Dark inclusions (DIs) are lithic fragments that form a volumetrically small, but important, component in carbonaceous chondrites. Carbonaceous clasts similar to DIs are also found in some ordinary chondrites and HEDs. DIs are of particular interest because they provide a record of nebular and planetary processes distinct from that of their host meteorite. DIs may be representative of the material that delivered water and other volatiles to early Earth as a late veneer. Here we focus on the oxygen isotopic composition of DIs in a variety of settings with the aim of understanding their formational history and relationship to the enclosing host meteorite

Buchanan, P. C.

Franchi, I. A.

Greenwood, R. C.

Zolensky, M. E.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe oxygen isotope composition of dark inclusions inHEDs, ordinary and carbonaceous chondritesConference or Workshop ItemHow to cite:Greenwood, R. C.; Zolensky, M. E.; Buchanan, P. C. and Franchi, I. A. (2015). The oxygen isotope composition ofdark inclusions in HEDs, ordinary and carbonaceous chondrites. In: 46th Lunar and Planetary Science Conference,16-20 Mar 2015, 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.ukTHE OXYGEN ISOTOPE COMPOSITION OF DARK INCLUSIONS IN HEDs, ORDINARY AND CARBONACEOUS CHONDRITES.  R. C. Greenwood1, M. E. Zolensky2, P.C. Buchanan3, I.A. Franchi1. 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: Dark inclusions (DIs) are lithic fragments that form a volumetrically small, but im-portant, component in carbonaceous chondrites [1,2]. Carbonaceous clasts similar to DIs are also found in some ordinary chondrites and HEDs [3,4]. DIs are of particular interest because they provide a record of nebular and planetary processes distinct from that of their host meteorite [1,5]. DIs may be representative of the material that delivered water and other volatiles to early Earth as a late veneer [6]. Here we focus on the oxygen isotopic composition of DIs in a variety of set-tings with the aim of understanding their formational history and relationship to the enclosing host meteorite.  Materials and methods: DIs and related materials were obtained from the following meteorites: CV3s (Allende, NWA 2140, NWA 2364): Samples from Al-lende cover all categories of the four-fold classification scheme [2] (inclusion numbers analyzed in brackets). Type A clasts (1a1, 4b1, 25s1-TW1) contain chon-drules, inclusions and matrix, but are somewhat finer grained than normal Allende material. Type A/B clasts (MZB) are transitional between Types A and B (Fig. 1). Type B clasts (12b1) contain opaque matrix and oli-vine-rich aggregates and may have experienced a hy-dration-dehydration cycle [2]. Type C clasts (5a1, ekpb4b1, MZ15, USNM 3876) consist of fine-grained, opaque material similar to Allende matrix. Full de-scriptions for of the Allende DI samples are given in [2]. DI material from NWA 2140 analyzed for this study is Type A/B and Type A for NWA 2364. HEDs: (Bholghati, PRA 04401, SCO 06040). DIs from the howardite Bholghati have not been analyzed by us for oxygen isotopes. Instead a sequence of 5-10 mg repre-sentative whole rock samples have been run to assess its carbonaceous chondrite content. PRA 04401 is an extremely coarse-grained howardite containing a high concentration (~40-50% in places) of nearly cm-sized angular carbonaceous clasts that texturally resemble CM2 material [7].  SCO 06040 is also a coarse-grained howardite breccia, contains a lower amount (~10%) of rounded, up to 2mm diameter, CM2-like clasts [7].  OCs: Sharps (H3.4) is a fragmental breccia containing accessory carbonaceous clasts up to 1 cm in diameter [8]. Oxygen isotope analysis was performed by infrared laser-assisted fluorination [9]. All analyses were ob-tained on untreated whole rock samples (0.5-2 mg). System precision, as determined on an internal obsidi-an standard is: ±0.05‰ for δ17O; ±0.09‰ for δ18O; ±0.02‰ for 17O (2σ).   Fig. 1 Mg Map of a Type A/B Allende clast (AMNH 4301 [1]). Remnants of primary material are present in the centre of some larger chondrules.   Results: All of the Allende DIs are displaced slightly to the right of the CCAM line in Fig.2 and de-fine a linear trend with a slope of y = -4.30 + 0.89x R2 = 0.99. The less altered chondrule-bearing clasts (A and A/B) plot closest to bulk Allende analysis in Fig. 2, with the most altered clast (Type B) (12b1) being ap-proximately 5‰ heavier with respect to 18O. Matrix-rich Type C clasts show a narrow range of oxygen iso-tope variation, plotting roughly halfway between the most and least altered DIs in Fig. 2. These results are in agreement with previous studies of Allende DIs [1].    