Thomas Staudacher

Rare gas systematics: formation of the atmosphere, evolution and structure of the Earth's mantle

Abstract To explain the rare gas content and isotopic composition measured in modern terrestrial materials we explore in this paper an Earth model based on four reservoirs: atmosphere, continental crust, upper mantle and lower mantle. This exploration employs three tools: mass balance equations, the concept of mean age of outgassing and the systematic use of all of the rare gases involving both absolute amount and isotopic composition. The results obtained are as follows: half of the Earths mantle is 99% outgassed. Outgassing occurred in an early very intense stage within the first 50 Ma of Earth history and a slow continuous stage which continues to the present day. The mean age of the atmosphere is 4.4 Ga. Our model with four main reservoirs explains quantitatively both isotopic and chemical ratios, assuming that He migrates from the lower to the upper mantle whereas the heavy rare gases did not. Noble gas fluxes for He, Ar and Xe from different reservoirs have been estimated. The results constrain the K content in the earth to 278 ppm. Several geodynamic consequences are discussed.

Neon isotopes in submarine basalts

Very large neon isotopic anomalies have been accurately measured in mid-ocean ridge basalt glassy samples from diverse locations worldwide. Values for20Ne/22Ne range up to ∼ 13 and21Ne/22Ne values range up to ∼ 0.07 (present atmospheric values are20Ne/22Ne= 9.8 and21Ne/22Ne= 0.029). The data are highly correlated in the20Ne/22Ne—21Ne/22Ne diagram, independent of sample location. Loihi seamount data show an atmospheric neon isotopic composition, which is consistent with previous He—Ar—Xe data and their interpretation as representative of a deep undegassed mantle source. Recent mixing within the mantle best explains the neon isotopic features, as well as the He—Ne—Ar—Xe relationships. We propose that the MORB21Ne/22Ne anomalies are due to nuclear reactions within the mantle. The origin of the20Ne/22Ne anomalies however cannot be explained in a straightforward way and we discuss several scenarios.

Recycling of oceanic crust and sediments: the noble gas subduction barrier

Abstract We have determined the concentrations and isotopic composition of noble gases in old oceanic crust and oceanic sediments and the isotopic composition of noble gases in emanations from subduction volcanoes. Comparison with the noble gas signature of the upper mantle and a simple model allow us to conclude that at least 98% of the noble gases and water in the subducted slab returns back into the atmosphere through subduction volcanism before they can be admixed into the earths mantle. It seems that the upper mantle is inaccessible to atmospheric noble gases due to an efficient subduction barrier for volatiles.

40Ar36Ar in MORB glasses: constraints on atmosphere and mantle evolution

We performed argon isotopic composition measurements of MORB glassy samples from the Pacific, Atlantic and Indian Oceans. There is a very large scatter in the40Ar36Ar ratio, from 980 up to 24,400 for bulk rock analyses, which is mainly due to atmospheric contamination. Using the stepwise heating technique, very high ratios are obtained, from 15,000 up to 25,250 which is the highest40Ar36Ar ratio ever measured in MORB. We establish a negative correlation between the highest40Ar36Ar results from stepwise heating and87Sr86Sr ratios, which is perfectly c consistent with a two-layered mantle structure. From both40Ar and129Xe [1] MORB systematics a model is proposed for the kinetics of degassing: a very early and extensive burst, with a time constant of≈ 4My, is followed by a slower process of present day type, with a time constant of≈ 0.5Gy. The mean age of the atmosphere is so determined to be around 4.4 Gy.

Noble gas systematics of deep rift zone glasses from Loihi Seamount, Hawaii

We report new noble gas fusion and crushing data for six pillow rim glasses, recovered between 3 and 5 km water depth on the south rift zone of Loihi Seamount, Hawaii. Helium abundances of the glasses vary from 0.3 to 2.3 μcc/g, with 4He/3He ratios between 30000 and 27000 (24–27 RA), similar to previously reported values. The neon data form a correlation line which is similar to the Loihi-Kilauea line reported by Honda et al. [1], but extends to much higher ratios, up to 12.9 and 0.0382 for the 20Ne/22Ne and 21Ne/22Ne ratios, respectively. This provides conclusive evidence for the suggestion that the Hawaiian plume, thought to originate in the lower mantle, has a solar-like 20Ne/22Ne composition [1], but a slightly higher 21Ne/22Ne ratio. 40Ar/36Ar ratios of the deep rift-zone glasses are as high as 2600, and show a positive correlation with neon isotopic ratios. In contrast to neon and argon, all xenon isotopic compositions are isotopically indistinguishable from air, which either suggests preferential atmospheric contamination of xenon, or could indicate an atmospheric xenon isotopic composition for the lower mantle.

Rare gas systematics on the southernmost Mid‐Atlantic Ridge: Constraints on the lower mantle and the Dupal source

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have been measured for mid-ocean ridge basalt glasses from the Mid-Atlantic Ridge Discovery section, centered at 47°30′S, thus extending the database for the 50°–53°S Shona section [Moreira et al, 1995]. The 44°–53°S part of the Mid-Atlantic Ridge includes the Discovery and Shona bathymetrie and geochemical ridge anomalies [Douglass et al, 1999], which also appear clearly in the rare gas isotopic record. In addition to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal-related component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degassed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal very consistent along-strike He, Ne, and Ar isotopic patterns, also consistent with Xe data. These systematics provide evidence that plume argon has low 40Ar/36Ar and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal-type, low 206Pb/204Pb component named LOMU (low μ, where μ = 238U/204Pb) by Douglass et al. [1999], and has radiogenic 4He/3He and 21Ne/22Ne, relatively elevated 20Ne/22Ne, mildly radiogenic 40Ar/36Ar, and low Xe isotopic ratios, possibly representing the Dupal rare gas signature. Interpretations of this component as either recycled oceanic crust, or delaminated subcontinental lithosphere are consistent with the rare gas systematics. In the former case, a maximum subduction age of 500 Ma can be calculated. In the latter case, the sublithospheric mantle should have a 40K/36Ar ratio 2–5 times lower than the convective mantle and a 238U/3He ratio 2–3 times higher.

