The process responsible for the mass independent oxygen isotope variation observed in Solar System materials remains poorly understood. While self-shielding of CO, either in the early solar nebula, or precursor molecular cloud, appears to be a viable mechanism, alternative models have also been proposed

Buchanan, P.C.

Franchi, I.A.

Greenwood, R.C.

Zolensky, M.E.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe origin and significance of the CCAM line: Evidencefrom chondrules and dark inclusions in Allende (CV3)Conference or Workshop ItemHow to cite:Greenwood, R.C.; Franchi, I.A.; Zolensky, M.E. and Buchanan, P.C. (2016). The origin and significance ofthe CCAM line: Evidence from chondrules and dark inclusions in Allende (CV3). In: 47th Lunar and Planetary ScienceConference, 21-25 Mar 2016, Houston, Texas.For guidance on citations see FAQs.c© [not recorded]Version: 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.ukTHE ORIGIN AND SIGNIFICANCE OF THE CCAM LINE: EVIDENCE FROM CHONDRULES AND DARK INCLUSIONS IN ALLENDE (CV3).  R. C. Greenwood1, I. A. Franchi1; M. E. Zolensky2, P. C. Buchan-an3. 1Planetary and Space Sciences, The Open University, Milton Keynes MK7 6AA, UK (r.c.greenwood@open.ac.uk).  2ARES, Johnson Space Center, Houston TX, USA.3Kilgore College, Kilgore, Texas 75662 USA.   Introduction: The process responsible for the mass independent oxygen isotope variation observed in Solar System materials remains poorly understood. While self-shielding of CO, either in the early solar nebula [1,2], or precursor molecular cloud [3], ap-pears to be a viable mechanism, alternative models have also been proposed [e.g. 4].  An important aspect of this problem relates to the interpretation of various reference lines on oxygen three-isotope diagrams. The Carbonaceous Chondrite Anhydrous Mineral (CCAM) line, derived predomi-nantly, but not exclusively, from analyses of compo-nents in the Allende (CV3) meteorite, is the most wide-ly used reference and has a slope of 0.94±0.01(2) [5,6]. However, the fundamental significance of the CCAM line has been questioned by [7]. Based on the results of a UV laser ablation study of an Allende CAI, it was suggested that a line of exactly slope 1 was of more fundamental significance. As pointed out by [7], almost all Solar System materials, with the exception of the R chondrites, plot to the right of the slope 1 line. This variation could be explained if the primitive oxy-gen isotope composition of the Solar System was rep-resented by a line of exactly slope 1, with subsequent mass fractionation, or isotopic exchange, shifting com-positions to the right [7]. The fact that a highly 17,18O-enriched phase (18O and17O = ~ +180‰) within the matrix of the primitive chondrite Acfer 094 plots on the extension of the slope 1 line lends additional sup-port to the primordial significance of this reference line [8]. With the aim of understanding the relationship between the CCAM and other reference lines we have undertaken a detailed study of chondrules and dark inclusions (DIs) from Allende.  Materials and methods: As part of this study in-tact chondrules (n=30) were extracted from Allende (CV3) whole-rock fragments under a binocular micro-scope. Adhering matrix was abraded from the chon-drules using stainless steel tools. Material from well characterized Allende DIs (n=11) have also been ana-lyzed. Preliminary results of the DI work were reported in [9]. In addition to Allende DIs, we have also ana-lyzed material from the Efremovka (CV3reduced) in-clusion E-80 [10]. Oxygen isotope analysis was performed by infrared laser-assisted fluorination [11]. All analyses were ob-tained on untreated whole rock samples (~2 mg). Sys-tem precision, as determined by replicate analyses of our internal obsidian standard, is: ±0.05‰ for δ17O; ±0.09‰ for δ18O; ±0.02‰ for 17O (2σ).  Results: Allende DIs plot on a well-defined linear trend with a somewhat shallower slope than the CCAM line [9] (Fig. 1). The Allende DI analyses of [6] plot on a similar trend (Fig. 1). Compared to Allende DIs, in-clusions from the CV3 reduced subgroup define a shal-lower slope (Fig. 2) (All analyses [6, 10], except E-80 (this study)). This trend is similar to that seen in CM2 chondrites (Fig. 2), to which DIs have sometimes been compared [12].    In contrast to the DIs, Allende chondrules (Fig. 3) plot to the left of the CCAM line. This feature is also seen in the Allende chondrule data of [13].  If the small cluster of chondrules plotting close to the TFL are ex-cluded, the remaining points define a distinct linear array (y = -3.45 + 1.01x R2 = 0.98) between the CCAM and PCM lines [14,15] (Fig. 3).  