Ureilite meteorites are partial melt residues of an asteroid-sized object. They record the differentiation process that transformed many asteroids during the earliest stages of solar system formation

Downes, H.

Franchi, I.A.

Hudon, P.

Mittlefehldt, D.W.

Romanek, C.S.

English

Ureilites are the second largest achondrite group. They are ultramafic achondrites that have experienced igneous processing whilst retaining some degree of nebula-derived chemical heterogeneity. They differ from other achondrites in that they contain abundant carbon and their oxygen isotope compositions are very heterogeneous and similar to those of the carbonaceous chondrite anhydrous mineral line. Their carbonaceous nature and some compositional characteristics indicative of nebular origin suggest that they are primitive materials that form a link between nebular processes and early periods of planetesimal accretion. However, despite numerous studies, the exact origin of ureilites remains unclear. Current opinion is that they represent the residual mantle of an asteroid that underwent silicate and Fe-Ni-S partial melting and melt removal. Recent studies of short-lived chronometers indicate that the parent asteroid of the ureilites differentiated very early in the history of the Solar System. Therefore, they contain important information about processes that formed small rocky planetesimals in the early Solar System. In effect, they form a bridge between nebula processes and differentiation in small planetesimals prior to accretion into larger planets and so a correct interpretation of ureilite petrogenesis is essential for understanding this critical step

Franchi, Ian

Mittlefehldt, David W.

Romanek, Christopher S.

Downes, Hilary

Hudson, Pierre

NASA Technical Reports Server

Workshop on Early Planetary Differentiation abstract 4013, submitted 
ASTEROIDAL DIFFERENTIATION PROCESSES DEDUCED FROM ULTRAMAFIC ACHONDRITE UREILITE METEORITES.  
Hilary Downes1,2, David W Mittlefehldt3, Pierre Hudon4, Christopher S. Romanek5 and Ian Franchi6 1Lunar and 
Planetary Institute, Houston, Texas, USA (Downes@lpi.usra.edu); 2School of Earth Sciences, Birkbeck, University 
of London, London, UK; 3NASA/Johnson Space Centre, Houston, Texas, USA; 4Dept. of Earth and Planetary Sci-
ences, McGill University, Montreal, Quebec, Canada; 5 Savannah River Ecology Laboratory, University of Georgia, 
Aiken, SC, USA; 6Planetary Sciences Research Institute, Open University, Milton Keynes, UK. 
 
 
Introduction:  Ureilites are the second largest 
achondrite group. They are ultramafic achondrites that 
have experienced igneous processing whilst retaining 
some degree of nebula-derived chemical heterogeneity 
[1, 2]. They differ from other achondrites in that they 
contain abundant carbon and their oxygen isotope 
compositions are very heterogeneous and similar to 
those of the carbonaceous chondrite anhydrous mineral 
line. Their carbonaceous nature and some composi-
tional characteristics indicative of nebular origin sug-
gest that they are primitive materials that form a link 
between nebular processes and early periods of 
planetesimal accretion. However, despite numerous 
studies, the exact origin of ureilites remains unclear. 
