Lunar Basalt - Lunar Homestead

Like the Earth, Luna is made of rocks. There are four distinct groups of rocks on Luna: 1) basaltic volcanic rocks (including pyroclastic and lava flows), 2) pristine (uncontaminated by impact mixing) non-basaltic highland rocks, 3) polymict breccias (the results of impacts) (includes impact melt rocks) that make up the bulk of the regolith and mega-regolith, and 4) and regolith fines (also incorrectly called Lunar soil) (<1 cm unconsolidated debris found in the Lunar regolith) (1, pg 184). Lunar basalt is found in groups 1, 3, and 4.
Basaltic volcanic and lava rocks are found primarily in the Lunar mares. Their darker color differentiates them from the lighter pristine highland rocks. They were formed 100-400 km below the surface, rose to the surface, and erupted (1, pg 184). The process is exactly the same as what happens on Earth.

Two types of Lunar volcanic rocks exist. They we both produced by partial melting of the Lunar mantle, creating basaltic magma:

Pyroclastic deposits (volcanic ash) – Caused by the explosive release of gases trapped in magma as it reaches the surface. These events often result in lava fountains that, due to the Lunar vacuum and low gravity, produce small glass beads.
- Pyroclastic deposits could be an important source of rare minerals and volatiles. There’s a good chance Homesteads will be located next to pyroclastic deposits just to mine them. But for SPORE purposes, these aren’t the basaltic rocks we’re looking for.
- Lunar pyroclastic deposits will eventually have their own page.
Lava flows – Magma erupting from fissures in the Lunar surface spread across many of the low-lying Lunar basins; eventually filling them up and creating the darker marias. It is thought that multiple flows occurred over time, creating multiple basaltic layers.
- This is the basalt we’re looking for.

The mineralogy of basaltic rocks is largely dependent on the chemical composition of the parent lava and how fast that lava cooled. Extremely fast cooling (lava fountains) doesn’t allow any minerals to form, resulting in pure glass. Very slow cooling causes crystallizing minerals to separate from the lava, allowing the remaining material to develop a wide range of chemical compositions. Several studies indicate that most mare basalt lava flow layers cooled at rates from 0.1℃ to 30℃ per hour and were less than 8m thick (1, pg 201).

Polymict breccias are the result of high energy impacts to the Lunar surface. A solid object hits the mare basalt, shatters and melts it, and sends it flying in all directions. The clump of rock fragments and melted glass cool on their way back down and create a breccia. The resulting breccia is composed mostly of the material that the impactor hit with a little impactor debris. Over time, and many repeated impacts, the regolith is formed.

Terminology

Basalt is hardened surface lava.
Gabbro is hardened subsurface lava.
Lava is molten surface rock is called lava.
Magma is molten subsurface rock.

The difference between basalt and granite

While both basalt and granite are igneous rocks (cooled from magma) that contain significant amounts of silicon and oxygen, there are important differences between the two. Luna’s basalt and granite are formed the same way as Earth’s and are effectively the same.

Basalt is extrusive while granite is intrusive. This simply means that basalt cooled outside the Earth’s crust and granite cooled deep within the crust.
Basalt cools much quicker that granite because it is formed outside the crust. When you see lava flowing out of a volcano you’re looking at basalt. Because it cools relatively fast, basalt is typically very fine grained as the minerals have little time to grow. Granite, on the other hand, forms easily visible large mineral grains.
Because of this, granite is harder than basalt. This is good for us because we want to break the rocks into powder so we can process them. Softer is better.
The second most important difference is that basalt is mafic, meaning it has a higher percentage of iron, titanium, magnesium, and other heavier elements than granite. Higher concentrations of metals is what we’re going after for ISRU operations.
The most important difference is that basalt is on the surface. We can easily get to it. Lunar granite has shown up is some samples but is thought to be relatively rare on the surface.

