Abstract
The Nomarski interference contrast technique is used to enhance the visibility of microtopographic relief on the surfaces of etched, polished sections and etched, polished thin sections. As an outgrowth of A. T. Anderson’s application of this technique to plagioclase, we have introduced modifications that permit the study of microstructures in olivine and clinopyroxene.
In thin section, titaniferous augite from an Arizona alkali basalt only faintly displays sector- and oscillatory-zoning, which is probably due tovariations in (Al + Ti) content. Nomarski images of HF-etched surfaces of this mineral reveal the zoning with extraordinary clarity, far surpassing in detail the normal thin-section image. In many aspects the oscillatory zonation in the augite appears as fine as that of plagioclase, which it superficially resembles.
Nomarski imaging of HCl-etched surfaces of olivines in basalts from three widely separated localities unexpectedly reveals delicate oscillatory zoning. Whereas some areas of the olivines show fine zoning apparently similar to that in plagioclase and augite, others show distinctive, relatively broad bands alternating with narrow incised troughs. Etch pits probably related to dislocations or inclusions are also developed in several olivines.
Textural features–such as tapering of zones, re-entrants at zone interfaces, and angular unconformities–leave little doubt that, in both clinopyroxene and olivine, the etched oscillatory zones are a reflection of primary growth features. We believe this to be the first documented report of oscillatory zoning in olivine. The nature and origin of the microstructures in this mineral are, as yet, problematic. Differential etching in the olivines could be due to either slight compositional variations or structural differences between zones. In the light of the known high rate of internal diffusion for the major elements Fe and Mg in this mineral, we find the preservation of such delicate growth-related structures remarkable.
