Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.265m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 265
Mathematics
Logic
Scientific paper
Introduction The ^129Xe/^132Xe ratio in Mid-Ocean Ridge Basalts (MORBs) is higher than in the atmosphere and Ocean Island Basalts. Enhanced ^129Xe/^132Xe ratios are widely regarded to be the result of ^129I decay (t(sub)1/2 = 16 m.y.) early in solar system history (e.g. Swindle et al., 1986). Allegre et al. (1983, 1988) proposed a catastrophic degassing scheme to explain this excess. Both Musselwhite et al. (1990) and Hiyagon and Ozima (1990) have noted that because mineral/melt partition coefficients (D) for I appear lower than for Xe, the I/Xe ratio may not be enhanced in the mantle by mineral/melt fractionation. Musselwhite et al. (1990) proposed recycling of I back into the mantle following outgassing, and Hiyagon and Ozima (1990) proposed impact degassing of the mantle as a way around this problem. Knowledge of the relative values of D(I) and D(Xe) is important to the discussion of early planetary outgassing models. Although the dataset for D(I) values is not complete, the known values so far are uniformly low. The dataset for Xe on the other hand is quite ambiguous. Experimentally determined values for D(Xe) vary widely--ranging from 0.05 to >> 1 (Hiyagon and Ozima, 1986; Broadhurst et al., 1992), and it is unclear which of the values is the geologically significant one. Particularly important is the question of whether D(Xe) is greater than or less than unity. Partitioning Experiments: We have undertaken to simultaneously determine the D(I) and D(Ar) values directly, then calculate the D(Xe) from D(Ar). This approach is possible because experiments investigating the mineral/melt partitioning of noble gases, while not consistent in an absolute sense between experiments, do display a consistent trend with the lightest noble gases being most incompatible and Xe most compatible. We are adapting our technique to determine D(Kr) and D(Xe) directly. Finely crushed silica glass (~100 micrometer grain size) was placed in a gas pressure vessel. The vessel was pressurized with a 50:50 mixture of Ar and Kr to 5 kbar at 600 degrees C and held for 20 hr. Electron microprobe analysis (Cameca sx50) of the resulting Ar concentrations in the silica glass ranged from 0.9 to 0.75 wt%. These are similar values to those of Carroll and Stolper (1991). Determination of Kr abundances is so far problematical. Our preliminary results indicate that Kr solubility in silica glass is about an order of magnitude lower than for Ar under the same conditions. The saturated silica glass was then used to mix starting material in the CAMS system (Presnall et al., 1978). Appropriate oxide mixtures with KI added were run at the desired pressures and temperatures in a piston-cylinder apparatus. Results from an 8 hr run at 15 kbar and 1480 degrees C are shown below. Discussion and Conclusions: The resulting concentration of Ar in melt is lower than in starting materials. The melt solubility is consistent with the results of White et al. (1989) and Lux (1987). The partitioning coefficient for Ar, while only an upper limit falls within the low range of values determined by Hiyagon and Ozima (1986) and Broadhurst et al. (1992). The correct value for D(Xe) should also be low, i.e., <0.6. Thus Xe may be incompatible, but its compatibility relative to I is unconstrained by this experiment. The value for D(I) is consistent with our earlier experiments (Musselwhite et al., 1990). It follows that the I/Xe ratio of the mantle cannot be significantly increased as a result of partial melting. The inability to remove Xe from the mantle by mineral/melt partitioning followed by eruption and outgassing of the melt would, in fact, favor the idea of impact outgassing as long as recycling of I occurred. Outgassing of the MORB source would have to be nearly complete before recycling began in order to raise its ^129Xe/^132Xe ratio. However, as long as D(Xe) <= 0.1, Xe will be nearly quantitatively removed from the solid phase along with I during partial melting of the mantle and, therefore, outgassed upon eruption. Thus, fractionation of I from Xe in liquid water, as proposed by Musselwhite et al. (1991) for Mars, may be required. Table 1. Experimental Data for Iodine and Argon Iodine Argon wt% in Forsterite 0.002+-0.001 0.001 wt% in Glass in equil w/Forsterite 0.459+-0.010 0.010+-0.001 (sub)DFo/Melt 0.004+-0.002 <= 0.1 References. Allegre et al. (1983) Nature, 303, 762-766; Allegre et al. (1988) EPSL 81, 127-150; Broadhurst et al. (1992) GCA 56, 709-723; Carroll and Stolper (1991) GCA 55, 211-226; Hiyagon and Ozima (1986) GCA 50, 2045-2057; Hiyagon and Ozima (1990) LPSC XXI, 518-519; Lux (1987) GCA 51, 2045-2057; Musselwhite et al. (1990) LPSC XXI, 833-834; Musselwhite et al. (1991) Nature 352, 697-699; Presnall et al. (1978) Cont. Min. Pet. 60, 203-220; Swindle et al. (1986) in Origin of the Moon 331-357; White et al. (1989) Am. Min. 74, 513-529.
Drake Michael J.
Musselwhite Donald Stanley
Swindle Timothy D.
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