The Growing Inventory of
3+
Ti -bearing
Results: L3 – L6 chondrites
Objects from the Solar Nebula
Some observations [9, 10] suggest that ordinary chondrites become more reduced
from grades 3.0 to 4 and oxidized from grades 4 to 5 while other work indicates nearconstant ƒO2s [11], leading to some uncertainty about the oxidation states and
temperatures of ordinary chondrites during metamorphism [12]. We are investigating
suites of L and LL chondrites to see how Ti valence varies with metamorphic grade,
possibly providing a new constraint on conditions of chondrite metamorphism. Results
for L chondrites are summarized in Fig. 5. More data are needed, but so far we have
found that the average valence of Ti in olivine decreases from 4.0±0.1 to 3.7±0.1 with
increase in grade from 3.0 through 3.6, and it is within 1s of 4 in the L3.8, L4 and L6
samples. Most of the Ti4+ is in the tetrahedral site, especially in the higher-grade
samples. Pyroxene valences are ~3.5 in grades 3.0 – 3.6 and range from 3.5 to 4 in
the L4 and L6 samples. As found in previous work [e. g., 4] there is no correlation
between Ti valence and FeO content in olivine or pyroxene.
Steven B. Simon1, Stephen R. Sutton1, 2 and Lawrence Grossman1, 3
1Dept.
of the Geophysical Sciences, 2Center for Advanced Radiation Sources (CARS), 3The Enrico Fermi Institute
The University of Chicago, Chicago, IL 60637
Results: Semarkona chondrules
Introduction
It has been known for some time that the Ti-rich phases
fassaite, hibonite and rhönite, found in coarse-grained
refractory inclusions, contain trivalent Ti, an indicator of
formation under very reducing conditions. The development of
Ti-XANES
(X-ray
absorption
near-edge
structure)
spectroscopy has allowed measurement of Ti valence to be
performed on relatively Ti-poor phases such as spinel and
olivine. These are important phases among early solar system
materials, and the valence of Ti in them can tell us whether
they formed in a reducing, nebular gas or under more
oxidizing conditions. We have investigated a wide variety of
materials for the occurrence of Ti3+. This report is a summary
of those findings. We have also recently begun an
investigation of the valence of Ti as a function of metamorphic
grade in L and LL chondrites, and some preliminary results are
reported.
Type I (FeO-poor olivine and pyroxene) and Type II (FeO-rich) chondrules in Semarkona (LL3.0) were
analyzed by XANES to see if they contain trivalent Ti [4]. This could help us determine whether the two
types derive from different precursors or different crystallization environments. Results are shown in
Fig. 2 below. Olivine in both chondrules shown contains Ti3+ but in different proportions and with
different spatial distributions. Grain OL8 in the Type I chondrule 18 (red arrow in Fig. 2a) is unzoned in
Fa but zoned with increasing Ti valence from ~3.0 in the core to ~3.6 at the rim (Fig. 2c). Grain OL5
(red arrow in Fig. 2b) in Type II chondrule 8 is strongly zoned in Fa but not in Ti valence (Fig. 2d). In
chondrule 18 there is a strong inverse correlation between grain size and Ti valence in the cores of
crystals (Fig. 2e). The systematics observed in the data for this chondrule are precisely what would be
expected for oxidation during crystallization. The Type II chondrules contain Ti3+ but generally in lower
proportions than the Type Is. The presence of Ti3+ in Type II chondrules suggests that their precursors
were reduced and that the valence of Ti is more resistant to resetting than that of Fe.
100 µm
Fig. 2a
En5
Methods
A
Samples were selected for analysis after characterization by
scanning electron microscope and electron microprobe.
Titanium K XANES spectra were collected using the
GSECARS X-ray microprobe in fluorescence mode, with a 3
µm X-ray beam. Results were determined following the
methods of [1, 2] and are reported as values from 3 to 4,
representing the average Ti valences of the analytical volume.
