Shallow depth, substantial change: fluid-metasomatism causes major compositional modifications of subducted volcanics (Mariana forearc)

Mass transfer at shallow subduction levels and its ramifications for deeper processes remain incompletely constrained. New insights are provided by ocean island basalt (OIB) clasts from the Mariana forearc that experienced subduction to up to ~25–30 km depth and up to blueschist-facies metamorphism; thereafter, the clasts were recycled to the forearc seafloor via serpentinite mud volcanism. We demonstrate that the rocks were, in addition, strongly metasomatized: they exhibit K2O contents (median = 4.6 wt.%) and loss on ignition (median = 5.3 wt%, as a proxy for H2O) much higher than OIB situated on the Pacific Plate, implying that these were added during subduction. This interpretation is consistent with abundant phengite in the samples. Mass balance calculations further reveal variable gains in SiO2 for all samples, and MgO and Na2O increases at one but the loss of MgO and Fe2O3* at the other study site. Elevated Cs and Rb concentrations suggest an uptake whereas low Ba and Sr contents indicate the removal of trace elements throughout all clasts.The metasomatism was likely induced by the OIBs’ interaction with K-rich fluids in the subduction channel. Our thermodynamic models imply that such fluids are released from subducted sediments and altered igneous crust at 5 kbar and even below 200°C. Equilibrium assemblage diagrams show that the stability field of phengite significantly increases with the metasomatism and that, relative to not-metasomatized OIB, up to four times as much phengite may form in the metasomatized rocks. Phengite in turn is considered as an important carrier for K2O, H2O, and fluid-mobile elements to sub-arc depths.These findings demonstrate that mass transfer from subducting lithosphere starts at low P/T conditions. The liberation of solute-rich fluids can evoke far-reaching compositional and mineralogical changes in rocks that interact with these fluids. Processes at shallow depths (<30 km) thereby contribute to controlling which components as well as in which state (i.e., bound in which minerals) these components ultimately reach greater depths where they may or may not contribute to arc magmatism. For a holistic understanding of deep geochemical cycling, metasomatism and rock transformation need to be acknowledged from shallow depths on.

Tracing the Evolution of Slab Fluids during Progressive Subduction: Insights from Serpentinite Mud Volcanoes in the Mariana Forearc

&lt;p&gt;Geological processes in subduction zones strongly influence seismicity, igneous activity, and geochemical cycling between the oceans, crust, and mantle. The down-going plate experiences metamorphism, and the associated dehydration and fluid flow alters the physical properties of the plate interface and overlying mantle wedge. Direct study of active slab evolution is inhibited by the great depths at which these processes occur and there is a dearth of physical samples to assess the state of water-rock-sediment reactions, thermal and pressure conditions, and physical properties of materials within the subduction channel.&lt;/p&gt;&lt;p&gt;The drilling of serpentinite mud volcanoes in the Mariana forearc provides a telescope into these deep processes and allows us to sample fluids and xenoliths from the subducting slab and forearc mantle.&amp;#160; Fluid-laden serpentinite is transported along active extensional faults in the upper plate and seeps out at mud volcano edifices. There is widespread evidence for episodic voluminous serpentine eruptions, likely related to seismic events. Mud volcanoes are found across the forearc and sample the slab interface from 13 to 19 km depth. Samples obtained over three Scientific ocean drilling legs (ODP Legs 125 and 195; IODP Leg 366) and additional ROV expeditions elucidate the evolution of fluid production, reaction and exchange, during the progressive subduction of the down-going plate.&lt;/p&gt;&lt;p&gt;Fluid analyses show clear trends in pore water chemical and isotopic composition with progressive subduction. These parameters can be used to assess the thermal state of the subduction channel at different depths, identify the reactions controlling fluid releases, and to estimate fluid fluxes. Pore waters from the shallowest depths-to-slab (13-16 km) are Ca and Sr-enriched compared to seawater, but otherwise solute poor, low alkalinity fluids of pH ~11. In contrast, more deeply derived fluids (&gt;18 km) have higher pH (12.5), reduced concentrations of Ca and Sr and elevated DIC, Na and Cl, as well as B and K compared to seawater &amp;#8211; these changes are associated with the breakdown of slab sheet silicate phases. These waters also have higher &amp;#948;D and &amp;#948;&lt;sup&gt;11&lt;/sup&gt;B values than shallower waters (&amp;#948;D values up to +16 &amp;#8240;; &amp;#948;&lt;sup&gt;11&lt;/sup&gt;B ~ 14-15 &amp;#8240; cf. &amp;#948;D &lt; 0&amp;#8240;; &amp;#948;&lt;sup&gt;11&lt;/sup&gt;B ~ 12-13 &amp;#8240;). PHREEQC modelling indicates pore water chemical evolution reflects mineralogical characteristics of a predominately basaltic source from the downgoing Pacific Plate; however, a component from sediment sources is a likely contributor, especially for those mud volcanoes near the trench.&lt;/p&gt;&lt;p&gt;Our new data indicate that the lawsonite-epidote mineral transformation boundary (~250 &amp;#176;C, &gt;18 km depth) is an important source of devolatilization waters and may also drive slab carbonate breakdown, despite its apparent thermodynamic stability at such temperatures and pressures. At shallower depths, the main reactions controlling fluid liberation are sediment compaction (&lt;13 km) followed by clay diagenesis and desorbed water release (&gt;13 km depth). This study thus provides direct evidence for the progressive mineralogical and chemical evolution of a subducting oceanic plate.&lt;/p&gt;

