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Bowles, JA, Gee JS, Jackson MJ, Avery MS.  2015.  Geomagnetic paleointensity in historical pyroclastic density currents: Testing the effects of emplacement temperature and postemplacement alteration. Geochemistry Geophysics Geosystems. 16:3607-3625.   10.1002/2015gc005910   AbstractWebsite

Thellier-type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 Novarupta (NV) and 1980 Mt. St. Helens (MSH) pyroclastic density currents (PDCs) with the intention of evaluating their suitability for geomagnetic paleointensity studies. PDCs are common worldwide, but can have complicated thermal and alteration histories. We attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of PDCs. Results demonstrate two types of nonideal behavior: unstable remanence in multidomain (MD) titanomagnetite, and nonideal behavior linked to fumarolic and vapor phase alteration. Emplacement temperature indirectly influences MSH results by controlling the fraction of homogenous MD versus oxyexsolved pseudo-single domain titanomagnetite. NV samples are more directly influenced by vapor phase alteration. The majority of NV samples show distinct two-slope behavior in the natural remanent magnetizationpartial thermal remanent magnetization plots. We interpret this to arise from a (thermo)chemical remanent magnetization associated with vapor phase alteration, and samples with high water content (>0.75% loss on ignition) generate paleointensities that deviate most strongly from the true value. We find that PDCs can be productively used for paleointensity, but thatas with all paleointensity studiescare should be taken in identifying potential postemplacement alteration below the Curie temperature, and that large, welded flows may be more alteration-prone. One advantage in using PDCs is that they typically have greater areal (spatial) exposure than a basalt flow, allowing for more extensive sampling and better assessment of errors and uncertainty.

Kent, DV, Kjarsgaard BA, Gee JS, Muttoni G, Heaman LM.  2015.  Tracking the Late Jurassic apparent (or true) polar shift in U-Pb-dated kimberlites from cratonic North America (Superior Province of Canada). Geochemistry Geophysics Geosystems. 16:983-994.   10.1002/2015gc005734   AbstractWebsite

Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (approximate to 30 degrees) gap in mean paleopoles in the Late Jurassic, between about 160 and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high-precision U-Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.91.1 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5 degrees of the reverse polarity magnetization of the 157.51.2 Ma Triple B kimberlite. The combined approximate to 156 Ma Triple B and Peddie pole (75.5 degrees N, 189.5 degrees E, A95=2.8 degrees) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites), showing that the polar motion was at a relatively steady yet rapid (approximate to 1.5 degrees/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria-Africa and Iran-Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander.

Horst, AJ, Varga RJ, Gee JS, Karson JA.  2014.  Diverse magma flow directions during construction of sheeted dike complexes at fast- to superfast-spreading centers. Earth and Planetary Science Letters. 408:119-131.   10.1016/j.epsl.2014.09.022   AbstractWebsite

Dike intrusion is a fundamental process during upper oceanic crustal accretion at fast- to superfast-spreading ridges. Based on the distribution of magma along fast-spreading centers inferred from marine geophysical data, models predict systematic steep flow at magmatically robust segment centers and shallow magma flow toward distal segment ends. Anisotropy of magnetic susceptibility (AMS) fabrics from 48 fully-oriented block samples of dikes from upper oceanic crust exposed at Hess Deep Rift and Pito Deep Rift reveal a wide range of magma flow directions that are not consistent with such simple magma supply models. The AMS is interpreted to arise from distribution anisotropy of titanomagnetite crystals based on weak shape-preferred orientation of opaque oxide and plagioclase crystals generally parallel to AMS maximum eigenvectors. Most dike samples show normal AMS fabrics with maximum eigenvector directions ranging from subvertical to subhorizontal. The distributions of inferred magma flow lineations from maximum eigenvectors show no preferred flow pattern, even after structural correction. We use a Kolmogorov Smirnov test (KS-test) to show that the distribution of bootstrapped flow lineation rakes from Pito Deep are not statistically distinct from Hess Deep, and neither are distinguishable from Oman and Troodos Ophiolite AMS data. Magma flow directions in sheeted dikes from these two seafloor escarpments also do not correlate with available geochemistry in any systematic way as previously predicted. These results indicate distinct compositional sources feed melt that is injected into dikes at fast- to superfast-spreading ridges with no preference for subhorizontal or subvertical magma flow. Collectively, results imply ephemeral melt lenses at different along-axis locations within the continuous axial magma chamber and either direct injection or intermingling of melt from other deeper ridge-centered or off-axis sources. (C) 2014 Elsevier B.V. All rights reserved.

