A new Technique for Probing Thermal Alteration in Paleointensity Studies: Double Thermal Demagnetization of 3-components of Anhysteretic Remanent Magnetization (ARM)

It is well known that changes in thermal remanent magnetization (TRM) due to thermal alteration of magnetic grains are the main reasons to result in failure in traditional type paleointensity studies. Blocking temperature (Tb) and/or unblocking temperature (Tb) of magnetic carriers are the comprehensive parameters in evaluating such changes. We have recently developed a double thermal demagnetization of 3-components ARM technique for checking thermal alteration in paleointensity determination experiments. By comparison thermal demagnetization curves of ARM before and after a given maximum temperature Tn, it is possible to check whether thermal alteration has occurred. It is also possible to carry out a thermal alteration check at every successive thermal demagnetization step Ti by acquiring a new ARM at room temperature. Three components thermal demagnetization of ARM technique is thus very useful for distinguishing magnetic behavior of microphenocryst grains (which are crystallized in early stage before eruption of lava) and intersertal titanomagnetite grains (which are crystallized in secondary stage after eruption of lava). We will present results from our case study on the Columbia River Basalt to illustrate these points.