Basalts incompatible elements

Incompatible-element ratios (e.g., Th/La, Nb/Zr, Ce/Yb in basalts) are therefore expected to be very insensitive to mineral separation from the melt and, for differentiated lavas, can be used as a parameter characteristic of their parent liquid see below). 

For small extents of crystallization, the maximum change, and thereby the most valuable information on F, will be obtained from elements with high Dt (compatible elements) such as Ni in basaltic olivine. Elements with ), 1 (incompatible elements), such as Th, Ba or rare-earth elements in basaltic systems, will provide basically no clue to F variations. In addition, information carried by incompatible elements, which do not fractionate with respect to each other, is entirely redundant. This is better shown by taking the relative change in the ratio of two elements il and i2 per increment of crystallization... 

Plots of uranium versus lanthanum (two refractory elements), and potassium versus lanthanum (a volatile element and a refractory element) for terrestrial and lunar basalts, HED achondrites (Vesta), and Martian meteorites. All three elements are incompatible elements and thus fractionate together, so their ratios remain constant. However, ratios of incompatible elements with different volatilities ( /La) reveal different degrees of volatile element depletion in differentiated bodies. After Wanke and Dreibus (1988). 

The Earth s crust and, indeed, the crusts of all differentiated bodies, are enriched in incompatible elements relative to their mantles. This reflects the partial melting of mantle material and extraction and transport of the basaltic melt to the surface. On Earth, further partial melting of the basaltic crust in the presence of water produces magma compositions even richer in silica (andesite and granite), which form the bulk of the continental crust. Because other differentiated bodies are effectively dry, this second level of differentiation did not occur. 

Anorthosites and basalts form two end members distinguished by FeO contents, and the impact melt breccias extend upward towards a KREEP end member with high thorium and intermediate FeO contents. Besides thorium, the KREEP end member is obviously enriched in the other incompatible elements that define its name. The impact melt breccias contain small clasts of KREEP basalt, which has not been sampled as large rocks. 

Differentiation of other terrestrial planets must have varied in important ways from that of the Earth, because of differences in chemistry and conditions. For example, in Chapter 13, we learned that the crusts of the Moon and Mars are anorthosite and basalt, respectively - both very different from the crust of the Earth. N either has experienced recycling of crust back into the mantle, because of the absence of plate tectonics, and neither has sufficient water to help drive repeated melting events that produced the incompatible-element-rich continental crust (Taylor and McLennan, 1995). The mantles of the Moon and Mars are compositionally different from that of the Earth, although all are ultramafic. Except for these bodies, our understanding of planetary differentiation is rather unconstrained and details are speculative. 

Fig. 3.4. Chondrite normalised REE patterns and mantle normalised incompatible element patterns of Intra-Apennine volcanic rocks. Patterns for mafic rocks from the Roman Province and the Virunga volcanoes (East Africa), for Enriched Mid Ocean Ridge Basalts (E-MORB) and for limestones from northern Apennines are also shown.

Fig. 4.8. (A). REE patterns of the Vulsini rocks. The ruled area encloses patterns of mafic rocks. (B) Incompatible element patterns of malic rocks from Vulsini. The pattern of the late erupted Lamone trachybasalt lava is shown in detail. Composition of Enriched Mid Ocean Ridge Basalt (E-MORB Sun and McDonough 1989) is reported for comparison.

Alicudi rocks range in composition from calc-alkaline basalts to high-K andesites (Fig. 7.3). Basalts exhibit the most primitive compositions over the Aeolian arc (Table 7.2). Mg (up to 0.72), Ni (up to 150 ppm), and Cr (up to 750 ppm) fall close to values of mantle equilibrated melts (Frey et al. 1978). There is an increase in K20, P205 and incompatible trace elements with increasing silica contents that is mirrored by a decrease in CaO, MgO, Ti02 and ferromagnesian trace elements (Fig. 7.4). REE are fractionated with flat HREE patterns (Fig. 7.5a). Incompatible element patterns of mafic rocks have negative anomalies of HFSE, and positive spikes... 

