Thermomagnetic analysis on all of the C3 and C4 chondrites, conducted under conditions of controlled oxygen fugacity, indicates the presence of a thermally unstable component in at least 5 of the C3 chondrites which upon heating results in magnetite production. This unstable component is most likely troilite (FeS). The presence of the unstable substance may affect the estimation of paleointensities in meteorites which contain it. Our results indicate that Grosnaja, Ornans, Kainsaz, Felix, and Warrenton are likely to be less complicated for paleointensity determinations than the other C3 chondrites. Both C4 chondrites should lead to reliable results

Herndon, J. M.

Larson, E. E.

Rowe, M. W.

Watson, D. E.

English

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(NASA-CR-141145) THER MOMAGNETIC ANALYSIS .N75- 14667
OF METEORITES. 3: C3 AND C4
CHONDRITES (Texas A&M Univ.) 15 p HC
$3.25 CSCL 03B Unclas
G3/91 05060
THERMOMAGNETIC ANALYSIS OF METEORITES, 3. C3 and C4 CHONDRITES
J. M. Herndon and M. W. Rowe
Department of Chemistry
Texas A&M University
College Station, Texas (USA)
and
E. E. Larson
Cooperative Institute for Research in Environmental Sciences
University of Colorado/NOAA
Boulder, Colorado (USA)
and
D. E. Watson
U. S. Geological Survey
Boulder, Colorado (USA)
https://ntrs.nasa.gov/search.jsp?R=19750006595 2020-03-23T00:27:57+00:00Z
Abstract - Thermomagnetic analysis on all of the C3 and C4 chondrites,
conducted under conditions of controlled oxygen fugacity, indicates the
presence of a thermally unstable component in at least 5 of the C3
chondrites which upon heating results in magnetite production. This
unstable component is most likely troilite (FeS). The presence of the
unstable substance may affect the estimation of paleointensities in meteor-
ites which contain it. Our results indicate that Grosnaja, Ornans, Kainsaz,
Felix, and Warrenton are likely to be less complicated for paleointensity
determinations than the other C3 chondrites. Both C4 chondrites should
lead to reliable results.
1. INTRODUCTION
This paper is the third in a series of reports of our saturation
magnetization vs. temperature (Js-T) analyses of samples of all the known
carbonaceous chondrites. We reported the results of our studies on samples
of the five C1 chondrites* in part 1 of this series [1]. The dominant
magnetic phase observed in the C1 chondrites was magnetic, for which we
obtained quantitative estimates ranging from 5 to 12 wt.% Fe30 4 for those
five meteorites.
In part 2 of this series we presented the results of our studies on
samples of all of the eighteen C2 chondrites [3]. Although all C2 chondrites
are chemically similar with one another, as based on whole rock elemental
analyses, we found considerable variation in the chemical state of iron in
these meteorites. This, of course, contrasts to the situation in the Cl
chondrites, where magnetite was the predominant state observed for iron.
In only five (Al Rais, Bells, Essebi, Kaba and Mokoia) of the eighteen
C2 chondrites was magnetite found to be the major magnetic phase. (A
second sample of Al Rais was found to contain metallic iron with about 6% Ni).
Metallic iron was observed in five samples (Al Rais, Crescent, Haripura,
Pollen and Renazzo). Eleven of the C2 chondrites, (including Crescent and
Pollen) were most strikingly characterized by a lack of magnetic minerals
(e.g.,<l% Fe304 ). Lacking a dominant magnetic phase, the thermomagnetic
behavior of those eleven C2 chondrites was characterized by a pronounced
increase in saturation magnetization which resulted from the formation of
magnetite from troilite during the experiment. In spite of their chemical
similarity, the thermomagnetic behavior of C2 chondrites was rather complex.
*As in Parts 1 and 2, we employ the classification scheme for chondrites of
Van Schmus and Wood [2].
The C3 chondrites appear to be considerably more heterogeneous within
their group than are either the Cl or the C2 chondrites. Only two C4
chondrites are known, the Coolidge and Karoonda meteorites, and we include
them with the C3's. In fact, Van Schmus [4] suggested that Karoonda
should be reclassified from C4 to C5. Since the C4 chondrites may be
derived from the C3 chondrites [2), it is reasonable to discuss them together.
