Title

Magnetization of oceanic crust near selected areas of hydrothermal activity on the Mid-Atlantic and Gorda Ridges

Date of Award

1990

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Marine Geology and Geophysics

First Committee Member

Christopher G. A. Harrison, Committee Chair

Second Committee Member

Peter A. Rona, Committee Member

Abstract

Magnetic properties and opaque mineralogy were determined for a variety of rock types collected from the rift valley at hydrothermal sites on the Mid-Atlantic and Gorda Ridges. Results of the magnetic properties were used to constrain parameters for magnetic inversion models used with sea surface magnetic and bathymetric data. Modeling results were used with magnetic property and opaque mineralogy data to determine the effect of hydrothermal activity on the magnetization of the oceanic crust.Evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts, but is pervasive in diabases, gabbros and ultramafic rocks. This lithologic distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low temperature alteration in young surface basalts to more pervasive high temperature alteration in diabases, gabbros and periodotites.Magnetization lows within the axial marine magnetic anomaly have been associated with some areas of active and relict high temperature hydrothermal venting. All of the sites examined show a low in the magnetic signature in varying degrees. A three-dimensional magnetic inversion was performed on data from the TAG hydrothermal area (26$\sp\circ$N) on the MAR and two-dimensional magnetic inversions were done on individual profiles from the other study sites. The TAG area shows a negative in the magnetization solution which encompasses the active vent area and lower magnetization continues up the eastern wall. Over the Sea Cliff hydrothermal field (Gorda Ridge) there is a distinct low in magnetization present within the Brunhes on the eastern wall which becomes less pronounced farther south of the known vent field. The profiles from 15$\sp\circ$N and 17$\sp\circ$N on the MAR show lows in magnetization over areas of inferred hydrothermal activity. The lows observed in the axial marine magnetic anomaly in these study sites all occur slightly off axis, on crust that is at least 200,000 years old on the Mid-Atlantic Ridge and at least 100,000 years old on the Gorda Ridge. On the basis of magnetic modeling, measurements of rock magnetic properties and opaque mineralogy, the major cause of the magnetization low is hydrothermal alteration of the oceanic crust. Possible subsidiary factors such as magma chamber/Curie isotherm effects, reversals of the Earth's magnetic field, locally thinner crust and non-hydrothermal crustal alteration may also lower the magnetization. These vary from site to site.

Keywords

Geophysics

Link to Full Text

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