An overview is presented on the efforts being made to estimate the time that the magnetic field(s) existed and acted on meteorite parent bodies. The basic premise is that if a sample exhibits a low K-Ar age, it is the requisite amount of Ar-40. This assumption is applied to a preliminary analysis of the Farmington chondrite

Rowe, M. W.

English

NASA Technical Reports Server

DURATION OF MAGNETIC FIELD(S ACtLG ON METEORITE PARENT BODY(S).
M. W. Rowe
Department of Chemistry
Texas A&M University
College Station, Texas 77843
Study of remanent magnetization in meteorites began 15 years ago when
Lovering and Stacey [1959] and Stacey and Lovering [1959] initiated their
studies. Although attempts have been made at estimating the intensities of
paleofields which acted on meteorite parent bodies [Stacey et al., 1961;
Weaving, 1962; Gus'kova, 1963; Banerjee and Hargraves, 1972; Brecher, 1972;
Butler, 1972], none have been directed toward estimating the temporal distri-
bution or duration of these fields. It is the purpose of this note to
undertake a preliminary attempt at estimating the time that the magnetic
field(s) existed and acted on meteorite parent bodies. Information necessary
for this preliminary analysis is available in the literature, although a
more careful check on the assumptions used will require further studies-both
on the diffusion of Ar40 and on the magnetic properties of meteorites.
For the present, I .will confine my comments to magnetic studies on
ordinary stony meteorites (chondrites) since recent studies indicate that
paleointensity determinations on carbonaceous chondrites may be more com-
plex than originally thought [Watson et al., 1974; Herndon et al., 1974] and
necessary data is not available on the iron meteorites. There are eight
chondrites which have had estimations of the intentsity of the ancient fields
implanting the magnetization in them: Brewster [Weaving, 1962], Farmington
and Mt. Browne [Stacey et al., 1961], and Mordvinoka, Ochansk, Pultusk,
Rakovka, and Zhovtnevyi [Gus'kova, 1963]. Of these, five have undergone
the necessary chronological studies, i.e., gas retention ages: Farmington,
Mt. Browne, Ochansk, Pultusk, and Zhovtnevyi [Zahringer, 1966]. K-Ar ages
reported woere o7- 4 1- 3-7- 31.9 and 4.0 Aeons (AE=109 years), respectively
for those meteorites.
'The essential argument is: If a sample exhibits a low K-Ar age,. I
assume that the sample was heated to a temperature high enough to drive off
the requisite amount of Ar40 . By examining the rare gas studies arbitrarily
40
selected from several laboratories on the thermal release of Ar from stone
meteorites and lunar samples, I make a crude estimate of the temperature
,jASA-CR-13 8 4 9 2 ) DUR ON OF - MAGNETIC 174-26286
FIELD(S) ACTING ON METEORITE PARENT
BODY(S) (Texas ASM Univ.) 4 p BC $4.00CSCL 03B Unclas
G3/30 40162
https://ntrs.nasa.gov/search.jsp?R=19740018173 2020-03-23T08:52:38+00:00Z
required to drive off a certain fraction of the Ar40 . After estimation of
the temperature reached, I examine the effect of this temperature on the
magnetization of the sample.
Of the five chondrites above which are dated, only Farmington is
much lower than the expected age of -4.6 AE with a K-Ar age of -0.7 AE.
Thus assuming a one-stage Ar40 loss I estimate that 287% of the Ar40 must
have been lost to result in such a low K-Ar age. To remove -87% of the
Ar40 from lunar samples by stepwise heating, temperatures of 1040 to 11000 C
were required in three independent studies [Alexander et al., 1972; Huneke
et al., 1972; Kaiser, 1972] and similar studies on four stone meteorites
required temperature of 750 to 12000 C [Reynolds and Turner, 1964;
Manuel et al., 1972; Miller et al., 1973]. The value of 7500 C stands alone;
all others were 10000C or more. I thus estimate from this data that a
temperature of -10000 C is generally necessary to remove 87% of the Ar40
from diverse samples in laboratory studies. This is obviously a long
extrapolation, i.e., from laboratory heating to parent body heating. But
Wood [1967] presented good petrologic evidence that Farmington had been
severely heated. Note, that the estimated temperature (-10000 C) is -2500 C higher
than the Curie point of iron. While more experimentation is obviously
desirable, new gas release studies via linear heating [Frick et al., 1973]
seem to support this as a reasonable means for estimating the temperature
40
necessary for Ar removal. For example, Frick et al. found that if a
sample is heated linearly to a certain temperature, T , allowed to cool
and once again linearly heated to even higher temperatures, the gas re-
lease exhibits the following characteristicc Upon the second heating,
40
only small amounts of addition Ar release are observed until about T is