New noble-gas data on glass samples from Loihi Seamount and Hualalai and on dunite samples from Loihi and Réunion Island

Abstract Ultra high-vacuum crushing and stepwise heating experiments on six Loihi seamount and Hualalai glass samples, and on one Loihi Seamount dunite and two Reunion dunites were performed. The noble gases were measured on ARESIBO I. The low 4 He 3 He and 40 Ar 36 Ar ratios of 20,000–40,000 and 360–410, respectively, and the atmosphere-type Xe isotopic composition for Hawaiian glasses confirm previously published analyses and the existence of an undegassed lower-mantle reservoir. The noble-gas pattern of the glass samples makes contamination of the samples by atmospheric noble gases dissolved in seawater unlikely. Dunites brought up from lithospheric upper mantle in the hotspot magma show high 40 Ar 36 Ar ratios, a signature of the degassed upper mantle, but intermediate 4 He 3 He ratios. This indicates that dunite residence time in contact with the hotspot magma was sufficiently short that complete noble-gas equilibration was not possible. Based on the pattern for heavy noble gases of Hawaiian samples we suggest that the primitive Earth had no chondritic noble-gas pattern and that the so-called “missing xenon” problem does not exist.

Noble gas systematics of Réunion Island, Indian Ocean

Abstract Noble gas concentrations and isotopic ratios have been analyzed in a suite of 40 samples from Reunion Island by crushing and stepwise heating. Olivine phenocrysts came from recently erupted oceanites, collected from Piton de la Fournaise as well as from 0.5- to ∼2-Ma-old eruptions from Piton des Neiges; xenolith nodules came from the Chisny crater, close to the Piton de la Fournaise; glass samples came from small seamounts from the Fournaise II region, east of Reunion. The Reunion isotopic noble gas signature “Re” is best preserved in the olivine phenocrysts and is characterized as follows: 4 He 3 He =55,400±3,100 , 20 Ne 22 Ne =9.90±0.11 , 21 Ne 22 Ne =0.0292±0.0002 , 40 Ar 36 Ar =450±70 and air-type Kr and Xe. These ratios and the systematics of the data are consistent with an origin of the noble gases by mixing from lower- and upper-mantle material and/or old subducted crustal material. There is no measurable change in the 4 He 3 He ratio between 2 Ma and present. The xenolith data are somewhat confusing since they have noble gas ratios that are consistent neither with a simple upper-mantle origin nor an origin in the source region of Reunion magma. Xenolith and olivine phenocryst 4 He 3 He ratios are identical but xenolith 40 Ar 36 Ar ratios of 1600–7970 are higher than phenocryst ratios, and would seem to imply an origin from the depleted upper mantle. The xenolith Ne isotopic ratios 20 Ne 22 Ne and 21 Ne 22 Ne are high and fall between those of typical MORB and the mass discrimination line. To resolve this contradiction, we propose a combination of mixing of upper-mantle-derived material and Reunion magma, and a diffusion process, with fractionation of the noble gases due to different diffusion coefficients.

4He/³He dispersion and mantle convection

Histograms of the 4He/³He ratios analysed in MORB glasses from individual ridge segments show a linar correlation between the ridge spreading rate and the standard deviation of the corresponding averaged 4He/³He ratio. We interpret the form of these histograms in terms of stirring time (τstir) of the upper mantle. This stirring time corresponds to the time that perturbations brought in from outside into the upper mantle, in the form of oceanic island source material or slabs, were attenuated by a factor of 1/e through the effect of convection. Using 4He/³He ratios of oceanic basalts, we calculate a stirring time close to 250 Ma, distinct from the residence time of ∼1 Ga. This may indicate the existence of two scale upper mantle convection, rapid convection being responsible for the homogeneisation of helium in the upper mantle, and the consequent uniformity of the 4He/³He ratio, and slower convection being responsible for mantle outgassing and plate tectonic motion.

April 2007 collapse of Piton de la Fournaise: A new example of caldera formation

Collapse calderas are frequent in the evolution of volcanic systems, but very few have formed during historical times. Piton de la Fournaise is one of the worlds most active basaltic shield volcanoes. The caldera collapse, which occurred during the April 2007 lateral eruption is one of the few large documented collapse events on this volcano. It helps to understand the mode and origin of caldera collapses in basaltic volcanoes. Field observations, GPS and seismic data show that the collapse occurred at an early stage of the eruption. The cyclic seismic signal suggests a step by step collapse that directly influenced the lateral eruption rate. Likely, the caldera results from the combined effect of (i) the progressive collapse of the plumbing system above the magma chamber since 2000, and (ii) the large amount of magma withdrawal during the early stage of the eruption by both a significant intrusion within the edifice and an important emission rate.