High-titanium mare basalt
The crystalline rocks returned from Tranquility Base (sample types A and B) are basalts that crystallized from lavas or near-surface melts, as shown by their mineral assemblages, crystal sizes, and gas cavities. Mineralogically and chemically they resemble terrestrial basalts but are distinguished by an exceptionally high content of opaque minerals, chiefly ilmenite, reflecting unusually high iron and titanium — hence “high-titanium mare basalt.” Terrestrial rock with 5–10% titanium would be considered an ore; these lunar basalts reach such levels, and are also enriched in zirconium, yttrium, and chromium. Bulk analyses bear this out: TiO₂ runs about 8–12 wt% (several times the ~1–4% of most terrestrial basalts) and FeO about 17–21 wt%, the iron and titanium carried largely by ilmenite. Texturally they range from fine-grained vesicular lava (gas bubbles up to a few mm, e.g. 10022, 10072) to coarser ophitic-to-subophitic rock with late-stage crystal-lined vugs — a gas-bearing magma that crystallized at or near the surface.
They are markedly depleted in alkalis and volatiles (sodium, potassium, rubidium, lead, bismuth) and contain very little water — the absence of hydrous minerals and the presence of free iron place a low limit on water in the parent melt.
Radiometric dating reported in the Preliminary Science Report gives a crystallization age of ~3.0 ± 0.7 × 10⁹ years (from radiogenic ⁴⁰Ar with potassium abundances) — older than had been expected for the maria — while cosmic-ray-exposure ages of ~20–160 × 10⁶ years date how long the rocks sat near the surface. The outer layers also hold solar-wind rare gases largely absent from rock interiors.
The Mission Report (§11.2) reaches the same picture from the first sample examination: the crystalline rocks date to 3–4 billion years by ⁴⁰K/⁴⁰Ar, are notably enriched in titanium and zirconium, and are depleted in alkalis and volatile elements.
Later high-precision dating compiled in the Lunar Sample Compendium splits these basalts into two suites — younger high-K (~3.55–3.6 Gyr) and older low-K (~3.7–3.85 Gyr) ilmenite basalts — distinct lavas rather than one cooling melt (see Apollo 11 basalt suites).
Related
Section titled “Related”- Apollo 11 lunar sample types
- Tranquility Base (Landing Site 2)
- Lunar sample collection and containers
- Solar Wind Composition experiment (S‑080)
- Apollo 11 basalt suites (high-K and low-K)
- Minerals first identified in Apollo 11 samples
Sources
Section titled “Sources”- Apollo 11 Preliminary Science Report
- Apollo 11 Lunar Sample Information Catalogue
- Apollo 11 Mission Report (MSC-00171)
- Apollo 11 Lunar Sample Compendium