Current opinion is that they represent the residual man-
tle of an asteroid that underwent silicate and Fe-Ni-S 
partial melting and melt removal. Recent studies of 
short-lived chronometers [3,4] indicate that the parent 
asteroid of the ureilites differentiated very early in the 
history of the Solar System. Therefore, they contain 
important information about processes that formed 
small rocky planetesimals in the early Solar System. In 
effect, they form a bridge between nebula processes 
and differentiation in small planetesimals prior to ac-
cretion into larger planets and so a correct interpreta-
tion of ureilite petrogenesis is essential for understand-
ing this critical step. 
Sampling the ureilite parent asteroid:  Polymict 
ureilites contain olivine and pyroxene clasts that cover 
an identical range of compositions to that shown by all 
known monomict ureilites [5,6], indicating derivation 
from a single parent asteroid. Each thin-section of a 
polymict ureilite may contain hundreds of clasts of 
ureilitic material, and multiple thin-sections greatly 
increase the number of clasts available for analysis. 
Therefore the potential for studying the parent asteroid 
of ureilites is much greater if polymict samples are 
analysed. We have analysed over 500 mineral or lithic 
clasts from polymict ureilites in order to compare the 
distribution of mineral compositions within samples 
and to shed light on ureilite petrogenesis. Although the 
basaltic material formed by partial melting is largely 
missing from the present-day meteorite collection, 
numerous melt clasts are present within polymict 
ureilites, indicating that such material was present on 
the surface of the asteroid [7]. Furthermore, our study 
shows that many individual mineral clasts in polymict 
ureilites have identical compositions to the unusual 
olivine and pyroxene grains from the Hughes 009 
augite-bearing ureilite, considered to have undergone 
interaction between melt and residue [8]. We have also 
done petrologic, bulk chemical and isotopic study of a 
suite of monomict ureilites. 
Two types of ureilite:  Our study has confirmed 
the observation that ureilitic olivine grains with mg#s 
< 85 are more common than those with mg# > 85 [9]. 
Recognition of the existence of two groups of ureilites 
(Fig. 1) may be of major significance. The ferroan 
group comprises a coherent suite of olivine composi-
tions with a strong negative correlation between Fe 
and Mn, whereas the magnesian group shows much 
more scatter and possibly even a positive correlation. 
There are also differences between the two groups in 
bulk C and siderophile element contents of monomict 
ureilites (Figs. 2, 3). Note that while there are two dis-
tinct groups, the actual boundary criterion that distin-
guishes the groups is uncertain. The two groups merge 
at mg# 85 for olivine compositions (Fig.1), but mg# 
83-84 works better for C and Ir (Figs. 2, 3). The two 
groups represent the products of contrasting processes 
or, more likely, the disruption of a single process by 
impact. 
0
5
10
15
20
25
0.35 0.40 0.45 0.50 0.55 0.60 0.65
MnO (wt%)
Fe
O
 (w
t%
)
polymict; mg# < 85
polymict; mg# > 85
monomict; mg# < 85
monomict; mg# > 85
Hughes 009 and other 
melt-related samples
 
