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.