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Bowles, JA, Gee JS, Burgess K, Cooper RF.  2011.  Timing of magnetite formation in basaltic glass: Insights from synthetic analogs and relevance for geomagnetic paleointensity analyses. Geochemistry Geophysics Geosystems. 12   10.1029/2010gc003404   AbstractWebsite

Absolute paleointensity estimates from submarine basaltic glass (SBG) typically are of high technical quality and accurately reflect the ambient field when known. SBG contains fine-grained, low-Ti magnetite, in contrast to the high-Ti magnetite in crystalline basalt, which has lead to uncertainty over the origin of the magnetite and its remanence in SBG. Because a thermal remanence is required for accurate paleointensity estimates, the timing and temperature of magnetite formation is crucial. To assess these factors, we generated a suite of synthetic glasses with variable oxygen fugacity, cooling rate, and FeO* content. Magnetic properties varied most strongly with crystallinity; less crystalline specimens are similar to natural SBG and have weaker magnetization, a greater superparamagnetic contribution, and higher unblocking temperatures than more crystalline specimens. Thellier-type paleointensity results recovered the correct field within 1 sigma error with 2 (out of 10) exceptions that likely result from an undetected change in the laboratory field. Unblocking and ordering temperature data demonstrate that low-Ti magnetite is a primary phase, formed when the glass initially quenched. Although prolonged heating at high temperatures (during paleointensity experiments) may result in minor alteration at temperatures <580 degrees C, this does not appear to impact the accuracy of the paleointensity estimate. Young SBG is therefore a suitable material for paleointensity studies.

Lawrence, K, Johnson C, Tauxe L, Gee J.  2008.  Lunar paleointensity measurements: Implications for lunar magnetic evolution. Physics of the Earth and Planetary Interiors. 168:71-87.   10.1016/j.pepi.2008.05.007   AbstractWebsite

We analyze published and new paleointensity data from Apollo samples to reexamine the hypothesis of an early (3.9-3.6 Ga) lunar dynamo. Our new paleointensity experiments on four samples use modern absolute and relative measurement techniques, with ages ranging from 3.3 to 4.3 Ga, bracketing the putative period of an ancient lunar field. Samples 60015 (anorthosite) and 76535 (troctolite) failed during absolute paleointensity experiments. Samples 72215 and 62235 (impact breccias) recorded a complicated, multicomponent magnetic history that includes a low-temperature (< 500 degrees C) component associated with a high intensity (similar to 90 mu T) and a high temperature (> 500 degrees C) component associated with a low intensity (2 [LT). Similar multi-component behavior has been observed in several published absolute intensity experiments on lunar samples. Additional material from 72215 and 62235 was subjected to a relative paleointensity experiment (a saturation isothermal remanent magnetization, or sIRM, experiment); neither sample Provided unambiguous evidence for a thermal origin of the recorded remanent magnetization. We test several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples. Specifically, an overprint from exposure to a small magnetic field (an isothermal remanent magnetization) results in multi-component behavior (similar to absolute paleointensity results) from which we could not recover the correct magnitude of the original thermal remanent magnetization. In light of these new experiments and a thorough re-evaluation of existing paleointensity measurements, we conclude that although some samples with ages of 3.6 to 3.9 Ga are strongly magnetized, and sometimes exhibit stable directional behavior, it has not been demonstrated that these observations indicate a primary thermal remanence. Particularly problematic in the interpretation of lunar sample magnetizations are the effects of shock. As relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities, the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable. Consequently, current paleointensity measurements do not support the existence of a 3.9-3.6 Ga lunar dynamo with 100 mu T surface fields, a result that is in better agreement with satellite measurements of crustal magnetism and that presents fewer challenges for thermal evolution and dynamo models. (c) 2008 Elsevier B.V. All rights reserved.

