ALTERATION of CAIS: TIMES and PLACES. S. S. Russell and G. J. Macpherson, Department of Mineral Sciences, MRC NHB-119, U.S
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Workshop on Parent-Body and Nebular Modification of Chondritic Materials 4054.pdf ALTERATION OF CAIS: TIMES AND PLACES. S. S. Russell and G. J. MacPherson, Department of Mineral Sciences, MRC NHB-119, U.S. Museum of Natural History, Smithsonian Institution, Washington DC 20560, USA. E-mail: [email protected]. Calcium- Aluminium- rich inclusions alkali- rich prior to incorporation in the parent (CAIs) commonly contain a distinctive suite of body. Veins cross-cutting CAIs typically do secondary minerals. The chemical and isotopic not extend into the meteorite matrix, indicating compositions of these minerals can be used to they did not form in situ. Euhedral wollastonite constrain the site and timing of the alteration whiskers, nepheline needles, and grossular in event. The style of alteration in CAIs is CAI cavities are indicative of condensation from strongly dependent on the meteorite group in a vapour, and these grains probably formed in which they are found. the nebula. An alternative viewpoint, CV meteorites: CAIs from the oxidised championed by Krot et al., argues that the subgroup (e.g. Allende) show extensive signs of alteration of CV CAIs can be explained by a secondary alkali- and iron- enrichment. The parent body process of alteration by alkaline- fine grained secondary minerals (typically <10- rich fluids followed by dehydration [5]. This 20µm) include nepheline, sodalite, monticellite, process is postulated to have affected the more hedenbergite, andradite, and grossular; these oxidised CV meteorites, such as Allende, more typically embay primary minerals and fill cross- than the other CVs, a conclusion also reached cutting veins within the CAIs. Some euhedral by some other studies [e.g. 6]. whiskers of wollastonite and nepheline are CO meteorites: CAIs in CO3 chondrites have located within cavities. In addition, multi- experienced considerable secondary alteration, layered Wark-Lovering rim sequences on CAIs both before and after accretion [7, 8]. The clearly postdate the CAI interior, and in that presence of altered CAIs in unmetamorphosed sense can be considered secondary. Fine- CO3s indicates some events occurred in the grained inclusions are typically more altered nebula: formation of Wark-Lovering rims, than coarser grained ones: alteration in these melilite and anorthite breakdown, and iron inclusions consists of feldspathoid layers enrichment of spinels in hibonite-rich surrounding primary spinel. The temperature of inclusions. In contrast, correlations between melilite + anorthite breakdown to grossular + petrologic type of the host meteorite with iron monticellite in type B Allende CAIs has been content and melilite breakdown in Type A and estimated to be 668oC [1]. Hutcheon and spinel-pyroxene CAIs suggest some alteration Newton argued that the temperature must have occurred during parent body metamorphism [8]. remained around this value for a “prolonged Hibonite seems to be unaffected by the period” to allow formation of large grossular metamorphism experienced by CO3s. grains, but the timing of the high temperature CM meteorites: CAIs in CM chondrites have alteration event was probably < ~100,000 years, suffered ubiquitous aqueous alteration. The otherwise Mg diffusion would ensure the CAI CAI primary mineralogy has been altered to anorthite no longer retains a 26Mg excess [2]. pyllosilicates (Fe- and Mg-serpentines) and The location of the alkali-iron alteration has tochilinite, calcite and calcium sulphate. been widely debated. Most workers believe the Secondary minerals typically occur in a layer alteration took place in a nebular setting. The immediately beneath the rim sequence. Some sequence of alteration is compatible with CM CAIs have also suffered fragmentation and equilibration with a cooling, oxidised solar recrystallisation. It is not clear what the nebula gas [3]. Wark [4] argued for pre- phyllosilicate is replacing: anorthite is a accretionary alteration because of the presence possibility. Greenwood et al. [9] suggest that of alkali-rich halos in the meteorite