Diagenetic alteration of impact spherules in the Neoarchean Monteville layer, South Africa

Title:
Diagenetic alteration of impact spherules in the Neoarchean Monteville layer, South Africa
Authors:
Kohl, Issaku; Simonson, Bruce M.; Berke, Melissa
Abstract:
Intercontinental correlation of distal Archean impact ejecta layers can be used to help create a global time-stratigraphic framework for early Earth events. For example, an impact spherule layer in the Neoarchean Monteville Formation (Griqualand West Basin, South Africa) may be correlated with layers in one or more formations in Western Australia. To help assess the degree to which diagenetic alteration would hinder such correlations, we performed a petrographic study of spherules in the Monteville layer. Most of the spherules in the Monteville layer have botryoidal rims composed of radial-fibrous K-feldspar, but compaction and replacement have greatly altered their appearance and mineralogy. Moreover, the Monteville spherule layer consists of three main subunits, and spherule compaction varies between subunits as well as across the Griqualand West region. Compaction is about three times greater in a medial spherule-rich subunit as compared to a basal subunit rich in large intraclasts, resulting in better preservation of the shapes of melt particles in the latter. However, spherule rims have comparable numbers of fractures in both subunits, indicating the melt particles were fractured prior to compaction. Some spherules contain mica ribbons with a septarian geometry. Fracturing via rapid thermal quenching could help explain all of these features. If hot spherules possessing crystalline rims were thermally shocked when they hit the ocean, fractures would have the observed geometries and provide pathways for fluid infiltration and local replacement of glass by mica. Although heavily distorted, impact spherules in the Monteville layer are very similar to those in the Hesta occurrence of the Neoarchean Jeerinah spherule layer of the Hamersley Basin, even showing similar diagenetic histories. In this instance, diagenetic alteration may actually help rather than hinder intercontinental correlation of impact spherule layers.
Citation:
Kohl, Issaku, Bruce M. Simonson, and Melissa Berke. 2006. "Diagenetic alteration of impact spherules in the Neoarchean Monteville layer, South Africa." Geological Society of America Special Papers 405: 57-73.
Publisher:
Geological Society of America
DATE ISSUED:
2006
Department:
Geology
Type:
article
PUBLISHED VERSION:
10.1130/2006.2405(04)
PERMANENT LINK:
http://hdl.handle.net/11282/309782

Full metadata record

DC FieldValue Language
dc.contributor.authorKohl, Issakuen_US
dc.contributor.authorSimonson, Bruce M.en_US
dc.contributor.authorBerke, Melissaen_US
dc.date.accessioned2013-12-23T16:17:44Z-
dc.date.available2013-12-23T16:17:44Z-
dc.date.issued2006en
dc.identifier.citationKohl, Issaku, Bruce M. Simonson, and Melissa Berke. 2006. "Diagenetic alteration of impact spherules in the Neoarchean Monteville layer, South Africa." Geological Society of America Special Papers 405: 57-73.en_US
dc.identifier.issn0072-1077en_US
dc.identifier.urihttp://hdl.handle.net/11282/309782-
dc.description.abstractIntercontinental correlation of distal Archean impact ejecta layers can be used to help create a global time-stratigraphic framework for early Earth events. For example, an impact spherule layer in the Neoarchean Monteville Formation (Griqualand West Basin, South Africa) may be correlated with layers in one or more formations in Western Australia. To help assess the degree to which diagenetic alteration would hinder such correlations, we performed a petrographic study of spherules in the Monteville layer. Most of the spherules in the Monteville layer have botryoidal rims composed of radial-fibrous K-feldspar, but compaction and replacement have greatly altered their appearance and mineralogy. Moreover, the Monteville spherule layer consists of three main subunits, and spherule compaction varies between subunits as well as across the Griqualand West region. Compaction is about three times greater in a medial spherule-rich subunit as compared to a basal subunit rich in large intraclasts, resulting in better preservation of the shapes of melt particles in the latter. However, spherule rims have comparable numbers of fractures in both subunits, indicating the melt particles were fractured prior to compaction. Some spherules contain mica ribbons with a septarian geometry. Fracturing via rapid thermal quenching could help explain all of these features. If hot spherules possessing crystalline rims were thermally shocked when they hit the ocean, fractures would have the observed geometries and provide pathways for fluid infiltration and local replacement of glass by mica. Although heavily distorted, impact spherules in the Monteville layer are very similar to those in the Hesta occurrence of the Neoarchean Jeerinah spherule layer of the Hamersley Basin, even showing similar diagenetic histories. In this instance, diagenetic alteration may actually help rather than hinder intercontinental correlation of impact spherule layers.en_US
dc.language.isoen_USen_US
dc.publisherGeological Society of Americaen_US
dc.identifier.doi10.1130/2006.2405(04)-
dc.subject.departmentGeologyen_US
dc.titleDiagenetic alteration of impact spherules in the Neoarchean Monteville layer, South Africaen_US
dc.typearticleen_US
dc.identifier.journalGeological Society of America Special Papersen_US
dc.subject.keywordPrecambrianen_US
dc.subject.keywordEjectaen_US
dc.subject.keywordReplacementen_US
dc.subject.keywordCompactionen_US
dc.subject.keywordStratigraphic correlationen_US
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