Bunburra Rockhole (BR) is the first recovered meteorite of the Desert Fireball Network. It was initially classified as a basaltic eucrite, based on texture, mineralogy, and mineral chemistry but subsequent O isotopic analyses showed that BR's composition lies significantly far away from the HED group of meteorites. This suggested that BR was not a piece of the HED parent body (4 Vesta), but other explanations could also account for the observed oxygen signatures. Possible scenarios include contamination by components from other bodies (chondrites or other achondrites) or that 4 Vesta may not be as equilibrated as hypothesized. After examining multiple pieces with different instruments (CT scans and x-ray maps), no obvious evidence of contamination was found. If BR is not from Vesta, a conundrum exists as no unusual features were found in mineral and bulk trace element chemistry as exist for other anomalous basaltic achondrites such as Ibitira or Asuka 881394. These meteorites have distinct petrological and geochemical characteristics, in addition to their anomalous O isotope compositions, that set them apart from eucrites. Thus, early results provided a somewhat ambiguous picture of BR's petrogenesis and parentage. To clarify the nature of the relationship, if any, between BR and eucrites, we have performed a correlated stable isotope and bulk chemical study of several lithologic fragments

Benedix, G. K.

Bevan, A. W. R.

Bland, P. A.

Franchi, I. A.

Friedrich, J. M.

Greenwood, R. C.

Mittlefehldt, D. W.

Perotta, G. C.

Sanborn, M. E.

Towner, M. C.