matrix nebula processes caused fragmentation of CAIs, surrounding some CAIs. Sodium mapping of whereas aqueous alteration took place over a Allende CAIs shows that the sodium is enriched protracted period of time on the parent body. In in accretionary rims, suggesting CAIs became contrast, MacPherson and Davis [10] argued Workshop on Parent-Body and Nebular Modification of Chondritic Materials 4054.pdf ALTERATION OF CAIS: S. S. Russell and G. J. MacPherson that the CAIs were not altered in the of altered plus pristine CAIs close together in environment in which they are now found, and some meteorites (e.g. CMs) suggests that some many are too fragile to have been moved to their alteration occurred before they reached their current location by recycling in the regolith, so current site in the parent body (although this they favoured formation of hydrous secondary may reflect post- accretionary brecciation). minerals in a nebula environment. Many features of alteration appear to have CR meteorites: CAIs in the CR chondrite occurred in the nebula. Wark-Lovering rims Acfer 059 shows no evidence of alteration [11], predate accretion into the present parent bodies. whereas inclusions in Renazzo and Al Rais Some primary minerals exchanged with a nebula contain some secondary calcite [12]. gas, and some secondary minerals condensed CH meteorites: Some CAIs in the CH from a vapour. Sodalite in Semarkona [15] chondrite ALH 85085 show evidence of probably formed in the nebula, since 26Al dating recrystallisation due to reheating [13]. In suggests it formed before the accretion of the contrast, inclusions from PCA 91467 and Acfer asteroids. In contrast, the long time span of 182 appear unaltered [14]. alteration suggested by I-Xe dating for Allende Unequilibrated ordinary chondrites: CAIs in CAIs has been used to argue in favour of ordinary chondrites are rare. UOC CAIs are alteration in a parent body [16]. In addition to often rimmed, and secondary feldspathoids are nebula processes, metamorphism in parent occasionally present. In one Semarkona CAI, bodies tended to equilibrate CAIs with their host melilite has been partially replaced by sodalite rock. Aqueous processing in some meteorites [15]. may have affected CAIs in parent bodies. The CONCLUSIONS: location of the event responsible for Times: While most CAIs are believed to have incorporation of alkalis into CAIs, however, is formed at around the same time, their alteration still highly contentious. was an on-going process that took place over References: [1] Hutcheon and Newton (1981) several million years. I-Xe dating suggests that LPSC XII 491-493; [2] LaTourette and Wasserburg the alteration took place up to >10Myr after (1997) LPSC 28 781-782; [3] Hashimoto and initial CAI formation [16]. Al-Mg studies of Grossman (1987) GCA 51 1685-1704; [4] Wark grossular in CV CAIs also indicate formation > (1981) LPSC XII 1145-1147; [5] Krot et al., (1995) 2.4 Myr after CAI production [17], and a Al-Mg Meteoritics 30 748-775; [6] McSween (1977) GCA analysis of a recrystallised CAI from CH 41 1777-1790; [7] Greenwood et al., (1992) chondrite indicates a heating event > 2 Myr after Meteoritics 27 229; [8] Russell et al., (1997) GCA, CAI production [13]. Chemical exchange submitted; [9] Greenwood et al., (1994) GCA 58 between anorthite and melilite in Type B 1913-1935; [10] MacPherson and Davis (1994) GCA 5599-5625; [11] Weber and Bischoff (1997) inclusions appears to have occurred >2-3 Myr 58 Chem. Erde 57 1-24; [12] Weisburg et al., (1993) after CAI formation [18]. Similarly, sodalite in GCA 57, 1567-1586; [13] Kimura et al., (1993) fine grained CV inclusions is postulated to have 26 GCA 57 2329-2360; [14] Weber et al., (1995) LPSC formed after Al decay, ie., several Myr after XXVI 1475-1476; [15] Russell et al., (1997) CAI formation [19]. In contrast, sodalite in a LPSC XXVIII, 1209-1210; [16] Swindle et al., Semarkona (LL3.0) inclusion apparently formed (1998) GCA 52 2215-2227; [17] Davis et al., (1994) very quickly after CAI formation [15]. LPSC XXV 315-316; [18] Podosek et al., GCA 55 Places: Many CAIs are pristine, but some 1083-1110; [19] Brigham et al., (1986) LPSC XVII underwent several heating events. The presence 85-86..