Small Degrees Of Partial Melting Research Articles

Rare earth element distribution in lavas and ultramafic xenoliths from the Comores Archipelago, Western Indian Ocean

Lavas and included xenoliths from the Comores Archipelago have been analysed for the rare earth elements (REE) La-Lu. Among basaltic lava types fractionation of REE rock/chondrite distribution patterns is more extreme with greater SiO2 undersaturation and contents of incompatible elements. Enrichment and slight fractionation of REE in the rock series basanite-phonolite is considered compatible with a model of fractional crystallisation at low pressures involving mainly olivine and clinopyroxene, and to a much lesser extent, plagioclase. Apatite is probably effective in curtailing further enrichment of REE. High level fractional crystallisation and eclogite fractionation at depth appear unlikely causes for the relative enrichment of light REE (La-Eu) in the undersaturated basalts. This effect is more probably due to mineralogical control during partial melting in the upper mantle. Lherzolite xenoliths are poor in REE, exhibiting a slight relative depletion in the light REE. These patterns are interpreted as those of possible mantle material subjected to small degrees of partial melting, although not necessarily related to those melts erupted as lava flows at the surface.

Special Papers-Apollo 11 Symposium: Petrogenesis of Apollo 11 Basalts and Implications for Lunar Origin

The origin of Apollo 11 basalts is discussed in terms of two hypotheses: (1) formation by a small degree of partial melting in the lunar interior, and (2) formation by prolonged near-surface crystallization differentiation in a lava lake. The second hypothesis is rejected on the following grounds: Most Apollo 11 magmas are far removed from the plagioclase-pyroxene-ilmenite cotectic; fractional crystallization cannot explain the large variations in concentrations of incompatible trace elements in conjunction with the small variations in major-element compositions, particularly, Mg/Fe ratios; experimentally determined partition coefficients show that the high abundances of Cr and V in Apollo 11 rocks cannot be reconciled with the previous separation of large quantities of ore minerals and pyroxenes. On the other hand, the major-element and trace-element contents of Apollo 11 rocks can be readily explained by partial melting of source material that buffers the major-element compositions and causes enrichments of incompatible elements according to the degree of partial melting (first hypothesis). Two alternative sources have been suggested for Apollo 11 basalts formed by partial melting: (1) unfractionated pyroxenite source region at depths of 200–600 km, and (2) fractionated source region with incompatible elements (e.g. Ba, U, and rare earths) strongly enriched over chondritic abundances and containing plagioclase (approximate eucritic composition). Mass-balance calculations and plagioclase-stability conditions show that the second hypothesis requires Apollo 11 basalts to be generated by partial melting in the outer 150 km of the moon. This is very difficult to achieve one billion years after the moon's formation, since the outer 200 km will have cooled well below the solidus by conduction. Furthermore, magmas generated by partial melting of a plagioclase-bearing source region should have plagioclase on the liquidus, which is contrary to observation. The second hypothesis accordingly appears improbable. The first hypothesis is capable of explaining the major-element chemistry and the trace-element abundances (Eu; see below) in terms of a simple, single-stage model that is consistent with the moon's density, moment of inertia, and inferred thermal history. A possible explanation of the europium anomaly is suggested on the basis of the first hypothesis. It will be necessary to determine the appropriate partition coefficients in order to test this explanation. If the lunar highlands are anorthositic, extensive differentiation of the outer 150 km of the moon is required. This may have been caused by heating arising from partial conservation of gravitational potential energy during the final stage of accretion. Formation of Apollo 11 basalts by partial melting 3.7 billion years ago was probably the result of radioactive heating (U, Th) in the deep interior of the moon. A two-stage magmatic history for the moon is thus required. Similarities between compositions of Apollo 11 and terrestrial basalts and between their respective source regions are suggestive of a genetic relationship between moon and earth. Nevertheless, important differences in trace-element abundances, major-element compositions, and oxidation states exist. These abundance patterns are unfavorable to the traditional fission, binary planet, and capture hypotheses of lunar origin. However, they may be explicable in terms of the precipitation hypothesis proposed by the author. This maintains that during the later stages of accretion of the earth, a massive primitive atmosphere developed that was hot enough to evaporate selectively a substantial proportion of the silicates that were accreting on the earth. Subsequently, the atmosphere was dissipated and the relatively nonvolatile silicate components were precipitated to form a swarm of planetesimals or moonlets, from, which the moon accreted.

