August 27, 2001

Research Report, Dr. Gray E. Bebout, Lehigh University, USA
For visit to the ISEI/PML August 8 to August 22, 2001
Part of my recent research has focussed on study of crust-ocean-mantle chemical cycling
at convergent margins, and has paid particular attention to the record of the cycling ofvolatile components in subducting sedimentary and oceanic crustal rocks. My recent workon subduction-zone cycling has turned to work examining records of devolatilization andelement mobility in deeply subducted rocks in the European Alps, specifically those in NW
Italy and Eastern France. These rocks have experienced complex metamorphic history not
only during their deep subduction, but also related to their complex exhumation histories.
This superposition of mineral assemblages (with complex textures) related to multiple
tectonometamorphic stages necessitates the use of analytical methods affording high degreesof spatial resolution, while maintaining high degrees of analytical precision and accuracy.
This PML possesses SIMS (ion microprobe) facilities that allow high spatial resolution
analyses of trace element concentration and stable isotope composition and produce data withquality exceeding that of any other laboratory.
On my visit August 8-22, 2001, I undertook detailed ion microprobe analyses of traceelement concentrations in UHP metamorphic rocks from the Lago di Cignana locality,
Valtournenche (Italy), Monviso (Italy), and exposures of the Schistes Lustres in the Cottian
Alps, also in Italy. The work focussed on analyses of porphyroblasts such as garnet andtourmaline, and their mineral inclusions (e.g., mica, clinopyroxene), and matrix phases (e.g.,micas, clinopyroxene, zoisite) with the goal of assessing whether or not a sufficientchemical/mineralogical record exists for reconstruction of element mobility during prograde
devolatilization of the rocks. The Lago di Cignana and Monviso rocks experienced peakmetamorphic pressures exceeding 2.4 GPa (perhaps as high as 2.9 GPa for the LDC rocks),
and these rocks can conceivably provide a record of devolatilization and element mobility at
depths approaching those beneath volcanic arcs (>80 km). Together, these suites provide theability to examine fluid-rock history and chemical evolution in both seafloor altered oceanic
crust and overlying seafloor sediment, lithologies thought to contribute chemical components
to arc lava source regions. The Schistes Lustres contains tectonometamorphic units oflargely metasedimentary compositions that have been peak-metamorphosed at conditions of~1.0 to 1.8 GPa, thus potentially affording assessment of devolatilization in units subductedto depths intermediate to those of the peak metamorphism of the Lago di Cignana andMonviso rocks and those experienced by the Catalina Schist and Sambagawa Belt suites wehave worked on previously (up to ~1.0 GPa represented by these two circum-Pacific suites).
While at the ISEI, I also worked briefly with Michael Walter in verifying the presence ofcoesite in some of the Italian rocks using Michael's laser Raman setup.
One of the primary goals of this visit was to further confer with Eizo Nakamura and othercolleagues at the PML (Katsura Kobayashi and Takuya Moriguti) in conceptualizing projects
to be undertaken by one or two of my new Ph.D. students. These students, formally Ph.D.
students at Lehigh University, would visit the PML for periods of 6-12 months each andwould undertake appropriate analytical work aimed at the problems above and under the
direction of Eizo Nakamura.