Abundance and distribution

Maximum indirect estimates have pyroclastic deposits and lava flows making up less than 1% of the Lunar crust (1, pg. 107).
Basaltic mare deposits occupy 17% of the Lunar surface (approximately 6.4 x 106 km2) (1, pg 105).
No direct measurements of the mare thickness have been made and Apollo active seismic profiling failed to yield good data (1, pg. 105).
- Electromagnetic sounding from orbit found Mare Serenitatis to be 1.6-2 km thick at the center of the Serenitatis Basin and 0.8-1.0 km thick at the peripheral shelf (1, pg 105).
- Indirect estimates are sketchy at best. The techniques require some pretty bold assumptions (ideal crater shapes and the shape of the basin before lava flooding occurred).
Pyroclastic deposits are thought to often occur on the edges of maria (1, pg 211).
Thick dark mantle deposits could also be pyroclastic deposits (1, pg 212).

Physical properties

Melting points
- Melting points (not specified so I assume at 1 standard atmosphere)]
  - 984°C to 1260°C [2]
  - 1000^oC to 1200^oC (1832^oF to 2192^oF) [3]
- Liquidus temperature (when the molten basalt begins to crystallize) at low pressure (0-1 bar). Basalts richest in FeO tend to have the lowest liquidus temperatures. (1, pg 206-207) (see Data section for more data).
  - High-Ti
    - 1095℃ – entire sample is liquid
  - High-Ti low-K
    - 1170℃ – entire sample is liquid
  - Very high-Ti
    - 1192℃ – entire sample is liquid
  - Orange glass
    - 1398℃ – entire sample is liquid
  - Low-Ti olivine
    - 1350℃ – entire sample is liquid
  - Low-Ti pigeonite
    - 11273℃ – entire sample is liquid
  - Low-Ti aluminous
    - 1285℃ – entire sample is liquid
  - Very low-Ti Luna 24
    - 1192℃ – entire sample is liquid
  - Very low-Ti Apollo 15 green glass
    - 1408℃ – entire sample is liquid
  - Very low-Ti Apollo 14 VLT glass
    - 1390℃ – entire sample is liquid
- Solidus temperature (when mare basalt lavas become totally solid upon cooling) has not been directly determined (1, pg 206).
  - The data implies that solid mare basalts begin to melt around 1050℃. This is about 100℃ higher than Earth basalts due to the lack of Lunar H2O. (1, pg 206)
  - Pressure is unspecified.
Viscosity
- 4.5 p at 1495℃ to 10 p at 1395℃. Based on measurements using a synthetic liquid with the composition of Apollo 11 high-Ti basalt. These viscosities are similar to heavy motor oil at room temperature. Earth lavas are 10x more viscous. (1, pg 193).
Texture (contributes to the rock’s physical properties)
- High-Ti mare basalts
  - Most Apollo 11 samples had intersertal textures, where pyroxene and ilmenite crystals form an open fretwork around plagioclase and glass (1, pg 195).
  - Other samples have ophitic textures (pyroxene crystals enclosed by plagioclase) (1 pg 195).
  - Some samples are vitrophyric and contain a glassy matrix with relatively large crystals embedded in it (1, pg 195).
  - Low-K, high-Ti have ophitic to subophitic textures (depends on pyroxene size). Ophitic is when the pyroxene is coarser and fully encloses individual plagioclase crystals. Subophitic is when pyroxene partially encloses the plagioglase. (1, pg 195)
- Low-Ti mare basalts
  - Show an extraordinary range of textures. Ranges from mostly glassy (vitrophyric) to coarse-grained completely crystallized (gabbroic) (1, pg 198).
  - Olivine becomes increasingly rare as grain size increases. This is the result of slow cooling reacting with the olivine to produce pyroxene (1, pg 198).
  - Most aluminous low-Ti basalts have ophitic to subophitic textures.
- Very low-Ti mare basalts
  - All know samples are small fragments and determining the texture is more difficult (1, pg 198).
  - General textures include subophitic to microporphyritic and both vitrophyric and ophitic features (1, pg 198).
Vesicles (frozen gas bubbles in the rock)
- Many samples contain numerous vesicles. These formed when gasses dissolved in the melt came out of solution. The rapidly cooling lava preserved these bubbles although the gas they contained is long gone.
- On Earth, the primary vesicle producing gasses are H2O and CO2. The lack of water in Lunar rocks means that CO2, along with CO, was probably the largest contributor to vesicle formation. (1, pg 201-202).

