Special Cases, Problems, and New Frontiers

Non-destructive analysis?

Heat

Deposition of charge

Beam sensitive materials:

Sample temperature rises during exposure to the electron beam

Leads to: Loss of water from hydrous phases (clays, micas, etc.)

Loss of CO2 from carbonates

Damage to halides, phosphates, glasses

Migration of alkalis in silicates

ΔT = 4.8E0i / kdE0 = beam accelerating potentiali = beam currentk = thermal conductivityd = beam diameter

Example: micak = 6 x 10-3 Wcm-1K-1

E0 = 20keV

i = 10nA

d = 1μm

Temperature rise = 160K

Moderate by:

Reduction in beam current

Increasing beam diameter

Using thicker coating of higher thermal conductivity material (Al, Cu, Ag)

Alkali migration

Establishment of space charge at depth as electrons enter the specimen can result in migration of alkali cations in some materials

Glasses

Feldspars, feldspathoids

Zeolites

Can be strong effect of Na+ Less so for K+

Also see for F and Cl in apatite

Minimize by lowering current density

lower beam current

lower exposure time

larger beam diameter

Can extrapolate back to initial (time zero) intensity

Morgan and London (2005)

0 1 2 3 4 5

Time (min)

0

10

5

Wt.% Na

Albite 1μm

10 nA30 nA70 nA

0 1 2 3 4 5

Time (min)

Wt.% Na

0

5

10Albite 50nA

1 μm

20 μm

10 μm

5 μm

Morgan and London (2005)

Fluorine intensity with time in wagnerite Mg(PO4)(F,OH)

Fialin and Chopin (2006)

F and Ca relative intensity with time in CaF2

Fialin and Chopin (2006)

Beam damage

Monazite

LGG246-5 Lower Granite Gorge - Grand Canyon

15kV, 200nA, 30 min

Trace elements

Geochronology

Geothermometry

Paleoclimatology

Speleothems

Enough counts can be acquired to realize detection limits of a few ppm in some cases

But accuracy is the real difficulty

Speleothems – Paleoclimate

StrontiumSulfur

Sr typically done on Sr L on TAP – note interference from Ca Ka 2nd order

Can be efficiently counted on PET, LPET and VLPET, which resolves the interference, and still yields excellent count rates

Five spectrometer integration = count rate > 2X single TAP15 kV, 100 nA ,10 m, 100 s counting time was used.

Single-point analysis of Sr = 29 ppm (2σ) at a concentration of200 ppm, with a detection limit of 50 ppm

0 10 20 30 40 50 60 70 80 90 100 110 120

20

25

30

35

40

45

Mg

/Ca

*(1

03 )

Age KY B.P.

0.2

0.4

0.6

0.8

1.0

1.2

Sr/C

a*(1

03)

980

960

940

920

900

880

860

840

820

Fe

b. in

sola

tion

(30

0S)

-6

-4

-2

0

(d)

(c)

(b)

(a)

18

O

Zr in rutile (thermometry)

Relatively low spatial resolution analysis

20 kV, 200nA, 5 spectrometer integration, 600 sec. acquisitions Integrating PET, two LPETs, and 2 VLPETs

= Single point detection limit of 14ppm (3σ) for Zr

= Grain average yields an overall detection limit of 3 ppm (3σ) for 15 points, and 4 ppm (3σ) for 10 points

Background determination is everything

Understand the spectrum

Interferences

Absorption edges

Understand the instrument and the analytical process

Time-integral effects

current drift

positional drift

direct beam damage to specimen

beam effects on the conductive coat

surface contamination

y = 0.679x-1

0

2

4

6

8

10

12

14

0.00 0.10 0.20 0.30 0.40 0.50 0.60

net intensity (cps/nA)

% e

rro

r o

f n

et in

ten

sity

Measured

Theoretical based on run5

At ~1000ppm Pb, 10% error can easily produce an age error of 35-40Ma (5 wt.% Th, 4000ppm U)

Becomes 50% error at ~ 0.015 net intensity

bkg net intensity (Pk-bkg)actual bkg 0.23544 0.059155lin fit 1 0.24223 0.052365diff 0.00679 -0.00679%error 2.883962 11.47832

bkg net intensity (Pk-bkg)actual bkg 0.23268 0.0956lin fit 1 0.2426 0.08568diff 0.00992 -0.00992%error 4.263366 10.37657

bkg net intensity (Pk-bkg)actual bkg 0.249807 0.131163lin fit 1 0.25831 0.12266diff 0.008503 -0.008503%error 3.403828 6.482773

bkg net intensity (Pk-bkg)actual bkg 0.29714 0.33756lin fit 1 0.30466 0.33004diff 0.00752 -0.00752%error 2.530794 2.227752

bkg net intensity (Pk-bkg)actual bkg 0.26367 0.21239lin fit 1 0.27187 0.20419diff 0.0082 -0.0082%error 3.109948 3.860822