DIs in CV3s NWA 2140 and NWA 2364  (Type A/B and A respectively) plot at the higher   δ18O values than their equivalents in Allende (Fig. 2). This is con-sistent with the results of previous studies [1] which found that Allende DIs are relatively 16O-enriched compared to inclusions in other CV3s (Fig.3).  Carbonaceous chondrite clast material from the howardite PRA 04401 plots at the extension of the CM2 field in Fig. 2 consistent with results of textural and mineralogical studies [7]. The other howardite samples analyzed in this study (SCO 06040 and Bhol-2975.pdf46th Lunar and Planetary Science Conference (2015)ghati) have 17O values that vary from -0.265 to -0.489‰, consistent with bulk carbonaceous  chondrite (CM2) contents of between about 1 and 10%.  Fig. 2 Oxygen isotopic composition of Dark Inclusions. The blue dashed line is the best fit line through the Allende DIs only. Bulk Allende [10]. TFL = Terrestrial Fractionation Line, CRML = CR Mixing Line [11], CCAM = Carbonaceous Chondrite Anhydrous Mineral line [12]. CM2 field (blue shading) [12], CV-CK field (grey shading) [10].   DIs analyzed in Sharps are clearly distinct from those in either the CV3s or howardites and plot on the CR Mixing Line [11]. The composition of these DIs is not well understood, although they do not appear to be mineralogically related to CR chondrites [8]. Discussion: Mineralogical studies of DIs have played a crucial role in highlighting the importance of parent body processes in modifying the composition of CV chondrites [2, 13]. In particular, DI studies have shown that CV3s, previously regarded as pristine nebu-lar condensates, underwent extensive aqueous altera-tion [2, 13]. Furthermore, textural evidence indicates that, following aqueous alteration, some DIs experi-enced a phase of thermal metamorphism, resulting in phyllosilicate dehydration and the formation of sec-ondary Fe-rich olivine (Type B inclusions) [13]. This model was later extended to explain the origin of CV3 Fe-rich matrix olivines in general [14].  The results of previous oxygen isotope studies of DIs have also pointed to the role of aqueous alteration in modifying their primary compositions [1]. As shown in this study, DIs define linear arrays with a shallower slope than the CCAM and with less altered material plotting at the 16O-rich end (Type As) and more altered material  (Type Bs) at the 16O-poor end [1]. However, with respect to phyllosilicate dehydration and second-ary Fe-rich olivine formation, oxygen isotope evidence appears less clear-cut [15]. Experimental evidence in-dicates that dehydration will result in heavy-isotope enrichments, which for Allende matrix olivines are not seen, if the CCAM line is used as a reference [15].  Fig. 3 Oxygen isotope composition of DIs in CV3 chondrites. Data this study and [1] (see text for details).  However, DIs in Allende may point to a possible weakness in this argument. In Fig. 2 and 3 DIs do not sit on the CCAM line, but instead define a line of lower slope (Fig. 2). In a similar way, chondrules in Allende plot on a discrete line with a steeper slope than the CCAM line. Analysis of 22 Allende chondrules (OU unpublished data) yield an array with a slope of: y = -3.45 + 0.97x (Fig. 3). If this reference line, rather than the CCAM, is used positive shifts of 7‰ along a mass fractionation line to the array defined by Vigarano DIs is feasible (Fig. 3). This is greater than required by earlier experiments [15]. More recent work has shown that shifts of 7‰ are produced during dehydration of serpentine [16]. Thus, phyllosilicate dehydration, lead-ing to the formation of CV3 Fe-rich matrix olivines, may be a viable mechanism, provided the relevant ref-erence line is steeper than the CCAM [17].  References: [1] Johnson C. A. et al. (1990) GCA, 54, 819-830. [2] Buchanan P. C. et al. (1997) GCA, 61, 1733-1743. [3] Zolensky M. E. et al. (1996) Meteo-ritics & Planet. Sci., 31, 518-537. [4] Zolensky M. E. et al. (1996) LPS XXVII, 1507-1508. [5] Weisberg M.K. et al. (1998) LPS XXIX, Absract #1882. [6] Gou-nelle M. et al. (2005) GCA 69, 3431-3443. [7] Herrin et al. (2011) LPS 42, abstract #2806. [8] Kebukawa Y. et al. (2012) MAPS 75,  abstract#5085. [9] Miller M. F. et al. (1999) Rapid Commun. Mass Spectrom. 13, 1211-1217. [10] Greenwood R.C. et al. (2010) GCA 74, 1684-1705. [11] Schrader D.L. et al. (2011) GCA 75, 308-325. [12] Clayton R.N. and Mayeda T.K. (1999) GCA  63, 2089-2104. [13] Kojima T. and Tomeoka K. (1994) Meteoritics 29, 484 (absr.). [14] Krot  A. N. et al. (1995) Meteoritics 30, 748 abstrct. [15] Mayeda T.K. and Clayton R.N. (1998) LPS XXIX abstract #1405.  [16] Ivanova et al. (2013) Meteoritics & Planet. Sci. 48, 2059-2070. [17] Young E.D. and Russell S. S. (1998) Science 282, 452-455.  2975.pdf46th Lunar and Planetary Science Conference (2015)

