Rocks crustal, trace elements

Trace elements are discharged into the ocean in particulate and dissolved form as a component of river runoff and groundwater seeps. They are introduced into these waters during the chemical and mechanical weathering of crustal rocks. Thus, the chemical composition of river water is dependent on the composition of the rocks in the... 

No rock samples have been collected from Mercury and Venus, and the arsenic chemistry of their crusts is unknown. Like the Moon, the crustal rocks on Mercury, Venus, and Mars are primarily basalts and other mafic rocks. If the trace element chemistry of their basalts is similar to lunar specimens, they should contain <1 mg kg-1 of arsenic. 

Hf-isotope ratios also fall close or within the field of crustal rocks (Fig. 2.4 Table 2.2). This has been interpreted as evidence that metasomatic modification of lamproitic mantle sources in Tuscany was provided by addition of crustal material (e.g. metapelites). Interestingly, the mantle-normalised incompatible element patterns of Tuscany lamproites are similar to those of gneisses and schists in almost eveiy detail. This has been interpreted as evidence for addition of bulk upper crustal material to the mantle, with little element fractionation during metasomatism and the subsequent partial melting (Peccerillo 2002). This makes the Tuscany Province a zone where the upper mantle magmas look very much like the upper crust in terms of trace element and radiogenic isotope compositions. 

The Tuscany Magmatic Province consists of an association of calc-alkaline to lamproitic mafic to intermediate magmas and silicic intrusive and effusive rocks. Silicic melts have been formed by crustal melting, with an important role of mixing with mantle-derived magmas. Mafic melts are of mantle origin but resemble closely some upper crustal rocks, such as metapelites, in terms of incompatible trace elements and radiogenic isotope... 

De Astis et al. (2000) and Calanchi et al. (2002b) noticed that calc-alkaline and HKCA basalts at Vulcano and Panarea have distinct trace element ratios (e.g. La/U, Rb/Zr, Zr/Nb) compared to the associated sho-shonitic and KS mafic volcanics. However, the rocks of the Calabro-Peloritano basement underlying the Aeolian volcanoes show compositions that resemble the calc-alkaline rather than shoshonitic and KS rocks this was interpreted to exclude a derivation of potassic rocks from calc-alkaline parents via crustal assimilation. The same conclusion was drawn by Frez-zotti et al. (2004), who modelled magma contamination processes using melt inclusions entrapped in metamorphic xenoliths as contaminants. 

Based on major and trace element variations, it has been argued that fractional crystallisation was the dominant evolutionary mechanism for Ustica magmas (Cinque et al. 1988). The lack of correlation between silica and Sr isotopic ratios has been interpreted as evidence that assimilation of crustal rocks played a minor role during magma evolution. The variable contents of K20, P2Os incompatible elements and radiogenic Sr in the mafic rocks most likely reflect the occurrence of various types of primary melts at Ustica. 

Table 1-1 (from Emsley, 1989) lists those elements with crustal concentrations >0.1% (10000 mg kg-1)- The table shows that only 8 elements account for over 99% of the mass of crustal rock and 12 account for 99.9% of crustal rocks. These are the major elements and the remaining elements are conveniently referred to as the trace elements. On earth, hydrogen and helium are rare because they are too volatile to be retained by terrestrial gravity and when released to the atmosphere they eventually diffuse back to space. 

The soil inherits from the parent material a stock of trace elements which, in turn, is controlled by the geochemical nature of the parent material (West, 1981). In global terms this relationship can be illustrated by comparing the commonly accepted mean contents of soil elemental constituents with crustal rock abundances. Reasonably reliable data are available for 67 elements. 

Trace element distribution rules. Attempts to explain the distributions of transition metal ions in crustal rocks and minerals by empirical rules based on... 

Rehnements of the Taylor and McLennan (1985) model are provided by McLennan and Taylor (1996) and McLennan (2001b). The latter is a modihcation of several trace-element abundances in the upper crust and as such, should not affect their compositional model for the bulk crust, which does not rely on their upper crustal composition. Nevertheless, McLennan (2001b) does provide modihed bulk-crust estimates for niobium, rubidium, caesium, and tantalum (and these are dealt with in the footnotes of Table 9). McLennan and Taylor (1996) revisited the heat-flow constraints on the proportions of mahc and felsic rocks in the Archean crust and revised the proportion of Archean-aged crust to propose a more evolved bulk crust composition. This revised composition is derived from a mixture of 60% Archean cmst (which is a 50 50 mixture of mahc and felsic end-member lithologies), and 40% average-andesite cmst of Taylor (1977). McLennan and Taylor (1996) focused on potassium, thorium, and uranium, and did not provide amended values for other elements, although other incompatible elements will be higher (e.g., rubidium, barium, LREEs) and compatible elements lower in a cmst composition so revised. 

The problem of crustal contamination is particularly acute for low mg continental flood basalts and smaller volume continental tholeiitic basalts, both of which have low trace-element concentrations (see Sections 3.03.3.2.3 and 3.03.3.3). The issue is less critical for many smaller volume continental rocks, such as kimberlites and alkali basalts, which have much higher abundances of many trace elements. As a result of their high strontium and neodymium content, for example, the isotopic compositions of these elements in kimberlites and alkali basalts are relatively insensitive to modification during crustal contamination. Conversely, the osmium and lead concentration of basaltic magmas are so low that these isotope systems are particularly vulnerable to modification by interaction with cmstal rocks (McBride et al, 2001 Chesley et al, 2002) hence these systems provide relatively sensitive indicators of crustal assimilation. 

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