Albert Jambon

Solubility of He, Ne, Ar, Kr and Xe in a basalt melt in the range 1250–1600°C. Geochemical implications

Abstract The solubility constant of Henrys law has been experimentally determined in a tholeiitic basalt melt. Equilibration with air, a noble gas mix, and various mixtures of the two permitted us to assess the validity of Henrys law under 1 bar ambient pressure in the temperature range 1250–1600°C. Mass spectrometric analyses of quenched products yield solubilities decreasing with increasing atomic size as: 56 (11.), 25 (3.), 5.9 (0.9), 3.0 (0.7) and 1.7 (0.4) in units of 10−5 cm3 STP/g-bar (with standard deviation) for He, Ne, Ar, Kr and Xe, respectively (1400°C). Partial pressures were varied by actors between 100 and 2 × 104. The dependence on temperature is very moderate. Comparison with earlier results confirms a significant dependence of solubility on melt composition. The data permit calculation of the distribution of noble gases between a melt and coexisting vesicles. Comparison with data obtained on MORB glasses shows that He, Ne and Ar display an equilibrium distribution while Kr and Xe in the vesicle-free glass are probably below analytical detection. The strong fractionation effects implied by the very different solubilities can explain most of the variations observed in MORB-glasses for He Ne and He Ar ratios.

Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications

Ion microprobe determination of boron content and δ11B values has been performed for a set of 40 oceanic basalt glasses (N-MORB, E-MORB, BABB and OIB) whose chemical characteristics (major and trace elements and isotopic ratios) are well documented. Boron contents, determined at ±10% relative, range from 0.34 to 0.74 ppm in N-MORB, whereas E-MORB, BABB and OIB extend to higher concentrations (0.5–2.4 ppm). After correction for crystal fractionation, this range is reduced to 0.5–1.3 ppm. N-MORB and E-MORB also exhibit different B/K ratios, 1.0 ± 0.3 × 10−3 and 0.2 to 1.4 × 10−3 respectively. This can be interpreted as resulting from the incorporation into the upper mantle of a K-rich and B-poor component (e.g., subducted oceanic crust having lost most of its initial boron). n nδ11B values range between −7.40 ± 2 and +0.6 ± 2‰, with no significant difference between N-MORB, E-MORB, OIB or BABB. The Hawaiian samples define a strong linear correlation between boron contents, δ11B values, MgO and water contents and δD values. This is interpreted as resulting from assimilation-fractionation processes which occurred within a water-rich oceanic crust, and which produced high δ11B values associated with high δD values. The low level of11B enrichment in the upper mantle constrains the amount of boron reinjected by subduction to a maximum of about 2% of the boron present in the subducted slab. This is turn corresponds to a maximum net Boron transfer of about 3 x 1010 g/a towards the surface reservoirs. Finally, a boron content of 0.25 ± 0.1 ppm is estimated for the bulk silicate Earth (i.e., primitive mantle), corresponding to a depletion factor relative to C1 chondrites of about 0.15 and suggesting that B was moderately volatile upon terrestrial accretion.

Water in oceanic basalts: evidence for dehydration of recycled crust

Analyses of water in MORB glasses have been performed using mass spectrometry techniques on speciments selected from worldwide localities (Mid Atlantic Ridge, East Pacific Rise, Red Sea, Lau Basin). Specimens belong to both normal and enriched types. Some OIB glasses have also been analyzed. The amounts of water found, comparable to those previously reported, are within the 1700–6000 ppm range. Positive correlations are observed with other trace elements, particularly K2O. Samples may be divided into two groups: (1) N-MORB (La/Sm 1) with K2O/H2O varying from 0.4 up to 1. The correlation found for N-MORB may be explained by variable degrees of partial melting (possibly complicated by fractional crystallization), water and potassium behaving as incompatible elements. The worldwide validity of the correlation demonstrates that K2O/H2O is constant in the source even though that source may have been depleted to a variable extent. The enriched samples though cannot be derived from the same source as the N-MORB through simple processes such as partial melting and fractional crystallization. n nThe respective importance of source composition and degree of partial melting may be assessed using additional data available in the literature. For sources with H2O contents between 70 and 550 ppm a 5–15% partial melting may be proposed. The positive correlation between (La/Sm)n and K2O/H2O requires the existence of (at least) two mantle components that may be variously depleted or enriched. One is the depleted MORB source with low La/Sm and K2O/H2O, while the other displays both high La/Sm and K2O/H2O ratios. The enriched source cannot be a primitive one because addition of an external composition (crust + ocean) with a K2O/H2O of 0.14 to a depleted MORB source (K2O/H2O= 0.25) cannot produce a source with an overall ratio close to 1. If a model of oceanic crust recycling is considered, then the recycled crust must be significantly dehydrated before mixing with the mantle source. This serves to explain why water is more abundant than K2O in the external reservoir, even though its incompatible behaviour is undistinguishable from that of K upon melting. An estimate of bulk earth water abundance is poorly constrained and strongly model dependent, and the proposed 1300 ppm is speculative. A value in the 550–1900 ppm range is however very plausible.

