Large fluxes of water-rich, slab fluids released during subduction inception
Abstract
Subduction zones have played a key role in regulating the water fluxes in and out exchanged between the surface of the Earth and its deeper interior over geological times. However, the processes that occur at the onset of a subduction zone remain poorly understood. Spreading of the pre-subduction lithosphere is believed to occur above the nascent descending plate, forming pillow magmas within a few kilometers from the trench. But the composition of the slab fluids that are released as the plate began sinking are often obscured by seawater alteration. Here, we further examine fresh samples from southeast Mariana fore-arc rifts (SEMFR) and the Matthew-Hunter (MH) convergent margin to better comprehend the processes that occurred during near-trench spreading. The MH convergent margin is believed to represent a modern, nascent subduction zone, which initiated ~ 2 Ma ago; while the SEMFR emplaced by large-scale and disorganized fore-arc stretching ~ 2-4 Ma ago above a long-lived subduction zone. Their formation within 90 km from the trench can shed some new important lights onto the processes that occurred during subduction inception. The SEMFR and MH magmas recorded the highest markers in slab dehydration (H2O/Ce = 0.04 - 17.79 , Rb/Th = 3 - 141 , Cs/Th = 436 - 23531 ) yet recorded in subduction zone magmas (worldwide arc magmas display Rb/Th ≤ 111, Cs/Th ≤ 4, H2O/Ce ≤ 10612), implying that large fluxes of water-rich, slab fluids can be released within 90 km from trench. The similar composition in slab-fluid proxies in the proto-arc basalts and boninites from the Troodos ophiolite and Izu-Bonin-Mariana proto-arc boninites further indicates that similar subduction processes occurred as the oceanic plate began sinking. In Troodos and in MH, the subducted slab dehydrated efficiently as the subducted oceanic plate began sinking. In the IBM proto-arc, the subducted Pacific slab retained most of its water-rich fluids 1-2 millions of years after subduction inception to produce the upper boninites. The water-rich, slab-derived fluids thus infiltrated the asthenospheric mantle to produce proto-arc magmas long before any arc magmatic activity initiated. Efficient dehydration of the nascent slab may thus have contributed to the Paleogene-Eocene and Late Cretaceous global warmings that preceded the onset of the IBM and Troodos subduction zones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV045...09R
- Keywords:
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- 8140 Ophiolites;
- TECTONOPHYSICS;
- 8147 Planetary interiors;
- TECTONOPHYSICS;
- 8410 Geochemical modeling;
- VOLCANOLOGY;
- 8413 Subduction zone processes;
- VOLCANOLOGY