Testing Deformation-Enhanced Element Mobility in PlagioclaseNaomi Barshi*, Christie Rowe, and Vincent van HinsbergDepartment of Earth and Planetary Sciences, McGill University, Montréal, QC, Canada

*naomi.barshi@mail.mcgill.ca

Why do we care?

How can deformation enhance element mobility?

Where can we study this?

What changes along the map-scale strain gradient? What should change at the grain scale?

Deformation at Meter to Micrometer Scales

Strain

Chemistry

Plagioclase Phenocrysts

Pressure and Temperature

What does this boil down to?

Where did the information come from?

Who helped us?

Variable gabbro,diorite and tonalite;some hypabyssal

Hornblende-biotite tonalite

Gabbro

Probable magmaticring complexes

Mafic cone-sheetassemblages

Ensenada

La Paz

115oW 110oW

29oN

33oN

Maparea

PacificOcean

San Diego

USA

México

Gulf of California

Baja California

Country rocks

Outer Unit

Central Unit

Sampling transects

1 km

115º40’W31º05’N

30º55’N

115º50’W

Other Intrusives

San José tonalite

Metasediments/Metavolcanics

106 Ma

103 Ma

105 Ma

Northwest Northeast

5 mm

0

0.5

1

1.5

2

2.5

3

640 660 680 700 720 740 760 780

Pres

sure

(kb

ar)

Temperature (ºC)

Results of Plagioclase-Hornblende Thermobarometry

MX23

MX05

MX08

MX16

MX11

MX14

MX13

-90

-75

-60

-45

-30

-15

0

15

30

45

60

75

90

1 2 3 4 5

-90

-75

-60

-45

-30

-15

0

15

30

45

60

75

90

1 2 3 4 5 6 7 8

-90

-75

-60

-45

-30

-15

0

15

30

45

60

75

90

1 2 3 4 5

Orientation and Shape in Samples

~115-97 Ma E

?

W

North America6-8 km depth

Strain

Com

posi

tiona

l Con

tras

ts

Detecti

on Li

mits

This study

Measurable strain enhanced diffusion

500 µm subgrains faint, straight twins

curved twins

l0

lb

100 µm

∆G

Distance

A B C D E

20

25

30

35

40

45

50

0.1

0.15

0.2

0.25

0.3

0 0.5 1 1.5 2 2.5 3 3.5 4

Var

iati

on in

Ori

enta

tion

(a

ngle

from

hor

izon

taal

in d

egre

es)

Phen

ocry

st P

ropo

rtio

n (%

of t

hin

sect

ion

area

)

Distance from Pluton Edge (log m)

Plagioclase Phenocryst Abundance and Orientation

Aspect Ratio

Angle Between Long Axis and

Reference Line

Rigid-body rotation

Reference Rotation and internal deformation

Idealized Relationships Between Orientation and Shape

0.30

0.35

0.40

0.45

-100 -75 -50 -25 0 25 50 75

Ano

rthi

te P

ropo

rtio

n

Distance from boundary (µm)

Calcium Diffusion Profiles (EPMA)

210

260

310

360

410

460

510

560

1350

1400

1450

1500

1550

1600

1650

1700

-60 -40 -20 0 20 40 60

Ba

Con

cent

rati

on (

ppm

)

Sr C

once

ntra

tion

(pp

m)

Distance from zone boundary (µm)

Tracel Element Profiles (LA-ICP-MS)

0 100 200 300 400 500 600 700

Ba La Li Mg

Pb Rb Sr

An100 with 0.004 wt% H2O

An100 with 0.07 wt% H2O

An60 with 0.3 wt% H2O

Ab100 with 0.2 wt% H2O

Dislocation Creep

Diffusion Creep

Diffusion

An100 with 0.004 wt% H2O

An100 with 0.07 wt% H2O

An60 with 0.3 wt% H2O

Ab100 with 0.2 wt% H2O (Ryb

acki

and

Dre

sen,

200

4)

(Agg

rega

ted

data

for

~A

n 30 fr

om B

rady

and

C

hern

iak,

201

0)

Activation Energy (kJ/mol)

Study area of fully-exposed strain gradient in tonalite with compositionally zoned plagioclase phenocrysts which serve as physical and chemical strain markers. Maps modified from Johnson et al.4

and sattelite image from Digital Globe/GoogleEarth 2014.

