Fig. 1.

Petrographic images of metavolcanic pillow lavas from a 183-m drill core in the approximately 3.5-Ga Hooggenoeg Formation (Barberton Greenstone Belt, South Africa), showing a continuum of titanite morphologies and filament sizes. The complex spectrum of titanite shapes, sizes, distributions, and mineral associations illustrated here and in Grosch and McLoughlin (2) support an abiotic metamorphic origin for the titanite, and contrasts with a bioalteration model of Staudigel et al. (1) that invokes delicate “signature” titanites of restricted morphology. (A) Titanite clusters with filaments of varying size contained within a quartz-carbonate band in interpillow breccia, unlike the chlorite-hosted variety reported in Staudigel et al. (1). (B) Titanite spheres with stubby, projections or spikes within an orthoclase feldspar band, not in chloritized glass. (C and D) Large, isolated, titanite porphyroblasts showing complex size and shape variation in their radiating filaments, which cross-cut matrix chlorite, quartz, epidote, and mica suggesting metamorphic titanite growth that postdates seafloor alteration. These textural observations support later titanite growth in one or more generations along with other metamorphic minerals, and do not support early titanite growth that filled in microtunnels on the seafloor. (E) A small titanite cluster with filaments of varying length and width with spotted hornfelsic texture titanite in the lower part of the image. (F) Titanite porphyroblasts in complex textural association with epidote. Late-stage approximately 2.9-Ga titanite filaments wrap around and intergrow earlier-formed epidote grain boundaries.