Selkin, PA, Gee JS, Meurer WP.  2014.  Magnetic anisotropy as a tracer of crystal accumulation and transport, Middle Banded Series, Stillwater Complex, Montana. Tectonophysics. 629:123-137.   10.1016/j.tecto.2014.03.028   AbstractWebsite

Fabric studies of layered mafic intrusions have led to improved understanding of the mechanical processes operating in large magma chambers, including crystal accumulation and crystal mush deformation. Such studies, however, are typically limited by a tradeoff between breadth (number of sites studied, characteristic of field-focused work) and sensitivity (ability to discern subtle fabric elements, characteristic of laboratory fabric analyses). Magnetic anisotropy, if analyzed in a systematic way and supported by single-crystal and petrofabric measurements, permits relatively rapid characterization of magmatic fabrics for large numbers of samples. Here we present the results of a study of remanence and susceptibility anisotropy from three transects through the Middle Banded Series of the Stillwater Complex, Montana. All three transects exhibit a magnetic foliation that increases with stratigraphic height up to the top of Olivine Bearing Zone III, consistent with crystal mush compaction. Perhaps more importantly, each tansect is characterized by a subtle lineation in the anisotropy of magnetic susceptibility with a consistent direction within that transect The magnetic lineation directions, which generally coincide with crystallographic preferred orientations of silicate minerals, likely record a pre-compaction fabric. Lineation directions differ from one transect to another, implying that the process generating the lineation - either slumping of a semiconsolidated crystal mush or magma transport - acted on length scales of at most a few km. These results demonstrate the sensitivity of magnetic anisotropy to petrofabric in mafic rocks. (C) 2014 Elsevier B.V. All rights reserved.

Vanderkluysen, L, Mahoney JJ, Koppers AAP, Beier C, Regelous M, Gee JS, Lonsdale PF.  2014.  Louisville Seamount Chain: Petrogenetic processes and geochemical evolution of the mantle source. Geochemistry Geophysics Geosystems. 15:2380-2400.   10.1002/2014gc005288   AbstractWebsite

The Louisville Seamount Chain is a similar to 4300 km long chain of submarine volcanoes in the southwestern Pacific that spans an age range comparable to that of the Hawaiian-Emperor chain and is commonly thought to represent a hot spot track. Dredging in 2006 recovered igneous rocks from 33 stations on 22 seamounts covering some 49 Myr of the chain's history. All samples are alkalic, similar to previous dredge and drill samples, providing no evidence for a Hawaiian-type tholeiitic shield-volcano stage. Major and trace element variations appear to be predominantly controlled by small but variable extents of fractional crystallization and by partial melting. Isotopic values define only a narrow range, in agreement with a surprising long-term source homogeneity-relative to the length scale of melting-and overlap with proposed fields for the "C" and "FOZO" mantle end-members. Trace element and isotope geochemistry is uncorrelated with either seamount age or lithospheric thickness at the time of volcanism, except for a small number of lavas from the westernmost Louisville Seamounts built on young (<20 Ma old) oceanic crust. The Louisville hot spot has been postulated to be the source of the similar to 120 Ma Ontong Java Plateau, but the Louisville isotopic signature cannot have evolved from a source with isotopic ratios like those measured for Ontong Java Plateau basalts. On the other hand, this signature can be correlated with that of samples dredged from the Danger Islands Troughs of the Manihiki Plateau, which has been interpreted as a rifted fragment of the "Greater" Ontong Java Plateau.

