Spinel-lherzolite xenoliths in alkali basalts from eastern China have porphyroclastic to equigranular textures displaying varying degrees of deformation and subsolidus re-equilibration. The proportions of minerals in these xenoliths vary from 52 to 72% homogeneous olivine (Fo88-91); 11 to 26% orthopyroxene (Wo0.9.1.6; En88-90; Fs8.7.10.7), with minor discontinuous variations of Al2O3, FeO, and CaO; 6 to 19% clinopyroxene (Wo43.47; En49.51; Fs3.7.6.7); and 1 to 5% spinel, with similar Mg# (79.6 to 82.6), but wider variations of Al2O3 and Cr2O3 (100Cr/(Cr + Al + Fe3+) = 8.1 to 23.6). Although previous trace-element and isotopic studies have shown that at least two distinctly different mantle sources were sampled by Cenozoic basalts, mineralogical heterogeneities seem to be minor within the spinel-peridotite-facies lithosphere beneath eastern China. These xenoliths experienced limited interaction with the host basaltic magma during eruption. Symplectites of secondary, minute silicates, titanomagnetite, and sulfide have replaced orthopyroxene—and to a lesser extent olivine—at the contact with the basalt. The spinel in the margin of the xenolith is continuously zoned by substitutions of Fe3O4 (magnetite) and Fe2TiO4 (ulvospinel) for MgAl2O3 (spinel), and is rimmed by titanomagnetite with a sharp boundary. However, the compositions of the interior clinopyroxenes were commonly modified by metasomatic partial melting, which resulted in “spongy-textured” rinds on primary clinopyroxene. This secondary assemblage is composed mainly of a refractory, jadeite-poor clinopyroxene, which is largely in optica! continuity with the primary clinopyroxene in addition to interstitial feldspars, with minor titanomagnetite and Fe-Ni sulfides. This assemblage was produced by the introduction of K-rich fluids from the enclosing basaltic magma. The intensity of these secondary reactions appears to have been a function of the residence time of the xenolith in the host basalt. Therefore, all secondary alteration of both external and internal primary minerals in these xenoliths are the result of near-surface metasomatic processes, rather than of mantle phenomena.
The properties of garnets in peridotite xenoliths from kimberlites in China are compared with garnets occurring as megacrysts (disaggregated peridotites) in alkali basalts. The pyropes in xenoliths from kimberlites in the Shandong and Liaoning provinces are distinctly purple, whereas the megacryst pyropes in basalts are considerably darker. Kelyphitic reaction rims are common on pyropes in peridotite xenoliths from kimberlites, but are rare on pyropes in basalts. The classification of pyropes according to the scheme of Dawson and Stephens (1975) demonstrates that pyropes in peridotite xenoliths from kimberlites are mainly Group-9 garnets, whereas those in basalts typically belong to Group 3 (calcic pyrope-almandine) and Group 1 (titanian pyrope). In China, pyropes in peridotite xenoliths of kimberlites, as compared with those in basalts, are richer in Cr2O3, Cr/(Cr + Al), knorringite, uvarovite, and Cr-component, but lower in Fe2O3 + FeO, Fe/(Fe + Mg), grossularite, and almandine molecules. Unlike the situation in basalts, there is a distinct negative correlation between Cr2O3 and Al2O3 contents of the pyropes in peridotite xenoliths from kimberlites. The formation pressures of pyropes in the peridotite xenoliths mainly plot within the diamond stability field (i.e., more than 4GPa). These garnets may be useful as an indicator mineral for diamond exploration.
With reflectance spectroscopy, one is measuring only properties of the fine‐grained regolith most affected by space weathering. The Lunar Soil Characterization Consortium has undertaken the task of coordinated characterization of lunar soils, with respect to their mineralogical and chemical makeup. It is these lunar soils that are being used as “ground truth” for all airless bodies. Modal abundances and chemistries of minerals and glasses in the finest size fractions (20–45, 10–20, and <10 μ m) of four Apollo 14 and six Apollo 16 highland soils have been determined, as well as their bulk chemistry and I S /FeO values. Bidirectional reflectance measurements (0.3–2.6 μ m) of all samples were performed in the Reflectance Experiment Laboratory. A significant fraction of nanophase Fe 0 (np‐Fe 0 ) appears to reside in agglutinitic glasses. However, as grain size of a soil decreases, the percentage of total iron present as np‐Fe 0 increases significantly, whereas the agglutinitic glass content rises only slightly; this is evidence for a large contribution to the I S /FeO values from the surface‐correlated nanophase Fe 0 , particularly in the <10 μ m size fraction. The compositions of the agglutinitic glasses in these fine fractions of the highland soils are different from the bulk chemistry of that size; however, compositional trends of the glasses are not the same as those observed for mare soils. It is apparent that the glasses in the highland soils contain chemical components from outside their terrains. It is proposed that the Apollo 16 soils have been adulterated by the addition of impact‐transported soil components from surrounding maria.
The Apollo 14 high-Al basalt, 14053, is the most reduced lunar rock examined to date. Both fayalite in the mesostasis and spinel minerals have been extensively reduced in the exterior of the rock, whereas the interior contains relatively limited reduction. It is shown that these products are the effects of solar-wind hydrogen that was implanted on the exterior of the .normal. 14053 basalt after it originally crystallized and was weathered to become part of the regolith. Subsequent reheating, probably in an impact-ejecta blanket, caused extreme subsolidus hydrogen reduction, particularly of the weathered exterior of this rock. The limited permeability of the rock prevented the entire rock from being subjected to the same degree of reduction. It is proposed that this extreme reduction, especially of the mesostasis, also affected the phosphate minerals, F-Cl apatite and merrillite, each of which vary greatly in REE contents. This effect on the phosphate minerals could relate to the upset Sm-Nd radiogenetic systematics, whereas the Rb-Sr system may have been largely immune to the subsolidus reduction.
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