[meteorite-list] Ordinary Chondrite Statistics

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:11 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C8698E578A_at_US-Torrance.mail.saic.com>

Hi All,

Thank you, Bernd, for your detailed breakdown of ordinary chondrite
finds and falls by type. There is a lot of interesting information
that can be gleened from such a table, provided you're careful about
interpreting the data. One thing people should be reminded of is
that the Antarctic meteorite totals do not take pairing into
consideration. (One need only look at the US Antarctic LL5 total
to realize something is amiss -- the vast majority of these LL5's
are all from the same fall.)

If you consider just the worldwide find totals for the various
ordinary chondrite classes (excluding Antarctica), they would be
ordered like this from rarest to most common:

H7 0000
L7 0001
LL7 0005
LL3 0016 (+subtypes)
H 0027
L 0031
LL4 0037
L3 0059
LL5 0062
H3 0119
LL6 0127
L4 0155
L5 0348
H6 0385
H4 0411
H5 0819
L6 0862

(Note: I'm not sure I understand the distinction between LL3
and LL3 (+subtypes) in Bernd's table. I've used the +subtypes
in the above ordering, figuring it includes all types of LL3's
excluding the exotic types like LL(L)3. But this may not be
right since the Japanese Antarctic value for LL3(+subtypes) is
less than its LL3 value.)

In the interests of making fair comparisons between the L's,
H's and LL's, I decided the best way to treat the H's and L's
without petrologic numbers was to add them in a weighted
distribution to the various types. This results in the
following new totals:

H7 0000 + 0.00 = 0000
L7 0001 + 0.02 = 0001
LL7 0005
LL3 0016 (+subtypes)
LL4 0037
L3 0059 + 1.28 = 0060
LL5 0062
H3 0119 + 1.85 = 0121
LL6 0127
L4 0155 + 3.37 = 0158
L5 0348 + 7.57 = 0356
H6 0385 + 5.99 = 0391
H4 0411 + 6.40 = 0417
H5 0819 + 12.75 = 0832
L6 0862 + 18.75 = 0881

Total: 3464

Converting to percentages:

H7 0.00%
L7 0.03%
LL7 0.14%
LL3 0.46%
LL4 1.07%
L3 1.73%
LL5 1.79%
H3 3.49%
LL6 3.67%
L4 4.56%
L5 10.28%
H6 11.29%
H4 12.04%
H5 24.02%
L6 25.43%
----------
Tot 100.0%

Grouped:

LL 7.13%
L 42.03%
H 50.84%

How do these find percentages compare with those for falls?
Interestingly different:

LL 0071 = 9.4%
L 0364 = 48.1%
H 0322 = 42.5%

We can see by comparing the falls to the finds that either a longer
lifetime or easier field recognition is favoring the H's over both
the L's and LL's. I wouldn't expect L's or LL's to weather faster
than H's -- if anything, the reverse. More likely it is a recognition
issue.

One hypothesis that could be explored is this: perhaps the L's
and LL's aren't found as often as their fall percentages would
predict because their average sizes are smaller than H's. Smaller
average size means smaller chance of being found. Perhaps this
size difference is due to the structural integrity of H's being
greater than L's or LL's (I don't know if this is true or not),
resulting in larger H meteorite fragments after passage through
the atmosphere than for L's or LL's. Or perhaps in the case of
the LL's it's their lower magnetism that makes them less likely
to be recognized as meteorites.

An interesting statistic would be the breakdown of meteorite finds
by method of discovery: plowed up (e.g. farming), visual recognition
based on exterior appearance (color, shape, texture), visual curiosity
leading to recognition based on magnetism, discovery with metal
detector, and so on.

Cheers,
Rob
Received on Tue 13 Nov 2001 02:58:41 PM PST