Centennial Honors College
Western Illinois University
Undergraduate Research Day 2014
Poster Presentation
Geochemical Trends in the Mineral Lake Intrusion, Iron County, Wisconsin
Morgan Stice
Faculty Mentor: Kyle Mayborn
Geology
The purpose of this project is to study the evolution of the Mineral Lake Intrusion (MLI).
The MLI represents an example of a layered mafic intrusion. The MLI has been rotated
by faulting, so that a complete cross-section of the intrusion is exposed. Because of
this, mineral layering and associated trends can be observed in relation to stratigraphic
height more easily. The layering in the MLI, and other layered intrusions, comes from
settling or rising of crystals as they grow, creating mineralogically distinct bands within
the intrusion. This particular project focuses on the liquid trapped in between crystals as
they settle, known as the interstitial liquid; as this liquid becomes trapped, it is unable to
maintain equilibrium with the rest of the intrusion, and evolves differently from the
remaining magmatic liquid. This project uses the compositional zoning of plagioclase to
determine the compositions of and changes to the interstitial liquid.
Compositional data for the MLI plagioclase were collected using the electron
microprobe at Washington University in Saint Louis. These data show anorthite
contents (Ca concentration) generally decrease 4% to 6% from the core to the rim of
individual crystals. This decrease is expected for magmas that undergo simple
crystallization, without mixing events. However, samples near the base or top of the
intrusion show greater decreases or increases in anorthite content, which is suggestive
of magma mixing events, where new magma is injected into the intrusion and mixes
with the old.