Bowles, JA, Jackson MJ, Berquo TS, Solheid PA, Gee JS.  2013.  Inferred time- and temperature-dependent cation ordering in natural titanomagnetites. Nature Communications. 4   10.1038/ncomms2938   AbstractWebsite

Despite years of efforts to quantify cation distribution as a function of composition in the magnetite-ulvo "spinel solid solution, important uncertainties remain about the dependence of cation ordering on temperature and cooling rate. Here we demonstrate that Curie temperature in a set of natural titanomagnetites (with some Mg and Al substitution) is strongly influenced by prior thermal history at temperatures just above or below Curie temperature. Annealing for 10(-1) to 10(3) h at 350-400 degrees C produces large and reversible changes in Curie temperature (up to 150 degrees C). By ruling out oxidation/reduction and compositional unmixing, we infer that the variation in Curie temperature arises from cation reordering, and Mossbauer spectroscopy supports this interpretation. Curie temperature is therefore an inaccurate proxy for composition in many natural titanomagnetites, but the cation reordering process may provide a means of constraining thermal histories of titanomagnetite-bearing rocks. Further, our theoretical understanding of thermoremanence requires fundamental revision when Curie temperature is itself a function of thermal history.

Tauxe, L, Gee JS, Steiner MB, Staudigel H.  2013.  Paleointensity results from the Jurassic: New constraints from submarine basaltic glasses of ODP Site 801C. Geochemistry, Geophysics, Geosystems.   10.1002/2013GC004704   AbstractWebsite

Tholeiite of the oldest oceanic crust was drilled during ODP Legs 129 and 185 at Hole 801C in the western Pacific. Fresh appearing submarine basaltic glass (SBG) was recovered from the tholetiites (~167 Ma; Koppers et al. [2003]) which has been shown to be nearly ideal for determining absolute paleointensity. Paleointensities of the younger, off-axis, alkalic basalts (~160 Ma; Koppers et al. [2003]), overlying the tholeiites, had been studied earlier [Tauxe, 2006]. Here we report results from the older tholeiitic (on-axis) sequence. We subjected a total of 73 specimens from 17 cooling units to absolute paleointensity experiments. Of these, 30 specimens and 6 cooling unit averages met our strictest reliability criteria, yielding an average of 11.9± 3.9 μT. The bulk of evidence suggests a paleolatitude of the site of 14°S (with an uncertainty of 10°). This translates the intensity to a value for the virtual axial dipole moment of 28 ZAm2, slightly lower than values determined from the plagio clase crystals in the three cooling units of the younger alkalic basalts over lying the tholeiites. This value is low when compared to the long-term median value of the field of 42 ZAm2. Our results and those of the published literature therefore support the contention of a low magnetic field strength in the Jurassic (average of 28 ± 14 ZAm2; N = 138 individual estimates), as initially suggested by Prévot et al. [1990]. Our interpretation of the body of available data argue for low field strengths for the entire Jurassic extending into the early Cretaceous.

Schoolmeesters, N, Cheadle MJ, John BE, Reiners PW, Gee J, Grimes CB.  2012.  The cooling history and the depth of detachment faulting at the Atlantis Massif oceanic core complex. Geochemistry Geophysics Geosystems. 13   10.1029/2012gc004314   AbstractWebsite

Oceanic core complexes (OCCs) are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults. We combine previously reported U-Pb zircon crystallization ages with (U-Th)/He zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the Atlantis Massif OCC (30 degrees N, MAR) and help establish cooling rates, as well as depths of detachment faulting and gabbro emplacement. We present nine new (U-Th)/He zircon ages for samples from IODP Hole U1309D ranging from 40 to 1415 m below seafloor. These data paired with U-Pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at Atlantis Massif as 2895 (+1276/-1162) degrees C Myr(-1) (from similar to 780 degrees C to similar to 250 degrees C); the lower 600 m of the borehole cooled more slowly at mean rates of similar to 500 (+125/-102) degrees C Myr(-1) (from similar to 780 degrees C to present-day temperatures). Rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of similar to 300 degrees C Myr(-1), possibly due to hydrothermal circulation to similar to 4 km depth through the detachment fault zone. Assuming a fault slip rate of 20 mm/yr (from U-Pb zircon ages of surface samples) and a rolling hinge model for the sub-surface fault geometry, we predict that the 780 degrees C isotherm lies at similar to 7 km below the axial valley floor, likely corresponding both to the depth at which the semi-brittle detachment fault roots and the probable upper limit of significant gabbro emplacement.