Figure 1.  FeO vs MnO in ureilitic olivine grains from 
monomict ureilites and clasts within polymict ureilites. 
The coherent trend shown by samples with mg# < 85 
is largely nebula-derived but overprinted by small de-
grees of melt extraction. Samples with mg#s > 85 were 
https://ntrs.nasa.gov/search.jsp?R=20060056235 2019-08-30T00:14:29+00:00Z
Workshop on Early Planetary Differentiation abstract 4013, submitted 
undergoing partial melting at the instant of disruption 
of the parent ureilite asteroid. 
0
10
20
30
40
50
60
70 75 80 85 90 95 100
mg# oliv
C
 (m
g/
g)
monomict ureilites
 
Figure 2.  Bulk C vs olivine mg# in monomict 
ureilites. Samples with olivine mg# < 85 exhibit a 
positive correlation. Samples with olivine mg#s > 85 
exhibit no coherent trend. 
0
100
200
300
400
500
600
700
70 75 80 85 90 95 100
mg# oliv
Ir 
(n
g/
g)
monomict ureilites
 
Figure 3.  Bulk Ir vs olivine mg# in monomict 
ureilites. Samples with olivine mg# < 85 exhibit a 
wide range in Ir and no correlation. Samples with oli-
vine mg#s > 85 have lower Ir contents on average, and 
possibly show a positive correlation. 
Interpretation:  We conclude that the proto-
ureilite asteroid accreted from the solar nebula, proba-
bly as aggregates of olivine, orthopyroxene, diopside, 
plagioclase, metal, sulfide and carbonaceous material, 
with a range in mg# from ~74 to ~90 and wide Δ17O 
variation. Ureilitic olivine compositions start abruptly 
at Fo74-75, the reason for which must relate to the 
minimum mg# of material in that part of the solar neb-
ula from which the parent asteroid accreted.  For com-
parison, the mantle of Mars also appears to have an 
mg# of ~75 [10]. The asteroid was heated by decay of 
radioactive 26Al [11], causing metamorphism, and 
transformation of carbonaceous material to graphite. 
Siderophile contents of bulk ureilites indicate that re-
moval of Fe-Ni-S melt may have occurred at this stage 
[12], perhaps forming a primitive core, but the high 
siderophile contents of some ureilites indicates that 
this process was incomplete (Fig. 3). As the inhomo-
geneous asteroid heated up, silicate melting would 
have commenced at the peritectic associated with the 
most iron-rich composition (mg# = 74-75), assuming a 
homogeneous Al distribution. Melting of each incre-
ment ceased when one (plagioclase) or more likely two 
(plagioclase + high-Ca pyroxene) of the original sili-
cate phases had been consumed in the melting reaction 
and the melt expelled. This process explains the deple-
tion of incompatible lithophile elements in ureilites [1]. 
It left behind a tri-mineralic olivine-pigeonite-graphite 
ureilite, essentially equivalent to a harzburgite in a 
more Mg-rich system (such as the Earth’s mantle; mg# 
= 89). Thus the more abundant and more coherent 
group of ferroan ureilites probably represents residues 
from partial melting. However, this partial melting had 
occurred in a series of events in regions of the asteroid 
with different bulk mg#, so there was no planetesimal-
wide magma ocean stage, and hence oxygen isotopes 
did not become homogenised. High precision oxygen 
isotope data for monomict ureilites can be interpreted 
as a series of small fractionation trends caused by par-
tial melting [13]. 
One possible scenario is that the parent asteroid 
was disrupted by a major impact when melting reached 
silicate compositions with mg#s of approximately 85, 
giving rise to the two types of ureilites: (a) those that 
were already residual after melting and (b) those that 
were still partially molten when disruption occurred. In 
several samples with mg# > 85, melt impregnation of 
the mantle of the asteroid occurred, forming a suite of 
augite-bearing ureilites including Hughes 009 [8]. The 
impact event formed the highly shocked ureilite clasts 
found in polymict ureilites and represented in the 
monomict samples. It also formed the unusual highly 
magnesian rims around silicate minerals by a short-
lived process akin to smelting. A daughter asteroid re-
accreted from the remnants of the proto-ureilite aster-
oid; regolith formed on the daughter asteroid is repre-
sented by polymict ureilites.  One problem with this 
scenario is that there is no quench-textured melt phase 
in mg# > 85 ureilites as might be expected from rapid 
cooling following disruption. 
References:  [1] Mittlefehldt D. W. et al. (1998) Plane-
tary Materials, RIM 36, ch. 4. [2] Clayton R. N. and Mayeda 
T. K. (1988) GCA 52, 1313. [3] Goodrich C. et al. (2002) 
MAPS 37, A54. [4] Lee D.-C. et al. (2005) LPSC 36, #1638. 
[5] Goodrich C. A. et al. (2004) Chemie der Erde 64, 283. 
[6] Downes H. and Mittlefehldt D. W. (2006) LPSC 37, 
#1150.[7] Cohen B. et al. (2004) GCA 68, 4249. [8] Good-
rich C. A. et al. (2001) GCA 65, 621. [9] Ikeda Y. et al. 
(2003) Antarctic Meteorite Res. 16, 105. [10] Dreibus G. and 
Wänke H. (1985) Meteoritics 20, 367. [11] Kita N. T. et al. 
(2003) GCA Suppl. 67, A220. [12] Warren P. et al. (2006) 
GCA 70, 2104. [13] Franchi I. A. et al. (1997) MAPS 32, 
A44. 


Asteroidal Differentiation Processes Deduced from Ultramafic Achondrite Ureilite Meteorites

