[BR-Crater] recognizing impact melts (was: Columnar jointed rocks [...])

[Ian Kluft]

On Sun, Jan 04, 2009 at 09:13:13PM -0600, Scot Wilcoxon wrote:
> Some information is actually available because of the melt process.  For  
> example, in some kinds of rock the process of radioactive decay creates  
> marks inside the rock.  Counting those marks can identify when the rock  
> cooled, because the tracks are destroyed above a certain temperature.

I'll bring this topic back up again because there was a question that
wasn't answered for me in this discussion.  So I went to look it up.

I never had any question that the age of the columnar jointed rocks could
be determined by normal means.  But the age is not a priority for this
project.  We will have handed off the research to universities before
the question of age becomes a priority.  Differentiation of impact vs
volcanic origin is the priority.  We have such limited resources that we
need to focus on what features can prove an impact well enough to include
in a scientific paper.

So the question is... do the columnar jointed rocks help with that?

I went to look up what "Traces of Catastrophe" (by Dr Bevan M French of
the Smithsonian Institution) says about this.  As usual, it provides some
useful advice.  The e-book is at
   http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html

I'll quote it below for your convenience.  Basically this says the
impact melt alone is difficult to tell from volcanic igneous rocks.
But they're usually is associated at the edges with inclusions of
non-melt rock which would be likely to have shock indicators.

The columnar jointed rocks at Black Rock, if they're remnants of a
melt pool as suspected, would be from an area which is pure melt.
The columnar jointing indicates it cooled slowly without movement.
It's less likely to yield any results there unless we find traces
of the impactor in it.

We can still go take a look to see how well they match that expectation.

The melt rock should be identical in composition to the target rocks in
the region (which is not normal for a volcano.)  But since the basement
rocks in the area are basalt according to the drill cores from the mining
papers, that wouldn't help to differentiate from a volcanic origin.

It lists iridium, osmium, platinum and gold as impact indicators.  We know
there's gold in the area because of Vista Gold Corp's strip mining operation
at Sulphur NV, within the outline of the suspected impact.  But iridium
would be of the most interest if traces were found - that was the trace
element in the K-T boundary used as evidence that it contains impact ejecta.

If someone has access to a lab that can test a sample for these rare
elements, then it would be worthwhile to quickly go grab some samples
from the columnar jointed rocks.  They're easily accessible at the side
of Jungo Road, which is usually accessible some of the time even during
Winter and Spring when the playa is not.

But other than analysis of the columnar jointed rocks for traces of
an impactor, I don't think we'll get much else from them.  The edges
where non-melt rocks would be included would be the places to focus.
With all the erosion in the area, it will still take some effort to
locate remnants of the edge of the melt pool.

See Chapter 6, beginning on Page 90, "Recognition of Impact Melt Rocks".
Hopefully this section shows why I consider that book such a useful
resource.  Here are the relevant quotes...

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   "6.5. RECOGNITION OF IMPACT MELT ROCKS

   "Despite their exotic origin, impact melt rocks are true
   igneous rocks that have formed by the cooling and crystallization
   of high-temperature silicate melts, and they often
   exhibit textures and mineral compositions that are identical
   to those of typical endogenic volcanic and intrusive rocks.
   [...]
   Because of these similarities in mineralogy and texture,
   it may often be impossible to distinguish between an
   isolated specimen of impact melt and a normal igneous rock
   on the basis of petrographic observations alone.

   "The similarities between impact melt rocks and endogenic
   igneous rocks have been one factor in the prolonged
   controversies over the origin of many now-accepted impact
   structures. Nevertheless, several decades of field and laboratory
   studies have now produced some generally reliable
   criteria for recognizing impact melt rocks and for differentiating
   them from endogenic igneous rocks.

   "The best field evidence for the origin of an impact melt
   unit is an intimate association with more distinctive shockmetamorphosed
   rocks. Impact melt bodies that occur as dikes
   cutting the subcrater basement rocks may be closely associated
   with pseudotachylite breccias or (more convincingly)
   with shatter cones and microscopic shock-deformation effects
   in the subcrater rocks that provide definite evidence of
   an impact origin. Above the crater floor, impact melt bodies
   of various sizes are intermingled with breccias that contain
   distinctively shock-metamorphosed rock and mineral fragments;
   such an association is also clear evidence for impact.

   "On the scale of individual hand specimens, the most definitive
   characteristic of impact melts is not the igneous crystallization
   textures, but the nature and appearance of included
   rock and mineral clasts. These fragments are derived from
   the target rocks; they do not resemble cogenetic volcanic
   materials. [...]

   "More convincing, preserved target rock fragments in impact
   melt rocks often contain definite shock features such as
   PDFs in quartz. Other textures, which reflect extremely high
   formation temperatures, also provide convincing evidence
   of impact. [...]

   "Impact melt samples that lack distinctive shock-metamorphic
   textures can still be identified by a variety of
   geochemical signatures. One test is to compare the impact
   melt composition with that of the target rocks. Because impact
   melts are produced predominantly from target rocks,
   with only a minor (usually >1%) projectile contribution, their
   chemical and isotopic compositions should correspond to
   the average compositions of the local bedrocks. The demonstration
   of such compositional matches, especially when the
   composition cannot be easily produced by endogenic processes,
   is a strong (although not absolute) indication of an
   impact origin. [...]

   "More definite evidence of an impact origin can be obtained
   by analyzing the impact melt for siderophile elements
   such as iridium, osmium, platinum, and gold. Such elements
   have extremely low abundances in terrestrial crustal rocks,
   but their abundances are much higher (100–1000×) in some
   meteorites. An anomalously high content of siderophile elements
   (especially iridium) in an impact melt indicates that
   the melt contains material (perhaps as much as a few percent)
   derived from the melted and vaporized impactor. [...]"