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Fluid Evolution in an Oceanic Core Complex: a Fluid Inclusion Study from IODP Hole U1309 D - Atlantis Massif, 30°N, Mid-Atlantic Ridge

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Fluid Evolution in an Oceanic Core Complex: a Fluid Inclusion Study from IODP Hole U1309 D - Atlantis Massif, 30°N, Mid-Atlantic Ridge This is a repository copy of Fluid evolution in an Oceanic Core Complex: a fluid inclusion study from IODP hole U1309 D - Atlantis Massif, 30°N, Mid-Atlantic Ridge. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/80181/ Version: Published Version Article: Castelain, T, McCaig, AM and Cliff, RA (2014) Fluid evolution in an Oceanic Core Complex: a fluid inclusion study from IODP hole U1309 D - Atlantis Massif, 30°N, Mid-Atlantic Ridge. Geochemistry, Geophysics, Geosystems, 15 (4). 1193 - 1214. ISSN 1525-2027 https://doi.org/10.1002/2013GC004975 Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. 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[email protected] https://eprints.whiterose.ac.uk/ PUBLICATIONS Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE Fluid evolution in an Oceanic Core Complex: A fluid inclusion 10.1002/2013GC004975 study from IODP hole U1309 D—Atlantis Massif, 30N, Key Points: Mid-Atlantic Ridge The TAG model is a good analog for fluid evolution at Atlantis Massif Teddy Castelain1, Andrew M. McCaig2, and Robert A. Cliff2 Fluid inclusion analyses reveal evidence for magmatic exsolution 1Institut des Sciences de la Terre d’Orleans, 1a rue de la Ferollerie, Orleans, France, 2School of Earth and Environment, Fluid inclusion analyses reveal Leeds University, Leeds, UK evidence for phase separation Correspondence to: Abstract In the detachment mode of slow seafloor spreading, convex-upward detachment faults take T. Castelain, up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the [email protected] seafloor. Large, long-lived hydrothermal systems such as TAG are situated off axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The Citation: Castelain, T., A. M. McCaig, and R. A. consequences of such deep circulation for the evolution of fluid temperature and salinity have not Cliff (2014), Fluid evolution in an previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro, and Oceanic Core Complex: A fluid trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30N, Mid-Atlantic Ridge), reveals inclusion study from IODP Hole U1309 D—Atlantis Massif, 30N, Mid-Atlantic evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and Ridge, Geochem. Geophys. Geosyst., 15, previously phase-separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 1193–1214, doi:10.1002/ to 35 wt % NaCl, although the most common inclusions have salinities close to seawater (3.4 wt % NaCl). 2013GC004975. Homogenization temperatures range from 160 to >400C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low-salinity inclusions trapped in quartz Received 2 AUG 2013 Accepted 14 JAN 2014 veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric Accepted article online 20 JAN 2014 data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal Published online 23 APR 2014 field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped. 1. Introduction In previous studies, oceanic and ophiolitic fluid inclusions have been interpreted in the context of purely magmatic ocean floor spreading, with the principle driving force for hydrothermal circulation being a shallow axial magma chamber [Kelley and Delaney, 1987; Kelley et al., 1992, 1993]. The consequences for hydrothermal fluid circulation in Oceanic Core Complexes (OCCs) of the recently established ‘‘detachment mode’’ [Escartin and Canales, 2011; McCaig and Harris, 2012] of seafloor spreading have not been investigated. In particular, this mode of spreading may involve deeper magma chambers (up to 7 km below seafloor) [deMartin et al., 2007], focussed fluid discharge along detachment faults [McCaig et al., 2007, 2010], and exhumation of gabbros and peridotites onto the seafloor [Cannat, 1993; Ildefonse et al., 2007; McCaig and Harris, 2012]. Fluids of variable salinities have been found in ocean crust. Processes responsible for such variation have been widely investigated in previous studies. Two-phase separation has been suggested as an important process to explain the observed salinity variation of hydrothermal fluids (10–200% of the seawater value) at different localities in the Mid-Atlantic Ridge such as the MARK area (Mid-Atlantic Ridge at Kane) [Kelley et al., 1993], and in the