the problem of secondary fluorescence in epma in the application of the ti-in-zircon geothermometer...

The Problem of Secondary Fluorescence in EPMA in the Application of the Ti-in-Zircon Geothermometer And the Utility of PENEPMA

Monte Carlo Simulations

John H. Fournelle

Eugene Cameron Electron Microprobe LabDepartment of Geology and Geophysics

University of Wisconsin Madison, Wisconsin

Zircon (ZrSiO4) is a small mineral highly resistant to chemical changes; this robustness has led it to be used for estimating earth conditions eons ago. It has been used to date geologic events, measuring its radiogenic Pb, U and Th isotopes.

The oldest dated mineral on Earth is a zircon from Australia, and its oxygen isotope value suggests Earth’s crust was cool and wet as long ago as 4.3 billion years.

Zircon…takes a licking, keeps on ticking…

• ZrSiO4, small ~100-200 microns, accessory mineral (~granites, rhyolites)

• highly resistant to chemical changes

• Th, U and Pb present: radiogenic decay

• used for estimating earth conditions eons ago

• oldest dated mineral on Earth is a zircon from Australia

• its oxygen isotope value suggests Earth’s crust was cool and wet as long ago as 4.3 billion years.

Zircon…takes a licking, keeps on ticking…

Taking the Earth’s temperature

• geologists can directly measure temperature of geothermal springs and lava flows

• but direct methods are not possible for most of Earth conditions …

• use various geochemical evidence -- element partitioning between coexisting minerals -- to infer conditions deep within the earth.

• these geothermometers and geobarometers have been developed with, and utilize, electron and ion probes.

Scan slide of ? At Kileaua re Therm

Watson and Harrison* and Watson et al** experimentally determined that the amount of Ti incorporated in zircon (~1 to 1000s of ppm), coexisting with a high-Ti mineral (e.g., rutile TiO2 or ilmenite FeTiO3), was proportional to the temperature at which the zircon crystallized and could be used as a geothermometer.

* Watson and Harrison, 2005, Science, 308, 841 ** Watson, Walk and Thomas, 2006, Contrib Mineral Petrol, 151, 413

A zircon thermometer?

However, there are situations where EPMA is used:

1. for original validation of Ti in synthesized zircons (i.e., for SIMS standards),

Measuring tiny levels of Ti …..

Where the level of Ti is very low (1-100 ppm), the preferred method is ion probe (SIMS).

SIMS

EPMA



2. by geologists who do not have ready access to SIMS, or



SIMS

EPMA



2. by geologists who do not have ready access to SIMS, or

3. where the large SIMS spot size (~25 microns) is prohibitive.



SIMS

EPMA

• the electron beam’s interaction volume is small relative to the volume excited by both characteristic and continuum x-rays generated • what is the amount of secondary fluorescence (SF) that is measured during EPMA???• critical for trace element EPMA measurements

Secondary Fluorescence may become an issue

The PENEPMA Monte Carlo program, based upon PENELOPE, has been shown to accurately predict the extent of secondary fluorescence and has been recently modified to explicitly state SF intensity.

One alternative: model the effect

2 Cases Modeled with Penepma

Both 15 kV, 40° take off angles, with only continuum secondary fluorescence

1. Rutiles (TiO2) in experimental runs for Ti in zircon solubility

2. Zircons in rock samples with nearby ilmenite and biotite (= Ti-bearing minerals)

Watson et al (2006) experimentally grew zircons where rutile (TiO2) were also present, and tried to account for secondary fluorescence….

Case 1 examines the potential for SF in experimentally grown zircons that have crystals of rutile either touching them or present in the vicinity, and all are in a glass which also has Ti present in it (6 wt%). Fig 1 shows the geometry (cross section and plan views), with a 30 m diameter zircon (0 wt% Ti) in a matrix, with 5 rutiles (30 m diameter) spread out 15 m away.

Rutile Distance Zirc Rim- Number Matrix

Spurious Ti

Diameter Rutile Rim Rutiles Material (ppm) 1 30 - 0 Glass w/ Ti 452 2 30 15 5 Glass w/ Ti 948 3 30 15 5 Pb-glass 390 4 30 15 5 epoxy 1179 5 30 60 5 Pb-glass 25 6 4 0 5 epoxy 61 7 4 0 10 epoxy 120

7 Geometries Modeled


Spurious Ti


7 Experimental Geometries Modeled

Geometry 1: 30 um zircon, no rutile, only Ti in surrounding glass (6 wt%) => 452 ppm SF Ti

Case 1 examines the potential for SF in experimentally grown zircons that have crystals of rutile either touching them or present in the vicinity.


Spurious Ti


Geometry 2: 30 um zircon, 5 large rutiles 15 um away, Ti in surrounding glass (6 wt%) => 948 ppm SF Ti


Spurious Ti


Geometry 3: 30 um zircon, 5 large rutiles 15 um away, NO Ti in surrounding glass, with Pb instead => 390 ppm SF Ti


Spurious Ti


Geometry 4: 30 um zircon, 5 large rutiles 15 um away, in epoxy => 1179 ppm SF Ti


Spurious Ti


Geometry 5: 30 um zircon, 5 large rutiles 60 um away, in Pb-glass => 25 ppm SF Ti


Spurious Ti


7 Experimental Geometries Modeled

Geometry 6: 30 um zircon, 5 tiny 4 um rutiles 0-1 um away, in epoxy => 61 ppm SF Ti


