Ocean Crustal Magnetization and Magnetic Anomalies Anomaly and magnetization basics Lavas as largest magnetic source what controls their magnetization? Do lower crustal layers contribute? Magnetic anomalies and plate reconstructions Skewness poles and anomalous skewness How does crustal accretion affect magnetization source and anomalies? Vector magnetic anomalies Does the ocean crust record intensity variations?
Anomaly Basics Source magnetization and sea surface and near bottom anomalies generated at pole (i.e. with remanence and ambient field both vertical). 55 km/m.y. half spreading rate. Earth filter: shorter wavelengths more attenuated Geomagnetic signal includes both direction and intensity Crustal accretion processes as filter
Shape (skewness) of anomalies depends on geometry of spreading lineation, Mdir, Bdir Skewness = phase shift to make similar to pattern at pole Anomalous skewness = not predicted by geometry
Magnetic Minerals Two properties of magnetic materials (ferromagnets) Long range ordering below Curie temperature Magnetic hysteresis = magnetization is a function of field history Curie temperature is a function of mineralogy Magnetic properties a strong function of grain size
Thermoremanent Magnetization Magnetization of ocean crust is primarily thermal in origin During cooling, thermal energy decreases and anisotropy barriers increase Small particles (< micron) are the most stable Tauxe (2010)
Lavas as Dominant Source Ridge parallel profile over R crust shows negative correlation with topography. Thin extrusive layer is dominant source Along axis profile fit with N magnetized bathymetry Cross axis profile to determine layer thickness (~400m) Talwani et al. (1971)
Magnetic Grain Size & Remanence Cooling at pillow/flow margins results in large gradient in grain size Smallest magnetic grains (near margin) have largest magnetization Composition of large titanomagnetites (TM60) but more variable near margin after Kent & Gee (1996); Zhou et al. (2000)
Remanence and Geochemistry Fractionation of melt in shallow magma chambers (FeO* increases) Higher FeO* should result in more titanomagnetite and higher magnetization (magnetic telechemistry) Signature of higher Fe at wakes of propagating rifts Magnetization pattern 9-10 N on (and near fracture zones) EPR. Maximum ~40 A/m. Carbotte & Macdonald (1992)
Magnetic Telechemistry Magnetization of axial lavas determined from slabs perpendicular to chilled margin Reasonable correlation (R=0.81) for FeO* & NRM Along axis magnetic anomaly variations generally match sample geochemistry after Gee & Kent (1997, 1998)
Along-Axis Magnetization Variations Variations in magnetization (calculated from anomaly data) are best modeled by lava thickness variations Williams et al. (2008)
Temporal Changes in Magnetization Anomaly amplitudes and sample remanence higher at ridge Attributed to low T oxidation of titanomagnetite Cracking in large titanomagnetite typical of low T oxidation sample NRM data from Irving (1970) oxidized TM (from Furuta, 1993)
Spreading Rate Dependence Large decrease in amplitude anomaly off-axis confined to full spreading rates less than about 30 km/m.y. Central Anomaly not such a pronounced feature at faster spreading rates. Gee & Kent (2007)
Evidence for Low-T Alteration Oxidation parameter measured on seafloor lavas. A value of 1 corresponds to complete oxidation and would be associated with a 4x reduction in magnetization. Direct determination of oxidation state of TM grains by TEM Very heterogeneous oxidation, with low values at oldest seafloor data from Wang et al. (2005, 2006), Zhou et al. (2001)
Temporal Magnetization Changes Drill sites with large n have lower NRM than axial dredges but no systematic change with age. Gee & Kent (2007) Age (Ma)
Alteration and Anomaly Amplitudes full rate 80 km/m.y. full rate 20 km/m.y. A 2x to 4x alteration-related reduction in magnetization is compatible with slow and fast spread profiles. The time scale of alteration is poorly defined. Gee & Kent (2007)
Magnetization of Sheeted Dikes Remanence in dikes ~5x lower than in lavas Uniformly high Curie temp. indicate pure magnetite NRM, inclination and Curie temperature variations in Hole 504B. Data compliled from various sources; see Gee & Kent (2007)
Gabbro Magnetization Lower crust has been sampled at several shallow holes and two holes penetrating ~1.5 km Remanence averages about 1 A/m data compiled from various sources; see Gee & Kent (2007)