Fig. 1 Oxygen isotopic composition of Allende DIs.  Discussion: In its original formulation the CCAM line was calibrated using analyses not only from Allen-de CAIs, but also Allende chondrules and dark inclu-sions [5]. In addition, a limited number of analyses of non-Allende carbonaceous chondrite components were also included [5]. The CCAM line includes materials with distinct formational and secondary histories. It is therefore not surprising that individual components in Allende, such as chondrules and dark inclusions, will display systematic deviations from it.  DI analyses from Allende present an interesting paradox in that they define a more limited and steeper trend than is displayed by inclusions from the less al-tered CV3 reduced samples (Fig. 2). Thus, CV3 re-duced DIs appear to preserve clearer evidence of aque-2153.pdf47th Lunar and Planetary Science Conference (2016)ous alteration than those from the more heavily altered Allende meteorite. Analysis of organic matter suggests that the CV3 chondrites experienced a significant range in thermal metamorphism, with Allende graded >3.6 and Efremovka, Leoville and Vigarano 3.1 to 3.4 [16]. One possibility is that the more intense thermal meta-morphism experienced by Allende resulted in relatively rapid expulsion of pore fluid and hence less intense hydrothermal alteration than CV3reduced DIs. Howev-er, this is at odds with mineralogical observations from Allende DIs which suggest that they experienced pro-longed aqueous alteration and dehydration [17, 18, 19], processes that would have produced heavy oxygen isotope shifts [20, 21]. It appears more likely that fol-lowing hydrothermal alteration Allende DIs experi-enced a phase of partial oxygen isotope reequilibration.  There is growing evidence that chondrules from relatively pristine carbonaceous chondrites (Acfer 094, MET 00426 and QUE 99177) define a distinct trend termed the PCM line, that lies between the CCAM and Y&R lines (Fig. 3) [14, 15]. Compared to the PCM line essentially all of the chondrules in Allende are shifted to the right on an oxygen three-isotope diagram (Fig. 3). This may reflect mass fractionation in re-sponse to secondary alteration [7].  Conclusions: Allende chondrules and DIs demon-strate that the CCAM line, while useful for reference purposes, is not of primary significance. In agreement with previous studies, we conclude that the primary oxygen isotope distribution is better represented by a line of steeper slope than the CCAM [7]. However, this does not preclude the possibility that multiple slope 1 lines may be valid. And raises the question as to what is controlling the slope of these different mixing lines. Additional detailed work using components from pris-tine samples is required to evaluate further the process-es that produced the mass-independent oxygen isotope variation observed in early Solar System materials.  References: [1] Clayton R. N. (2002) Nature 415, 860-861. [2] Lyons J. R. and Young E. B. (2005) Na-ture 435, 317-320. [3] Yurimoto H. and Kuramoto K. (2004) Science 305, 1763-1766. [4] Dominguez G. (2010) Ap. J. Lett. 713, L59-L63. [5] Clayton R. N. et al. (1977) EPSL 34, 209-224. [6] Clayton R. N. and Mayeda T. K. (1999) GCA 63, 2089-2104. [7] Young E. D. and Russell S. S. (1998) Science 282, 452-455. [8] Sakamoto N. et al. (2007) Science 317, 231 – 233. [9] Greenwood R. C. et al. (2015) 46th LPSC #2975. [10] Krot A. N. et al. (1999) MAPS 34, 67-89. [11] Miller M.F. et al. (1999) Rapid. Commun. Mass Spec-trom. 13, 1211-1217. [12] Gordon S. H. et al. (2009) 40th LPSC, #1713. [13] Clayton R. N. et al. (1983) in Chondrules and their origins LPI, 37-43. [14] Ushiku-bo T. et al. (2012) GCA 90, 242-264. [15] Tenner T. J. et al. (2015) GCA 148, 228-250. [16] Bonal L. et al. (2006) GCA 70, 1849-1863. [17] Johnson C. A. et al. (1990) GCA 54, 819-830. [18] Kojima T. and Tomeoka K. (1996) GCA 60, 2651-2666. [19] Bu-chanan P. C. et al. (1997) GCA 61, 1733-1743. [20] Mayeda T. K. and Clayton R. N. (1998) LPS XXIX #1405. [21] Ivanova et al. (2013) MAPS 48, 2059-2070. Acknowledgments: Sasha Krot and Mike Weis-berg are thanked for providing samples for this study.    Fig. 2 Oxygen isotopic composition of Allende DIs (This study) [6]; CM2 falls and finds (all data OU); CV3 reduced DIs [6, 10] (E-80: this study).   Fig. 3 Oxygen isotopic composition of Allende chon-drules and DIs. PCM = Primitive Chondrule Minerals line [14, 15]. 2153.pdf47th Lunar and Planetary Science Conference (2016)

The origin and significance of the CCAM line: Evidence from chondrules and dark inclusions in Allende (CV3)

Buchanan, P. C.

