From:    James G. Acker                         
Subject: Permo/Triassic quandary
Organization: NASA Goddard Space Flight Center - Greenbelt, MD USA


      O.K.  I went back to the library and read the article (from the
N.Y. Times) closely.  Basically, the investigators used ion microprobe
dating analysis (SHRIMP) of zircons from the Noril'sk I intrusion.
The analysis provided a date of 251.6 +/- 3.6 million years for the
Noril'sk I intrusion.  According to the article, the Siberian Trap
volcanism occurred for approximately 600,000 years.
      I copied the article in case questions arise.

      "Synchronism of the Siberian Traps and the Permian-Triassic
Boundary", I.H. Campbell, G.K. Czamanske, V.A. Fedorenko, R.I. Hill,
and V. Stepanov, _Science_ Vol. 258, 11 Dec 92.

      Now here's the fun stuff.  Czamanske also presented at the
AGU and has an abstract in _EOS_, entitled "The Siberian Traps:  A Flip
of the Dynamo and 600,000 Years of Hell on Earth".  The eruptions all
appear to have occurred in one 600,000 year interval, as alluded to
earlier.  Thought to have been caused by a mantle plume.  He also notes
the K/T event and the P/Tr event need not have a common cause (quoting
Holser and Magaritz, Geochim. Cosmochim. Acta, 1992, 56, 3297-3309.
Because...  the article ALSO notes that Rampino and Oberbeck propose
an asteroid impact for the Permian/Triassic boundary, causing the
breakup of Gondwanaland and puncturing the crust, allowing the Siberian
Trap volcanism!  Czamanske is quoted in the article as skeptical.

      Here's the Rampino abstract from _EOS_  (all this was going on
at the meeting and I was doing oceanography!):

      (Sorry, I wrote ahead.  The _Times_ article put Rampino
with Oberbeck, but the paper was authored by Oberbeck, Marshall, and
Agarwal):

      "Did Impacts Initiate Breakup of Gondwanaland?"

      Impacts (1,2,3,4) may have triggered flood basalt eruptions
and they may have caused crustal rifting (3,4).  Additionally, volcanic
Precambrian greenstone belts may have resulted from partial melting of
mantle sources after impacts (1).  Formation of impact basins before
3.9 Ga may have formed vast basalt flows and initiated crustal rifting
(2,3).  Impacts may even have initiated formation of mantle plumes
(3,4).  We propose that tillites/diamictites provide clues to the role
of impacts in Gondwanaland flood basalt volcanism and rifting.

      During Carboniferous and Permian time, extensive tillite/diamictite
deposits were formed on the  interior of Gondwanaland and this was followed
by the most extensive eruption of flood basalts in geologic history and by
the breakup of Gondwanaland.  Textures of tillites/diamictites resemble
those of impact craters (5,6,7) and the thickness distribution of
tillites/diamictites produced in the last 2 billion years is the same as
that predicted from impact models (6,8,9).  This suggests that some
tillites/diamictites could be of impact origin.

      Flood basalts postdate, and are associated spatially, with
tillites and diamictites on the interior of Gondwanaland.  We suggest that
a long sequence of impact cratering preceding breakup of Gondwanaland
could have extensively fractured the crust, triggered flood basalt
eruptions, and facilitated final continental fragmentation.

      (1) Green, 1972, Earth. Plan. Sci. Lett. 15, 263-270.  (2) Frey,
1980, Precam Res. 10, 195-216.  (3)  Grieve 1980, Precam Res. 10, 217-247.
(4) Alt et al. 1988, J. Geol 96, 647-662.  (5) Oberbeck & Marshall, 1992,
LPSC XXIII, 1013-1014.  (6)  Oberbeck et al., 1992, J. Geol (in press)
(7)  Marshall and Oberbeck, 1992, AGU Abstr., this volume.  (8) Oberbeck
and Aggarwal, 1992, LPSC XXIII, 1011-1012.  (9)  Aggarwal and
Oberbeck, 1992, AGU Abstr., this volume.

      Marshall and Oberbeck, the previous abstract, discusses
terrestrial impact geology -- Ries crater Bunte Breccia, the Caribbean
region of Chicxlub (the K/T "smoking gun") and an area in Australia.