Spatial variation of subduction zone fluids during progressive subduction: Insights from Serpentinite Mud Volcanoes

Geological processes at subduction zones control seismicity, plutonism and volcanism, 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 mantle wedge, as well as controlling the composition of material descending into the mantle. Any direct study of slab evolution during subduction is inhibited by the prohibitive depths at which these processes occur. To examine these processes we use serpentinite mud volcanoes in the Mariana forearc, that permit sampling of serpentinite materials and their pore waters that ascend from the subduction channel. We present new pore water chemical data from the summit and flanks of three serpentinite mud volcanoes that were drilled during International Ocean Discovery Program Expedition 366 which are reflective of reactions within the crust and mantle during the early, shallow (<20 km) stages of subduction. We show, via thermodynamic modelling, that our new data on the evolution of pore water chemical compositions reflect mineralogical characteristics of a predominately basaltic source from the downgoing Pacific Plate. However, a component from sedimentary sources is likely, especially for those mud volcanoes near the trench. Other potential slab-derived constituents, such as lithospheric serpentinite, carbonate-rich sediments, or seamount basalts with an intraplate geochemical character, are not required to explain our results. Our results indicate that with progressive subduction the lawsonite-epidote mineral transformation boundary at ∼250 °C may help drive slab carbonate destabilisation, despite its apparent thermodynamic stability at such temperatures and projected pressures (∼300 °C and ∼0.6 GPa). New dissolved gas data also point to primary thermodynamic controls over methane/ethane production within the subduction channel as depths-to-slab increase. Our findings provide direct evidence for the progressive mineralogical and chemical evolution of a subducting oceanic plate, which liberates a progressively evolving fluid phase into the subduction channel

Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand

C.D.M. was supported by NERC CASE PhD studentship award NE/G524160/1 (GNS Science, NZ, CASE partner). D.A.H.T. acknowledges support from research grants NE/H012842/1 and NE/J022128/1 and a Royal Society Wolfson Research Merit Award (WM130051). S.C.C. was funded under GNS Science's “Impacts of Global Plate Tectonics in and around New Zealand Programme” (PGST Contract CO5X0203). J.C.A. was supported by NSF OCE1334758. We also thank Matthew Cooper, Andy Milton, Darryl Green and Lora Wingate for laboratory assistance. We thank Mike Bickle for editorial advice and comments, and reviews from two anonymous reviewers that improved this manuscript.Peer reviewedPublisher PD

Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone

International audienceFault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging‐wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP‐2). We present observational evidence for extensive fracturing and high hanging‐wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP‐2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging‐wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off‐fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

<p>During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.</p

Incursion of meteoric waters into the ductile regime in an active orogen

Rapid tectonic uplift on the Alpine Fault, New Zealand, elevates topography, regional geothermal gradients, and the depth to the brittle ductile transition, and drives fluid flow that influences deformation and mineralisation within the orogen. Oxygen and hydrogen stable isotopes, fluid inclusion and Fourier Transform Infrared (FT-IR) analyses of quartz from veins which formed at a wide range of depths, temperatures and deformation regimes identify fluid sources and the depth of penetration of meteoric waters. Most veins formed under brittle conditions and with isotope signatures (?18OH2O = ?9.0 to +8.7‰VSMOW and ?D=?73 to ?45‰VSMOW?D=?73 to ?45‰VSMOW) indicative of progressively rock-equilibrated meteoric waters. Two generations of quartz veins that post-date mylonitic foliation but endured further ductile deformation, and hence formation below the brittle to ductile transition zone (&gt;6–8 km&gt;6–8 km depth), preserve included hydrothermal fluids with ?D?D values between ?84 and ?52‰?52‰, indicating formation from meteoric waters. FT-IR analyses of these veins show no evidence of structural hydrogen release, precluding this as a source of low ?D?D values. In contrast, the oxygen isotopic signal of these fluids has almost completely equilibrated with host rocks (?18OH2O = +2.3 to +8.7‰). These data show that meteoric waters dominate the fluid phase in the rocks, and there is no stable isotopic requirement for the presence of metamorphic fluids during the precipitation of ductilely deformed quartz veins. This requires the penetration during orogenesis of meteoric waters into and possibly below the brittle to ductile transition zone

Spatial Variation of Subduction Zone Fluids during Progressive Subduction: Insights from Serpentinite Mud Volcanoes

Geological processes at subduction zones control seismicity, plutonism and volcanism, 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 mantle wedge, as well as controlling the composition of material descending into the mantle. Any direct study of slab evolution during subduction is inhibited by the prohibitive depths at which these processes occur. To examine these processes we use serpentinite mud volcanoes in the Mariana forearc, that permit sampling of serpentinite materials and their pore waters that ascend from the subduction channel. We present new pore water chemical data from the summit and flanks of three serpentinite mud volcanoes that were drilled during International Ocean Discovery Program Expedition 366 which are reflective of reactions within the crust and mantle during the early, shallow (\u3c20 km) stages of subduction. We show, via thermodynamic modelling, that our new data on the evolution of pore water chemical compositions reflect mineralogical characteristics of a predominately basaltic source from the downgoing Pacific Plate. However, a component from sedimentary sources is likely, especially for those mud volcanoes near the trench. Other potential slab-derived constituents, such as lithospheric serpentinite, carbonate-rich sediments, or seamount basalts with an intraplate geochemical character, are not required to explain our results. Our results indicate that with progressive subduction the lawsonite-epidote mineral transformation boundary at ∼250 °C may help drive slab carbonate destabilisation, despite its apparent thermodynamic stability at such temperatures and projected pressures (∼300 °C and ∼0.6 GPa). New dissolved gas data also point to primary thermodynamic controls over methane/ethane production within the subduction channel as depths-to-slab increase. Our findings provide direct evidence for the progressive mineralogical and chemical evolution of a subducting oceanic plate, which liberates a progressively evolving fluid phase into the subduction channel

