1Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing
(Received 01 April 2015; revision accepted 07 October 2015)
Abstract–Bulk major element composition, petrography, mineralogy, and oxygen isotopecompositions of twenty Al-rich chondrules (ARCs) from five CV3 chondrites (NorthwestAfrica [NWA] 989, NWA 2086, NWA 2140, NWA 2697, NWA 3118) and the Ningqiangcarbonaceous chondrite were studied and compared with those of ferromagnesian chondrulesand refractory inclusions. Most ARCs are marginally Al-richer than ferromagnesianchondrules with bulk Al2O3 of 10–15 wt%. ARCs are texturally similar to ferromagnesianchondrules, composed primarily of olivine, pyroxene, plagioclase, spinel, Al-rich glass,and metallic phases. Minerals in ARCs have intermediate compositions. Low-Ca pyroxene(Fs0.6–8.8Wo0.7–9.3) has much higher Al2O3 and TiO2 contents (up to 12.5 and 2.3 wt%,respectively) than that in ferromagnesian chondrules. High-Ca pyroxene (Fs0.3–2.0Wo33–54)contains less Al2O3 and TiO2 than that in Ca,Al-rich inclusions (CAIs). Plagioclase (An77–99Ab1–23) is much more sodic than that in CAIs. Spinel is enriched in moderately volatileelement Cr (up to 6.7 wt%) compared to that in CAIs. Al-rich enstatite coexists with anorthiteand spinel in a glass-free chondrule, implying that the formation of Al-enstatite was not due tokinetic reasons but is likely due to the high Al2O3/CaO ratio (7.4) of the bulk chondrule. ThreeARCs contain relict CAIs. Oxygen isotope compositions of ARCs are also intermediatebetween those of ferromagnesian chondrules and CAIs. They vary from �39.4& to 13.9& ind18O and yield a best fit line (slope = 0.88) close to the carbonaceous chondrite anhydrousmineral (CCAM) line. Chondrules with 5–10 wt% bulk Al2O3 have a slightly more narrowrange in d18O (�32.5 to 5.9&) along the CCAM line. Except for the ARCs with relict phases,however, most ARCs have oxygen isotope compositions (>�20& in d18O) similar to those oftypical ferromagnesian chondrules. ARCs are genetically related to both ferromagnesianchondrules and CAIs, but the relationship between ARCs and ferromagnesian chondrules iscloser. Most ARCs were formed during flash heating and rapid cooling processes like normalchondrules, only from chemically evolved precursors. ARCs extremely enriched in Al andthose with relict phases could have had a hybrid origin (Krot et al. 2002) which incorporatedrefractory inclusions as part of the precursors in addition to ferromagnesian materials. Theoccurrence of melilite in ARCs indicates that melilite-rich CAIs might be present in theprecursor materials of ARCs. The absence of melilite in most ARCs is possibly due to high-temperature interactions between a chondrule melt and the solar nebula.
INTRODUCTION
Chondrules and refractory inclusions are importantcomponents of most chondrites. Although there hasbeen wide consensus that chondrules and refractory
inclusions were formed under different physico-chemicalconditions, the genetic relationship between them is stillcontroversial. Considerable lines of evidence from short-lived radionuclides, especially 26Al, have suggested thatthe onset of chondrule formation postdated the
Meteoritics & Planetary Science 51, Nr 1, 116–137 (2016)
formation of calcium-aluminum-rich inclusions (CAIs)by at least 1–2 Myr (MacPherson et al. [2005] andreferences therein). However, some chondrules yieldnearly consistent formation ages with CAIs according toabsolute 206Pb/207Pb and relative 26Al chronometers(Bizzarro et al. 2004; Connelly et al. 2012). Theexistence of CAI-bearing chondrules and chondrule-bearing CAIs (e.g., Maruyama et al. 1999; Itoh andYurimoto 2003) suggests possible spatial and temporalconnections between chondrule and CAI formation.
Aluminum-rich chondrules (ARCs), as defined byBischoff and Keil (1984), are a subset of chondruleswith more than 10 wt% bulk Al2O3. ARCs havemineralogies and bulk compositions intermediatebetween CAIs and “normal” ferromagnesian chondrules(Sheng et al. 1991; MacPherson and Huss 2005). MostARCs consist of both Ca,Al-rich minerals such asspinel, anorthositic plagioclase, and high-Ca pyroxene,and Mg,Fe-rich minerals like olivine and low-Capyroxene. Glassy or microcrystalline phases also exist.ARCs have relatively 16O-enriched oxygen isotopecompositions compared to their ferromagnesiancounterparts in ordinary, carbonaceous, and enstatitechondrites (Maruyama et al. 1999; Russell et al. 2000;Guan et al. 2006; Krot et al. 2006a, 2006b). Evidencefor 26Al was found in ARCs (Sheng et al. 1991; Russellet al. 1996; Galy et al. 2000; Hsu et al. 2003). And over15% of ARCs in carbonaceous chondrites contain CAIrelicts (Krot et al. 2006c). The distinct characteristics ofARCs make them a unique window to study the originof chondrules and CAIs and their petrogeneticrelationship (Krot and Keil 2002; Krot et al. 2002;Russell et al. 2005; Akaki et al. 2007).
Laboratory crystallization experiments have shownthat Al-rich and ferromagnesian chondrules can beformed under similar thermal conditions (Tronche et al.2007). However, the origin of ARCs is still underdebate. On the basis of petrographic, mineralogical,chemical, and oxygen isotopic studies, ARCs are widelybelieved to have formed from hybrid precursors offerromagnesian chondrules and spinel-anorthite-pyroxene-rich CAIs (Krot and Keil 2002; MacPhersonand Huss 2005; Krot et al. 2006a; Zhang and Hsu2009). However, this model is faced with at least twoproblems. First, oxygen isotope compositions of ARCsare too 16O-poor to be formed from simple mixing offerromagnesian chondrules and CAIs (Russell et al.2000; Guan et al. 2006). Second, the mineralogy ofpotential precursor CAI is restricted. Melilite, acommon phase in CAIs, is barely present in ARCs.Among all chondrites, the CV group has abundant (10vol%, Scott and Krot 2005) and the most diversepopulation of refractory inclusions, among whichmelilite-rich type A CAIs are very common. The
motivation of our study is to conduct an extensivesurvey of ARCs and search for melilite-bearing ARCsin CV3 chondrites, and to obtain systematic oxygenisotope data of high accuracy and precision with aCameca IMS 1280. Here, we report our detailedpetrographic, mineralogical, chemical, and oxygenisotopic studies of twenty ARCs from five CV3chondrites (Northwest Africa [NWA] 989, NWA 2086,NWA 2140, NWA 2697, NWA 3118), and theNingqiang carbonaceous chondrite and provide newevidence for the origin of ARCs. Ningqiang is a uniquemeteorite which has been classified as CV3 anomalous,CK3 anomalous, and a type 3 ungrouped chondrite(Wang and Hsu [2009] and references therein). It closelyresembles CV3.
SAMPLES AND METHODS
Thirty polished thick and thin sections of five CV3chondrites (NWA 989, NWA 2086, NWA 2140, NWA2697, NWA 3118) and Ningqiang were examined with aNikon E400 POL optical microscope and a Hitachi S-3400N scanning electron microscope (SEM) equippedwith an Oxford X-Max20 energy dispersive spectroscope(EDS). ARCs and CAIs were located by automaticlarge area X-ray mapping with a resolution of 2–5 lm pixel�1.
