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Solubilities and partitioning of Ne, Ar, Kr, and Xe in anorthite, forsterite, diopside and coexisting melts with implications for terrestrial planet atmospheric origin and evolution
36 CARBON AND HELIUM ISOTOPIC CONSTRAINTS ON THE ORIGIN OF VOLCANIC CARBON FROM SUBDUCTION ZONES.
•¢;OLUBILITIES AND PARTITIONING OF Ne, At, Kr, AND Xe IN ANORTHITE, FORSTERITE, DIOPSIDE AND COEXISTING MELTS WITH IMPLICATIONS FOR TERRESTRIAL PLANET ATMOSPHERIC ORIGIN AND EVOLUTION
P. ALLARD (Centre des Faibles Radioactivit4s, CNRS, 91190 Gif/Yvette, France).
C.L BROADHURST, M.J. DRAKE, B.E. HAGEE, AND T. J. BERNATOWlCZ (Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 USA)
In order to elucidate the origin of carbon dioxide released by subduction volcanism (SV), we have compared the carbon and helium isotopic ratios of volcanic gas samples (T - 100°1000°C) collected from various representative areas, using ]itterature data and our own results on more than 200 gas samples. The 313C range from -12 to 0 (%° /PDB), most of them clustering between -7 and 0, which are typical mean values for mantle carbon and limestones, respectively. NO simple relationship was found with the corresponding gas emission temp_erature, composition of the m a g m a s (including their 87Sr and 10Be contents), or subduction rates.
The noble gas abundance and isotopic composition patterns of the atmospheres of Earth, Mars, and Venus differ from each other as well as from the average ¢hondritic trend. In the absence of primordial heterogeneities, these differences must result from different degassing histories and/or efficlencias among the planets. There are three major mechanisms for non-catastrophic degassing of planetary mantles: (1) diffusion, (2) mass transport by convective overturn, (3) magmatic transport. Of these, only (3) can be demonstrated to be important on all three terrestrial planets. To investigate the efficacy of magmatlo transport we. have developed a technique of synthesis and reversal experiments. The syntheses generate noble gas solubilities and partition coefficients for equilibrium mineraVmelt pairs at 1300°C in a 1 bar noble gas atmosphere (100% Ar, or 5% Ne, 93% Ar, 1% Kr, 1% Xe). The reversals have established that we have attained equilibrium solubilities of Ar in anorthite. Thus far, we have investigated forsterite, diopside, 5 anorthltes, and coexisting melts. The mineral solubility data show a clear trend of increasing solubility with increasing noble gas atomic number, however, the absolute solubilities are extremely variable. This variation is both real and reproducible, and indicates that prior sample history can affect noble gas solubilityin minerals. In contrast, the molt solubilities show a clear trend of decreasing solubility with increasing atomic number, and relatively little variation over a large compositional range. The spread in mineral solubilities affects the absolute values of the partition coefficients, but not the partition coefficient patterns. These patterns exhibit increasing compatibility with increasing atomic number for all mineral/melt pairs. Our results indicate that the noble gases as a group are not uniformly incompatible, thus they will be fractionated by igneous processes. If the observed behavior is characteristic of other common minerals, then a planetary atmosphere will contain the bulk planetary inventory of noble gases only if the planet is substantially outgassed. A more general conclusion is that the noble gas abundance patterns should not be interpreted in terms of bulk planet inventories without a detailed understanding of planetary differentiation.
lannd i h ter~H e ;4 Heeg~itv; : f [h:l at~°:cieatieSdshb~we e(nw~ii°ht rOa13gi from 8 to 2 times the ratio in air): the poorer in 3He the He, the higher (closer to carbonate values) the 313C of carbon, and vice versa• SV gases thus tend to differ from MORB volatiles and volcanic gases from rift zones in being; enriched in 13C and in having a higher (correlated) CO2/°He ratio• This pattern may result from the combined effects of (1) a variable dilution of mantle gas by 13C- and 4He-rich crustal fluids (derived from either subducted sediments or/and the crust in place) and (2) fractionation of carbon isotopes during subduction and subsequent magmatic processes• Positive variations of 313C with the thickness of the crust beneath arcs and, to a lesser extent, with the focal depth of subducted slabs suggest that the extent of these effects may be controlled in part by the depth of generation of magmas and by their residence time within the crust during ascent.
