Apollo 11 basalt suites (high-K and low-K)
The crystalline mare basalts returned from Tranquility Base are not a single rock type. As the per-sample Lunar Sample Compendium makes explicit, they fall into two chemically and chronologically distinct suites — both high-titanium ilmenite basalts, but with different potassium contents and different ages:
- High-K ilmenite basalts — higher in potassium, rubidium, and the rare-earth elements. They are the younger suite, with crystallization ages clustering around ~3.55–3.63 billion years (e.g., 10017 at 3.59 Gyr, 10057 at 3.63 Gyr, plus 10022, 10024, 10049, 10069, 10071).
- Low-K ilmenite basalts — lower in potassium and incompatible trace elements, and older, with ages around ~3.7–3.85 billion years (e.g., 10003, 10020 at 3.77 Gyr, 10044, 10058, 10062, 10045, 10047, 10050, 10029, 10092).
The age gap is the key point: the high-K and low-K basalts are separate batches of lava, erupted perhaps ~100–200 million years apart, not two products of a single cooling magma. They cannot be related by simple crystal fractionation — their trace- element patterns require different source regions or melting histories. The Compendium reads it the same way: because Apollo 11 sampled only a small area, it appears to have tapped just two lava flows, the two told apart by potassium (and slightly higher REE) and even correlating with cosmic-ray exposure age (10057). A few small fragments resist tidy assignment (e.g., the vitrophyre 10031, a rapidly quenched glassy basalt, and 10032). All share the type A/B high-titanium character — ilmenite-rich, refractory-enriched, alkali- and volatile-poor — that distinguishes them from terrestrial basalts.
This two-suite structure refined the first-look picture: the 1969 Preliminary Science Report reported a single ~3.0 ± 0.7 Gyr age for “the” basalt, but later high-precision dating compiled in the Compendium resolved the older, tighter ~3.6–3.85 Gyr ages and the high-K / low-K split.
Measured ages
Section titled “Measured ages”High-precision dating in the Compendium bears out the two-suite picture (representative crystallization ages):
| Suite | Representative crystallization ages (Gyr) |
|---|---|
| High-K | 10017 3.59 · 10022 3.58 · 10057 3.63 · 10032 3.58 |
| Low-K | 10044 3.71 · 10047 3.72 · 10020 3.77 · 10062 3.83 · 10003 3.84–3.91 |
Separately, cosmic-ray exposure ages — how long each rock sat within ~1 m of the surface before collection — vary widely (10044 ~80 Myr, 10003 137 Myr, 10072 235 Myr, 10017 480 Myr), showing the rocks were excavated by different impacts at different times rather than all at once. The Compendium reads the ~80–90 Myr ages of 10044 and 10062 as possibly dating West crater itself — the impact that excavated the boulder field Armstrong avoided.
Within the low-K suite, Beaty & Albee judged 10044, 10047, and 10058 “so similar to one another that it seems quite likely that these rocks are fragments of a larger block” — and the container record quietly supports them: all three were returned in the bulk box, gathered from one ~14-minute scoop zone near the MESA.
Bulk chemistry
Section titled “Bulk chemistry”The split shows in the rocks’ chemistry too — it is potassium that names the suites. Representative bulk analyses from the Compendium:
| Suite | K₂O (wt%) | TiO₂ (wt%) | FeO (wt%) |
|---|---|---|---|
| High-K | ~0.21–0.32 (10017 0.22 · 10057 0.32 · 10022 0.21) | ~10–11 | ~19–21 |
| Low-K | ~0.05–0.11 (10003 0.06 · 10020 0.06 · 10044 0.11) | ~9–12 | ~18–20 |
The roughly threefold K₂O contrast (carried along with rubidium and the rare-earth elements) is the defining difference; otherwise the two suites are alike — strongly high-titanium (~8–12 wt% TiO₂, several times terrestrial values) and iron-rich (~17–21 wt% FeO).
Related
Section titled “Related”- Apollo 11 sample inventory
- High-titanium mare basalt
- Apollo 11 lunar sample types
- Minerals first identified in Apollo 11 samples
- Surface exposure and space weathering (Apollo 11 samples)