Bulk major element compositions of chondruleswere obtained with EDS (Oxford INCA). Theaccelerating voltage was 15 kV and live time was 200 s.With the random-area-select tool of the attachedsoftware, the outline of a given chondrule was drawn upcarefully and then the average composition of thechondrule was obtained. Standard minerals wereanalyzed for calibration. Due to the relatively lowaccuracy of EDS compared to wavelength dispersivespectroscope (WDS) analysis, only results with totals of95–105 wt% were accepted and normalized to 100%.Metallic Fe and FeO cannot be discriminated with themethod, therefore total Fe contents are reported in theform of FeO. The induced error is minor for our Al-rich and subaluminum chondrules because metallic ironaccounts for a very small proportion (2–5 vol%) inthem. The total error of EDS analysis is estimated to be<10%, which is acceptable compared with theuncertainty introduced by using 2-D analysis torepresent 3-D situation (errors from <1% up to 30%,Hezel and Kiebwetter 2010). Similar method has beenemployed previously (e.g., Zhang and Hsu 2009).
Quantitative analyses of minerals and glasses werecarried out with an electron microprobe JEOL JXA-8100M at China University of Geosciences, Wuhan.The accelerating voltage was 15 kV and the beamcurrent was 10–20 nA. Both synthetic and natural
Aluminum-rich chondrules from CV3 chondrites 117
mineral standards were used, and ZAF corrections wereapplied.
In situ oxygen isotope analysis was performed witha Cameca IMS 1280 ion microprobe at Institute ofGeology and Geophysics, Chinese Academy of Sciences,Beijing. A Cs+ primary beam of ~200 pA and ~5 lmdiameter was accelerated to 10 kV. The sample voltagewas set to �10 kV and extracted to real ground. Anormal-incident electron gun was used to compensatefor sample charging. Secondary ions were measured inmulticollection mode: using multicollector Faraday cup(FC) and electron multiplier (EM) for 16O and 18O,respectively, with mass resolving power (MRP) of 2500,and using monocollector EM for 17O with MRP of8500. Interference from 16OH can be resolvedcompletely. Carbon-coated samples were presputteredover 25 9 25 lm area for 150 s, and then rastered overcentral 10 9 10 lm area. Secondary ions wereintegrated for 200 s (4 s 9 50 cycles). Each analysistook ~7 min. The typical secondary 16O intensity is~4 9 107 cps. Measured 18O/16O and 17O/16O ratioswere reduced for dead time, background, and relativeyield corrections, and normalized to the Viennastandard mean ocean water composition (VSMOW,18O/16O = 0.0020052 and 17O/16O = 0.000383). Oxygenisotope compositions are reported as d17O and d18O:
d17Oð&Þ ¼ ð17O=16OÞsample
ð17O=16OÞVSMOW
� 1
" #� 1000;
d18Oð&Þ ¼ ð18O=16OÞsample
ð18O=16OÞVSMOW
� 1
" #� 1000
Instrumental mass fractionation (IMF) wascalibrated with terrestrial mineral standards: San Carlosolivine (d18O = 5.3&, d17O = 2.7&; Eiler et al. 1995)for olivine; enstatite (d18O = 1.9&, d17O = 0.9&;measured by R. N. Clayton at Chicago University inOctober, 1997) for pyroxene; Meyakejima anorthite(d18O = 6.4&, d17O = 3.3&; Yurimoto et al. 1994) forplagioclase and glass; and Burma spinel (d18O = 24.1&,d17O = 12.2&; Jabeen et al. 1998) for spinel andhibonite. It is not easy to have standards that cover allunknowns in composition and mineral structure.However, IMF is correlated with the molar Mg/(Mg+Fe) ratio of olivine, Ca/(Ca+Na+K) ratio ofplagioclase, and Ca/(Ca+Fe+Mg) ratio of pyroxene(Ushikubo et al. 2012). Under our analytical conditions,the differences in matrix effect between standards andunknowns are estimated to be ~1–2& in D17O.Analytical errors of individual analyses are 2–3& (2r,for D17O), including both internal precision of unknownsamples and external precision of standard analyses.
The reproducibility of repeated standard analyses is~1.0& and ~1.5& (2r) for d18O and d17O, respectively.Nuclear magnetic resonance (NMR) controller was usedto stabilize the magnetic field to an instrumental drift(DM/M) within 2.5 ppm over 16 h. After SIMSanalysis, the phases being analyzed were verified byexamining the sputtered areas with SEM undersecondary and backscattered electron modes.
RESULTS
Bulk Major Element Composition
Bulk major element compositions of ARCs arelisted in Table 1. Most ARCs (90%) have a bulk Al2O3
content of 10–15 wt%. Only two chondrules are highlyAl-enriched, with 27.5 and 31.0 wt% Al2O3,respectively. NWA 3118_ARC#1 with 9.7 wt% Al2O3
was loosely defined as an ARC because it has a relictCAI which might give hint on the petrogenesis ofARCs. A minor amount of Na2O (<3.5 wt%) is present.FeO content (4–22 wt%) of ARCs in CV chondrites ismuch higher than that in ordinary chondrites (0.2–12%;MacPherson and Huss 2005).
Among the chondrules with <10 wt% bulk Al2O3,nearly one half (20 out of 42 chondrules) has bulkAl2O3 >5 wt%, with an average of 7.3 wt% (Table 2).Because the definition of ARCs (>10 wt% Al2O3) isarbitrary, we designate chondrules with 5–10 wt% bulkAl2O3 as subaluminum chondrules (SACs), andcompare them with ARCs in this paper. Subaluminumchondrules have comparable Na2O (<2.1 wt%) and FeO(3.8–19 wt%) contents with ARCs, but their MgOcontents (24–40 wt%) are marginally higher and CaOcontents (3.0–8.3 wt%) are lower than those of ARCs(Table 2).
Petrography and Mineralogy
The textures and mineralogies of ARCs aresummarized in Table 1. ARCs in CV3 chondrites arespheroidal and mostly millimeter-sized, with an averagediameter of 1.1 mm, almost identical to the mean of CVchondrules (1.0 mm, Scott and Krot 2005).
Typical porphyritic ARCs are shown in Figs.S1–S3. NWA 2697_ARC#4 is a porphyritic olivine (PO)ARC (Figs. S1a and S1b). Euhedral to subhedral olivinephenocrysts up to 500 lm were embedded in the coarse-grained mesostasis composed of dendritic plagioclaseand interstitial diopside. Ningqiang_ARC#1 is thelargest (2 mm in diameter) PO ARC and has a core-mantle structure (Figs. S1c and S1d). The core iscomposed of plagioclase laths (up to 1 mm long) andovergrown diopside and pigeonite, with sporadic
118 Y. Wang et al.
Table 1. Bulk major element compositions (wt%) and petrographic characteristics of Al-rich chondrules from CV3 chondrites.
