Distinguishing contributions to diffuse CO2 emissions in volcanic areas from magmatic degassing and thermal decarbonation using soil gas 222Rn–δ13 C systematics: Applicationto Santorini volcano,Greece

Between January 2011 and April 2012, Santorini volcano (Greece) experienced a period of unrest
characterised by the onset of detectable seismicity and caldera-wide uplift. This episode of inflation
represented the first sizeable intrusion of magma beneath Santorini in the past 50 years. We employ
a new approach using
222
Rn–
δ
13
C systematics to identify and quantify the source of diffuse degassing
at Santorini during the period of renewed activity. Soil CO
2
flux measurements were made across
a network of sites on Nea Kameni between September 2010 and January 2012. Gas samples were
collected in April and September 2011 for isotopic analysis of CO
2
(
δ
13
C), and radon detectors were
deployed during September 2011 to measure (
222
Rn). Our results reveal a change in the pattern of
degassing from the summit of the volcano (Nea Kameni) and suggest an increase in diffuse CO
2
emissions
between September 2010 and January 2012. High-CO
2
-flux soil gas samples have
δ
13
C
∼
0
.Using
this value and other evidence from the literature we conclude that these CO
2
emissions from Santorini
were a mixture between CO
2
sourced from magma, and CO
2
released by the thermal or metamorphic
breakdown of crustal limestone. We suggest that this mixing of magmatic and crustal carbonate sources
may account more broadly for the typical range of
δ
13
CvaluesofCO
2
(from
∼−
4
to
∼+
1
)in
diffuse volcanic and fumarole gas emissions around the Mediterranean, without the need to invoke
unusual mantle source compositions. At Santorini a mixing model involving magmatic CO
2
(with
δ
13
C
of
−
3
±
2
and elevated (
222
Rn)/CO
2
ratios
∼
10
5
–10
6
Bqkg
−
1
)andCO
2
released from decarbonation
of crustal limestone (with (
222
Rn)/CO
2
∼
30–300 Bqkg
−
1
,and
δ
13
Cof
+
5
) can account for the
δ
13
C
and (
222
Rn)/CO
2
characteristics of the ‘high flux’ gas source. This model suggests
∼
60% of the carbon
in the high flux deep CO
2
end member is of magmatic origin. This combination of
δ
13
Cand(
222
Rn)
measurements has potential to quantify magmatic and crustal contributions to the diffuse outgassing
of CO
2
in volcanic areas, especially those where breakdown of crustal limestone is likely to contribute
significantly to the CO
2
flux