Noble gas isotopes in hydrothermal volcanic fluids of La Soufrière volcano, Guadeloupe, Lesser Antilles arc

TLDR: In this paper, the ascending magmatic component in La Soufriere volcano (Guadeloupe, Lesser Antilles) was investigated by measuring noble gas concentrations and isotopic ratios in thermal springs and fumaroles.

About: This article is published in Chemical Geology.The article was published on 2012-04-18 and is currently open access. It has received 28 citations till now. The article focuses on the topics: Magma chamber & Magma.

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Table 2 Concentrations (in ppm per volume) and isotopic ratios measured in South Crater fumarole in 2007 and 2010. * Gas1995 data from Pedroni et al. (1999). The fractionation factor is expressed as F(X)=(X/36Ar)sample/(X/36Ar)air.

Fig. 3. Sketch representing a scenario for the activity of La Soufrière Volcano since its last magmatic eruption (1530 AD). Each vertical arrow represents phreatic activity: light arrows are intense phreatic activity and dark arrows are well-identified phreatic eruptions. Since 1635 AD, when the first settlers arrived, historical eruptive activity has exclusively consisted of phreatic eruptions (Komorowski et al., 2005). Very intense phreatic activity has been reported for 1645 (DuTertre, 1654) and 1696 (Labat, 1722). An additional phreatic eruption seems to have occurred in 1680 or 1690 (Ballet, 1896). After 100 years of rest, phreatic activity started again in 1797 with a huge crisis (Amic et al., 1798) followed by smaller crises in 1812 and 1837 (Lherminier, 1837). After 120 years of rest, a new cycle started again in 1956 (Jolivet, 1956) with the last phreatic eruption occuring in 1976–1977. Our interpretation of magma chamber replenishments is based on geophysical data for the 1976/77 crisis and literature for the 1797 crisis (Amic et al., 1798).

Fig. 1. A) Map of the Lesser Antilles Volcanic Arc. B) Schematic E–W cross-section showing the main geological structures of the volcanic growth (modified from Villemant et al., 2005). C) Structural map of La Grande Découverte/Soufrière volcanic complex showing the locations of the surveyed thermal springs (except Habitation Revel, which lies 5 km from the dome towards the west), Crater South fumarole and Col de l'Echelle borehole (modified from Boichu et al., 2008).

Fig. 4. Temperature variationswith time between 1968 and 2010 in Col de l'Echelle borehole (at -76 m) and histogramof the number of recorded earthquakes per year from1956 to 2010 The two phreatic eruptions (1956 and 1976–77) are indicated with grey bars. After May 1977, fumarolic activity decreased until mid-1980's. In 1992, fumarolic activity increased and three plumes appeared through time: South Crater since 1998, Tarissan vent in 2000 and Napoléon vent in 2002 (Bernard et al., 2006; Nicollin et al., 2006, 2007).

Table 1 Helium and neon concentrations and isotopic ratios for water samples. Year of sampling is indicated in sample name (e.g. GUA07 for 2007).

Fig. 2. Plot of measured R/Ra against 22Ne/4He. BJ is for Bains Jaunes, PR Pas du Roy, RM Ravine Marchand, CC Chute du Carbet, GaB Galion Bis, Ga Galion, BCM Bains Chauds du Matouba, TA Tarade, CE Carbet Echelle, HR Habitation Revel. The grey shaded region shows mixing between ASW 20 °C and MORB source mantle (R/Ra 8±1). Air end-member seems to strongly influence Bains Jaunes.

Frequently asked questions

Q1. What are the contributions in "Noble gas isotopes in hydrothermal volcanic fluids of la soufrière volcano, guadeloupe, lesser antilles arc" ?

Ruzié et al. this paper used helium isotopes in fumaroles and thermal springs to estimate volumetric fluxes at the summit of La Soufrière volcano in Guadeloupe Island ( Lesser Antilles ) in Haiti. 

Q2. How many years are needed to degas the helium in a 0.1-km?