Franchi, I. A.

Greenwood, R. C.

Zolensky, M. E.

NASA Technical Reports Server

THE ORIGIN AND SIGNIFICANCE OF THE CCAM LINE: EVIDENCE FROM CHONDRULES AND 
DARK INCLUSIONS IN ALLENDE (CV3).  R. C. Greenwood1, I.A. Franchi1; M. E. Zolensky2, P.C. Buchanan3. 
1Planetary and Space Sciences, The Open University, Milton Keynes MK7 6AA, UK (r.c.greenwood@open.ac.uk).  
2ARES, Johnson Space Center, Houston TX, USA.3Kilgore College, Kilgore, Texas 75662 USA. 
 
 
Introduction: The process responsible for the 
mass independent oxygen isotope variation observed 
in Solar System materials remains poorly understood. 
While self-shielding of CO, either in the early solar 
nebula [1,2], or precursor molecular cloud [3], ap-
pears to be a viable mechanism, alternative models 
have also been proposed [4].  
An important aspect of this problem relates to the 
interpretation of various reference lines on oxygen 
three-isotope diagrams. The Carbonaceous Chondrite 
Anhydrous Mineral (CCAM) line, derived predomi-
nantly, but not exclusively, from analyses of compo-
nents in the Allende (CV3) meteorite, is the most 
widely used reference and has a slope of 
0.94±0.01(2) [5,6]. However, the fundamental signif-
icance of the CCAM line has been questioned by [7]. 
Based on the results of a UV laser ablation study of an 
Allende CAI, these authors suggested that a line of 
exactly slope 1 was of more fundamental significance. 
They pointed out that almost all Solar System mate-
rials, with the exception of the R chondrites, plot 
either on, or to the right of the slope 1 line. They went 
on to suggest that this variation could be explained if 
the primitive oxygen isotope composition of the Solar 
System was represented by the slope 1 line, with sub-
sequent mass fractionation or isotopic exchange shift-
ing compositions away from this line to the right. The 
fact that a highly 17,18O-enriched phase (18O and17O 
= ~ +180‰) within the matrix of the primitive chon-
drite Acfer 094 plots on the extension of the slope 1 
line lends additional support to the primordial signifi-
cance of this reference line [8]. With the aim of under-
standing the relationship between the CCAM and other 
reference lines we have undertaken a detailed study of 
chondrules and dark inclusions from Allende.  
Materials and methods: As part of this study in-
tact chondrules (n=30) were extracted from Allende 
(CV3) whole-rock fragments under a binocular micro-
scope. Adhering matrix material was abraded from the 
chondrules using stainless steel hand tools. Material 
from well characterized Allende Dark Inclusions (DIs) 
(n=11) have also been analyzed. Preliminary results of 
this DI work were reported in [9]. In addition to Al-
lende DI samples, we have also analyzed material from 
the Efremovka (CV3reduced) inclusion E-80 [10]. 
Oxygen isotope analysis was performed by infrared 
laser-assisted fluorination [11]. All analyses were ob-
tained on untreated whole rock samples (0.5-2 mg). 
System precision, as determined by replicate analyses 
of our internal obsidian standard, is: ±0.05‰ for δ17O; 
±0.09‰ for δ18O; ±0.02‰ for 17O (2σ).  
Results: Allende DIs analyzed in this study plot on 
a well-defined linear trend with a somewhat shallower 
slope than the CCAM line (Fig. 1). The Allende DI 
analyses of [6] are also shown in Fig. 1 and plot on a 
similar trend. Less altered chondrule-bearing clasts 
(Types A and A/B) tend to plot closer to bulk Allende 
than the more altered Type B inclusion, with the ma-
trix-rich Type C clasts plotting in a relatively narrow 
intermediate position along the array (Fig. 1). 
 
 
Fig. 1 Oxygen isotopic composition of Allende Dark 
Inclusions. 
 