Forearc magmatic evolution during subduction initiation: Insights from an Early Cretaceous Tibetan ophiolite and comparison with the Izu-Bonin-Mariana forearc

Subduction initiation is a key process in the operation of plate tectonics. Our understanding of melting processes and magmatic evolution during subduction initiation has largely been developed from studies of the Izu-Bonin-Mariana forearc. Many suprasubduction zone ophiolites are analogous to the Izu-Bonin-Mariana forearc sequence. However, whether there are differences between Izu-Bonin-Mariana subduction initiation sequences and suprasubduction zone ophiolites remains unclear. Here, we report field geological, geochemical, and geochronological data from mafic and felsic rocks in the Xigaze ophiolite (southern Tibet) mantle and crustal section; the same types of published data from both this ophiolite and the Izu-Bonin-Mariana forearc are compiled for comparison. The ophiolite section is intruded by various late-stage dikes, including gabbroic pegmatite, diabase, basalt, and plagiogranite. The compositions of clinopyroxene and amphibole suggest that gabbroic pegmatite formed from hydrous high-SiO2 depleted melts, while whole-rock compositions of basaltic and diabase dikes show negative Nb and Ta anomalies, suggesting flux melting of depleted mantle. Along with the mafic rocks, plagiogranite has a roughly constant content of La and Yb with increasing SiO2 contents, implying hydrous melting of mafic amphibolite. Early-stage pillow basalts exhibit geochemical affinities with Izu-Bonin-Mariana forearc basalts, but they are slightly enriched. Synthesized with the regional geological setting and compared with Izu-Bonin-Mariana forearc magmatism, we propose that the transition from mid-ocean ridge basalt−like lavas to subduction-related mafic and felsic dikes records an Early Cretaceous subduction initiation event on the southern flank of the Lhasa terrane. However, the mantle sources and the magmatic evolution in the Xigaze ophiolite are more variable than those for the Izu-Bonin-Mariana forearc.

Supplemental Material: Forearc magmatic evolution during subduction initiation: Insights from an Early Cretaceous Tibetan ophiolite and comparison with the Izu-Bonin-Mariana forearc

Detailed analytical methods in Text S1, major- and trace-element compositions of clinopyroxene, orthopyroxene, and amphibole, whole-rock major and trace elements, Sr-Nd isotopic data, and zircon U-Pb and Lu-Hf data in Tables S1–S7; Figures S1–S5.

Supplemental Material: Forearc magmatic evolution during subduction initiation: Insights from an Early Cretaceous Tibetan ophiolite and comparison with the Izu-Bonin-Mariana forearc

Detailed analytical methods in Text S1, major- and trace-element compositions of clinopyroxene, orthopyroxene, and amphibole, whole-rock major and trace elements, Sr-Nd isotopic data, and zircon U-Pb and Lu-Hf data in Tables S1–S7; Figures S1–S5.