ms defense talk

Oxygen Isotopes, TitaniQ, and Cathodoluminescence analyses in the Alta Stock, UT: Insights into pluton assembly and

early history

Benjamin W. JohnsonM.S. Candidate

May 2009

Outline

•Motivation-Who cares about granites•Geology of Alta•Background/methods

Oxygen IsotopesCathodoluminescence (CL)TitaniQ temperatures

•Results/DiscussionOxygen IsotopesCL/TitaniQ temperatures

•Conclusions

Motivation• Granite is what makes continents

Some picture of granites here

Keith Beisner

• Traditional emplacement model has been put into question

Figures from Glazer et al., 2004

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It is possible that magma increments leave behind traces ofthemselves geochemically/texturally

A significant difficulty in the incremental emplacement model is that it is difficult to map individual increments.

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Geology of the Alta Stock

BP

BP

CP

BP

CP

Two end-member emplacement models

Incrementally in an extensional duplex Instantaneously (<5,000yrs)

Vogel et al. (2001) Cook et al. (1997)

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Sampling strategy• We wanted to try and detect variation at a

variety of scales

Hand sample/thin section

Outcrop

Stock-wide

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Techniques utilized

• Oxygen Isotopes• Cathodoluminescence (CL)• TitaniQ

Oxygen Isotopes

δ18O=[(18O/16Osm-18O/16Ostd)/ 18O/16Ostd] x 1000=X‰

•Isotopes, including oxygen, primarily reflect variations in the source of magma•The size of fractionations are functions of temperature, so under igneous conditions (high temps), fractionations will be small

Delta notation:

Precision of ± 0.1‰

What is CL?Weak visible light emitted by a semiconductor (minerals)when excited by an electron beam.

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(From D’Lemos et al., 1997)

TitaniQ Geothermometry Calibrated by Wark and Watson (2006)

• Ti in quartz varies as a function of the activity of Ti and temperature.

• Calibrated in the presence of rutile

• The Alta stock has no rutile, but it does have titanite.

• Previous work has found that an activity of 0.6 is reasonable for a titanite-bearing rock. Changes in Ti concentration can be

interpreted as temperature changes.

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CL/TitaniQCL intensity is a function of Ti concentration.

This thermometer is very sensitive, our sampling technique has an uncertainty of ±10ºC.

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Quartz from the Vinalhaven Granite, ME(Wiebe et al., 2007)

A

A’

A’A

B

B’

B B’

Oxygen Isotopes

Similar WR but Qtz varies in granodiorite from the border and central phases

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Minerals have exchanged with each other to sub-solidus temperatures of 550-650°C.

The solidus is estimated at 675 °C (Hanson)

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Solidus

Additional plots suggest that all minerals analyzed have equilibrated to 550-650°C

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Solidus

Whole rock oxygen isotope values are ~consistent, but

quartz isotope values vary by 1.1‰

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What is the cause of variation in quartz?

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9.31

9.45

9.16

9.19

9.20

9.37

9.52

9.119.17

9.36

7.82

7.72

7.75

7.38

Model granodiorite as evolution from a parent magma.

“Amount” effect

6.00

6.50

7.00

7.50

8.00

8.50

9.00

9.50

10.00

10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00

δ18O

Qtz

%Qtz

May account for 0.4‰

BP

CP

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

3.50 3.70 3.90 4.10 4.30 4.50 4.70 4.90 5.10 5.30 5.50

“Temperature” Effect

Can account for 0.25‰

∆Qtz-Bi

δ18O

Qtz

BP CP

“Hydrothermal” Effect

•Hard to quantify.

•It is possible that the variation in WR represents the alteration due to hydrothermal activity.

•If so, the “hydrothermal” effect couldbe ~0.3‰

Oxygen Isotope Conclusions

• Minerals have exchanged oxygen isotopes to sub-solidus temperatures (550-650°C)

• Quartz varies by 1.1‰• Up to 0.9‰ may be due to amount,

temperature, and/or hydrothermal effects• Remaining variations (0.2‰) cannot be

clearly ascribed to distinct magma increments • Oxygen isotope data do not clearly identify

magma increments, but do not disprove the existence of increments

CL/Ti temperatures

BSE

CL

• Some are quite complex!• Correlates positively (Bright CL with High Ti)• White bars are 200µm

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Cathodoluminescence/TitaniQ temperatures

Most phenocrysts display CR zoning in CL with higher temperatures in the core of the grain, decreasing temperatures towards the edge, and a bright CL, hot Ti thermal rejuvenation rim.

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Some grains in the border phase show relic CR zoning and a thermally rejuvenated edge.

Core

Rim

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Border phase samples show a variety of textures suggesting grain replacement and synkinematic emplacement

The central phase has higher maximum temperatures than the border phase.

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CL/Ti Conclusions• CL images reveal a complex history of crystal

growth/alteration not visible by standard microscopic techniques

• CL shows three main textures: CR zoning (growth zoning), bright CL rims (thermal rejuvenation), and tile-like patterns (annealed cracks)

• The presence of bright CL, high Ti temperature rims in phenocrysts from the central phase shows evidence for either one large scale thermal rejuvenation or many local rejuvenations from multiple emplacements

Summary• Minerals have exchanged oxygen isotopes to

sub-solidus temperatures (550-650°C)• Variations in δ18O of quartz cannot clearly

identify discreet magma increments• CL textures support synkinematic

emplacement• Phenocrysts throughout the central phase

record widespread thermal rejuvenation either from one large-scale event or emplacement of individual magma aliquots

• Both CL and Ti temperatures are consistent with, but do not prove, the growth of the Alta stock from discreet magma increments

Acknowledgments

Committee: John Bowman, John Bartley, Barb NashField Work: Jared Singer, Will Gallin, Keith Christianson, Melinda Hilber, Neil Lareau, Krysia Skorko

Lab Work: Henny Cathey, John Valley, Mike Spicuzza, Mike DePhanger, Mike Spilde, Quintan Sahratian, Eric Thomas, Erich Peterson

Mental Support: Matt Heumann, Michelle Mary, Mike Stearns, Kit Clemmons, Keith Beisner, Jared Gooley, Ian Semple

Family: Dad, Mom, Tom and Emily!

Thanks and questions?