Transcript t 1 m - North Dakota State University

STRUCTURAL STATE OF
PLAGIOCLASE PHENOCRYSTS
IN PORPHYRITIC ROCKS OF
THE MIDCONTINENT RIFT,
NORTHEASTERN MINNESOTA
Damion Knudsen and Bernhardt Saini-Eidukat
Department of Geosciences, North Dakota State University, Fargo, ND
James D. Miller, Jr.
Minnesota Geological Survey
Peter Daniels
Markstrasse 123, 44803 Bochum, Germany
Steven Dutch, U W - Green Bay
Area of study
N
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
What do we mean by structural state?
2.50
Δ131= 2θ(131)-2θ(131)
(Smith and Yoder, 1956)
Split Rock
2.00
+ Samples from anorthositic
1.50
masses near the tops of gabbroic
sills, Northern Minnesota
1.00
0
20
60
40
Mol Percent Anorthite
80
100
Kroll and Ribbe (1980)
Kroll and Ribbe’s (1980) g method
Plagioclase crystal structure
An0 = 1:3 Al:Si C1
An100 = 1:1 Al:Si I1
R J. Harrison, University of Cambridge, England
The feldspar
“crankshaft”
structure
Steven Dutch, Dept. of Natural and
Applied Sciences, University of
Wisconsin - Green Bay
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
An0 = 1:3 Al:Si C1
An100 = 1:1 Al:Si I1
R J. Harrison, University of Cambridge, England
(Smith and Brown, 1988)
High
5 mm
Intermediate
5 mm
Low
5 mm
Diagrams illustrating High, Intermediate, and Low disorder.
Temperature and order
An0 = 1:3 Al:Si C1
An100 = 1:1 Al:Si I1
(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.
Kroll and Ribbe’s (1980) g method
Methods used in this study
Philips X’pert PW 3040MPD diffractometer at
NDSU
Anorthite contents determined
by electron microprobe at
the Department of
Geology and Geophysics,
University of MinnesotaTwin Cities
Assigning reflections to peaks
Which 2θ
reflection(s)
should be used
here?
Profile Fitting using WinPlotr
Unit Cell Refinement using “Unit Cell”
Croftville
Cabin Creek
Scott Creek Katydid Lake
PDD
Results using
g method of Kroll and
Ribbe, 1980
A new method: t10-<t1m> vs. An
t10-<t1m> =
γ-2.011-0.2471*(An % /100)-0.35*0.2012|(An % /100)-0.33|
-0.86*0.2012|(An % /100)-0.33|
Kroll and Ribbe’s (1980) g method
Results using
our 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 Albers
University of Minnesota Duluth
Ellery Frahm
Electron microprobe laboratory at the Department of
Geology and Geophysics, University of Minnesota-Twin
Cities.