Since the 1980s, more than 20 thermally metamorphosed carbonaceous chondrites (TMCCs) have been found in Antarctica and in hot deserts. The petrology of TMCCs suggests that some C-type asteroids were heated and dehydrated after aqueous alteration. Besides, previous studies indicate that the conditions of thermal metamorphism experienced by these meteorites may have been quite variable. It reflects that metamorphism of the TMCCs was complex

Chan, Q. H. S.

Kebukawa, Y

Nakamura, T.

Nakato, A.

Zolensky, M. E.

NASA Technical Reports Server

MINERALOGY OF EXPERIMENTALLY HEATED TAGISH LAKE.   
A. Nakato1, Q. H. S. Chan2, T. Nakamura3, Y. Kebukawa4, M. E. Zolensky2, and Astromaterials Science Research 
Group1, 1JAXA/ISAS (nakato@planeta.sci.isas.jaxa.jp), 2NASA/JSC, 3Tohoku Univ., 4YNU. 
 
Introduction: Since the 1980s, more than 20 
thermally metamorphosed carbonaceous chondrites 
(TMCCs) have been found in Antarctica [e. g., 1, 2] 
and in hot deserts [3]. The petrology of TMCCs sug-
gests that some C-type asteroids were heated and de-
hydrated after aqueous alteration [e. g., 1]. Besides, 
previous studies indicate that the conditions of thermal 
metamorphism experienced by these meteorites may 
have been quite variable. It reflects that metamorphism 
of the TMCCs was complex [e.g., 4, 5].  
The Belgica group represents strongly heated 
TMCCs consisting of secondary silicates formed by 
decomposition of hydrous minerals during heating [3]. 
The estimated heating conditions for this group are 10 
to 103 days at 700 ºC to 1 to 100 hours at 890 ºC [6]. 
These relatively short heating durations of thermal 
metamorphism are inconsistent with internal heating 
by the decay of the short-lived radionuclide, 26Al and 
60Fe, and suggest that the heat source might be im-
pact-induced heating and/or solar radiation heating. 
Although the petrography of the Belgica group is ap-
parently similar to typical CM2 chondrites, they are 
also similar to the Tagish Lake carbonate-poor lithol-
ogy with respect to their bulk oxygen isotopic compo-
sitions and the chemical composition of the matrix. 
The oxygen isotopic compositions of the Belgica group 
lie on or close to the TF line and have lower δ16O val-
ues than CMs [7], and the matrix shows an enrichment 
in Si compared to CM chondrites [6] because of sapo-
nite enrichment in the matrix. To understand the for-
mation process of the Belgica group TMCCs, we have 
performed heating experiments on fragments of the 
Tagish Lake meteorite and observed the mineralogical 
changes as a function of temperature and duration of 
heating. 
Experimental procedures: X-ray computed to-
mography was obtained at the University of Texas to 
identify the Tagish Lake carbonate-poor lithology. The 
carbonate-poor lithology sample was then divided into 
eight fragments each weights approximately 100 mg. 
The heating experiments were performed at four dif-
ferent conditions: 600 ºC for 1 hour (hereafter 600 
ºC/1 h), 600 ºC/96 h, 900 ºC/1 h, and 900 ºC/96 h. 
Two Tagish Lake sample fragments were used for each 
experiment. During the heating period, the oxygen 
fugacity was kept at the IW buffer in order to repro-
duce the secondary iron-bearing minerals in the Bel-
gica group meteorites that contain both Fe and Fe2+.  
	 The Tagish Lake samples were weighed before 
and after the heating experiments. In addition, the 
mineralogical, structural, and chemical characteristics 
of the matrix were determined using synchrotron X-ray 
diffraction (XRD) analysis at KEK BL-3A and 
SEM/EDS installed in JAXA/ISA. Organic materials 
of the heating experiment products were studied using 
the STXM technique and XANES analysis at KEK 
BL-13A.  
Results: Tagish Lake heated at 600 ºC/1 h: The 
products show weight loss of about 11 % of the un-
heated Tagish Lake despite the heating duration. The 
constituent minerals of 600 ºC/1 h matrix obtained by 
XRD are magnetite, pentlandite, pyrrhotite, and olivine 
with high crystallinity. The absence of saponite sug-
gests that the matrix phyllosilicate has been already 
decomposed. Based on SEM observations, no signifi-
cant petrographic changes have occurred during the 
heating. The matrix consists of the Tagish Lake origi-
nal materials, which are the heterogeneous mixture of 
submicron and micron-sized mineral fragments, and 
framboidal magnetite that is common in the car-
bonate-poor lithology can be observed throughout the 
matrix. We could observe tiny compacted framboidal 
magnetite, which consists of less than 150 nm in diam-
eter particles.  
Tagish Lake heated at 600 ºC/96 h: The XRD pat-
terns of magnetite and saponite in the 600 ºC/96 h 
sample are wider and smaller than unheated and 600 
ºC/1 h samples, suggesting its low crystallinity. SEM 
observation shows a similar texture and petrography 
for the unheated and 600 ºC/1 h samples. Framboidal 
magnetite can be observed throughout the entire matrix, 
and the fine-grained framboidal magnetite consisting 
of less than 150 nm particles still survives.   
Tagish Lake heated at 900 ºC/1 h: The sample lost 
about 17 % of its original weight during heating. The 
constituent minerals determined by XRD is quite dif-
ferent from that in the samples heated at 600 ºC. Pent-
landite, which decomposes at 610 ºC, magnetite, and 
phyllosilicates were not detected. Instead of these 
common phases, Fe-Ni metal, troilite, and low crystal-
lized secondary silicates, olivine and pyroxene, domi-
nate in the matrix. Based on the SEM observation, 
troilite and metal were found as assemblage and also 
isolated grains. Although framboidal magnetite is 
rarely observed, aggregate of Mg, Si-bearing iron ox-
ide with partially-melted framboidal texture distributes 
throughout the matrix (Fig. a). Chemical composition 
of the iron oxide aggregate varies from the grain tex-
ture. Smooth and massive grains are iron oxide with 
several wt % of Mg. On the other hand, porous grains 
https://ntrs.nasa.gov/search.jsp?R=20160002403 2019-08-31T03:51:59+00:00Z
contain several wt % of Si in addition to the composi-
tion of smooth grains. Fine-grained, anhydrous silicate 
minerals, such as olivine or pyroxene, as a result of the 
heating experiment were not found in the sample ma-
trix at the SEM scale even the minerals were detected 
by XRD. 
Tagish Lake heated at 900 ºC/96 h: The weight 
loss of the matrix during heating is about 20 %, which 
is the largest sample loss observed among the other 
heating products. XRD result shows that the major 
components in the matrix are similar to that in the 900 
ºC/1h sample. Sharp and narrow diffraction patterns of 
secondary silicates suggest that these silicates are well 
developed and crystallized with increasing heating 
duration. No framboidal texture has been observed so 
far (Fig. b). Troilite and Fe-Ni metal aggregates domi-
nate in the matrix. The Mg, Si-bearing iron oxide as 
observed in the 900 ºC/1 h sample was rarely found in 
the aggregate. The microstructure of the matrix is more 
homogeneous and fine-grained than the other samples.  
Discussion: The weight loss of the samples during 
heating reflects dehydration of volatile-rich materials, 
like phyllosilicate. Decomposition of phyllosilicate 
was also detected as decrystallization, as suggested by 
the broad peak of XRD pattern even in the 600 ºC/1h 
sample. On the other hand, phyllosilicates have not 
been observed in the 900 ºC samples. Instead of phyl-
losilicate, secondary olivine and pyroxene are domi-
nant in the matrix. The crystallinity of secondary sili-
cates increases with increasing heating duration. 
Framboidal magnetite that is common in the Tagish 
Lake carbonate-poor lithology still survives at 600 ºC.  
All results taken together, significant mineralogical 
changes are only evident after heating at 900 ºC, and 
the main changes are reduction of magnetite to form 
Fe-Ni metal and the development of troilite. Consistent 
with previous experimental heating studies of hydrous 
minerals [8], our experiments demonstrate that the 
degree of mineralogical transformation in carbona-
ceous chondrites as a result of heating is more affected 
by temperature rather than the heating duration.  
The relative degrees of transformation of the 
Tagish Lake meteorite can be estimated as following; 
600 ºC/1 h ≦ 600 ºC/96 h < 900 ºC/1 h < 900 ºC/96 
h. These degrees are estimated by observation of min-
eralogical changes of opaque minerals and crystallinity 
of secondary silicates as a heating parameter based on 
this study. A comparison of the mineralogy of our ex-
perimental results to the Belgica group meteorites 
shows that the sample heated at 900 ºC reproduces the 
mineralogical and textural characteristics of the Bel-
gica group meteorites. According to our previous study 
[9], these meteorites contain partially transformed 
phyllosilicates with a fibrous texture, no secondary 
grains of olivine and pyroxene more than 100 nm in 
size, and Fe-Ni metal and troilite assemblage instead of 
magnetite. We will present result of XANES analysis 
and STXM observation of these heating products. The 
information of organics will provide additional insights 
into how the organics in heated chondrites evolves as 
function of time and temperature.  
 