NWA 2140_ARC#1 12.4 0.7 28.0 39.8 3.7 15.4 1.1c Porphyritic, with Pl, Di-rich core + Ol, Pyx-rich mantle Pl+Pyx+OlNWA 2140_ARC#2 10.5 0.5 23.1 44.4 8.3 13.2 0.9 Porphyritic, with Pl groundmass Ol+Pl+Pyx+SpNWA 2140_ARC#3 10.9 1.0 32.4 43.8 7.3 4.5 1.5 Barred olivine with interstitial glass Ol+Gls+PyxNWA 2140_ARC#4 11.8 0.6 20.8 43.2 7.3 16.4 0.8 Incompact and altered, with Pl- and En, Ol-rich patches Pyx+Pl+Ol+SpNWA 2140_ARC#5 10.6 2.9 23.4 42.4 4.8 16.1 0.7 Porphyritic, with Pl laths in the center Ol+Pl+PyxNWA 2140_ARC#6 10.0 1.0 33.7 43.2 6.2 6.0 0.7 Barred olivine with interstitial glass and Di Ol+Gls+Di
NWA 2697_ARC#1 11.2 0.9 21.4 46.1 10.3 10.1 1.0 Granular, with Pl groundmass Cpx+Pl+OlNWA 2697_ARC#2 12.8 2.8 21.0 44.8 5.2 13.5 1.0 Porphyritic, with Pl laths in the center Pyx+Pl+OlNWA 2697_ARC#3 12.6 2.8 21.4 45.1 7.4 10.7 1.2 Porphyritic, with Pl laths and groundmass Pyx+Ol+PlNWA 2697_ARC#4 10.1 1.8 32.6 42.5 5.2 7.8 1.7c Porphyritic, with Pl laths in the center Ol+Pl+PyxNWA 2697_ARC#5 14.6 0.4 17.7 44.9 13.2 9.3 0.7 Porphyritic, with Pl, Di-rich core + Di-rich mantle Pl+Di+Ol+SpNWA 989_ARC#1 12.4 0.9 23.1 47.1 7.5 9.0 1.6 Porphyritic, with Pl laths Pyx+Pl+OlNWA 989_ARC#2 13.7 3.5 21.5 44.5 8.6 8.3 1.8 Granular, with Pl laths and groundmass Pyx+Pl+Ol+SpNWA 989_ARC#3 11.1 nd 40.0 38.1 5.5 5.3 0.3 Porphyritic, with a spinel phenocryst Ol+Di+SpNWA 2140_ARC#7 27.5 2.3 15.6 34.0 11.2 9.5 0.6 Incompact and altered, with Sp, Mel-rich CAI relicts Cpx+Sp+Ol+Mel+Pl+PervNWA 989_ARC#4 12.6 0.8 20.7 38.1 5.5 22.3 0.9c Porphyritic, with a Pl, Sp-rich CAI relict Pyx+Ol+Pl+SpNWA 3118_ARC#1 9.7 nd 34.8 39.7 4.6 11.2 1.0 Porphyritic, with a Pl, Sp-rich CAI relict Ol+Pl+Sp+PyxaTotal Fe content, with a small portion of metallic Fe normalized to FeO.bMinor phases include FeNi metal, oxide, sulfide, and secondary minerals of nepheline and sodalite.cFor incomplete ARCs, their diameters were estimated on the remaining outlines.
nd = not detected; Pl = plagioclase; Ol = olivine; Pyx = pyroxene; En = enstatite; Sp = spinel; Cord = cordierite; Gls = glass; Di = diopside; Cpx = clinopyroxene;
Mel = melilite; Perv = perovskite.
Aluminum-rich
chondrules
from
CV3chondrites
119
euhedral olivine crystals and the mantle is composed ofolivine and minor pyroxene. NWA 2697_ARC#2 andNWA 989_ARC#1 (Fig. S2) are porphyritic pyroxene(PP) ARCs dominated by euhedral to subhedralenstatite phenocrysts. Plagioclase exists as lathy crystalsor blocky groundmass. Finer grained olivine is usuallyenclosed by enstatite or plagioclase. NWA2140_ARC#5 and NWA 2697_ARC#3 are porphyriticolivine-pyroxene (POP) ARCs (Fig. S3). Plagioclaseexhibits lathy or blocky morphologies.
Ningqiang_ARC#2 and NWA 989_ARC#2 are twogranular olivine-pyroxene (GOP) ARCs.Ningqiang_ARC#2 consists of an Al,Ca-rich core and aMg,Fe-rich mantle (Fig. S4). Euhedral to subhedralolivine and enstatite grains are embedded in plagioclasegroundmass in the core. Olivine, pyroxene, FeNi metal,and sulfide constitute the mantle. In NWA 989_ARC#2,several patches of spinel were found in blockyplagioclase. Spinel (Fig. S5b) has corroded outlines andFe-rich haloes, and might be a relict phase.
Glass accounts for most Al of bulk chondrules inbarred olivine (BO) ARCs. In NWA 2140_ARC#6,olivine bars are parallel (Figs. S6c and S6d). However,in NWA 2140_ARC#3, olivine bars made up concentricpolygons, with some intersecting at 120° triple junctions(Figs. S6a and S6b).
Ningqiang_ARC#3 is the most Al-rich chondrule(31 wt% Al2O3) in this work. It consists of 33 vol%plagioclase, 25% spinel, 25% Al-rich enstatite, 6%cordierite, 1% olivine, and 10% of metallic andsecondary phases (Fig. 1). The chondrule has a Mg,Fe-rich mantle of Al-enstatite, olivine, and metallic phases,and an Al,Ca-rich core where euhedral to subhedralspinel grains (5–15 lm) are poikilitically enclosed by Al-enstatite, plagioclase, and cordierite (Figs. 1b–d).
It is noteworthy that some ARCs are texturallyand/or compositionally heterogeneous. Take NWA2697_ARC#3 as an example. Its upper left part containsa barred olivine area (boxed in Fig. S3b); the upper andlower right quarters are dominated by pyroxenephenocrysts; and the lower left is porphyritic olivineand pyroxene embedded in the groundmass ofplagioclase. In NWA 2140_ARC#4, there are twodistinct patches surrounded by heavily altered pyroxenes(Fig. S7d): one is plagioclase-dominated with spinelinclusions (Fig. S7b); the other is enstatite and olivinerimmed by diopside (Fig. S7c).
Three ARCs have relict CAIs. NWA 2140_ARC#7 isa 600 lm round chondrule (Figs. 2a–e). CAI componentsin this chondrule include scattered spinel grains(<15 lm); spinel nodules; and a type A CAI relictcomposed mainly of spinel, melilite, and perovskite. Mostprimary silicate phases of the chondrule have beenaltered. NWA 989_ARC#4 (Figs. 2f–h) is a chondrule-
Table 2. Bulk major element compositions (wt%) ofsubaluminum and ferromagnesian chondrules fromCV3 chondrites.
CAI complex. Not only is the CAI included in thechondrule, but the chondrule has also partly mergedinto the CAI (Fig. 2h). The chondrule part isdominated by low-Ca pyroxene, with minor olivine,clinopyroxene, FeNi metal, and sulfide. The CAI partis blocky plagioclase with included olivine and spinel.The chondrule–CAI boundaries are basically sharp.Most spinel grains have corroded outlines and Fe-richhaloes. In NWA 3118_ARC#1 (Figs. 2i–k), a bean-shaped CAI composed of euhedral to subhedral spinel,plagioclase, and dendritic pigeonite is enclosed by aporphyritic olivine chondrule. Abundant triplejunctions are present among olivine crystals (Fig. 2k).The chondrule-CAI boundary is not as clear as that inNWA 989_ARC#4, indicating a higher fusion degreeof this complex.
Secondary alteration is ubiquitous in ARCs. InNWA 2140_ARC#4 (Fig. S7), enstatite is partlyreplaced by fayalitic olivine. Fractures spread all overthe chondrule, filled by secondary aluminosilicate. InNWA 2140_ARC#7 (Fig. 2), secondary nepheline,sodalite, and hedenbergite constitute a considerableportion of the chondrule, indicating the occurrence ofsignificant iron-alkali-halogen metasomatism.
