Quantitative description of oscillatory zoning in basaltic to dacitic plagioclases from the Shirahama Group, Japan

Akira Tsune,Atsushi Toramaru

doi:10.1186/bf03353129

Abstract

Abstract New criteria related to the origin of oscillatory zoning (OZ) in plagioclase are presented. We compare the OZs in basaltic to those of dacitic plagioclases in the tholeiitic series volcanic rocks of the Shirahama Group, Izu Peninsula, Japan. Nomarski differential interference contrast (NDIC) images of the etched thin sections are used to measure zone thicknesses of the OZs. The normalized standard deviations per data series of the thicknesses are then calculated. We found that the average thicknesses are almost constant (mostly from 2 to 3 μm) through all the rock samples. This constancy corresponds to our idea that the length of oscillation, D/V (D: diffusivity in the melt; V: growth velocity of plagioclase) is almost constant for a variety of melt viscosity because strong dependences on the viscosity of D and V are canceled out in D/V. Consequently, we concluded that the growth of the OZ is basically controlled by an interface kinetics mechanism. In contrast, the plagioclases in SiO2-rich rocks have the following features: (1) larger standard deviations, (2) abundant erode-like zones, and (3) large oscillation amplitudes. These features reveal that the OZ patterns of plagioclases in more silicic magmas are disturbed due to change of the environmental parameters under the magma dynamics.

Highlights

The oscillatory zoning (OZ) found in magmatic plagioclases has been of scientific interest
Since the dissolution features are commonly observed in both type 1 and 2, Pearce and Kolisnik consider that various-scale dynamics of magma are involved in the mechanisms of the formations of type 1 and 2 zones; type 1 is formed through small-scale convection in a diffusion-controlled chemical layer, whereas type 2 is formed in large-scale convective magma
We describe the characteristics of the observed OZs

Summary

Introduction

The oscillatory zoning (OZ) found in magmatic plagioclases has been of scientific interest. The most recent approach, which uses two-dimensional grayscale data of BSE images with the electron microprobe calibration (Ginibre et al, 2002), has contributed to our knowledge of the chemical and morphological characteristics of the zones and a more detailed understanding of the magmatic system (e.g., Wallace and Bergantz, 2002, 2005; Perugini et al, 2005). This approach has been used to perform chaotic analysis, such as the G-P method (Grossberger and Procaccia, 1983) which requires many data points or de-. Ginibre et al (2002) report the existence of small-scale OZs with low amplitudes, small thicknesses, and no-truncated shapes in nature—in addition to the types 1 and 2 of Pearce and Kolisnik (1990). Ginibre et al (2002) report that the newly found zones may be formed by a mechanism of interface kinetics

Methods

Discussion

Conclusion