Chemical composition

Using remote spectral mapping we are able to determine the chemical composition of the Lunar maria without taking physical samples. Only about 1/3 of the nearside maria basalts have compositions similar to those sampled by the Apollo and Luna missions. (1, pg 210-211).
Low in silica (<54% SiO2) (1, pg 186).
Three major groups of Lunar basalt based on titanium concentrations (1, pg 186). They can be further divided based on other chemical parameters.
- High-Ti (>9% by weight TiO2) (Apollo 11 and 17) (Apollo 16 had a few fragments but they were probably the result of impacts on distant mare (1, pg 187)) .
  - High-K (>0.3% by weight K2O)
    - Apollo 11
  - Low-K (<0.1% by weight K2O)
    - Apollo 11 and 17
  - Contains a relatively higher abundance of KREE elements than L-Ti and VL-Ti (1, pg 188).
- Low-Ti (1.5 – 9% by weight TiO2) (Apollo 12, 14, and 15) (Luna 6)
  - Aluminous low-Ti – richer in Al2O3
    - Very high-K (VHK) (0.9% by weight K2O) (Apollo 14)
  - Olivine basalts
    - More MgO and less CaO, Al2O3, and TiO2 than pigeonite (1, pg 188).
  - Pigeonite basalts (low-Ca pyroxene)
  - Ilmenite basalts (Apollo 12)
- Very low-Ti (<1.5 % by weight TiO2) (Apollo 17) (Luna 24)
Remote-sensed spectral data suggests that there is a continuous gradation from VL-Ti to H-Ti (1, pg 186).
Differs from Earth basalts.
- Lunar basalts are generally higher in titanium and chromium and have a higher iron to magnesium ratio (1, pg 192).
- Sulfur is more abundant (1, pg 192).
- Aluminum, sodium, and potassium are lower (1, pg 192).
- Depleted in volatile elements (K, Na, Rb, Pb, C, H, etc.) (1, pg 192).
- Depleted in siderophile elements (these tend to concentrate iron) (Ni, Co, Au, Ir, etc.) (1, pg 192).
- Almost always contain metallic iron (very rare for Earth basalts). Most metallic iron was formed from the reduction of FeO during the crystallization of the basalts. (1, pg 193)
- Lunar basalts are composed of four major minerals: (IMAGE 1, pg 188)
  - Pyroxene
  - Plagioclase
  - Olivine
  - Metal oxides (abundance varies directly with TiO2 content)
    - Ilmenite
    - Armalcolite
    - Spinel

Data

Liquidus temperature (when the molten basalt begins to crystallize) at low pressure (0-1 bar) (1, pg 206-207)

Major element concentrations in representative Lunar mare basalt samples (% by weight) (1, pg 261)

Trace element concentrations in representative Lunar mare basalt samples (µg/g) (1, pg 262)

Trace element concentrations in representative Lunar mare basalt samples – continued (µg/g) (1, pg 263)

Resources

Lunar Sourcebook: A user’s guide to the moon. Grant H. Heiken, David T. Vaniman, Bevan M. French. 1991. [www.lpi.usra.edu/publications/books/lunar_sourcebook/]. Last checked Feb 25, 2019.
American Mineralogist (www.minsocam.org/msa/collectors_corner/arc/tempmagmas.htm)
National Geographic Encyclopedia – Magma (www.nationalgeographic.org/encyclopedia/magma/)

Relevant literature (but not used)

https://curator.jsc.nasa.gov/lunar/letss/mare3.pdf