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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.

Selkin, PA, Gee JS, Tauxe L, Meurer WP, Newell AJ.  2000.  The effect of remanence anisotropy on paleointensity estimates: a case study from the Archean Stillwater Complex. Earth and Planetary Science Letters. 183:403-416.   10.1016/s0012-821x(00)00292-2   AbstractWebsite

Paleomagnetism of Archean rocks potentially provides information about the early development of the Earth and of the geodynamo. Precambrian layered intrusive rocks are good candidates for paleomagnetic studies: such complexes are commonly relatively unaltered and may contain some single-domain magnetite 'armored' by silicate mineral grains. However, layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy. Magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected. Here we examine the magnetic anisotropy of an anorthosite sample with a well-developed magmatic foliation. The effect of the sample's remanence fabric on paleointensity determinations is significant: paleointensities estimated by the method of Thellier and Thellier range from 17 to 55 muT for specimens magnetized in a field of 25 muT. We describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the Stillwater Complex (MT, USA) of similar to 32 muT (adjusted for the effects of slow cooling). (C) 2000 Elsevier Science 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.

Selkin, PA, Gee JS, Tauxe L.  2007.  Nonlinear thermoremanence acquisition and implications for paleointensity data. Earth and Planetary Science Letters. 256:81-89.   10.1016/j.epsl.2007.01.017   AbstractWebsite

In paleointensity studies, thermoremanence is generally regarded as a linear function of ambient inagnetic field at low fields comparable to that of the present-day Earth. We find pronounced nonlinearity at low fields for a class of materials with silicate-hosted magnetite that otherwise perforin well in paleointensity experiments. We model this nonlinearity with narrow size ranges of large, acicular single domain grains, which are most likely in a vortex state (i.e. nonuniformly magnetized, sometimes labeled pseudosingle domain). Simple TRM theory predicts that even certain single domain particles will also exhibit a nonlinear response, saturating in fields as low as the Earth's. Such behavior, although likely to be rare, may bias some paleointensity estimates. The bias is especially pronounced when the laboratory field is higher than the ancient field. Fortunately, the fundamental assumption that thermoremanence is proportional to applied field can (and should) be routinely checked at the end of successful paleointensity experiments by adding two extra heating steps. (c) 2007 Elsevier B.V. All rights reserved.

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.

Sempere, JC, Gee J, Naar DF, Hey RN.  1989.  3-Dimensional Inversion of the Magnetic-Field Over the Easter-Nazca Propagating Rift Near 25-Degrees-S, 112-Degrees-25 W. Journal of Geophysical Research-Solid Earth and Planets. 94:17409-17420.   10.1029/JB094iB12p17409   AbstractWebsite

The Easter microplate boundary configuration is being reorganized by rift propagation. A Sea Beam survey of the Easter-Nazca spreading center, which forms the eastern boundary of the microplate, has revealed the presence of a young propagating rift growing northward (Naar and Hey, 1986). The tip of the propagating rift is associated with a high-amplitude positive magnetic anomaly. We have performed a three-dimensional inversion of the magnetic field over the propagating rift tip area. The magnetization solution suggests that the western and eastern pseudofaults strike 014° and 338°, respectively, and converge near the rift tip. These orientations yield a propagation to spreading rate ratio of 1.5, slightly higher than the estimate of Naar and Hey (1986). Using the revised estimate of the full spreading rate along the Easter-Nazca spreading center near 25°S (80 mm/yr) (D. F. Naar and R. N. Hey, unpublished manuscript, 1989), we obtain a propagation rate of 120 mm/yr. Within 27–30 km of the rift tip, the propagating rift curves by about 15° to the east toward the failing rift, probably as a result of the interaction between the two offset spreading centers. As at the Galapagos propagating rift, rift propagation appears to be a very orderly process along the Easter-Nazca spreading center. The magnetization distribution that we obtain exhibits a high at the propagating rift tip. At other large ridge axis discontinuities, similar magnetization highs have been interpreted as being the result of the eruption of highly differentiated basalts enriched in iron. The origin of the high magnetization zone in the case of the Easter-Nazca propagating rift near 25°S may be more complex. Preliminary rock magnetic measurements of basalts recovered in the vicinity of the propagating rift confirm the presence of highly magnetized basalts but suggest that the relationship between high magnetization intensities and high Fe content is not straightforward.

