『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