The movement of water bound in the mineral chlorite from the earth’s surface to the mantle via subduction zones
When two of the earth’s tectonic plates collide, the heavier plate is subducted – beginning a long journey down into the mantle. The subducting plate contains numerous water-bearing minerals. Due to increasing pressure (P) and temperature (T) during descent, these minerals progressively transform and release ‘free’ water. By transforming, the mineral volume reduces and this registers as an earthquake. Free water is an essential ingredient in magma which feeds surface volcanoes. Deep mantle plumes are known to contain water. The addition of water to high-P, high-T rocks in the subduction zone has many other effects. Therefore, a thorough understanding of hydrous minerals in the subduction zone is essential if we are to sharpen our knowledge of these phenomena.
Geophysical models estimate that the temperatures within many subduction zones would render most hydrous minerals unable to journey beyond a depth of ~150kms. However, these models use published geochemical data and not all hydrous minerals have been well-studied. One of these is chlorite, a common water-bearing mineral which contains ~13 wt% water. Chlorite forms contrasting rock types in the subduction zone, including chlorite peridotite and chlorite schist. Little is known of chlorite stability in these rocks at high pressure. This study aimed to remedy this situation.
Laboratory experiments were conducted at a range of pressures (1.0 GPa-6.2 GPa) and temperatures (500°C to 860°C) using piston cylinder apparatus to simulate actual P,T conditions experienced in a subduction zone. Experiments used only natural rocks. Microphotographs and chemical analyses were used to examine mineral textures and chemical reactions. Some very interesting conclusions were obtained.
Chlorite possessed a maximum stability of 6.2 GPa, 675°C (in chlorite schist) and 6.2 GPa, 635°C (in chlorite peridotite). This is equivalent to ~190 kms depth, over 40kms deeper than determined by previous research, and places stable chlorite within the upper mantle. Upon breakdown, chlorite schist formed garnet, olivine and two hydrous minerals – 11.5Å-phase (12.1 wt% water) and Mg-sursassite (7.2 wt% water). The transformation of chlorite peridotite formed olivine and Mg-sursassite. Both hydrous minerals are stable to high P,T conditions. These results show that in a cooler subduction setting such as Tonga, these hydrous minerals would act as transport vehicles, quickly moving significant quantities of water to the deep mantle thus providing a potential water source for mantle plumes.