CENTENNIAL HONORS COLLEGE
Western Illinois University
Undergraduate Research Day 2016
PosterPresentation
InvestigationofaMarginalReversalattheBaseoftheMineralLakeIntrusion,Northern
Wisconsin
LukasSmith
FacultyMentor:KyleMayborn
Geology
TheMineralLakeIntrusionisamagmaticintrusionlocatedinnorthernWisconsin.Themagmabody
intrudedintotheshallowcrust1.2billionyearsagoandsubsequentuplifthasexposeda6000mcrosssectionthroughthewholeintrusion.Elevensampleswerecollectedalongatraverseofthelowermost
400moftheintrusiontoconfirmpreviousresultsthatsuggestedthepresenceofamarginalreversal.
Thechemicalcompositionsofclinopyroxene,olivine,andplagioclasefromthesesampleswere
determinedbyelectronmicroprobeanalyses,whichwerethenplottedagainsttheirstratigraphicheight
fromthebaseoftheintrusion.Inaddition,thechemistryofsixwholerocksampleswereobtained
throughx-rayfluorescence.Fromthebaseto120mintotheintrusion,thedatashowssystematic
variationinmineralandwholerockcompositions.Forinstance,theclinopyroxeneMg#increasesfrom
0.45to0.70.PlagioclaseCacontentincreasesfrom49to59.Olivine’sMgcontentincreasesfrom24to
58.ThewholerockMg#was0.19atthebaseandincreasedto0.40at120m.
Thechangeinmineralcompositionsfromthebaseto120misinterpretedtorevealamarginalreversal,
becausetheyareoppositeofnormalgeochemicaltrendsseenduringcrystallization.Themechanism(s)
thatcausedthisreversaliscurrentlynotwellconstrained.However,previousresearchhasfound
evidenceofmultiplemagmainjectionsintotheintrusion,whichmayhavecontributedtotheformation
ofthemarginalreversal.