CHARACTERIZATION OF MAGNETIC PROPERTIES AND MAGNETIC MINERALOGY OF MANTLE XENOLITHS FROM THE RIO PUERCO VOLCANIC FIELD, NEW MEXICO

Xenolith suites from the Rio Puerco volcanic field were collected to test the hypothesis that metasomatic processes related to the perturbation of the North American lithosphere are recorded by the overall magnetic properties of these materials. Twenty-two oriented spinel websterites with no apparent metasomatic alteration were analyzed from Cerrito Negro. In addition, two unoriented spinel lherzolites with conspicuous red discoloration were analyzed from Cerro de Santa Rosa (CSR). Field observations at CSR show a close spatial association of altered and unaltered xenoliths, and thus suggest that the red alteration developed in the spinel lherzolites prior to eruption in the basalt host. Based on in situ information, the unaltered samples all yield well-grouped magnetizations with north-northeast declination and steep positive inclination. We suggest that these rocks can record a thermoremanent magnetization that defines an ancient field with high fidelity. Bulk susceptibility ranges from 339x10^-6 to 1.3x10^-3 (SI volume), with a cluster between 400x10^-6 to 600x10^-6. Response to alternating field demagnetization suggests that the magnetic carrier is dominated by a high coercivity phase (median destructive fields greater than 40 mT). Overall data indicate that fine-grained magnetite is a dominant magnetic carrier within the spinel websterites, despite the fact that the rocks exhibit a wide range of grain sizes and fabric types. CSR xenoliths are characterized by lower coercivity magnetic carriers (MDF’s of about 20 mT). Notably, bulk susceptibility values for the red xenoliths fall within the same range as in the unaltered xenoliths. Preliminary TEM data suggest the presence of a Mg-bearing iron oxide; additional TEM analyses and magnetic tests will be used to characterize further the different magnetic phases within these xenolith suites. This combination of techniques has not typically been utilized to study the magnetic properties of xenoliths and has the potential to offer new insights into mantle metasomatism.