South West Indian Ridge [Kelley and Fruh-Green€ , 2001]. Nevertheless, other processes such as exsolution from a melt [Kelley et al., 1992; Kelley and Malpas, 1996] and hydration/dehydration reactions with precipitation/dissolution of associated chloride-bearing minerals [Kelley and Robinson, 1990] need to be considered in certain cases. This is an open access article under the The Atlantis Massif (AM) is an Oceanic Core Complex (OCC) located at 30N at the inside corner of the Mid- terms of the Creative Commons Atlantic Ridge (MAR) and the Atlantis Transform Fault (ATF). Gabbros and mantle peridotite were exhumed Attribution License, which permits use, to the seafloor by the movement of a detachment fault that is the locus of hydrothermal fluid discharge distribution and reproduction in any medium, provided the original work is [Boshi et al., 2006; McCaig et al., 2007, 2010]. IODP Hole U1309D has been drilled in the central dome of the properly cited. AM which is believed to be the footwall of this major detachment fault. The drillcore shows evidence of fluid CASTELAIN ET AL. VC 2014. The Authors. 1193 Geochemistry, Geophysics, Geosystems 10.1002/2013GC004975 Figure 1. 3-D view of the Atlantis Massif showing morphotectonic features and the location of IODP holes U1309A-E. Illumination comes from SE. Vertical exaggeration is about 1.8. A color scale is shown to indicate the bathymetry [Blackman et al., 2006]. circulation, especially at depths <350 mbsf [Blackman et al., 2011]. Gabbros, diabases, and trondjhemite have trapped samples of hydrothermal fluids that might represent different fluids circulating in the AM dur- ing the exhumation and thermal evolution of the massif. In this paper, we present analyses of fluid inclusions in gabbro, trondjhemite, and in quartz veins cutting diabase and trondjhemite recovered from IODP Hole U1309D from the central dome of the AM during the two consecutive IODP expeditions 304 and 305. Results are interpreted in a geodynamical context sug- gested by a model of the trans-Atlantic geotraverse (TAG) area [deMartin et al., 2007; McCaig et al., 2007, 2010]. They provide new insights on generation of brine and pressure-temperature constraints on the evo- lution of the hydrothermal fluid circulation in the Atlantis Massif and in OCCs [Kelley, 1996, 1997; Vanko and Stakes, 1991] in general. 2. Geological Setting The Atlantis Massif is an OCC, located at the inside corner of the MAR and the ATF (30N) (Figure 1). An OCC is a dome-like exposure of variably deformed and metamorphosed lower crustal and upper mantle rocks that has been unroofed by movement on a major detachment fault (Figure 2) [Blackman et al., 2002, 2011; Ildefonse et al., 2007; Tucholke et al., 1998; McCaig and Harris, 2012]. Gabbros from the massif have been dated between 1.08 6 0.07 and 1.28 6 0.05 Myr [Grimes et al., 2008]. The AM is composed of three different parts: (i) the central dome in which two deep holes (U1309B and U1309D) and five shallow-penetration holes (U1309A and U1309E-H), recovering upper sediment cover and fragments of detachment fault schist have been drilled (Figure 1), (ii) the southern wall which is dominated by serpentinized peridotite capped by a 100 m thick detachment shear zone rich in talc and tremolite [Boschi et al., 2006]; it is the host of the Lost City hydrothermal field, and (iii) the eastern block, interpreted as a fault-bounded block of basaltic material lying structurally above the central dome. The central dome is characterized by a corrugated surface believed to be an exposure of the major detachment fault responsi- ble for the uplift of the massif. The corrugations are parallel to the spreading directions and have a wave- length of approximately 1000 m, amplitude of tens of meters, and length of several kilometers [Cann et al., 1997]. Hole U1309B (3010.110N, 4207.110W; 1642 mbsl) was drilled up to 101.8 mbsf with an average recovery of about 50%. Hole U1309D, located at 3010.120N, 4007.110W, 1645 mbsl, (20 m from Hole U1309B) pene- trated 1415.5 mbsf, with a recovery of 75% comprising intrusive basalt and diabase (3%), gabbroic (91%), and olivine-rich rock (5%) consisting of dunites, wehrlites, troctolites, as well as a few mantle peridotites CASTELAIN ET AL. VC 2014. The Authors. 1194 Geochemistry, Geophysics, Geosystems 10.1002/2013GC004975 Figure 2. Schematic 3-D block diagram of the Atlantis Massif. Detachment fault is shown as a curved white line which steepens at depth. Location of U1309D is also shown in white.
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