Morphologically based sampling designs for zoned orthopyroxenes

Abstract

Compositional data of orthopyroxene crystals obtained by the electron microprobe may be biased unless zoning morphologies are taken into account in the sampling design. Statistical models (predictive or interpretative) based on such data are open to question because the input data, such as correlations, are spurious. In an attempt to propose sampling designs two established crystal zoning morphologies are considered which are (a) the concentric shell-type and (b) sectoral zonations. These zonational types lend themselves well to statistical population modeling and graphical interpretation provided assumptions of chemical invariance hold or are approximated by a stratified sampling design. The statistical designs may be regarded as limiting cases in the sense that relaxations of certain assumptions lead to other population models. Hypersthene crystals from a central portion of the mafic norite formation at the Strathcona Mine, Sudbury irruptive, are examined for zonation. It appears that the trace elements have a different zoning morphology than the major elements of hypersthene.