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Subject:

Re: wollastonite in volcanic ash

From:

obenholzner <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Mon, 19 Feb 2001 23:14:45 +0100

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text/plain

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To Eric and Bob and whoever is interested:
1. Thanks for comments.
2. Eric asked, if EMP is possible. For some fragments, yes. Most of the
fragments
show intergrowth features at the smaller 5 Ám scale (Fe-rich, Fe-poor
wo, and
sulfides - pyrite, rare MoS2, fluorite is also present). Who wants do do
the EMP
work? It is hard to relocate the fragments on a polished TS.
Anhydrite is not known from lavas. We could find anhydrite in pumice of
the
so-called "ocre surge". It does not look like the i.e. Pinatubo
anhydrite, it might
be of secondary origin. The anhydrites of the buchites are phenocrysts,
however.
The gas monitoring people of the Popo crew are working with COSPEC and
FTIR. There
are indications that excursions of the CO2/SO2 emissions are linked to
assimilation
of limestone in the magma chamber/storage.
We have  also a set of FE SEM data on aerosols from the plume, particles
hard to
describe with traditional terms. S- and Cl-rich particles are wide
spread.
Question1: Is it possible that aerosol formation can reflect contact
metamorphism
underneath a volcano? The base of Popo consists of carbonates and
evaporites, ca. 3
km thick.
Question 2 : Is there somebody in this discussion group working with an
ion
microprobe and got spar time to test if the S of the anhydrites/aerosols
is
magmatic or sedimentary in origin? I could provide polished thin
sections for the
buchites. AEROSOLS ARE COLLECTED AS 3D objects on a teflon filter. I am
more than
willing to go to a high-tech lab to assist.
Preliminary SEM work could be send to interested researchers.
Cheers Johannes



Robert Tracy wrote:

> Eric's response over the weekend prompts me to mention that we have
> found similar material, in this case products from combustion inside
> country-rock spoil piles at an abandoned coal mine near here (SW
> Virginia).  The pile apparently contained quite a bit of coal,
> spontaneously ignited and smoldered for a long time and probably
> reached >1000C temperatures, sufficiently high to promote substantial
> low-P melting.  It's superficially similar to the Mottled Zone rocks
> from the Negev that Eric mentioned in his message and the
> coal-combustion products Mike Cosca studied some years ago.  I guess
> it's a philosphical issue as to whether you call this occurrence
> "geological" or not - the ignition was apparently natural, as was the
> formation of igneous and metamorphic minerals, but the starting
> material was obviously there through human intervention.  The
> resemblance to Obenholzner's xenoliths in El Popo ash is striking.
>
> The "rock" occurs as a fine-grained, highly vesiculated and slag-like
> matrix cementing fragments of various size of fissile limestone and
> carbonaceous shale.  These aggregate masses occur in huge chunks up
> to several meters in size.  In thin section, the vesicular material
> is texturally much like a basalt, with laths of plagioclase in a
> heterogenous fine-grained matrix that includes Fe-Ti oxides and
> pyroxene-like minerals (based on very cursory EDS examination).  The
> baked limestone and shale fragments have a wide range of high-T
> minerals (including wollastonite, a ferrobustamite-like phase, calcic
> pyroxene, spinel and other as-yet unidentified minerals) that we have
> not studied in any detail, as well as apparent amazing short-range
> fO2 gradients.  We hope to study our samples in much greater detail
> in the next year or so.  Sadly, the US Bureau of Mines came in last
> year and "remediated" the property and the spoil piles now look like
> a golf course, so a good "hard-rock" field trip locality in the midst
> of a sea of soft rocks has gone away.
>
> Bob T.
>
> >Dear colleagues,
> >I am writing a manuscript on new textural, mineralogical and chemical results
> >on volcanic ash and aerosols: SEM and FE SEM examinations. All data are from
> >the recent activity of Popocatepetl volcano, Mexico. One part is dedicated to
> >the occurrence of Fe-rich wollastonite, hercynite and buchites
> >(glasses with an
> >origin of contact metamorphism [CM]) as a population of fragments of the
> >volcanic ash. The wollastonite is not homogeneous, it shows Fe-rich
> >and Fe-poor
> >parts which are intergrown. Some grains show  vesiculation-like textures.
> >Does anybody know SEM studies of similar wollastonites etc.? Also the buchites
> >are highly heterogeneous.
> >The main question is: Are these fragments products of ongoing CM underneath
> >Popocatepetl, or are they fragments of an old aureol. Popocatepetl is situated
> >on a ca. 3 km thick sequence of lime/dolostones/evaporites etc. of Cretaceous
> >age.
> >Buchites show wild compositions, they are rich in S, Cl and P. Anhydrite and
> >inheritated zircons are enbedded in this glassy/devitrified matrix.
> >Ideas, references and coauthorship for essential contributions are welcome.
> >Thanks and best wishes
> >Johannes H. Obenholzner, PhD
> >MNH-Mineralogy- Vienna/Austria
> >email: [log in to unmask]
>
> --
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Dr. Robert J. Tracy
> Professor of Geological Sciences
> Virginia Tech
> Blacksburg VA 24061-0420
>
> 540-231-5980
> [log in to unmask]
> (FAX: 540-231-3386)
>
> "We can't solve problems by using
> the same kind of thinking we used
> when we created them."
> -- Albert Einstein

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