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Catastrophes and Prehistory


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Author Topic: Catastrophes and Prehistory  (Read 6321 times)
Troy Exeter
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« Reply #15 on: March 18, 2007, 10:25:31 pm »

Impacts and vulcanism
CCNet 15 Feb 2001
P/T BOUNDARY: NO IRIDIUM

From Hermann Burchard <burchar@mail.math.okstate.edu>


Dear Benny,

In Permian/Triassic boundary strata in South China, the element iridium is not present or at most only in trace amounts, according to Doug Erwin, who kindly responded to my e-mail question. This can be understood, as I would like to suggest, by noting certain connections with the iridium-rich Hawai'i hotspot, which has been moving in a SE direction across the Pacific for >100Ma, probably 225Ma, starting off from Sibiria.

As mentioned by Victor Clube and Bill Napier in their book "Cosmic Winter", magmas from the great Hawai'i volcanoes are rich in iridium. They discuss this, because it's an argument against cometary impact as a cause of the abundance of the element in extinction layers, such as the famous K/T-boundary.

There is a clear trace on the floor of the Pacific ocean beginning with the Emperor Seamount chain from the Kamchatka Peninsula to Midway Island, then angling off in a slight left turn along the Hawai'ian island chain. Although the trace possibly is now partly subducted in the Kamchatka - Aleutian trench, it seems clear enough that the hotspot was originally positioned in Eastern Sibiria.

Underlying the hotspot is a mantle plume which presumably was created when a cosmic body hit Sibiria and created the vast flood basalts of Yakutia (Sakha).  See the article by Renne et al. in "Science", 1995, 269:1314, for a map of the conjectured extent of the original lava beds, which may not have been fully explored.  These cover Yakutia (Sakha), bordering directly on the Sea of Okhotsk near Magadan, immediately adjacent to the present day NW-terminus of the Emperor Seamount chain. From my less than adequate maps, the basalt beds seem to abut on or even include the Kolyma gold and diamond fields; diamonds have been studied in connection with impact sites e.g. by Christian Koeberl.)

Therefore, little doubt can exist concerning the essential identity of the following events:

          1.  Inception of Hawai'i hotspot in Sibiria.
          2.  Sibirian flood basalt eruption.
          3.  Cause of P/T mass extinction.

We owe the identity of 2. and 3. to the work of paleobiologists like Doug Erwin. Here, we wish to explain that event 1. probably was a cosmic body impacting in Sibiria - more precisely a spot in Gondwana-land which became present-day Eastern Sibiria.

Much of the meteoritic material from the comet or asteroid, that struck Earth at the P/T transition, appears to remain still in the hole punched in the upper mantle by the cosmic impact body, the Hawai'i hotspot (I sincerely doubt that this will seem like a very novel idea in the minds of many geologists).

Hence we may conclude:

   [A] Iridium continues to be pumped upward with deep mantle material in Hawai'i volcanoes to this day.
    Little of the cosmic material was thrown into orbit at impact time, because of uniquely deep penetration of the giant P/T impactor.
   [C] Iridium cannot be traced in the layers separating Paleozoic and   Mesozoic rocks, never having been dispersed to a great extent.
   [D] Rather than refute it, as Clube-Mapier feared, abundant ir in the  magmas from Hawai'i confirms the impact theory of mass exinctions.

The relationship between impacts and hotspots is perhaps still somewhat controversial, so I will attempt to elaborate on this. Hotspot physics and geology is probably not a perfect science.  If I understand it correctly, the main mechanism is the same as in spreading or rift zones:
Pressure on the upper mantle is relieved as the minerals rise with reduced overburden, causing a phase transition which we see as melting. The causes of pressure release are somewhat different in the two arrangements of a) impact related hotspot and b) rift zone.
In case b) of a rift zone one possible initial cause of reduced pressure seems to be thinning of continental crust due to erosion of a stable craton over many 100Ma. The African rift valley is a case in point. In North America, at the beginning of the Jurassic era, the Atlantic ocean first began as a rift, with the margin seen today e.g. in the New York palisades
rock facade. At present, the New Madrid fault along the middle Mississippi may exemplify the same phenomena at an early stage. However, a rift may begin with a hotspot, as one other interpretation of the Great African Rift suggests! One hotspot is in the Afar region.

