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