ALBERT

Description: The gabbronoritic cumulates drilled at DSDP Site 334 (Mid-Atlantic Ridge off the FAMOUS area) are neither crystallization products of the associated basalts, nor from any MORB composition documented along ocean ridges. Their parent melts are richer in SiO2 than MORB at a given MgO content, as attested by the crystallization sequence starting with an olivine+calcic and sub-calcic pyroxene assemblages. These melts are issued from a source highly depleted in incompatible elements, likely residual peridotite left after MORB extraction. To understand the role of water in the genesis of these lithologies whose occurrence in a mid-ocean ridge setting is rather puzzling, we performed a geochemical study on clinopyroxene separates following an analytical protocol able to remove the effects of water rock interactions post-dating their crystallization. Accordingly, the measured isotopic signatures can be used to trace magma sources. We find that Site 334 clinopyroxenes depart from the global mantle correlation: normal MORB values for the 143Nd/ 144Nd ratio (0.51307-0.51315) are associated to highly radiogenic 87Sr / 86Sr (0.7034-0.7067) ratios. This indicates that the parent melts of Site 334 cumulates are issued from a MORB source but that seawater contamination occurred at some stage of their genesis. The extent of contamination, traced by the Sr isotopic signature, is variable within all cumulates but more developed for gabbronorites sensus stricto, suggesting that seawater introduction was a continuous process during all the magmatic evolution of the system, from partial melting to fractional crystallization. Simple masse balance calculations are consistent with a contaminating agent having the characters of a highly hydrated (possibly water saturated) silica-rich melt depleted in almost all incompatible major, minor and trace elements relative to MORB. Mixing in various proportions of contaminated melts similar to the parent melts of Site 334 cumulates with MORB can account for part of the variability in the Sr isotopic signature of oceanic basalts, among other to the short wavelength isotopic ,,noise" superimposed on regional trends. We conclude that seawater introduction into residual peridotite at shallow depth beneath mid-ocean ridges can lead mantle rocks and their melts to follow complex P-T-fH2O paths that mimic petrogenetic contexts classically attributed to subduction zone environments, like the production of boninitic-andesitic magmas.

Description: The database reports the results of bulk rock geochemical measurements realized on 105 ultramafic lithologies (harzburgites and dunites) samples collected from Holes CM1A (46 samples) and CM2B (59 samples) drilled in the Wadi Tayin massif in the SE of the ophiolite during Phase 2 of the ICDP OmanDP (Nov. 2017-Jan. 2018) (Kelemen et al. [2020]). The studied samples were selected following two strategies. First, a homogeneous sample was selected every 10 m downhole cores during the OmanDP Phase 2 drilling operations, onsite in Oman, in order to get a petrological and geochemical overview continuously along the cores. Second, additional samples have been selected during the daily ChikyuOman Leg 3 sampling meetings in consultation with the core description teams, to focus on more specific facies or levels. These samples are referred to as onsite samples and shipboard samples respectively. Adjacent to each onsite and shipboard sample an oriented thin section billet was taken for mineralogical and lithological characterization. Geochemical data of onsite and shipboard samples were measured both aboard the D/V Chikyu during the ChikyuOman Phase 2 Leg 3 for major element and volatile contents for part of the samples, and at Institute of Earth Science, Academia Sinica, Taiwan (IES-AS), the University of Edinburgh, Scotland (EU), Université Toulouse III - Paul Sabatier, France (TU), and Niigata University, Japan (NU) for trace element contents and additional major element and volatile contents. The purpose of the study was to obtain a high-density and high analytical quality bulk geochemical characterization along continuous cores recovered from OmanDP Holes CM1A and CM2B, from the crust to the mantle through the crust-mantle transition zone. Loss on ignition (LOI) of all onsite and shipboard samples were determined onboard the D/V Chikyu, using the OHTI (Ocean High Technology Institute, Inc., Tokyo, Japan) motion compensated balance system into a pre-weighed ceramic crucible using a spatula (that was never in contact with lithium metaborate flux). Duplicate LOI measurements were done on the onsite samples at EU, following the same steps and procedures. Major element abundances (wt.% oxides) in powdered rock samples were determined using the RIGAKU Supermini wavelength dispersive X-ray fluorescence spectrometer equipped with a 200 W Pd anode tube at 50 kV and 4 mA onboard DV Chikyu during OmanDP Phase 2 Leg 3. Major element analyses were determined to be acceptable if the sum of the anhydrous oxide concentrations totaled to between 99 and 101 wt.%. Precision and accuracy are better than 2.5 % for all oxides except for TiO2 for reference materials DTS-2B and JP-1 (better than 11%) and Na2O, P2O5 and K2O for JGb-2 (3.40, 17.60, and 7.49% respectively). Duplicates of onsite samples whole rock major element analyses were performed at EU, using the Panalytical PW2404 wavelength-dispersive sequential X-ray spectrometer. Gas chromatographic separation was undertaken on non-ignited powders to determine their volatile element contents (total carbon, CTotal and water recalculated from hydrogen) using the Thermo Finnigan™ FlashEA® 1112 elemental analyzers (EA) onboard D/V Chikyu. Whole rock trace element analyses were measured by ICP-MS using acid digestion of powder samples after ChikyuOman 2018 Leg 3. Sample powders were divided into three batches. One batch was sent to each IES-AS, TU and NU laboratory for trace element measurements. The measurements were conducted at IES-AS using an Agilent 7500s inductively coupled plasma‐mass spectrometer (ICP‐MS); at TU using a Thermo Scientific™ Element XR™ HR-ICP-MS; and at NU using Yokogawa HP4500 ICP-MS. To compare the accuracy and the precision in the three different laboratories, trace element measurements were performed on a selection of duplicate samples, and on the same reference materials (DTS-2B and JP-1a).

Description: The Antarctic Circumpolar Current (ACC) has a pivotal role in global climate through its strong influence on the global overturning circulation, ocean heat and CO~2~ uptake and storage. When and how the ACC reached its modern-like conditions (i.e., vigorous, and deep-reaching) after the opening and deepening of the Drake Passage and the Tasmanian Gateway remains highly controversial. We present mean sortable silt and neodymium isotope records from the Deep Sea Drilling Project (DSDP) Leg 29 Site 278 located in the southern Emerald Basin (56°33.42'S, 160°04.29'E, 3675 m water depth) and Ocean Drilling Program (ODP) Leg 119 located in the southern Kerguelen Plateau (61°34.66'S, 80°35.43'E, 2307 m water depth) spanning the last 31 million years. These records were used to determine the timing of the onset of diagnostic features of today's ACC: a modern-like deep-reaching strong flow, and homogenisation (i.e., common water mass signature) of Circumpolar Deep Water within the ACC. Grain size was measured using the laser microgranulometer Malvern mastersizer hydro 2000G. Neodymium isotopes from Site 29-278 were measured on a Nu Plasma multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS).Neodymium isotopes from Site 119-744 were measured on a Thermo Scientific Triton Plus Thermal Ionization Mass Spectrometer (TIMIS).