Two Major Igneous Events in the Evolution of the Moon [and Discussion]
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
An understanding of the origin of the Moon is strongly dependent upon a knowledge of its bulk composition and thermal history. Both aspects require a detailed consider ation of the composition and origin of the lunar crust and of the mantle-derived lunar basalts. The evidence for two major igneous events is discussed, the first being a large-scale melting and fractionation into crust and mantle at -4.6 to -4.5 Ga, and the second a partial melting of the uppermost mantle at -3.8 to -3.2 Ga. The
... .8 to -3.2 Ga. The distribution of uranium is used to place constraints on the minimum extent of initial melting and on the depth at which the mare basalts were generated, using recent lunar heatflow data for a bulk-Moon uranium content of 30 parts/109. The model favours melting of at least 90 % by volume, and a concentration of the high U-contents of the crust and upper mantle by formation of a thick lower mantle of mafic adcumulates 'barren' in heat-producing elements. The 'fertile' mafic orthocumulates from which the mare basalts were generated are restricted by the model to depths of less than 200 km. A downward revision of the bulk U-content of the Moon results in down-scaling of the other refractory lithophile elements by analogy with the solar-nebula conden sation models. This means that the bulk Moon is fairly close in composition to that of the Earth's mantle, including its iron content but excluding the volatile elements which are strongly depleted in the Moon. Low contents of siderophile and chalcophile elements, and high contents of lithophile refractory elements in the lunar basalts are attributable to the large-scale fractionation into a core, mantle and crust. The hypothesis of an origin for the Moon by fission from a proto-Earth is revived. Earth layering by a heterogeneous accretion sequence would account for non equilibrium between core and mantle (e.g. nickel distribution) and an outer veneer of volatile-rich condensate that would contribute to subsequent generation of a granitic crust. Early collision with a large body may have caused fission and formation of a proto-Moon from the Earth's iron-poor, proto-mantle, with loss of volatiles. Early melting of most of the proto-Moon led to strong fractionation such that the crust and mantle-derived basalts appear to have more extreme compositions, relative to Earth basalts, than is indicated by the likely bulk composition of the Moon. 1 D ifferences betw een lunar and terrestrial basalts The term 'basalt' is applied to those crystalline, igneous-textured lunar rocks that have closer affinities with terrestrial basalt lavas than with any other terrestrial rock type. They occur chiefly as lava infillings of the large mare basins but more aluminous varieties occur as rare fragments in the breccias that result from impact-reworking of the older feldspathic crust. The coarser-grained anorthositic gabbros and troctolites found in the crustal breccias are not grouped with the basalts. The lunar basalts are chemically distinct from terrestrial basalts in having low SiOa contents associated with high FeO/FeO + MgO ratios, high but variable TiOz, and low NaaO and K aO. The low alkalis give rise to the calcic plagioclases (average An95) that contrast most [ 205 ] 43 Vol. 286. A.