Koppers, AAP, Yamazaki T, Geldmacher J, Gee JS, Pressling N, Hoshi H, Anderson L, Beier C, Buchs DM, Chen LH, Cohen BE, Deschamps F, Dorais MJ, Ebuna D, Ehmann S, Fitton JG, Fulton PM, Ganbat E, Hamelin C, Hanyu T, Kalnins L, Kell J, Machida S, Mahoney JJ, Moriya K, Nichols ARL, Rausch S, Sano SI, Sylvan JB, Williams R.  2012.  Limited latitudinal mantle plume motion for the Louisville hotspot. Nature Geoscience. 5:911-917.   10.1038/ngeo1638   AbstractWebsite

Hotspots that form above upwelling plumes of hot material from the deep mantle typically leave narrow trails of volcanic seamounts as a tectonic plate moves over their location. These seamount trails are excellent recorders of Earth's deep processes and allow us to untangle ancient mantle plume motions. During ascent it is likely that mantle plumes are pushed away from their vertical upwelling trajectories by mantle convection forces. It has been proposed that a large-scale lateral displacement, termed the mantle wind, existed in the Pacific between about 80 and 50 million years ago, and shifted the Hawaiian mantle plume southwards by about 15 degrees of latitude. Here we use Ar-40/Ar-39 age dating and palaeomagnetic inclination data from four seamounts associated with the Louisville hotspot in the South Pacific Ocean to show that this hotspot has been relatively stable in terms of its location. Specifically, the Louisville hotspot-the southern hemisphere counterpart of Hawai'i-has remained within 3-5 degrees of its present-day latitude of about 51 degrees S between 70 and 50 million years ago. Although we cannot exclude a more significant southward motion before that time, we suggest that the Louisville and Hawaiian hotspots are moving independently, and not as part of a large-scale mantle wind in the Pacific.

Blackman, DK, Ildefonse B, John BE, Ohara Y, Miller DJ, Abe N, Abratis M, Andal ES, Andreani M, Awaji S, Beard JS, Brunelli D, Charney AB, Christie DM, Collins J, Delacour AG, Delius H, Drouin M, Einaudi F, Escartin J, Frost BR, Fruh-Green G, Fryer PB, Gee JS, Godard M, Grimes CB, Halfpenny A, Hansen HE, Harris AC, Tamura A, Hayman NW, Hellebrand E, Hirose T, Hirth JG, Ishimaru S, Johnson KTM, Karner GD, Linek M, MacLeod CJ, Maeda J, Mason OU, McCaig AM, Michibayashi K, Morris A, Nakagawa T, Nozaka T, Rosner M, Searle RC, Suhr G, Tominaga M, von der Handt A, Yamasaki T, Zhao X.  2011.  Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30 degrees N. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb007931   AbstractWebsite

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

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.

Mitra, R, Tauxe L, Gee JS.  2011.  Detecting uniaxial single domain grains with a modified IRM technique. Geophysical Journal International. 187:1250-1258.   10.1111/j.1365-246X.2011.05224.x   AbstractWebsite

Mid-ocean ridge basalt (MORB) specimens have often been found to have high ratios of saturation remanence to saturation magnetization (M(rs)/M(s)). This has been attributed either to dominant cubic anisotropy or to insufficient saturating field leading to overestimation of M(rs)/M(s) of a dominantly uniaxial single domain (USD) assemblage. To resolve this debate, we develop an independent technique to detect USD assemblages. The experimental protocol involves subjecting the specimen to bidirectional impulse fields at each step. The experiment is similar to the conventional isothermal remanent magnetization (IRM) acquisition experiment but the field is applied twice, in antiparallel directions. We define a new parameter, IRAT, as the ratio of the remanences at each field step and show it to have characteristic behaviour for the two assemblages; IRAT similar to 1 at all field steps for USD and <1 with a strong field dependence for multi-axial single domain (MSD) grains. We verified the theoretical predictions experimentally with representative USD and MSD specimens. Experiments with MORBs gave low IRATs for specimens having high M(rs)/M(s). This argues for a dominant MSD assemblage in the MORBs, possibly cubic in nature. Although undersaturation of the samples can indeed be a contributing factor to the exceptionally high M(rs)/M(s), this study shows that the nature of the assemblage cannot be dominantly USD.