NUCLEATION AND GROWI~ OF CO 2 BLgBLES £g MOF~ Y. BOTTINGA and M. JAVOY, I.P.G.P., 4 place Jussieu, 75252 pARIS C E D ~ 05. We will discuss the theoretical asr~cts of CO < s , 2 cape from the mantle. Of the ma]or gases dzssol\ed in MORB CO has the smallest solubility, consequently it fomns ~ e first bubble in rising .MOB. ~ e fairly large surface energy of basalt melts is d~e cause of a large degree of supersaturation needed before homogeneous nucleation occurs. But the size and coofigurational distributions of bubbles in 5~RB suggest strongly that nucleation is homogeneous, contrary to popular belief. Calculation of the free energy of COgbubble nucleus formation shows that the CO 2 concentration should be at least three timas greater than the equilibrium concentration for nucleation to take place. This is compatible with the known CO?solubility and the observed concentrations in ~ORB. Initially the small bubble are stationary, they grow by diffusion and buoymncy causes them to move in the melt. Frc~ then onwards equilibriumbetween the melt and bubble is unattainable. Because of the large degree of supersatu/ation required for nucleation, solubility determ/nations based on the appearance of bubbles are usually non valid, erupting MORB will be supersaturated in COg, and the presence of different generations of bubbIes becomes understandable. Most of the rare gases in MORB are in CO bubbles. According to chemical and • . . 2 lSOtOplC evldence ~ R B starts to e~xsolve CO , when it contains between 5000 and 13000 ppmCOg.2Hence Mf)RB is already saturated with CO~ when i£ is formed by partial malting. Hence the ini[ial CO content of MORB is restricted by a solubility licit. The ~He concentration in MORB should be a s i d l e function of the degree of partial melting, if 3He is uniformly distributed in the mantle, with the great majority of ~He being in CO 2 bubbles, it is to be
hely constant zn MORB as has been r e p e a t e d l y o b s e r ved
NUCLEOGENIC NE AND AR IN PITCHBLENDES J. EIKENBERG~ P. SIGNER, and H. BAUR (ETH Z~rich, Labor f~r Isotopengeochemie, NO C 61 CH-8092 ZOrich, Switzerland)
Fissiogenic Xe and Kr as well as Ne and Ar and radiogenic He have been measured in pitchblendes. The isotopic ratios of Ne and Ar d i f f e r significantly from their atmospheric values. This study shows that nucleogenic Ne and Ar results neither from strong asymmetric fission nor from (n,~)- or (~,n)-reactions, respectively. The sources a~e the^nuclear reactions [[) 180(e~n)21Ne, ~2) 1~F(e,n)L~Ne, (3) ~bCl(e,n)J~Ar or ~oCl(e,p)~OAr and (4) 35Cl(n,~)36Ar as already proposed 1954 by Wetherill. Concentrations of the target elements F and Cl have been determined by the electron microprobe. Since these are distributed homogenously in all samples, the target element normalized production probabilities for 21Ne, 22Ne und 38Ar may be determined. S i n c e He-loss is common in these samples, 136Xe formed during the spontaneous fission was used for determination of the ~-dosis. The results are: Ne/~ = (5.23±0.50)'10-8 [ccSTP/g, e, gO] ~Ne/~ (7.54±1.23)'10-~ [ccSTP/g, e, gF] OAr/a (8.67±0.93)'I0 -I [ccSTP/g, e, gCl] From these production probabilities, together with concentrations of U, Th, O, F, and Cl in earth's crust and mantle the maximum fraction of nucleogenic Ar and Ne in the atmosphere can be estimated. Atmospheric 21Ne contains up to 2.4% nucleogenic 21~Je, while the fractions of nucleogenic 22Ne and 38Ar in the atmosphere are neglegible.