In case a) of an impact-induced hotspot the initial step is that the impact events destroy the phase equilibrium of the upper mantle in a narrow region underlying the crater.  This may be due partly to the shock wave of impact upsetting crystalline structures, or partly because surface rocks are excavated and removed by the impact explosion.  A massive melt results in the form of flood basalts from the suddenly relieved pressure, and/or from impact shock wave induced phase transition. The effect is a snowballing phase transition and melting.

Again, to avoid misconceptions, and because this does seem to remain controversial in some circles, it should be emphasized that:
       The melt in the plume after impact is _NOT_ caused by the initial  energy yield of impact, but rather by the reduced pressure which  forces a phase transition to take place that ends up in a phase  equilibrium at a lower Gibbs energy.

(Other views [as in Renne et al.] present a picture of a spontaneous rise of mantle, liquifying over a huge area, for which no account of origin can be given.  This can be considered for basalt floods, but could not explain narrow-bounded hotspots.)

Once in operation, lower mantle material appears to be resupplied continually from the sides to the punch hole, which maintains a pore where the pressure remains lower than in the surrounding mantle.  Thus the plume can rise indefinitely, as we see happen today in Hawai'i.

Any computations of effect of impact on the mantle not modeling phase equilibria and transitions should be treated with suspicion. Above description of mantle plumes, to make a disclaimer, is conjectural, not substantiated by actual computation.  My limited understanding of these things is based on a study of stable computation of phase equilibria, working with a petroleum engineer, on computing "flash" crude oil separation.

Although apparently still controversial, years ago already I have heard mention made by geologists of the connection of hotspots and impacts, as in the example of the Yellowstone hotspot, now in Wyoming, that has travelled East along the Snake river plateau for > 10Ma, and that is implicated in the flood basalts in Western Idaho and probably Washington State (?).

(Unfortunately, I was unable to attend the February 9 RAS conference on impacts, where Christian Koeberl was keynote speaker. I missed talks by Adrian Jones and Simon Kelly on impact, flood basalt, & hotspot related topics, that might have led me to improve this account).

Best regards,

Hermann G.W. Burchard
burchard@math.okstate.edu

See a response by Andrew Glikson, Item 12, CCNet 23 Feb 01. He questions the P/T link.

IMPACT VOLCANISM CLARIFICATION

>From Mark Boslough <mbboslo@sandia.gov> CCNet 1 Mar 01

A few inaccuracies crept into the March Scientific American article.

1) Our seismic focusing calculations showed that the peak in seismic energy
dissipation is in the asthenosphere both antipodal and directly beneath the
point of impact. We suggested that for a sufficiently large impact the
increased melting in the asthenosphere would be a significant contributor to
any impact-induced volcanism, but we did not speculate about effects on
pre-existing plumes or extinctions (although these ideas are worth
considering). Our idea was that a narrow column of hotter mantle could
create an instability that *looks* like a plume (as opposed to a classic
fluid plume that pushes its way up from the CM boundary).

2) I'm not sure where the "may not have been antipodal" phrase came from.
The impact antipode was clearly something like 30 degrees from the Deccan
Traps at the time of the K/T boundary.  If the Deccan Traps are
impact-induced it was not the Chicxulub impact (which came too late and in
the wrong place!) but an earlier impact either into the east Pacific or into
India.

3) We suggested that an impact might generate the same surface
manifestations normally associated with mantle plumes (i.e. flood basalts
and long-lived hotspots). We did not connect them to superplumes which is
what Dallas Abbott proposed. It was Jon Hagstrum of the USGS who suggested
the connection to sea level, weathering, ocean chemistry, sediments, etc.
The ideas of Abbott and Hagstrum are also interesting worth considering--but
they're not mine as the article implies.

Mark Boslough
Sandia National Laboratories
 
 

Can Impacts Induce Volcanic Eruptions?
 Authors: Melosh, H. J.
 Journal: International Conference on Catastrophic Events and Mass Extinctions: Impacts and Beyond, 9-12                   July 2000, Vienna, Austria, abstract no.3144
Available from ADS Abstract service.
Conclusions: The bottom line of this discussion is that there is not a
single clear instance of volcanism induced by impacts, either in the
near vicinity of an impact or at the antipodes of the planet. This
accords well with theoretical expectation from our current understanding
of the impact cratering process. The possibility of impact-induced
volcanism must thus be regarded with extreme skepticism.


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