DIAGENETIC ALTERATION OF IMPACT SPHERULES IN A NEOARCHEAN EJECTA LAYER IN SOUTH AFRICA

Distal Archean impact ejecta layers could be used to help create a global time-stratigraphic framework for early earth events, provided they can be confidently correlated from one continent to another. We did a petrographic study of a well-preserved Neoarchean spherule layer in the Monteville Formation (Griqualand West basin, South Africa) to assess the role of diagenetic alteration in attempting such correlations because it may have correlatives in the Hamersley basin of Western Australia. Although many spherules in the Monteville layer have botryoidal rims composed of radial-fibrous K-feldspar, compaction and replacement have greatly altered the appearance and mineralogy of these and other former melt particles. Moreover, the two main subunits of the Monteville spherule layer (an upper one rich in spherules and a lower one rich in intraclasts) show differing degrees of compaction. Compaction is about three times greater in the spherule-rich subunit, probably due to preferential early carbonate cementation in the intraclast-rich subunit, resulting in much better preservation of the shapes of melt particles in the latter. However, spherule rims have been fractured to uniform degrees in both subunits, indicating the melt particles were fractured prior to compaction, most likely due to thermal quenching upon entering the ocean. Some spherules also contain mica ribbons with a septarian geometry, again suggesting pre-compaction fracturing via thermal shock. This was followed by fluid infiltration and local replacement of glass by mica. We have observed similar ribbons in spherules in the K/T boundary layer from ODP Site 1049 drilled on the Blake Nose. Although diagenetic processes obscured some of the original textures of the Monteville impact spherules, it appears to match the diagenetic history of one Hamersley layer in particular, the Hesta occurrence of the Neoarchean Jeerinah spherule layer. Hence a better understanding of diagenetic histories may actually help rather than hinder transcontinental correlation.