300 million years of komatiite evolution of the Barberton Greenstone Belt
Abstract
The 3.5-3.2 Ga Barberton greenstone belt in South Africa is the type locality of komatiite and ultramafic lavas erupted throughout most of its 300 m.y. history. Despite 30 years of investigation, there are very few complete geochemical analyses of these rocks and we are undertaking a detailed analytical study, including major and trace elements, Nd, Hf, Os, O isotopes and melt inclusion studies, of samples from four stratigraphic formations that span the complete history of the belt. The oldest well preserved komatiites are the in 3.5 Ga Komati Formation near the base of the sequence. These have sub-chondritic Al/Ti and depleted HREE and are thought to have formed during deep (>300km) partial melting in a hot mantle plume. The majority are non-vesicular and show no evidence of having erupted as hydrous lavas, but some uncommon examples contain large amygdales and are coarse-grained, and they may have contained some water. The 3.3 Ga Weltevreden komatiites have slightly super-chondritic Al/Ti and a positively sloping REE profiles, characteristics consistent with shallower melting of a source that either was enriched in garnet or had previously undergone extensive partial melting. Some Weltevreden komatiites contain olivine with the composition Fo96. The liquid composition inferred using this composition and whole-rock compositions contains 33% MgO. If anhydrous, as supported by analysis of melt inclusions, this magma erupted at about 1660° C making it the hottest magma on earth. Two types of komatiites occur in the 3.3 Ga Mendon Formation. The first has Al/Ti and HREE similar to Komati formation; the second has highly super-chondritic Al/Ti which may indicate a garnet enriched source.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V13B2116R
- Keywords:
-
- 1037 Magma genesis and partial melting (3619);
- 1038 Mantle processes (3621);
- 1065 Major and trace element geochemistry;
- 8125 Evolution of the Earth (0325);
- 9623 Archean