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| Titel |
Lunar Orientale Basin Melt Lake: Depth and Differentiation |
| VerfasserIn |
W. M. Vaughan, J. W. Head, P. C. Hess, L. Wilson, G. A. Neumann, D. E. Smith, M. T. Zuber |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069156
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| Zusammenfassung |
| Impact melt emplacement and evolution in lunar multi-ring basins is poorly understood since
impact melt deposits in basins are generally buried by mare basalt fill and obscured by
subsequent impact cratering. The relatively young Orientale basin, which is only
partially flooded with mare basalt, opens a rare window into basin-scale impact
melts.
Depth of the Orientale melt lake. The smooth inner plains facies that fills the ~350 km
diameter central depression of the Orientale basin has been interpreted as a pure impact melt
sheet. Recent Lunar Orbiter Laser Altimeter (LOLA) altimetry reveals that the ~1.75 km
average vertical subsidence of the central depression concentrates almost entirely along ~25
km of marginal normal faults. This abrupt subsidence is not predicted by models that relate
subsidence to thermal stresses resulting from impact-generated heat and uplift of crustal
isotherms. However, since the fractured surface of the smooth facies suggests that it has
undergone lateral shrinkage upon solidification and cooling, the vertical subsidence of
the central depression could similarly result from solidification and cooling of the
impact melt sheet. This end-member assumption constrains the depth of the Orientale
melt sheet: a body of hot magma emplaced on the lunar surface should undergo
~11-14% vertical subsidence upon solidification and cooling; ~1.75 km average
vertical subsidence is observed, implying the melt sheet may be up to ~12.5-16 km
deep.
Differentiation of the Orientale melt lake. The Orientale melt sheet (which,
volumetrically, may be better described as a lake) is ~350 km in diameter and may be up to
~12.5-16 km deep, implying a volume of ~106 km3, far greater than the largest
differentiated igneous intrusions known on Earth. Could the Orientale melt sheet have
differentiated? Previous work has argued that impact melt sheets do not differentiate since 1)
few or no differentiated impact melt sheets are known on Earth; 2) impact “melt” sheets are
better described as magmas carrying cold clasts, assimilation of which rapidly depresses
liquid temperature. However, mounting evidence suggests that several large terrestrial impact
melt sheets have differentiated (including the Sudbury Igneous Complex, Manicouagan, and
Norokweng), and the volume of shock melt produced by an Orientale-size impact is so
enormous that huge volumes of melt uncontaminated by clasts during crater excavation
and modification seem likely to exist. We develop a simple model to predict the
lithologies that might crystallize from the Orientale melt lake and other solidified
multi-ring basin impact lakes based on 1) the bulk composition of the melt lake, 2) the
operation of melt mixing in the melt lake, and 3) the chemical evolution of the
resulting liquids on the An-Fo-Qz ternary. We investigate whether the resulting
differentiates could explain puzzling lithologies observed in remotely-sensed data
and the lunar sample suite. Young anorthosites could have crystallized from melt
sheets. Mg-suite norites and troctolites could form in melt sheets, although their
distinctive geochemical signature is hard to reproduce in this context. Mg-spinel
lithologies could form from mixing of anorthosite and olivine-rich mantle liquids. |
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