Chemistry of Slab-Derived Fluids in the Mariana Forearc

Geological processes at subduction zone margins control seismicity, plutonism/ volcanism, and ocean-crust-mantle geochemical cycling. The down-going plate experiences dehydration, fluid release and associated metamorphism alters the physical properties of the plate interface. The Mariana convergent margin is non-accretionary, and serpentinite mud volcanoes in the pervasively faulted forearc permit sampling of fluids and materials from the subducting slab and forearc mantle. IODP Expedition 366 drilled into three serpentinite mud volcanoes: Yinazao (13 km depth-toslab); Fantangisña (14 km) and Asùt Tesoru (18 km), allowing comparison with the previously drilled South Chamorro (18 km) and Conical (19 km) seamounts. The shallowest depth-to-slab seamounts are associated with Ca and Sr enriched, but otherwise solute poor, low alkalinity fluids of pH ~11, at equilibrium with gypsum. The Asùt Tesoru seamount fluids are markedly higher in both Na and Cl, as well as in species like B and K which are associated with the breakdown of slab sheet silicate phases, and are dramatically depleted in Ca and Sr. Higher DIC at this site is attributed to slab carbonate decomposition, while the observed elevated pH (up to 12.5) is likely caused by serpentinization reactions during which released iron is oxidised, producing H2 and OH- . Asùt Tesoru porefluids are similar to those studied at South Charmorro and Conical Seamounts, but display distinctly higher Na and Cl, and 3-4 times lower B contents. Changes in chemistry between sites reflect changes in metamorphic prograde reactions on the downgoing plate with increasing depth (P-T°). At shallowest depths sediment compaction and opal CT dehydration dominate; intermediate depths are characterised by clay diagenesis and desorbed water release; and at greater depths decarbonation and clay decomposition are dominant

The significance of heat transport by shallow fluid flow at an active plate boundary; the Southern Alps, New Zealand

Fluid flow can influence fault behavior. Here we quantify the role of groundwater heat advection in establishing the thermal structure of the Alpine Fault, a major tectonic boundary in southern New Zealand that accommodates most of the motion between the Australian and Pacific Plates. Convergence on the Alpine Fault has rapidly uplifted the Southern Alps, resulting in high geothermal gradients and a thin seismogenic zone. A new equilibrium temperature profile from the 818 m‐deep Deep Fault Drilling Project 2B (DFDP‐2B) borehole has been interrogated using one‐dimensional analytical models of fluid and rock advection. Models indicate a total heat flux of 720 mW·m‐2 results from groundwater flow with Darcy velocities approximating to 7.8×10‐10 m·s‐1. Groundwaters advect significantly more heat than rock advection in the shallow orogen (&lt;6 km depth) and are the major control on the subsurface temperature field

Compositional Variability in Serpentinite Solids, IODP Expedition 366: Insights into a Developing Subduction Channel

Recovered rocks and sediments from IODP Expedition 366 sites include serpentinite muds, serpentinized ultramafic rocks, and altered mafic and sedimentary rocks. In terms of major and compatible trace elements (Mg, Fe, Al, Si; Mn, Ni, Cr), serpentinites from Yinazao, Fantangisna and Asut Tesoru seamounts are similar to those encountered during ODP Legs 195 and 125 (S. Chamorro and Conical Smts.) in reflecting depleted mantle protoliths. However, for elements mobile in Mariana forearc porefluids, the samples are highly variable. Yinazao summit (Site U1492) samples are high in Ca and Sr, consistent with their enriched (7-10x seawater) porefluids. These samples and fluids are in thermodynamic equilibrium with gypsum, which occurs as an abundant precipitate in several core segments. Asut Tesoru Site U1496 summit samples have elevated Na2O, consistent with the very high pore fluid Na concentrations (\u3e700mM Na) from this Site. As seen in past ODP Legs, all Exp. 366 serpentinite muds have elevated CaO and Al2O3, consistent with the presence of a mafic/sedimentary component. However, the summit Site muds also show substantial enrichments in elements high in their entrained pore fluids, the compositions of which appear to vary with slab metamorphic conditions (e.g., Hulme et al 2010). The Mariana serpentinite muds thus comprise a developing reservoir of “fertile” hydrated ultramafic materials forming near the slab-mantle interface starting at very shallow (13-19 km) depths, that reflect both the evolving metamorphic state of the downgoing plate, as well as (potentially) its along-strike variability (e.g., Pearce et al 2005)