Yin, Q. - Z.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsBunburra Rockhole: Exploring the geology of a newdifferentiated basaltic asteroidConference or Workshop ItemHow to cite:Benedix, G. K.; Bland, P. A.; Friedrich, J. M.; Mittlefehldt, D. W.; Sanborn, M. E.; Yin, Q. - Z.; Greenwood,R. C.; Franchi, I. A.; Bevan, A. W. R.; Towner, M. C. and Perotta, G. C. (2014). Bunburra Rockhole: Exploring thegeology of a new differentiated basaltic asteroid. In: 45th Lunar and Planetary Science Conference, 17-21 Mar 2014,The Woodlands, TX, USA.For guidance on citations see FAQs.c© UnknownVersion: Version of RecordCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukBUNBURRA ROCKHOLE: EXPLORING THE GEOLOGY OF A NEW DIFFERENTIATED BASALTIC ASTEROID.  G.K. Benedix1,7, P.A. Bland1,7, J. M. Friedrich2,3, D.W. Mittlefehldt4, M.E. Sanborn5, Q.-Z. Yin5, R.C. Greenwood6, I.A. Franchi6, A.W.R. Bevan7, M.C. Towner1 and Grace C. Perotta2. 1Dept. Applied Geology, Curtin University, GPO Box U1987, Perth, WA, 6845 Australia (g.benedix@curtin.edu.au), 2Dept. Chem., Fordham University, 441 East Fordham Road, Bronx, NY 10458 USA, 3Dept. Earth & Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA, 4NASA/Johnson Space Center, Houston, TX, USA, 5Dept. of Earth and Planetary Sciences, University of California at Davis, Davis, CA 95616, USA, 6Planetary & Space Sci., The Open University, Milton Keynes, MK7 6AA, UK, 7Western Australia Museum, Locked Bag 49, Welshpool, WA, 6986, Australia.  Introduction:  Bunburra Rockhole (BR) is the first recovered meteorite of the Desert Fireball Network [1].  It was initially classified as a basaltic eucrite, based on texture, mineralogy, and mineral chemistry [2] but subsequent O isotopic analyses showed that BR’s composition lies significantly far away from the HED group of meteorites (fig. 1) [1].  This suggested that BR was not a piece of the HED parent body (4 Vesta), but other explanations could also account for the ob-served oxygen signatures. Possible scenarios include contamination by components from other bodies (chondrites or other achondrites) or that 4 Vesta may not be as equilibrated as hypothesized [3]. After examining multiple pieces with different in-struments (CT scans and x-ray maps), no obvious evi-dence of contamination was found.  If BR is not from Vesta, a conundrum exists as no unusual features were found in mineral and bulk trace element chemistry [4] as exist for other anomalous basaltic achondrites such as Ibitira or Asuka 881394 [5,6]. These meteorites have distinct petrological and geochemical characteris-tics, in addition to their anomalous O isotope composi-tions, that set them apart from eucrites.  Thus, early results provided a somewhat ambiguous picture of BR’s petrogenesis and parentage.  To clarify the nature of the relationship, if any, between BR and eucrites, we have performed a correlated stable isotope and bulk chemical study of several lithologic fragments. Samples and Analytical Methods:  Earlier O iso-tope [1] and bulk trace element studies [4] focused on analyzing the different lithologies characterized by different grain sizes to look at chemical variability. A 1.81g, slightly fusion-crusted, piece of BR (measuring a little over 2 cm in long dimension) was allocated to the Open University for follow-up oxygen isotope analyses. The aliquots analysed here have a dominant grain size, but overall contained mixed lithologies. The main piece was divided into 4 smaller pieces (labeled A, B, C, and D), from which further fragments were taken. O isotopic compositions were measured on two fragments of piece A (A and A/1) and three fragments of piece C (C/A/1, C/A/2, and C/B/3). After O analysis, four powders (A/1, C/A/1, C/A/2, C/B/3) and two uncrushed (A, B) pieces were sent on to Fordham Uni-versity for chemical analysis. Four of these were ana-lysed for major and trace elements and two for trace elements only. Two of these subsamples (A/1 and C/A/2) were analysed at UC Davis to investigate the Cr isotopic systematics.  Homogenized powders from the two chips were dissolved in Parr bombs for a 96 hour period at 200°C to insure complete dissolution of re-fractory minerals such as spinel.  Cr separation was completed following the methods described in [7].  