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| Titel |
A sulfide-saturated lunar mantle? |
| VerfasserIn |
James M. Brenan, James E. Mungall |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143139
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| Publikation (Nr.) |
 EGU/EGU2017-6838.pdf |
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| Zusammenfassung |
| Although much work has been done to understand the controls on the sulfur content at sulfide
saturation (SCSS) for terrestrial melt compositions, little information exists to evaluate the
SCSS for the high FeO compositions typical of lunar magmas, and at the reduced conditions
of the Moon’s interior. Experiments were done to measure the SCSS for a model
low Ti mare basalt with 20 wt% FeO at 1400oC as a function of fO2 and pressure.
Synthetic lunar basalt was encapsulated along with stoichiometric FeS in capsules made
from Fe-Ir alloy. The fO2 of the experiment can be estimated by the heterogeneous
equilibrium:
Femetal + 1 ∕2 O2 = FeOsilicate
Variation in the metal composition, by addition of Ir, serves to change the fO2 of the
experiment. Capsule compositions spanning the range Fe25Ir75 to Fe96Ir4 (at%) were
synthesized by sintering of pressed powders under reducing conditions. Fe100 capsules were
fabricated from pure Fe rod. For a melt with 20 wt% FeO, this range in capsule
composition spans the fO2 interval of ∼IW-1 (Fe100, Fe96Ir4) to IW+2.2 (Fe25Ir75).
Experiments were done over the pressure interval of 0.1 MPa to 2 GPa. Results for
experiments involving Fe100capsules indicate that the SCSS decreases from ∼2000 ppm
(0.1 MPa) to 700 ppm (2 GPa). Experiments done thus far at 1 GPa, involving the
range of capsule compositions indicated, show a marked decrease in SCSS as the Fe
content of the capsule increases (fO2 decreases). Complementary to the decrease
in SCSS is a drop in the sulfur content of the coexisting sulfide melt, from ∼50
at% at ΔIW = +2.2 to ∼20 at% at ΔIW-1. In fact, both the composition of the
sulfide melt and the SCSS are essentially indistinguishable for Fe96Ir4 and Fe100
compositions. Results thus far indicate that at reduced conditions and high pressure, the
SCSS for high FeO lunar compositions is low, and overlaps with Apollo 11 melt
inclusion data. Importantly, such low SCSS does not require Fe metal saturation, and
suggests that some lunar source regions could be saturated in a low sulfur, sulfide
melt. Additional sulfide-silicate partitioning experiments for the PGE and Re have
also been done at Fe-metal saturation at 1400oC, 0.1 MPa using chromite capsules
sealed in silica ampoules. Results confirm that the highly siderophile elements
(HSE) will be strongly sequestered by residual sulfide, and that the concentrations of
these elements will be strongly depleted in lunar basalts. Hence, estimates of the
HSE content of the lunar mantle from basalt compositions must take into account
the fractionation imposed by sulfide-silicate partitioning at reduced conditions. |
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