Download Report

V43A
2822
TOWARDS AN UNDERSTANDING OF DEEP BORON:
STUDY OF TYPE IIB BLUE DIAMONDS.
Department of Mineral Sciences, NHMLAC, Los Angeles, CA 90007, USA. [email protected]
2 Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560, USA.
3 The University of Chicago, Department of Geophysical Sciences, Chicago, IL 60637, USA.
1
1
DIAMOND: A CLUE TO DEEP BORON
Boron concentration and isotopic signature are known as tracers of
recycled crustal material from subduction zones inside the Earth’s mantle.
Thus far, the focus has been on analyzing B in volcanic rocks and olivine
inclusions. However, these materials always experience some degree of
late processing on their way to the surface (alteration, crystallization,
change in structure, etc.). As of now, the B content and isotopic ratio of the
mantle end-member is only assumed through mass balance calculations
(Chaussidon & Marty, 1995). Diamonds, on the other hand, are more ideal
to analyze for B, as they do not undergo significant processing while on
their way to the surface.
B-containing diamonds are well known, but extremely rare, and are
referred to as type IIb diamonds. They are highly valuable in the gem
market, as the presence of B in the diamond structure often rise to the blue
color, such as in the Hope diamond. Only a few B analyses have been
undertaken on type IIb natural diamonds, however, it is generally accepted
that their B concentration is ~0.5 ppm or lower. This combination of
rarity, high value, and low B content is the most likely reason why
geologists have not yet performed B analyses on blue diamonds.
2
HOW TO ANALYZE
FOR BORON?
Boron can be quantified by indirect measurements, such as Fourier
Transform Infrared (FTIR), as used in this study. IR spectra were obtained
using a microscope, with a 250 µm x 250 µm focused beam spot size and
up to several mm path length. B contents were determined using different
B absorption peaks, following Collin’s (2010) equations. However, this
method measures only uncompensated B (Bunc i.e. IR active B), therefore
underestimating the total B content. We developed a technique to measure
total B (Btotal) by time-of-flight SIMS (ToF-SIMS, Fig. 1), using synthetic Bdoped diamond standards with known B content. Measurements over a
few hours (up to 6) were done on a 50 x 50 µm x few nm area. Limits of
detections and errors vary with the measurements (Table 1). Eight of the
78 natural blue type IIb diamonds studied here were analyzed by ToFSIMS.
Figure 1: Co-author D. Rost inserting the Hope diamond in the ToF-SIMS
(left). Close-up on the Hope diamond under the ion guns of the ToF-SIMS
(right).
GAILLOU E. 1,2*, ROST D. 3, POST J.E. 2, BUTLER J.E.2
3
3
RESULTS
Bunc ranges from 0.03 ± 0.0028
to 1.72 ± 0.15 at. ppm with an
average of ~0.4 ppm. 25 % of the
measured diamonds are above the
nominally assumed maximum of
0.5 ppm for Bunc (Collins, 2010),
and 5 % (i.e. 4 diamonds) have
values above 1 ppm.
Btotal values range from 8.4 ± 1.1
ppm for the Hope diamond to less
than 0.08 ppm in other blue
diamonds, with an overall average
value of ~1 ppm (Table 1). The
Hope diamond showed the
greatest range in B content from
region to region, from 0.19 ± 0.09
to 8.4 ± 1.1 ppm. Similarly several
of the total B concentrations in
table 1 are significantly greater
than the maximum of 0.26 ppm
of B in natural type IIb diamonds
reported in the previous studies
analyzing Btotal (Lightowlers and
Collins 1976; Von Windheim et al.
1993; Wynands et al. 1994). The
type IaAB diamond (EG19) did
not show B above the detection
limit (Table 1).
When comparing Btotal (by ToFSIMS) to Bunc (FTIR) there are
some inconsistencies (Table 2).
These discrepancies are explained
by the fact that the sample
volumes analyzed by these two
methods
are
significantly
different (>0.16 mm3 for FTIR
and <3 × 10−7 mm3 for ToFSIMS).
Both ToF-SIMS and, to a lesser
extent, FTIR analyses revealed
strong zoning of B in some
diamonds, which was confirmed
by
mapping
Bunc
using
synchrotron FTIR (Fig. 2). The
zoning does not correlate to the
cathodoluminescence
(CL)
features, which are known to be
related to plastic deformation
(Kanda et al. 2005).
Sample
B [ atomic ppm]
LOD [ppm]
mode
0.08
chopped
0.05
chopped
0.06
chopped
0.05
chopped
0.09
chopped
0.29
bunched
0.17
chopped
0.06
chopped
0.06
bunched
-
-
0.08
bunched
0.31
bunched
0.29
bunched
0.39
chopped
0.26
chopped
0.17
chopped
0.35
bunched
0.04
bunched
0.14
bunched
0.12
bunched
0.32
chopped
RESULTS
Figure 2: Steinmetz diamond
slice showing: (a) Synchrotron
FTIR map of the integrated
intensities of the main B peak
(darkest blue: Bunc = 0.13 ppm;
darkest red: Bunc = 0.20 ppm).
