Formation and alteration of CAIs in Cold Bokkeveld (CM2)

Greenwood, Richard C.; Lee, Martin R.; Hutchison, Robert and Barber, David J. (1994). Formation and alteration of CAIs in Cold Bokkeveld (CM2). Geochimica et Cosmochimica Acta, 58 pp. 1913–1935.



Calcium aluminium-rich inclusions (CAIs) from the carbonaceous chondrite Cold Bokkeveld (CM2) have been located and characterised in situ; the textural environment of CM2 CAIs has been studied for the first time. Based on their primary mineralogy, 345 CAls have been classified into four groups:

(1) spine1 inclusions
(2) a hibonite inclusion
(3) spinel-pyroxene (sp-py) inclusions
(4) spinel-pyroxene-olivine (sp-py-01) inclusions.

In addition, refractory spherules and spinel-bearing isolated olivines are also recognised. CAIs in Cold Bokkeveld underwent extensive aqueous alteration and contain secondary minerals, including calcite and various phyllosilicates. These assemblages are similar to those in the chondrules and matrix of the meteorite, and it is concluded that all were altered more or less simultaneously within a parent body regolith. As is common in CM2s, most CAIs in Cold Bokkeveld have a rim sequence containing a prominent Fe-rich phyllosilicate layer. This formed early in the alteration sequence by the dissolution of melilite and subsequent precipitation of Fe-rich phyllosilicates from fluids with a high Fe/Mg ratio.

CAIs in Cold Bokkeveld experienced two stages of deformation. An earlier nebula stage predated the formation of the dust mantles that surround most CAIs, and a later stage occurred in an asteroidal regolith. The CAIs in CM2s are smaller than those in CV3 chondrites because of the greater intensity of the earlier fragmentation episode that they experienced. Aqueous alteration of CAIs and regolith deformation appear to have been simultaneous.

Phase relations and pyroxene compositional trends in sp-py and sp-py-01 inclusions indicate that they crystallised from liquids. Most spinel-bearing inclusions in Cold Bokkeveld have a normal Mg isotopic composition, so they could not have formed by the simultaneous melting and evaporation of primitive dust. One inclusion is enriched in the heavy isotopes of Mg and may have formed in this way. Two (or more) events are required to form most of the inclusions. The first produced the refractory compositions, either by vapour/solid condensation or slow evaporation of primitive dust. The second event melted the refractory precursors which later crystallised to form the observed textures and mineral assemblages. It has been suggested that refractory spherules with inwardly radiating hibonites formed from molten droplets at temperatures as high as 2135°C. However, the crystallisation sequences of these inclusions inferred from textural evidence are incompatible with those derived from phase relations and estimates of bulk composition. It is concluded that CM2 refractory spherules did not form by crystallisation of molten droplets, but were derived by fragmentation of larger inclusions in which they formed by partial melting.

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