Subaluminum chondrules have similar textures toARCs, including 15 porphyritic and 4 barred olivinechondrules. All subaluminum chondrules contain lath-shaped plagioclase or Al-rich mesostasis of plagioclaseor glass. Two BO chondrules have spinel inclusions inthe mesostasis. And NWA 989_SAC#3 has a relict CAIconsisting of spinel, plagioclase, and armalcolite(Fig. S8).
Fig. 1. BSE images of Ningqiang_ARC#3, the most Al-rich chondrule in this work. It has a core-mantle structure. The mantle isMg,Fe-rich and composed of Al-enstatite and minor metallic phases and olivine. The core is Al,Ca-rich with spinel crystalspoikilitically enclosed by Al-enstatite, plagioclase, and cordierite. Please refer to Tables 1 and 4 for abbreviations.
Aluminum-rich chondrules from CV3 chondrites 121
122 Y. Wang et al.
Mineral Chemistry
Olivine in ARCs is mostly forsteritic with anaverage Fa (mole% of Fe to Fe+Mg) of 7.8 (Table 3).Secondary olivine is fayalitic with Fa up to 43.Concentrations of minor elements are very low (0.10 wt%TiO2, 0.24 wt% Al2O3, 0.11 wt% Cr2O3, 0.09 wt%MnO, and 0.34 wt% CaO on average). No systematicdifference has been found between compositions ofolivine from ARCs, ferromagnesian chondrules, andCAIs.
Enstatite (Fs0.6–3.6Wo0.7–5.0) in ARCs is characterizedby high concentrations of Al2O3 and TiO2 (3.5 and 0.7wt% on average, respectively) (Table 3). Figure 3 showsthat Al2O3 and TiO2 contents of enstatite in ARCs canreach up to 12.5 and 2.3 wt%, respectively, which aremuch higher than those of enstatite in ferromagnesianchondrules. Pigeonite (Fs0.7–8.8Wo5.3–9.3) in ARCs alsocontains significant amounts of Al2O3 (2.2 wt%) andTiO2 (0.7 wt%).
High-Ca pyroxene (Fs0.3–2.0Wo33–54) in ARCsprimarily includes augite and diopside. Hedenbergite is
mostly secondary. High-Ca pyroxene has higherconcentrations of Al2O3 (1.2–17.1 wt%) and TiO2 (0.6–3.3 wt%) than low-Ca pyroxene. Other minor elementsinclude Cr2O3 (0.2–1.4 wt%) and MnO (<0.6 wt%).
Fig. 2. BSE and X-ray images of ARCs with relict CAIs. a–e) NWA 2140_ARC#7 contains CAI components of spinel nodulesand scattered grains (c), and a type A CAI fragment composed primarily of spinel, melilite, and perovskite (b). The chondrule isrimmed by high-Ca pyroxene (d). Na-rich secondary phases (e) account for a large proportion of the chondrule. Note that theX-ray images are rotated to the left for � 30� from the position of image (a). f–h) NWA 989_ARC#4. The relict CAI iscomposed of plagioclase, spinel, and minor olivine (g) and the host is a POP chondrule (Ch). The boundaries between chondruleand CAI are largely sharp. In some parts, however, chondrule materials have been partially mixed with CAI. The arrowed areaof chondrule materials in (f) is shown by (h). i–k) NWA 3118_ARC#1 is a PO chondrule (k) with a relict CAI (j) composed ofplagioclase, spinel, and minor pigeonite. The chondrule–CAI boundary is dimmer than that in NWA 989_ARC#4. The arrows in(k) point to triple junctions. Note that images (j) and (k) are rotated to the right for 90° from original position.
Table 3. Average compositions (wt%) of major components in Al-rich chondrules.
En 95.6 (1.7) 91.6 (2.6) 57.4 (5.1)An 88.5 (5.1)Ab 11.5 (5.1)aGlass-1: from NWA 2140_ARC#3.bGlass-2: from NWA 2140_ARC#6. Results of EDS analysis.c1r error.
Fig. 3. Concentrations (in wt%) of Al2O3 versus TiO2 inenstatite from Al-rich and ferromagnesian chondrules in CV3chondrites.
Aluminum-rich chondrules from CV3 chondrites 123
Plagioclase (An77–99Ab1–23) in ARCs is anorthiticwith an average An (mole% of Ca to Ca+Na+K) of88.5 (Table 3). Minor elements include FeO (0.59 wt%)and MgO (0.57 wt% on average). It is evident from theAn histogram (Fig. 4) that plagioclase in ARCs coversthe combined range of ferromagnesian chondrules andCAIs. Plagioclase in ARCs is much more sodic thanthat in CAIs and marginally more calcic than that inferromagnesian chondrules. Plagioclase in CAI relictshas a similar An range to that in ARCs.
Spinel in ARCs is essentially (Mg,Fe)Al2O4 with0.3–6.7 wt% Cr2O3, 0.2–1.5 wt% TiO2, and <0.4 wt%CaO. The Fe/(Fe+Mg) ratio (in mole%) variesdramatically from 1.5 to 45. Spinel in ARCs has muchhigher Cr2O3 and slightly lower CaO content than inCAIs (Fig. 5). Most spinel in ARCs has comparablecontent of TiO2 with that in CAIs. Compared to thosefrom ARCs and CAIs, spinel from CAI relicts hasintermediate concentrations of Cr2O3 and CaO. Thearmalcolite grain in a relict CAI is mainly composed of72.9 wt% TiO2, 19.4 wt% FeO, and 7.7 wt% MgO,corresponding to a structural formula of (Fe0.6Mg0.4)Ti2.0O5.0.
Cordierite in Ningqiang_ARC#3 is composed of52.0 wt% SiO2, 30.3 wt% Al2O3, 10.5 wt% MgO, 5.39wt% Na2O, and 1.05 wt% K2O (average of 10 electronmicroprobe analyses). It is Na-rich and has an excess ofcations (11.6 relative to 18 O2�) in stoichiometry, which
is similar to Na-rich cordierites from Allendechondrules (Fuchs 1969; Akaki et al. 2007).
Oxygen Isotope Compositions
Oxygen isotope compositions of 14 ARCs weremeasured with an ion microprobe. CAIs, subaluminumand ferromagnesian chondrules were also measured forcomparison. The data are listed in Table 4 and plottedin Figs. 6 and 7. In Fig. 6, phases within an individualARC are plotted in one graph. In Fig. 7, oxygenisotope data of all components are plotted together forcomparison.
Except two points of plagioclase inNingqiang_ARC#1, all data of minerals in porphyriticARCs plot along the CCAM (carbonaceous chondriteanhydrous mineral) line on the three-oxygen-isotopediagram, spanning a wide range from �32.1 � 1.2& (2r)to 13.9 � 0.9& in d18O. In a given chondrule, olivine andpyroxene are usually on the 16O-rich end, and plagioclaseis always on the 16O-poor end (Fig. 6a). Spinel hasgenerally the lightest oxygen isotope among mineralphases (Figs 6c–e). Most ARCs have relatively uniformO isotope compositions with D17O varying within 4–6&,while spinel-bearing ARCs are more heterogeneous withD17O varying up to 16& (Table 4). Spinel and olivine inNWA 989_ARC#2 (Fig. 6d) are highly 16O-enriched withD17O of �18&, suggesting that they are probably relicts
76 78 80 82 84 86 88 90 92 94 96 98 1000
10
20
30
40
Counts
An (mol%)
(d) CAIs (N=40)0
2
4
6
8
Counts
(c) FMCs (N=24)0
2
4Counts
(b) CAI Relicts in Chondrules (N=5)0
5
10
15
20
Counts
(a) ARCs (N=65) Plagioclase
Fig. 4. Histogram of anorthite (An) contents (in mole%) of plagioclase in ARCs (a), CAI relicts in chondrules (b),ferromagnesian chondrules (FMCs) (c), and CAIs (d). Data are from this work and Sheng et al. (1991) and Brearley and Jones(1998).