Smith, GM, Gee J, Klootwijk CT.  1991.  Magnetic petrology of basalts from Ninetyeast Ridge. Proceedings of the Ocean Drilling Program, Scientific Results. 121:525-545.   10.2973/odp.proc.sr.121.154.1991   Abstract

Given the importance of the inversion of seamount magnetic anomalies, particularly to the motion of the Pacific plate, it is important to gain a better understanding of the nature of the magnetic source of these features. Although different in detail, Ninetyeast Ridge is composed of submarine and subaerial igneous rocks that are similar to those found at many seamounts, making it a suitable proxy. We report here on the magnetic petrology of a collection of samples from Ninetyeast Ridge in the Indian Ocean. Our purpose is to determine the relationship between primary petrology, subsequent alteration, and magnetic properties of the recovered rocks. Such information will eventually lead to a more complete understanding of the magnetization of seamounts and presumably improvements in the accuracy of anomaly inversions. Three basement sites were drilled on Ninetyeast Ridge, with recovery of subaerial basalt flows at the first two (Sites 756 and 757) and submarine massive and pillow flows at the final one (Site 758). The three sites were distinctly different. Site 756 was dominated by ilmenite. What titanomagnetite was present had undergone deuteric alteration and secondary hematite was present in many samples. The magnetization was moderate and stable although it yielded a paleolatitude somewhat lower than expected. Site 757 was highly oxidized, presumably while above sea level. It was dominated by primary titanomagnetite, which was deuterically altered. Secondary hematite was common. Magnetization was relatively weak but quite stable. The paleolatitude for all but the lowermost flows was approximately 40° lower than expected. Site 758 was also dominated by primary titanomagnetite. There was relatively little oxidation with most primary titanomagnetite showing no evidence of high-temperature alteration. No secondary hematite was in evidence. This site had the highest magnetization of the three (although somewhat low relative to other seamounts) but was relatively unstable with significant viscous remanence in many samples. Paleolatitude was close to the expected value. It is not possible, at present, to confidently associate these rocks with specific locations in a seamount structure. A possible and highly speculative model would place rocks similar to Site 757 near the top of the edifice, Site 756 lower down but still erupted above sea level, and Site 758 underlying these units, erupted while the seamount was still below sea level.

Staudigel, H, Gee J, Tauxe L, Varga RJ.  1992.  Shallow Intrusive Directions of Sheeted Dikes in the Troodos Ophiolite - Anisotropy of Magnetic-Susceptibility and Structural Data. Geology. 20:841-844.   10.1130/0091-7613(1992)020<0841:sidosd>2.3.co;2   AbstractWebsite

Sheeted dikes play a central role in the formation of oceanic crust. It is commonly assumed that sheeted dikes intrude vertically upward, from elongated mid-ocean ridge (MOR) magma chambers, but there are no direct observational data bearing on this hypothesis. This assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes. We have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust, the Troodos ophiolite. Structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow (<20-degrees) to nearly vertical, upward magma-transport directions. These data suggest that horizontal emplacement has to be considered for sheeted dikes at MORs, implying more centralized MOR plumbing systems than previously thought. Such plumbing systems provide ample opportunity for complex mixing, fractionation, and contamination of MOR lavas in magma chambers and tabular magma-storage volumes. Whether the MOR magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems.

Staudigel, H, Tauxe L, Gee JS, Bogaard P, Haspels J, Kale G, Leenders A, Meijer P, Swaak B, Tuin M, Van Soest MC, Verdurmen EAT, Zevenhuizen A.  1999.  Geochemistry and Intrusive Directions In Sheeted Dikes in the Troodos Ophiolite: Implications for Mid-Ocean Ridge Spreading Centers. Geochemistry Geophysics Geosystems. 1 AbstractWebsite

Sheeted dikes at mid-ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity. As such, they are crucial for the interpretation of many observations at mid-ocean ridges or other volcanoes with pronounced rift zones, including topography, hydrothermal systems, petrology, and geochemistry. We carried out a structural, magnetic, and chemical investigation of a 4 x 10 km sheeted dike section in the Troodos ophiolite, Cyprus. On the basis of major and trace element geochemistry, we distinguish dikes that may be correlated with the basal high-Ti series (HTS) lavas from those of the overlying low-Ti series (LTS) lavas. All dikes studied are nearly parallel to each other, with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center. Anisotropy of magnetic susceptibility measurements suggests that the HTS and LTS dikes intrude in fundamentally different ways. HTS dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber. LTS dikes show a bimodal, orthogonal set of intrusive directions, one shallow and one near vertical. Near-lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system, and near-vertical dike propagation directions are probably associated with feeder dikes to down-rift surface flows. Our study suggests that the types of dike intrusive behavior in the Troodos ophiolite may also be typical for "normal" mid-ocean ridges or other major shield volcanoes with well-developed rift zones.