Geology Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 4.20. (a) Vesicular mare basalt collected by the Apollo 15 astronauts at the Hadley Rille site (sample 15016, about 15 cm wide;
NASA S71
-
46632). (b) A view of the
genesis rock,
a sample of anorthosite collected at the Apollo 15 site (sample 15415; NASA 571
-
42951).
(c) A scanning electron micrograph of a composite volcanic orange glass droplet from Apollo 17 (from McKay et al., 1991 ). (d) Lunar
breccia from the Apollo 16 landing site (sample 67015; NASA 72
-
37216).
an igneous rock composed of plagioclase feldspar. Similar
occurrences were found at highland sites, such as Apollo 14.
Extension of these ground-truth sites to the global
Clementine maps shows that most of the highlands on the
near side and far side are composed of anorthosite. Dated at
4.2 Ga, anorthosite represents much of the ancient lunar crust
and is considered to have formed from the cooling of a
massive magma ocean that covered the Moon to a depth
of more than 100 km early in its history. In this model, the
lower-density feldspar crystals rose toward the surface of the
ocean to form a crust, while the denser olivine and pyroxene
crystals sank to a lower level and served as the source for
later-stage eruptions of basalt.
One of the discoveries in the lunar samples is the wide-
spread occurrence of a chemical group of incompatible
elements, termed KREEP (K for potassium, REE for rare-
earth elements, and P for phosphorus). The age of KREEP
is uniformly found to be 4.35 Ga and is considered to
re ect the crystallization of the lunar magma ocean.
In addition to mare basalts and anorthositic crustal
materials, dark mantle deposits ( Figs. 2.8 and 4.21 ) are
seen in many areas of the Moon, including the margins of
the Serenitatis basin and the Apollo 17 landing site. Apollo
17 samples collected by geologist-astronaut Harrison
Jack Schmitt revealed that this material consists of
orange glass beads that are coated with volatile elements
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