Considering a constant flux for a single fumarole (3He=400 cc/ year), a non-degassed and non-crystallized magma with a density of 2.5 g/cm3 and no phreatic eruptions since the last magmatic eruption, a simple mass balance calculation indicates that 800 years are needed to fully degas the helium in a 0.1-km3 magma chamber. 

Q3. How many neon isotopic ratios are close to the air?

neon isotopic ratios are very close to the air value (20Ne/22Ne=9.8 and 21Ne/22Ne=0.0290) suggesting that neon is purely of atmospheric origin and that the mantle neon is negligible. 

Q4. What is the effect of the cooling of the mafic melt on the stability of the gas?

When a dense mafic magma is injected into an intermediate or silicic reservoir, the induced cooling and crystallization of the mafic melt causes volatile exsolution. 

Q5. What is the role of hydrothermal systems in volcanic eruptions?

However hydrothermal systems on some volcanoes, particularly in tropical areas, can buffer the magmatic signal, thereby complicating its interpretation. 

Q6. What happens when a large batch of magma is emplaced?

When a bigger batch is emplaced, mechanical mixing occurs between fresh and andesitic magma andleads to chamber instabilities, triggering the eruption. 

Q7. What is the effect of magmatic degassing on the helium in the fum?

During periods of volcanic inactivity, magmatic degassing is mainly controlled by system cooling which induces crystallization and/or by magma mixing affecting the stability of volatile species. 

Q8. What is the common scenario for the lava chamber?

When a large batch is emplaced, mechanical mixing occurs between fresh and andesitic magma leading to chamber instabilities triggering the eruption. 

Q9. When did the pressure and temperature in the aquifers rapidly increase to trigger a?

Only when the cracks became completely clogged and sealed did the pressure and temperature in the aquifers rapidly increase to trigger a crisis. 

Q10. What is the helium and neon concentrations in water samples?

Helium and neon measurements were conducted on water samples using a Quadrupole Mass-Spectrometer (QMG220, Pfeiffer©) for concentration and a Noblesse (Nu Instruments©) mass-spectrometer for helium and neon isotopic compositions. 

Q11. What are the different forms of volcanic emissions?

Volcanic emissions of gases to the atmosphere take many different forms— from hydrothermal manifestations tomassive syn-eruptive releases. 

Q12. How is the helium incompatible during melting?

As helium is incompatible duringmelting (Heber et al., 2007), for partial melting of 10% in the mantle wedge (Turner et al., 1996), 3He concentration is estimated around 10−9 cc/g in the magma considering a 4He/3He ratio of 89,000 (3He/4He=8 Ra). 

Q13. How long does it take to degas the magma chamber?

For two extreme cases of magmatic degassing before emplacement in the magma chamber, 90% and 10%, the authors obtain respectively 80 years and 700 years to fully degas the magma chamber with the observed helium flux in one fumarole. 

Q14. What is the average concentration of 3He in the fumarole?

In the total fumarole, this corresponds to 1.2 -2.9×10−12 ppm, depending on the water content (between 95% and 98%), and represents a 3He flux between 1.2×10−5 and 3×10−5 cc/s. 

Q15. How do the authors show that fumaroles are a mixture of meteoric water and?

Using noble gas systematics, the authors have shown that summit fumaroles represent a mixture between meteoric water and a magmatic flux component. 

Q16. What is the efficient pathway to degas the magmatic system?

The authors have calculated that 3He flux is much higher in fumaroles than in springs, showing that fumaroles are the most efficient pathways to degas the magmatic system. 

Q17. What was the volume of the La Soufrière magma chamber?

Semet et al. (1981) estimated the volume of La Soufrière magma chamber to be 0.08–0.1 km3 based on volumes of eruptive products of that last magmatic eruption. 

Q18. What is the role of gases in volcanic eruptions?

Understanding the relationship between the gases released and magma evolution is central for monitoring and forecasting volcanic eruptions. 

Q19. How many times could the degassing time be overestimated?

the estimated initial magmatic 3He concentrations may be overestimated by at least a factor of 10, implying that the degassing times could also be overestimated by a factor of ten.