Chapter 13 - Crystallization remanent magnetization

Summary

Introduction

Crystallization remanent magnetization (CRM) results from the formation of a new magnetic mineral in the presence of a magnetic field, either by nucleation and growth to a stable blocking volume V_B (single-phase or growth CRM) or through alteration of an existing magnetic phase (two-phase or parent-daughter CRM). The commonly used term chemical remanent magnetization is not always strictly accurate, e.g., in the γFe₂O₃ → αFe₂O₃ (spinel → rhombohedral) transformation, where no chemical change occurs, only a restacking of the lattice.

CRM is usually thought of as being blocked when grains grow from superparamagnetic (SP) to thermally stable SD size at V = V_B. This simple picture breaks down in the case of two-phase CRM because the growing daughter phase is influenced not only by an external field H₀ but also by its magnetic parent phase, to which it may be magnetostatically or exchange coupled with varying degrees of efficiency. Exchange coupling can occasionally result in self-reversed CRM (§13.4.6, 14.5.3).

CRM is usually regarded as a contaminant by paleomagnetists because the time of secondary mineral formation is difficult to date. Unfortunately CRM is not always easy to recognize because its unblocking temperatures and coercivities overlap those of primary TRM and DRM.

Many processes generate growth CRM in low-temperature sedimentary environments: precipitation of hematite cement from iron-rich solutions in red beds (Larson et al., 1982); microbially mediated production of authigenic magnetite in the iron-reducing zone of recent marine sediments (Karlin et al., 1987); biogenic magnetite production in calcareous sediments that eventually form limestones (Chang et al., 1987); and inorganic authigenesis of magnetite in soils (Maher, 1986; Maher and Taylor, 1988).