OL3
En3
OL1
En2
OL5
En7
OL8
OL9
Meso1-4
OL2
En11
En1
meso5
O4
En10
meso6
Dusty
meso5
OL8
meso4
OL1
meso1
En9
Chondrule 18
Type IAB
meso6 meso7
meso3
OL7
G1
Isolated refractory forsterite (RF) grains (RF has CaO>0.4 wt%
and Al2O3>0.2 wt%) and RF-bearing chondrules from two CM
chondrites were studied by [3]. At least 75% of the Ti is
trivalent in RF, whether it is found as isolated grains or in
chondrules; RF has higher Ti3+/Ti4+ ratios than fassaite from
refractory inclusions [3]. Trivalent Ti is also found in “typical”
olivine in chondrules, but in lower proportions than in RF [4].
OL4
OL3
meso7
Results: Refractory forsterite
meso2
En6
En4
Fig.2b
200 µm
Fig. 5
Semarkona
USNM 1805-18
Discussion
Type IIA
Type IAB
That RF might contain Ti3+ was suggested by [13], who calculated refractory
lithophile element contents of a melt that would be in equilibrium with RF. They
predicted enrichments of ~10-40xC for most elements, but Ti enrichments of 5065xCI would have to be inferred if it were only present as Ti4+. Our measurements
of Ti3+/Ti4+ in RF combined with an olivine/liquid D for Ti3+ of 0.355 interpolated
from Ds for V3+ and Sc3+ [14] indicate a liquid with 28xCI, well within the range
predicted by [13] for other refractory lithophiles, supporting their model for the
formation of RF.
Fig. 2c
Fig. 2d
Fig. 2e
Fig. 2. (a, b) Backscattered electron images of a Type IAB (a) and a Type IIA chondrule from the Semarkona LL3
chondrite, showing locations of XANES analysis spots. Brightness of a mineral is proportional to the average atomic
number of the elements in it. (c, d) Plots of mole % fayalite (Fa) and Ti valence as a function of distance across
single olivine grains in Semarkona chondrules. No correlation is observed between Fa content and Ti valence within
either grain. (e) Plot of Ti valence in phenocryst cores as a function of grain size.
Ti valence in AOA olivine
Fig. 1
This is a common type of inclusion found in many
carbonaceous chondrite groups. They have long been
interpreted as nebular condensates [5, 6] and the presence of
Ti3+ in them would support that idea. Analyses of olivine in
AOAs in two CV3 chondrites, Allende and Efremovka, show
that AOAs do contain Ti3+. As in chondrule olivine, Ti valence is
not correlated with Fa content. As shown in Fig. 3 (left), the Ti
in the relatively Fa-poor Efremovka olivine has a higher
valence (avg. 3.7±0.1) than that in the more fayalitic Allende
olivine (avg. 3.5±0.1). The presence of Ti3+ in both chondrule
and AOA olivine is consistent with a common source, or
perhaps related precursor assemblages, for these objects [7].
Fig. 1. Plot of normalized intensity of pre-edge peak vs. energy;
these parameters vary systematically as a function of valence and
coordination number. Mixing lines are calculated from endmember
values shown and are used to calculate valences in samples. Fig.
1 shows that almost all of the Ti in the isolated RF grains (open
squares) is trivalent, Ti in the chondrule RF (open circles) is 8090% trivalent, and that in forsterite-bearing inclusions is tetravalent.
Results: Forsterite-bearing refractory
inclusions
Fig. 3
Two of these objects (TS45, CG14) were investigated as a
possible source of RF grains because they contain abundant,
coarse, Ca-rich forsterite. It was found, however, as shown in
Fig. 1, that neither the olivine nor the pyroxene in these
inclusions contains Ti3+.
Results: Murchison inclusions
References
[1] Simon S. B. et al. (2007) GCA, 71, 3098–3118. [2] Farges F. et al.
