1. Quantitative EPMA Mapping of Minor and Trace Elements in Zircons
from Yellowstone Tuffs and Lavas
John H. Fournelle*, Ilya Bindeman*,
John J. Donovan**, John W. Valley*
* Department of Geology & Geophysics, University of Wisconsin-Madison, Madison, WI
** Department of Geology & Geophysics, University of California-Berkeley, Berkeley, CA
USGS Photo by David E. Wieprecht

2. Zircons: important accessory minerals
Geochronology of earth events
Preservation of stable isotope signatures, e.g. low d18O magmas
Preservation of original trace element partitioning with host rock or magma

3. Photogenic Yellowstone Zircons…
From Izett (JGR, 1981) “Volcanic Ash Beds…”
Huckleberry Ridge zircons: “U maps” by induced fission tracks in muscovite detectors

4. Background:
Zircons from Yellowstone rhyolites preserve history of generation of both normal and low d18O magmas (Bindeman and Valley, Geology 2000)

5. Yellowstone Background:
Caldera forming tuffs -- HRT, 2500 km3 @ 2.0 Ma and LCT, Ma -- have normal d18O values
The HRT and LCT eruptions are two of Earth’s largest known volcanic eruptions
Some post-LCT lavas (~ 0.5 Ma) have lowered d18O values

6. CL Y
Pre-LCT YL zircon 6 P Th U
Sector zoning
Bright CL ~ high P; Lowered Y(?); Low U, Th … obviously complex relationship

7. CL & BSE images of Post-LCT YL96-18 zircon 62 3
BSE
High in U and Th, with distinctive core
Bright CL: high Dy, Yb, Y; Dark CL ~ quenched by high U,Th?
X-ray mapping of minor/trace elements desirable

8. Zircon in “Fried” Epoxy
But X-ray Mapping zircons mounted in epoxy?
150 um
Zircon in “Fried” Epoxy
(accidental rastering at 100 nA for ~ 1 sec)
Normal x-ray mapping is rectangular or square
High currents necessary for low detection levels —not desirable for electron microprobe chambers

9. High Temperature Epoxy
Semiconductor high temp-erature (nominal 445°C) epoxy
Epoxy takes the heat well
Has alumina filler (trace element analysis shows only Fe at ~0.02 wt% )
Secondary fluorescence of Al apparent in adjacent zircon rims
100 um
Al2O3
Zircon

10. Column conditions for mapping
Detection Limits etc
18 kV, 400 nA for minor and trace elements, peak counting times sec, equal for bkgs
18 kV, 20 nA for Zr, Si, Hf
Mounting: epoxies
Standards: U, Th-diopside glasses (JJD); Hafnon (Hanchar); Y, REE-phosphates (Jarosewich & Boatner): Zircon (USNM)

11. Hf (wt%) ?
Y (um)
X (um)

12. X-ray mapping of irregularly positioned/shaped zircon grains
Define polygon boundary
Select point spacing interval
Fully automated quantitative EPMA
Software contouring or 3D surface mapping (“Surfer”)

13. Post-LCT YL96-18-7 Uranium ... Thorium at similar levels CL BSE
…over 2 wt%

14. Post-LCT YL96-18-7 Yttrium … Hf (wt%) Over 9000 ppm Th/U (wt %) BSE CL
…over 0.93 wt%
Th/U (wt %)

15. Post-LCT YL96-18-7 Very high U and Th, as well as Y and REE
BSE CL
Post-LCT YL
Very high U and Th, as well as Y and REE
Rim enrichment in U not obvious in BSE or CL, observed by x-ray mapping
Dark CL cores correlate with high U and Th, high Th/U, and lower Hf
Brighter CL (with sharp boundary) correlates with high Y
…over 0.93 wt%

16. Zircons from low d18O lava have distinctive trace elements abundances: How do they compare with the zircons from the 2 Ma HRT tuffs (with normal d18O)?

17. Huckleberry Ridge Tuff Zircon grain A (2 Ma, 2500 km3, normal d18O)
BSE
CL
Huckleberry Ridge Tuff Zircon grain A (2 Ma, 2500 km3, normal d18O)
U wt%
Th wt%
94 points
10 elements
11 um spacing
8 hours
seconds
mdl = 130 ppm

18. Huckleberry Ridge Tuff Zircon grain A (2 Ma, 2500 km3, normal d18O)Yb
mdl=170 ppm
30+30 sec
CL
BSE
Huckleberry Ridge Tuff Zircon grain A (2 Ma, 2500 km3, normal d18O) Y
mdl=40 ppm
sec Dy
mdl=150 ppm
30+30 sec

19. HRT Zircon grain A: CL Intensity - Elemental Correlations?Yb Hf Er Dy Y U Th
Suppression of Dy activation?

20. Range of compositions of randomly chosen zircons from 3 distinct volcanics
One YL96-18 xtal
One YL96-18 xtal
Histogram = ~50 random xtals from 3 units
…one post-LCT zircon shows wide range of values, overlapping the HRT, LCT and post-LCTzircon ranges

21. Th/U ~Half Th/U values are below detection limits
For detectable values, Th/U ratios span the range from ‘normal’ igneous values ( ) to higher values >2
Higher Th/U might signify either changes in source composition, or process effects (U or Th lovers)
One mapped zircon spans ~ whole Th/U range
YL96-18B1
Data: ~15 randomly chosen grains of each type, with 3 analyses from each grain (c,m,r)

22. Y+REE / P REE analyzed here were: Yb, Er, Dy, Gd and Ce
Xenotime-type (YPO4) substitution would have atomic Y+REE/ P ~ 1
Values here generally are much higher, mainly due to large Y content
Consistent with suggest-ion by Rizzo et al (GS31A -05) for need of alternate mechanism in zircon
YL96-18

23. Polygon Grid Mapping: “Economics”
Time for acquisition = f ( grid spacing, # elements, mdl desired)
Examples (for 2 column conditions, ~180 ppm mdl, 400 nA):
~50 um grain: 50 spots, 7 um spacing, 10 elements, 230 minutes
~80 um grain: 137 spots, 6 um spacing, 14 elements, 920 minutes

24. Conclusions
X-ray mapping of individual crystals mounted in epoxy is feasible, via polygon gridding of samples
This is very good for low detection limits ... but shows only coarse features if wide spacing used
It does take time … Data is totally quantitative … Range of analyses within one crystal can be compared with spread of data in a suite of crystals

25. d18O zonation in post-LCT zircons (Bindeman and Valley, Geology 2000

27. Quantitative EPMA Mapping of Minor and Trace Elements in Zircons
from Yellowstone Tuffs and Lavas
John H. Fournelle*, Ilya Bindeman*,
John J. Donovan**, John W. Valley*
* Department of Geology & Geophysics, University of Wisconsin-Madison, Madison, WI
** Department of Geology & Geophysics, University of California-Berkeley, Berkeley, CA
USGS Photo by David E. Wieprecht

28. Problem: How to produce x-ray maps of minor and trace elements in mineral separates mounted in epoxy

29. In their oxygen isotopic study of LCT and post-LCT zircons, Bindeman and Valley examined the zircons by CL and BSE, with some WDS traverses
Most grains showed normal oscillatory or sector zonation
Some trace element profiles acquired

30. Post-LCT
YL96-4
zircon 5 CL Y Th U
Normal oscillatory zoning
CL correlation with Y, U, Th suggested

31. Zircons: usually separated from their host rock and concentrated

32. HRT Zircon grain A: CL Intensity - Elemental Correlations?Hf Y
Line 1
Max CL
zone