A 1.1 billion-year-old anisotropy experiment: a study of anorthosite
xenoliths within the Beaver River diabase
Abstract
Anorthosites are attractive paleomagnetic recorders as silicate-hosted
magnetite inclusions can be single-domain and be shielded from
alteration. However, petrofabrics within anorthosites may result in
magnetic remanence anisotropy that is potentially detrimental to
recovering paleomagnetic direction and intensity. The Beaver River
diabase of the North American Midcontinent Rift contains abundant nearly
100 percent plagioclase anorthosite xenoliths that are hypothesized to
have been liberated from the lower crust by the magma enroute to
becoming embedded in shallow crustal sills. In this study, we compare
the remanent paleomagnetic directions recorded by anorthosite xenoliths
to those of the Beaver River diabase host rocks. Given that both
lithologies should record the same thermal remanent magnetization, this
comparison provides a means to assess the effects of remanence
anisotropy on the paleodirection recorded by the anorthosites. Thermal
and anhysteretic remanence (TRM and ARM) anisotropy experiments, which
are typically used to assess for anisotropy, can be compared to the
natural remanence of the diabase and anorthosite in this geologic
experiment that was conducted 1.1 billion years ago. Paleodirection data
from the interior of the largest (>300 m) anorthosite
xenoliths also have the potential to test their hypothetical lower
crustal origin. An origin below the Curie depth would result in a full
thermal remanence from the time of diabase emplacement, while a
shallower origin from above the Curie depth could have resulted in a
distinct remanence direction in the xenolith interior that predates the
intrusion (with samples from the exterior having acquired a Beaver River
diabase coeval thermal remanence in either scenario). Overall, this
novel geological association between diabase and anorthosite provides a
means to assess the effects of remanence anisotropy providing valuable
context for efforts to use anorthosites to understand the ancient
geomagnetic field.