Description: International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

Description: IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (~ 〉 70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poikilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82-88 in olivine-rich troctolites, to 78-83 in gabbros) and is in Mg-Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Ybcpx ~ 5-11 * Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges.

Description: This database reports the results of bulk rock geochemical measurements realized on 84 rock samples collected from Hole BT1B drilled during ICDP Oman Drilling Project (OmanDP, Kelemen et al. [2020]). 15 samples were collected on-site every 20m during the drilling operations (February-March 2017). 59 samples were selected by the shipboard science party as representative of the different lithologies recovered from Hole BT1B during the description of the cores, on board D/V Chikyu (August 2017). 10 additional listvenites and serpentinites were selected from Sections C5704B-73Z-1 to -75Z-2 (180.01-186.945mbg) for a coordinated on-shore study of the lower serpentinite intervals and neighboring listvenites (thereafter referred to as consortium samples). The purpose of the study was to obtain a high-density and high analytical quality bulk geochemical characterization along the 300.05 meters of continuous cores recovered from OmanDP Hole BT1B, through the transition from the variously carbonated peridotites at the base of the Semail ophiolite mantle section to the underlying metamorphic lithologies. 51 listvenites, 14 serpentinites, and 19 greenshists and greenstones were analyzed. The rock names and grouping by Units were determined on-board D/V Chikyu from macroscopic observations (Visual Core Description; Kelemen et al. [2020]). Major and trace element concentrations were measured by X-ray fluorescence (XRF). XRF analyses of shipboard and on-site samples noted * in the Method columns were realized on-board D/V Chikyu (Note that major oxide concentrations in Kelemen et al. [2020] are recalculated to 100 wt.%) and those noted † in the Method columns were realized at the University of St. Andrews (see Table BT1-T12 in Kelemen et al. [2020]). XRF analyses of consortium samples were realized at Geolabs. FeO concentrations were measured by titration at the University of Lausanne (Switzerland). Total H and C concentrations (noted TH and TC) were determined on-board D/V Chikyu by combustion CHNS elemental analysis (EA) and used to recalculate H2O and CO2 contents. Concentrations of carbon in Ca-carbonates (total inorganic carbon; noted TIC) were determined by coulometry. Trace element compositions were determined using a Quadrupole Inductively-Coupled-Plasma-Mass Spectrometer (Q-ICP-MS) at the University of Montpellier (France). All analyses were performed on samples prepared from non ignited rock-powders, except for XRF major element analyses realized on beads on-board D/V Chikyu. Concentrations are reported in wt.% (10-2g/g) and in ppm (10-6 g/g). Abbreviations: mbg: meters below ground (Chikyu curated depth); Fu-listvenite : fuchsite-bearing listvenite; LOI : Loss on ignition; XRF B : XRF analyses on beads; XRF P : XRF analyses on powder pellets; XRF B/P : XRF major element analyses on beads except for K measured on pellets and recalculated as volatile free; n.a.: not analysed; n.d.: not determined. (Notes, abbreviations & reference at the bottom of the file) ‡ Sample C5704B-60Z-4-1, 24.0--29.0 cm: Green matrix (Host: Sample C5704B-60Z-4-1, 24.0--29.0 cm - H) crosscut by pink vein (Vein : Sample C5704B-60Z-4-1, 24.0--29.0 cm - V) Reference : Kelemen, P. B., J. M. Matter, D. A. H. Teagle, J. A. Coggon, and the Oman Drilling Project Science Team (2020), Proceedings of the Oman Drilling Project, College Station, TX.

Description: IODP Site U1309 was drilled at Atlantis Massif, an oceanic core complex, at 30°N on the Mid-Atlantic Ridge (MAR). We present the results of a bulk rock geochemical study (major and trace elements) carried out on 228 samples representative of the different lithologies sampled at this location. Over 96% of Hole U1309D is made up of gabbroic rocks. Diabases and basalts cross-cut the upper part of the section; they have depleted MORB compositions similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic and geochemical evidence of extensive melt-rock interactions. Gabbroic rocks comprise: olivine-rich troctolites (〉 70% modal olivine) and troctolites having high Mg# (82-89), high Ni (up to 2300 ppm) and depleted trace element compositions (Yb 0.06-0.8 ppm); olivine gabbros and gabbros (including gabbronorites) with Mg# of 60-86 and low trace element contents (Yb 0.125-2.5 ppm); and oxide gabbros and leucocratic dykes with low Mg# (〈 50), low Ni (~65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros are amongst the most primitive and depleted oceanic gabbroic rocks. The main geochemical characteristics of Site U1309 gabbroic rocks are consistent with a formation as a cumulate sequence after a common parental MORB melt, although (lack of systematic) downhole variations indicate that the gabbroic series were built by multiple magma injections. In detail, textural and geochemical variations in olivine-rich troctolites and gabbronorites suggest chemical interaction (assimilation?) between the parental melt and the intruded lithosphere. Site U1309 gabbroic rocks do not represent the complementary magmatic product of 30°N volcanics, although they sample the same mantle source. The bulk trace element composition of Site U1309 gabbroic rocks is similar to primitive MORB melt compositions; this implies that there was no large scale removal of melts from this gabbro section. The occurrence of such a large magmatic sequence implies that a high magmatic activity is associated with the formation of Atlantis Massif. Our results suggest that almost all melts feeding this magmatic system stays trapped into the intruded lithosphere.