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Supplementary Information
Robustness of Ne Isotope Intersection with the MORB-Air Line
We present here an assessment of the robustness of the best-fit Ne isotope intersection with the AirMORB mixing line (Fig. 3). We make the a-priori assumption that the crustal and air components
are pre-mixed, probably in the groundwater system, before contact and mixing with the magmatic
CO2 phase. In Ne space, data in a perfectly mixed crust+air mantle system would fall on a single
straight line (Fig. 3a). Variance in the proportion of crustal Ne and air-derived Ne has resulted in a
mixing ‘wedge’ (Fig. 3b). We can determine the variance in the crust+air Ne component using a
semi-independent technique, and assess the fit of the observed data to the data ‘wedge’ predicted by
this variance for different estimates of the mantle Ne isotopic end-member.
For any estimate of the mantle Ne isotopic end-member, with the air and crustal values defined, the
air component 21Ne and 22Ne is also defined (e.g. Supplementary information Tables 1 and 2) 49.
For all samples except BD11, BD12 and WBD2 (the high 20Ne/22Ne samples), the resolved 21Neair
and 22Neair component is insensitive to the mantle end-member choice, changing by 12% from
mantle 20Ne/22Ne = 12.5 to 13.8. While the crustal 21Ne can also be estimated from the Ne isotopic
composition, there is then no component independent of the choice of mantle Ne end-member value
and this provides a non-critical test. The crustal 21Ne can however, also be estimated from the
crustal 4He concentration and the crustal 4He/21Ne value resolved in Figure 5. The 4He/21Necrust still
has some dependency on the choice of mantle 20Ne/22Ne but this is small, changing by only 4.5%
from mantle 20Ne/22Ne = 12.5 to 13.8. As the net change in both air and crustal components is in the
same direction, the change in resolved (21Ne/22Ne)(crust+air) is lower yet. The average of the
(21Ne/22Ne)(crust+air) derived from the 4Hecrust and the model mantle end member then provides the
average model mixing line, and the variance in the (21Ne/22Ne)(crust+air) - the 1 confidence baseline
for the mixing wedge. This is shown for a mantle 20Ne/22Ne=12.2 end-member on the x-axis in
Figure 3b. The fit of the observed data set can then be simply compared with the model best fit Ne
isotopic composition and predicted variance. Because of the low dependency of (21Ne/22Ne)(crust+air)
on the choice of mantle Ne isotopic composition, this approach provides a critical test of the mantle
Ne isotopic composition (Supplementary information Fig. 1).
Equations for the planes defined by the data:
4
He/22Ne = 129800 – 141110* 20Ne/22Ne + 14088000* 21Ne/22Ne
3
He/22Ne = -8.0253 + 0.72028* 20Ne/22Ne + 30.664* 21Ne/22Ne
40
Ar/22Ne = -921860 + 68810* 20Ne/22Ne + 6589000* 21Ne/22Ne
36
Ar/22Ne = 47.43 – 5.7824* 20Ne/22Ne + 588.3* 21Ne/22Ne
84
Kr/22Ne = 1.355 – 0.1516*20Ne/22Ne + 20.573*21Ne/22Ne
Input of the resolved mantle 20Ne/22Ne and 21Ne/22Ne values therefore defines the I/22Ne values.
Care must be taken with these values as the plane fits are neither error weighted nor do they have an
error assessment of extrapolated values.
Phase Fractionation and the Mantle Elemental Abundance Pattern
Resolution of the noble gas mantle elemental abundance pattern also plays a key role in assessing
the viability of mixing models often advocated to account for Ne isotope variance in mantle
samples. However, we need to consider the possibility that the resolved elemental pattern has been
fractionated during magmatic degassing21 but before mixing with the crust and air-derived species.
For example, while similar, the well gas mantle radiogenic 4He/21Ne* (where 21Ne* is the mantle
21
Ne corrected for the solar component)2 and 4He/40Ar ratios are ~25-30% lower than those in the
reference popping rock. Non-radiogenic 3He/22Ne and 3He/36Ar in the well-gas are ~ 40-50% lower
than the (36Ar corrected) popping rock values. In contrast, 22Ne/36Ar and 84Kr/36Ar ratios are similar
for both sample types. In part, the apparent deficit of mantle He in the well-gas samples may be due
to phase fractionation.
The magnitude of fractionation that may have affected the well gas samples can be assessed by
considering the 4He/21Ne*, as its production rate is constant. Theoretical estimates of 4He/21Ne*
range between 2.15x107 to 2.79x107 [A1,A2]. The observed average in the continental crust, with a
similar theoretical range, is lower at 1.70±0.09x107 31. The latter value is indistinguishable from the
value resolved in popping rock. In this case, the popping rock can be considered to be
unfractionated and therefore gives the lower limit for the mantle He/Ne, He/Ar, Ne/Ar ratios, and an
upper limit for Kr/Ar (and Xe/Ar). The upper limit for mantle He/Ne, He/Ar, Ne/Ar ratios, and a
lower limit for Kr/Ar can then be obtained by considering the fractionation required to alter the
average theoretical 4He/21Ne*=2.5x107 to the resolved value in the well gas of 1.23x107. Taking the
noble gas solubilities for a melt with a reasonable ionic porosity of 46% (~2.6g/cm3)[A3], a
gas/melt volume ratio that approaches zero accounts for this difference and enables the prefractionated (degassing corrected) abundance pattern limit for the other noble gases to be estimated
(3He/36Ar<3.2; 22Ne/36Ar<0.62; 84Kr/36Ar>0.038). This approach provides the limit on the range of
elemental abundances to be considered (Fig. 5).
A1 Yatsevich, I. & Honda, M. Production Of Nucleogenic Neon In the Earth From Natural
Radioactive Decay. J. Geophys. Res. Solid Earth 102 , 10291-10298. (1997).
A2 Leya, I. & Wieler, R. Nucleogenic production of Ne isotopes in Earth's crust and upper mantle
induced by alpha particles from the decay of U and Th. J. Geophys. Res. Solid Earth 104,
15439-15450 (1999).
A3 Carroll, M. R. & Draper, D. S. Noble gases as trace elements in magmatic processes.
Chemical Geology 117, 37-56 (1994).
Caption for Supplemental Material
Supplemental information Fig 1: 2 vs Model Mantle 20Ne/22Ne end-member. The average and
variance of the 21Ne/22Ne crust+air end-member for each model mantle 20Ne/22Ne was calculated to
define a model ‘wedge’ between the crust and mantle+air range and mantle value. The orthogonal
distance of each measured point from the central line and the error envelope provided by the model
data wedge are used to derive a  value. The 2min = 38, which for 30 degrees of freedom (15
samples, 2 variables) gives a p value of 0.15 ( MSWD=1.26). The data set provides an acceptable
best fit to the model used and critically distinguishes between different model isotopic compositions
of the mantle. For two variables 2min +2.3 provides the 68.3% confidence limit on the best fit.
The best fit mantle 20Ne/22Ne=12.2 ±0.05.
Supplementary information Table 1: 21Ne Component Concentrations
(cm3(STP)/cm3) based on mantle 20Ne/22Ne = 12.20.
Ne
(Air)

