There's a new paper in press with Geochimica et Cosmochimica Acta that is fairly interesting:

Comparative planetary mineralogy: Implications of martian and terrestrial jarosite. A crystal chemical perspective
J.J. Papike, J.M. Karner and C.K. Shearer
Geochim. Cosmochim. Acta, In Press, Corrected proof available online January 5, 2006
Abstract

That was a surprisingly substantive abstract. I'm sure the full text would have left me in the dust. I can almost understand how jarosite would be a useful indicator sulfate, but just barely. I guess I need to rethink my ideas about oxidizing environments. The description of highly oxidizing conditions initially surprised me, as I was really convinced they were only mildly oxidizing at best. SO2 is a pretty strong oxidant in my experience, but I always thought it took SO3 to create H2SO4.

I probably shouldn't be venturing into this territory.

The Jarosite is interesting since in my work with acid mine drainage it can be associated with the weathering of iron pyrite (FeS) (pyrite+H2O+O2= free iron+ H2SO4, in a nutshell). There is a whole 'nuther science built on AMD.

Interesting, too, is that there is a Alunite-Jarosite series wherein aluminium can substitute for the iron and/or vice versa and I'm thinking that this _can_ be associated with the weathering of feldspar (but don't quote me, I may be wrong).

--Bill


PS-- thanks for the references, Alex.

Bacteria are an important ingredient ( though not essential) to this process on Earth

Indeed, and in fact can very greatly facilitate the reaction. Years ago there were studies that demonstrated that if the bacteria could be killed off, the reaction would slow to almost zero. It was found that this could be done by adding a detergent to the minespoil, which affected the cell membranes and made the bugs sensitive to water pH.

Yes, it was, as all good abstracts are, a fairly concise summary of what's in the paper, which contains all the "messy" details.

For what it's worth, I've always been as interested in the findings of jarosite at Meridiani as I have been with hematite, even apart from what's been the big story to date (viz., the role of the former's diagentic breakdown to form the latter). For example, I found some of the conclusions in the recent McCollom and Hynek paper in Nature interesting.

At any rate, as Papike et al. note, more extensive phase analysis of martian jarosite (almost certainly requiring returned samples) may be able to unlock a larger geologic and atmospheric history.

I think the most important of the potential uses of martian jarosite may be that the time of precipitation/crystallization can be dated, as the authors mention. This may greatly simply remote/robotic age dating of the Meridiani deposits. Instead of looking for, and processing, great amounts of possibily volcanogenic materials looking for datable zircons or sanidines (needles in a huge haystack), a robotic lab may need only to scoop up and process some known jarosite-rich deposits to do in situ isotopic dating.

Anyone want to be a co-PI with me on this one... ?

--
Tim Demko
http://umn.edu/home/tdemko

Did you intend to say zircons? Well then you are right, it would really be difficult if not techically possible to date such in a robotic lab.

Yes, Myran...post edited...thanks!

For all of the non-geologists out there, the traditional methods for processing rocks for datable zircons is a very time- and work-intensive task, with much hands-on and microscope work (including final picking and separating very small grains with tweezers, needles, and brush hairs). Even in the best cases, zircon and sanidine crystals that may provide reliable absolute age dates of crystallization are a very small percentage of rock volume.

Jarosite in the Opportunity-analyzed Meridiani deposits seems to be quite common in some beds, and processing for datable separates may prove to be much simpler...

I will be perusing the terrestrial literature for some examples of jarosite dating...I can post results/key references, if anyone is interested...

And we need to come up with stratigraphic markers so we can figure out where we are in the section.

Yes, post anything you find.


--Bill

Say the Magic Word -"NANOMACHINES " and win a hundred dollars

Seriously, we'll probably get the samples home to earth before that halcyon era dawns.

Hehe, that tweezer picking image is precisely the one I had in mind when I posted a comment in another topic about the difficulty of building a robotic age-dating lab. I too, would like to learn more about the jarosite method.


Thinking about marker beds, we really don't have anything solid in the way of such things, do we? We only have a tiny section from Eagle crater, and a larger one from Endurance. The closest thing to a marker I have seen is the Wellington contact observed at the base of Burns Cliff. That is the break between the high-angle bedding and the lower angle stuff. Considering the relatively short section we have seen, it would be hard to imagine we could call that a marker one could be confident in regionally. Unless we come across an ash bed that was deposited in a geological instant, we probably have to pray that the Opster hobbles up to Victoria for a closer look at deeper strata.

Finding a few index fossils would sure be nice...

Finding index fossils would be "iffy". There needs to be the correct diversity, environment and preservation, which might be difficult to achieve here.

A marker be is going to be difficult. The textural changes would work in a very broad sense, but could change over the same isochron. Subtle chemistry changes could be too dependent on the weathering at that point and not be time-related.
As Tom says, we need an ashfall or dustfall.

We also need some core drilling on our wish list...

--Bill

Do the MER's have the ability to identify (for example) a layer similar to the Chixulub Iridium layer?