Spurious Ti


Geometry 7: 30 um zircon, 10 tiny rutiles 0 um away, in epoxy => 120 ppm SF Ti


Spurious Ti


Conclusions from Penepma 7 geometries

• easy to get several hundred ppm of Ti if large Ti-rich phases are within tens of microns of analysis point

• conceptually, “solid angle” that Ti-rich phases present to analysis point

• continuum SF is not insignificant

Table 1 presents the results of using different matrix materials, and 2 different sizes of the surrounding rutiles, following various configurations described in [2]’s experimental zircons. Row 1 shows that if the experimental glass were left present during EPMA of a zircon core, over 400 ppm of fictitious Ti could result from SF of it alone. If 5 equal size rutiles were scattered in the glass matrix 15 m away, an additional 500 ppm of spurious Ti counts would result by SF. The researchers in [2] recognized the problem of the glass and dissolved it, replacing it with a Pb-glass to attempt to minimize the SF of Ti with the commingled rutile. Row 3 shows that the Pb-glass moderates the SF effect but does not eliminate it. If the same number and size rutiles are moved back further from the zircon (60 m away) the SF effect drops drastically, down to 25 ppm. Simulations were also done in an epoxy matrix: the effect of replacing a glass matrix with epoxy caused an increase in the SF effect (row 4), apparently due to a lowered absorption of the continuum x-rays moving out from the zircon, and also a lowered absorption of SF-produced Ti Ka x-rays. The last simulations of much smaller rutiles (4 m diameter) attached to the edge of the zircon yielded ~10 ppm of fictitious Ti per attached grain.

Case 2 examines the potential for SF in a rock where zircons are surrounded by ilmenite, hematite and biotite (suggested by C. Morisset, Univ. British Columbia). Here it is less easy to model a realistic geometry as the zircons are irregular in shape.

Case 2 examines the potential for SF in a rock where zircons are surrounded by ilmenite, hematite and biotite (suggested by C. Morisset, Univ. British Columbia). Here it is less easy to model a realistic geometry as the zircons are irregular in shape. A simple planar geometry (Fig 2) was modeled for a first approximation of the effect. Fig 3 shows the extent of SF in zircon from 5 to 100

m away from the ilmenite boundary. In the actual case, the distance to the boundary is not always easy to determine, but generally is 10-40 m, for which PENEPMA gives SF values between 100-1000 fictitious ppm Ti. The actual EPMA measurements in one sample range from 162-645 ppm Ti in zircon, increasing as the analysis point gets closer to the ilmenite—matching fairly well the PENEPMA simulation. In a second sample, there is a single measured value of 2559 ppm at ~20 m distance, 6X that from SF, but adjacent ilmenite may be plunging below the surface of the zircon.

Ti (ppm) from Secondary Fluorescence

10

100

1000

10000

0 20 40 60 80 100

microns into Zircon from FeTiO3

Ti (ppm)

Complications of Secondary Fluorescence in EPMA: The “Size Discrepancy

Issue”

Difference between small sample and large standardSample = 10 um polished sphere Cr2O3 embedded in plastic

In troubleshooting low totals, the question arose: if there is a several order magnitude size difference between unknowns (small grain separates) and standard (large), what could result?

Standard = 2 mm polished sphere

PMM (plastic) PMM (plastic)

Standard not to scale with unknown, would be much larger if true scale.

Electron beam

Electron beam

Difference between small sample and large standardSample = 10 um polished sphere Cr2O3 embedded in plastic

If the primary electron “interaction volume” is confined within the material, and therefore the primary x-ray generation is also confined therein, is the lack of “additional” Cr x-ray counts resulting from secondary fluorescence outside the primary electron interaction volume of any importance in “normal” EPMA???


PMM (plastic) PMM (plastic)

Standard not to scale with unknown, would be much larger if true scale.

Electron beam

Electron beam

z

Cr2O3

PMM (plastic)

Sample = 10 um polished sphere embedded in plastic


Set up a PENELOPE Monte Carlo simulation:

Standard of “huge size”, 2 mm

Unknowns of much smaller size

Accelerating voltage of 20 keV, takeoff angle 40°

Yes, Secondary Fluorescence can cause problems

Standard=2000 m Cr2O3 Unknown = smaller Cr2O3

A 100 m grain of pure Cr2O3 will have 1% low Cr K-ratio, and a 10 m grain will have a K-ratio 2.5% low.

(plots show K-ratios produced in centers of various discrete sized diameter cut-off spheres imbedded in epoxy simulations)

Electron range (K-O): 1.7 micron Cr Ka X-ray range (A-H): 1.6 micron

0.74

0.76

0.78

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

0 10 20 30 40 50

diameter in microns

fraction of max x-ray intensity

4

0.7

0.75

0.8

0.85

0.9

0.95

1

1 10 100 1000 10000

diameter in microns

fraction of max x-ray intensity

Conclusion

Discrepancies in size between unknown and standard can lead to small, but noticeable errors, because secondary fluorescence yields

• additional x-rays beyond the primary electron impact-x-ray production volume in the same phase if the phase is large,

or

• a lack of additional x-rays if the phase is small and mounted in epoxy.

PENELOPE

• created to model high energy radiation in bodies of complex geometries

• simulates x-ray generation and x-ray absorption/secondary fluorescence

• a new version developed for EPMA, with EDS-like spectral output

• a FORTAN program, runs with G77 compiler under OS X, Linux, Windows

• developed by Salvat, Llovet et al. of Universitat de Barcelona … and free

the problem of secondary fluorescence in epma in the application of the ti-in-zircon geothermometer...

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