5th Swiss Geoscience Meeting, Geneva 2007
Fluid inclusions during plastic deformation of quartz:
An experimental study
Tarantola Alexandre*, Diamond Larryn W.* & Stünitz Holger**
* Institute of Geological Sciences, University of Bern, Baltzerstr. 3, CH-3012 Bern
(alexandre.tarantola@geo.unibe.ch)
** Institute of Geology and Paleontology, University of Basel, Bernoullistr. 32, CH-4056
Basel
Several experimental studies have characterized re-equilibration of fluid
inclusions subjected to hydrostatic under- or overpressures (e.g. Sterner and
Bodnar 1989; Bakker and Jansen 1991; Vityk and Bodnar 1998). However, little
is known experimentally about the effects of plastic deformation on the
properties of fluid inclusions. For want of a clear interpretative framework, the
potentially useful information from inclusions within sheared hydrothermal veins
and metamorphic rocks remains largely inaccessible.
To elucidate the behaviour of fluid inclusions during plastic deformation of their
host crystals we are undertaking experiments using a Griggs-type pistoncylinder apparatus. In addition to offering higher experimental pressures than
conventional cold-seal autoclaves, the apparatus permits experiments under
deviatoric stress. Our samples are of natural CO2–H2O–NaCl inclusions in
large, undeformed quartz crystals. At Tlab, the volume fraction of the carbonic
phase (car) is ~0.2.
Prior to the experiments numerous inclusions were mapped, photographed and
analysed by microthermometry and Raman spectroscopy to determine their
molar volumes and compositions. The corresponding isochores were calculated
and found to span a range of pressures at 700 °C, the mean internal pressure
being ~600 MPa. Based on this information a first set of control experiments
was conducted under hydrostatic conditions. The samples were placed at 700
°C and 500, 600 or 800 MPa (all within the -quartz field) for 16 hours, in order
to induce static re-equilibration of the inclusions under internal under- or
overpressure. At these conditions only one homogeneous phase is stable in the
CO2–H2O–NaCl system. Following the experiments the inclusions were
relocated and reanalysed. In each case, irreversible changes in the shape of
the inclusions were observed, similar to those reported in the cited earlier
studies. The initial variation in molar volumes of the inclusions was considerably
reduced during the experiments, converging on the values expected for the
experimentally imposed P–T conditions.
With the effects of hydrostatic re-equilibration thus known for these samples, a
second set of experiments was conducted at 700 °C and 600 MPa and at 50 to
100 MPa deviatoric stress for 12 to 133 hours, leading to mean strain rates of
10-6 to 10-7 s-1. New inclusion shapes were observed, often with branches
emanating from the inclusions. Inclusions initially aligned along healed fractures
were transposed. Most interestingly, new gas-free aqueous inclusions
(homogeneous liquid at Tlab) were formed, close to but separate from relicts of
the original inclusions. The relicts themselves have highly enriched gas
contents (car up to 0.8), indicating partitioning of H2O and CO2 from the
originally homogeneous CO2–H2O–NaCl mixture into separate inclusions.
Isochores calculated for the transposed inclusions lie close to the P-T
5th Swiss Geoscience Meeting, Geneva 2007
conditions of the experiments, whereas the newly formed inclusions show a
span of isochores.
The observed ranges in textures, compositions and molar volumes of the
inclusions in the latter experiments are remarkably similar to those in naturally
deformed quartz in ductile shear zones. This similarity promises an approach to
interpret such natural samples.
REFERENCES
Sterner, S.M. & Bodnar, R.J. 1989: Synthetic fluid inclusions - VII. Reequilibration of fluid inclusions in quartz during laboratory-simulated
metamorphic burial and uplift. J. Metam. Geol., 7, 243-260.
Vityk, M.O. & Bodnar, R.J. 1995: Textural evolution of synthetic fluid inclusions
in quartz during reequilibration, with applications to tectonic reconstruction.
Contrib. Mineral. Petrol., 121, 309-323.
Bakker, R.J. & Jansen, J.B.H. 1991: Experimental post-entrapment water loss
from synthetic CO2-H2O inclusions in natural quartz. Geochim. Cosmochim.
Acta, 55, 2215-2230.