Thus we present in this paper the thermomagnetic analyses of all thirteen C3
chondrites and both C4 chondrites.
The equipment and technique used to measure the saturation magnetization
as a function of temperature is the same as used previously in this series
[1,3] and is discussed in detail by Larson, et al. [5]. The apparatus
consists basically of a Cahn electrobalance, a furnace to heat the sample to
~800'C, and a moveable, 2500 - oersted permanent magnet. An essential feature
of the system is the fugacity control. To minimize oxidation or reduction
of the magnetic minerals that might occur during the experiment, H2 and CO2
gases are flowed past the sample: the oxygen fugacity at any moment can be
altered and monitored. The oxygen fugacity is monitored by measurement of
the electrical potential developed across a solid-state ceramic electrolyte
(yttria-doped zirconia) that is positioned adjacent to the sample. Typically
heating-cooling cycles to 6000 C or 8000C required ~45 minutes and 60 minutes,
respectively.
2. RESULTS AND DISCUSSION
In part 2, we found it convenient to arbitrarily subdivide the C2
chondrites into three groups in order to discuss the thermomagnetic behavior.
We can also benefit from such a subdivision within the C3 and C4 chondrites
according to criteria similar to that used for the C2 chondrites [3]: (A)
those meteorites characterized by their low initial magnetite content and
by pronounced magnetite production observed during the course of the
experiment, (B) those meteorites, thermomagnetically similar to C1 chondrites,
indicating magnetite as their major magnetic component and, (C) those
meteorites which do not readily lend themselves to classification in either
of the first two groups. Though we choose to subdivide C3 and C4 chondrites
according to the same criteria as the C2 chondrites, there are both subtle
and distinct differences observed within any group. We will discuss the
above mentioned groups separately, and in doing so, will compare and contrast
them to the thermomagnetic behavior observed in C2 chondrites in similar
subgroups.
Group A. The Js-T curves for those meteorites displaying magnetite
production during the course of the experiment are shown in Fig. la-f.
We begin our discussion with the thermomagnetic data for the Allende meteorite
which is shown in Figs. la and lb. The normal J -T curve for this meteorite
is shown in Fig. la with two complete thermal cycles indicated, both run in
the oxygen fugacity field for magnetite. The increase in saturation magnetiz-
ation upon heating is characteristic of the oxidation of troilite to magnetite
and pyrrhotite. Indeed, we observed this behavior in over half of the C2
chondrites [3]. Butler [6] and Banerjee and Hargraves [7] noted similar
behavior in their thermomagnetic studies (conducted under vacuum) of the
Allende meteorite. However, Butler attributed the increase in saturation
moment to a "high temperature homogenization" of Ni-Fe alloys of grossly
different composition. Butler stated, "Thermomagnetic analysis indicates that
the ferromagnetic minerals in Allende consist of 95 wt.% taenite containing
67% Ni plus 5 wt. % taenite with 36% Ni." To determine whether the increase
in saturation magnetization in our sample of Allende was the result of
magnetite formation as we had observed in the C2 chondrites or "high temperature
homogenization" of Ni-Fe alloys as proposed by Butler [6], we completed a
normal Js-T analysis on a powdered sample of Allende (Fig. lb). Following
4that run, we heated the same Allende powder to 7000C in an intentionally
produced reducing atmosphere and maintained those conditions for about
fifteen minutes. The broken curve in Fig. lb shows the high temperature
portion of the Js-T curve which was obtained following the reduction
process. The broken curve indicates that the magnetic component now in
Allende (after reduction) is metallic iron of low nickel content (< 2% Ni)
which was derived from the reduction of the magnetite (produced from oxidation
of FeS) and by reduction of any FeS which remained after the normal Js-T
cycle. The magnitude of the saturation moment of the high temperature reduced
component is greater than that which would have been produced from the Allende
sample if the initial saturation magnetization had resulted entirely from
magnetite. This experiment thus demonstrates that magnetite was produced
during the J -T run prior to reduction. Several characteristics of the broken
curve in Fig. lb are inconsistent with Butler's hypothesis. If metallic
iron with ~65% Ni was the result of the reduction (as expected from Butler's
proposal): (1) The Curie temperature would not be expected to be ~770 0 C
(the Curie point of pure iron) as we observed. 65% Ni in iron would yield
a Curie temperature of ~610 0 C. (2) A y - a transition in the high-temperature
portion of Fig. lb is expected from Butler's proposal. The lack indicates
iron with < 2% Ni. (See Al Rais data, Fig. 5 in Watson, et al. [3] or Fig. 3f
here for good examples of the y -- ca transition reflected in a Js-T plot).