error
2.28E-12
1.59E-11
1.12E-11
2.67E-12
9.98E-12
3.10E-12
1.58E-12
4.72E-12
2.24E-12
5.86E-12
2.48E-13
8.4E-14
4.2E-13
1.7E-13
1.1E-13
1.6E-13
1.2E-13
1.1E-13
1.3E-13
1.3E-13
1.3E-13
5.5E-14
2.42E-12
1.18E-11
9.14E-12
2.43E-12
8.67E-12
3.26E-12
1.33E-12
5.44E-12
2.68E-12
5.34E-12
3.21E-13
9.9E-14
4.5E-13
1.5E-13
1.3E-13
1.8E-13
1.5E-13
1.3E-13
1.6E-13
1.7E-13
1.5E-13
7.3E-14
4.19E-12
7.46E-12
6.38E-12
4.13E-12
4.96E-12
4.44E-12
5.79E-12
4.76E-12
4.77E-12
5.00E-12
4.33E-12
3.8E-13
1.8E-12
5.9E-13
4.8E-13
8.2E-13
6.2E-13
5.4E-13
6.7E-13
6.7E-13
6.7E-13
3.1E-13
1.83E-12
3.25E-12
2.78E-12
1.80E-12
2.16E-12
1.94E-12
2.52E-12
2.07E-12
2.08E-12
2.18E-12
1.89E-12
3.8E-13
1.8E-12
5.9E-13
4.8E-13
8.2E-13
6.2E-13
5.4E-13
6.7E-13
6.7E-13
6.7E-13
3.1E-13
2.54E-12
4.18E-12
6.32E-13
5.28E-13
2.9E-13
1.7E-13
7.9E-14
2.2E-13
3.09E-12
4.87E-12
4.14E-13
4.82E-13
2.5E-13
2.1E-13
8.3E-14
2.8E-13
4.21E-12
4.48E-12
4.48E-12
4.22E-12
5.6E-13
7.8E-13
2.6E-13
1.1E-12
1.95E-12
1.84E-12
1.95E-12
1.87E-12
7.8E-13
5.6E-13
2.6E-13
4.9E-13
21
Sample
BD01
BD02
BD03
BD04
BD05
BD06
BD07
BD08
BD09
BD10
BD11
BD12
BD13
BD14
BD12repeat
WBD2
21
Ne
(Crust)
End-member
Air
Crust
Mantle
20
Ne/22Ne
9.80
0.3
12.20
21
Ne/22Ne
0.0290
0.52
0.0558
§ 21
Ne*mantle is 21Nemantle corrected for solar 21Ne [2]

