Publications

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2011
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.

1999
Gee, J, Kent DV.  1999.  Calibration of magnetic granulometric trends in oceanic basalts. Earth and Planetary Science Letters. 170:377-390.   10.1016/s0012-821x(99)00125-9   AbstractWebsite

The validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains. For example, populations with dominantly superparamagnetic (SP) or multidomain (MD) grains both are characterized by low remanence and coercivity, and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain (SD) size range. The natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size, from SP-SD mixtures in submarine basaltic glass to SD-MD mixtures in flow interiors. Based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the Mid-Atlantic Ridge (ages <1 Ma to 70 Ma), a model of preferential dissolution with time of the finest-grained titanomagnetites has recently been suggested as the major process contributing to long-term temporal changes in remanent intensity of mid-ocean ridge basalts. We evaluated the local and long-term temporal trends in effective magnetic grain size predicted by this model using hysteresis data from a large number of submarine basalt samples which span a range of apes from similar to 0 to similar to 122 Ma. Specimens were systematically taken along transects perpendicular to the chilled margin of each sample. The large number of data (similar to 750 loops) and the inferred progressive change in grain size approaching the chilled margin allow recognition of mixing trends between MD and SD grains and between SD and SP grains on a Day-plot. These trends in hysteresis parameters are crucial to resolving the inherent, but frequently overlooked, ambiguity in inferring grain size from hysteresis parameters. We illustrate that two additional rock magnetic tests (warming of a low-temperature isothermal remanence and hysteresis loop shapes) often used to address these ambiguities are inconclusive, requiring some independent knowledge of whether SP or MD grains are likely to be present. Even with a considerably larger data set the substantial intrasample variability in oceanic basalts precludes recognition of any systematic trend in magnetic grain size with age. (C) 1999 Elsevier Science B.V. All rights reserved.

1995
Gee, J, Kent DV.  1995.  Magnetic Histeresis in Young Midocean Ridge Basalts - Dominant Cubic Anisotropy. Geophysical Research Letters. 22:551-554.   10.1029/95gl00263   AbstractWebsite

Magnetic hysteresis data from young mid-ocean ridge basalts include samples with saturation remanence to saturation magnetization (Mrs/Ms) ratios greater than 0.5, the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy. Under the usual assumption of dominant uniaxial anisotropy, the narrow single domain grain size distribution implied by these high Mrs/Ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains. Dominant cubic anisotropy provides a plausible explanation for the high Mrs/Ms ratios, and if generally valid, requires reinterpretation of granulometric and domain state inferences made from hysteresis data.