Precambrian target rock mystery
Jack wrote in Precambrian target rock mystery at
http://www.mail-archive.com/meteorite-list@meteoritecentral.com/msg115243.html
"I am thinking an ET origin regarding these targets and
throwing out idea for comments."
and
"Phillis Hargrave et al lead author of the above paper with
the Montana Bureau of Mines and Geology has sent me
additional photos to share. The paper should be online."
One publication, Hargarve et al. (2011) is online at:
http://www.mbmg.mtech.edu/pdf-open-files/mbmg602_SilverCity.pdf
Yes, they are interesting and quite curious. Below is
specifically what Hargarve et al. (2011) stated about them.
"Ys Spokane Formation – Grayish red, dark greenish red, and
purplish red, microlaminated argillite and siltite; contains abundant
mud cracks, ripple marks, etc. The Spokane Formation in this
quadrangle contains many areas where “target” rocks, areas of
concentric bleaching around a central grain, can be found,
especially in the hills west of Little Prickly Pear Creek. These
bleached circles are sometimes single occurrences and are locally
also “constellations” of numerous small circles. The target rocks
are the result of radioactive decay. According to Bregman
(written comm, 2009 the central nodule is high in strontium,
rubidium or any one of a number of radioactive elements derived
from the Grenville Highlands way off to the "east (?)" of the
deposition area. The "target rocks" are usually found in the
Spokane which is normally maroon (though there are some
relatively thin light green beds throughout). The age of the rocks,
about 1.2 (?) billion years old. Enough time for the heat generated
by the radioactive decay to cook (bleach) the color out of the
Spokane. Because heat radiates out in a sphere from the nodules,
the circles are actually spheres (or ellipsoids) sliced by the shale
layers. There probably are the same element nodules in the other
Belt rocks (Greyson, Empire, etc.), but since their normal color,
or weathered color can be not much different than the bleached
out portions, they would be veritably invisible."
The publication is:
Hargrave, P., J. Lonn and M. Bregman, 2011, Geologic Map of
the Silver City 7.5' Quadrangle,West-Central Montana. Montana
Bureau of Mines and Geology Open-File Report no. MBMG 602.
http://www.mbmg.mtech.edu/pdf-open-files/mbmg602_SilverCity.pdf
http://www.mbmg.mtech.edu/mbmgcat/public/ListCitation.asp?pub_id=31402&
Looking at the literature, the Spokane Formation (formerly
Grinnell Formation) has gone through significant metamorphism,
diagenetic alteration, and, in places, copper and other mineralization,
which will make understanding what created them quite complicated.
Jack asked:
"Another geologist suggested the reaction is a type of
geo-chromotography but by what?"
They remind me of and look a lot like reduction haloes, which are
commonly found in redbeds, paleosols, and other strata. Reduction
haloes have also been called "reduction spheroids," "reduction
spots," "fish eyes," "reduction mottling," "bleaching haloes,"
"(radioactive) concretions," and "(radioactive) nodules." There are
various processes by which they have been explained. Some of
them are discussed in:
Dyck, W., and R. H. McCorkell, 1983, A study of uranium-rich
reduction spheroids in sandstones from Pugwash Harbour, Nova
Scotia. Canadian Journal of Earth Sciences. vol. 20, no. 11,
pp. 1738-1746.
http://www.nrcresearchpress.com/doi/abs/10.1139/e83-163
Hofmann, B. A., 1991, Mineralogy and Geochemistry of
Reduction Spheroids in Red Beds. Mineralogy and Petrology.
vol. 44, pp. 107-124.
http://link.springer.com/article/10.1007%2FBF01167103
Hofmann, B., J. P. L. Dearlove, M. Ivanovich, D. A. Lever, D. C.
Green, P. Baertschi, and Tj. Peters, 1987, Evidence of Fossil and
Recent Diffusive Element Migration in Reduction Haloes from
Permian Red-Beds of Northern Switzerland. in B. Come and N. A.
Chapman. eds., pp, 217-238, Natural Analogues in Radioactive
Waste Disposal. Springer, New York, New York.
http://link.springer.com/chapter/10.1007%2F978-94-009-3465-8_21
Kemp, A. J., M. R. Palmer, and K. V. Ragnarsdottir, 1994, The
Uranium-Thorium and rare earth element geochemistry of reduced
nodules from Budleigh Salterton, Devon. Proceedings of the
Ussher Society. vol. 8, pp. 214-218.
http://www.ussher.org.uk/journal/90s/1994/documents/Kemp_et_al_1994.pdf
Lines, A. W., J. Parnell, and D. J. Mossman, 1996, Reduction
spheroids from the Upper Carboniferous Hopewell Group,
Dorchester Cape, New Brunswick: notes on geochemistry,
mineralogy and genesis. Atlantic Geology. vol. 32, no. 2, pp. 159-172.
http://journals.hil.unb.ca/index.php/ag/article/view/2085
http://journals.hil.unb.ca/index.php/ag/article/download/2085/2449
van Panhuys-Sigler, M., N. H. Trewin and J. Still, 1996, Roscoelite
associated with reduction spots in Devonian red beds, Gamrie Bay,
Banffshire. Scottish Journal of Geology. vol. 32, pp. 127-132.
http://sjg.lyellcollection.org/content/32/2/127.abstract
They have been found to have form around a variety of objects,
including roots, "organic matter," and, even in one case in
Swedish limestone, meteorites. In many cases, the identity of
what, if anything, was at the center of a reduction haloes remains
unknown. If a “self-organizing” geochemical processes were involved,
there likely never was either a nucleolus or anything else at the center
of a reduction haloe at any time.
Some pictures of the Spokane Formation (Grinnell Formation) can
be seen in “The Rocks around Glacier National Park, Montana:
Introduction to the formations” at
http://mountainbeltway.wordpress.com/2010/08/23/rocks-of-glacier-national-park/
A ways down there is a picture of “Sometimes there are areas of
low oxidation called reduction spots.” as at:
https://farm5.static.flickr.com/4098/4910489039_dac036d1d2.jpg
Yours,
Paul H.