error
21
Ne
(Mantle)

error
Ne* §
(Mantle)
21

error
Supplementary information Table 2: 21Ne Component Concentrations
(cm3(STP)/cm3) based on mantle 20Ne/22Ne = 12.50.
Ne
(Air)

error
2.47E-12
1.62E-11
1.15E-11
2.86E-12
1.0E-11
3.30E-12
1.84E-12
4.93E-12
2.45E-12
6.0E-12
4.47E-13
7.9E-14
4.0E-13
1.7E-13
1.0E-13
1.6E-13
1.1E-13
1.0E-13
1.2E-13
1.2E-13
1.2E-13
5.1E-14
3.80E-12
6.76E-12
5.78E-12
3.74E-12
4.50E-12
4.03E-12
5.25E-12
4.31E-12
4.32E-12
4.53E-12
3.92E-12
3.4E-13
1.6E-12
5.3E-13
4.4E-13
7.5E-13
5.6E-13
4.9E-13
6.1E-13
6.1E-13
6.1E-13
2.8E-13
2.62E-12
1.22E-11
9.44E-12
2.63E-12
8.91E-12
3.48E-12
1.61E-12
5.67E-12
2.91E-12
5.58E-12
5.27E-13
9.5E-14
4.3E-13
1.4E-13
1.2E-13
1.7E-13
1.4E-13
1.2E-13
1.5E-13
1.6E-13
1.4E-13
7.0E-14
1.66E-12
2.94E-12
2.52E-12
1.63E-12
1.96E-12
1.76E-12
2.29E-12
1.88E-12
1.88E-12
1.97E-12
1.71E-12
1.52E-13
7.38E-13
2.34E-13
1.93E-13
3.27E-13
2.48E-13
2.16E-13
2.68E-13
2.67E-13
2.67E-13
1.23E-13
4.38E-12
2.74E-12
8.38E-13
7.69E-13
1.6E-13
2.6E-13
7.4E-14
2.0E-13
4.06E-12
3.82E-12
4.06E-12
4.83E-13
7.1E-13
5.1E-13
2.3E-13
2.8E-13
5.08E-12
3.29E-12
6.27E-13
3.97E-12
2.0E-13
2.3E-13
7.8E-14
1.0E-12
1.77E-12
1.67E-12
1.77E-12
1.89E-12
3.10E-13
2.24E-13
1.03E-13
4.91E-13
21
Sample
BD01
BD02
BD03
BD04
BD05
BD06
BD07
BD08
BD09
BD10
BD11
BD12
BD13
BD14
BD12repeat
WBD2
21
Ne
(Crust)
End-member
Air
Crust
Mantle
20
Ne/22Ne
9.80
0.3
12.50
21
Ne/22Ne
0.0290
0.52
0.0558
§ 21
Ne*mantle is 21Nemantle corrected for solar 21Ne [2]

error
21
Ne
(Mantle)

error
Ne* §
(Mantle)
21

error