 |
| Titel |
Fluid-mobile element behaviour during subduction dehydration of serpentinites from the Voltri Massif (Ligurian Alps, Italy) |
| VerfasserIn |
Jan C. M. De Hoog, Keiko Hattori, Haemyong Jung |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076395
|
|
|
|
|
|
| Zusammenfassung |
| Large amounts of water are stored in the hydrated abyssal peridotites of oceanic lithospheric
slabs. During their subduction much of this water will be released at depths of 120-240 km
due to dehydration of serpentinite. This dehydration is accompanied by the crystallisation of
secondary olivine. It is typically very poor in water and fluid-mobile elements and it is thus
assumed that the water and fluid-mobile elements are released to the overlying mantle
wedge.
To evaluate the behaviour of elements in the mantle during the dehydration of
serpentinite, we examined the compositions of secondary olivine grains from the
high-pressure Erro-Tobbio unit (Voltri complex, Ligurian Alps) by ionprobe (SIMS), electron
backscatter diffraction (EBSD) detector and electron microprobe. Secondary olivine
(Fo86-88) is MnO-rich (0.26–0.43), has high NiO (0.23-0.34) and contains abundant
inclusions of magnetite, which attest to its secondary origin. It has variable but very high
water contents (up to 0.7 wt.%) which correlate with high TiO2 contents (up to 0.85 wt.%).
These values are much higher than typical primary mantle olivine. Olivine grains with
the highest H2O and TiO2 contents show colourless to yellow pleochroism in thin
section. They commonly occur as large porphyroblastic grains within the serpentine
matrix but locally form rims around low-Ti olivine. Recrystallised fine-grained
olivine is always poor in TiO2 and water. The sample also contains Ti-clinohumite
((Mg2SiO4)n.xTiO2.(1-x)Mg(OH)2 with n=4 and x=0.34-0.46) and minor Ti-bearing humite
(n=3) and chondrodite (n=2). Olivine closely associated with Ti-Chu contains low Ti
contents, suggesting that Ti-rich olivine is not the breakdown product of Ti-Chu.
Furthermore, detailed examination of olivine by EBSD shows no lamellae of Ti-Chu in
Ti-rich olivine.
To further determine the origin of the high Ti and H2O contents of olivine, several grains
were studied by FTIR absorption spectroscopy. The spectra show peaks of serpentine
inclusions, Ti-Chu-like defects (MgTi[]O2(OH)2), Si vacancies (Mg2[](OH)4) plus an
unidentified but prominent peak which could not be associated with any known water
substitution mechanism in olivine. As serpentine contains little TiO2 it cannot contribute to
high TiO2 and H2O in olivine as measured by SIMS. The data also suggests that high TiO2
and H2O contents of olivine are not related to submicroscopic Ti-Chu inclusions, as the
atomic ratios of H/Ti of olivine (8-17) are higher than those of Ti-Chu grains in
the sample (2.8-4.0). They are also much higher than the value of 2 expected for
Ti-Chu defects in olivine crystals. Combining FTIR and SIMS data we conclude
that TiO2 and H2O are attributed to Ti-Chu-like defects in olivine structure that
are more water-rich than co-existing Ti-Chu grains outside olivine. Hence, these
defects may not necessarily have the ideal composition MgTi[]O2(OH)2. Future work
by TEM may provide more evidence about the type of defects in Ti-rich olivine.
Irrespective of the incorporation mechanism of water, secondary olivine may serve as an
efficient host for H2O to be transported beyond the stability field of antigorite in the
mantle.
Previous work on the area established that B, Cl and Li are enriched in secondary olivine
compared to mantle reservoirs and therefore subduction of oceanic mantle may introduce
anomalies of light elements into the upper mantle [4]. We note that compared to serpentine
from the same sample, secondary olivine is enriched in Li (2-60 ppm) and B (10-20 ppm)
independent of its water and Ti content. In addition, Ti-rich olivine has high F contents
(10-130 ppm) although less than serpentine (20-350 ppm). We thus confirm that secondary
olivine may play a significant role during redistribution of fluid-mobile elements
such as Li, B and F upon the dehydration of serpentinites in subduction zones. |
|
|
|
|
|
|