Janet E. Pariso

Variations in oceanic crustal magnetization: Systematic changes in the last 160 million years

The amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age. Globally, the amplitudes decrease with age over the first 20 to 30 million years, a phenomenon that has been attributed to the low-temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks. In oceanic crust older than 40 million years, however, the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years. In order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust, we compiled all of the existing rock magnetic data from relevant Deep Sea Drilling Project and Ocean Drilling Program sites that sampled extrusive rocks from “normal” ocean crust. This compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes, both in the decrease due to oxidation in the first 20 to 30 million years, and in the increase in magnetization in older (>80 million years) crust. This positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks. Further, the Curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years, and does not increase significantly beyond that point. Finally, the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust. Specifically, we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic FeTi oxides in the older crustal rocks. This large increase in abundance of the FeTi oxides may be related to a systematic increase in bulk FeTi content in older tholeiitic basalts, or, more likely, is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases.

Alteration processes at Deep Sea Drilling Project/Ocean Drilling Program Hole 504B at the Costa Rica Rift: Implications for magnetization of oceanic crust

Magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504B during Ocean Drilling Program leg 111. Our results, combined with those from previous studies, show abrupt first-order changes in magnetic properties at alteration boundaries. Within the 504B crustal section, changes in style and degree of alteration fall near the boundaries of the three well-defined lithologic units: the extrusive basalts, the transition zone, and the sheeted dike complex. This postemplacement alteration heavily influences magnetic properties and is observed to change, in both style and degree, with depth within each lithologic unit. Our results indicate that, at 504B, the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration. The upper, more permeable basalts above this critical depth have not experienced this reheating. Within the sheeted dike complex, the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth. The changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth. Overall, these observations suggest that the effective penetration of water into layer 2C decreased with increasing depth and resulted in a lower rate of convective cooling. Magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration, the upper dikes have become magnetically more stable than the lower dikes. A review of all magnetic properties indicate that the dike section at 504B carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict. We suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions. Despite a magnetization which is lower than expected, it appears that the sheeted dike complex at hole 504B is capable of making a substantial contribution to the overlying marine magnetic anomaly. Because of the systematic decrease in hydrothermal alteration of magnetic minerals, the ability of the dike section to contribute decreases as a function of depth.

Do layer 3 rocks make a significant contribution to marine magnetic anomalies? In situ magnetization of gabbros at Ocean Drilling Program hole 735B

We estimate the average magnetization of a vertical section of oceanic gabbros using paleomagnetic and downhole magnetic logging techniques and evaluate the ability of these crustal rocks to contribute to marine magnetic anomalies. Results from Ocean Drilling Program Hole 735B show that the drilled crustal section has a mean effective remanent magnetization of 2.5 A/m. Olivine gabbros, which make up 60% of the section, have an average effective magnetization between 1 and 2 A/m. We used two-dimensional forward modeling and the average effective magnetization of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface. These results indicate that if polarity boundaries within oceanic layer 3 are near-vertical, crustal sections similar to 735B would contribute from 25% to 75% of the overlying marine magnetic anomalies at the sea surface. The shape of the polarity boundaries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitude and skewness of marine magnetic anomalies at individual spreading centers.

Temporal and spatial variations in crustal accretion along the Mid-Atlantic Ridge (29°–31°30′N) over the last 10 m.y.: Implications from a three-dimensional gravity study

We have conducted a three-dimensional gravity study of the Mid-Atlantic Ridge near the Atlantis Transform to study the evolution of accretionary processes at this slow-spreading center over the last 10 m.y. We have removed from the free-air gravity anomaly the gravity contribution of the density contrast at the seafloor and the gravity contribution of the lateral density variations associated with the cooling of the lithosphere. The resulting residual gravity anomaly exhibits substantial variation along and across the ridge axis. The residual gravity anomaly can be accounted for by variations in crustal thickness of up to 3 km. For the first two segments south of the Atlantis Transform, the midportions of the segments have been associated with thick crust and the segment discontinuities have been associated with thin crust for the last 10 m.y., suggesting the segment discontinuities act as long-term boundaries in the delivery of melt to the individual segments. In contrast, our calculations indicate that for the segments north of the fracture zone, thick crust is associated with the midportions of segments and thin crust is associated with segment discontinuities only in crust less than ∼3 m.y. This result suggests that focused mantle upwelling has only recently developed north of the fracture zone. The onset of focused mantle upwelling at approximately 2–3 m.y. may be related to a change in the spreading direction which occurred between magnetic anomalies 5 and 3 (Figure 1) and resulted in changes in the geometry of the plate boundary north of the fracture zone. Cross sections of crustal thickness extracted along the midpoint traces of paleosegments show that, for a few segments, up to 2 km of gradual crustal thinning is observed. We suggest that the “apparent” crustal thinning is a result of lateral changes in mantle density associated with buoyant upwelling not predicted by the passive flow model used in our study. Variations in computed crustal thickness are observed across axis in all of the paleosegments in our study area, but are not correlated between individual segments. If these computed crustal thickness variations are due to temporal variations in melt production, this implies that there is little interdependence in the amount of melt supplied to adjacent segments.

