1, Toru INOUE *1, Hideki SUENAMI1 and Takumi KIKEGAWA2
1
Geodynamics Research Center, Matsuyama, Ehime 790-8577, Japan
2
High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
Introduction
Carbon is one of the most important volatile elements
on Earth. It is found as CO2 in the atmosphere, as
dissolved ions in seawater (e.g., CO32-, HCO3-), and as
carbonate minerals deposited from seawater (e.g., calcite,aragonite). Further, it is conceived that carbonate
minerals, C-O-H fluid, and diamond exist in the deepmantle. The carbonate minerals (CaCO3: calcite,aragonite) are transported into the mantle within marine
sediments by subducting slabs.
In the MgCO3-SiO2 system, it is reported that thefollowing reaction occurs up to pressures of 80 GPa:MgCO3 + SiO2 → MgSiO3 + CO2 (e.g. [1-4]). The P-T
slope of the boundary showed as negative above 30 GPa.
In addition, the CO2 generated decomposes to diamondand oxygen (ε-phase) [4]. On the other hand, Litasov [5]indicated that the MgCO3-SiO2 system shows a melting
reaction in the pressure range of 6-30 GPa. Thus, theseresults are inconsistent with each other. In this study, wethoroughly investigated the stability and the reaction ofmagnesite with silica (SiO2) up to 26 GPa to clarify
whether decarbonation or melting occurs under these
conditions.
Experimental
High-pressure in situ X-ray experiments wereperformed at the AR-NE5C beam line of PF-AR in KEK,Tsukuba, Japan, using the MAX 80 cubic-type highpressure apparatus. The 6-6 type compression method
was used. The TEL of the second stage anvils made by
WC was 6 mm and 4 mm. We used boron epoxy as thepressure medium and cylindrical graphite as a heater. The
experiments were conducted at pressures of 4-6 GPa andtemperatures up to 1400 °C. First, we compressed the cellto a certain pressure at room temperature and thentemperature was gradually increased to ~ 1000 °C. The
diffraction data were collected at 50 °C intervals as thetemperature increased. A mixture of magnesite and αquartz (1:1 molar ratio) was used as the starting material.
Results and discussion
We conducted three in-situ runs up to 6 GPa and1400 °C. In run AR288, the sample was compressed to ~8 GPa, and then temperature was increased gradually to1350 °C. The main peaks of magnesite and coesite
remained for 30 min at 1350 °C. Then, the temperature
was increased to 1400 °C. After 30 min, the magnesite
and coesite peaks disappeared and enstatite peaks
appeared. This indicates that decarbonation started tooccur at ~5.8 GPa and 1350-1400 °C.

Combined the present in situ X-ray results with thequench experiments in Ehime University, we could
determine that the decarbonation reaction (MgCO3 + SiO2→ MgSiO3 + CO2) occurs below 8 GPa, but it becomes amelting reaction above 8 GPa. This work is published in[6].

Fig.1 X-ray diffraction patterns of the in situ X-rayexperiment (AR228). Mgs, magnesite; Coe, coesite; En,enstatite.

Fig.2 P-T diagram of MgCO3-SiO2 (MgCO3: SiO2 = 1:1).
References[1] Koziol&Newton, Am. Mineral., 80, 1252 (1995)[2] Takafuji et al., Phys. Chem. Minerals, 33, 651 (2006)[3] Litasov et al., Phys. Earth Planet. Inter., 168, 191 (2008)[4] Seto et al., Phys. Chem. Minerals, 35, 223 (2008)[5] Litasov, Russian Geol. Geophys., 52, 475 (2011)[6] Kakizawa et al., J. Mineral. Petrol. Sci. (2015, in press)*inoue@sci.ehime-u.ac.jp