REE patterns are fractionated silicic rocks contain negative Eu anomalies (Fig. 7.14a), which are much smaller than observed for the Lipari and Vulcano rhyolites. Mantle normalised incompatible element patterns of mafic rocks show high LILE/HFSE ratios and a positive anomaly of Pb a small positive Sr spike is observed in the calc-alkaline basaltic andesites (Fig. 7.14b). HKCA and shoshonitic rocks have higher incompatible element abundances than the associated CA products. 

Calc-alkaline rocks at Stromboli have lower K20, P2O5 and incompatible trace element contents than the associated shoshonitic and KS rocks. However, REE and the mantle-normalised incompatible element patterns have similar shapes for all rocks, with positive spikes of Ba and negative anomalies of HFSE (Fig. 7.14). Glass inclusions with primitive compositions (MgO 7.8 wt %) contained in olivine (F091-84) from basaltic scoriae of the present-day activity, have the same type of incompatible element patterns as the whole rocks (Bertagnini et al. 2003). Sr-isotope ratios are... 

REE patterns are fractionated for all the rocks, but tholeiites show lower La/Yb ratios than alkaline products (Fig. 8.5a). Incompatible element patterns normalised to primordial mantle compositions for mafic rocks are very different from the Aeolian arc and central-southern Italian peninsula. Both tholeiitic and alkaline basalts show a marked upward convexity, with negative spikes of K (Fig. 8.5b). Note, however, that there are also negative anomalies for Hf and Ti, which are uncommon in most Na-alkaline basalts from intraplate settings (e.g. Wilson 1989). Overall, the Etna magmas have been found to be more enriched in volatile components than common intraplate magmas, and water contents up to 3-4 wt % have been found by melt inclusion studies (Corsaro and Pompilio 2004 Pompilio, personal communication). 

REE patterns for basalts show moderate fractionation and a slightly positive Eu anomaly. The silicic rocks have much higher REE contents than basalts and display negative Eu anomalies (Fig. 8.15a). Extended incompatible element diagrams of mafic rocks show an upward convex pattern with a maximum at Ta and Nb, and a positive spike of Ba, but negative Sr, Hf and, to a lesser extent, Ti (Fig. 8.15b). 

Trace element abundances of rocks dredged from the Sicily Channel seamounts are scarce (Beccaluva et al. 1981 Calanchi et al. 1989). They show variable concentrations, with incompatible element abundances increasing from tholeiitic to alkaline basalts and basanites (Fig. 8.17). Mantle normalised incompatible elements define bell-shaped patterns (not shown), which resemble those for the exposed rocks in the Sicily Channel. 

The Island of Ustica and the nearby Prometeo lava field have been less extensively studied than Etna. However, the available data have shown close compositional similarities among these volcanoes. Cinque et al. 1988) found that the Ustica basalts have incompatible element and Sr-isotope ratios that are intermediate between the intraplate basalts from the African plate (e.g. Pantelleria) and the calc-alkaline mafic rocks from Alicudi (Aeolian arc). This has led to the conclusion that the source of Ustica magmas is an intraplate-type mantle contaminated by subduction fluids or melts. A similar conclusion has been reached by Trua et al. (2003). 

Mafic rocks from the Sicily Province display variable abundances of incompatible elements, which increase from tholeiites to alkali basalts and nephelinites. However, all the rocks show low LILE/HFSE ratios, typical of intraplate basalts, in contrast with the high LILE/HFSE compositions of volcanism from the Aeolian arc and the Italian peninsula. They have a relatively restricted range of Sr-Nd isotopic compositions, but variable Pb isotopic ratios, which fall along a trend connecting HIMU and intermediate compositions between DMM and EMI mantle. 

Magnaghi and Vavilov seamounts. Magnaghi (3.0 to 2.7 Ma) and Vavilov (Late Pliocene to 0.1 Ma) seamounts are two N-S elongated volcanoes. The few rocks recovered to date have a basaltic composition with moderate to high TiC>2 (1.9 to 3.2 wt %), variable Ni (about 30 to 180 ppm) and Cr (about 30 to 350 ppm Selli et al. 1977 Beccaluva et al. 1982 Robin et al. 1987 Kastens et al. 1988, 1990 Savelli 1988). Incompatible elements are moderately enriched and mantle-normalised patterns show a small upward convexity, a moderate spike of Nb, and no LILE spikes or HFSE depletion (Fig. 9.12). 

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