Accretionary phase

Berman, R.G., Davis, W.J., Pehrsson, S., 2007. Collisional Snowbird tectonic zone resurrected Growth of Laurentia during the 1.9 Ga accretionary phase of the Hudsonian orogeny. Geology, 35, 911-914. 

Four principal models have been proposed to explain the presence of alteration phases in chondritic meteorites. These models can be summarized as follows (i) reaction of anhydrous, high-temperature condensate phases with water vapor as the solar nebula cooled to temperatures below —375 K (e.g., Grossman and Larimer, 1974]) (ii) hydration of anhydrous dust in icy regions of the nebula during the passage of shock waves (Ciesla et al., 2003) (iii) alteration within small (tens of meters), ephemeral parent bodies that were subsequently disrupted and their altered components accreted with unaltered materials into the final asteroidal parent bodies (pre-accretionary alteration) (Metzler et al., 1992 Bischoff, 1998) and (iv) alteration within asteroidal parent bodies (Kerridge and Bunch, 1979 Zolensky and McSween, 1988). 

Evidence to constrain the location of alteration for the CO chondrites is limited. However, most data suggest a late-stage event that postdated metamorphism, implying that it was probably a parent-body process. Development of alteration phases in CO matrices has occurred interstitially to matrix phases and there is no evidence of distinct hydrous phases intermixed with unaltered phases that would support pre-accretionary alteration. However, formation of phyllosilicates in chondmles in ALH77307 may have occurred prior to accretion, based on the fact that the ALH77307 matrix shows essentially no evidence of aqueous alteration (Brearley, 1993). 

Most workers have argued that alteration of the Bali group CV chondrites occurred in an asteroidal environment, although Kimura and Ikeda (1998) did not rule out the possibility of pre-accretionary alteration. In the case of Allende, the very heterogeneous character of the phases present in CAIs, in addition to the fact that they are extremely rare and are heterogeneously distributed, has led most workers to favor formation of these hydrous phases in the solar nebula prior to accretion. 

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