Koppers, AAP, Gowen MD, Colwell LE, Gee JS, Lonsdale PF, Mahoney JJ, Duncan RA.  2011.  New Ar-40/Ar-39 age progression for the Louisville hot spot trail and implications for inter-hot spot motion. Geochemistry Geophysics Geosystems. 12   10.1029/2011gc003804   AbstractWebsite

In this study we present 42 new Ar-40/Ar-39 incremental heating age determinations that contribute to an updated age progression for the Louisville seamount trail. Louisville is the South Pacific counterpart to the Hawaiian-Emperor seamount trail, both trails representing intraplate volcanism over the same time interval (similar to 80 Ma to present) and being examples of primary hot spot lineaments. Our data provide evidence for an age-progressive trend from 71 to 21 Ma. Assuming fixed hot spots, this makes possible a direct comparison to the Hawaiian-Emperor age progression and the most recent absolute plate motion (APM) model (WK08G) of Wessel and Kroenke (2008). We observe that for the Louisville seamount trail the measured ages are systematically older relative to both the WK08G model predictions and Hawaiian seamount ages, with offsets ranging up to 6 Myr. Taking into account the uncertainty about the duration of eruption and magmatic succession at individual Louisville volcanoes, these age offsets should be considered minimum estimates, as our sampling probably tended to recover the youngest lava flows. These large deviations point to either a contribution of inter-hot spot motion between the Louisville and Hawaiian hot spots or to a more easterly location of the Louisville hot spot than the one inferred in the WK08G model. Both scenarios are investigated in this paper, whereby the more eastern hot spot location (52.0 degrees S, 134.5 degrees W versus 52.4 degrees S, 137.2 degrees W) reduces the average age offset, but still results in a relatively large maximum offset of 3.7 Myr. When comparing the new ages to the APM models (S04P, S04G) by Steinberger et al. (2004) that attempt to compensate for the motion of hot spots in the Pacific (Hawaii) or globally (Hawaii, Louisville, Reunion and Walvis), the measured and predicted ages are more in agreement, showing only a maximum offset of 2.3 Myr with respect to the S04G model. At face value these more advanced APM models, which consider both plate and hot spot motions, therefore provide a better fit to the new Louisville age data. The fit is particularly good for seamounts younger than 50 Ma, a period for which there is little predicted motion for the Louisville hot spot and little inter-hot spot motion with Hawaii. However, discrepancies in the Louisville age-distance record prior to 50 Ma indicate there is an extra source of inter-hot spot motion between Louisville and the other Pacific hot spots that was not corrected for in the global S04G model. Finally, based on six new Ar-40/Ar-39 age dates, the 169 W bend in the Louisville seamount trail seems to have formed at least 3 Myr before the formation of the Hawaiian-Emperor bend. The timing of the most acute parts of both bends thus appears to be asynchronous, which would require other processes (e. g., plume motions) than a global plate motion change between 50 and 47 Ma to explain these two observations.

Horst, AJ, Varga RJ, Gee JS, Karson JA.  2011.  Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008268   AbstractWebsite

The uppermost oceanic crust produced at the superfast spreading (similar to 142 km Ma(-1), full-spreading rate) southern East Pacific Rise (EPR) during the Gauss Chron is exposed in a tectonic window along the northeastern wall of the Pito Deep Rift. Paleomagnetic analysis of fully oriented dike (62) and gabbro (5) samples from two adjacent study areas yield bootstrapped mean remanence directions of 38.9 degrees +/- 8.1 degrees, -16.7 degrees +/- 15.6 degrees, n = 23 (Area A) and 30.4 degrees +/- 8.0 degrees, -25.1 degrees +/- 12.9 degrees, n = 44 (Area B), both are significantly distinct from the Geocentric Axial Dipole expected direction at 23 degrees S. Regional tectonics and outcrop-scale structural data combined with bootstrapped remanence directions constrain models that involve a sequence of three rotations that result in dikes restored to subvertical orientations related to (1) inward-tilting of crustal blocks during spreading (Area A = 11 degrees, Area B = 22 degrees), (2) clockwise, vertical-axis rotation of the Easter Microplate (A = 46 degrees, B = 44 degrees), and (3) block tilting at Pito Deep Rift (A = 21 degrees, B = 10 degrees). These data support a structural model for accretion at the southern EPR in which outcrop-scale faulting and block rotation accommodates spreading-related subaxial subsidence that is generally less than that observed in crust generated at a fast spreading rate exposed at Hess Deep Rift. These data also support previous estimates for the clockwise rotation of crust adjacent to the Easter Microplate. Dike sample natural remanent magnetization (NRM) has an arithmetic mean of 5.96 A/m +/- 3.76, which suggests that they significantly contribute to observed magnetic anomalies from fast- to superfast-spread crust.

