HR: 1340h
AN: V43A-1106    [Abstracts]
TI: The Magnetic Stratification of Mafic Magma Chambers: Natural Examples and Numerical Models
AU: * Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University, Department of Geology, Carbondale, IL 62901, United States
AU: Maes, S M
EM: maess@strose.edu
AF: The College of Saint Rose, Department of Physical and Biological Sciences, 432 Western Avenue, Albany, NY 12203, United States
AU: Butak, K C
EM: kbutak@siu.edu
AF: Southern Illinois University, Department of Geology, Carbondale, IL 62901, United States
AB: Mafic plutons commonly display a prominent petrographic and geochemical layering attributed to differentiation in a magma chamber. This layering occurs at scales from several hundreds of meters in large intrusions down to scales of a few mm at the hand specimen scale. The processes responsible for development of such layering are numerous and include, both internal processes, such as crystal settling, flow seggregation and compaction, and external processes such as magma recharge, magma extraction or magma mixing. An additional distinction can be made between mafic magma chambers that operated as a closed system and those that operated as an open system. Several recent investigations have been conducted on borehole cores through the Great Dyke (Zimbabwe), the Bushveld Complex (South Africa), the Stillwater Complex (Montana), the Sonju Lake intrusion (Minnesota) and the Insizwa sill (South Africa). We present a synthesis of magnetic results on these five intrusions. The magnetic properties of mafic rocks are dominated by the presence of multi-domain magnetite in most layers. Pyrrhotite, the only ferromagnetic sulfide, is restricted to sulfide-rich reefs (Merensky, J-M) or to massive sulfide deposits at the base of the intrusion. Single-domain magnetite (characterized by an inverse magnetic fabric) occurs preferentially in cumulate layers and in the peridotitic basal layers. All intrusions display remarquable variations of their magnetic properties (magnetic susceptibility, degree of anisotropy, shape factor, AMS, magnetic remanence, magnetic saturation, high field magnetic susceptibility) across the layering. The significance of these magnetic zones varies depending on the nature of variations from zone to zone. For example, a drop in magnetic susceptibility does not necessarily correspond to a new magma batch, unless it is also associated with a change in AMS directions. In most cases, the magnetic susceptibility and the total iron content (estimated from high field measurements) increase towards the top of these intrusions suggesting a general iron enrichment. Numerical petrologic models for tholeiitic magma compositions predict absolute magnetic susceptibility variations across a statically-differentiated, closed system intrusion (no convection). The effects of sequential magma recharge and discrete magma extraction are investigated from the magnetic properties point of view. Conversely, these models can be used to interpret the magnetic stratification pattern in natural examples and test for its possible causes.
DE: 1036 Magma chamber processes (3618)
DE: 8145 Physics of magma and magma bodies
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting