Adventures of an Igneous Petrologist in a Probe Lab John Fournelle University of Wisconsin Madison, Wisconsin

Mt. Pinatubo June 15, 1991: 20th century’s 2nd largest (volumetric) eruption largest observed SO 2 cloud, 20 megatons

Anhydrite (CaSO 4 )…in a volcanic rock??!!! Anhydrite had been observed in the pumices from the SO 2 -rich eruption of El Chichon in 1982 Experiments had proven that it truly was an equilibrium phase in a hydrous oxidized dacitic magma It is rarely observed in pumices which have been rained upon/weathered When I learned of the SO 2 -rich Pinatubo eruption, I guessed that anhydrite might be present

Optically, high birefringence (~like calcite) BSE: brighter than plag, darker than apatite; Reflected light: distinctive surface (rough, ~soft/scratches?) Above 3 BSE images: anhydrite as microphenos (with apatite!); and as inclusions in plag and hb

A student to do single crystal XRD… to confirm same crystal structure (there are several forms between gypsum and anhydrite Plan: -- First separate CaSO 4 from Ca 5 (PO 4 ) 3 grains by EDS (heavy liquid separation from pumice) -- But we found “bumps” all over the CaSO 4 when we looked at SEM images with the electron microprobe!

Project Redefined: 1.Are the pyramids laboratory artifacts? Are they produced during eruption? Are they produced before the eruption, at depth? 2.Is the anhydrite “true” high temperature orthorhombic anhydrite? 3.What are the pyramids? What caused them?

Pyramids formed before glass vesciculated, so not laboratory artifact -- and also before eruption

Single crystal XRD: Yes, phenocrysts true high temperature orthorhombic anhydrite

Anhydrite -- trapped inside large plagioclase…anhydrite has pyramids that had formed prior to capture… evidence that pyramids formed at depth

Electron Backscatter Diffraction: both pyramid and host anhydrite inclusion in plagioclase are orthorhombic CaSO4 1=Pyramid 2=Host

Comparison: GaN grown in laboratory vs Pinatubo magma chamber

Gas Modeling

Conclusion Textures suggest that anhydrite pyramids grew from vapor deposition in the magma chamber prior to eruption This is consistent with temperatures, SO 2 content of eruptive gas, magma fO 2 and water contents This also is consistent with the evidence for “excess” sulfur that cannot be accounted for strictly from magma sulfur solubility; i.e. this supports the suggestion of Gerlach that there was a previously separated sulfur-rich gas below the volcano We give this the name “magmatic vapor deposition” Jakubowski, Fournelle, Welch, Swope and Camus (2002) Evidence for magmatic vapor deposition of anhydrite prior to the 1991 climactic eruption of Mount Pinatubo, Philippines. American Mineralogist, 87,

Problems of Secondary Fluoresence in EPMA Issue: Is there ~10 wt% Nb in the Pd3Hf and Pd2HfAl?

Results from other research group using EDS (Nb Ka) My results using WDS (Nb La)

Results from other research group using EDS (Nb Ka) My results using WDS (Nb La)

Nb Ka is really there in the spectrum! But it shouldn’t be, right??? Well, yes and no…

Two approaches to understanding the problem of “secondary fluorescence across phase boundaries”: Experimental: create a “non-diffused couple”, putting two perfectly flat pieces of A and B together …but not always easy nor possible Simulate with a model that reproduces the physics of electron- material interactions and follows the x-rays; i.e. the Monte Carlo PENEPMA program (based upon PENELOPE)

Nondiffused couple: Nb to left / Pd2HfAl to 28 kV

PENEPMA simulation of secondary fluorescence: 5 microns away from the Nb, inside the Pd2HfAl

Comparing experimental (non-diffused couple) with PENEPMA: geometry effects also

Hot off the press

Al-Ir compounds and the problem of light element- heavy element matrix corrections in EPMA Figure: BSE image of alloy Ir Al annealed at 1130 o C for more than 1000 hours The bright phase is fcc-(Ir) and the dark one is B2-AlIr Problem: with such a large difference in Z (13- 77), using pure end member standards, there is a potential for major errors in the matrix correction. This has been noted by some researchers, but not systematically addressed before.

Compounding this problem are at least two issues MAC’s not known well: Al ka by Ir has 26% range Heinrich (1966): 2351 Henke (1985): 2057 FFAST (Chandler 2005): 2057 Mean atomic number of the intermediate alloy also controversial (important for backscatter correction)

PENEPMA Simulations for 6 intermediate compounds were run, to find which one closest fit the actual experimental EPMA data. -- Ir matches for 50, but AL matches for 53. Split the difference: 51.5 as the most probable true composition… -- Next step is to set up a geometry for secondary fluorescence to see if that would improve the fit