Axel D. Renno

Primary clinopyroxene spherulites in basaltic lavas from the Pacific-Antarctic Ridge

Fresh glassy basaltic andesite samples recovered from the northern part of the Pacific–Antarctic Ridge contain abundant spherulites consisting of arrays of closely packed clinopyroxene fibers. The spherulites frequently enclose elongated vesicles that are tear drop-shaped or tailed. The long axes of the elongated vesicles were found to be always parallel to the orientation of the surrounding crystal fibers. In several cases, elongated vesicles having different orientations are hosted by a single spherulite. The existence of elongated vesicles provides unequivocal evidence that a significant proportion of the clinopyroxene fibers must have crystallized directly from a supercooled liquid at temperatures above the glass transition. Moreover, the absence of fractures within the spherulites and the surrounding glass as well as the observed nucleation of clinopyroxene fibers at vesicle walls are interpreted to be consistent with a primary origin of the clinopyroxene spherulites. Based on these textural observations it has to be concluded that spherulites do not represent a diagnostic texture for the devitrification of volcanic glass that occurs below the glass transition temperature.

Discussion on Wang et al. (2000) ‘Chemistry of Hf-rich zircons from the Laoshan I- and A-type granites, Eastern China’, Mineralogical Magazine, 64, 867–877

Abstract Wang et al. (2000) presented interesting new data on the occurrence of Hf-rich zircon in peralkaline granitic rocks. It is shown that the knowledge of the Hf distribution in zircon is important for the understanding of Zr/Hf ratios defined by whole-rock analysis. However, the processes leading to the unusual Zr/Hf fractionation are still poorly constrained. Some aspects are discussed further here. Analysis of available data demonstrates that extremely low Zr/Hf ratios found in topaz- and rare-metal bearing granites may be either a primary signature of some evolved granitic melts or, alternatively, they are produced during albitization of the rocks. Topaz- and rare-metal bearing granites may be P-rich and contain zircon moderately enriched in Hf (2-9 wt.% HfO2). Another group is P-poor and the Hf content in accessory zircon is high (up to 35 wt.% HfO2). Both types of intrusions may occur within a single orogen as demonstrated by the example of the Hercynian magmatism in the Erzgebirge/Krusne hory region (Germany/Czech Republic).

Sub-pixel resolution with a color X-ray camera

The color X-ray camera SLcam® is a full-field, single photon detector providing scanning-free, energy and spatially resolved X-ray imaging. Spatial resolution is achieved with the use of polycapillary optics guiding X-ray photons from small regions on a sample to distinct energy dispersive pixels on a charged-coupled device detector. Applying sub-pixel resolution, signals from individual capillary channels can be distinguished. Therefore, the SLcam® spatial resolution, which is normally limited to the pixel size of the charge-coupled device, can be improved to the size of individual polycapillary channels. In this work a new approach to a sub-pixel resolution algorithm comprising photon events also from the pixel centers is proposed. The details of the employed numerical method and several sub-pixel resolution examples are presented and discussed.