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| Titel |
Behavior of major and trace elements upon weathering of peridotites in New Caledonia : A possible site on ultramafic rocks for the Critical Zone Exploration Network (CZEN) ? |
| VerfasserIn |
Farid Juillot, D. Fandeur, E. Fritsch, G. Morin, J. P. Ambrosi, Luca Olivi, A. Cognigni, J. L. Hazemann, O. Proux, S. Webb, G. E. Jr. Brown |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250045360
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| Zusammenfassung |
| Ultramafic rocks cover about 1% of the continental surfaces and are related to ophiolitic
bodies formed near convergent plate boundaries (Coleman, 1977). The most typical
ultramafic rocks are dunite and harzburgite, which are composed of easily weatherable
ferromagnesian mineral species (olivines and pyroxenes), but also of more resistant spinels
(chromite and magnetite). Oceanic serpentinization of these ultramafic rocks usually lead to
partial transformation of these initial mineral assemblages by forming hydrous layer silicates
such as serpentine (lizardite, chrysotile and antigorite) talc, chlorite and actinolite (Malpas,
1992). It also lead to the formation of highly sheared textures, which favor meteoric
weathering through preferential water flows. Compared to their crystalline rock counterpart
that covers most of the continental surfaces, these ultramafic rocks mainly differ by their
lower SiO2, Al2O3 and K2O contents (less than 50%, 10% and 1%, respectively)
and, on the opposite, much higher MgO content (more than 18%). Moreover, they
commonly have higher concentrations in FeO and other trace elements, such as Ni, Cr,
Mn and Co. Weathering of these rocks is then at the origin of major geochemical
anomalies on continental surfaces, especially when they occur in tropical and subtropical
regions.
Such conditions are encountered in New Caledonia where one third of the surface is
covered with peridotites (mainly harzburgite with small amounts of dunite) obducted about
35 millions years ago during large tectonic events in the Southwest Pacific at the Late
Eocene (Cluzel et al., 2001). Tropical weathering of these ultramafic rocks lead to the
development of thick lateritic regoliths where almost all Mg and Si have been leached out
and Fe, Mn, Ni, Cr and Co have been relatively concentrated. In these oxisols, Ni,
Cr and Co can exhibit concentration up to several wt%, which make them good
candidates for ore mining (New Caledonia is the third Ni producer in the world).
However, these high concentration of potentially toxic elements can represent a
serious hazard for the environmental quality of the Caledonian ecosystem which
is a ‘’ biodiversity hotspot” (Myers, 2000), which emphasize the strong need for
characterizing the natural cycling of these elements upon weathering of ultramafic
rocks.
To reach this goal, we have studied the mineralogical distribution, crystal-chemistry and
mass balance modelling of major (Si, Mg, Al, Fe, Mn) and trace elements (Ni, Cr
and Co) in the freely-drained weathering profile developed in the serpentinized
harzburgites of Mt Koniambo (West Coast of New Caledonia). Results show that both
hydrothermal and meteoric processes contributed to the vertical differentiation of
this freely drained weathering profiles in serpentinized ultramafic rocks. Finally,
they also emphasize the importance of both redox reactions and interactions with
Mn- and Fe-oxyhydroxydes (Fandeur et al., 2009a; 2009b) to explain the opposite
behavior observed between very mobile Ni and almost immobile Cr (Fandeur et al.,
2010).
These results bring new insights on the geochemical behavior of trace elements upon
weathering of ultramafic rocks under tropical conditions leading to the formation of
supergene ore deposits. They also emphasize the interest of such a weathering site on
ultramafic rocks under tropical climate to complemente the reference sites of the Critical
Zone Exploration Network (CZEN).
References
Cluzel D., Aitchinson J.C. and Picard C. (2001) Tectonic accretion and underplating of
mafic terranes in the Late Eocene intraoceanic fore-arc of New-Caledonia (Southwest
Pacific): geodynamic implications. Tectonophysics, 340, 23-59.
Coleman, R.G. (1977) Ophiolites: Ancient oceanic lithosphere?: Berlin, Germany,
Springer-Verlag, 229p.
Fandeur D., Juillot F., Morin G., Olivi L., Cognigni A., Fialin M., Coufignal F., Ambrosi
J.P., Guyot F. and Fritsch E. (2009a). Synchrotron-based speciation of chromium in an Oxisol
from New-Caledonia : Importance of secondary Fe-oxyhydroxydes. American Mineralogist,
94, 710-719.
Fandeur D., Juillot F., Morin G., Olivi L., Cognigni A., Webb S., Ambrosi J.P., Fritsch E.
and Brown Jr. G.E. (2009b). XANES evidence for oxidation of Cr(III) to Cr(VI) by
Mn-oxides in a lateritic regolith developed on serpentinized ultramafic rocks in New
Caledonia. Environmental Science and Technology, 43, 7384-7390.
Fandeur D., Fritsch E., Juillot F., Morin G., and Ambrosi J.P. (2010). Influence of
mineralization and weathering on the distribution and mobility of major and trace elements
along a freely-drained lateritic regolith developed in ultramafic rocks in New-Caledonia.
Chemical Geology, submitted.
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Myers, N., R. A. Mittermeier, C. G. Mittermeier, G. A. B. da Fonseca, J. Kent
(2000) Biodiversity hotspots for conservation priorities. Nature, 403, 853-858. |
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