1.21 The Moon - Harold Connolly Jr.

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concentration of thorium (an exemplary incompatible
trace element; on the Moon such elements are
strongly concentrated into a potassium, REE- and
phosphorus-rich component known as KREEP) is
3.5 times higher on the hemisphere centered over
Oceanus Procellarum compared to the hemisphere
antipodal to Procellarum, although it should be
noted that this conclusion is based on a correction to
the initial (e.g., Lawrence et al., 1998) calibration
for the Prospector thorium data (Warren, 2003).
1.21.3 MARE VOLCANISM
1.21.3.1 Classification of Mare Rocks
The dark basalts that erupted to veneer the
Moon’s flat, low-lying “seas” (maria) during
the waning of lunar magmatism are compositionally
distinctive, even compared to other
lunar samples. Mare basalts have high
(mostly .16 wt.%) FeO, low to moderate mg
(¼MgO/[MgO þ FeO]), low to high TiO2, and
low Al 2O 3 contents (Figure 1 and Table 2).
Calling these rocks “basalt” is potentially misleading.
Most mare basalts are far more melanocratic
(mafic silicate-rich feldspar-poor) than
terrestrial basalt, and arguably more analogous
to a relatively leucocratic and low-mg komatiite.
The contrast with the rest of the Moon’s crust is
stark. The typical composition of highland crust,
as represented by lunar-meteoritic regolith breccias
and regolith averages for the Apollo 14,
Apollo 16, and Luna 20 sites (Table 3), features
,5 wt.% FeO, ,0.4 wt.% TiO2, and ,27 wt.%
Al2O3. The distinction between mare and highland
The Moon
materials is seldom controversial, but for quickand-dirty
mare versus highland classification
based exclusively on bulk-analysis data, Wood
(1975) proposed using a combination of wt.%
TiO2 and Ca/Al ratio (Figure 2).
The various subclassifications for mare basalt we
describe below may seem arbitrary but, as will be
discussed in the next section, mare basalt compositional
diversity is in general not systematic, but
haphazard. On Earth, volcanic diversity is largely a
function of systematic global tectonics, such as
upwelling at mid-ocean ridges to generate MORB.
But plate tectonics probably never occurred on the
Moon; and it surely never occurred during the
comparatively late era of mare volcanism.
Mare basalts are classified primarily on the
basis of their highly diverse bulk TiO2 contents.
Those with ,1.5 wt.% TiO2 are termed very-low-
Ti (VLT), those with 1.5 , TiO2 , 6 wt.% as
medium-Ti, and those with .6 wt.% TiO2 as high-
Ti. In pre-2001 literature (e.g., Neal and Taylor,
1992), these same classes were called “VLT,”
low-Ti, and high-Ti, respectively. The bizarre
usage of “low-Ti” for basalts with up to 6 wt.%
TiO2 was a historical accident: in the first studies
of lunar rocks from Apollo 11, the only large
igneous rocks happened to all be “high-Ti.” Le Bas
(2001) has suggested calling these three groups
titanium-poor, medium-Ti, and titanium-rich, but
judging from more recent publications (e.g.,
Hiesinger et al., 2002; Gillis et al., 2003; Jolliff
et al., 2003), this terminology has not been widely
adopted. To minimize confusion, I choose to
adopt the Le Bas “medium-Ti” replacement for
“low-Ti,” but continue using the old and logical
terms VLT and high-Ti.
Figure 1 Mare basalt major elements: mg versus TiO 2. Note that the apparent bimodality in TiO 2 may be
misleading (see text) (data are mainly from the compilation of Haskin and Warren, 1991).