124 Y. Wang et al.
of refractory inclusions. In NWA 2697_ARC#5 (Fig. 6e),the oxygen isotopes (D17O ~ �14&) of fine-grained spinelwere infected by adjacent plagioclase. Nevertheless, spinelmight also be a relict phase because it has corrodedoutlines and Fe-rich haloes similar to spinel in NWA989_ARC#2 (Fig. S5). In Ningqiang_ARC#3, bycontrast, spinel grains were crystallized from a chondrulemelt and are less 16O-enriched with D17O of �10&(Fig. 6c). Oxygen isotope compositions of the BOchondrule NWA 2140_ARC#3 plot between the TF(terrestrial fractionation) and CCAM line (Fig. 6b), withd18O ranging from �9.7 � 1.2& of olivine to3.1 � 0.9& of glass. The deviation from CCAM line is
likely due to the isotopic exchange between the chondrulemelt and the solar nebula.
ARCs with relict CAIs are more 16O-enriched thanthose without (Table 4). Oxygen isotopic exchangebetween CAI relicts and the host chondrule was limited.Plagioclase in relict CAIs has similar D17O values tothat in chondrules; however, spinel in relict CAIs isdistinctly more 16O-rich than chondrule minerals. Spinelin NWA 2140_ARC#7 and NWA 3118_ARC#1 hasd18O values (�39.4 and �35.3&, respectively)comparable to spinel in “normal” CAIs. In NWA989_ARC#4, the apparent D17O values of spinel couldbe higher than reality because the analyses wereconducted on spinel–plagioclase mixtures due to thesmall grain size of spinel. Plagioclase is the most 16O-poor phase no matter when it occurs in a CAI orchondrule. Olivine and pyroxene have similar oxygenisotopes to those in other ARCs.
Oxygen isotope compositions of subaluminumchondrules generally plot along the CCAM line, spanninga slightly narrower d18O range (�32.5 to 5.9&) than thatof ARCs (Fig. 7). Spinel in NWA 989_SAC#3 retainedmost of its oxygen isotope signature as a relict CAIcomponent, being highly 16O-enriched with d18O of�32.5& and d17O of �40.6& (Table 4).
It is shown in Fig. 7 that the oxygen isotopecompositions of ARCs, CAIs, AOAs, andferromagnesian chondrules all spread along the CCAMline. Best fit line of the ARC data yields a slope of0.88 � 0.02, which is close to the slopes of the CCAMline (0.94) and the regression line (0.83) of ARCs fromUOCs (Russell et al. 2000). Refractory inclusions arethe most 16O-enriched with d18O extending to �45&.Ferromagnesian chondrules have d18O ranging between�8.4 and 4.3&. ARCs have intermediate compositions,overlapping with ferromagnesian chondrules on the 16O-poor end and intersecting with refractory inclusions onthe 16O-rich end. Due to the limited number of analyses,oxygen isotope compositions of ferromagnesianchondrules obtained here are far from representative.Typical ferromagnesian chondrules in CV3 chondriteshave a wider d18O range between �20 and 5&(Yurimoto et al. 2008). Except the ARCs with relictCAIs (Figs. 6f and 6g) and relict grains (Figs. 6d and6e), most ARCs have oxygen isotope compositions(>�20& in d18O) consistent with ferromagnesianchondrules.
DISCUSSION
Comments on Nomenclature for ARCs
Different schemes have been applied to classifyARCs with diverse characteristics. On the basis of
Fig. 5. Concentrations (in wt%) of Cr2O3 versus TiO2 (a) andCaO (b) in spinel from ARCs, CAI relicts in ARCs, and CAIs inCV3 chondrites. Data are from this work and Sheng et al.(1991), Brearley and Jones (1998), and Srinivasan et al. (2000).
Aluminum-rich chondrules from CV3 chondrites 125
Table 4. Oxygen isotope compositions of Al-rich chondrules and other components.
chemical composition, ARCs had been divided intoCa-Al-rich, Ca-Na-Al-rich, Na-Al-rich, Na-Cr-Al-rich,and Ti-Al-rich chondrules by Bischoff and Keil (1984).Mineralogy-based names such as plagioclase-olivineinclusion (POI), anorthite-rich, plagioclase-rich, oraluminum-diopside-rich chondrules have also beenused (Krot et al. 2001, 2002; Krot and Keil 2002; Hsuet al. 2003). Recently, MacPherson and Huss (2005)proposed a new classification system, based on thebulk composition and phase equilibria of ARCs.According to the earliest-crystallizing silicate phase,ARCs were divided into olivine-phyric [Oliv] andplagioclase-phyric [Plag] Al-rich chondrules, and athird variety, glass-rich [glass] chondrule. Comparingwith previous methods, this scheme seems morerational and has been applied well to ARCs fromordinary and some carbonaceous chondrites(MacPherson and Huss 2005). However, when we tryto classify ARCs studied here with this method, weencountered some problems.
First, the phase diagram used by MacPherson andHuss (2005) to infer theoretical crystallization sequenceof a chondrule is only valid for a melt with low ironand sodium, presumably less than 4 wt% FeO and 3 wt%Na2O (Sheng 1992). The addition of 6% FeO to thestarting composition can shift phase boundariesdramatically (Sheng 1992). However, due to thepresence of secondary phases (e.g., fayalite, hedenbergite,
nepheline, and sodalite), most ARCs in this workcontain significant amounts of FeO (4–22 wt%, with avery small part contributed by metallic Fe) and Na2O(<3.5 wt%). Contributions of secondary and sometimesrelict phases are not readily subtracted from the bulkcomposition. Therefore, for most of the ARCs, theearliest-crystallizing phase cannot be determineddirectly from their bulk composition. In addition, theearliest-crystallizing silicate phase may not be phyric.NWA 989_ARC#1 might be a case (Fig. S2b), wheresmall olivine grains were enclosed in enstatitephenocrysts and plagioclase groundmass. Olivineprobably crystallized earlier than enstatite andplagioclase. However, naming this chondrule as an[Oliv] ARC is not reconciled with its enstatite-dominantpetrography. This conflict is easily understood on thephase diagram. For a chondrule with a bulkcomposition in the Fo+L region and falling close to theFo+An+L boundary curve, only a small amount ofolivine could be crystallized before the evolving melt hitthe boundary curve and started to crystallize anorthite.Therefore, the first crystallizing phase might be a minorphase in texture. Similarly, a [Plag] ARC might betexturally dominated by phases other than plagioclase.Third, from the perspective of petrography, besidesolivine-, plagioclase-, and glass-rich ARCs proposed byMacPherson and Huss (2005), some ARCs aredominated by pyroxene (Fig. S2).
Table 4. Continued. Oxygen isotope compositions of Al-rich chondrules and other components.
