Lunar mare basalts

Samarium-neodymium evolution diagram for lunar mare basalt 75075. Data points for the total rock, plagiodase, ilmenite, and pyroxene form a precise linear array, the slope of which gives a crystallization age of 3.70 0.07 Ga for this rock (7= 6.54 x 10 12 yr 1). The insert shows the deviations from the best-fit line in parts in 104. After Lugmair et al. (1975a). 

Lunar mare basalts from the various Apollo and Luna missions are classified by their titanium and potassium contents. After Lucey et al. (2006). 

Comparison of titanium contents of lunar mare basalts, as measured from neutrons by the Lunar Prospector gamma-ray spectrometer and from reflectance spectra by the Clementine spacecraft. After Prettyman et al. (2006). 

The elemental abundance of the lunar mare rocks as compared to that of carbonaceous chondrites vary up to 6 orders of magnitude (Fig. 3a). The strongly siderophile elements and the very volatile elements are highly depleted, while the refractory elements Al, Ca, Ti, REE, Th, U. etc. are enriched. Hence, it is rather difficult to explain the fractionation of the lunar mare basalts by... 

The basaltic achondrites (eucrites as well as howardites) show the greatest similarity to the lunar mare basalts (Fig. 4). We will come back to this relation later on. 

Hiesinger H., Head J. W., Wolf U., and Neukum G. (2000) Lunar mare basalts in oceanus procellarum initial results on age and composition. In Lunar Planet. Sci. XXXI, 1278. The Lunar and Planetary Institute, Houston (CD-ROM). 

Horz L. (1978) How thick are lunar mare basalts Proc. 9th Lunar Planet. Sci. Conf, 3311—3331. 

Unruh D. M., Stille P., Patchett P. J., and Tatsumoto M. (1984) Lu-Hf and Sm-Nd evolution in lunar mare basalts. Proc. 14th Lunar Planet. Sci. Conf, B459-B477. 

Zhong S., Parmentier E. M., and Zuber M. T. (2000) A dynamic origin for the global asymmetry of lunar mare basalts. Earth Planet. Sci. Lett. 177, 131-140. 

Rare earth element abundances [in ppm (wt)] in low-Ti lunar mare basalts. (Adapted from Taylor 1982.)... 

Typical Pyroxenes Komatiite Alkali Olivine Basalt MORB Andesite Lunar Mare Basalt Eucrite Shergottite... 

Ring wood, A. E., Some aspects of the minor element chemistry of lunar mare basalts. The Moon. 12, 127-157, 1975. 

Lunar Meteorites Highlands breccias Mare basalts... 

Internal 87Rb-87Sr isochrons were particularly useful in unraveling the history of the Moon and Mars. As an example, consider mare basalts from the Sea of Tranquility collected by Apollo 11. These lunar basalts consist primarily of calcium-rich (anorthitic) plagioclase and low-calcium pyroxene, along with ilmenite, crystobalite, and other minor phases. Strontium is concentrated in the calcium sites in plagioclase and is excluded from most other phases, resulting in a variation of a factor of several hundred in the strontium... 

Lunar surface materials (Apollo and Luna returned samples and lunar meteorites) are classified into three geochemical end members - anorthosite, mare basalt, and KREEP. These components are clearly associated with the various geochemically mapped terrains of different age on the lunar surface. The composition of the lunar interior is inferred from the geochemical characteristics of basalts that formed by mantle melting, and geochemistry provides constraints on the Moon s impact origin and differentiation via a magma ocean. 

A gross subdivision of the Moon s surface is based on albedo. The bright regions are the highlands, which are clearly ancient because of their high densities of impact craters. The dark regions are younger maria, basaltic lavas that flooded the impact basins on the nearside of the Moon. Mare basalts are exposed over 17% of the lunar surface and probably comprise only 1% of the crustal volume. 

Mare basalts include lavas that erupted from fissures and pyroclastic deposits that produced glass beads. Six of the nine missions to the Moon that returned samples included basalts. The mare basalts from different sites have distinctive compositions and are classified based on their Ti02 contents, and to a lesser extent on their potassium contents (Fig. 13.3). A further subdivision is sometimes made, based on A1203 contents. Mare basalts are compositionally more diverse than their terrestrial counterparts. Volcanic glass beads, formed by fire fountains of hot lava erupting into the lunar vacuum, were found at several Apollo sites and eventually were shown to be a constituent of virtually every lunar soil. The glasses are ultramafic in composition and formed at very high temperatures. 

Lunar meteorites (see review by Korotev et al., 2003) are mostly brecciated samples of highlands crust (FAN) and regolith, although a few mare basalts are included in this collection. It is likely that the source craters for the meteorites are randomly distributed and thus include materials from the lunar farside. As we will see, these meteorites provide a better estimate of the crustal composition than do the geographically biased samples returned by spacecraft. 

Samples returned by the Apollo and Luna missions can be readily distinguished based on their contents of FeO and thorium. This may seem like an unlikely choice of chemical components for classification, but they nicely discriminate rock types and are easily measured by remote sensing. The FeO and thorium contents of ferroan anorthosites, mare basalts, impact melt breccias, and lunar meteorites are shown by various symbols in Figure 13.4. 

Analyses of thorium and FeO in lunar rocks and lunar meteorites can be described by three compositional end members - ferroan anorthosite, KREEP, and mare basalt. The various lunar terranes defined by orbital measurements of Th and FeO, illustrated by shaded and hatched fields, can also be explained by mixtures of these components. Terrane abbreviations are PKT (Procellarum KREEP Terrane) FHT (Feldspathic Highlands Terrane) SPA (South Pole-Aiken Terrane). Modified from Jolliff etal. (2000). 

The same ferroan anorthosite-mare basalt-KREEP components also define the compositions of lunar soils. The soils from each site contain different proportions of these end members. For example, Apollo 12 soils are mixtures of mare basalt and KREEP, whereas Apollo 15 soils contain all three components. 

As noted earlier, lunar meteorites are mostly breccias of ferroan anorthosite and related early crustal rocks, although a few mare basalt meteorites are known. The lunar meteorites likely sample the whole Moon. The absence of KREEP-rich breccias so common among Apollo samples collected from the nearside in the lunar meteorite collection implies that KREEP-rich rocks cover only a small area on the Moon. In fact, the lunar highlands meteorites appear to provide a closer match to the average lunar crust than do the Apollo highlands samples (Fig. 13.5), as measured by geochemical mapping (see below). 

The Feldspathic Highlands Terrane (FHT) is characterized by its relatively low FeO and Th contents (Fig. 13.4), which are consistent with ferroan anorthosite. The FHT is the most extensive terrane and is concentrated on the lunar farside (Fig. 13.7). The Procellarum KREEP Terrane (PKT) occupies a large oval-shaped portion (Oceanus Procellarum) of the nearside (Fig. 13.7). The PKT has both light and dark areas, corresponding to highlands (non-mare) rocks and mare basalts. All of these materials... 

Ryder, G. (1991) Lunar ferroan anorthosites and mare basalt sources the mixed connection. Journal of Geophysical Research, 118, 2065-2068. 

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