The thermomagnetic behavior of samples of the Arch, Efremovka, Leoville
and Vigarano C3 chondrites (Figs. lc-f, respectively) is similar to that of
the Allende meteorite, indicative of no metallic iron component and being
characterized by an increase in saturation magnetization during the experiment.
Since these samples become more magnetic with heating, it is not possible here
to identify the components responsible for the initial magnetization.
As we did with the C2 chondrites [3], we can ascribe upper limits of
Fe304 content for those C3 chondrites illustrated in Fig. 1 on the basis
of saturation magnetization measurements at ambient temperature by assuming
that the initial saturation moment is due entirely to magnetite. For
Allende and Vigarano,determinations were made on aliquots taken from gently
crushed 50-200 mg samples. The other meteorite samples were measured as
small whole pieces. The upper limits of Fe304 content are shown in Table 1.
Of these five meteorites only Allende and Vigarano are falls. The relatively
high upper limits of magnetite content for Arch, Efremovka and Leoville may reflect
magnetite formation from terrestrial weathering. Similarly, the increase in
saturation magnetization for the finds may result in part from the reduction
of terrestrially acquired hematite (Fe20 3) to magnetite (Fe304 ) since the
samples were measured in the oxygen stability field for magnetite. The
increase in saturation moment in the Allende and Vigarano samples most likely
results from the oxidation of troilite during the experiment.
Group B. Our thermomagnetic data for samples of the Bali, Grosnaja and
Ornans C3 chondrites and the Karoonda C4 chondrite are shown in Fig. 2a-d,
respectively. The Js-T curves for all these meteorites are quite reversible
and their shape and Curie temperature (~5900C) indicate that magnetite is the
dominant magnetic phase. Similar curves were seen for all five Cl chondrites
and for five samples of the C2 chondrites [1,3]. Quantitative estimates of the
magnetite contents of these C3 chondrites, as determined from ambient temper-
ature saturation magnetization measurements are shown in Table 2. Also
included are similar estimates for the Cl and C2 chondrites for comparison
[1,3].. The Ornans and Karoonda data were obtained on aliquots taken from
gently crushed samples. The Bali and Grosnaja samples were measured as small
whole pieces. It should be noted that the grain sizes observed in the
Grosnaja meteorite are comparatively large and as a consequence our data may
not be representative of the meteorite as a whole. Error estimates recorded
6are based only on deviations observed when measuring a standard sample.
The reported error does not reflect the possibility that some of the
magnetization is affected by some magnetic component other than Fe304,
e.g. pyrrhotite, nor was any attempt made to take into account sample
inhomogeneity. We estimated in part 1 that for Orgueil , at least, pyrrhotite
contributes 7% or less to the total saturation moment.
Group C. The thermomagnetic data shown in Fig. 3a-f respectively, for
the Felix, Isna, Kainsaz, Lance, and Warrenton C3 chondrites and the Coolidge
C4 chondrite are all indicative of the presence of magnetite in addition to
metallic iron. Of these meteorites the Isna sample (Fig. 3b) and the Lance
sample (Fig. 3c) are the only ones showing pronounced magnetite formation during
the experiment with a suggestion of magnetite production appearing in the Felix
data as well (Fig. 3a). The metallic iron components in our samples of Kainsaz
(Fig. 3c) and Lance (Fig. 3d) contain < 2% Ni as indicated by the absence of a
lag in reacquisition of magnetization (no y ~ a transition) upon cooling as well
as the observed Curie temperature of ~770 0C. This low nickel content in the
metal is probably not representative of all of the metal in those two meteorites.