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•¢;OLUBILITIES AND PARTITIONING OF Ne, At, Kr, AND Xe IN ANORTHITE, FORSTERITE, DIOPSIDE AND COEXISTING MELTS WITH IMPLICATIONS FOR TERRESTRIAL PLANET ATMOSPHERIC ORIGIN AND EVOLUTION
P. ALLARD (Centre des Faibles Radioactivit4s, CNRS, 91190 Gif/Yvette, France).
C.L BROADHURST, M.J. DRAKE, B.E. HAGEE, AND T. J. BERNATOWlCZ (Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 USA)
In order to elucidate the origin of carbon dioxide released by subduction volcanism (SV), we have compared the carbon and helium isotopic ratios of volcanic gas samples (T - 100°1000°C) collected from various representative areas, using ]itterature data and our own results on more than 200 gas samples. The 313C range from -12 to 0 (%° /PDB), most of them clustering between -7 and 0, which are typical mean values for mantle carbon and limestones, respectively. NO simple relationship was found with the corresponding gas emission temp_erature, composition of the m a g m a s (including their 87Sr and 10Be contents), or subduction rates.
The noble gas abundance and isotopic composition patterns of the atmospheres of Earth, Mars, and Venus differ from each other as well as from the average ¢hondritic trend. In the absence of primordial heterogeneities, these differences must result from different degassing histories and/or efficlencias among the planets. There are three major mechanisms for non-catastrophic degassing of planetary mantles: (1) diffusion, (2) mass transport by convective overturn, (3) magmatic transport. Of these, only (3) can be demonstrated to be important on all three terrestrial planets. To investigate the efficacy of magmatlo transport we. have developed a technique of synthesis and reversal experiments. The syntheses generate noble gas solubilities and partition coefficients for equilibrium mineraVmelt pairs at 1300°C in a 1 bar noble gas atmosphere (100% Ar, or 5% Ne, 93% Ar, 1% Kr, 1% Xe). The reversals have established that we have attained equilibrium solubilities of Ar in anorthite. Thus far, we have investigated forsterite, diopside, 5 anorthltes, and coexisting melts. The mineral solubility data show a clear trend of increasing solubility with increasing noble gas atomic number, however, the absolute solubilities are extremely variable. This variation is both real and reproducible, and indicates that prior sample history can affect noble gas solubilityin minerals. In contrast, the molt solubilities show a clear trend of decreasing solubility with increasing atomic number, and relatively little variation over a large compositional range. The spread in mineral solubilities affects the absolute values of the partition coefficients, but not the partition coefficient patterns. These patterns exhibit increasing compatibility with increasing atomic number for all mineral/melt pairs. Our results indicate that the noble gases as a group are not uniformly incompatible, thus they will be fractionated by igneous processes. If the observed behavior is characteristic of other common minerals, then a planetary atmosphere will contain the bulk planetary inventory of noble gases only if the planet is substantially outgassed. A more general conclusion is that the noble gas abundance patterns should not be interpreted in terms of bulk planet inventories without a detailed understanding of planetary differentiation.
lannd i h ter~H e ;4 Heeg~itv; : f [h:l at~°:cieatieSdshb~we e(nw~ii°ht rOa13gi from 8 to 2 times the ratio in air): the poorer in 3He the He, the higher (closer to carbonate values) the 313C of carbon, and vice versa• SV gases thus tend to differ from MORB volatiles and volcanic gases from rift zones in being; enriched in 13C and in having a higher (correlated) CO2/°He ratio• This pattern may result from the combined effects of (1) a variable dilution of mantle gas by 13C- and 4He-rich crustal fluids (derived from either subducted sediments or/and the crust in place) and (2) fractionation of carbon isotopes during subduction and subsequent magmatic processes• Positive variations of 313C with the thickness of the crust beneath arcs and, to a lesser extent, with the focal depth of subducted slabs suggest that the extent of these effects may be controlled in part by the depth of generation of magmas and by their residence time within the crust during ascent.