High-precision Cr isotope ratios were obtained using the Thermo Triton-Plus TIMS at UC Davis with the 54Cr/52Cr ratio expressed as ε-notation (parts per 104) deviation from a terrestrial standard. Oxygen isotope analysis was undertaken by laser fluorination using the methods described in [8]. Sys-tem precision, as determined on an internal obsidian standard is: ±0.05‰ for δ17O; ±0.09‰ for δ18O; ±0.02‰ for Δ17O (2σ). Δ17O values were calculated using the linearized format of [9], with λ = 0.5247.  All error values listed below are 2σ. Bulk trace and major elements were analysed fol-lowing the methods described in [10, 11] respectively. Results and Discussion:  Stable isotopes The oxygen isotope compositions of 25 different fractions (n = 42) of BR are shown in Fig. 1. The mean Δ17O of BR is -0.127 ± 0.044‰ and the mean δ18O is 3.98 ± 0.22‰[12].  This is signifi-cantly different from the HED mean Δ17O value of -0.246 ± 0.014 [3].  The five pieces for which bulk chemistry has also been determined have a much tight-er mean Δ17O of  -0.133 ± 0.016‰, at the upper end of the δ18O range for BR (4.09 ± 0.16‰).  The measured ε54Cr for sample A/1 is -0.37 ± 0.11 and for C/A/2 is -0.35 ± 0.08, identical within error.  These values are resolvably different from the average ε54Cr value range observed in eucrites of -0.7ε (Fig. 2). The stable isotope systematics indicate that BR does not come from the same parent asteroid as eu-crites. Interestingly, BR plots in the same field with Asuka 881394 [13] in the Δ17O-ε54Cr isotopes space – indicating they may be sampling the same differentiat-ed body.  1650.pdf45th Lunar and Planetary Science Conference (2014)Bulk composition Like previous studies [3], BR ap-pears similar in chemistry to eucrites. Bulk trace ele-ments for all follow a volatility pattern (Fig. 3) very similar to basaltic and polymict, and distinct from cu-mulate, eucrites. Further, siderophile element contents in BR are low, at nearly the same level as in basaltic eucrites.  Bulk rare earth elements show a relatively flat pat-tern with a slight negative Eu anomaly for all samples, except piece C/B/3, which has a slight LREE enriched pattern without the Eu anomaly.  Concentrations are 10-15xCI.  The BR mode is dominated by plagioclase and thus the negative Eu anomaly may be evidence of earlier fractionation of a plagioclase rich component prior to formation of BR melts, similar to the case for lunar basalts.   The distinct O and Cr isotopes suggest that BR is sampling a different differentiated body to the HED parent body (4 Vesta).  The bulk trace elements and low siderophile concentrations argue against contami-nation.  BR is a jumble of lithologic fragments from a differentiated parent asteroid which may be the same body represented by Asuka 881394. This implies that they have been mixed on a fine scale, most likely by impact.  The next step is to determine the relationship be-tween BR and A881394 and their possible parent body and to seek a better understanding of how trace ele-ment and isotope geochemistry co-varies with the ob-served variation in oxygen isotopic composition. References: [1] Bland P. A., et al. (2009) Science 325, 1525–1527 [2] Weisberg M. K., et al. (2009) Me-teoritics and Planet. Sci. 44, 429-462 [3] Greenwood et al. 2014 accepted EPSL. [4] Spivak-Birndorf L.J. et al. (2014) accepted Meteoritics and Planet. Sci. [5] Mittlefehldt D. (2005) Meteoritics and Planet. Sci. 40, 665-677. [6] Scott E. R. D., et al. (2009) GCA 73, 5835–5853. [7] Yamakawa et al. (2009) Analytical Chem. 81, 9787-9794  [8] Miller et al. (1999) Rapid Comm. Mass Spec. 13, 1211-1217. [9] Miller (2002) GCA 66, 1881-1889. [10] Friedrich J. M. et al. (2003) GCA 67, 2467-2479. [11] Wolf S. F. et al. (2012) Ta-lanta 100, 276-281.  [12] Benedix et al. (2013) Abs-tract#5142. MetSoc 76. [13] Sanborn M.E. et al. (2014) 45th LPSC. [14] Greenwood et al. (2005) Na-ture 435, 916-918.  [15] Trinquier et al. (2008) ApJ 655, 1179-1185. [16]Mittlefehldt D. (2014) EPSL submitted.    Fig. 1.  Δ17O vs δ18O for BR.  Also shown are the HED [4] and angrite fractionation lines [14].  The five pieces analysed for bulk composition in this study are shown in coloured symbols Grey boxes show the 2σ variation for the HED and angrite data [3,14]. Anomalous eu-crite data from [6].    Fig. 2.  Δ17O vs ε54Cr for BR (red symbols) and normal eucrites (green symbols).  Data for eucrites from [15] and [7] for ε54Cr and Δ17O, respectively.  Fig. 3.  Bulk trace lithophile elements (in order of in-creasing volatility) in different pieces of BR compared to average compositions for eucrites [data from 16].   1650.pdf45th Lunar and Planetary Science Conference (2014)

