Canet,C., Prol-Ledesma,R.M., Bandy,W.L., Schaaf,P., Linares,C., Camprubi(iの頭は´),A., Tauler,E. and Mortera-Gutierrez(最初のeの頭に´),C.(2008): Mineralogical and geochemical constraints on the origin of ferromanganese crusts from the Rivera Plate (western margin of Mexico). Marine Geology, 251, 47-59.

『リベラ・プレート(メキシコの西縁)からの鉄マンガンクラストの起源に関する鉱物学的および地球化学的制約』


Abstract
 Marine hydrogenetically precipitated ferromanganese crusts are widespread in the Pacific Ocean. They occur as pavements coating volcanic or sedimentary hard-rock substrate, mainly on the slopes of seamounts, plateaus and hills in ocean basins and continental margins. we studied three ferromanganese crusts from one dredge haul from the Rivera Plate (western margin of Mexico), which are up to 15 mm thick and grow directly on a substrate of pillow basalt fragments. They consist of laminated, botryoidal, porous aggregates mostly of Fe and Mn oxyhydroxides and up to 10% silicates. XRD analysis showed the predominance of poorly crystallized mineral phases in the crusts that include Mn-feroxyhyte and vernadite, and an authigenic smectite-like clay. Detrital minerals probably derive from granodiorites of the eastern wall of the neighboring Middle America Trench. Scattered barite grains occur on the crust surface and suggest plume fall-out derived from hydrothermal vents, although a possible pelagic source cannot be dismissed. Ratios between major (Fe, Mn, Si) and trace (Co, Ni, Cu) elements reveal that such crusts are predomonantly hydrogenetic in origin, although they show a hydrothermal influence that increases in the outer layers. Iron contents range from 16.2 to 25.2 wt.%, and manganese from 3.4 to 14.5 wt.%. The Fe/Mn ratio ranges from 1.6 to 7.90. The SiO2/Al2O3 ratio ranges from 4.3 to 6.6, indicating the presence of biogenic silica (radiolarians). The concentrations of copper (up to 383 ppm) and cobalt (up to 534 ppm) are significantly below those of the typical hydrogenetic crusts, whereas Ni (up to 1320 ppm) is about the same or slightly lower. REE normalized profiles and ΣREE values (486 to 732 ppm) match those reported for hydrogenetic crusts, but suggest a hydrothermal contribution for the later crust layers. The inferred crust growth rates using the Co-chronometer (44 and 229 mm/Myr) are higher than those reported for pure hydrogenetic crusts (mostly 1 to 6 mm/Myr), thus indicating an increasing hydrothermal influence. The highest growth rate (229 mm/Myr, corresponding to the outer crust layers) suggests a regional hydrothermal input of iron and manganese to seawater. The intensification of regional submarine hydrothermal activity began about 13,000 yr ago, and may be related to the tectonic activity in the complex junction of the Rivera, Cocos, Pacific and North America plates.

Keywords: ferromanganese crusts; hydrogenetic; hydrothermal; vernadite; smectite; Rivera Plate』

1. Introduction
2. Geotectonic setting
3. Sampling and analytical methods
4. Paragenesis and mineral chemistry
 4.1. Characteristics of the basaltic substrate
 4.2. Bulk mineralogy determined by X-ray diffraction
 4.3. Microscopic description and mineral chemistry
5. Whole-rock chemical composition
 5.1. Major element geochemistry
 5.2. Minor and trace-element geochemistry
6. Discussion
 6.1. Mineral associations
 6.2. Geochemistry and growth rates for Fe-Mn oxide crusts
 6.3. Formation of Fe-Mn crusts: age and geologic implications
7. Conclusions
Acknowledgments
References


ホーム