Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

S (Solid Earth Sciences) » S-VC Volcanology

[S-VC44] Magmatism and volcanic dynamics on subduction zone

Thu. May 24, 2018 9:00 AM - 10:30 AM A08 (Tokyo Bay Makuhari Hall)

convener:Yujiro Suzuki(Earthquake Research Institute, The University of Tokyo), Hitomi Nakamura(Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology), Yu Iriyama(国立研究開発法人 防災科学技術研究所), Chairperson:Suzuki Yujiro(Earthquake Research Institute, The University of Tokyo), Iriyama Yu(National Research Institute for Earth Science and Disaster Resilience (NIED))

9:30 AM - 9:45 AM

[SVC44-03] Parameterization of melting phase relations in CO2-H2O-silicate system toward understanding carbon cycle

*Masaki Ohno1, Hikaru Iwamori1,2 (1.Tokyo Institute of Technology, 2.Japan Agency for Marine-Earth Science and Technology)

Keywords:material cycling, subduction zone, dehydration-hydration, carbon dioxide, Earth history, solidus

Carbon dioxide in the atmosphere acts as a greenhouse gas together with H2O vapor, and its increase by artificial degassing during the past decades has attracted societal and scientific attention. Through the Earth's history over the past several giga years, the atmospheric CO2 partial pressure drastically decreased, during which partitioning of CO2 between the surface environment and the Earth's interior has controlled the budget. Behavior of CO2 in subduction zones is important for the partitioning as subduction zones are the site where the surface materials enter the mantle. Fluid and magma play crucial roles in material cycling, including CO2, in subduction zones. Therefore, how CO2 is transported by and interacts with the processes of hydration-dehydration and melting is key to understanding the carbon cycle in the Earth system (Dasgupta & Hirschmann, 2010, EPSL; Kelemen & Manning, 2015, PNAS). For this sake, phase relations in the wide and continuous compositional space of CO2-H2O-silicate system and their quantitative description are necessary, which, however, are not available at present. In this study, we compile the relevant phase relations based on the previous experiments, and attempt to establish their quantitative parameterization, ultimately aiming at revealing the carbon cycle and its evolution in the Earth's system.
Material cycling and melting in subduction zones have been discussed long, in which volatile components are critical. For, volatile components decrease solidus temperatures of mantle rocks. Carbon exists mostly as CO2 in the shallow part above ~300 km depth of subduction zones. Both H2O and CO2 are released from slabs, and affect melting. The effect of H2O on melting has been parameterized in the previous studies (e.g., Iwamori, 1998, EPSL). Utilizing the parameterized phase relation, numerical simulation of mantle-fluid flow, melting and elemental transport in subduction zones has been performed (e.g., Ikemoto & Iwamori, 2014, EPS). On the other hand, the effect on melting has not been parameterized for CO2, and even more so for the combination of CO2-H2O. In this study, on the basis of previous experimental studies, the phase relations within the range of CO2=0-5 wt.%, H2O=0-1 wt.%, pressure=0-12 GPa, temperature=0-2000 degree C are parameterized.
Below 2.2 GPa CO2 constitutes a gas-fluid phase, and does not dissolve much to melt or solid phase, which minimize its effect on melting (e.g., several degree C at most). Above 2.2 GPa, solidus temperature of mantle peridotite decreases by ~200 degree C, due to occurrence of carbonate minerals (ex., magnesite) and carbonatite melt. Carbonate and carbonatite melt are incompatible with silicates mainly consisting of the peridotite. Because of this nature when melting occurs, carbonatite melt appears at once on the solidus temperature, whereas silicate melt is produced on higher temperature than solidus temperature.
In the system involving both CO2 and H2O, the role of volatiles is complicated. Below 2.2 GPa, CO2 is mixed with H2O in the gas-fluid phase and prevents H2O from decreasing solidus temperature by suppressing the activity of H2O. In this case, the solidus temperature of peridotite is higher than that only with H2O. On the other hand, above 2.2 GPa, both CO2 and H2O increase their solubilities into the melt and solid phases, affecting significantly the phase relations, including peridotite melting. As a result, melting occurs at lower temperatures: under the presence of the two components, the lowest solidus temperature is ~850 degree C at pressure=~3 GPa. This new parameterization enables us to perform quantitative evaluation and numerical simulation concerning the carbon cycling in the upper mantle, including subduction zones.