Vacancy and atomic migration amount to lattice deformation. (Figure after Porter and Easterling1, Fig. 2.6.)

Strain gradient sample locations, with ranch for scale.

Magmatic alignments, Hbl dominates mafic phases

Bt flattening, increasing Plg alignment

Connected Bt, fewer Plg phenocrysts, smaller Hbl

Solid-state, Bt foliation, Plg phenocrysts rare

Strain adds energy to the lattice, making the atomic jumps easier (cyan line on graph above). Activation energies for diffusion and deformation are similar, for example in plagioclase2,3.

For this small strain, deformation-enhanced element mobility cannot be measured in plagioclase with present methods. However, previous work6,7 in silicates indicates it can be measured in natural samples. Future work should explore variables such as compositional contrasts, strain, time, temperature, pressure, thermal history, and analytical methods to constrain the realm of deformation enhanced diffusion.

Dislocations provide efficient travel paths that mobilize during deformation.

Thanks to the Field Rheology and FlexPet groups at McGill University for support and assistance. Thanks to Lang Shi for help with the electron microprobe at McGill, André Poirier for help with the LA-ICP-MS at UQÀM/GEOTOP, and Matt Paulson for help in the field. This project was funded by NSERC (van Hinsberg and Rowe), FQRNT (Rowe), GEOTOP and GREAT (Barshi), and a Robert Wares Fellowship (Rowe). Thanks also to Scott Johnson and John Brady for invaluable advice and discussions.

Point Defects

Activation EnergyLine Defects

1. Porter, D. A., Easterling, K. E., 1989. Phase Transformations in Metals and Alloys. 2. Brady, J. B., Cherniak, D. J., 2010. Reviews in Mineralogy and Geochemistry 72 (1), 899–920.3. Rybacki, E., Dresen, G., 2000. Tectonophysics 382, 173–187.4. Johnson, S. E., Tate, M. C., Fanning, C. M., 1999. Geology 27, 743–746. 5. Holland, T., Blundy, J., 1994. Contributions to Mineralogy and Petrology 116 (4), 433–447.6. Büttner, S. H., Kasemann, S. A., 2007. American Mineralogist 92 (11), 1862–1874.7. Reddy, S. M., Timms, N. E., Trimby, P., Kinny, P. D., Buchan, C., Blake, K., 2006. Geology 34 (4), 257–260.

Strongly deformed, phenocrysts rare, strong alignment in foliation

Ellipses fit to Plg phenocrysts circled in thin sections show rotation with little internal deformation. Here the reference line is foliation.

Phenocrysts decrease in abundance and size with increasing strain toward the edge of the pluton. Decreasing variation of phenocryst long axes shows increasing preferrent alignment.

Pressures and temperatures measured in plagioclase and hornblende5 do not vary with strain indicating no systematic fluid overprint nor divergent thermal history.

• Total integrated elongation: 1.5±.5% at the composition zone boundary (cyan line)

• Localized strain in the hinge of the bend: total elongation of 11±2%.

Three intragrain regions accommodate extension with different deformation mechanisms. The shortening direction is approximately vertical to this grain, consistent with the elongation direction (measured in oriented thin section) and flattening of the pluton carapace.

Thin section scan, xpl Photomicrograph, xpl

We anticipated that diffusion profiles would be more smooth with increasing strain for the same elements (here Ca) and that more slowly diffusing species (Ba) would show more sharp steps than more rapid diffusers (Sr). Rather, we see perfect, consistent major element profiles. Trace element profiles have variation which cannot be accounted for.

We measured diffusion profiles and strain in zoned plagioclase phenocrysts with complete internal strain gradients. This way, we control for variables such as starting concentrations, magmatic history, and crystallographic orientation effects. We present results from the grain with highest strain, nicknamed “MTL”.

Undeformed, little to no magmatic alignment

Very low strain, strong magmatic alignment, long

phenocrysts

Least deformed

Most deformed

Optical and compositional image overlay

If deformation enhances element mobility in minerals:

• Geochronometers, thermometers, and barometers based on static diffusion models cannot be applied to deformed rocks.

• Deformation can be directly linked to a time-dependent process as a basis for a strain speedometer.

EAct EAct