Engels, M, Barckhausen U, Gee JS.  2008.  A new towed marine vector magnetometer: methods and results from a Central Pacific cruise. Geophysical Journal International. 172:115-129.   10.1111/j.1365-246X.2007.03601.x   AbstractWebsite

This paper focuses on new instrumental and methodological aspects of the acquisition, processing and interpretation of marine magnetic data. Between two Overhauser sensors towed as a longitudinal gradiometer, a new fluxgate vector magnetometer was employed. A second independent vector magnetometer system operated simultaneously. This equipment was used during research cruise SO-180 of R/V SONNE in the Central Pacific at about 120 degrees W just south of the equator. The survey area on the Pacific Plate is the mirror image to the currently subducting Cocos plate off Central America. The oceanic crust was formed around 23 Ma at the East Pacific Rise when the Farallon plate broke up into the Cocos and Nazca plates. The magnetic seafloor anomalies in the survey area strike approximately north-south almost parallel to the main field, resulting in very low anomaly amplitudes which had hindered detailed anomaly identification so far. A new processing scheme was applied to the data which identifies the weak anomalies in the total field and those in the vertical component that, as a consequence of the source body geometry, have about doubled amplitude. The vertical component constrains 2-D modelling much better than the total field alone. Processed fluxgate total field data are practically identical to the Overhauser reference and even provide a reliable gradient when combined with one Overhauser. Although towed vector magnetometers typically provide no independent estimate of yaw, we illustrate that a numerical yaw (bandpass filtered magnetic heading) can provide reasonable estimates of the horizontal field components. These component data open additional analysis tools: the strike direction of magnetic lineations can be estimated from single profiles by either magnetic boundary strike ellipses in the space domain or by coherences between vertical and horizontal components in the wavenumber domain. Auto power spectra of the total field provide an approximate depth to the anomaly source or, if in obvious contradiction to the bathymetric depth, allow the detection of distortions, for example, by external temporal geomagnetic variations.

Yu, YJ, Gee JS.  2005.  Spinel in Martian meteorite SaU 008: implications for Martian magnetism. Earth and Planetary Science Letters. 232:287-294.   10.1016/j.epsl.2004.12.015   AbstractWebsite

Shergotty-Nakhla-Chassigny (SNC) meteorites provide the only available samples of Martian material. The stable permanent magnetization of SNC meteorites has been traditionally attributed to magnetite (Fe3O4) or pyrrhotite (Fe7S8). On the basis of rock magnetic, microscopic, and electron microprobe analyses on rock chips and mineral separates, we suggest that a new material (Fe-Cr-Ti spinel) is responsible for the stable paleomagnetic record of Martian meteorite SaU 008. It is possible that SaU 008 acquired a primary remanence of thermal origin from the Martian crustal field. However, this proposition requires further testing because the effect of shock events on Fe-Cr-Ti spinel is unknown. (c) 2004 Elsevier B.V. All rights reserved.

Gee, JS, Cande SC.  2002.  A surface-towed vector magnetometer. Geophysical Research Letters. 29   10.1029/2002gl015245   AbstractWebsite

[1] We have tested the feasibility of using a commercial motion sensor as a vector magnetometer that can be towed at normal survey speeds behind a research vessel. In contrast to previous studies using a shipboard mounted vector magnetometer, the towed system is essentially unaffected by the magnetization of the towing vessel. Results from a test deployment compare favorably with an earlier vector aeromagnetic survey, indicating that the towed instrument can resolve horizontal and vertical anomalies with amplitudes >30-50 nT. This instrument should be particularly useful in equatorial regions, where the vector anomalies are substantially greater than the corresponding total field anomalies.

Gee, JS, Cande SC, Hildebrand JA, Donnelly K, Parker RL.  2000.  Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies. Nature. 408:827-832.   10.1038/35048513   AbstractWebsite

Knowledge of past variations in the intensity of the Earth's magnetic field provides an important constraint on models of the geodynamo. A record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials, and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences. But a long-term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust. Here we show that near-seafloor magnetic anomalies recorded over the southern East Pacific Rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr. Moreover, the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well-documented dipole intensity pattern for the past 50 kyr. A comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region. The record of geomagnetic intensity preserved in the oceanic crust should provide a higher-time-resolution record of crustal accretion processes at mid-ocean ridges than has previously been obtainable.

Varga, RJ, Gee JS, Bettison-Varga L, Anderson RS, Johnson CL.  1999.  Early establishment of seafloor hydrothermal systems during structural extension: paleomagnetic evidence from the Troodos ophiolite, Cyprus. Earth and Planetary Science Letters. 171:221-235.   10.1016/s0012-821x(99)00147-8   AbstractWebsite

