The high-Z phases in your BSE image appear to be late, intergranular chromite. Alternatively they may represent a reaction product forming along the margins of the larger phases. 

 Thanks, Hamish. Can you tell me the context in which you have seen these. Was it in formations on the Canadian Shield?

Hi Allen.

The chromite appears to be from a ferrous-chromian mafic-ultramafic rock. I have not seen these in Archean assemblages, but it might occur in appropriate Proterozoic rocks. What is the sample exactly?

Kenneth and Hamish. Our material came from Chile and appears originally to have been volcanic in origin. Later, it appears to have been altered by melting and fairly rapid quenching that also produced many large vesicles. Much more needs to be done, but we are exploring the possibility that the volcanic source material was re-melted and ejected during impact by a meteorite. The paper is very helpful, Kenneth, because it shows sub-euhedral chromite that appears similar. After a limited search of the literature, I can't find volcanic grains that look very similar -- nearly all are euhedral.

Thanks to both of you.

Allen,

These textures are reminiscent of a material very rapidly precipitated from a vapour phase, although I have never heard of such a process for chromites. However, you can get strongly skeletal shaped chromites crystallizing from quenched volcanic material, for example the rinds of MORB pillows or in the upper A-zone (random spinifex textured) zones of komatiite flows. See for example the photos on page 1691 of Barnes  S J 1998, Journal of Petrology Vol 39, 1689-1720. Also, when conducting experiments on fusion of chromitites for an improved fire assay for PGE, I frequently found small neocrystallized sub-hexagonal flakes or thin plates of a Cr-rich material, probably Cr2O3, within the quenched slag. Neither of these cases are quite analogous to what you have shown, but they may provide some leads to follow-up.

Dave

Dr. Allen West.

Sorry. I never seen anything similar like this. Very interesting. So I prefer read the good opinions of the collegues Hamish, Kenneth and D.M Evans.

Valter Salino 

DM and Valter, thanks for your comments. Very helpful.

Dear Dr. West,

chromite tends to occur in euhedral to subhedral XX and used to prevail its X morphology against the remaining rock-forming minerals, particularly when being among the first minerals to come. There are other mineral aggregates which are reaction products formed in the course of weathering such as the so-called “merumites” described from weathering crusts. They are close to goethite (better call it here “limonite”). It is CrOOH, CrHO2, CrOOCu eskololaite, guyanaite (locus typicus), bracevillite) intergrown with FeOOH and the Cr-bearing phyllosilicate kämmererite. These aggregates were described by the “Father of ore microscopy” P. Ramdohr. In my opinion it is such an alteration product which can be originated from supergene and/or hypogene alteration. Chromite does not fit at all, neither from the chemical nor crystallographic point of view.

With kind regards

H.G.Dill

Dr. Dill, thank you very much for your very helpful answer.

Regards,

Allen

Allen, is there any reason why the Chromite in your sample would not have been from the meteorite that impacted the site? Chromite is far more common in stony meteorites than in terrestrial rocks, according to this website (my apologies for butting in as a non-expert on minerals):

http://meteorites.wustl.edu/metcomp/cr.htm

Quoting: "One of the best elements for distinguishing meteorites from Earth rocks is Cr. Nearly all stony meteorites have high concentrations of Cr compared to most Earth rocks. The most Cr-poor meteorites are the feldspathic lunar meteorites, which have high concentrations of Al2O3 and CaO. Cr-rich terrestrial rocks tend to be ultramafic rocks like dunites, peridotites, pyroxenites, and serpentinites."

Hi, Hermann. Good question -- we wondered that, too. However, the raw Cr-rich grains are found everywhere throughout the profile, spanning ~10,000 years, so it doesn't seem they are meteoritic. On the plus side, Cr-rich spherules occur only in the 12,800-year-old layer, so they are possibly impact-related, probably derived from melting of the Cr-rich grains during impact.

Allen

Hi Allen, great reply from you. Can you send me details of the profile? Are you making a distinction between Cr spherules in narrow field vs. grains more widely ranging? –– 12,800 BP says this is almost surely the same YD Comet of our mutual acquaintance ...

Hi, Hermann. Can't say anything more because the data are in the final stages of review, and we hope to have them published soon. We will post the paper here on Researchgate as soon as it is available.

Allen