Compared to Allende DIs, inclusions from the CV3 
reduced subgroup (Efremovka, Leoville, Vigarano) 
fall on a distinct, well-defined trend with a relatively 
shallow slope (Fig. 2) (All analyses from the literature 
[6, 10], except E-80 (this study)). This trend is similar 
to that seen in CM2 chondrites (Fig. 2), to which DIs 
have sometimes been compared [12]. 
   In contrast to the DIs, chondrules extracted from 
Allende (Fig. 3) plot to the left of the CCAM line. This 
feature is also clearly seen in the Allende chondrule 
data of [13].  If chondrules that plot close to the TFL 
are included our Allende chondrule data define a trend 
with a slope that is somewhat steeper than 1 (Fig. 3). 
Excluding this small cluster of 16O-poor chondrules, 
the remaining data points lie roughly equidistant be-
tween the CCAM and PCM lines (Fig. 3) and define a 
distinct linear array ((y = -3.45 + 0.97x R2 = 0.98).    
Discussion: In its original formulation the CCAM 
line was calibrated using analyses not only from Al-
lende CAIs, but also Allende chondrules and dark in-
clusions [5]. In addition, a limited number of analyses 
https://ntrs.nasa.gov/search.jsp?R=20160002397 2019-08-31T03:52:18+00:00Z
of non-Allende carbonaceous chondrite components 
were also included [5]. The CCAM line includes mate-
rials with distinct formational and secondary histories. 
It is therefore not surprising that individual compo-
nents in Allende, such as chondrules and dark inclu-
sions, will display systematic deviations from it.  
 
Fig. 2 Oxygen isotope composition of Allende DIs 
(This study) [6]; CM2 Falls and finds (all data OU); 
CV3 reduced DIs [6, 10] (E-80: This Study).  
    
Fig. 3 Oxygen isotopic composition of chondrules and 
DIs from Allende shown in relation to various refer-
ence lines. PCM = Primitive Chondrule Minerals line 
[14, 15]. 
 
DI data from Allende presents an interesting para-
dox in that it defines a more limited and steeper trend 
then that of inclusions from the less altered CV3 re-
duced samples (Fig. 2). Thus, CV3 reduced DIs appear 
to preserve more clear cut evidence of aqueous altera-
tion than those from the more heavily altered Allende 
meteorite. Raman spectroscopic analysis of organic 
matter suggests that the CV3 chondrites experienced a 
significant range in thermal metamorphism, with Al-
lende >3.6 and Efremovka, Leoville and Vigarano = 
3.1 to 3.4 [16]. One possibility is that the more intense 
thermal metamorphism experienced by Allende result-
ed in relatively rapid expulsion of pore fluid and hence 
less intense hydrothermal alteration compared to CV3 
reduced DIs. However, this explanation seems at odds 
with detailed mineralogical observations from Allende 
DIs which suggest that they experienced prolonged 
aqueous alteration followed by dehydration [17, 18, 
19], processes that should both have produced signifi-
cant heavy oxygen isotope shifts [20, 21]. It appears 
more likely that subsequent to significant hydrother-
mal alteration Allende DIs experienced a phase of par-
tial oxygen isotope reequilibration.  
There is growing evidence that chondrules from 
relatively pristine carbonaceous chondrites such as 
Acfer 094, MET 00426 and QUE 99177 define a dis-
tinct trend termed the PCM line, that is roughly equi-
distant from the CCAM and Y&R lines (Fig. 3) [14, 
15]. Compared to the PCM line the majority of chon-
drules in Allende are shifted to the right on an oxygen 
three-isotope diagram (Fig. 3). This most likely re-
flects mass fractionation in response to secondary al-
teration.  
Conclusions: Chondrules and DIs in Allende 
demonstrate that the CCAM line, while useful for ref-
erence purposes, is not in itself of primary signifi-
cance. In agreement with previous studies, we con-
clude that primary oxygen isotope distribution is better 
represented by a line of steeper slope than the CCAM 
[7]. However, this does not preclude the possibility 
that multiple slope 1 lines may be valid. Additional 
detailed work using components from pristine samples 
is required to evaluate further the processes that pro-
duced the mass-independent oxygen isotope variation 
observed in early Solar System materials.    
References: [1] Clayton R. N. (2002) Nature 415, 
860-861. [2] Lyons J. R. and Young E. B. (2005) Na-
ture 435, 317-320. [3] Yurimoto H. and Kuramoto K. 
(2004) Science 305, 1763-1766. [4] Dominguez G. 
(2010) Ap. J. Lett. 713, L59-L63.[5] Clayton R. N. et 
al. (1977) EPSL 34, 209-224. [6] Clayton R. N. and 
Mayeda T. K. (1999) GCA 63, 2089-2104. [7] Young 
E. D. and Russell S. S. (1998) Science 282, 452-455. 
[8] Sakamoto N. et al. (2007) Science 317, 231 – 233. 
[9]  Greenwood R. C. et al. (2015) 46th LPSC abstract 
#2975. [10] Krot A. N. et al. (1999) MAPS 34, 67-89.  


The Origin and Significance of the CCAM Line: Evidence from Chondrules and Dark Inclusions in Allende (CV3)

The origin and significance of the CCAM line: Evidence from chondrules and dark inclusions in Allende (CV3)

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