Statistical Systematic
error
error
F321
F32
n.d.
2
0.10
±0.03
F323
0.12 ±0.03
H261
0.08 ±0.02
Apollo Blue
1
0.55
±0.06
Apollo Blue2 0.44 ±0.26
Apollo Blue3 1.91 ±0.14
Berman1
0.20 ±0.04
Cullinan1
0.56 ±0.14
Steinmetz1
1.92 ±0.53
Steinmetz2
1.37 ±0.25
Blue Heart1
2.11 ±0.59
Hope1
8.4 ±1.1
Hope2
0.39 ±0.27
Hope3
0.47 ±0.19
Hope
1.2
0.19 ±0.09
Hope4
4.65 ±0.94
Hope5
0.32 ±0.09
Hope6
0.21 ±0.12
Hope8
0.31 ±0.14
EG191
+0.02
−0.01
+0.02
−0.02
+0.01
−0.01
+0.10
−0.08
+0.08
−0.07
+0.35
−0.29
+0.04
−0.03
+0.10
−0.09
+0.35
−0.30
+0.25
−0.21
+0.38
−0.32
+1.5
−1.3
+0.07
−0.06
+0.09
−0.07
+0.04
−0.03
+0.84
−0.71
+0.06
−0.05
+0.04
−0.03
+0.06
−0.05
n.d.
Table 1: ToF-SIMS results.
Superscript numbers refer to
analysis acquired in different places
on the same sample. EG19 is a type
IaAB diamond for reference.
FTIR
B content (ppm)
for sample:
Hope
Steinmetz
ApolloBlue
Berman
F32
H26
Blue Heart
Cullinan1
0.36
0.19
0.17
0.09
0.24
0.30
0.24
0.31
error
ToF SIMS
(range)
± 0.06
± 0.014
± 0.012
± 0.008
± 0.008
± 0.008
± 0.04
± 0.0028
0.19 - 8.4
1.37 - 1.92
0.44 - 1.91
0.20
0.10 - 0.12
0.08
2.11
0.56
Table 2: Comparison between FTIR
and ToF-SIMS results. The ToFSIMS results represent the Btotal and
FTIR the Bunc (Btotal should be equal
to or higher than Bunc ).
(b) Visible CL image
over the same area as
mapped by FTIR shows
typical
blue
mosaic
patterns that do not
correlate with the B
distribution.
(c) Secondary electron
image of the laser cut
bottom surface of the
same area.
4
WHAT IS NEXT?
We have shown that ToF-SIMS is an excellent method for analyzing B
content in blue diamonds, as it was just above the detection limit, and
viable standards were “easy” to make. Also, as Tof-SIMS simultaneously
measures all masses, any other trace elements in the C structure would
be detected. Although both 11B and 10B were detected, the counts on 10B
were too low even for the counting times we used (several hours) to
provide significant δ11B measurements. As we develop the ToF-SIMS
method, we believe that, even for B concentrations in the lower range of
detectability, such instruments should soon be able to provide boron
isotopic measurements. Future results will give insights into the origin
of B in diamonds, e.g. from a subducted slab and/or from primitive
mantle reservoir.
REFERENCES
Chaussidon M., Marty B. (1995) Primitive boron isotope composition of the mantle. Science, 269,
383-386.
Collins A. (2010) Determination of the boron concentration in diamond using
Gaillou E., Rost D., Post J.E., Butler J.E. (2012) Boron in natural type IIb blue
diamonds: chemical and spectroscopic measurements. Am. Min., 1, 1-18.
Kanda H., Ahmadjan A., and Kitawaki H. (2005) Change in cathodoluminescence spectra and
images of type II high-pressure synthetic diamond produced with high pressure and
temperature treatment. DRM, 14, 1928–1931.
Lightowlers E.C., Collins A.T. (1976) Determination of boron in natural semiconducting diamond by
prompt particle nuclear microanalysis and Schottky barrier differential capacitance
measurements. J. Appl. Phys., 9, 951-963.
Von Windheim J.A., Venkatesan V., Malta D.M., Das K. (1993) Electrical characterization of
semiconducting diamond thin films and single crystals. JEM, 22, 4, 391-398.
Wynands H.A., Malta D.M., Fox B.A., Von Windheim J.A., Fleurial J.P., Irvine D., Vandersande J.
(1994) Impurity-characterization agreement in type IIb single-crystal diamond by hightemperature Hall-effect, capacitance-voltage, and secondary-ion mass-spectroscopy
measurements. Phys. Rev.B., 49, 8, 5745-5748.