The oxygen isotope composition of dark inclusions in HEDs, ordinary and carbonaceous chondrites

NASA Technical Reports Server

THE OXYGEN ISOTOPE COMPOSITION OF DARK INCLUSIONS IN HEDs, ORDINARY AND 
CARBONACEOUS CHONDRITES.  R. C. Greenwood1, M. E. Zolensky2, P.C. Buchanan3, I.A. Franchi1. 
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: Dark inclusions (DIs) are lithic 
fragments that form a volumetrically small, but im-
portant, component in carbonaceous chondrites [1,2]. 
Carbonaceous clasts similar to DIs are also found in 
some ordinary chondrites and HEDs [3,4]. DIs are of 
particular interest because they provide a record of 
nebular and planetary processes distinct from that of 
their host meteorite [1,5]. DIs may be representative of 
the material that delivered water and other volatiles to 
early Earth as a late veneer [6]. Here we focus on the 
oxygen isotopic composition of DIs in a variety of set-
tings with the aim of understanding their formational 
history and relationship to the enclosing host meteorite.  
Materials and methods: DIs and related materials 
were obtained from the following meteorites: CV3s 
(Allende, NWA 2140, NWA 2364): Samples from Al-
lende cover all categories of the four-fold classification 
scheme [2] (inclusion numbers analyzed in brackets). 
Type A clasts (1a1, 4b1, 25s1-TW1) contain chon-
drules, inclusions and matrix, but are somewhat finer 
grained than normal Allende material. Type A/B clasts 
(MZB) are transitional between Types A and B (Fig. 1). 
Type B clasts (12b1) contain opaque matrix and oli-
vine-rich aggregates and may have experienced a hy-
dration-dehydration cycle [2]. Type C clasts (5a1, 
ekpb4b1, MZ15, USNM 3876) consist of fine-grained, 
opaque material similar to Allende matrix. Full de-
scriptions for of the Allende DI samples are given in 
[2]. DI material from NWA 2140 analyzed for this 
study is Type A/B and Type A for NWA 2364. HEDs: 
(Bholghati, PRA 04401, SCO 06040). DIs from the 
howardite Bholghati have not been analyzed by us for 
oxygen isotopes. Instead a sequence of 5-10 mg repre-
sentative whole rock samples have been run to assess 
its carbonaceous chondrite content. PRA 04401 is an 
extremely coarse-grained howardite containing a high 
concentration (~40-50% in places) of nearly cm-sized 
angular carbonaceous clasts that texturally resemble 
CM2 material [7].  SCO 06040 is also a coarse-grained 
howardite breccia, contains a lower amount (~10%) of 
rounded, up to 2mm diameter, CM2-like clasts [7].  
OCs: Sharps (H3.4) is a fragmental breccia containing 
accessory carbonaceous clasts up to 1 cm in diameter 
[8]. 
Oxygen isotope analysis was performed by infrared 
laser-assisted fluorination [9]. All analyses were ob-
tained on untreated whole rock samples (0.5-2 mg). 
System precision, as determined on an internal obsidi-
an standard is: ±0.05‰ for δ17O; ±0.09‰ for δ18O; 
±0.02‰ for '17O (2σ).  
 
