We have examined suites of heated Murchison (CM2) and Allende (CV3) samples heated in the range 400-1200 C, in a H2 atmosphere with a pressure of 10(exp 5) bar for periods of 1 to 4 weeks. We used a combination of X-ray diffraction, electron microprobe and TEM analyses to determine the mineralogy of these samples

Hiroi, T.

Lipschutz, M. E.

Zolensky, M. E.

English

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A, N94- 35477 LPsc 'vv 1567
MINERALOGY OF ARTIFICIALLY HEA rED CARBONACEOUS CHONDRITES
M.E. Zolensky 1, M.E. Lipschutz 2 and T. Hiroil; INASA Johnson Space Center, Houston, TX 77058;
2Dept. Chemistry, Purdue University, West Lafayette, IN 47907.
Abstract: We have examined suites of heated Murchison (CM2) and Allende (CV3) samples heated in
the range 400-1200°C, in a H 2 atmosphere with a pressure of 10.5 bar for periods of 1 to 4 weeks. We
used a combination of X-ray diffraction, electron microprobe and TEM analyses to determine the
mineralogy of these samples.
Introduction: Several carbonaceous (C) chondrites have been discovered which exhibit evidence of
having undergone aqueous alteration followed by a parent body heating event, possibly related to the
former process [1]. Our understanding of these heating events is limited by a paucity of information
concerning the compositional and mineralogical effects of C-chondrite heating under different
environmental conditions. Thus, we have been characterizing suites of C chondrites subjected to
laboratory heating under plausible parent body conditions. To date we have examined Murchison (CM2)
and Allende (CV3), which together represent the most abundant types of C chondrites. Both suites have
been heated in the range 400-1200°C, in a H2 atmosphere with a pressure of 10.5 bar for periods of 1 to 4
weeks. We used a combination of X-ray diffraction, electron microprobe and TEM analyses to determine
the mineralogy of these samples. This abstract complements another in this volume which describes the
mineralogy of the Murchison samples as determined by MiSssbauer spectroscopy [2], and earlier work on
their reflectance spectroscopy [3]. Akai has been performing complementary heating experiments on
samples maintained in a vacuum [41.
Murchison The CM2 ehondrites experienced extensive aqueous alteration, and they typically consist
predominantly of phyllosilicates (serpentines + chlorite), tochilinite, olivine, glass, pyrrhotite and minor
pyroxene and Fe-Ni metal. Olivine and orthopyroxene compositions huddle around Fo99 and En99 (see
Fig. 1). We observed the following mineralogical changes in heated Murchison samples (although we
examined samples at 100°C steps, only the most interesting changes are noted here).
400°C: Well-crystalline serpentine is now a minor phase; some has been replaced by a poorly
crystalline, so-called "intermediate phase" [4]. Tochilinite has been destroyed, some pyrrhotite
has been replaced by Fe-sulfate. Olivine is still a minor component of matrix.
500°C: Serpentines are entirely replaced by the intermediate phase.
600°C: Recrystallization (and coarsening) of matrix and chondrule rims is evident.
700°C: The abundance of Fe-Ni metal in the matrix has substantially increased.
800°C: The intermediate phase has now been replaced entirely by olivine. Olivine, now the most
abundant mineral (including matrix), has a heterogeneous composition, with a greatly reduced
population peak at F099; likewise orthopyroxene.
1000°C: Olivine compositions are evenly distributed between Fo99-Fo50; likewise for
orthopyroxene. Chondrules are beginning to recrystallize. Matrix olivines and Fe-Ni metal
grains continue to coarsen.
Allende The CV3 chondrites experienced minimal aqueous alteration (compared to the CMs), and they
typically consist predominantly of olivine, with minor pyrrhotite and/or troilite, pyroxene, Fe-Ni metal,
saponite, glass and refractory phases in CAIs. Olivine and orthopyroxene compositions bunch around
Fo55 and En55 (see Fig. 1). The mineralogical changes in heated Allende samples were not as dramatic as
those observed in Murchison, until 1200°C. The major observed mineralogical effect (until 1200°C) was a
progressive increase in the amount of Fe in olivine, beginning at grain rims and spreading core-ward with
increasing temperature, which suggests that the Fe was acquired from matrix phases.
800°C: All olivines have Fe-rich rims; no olivines are observed to be more Mg-rich than Fo92.
1000°C: Fe-Ni metal grains are very abundant in matrix. Chondrules are beginning to
recrystallize.
1200°C: The meteorite has completely recrystallized to coarse-grained olivine (Fo64_67), with
interstitial augite, chromite, Fe-Ni metal and rare FeS. Large vesicles are evident, presumably
from the volatilization of S.
https://ntrs.nasa.gov/search.jsp?R=19940030971 2020-06-16T11:03:32+00:00Z
1568 HEATED CARBONACEOUS CHONDRITES; Zolensky M.E. et al.
These mineralogical data are being compared to those for naturally heated C chondrites. We are also
examining the mineralogy of the Allende samples by Mt_ssbauer spectroscopy, and collecting their optical
reflectance spectra. These latter results will then be compared to optical reflectance data of asteroids, in
an effort to better understand their evolution.
References: [1] Ikeda (1992) NIPR Syrup. 5, 49; [2] Morris et al. (1994) LPSC.XXV, this volume; [3] Hiroi
¢t al. (1993) Science 261, 1016; Akai (1992) NIPR Syrup. 5, 120.
l O
ALLENDE 1200
O
ALLENDE 1000
1 I1!!_____.. i
| , ;ALLENDE 800 °
s - ALLENDE UNHEATED I I
_ __lie n n_ ill • a l_ ndlmJ l 
MURCHISON 800 °
IL__--._
455_N UNHEATED
o
100 90 80 70 60 50
Figure 1. Olivine analyses from unheated and heated samples of Murchison and Allende. Note the
aeiglat of the Fooo peak for unheated Murchison, which is shown here in a greatly abbreviated fashion.


Mineralogy of artificially heated carbonaceous chondrites

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