reached, where the Ar40 release increases rapidly until the rate of Ar
release is virtually identical with the rate observed during the initial
heating. Gas release at temperatures below T during the second heating
m
is less than expected if the mechanism were purely classical diffusion loss.
Stacey, et al. [1961] demonstrated that the magnetization in Farmington
was carried primarily by nickel-iron (as it was in all chondrites studied).
Since the remanent magnetization in nickel-iron disappears at temperatures
> 770*C, no remanent magnetization is expected in Farmington since the
time of outgassing (-0.7AE), unless it re-cooled in the presence of a
magnetic field. Therefore I tentatively suggest that the Farmington
chondrite parent body was outgassed -0.7 AE ago (or more recently)
losing .87% of its total Ar4 0 and most likely reached at a temperature
of >770 0C, thus erasing whatever magnetic remanence was previously re-
corded. However, a magnetic field seems to have been present on the
Farmington parent body as recently as -0.07 AE ago in view of the rema-
nence found in the Farmington chondrite by Stacey, et al. Fig. 1 shows a
schematic history of the Farmington meteorite.
That a magnetic field was associated with the Farmington meteorite as
recently as -0.7 AE ago should not be taken as evidence that a steady
magnetic field existed in the early history of the solar system and extended
to at least 0.7 AE ago. As Stacey [1967] aptly stated, "While alternative
explanations of the primary magnetizations of chondrites cannot be finally
excluded, the most satisfying ones at the present time require a parent
body with a magnetic field. However, there is no experimental basis for
asserting that it was a steady (terrestrial-type) field, generated by
a well-developed core, and not a transcient field accompanying the break-up
of the body." Perhaps, in view of the difficulties involved in maintaining
an -4 AE long core on meteorite parent bodies, [Fish et al., 1960; Wood, 1967;
Fricker et al., 1970; Herndon and Rowe, 1973], a transcient field accompanying
the break-up of the parent body is more probable than a steady field.
Acknowledgements. This work was supported by the National Science
Foundation Grant No. GP-18716 and the National Aeronautics and Space
Administration Grant No. NGR-44-001-152.
REFERENCES
Alexander, Jr., E.C., P.K. Davis and R.S. Lewis, Science, 175, 417-419,
1972.
Banerjee, S.K. and R.B. Hargraves, Earth Planet.Sci. Letters, 17, 110-
119, 1972.
Brecher, A., Proc. of Sym. "On the Origin of the Solar System", Nice,
April 3-8, 260-273, 1972.
Butler, R.F., Earth Planet. Sci.Letters, 17, 120-128, 1972.
Fish, R.A., G.G. Goles and E. Anders, Astrophys. J., 132, 243-248,
1960.
Frick, U., H. Baur, H. Funk, D. Phinney, C. Schafer, L. Schultz and
P. Signer, Geochim. Cosmochim. Acta, Suppl. 4, 1987-2002, 1973.
Fricker, P.E., J.I. Goldstein and A.L. Summers, Geochim. Cosmochim.
Acta, 34, 475-491, 1970.
Gus'kova, E.G., Geomagn. and Aeron., 3, 308-312, 1963.
REFERENCES (Continued)
Herndon, J.M. and M.W. Rowe, Nature (Phys. Sci.), 244, 40-41, 1973.
Herndon, J.M., M.W. Rowe, E.E. Larson and D.E. Watson, to be submitted
to Earth Planet Sci. Letters, 1974.
Huneke, J.C., F.A. Podosek, and G.J. Wasserburg, Earth Planet. Sci.
Letters, 13, 375-383, 1972.
Kaiser, W.A., Earth Planet. Sci. Letters, 13, 387-399, 1972.
Lovering, J.F., Am. J. Sci., 257, 271-275, 1959.
Manuel, O.K., R.J. Wright, D.K. Miller and P.K. Kuroda, Geochim.
Cosmochim. Acta, 36, 961-983, 1972.
Miller, D.K., D.W. Efurd, R.H. Thompson, M.A. Reynolds, K. Sakamoto
and P.K. Kuroda, Personal communication, 1973.
Reynolds, J.H. and G. Turner, J. Geophys. Res., 69, 3263-3281, 1964.
Stacey, F.D., International Dictionary of Geophysics, Pergamon Press,
N.Y., 1141-1143, 1967.
Stacey, F.D. and J.F. Lovering, Nature, 183, 529-530, 1959.
Stacey, F.D., J.F. Lovering and L.G. Parry, J. Geophys. Res., 66,
1523-1534, 1961.
Watson, D.E., E.E. Larson, J.M. Herndon and M.W. Rowe, submitted to
Earth Planet. Sci. Letters, 1974.
Weaving, B., Geophys. J. Roy. Astron. Soc., 7, 203-211, 1962.
Wood, J.A., Icarus, 6, 1-49, 1967.
Zahringer, J., Meteoritika, 27, 25-40, 1966, in Russian.
4.7 AE 4.6 AE < 0.7 AE 10-20 m.y. <0.2m.y.
Formation PRESENT
of Elements (not to scale) TIME
Nucleo- Solid Bodies Meteorite
synthesis of Solar Major Parent
Ends System Form Planetary BodiesCollision, generally
MAGNETIC involving break
FIELDS WERE Farmington into meter
PRESENT HERE Parent Body sizes, but
Farmington /
broke up here
A MAGNETIC
FIELD EXISTED
Fig. 1. Schematic representation FIHERE, TOO.
of the history of the Farmington
meteorite. Time axis grossly
distorted.


Duration of magnetic field(s) acting on meteorite parent body(s)

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server