Volatile elements basalts

Plots of uranium versus lanthanum (two refractory elements), and potassium versus lanthanum (a volatile element and a refractory element) for terrestrial and lunar basalts, HED achondrites (Vesta), and Martian meteorites. All three elements are incompatible elements and thus fractionate together, so their ratios remain constant. However, ratios of incompatible elements with different volatilities ( /La) reveal different degrees of volatile element depletion in differentiated bodies. After Wanke and Dreibus (1988). 

Trace element measurements in lunar basalts also indicate that the Moon is depleted in highly volatile elements (Taylor et al., 2006a). Estimates of some of the Moon s volatile element concentrations are compared with the Earth in Figure 13.11 a. The absence of water in lunar basalts suggests that the mantle is dry. The Moon may also be enriched in refractory elements (Fig. 13.11b). Volatile element depletion and refractory element enrichment are expected consequences of the giant impact origin and subsequent high-temperature accretion of the Moon. 

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 angrites are basaltic in composition, and critically silica undersaturated—all have normative olivine, Ca2Si04, and nepheline and lack orthopyroxene (Mittlefehldt et al., 2002). Angrites are also characterized by extreme depletions in moderately volatile elements (Figure 1). They have very low abundances of all the alkali elements, and have Ga/Al ratios lower than for any other achondrite, lunar sample, or martian meteorite... 

Wolf R., Woodrow A., and Anders E. (1979) Lunar basalts and pristine highland rocks comparison of siderophile and volatile elements. Proc. 10th Lunar Planet. Sci. Conf, 2107-2130. 

Returned lunar basalt samples presented a few surprises when their chemical compositions were first determined, for their chemistry turned out to be rather different from their terrestrial counterparts. Particular chemical features of lunar basalts are their strong depletion in highly volatile elements (Fig. 2.11), and modest depletion of moderately volatile elements, relative to the Earth s mantle. 

Saal et ah, 2008 [286] discussed volatile content of lunar volcanic glasses and the presence of water in the Moon s interior. Their conclusion from measurements of contents of the most primitive basalts in the Moon—the lunar volcanic glasses— is that water (and other volatiles like CO2) must be considered in models constraining the Moon s formation and its thermal and chemical evolution. Chaudisson, 2008 [65] also deduced from analyzes of lunar volcanic glasses that they are rich in volatile elements and water. If the moon was formed by a huge impact on the early earth by a planet sized object this would have also consequences for models of early Earth formation. 

In most respects, asteroid 4 Vesta is geochemically similar to the Moon. As judged from howardite-eucrite-diogenite (HED) meteorites (see Chapter 6), Vesta is an ancient, basalt-covered world (Keil, 2002). Its rocks are highly reduced, and its depletions in volatile and siderophile element abundances resemble those of lunar basalts. And like the Moon, Vesta is hypothesized to have had an early magma ocean. The exploration of Vesta is now in progress, and within a few years we may have enough data to discuss it in a similar way that we have considered the Moon. 

REE patterns are fractionated for all the rocks, but tholeiites show lower La/Yb ratios than alkaline products (Fig. 8.5a). Incompatible element patterns normalised to primordial mantle compositions for mafic rocks are very different from the Aeolian arc and central-southern Italian peninsula. Both tholeiitic and alkaline basalts show a marked upward convexity, with negative spikes of K (Fig. 8.5b). Note, however, that there are also negative anomalies for Hf and Ti, which are uncommon in most Na-alkaline basalts from intraplate settings (e.g. Wilson 1989). Overall, the Etna magmas have been found to be more enriched in volatile components than common intraplate magmas, and water contents up to 3-4 wt % have been found by melt inclusion studies (Corsaro and Pompilio 2004 Pompilio, personal communication). 

Mittlefehldt D. W. (1987) Volatile degassing of basaltic achondrite parent bodies evidence from alkali elements and phosphorus. Geochim. Cosmochim. Acta 51, 267-278. 

Figure 7 Siderophile element concentrations in averaged medium-Ti mare and terrestrial basalts, normalized to Cl chondrites. The elements are plotted in order of Cl-depletion factors in average medium-Ti mare basalt, but for some elements volatility may account in large part for the depletion. Data are from same sources as for Figure 6. To avoid an over-complex diagram, individual terrestrial compositions are not plotted, but all show similar patterns at the scale of this diagram the most noteworthy exceptions being low iridium (9 X 10 times Cl) and nickel (10 times Cl) in BCR-1, and relatively low osmium and iridium (virtually identical to mare basalts) and antimony (0.11 times Cl)...

The role of sulfide-driven (chalcophile) fractionations in lunar magmatism is difficult to constrain, but sulfides presumably play a lesser role on the Moon than on Earth, because the solubility of sulfide in mafic melt increases with decreasing/o (Peach and Mathez, 1993). Compared to lithophile elements of similar volatility, sulfur is exceptionally depleted in terrestrial and martian basalts, but not so depleted in lunar basalts (Figure 6). Medium-Ti mare basalts are clearly unsaturated with sulfide, and although high-Ti mare basalts were originally believed to be sulfide-saturated (Gibson et al., 1977), Danckwerth et al. (1979) found them to be unsaturated, as well. 

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