Minerals anorthosite

Anorthositic deposits - nearly all of the known commercially important rock deposits of titanium minerals are associated with anorthositic or gabbroic rocks. There are three main types (a) ilmenite-magnetite (titanoferous magnetite), (b) ilmenite-haematite, and (c) ilmenite-rutile. 

Example 5.7. The oldest lunar rock known to date is a lunar highland anorthosite 60025. The data for different minerals in a single rock are shown in the table below (Carlson and Lugmair, 1988). Find the age. 

The formation of the Moon s crust, composed primarily of feldspar (the rock is called anorthosite) illustrates how physical fractionation can occur during differentiation. Early in its history, a significant portion of the Moon was melted to form a magma ocean. The first minerals to crystallize, olivine and pyroxene, sank because of their high densities and formed an ultramafic mantle. Once feldspar began to crystallize, it floated and accumulated near the surface to produce the crust. 

Lahradoriie is the characteristic feldspar of the more basic rock types like diorite. gahbro. andesite or basalt and it is usually associated with some one of the pyroxenes or amphiholes. Lahradoriie frequently shows a heauliful play of iridescent colors due lo minute inclusions of another mineral. However, the labradorescent phenomenon has not been fully determined. The classic location for this mineral is. of course. Labrador, whence its name. Il is a constituent there of the rock anorthosite and is found in the anorthosites of the Provinces of Quebec and Ontario, and in the Adirondack region in New York Stale. 

Positive Eu anomalies were found first in a few grains of Apollo 11 soil35) and later on in various anorthosite samples where calcic feldspar (anorthite) is the dominant mineral. 

The silicates consist of mineral and lithic clasts set in a fine-grained fragmental to impact-melt matrix. The most common lithic clasts are basalts, gabbros, and orthopyroxenites, while dunites are minor and anorthosites are rare. The most common mineral clasts are centimeter-sized orthopyroxene and olivine fragments, while millimeter-sized plagioclase fragments are less common. 

Shervais J. W. and McGee J. J. (1999) KREEP cumulates in the western lunar highlands ion and electron microprobe study of alkali-suite anorthosites and norites from Apollo 12 and 14. Am. Mineral. 84, 806-820. 

Warren P. H. (1990) Lunar anorthosites and the magma ocean hypothesis importance of FeO enrichment in the parent magma. Am. Mineral. 75, 46-58. 

Longhi et al. 1999) and amphibolite (Christensen Mooney 1995 Rudnick Fountain 1995). For any of these compositions, partial melting and extraction of the anorthosite suite had to result in a general increase of restitic minerals (pyroxene and garnet). The occurrence of this process at a large scale, as the volume of mid-Proterozoic meta-anorthosites would require, would have produced a restitic bulk composition of the lower crust and a decrease of VpjVs ratios to values inconsistent with our observations. 

Figure 11. Isotope Exchange Trajectories (lET) for magnetite from six rocks from the Sybille Fe-Ti-oxide quarry, Laramie anorthosite complex. The average temperature of intersection, 1077+25°C, is higher than Tc for any mineral in these rocks, and it is interpreted as the solidus temperature of the magma (from Farquhar et al. 1993).

Dymek RF, Owens BE (2001) Petrogenesis of apatite-rich rocks (nelsonites and oxide-apatite gabbronorites) associated with massif anorthosites. Econ (Jeol 96 797-815 Edgar AD (1989) Barium- and strontium-enriched apatites in lamproites from West Kimberley, Western Australia. Am Mineral 74 889-895... 

The radar-echo surveys of the Antarctic ice sheet by Drewry (1975) provided information about the sub-ice topography but did not detect mineral deposits. More recently, Behrendt et al. (1980) combined the radar-echo sounding technique of Drewry (1975) with an airborne magnetometer survey which revealed that a laige part of the Dufek intrusion in the Pensacola Mountains is covered by the East Antarctic ice sheet. The enlargement of the area of this intrusion increased the amount of potential ore of iron, titanium, and vanadium in this differentiated body of gabbro, pyroxenite, and anorthosite. 

Fig. 13.51 The concentrations of copper in the Forrestal Range vary stratigraphically and reach high values in two layers at 1,080 m (Cu = 2,000 ppm) and at 1,343 m (Cu= 460 ppm) both of which have high modal pyroxene concentrations (71% at 1,080 m 46% at 1,343 m, respectively). The copper concentrations of the Stephens Anorthosite and the Lexington Granophyre are less than 100 ppm. The association between copper and pyroxene is caused by the association of sulfide minerals with layers of pyroxenite in the Forrestal Range (Data from Ford et al. 1983)...

In 2004, nearly 90% of the titanium minerals mined in the world today came either directly from igneous magnetite-ilmenite ore bodies associated with primary anorthosite complexes or secondary placer deposits derived from the weathering of these complexes. 

The nature of the lunar crust is imperfectly known. It is on a firm basis inasmuch as it was derived mainly from studies of samples. The samples, however, were collected from eight locations in a region on the lunar frontside that is not characteristic of the lunar highlands. An important characteristic of the lunar crust is its enrichment, relative to the remainder of the moon, in the constituents of Ca-rich feldspar (CaAl2Si20s). Presumably, feldspar-rich materials crystallized from the molten lunar exterior along with more mafic (Fe, Mg rich) minerals and collected selectively at the lunar surface. Samples of nearly pure feldspar rock, called anorthosite, were included with the lunar samples. Lanthanide distributions (fig. 21.14) in these rocks reflect the strong affinity of feldspar for Eu(II) and the relative preference for lighter members of the lanthanide series. 

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