Three-dimensional inversion of marine magnetic anomalies: Implications for crustal accretion along the Mid-Atlantic Ridge (28°–31°30′ N)

We present magnetic field data collected over the Mid-Atlantic Ridge in the vicinity of the Atlantis Fracture Zone and extending out to 10 Ma-old lithosphere. We calculated a magnetization distribution which accounts for the observed magnetic field by performing a three-dimensional inversion in the presence of bathymetry. Our results show the well-developed pattern of magnetic reversals over our study area. We observe a sharp decay in magnetization from the axis out to older lithosphere and we attribute this decay to progressive low temperature oxidation of basalt. In crust which is ∼ 10 Ma, we observe an abrupt increase in magnetic field intensity which could be due to an increase in the intensity of magnetization or thickness of the magnetic source layer. We demonstrate that because the reversal epoch was of unusually long duration, a two-layer model comprised of a shallow extrusive layer and a deeper intrusive layer with sloping polarity boundaries can account for the increase in the amplitude of anomaly 5. South of the Atlantis Fracture Zone, high magnetization is correlated with bathymethic troughts at segment end points and lower magnetization is associated with bathymetric highs at segment midpoints. This pattern can be explained by a relative thinning of the magnetic source layer toward the midpoint of the segment. Thickening of the source layer at segment endpoints due to alteration of lower oceanic crust could also cause this pattern. Because we do not observe this pattern north of the fracture zone, we suggest it is a result of the nature of crustal formation process where mantle upwelling is focused. South of the fracture zone, reversals along discontinuity traces only continue to crust ∼2 Ma old. In crust >∼2 Ma, we observe bands of high, positive magnetization along discontinuity traces. We suggest that within the discontinuity traces, a high, induced component of magnetization is produced by serpentinized lower crust/upper mantle and this masks the contribution of basalts to the magnetic anomaly signal.

Do lower crustal rocks record reversals of the Earth's magnetic field? Magnetic petrology of oceanic gabbros from Ocean Drilling Program hole 735B

Ocean Drilling Program (ODP) Leg 118 at the Southwest Indian Ridge recovered the first vertically oriented, continuously cored gabbros from in situ ocean crust. Magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the Earths magnetic field. Primary, igneous magnetite is common only in evolved, Fe-Ti oxide-rich gabbros and averages, by volume, 2% of the rock, although substantially higher contents are common. Secondary magnetite, produced by high-temperature exsolution and hydrous alteration of olivine and pyroxene, is observed in all gabbros studied and is the most important magnetic phase in the majority of recovered gabbros (cumulate olivine gabbros). Because of the high temperatures associated with this alteration, it is likely that the primary magnetization of this crustal section was a thermal remanent magnetization, or a very high temperature chemical remanent magnetization acquired very near the ridge crest. Hysteresis loop parameters indicate that all of the studied samples are capable of carrying significant, stable remanent magnetization. Finally, previously published drilling results and field studies from ODP Site 735B indicate that although this crustal section has been tectonically uplifted, it formed at a mid-ocean ridge spreading center, similar to much of the oceanic crust. Therefore the results of this magnetic study can be extrapolated to include the oceanic gabbro section, in general, and argue strongly that layer 3 crustal rocks are capable of recording reversals of the Earths magnetic field.

A downhole magnetic logging tool for the Ocean Drilling Program

In October 1986 the Ocean Drilling Program (ODP) program of the National Science Foundation funded the University of Washington (UW), Seattle, to construct, test, and initially deploy a downhole magnetometer as part of the logging program for the drilling ship JOIDES Resolution. The success of the initial deployment of the magnetometer in a 500-m gabbroic crustal section during Leg 118 in December 1987 (Leg 118 Scientific Party, 1988), and the general availability of this tool as part of the ODP suite of logging tools, prompted this brief announcement. The UW/ODP magnetic logging tool was designed by IFG Corporation of Toronto, Canada, and underwent several design modifications specifically for ODP use by both URTEC Instrument Sales Limited, of Toronto, and the Borehole Research Group (BRG) at Lamont-Doherty Geological Observatory (LDGO), Palisades, N.Y. The tool consists of three orthogonally oriented ring core fluxgate magnetometers, a susceptibility sensor, and two thermocouples which monitor the internal temperature of the probe. The fluxgate magnetometers each have a range of +/−100,000 n T and a sensitivity of 0.1 nT. The fluxgates have been calibrated in magnetic fields ranging from 3000 to 100,000 n T and at a range of temperatures between 25° and 90°C. The susceptibility coil has a sensitivity of approximately 10∼5 SI units when run in a borehole 10″ (25 cm) in diameter. In addition to magnetic susceptibility, the coil measures electrical resistivity, but with this particular type of sensor, the measurement is only useful in formations with very high electrical conductivities. The entire magnetometer syst em is currently capable of being run at operating temperatures up to 125°C without damage. Although the fluxgate magnetometer output remains linear within this range, the susceptibility probe becomes nonlinear and exhibits some thermal hysteresis at elevated temperatures. T h e susceptibility probe will require more detailed calibration in order to utilize measurements made at these higher (>60°C) temperatures. Although the magnetometer probe does not currently have an independent system for azimuthal orientation in highly magnetic media (e.g., accelerometers or a gyroscope), this is a desirable feature which is hoped to eventually be included in the magnetic logging package.