4th Swiss Geoscience Meeting, Bern 2006
Trace elements in serpentine minerals of hydrated peridotites
from different tectonic environments: the role of precursor
mineralogy and source of serpentinizing fluid
Kodolányi, J., Spandler, C. & Pettke, T.
Institute of Geological Sciences, University of Bern, CH 3012 Bern, Switzerland
janos@geo.unibe.ch
Hydrated oceanic mantle is a significant carrier of H2O in subduction zones.
Experimental studies and field observations demonstrate that serpentinites undergo
major dehydration over a restricted -P-T interval (ca. 2-5 GPa, 650-700 °C; Ulmer and
Trommsdorff, 1995; Trommsdorff et al., 1998). Fluids released during serpentinite
dehydration may have a considerable effect on element recycling through subduction
zones. Yet little is known about the ability of serpentinite minerals, commonly found in
hydrated mantle rocks, to take up and host trace elements. We used EMPA and LA-ICPMS to learn about the major and trace element contents of serpentine minerals found in
different textural positions of hydrated mantle rocks from ODP drill cores.
The degree of serpentinization in the samples varies between 65 and 100 vol%. Besides
serpentine minerals (srp) and magnetite (mag), the most common secondary phases
are chlorite (chl), brucite (brc), calcite (cal), aragonite (ar), iowaite (iow) and sulfides
(sul). Excess brc component in srp is commonly observed in the rims and cores of srp
mesh structures formed after olivine. Minor and trace element compositions of srp in
different textural settings generally reflect that of the precursor mineral (olivine and
orthopyroxene). However, there are considerable variations due to varying degrees of
element mobility and fluid source during serpentinization. Boron and Sr are extremely
enriched in the srp pseudomorphs compared to the original mantle minerals (B: 6-250
ppm, Sr: 0.14-23.0 ppm). Serpentine forming after orthopyroxene tends to have about
1.5-4 times higher trace-element concentrations than serpentine after olivine. This is
probably related to local fluid chemistry, with relatively high dissolved SiO2 and relatively
low pH around orthopyroxene during serpentinization. Differences exist among samples
from different tectonic settings, too: srp of supra-subduction zone (SSZ) serpentinites is
richer in Sr, Rb and Cs and has lower Mo contents than srp in samples from mid-ocean
ridges and passive margins. This can be explained by differences in fluid chemistry
related to different fluid source during serpentinization (seawater in mid-ocean ridges vs.
seawater plus water released from the subducting lithospheric slab in SSZ peridotites).
Our results show that srp chemistry is a strong function of the precursor mineralogy and,
more importantly, of the chemical nature of the serpentinizing fluid.
REFERENCES
Ulmer, P. & Trommsdorff, V. (1995). Serpentine Stability to Mantle Depths and
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Trommsdorff, V., López Sánchez-Vizcaíno, V., Gómez-Pugnaire, M. T. & Müntener, O.
(1998). High pressure breakdown of antigorite to spinifex-textured olivine and
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