Fig. 1 Mg Map of a Type A/B Allende clast (AMNH 
4301 [1]). Remnants of primary material are present in 
the centre of some larger chondrules. 
  
Results: All of the Allende DIs are displaced 
slightly to the right of the CCAM line in Fig.2 and de-
fine a linear trend with a slope of y = -4.30 + 0.89x R2 
= 0.99. The less altered chondrule-bearing clasts (A 
and A/B) plot closest to bulk Allende analysis in Fig. 2, 
with the most altered clast (Type B) (12b1) being ap-
proximately 5‰ heavier with respect to G18O. Matrix-
rich Type C clasts show a narrow range of oxygen iso-
tope variation, plotting roughly halfway between the 
most and least altered DIs in Fig. 2. These results are in 
agreement with previous studies of Allende DIs [1].    
DIs in CV3s NWA 2140 and NWA 2364  (Type 
A/B and A respectively) plot at the higher   δ18O values 
than their equivalents in Allende (Fig. 2). This is con-
sistent with the results of previous studies [1] which 
found that Allende DIs are relatively 16O-enriched 
compared to inclusions in other CV3s (Fig.3).  
Carbonaceous chondrite clast material from the 
howardite PRA 04401 plots at the extension of the 
CM2 field in Fig. 2 consistent with results of textural 
and mineralogical studies [7]. The other howardite 
samples analyzed in this study (SCO 06040 and Bhol-
https://ntrs.nasa.gov/search.jsp?R=20150001927 2019-08-31T14:06:55+00:00Z
ghati) have '17O values that vary from -0.265 to -
0.489‰, consistent with bulk carbonaceous  chondrite 
(CM2) contents of between about 1 and 10%. 
 
Fig. 2 Oxygen isotopic composition of Dark Inclusions. 
The blue dashed line is the best fit line through the 
Allende DIs only. Bulk Allende [10]. TFL = Terrestrial 
Fractionation Line, CRML = CR Mixing Line [11], 
CCAM = Carbonaceous Chondrite Anhydrous Mineral 
line [12]. CM2 field (blue shading) [12], CV-CK field 
(grey shading) [10]. 
  