Burgess, K, Cooper RF, Bowles JA, Gee JS, Cherniak DJ.  2010.  Effects of open and closed system oxidation on texture and magnetic response of remelted basaltic glass. Geochemistry Geophysics Geosystems. 11   10.1029/2010gc003248   AbstractWebsite

As part of an experimental and observational study of the magnetic response of submarine basaltic glass (SBG), we have examined, using ion backscattering spectrometry (RBS), transmission and scanning electron microscopy, energy dispersive X-ray spectrometry, and surface X-ray diffraction, the textures wrought by the controlled, open and closed system oxidation of glasses prepared by the controlled environment remelting and quenching of natural SBG. Initial compositions with similar to 9 wt % FeO* were melted at 1430 degrees C with the oxygen fugacity buffered at fayalite-magnetite-quartz; melts were cooled at a rate of 200 degrees C min(-1) near the glass transition (T(g) = 680 degrees C). In open system experiments, where chemical exchange is allowed to occur with the surrounding atmosphere, polished pieces of glass were reheated to temperatures both below and above T(g) for times 1-5000 h; undercooled melts were oxidized at 900 degrees C and 1200 degrees C for 18 and 20 h, respectively. RBS demonstrates unequivocally that the dynamics of open system oxidation involves the outward motion of network-modifying cations. Oxidation results in formation of a Fe-, Ca-, and Mg-enriched surface layer that consists in part of Ti-free nanometer-scale ferrites; a divalentcation- depleted layer is observed at depths >1 mu m. Specimens annealed/oxidized above T(g) have magnetizations elevated by 1-2 orders of magnitude relative to the as-quenched material; this does not appear to be related to the surface oxidation. Quenched glass (closed system, i.e., no chemical exchange between sample and atmosphere) exhibits very fine scale chemical heterogeneities that coarsen with time under an electron beam; this metastable amorphous immiscibility is the potential source for the nucleation of ferrites with a wide range of Ti contents, ferrites not anticipated from an equilibrium analysis of the bulk basalt composition.

Gee, JS, Yu YJ, Bowles J.  2010.  Paleointensity estimates from ignimbrites: An evaluation of the Bishop Tuff. Geochemistry Geophysics Geosystems. 11   10.1029/2009gc002834   AbstractWebsite

Ash flow tuffs, or ignimbrites, typically contain fine-grained magnetite, spanning the superparamagnetic to single-domain size range that should be suitable for estimating geomagnetic field intensity. However, ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history. We examined three stratigraphic sections through the similar to 0.76 Ma Bishop Tuff, where independent information on postemplacement cooling and alteration is available, as a test of the suitability of ignimbrites for paleointensity studies. Thermomagnetic curves suggest that low-Ti titanomagnetite (T(c) = 560 degrees C-580 degrees C) is the dominant phase, with a minor contribution from a higher Tc phase(s). Significant remanence unblocking above 580 degrees C suggests that maghemite and/or (titano)maghemite is an important contributor to the remanence in most samples. We obtained successful paleofield estimates from remanence unblocked between 440 degrees C and 580 degrees C for 46 of 89 specimens (15 sites at two of three total localities). These specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43.0 mu T (+/- 3.2), equivalent to a VADM of 7.8 x 10(22) Am(2). The most densely welded sections of the tuff have emplacement temperatures inferred to be as high as similar to 660 degrees C, suggesting that the remanence may be primarily thermal in origin, though a contribution from thermochemical remanence cannot be excluded. These results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations.

Morris, A, Gee JS, Pressling N, John BE, MacLeod CJ, Grimes CB, Searle RC.  2009.  Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples. Earth and Planetary Science Letters. 287:217-228.   10.1016/j.epsl.2009.08.007   AbstractWebsite

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the "rolling-hinge" model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46 degrees +/- 6 degrees anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011 degrees +/- 6 degrees. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases. (C) 2009 Elsevier B.V. All rights reserved.

