Approach to saturation analysis of hysteresis measurements in rock magnetism and evidence for stress dominated magnetic anisotropy in young mid-ocean ridge basalt

Abstract

Young mid-ocean ridge basalts contain titanomagnetite crystals of varying size and composition. Many studies of their hysteresis properties have found M rs / M s ratios considerably above the theoretical limit of 0.5 for uniaxial single domain particles. Since titanomagnetite is a cubic mineral, high M rs / M s could occur due to cubic anisotropy which allows for M rs / M s values up to 0.866. On the other hand, titanomagnetites with high Ti content possess extremely large magnetostriction constants. Already slight internal stress easily outweighs cubic anisotropy and enforces uniaxial behavior. Are high M rs / M s ratios now a proof for very low internal stress? No! On the contrary, previous studies on synthetic titanomagnetite with high M rs / M s show that after annealing this ratio decreases. A possible explanation is that insufficient saturation of the hysteresis loop, used to infer M rs / M s , leads to underestimation of M s . Here, a systematic experimental study on a young mid-ocean ridge basalt using fields of up to 7 T demonstrates that indeed the M rs / M s ratio of the single domain fraction does not significantly deviate from the theoretical value of 0.5 for uniaxial anisotropy. It is further estimated that internal stress above 200 MPa is necessary to explain the observed hysteresis behavior – a value which is consistent with recent independent approximations. On the other hand, theoretical loops for cubic minerals do not fit the observed data. In order to assess the validity of M s determinations from hysteresis measurements, an improved method to evaluate the approach to saturation behavior of hysteresis loops is developed. It allows to recognize insufficiently saturated loops and thereby helps to avoid misinterpretation of standard M rs / M s measurements when high fields are not accessible.