Origin of the Moon: The precipitation hypothesis

The basaltic rocks returned by Apollo 11 were probably formed by a small degree of partial melting in the lunar interior. Their compositions overlap the terrestrial basalt ranges for most elements; however, there are important differences in the abundances of another group of elements and also in oxidation states. These similarities and differences are believed to be characteristic of the source regions of the lunar basalt and terrestrial basalts. In particular they demonstrate the occurrence of a fractionation between lunar and terrestrial mantles dominated by selective volatility of some elements. They are not readily explained by current versions of fission, binary planet and capture hypotheses. It is therefore necessary to examine other theories of lunar origin. Öpik has argued that the moon formed by coagulation of a terrestrial “sediment-ring” but did not explain the origin or composition of the latter. A possible explanation arises from an extension of the single-stage hypothesis for the origin of the earth previously advanced by the author. This maintains that during the later stages of accretion of the earth, a massive primitive atmosphere developed which was hot enough to selectively evaporate a substantial proportion of the silicates which were accreting upon the earth. Subsequently, the atmosphere was driven away by particle radiation from the sun as it passed through a T-Tauri phase. The relatively non-volatile silicate components were precipitated close to the earth to form a swarm of planetesimals or moonlets, as the atmosphere was dissipated, and the moon accreted from these chemically fractionated planetesimals. The more volatile components of the terrestrial atmosphere were precipitated at lower temperatures, further from the earth, as fine smoke particles and were lost from the earth-moon system with the escaping gases. The hypothesis appears capable of explaining the low density of the moon, the inferred fractionation of relatively volatile elements between earth and moon, and the different oxidation states of the terrestrial and lunar mantles. The above “precipitation” hypothesis thus implies a close genetic relationship between earth and moon. The precipitation and fission hypotheses are also closely connected since the material now in the moon is regarded as having been derived ultimately from the earth — not from the solid mantle, but from the massive primitive terrestrial atmosphere.

Petrogenesis of Lunar Basalts and the Internal Constitution and Origin of the Moon

Petrographic and electron-microprobe studies combined with high pressure-temperature investigations of phase relationships in average Apollo 11 basalt and possible source material show that the lower parts of maria may be composed of eclogite (density 3.74 grams per cubic centimeter), thus explaining the existence of mascons. The Apollo 11 basalt was probably formed at depths of 200 to 400 kilometers by a small degree of partial melting from pyroxenitic source material [FeO/(FeO + MgO) = 0.25, A1(2)O(3) 4 percent, CaO 3 percent]. This composition may be representative of the lunar interior and yields the observed mean lunar density and moment of inertia. Present data are in conflict with fission, binary planet, and capture hypotheses of lunar origin but are consistent with Ringwood's (1966) precipitation hypothesis.

Transitional basalts from the eastern Australian tertiary province

Study of Tertiary volcanic rocks from Mt. Tamborine in S. E. Queensland, Australia, suggests that some basalts from the large Tweed vent are intermediate in character between true tholeiites and well documented alkaline basalts of the Australian Tertiary province. Geochemical evidence and comparison with synthetic systems suggests that the transitional lavas are derived by continuous fractional crystallization during the ascent of a parent magma produced by substantial partial melting of the upper mantle at moderate depths and pressures. Alkaline basalts that underlie the transitional basalts are the result of relatively rapid, intermittent extrusion of magma generated by a smaller degree of partial melting of the upper mantle. This interpretation resolves the difficulties posed by an apparent inversion of the usual stratigraphic relationship of tholeiites capped by alkali basalts, and is in accord with recent seismic studies of the continental margin of eastern Australia.