Department of Earth Sciences
University of Southern California
The Stable Isotope
Laboratory at USC houses a VG PRISM triple collector gas ratio mass spectrometer
for the analysis of oxygen, carbon, hydrogen, sulfur and nitrogen isotopic
ratios. The instrument is fully automated and can analyze CO2 samples as
small as 0.1µmoles. On-line with the mass spectrometer is a new laser-based
silicate extraction system which enables us to determine oxygen isotope
ratios in silicate and oxide minerals. This laser extraction system includes
a 20 Watt Melles Griot CO2 laser which is mounted on a X-Y stage. BrF5
is our oxidizing reagent. Approximately 30 samples can be analyzed in one
day. We routinely analyze samples as small as 0.05mg. (See Current Research
below for more information). The lab also houses conventional vacuum extraction
lines for carbonates (e.g. calcite and dolomite), graphite, organic material
and sulfides. Hydrogen isotope ratios of waters and water in hydrous minerals
is extracted using the Zn-technique. The lab is maintained by Leslie Ann
Driver (M.Sc. Geology). Leslie has been running the lab since December
1998.
Sample preparation area |
Carbonate extraction line |
Silicate laser line |
 VG Prism
Series II mass spectrometer
|
Current
Research
Epidote: magmatic vs. subsolidus?
Laser probe measurements
of oxygen isotope ratios of epidote and adjacent minerals suggest that
oxygen isotope fractionations may provide a tool for distinguishing magmatic
from subsolidus epidote. Textural criteria (Zen and Hammarstrom, 1984)
have been used to make the distinction, but textures alone can be misleading.
In the Mt. Lowe intrusion of S. California, oxygen isotopic compositions
confirm that large euhedral epidote crystals are magmatic in origin. Texturally
ambiguous coarse anhedral epidotes have also been shown to be magmatic
in origin. Surprisingly, intergrowths of epidote with biotite and/or hornblende
are subsolidus. These textures were thought to be indicative of crystallization
from a magma. Work is continuing on many of the classic "magmatic" epidote
occurrences to assess the reliability of oxygen isotopes in distinguishing
magmatic from subsolidus epidote.
Fluids in metamorphic core complexes
The role of fluids
in the evolution of the Whipple Mountains metamorphic core complex (SE
California) is a subject of on going research. Our initial studies in the
Whipple Mountains have shown that lower-plate lithologies have exchanged
with surface-derived or meteoric waters. However, this fluid-rock exchange
occurred after the mylonitized lower-plate rocks were brought up to depths
of <~4km. There is no evidence to suggest that meteoric fluids circulated
down into the zone of ductile deformation, below the brittle-to-ductile
transition. Recent work has focused on detailed oxygen isotope analysis
of hornblende + potassium feldspar filled tension gashes in lower-plate
rocks. These tension gashes are thought to document the earliest stages
of extension. Hornblende d18O values range from 5.17 to 6.69 o/oo (average
= 5.64 ± 0.35 o/oo). We are currently analyzing coexisting feldspar,
and the data will be used to constrain the temperatures of mineralization
and the source of the mineralizing fluid.
Come
see the
King
of the West Gold Nugget!!
Click
here for more information and photos
Links
Department
of Earth Sciences Homepage
University
of Southern California Hompage
ISOGEOCHEM
Homepage
Jean
Morrison's Homepage
Lawford
Anderson's Homepage
Please send e-mail to earthsci@usc.edu.