Bunburra Rockhole: Exploring the geology of a new differentiated basaltic asteroid

Yin, Q.-Z.

Benedix, G.K.

Greenwood, R. C

Franchi, L. A.

Perotta, Grace C.

Mittlefehldt, D.

NASA Technical Reports Server

BUNBURRA ROCKHOLE: EXPLORING THE GEOLOGY OF A NEW DIFFERENTIATED BASALTIC 
ASTEROID.  G.K. Benedix1,7, P.A. Bland1,7, J. M. Friedrich2,3, D.W. Mittlefehldt4, M.E. Sanborn5, Q.-Z. Yin5, 
R.C. Greenwood6, I.A. Franchi6, A.W.R. Bevan7, M.C. Towner1 and Grace C. Perotta2. 1Dept. Applied Geology, 
Curtin University, GPO Box U1987, Perth, WA, 6845 Australia (g.benedix@curtin.edu.au), 2Dept. Chem., Fordham 
University, 441 East Fordham Road, Bronx, NY 10458 USA, 3Dept. Earth & Planetary Sciences, American Museum 
of Natural History, New York, NY 10024, USA, 4NASA/Johnson Space Center, Houston, TX, USA, 5Dept. of Earth 
and Planetary Sciences, University of California at Davis, Davis, CA 95616, USA, 6Planetary & Space Sci., The 
Open University, Milton Keynes, MK7 6AA, UK, 7Western Australia Museum, Locked Bag 49, Welshpool, WA, 
6986, Australia. 
 
