Eastern sites

Abstract

Operations 3 Lithology, alteration, and structure 15 Bulk rock geochemistry 20 Fluid chemistry 23 Microbiology 24 Sensor package data 29 Physical properties 28 Borehole plugs 32 Downhole logging 34 Paleomagnetism 35 References Operations During Expedition 357, cores were recovered from two sites in the eastern area of Atlantis Massif: Sites M0068 and M0075 (Figure F1; Table T1). Newly acquired multibeam data, combined with preexisting data sets, were evaluated prior to each site to guide the drill teams with regard to anticipated seabed conditions and slope. Site M0068 Cores were recovered from two holes at Site M0068 (proposed Site AM-02A), with an average site recovery of 58.88%. The mean water depth for the two holes was 1102.35 m with no tidal range. The total time spent on station was 2.36 days. Hole M0068A The RSS James Cook moved onto Site M0068 at 2208 h on 5 November 2015, with the site position centered over the British Geological Survey RockDrill2 (RD2) port-side deployment position. The vessel settled into dynamic positioning (DP) mode. Prior to coring at each location, a conductivity, temperature, and depth (CTD) cast was conducted to quantify the background geochemistry and microbiology within the water column. The CTD was deployed at 2248 h and held at 10 m water depth for a full systems check. At 2332 h, the CTD was 1.6 m off the bottom, and the first three Niskin bottles were fired. Two more Niskin bottles were fired on the way up, with a final one fired near the surface (see Table T5 in the Expedition 357 summary chapter [Früh-Green et al., 2017c]). The CTD was recovered to deck at 0005 h on 6 November. During the ship CTD cast, the RD2 was made ready for deployment, but a communications problem with the CTD sensor mounted on the drill delayed launch. The RD2 was deployed at 0124 h; however, problems with the winch spooling mechanism and the methane sensor on the water sensor package meant the drill was recovered at 0146 h before landing on the seafloor. At 0234 h, the RD2 was launched again, landing on the seafloor at 0356 h after a full mid-water column systems check. Coring operations commenced at 0418 h. The first core run was completed at 0602 h. However, a problem with the arm shift meant the RD2 had to be recovered after a total penetration of 1.97 m. Prior to recovery, the Niskin bottles on the water sampling package were fired. The RD2 was on deck and secure by 0835 h. The vessel began to transit to Site M0069 at 0840 h on 6 November. Preparations began for deploying the Meeresboden-Bohrgerät 70 (MeBo) drill. One coring attempt was made in Hole M0068A to a maximum depth of 1.97 m with 23.92% recovery. Hole M0068B The vessel arrived on station at 1545 h on 23 November 2015. RD2 preparations continued until 1757 h, when the drill was deployed. At 1836 h, the RD2 was brought back on deck after a midwater column systems check revealed two sensor inputs were G.L. Früh-Green et al. Eastern sites IODP Proceedings 2 V o l u m e 3 5 7

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“…Samples from Hole M0068B exhibit the highest SiO 2 , CaO, and Na 2 O contents but the lowest Al 2 O 3 and FeO contents. Mafic samples (gabbros and dolerite) also divide in two groups ( Figure F12; see Table T5 in the Eastern sites chapter [Früh-Green et al, 2017b]). In this case, the division relates to the degree of hydration and type of alteration (LOI of 0.4, 0.3, 3.3 wt% for the samples from Hole M0068B as opposed to LOI around 8 wt% for samples from the central and western sites).…”

“…Independent of the site location, altered ultramafic lithologies and talc-amphibole-chlorite schists form two groups within the SiO 2 -MgO space ( Figure F11; see Table T5 in the Eastern sites chapter [Früh-Green et al, 2017b]). Talc-amphibole-chlorite schists are associated with the highest SiO 2 /MgO ratios (SiO 2 > 55.1 wt% [normalized concentrations] and MgO < 28.9 wt%), whereas the serpentinized ultramafic rocks are associated with the lowest ratios (SiO 2 < 49.3 and MgO > 37.6 wt%).…”

“…Rare earth element (REE) patterns group by lithology (see Figure F21 in the Eastern sites chapter, Figure F23 in the Central sites chapter, and Figure F10 in the Western sites chapter [Früh-Green et al, 2017b, 2017a). Serpentinized ultramafic rocks have relatively flat to slightly light rare earth element (LREE) depleted chondrite-normalized patterns (typically centered around 1 or below).…”

“…Primitive mantle (PM)-normalized patterns are relatively smooth (see Figure F21 in the Eastern sites chapter, Figure F24 in the Central sites chapter, and Figure F10 in the Western sites chapter [Früh-Green et al, 2017b, 2017a). Along with correlated Mg# and Ni abundances (Figure F13), other geochemical trends in serpentinized ultramafic rocks can be identified, including a common uranium positive anomaly (the intensity of which decreases in impregnated/metasomatized samples) (Figure F14), enriched lithium, cerium, and strontium anomalies in the central sites (Table T4).…”

“…As a substitute for shipboard pH measurements, values were used from the in situ sensor packages for the period immediately prior to landing the drill on the seafloor and commencing operations (see Water sampling and sensor package data in the Expedition 357 methods chapter (Früh-Green et al, 2017c). These pH values vary from 7.8 to 7.9 and are reported in Table T11 in the Eastern sites chapter (Früh-Green et al, 2017b) as shipboard pH data for bottom water.…”