Granot, R, Cande SC, Gee JS.  2009.  The implications of long-lived asymmetry of remanent magnetization across the North Pacific fracture zones. Earth and Planetary Science Letters. 288:551-563.   10.1016/j.epsl.2009.10.017   AbstractWebsite

Large marine magnetic anomalies accompany the Pacific fracture zones (FZs) for thousands of kilometers. Although the origin of these anomalies is poorly understood, their underlying magnetization contrasts should reflect the temporal record of crustal accretion as well as geomagnetic field variations. Here we present an analysis of archival and newly collected magnetic anomaly profiles measured across three FZs from the North Pacific Cretaceous Quiet Zone (120.6 to 83 Ma) that are characterized by a remarkably uniform shape. Forward and inverse modeling indicate that these anomalies arise from remanent magnetization, with enhanced remanence located on one side of each FZ along the entire studied area. A comparison of geochemical and magnetic data from active ridge discontinuities and transform faults suggests that elevated iron content near segment ends is likely responsible for the observed anomalies in the Cretaceous Quiet Zone as well. A more complex magnetization setting is observed where the FZs contain multiple faults. There, the simple model of one-sided enhancements is only partly valid. Comparison between 3D forward modeling of the Quiet Zone magnetization and the calculated magnetization contrasts found across the Pioneer and Pau FZs suggests that the intensity of the geomagnetic field during the Cretaceous superchron had less than 50 percent variability about its average value. No major trends in the strength of the geomagnetic field during the superchron are observed. The presence of long-duration (> 30 m.y.) zones of enhanced magnetization along the young/old sides of the Pioneer/Pau FZs (both left-stepping) requires some long-lived asymmetry in crustal construction processes near ridge-transform intersections. Although the underlying mechanism that controls this long-lived asymmetry remains unclear, absolute plate motions might explain this asymmetry. Shorter period (few m.y.) variations in the amplitudes of the enhancements probably result from oscillations in crustal construction. (C) 2009 Elsevier B.V. All rights reserved.

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.

Selkin, PA, Gee JS, Meurer WP, Hemming SR.  2008.  Paleointensity record from the 2.7 Ga Stillwater Complex, Montana. Geochemistry Geophysics Geosystems. 9   10.1029/2008gc001950   AbstractWebsite

The record of geomagnetic intensity captured in the 2.7 Ga Stillwater Complex (Montana, USA) provides a statistical description of the Archean geodynamo. We present results of modified Thellier paleointensity experiments on 441 core specimens, 114 of which pass strict reliability criteria. The specimens are from 53 sites spanning most of the Banded Series rocks in the Stillwater Complex. On the basis of thermochronologic and petrologic evidence, we interpret the highest temperature component of remanence to be a late Archean thermoremanence, though the possibility remains that it is a thermochemical remanence. Thermal models indicate that the highest temperature magnetization component at each of the sites averages similar to 20-200 ka of geomagnetic secular variation. The suite of sites as distributed through the Banded Series samples a roughly a 1 Ma time interval. The average of the most reliable paleointensity measurements, uncorrected for the effects of anisotropy or cooling rate, is 38.2 +/- 11.3 mu T (1 sigma). Remanence anisotropy, cooling rate, and the nonlinear relationship between applied field and thermoremanence have a significant effect on paleointensity results; a corrected average of 30.6 +/- 8.8 mu T is likely a more appropriate value. Earth's average dipole moment during the late Archean (5.05 +/- 1.46 x 10(22) Am(2), lambda(pmag) = 44.5 degrees) was well within the range of estimates from Phanerozoic rocks. The distribution of site-mean paleointensities around the mean is consistent with that expected from slow cooling over timescales expected from thermal models and with secular variation comparable to that of the Phanerozoic field.

Gee, JS, Tauxe L, Constable C.  2008.  AMSSpin: A LabVIEW program for measuring the anisotropy of magnetic susceptibility with the Kappabridge KLY-4S. Geochemistry Geophysics Geosystems. 9   10.1029/2008gc001976   AbstractWebsite

Anisotropy of magnetic susceptibility (AMS) data are widely used as a petrofabric tool because the technique is rapid and nondestructive and because static measurement systems are capable of determining small degrees of anisotropy. The Kappabridge KLY-4S provides high resolution as a result of the large number of measurements acquired while rotating the sample about three orthogonal axes. Here we describe a graphical-based program called AMSSpin for acquiring AMS data with this instrument as well as a modified specimen holder that should further enhance the utility of this instrument. We also outline a method for analysis of the data (that differs in several ways from that of the software supplied with the instrument) and demonstrate that the measurement errors are suitable for using linear perturbation analysis to statistically characterize the results. Differences in the susceptibility tensors determined by our new program and the SUFAR program supplied with the instrument are small, typically less than or comparable to deviations between multiple measurements of the same specimen.

