The isotopic compositions of elements are often used to characterize nucelosynthetic contributions in early Solar System objects. Coordinated multiple middle-mass elements with differing volatilities may provide information regarding the location of condensation of early Solar System solids. Here we detail new procedures that we have developed to make high-precision multi-isotope measurements of chromium and calcium using thermal ionization mass spectrometry, and characterize a suite of chondritic and terrestrial material including two fragments of the Chelyabinsk LL-chondrite

Mills, R. D.

Simon, J. I.

Tappa, M. J.

Ware, B.

NASA Technical Reports Server

A PROCEDURE TO DETERMINE THE COORDINATED CHROMIUM AND CALCIUM ISOTOPIC 
COMPOSTION  OF  ASTROMATERIALS  INCLUDING  THE  CHELYABINSK  METEORITE.  M. J. 
Tappa
1
, R. D. Mills
2
, B. Ware
3
, and J. I. Simon
2
. 
1
Jacobs, NASA-Johnson Space Center, Houston, TX 77058 (mi-
chael.j.tappa@nasa.gov), 
2
Center for Isotope Cosmochemistry and Geochronology, NASA-ARES, Johnson Space 
Center, Houston TX 77058, 
3
Dept of Applied Geology, Curtin University, Perth, WA 6845, Australia 
 
 
Introduction: The isotopic compositions of ele-
ments are often used to characterize nucelosynthetic 
contributions in early Solar System objects. Coordinat-
ed multiple middle-mass elements with differing vola-
tilities may provide information regarding the location 
of condensation of early Solar System solids. Here we 
detail new procedures that we have developed to make 
high-precision multi-isotope measurements of chromi-
um and calcium using thermal ionization mass spec-
trometry, and characterize a suite of chondritic and 
terrestrial material including two fragments of the 
Chelyabinsk LL-chondrite. 
Analytical Procedure: Powders of whole-rock 
samples  (Chelyabinsk, Allende) and silicate mineral 
separates (Akermanite NMNH 103104, Augite NMNH 
164905) were dissolved in concentrated HF+HNO3 
using Parr digestion vessels heated to 180°C for 48 to 
72 h. Chromite separates (Lundsgård L6 chondrite, 
Winona primitive achondrite, NMNH 117075) were 
dissolved in loosely-capped precleaned 3mL Savillex 
beakers using HCl+HNO3 (3:1) placed within Parr di-
gestion vessels and heated to 180°C for 48 h. All sam-
ples were visually inspected using a microscope to 
ensure complete dissolution. If undissolved material 
remained the procedure was repeated.  
Chemical separation of Cr: Dissolved Cr sample 
solutions, either Cr eluted from the TODGA columns 
described below or the dissolved chromite samples 
(not processed for Ca) are evaporated, dissolved in 
HCl and fluxed at 180°C for 12 h to ensure complete 
reduction prior to chemical separation. Cr is separated 
via a two-stage micro column procedure based off pre-
vious work [1]. The 1
st
 stage anion exchange resin 
(Bio-Rad AG1-X8 200-400 mesh) separates Fe, the 2
nd
 
stage cation exchange resin (Bio-Rad AG 50W-X8 
200-400 mesh) purifies Cr. The combined Cr yield of 
this procedure is ~85%. 
Chemical separation of Ca: Following dissolution, 
sample solutions are evaporated, and concentrated 
HNO3 was added to samples dissolved using HF. This 
solution is ultrasonicated, fluxed, evaporated, and re-
peated 3 times to ensure no precipitation of fluoride 
compounds. Samples are then dissolved in dilute 
HNO3 and processed using a single-step procedure 
using Eichrom TODGA resin cartridges and a vacuum 
box. The method, developed using the elemental dis-
tribution coefficients from
 
[2], was designed to specifi-
cally separate and purify Ca while also maintaining 
high Cr yield. Ca yield from this column is consistent-
ly >95% for the wide range of sample compositions 
investigated (including a range of whole-rock compo-
sitions, oxides, refractory ceramics, Fe-Mg silicates, 
and framework silicates). 
Isotope Analysis:  All isotope measurements are 
made using a Thermo Scientific TRITON mass spec-
trometer housed in the Center for Isotope Cosmochem-
istry and Geochronology, ARES, NASA-JSC. 
Cr isotope measurements: Cr samples are loaded in 
dilute HCl on outgassed single Re filaments along with 
silica gel and saturated boric acid. Sample load sizes 
are generally estimated to be 1 µg, though some exper-
imentation was conducted using smaller load sizes. Cr 
is analyzed using a multi-line method modified from 
[1]. This method is preferred over a static analysis due 
to the ability to monitor Faraday cup degradation. Each 
sample was loaded on 2-3 filaments and each filament 
was analyzed 2-3 times per session. A single run con-
sists of 21 blocks of 10 ratios through 4 cup settings 
for a total of 840 ratios per analysis. Virtual amplifier 
rotation and baseline calibration is preformed preced-
ing each block. The 
52
Cr beam intensity is normally 
between 5-10 V using a 10
11
 Ω resistor. Instrumental 
mass fractionation is corrected using an exponential 
law and a 
50
Cr/
52
Cr value of 0.051859 [3]. Isobaric 
interfering elements are monitored on 
49
Ti, 
51
V, and 
56
Fe, respectively, using an ion counter prior to the 
analysis. Normally, only 
56
Fe requires a correction. 
Ca isotope measurements: Purified Ca is dissolved 
in dilute HCl and loaded in 2-3 µg aliquots with dilute 
H3PO4 on outgassed Re filaments. A Parafilm dam is 
applied to the filament prior to loading the sample in 
an effort to reduce spreading and improve the con-
sistency of loading. Filaments are slowly heated to 
evaporate loading acid and residual Parafilm. Ca is 
analyzed using a multi-dynamic method, which is pre-
ferred to better understand cup degradation effects. 
One standard (SRM 915a) is run for ~ every 2 un-
knowns; normally 3 standards per barrel. A single run 
consists of 7 blocks of 15 ratios through four cup set-
tings. Virtual amplifier rotation and baseline calibra-
tion are preformed preceding each block. The 
40
Ca 
beam intensity is usually between 20-30 V using a 10
11
 
