1. Steven Dutch, U W - Green Bay
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, Bochum, Germany
Good Morning.
My name is Damion Knudsen and I am a recent graduate of North Dakota State University with my major in Geology and minor in Chemistry. I would like to thank my co authors Dr. Bernhardt Saini-Eidukat, Dr. Miller, and Dr. Peter Daniels a crystallographer from Germany.
Steven Dutch, U W - Green Bay

2. Area of study
In this talk I will present what information we can extract from plagioclase phenocrysts found along the North Shore of Lake Superior in Eastern Minnesota as also how they may relate to anorthosite and matrix plagioclases found in the same area. N

3. BEAVER BAY AREA
This Geologic map of N.E. Minnesota shows the plutonic Duluth Complex, the NSVG, and the shallower Beaver Bay Complex which intrudes the volcanic pile. These hypabyssal intrusions emplaced near the top of the volcanic pile are referred to collectively as the Beaver Bay Complex. In this 600 km2 area, thirteen major units have been identified. The rock units vary in composition due to assimilation and crystal fractionation processes that generated gabbroic to felsic parental magmas, and from mixing with magmas of deeper chambers. Mapping by Grout and Schwartz in the 1930’s identified large numbers of anorthosite xenoliths, some of which prominently crop out at topographic highs along the north shore of Lake Superior. Many of these inclusions occur in a particular intrusive phase of the Beaver Bay Complex, the Beaver River Diabase. N

4. In addition to the anorthositic xenoliths, detailed mapping by Miller, Green, and colleagues identified NSVG basaltic flows and Beaver Bay Complex intrusives with plagioclase porphyritic textures. In some cases, chemistries of the plagioclase phenocrysts and their host matrices indicated they are not related to typical Beaver Bay Complex magmas. Rather, they appeared to originate from magmas that produced the anorthositic rocks of the stratigraphically much lower Duluth Complex.

5. 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?
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?

6. Separation between 131 and 131 peaks
The earliest powder X-ray diffraction studies of plagioclase showed that the separation between two distinctive reflections positions in the powder diffraction pattern for 131 and 1-31 varied depending on the degree of ordering of the Si and Al in tetrahedral sites of the plagioclase crystal structure.
131
131

7. What do we mean by structural state?
2.50
(Smith and Yoder, 1956)
Split Rock
2.00
Δ131= 2θ(131)-2θ(131)
1.50
+ Samples from anorthositic masses near the tops of gabbroic sills, Northern Minnesota
Smith and Yoder studied variation of X-ray powder patterns of plagioclase in 1956 and were one of the first groups to show there are two end-member series. Their samples included seven anorthositic inclusions from northeastern Minnesota. Using the angular separation between the (131) and (1-31) reflections as a measure of structural state, they noted these samples, circles in red, plotted between their high-temperature synthesized plagioclases and lower-temperature and/or slower cooled “Bushveld-type” plagioclases. They concluded that the inclusion plagioclases were “frozen” in higher temperature states perhaps due to “more rapid cooling of the small extrusive and hypabyssal bodies in which they occur.”
1.00 20 40 60 80
100
Mol Percent Anorthite

8. Kroll and Ribbe (1980)
Kroll and Ribbe concluded that the 131 method is limited to 1% or less Orthoclase without correction. In this diagram they show the differences between corrected and un-corrected delta 131. As a next step to the 131 method Kroll and Ribbe developed a technique that is unsensitive to orthoclase content using unit cell refinement from diffraction patterns.

9. Kroll and Ribbe’s (1980) g method
When using the 131 method orthoclase correction is an amount that can be found by plotting Or vs An content on these graphs.
With the gamma method we are introduced to the value of t10-<t1m> “but what exactely is this?”

10. Plagioclase crystal structure
An0 = 1:3 Al:Si C1
An100 = 1:1 Al:Si I1
R J. Harrison, University of Cambridge, England
With the crankshaft arrangement of the tetrahedra in Feldspars not all of the tetrahedral sites are equivalent.
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.
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.

