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Impact melt differentiation in South Pole-Aitken basin. Will Vaugha n Department of Geological Sciences, Brown University With thanks to his co-authors:. Will Vaughan and James Head (2013), Modeling impact melt differentiation in the South Pole-Aitken Basin, GRL , in review.

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impact melt differentiation in south pole aitken basin

Impact melt differentiation in South Pole-Aitken basin

Will Vaughan

Department of Geological Sciences, Brown University

With thanks to his co-authors:

Will Vaughan and James Head (2013), Modeling impact melt differentiation in the South Pole-Aitken Basin, GRL, in review.

Will Vaughan, James Head, Lionel Wilson, and Paul Hess (2013), Geology and petrology of enormous volumes of impact melt on the Moon: A case study of the Orientale basin melt sea, Icarus, 223, 749–765.



Massive melting

Igneous differentiation



Massive melting

Igneous differentiation


Historical introduction to the problem of the felsic floor of the South Pole-Aitken basin (SPA) and one proposed solution, impact melt differentiation.
  • Spectroscopic and geophysical constraints on SPA stratigraphy.
  • Modeling impact melt differentiation in SPA.
  • Implications of impact melt differentiation in SPA for geophysics, geochemistry, and sample return.
  • Upcoming LPSC presentations concerning impact melt differentiation.
Impacts melt their target rocks (Dence, 1971).
  • Impact melting is an important petrogenetic process on the Moon.Many lunar glasses and “basalts” are impact melts, not volcanic (e.g., Dowty, 1974).
  • Giant basin-forming impacts melt deep into the Moon’s mafic interior and therefore produce mafic impact melt.
  • The bigger the basin, the more mafic the impact melt.

Top left: mafic impact melt breccia 15455

Bottom left: from Ryder and Wood (1977)

Impact melt deposits constitute geologic units. Impact melt deposits are associated with small craters (Hawke and Head, 1977) as well as large basins such as Orientale (Head, 1974).
  • Conclusion integrating Apollo-era geochemical and geological analyses: very large lunar basins such as the South Pole-Aitken Basin, which melt deeply into the Moon’s mantle, should host extensive mafic deposits of impact melt in their interiors, more mafic than norite 15455.

Right: facies of the Orientale basin (Head, 1974)

Yet the floor of the South-Pole Aitken basin (which ought to have excavated 200 km into the Moon and melted even deeper) is norite, not mantle olivine pyroxenite!

So much for the

“mafic anomaly”!

15-20 wt. % Al2O3 = norite, not mantle pyroxenite!

Wu (2012), Major elements and Mg# of the Moon: Results from Chang’E-1 Interference Imaging Spectrometer (IIM) data, GCA

why is the interior of spa so felsic
Why is the interior of SPA so felsic?

(The felsic interior of SPA is probably primary. Later mixing with feldspathic basin ejecta could explain the floor’s composition, but not its thickness: Petro and Pieters (2004) estimate <1.5 km of infill, but the norite layer thickness in SPA is at least 10x greater (Wieczorek and Phillips, 1999). Norite volcanism is unknown on the Moon, much less >10 km of norite volcanism—though see Whitten et al., #2461.)

  • Anomalously shallow excavation. Perhaps the SPA-forming impact was oblique (Schultz, 1997); perhaps proportional scaling breaks down for very large basins (Wieczorek and Phillips, 1999).
  • Impact melt differentiation (Morrison, 1998).
Morrison (1998) proposed that igneous differentiation of a massive SPA impact melt sheet by crystal settling would concentrate felsic components to form a norite floor overlying ultramafic cumulates.
  • Impact melt differentiation is a mechanism by which felsic compositions could be produced from SPA impact melt with a very mafic bulk composition.

Morrison (1998), Did a Thick South Pole-Aitken Basin Melt Sheet Differentiate to Form Cumulates?, LPS, 29, #1657.

The hypothesis of impact melt differentiation in SPA has enjoyed a recent resurgence: spectroscopists have interpreted ultramafic materials exhumed from the SPA subsurface in terms of impact melt differentiation (e.g., Nakamura et al., 2009; Yamamoto et al., 2012).

But does impact melt differentiation really work? The predictions of the impact melt differentiation hypothesis need to be made quantitative and tested against the stratigraphy of the SPA interior.

We’ll 1. establish the stratigraphy of the SPA interior from spectroscopic and geophysical constraints; 2. model the cumulate stratigraphy produced by impact melt differentiation; and 3. consider the implications for the geochemistry and geophysics of SPA.

The surface of SPA is norite (Pieters et al., 2001; Uemoto et al., 2011). What lithologies are present at depth?
  • Complex craters exhume subsurface material for spectroscopic inspection.
  • Wecompiled a corpus of complex craters with previously characterized central peaks (at left).

