Data report: geochemistry of rocks and minerals of the gabbro complex from the MARK area

Abstract

Leg 153 represents an Ocean Drilling Program (ODP) offset drilling leg in the MARK area (Mid-Atlantic Ridge at the Kane Transform), around 22°33' N, 45° 1.9' W. Shipboard documentation has shown that the gabbroic rocks from Sites 921, 922, and 923 are part of a layered complex and are generally similar, although cumulus textures are particularly well developed in the rocks from Site 922. These rocks also exhibit some special geochemical features. Major-and trace-element chemistries were studied from a limited pool of 26 samples from nine holes. In some polished thin sections, the chemical compositions of olivines, orthopyroxenes, clinopyroxenes, amphiboles, and plagioclases were determined using an electron microprobe. These data are presented here as a data report; a detailed paper will be prepared, if additional rock samples are analyzed, and if fluid inclusions and oxygen isotope data become available. SAMPLE MATERIAL All gabbroic rock samples in this study are from Holes 153-921 A, 921B, 921C, 921E, 922A, 922B, and 923A. Two diabase vein rocks are from Holes 920B and 920D. The special designations correspond to shipboard documentation and to standard ODP sample nomenclature conventions, but, for clarity, single numbers are generally used in the data tables (Table 1). The individual rock names were determined after microscopic investigation and calculation of mineral norms in accordance with the CIPW system (after the authors Cross, Iddings, Pearson, and Washington). ANALYTICAL METHODS Whole-Rock Chemistry Major-element oxides and the trace elements Cl, Co, Cr, Cu, F, Ga, Nb, Ni, Pb, S, Se, V, Y, Zn, and Zr were measured by X-ray fluorescence spectroscopy using a Phillips PW 1480 sequential spectrometer. Glass discs were prepared by melting the powdered, but not ignited, whole-rock material with lithium tetraborate. International reference rocks, as well as two internal mafic standard samples, were used for calibration of the method. K, Li, Na, Rb, and Zn were analyzed using flame photometry, and Ba and Sr were analyzed using inductively coupled plasma-atomic emission spectroscopy using a Perkin-Elmer PLASMA 1000 spectrometer. B was determined by optical spectroscopy, with boron-free spectral carbons, using a Carl Zeiss PGS 2 spectrograph. Table 1. Sample information. Sample name 2D 7M-D

Slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the Mid‐Atlantic Ridge south of the Kane fracture zone (MARK) area

“…(a) NRM variation with whole rock Mg# (where Mg# = 100* Mg/[Mg + 0.85* Fe (total)]). Data from the MARK area include shipboard samples used for both X‐ray fluorescence (XRF) and magnetic studies [ Cannat et al , 1995], as well as geochemical data acquired during postcruise studies [ Casey , 1997; Barling et al , 1997; Werner , 1997; Agar et al , 1997; Meurer and Gee , 2002]. Data for hole 894G (Hess Deep) and hole 735B are from shipboard samples with XRF data [ Gillis et al , 1993; Dick et al , 1999].…”

Palaeozoic and Proterozoic zircons from the Mid-Atlantic Ridge

“…Leg 153 recovered harzburgites of the upper mantle which are depleted by partial melting followed by the formation of gabbroic melts [1][2][3][4] . The peridotite-harzburgite-gabbro complex is interpreted to be uplifted and exhumed by normal faults [1][2][3][4]10,11 . The gabbros comprise normal gabbro, olivine gabbro, gabbronorite, olivine gabbronorite, olivine norite, and troctolite.…”