Helium and argon from an Atlantic MORB glass: concentration, distribution and isotopic composition

Data are reported for detailed He and Ar measurements on a single MORB glass from the Atlantic (CH 98-DR 11; 30°41′N, 41°49′W; depth ≈ 3500 m). Grain-size fractions prepared under ambient conditions are strongly affected by diffusive loss of He from grains and by adsorption of atmospheric argon on grain surfaces. Both effects could be controlled by crushing gram-sized chunks of glass under vacuum and analyzing the powder without any further handling, in particular without its exposure to the atmosphere. n nGases from vesicles, released upon crushing, are characterized by a4He/40Ar ratio of6 ± 1 and a40Ar/36Ar ratio of up to 22,600. In dissolved gases the4He/40Ar ratio was found to be (7.2 ± 1.6) times higher and the40Ar/36Ar ratio to be about ten times lower than in vesicles. The difference of theelemental ratio He/Ar is as anticipated from the ratio of the solubilities (ca. nine) in the investigated basalt of He and Ar. Thus, whileelemental abundance ratios are compatible with equilibrium between vesicles and basalt the grossly different argonisotopic ratios are not. It is proposed that two basalts, chemically very similar but with different40Ar/36Ar ratios, were mixed shortly before eruption. n nThe overall4He/40Ar ratio in the vesiculated basalt is12 ± 2 so that, for the ratio in the primary magma, we find6 ⩽4He/40Ar⩽ 12 which is considerably higher than the radiogenic production ratio in all conceivable sources of MORB. Preferential removal from a melt of argon via vesicles is suggested to be the most likely explanation, whereas any metasomatic transfer seems unrealistic.

Lithium diffusion in silicate glasses of albite, orthoclase, and obsidian composition: An ion-microprobe determination

Abstract Tracer diffusion coefficients for Li in glasses of albite, orthoclase, and obsidian composition have been determined by a method involving deposition of a thin source on polished glass wafers, anneal under controlled temperature conditions (300–900°C), and ion-microprobe determination of the concentration profile. All results conform to an Arrhenius-type relationship,D = D0exp(−Q/RT), whereQ is 23, 17, and 22 kcal mol−1;D0 is 0.2, 0.003, and 0.03 cm2s−1 for albite and orthoclase glasses, and obsidian respectively. Lithium is thus a fast diffusing ion and behaves similarly to sodium in the same glasses. A mechanism involving jumps of the diffusing ions through oxygen hexagonal rings is suggested by consideration of ionic radii ratio of alkali (H, Li, Na, K, Rb, and Cs) ions to the oxygen anions.

Low argon solubility in silicate melts at high pressure

The solubility of rare gases in silicate melts and minerals at high pressure is of importance for understanding the early history of the Earth and its present day degassing. Helium, neon, argon, krypton and xenon were originally incorporated into the Earth during its accretion, and have also been produced by radioactive decay. These elements have been used as tracers for deciphering mantle structure and constraining the number and size of geochemical reservoirs. In particular, it has been proposed that the budget of 40Ar, produced by the radioactive decay of 40K, provides the strongest argument for chemical layering within the mantle,. The geochemical models used to arrive at this conclusion are, however, currently under re-examination, with a large source of uncertainty being the lack of data on argon partitioning during melting. It has previously been assumed, on the basis of low-pressure data, that noble gases are highly soluble in melts at all pressures. But here we present solubility data of argon in olivine melt at very high pressure that indicate that argon solubility is strongly dependent on pressure, especially in the range of 4–5 gigapascals.

