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In situ Rb-Sr and K-Ca dating by LA-ICP-MS/MS: an evaluation of N2O and SF6 as reaction gases
Ist Teil von
Journal of analytical atomic spectrometry, 2017-02, Vol.32 (2), p.35-313
Erscheinungsjahr
2017
Link zum Volltext
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
In situ
dating of K-rich minerals,
e.g.
micas and K-feldspar, by the Rb-Sr isotopic system is a new development made possible by the ICP-MS/MS technique. Online chemical separation of Rb and Sr is possible in an O
2
-filled reaction cell, wherein a portion of the Sr reacts to SrO
+
while simultaneously no RbO
+
is formed. O
2
reactions provide stable analytical conditions sufficient for precise and accurate determination of Rb/Sr and Sr/Sr isotopic ratios using 80 micron laser ablation spots. However, to date <10% of the Sr reacts with O
2
as reaction gas, leaving room for improvement using more potent reaction gases. With a more efficient reactive transfer, it should be possible to obtain similar results with a smaller laser spot size, hence gaining higher spatial resolution. In this study, we have evaluated N
2
O and SF
6
as reaction gases since they have previously been shown to react strongly with Sr
+
, without affecting Rb
+
. Analytical conditions, including cell parameters and reaction gas flow rate were optimized while ablating NIST SRM 610. The main reaction product is SrO
+
for N
2
O reaction and SrF
+
for SF
6
reaction. Both gases show significantly higher reaction product formation compared to O
2
with >85% of Sr reacting with N
2
O and >70% Sr reacting with SF
6
; Rb does not react with either gas. As a result, the sensitivity for Sr reaction products is ∼10 times higher with N
2
O and ∼8 times higher with SF
6
compared to O
2
. With these more reactive gases, the error of mica isochron ages, calibrated against a newly developed nano-particulate pressed powder tablet of mica-Mg, is ∼1% using a 50 μm laser spot. Our tests show that both N
2
O and SF
6
form interfering reaction products,
e.g.
, SrOH (N
2
O), SiF
3
and TiF
3
(SF
6
) that are difficult to handle using single mass spectrometer instruments, but which can be overcome using MS/MS. Using SF
6
combined with H
2
, it is possible to measure
40
Ca
+
as
40
Ca
19
F
+
, free from interference of
40
Ar
+
and
40
K
+
. This facilitates the dating of micas by the K-Ca isotopic system; we present the first
in situ
K-Ca age determination.
Significant improvements to the analytics and calibration of
in situ
Rb-Sr dating; extending the concept to
in situ
K-Ca dating.