Microsymposium 38, MS062, 2003 (A4 format)
POSSIBLE REASONS OF LOW Fe3+/Fe2+ RATIOS IN TEKTITES IN COMPARISON WITH THAT OF INITIAL TARGET MATTER INVOLVED IN THE IMPACT PROCESS. O.A Lukanin and A.A. Kadik , Vernadsky Institute of Geochemistry and Analytical Chemistry of RAS, Kosygin st. 19, Moscow, 119991 GSP-1 Russia. e-mail:
[email protected]
Introduction: The composition of tektitic glasses formed as the result of impact events is characterized by significantly low Fe3+/Fe2+ in comparison with that of target rocks that are the initial material for tektites. Possible reasons of iron valency change during impact process are the object of discussion: (1) oxygen removal from the system together vapor phase in the process of melting and vaporization; (2) degassing of tektitic melts, (3) the presence of such inherent (intrinsic) reducers in initial target matter as carbon, sulfur and their compounds , (4) fractionation of iron ions at the vapor phase condensation during of tektitic melts formation etc. [1-3].Authors of this communication suppose that the reducing reactions with the assistance of ions of iron and other elements may be the result of appropriate change of oxygen regime in the process of adiabatic decompression of matter at very high temperatures after its impact compression. Oxygen regime and Fe3+/Fe2+ in impact melts: The suggested model of impact process oxygen regime assumes that with temperature and pressure increase the pO2 value of impact melt with given Fe3+/Fe2+ ratio increases just in the same manner as pO2 of solid phase buffers such as magnetite-wustite (MW). This assumption is based on two observations. 1) Electrochemical measurements of intrinsic fO2 of tektitic glasses within range of 8001050oC show that temperature dependence of tektite pO2 is similar to that for MW buffer [3]. 2) The data on the redox state of iron ions in basic silicate melts evidence that pressure increase (T and Fe3+/Fe2+ in the melt are constant) leads to increase of pO2 value in the melt approximately to the same magnitude as the one for QFM and MW buffer [4,5]. The main condition for reducing reactions to proceed is full melting of matter, involved in the impact process, and the attainment of very high temperatures (>1700-2000oC) that are characteristic for tektite formation at the unloading stage. In this case oxygen partial pressure (pO2) during the adiabatic decompression of the melt approaches the value of total pressure in the system (Ptot). Starting from some value of total pressure its subsequent decrease causes inescapable decrease of pO2 that accordingly leads to partial reduction of Fe3+ in the melt. It should be noted, that in this case reducing reactions run in closed system and they don’t require oxygen to move away from the system. Fig. 1 explains the decompression mechanism of Fe3+ reduction in impact melt. This figure shows the change of impact melt pO2 depending on Ptot, T
and Fe3+/Fe2+ ratio in the melt. Let the temperature of impact matter at the unloading stage under ≥ 3040 kbar is ≈2250oC and the matter is completely melted. The Fe3+/Fe2+ ratio in the melt is equal to R(mw) and corresponds to pO2 value of MW buffer. Adiabatic gradient of silicate melt is ~ 1oC/kbar because the melt adiabatic decompression can be considered to the first approximation as isothermal process. Consequently as Ptot decreases, pO2 value of the melt with Fe3+/Fe2+ = R(mw) changes accordingly to the trajectory that is similar to the one on the fig.1A (thick line). The calculated value of pO2 becomes close to Ptot, when Ptot is ~ 50 bar. However the condition pO2=Ptot is not realized in fact, because of the presence of other components besides oxygen in forming vapor phase. Total (summarizing) vapor pressure Pv = Σpi , where pi is partial pressure of every component of vapor phase including oxygen. There are no experimental and thermodynamic data on estimations of these values under given PT conditions. Therefore it is not possible to calculate exactly the Pv=Ptot equality conditions during the melt decompression, although it can be proposed that this equality is achieved under pressures somewhat higher than 50 bar. Subsequent Ptot decrease should be accompanied with pO2 decrease and consequent reducing of Fe3+ in the melt resulting in Fe3+/Fe2+ change from R(mw) to R(mw-2). The higher the temperature is the higher is Ptot when the Рtot = Pv ≈ рO2 condition is realized, and the degree of melt reduction can be more significant at final stages of adiabatic decompression. But the iron oxidation degree of impact melt with given Fe3+/Fe2+ = R(mw) is not change during decompression if its temperature is ≤ 1750 oC (fig 1B). If residual temperature at the certain stage of decompression reaches the values of full vaporization of impact melt (>2500-3000oC), the melt, that is formed in the process of condensation during subsequent decompression and cooling of the system (under Ptot < 102-103 bar), should be more reduced than initial impact melt was before its vaporization. It is possible that during the processes of full vaporization and subsequent condensation of impact melt the proposed reducing mechanism is realized the most effectively, because the reactions in vapor phase proceed essentially faster and redox state of the condensed liquid is closer to equilibrium conditions. Conclusion: Thus essential reducing of hightemperature impact melts can be expected under adiabatic decompression without resorting to the
1
Fig.1. Scheme of "decompressional” reduction of impact melt. A - The change of pO2 depending on total pressure (Ptot) for melts with various Fe3+/Fe2+ ratios under isothermic conditions (T=2250oC) is shown. R(mw), R(mw ± n) - Fe3+/Fe2+ ratios in the melt correspond to (or n orders more/less as that of) рО2 of MW – buffer. (R(mw+2)>R(mw+1)>R(mw)>R(mw-1)). Thick lines with arrows are trajectories of рО2 in the process of decompression for melt with initial Fe3+/Fe2+ = R(mw). B - The change of pO2 depending on temperature of melt with Fe3+/Fe2+ = R(mw). Numbers at curves are temperature in oC. Additional explanations see in the text. Acknowledgments: The work was financially supported by RFBR (project 02-05-64735) and Department of Earth Science RAS (project 10-7, 2003). References: [1] Fudali R.F.et al.(1987) Geochim. Cosmochim. Acta, 51. 2749-2756, [2] Feldman V.I. Petrology of impactites. M., MSU Publishing house, 1990. 299 p.(In Russ.), [3] Kadik A.A., et al. (2003) Geokhimiya, No 9 (in press). [4] Lukanin O.A. et al. (2002) Petrology, 10, No 4, 299320, [5] Kress V. and Carmichael I.S.E. (1991) Contrib. Mineral. and Petrol.,. 108. 82-92.
A PO 2=
P to t
T = 2250 oC
4
lg(PO2, bar)
1 atm
R(mw+1)
2
R(mw) R(mw-1)
0
R(mw-2) R(mw-3)
-2 -2
-1
0
1
2
lg(Ptot, bar)
3
4
B Fe3+/Fe2+ = R(mw)
4
2500
2 lg(PO2, bar)
assumption that oxygen is removed from the system together with vapor phase or selectively dissipates from the vapor. During the final stage of impact process, that is characterized by catastrophic increase of the volume of explosion cloud, scattering and cooling of the substance, the system is not closed. Quenching glasses formed under these nonequilibrium conditions keep reduced state of tektite melts that were formed mainly at previous stages of decompression. The vaporization process of impact melts at this stage is able to perform an additional contribution to reducing of Fe3+ as a result of oxygen escape together with vapor phase in upper rarefied layers of the atmosphere under very low pressures.
PO
2
=
5
t P to
2250
0
2000
-2
1750
-4
1500
-6 -2
-1
0
1
2
lg(Ptot, bar)
3
4
5
2