Because of the high saturation moment of metallic iron and the extreme sensitivity
of the thermomagnetic balance used, we were constrained to use quite small
samples (<1 mg). For that reason sample inhomogeneity is likely to be more
important. The low nickel content of the metal observed in the Kainsaz and
Lance samples suggests that this component may be of secondary origin, possibly
produced from the reduction of magnetite prior to accretion.
If as Van Schmus and Wood [2] believe, the C4 chondrites were derived from
the C3 chondrites, then we would perhaps expect that the unstable component may
well have been entirely altered to magnetite during the process of converting
7C3 to C4 material. We do not observe any unstable magnetic components
(i.e., no magnetite production upon heating) in the C4 chondrites but with
statistics involving only two samples, no conclusions can be drawn. It
would be of interest to examine other more numerous classes (the LL, L or
H chondrite groups) to see whether this suspected trend actually holds.
In view of the recent interest in paleointensity determinations on
carbonaceous chondrites [6-8], we note that the data presented here and
in paper 2 [3] suggest that considerable care be exercised in conducting
paleointensity studies on C2 and C3 chondrites. Obviously, those samples
containing the unstable component will be less suitable for paleointensity
determinations on the magnetite component than samples lacking the noted
magnetite production. The Ni-Fe component may yield valuable information;
however, considerable care in experiment design should be exercised in
regard toy - a recovery during remagnetization.
ACKNOWLEDGEMENTS
This research was supported in part by the National Aeronautics and
Space Administration Grants No. NGR-06-003-181 and NGR-44-001-152 and the
National Science Foundation Grant GP-18716. For supplying samples we are
grateful to the following: Dr. R. S. Clarke, Jr., U. S. National Museum,
Smithsonian Institution, Washington, D.C.; Dr. J. Fabries, Museum National
d'Histoire Naturelle, Paris; Dr. E. A. King, Jr., Department of Geology,
University of Houston; Dr. E. L. Krinov, Academy of Sciences, U.S.S.R.,
Moscow; Dr. C. B. Moore, Center for Meteorite Studies, Arizona State
University, Tempe; and Dr. E. Olsen, Chicago Museum of Natural History. We
benefitted from discussion with Dr. E. Olsen.
REFERENCES
[1] E. E. Larson, D. E. Watson, J. M. Herndon, and M. W. Rowe, Earth Plan.
Sci. Letters 21 (1974) 345.
[2] W. R. Van Schmus and J. A. Wood, Geochim. Cosmochim. Acta 31 (1967) 747.
[3] D. E. Watson, E. E. Larson, J. M. Herndon and M. W. Rowe, Earth Plan.
Sci. Letters, submitted (1974).
[4] W. R. Van Schmus, Meteorite Research, P. Millman (Ed.), Reidel, Dordrecht
(1969) 480.
[5] E. E. Larson, R. P. Hoblitt and D. E. Watson, Geophys. J. (1974) in press.
[6] R. F. Butler, Earth Plan. Sci. Letters 17 (1972) 120.
[7] S. K. Banerjee and R. B. Hargraves, Earth Plan. Sci. Letters 17 (1972)
110.
[8] A. Brecher, Sym. on Origin of Solar System, Nice, April 3-8 (1972) 260.
Table 1 Upper Limit Determination of Magnetite Content
of Some C3 Chondrites
Upper Limit for Sample Weight (mg)
Wt. % Fe3 04
ALLENDE
< 0.72 + 0.04 1.0.07
< 0.73 + 0.04 7.93
Weighted average,< 0.72 ± 0.04
ARCH
<2.20 + 0.15 2.97
**
EFREMOVKA
< 10.7 + 0.6 5.37
< 7.0 0.4 3.34
Weighted average < 9.3 - 0.6
LEOVILLE
< 7.5 + 0.5 2.64
6.7 + 0.4 2.50
Weighted average < 7.12 + 0.4
VIGARANO
< 8.5 + 0.5 3.20
< 7.1 + 0.4 1.33
< 9.1 + 0.5 2.95
Weighted average < 8.5 + 0.5
*
Error is analytical only and does not include any attempt to evaluate
sample inhomogeneity.
