Systematic Variation of Volatile Elements in a Petrologic Suite of R

46th Lunar and Planetary Science Conference (2015)
CHONDRITES. Rahat Khan, Naoki Shirai and Mitsuru Ebihara, Department of chemistry, Tokyo Metropolitan
University, Hachioji, Tokyo 192-0397, Japan ([email protected]).
able (Fig. 2). Variation in Zn abundances is not significant whereas Pb (0.33 – 0.64 ×CI), In (0.15 – 0.56
×CI), Bi (0.15 – 0.63 ×CI), Cd (0.03 – 0.81 ×CI) and
Tl (0.02 – 0.79 ×CI) vary over a wide range. Excluding
MIL 11207.8 and Y 793575.44, volatile elemental
abundances in RCs show a depleting trend with the
increasing degree of metamorphism (Fig. 2). Sometimes it is difficult to separate the petrologic types 3
from 4, or petrologic types 4 from 5/6, but petrologic
types 3 and 6 can easily be distinguished from their
volatile elemental abundances.
Fig. 1 Literature data-normalized Zn, Pb, In, Bi, Cd
and Tl abundances from replicate measurements of
Allende (A1~ A4) and the mean (n=4) value of our
study as well as compiled data [9] and purdue mean
data [10]. For normalization Chicago mean values [10]
were used for all elements, except for Pb [11].
CI-normalized abundances
Introduction: Chondrites are the most pristine extraterrestrial objects and representing the history of the
early stage of our solar system. In this study we have
analyzed 15 Rumuruti-like chondrites (RCs) covering
all petrologic types (3-6). From the previous bulk
chemical studies [e.g., 1], RCs are compositionally
relatively uniform regardless of their petrologic types
and their bulk chemical compositions are comparable
with those of ordinary chondrites (OCs), except for
moderately volatile elements (e.g., Zn and Se). Highly
volatile elemental abundances such as Pb, In, Bi, Cd
and Tl have not hitherto been analyzed in a petrologic
suite of RCs. These elements are strongly depleted in
chondritic meteorites [2] and often vary greatly [3]. The
unique factors about the cosmochemistry of volatile
elements are that they are thermally more labile [4] and
are important tool for explaining the nebular and(/or)
parent body processes [2-3]. In ordinary chondrites [5],
volatile elements abundances anti-correlate with the
petrologic types, which is not conspicuous in the case
of thermally metamorphosed Karoonda-like carbonaceous (CK) chondrites [6]. RCs and CKs both are the
highly oxidized chondrites in terms of their negligible
metallic Fe-Ni [1]. Presence of amphibole and biotite
indicates some degree of aqueous alteration [7] in RCs.
Selenium abundances in RCs [1] are comparable with
those of aqueously altered CM chondrites [8]. Due to
some similarities with different classes of chondritic
(i.e., OCs, CKs and CMs) meteorites, RCʼs parent
body must have a diverse formation history which can
be investigated by volatile elemental abundances. So,
the aim of this study is to present the accurate and precise volatile elemental abundances in a petrologic suite
of RCs to study their formation process(es).
Analytical methods: Isotope dilution inductively
coupled plasma mass spectrometry (ID-ICP-MS) was
used for the determination of Zn, Cd, In, Tl and Pb,
whereas for Bi, Pb/Bi ratio was used. Solvent extruction (for Tl) and anion exchange column chromatography (for Zn, Cd, In) were used for separating the
matrix elements. Accuracy and precision were ensured
by the repeated analysis of Smithsonian Allende powder (split/position: 22/6) and comparing our data with
those of previously published literature data (Fig. 1).
Our data are consistent with literature data [9-11] and
relative standard deviations (RSDs) are less than 3%
(n=4) for Zn, In, Cd and Tl whereas for Pb and Bi
RSDs were 9-15% (n=4).
