Open Session - SWISS GEOSCIENCE MEETINGs

86
Symposium 2: Mineralogy-Petrology-Geochemistry
sults demonstrate that Ra is a compatible element in leucite, K-feldspar and phlogopite, whereas it is incompatible in plagioclase.
In leucite, Onuma-type element partitioning parabola peak near Cs (and not the major cation K) for the alkalis and
near Ra for the earth alkalis. In phlogopite Ra is hosted in the interlayer site and has a cation radius close to the optimum
of this site, leading to high partition coefficients. In all cases DBa and DRa are significantly different, DRa/DBa ranging from
0.2-0.6 in the feldspars to 1.3 in phlogopite to 4.2 in leucite. The knowledge of DRa then allows us to recalculate correct
crystal ages of various volcanic systems (e.g. Vesuvius, Mt St Helens, Mt Shasta, Nevado del Ruiz, Mt Erebus) obtaining a change
from 2 to 10-fold in the ages derived from minerals (leucite, plagioclase) in which the difference between DRa and DBa is
high.
REFERENCES
Black, S., Macdonald, R., DeVivo, B., Kilburn, C.R.J., Rolandi, G. 1998: U-series disequilibria in young (A.D. 1944) Vesuvius
rocks: Preliminary implications for magma residence times and volatile addition. Journal of Volcanology and
Geothermal Research, 82, 97-111.
Cooper, K.M. & Reid, M.R. 2003: Re-examination of crystal ages in recent Mount St. Helens lavas: implications for magma
reservoir processes. Earth and Planetary Science Letters, 213, 149-167.
Cooper, K.M., Reid, M.R., Murrell, M.T., Clague, D.A. 2001: Crystal and magma residence at Kilauea Volcano, Hawaii: 230 Th-
226 Ra dating of the 1955 east rift eruption. Earth and Planetary Science Letters, 184, 703-718.
Reagan, M.K., Volpe, A.M., Cashman, K.V. 1992: 238 U- and 232 Th-series chronology of phonolite fractionation at Mount Erebus,
Antarctica. Geochimica et Cosmochimica Acta, 56, 1401-1407.
Schaefer, S.J., Sturchio, N.C., Murrell, M.T., Williams, S.N. 1993: Internal 238 U-series systematics of pumice from the November
13, 1985, eruption of Nevado del Ruiz, Colombia. Geochimica et Cosmochimica Acta, 57, 1215-1219.
Volpe, A.M., & Hammond, P.E. 1991: 238 U- 230 Th- 226 Ra disequilibria in young Mount St. Helens rocks: time constraint for magma
formation and crystallization. Earth and Planetary Science Letters, 107, 475-486.
Volpe, A.M. 1992: 238 U- 230 Th- 226 Ra disequilibria in young Mt. Shasta andesites and dacites. Journal of Volcanology and
Geothermal Research, 53, 227-238.
2.13
On mass fluxes and metasomatic processes above subducting slabs: An
experimental solubility and melting study of Enstatite + Quartz + H 2 O
Hack Alistair*, Ulmer Peter, Thompson Alan
Institute for Mineralogy and Petrology
ETH Zürich Clausiusstrasse 25, Zürich 8092, Switzerland
(Alistair.Hack@erdw.ethz.ch)
We have conducted diamond-trap type experiments to determine the solubility and melting behaviour in the poorly investigated
silica-rich part of the MgO-SiO 2 -H 2 O (MSH) system to 700 to 1250 ˚C, 1 to 3.5 GPa. The results bear on the occurrence of
complete melt-fluid miscibility (or so-called supercriticality) and other general aspects of fluid-mediated mass transport in
metasomatic regions, for example: above subduction zones where slab dehydration fluids ascend and interact with overlying
mantle.
Our experimental results for the solubility of enstatite + quartz in water suggest that fluids related to subsolidus metasomatic
assemblages which are intermediate to model crust (quartz) and mantle (forsterite + enstatite) remain distinctly silica-rich
(Si/Mg molar ratio from ≈5 to 15) with increasing P to > 3.5 GPa, and thus distinct from more dilute and higher Mg/Si fluids
associated with forsterite + enstatite at equivalent depth (related to P). We find Ens + Qtz solubility in H 2 O increases more
rapidly with increasing P (∂X/∂P) compared to Qtz in H 2 O; at 950 °C, ≈ 3 GPa the relative solubilities of Qtz and Qtz+Ens
crossover. This solubility behaviour suggests that in MSH, fluids moving from Qtz to Ens + Qtz saturation precipitate where
P < ≈ 3 GPa, whereas dissolution is predicted along a similar path at higher P.
The results suggest that metasomatic assemblages may be associated with enhanced solubility at certain conditions. This is
significant because lower time-integrated fluid fluxes are required where mass transport occurs at higher concentrations.
In subduction zones, for example, metasomatic processes may play a significant role in maximising transfer of slab components
to melting regions in overlying wedge.