**Run on whole samples.
Table 2 Magnetite Content of the Bali, Grosnaja and Ornans
C3 Chondrites and the Karoonda C4 Chondrite. The
Magnetite Contents of the Cl Chondrites [1] and
Those C2 Chondrites [3] with Magnetite as the Prin-
cipal Magnetic Phase are Included for Comparison.
Fe304 (Wt. %) Sample Weight (mg)
Cl Chondrites [1]
Alais 5.3 ± 0.4 (Weighted Average of 11 determinations)
Ivuna 12.2 ± 0.9 ( " " " 5 " )
Orgueil 11.9 ± 0.8 ( " " " 7 " )
Revelstoke 7.2 ± 0.5 0.16
Tonk 9.4 ± 0.6 0.67
C2 Chondrites [3]
Bells 13.4 ± 0.8 (Weighted Average of 3 determinations)
Essebi 9.4 + 0.6 ( " " " 5 " )
Kaba 11.3 + 0.7 ( " " " 2 ")
Mokoia 4.1 ± 0.3 ( " " " 3 " )
C3 Chondrites
BALI 3.0 + 0.2 0.36
GROSNAJA 2.1 ± 0.1 3.42
1.9 ± 0.1 9.62
Weighted Av. 2.0 + 0.1
ORNANS 4.5 ± 0.3 0.85
4.3 ± 0.3 1.57
4.2 ± 0.3 2.59
Weighted Av. 4.3 ± 0.3
C4 Chondrites
KAROONDA 8.2 ± 0.5 0.97
7.6 ± 0.5 5.52
Weighted Av. 7.7 1 0.5
Error is analytical only and does not include any attempt to evaluate
sample inhomogeniety.
Run on whole samples.
ALLENDE ALLENDE
2-
JO(T)
o 200 400 600 800 0 200 400 600 8oo
EFREMOVKA LEOVILLE
Js(T) '
Jo
00 200 40 0(T0 80 0 200 400 600 800
VIGARANO
Jo
% 200 400 600 800
TEMPERATURE, 0C
Fig. I. Saturation magnetization vs. temperature (J -T) curves for four
C3 chondrites. Note the large increase in Aagnetization as the
samples cool, indicative of magnetite production. All of these
except Eframovka were subjected to two heating - cooling cycles
(broken curves). As with the Group A C2 chondrites [2], one cycle
is not adequate to allow complete production of magnetite. Five
of the eleven C2 chondrites show production of magnetite upon
heating (see also Fig. 3).
GROSNAJABALI 1
Js(T)
Jo
200 40 6000
0 200 400 600 800 ) 2000 0 0 800 800
ORNANS KAROONDA
Js(T)
Jo
0 -
0 200 400 600 800 200 400 600 800
TEMPERATURE, OC TEMPERATURE, OC
Fig. 2. Saturation magnetization vs. temperature (Js-T) curves for three
C3 chondrites and one C4 chondrite. Magnetite is quite dominant.
No evidence for significant magnetite production is indicated in
contrast to Figs. 1 and 3.
FELIX KAINSAZ
JS(T) 1
Jo
0 200 400 600 800 0 200 400 600 800
LANCE 1 WARRENTON
Js(T) 1
Jo
0 200 400 600 800 0 200 400 600 800
COOLIDGE
Js(T) -1
Jo
0200 400 600 800
TEMPERATURE, OC
Fig. 3. Saturation magnetization vs. temperature (Js-T) curves for fourC3 chondrites and the remaining C4 chondrite. Felix and Lance'
show evidence for slight magnetite production. No magnetiteproduction was seen in the Kainsaz and Warrenton C3 chondritesor in the Coolidge C4 chondrite.


Thermomagnetic analysis of meteorites. 3: C3 and C4 chondrites

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