Ω resistor. Instrumental mass fractionation is corrected 
using an exponential law and a 
42
Ca/
44
Ca value of 
https://ntrs.nasa.gov/search.jsp?R=20140012794 2019-08-31T18:52:53+00:00Z
0.31221 [4]. Isobaric interfering elements are moni-
tored on 
39
K and 
47
Ti during the analysis, and a correc-
tion is made for K.  
Data reduction: For each element, instrumental 
mass fractionation, and interference corrections for Fe 
and K, respectively, are performed during acquisition 
using Thermo software. For both 
53
Cr/
52
Cr and 
54
Cr/
52
Cr ratios, the value for a single run is calculated 
as the cycle average of the four lines and the sample 
mean is taken as the average of all of the analyses dur-
ing a session. For Ca, we empirically determined that 
the exponential law does not properly account for in-
strumental mass fractionation outside of a narrow 
range (0.315-0.310) of 
42
Ca/
44
Ca values, which is simi-
lar to what has been found by others [5]. To account 
for this poor correction, we exclude data collected dur-
ing cycles that yield values outside of this range. For 
ratios measured on multiple lines, 
40
Ca/
44
Ca and 
43
Ca/
44
Ca, we perform an offline multidynamic calcu-
lation which we report as the mean for a run. The nor-
malized cycle values of the remaining Ca ratios 
(
46
Ca/
44
Ca, 
48
Ca/
44
Ca) are reported as the mean. For 
each sample we report the weighted mean of n repli-
cates for each ratio, and 2σ uncertainty on the weighted 
mean. All ratios are presented in epsilon notation (ε) 
which is calculated as the deviation from session-
averaged standard value in parts per 10,000.  
Cr isotopic results: Two chromium standards, in-
house legacy standard Cr Std 361 and SRM 3112a, 
were run in each session. Due to commercial availabil-
ity and more widespread use, SRM 3112a was used as 
the reference here. The results we report here for the 
Chelyabinsk, Lundsgård, and Allende samples agree 
well with data previously reported [6-8]. Interestingly, 
an observable difference was measured between the 
two standards. We attribute this discrepancy to be the 
result of residual uncorrected isotopic fractionation, 
either produced during production of the standard [7], 
or during the analysis as has been observed in earlier 
studies [1,7,9]. 
Ca isotopic results: Recent comprehensive studies 
of oceanic basalts and peridotites [10,11] indicate an 
observable excess of 0.78ε between the ε40Ca value of 
the standard (SRM 915a) and that of Earth’s mantle, 
thus here we reference our samples to the mantle value 
[10,11]. 
Nearly all measured Ca ratios yield mantle values. 
Both samples of the Chelyabinsk meteorite yield posi-
tive ε40Ca values, even when accounting for radiogen-
ic ingrowth. These values agree well with previously 
reported values for other LL chondrites [11]; consistent 
with the material originating in a similar source region. 
All ε48Ca measurements, including those for Chelya-
binsk samples, are within uncertainty of the mantle. 
 
Figure 1: Cr data for all samples measured here (solid 
symbols) and comparative literature values (open sym-
bols). Gray box represents 2SD of SRM 3112a. 
 
Figure 2: Ca data for all samples measured here (solid 
symbols) and comparative literature values (open sym-
bols). Gray box represents 2SD of SRM 915a. Square 
symbols are corrected for radiogenic ingrowth. 
Acknowledgements: Chelyabinsk samples provid-
ed by the consortium [12]. 
References: [1] Trinquier A. et al. (2008) JAAS, 
23, 1565-1574. [2] Pourmand A. and Dauphas N. 
(2010) Talanta, 81, 741-753. [3] Shields W. R. et al. 
[1966] J. Res. Natl. Bur. Stand., 70A,193-197. [4] Rus-
sell W. A. et al. (1978) GCA, 42, 1075 [5] Caro G. et 
al. (2010) EPSL, 296, 124-132. [6] Trinquier A. et al. 
(2007) Astrophys. J., 655, 1179-1185. [7] Qin L. et al. 
(2010) GCA, 74, 1122-1145. [8] Popova O. P. et al. 
(2013) Science, 342, 1069-1073. [9] Lugmair G. W. 
and Shukolyukov, A. (1998) GCA, 62, 2863-2886. [10] 
Simon J. I. et al. (2009) Astrophys. J., 702, 707-715. 
[11] Mills R. D. et al. (2014) in prep. [12] Righter K. 
(2013) MAPS Supp.,76, 5235. 


A Procedure to Determine the Coordinated Chromium and Calcium Isotopic Composition of Astromaterials Including the Chelyabinsk Meteorite

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