Composition of Co-rich ferromanganese crusts and substrate rocks from the NW Marshall Islands and international waters to the north, Tunes 6 cruise

Abstract

This report is preliminary and has not been reviewed for conformity with the U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. plasma-atomic emission spectrometry, except those of K, Zn, and Pb, which were determined by flame atomic-absorption spectroscopy, and those of As, Cr, and Cd, determined by graphite-furnace atomic absorption spectroscopy on air-dried samples (Aruscavage et al., 1989). Concentrations of platinum-group elements (PGEs) and rare earth elements (REEs) for Fe-Mn crusts were determined by inductively coupled plasmamass spectrometry (Lichte et al., 1987a,b). The usual Pearson product moment correlation coefficient was used to calculate the correlation coefficient matrices. For Q-mode factor analysis, each variable percentage was scaled to the percent of the maximum value before the values were row-normalized and cosine theta coefficients calculated. Factors were derived from orthogonal rotations of principal component eigenvectors using the Varimax method (Klovan and Imbrie, 1971). All communalities are >0.93. Low factor scores, < 10.21, were discarded because they are not statistically significant. SUBSTRATE ROCK DESCRIPTIONS AND COMPOSITION Substrate rocks in decreasing order of abundance are basalt, breccia, phosphorite, hyaloclastite, limestone, volcaniclastic siltstone and sandstone, and mudstone (Table 1). As many as five different rock types were recovered in a single dredge haul. As seen in hand samples, breccias most commonly consist of basalt clasts in a hyaloclastite matrix, and/or carbonate fluorapatite (CFA) cement. Many samples have Mn oxide impregnations that commonly form dendrites. Volcanogenic clasts in breccias and volcaniclastic rocks are commonly replaced by clay minerals and iron oxides. Basalts are aphanitic with common plagioclase phenocrysts and CFA infilling fractures and vesicles. Some samples have vesicles filled with carbonate mud or Fe-Mn oxides. Reef limestones consist of rounded carbonate clasts with calcite cement and moldic porosity; pelagic limestones are micritic and bioturbated to massive. Mudstones are mostly bioturbated with Fe-Mn oxides lining some burrows. Phosphorites have carbonate mud in cracks and Fe-Mn oxide impregnations. X-ray Diffraction Mineralogy and Petrography Primary Volcanogenic and sedimentary minerals include plagioclase, pyroxene, magnetite, and calcite; and secondary minerals include CFA, smectite, calcite, phillipsite, clinoptilolite, goethite, hematite, barite, and potassium feldspar (Table 2). Secondary CFA is the most abundant mineral in these samples and occurs in 73% of the samples as a major or moderate constituent, regardless of rock type (Table 2). Hyaloclastite and hyaloclastite breccias consist chiefly of CFA and phillipsite (Table 2), with moderate to minor amounts of plagioclase, pyroxene, and magnetite. Most samples contain minor amounts of smectite. One sam...

Composition of Co-rich ferromanganese crusts and substrate rocks from the Marshall Islands, cruise KODOS 97-4

“…These various trends are not related to present-day water depth of crust collection nor to phosphatization of the crusts. For crusts from the area northwest of the Marshall Islands, 11 of 13 crusts analyzed showed decreases in REE contents from the substrate to the crust surface (Hein et al, 1997a), as did crust D2-1 here.…”

“…Bulk Crusts: The mean Fe and Mn contents of the 27 bulk crust samples are 15.1% and 21.4%, respectively (Tables 13,15). The mean Fe/Mn ratio (0.71) is comparable to the mean ratio for the entire central Pacific region (0.72; Hein et al, 1992), but is higher than the ratio for the entire Marshall Islands EEZ (0.64; Hein et al, 1998), and lower than the ratio for the area to the northwest of the Marshall Islands EEZ (0.76; Hein et al, 1997a). The mean Mn content is less and the Fe content somewhat less compared to bulk crusts from the entire Marshall Islands EEZ, 16.9% and 26.3%, respectively.…”