Paleomagnetic data from the Troodos ophiolite are used to help constrain models for the relationship between extensional normal faulting and hydrothermal alteration related to production of large-tonnage sulfide deposits at oceanic ridges. We have sampled dikes from the Troodos sheeted complex that have been subjected to variable hydrothermal alteration, from greenschist alteration typical of the low water/rock mass ratio interactions outside of hydrothermal upflow zones as well as from severely recrystallized rocks (epidosites) altered within high water/rock mass ratio hydrothermal upflow zones in the root zones beneath large sulfide ore deposits. These dikes are moderately to highly tilted from their initial near-vertical orientations due to rotations in the hangingwalls of approximately dike-parallel, oceanic normal faults. Comparison of characteristic remanence directions from these dikes with the Late Cretaceous Troodos reference direction, therefore, allows a tilt test to determine whether remanent magnetizations were acquired prior to or subsequent to tilting. Remanence directions for both greenschist and epidosite dikes show similar magnitudes of tilting due to rotational normal faulting and restore to the Late Cretaceous Troodos reference direction upon restoration of dikes to near-vertical positions about a NNW-trending, horizontal axis. These data, along with field observations of focused alteration along normal faults, suggest that epidosite alteration occurred during the early stages of extensional tilting and prior to significant rotation. This sequence of events is similar to that observed for creation of large-tonnage sulfide bodies at intermediate to slow spreading centers which form soon after cessation of magmatism and during the early stages of structural extension. We suggest that the dike-parallel normal faults were initiated as extensional fractures during this early stage of crustal extension, thus providing the necessary permeability for focused fluid flow, and that later slip along these structures during rotational-planar normal faulting caused reduction in permeability due to gouge formation. (C) 1999 Elsevier Science B.V. All rights reserved.

Gee, J, Schneider DA, Kent DV.  1996.  Marine magnetic anomalies as recorders of geomagnetic intensity variations. Earth and Planetary Science Letters. 144:327-335.   10.1016/s0012-821x(96)00184-7   AbstractWebsite

In addition to providing a robust record of past geomagnetic polarity reversals, marine magnetic anomalies often show shorter wavelength variations, which may provide information on geomagnetic intensity variations within intervals of constant polarity. To evaluate this possible geomagnetic signal, we compare sea surface profiles of the Central Anomaly with synthetic profiles based on Brunhes age (0-0.78 Ma) paleointensity records derived from deep sea sediments. The similarity of the synthetic profiles and observed profiles from the ultra-fast spreading southern East Pacific Rise suggests that geomagnetic intensity variations play an important role in the magnetization of the oceanic crust. This interpretation is further supported by systematic variations in the pattern of the Central Anomaly at slower spreading ridges, which are entirely consistent with a progressively smoother record of the sediment-derived paleointensity. If the sedimentary records, as calibrated to available absolute paleointensity data, accurately record variations in dipole intensity over the Brunhes, it follows that much of the Brunhes was characterized by geomagnetic intensities lower than either the mean dipole moment for the past 10 ka or the average for the period from 0.05 to 5.0 Ma. Furthermore, the sediment paleointensity records reflect the significant increase in geomagnetic intensity, from a low of similar to 2 x 10(22) Am-2 near 40 ka to a peak value (11 x 10(22) Am-2) at similar to 3 ka, that has been well documented from absolute paleointensity determinations, We suggest that geomagnetic intensity variations may be the most important cause of the rapid changes in the source layer magnetization near the ridge crest and the resultant Central Anomaly Magnetic High.

Tarduno, JA, Gee J.  1995.  Large-Scale Motion Between Pacific and Atlantic Hotspots. Nature. 378:477-480.   10.1038/378477a0   AbstractWebsite

STUDIES of true polar wander (TPW), the rotation of the solid Earth with respect to the spin axis(1), have suggested that there has been 10-15 degrees of relative motion over the past 130 Myr (refs 2-4). In such studies, the orientation of the spin axis is recovered from continental palaeomagnetic poles (corrected for relative plate motions), and compared with a deep-mantle reference frame defined by hotspot locations. But deducing relative plate motions becomes increasingly difficult for older (Mesozoic) time periods, hindering tests of TPW on timescales comparable to those of large-scale mantle convection; moreover, the assumption of hotspot fixity is controversial(5,6). We examine here a more direct approach(7,8), using palaeolatitudes derived from Pacific guyots. Contrary to predictions from TPW models, these data suggest only minor latitudinal shifts of Pacific hotspots during the Cretaceous period. Instead of TPW, relative motion between the Atlantic and Pacific hotspot groups(9) is required at a velocity of approximately 30 mm yr(-1), more than 50% larger than previously proposed(5).