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 SO2

cloud, 20 megatons

Anhydrite (CaSO4)…in a volcanic rock??!!!

• Anhydrite had been observed in the pumices from the SO2-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 SO2-rich Pinatubo eruption, I

guessed that anhydrite might be present

A

150 um

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 CaSO4 from Ca5(PO4)3 grains by EDS (heavy liquid separation from pumice)

-- But we found “bumps” all over the CaSO4 when we looked at SEM images with the electron microprobe!

c

e

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?

Gl

pyr

Glc

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, SO2 content of eruptive gas, magma fO2 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, 1029-1045.

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 right

@ 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-46.20Al annealed at 1130oC for more than 1000 hoursThe 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