Impossible Lunar Magma Chambers

One of my very first science microblogs from the Lunar and Planetary Science Conference  last month was about a talk given by Katelyn Lehman, an undergraduate student, suggesting the existence of lunar magma chambers. At the time, I was skeptical, because current thinking is that these can't occur on the Moon. But, new work is suggesting that they can, and do.  So, as promised, here is a full discussion on the topic.

On the Earth, hot magma often stalls in shallow rocks rather than erupting at the surface. There it forms a magma chamber, where it sits and cools.

But such magma chambers are thought to be unlikely on the Moon. The density of the lunar crust is very low, so hot magma is not buoyantly stable within this crust. Just like water under ice, the neutral buoyancy zone of magma is just below the lunar crust (which is about 40 kilometers thick). So, that is where the magma stays and collects, deep in the lunar interior.

The magma that makes basaltic rocks is less dense than the lunar crust. On the Moon, therefore, magma stalls just below the crust. As the Moon cools, the mantle below the crust becomes more solid, and magma stalls even further below the crust.
Image credit: Head and Wilson, 1992

If a lot of magma collects in one place below the crust, it can build up enough pressure to fracture the overlying rocks and rise up through the crust in thin vertical sheets, called dykes.  If the pressure of the magma is high enough, it will make it to the surface and extrude as lava, forming dark, smooth mare flows. But, if the pressure is too low, the magma will not make it to the surface. It will freeze in the dykes, or if the dyke is very thick, the dense magma could sink back down below the less dense crust. This is why magma chambers are not expected to occur in the Moon's crust. The physical properties of the magma and crust don't support them.

However, recent research is suggesting that some magma chambers may be found on the Moon.  Walter Kiefer, a colleague of Katelyn Lehman, has used high resolution gravity data to show that very dense material must be located not too far below the surface in the Marius Hills region of the Moon.  This material is too dense to represent the crust of the Moon, so it must be something else.

Gravity data has been used since the Apollo era to show that large concentrations of massive, dense material (called mascons) are located at depth on the Moon. But mascons are very large features, associated with big basins, and so assumed to represent thinning of the crust due to huge impacts. The new gravity features are much smaller and shallower, and so cannot be related to crustal-scale processes. They must, therefore, represent shallow magma chambers, where basalt magma intruded into the Moon's crust.

Very high resolution gravity data of the Marius Hills region of the Moon shows that there are two small positive gravity anomalies located in this region.  These are thought to represent relatively shallow magma chambers,  which are quite rare on the Moon.
Image credit: Kiefer, 2012

Backing up this interpretation, Katelyn Lehman's research has shown that the Marius Hills region of the Moon contains a number of regions that are rich in the mineral plagioclase. Plagioclase-rich materials are not expected in a mare area like the Marius Hills, where thick basalt lavas mean that the crust is not likely to be exposed, even by impact events.  Katelyn and her team, therefore, interpret these plagioclase areas to represent a different type of volcanism, one that would have resulted when magmas evolved after sitting in a shallow magma chamber for some time.

So, while the physics of magma and lunar crust means that magma chambers are not likely to occur on the Moon, it appears that it is not impossible. And, at least in the Marius Hills region, one of these rare lunar magma chambers may have in fact been formed.

Sources:
Head and Wilson, 1992. Lunar mare volcanism: Stratigraphy, eruption conditions, and the evolution of secondary crusts, Geochim. et Cosmochim. Acta, V56, 2155-2175. DOI: 10.1016/0016-7037(92)90183-J.

Kiefer, 2012. Gravity constraints on the subsurface structure of the Marius Hills: The magmatic plumbing of the largest lunar volcanic dome complex, J. Geophys. Research - Planets, DOI: 10.1029/2012JE004111.

Lehman, et al., 2013, Composition analysis of the Marius Hills volcanic complex using Diviner Lunar Radiometer Experiment and the Moon Mineralogy Mapper, LPSC 44, Abstract #1225.