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsAsteroidal differentiation processes deduced fromultramafic achondrite ureilite meteoritesConference or Workshop ItemHow to cite:Downes, H.; Mittlefehldt, D.W.; Hudon, P.; Romanek, C.S. and Franchi, I.A. (2006). Asteroidal differentiationprocesses deduced from ultramafic achondrite ureilite meteorites. In: Workshop on Early Planetary Differentiation: AMulti-Planetary and Multi-Disciplinary Perspective, 8-10 Dec 2006, Sonoma County, California, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/epd2006/pdf/4013.pdfCopyright 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.ukASTEROIDAL DIFFERENTIATION PROCESSES DEDUCED FROM ULTRAMAFIC ACHONDRITE UREILITE METEORITES.  Hilary Downes1,2, David W Mittlefehldt3, Pierre Hudon4, Christopher S. Romanek5 and Ian Franchi6 1Lunar and Planetary Institute, Houston, Texas, USA (Downes@lpi.usra.edu); 2School of Earth Sciences, Birkbeck, University of London, London, UK; 3NASA/Johnson Space Centre, Houston, Texas, USA; 4Dept. of Earth and Planetary Sci-ences, McGill University, Montreal, Quebec, Canada; 5 Savannah River Ecology Laboratory, University of Georgia, Aiken, SC, USA; 6Planetary Sciences Research Institute, Open University, Milton Keynes, UK.   Introduction:  Ureilites are the second largest achondrite group. They are ultramafic achondrites that have experienced igneous processing whilst retaining some degree of nebula-derived chemical heterogeneity [1, 2]. They differ from other achondrites in that they contain abundant carbon and their oxygen isotope compositions are very heterogeneous and similar to those of the carbonaceous chondrite anhydrous mineral line. Their carbonaceous nature and some composi-tional characteristics indicative of nebular origin sug-gest that they are primitive materials that form a link between nebular processes and early periods of planetesimal accretion. However, despite numerous studies, the exact origin of ureilites remains unclear. Current opinion is that they represent the residual man-tle of an asteroid that underwent silicate and Fe-Ni-S partial melting and melt removal. Recent studies of short-lived chronometers [3,4] indicate that the parent asteroid of the ureilites differentiated very early in the history of the Solar System. Therefore, they contain important information about processes that formed small rocky planetesimals in the early Solar System. In effect, they form a bridge between nebula processes and differentiation in small planetesimals prior to ac-cretion into larger planets and so a correct interpreta-tion of ureilite petrogenesis is essential for understand-ing this critical step. Sampling the ureilite parent asteroid:  Polymict ureilites contain olivine and pyroxene clasts that cover an identical range of compositions to that shown by all known monomict ureilites [5,6], indicating derivation from a single parent asteroid. Each thin-section of a polymict ureilite may contain hundreds of clasts of ureilitic material, and multiple thin-sections greatly increase the number of clasts available for analysis. Therefore the potential for studying the parent asteroid of ureilites is much greater if polymict samples are analysed. We have analysed over 500 mineral or lithic clasts from polymict ureilites in order to compare the distribution of mineral compositions within samples and to shed light on ureilite petrogenesis. Although the basaltic material formed by partial melting is largely missing from the present-day meteorite collection, numerous melt clasts are present within polymict ureilites, indicating that such material was present on the surface of the asteroid [7]. Furthermore, our study shows that many individual mineral clasts in polymict ureilites have identical compositions to the unusual olivine and pyroxene grains from the Hughes 009 augite-bearing ureilite, considered to have undergone interaction between melt and residue [8]. We have also done petrologic, bulk chemical and isotopic study of a suite of monomict ureilites. Two types of ureilite:  Our study has confirmed the observation that ureilitic olivine grains with mg#s < 85 are more common than those with mg# > 85 [9]. Recognition of the existence of two groups of ureilites (Fig. 1) may be of major significance. The ferroan group comprises a coherent suite of olivine composi-tions with a strong negative correlation between Fe and Mn, whereas the magnesian group shows much more scatter and possibly even a positive correlation. There are also differences between the two groups in bulk C and siderophile element contents of monomict ureilites (Figs. 2, 3). Note that while there are two dis-tinct groups, the actual boundary criterion that distin-guishes the groups is uncertain. The two groups merge at mg# 85 for olivine compositions (Fig.1), but mg# 83-84 works better for C and Ir (Figs. 2, 3). The two groups represent the products of contrasting processes or, more likely, the disruption of a single process by impact. 05101520250.35 0.40 0.45 0.50 0.55 0.60 0.65MnO (wt%)FeO (wt%)polymict; mg# < 85polymict; mg# > 85monomict; mg# < 85monomict; mg# > 85Hughes 009 and other melt-related samples Figure 1.  