On the controls of mineral assemblages and textures in alkaline springs, Samail Ophiolite, Oman

Interactions between meteoric water and ultramafic rocks in the Oman Ophiolite generate waters of variable physicochemical characteristics. The discharge of these waters forms complex alkaline pool networks, in which mineral precipitation is triggered by mixing, evaporation, and uptake of atmospheric CO2. A systematic and colocalized sampling of waters and solids in two individual spring sites allowed us to determine the saturation state of a range of minerals and correlate them to the different water and precipitate types. We subdivided the waters of the spring sites into three distinctive types: i) Mg-type; moderately alkaline (7.9<pH<9.5), Mg2+–HCO3 −- rich waters, ii) Ca-type; hyperalkaline (pH>11.6), Ca2+–OH−-rich waters, and iii) Mix-type; alkaline to hyperalkaline (9.6 < pH < 11.5) waters with intermediate chemical composition. We first report the occurrence of hydrated magnesium (hydroxy-) carbonate phases in Mg-type waters. Nesquehonite forms in these waters via evaporation and transforms into dypingite and hydromagnesite under CO2-rich conditions. In Ca-type waters, the coupling of atmospheric CO2 uptake with evaporation leads to the formation of a calcitic crystalline crust on the air-water interface. The crusts are aragonite- and brucite-bearing, where Mg-type and Ca-type waters discharge and vigorously mix at the same pool. Unlike the Mg-type and Ca-type waters, the pools of Mix-type waters host massive aragonite-dominated deposits due to high Mg/Ca ratio that favors the growth of aragonite over calcite. The hydrodynamics during mixing spatially control brucite precipitation and restrict its formation and accumulation around specific mixing zones, where a continuous supply of Mg of inflowing Mg-type waters takes place. Crystal morphologies record the effect on the values of supersaturation and supersaturation rates in the pools due to mixing processes, evaporation and CO2 uptake. In Ca-type waters, CO2 uptake and evaporation dictate the textural characteristics of calcite both in crystalline crusts and rock coatings. Textural evolution of aragonite from crystalline sheaves to spheroidal shapes underlines the different supersaturation rates of calcium carbonate crystallization in flocculent material of Mix-type waters. Geochemical models of mixing between Mgtype and Ca-type waters revealed the evolution of mineral saturation indices under various mixing proportions, and their relation to the observed mineralogy and geochemistry of the pool waters. The thorough documentation of mineral assemblages and crystal morphologies enabled us to provide a more detailed account of how water composition, mixing, and mineral precipitation co-evolve in the alkaline spring systems, where CO2 is sequestered.The present research has been funded by the European Union Seventh Framework Programme FP7/2007-2013 under the FP7 People Program (Marie Curie Action– ITN “Abyss”) under REA grant agreement no. 608001, and the F7 Ideas: European Research Council grant PROMETHEUS under grant agreement no. 340863

Magnesium and calcium carbonate precipitation in serpentinite-hosted alkaline environments: natural and experimental constraints

In continental settings, the interaction ofmeteoric water with ultramafic rocks generates waters of variable physicochemical characteristics owing to serpentinization and weathering. The discharge of these waters forms aerial alkaline to hyperalkaline spring systems where waters mix, undergo evaporation, and take up atmospheric CO2, leading to the formation of carbonate minerals. The understanding of natural carbonation taking place in such serpentinite-hosted alkaline environments is critical for assessing the role of this potentially significant sink in the global carbon cycle, and the viability of CO2 sequestration techniques for safe carbon storage. Serpentinization-driven, alkaline environments provide critical insights into the natural conditions regarding the capture of atmospheric carbon dioxide through carbon mineralization. The main objective of this Ph.D. thesis is to advance our understanding of serpentinization-related alkaline spring systems and the associated precipitation of carbonate minerals under alkaline conditions. To contribute to this main research goal, the present Ph.D. thesis aims to (i) provide an additional account of how water composition, mixing, and mineral precipitation and textures co-evolve in serpentinization-driven alkaline spring systems in ophiolites, (ii) investigate alkaline spring sites in subcontinental mantle peridotites and associated mineralizations formed by the interaction between hyperalkaline fluids and river waters, and (ii) experimentally investigate the crystallization sequence and morphologies of hydrated magnesium carbonates, and define the conditions under which their nucleation, crystal growth, and transformation take place. These aims have been addressed through the study of natural alkaline springs hosted in exposed oceanic (Samail Ophiolite, Oman) and subcontinental mantle serpentinized peridotites (Ronda peridotites, Spain), and through carbonate crystallization experiments, to fill gaps in our current knowledge on the mechanisms and the conditions characterizing carbonate mineral precipitation in such systems.Tesis Univ. Granada.Funded by the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7 (People) under REA grant agreement n° 608001F7 (Ideas) European Research Council grant “PROMETHEUS” under REA grant agreement nº 340863Junta de Andalucía research group GIMPY (grant RNM–131)Research and infrastructure grants used in this research have been (co)funded by the European Regional Development Fund (ERFD

Lithology, volatile, major and trace element composition of Holes CM1A and CM2B ultramafic samples drilled in Wadi Tayin massif, SE Oman ophiolite (ICDP Oman drilling project Phase 2 Leg 3)

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)