Varga, RJ, Horst AJ, Gee JS, Karson JA.  2008.  Direct evidence from anisotropy of magnetic susceptibility for lateral melt migration at superfast spreading centers. Geochemistry Geophysics Geosystems. 9   10.1029/2008gc002075   AbstractWebsite

Rare, fault-bounded escarpments expose natural cross sections of ocean crust in several areas and provide an unparalleled opportunity to study the end products of tectonic and magmatic processes that operated at depth beneath oceanic spreading centers. We mapped the geologic structure of ocean crust produced at the East Pacific Rise ( EPR) and now exposed along steep cliffs of the Pito Deep Rift near the northern edge of the Easter microplate. The upper oceanic crust in this area is typified by basaltic lavas underlain by a sheeted dike complex comprising northeast striking, moderately to steeply southeast dipping dikes. Paleomagnetic remanence of oriented blocks of dikes collected with both Alvin and Jason II indicate clockwise rotation of similar to 61 degrees related to rotation of the microplate indicating structural coupling between the microplate and crust of the Nazca Plate to the north. The consistent southeast dip of dikes formed as the result of tilting at the EPR shortly after their injection. Anisotropy of magnetic susceptibility of dikes provides well-defined magmatic flow directions that are dominantly dike-parallel and shallowly plunging. Corrected to their original EPR orientation, magma flow is interpreted as near-horizontal and parallel to the ridge axis. These data provide the first direct evidence from sheeted dikes in ocean crust for along-axis magma transport. These results also suggest that lateral transport in dikes is important even at fast spreading ridges where a laterally continuous subaxial magma chamber is present.

Garces, M, Gee JS.  2007.  Paleomagnetic evidence of large footwall rotations associated with low-angle faults at the Mid-Atlantic Ridge. Geology. 35:279-282.   10.1130/g23165a.1   AbstractWebsite

Exposures of gabbros and mantle-derived peridotites; at slow-spreading oceanic ridges have been attributed to extension on long-lived, low-angle detachment faults, similar to those described in continental metamorphic core complexes. In continental settings, such detachments have been interpreted as having originated and remained active at shallow dips. Alternatively, currently shallow dipping fault surfaces may have originated at moderate to steep dips and been flattened by subsequent flexure and isostatic uplift. While the latter interpretation would be more consistent with Andersonian faulting theory, it predicts large footwall tilts that have not been observed in continental detachment faults. Here we use the magnetization of oceanic gabbro and peridotite samples exposed near the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge to demonstrate that substantial footwall rotations have occurred. Widespread rotations ranging from 50 degrees to 80 degrees indicate that original fault orientations dipped steeply toward the spreading axis.

Yu, YJ, Tauxe L, Gee JS.  2007.  A linear field dependence of thermoremanence in low magnetic fields. Physics of the Earth and Planetary Interiors. 162:244-248.   10.1016/j.pepi.2007.04.008   AbstractWebsite

We tested a linear field-dependence of thermoremanent magnetization (TRM) to saturation isothermal remanent magnetization (SIRM) ratio for magnetite-containing natural samples. The TRM/SIRM shows a linear field-dependence to very low field ranges (<1 mu T). This observation is at odds with a claim of limited sensitivity at low fields in TRM acquisition documented in previous studies. We attribute the difference to poor field control in the ovens used in previous studies. The TRM/SIRM ratio shows a grain-size dependence. For magnetite-containing samples with insignificant anisotropy, the TRM/SIRM is most efficient in pseudo-single-domain magnetites. These results suggest that while the TRM/SIRM ratio is linear at low field strengths, the ratio provides only a crude estimation on the actual paleo-field within two orders of magnitude, suggesting that a careful sample characterization is necessary in applying the TRM/SIRM as a paleointensity proxy. (c) 2007 Elsevier B.V. All rights reserved.