Database of mineral compositions within the lower oceanic crust from the ultraslow-spreading Gakkel Ridge

This database contains mineral major and trace element compositions of gabbroic rocks composing the lower oceanic crust from the ultraslow-spreading Gakkel Ridge. The lithological suite analysed in this research was recovered during the Artic Mid-Ocean Ridge Expedition (AMORE 2001) with the American research vessel USCGC Healy (HLY102) and the German RV Polarstern (PS59) (Michael et al., 2003). While these missions successfully sampled a diverse range of lithologies, including significant quantities of basalts and peridotites distributed in varying proportions across the ridge’s provinces, the gabbroic component has remained largely overlooked and has never been systematically studied until now.The gabbroic rocks included in this database are primitive in composition and range from troctolites to olivine gabbros, gabbros s.s., gabbronorite and oxide gabbro with some host peridotite and basaltic association.

EPMA: Mineral major element compositions (SiO2, TiO2, Al2O3, Cr2O3, FeO, NiO , MnO, MgO, CaO, Na2O, K2O) of all 57 selected samples were determined using electron probe micro-analysis (EPMA), at the Institut für Mineralogie, Universität Münster in Germany using a JEOL JXA 8530F Hyperprobe and at the Department of Earth Science Ardito Desio of Milano University using a JEOL JXA8200 Superprobe. The analyses were performed with a 3-micron spot size for plagioclase, pyroxene and olivine and 1-3-micron spot size for amphiboles and oxides at 15 KV accelerating potential and 20 nA beam current. Different peak and background time analysis have been defined for each element, depending on the analysed phase, to maximize accuracy and sensitivity. A set of in‐house silicate and spinel standards were measured regularly to check for precision and accuracy, and the quality of analyses was further assessed based on the atom per formula units. Detection limits were <300 ppm for Al, <500 ppm for Na, Mg, Si, K, <600 ppm for Ca, Cr, Mn, <700 ppm for P, Fe, <800 ppm for Ni and <1100 ppm for Ti. A mixture of natural and synthetic minerals and oxides were used as standards and the quality of analyses was assessed on the basis of the atom per formula units.
LA-ICP-MS: Trace element compositions of clinopyroxene, plagioclase were determined on mineral separates by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometre (ICP-MS) using a triple quadrupole ICP-MS Agilent Series 8900 interfaced to a GeoLas 193 nm excimer ablation system (Lambda Physik, Germany) at the Istituto Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche (CNR-IGG) in Pavia (Italy) and a Thermo Scientific Element 2 ICP‐MS interfaced with a 193 nm ArF excimer laser (Analyte G2, Photon Machines) at the Institut für Mineralogie, Universität Münster. The spot size of the laser ablation system was set to 80μm for clinopyroxene and plagioclase with a repetition rate of 10 Hz and an energy fluence of 3.5 J/cm2. Helium mixed downstream of the ablation cell with Ar was used as the carrier gas. NIST 612 synthetic glass was used as an external standard and the BCR-1, BIR-1G, BHVO-2G and BCR-2G standards were used as internal standards to assess the precision and accuracy of the analyses better than ±5% and 10%.

Data processing:
EPMA:
A mixture of natural and synthetic minerals and oxides were used as standards and the quality of analyses was assessed on the basis of the atom per formula units.
LA-ICP-MS:
NIST SRM 612 synthetic glass was used as external standard, SiO2 concentrations previously measured by EPMA was used as internal standard. The precision and accuracy of the trace element concentrations were assessed by repeated analyses of the BIR-1G, BHVO-2G and BCR-2G standard and were better than ±7% and ±10%, respectively. Average element abundances in synthetic glasses NIST SRM 612 were taken from Pearce et al. (1997).
Background and signal were measured for about 60 seconds; signal of standards and unknowns were carefully checked, and the raw data were reduced using the software package GLITTER® (Van Achterberg et al., 2001).