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Lunar Magmatic Processes

Assimilation in the lunar crust: Assimilation is a natural consequence of melt-rock reaction in a multicomponent system. It is a complicated fluid dynamic process involving melting, dissolution, precipitation as well as crystallization. Figures 1a and 1b illustrate the essential features of melt-rock reaction in the lunar anorthositic crust. As hot olivine-bearing picritic magma (P in Figs. 1a and 1b) travels through the lunar crust, via a large dike, say, it losses heat to the surrounding wallrock, resulting in wallrock melting. Addition of anorthite-rich components to the reacting magma results in spinel crystallization near the melt-rock interface (blue region in Fig. 1a). Addition of anorthite-rich components to the reacting magma also lowers olivine liquidus temperature, which suppresses olivine crystallization and dissolves existing olivine in the picritic magma (Fig. 1b), provided the heat flux brought by the melt is sufficiently large to maintain the temperature above the olivine-anorthite cotectic at x = b in Fig. 1a. Hence an olivine-free thermo-chemical boundary layer d m is developed on the melt-side of the melt-rock interface and a thermal boundary d a on the rock-side of the interface (Fig. 1a). The thickness of the thermo-chemical boundary layer d m depends on conduit geometry, vigor and styles of convection in the picritic melts.

Figure 2 shows an experimental example of anorthosite dissolution in an Apollo 15 picritic yellow glass. At 1250°C and 0.6 GPa, olivine is on the liquidus of the dissolving picritic melt. Dissolution of anorthosite produced a spinel-bearing melt layer and a crystal-free melt layer, similar to that predicted by the simple Fo-An phase diagram (Figs. 1a and 1b). Dissolution or melting of anorthosite primarily adds CaO and Al2 O3 to the picritic melts, diluting the abundance of MgO, FeO and other elements not contained within anorthosite while leaving the Mg# of the melt largely unchanged. A potentially important implication of our studies is that dissolution of anorthosite may not induce as significant amount of olivine crystallization as previously thought. We believe that this (thermo-) chemical boundary layer is likely to play an important role in the lunar crustal assimilation processes.

Melt-Rock Reaction in the Lunar Mantle: The lunar mantle probably consists of mixtures of cumulate dunite, harzburgite, and lherzolite after the lunar mantle overturn. Once segregatedfrom their source regions, the picritic magmas and mare basalts are likely to be out of thermal and chemical equilibrium with their surrounding mantle at shallow depth. Hence melt-rock reaction is inevitable during the transport of these melts through the lunar mantle. The mechanisms of melt transport in the lunar mantle depend on the thermal state of the mantle. Figure 3 is a cartoon illustrating one possible scenario of melt transport and melt-rock reaction in the lunar mantle. Picritic magmas are generated at some depth in the moon, greater than depths of multisaturation (Fig. 3a). As they rise, melts will remain saturated with olivine and orthopyroxene until a dunite channel can form. Melts rising in the interiors of dunite channels have minimal interaction with the wall rock, and may retain a high pressure chemical signature (Fig. 3b). At shallower levels, when the mantle is cold and brittle, fractures may form allowing the melt to rise at very high velocities (Fig. 3c). This figure is modified from Beck, Morgan, Liang, and Hess (2006). Currently, we are investigating the consequences of melt transport and melt-rock reaction in the lunar mantle.

Related References:
Beck, A. R., Morgan, Z. T., and Liang, Y., and Hess, P. C. (2006) Dunite channels as viable pathways for mare basalt transport in the deep lunar mantle. Geophys. Res. Lett. 33, doi: 10.1029/ 2005GL024008.

Morgan, Z. T., Liang, Y., and Hess, P. C. (2006) An experimental study of the kinetics of anorthosite dissolution in lunar picritic magmas with applications to lunar crustal assimilation processes. Geochim. Cosmochim. Acta 70, 3477-3491.

Thacker, C., Liang, Y., Peng, Q., and Hess, P. C. (2009) The stability and major element partitioning of ilmenite and armalcolite during lunar cumulate mantle overturn. Geochim. Cosmochim. Acta 73, 820-836 (doi: 10.1016/j.gca.2008.10.038).

Zhang, N., Parmentier, E. M., and Liang, Y. (2012) Instability and distribution of ilmenite-rich cumulates after the overturn of an initially stratified lunar mantle. Lunar and Planetary Science Conference, 43th, #2641.

Dygert, N., Liang, Y., and Hess, P. C. (2012) The effect of melt TiO2 on Fe-Ti oxide-picritic basalt HFSE partitioning: Parameterized models, lunar applications. Lunar and Planetary Science Conference, 43th, #2033.

Yao, L. and Liang, Y. (2012) An experimental study of the solidus of a hybrid lunar cumulate mantle: Implications for the temperature at the core-mantle boundary of the moon. Lunar and Planetary Science Conference, 43th, #2258.

Sun, C. and Liang, Y. (2012) Trace element partitioning between low-Calcium pyroxene and lunar picritic glass melts at multiple-saturation points with applications to melting and melt migration in a heterogeneous lunar cumulate mantle. Lunar and Planetary Science Conference, 43th, #1952.

Brown faculty collaborators:

Paul Hess
Marc Parmentier

Other project collaborators:

Nick Dygert, Michelle Graf, Chengguang Sun, Lijing Yao

Figure 1. Thermo-kinetics of lunar picritic magma-anorthosite reaction. This figure is modified from Liang, Morgan, and Hess (2005).

Figure 2. BSE image of part of an anorthosite-Apollo 15 yellow glass dissolution run conducted at 1250°C and 0.6 GPa for 0.5 hrs. The anorthosite is on the top and yellow glass is on the bottom of the dissolution couple. The offsets or steps near the top of this image mark the anorthosite-melt interface at the onset of dissolution run. Large olivine crystals can be easily distinguished from the fine quench growth matrix in the lower portion of this image. Inset shows the spinel + melt layer below the dissolving anorthosite. This figure is modified from Morgan, Liang, and Hess (2006).

Figure 3. Cartoon illustrating the melt transport mechanism and melt-rock reaction in the lunar mantle. This figure is from Beck et al. (2006).

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