STRUCTURAL STATE OF PLAGIOCLASE PHENOCRYSTS
IN PORPHYRITIC ROCKS OF THE MIDCONTINENT RIFT,
NORTHEASTERN MINNESOTA
Damion Knudsen and Bernhardt Saini-EidukatDepartment of Geosciences, North Dakota State University, Fargo, ND
James D. Miller, Jr.Minnesota Geological Survey
Peter DanielsMarkstrasse 123, 44803 Bochum, Germany
Steven Dutch, U W - Green Bay
N
Area of study
BEAVER BAY AREA
N
Can we use the structural state of plagioclase to
determine if there is a petrogenetic relationship
between anorthosite inclusions in Beaver Bay
Complex intrusives and plagioclase phenocrysts
in NSVG basalts?
Separation between 131 and 131 peaks
131 131
2.50
2.00
1.50
1.001000 20 40 60 80
Δ13
1= 2
θ(13
1)-2θ(
131)
Mol Percent Anorthite
+ Samples from anorthositic masses near the tops of gabbroic sills, Northern Minnesota
Split Rock
What do we mean by structural state?
(Smith and Yoder, 1956)
Kroll and Ribbe (1980)
Kroll and Ribbe’s (1980) method
Plagioclase crystal structure
The feldspar “crankshaft”
structureSteven Dutch, Dept. of Natural and Applied Sciences, University of Wisconsin - Green Bay
R J. Harrison, University of Cambridge, England
An0 = 1:3 Al:Si C1
An100 = 1:1 Al:Si I1
In feldspars, both Al and Si occur in tetrahedrally coordinated sites, known as T sites. With the value t10-<t1m>, t1O = the probability that an Al atom will be found on the t1O site. <t1m> is the average probability of finding an Al atom at any one of these three sites and t10-<t1m> is simply the difference between the two probablities.
Phase diagram of plagioclase
R J. Harrison, University of Cambridge, England
An0 = 1:3 Al:Si C1
An100 = 1:1 Al:Si I1
Diagrams illustrating High, Intermediate, and Low disorder.
High
Intermediate Low
(Smith and Brown, 1988)
5 mm5 mm
5 mm
Temperature and orderAn0 = 1:3 Al:Si C1
An100 = 1:1 Al:Si I1
Kroll and Ribbe’s (1980) method
(Smith and Brown, 1988)
?
Looking at these two different graphs may allow one to interpret a relationship between the two. My idea is that the area between the high and low plagioclase lines on this graph is essentially a solid solution. Once you drop below this line you start to encounter exsolution lamallae and higher states of order.
Methods used in this study
Philips X’pert PW 3040-MPD diffractometer at
NDSU
Anorthite contents determined by electron microprobe at
the Department of Geology and Geophysics, University of Minnesota-
Twin Cities
Assigning reflections to peaks
Which 2θ reflection(s) should be used here?
Profile Fitting using WinPlotr
Unit Cell Refinement using “Unit Cell”
Croftville
Scott Creek Katydid LakeCabin Creek
PDD
Results using method of Kroll and
Ribbe, 1980
Kroll and Ribbe’s (1980) method
A new method: t10-<t1m> vs. An
γ-2.011-0.2471*(An % /100)-0.35*0.2012|(An % /100)-0.33| -0.86*0.2012|(An % /100)-0.33|
t10-<t1m> =
t 1O-<t 1
m>
Results usingour method of t10-
<t1m>
Conclusions• Plagioclase phenocrysts in basalts exhibit intermediate to high
structural state
• The Leveaux Porphyry phenocrysts have intermediate structural states and plot near Duluth Complex anorthositic series samples, while the Cabin Creek phenocrysts have distinctly more disordered structures.
• Plagioclase phenocrysts from the Scott Creek leucogabbro and the Katydid Lake gabbroic anorthosite are intermediate to ordered and plot within the field of Duluth Complex anorthositic series samples.
• IN ADDITION:
• Large plagioclase rich xenoliths in the Beaver Bay Complex vary in their structural state with texture type, and Anorthositic series rocks of the Duluth Complex are generally in low structural state.
Acknowledgements
Paul AlbersUniversity of Minnesota Duluth
Ellery FrahmElectron microprobe laboratory at the Department of
Geology and Geophysics, University of Minnesota-Twin Cities.
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