(1997) Phys. Rev. B, 56, 1809-1819. [3] Simon S. B. et al. (2007) LPS
XXXVIII, Abstract #1892. [4] Simon S. B. et al. (2008) LPS XXXIX, Abst.
#1352. [5] Grossman L. & Steele I. (1976) GCA 40, 149-155. [6] Krot A.
et al. (2004) Chemie der Erde 64, 185-239. [7] Simon S. B. et al.
(2010) M&PS 45, A189. [8] Simon S. et al. (2009) LPS XL, Abst. #1626.
[9] McSween H. Y. & Labotka T. (1993) GCA 57, 1105-1114. [10]
Menzies O. et al. (2005) M&PS 38, 1023-1042. [11] Kessel R. et al.
(2004) M&PS 39, 1287-1305. [12] Huss G. et al. (2006) MESS II, 567586. [13] Pack A. et al. (2005) GCA 69, 3159-3182. [14] Kennedy A. K.
et al. (1993) EPSL 115, 177-195. [15] Simon S. B. et al. (2011) LPS
XLII, Abst. #1271. [16] Berry A. et al. (2007) Chem. Geol. 242, 176-186.
Results: Amoeboid olivine
aggregates (AOAs)
Fig. 4
Refractory inclusions in CM2 carbonaceous chondrites, such
as Murchison, are dominated by oxides, mainly spinel and
hibonite, unlike inclusions in CV3 chondrites, which are
silicate-rich. Results [8] for five typical Murchison samples are
shown in Fig. 4. Hibonite, spinel and pyroxene all contain Ti3+,
and there is a good correlation of average Ti valence in spinel
with that of coexisting pyroxene or hibonite. Pyroxene in
Murchison inclusions generally has much lower Ti, Sc and V
contents than fassaite in coarse-grained refractory inclusions,
suggesting different sources, but the Murchison pyroxene has
Ti3+/Ti4+ ratios within the range of fassaite from CV3 inclusions.
The lack of Ti3+ in Fo-bearing refractory inclusions (FOBs) is not understood. One
of the inclusions we studied is an isotopically fractionated (FUN) inclusion, a type
thought to have undergone evaporation, so it may have been exposed to oxidizing
conditions, but the other FOB we studied is not a FUN inclusion.
The occurrence of Ti3+ in Type I and Type II chondrules suggests that both types
had reduced precursors. The valence of Ti in chondrules does not reflect the nearIW conditions under which FeO-bearing olivine forms, and the Fa contents of
chondrule olivine do not reflect the reducing conditions (ƒO2≤IW-3) that stabilize
Ti3+ in chondrule-composition melts [15]. Either Fe attains redox equilibrium more
rapidly than Ti, or the Fe/Mg ratio of olivine and pyroxene can increase by inward
diffusion of Fe without disturbance of the Ti3+/Ti4+ ratio.
Measurements on L chondrites show that variation in Ti valence in olivine and
pyroxene persists to metamorphic grades at which Mg/Fe ratios are equilibrated.
The high proportions of tetrahedral Ti4+ indicate formation under anhydrous
conditions [16]. These results and the chondrule data show that the Ti valence of
olivine and pyroxene is not easily disturbed.
Conclusions
1. The occurrence of Ti3+ is not restricted to refractory inclusions; it is
also found in olivine-rich objects such as chondrules and AOAs.
2. The proportions of trivalent Ti in refractory forsterite and the resulting
crystal/liquid distribution coefficient for Ti yield calculated residual liquid
compositions with Ti contents within the range of other lithophile element
enrichments predicted by Pack et al. [13], supporting their model.
3. Ordinary chondrites retain Ti3+ at least through grade 6. The valence of
Ti is not as easily reset as that of Fe.
Acknowledgments
This work was supported by NASA Grant NNX08AE06G (LG). Portions of this
work were conducted at GeoSoilEnviroCARS (Sector 13), Advanced Photon
Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported
by NSF (EAR-0622171) and DOE-Geosciences (DE-FG02-94-ER14466).