Thanks everyone for the discussion. I’ve summarized the main points (way) below.
Here are some additional points regarding our Archean paragneiss.
The purpose of my original question was mostly about “how bulletproof must we make our cordierite case?” for publication. As many of you have highlighted: cordierite-staurolite co-existence has interesting implications in terms of kinetics or P-T. Sounds like we won’t run into problems if we assume cordierite stability but can’t find a fresh piece; the alteration pattern is “classic” and “typical”.
Note: We do observe haloes in altered cordierite, but they are not pleochroic (anymore). We have not looked at individual grains in haloes but we know (from geochron) that there’s both abundant monazite and detrital zircon in the rocks.
The geochron we’ve done suggests an early lowP medium T subsolidus event predating barrovian metamorphism. So I have one additional question: has anyone ever described staurolite overgrowing cordierite? That would be either a counterclockwise P-T path or polymetamorphism, like Dave Pattison mentioned.
We have built pseudosections for our rocks and in such bulks (Archean plag-rich wackes), isothermal (roughly 600C) pressure shifts (3-5 kbar) will cause opposed dramatic changes in staurolite and cordierite modal proportions in a very narrow field, i.e. they directly replace each other. In a clockwise loop, it’s straightforward to picture syn-kinematic cordierite replacing staurolite in pressure-shadows. On the other hand, what textures would result from early kinematic staurolite replacing pre-kinematic cordierite? Cordierite will not produce pressure shadows, and staurolite will always be prismatic. I’m under the impression both the clockwise and counterclockwise cases could result in a quite similar texture, especially because deformation post-dates staurolite growth; that is pre-kinematic staurolite porphyroblasts cross-cutting altered cordierite moats aligned in the foliation.
Thanks
Carl
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Summary of discussion
Identifying altered cordierite
Cordierite alters to 50 shades of pinite (m, I, f-type etc., Ogiermann thesis, Kelsey, Marschall): classic yellow patchy (Waters, Harley, Vry) sheet silicate agregates with some birefregence but can be amorphous (Waters), isotropic (Vry). Bulk composition ranges between Ca-, K- and Na-rich. Al:Si atomic ratio should remain close to that of cordierite (4:5). Isocon diagrams (Grant 1986) can help constrain mass gain and loss during alteration (Waters). Altered cordierite can preserve pleochroic haloes (Mezger, Verdecchia).
Cordierite pleochroic haloes
Yellow pleochroic haloes around radiogenic element bearing phases can be preserved in altered cordierite (Mezger). A pioneering study describes haloes around zircon, apatite, epidote, rutile and sphene (Holmes, 1913).
In fresh cordierite, haloes are found more often around monazite and/or allanite (Tracy) than around low-alpha particle oomph-lacking zircon (Tracy). Energy of alpha particles emission (Th vs. U) might be more important than concentration of radioactive element (Waters). 1Ga monazites produce large annuli, as opposed to compact haloes around zircon. Haloes observed around U-bearing Th-free rutile as well (Zack)
Haloes also observed in biotite and chlorite. Shape of haloes appears to be controlled by host crystallography, suggesting anisotropic susceptibility to radiation damage (Tracy). Or it is related to anisotropic annealing, as in apatite-zircon fission-track (Mark).
Haloes around zircons are common in cordierite replacing garnet, because garnet breakdown generates zircons (Taylor). The pleochroic halo is not related to crystal lattice damage but to odd excitation effects (Fe3+) in hydrous phases (Taylor). Doesn’t need serious alpha bombing.
Meaning of staurolite-cordierite association
St + Crd assemblages are uncommon, not typical of Barrovian sequence (Cesare). In muscovite-bearing rocks, cannot be stable together, involves decompression OR counterclockwise (Pattison). Possible in a clockwise P-T path (Waters) with isothermal decompression from 600 – 5.5 to 600 – 3 kbar (Lhotse schists, Everest, Jessup et al. 2008; Sierra de Ancasti, Argentina, Verdecchia et al. 2013). In such a P-T path, depending on the protolith, Staurolite can be replaced by Cordierite and/or andalusite. Crd-St stability also possible at low P for some low-K bulk (Mezger and Regnier, 2016, Spear 1993 p. 483; Diener et al. 2008, Mezger, Pattison), metasomatomatized protoliths (Tracy) or contact metamorphism (Tracy).
Documented examples
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Carl Guilmette, PhD. Eng.
Professeur Adjoint
Chaire de Leadership en Enseignement Virginia-Gaumond
Département de Géologie et Génie Géologique
Université Laval
1-418-656-2131 poste 3137
Hi Bob
Arthur Holmes talked about pleochroic haloes in “The Age of the Earth” (1913). The book is available free online from Google. He shows nice concentric haloes in figures 1-4 and on p 107 he says that haloes are usually around zircon, but sometime apatite, epidote, rutile or sphene. That’s 105 years ago. I think we can cut him slack if he missed monazite! I don’t normally stick pleochroic haloes in the probe, fun though that would be, but I think I have seen a fair number of square cross sections optically. Surely these are not "virtually always monazite or allanite”. I assume this varies from rock to rock.
Your point that its not always zircon is well taken, as Holmes said.
Best, John
John W. Valley,
Charles R. Van Hise Distinguished Professor
Department of Geoscience
1215 W. Dayton St.
University of Wisconsin
Madison, WI 53706, USA
phone: 608-263-5659, fax: 608-262-0693
Jochen,
No implications - just sayin’ it outright. I remember in many courses as an undergrad and grad student being told by profs who were pretty good petrographers that those little high-relief guys were always zircons. Then when I started getting interested in monazite and began hitting them with the probe beam while doing recon, and finding that they were nearly all monazite, I reassessed.
When you think about monazite chemistry versus zircon chemistry, in terms of concentrations of radioactive elements, it’s pretty obvious. I’ve also observed that haloes in biotite and chlorite especially, but also in cordierite, are different shapes in thin sections and typically correlate with the host crystal orientation being observed. Someone may have written something on this, but I’ve never seen anything. The obvious implication is that host phases seem to have a non-isotropic susceptibility to radiation damage. For example, you will usually see haloes in basal-section, c-axis orientations of biotite that are perfectly circular, but haloes are more elliptical in sections that are more nearly perpendicular to the optic axis (or more correctly, the acute bisectrix of a mineral with 2V of 5° or so). This might make a nice little research project for a student.
Bob
Dr. Robert Tracy
Professor of Geosciences
Director, Museum of Geosciences
Virginia Tech
Blacksburg VA 24061-0420
540-231-5980
540-231-3386 (F)
Really Bob? Do you want to imply that all these years I or we've been living a lie? (pleochroic halos around zircons...)
Good point, though.
:) Jochen