11. Phase diagram of plagioclase
An0 = 1:3 Al:Si C1
An100 = 1:1 Al:Si I1
To really determine structure vs. order which we are doing in this project, one would carry out single crystal structure determinations. This is because “e” plagioclases show reflections that cannot be seen well in powder patterns. Generally though if you have “e” plagioclase you will be plotting in the low intermediate to low structural states.
R J. Harrison, University of Cambridge, England

12. High Intermediate Low (Smith and Brown, 1988)
5 mm
(Smith and Brown, 1988)
High
The Clockwise arrow in this diagram demonstrates an increase in temperature as also a decrease in cooling rate.
High disorder can represent volcanic or rapidly quenched hypabyssal plagioclase.
An intermediate disorder can represent conditions of slower cooling rates or longer annealing times in shallow plutons or intrusions such as the Leveux and Cabin Cabin Creek Ferrodiorites .
Low-intermediate to low disorder can represent plagioclases such as deeper seated plutonic or metamorphic rocks such as is found in this granoblastic textured Carlton Peak Anorthosite with An approximatly 75%.
Intermediate
Low
Diagrams illustrating High, Intermediate, and Low disorder.

13. Kroll and Ribbe’s (1980) g method
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 interpretation 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.
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

14. Methods used in this study
Now I would like to go over my methods. Plagioclase phenocrysts were separated for XRD using common magnetic and heavy liquid methods. Patterns were collected in a 3 hour step scan mode on a Philips X’pert diffractometer. Anorthite mol percent was found by microprobe analysis of sample thin sections.
Anorthite contents determined by electron microprobe at the Department of Geology and Geophysics, University of Minnesota-Twin Cities
Philips X’pert PW 3040-MPD diffractometer at NDSU

15. Assigning reflections to peaks
Which 2θ reflection(s) should be used here?
The most important step with both the 131 and gamma methods is to properly assign reflections to observed peaks.
Which 2θ reflection(s) should be used here?

16. Profile Fitting using WinPlotr
Well to answer this question Dr. Daniels introduced me to the program WinPlotr. This allowed me to identify reflections that overlap. In this case the program identified three over lapping peaks.
For each different sample pattern I used the same 2 theta positions. Once I was finished profile fitting my patterns I imported the data into a simple text file.

17. Unit Cell Refinement using “Unit Cell”
This data was then used to do a unit cell refinement using the program Unit Cell. This program would give me my unit cell dimensions but most importantly a gamma value was assigned with corresponding standard deviations and from here I could determine structural state.

18. Croftville
Cabin Creek
Scott Creek Katydid Lake
PDD
The samples analyzed for this study are: (a) porphyritic, ophitic basalts from near Silver Bay and Croftville, (b) porphyritic ferrodiorites of the Cabin Creek and the Leveaux Porphyry intrusions, (c) porphyritic leucogabbros and gabbroic anorthosites of the Scott Creek and Katydid Lake intrusions and (d) a porphyritic diabase dike, possibly an offshoot of the main Scott Creek intrusion.

19.  method of Kroll and Ribbe, 1980
Results using
 method of Kroll and Ribbe, 1980
XRD results show the Croftville basalt phenocrysts, along with those of the porphyritic dike, have intermediate to high disorder, plotting near the curve for disordered plagioclase. In contrast, the Silver Bay basalt phenocryst has intermediate structure.
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. Anorthosite xenoliths in the BBC are intermediate to ordered.
Plagioclase separated from the matrix of selected samples all have lower An contents than the phenocrysts they host, and show intermediate to high disorder. The relationship of the matrix t10-<t1m> values to those of the phenocrysts they host reverses in a manner similar to that noted by Hoffer (1968) for the Rock Creek porphyritic basalt flow of Idaho.

20. A new method: t10-<t1m> vs. An
γ *(An % /100)-0.35*0.2012|(An % /100)-0.33|
-0.86*0.2012|(An % /100)-0.33|
t10-<t1m> =
Kroll and Ribbe’s (1980) g method
t1O-<t1m>
EXPLAIN NOVELTY OF THE “DERIVED METHOD”
The graph produced by Kroll and Ribbe (1980) was used to derive a t10-<t1m> vs. An content graph. Essentially the gamma vs. An graph was referenced for 6 points above 35% An (green dots). Then the high and low trend lines were digitized and this data was plotted on a standard XY graph with the t10-<t1m> values obtained using this equation.

21. our method of t10-<t1m>
Results using
our method of t10-<t1m>
This was the result. One group of large anorthosite xenoliths (those with granoblastic textures) plot together with low structural state, and another group (with igneous or lath-like textures, and lower An) plot with intermediate structures.
Duluth Complex Anorthositic Series samples again are low to intermediate.

22. 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.
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.

23. Acknowledgements Paul Albers Ellery Frahm
University of Minnesota Duluth
Ellery Frahm
Electron microprobe laboratory at the Department of Geology and Geophysics, University of Minnesota-Twin Cities.