FeO vs MnO in ureilitic olivine grains from monomict ureilites and clasts within polymict ureilites. The coherent trend shown by samples with mg# < 85 is largely nebula-derived but overprinted by small de-grees of melt extraction. Samples with mg#s > 85 were Workshop on Early Planetary Differentiation 2006 4013.pdfundergoing partial melting at the instant of disruption of the parent ureilite asteroid. 010203040506070 75 80 85 90 95 100mg# olivC (mg/g)monomict ureilites Figure 2.  Bulk C vs olivine mg# in monomict ureilites. Samples with olivine mg# < 85 exhibit a positive correlation. Samples with olivine mg#s > 85 exhibit no coherent trend. 010020030040050060070070 75 80 85 90 95 100mg# olivIr (ng/g)monomict ureilites Figure 3.  Bulk Ir vs olivine mg# in monomict ureilites. Samples with olivine mg# < 85 exhibit a wide range in Ir and no correlation. Samples with oli-vine mg#s > 85 have lower Ir contents on average, and possibly show a positive correlation. Interpretation:  We conclude that the proto-ureilite asteroid accreted from the solar nebula, proba-bly as aggregates of olivine, orthopyroxene, diopside, plagioclase, metal, sulfide and carbonaceous material, with a range in mg# from ~74 to ~90 and wide Δ17O variation. Ureilitic olivine compositions start abruptly at Fo74-75, the reason for which must relate to the minimum mg# of material in that part of the solar neb-ula from which the parent asteroid accreted.  For com-parison, the mantle of Mars also appears to have an mg# of ~75 [10]. The asteroid was heated by decay of radioactive 26Al [11], causing metamorphism, and transformation of carbonaceous material to graphite. Siderophile contents of bulk ureilites indicate that re-moval of Fe-Ni-S melt may have occurred at this stage [12], perhaps forming a primitive core, but the high siderophile contents of some ureilites indicates that this process was incomplete (Fig. 3). As the inhomo-geneous asteroid heated up, silicate melting would have commenced at the peritectic associated with the most iron-rich composition (mg# = 74-75), assuming a homogeneous Al distribution. Melting of each incre-ment ceased when one (plagioclase) or more likely two (plagioclase + high-Ca pyroxene) of the original sili-cate phases had been consumed in the melting reaction and the melt expelled. This process explains the deple-tion of incompatible lithophile elements in ureilites [1]. It left behind a tri-mineralic olivine-pigeonite-graphite ureilite, essentially equivalent to a harzburgite in a more Mg-rich system (such as the Earth’s mantle; mg# = 89). Thus the more abundant and more coherent group of ferroan ureilites probably represents residues from partial melting. However, this partial melting had occurred in a series of events in regions of the asteroid with different bulk mg#, so there was no planetesimal-wide magma ocean stage, and hence oxygen isotopes did not become homogenised. High precision oxygen isotope data for monomict ureilites can be interpreted as a series of small fractionation trends caused by par-tial melting [13]. One possible scenario is that the parent asteroid was disrupted by a major impact when melting reached silicate compositions with mg#s of approximately 85, giving rise to the two types of ureilites: (a) those that were already residual after melting and (b) those that were still partially molten when disruption occurred. In several samples with mg# > 85, melt impregnation of the mantle of the asteroid occurred, forming a suite of augite-bearing ureilites including Hughes 009 [8]. The impact event formed the highly shocked ureilite clasts found in polymict ureilites and represented in the monomict samples. It also formed the unusual highly magnesian rims around silicate minerals by a short-lived process akin to smelting. A daughter asteroid re-accreted from the remnants of the proto-ureilite aster-oid; regolith formed on the daughter asteroid is repre-sented by polymict ureilites.  One problem with this scenario is that there is no quench-textured melt phase in mg# > 85 ureilites as might be expected from rapid cooling following disruption. References:  [1] Mittlefehldt D. W. et al. (1998) Plane-tary Materials, RIM 36, ch. 4. [2] Clayton R. N. and Mayeda T. K. (1988) GCA 52, 1313. [3] Goodrich C. et al. (2002) MAPS 37, A54. [4] Lee D.-C. et al. (2005) LPSC 36, #1638. [5] Goodrich C. A. et al. (2004) Chemie der Erde 64, 283. [6] Downes H. and Mittlefehldt D. W. (2006) LPSC 37, #1150.[7] Cohen B. et al. (2004) GCA 68, 4249. [8] Good-rich C. A. et al. (2001) GCA 65, 621. [9] Ikeda Y. et al. (2003) Antarctic Meteorite Res. 16, 105. [10] Dreibus G. and Wänke H. (1985) Meteoritics 20, 367. [11] Kita N. T. et al. (2003) GCA Suppl. 67, A220. [12] Warren P. et al. (2006) GCA 70, 2104. [13] Franchi I. A. et al. (1997) MAPS 32, A44. Workshop on Early Planetary Differentiation 2006 4013.pdf

Asteroidal differentiation processes deduced from ultramafic achondrite ureilite meteorites

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server

Open Research Online (The Open University)