Results and discussion: CI-normalized [12] Zn,
Pb, In, Bi, Cd and Tl abundances in RCs are quite vari-
PRE 95411.21 (R3)
ALH 85151.41 (R3.6)
Y 793575.44 (R3.8)
Y 983270.56 (R4)
A 881988.68 (R4)
MIL 07440.8 (R4)
LAP 03639.33 (R4)
Y 983720.81 (R4)
Y 983097.81 (R5)
LAP 04840.12 (R6)
MIL 11207.8 (R6)
Y 980702.61 (R6)
Y 980703.71 (R6)
LAP 02238.13 (R)
PCA 91002.64 (R3.8-6)
Fig. 2 CI-normalized Zn, Pb, In, Bi, Cd and Tl abundances in a petrologic suit of RCs.
Volatile elements fractionation. In Fig. 3, average
CI-normalized abundances for type 3, 4 and 5/6 are
plotted. LAP 02238.13 (R) and PCA 91002.64 (R3.86) are included in petrologic type 3, as their volatile
abundances are consistent with the type 3 RCs. MIL
11207.8 and Y 793575.44 were excluded from the re-
46th Lunar and Planetary Science Conference (2015)
spective mean values because of their unexpectedly
high and low volatile elements abundances, respectively. A clear fractionation of volatile elements among the
petrologic types is observed in Fig. 3.
CI-normalized abundances
contain lower amounts of low temperature condensates
than the outer portion of RC parent body. Consequently the outer portion (less metamorphosed) will contain
the higher amount of volatile elements. For such scenario, we need to consider a parent body of onion-shell
Fig. 3 CI-normalized average Zn, Pb, In, Bi, Cd and Tl
abundances in RCs of different petrologic types. Error
bars are due to the standard deviations.
Considering that the interior chips for all the RCs
were used in this study and that our data quality was
ensured by the repeated analysis of Smithsonian Allende powder, the systematic variations of volatile elements in RCs are neither due to the experimental artefact nor due to the terrestrial weathering. We have investigated a total of 15 RCs, which are about 10% of
total RCs inventory. So, we can confirm that the systematic variation of volatile elements with the petrologic types is an intrinsic chemical characteristic of
RCs, inherited from nebular and(/or) parent body process(es).
Nebular and(/or) parent body process(es). Thermal
labiality can provide a provisional explanation for the
distribution of volatile trace elements according to the
metamorphic grade. Radioactive decay (internal heating) and(/or) impact (external heating) can be a good
source of parent body heating for thermal metamorphism. Parent body heating can drive the volatile elements from the interior to the surface which could results the volatile elements fractionation shown in Fig. 3.
But the redistribution of volatiles requires the degassing followed by transportation. But, if the system was
not opened, transportation will not occur. Then we
need to invoke some nebular processes for a possible
explanation of volatile elements fractionation in RCs.
In the fractional condensation process, high temperature condensates are supposed to be depleted in
volatile elements while the low temperature condensates are comparatively enriched in volatile elements.
If the accretion and sequential condensation with the
temperature gradient were taken place simultaneously,
then the inner portion of the RCʼs parent body would
Fig. 4 CI-normalized average In abundances in RCs
and OCs [13-17] of different petrologic types.
Comparison with OCs. Volatile elements abundances in unequilibrated RCs are comparable with
those of unequilibrated OCs, but the volatile elements
abundances in equilibrated RCs are comparatively
higher than the equilibrated OCs (e.g., Indium in Fig 4).
Assuming a simultaneous accretion followed by condensation and an onion-shell structure for both the RC
and OC parent body, a lower accretion temperature of
RCʼs parent body compared with those of OCs is suggested.
Conclusion: Unlike refractory and moderately volatile elements in RCs, volatile elements vary by a factor of 2 to 40. Variations of volatile elements show
anti-correlation with the petrologic types. Sequential
condensations of high-temperature and lowtemperature condensates with simultaneous accretion
of RCs parent body are apparently conceivable for explaining the systematic variation of volatile elements.
A lower accretion temperature is suggested for RCs
compared with those of OCs.
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