Rim outlines from Garrick-Bethell and Zuber (2009)

(We calculate the depth from which central peak material is derived using a formula of Cintala and Grieve (1998). Central peak lithologies determined spectroscopically by Tompkins and Pieters (1999), Nakamura et al. (2009), and Yamamoto et al. (2012). Radial symmetry of subsurface strata is assumed.)
  • Olivine-bearing lithologies are present only at depth (~40 km?) in the SPA subsurface (in Zeeman and Schrödinger).
  • Orthopyroxene-bearing lithologies overlie olivine-bearing lithologies, and the proportion of orthopyroxene decreases with decreasing depth—but are deep peaks norite (Tompkins and Pieters, 1999) or pyroxenite (Nakamura et al., 2009)?
  • Clinopyroxene-bearing lithologies are probably secondary.
VNIR spectroscopy poorly constrains the depth at which noritic materials transition to pyroxenite.
  • This depth is well-constrained by gravity data: it’s simply the crustal thickness.
  • Wieczorek and Phillips (1999) find ~40 km thick norite layer in the SPA interior, inconsistent with spectroscopic results.
  • New GRAIL-derived crustal thickness models (Wieczorek et al., 2013) indicate 12.5 km of norite.

Rim outlines from Garrick-Bethell and Zuber (2009)

Crustal thickness model 1 of Wieczorek et al. (2013)

Primary stratigraphy of SPA according to spectroscopic and geophysical constraints:

12.5 km of norite overlying orthopyroxene-rich and olivine-rich lithologies.

Modeling impact melt differentiation is different from magma ocean modeling (Snyder et al., 1992): in this case, we know relatively little about cumulate layer composition but a lot more about layer thickness / position.
  • Strategy: simple phase equilibria combined with mass balance of components between initial melt volume / final cumulate layers.
  • Crust / mantle proportion of melted material controls melt composition.
  • Is there enough crustal Al2O3 in mantle-dominated (?) SPA impact melt to make 12.5 km of norite?

Top right: magma oceans in Snyder et al. (1992)

Bottom left: SPA melt sea from Morrison (1998)

Vertical impacts melt mostly mantle. Oblique impacts melt more crust.
  • Compositional evidence for an oblique SPA-forming impact.

From Pierazzo and Melosh (2000)

Excavation of melt must change crust / mantle proportion of melt and melt volume geometry.
  • Previous assumption is that excavation flow skims almost all crustal material off melt. This makes forming ~12.5 km of norite impossible. We take the excavation flow to remove no crustal material (the result of pre-excavation mixing, melt mixing from the excavation flow, or an anomalously shallow excavation flow?) as a bound.
  • Final melt sheet: ~50 km thick prism (~108 km3 of melt spread over the SPA interior of Garrick Bethell and Zuber (2009)).

From Vaughan et al. (2013)

did the spa melt sheet differentiate
Did the SPA melt sheet differentiate?
  • Melt sheet cooling timescale proportional to melt sheet thickness L2, and crystal settling timescale proportional to L. Some minimum Lcrwhere these are equal. What is this Lcr?
  • Terrestrial Lcr ~ 1 km (Manicouagan, Sudbury).
  • Scales as gravity, 1/viscosity: lunar Lcr< 10 km.
  • SPA melt sheet ~50 km thick – yes, it differentiated.

Manicouagan cross-section from Spray and Thompson (2008)

SPA melt sheet an opx- and ol-rich mixture of plag, opx, and ol. (Ilmenite minor—low Ti-mare basalts, cpx minor?)
  • Crystallization sequence: olivine; orthopyroxene; plagioclase and opx.
  • Stratigraphic sequence (all crystals sink): dunite and pyroxenite below norite.
  • Mass balance for layer proportions: 20 vol. % crust × 25 wt. % Al2O3/ 20 wt. % Al2O3× 50 km melt sheet = 12.5 km norite.
  • Modeled stratigraphy fits!
implications of impact melt differentiation in spa
Implications of impact melt differentiation in SPA
  • Geophysical. The ~12.5 km crustal thickness of the SPA interior is not the thickness of primary crust (which may well be 0 km), but of a noritic impact melt differentiate. Nonzero crustal thickness does not mean that basin-forming impacts did not totally excavate the local primary crust.
  • Geochemical. The noritic floor of SPA need not be lower crust—it can be an impact melt differentiate. This explains Th concentrations of the floor (125 ppb × 50 (for top km) × 1/2 = ~3 ppm Th, similar to 2 ppm observed)—μKREEP?
  • Sample return. SPA impact melts have the potential to date the SPA-forming impact and test the lunar cataclysm hypothesis. Well, the whole noritic floor is an impact melt differentiate—you can land MoonRise almost anywhere!
Hurwitz and Kring (#2224), Friday afternoon: “Composition and Structure of the South Pole-Aitken Basin Impact Melt Sheet”
  • Moriarty et al. (#3039), Monday morning: “NW-Central South Pole-Aitken: Compositional Diversity, Geologic Context, and Implications for Basin Evolution”
  • Nakamura et al. (#1988), Thursday evening: “Differentiation of Impact-Generated Magma Seas on the Moon as Revealed by Spectral Profiler Onboard Kaguya”
  • Wittmann et al. (#2061), Thursday afternoon: “Feldspathic Granulite Clasts in Lunar Meteorite Shişr 161 – Cumulates From a Differentiated Basin Melt Sheet?”
  • Vaughan and Head (#2012), Thursday evening: “Modeling the South Pole-Aitken Subsurface”
  • Impact melt differentiation probably took place on the Moon: the interior stratigraphy of SPA is consistent with that of a differentiated melt sheet.
  • Impact melt differentiation clarifies the geochemistry and geophysics of SPA.
  • The same goes for several lunar basins: by the >10 km criterion, Nectaris, Orientale, Serenitatis, and Imbrium in addition to SPA contain differentiated impact melt sheets. Keep this in mind looking at samples and spectra!