 
Figure. BSE images of heated Tagish Lake. Tr: 
Troilite, M: Fe-Ni metal, IO: Mg, Si-bearing iron oxide. 
(a) Aggregate of Mg, Si-bearing iron oxide with par-
tially melted framboidal texture in the matrix of 900 
ºC/1 h sample. (b) Troilite and Fe-Ni metal aggregate 
in 900 ºC/96 h sample.  
Acknowledgement: We thank Mr. Jim Brook who 
provided the meteorite for analysis. We are grateful to 
Drs. Takahashi, Mase, Takeichi, Mr. Suga, and Ms. 
Miyamoto for supports of Synchrotron X-ray experi-
ments. 
References: [1] Kimura, M. and Ikeda Y. (1992) 
Proc. NIPR Symp. on Ant. Met., 5, 74-119. [2] 
Tomeoka K. (1989) Proc. NIPR Symp. on Ant. Met., 2, 
55-74. [3] Ivanova M. A. et al. (2004) 67th Met. Soc., 
#5013. [4] Nakamura T. (2005) J. of Min. and Pet. Sci., 
100, 260-272. [5] Nakato A. et al. (2009) 72nd Met. 
Soc., #5336. [6] Nakato A. et al. (2008) Earth, Planets 
and Space, 60, 855-854. [7] Clayton R. N. and Mayeda. 
T. K. (1984) Earth and Planetary Science Letters, 67, 
151-161. [8] Akai J. (1992) Proc. NIPR Symp. on Ant. 
Met., 5, 120-135. [9] Nakato A. et al. (2014) 45th LPSC. 
#2355. 


Mineralogy of Experimentally Heated Tagish Lake

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