Hib, hibonite; Mel, melilite; Chon, chondrule; Inter, chondrule-CAI interface. Phases in parentheses are minor components in the ion
microprobe craters.
Fig. 6. Oxygen isotope compositions of representative ARCs from NWA 2140 (a, b), Ningqiang (c), NWA 989 (d, f), NWA2697 (e), and NWA 3118 (g). All data of minerals in porphyritic ARCs plot along the CCAM line, while the data of barredolivine ARC scatter between the TF and CCAM line (b). Olivine and pyroxene are always more 16O-rich than plagioclase in agiven chondrule, and ARCs with relict grains (d, e) or CAIs (f, g) extend further to the 16O-rich end than normal ARCs (a–c).Gray symbols represent analyses with minor mixing phases (see Table 4 for detail), mostly due to small grain sizes. Error barsare 2r. TF = terrestrial fractionation line, CCAM = carbonaceous chondrite anhydrous mineral line.
130 Y. Wang et al.
Aluminum-rich chondrules from CV3 chondrites 131
Therefore, ARCs are not subdivided here on theirbulk composition. We choose to depict them in theway we do for normal chondrules, e.g., porphyriticolivine (PO) ARC, porphyritic pyroxene (PP) ARC,porphyritic olivine-pyroxene (POP) ARC, barredolivine (BO) ARC, and granular olivine-pyroxene
(GOP) ARC, etc. When using these names, however,one must bear in mind that ARCs usually contain Al-rich components as anorthite, spinel, or glass besidesolivine, pyroxene, and metallic phases. It seems thatARCs from different chondritic groups might suitdifferent classification schemes because of the diversityin their characteristics.
Comparison between Subaluminum and Al-Rich
Chondrules
This might be the first time that chondrules with 5–10 wt% bulk Al2O3 are defined as subaluminum andcompared with chondrules with >10 wt% Al2O3. Ourpurpose is not to set a new Al content boundarybetween Al-rich and ferromagnesian chondrules, but tofind out factors that might account for characteristics ofARCs.
Subaluminum chondrules do not differ significantlyfrom most ARCs in bulk compositions. Except for two
with >25 wt% Al2O3, ARCs have an average Al2O3
content of 11.8 wt%. Nineteen subaluminum chondruleshave average Al2O3 content of 7.3 wt%, with eightchondrules above 8 wt%. Al-rich phases, such asplagioclase, spinel, and glass, are also present insubaluminum chondrules, although in less abundancethan in ARCs. NWA 989_SAC#3 has a relict CAIconsisting of spinel (d18O: �32.5&), plagioclase, andarmalcolite. Similar to minerals in ARCs, enstatite insubaluminum chondrules has high concentrations ofAl2O3 and TiO2 (up to 10.6 and 0.7 wt%, respectively);spinel has a minor amount of Cr2O3 (0.3–4.7 wt%); andplagioclase is anorthitic (An82–95). Subaluminumchondrules span a slightly narrower range (d18O: �32.5to 5.9&) than ARCs (�39.4 to 13.9&) on the three-oxygen-isotope diagram. Adding the data ofsubaluminum chondrules to the regression line of ARCswould barely change the slope from 0.88 � 0.02 to0.90 � 0.02. If the relict phases in subaluminumchondrules were not taken into account, the oxygenisotopes of subaluminum chondrules would be consistentwith those of typical ferromagnesian chondrules.
Therefore, we conclude that subaluminumchondrules have similar characteristics with ARCs inpetrography, mineral chemistry, and oxygen isotopecompositions. The 5 wt% deviation of bulk Al2O3
content would not affect the nature of chondrulesdramatically. Therefore, the definition of Al-richchondrule could be more flexible. From the O isotopeperspective, chondrules that have relict components ofrefractory inclusions might have similar origins,regardless of their bulk compositions.
Origin of Al-Rich Low-Ca Pyroxene in Chondrites
Al-rich low-Ca pyroxene is extremely rare in theterrestrial system. The limited occurrences are usuallyrelated to mantle xenolith, high-grade metamorphism,or hydrous environment (Arai and Abe 1995; Rauchand Keppler 2002; Brandt et al. 2003). However, Al-rich low-Ca pyroxene is common in ARCs andrefractory inclusions of primitive chondrites (e.g., Fuchs1969; Krot and Keil 2002; Rubin 2004; Zhang and Hsu2009), and sometimes present in ferromagnesianchondrules (Tachibana et al. 2003). Unlike theirterrestrial counterparts, Al-rich low-Ca pyroxene inprimitive chondrites could have formed under low-pressure and anhydrous settings. Al-rich low-Capyroxene has been found in a Ca-Al-rich chondrulefrom an ordinary chondrite (Rubin 2004). Thechondrule has 20.8 wt% bulk Al2O3 and is primarilycomposed of 73 vol% glass and 22% olivine. Accordingto Rubin (2004), the occurrence of Al-rich low-Capyroxene was due to the kinetic failure of anorthite
Fig. 7. Oxygen isotope compositions of all chondrules andrefractory inclusions analyzed in this work. ARCs haveintermediate oxygen isotope compositions overlapping withferromagnesian chondrules on the 16O-poor end andintersecting with refractory inclusions on the 16O-rich end. Thedashed line with slope of 0.88 � 0.02 is the result of linearregression of the data of ARCs.
132 Y. Wang et al.
crystallization during quenching of chondrule melt. InARCs of a CH chondrite, Al-rich enstatite (Al-enstatite)did not coexist with anorthite and its formation wasalso ascribed to kinetic effects (Zhang and Hsu 2009).
We found Al-enstatite in two ARCs and onesubaluminum chondrule in CV chondrites. However, theARCs are glass free and contain considerable amountsof anorthite. Bulk Al2O3 contents of three Al-enstatite-bearing chondrules vary from 7.3 to 31 wt%, implyingthat bulk composition was not a critical factor for Al-enstatite formation, although precursors relativelyenriched in Al are the substantial and ultimate basis.
Al-enstatite in Ningqiang_ARC#3 has 11 wt% Al2O3
on average and constitutes ~25 vol% of the chondrule.Compact core-mantle structure of the chondrule indicatescrystallization from a melt droplet. Al-enstatite andspinel crystals are euhedral to subhedral, and Al-enstatitehas a large grain size up to 800 lm in length. Thesetextures are not reconcilable with rapid quenchingprocesses that produced Al-enstatite as suggested byRubin (2004). Except for spinel and glass, Al usuallyresides in plagioclase and/or high-Ca pyroxene inchondrules. However, Ningqiang_ARC#3 has a very lowcontent of bulk CaO (4.2 wt%). Its bulk Al2O3/CaO ratio(7.4) is almost six times of the chondritic value (1.25) andis much higher than most ARCs (1.8 on average).Therefore, high-Ca pyroxene is absent and plagioclase isinsufficient to accommodate abundant Al in thechondrule, which resulted in the access of Al to enstatite.We believe that the high Al2O3/CaO ratio of bulkchondrule is the primary reason for the formation of Al-enstatite. And crystals formed in the nebula were likelymore insusceptible to lattice deformation due to theextremely low pressure of the nebula compared withterrestrial environments.