Magnetization of Dikes and Gabbros Two generations of magnetite exsolved from CPX (from Feinberg et al.,2004). Ilmenite lamellae in titanomagnetite Lower crustal rocks typically have nearly pure magnetite (Curie temperature 580C) either as a result of subdivision of larger grains or as tiny magnetite grains included in silicate minerals. Pang et al. (2007)
Three Layer Model Lavas have highest magnetization (~5 A/m) Thicker dikes (2.3 A/m) and gabbro (1.2 A/m) generate comparable anomalies Gee & Kent (2007)
Lava Accumulation Pattern Transition zone width (4 sigma) of 1.5 to 3 km limits the ultimate temporal resolution Brunhes/Matuyama boundary at 21 N on EPR illustrating lava spillover. Brunhes lavas extend 250-500m beyond the vertically averaged reversal boundary determined from near bottom data. Macdonald et al. (1983)
Magnetic anomalies from modern GPTS From Gee and Kent, 2007, Treatise on Geophysics, v. 5.12
Marine magnetic anomaly skewness Shape (skewness) a function of the remanent and ambient field projections and spreading lineation θ meas = I r ʹ + Iʹ f 180 I r ʹ = tan( I ) r sin( Az Dec)
Effective Remanent Inclination Note that effective inclination changes most rapidly for spreading centers near the paleoequator Variation of effective remanent inclination as a function of lineation and paleolatitude (from Acton et al., 1996)
Skewness Lunes of Confidence A single Ir estimate could be produced by a family of remanent declination, inclination pairs Example of lunes of confdence from Late Cretaceous anomalies in the north and south Pacific (these have been adjusted for anomalous skewness of 14 ; Cande, 1976)
Anomalous Skewness Anomalous skewness not predicted by block model geometry Wharton Basin: different phase shift needed for two flanks Dyment et al. (1994)
Anomalous skewness estimates from Indian Ocean (half difference of conjugate flanks) and an example of the sequence effect (right). From Dyment et al. (1994)
Possible causes of anomalous skewness 1) outward tilt of source blocks 2) sloping boundary in the lower crust 3) systematic variations in field intensity
Tilting of Lavas Lava tilting model for ODP Hole 801C. Dips inferred from logging data are used to suggest tilt from lava loading. Tivey et al. (2005) Lava tilt from loading plus block rotations inferred for Sites 417/418. Schouten (2002)
Lava Tilt and Skewness Anomaly skewness is sensitive indicator of such tilting (1 tilt corresponds to 1 difference in skewness) Lava tilt from loading results in anomalous skewness opposite to that observed Gee & Kent (2007)
Nonvertical Boundaries in Gabbro Sloping isochrons in lower crust can significantly affect anomaly shape Phase shift of ~45, in same sense as global observations of anomalous skewness Gee & Kent (2007)
Best-fitting paleopoles from anomaly 12r (~32 Ma) for the Pacific plate from anomaly skewness (blue) compared to estimates from sedimentary facies, paleomagnetic data and hotspots Richard Gordon s video presentation provides some examples of application of skewness poles to Pacific plate motion
Example of aeromagnetic vector data at low latitude Tests for two dimensionality Horner-Johnson and Gordon (2003)
Gee and Cande (2002) Note that magnitude of vector anomalies is always greater than or equal to total field anomaly.
Vector anomalies can allow identification of anomalies in geometries with very low amplitude total field anomalies Engels et al. (2008)
Engels et al. (2008) Vector data allow orientation of magnetic contrasts to be estimated from a single profile
Does Ocean Crust Record Intensity Fluctuations? 82 mm/yr 54 mm/yr 31 mm/yr 82 mm/yr 54 mm/yr Potential higher resolution time markers Consistent pattern in sea surface anomalies (anom 5) Wavelengths a function of earth filter Bowers et al. (2001) Cande and Kent (1992)
Near Bottom Anomaly Data Anomaly 5 profiles acquired a few 100m above source Near bottom anomalies also lineated but more complex pattern Unlikely to represent reversals Globally present? after Bowers et al. (2001)
An Independent Record of Intensity Fluctuations in the Brunhes Sedimentary cores provide relative intensity record, that match sea surface anomaly profiles reasonably well. Tauxe and Yamazaki (2007) after Gee et al. (1996)
Brunhes Near Bottom Anomalies Stack of 8 profiles separated by up to 50 km Topographic variations can be accounted for with inversions Several similarities with sedimentary relative intensity records Gee et al. (2000)
An Additional Check Can determine absolute intensity of the field from surface (glass)samples Similar pattern with axial low and bounding highs in magnetic anomalies and glass paleointensities Gee et al. (2000)