DIs analyzed in Sharps are clearly distinct from 
those in either the CV3s or howardites and plot on the 
CR Mixing Line [11]. The composition of these DIs is 
not well understood, although they do not appear to be 
mineralogically related to CR chondrites [8]. 
Discussion: Mineralogical studies of DIs have 
played a crucial role in highlighting the importance of 
parent body processes in modifying the composition of 
CV chondrites [2, 13]. In particular, DI studies have 
shown that CV3s, previously regarded as pristine nebu-
lar condensates, underwent extensive aqueous altera-
tion [2, 13]. Furthermore, textural evidence indicates 
that, following aqueous alteration, some DIs experi-
enced a phase of thermal metamorphism, resulting in 
phyllosilicate dehydration and the formation of sec-
ondary Fe-rich olivine (Type B inclusions) [13]. This 
model was later extended to explain the origin of CV3 
Fe-rich matrix olivines in general [14].  
The results of previous oxygen isotope studies of 
DIs have also pointed to the role of aqueous alteration 
in modifying their primary compositions [1]. As shown 
in this study, DIs define linear arrays with a shallower 
slope than the CCAM and with less altered material 
plotting at the 16O-rich end (Type As) and more altered 
material  (Type Bs) at the 16O-poor end [1]. However, 
with respect to phyllosilicate dehydration and second-
ary Fe-rich olivine formation, oxygen isotope evidence 
appears less clear-cut [15]. Experimental evidence in-
dicates that dehydration will result in heavy-isotope 
enrichments, which for Allende matrix olivines are not 
seen, if the CCAM line is used as a reference [15]. 
 
Fig. 3 Oxygen isotope composition of DIs in CV3 
chondrites. Data this study and [1] (see text for details). 
 
However, DIs in Allende may point to a possible 
weakness in this argument. In Fig. 2 and 3 DIs do not 
sit on the CCAM line, but instead define a line of lower 
slope (Fig. 2). In a similar way, chondrules in Allende 
plot on a discrete line with a steeper slope than the 
CCAM line. Analysis of 22 Allende chondrules (OU 
unpublished data) yield an array with a slope of: y = -
3.45 + 0.97x (Fig. 3). If this reference line, rather than 
the CCAM, is used positive shifts of 7‰ along a mass 
fractionation line to the array defined by Vigarano DIs 
is feasible (Fig. 3). This is greater than required by 
earlier experiments [15]. More recent work has shown 
that shifts of 7‰ are produced during dehydration of 
serpentine [16]. Thus, phyllosilicate dehydration, lead-
ing to the formation of CV3 Fe-rich matrix olivines, 
may be a viable mechanism, provided the relevant ref-
erence line is steeper than the CCAM [17]. 
 
References: [1] Johnson C. A. et al. (1990) GCA, 
54, 819-830. [2] Buchanan P. C. et al. (1997) GCA, 61, 
1733-1743. [3] Zolensky M. E. et al. (1996) Meteo-
ritics & Planet. Sci., 31, 518-537. [4] Zolensky M. E. 
et al. (1996) LPS XXVII, 1507-1508. [5] Weisberg 
M.K. et al. (1998) LPS XXIX, Absract #1882. [6] Gou-
nelle M. et al. (2005) GCA 69, 3431-3443. [7] Herrin 
et al. (2011) LPS 42, abstract #2806. [8] Kebukawa Y. 
et al. (2012) MAPS 75,  abstract#5085. [9] Miller M. 
F. et al. (1999) Rapid Commun. Mass Spectrom. 13, 
1211-1217. [10] Greenwood R.C. et al. (2010) GCA 
74, 1684-1705. [11] Schrader D.L. et al. (2011) GCA 
75, 308-325. [12] Clayton R.N. and Mayeda T.K. 
(1999) GCA  63, 2089-2104. [13] Kojima T. and 
Tomeoka K. (1994) Meteoritics 29, 484 (absr.). [14] 
Krot  A. N. et al. (1995) Meteoritics 30, 748 abstrct. 
[15] Mayeda T.K. and Clayton R.N. (1998) LPS XXIX 
abstract #1405.  [16] Ivanova et al. (2013) Meteoritics 
& Planet. Sci. 48, 2059-2070. [17] Young E.D. and 
Russell S. S. (1998) Science 282, 452-455.  


The Oxygen Isotope Composition of Dark Inclusions in HEDs, Ordinary and Carbonaceous Chondrites

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The oxygen isotope composition of dark inclusions in HEDs, ordinary and carbonaceous chondrites

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