The second chondrule with Al-enstatite is NWA2140_ARC#2. Medium- to coarse-grained enstatite hasan average Al2O3 content of 8 wt%. The third one is asubaluminum barred olivine chondrule in NWA 2140.Enstatite patches are interstitial between olivine barsand contain up to 10.6 wt% Al2O3. The origin of Al-enstatite in the BO chondrule might be related to rapidcooling of the chondrule melt, as suggested by Rubin(2004) and Zhang and Hsu (2009). However, Al-enstatite in NWA 2140_ARC#2 would have formeddifferently because it is porphyritic-textured and itsbulk Al2O3/CaO ratio (1.3) is roughly chondritic.Aluminum can be incorporated in orthopyroxenes byTschermak’s substitution [4]Al[6]Al(Mg,Fe)�1Si�1 underappropriate P-T conditions (Zang et al. 1993). It islikely that Al-enstatite in NWA 2140_ARC#2 formedfrom subsolidus ion exchange with a nebular gasthat is locally concentrated in Al, probably beforeaccretion.
Genetic Relationships among ARC, Ferromagnesian
Chondrule, and CAI
The petrographic characteristics of ARCs areclosely related to those of ferromagnesian chondrules.All the ARCs from carbonaceous, unequilibratedordinary and enstatite chondrites (UOCs and UECs)have igneous textures. The average diameter of 20ARCs studied here is 1.1 mm, almost identical to thatof chondrules in CV3 (1.0 mm, Scott and Krot 2005).The proportion of nonporphyritic ARCs is 2 out of 20,which is also close to the mean value of CV chondrules(6%, Scott and Krot 2005). Similarly, ARCs in CHchondrites are small (roughly within 100 lm) like theferromagnesian CH chondrules (Zhang and Hsu 2009).Most ARCs in UOCs have diameters of 300–800 lm(Russell et al. 2000; MacPherson and Huss 2005)comparable with the average of UOC chondrules (0.3–0.6 mm, Scott and Krot 2005).
On the other side, ARCs and CAIs are also closelyrelated. According to Krot et al. (2006c), over 15% ofARCs have relict CAIs. Three ARCs found in thisstudy have relict CAIs, which are composed of spinel,plagioclase, and minor melilite and perovskite. The16O-enriched spinel and olivine (with d18O up to�32&) in NWA 989_ARC#2 and SAC#3 are probablyrelict grains of refractory inclusions. Furthermore,chondrule-bearing CAIs had also been found in COchondrites (Itoh and Yurimoto 2003). The existence ofa chondrule-CAI complex indicates that there shouldbe spatial and temporal overlapping of chondrule andCAI formation.
It is evident that ARCs have affinity with bothferromagnesian chondrules and CAIs. The bulkcompositions of ferromagnesian chondrules and type CCAIs are linked by ARCs. Minerals in ARCs haveintermediate compositions between ferromagnesianchondrules and CAIs. Low-Ca pyroxene in ARCs hasmuch higher Al and Ti contents than that inferromagnesian chondrules (Fig. 3). The Al2O3 andTiO2 concentrations of high-Ca pyroxene in ARCs arelower than those in CAIs (Brearley and Jones 1998).Plagioclase in ARCs (An77–99) is much more sodic thanplagioclase in CAIs (An96–100) and marginally morecalcic than plagioclase in ferromagnesian chondrules(An79–95) (Fig. 4). Spinel in ARCs is more enriched inCr (0.3–6.7 wt%) than spinel in CAIs, but the Cacontent of spinel in CAIs is higher than that in ARCs(Fig. 5). Similar characteristics have been observed inARCs from CR, CH, CO, and other CV chondrites(Jones and Brearley 1994; Krot and Keil 2002; Krotet al. 2002; Zhang and Hsu 2009). Oxygen isotopiccompositions of ARCs in CV chondrites, ranging from�39.4& to 13.9& in d18O, are also intermediate
Aluminum-rich chondrules from CV3 chondrites 133
between those of ferromagnesian chondrules and CAIs,being lighter than ferromagnesian chondrules andheavier than CAIs. The intermediate nature of bulkmajor element, mineral chemistry, and O isotopes ofARCs manifests that ARCs are genetically related toboth ferromagnesian chondrules and CAIs.
We note that there are two categories of ARCs: onecontains relict CAIs or individual relict grains ofrefractory inclusions, and the other does not. Theformer has intersecting oxygen isotopes (as low as�39.4& in d18O) with refractory inclusions, while thelatter (>�20&) has oxygen isotopes similar to those offerromagnesian chondrules. The less 16O-enriched ARCsare the majority of all ARCs. In this sense, ARCs mightbe more closely related to ferromagnesian chondrulesthan to CAIs. Similar conclusions have been drawn forARCs in UOCs and UECs based on their oxygenisotope compositions (Russell et al. 2000; Guan et al.2006). The petrography of ARCs is also compatiblewith formation from flash heating and rapid coolingprocesses like ferromagnesian chondrules, which hasbeen verified by analog crystallization experiments(Tronche et al. 2007).
Origin of Al-Rich Chondrules
Several models have been proposed to account forthe origin of ARCs. On the basis of the fractionatedpattern of incompatible elements in ARCs, Bischoffet al. (1989) suggested that ARCs were crystallized froman Al-rich melt splashed from partly moltenferromagnesian chondrules during collision. If themodel was correct, ARCs should have a smallerdiameter than ferromagnesian chondrules on average.However, the Al-rich and ferromagnesian chondruleshave a comparable size in CV (this work) and otherchondritic groups (Russell et al. 2000; MacPherson andHuss 2005; Zhang and Hsu 2009). MacPherson andHuss (2005) studied the bulk compositions of ARCsand proposed that the precursors of ARCs could nothave formed from equilibrium condensation of the solarnebula or evaporation of a chondritic liquid. Oxygenisotopes of ARCs in CV3 chondrites are compatiblewith this argument because ARCs are more 16O-enriched than ferromagnesian chondrules and deviatedfrom the mass fractionation line on the three-O-isotopediagram. Due to the intermediate nature of ARCsbetween ferromagnesian chondrules and CAIs, hybridmodels were proposed that ARCs formed fromprecursors of ferromagnesian chondrule- and CAI-likematerials via processes that produced normalchondrules (Sheng et al. 1991; Krot et al. 2002, 2004).Nagahara et al. (2008) have recently proposed a variantof hybrid model that ARCs formed from type C
CAI-like precursors and the elements of Si and Mgwere condensed into chondrule melt from ambientgases. This model was derived from two atypical ARCswhich have high abundances of glass (70–80 vol%) andCaO (13–14 wt%), and hence is hardly applicable tocommon ARCs with lower Ca and higher Si, Mg, Fecontents. When we talk about hybrid models in thefollowing text, this variant model is not considered.
The results of chondrule precursor modeling haveproven that CAI-like material is one precursor type ofARCs (Hezel and Palme 2007). And the occurrence ofchondrule-CAI complexes (Itoh and Yurimoto 2003;Krot et al. 2006c) is direct evidence for hybrid models.ARCs with relict CAIs might represent the initial stageof chondrule-CAI mixing, where CAI componentsremained largely unmelted. Compositionally ortexturally heterogeneous ARCs like NWA 2140_ARC#4and NWA 2697_ARC#3 might represent the incompleteblending of diverse components. The intermediatenatures of composition, mineralogy, and oxygenisotopes of ARCs in CV3 chondrites are compatiblewith hybrid precursors of ARCs composed of 16O- andrefractory element-rich and 18O- and volatile element-rich materials, probably corresponding to CAIs andferromagnesian chondrules, respectively.