Ultra-light elements

Be, B, C, N, O, F

Low energies, long wavelengths

Requires the use of large d-spacing monochromator

TAP

Pb-stearate

Multilayers

MonochromatorsUse different crystals (or synthetic multilayers) with different d-spacings to get different ranges in wavelength

Smaller d = shorter λ detection and higher spectral resolution

synthetic crystals

pseudocrystals (e.g., stearate films on mica)

layered synthetic microstructures (multilayers) - LSM

“crystal” 2d(Å)LIF Lithium flouride 4.0

PET Pentaery thritol 8.7

TAP (TlAP) Thallium acid phthalate 25.76

Ge Germanium 6.532

LAU Lead laurate 70.0

STE Lead stearate 100.4

MYR Lead myristate 79.0

RAP Rubidium acid phthalate 26.1

CER Lead cerotate 137.0

LSM W / Si W / C 45

60

80

90

98

Comparison of Osmic - Ovonyx LSMs to STE, MYR, CER

Problems:

Interferences from low energy X-rays from heavy elements

High order interferences

Coating thickness variation

Bonding – coordination effects

Carbon contamination

Reduction / oxidation effects

High absorption

Interference of Til on NK

And TiLβ3 on OK

Coating thickness and Carbon contamination

Bonding – coordination effects

Absorption

CK in Fe3C

For best results:

Samples and standards should be similar in composition and physical properties

Try to ensure constant coating thickness

Can coat samples and standards at the same time to help

Use multilayers whenever possible to minimize high order interferences and maximize count rates

Peak distortions due to bonding effects can be accounted for by using integrated peak intensities rather than peak heights

Minimize carbon contamination by

Use of oil-free vacuum pumps

can use vapor trap on backing line

Use O2 gas jet and / or cold plate to remove carbon

If possible, use lower kV to decrease depth of interaction volume

minimize absorption corrections

maximize count production near surface

Using peak shape and position effects…Fe oxidation state

Examine shape of

Fe-L emission

Fe electronic structure = Ar + 3d64S2

N-I

Using peak shape and position effects…Fe oxidation state

Examine shape of Fe-L emission

Flank Method…

Almandine:Fe3

2+Al2Si3O12

Andradite:Ca3(Fe3+ ,Ti)2Si3O12

A=8 B=6

B=6 A=8

cps L cps L

LLratio

Peak energy with increasing Fe3+

Low energy analysis

LEXES (Low Energy X-Ray Emission Spectrometry)

Excitation volume

Sample

Low energy beam

5 – 100 μm

1 – 500 nm

detector

Some other applications:

Particle analysis

Thin films

Rough surfaces

Garnet - Moretown Formation, MA CaKα

S-shaped trails of minerals in the inner part of the garnet were trapped as the mineral grew, and were part of an earlier fabric. A higher Ca internal zone ends at the edge of the zone of inclusions defining the older fabric. Subsequent growth of inclusion –free garnet occurred first, with little Ca, then much more, then little again.

Records either:

1) smooth single-stage metamorphic history (excursion in P and T) or;

2) A multi-stage history

Chemical equilibria involving the outermost rim and the matrix minerals (biotite, muscovite, paragonite, chlorite, plag, ilmenite, and quartz) records final equilibration at ca. 5200 C and 7kb, or about 23 km depth.

Garnet - Italy MgKα

Lago di Cignana locality, Valtournenche, Italy

Very high pressure metamorphism (>25kb and 6000C) and uplift of coesite-bearing metasediments from the Zermatt-Saas zone, Western Alps.

The matrix assemblage includes quartz (after coesite), phengite (Si ~ 3.4pfu), Mn-rich phlogopite, piemontite, and Mn-rich calcite. Inclusions in garnet are piemontite and quartz.

Note the angular unconformity between core and overgrowth

Originalopx

Cpx + qtz

opx

plag

opx+plag+ mt

garnet

matrixplag

Corona texture - CaKα Saskatchewan

Orthopyroxene core and surrounding mantle from 2.6 Ga East Athabaska mylonite triangle

Multi-stage coronitic overgrowths on OPX in mafic granulite

Sequence:

original opx core (lower-right)→

Mantle of cpx+qtz→

2nd generation opx→

Moat of plagioclase→

Symplectitic intergrowth of opx+plag+magnetite→

Outer shell of garnet →

Matrix plagioclase (upper left)

Proposed reaction history:

Prograde growth of a cpx+garnet+qtz assemblage at the expense of opx+plag,

and retrograde growth of opx+plag+oxide from the peak assemblage

Summary P-T path