Introduction:  Bunburra Rockhole (BR) is the first 
recovered meteorite of the Desert Fireball Network [1].  
It was initially classified as a basaltic eucrite, based on 
texture, mineralogy, and mineral chemistry [2] but 
subsequent O isotopic analyses showed that BR’s 
composition lies significantly far away from the HED 
group of meteorites (fig. 1) [1].  This suggested that 
BR was not a piece of the HED parent body (4 Vesta), 
but other explanations could also account for the ob-
served oxygen signatures. Possible scenarios include 
contamination by components from other bodies 
(chondrites or other achondrites) or that 4 Vesta may 
not be as equilibrated as hypothesized [3]. 
After examining multiple pieces with different in-
struments (CT scans and x-ray maps), no obvious evi-
dence of contamination was found.  If BR is not from 
Vesta, a conundrum exists as no unusual features were 
found in mineral and bulk trace element chemistry [4] 
as exist for other anomalous basaltic achondrites such 
as Ibitira or Asuka 881394 [5,6]. These meteorites 
have distinct petrological and geochemical characteris-
tics, in addition to their anomalous O isotope composi-
tions, that set them apart from eucrites.  Thus, early 
results provided a somewhat ambiguous picture of 
BR’s petrogenesis and parentage.  To clarify the nature 
of the relationship, if any, between BR and eucrites, 
we have performed a correlated stable isotope and bulk 
chemical study of several lithologic fragments. 
Samples and Analytical Methods:  Earlier O iso-
tope [1] and bulk trace element studies [4] focused on 
analyzing the different lithologies characterized by 
different grain sizes to look at chemical variability. A 
1.81g, slightly fusion-crusted, piece of BR (measuring 
a little over 2 cm in long dimension) was allocated to 
the Open University for follow-up oxygen isotope 
analyses. The aliquots analysed here have a dominant 
grain size, but overall contained mixed lithologies. The 
main piece was divided into 4 smaller pieces (labeled 
A, B, C, and D), from which further fragments were 
taken. O isotopic compositions were measured on two 
fragments of piece A (A and A/1) and three fragments 
of piece C (C/A/1, C/A/2, and C/B/3). After O analysis, 
four powders (A/1, C/A/1, C/A/2, C/B/3) and two 
uncrushed (A, B) pieces were sent on to Fordham Uni-
versity for chemical analysis. Four of these were ana-
lysed for major and trace elements and two for trace 
elements only. Two of these subsamples (A/1 and 
C/A/2) were analysed at UC Davis to investigate the Cr 
isotopic systematics.  Homogenized powders from the 
two chips were dissolved in Parr bombs for a 96 hour 
period at 200°C to insure complete dissolution of re-
fractory minerals such as spinel.  Cr separation was 
completed following the methods described in [7].  
High-precision Cr isotope ratios were obtained using 
the Thermo Triton-Plus TIMS at UC Davis with the 
54Cr/52Cr ratio expressed as ε-notation (parts per 104) 
deviation from a terrestrial standard. 
Oxygen isotope analysis was undertaken by laser 
fluorination using the methods described in [8]. Sys-
tem precision, as determined on an internal obsidian 
standard is: ±0.05‰ for δ17O; ±0.09‰ for δ18O; 
±0.02‰ for Δ17O (2σ). Δ17O values were calculated 
using the linearized format of [9], with λ = 0.5247.  All 
error values listed below are 2σ. 
Bulk trace and major elements were analysed fol-
lowing the methods described in [10, 11] respectively. 
Results and Discussion:  
Stable isotopes The oxygen isotope compositions 
of 25 different fractions (n = 42) of BR are shown in 
Fig. 1. The mean Δ17O of BR is -0.127 ± 0.044‰ and 
the mean δ18O is 3.98 ± 0.22‰[12].  This is signifi-
cantly different from the HED mean Δ17O value of -
0.246 ± 0.014 [3].  The five pieces for which bulk 
chemistry has also been determined have a much tight-
er mean Δ17O of  -0.133 ± 0.016‰, at the upper end of 
the δ18O range for BR (4.09 ± 0.16‰).  
The measured ε54Cr for sample A/1 is -0.37 ± 0.11 
and for C/A/2 is -0.35 ± 0.08, identical within error.  
These values are resolvably different from the average 
ε54Cr value range observed in eucrites of -0.7ε (Fig. 2). 
The stable isotope systematics indicate that BR 
does not come from the same parent asteroid as eu-
crites. Interestingly, BR plots in the same field with 
Asuka 881394 [13] in the Δ17O-ε54Cr isotopes space – 
indicating they may be sampling the same differentiat-
ed body.  
https://ntrs.nasa.gov/search.jsp?R=20140012870 2019-08-31T18:30:42+00:00Z
Bulk composition Like previous studies [3], BR ap-
pears similar in chemistry to eucrites. Bulk trace ele-
ments for all follow a volatility pattern (Fig. 3) very 
similar to basaltic and polymict, and distinct from cu-
mulate, eucrites. Further, siderophile element contents 
in BR are low, at nearly the same level as in basaltic 
eucrites.  
Bulk rare earth elements show a relatively flat pat-
tern with a slight negative Eu anomaly for all samples, 
except piece C/B/3, which has a slight LREE enriched 
pattern without the Eu anomaly.  Concentrations are 
10-15xCI.  The BR mode is dominated by plagioclase 
and thus the negative Eu anomaly may be evidence of 
earlier fractionation of a plagioclase rich component 
prior to formation of BR melts, similar to the case for 
lunar basalts.   
The distinct O and Cr isotopes suggest that BR is 
sampling a different differentiated body to the HED 
parent body (4 Vesta).  The bulk trace elements and 
low siderophile concentrations argue against contami-
nation.  BR is a jumble of lithologic fragments from a 
differentiated parent asteroid which may be the same 
body represented by Asuka 881394. This implies that 
they have been mixed on a fine scale, most likely by 
impact.  
The next step is to determine the relationship be-
tween BR and A881394 and their possible parent body 
and to seek a better understanding of how trace ele-
ment and isotope geochemistry co-varies with the ob-
served variation in oxygen isotopic composition. 
References: [1] Bland P. A., et al. (2009) Science 
325, 1525–1527 [2] Weisberg M. K., et al. (2009) Me-
teoritics and Planet. Sci. 44, 429-462 [3] Greenwood 
et al. 2014 accepted EPSL. [4] Spivak-Birndorf L.J. et 
al. (2014) accepted Meteoritics and Planet. Sci. [5] 
Mittlefehldt D. (2005) Meteoritics and Planet. Sci. 40, 
665-677. [6] Scott E. R. D., et al. (2009) GCA 73, 
5835–5853. [7] Yamakawa et al. (2009) Analytical 
Chem. 81, 9787-9794  [8] Miller et al. (1999) Rapid 
Comm. Mass Spec. 13, 1211-1217. [9] Miller (2002) 
GCA 66, 1881-1889. [10] Friedrich J. M. et al. (2003) 
GCA 67, 2467-2479. [11] Wolf S. F. et al. (2012) Ta-
lanta 100, 276-281.  [12] Benedix et al. (2013) Abs-
tract#5142. MetSoc 76. [13] Sanborn M.E. et al. 
(2014) 45th LPSC. [14] Greenwood et al. (2005) Na-
ture 435, 916-918.  [15] Trinquier et al. (2008) ApJ 
655, 1179-1185. [16]Mittlefehldt D. (2014) EPSL 
submitted.  
 
 
Fig. 1.  Δ17O vs δ18O for BR.  Also shown are the HED 
[4] and angrite fractionation lines [14].  The five pieces 
analysed for bulk composition in this study are shown 
in coloured symbols Grey boxes show the 2σ variation 
for the HED and angrite data [3,14]. Anomalous eu-
crite data from [6].  
 
 
Fig. 2.  Δ17O vs ε54Cr for BR (red symbols) and normal 
eucrites (green symbols).  Data for eucrites from [15] 
and [7] for ε54Cr and Δ17O, respectively. 
 
Fig. 3.  Bulk trace lithophile elements (in order of in-
creasing volatility) in different pieces of BR compared 
to average compositions for eucrites [data from 16].   


Bunburra Rockhole: Exploring the Geology of a New Differentiated Basaltic Asteroid

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Bunburra Rockhole: Exploring the geology of a new differentiated basaltic asteroid

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