Although the idea of hybrid precursors iscompatible with various studies from compositional,isotopic, and petrologic perspectives (e.g., Kennedy andHutcheon 1992; Hsu et al. 2003; Wang and Hsu 2009),some challenges have been raised recently. Based on theO isotope compositions of ARCs in UOCs, it waspointed out that if ARCs were formed from mixtures offerromagnesian chondrules and CAIs, they should bemore 16O-enriched than observed (Russell et al. 2000).Similar conclusions were reached in UECs and CHchondrites that ARCs are not simple mixtures ofmaterials from ferromagnesian chondrules and CAIs(Guan et al. 2006; Zhang and Hsu 2009). Open systemconditions like gas-melt exchange of O isotopes havebeen invoked to account for the problem. MacPhersonand Huss (2005) presented “the most serious problem”for the hybrid model that the CAI precursor wasrestricted to the pyroxene + plagioclase + spinel typeand melilite was largely missing.
One motivation of this work was to search formelilite-bearing ARCs in CV3 chondrites. It wasobserved that melilite occurs as a relict CAI componentin an ARC (Fig. 2b), but barely occurs as liquidusphase in chondrules. MacPherson and Huss (2005)proposed as a possible explanation for missing melilitein ARCs that melilite-rich CAIs were basically absent inthe CAI-chondrule hybridization region when ARCswere forming. On the basis of the finding of type ACAI residues in chondrules, however, we are inclined to
134 Y. Wang et al.
seek alternative explanations. It is common in CAIsthat melilite was replaced by calcic pyroxene, spinel,and anorthite. The alteration mechanism was suggestedas following reactions between solid melilite and thesolar nebula (Krot et al. 2004):
Experimental and meteoritic studies have shownthat high-temperature interactions between a chondrulemelt and the solar nebula played an important role forthe chondrule formation (Hewins et al. 2005; Libourelet al. 2006). It is likely that melilite in the hybrid meltreacted with an ambient hot gas and became anorthite,spinel, and diopside. The reactant of SiO and Mg couldalso come from the breakdown or reduction of mineralsin the melt (Hewins et al. 2005).
Hybrid models are probably reconcilable with theabsence of melilite in ARCs. However, this does notmean that all ARCs have to be formed from precursorsof chondrule-CAI mixture. First, most ARCs in CV3chondrites, which do not have relict CAIs or individualrelict grains, are oxygen isotopically consistent withtypical ferromagnesian chondrules. If CAIs have to beincorporated into precursors of these ARCs, they shouldbe 16O-richer than observed. Second, spinel and otherrefractory minerals only appear as minor components ofsome ARCs; most ARCs are composed of olivine,pyroxene, plagioclase, and metallic phases as someferromagnesian chondrules. Third, subaluminumchondrules are similar to ARCs in petrography,mineralogy and oxygen isotopes. An Al-rich bulkcomposition is not critical for all natures of ARCs. Forexample, the formation of Al-enstatite does not have tobe related with an Al-rich melt, but could also be theresult of kinetic factors or subsolidus ion exchange.Fourth, the extremely 16O-enriched olivine with d17O andd18O down to ~�50& has been found in aferromagnesian chondrule of CV3 chondrite (Jones et al.2004). This implies that chondrule precursors thatcondensed earliest from the solar nebula might be as 16O-rich as refractory inclusions. Last but not least, CAIs areextremely rare in ordinary and enstatite chondrites (<0.1–0.2 vol%), but ARCs are common in all chondrites (Scottand Krot 2005). If all ARCs were derived fromchondrule-CAI mixtures, a positive correlation betweenthe abundances of ARCs and CAIs should be observed.Otherwise ARCs were not formed separately inaccretionary regions of different chondritic parent bodies.
In conclusion, we suggest that ARCs may havemultiple origins. The ARCs with relict CAIs orrefractory minerals could have formed from hybridprecursors of ferromagnesian chondrules and refractoryinclusions. Due to the presence of 16O-enriched olivinein ARCs, we concur with Rubin (2004) that AOA is apossible precursor for ARCs. For ARCs with extremelyhigh bulk content of Al or Ca, e.g., Ningqiang_ARC#3,CAI components were probably involved in theirprecursors. For the majority of ARCs, however, theformation processes are similar to those of normalchondrules, only from more chemically evolvedprecursors. The evolution of bulk compositions fromferromagnesian to aluminous might have been inducedby volatility-controlled gas–solid reactions as suggestedby MacPherson and Huss (2005).
The similarities in size and texture between ARCsand their ferromagnesian cousins in different chondriticgroups indicate that ARCs might have formed indisparate nebular regions where distinct groups ofchondrules were formed. The alternative is that ARCswere formed in restricted areas of the turbulent solarnebula and then spread to wider areas by aerodynamicsorting, which is based mostly on the radius and densityof particles.
Acknowledgments—We are grateful to three reviewersDrs. Alan E. Rubin, Dominik C. Hezel, Noriko T. Kita,and Christine Floss (A.E.) for their constructive commentsand critical suggestions which have helped to improve thequality of this work. This work was supported by theNational Natural Science Foundation of China (GrantNo. 41003026, 41173076, 41273079), the Natural ScienceFoundation of Jiangsu Province (BK20131040), and theMinor Planet Foundation of China.
Editorial Handling—Dr. Christine Floss
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Fig. S1. Backscattered electron (BSE) images ofporphyritic olivine (PO) ARCs. (a, b) NWA2697_ARC#4, a PO ARC composed of euhedral tosubhedral olivine phenocrysts and mesostasis ofdendritic plagioclase and interstitial diopside. (c, d)Ningqiang_ARC#1, a PO ARC composed of aplagioclase-clinopyroxene-dominant core and an olivine-rich mantle. Please refer to Tables 1 and 4 forabbreviations.
Fig. S2. BSE images of two porphyritic pyroxene(PP) ARCs, NWA 2697_ARC#2 and NWA989_ARC#1, composed primarily of euhedral enstatiteand lathy or blocky plagioclase.
Fig. S3. BSE images of two porphyritic olivine-pyroxene (POP) ARCs, NWA 2140_ARC#5 and NWA2697_ARC#3. The boxed area in (b) appears to have abarred olivine (BO) texture.
Fig. S4. BSE and X-ray images of a granularolivine-pyroxene (GOP) ARC, Ningqiang_ARC#2. Ithas an Al,Ca-rich core and a Mg,Fe-rich mantle (e–g).
Granular olivine and pyroxene are embedded in themesostasis of plagioclase with overgrown diopside (b).The abundance of plagioclase decreases from core tomantle which consists mostly of olivine, pyroxene, andmetallic phases (c, d).
Fig. S5. BSE images of the GOP ARC, NWA989_ARC#2. The enlarged area (b) shows a cluster ofpartially resorbed spinel grains with Fe-rich haloes.
Fig. S6. BSE images of barred olivine ARCs. (a, b)NWA 2140_ARC#3. Olivine bars form concentricpolygons with 120° intersecting angles. (c-d) NWA2140_ARC#6 composed of parallel olivine bars andglass mesostasis and minor diopside exolutions.
Fig. S7. BSE images of the altered chondrule, NWA2140_ARC#4. It has a porous texture with fracturesfilled by secondary aluminosilicate. Two distinct areasare surrounded by heavily altered pyroxenes (d): one isplagioclase-dominated with included spinel and olivine(b); the other is enstatite and olivine rimmed bydiopside (c).
Fig. S8. BSE images of NWA 989_SAC#3. Theporphyritic olivine chondrule contains a CAI relictcomposed of spinel, plagioclase, and armalcolite (Am)(b).
