Re: Mineral analysis may reveal life on Mars

George wrote:

"Jo Schaper" <joschapern4ospam@xxxxxxxxxxxxxxxxx> wrote in message news:11rt87krgp7i3c1@xxxxxxxxxxxxxxxxxxxxx

George wrote:


http://www.newscientistspace.com/article.ns?id=dn8534

 a.. 11:11 05 January 2006
 b.. NewScientist.com news service
 c.. Maggie McKee

Early degradation
The researchers took 12 terrestrial calcite samples - from sources representing all three types of the mineral - to see if they could detect differences among them. Using X-ray diffraction and electron scanning microscopy to study the samples' mineralogical and chemical composition, they found the abiotic samples were pure. But the biotic samples contained impurities - they grew faster and sometimes substituted magnesium atoms for calcium in their crystalline structures.

These defects "structurally weaken the calcite", the authors write in Geophysical Research Letters. This weakening was detected when the researchers steadily heated the samples and noted when they began to lose mass by decomposing into gaseous carbon dioxide.

What dummies. Substitution of magnesium atoms turns it to high-mag calcite, then, if it continues, ultimately to secondary dolomite. Calcite-to-dolomite is a chemical continuum, not just a way to weaken calcite...and dolomitization often destroys any fossil evidence present in the original limestone. Naturally, because of the structure of dolomite, it is going to have a looser structure than CaCO3...

Now, evidence of high mag CaCO3 to dolomite would indicate the presence at some time of liquid water...at the same time it destroys the primary evidence of life.


While it is generally true that the dolomitization process destroys biologic structures, there are a lot of dolomites that contain abundant fossils.

You may have noticed my waffle words in what I wrote, George. I did not imply that dolomitization *always* destroys the fossils. It also seems to depend somewhat on the original fossils' composition. In my experience(all non-tidal flat secondary dolomitization)looking stromatolite and cryptozooan reef structures (lots of blue green algal colonies/mats) dolomitization occurs concurrently with the silicification of these fossils, preserving them, but as quartz/chert, not as calcite or dolomite. These same structures still exist as calcite structures in nearby, non-altered limestones. Sometimes you can follow altered to unaltered zones (Don't know why. Haven't a clue. But simply my repeated observations.) Obviously, based on your study sites, nothing is the same in Ky...

(dolomite containing fossil examples snipped)
Since dolomitization typically occurs in tropical tidal flats, one has to wonder how much of the dolomitization is chemically induced, and how much, if any, is biologically induced. I'm not suggesting that dolomite IS biologically induced, because dolomitization is certainly not one of my specialties. It would, however, make for an interesting research project. I wonder if there are any species of algae that convert calcite to dolomite. Anyone?

One of the theories floating around here on the other side of the Mississippi is the secondary dolomitization of many/most of the thick Missouri layers (Potosi, Eminence, Gasconade) were related to the movement of 100 degree C+ highly mineralized fluids squeezed through the subsurface in association with the precipitation of the MVT lead/zinc deposits concurrent with the Ouachita uplift further south in AR and OK. Now, there are areas where the MVT itself does NOT come from dolomitic layers, but from unaltered, older limestones,(which throws a wrench into the mix, especially since the lead dates as younger than the limestone) and some of these layers (such as the Bonne Terre (labeled Limestone) and Potosi (labeled Dolomite) where the actual rock may be either, or a mix of both. Many of the cherts in these layers are either biogenic or preserve biogenic markers (trace and actual fossils). There is this whole CaCO3, PbCl, MgCO3, and Mg(sulfide-sulfate) chemistry interaction as a low temp geochem process which escapes me at the moment; the bottom line is that the heated/injected seawater somehow facilitates the conversion of PbCl to PbS.

These dolomitic layers are not the thin, finegrained, yellow to ochre dolomite typical of tidal flat deposits. In TriState, well formed pink dolomite crystals and clean, tight gray dolostone associated with microcrystalline chert breccias are common; in the SE District, the dolomite is granular/sandy, massive, white/tan and somewhat 'dirty' and crumbly--cherts tend to be fossiliferous and either in layers or nodules.

My 'wild-a' guess about any biotic magnesium fixing or acceleration is that it would be bacterial, not algal, esp. since magnesium nodule formation in the ocean is often below the light zone at the bottom. However, as noted, this is just a guess--

.

Relevant Pages

  • Re: Mineral analysis may reveal life on Mars
    ... >>>calcite, then, if it continues, ultimately to secondary dolomite. ... >>>calcite...and dolomitization often destroys any fossil evidence present ... > imply that dolomitization *always* destroys the fossils. ... > layers, but from unaltered, older limestones,(which throws a wrench into ...
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