NUCLEATION AND GROWI~ OF CO 2 BLgBLES £g MOF~ Y. BOTTINGA and M. JAVOY, I.P.G.P., 4 place Jussieu, 75252 pARIS C E D ~ 05. We will discuss the theoretical asr~cts of CO < s , 2 cape from the mantle. Of the ma]or gases dzssol\ed in MORB CO has the smallest solubility, consequently it fomns ~ e first bubble in rising .MOB. ~ e fairly large surface energy of basalt melts is d~e cause of a large degree of supersaturation needed before homogeneous nucleation occurs. But the size and coofigurational distributions of bubbles in 5~RB suggest strongly that nucleation is homogeneous, contrary to popular belief. Calculation of the free energy of COgbubble nucleus formation shows that the CO 2 concentration should be at least three timas greater than the equilibrium concentration for nucleation to take place. This is compatible with the known CO?solubility and the observed concentrations in ~ORB. Initially the small bubble are stationary, they grow by diffusion and buoymncy causes them to move in the melt. Frc~ then onwards equilibriumbetween the melt and bubble is unattainable. Because of the large degree of supersatu/ation required for nucleation, solubility determ/nations based on the appearance of bubbles are usually non valid, erupting MORB will be supersaturated in COg, and the presence of different generations of bubbIes becomes understandable. Most of the rare gases in MORB are in CO bubbles. According to chemical and • . . 2 lSOtOplC evldence ~ R B starts to e~xsolve CO , when it contains between 5000 and 13000 ppmCOg.2Hence Mf)RB is already saturated with CO~ when i£ is formed by partial malting. Hence the ini[ial CO content of MORB is restricted by a solubility licit. The ~He concentration in MORB should be a s i d l e function of the degree of partial melting, if 3He is uniformly distributed in the mantle, with the great majority of ~He being in CO 2 bubbles, it is to be
hely constant zn MORB as has been r e p e a t e d l y o b s e r ved
NUCLEOGENIC NE AND AR IN PITCHBLENDES J. EIKENBERG~ P. SIGNER, and H. BAUR (ETH Z~rich, Labor f~r Isotopengeochemie, NO C 61 CH-8092 ZOrich, Switzerland)
Fissiogenic Xe and Kr as well as Ne and Ar and radiogenic He have been measured in pitchblendes. The isotopic ratios of Ne and Ar d i f f e r significantly from their atmospheric values. This study shows that nucleogenic Ne and Ar results neither from strong asymmetric fission nor from (n,~)- or (~,n)-reactions, respectively. The sources a~e the^nuclear reactions [[) 180(e~n)21Ne, ~2) 1~F(e,n)L~Ne, (3) ~bCl(e,n)J~Ar or ~oCl(e,p)~OAr and (4) 35Cl(n,~)36Ar as already proposed 1954 by Wetherill. Concentrations of the target elements F and Cl have been determined by the electron microprobe. Since these are distributed homogenously in all samples, the target element normalized production probabilities for 21Ne, 22Ne und 38Ar may be determined. S i n c e He-loss is common in these samples, 136Xe formed during the spontaneous fission was used for determination of the ~-dosis. The results are: Ne/~ = (5.23±0.50)'10-8 [ccSTP/g, e, gO] ~Ne/~ (7.54±1.23)'10-~ [ccSTP/g, e, gF] OAr/a (8.67±0.93)'I0 -I [ccSTP/g, e, gCl] From these production probabilities, together with concentrations of U, Th, O, F, and Cl in earth's crust and mantle the maximum fraction of nucleogenic Ar and Ne in the atmosphere can be estimated. Atmospheric 21Ne contains up to 2.4% nucleogenic 21~Je, while the fractions of nucleogenic 22Ne and 38Ar in the atmosphere are neglegible.
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