[meteorite-list] [Announced June 8, 2010] NWA 5400: Earth-Related Meteorite

From: Richard Kowalski <damoclid_at_meteoritecentral.com>
Date: Tue, 28 Sep 2010 23:52:22 -0700 (PDT)
Message-ID: <470065.550.qm_at_web113610.mail.gq1.yahoo.com>

I think there is a misunderstanding on what the CRE dating represents.

As Shawn pointed out, the abstract clearly states that "The meteorite shows a large 129Xe excess (~3?10-10 cm3/g) derived from 129I decay (T1/2 = 16 Ma), indicative of its formation very early in Solar System history."

That is a clear statement that the material is over 4 billion years old.

The Cosmic Ray Exposure (CRE) denotes the period of time the material spent in space *after* a disruptive event.

IOW, a parent body, very possibly formed during the impact event that formed the moon, was disrupted at some time in the past. The material from this parent body that eventually came to earth and became this meteorite has spent between 3 and 57 million years in space, which is typical of brachinites. Note the range of 54 million years. There are a number of variables in determining the CRE. Part of it is estimating the cosmic ray flux over that period of time and the other is the original depth of the material in the parent body and the diameter of the meteoroid that eventually landed on earth's surface.

Cosmic rays penetrate rock only several kilometers, so a parent body with a diameter as little as two or three times this distance could effectively shield large quantities of its interior from CRE over billions of years.

Such a body could last in a Main Belt like orbit for billions of years, be disrupted some time between 3 & 57 million years ago, sending fragments into earth crossing orbits, eventually bringing meteorites like NWA 5400 to earth's surface.

A formation age of 4.5+ billion years and a CRE of only 3 -57 million years are perfectly consistent with each other for asteroidal meteorites.

--
Richard Kowalski
Full Moon Photography
IMCA #1081
      

Received on Wed 29 Sep 2010 02:52:22 AM PDT