Gas geochemistry of geothermal fluids, the Hengill area, southwest rift zone of Iceland

A systematic study of the chemical and isotopic (C, S, N, He) composition of fumarole and geothermal well gases has been undertaken along a 15 km section of a subaerial spreading center (Hengill area, southwest Iceland). The highest geochemical (H2) temperatures and highest helium contents are found along the rift axis and tend to decrease with distance from this axis, as a result of cooling and gas loss from underground fluids. In terms of components, these gases appear to be derived mostly from the mantle, with minor addition from the atmosphere. Chemical ratios between species do not support mineral control of CO2 fugacity in this kind of geothermal reservoir. δ15N values range from −10.4‰ to −0.2‰ (versus ATM) which may result either from kinetic fractionation of atmospheric nitrogen or from the occurrence of a deep component. Values of δ34S close to 0‰ (versus CDT) and constant helium isotope ratios, typical of hot spot helium (RRA = 14.4), confirm the deep origin of the gases, δ13C values varying from −9‰ to −1.6‰ (versus PDB) are attributed to aqueous phase changes in geothermal aquifers. When corrected for fractionation, a δ13C mean of −4.1‰ may represent the magmatic signature in the rift area. Heat3He ratios (1.3–8.4) × 1016 J/mol are intermediate between Loihi Seamount (Hawaii) and mid-ocean ridge values. High enthalpy geothermal systems in rift zones of Iceland provide a means for studying volatile transfers at spreading centers and of supplementing data derived from oceanographic studies.

Argon solubility in silicate melts at very high pressures. Experimental set-up and preliminary results for silica and anorthite melts

A CO2 laser-heated diamond anvil cell was used for performing argon solubility experiments in silicate melts. This technique allowed solubility experiments to be carried out at much higher pressures than in a piston-cylinder-type apparatus. When the beam of the CO2 laser is focused on the silicate sample, argon, acting as a pressure-transmitting medium, melts by conductive heating from the molten sample and can disserve into the melt. Preliminary results for argon solubility in silica and anorthite melts up to 10 GPa are presented. In anorthite melt, the Ar content levels up at 0.5 wt% above 5 GPa. For silica melt, Ar contents increase up to nearly 5 wt% at 5 GPa and decrease below 1% at higher pressures. This behaviour is interpreted as resulting from a profound change in the structure of the melt above 5 GPa, probably related to an increase in the proportion of 4- and 3-membered SiO4 rings.

Major volatiles from a North Atlantic MORB glass and calibration to He: A size fraction analysis

The analysis of major volatiles in a North Atlantic mid-ocean ridge basalt (MORB) glass, has been performed using a new procedure based on the analysis of several size fractions of one single sample. It enables us to separate the contributions of volatiles from the glass phase, from the vesicle gas phase and eventually gas adsorbed on the grain surface. The vesicle composition (mole fractions) obtained upon crushing is 0.94 CO2, 0.002 H2, 0.06 H2O. Volatiles extracted from the glass at 1150°C are 9.7 · 10−6 mol g−1 CO2, 156 · 10−6 mol g−1 H2O. The gases released at 500°C are mostly adsorbed contaminants but contribute marginally to the total amount except for numerous organic compounds which are destroyed at higher temperature. n nWhen previous noble-gas data for the same sample are considered, one obtains the evidence that: (1) the HeCO2 ratio does not fractionate between glass (4.1 · 10−5) and vesicles (4.6 · 10−5); and (2) water is not quantitatively degassed from MORB glasses. The ratio He : CO2 : H2O of 10−4 : 2.4 : 13 for primitive magma has to be confirmed by further results to be considered as a typical value for a MORB source.

He solubility in silicate melts: A tentative model of calculation

Abstract Published data of He solubility in silicate melts of simple composition enable the calculation of He solubility of any melt of natural composition with a fair accuracy. The model proposed relates Henrys law solubility constant K to the mole fraction of oxide component in the melt according to: ln K= ∑ i X i k i where k i is a constant, to be determined and X i the oxide mole fraction of component i . k i -values are obtained, for SiO 2 , Al 2 O 3 , K 2 O, CaO, Na 2 O, Li 2 O and MgO, and increase in that order. These k i -values and reasonable assumptions (concerning TiO 2 , FeO and Fe 2 O 3 ) make it possible to calculate a model He solubility for a tholeiitic basalt melt which compares fairly well with the previously published result.