Magmas, early studies

Despite the close association of intnisions with many ore deposits, there is still debate about the extent to which magmas contribute water and metals to ore-forming fluids. Many early studies of the stable isotope composition of hydrothermal minerals indicated a dominance of meteoric water (Taylor 1974), more recent studies show that magmatic fluids are commonly present, but that their isotopic compositions may be masked or erased during later events such as the influx of meteoric waters (Rye 1993 Hedenquist and Lowenstem 1994). 

Stable isotopes have become an integral part of ore deposits studies. The determination of light isotopes of H, C, O, and S can provide information about the diverse origins of ore fluids, about temperatures of mineralfration and about physico-chemical conditions of mineral deposition. In contrast to early views, which assumed that almost all metal deposits owed their genesis to magmas, stable isotope investigations... 

Mineral composition and structure are the primary intrinsic factors controlling weathering rates. Based on early weathering studies, Goldich (1938) observed that the weathering sequence for common igneous rocks in the field was the reverse of Bowen s reaction series that ranked minerals in the order of crystalhzation from magma. Amphi-boles and pyroxenes are expected to weather faster than feldspars which weather faster than... 

The recognition of a 142Nd anomaly within the mantle implies that the Earth experienced a major, very early differentiation event. The study by Boyet and Carlson (2005) showed that lunar basalts also have elevated 142Nd/144Nd ratios relative to primitive chondrites, implying that the Moon was formed from an Earth that had already experienced major differentiation. This means that the early differentiation of the Earth took place within 30 Ma of the formation of the solar system. Whilst the precise nature of this differentiation event is not known, a favored model is the formation of an Fe- and trace element-enriched basaltic crust, perhaps as an initial crust to a magma ocean. It is postulated that this crust is now isolated from the convecting mantle and is located deep within the lower mantle. 

A number of models for the very early Earth propose the existence of a Hadean (ca. 4.5 Ga) basaltic crust. The evidence for such a model comes from the study of 142Nd and is consistent with the observed extreme fractionation of 207Pb (Kamber et al., 2003), and with Hf-isotope studies on zircons (Bizzarro et al., 2003). For this reason it has been proposed that an early, trace element enriched mafic crust, created perhaps as a lid to a Hadean magma ocean, was subducted and buried deep in the mantle, where it is thought to be still stored (Galer Goldstein, 1988, 1991). Hence, this material is a candidate for a lower mantle layer and is another possible explanation for a layered mantle created very early in Earth history. 

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