Timing and rates of fluid release during the dehydration of subconducting oceanic crust: reactive fluid flow und high-pressure conditions
Final Report Abstract
Element cycling at subduction zones controls the global geochemical budget of many elements and is a crucial process for the evolution of Earth's mantle-crust-atmosphere system. As subducting plates heat up during their passage into the deeper mantle, hydrous minerals within the different rocks of the subducted slab become progressively unstable, eventually break down, and release water. In order to develop reliable quantitative models of fluid-mediated mass transfer in a subducting slab, the general relationships between mineral reactions, fluid flow and element mobilization during fluid-rock interaction need to be quantified. The dimensions and timescales of distinct fluid flow events and the physicochemical processes triggered by fluid flow are crucial parameters of such models. As direct investigation of dehydration-related fluid flow in subducting slabs is impossible, rocks exhumed long after (i.e. millions of years) fluid-rock interaction occurred provide the only observational evidence for these processes. A complex HP/LT vein system cross-cutting an eclogitic host rock of the Pouébo Eclogite Mélange, New Caledonia, records the prograde blueschist-to-eclogite transition and includes an extensive network of veins with no or only poorly developed reaction haloes, as well as few larger veins that are associated with distinct omphacite-rich metasomatic selvages. The formation of these veins can be attributed to internal and external fluid flow processes that occurred on largely different time scales. Type I or “dehydration” veins collected internally-derived fluids related to the breakdown of hydrous phases (amphibole, chlorite, epidote) in the host rock during prograde eclogite-facies metamorphism. Pseudosection calculations indicate that internal fluids are initially derived from amphibole breakdown and fluid flow is initially low. Rapid breakdown of chlorite released additional fluids into the system and fluids accumulated in local zones with increased porosity allowing higher fluxes by channelized flow. Type II veins are attributed to fluid influx from an external source and are considered to represent “transport” veins which record superimposed influx of an external Ca-, Na- and Li-rich fluid that is most likely related to similar dehydration reactions in other parts of the subducting slab. Such veins are surrounded by a metasomatic selvage which formed by fluid-rock interaction along a reaction front due to chemical disequilibrium between internal (from in situ fluid production) and external (from fluid production elsewhere in the same slab) fluids. The chemical gradients between the dehydration-related wall rock fluid system and this external fluid caused diffusional element exchange. Chemical changes in the immediate wall-rock resulted in mineral dissolution-precipitation processes along a reaction front and the selvage formation. Various chemical and textural characteristics of garnets (zonation, inclusion patterns, REE peaks associated to titanite breakdown) along with pseudosection calculations indicate that the selvage formation took place at >500°C, but shortly prior to peak metamorphic conditions. Mass-balance calculations demonstrate that most of the LILE (K, Rb, Ba) were effectively mobilized and removed from the selvage by the fluid passing through the type II vein. The overall dehydration, fluid accumulation and fluid migration documented by the type I veins is a continuous process and occurred on a timescale of 10^5 -10^6 years that is mainly given by the geometry and convergence rate of the subduction system. In contrast, individual fluid release steps documented by type II veins are much faster and highly localized in space and time. As indicated by our results from the Chinese Tianshan Mountains and from New Caledonia the channelized fluid pulses associated with metamorphic slab dehydration can result in accumulation of high fluid volumes within short time intervals to allow for a fast rise of pore fluid pressures. The short-lived character of this process (thousands of years to years) suggests that the fluid flow related to oceanic plate dehydration is likely responsible for episodic pore fluid pressure increases at the plate interface. These occur localized in space and time and may trigger various seismic to nonseismic slip phenomena at the plate interface.
Publications
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Fast dehydration linked to pulses of high pore fluid pressure at the subducted plate interface. Submitted to Earth Planetary Science Letters
Taetz S, John T, Bröcker M, Spandler C, Stracke A
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(2012). Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs. Nature Geoscience, 5, 489– 492
John, T., Gussone, N., Podladchikov, Y. Y., Bebout, G. E., Dohmen, R., Halama, R., et al.
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(2013). Fluid-rock interaction along a HP/LT vein system on New Caledonia. DMG-GV Conference Abstracts 2013: 300
Taetz S, John T, Bröcker M
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(2015). Fluid-Rock Interaction along a HP/LT Vein System (New Caledonia). Goldschmidt Abstracts: 2015 3057
Taetz S, John T, Bröcker M, Spandler C
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(2016). Fluid–rock interaction and evolution of a highpressure/low-temperature vein system in eclogite from New Caledonia: insights into intraslab fluid flow processes. Contributions to Mineralogy and Petrology, 171, 1–27
Taetz S, John T, Bröcker M, Spandler C
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(2017). Fast fluid-flow events within a subductionrelated vein system in oceanic eclogite: implications for pore fluid pressure at the plate interface. Geophys Res Abstr: EGU2017-12617
Taetz S, John T, Bröcker M, Spandler C, Stracke A
