Full vector low-temperature magnetic measurements of geologic materials

GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS, VOL. ???, XXXX, DOI:10.1029/, Full vector low-temperature magnetic measurements of geologic materials Joshua M. Feinberg 1 , Peter A. Solheid 1 , Nicholas L. Swanson-Hysell 1,2 , Mike J. Jackson 1 , and Julie A. Bowles 1,3 The magnetic properties of geologic materials offer insights into an enormous range of important geophysical phenomena ranging from inner core dynamics to pale- oclimate. Often it is the low-temperature behavior (<300 K) of magnetic minerals that provides the most useful and highest sensitivity information for a given problem. Con- ventional measurements of low-temperature remanence are typically conducted on instru- ments that are limited to measuring one single axis component of the magnetization vec- tor and are optimized for measurements in strong fields. These instrumental limitations have prevented fully optimized applications and have motivated the development of a low-temperature probe that can be used for low-temperature remanence measurements between 17 and 300K along three orthogonal axes using a standard 2G Enterprises SQuID rock magnetometer. In this contribution, we describe the design and implementation of this instrument and present data from five case studies that demonstrate the probe’s con- siderable potential for future research: a polycrystalline hematite sample, a polycrystalline hematite and magnetite mixture, a single crystal of magnetite, a single crystal of pyrrhotite and samples of Umkondo Large Igneous Province diabase sills. Abstract. Measurement System (MPMS), most often with the inten- tion of revealing information about the dominant magnetic mineral phases and grain size distribution. While these in- struments are adept at a range of low-temperature exper- iments, understanding the full behavior of a rock’s natu- ral remanence at low-temperature is hampered by the mea- surement capabilities being limited to a single axis and the instrument not providing an ultra-low field environment. If low-temperature measurements of a natural remanence (NRM) are desired using such instrumentation, great care must be taken to align the NRM with the measurement axis, and any directional change during thermal cycling will not be captured. In such an instrument, deviation from the single-axis will result in a measured magnetization that is less than the specimen’s actual total magnetization. In this contribution, we describe a low-temperature probe developed for use with superconducting rock magnetometers (SRM) at the Institute for Rock Magnetism (IRM ), Uni- versity of Minnesota. This instrument allows for three-axis full-vector measurements of magnetic remanence at temper- atures between 300 and 17 K in low-field environments (<10 nT). It was developed with different engineering, but the same intent, as a low-temperature insert that has previously been implemented at the University of Rochester Paleomag- netic Laboratory [Smirnov and Tarduno, 2011]. 1. Introduction Magnetic behavior at low temperatures (<300 K) is one of the most sensitive indicators of the iron mineral phases and their concentrations and grain size distributions in nat- ural samples. Changes in magnetocrystalline anisotropy and crystallographic structure give rise to low-temperature tran- sitions that are diagnostic of specific mineral phases. The Morin transition of hematite (at ∼262 K; Morin [1950]), the Verwey transition of magnetite (at ∼122 K; Verwey [1939]) and the Besnus transition of pyrrhotite (at ∼32 K, Besnus and Meyer [1964]) are all diagnostic of common magnetic minerals that carry remanence at Earth surface tempera- tures. Other phases that acquire remanence at low tem- perature, such as siderite (with a Ne´el temperature of 38 K; Frederichs et al. [2003]) and superparamagnetic grains [Worm and Jackson, 1999], can also be readily identified through their low-temperature behavior. In addition to the utility of low-temperature data as a diagnostic tool for mag- netic mineral identification and characterization, irreversible changes in remanence that are associated with cycling to low temperatures are often used as a tool in paleomagnetic stud- ies. Low-temperature steps in paleodirectional and paleoin- tensity study are applied in some protocols with the goal of preferentially removing magnetic remanence held by mul- tidomain grains and thereby isolating magnetizations held by single-domain grains [e.g., Schmidt [1993]; Dunlop [2003]; Yamamoto et al. [2003]]. Low-temperature remanence experiments are routinely conducted on the Quantum Designs Magnetic Properties 2. Instrument design In order to develop the capacity to make three-axis mea- surements of remanence in an ultra-low-field environment, a cryostat insert was developed at the IRM in coopera- tion with ColdEdge Technologies (Allentown, PA) (Figure 1). This low-temperature instrument (IRM-LTI) allows for three-axis measurements to be made between room temper- ature and ∼17 K using horizontal-loading SRMs. There are many advantages to outfitting a superconducting rock mag- netometer for measurements at low-temperatures. First, these instruments are specifically designed for three-axis re- manence measurements, and ambient fields are minimized using a superconducting lead shield. Nulling fields are ap- plied by external coils while the shield cools to superconduct- ing temperatures, ultimately trapping a ∼2-3 nT field along 1 Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota, USA 2 Department of Earth and Planetary Science, University of California, Berkeley, California, USA 3 Department of Geosciences, University of Wisconsin, Milwaukee, WI, USA Copyright 2015 by the authors.