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

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

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

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

Yellowstone Background: Caldera forming tuffs -- HRT, 2500 km3 @ 2.0 Ma and LCT, 1000 km3 @ 0.6 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

CL Y Pre-LCT YL96-8 zircon 6 P Th U Sector zoning Bright CL ~ high P; Lowered Y(?); Low U, Th … obviously complex relationship

CL & BSE images of Post-LCT YL96-18 zircon 6 2 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

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

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

Column conditions for mapping Detection Limits etc 18 kV, 400 nA for minor and trace elements, peak counting times 30-60 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)

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

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”)

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

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

Post-LCT YL96-18-7 Very high U and Th, as well as Y and REE BSE CL Post-LCT YL96-18-7 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%

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)?

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 50 + 50 seconds mdl = 130 ppm

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 110+110 sec Dy mdl=150 ppm 30+30 sec

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

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

Th/U ~Half Th/U values are below detection limits For detectable values, Th/U ratios span the range from ‘normal’ igneous values (0.1 - 0.7) 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)

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

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

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

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

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

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

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

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

Zircons: usually separated from their host rock and concentrated

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