[1] Superconducting quantum interference device (SQUID) microscopes are a new generation of instruments that map magnetic fields with unprecedented spatial resolution and moment sensitivity. Unlike standard rock magnetometers, SQUID microscopes map magnetic fields rather than measuring magnetic moments such that the sample magnetization pattern must be retrieved from source model fits to the measured field data. Here we present the first direct comparison between paleomagnetic analyses on natural samples using joint measurements from SQUID microscopy and moment magnetometry. We demonstrate that in combination with a priori geologic and petrographic data, SQUID microscopy can accurately characterize the magnetization of lunar glass spherules and Hawaiian basalt. The bulk moment magnitude and direction of these samples inferred from inversions of SQUID microscopy data match direct measurements on the same samples using moment magnetometry. In addition, these inversions provide unique constraints on the magnetization distribution within the sample. These measurements are among the most sensitive and highest resolution quantitative paleomagnetic studies of natural remanent magnetization to date. We expect that this technique will be able to extend many other standard paleomagnetic techniques to previously inaccessible microscale samples.

Mars today has no core dynamo magnetic field. However, the discovery of remanent magnetization in Martian meteorites and intense crustal magnetization suggests that Mars once had a global field. Here we present high resolution maps of the magnetic field of Martian meteorite ALH 84001. These maps are the most sensitive yet quantitative study of natural remanent magnetization (with resolved anomalies as weak as 1 × 10−14 Am2). ALH 84001 likely contains a 4 billion year old (Ga) thermoremanence partially overprinted by one or more poorly understood secondary components. Our data suggest that the paleointensity of the local paleofield was within an order of magnitude of that of the present‐day Earth. If this field were global in extent, it should have played a key role in Martian atmospheric and climatic evolution. However, it is still too weak to easily explain the intensity of Martian crustal paleomagnetic anomalies.

We have developed a scanning superconducting quantum interference device ͑SQUID͒ microscope system with interchangeable sensor configurations for imaging magnetic fields of room-temperature ͑RT͒ samples with submillimeter resolution. The low-critical-temperature ͑T c ͒ niobium-based monolithic SQUID sensors are mounted on the tip of a sapphire and thermally anchored to the helium reservoir. A 25 m sapphire window separates the vacuum space from the RT sample. A positioning mechanism allows us to adjust the sample-to-sensor spacing from the top of the Dewar. We achieved a sensor-to-sample spacing of 100 m, which could be maintained for periods of up to four weeks. Different SQUID sensor designs are necessary to achieve the best combination of spatial resolution and field sensitivity for a given source configuration. For imaging thin sections of geological samples, we used a custom-designed monolithic low-T c niobium bare SQUID sensor, with an effective diameter of 80 m, and achieved a field sensitivity of 1.5 pT/ Hz 1/2 and a magnetic moment sensitivity of 5.4ϫ 10 −18 A m 2 /Hz 1/2 at a sensor-to-sample spacing of 100 m in the white noise region for frequencies above 100 Hz. Imaging action currents in cardiac tissue requires a higher field sensitivity, which can only be achieved by compromising spatial resolution. We developed a monolithic low-T c niobium multiloop SQUID sensor, with sensor sizes ranging from 250 m to 1 mm, and achieved sensitivities of 480-180 fT/ Hz 1/2 in the white noise region for frequencies above 100 Hz, respectively. For all sensor configurations, the spatial resolution was comparable to the effective diameter and limited by the sensor-to-sample spacing. Spatial registration allowed us to compare high-resolution images of magnetic fields associated with action currents and optical recordings of transmembrane potentials to study the bidomain nature of cardiac tissue or to match petrography to magnetic field maps in thin sections of geological samples.

We have developed a monolithic low-temperature superconducting quantum interference device (SQUID) magnetometer and incorporated the device in a scanning microscope for imaging magnetic fields of room temperature samples. The instrument has a ∼100 μm spatial resolution and a 1.4 pT/Hz1/2 field sensitivity above a few hertz. We discuss design constraints on and potential applications of the SQUID microscope.

12Hydrogenetic ferromanganese crusts are iron-manganese oxide chemical 13 precipitates on the seafloor that grow over periods of tens of millions of years. Their 14 secular records of chemical, mineralogical, and textural variations are archives of deep-15 sea environmental changes. However, environmental reconstruction requires reliable 16 high-resolution age dating. Earlier chronological methods using radiochemical and stable 17 isotopes provided age models for ferromanganese crusts, but have limitations on the 18 millimeter scale. For example, the reliability of 10 Be/ 9 Be chronometry, commonly 19 considered the most reliable technique, depends on the assumption that the production 20 and preservation of 10 Be are constant, and requires accurate knowledge of the 10 Be half-21 life. To overcome these limitations, we applied an alternative chronometric technique, 22 2 magnetostratigraphy, to a 50-mm-thick hydrogenetic ferromanganese crust (D96-m4) 23 from the northwest Pacific. Submillimeter-scale magnetic stripes originating from 24 approximately oppositely magnetized regions oriented parallel to bedding were clearly 25 recognized on thin sections of the crust using a high-resolution magnetometry technique 26 called scanning SQUID (superconducting quantum interference device) microscopy. By 27 correlating the boundaries of the magnetic stripes with known geomagnetic reversals, we 28 determined an average growth rate of 5.1 ± 0.2 mm/m.y., which is within 16% of that 29 deduced from 10 Be/ 9 Be method (6.0 ± 0.2 mm/m.y.). This is the finest-scale 30 magnetostratigraphic study of a geologic sample to date. Ultrafine-scale 31 magnetostratigraphy using SQUID microscopy is a powerful new chronological tool for 32 estimating ages and growth rates for hydrogenetic ferromanganese crusts. It provides 33 chronological constraints with the accuracy promised by the astronomically calibrated 34 magnetostratigraphic time scale (1-40 k.y.). 35 INTRODUCTION 36