Introduction: Gold Basin is a large L4 chondrite shower
that was discovered in 1995 in Mohave County, Arizona [1]. The extent of this strewn field is still
being determined, but it has already exceeded 200 sq. km [2]. The pre-atmospheric radius for the meteoroid
that produced this shower has been estimated to be 3-4 meters [3]. Additional studies of cosmogenic
radionuclides and noble gases taken from “eight samples of the Gold Basin
shower” suggested that, while some of these samples were subjected to a
two-stage exposure history, most were a one-stage exposure. [4].
During the UofA field effort to
determine the extent of the Gold Basin strewn field, the survey team recovered
other meteorites unrelated to the Gold Basin fall [e.g., Golden Rule, King Tut,
Temple Bar, White Hills]. The more
recent finds, White Elephant and Hualapai Wash, pose more of a problem in that
their exterior appearance is no different from the Gold Basin L4 stones. In fact, White Elephant is identical to Gold
Basin in all respects except terrestrial age, and was only recognized
serendipitously during isotopic age-dating tests [5]. The Hualapai Wash stone was “recognized” by this writer, but only
after the stone had been cut and polished as a favor to the finder, Mr. Donald
C. O’Keeffe. Until the cut and polished
surface revealed chondrules and a groundmass more recrystallized than expected,
I had assumed this weathered stone to be another Gold Basin. A type specimen was submitted to UCLA, that
Alan Rubin characterized as being L6 S4 W3 (Fa 24.6+/-0.1%).
Results and Discussion: A
survey was initiated by this writer that endeavored to purposely recognize or
recover more stones like Hualapai Wash, a meteorite from the Gold Basin area,
but unrelated to the Gold Basin L4 shower. Specimens of “Gold Basin” meteorites
from various collections would be evaluated as potential candidates for closer
inspection. Candidate stones were obtained from collections in the U.S.,
Canada, and Italy, but because of difficultly in verifying provenance, only
finds directly from the field were used in this study. In all, 140 fragments/individual stones,
totaling ~2kg (avg. wt. per stone ~15grams) were evaluated. Cutting and closer inspection was conducted
on 58 stones. A total of five candidate
stones were submitted for classification.
Table 1.
Field/Sample ID# - Fa (mol.wt.)
- Classification
---------------------------------------------------------------------------------
MRF-RSV-01a 24.1+/-0.4% L6
S5 W3
MRF-DCO-02a 24.0+/-0.4% L5
S4 W2
MRF-DCO-03a 24.7+/-0.3% L5
S3 W2
MRF-RSV-21.8 24.5+/-0.6% L6
S3 W2
MRF-RSV-02.1 24.5+/-0.4% L6
S4 W2
[Fig 1. Map of Gold Basin area,
Mohave Co., AZ.
The locations for the 140 finds
used in this survey]
Alan Rubin characterized all five
of the stones as being something other than L4. These five stones, totaling 200 grams, represent ~10% of the
sample population of finds inspected.
This begs the question, are these stones from overlapping strewn fields
or did these stones originate from a single meteoroid composed of chondritic
material comprising several petrologic grades and shock stages?
It would be a convenient explanation
to say that all these stones come from a genomict brecciated meteoroid, but
there has been no evidence shown to date that suggests the Gold Basin L4
meteorite is brecciated. Problematic is
that the majority of “Gold Basin” stones are of a size that would not exhibit
anymore than one petrologic grade or shock stage, were the transition
gradational. For all the several thousands of fragments recovered from the Gold
Basin shower, very few are even as large as 500 grams. The few that are larger
quickly become display pieces. Of
interest would be individual stones that, within, exhibit a variation of
petrologic grade or shock stage. These would be excellent candidate specimens
for further study.
Inspection of cut surfaces on
several of the sampled stones have shown a fabric of uniformly disseminated
metal grains in an even groundmass that transitions to a fractured groundmass
with deformation of metal grains forming thin veins of metal trending several
centimeters in length, corroborating observations by other researchers [6].
This deformation occurs in the two “L5” samples listed in Table 1. This could be interpreted as evidence that a
range of petrologic grades and shock stages exist for the “Gold Basin”
meteorite, at least ranging up to L5 and S4.
No evidence of a “transition” was observed in the “L6” samples.
Conclusions: A sampling of meteorite finds from the Gold
Basin area has produced stones with classifications that are different from the
type specimen of the Gold Basin L4 shower. This can best be explained as,
partly the result of overlapping strewn fields, as well as, partly the result
of a shower from a heterogeneous meteoroid. The occurrence of chondritic stones
which exhibit a transition from uniform to deformed metal grains, coupled with
an increase in petrologic grade, is the best evidence that a range of petrologic
grades and shock stages might exist for stones from the Gold Basin “L4
shower”. Although an effort should be
made to find additional “transitional” stones in order to better characterize
this “heterogeneous meteoroid”, the more conventional explanation of “overlapping
strewn fields” should be initially invoked in order to explain these non-L4 meteorites.
Even after a liberal adjustment for ablation of mass during
atmospheric entry, the present Total Known Weight (TKW) for the “Gold Basin L4
shower” cannot account for a 3-4 meter wide pre-atmospheric meteoroid. By necessity, the strewn field for this
shower will continue to enlarge, and as a result, will encompass even more overlapping
strewn fields. This should not be unexpected, since similar areas with intense
meteorite recovery (i.e., Lucerne Dry Lake) have resulted in a high ratio of
unpaired meteorites [7]. Given the similar
conditions of sparse vegetation and broad areas that are dominantly erosive,
the recovery of numerous meteorites should be occurring at Gold Basin, as it is
occurring elsewhere in the Mojave Desert [8].
If the Gold Basin L4 shower were absent from the area, there would still
be many unpaired meteorite specimens.
Acknowledgements: I wish to thank Alan Rubin for providing his
unpublished classification results.
References: [1] Kring D. A. et al. (1998) LPS XXIX, Abstract
#1526.
[2] Kring D. A. et al. (2000) LPI Contrib. No. 997, 44, Houston,
TX. [3] Kring
D. A. et al. (2001) MAPS 36 (submitted). [4] Welten K. C. et al.
(2001) LPS XXXII, Abstract #2110. [5] Kreigh J. (2001) pers.
comm. [6] Kring D.A. (2001) pers. comm. [7]
Verish R. S. et al. (1999) LPI Contrib. No. 997, 74, Houston, TX. [8]
Rubin A.E. et
al. (2000) Meteoritics & Planet. Sci., 35, A183.
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