[ Pobierz caÅ‚ość w formacie PDF ] Cape Verde lithosphere (which we discuss in the next section) and suggested this mate- Cameroon Line rial may occasionally become delami- nated. Because it is cold, it would also St Helena sink to the deep mantle. As in the case of the Hofmann and White model, it Mangaia would be stored in a thermal boundary MORB layer, heated, and rise in the form of mantle plumes. However, recent stud- DMM HIMU ies have shown that the Os isotope 143 composition of the subcontinental Figure 11.30. Three dimension plot of 87Sr/86Sr, Nd/144Nd, lithosphere is quite distinctive, and and 206Pb/204Pb. Most oceanic basalt data plot within a tet- quite different from that of mantle rahedron defined by the composition of EMI, EMII, HIMU, plumes, as we shall see in the next sec- and DMM components. Oceanic islands and island chains tion. This rather strongly suggests tend to form elongate isotopic arrays, many of which seem to that ÒdelaminatedÓ subcontinental point toward a focal zone (FOZO) at the base of the tetrahe- lithosphere does not contribute to man- dron. Adapted from Hart et al. (1992). tle plumes. Because mantle plumes come in several geochemical varieties, it is possible that both mechanisms operate. Indeed, other as yet unknown processes may be involved as well. Most oceanic islands show some variability in their isotopic compositions, defining elongated ar- rays on plots of isotope ratios. Such elongated arrays suggest mixing. This raises the rather obvious question of what is mixing with what. In a few cases, the Comores are a good example, the elongate arrays seems to reflect mixing between different plume reservoirs. The Comores data defines a trend in isotopic space that appears to be the result of mixing between an EMI and a HIMU component. In other cases, such as the Galapagos, the plume is clearly mixing with the depleted upper mantle. However, in many cases, the cause of the isotopic variation is less clear. Hart et al. (1992) plotted oceanic basalt isotope data in three dimensions, with axes of 87Sr/86Sr, 143 Nd/144Nd, and 206Pb/204Pb (Figure 11.30). Principal component analysis confirmed that 97.5% of the variance in the oceanic basalt isotope data could be accounted for by these ratios (leaving 2.5% to be accounted for by 207Pb/204Pb, 208Pb/204Pb, and 176Hf/177Hf). They found that most of the data plotted within a tetrahedron defined by the hypothetical end members EM1, EM2, HIMU, and DMM. They also noticed that many arrays were elongated toward the base of this tetrahedron on the DMM- HIMU join. From this they concluded that in many, if not most cases, mantle plumes appear to mixing with a previously unidentified component, which they named ÒFOZOÓ (an acronym for Focal Zone), that has the approximate isotopic composition of 87Sr/86Sr = 0.7025, µ = +9, and 206Pb/204Pb = 19.5. Nd They suggested that FOZO is the isotopic composition of the lower mantle and that plumes rising from the core mantle boundary entrain and mix with this lower mantle material. It is unclear, how- ever, whether such a composition for the lower mantle can be fitted to reasonable isotopic mass bal- ances for the Earth. A rather similar idea was presented by Farley et al. (1992), who point out that 3 this additional component, which they called ÒPHEMÓ, seems to be associated with high He/4He. White (1995) concurred with these ideas, but argued that the 87Sr/86Sr of FOZO is higher, and the µ Nd 503 November 25, 1997 ir s i v al W n P d a b N c s t e i i t P e i s c e o r i o S i z a A w a H Z O O F W. M W hit e Geochemistry Chapter 11: The Mantle and Core lower, than estimated by Hart et al. (1992) and probably closer to the values chosen by Farley et al. (1992). If Hart et al. (1992) are correct, this may explain why the isotopic composition of some volcanos, notably those of Hawaii and the Society Islands, change over time. Hawaiian volcanos commonly go through several evolutionary stages: a shield-building stage, during which the volcanic edifice is rapidly constructed and a relatively uniform tholeiitic basalt is erupted, followed by a late stage, during which eruption rates drop dramatically and compo- sition shift to alkali basalt, and, following a significant Shield-building Phase hiatus in activity, a post-erosional stage in which small volumes of basanite and nephelinite are erupted. These latter magmas types are thought to be produced by small degrees of melting. Through this sequence, isotope ratios 87 evolve, with Sr/86Sr decreasing and µ increasing (e.g., Nd Chen and Frey, 1985). White and Duncan (1996) observed a similar pattern in volcanos of the Society Islands. The argued that this evo- lution in isotopic composition reflects the passage of the volcano over a compositionally zoned mantle plume (Figure 11.31). The most incompatible element enriched material, which is also the hotest, is located in the center, and formed the core of the plume. This is surrounded by a Post-erosional Phase sheath of material that is viscously entrained by the plume as it rises. It is cooler and also not as enriched in in- compatible elements. When the volcano is over the core of the plume, the degree of melting is high, giving rise to 87 tholeiitic basalt with high Sr/86Sr and high eruption rates during the shield building stage. When the volcano is over the sheath, the extent of melting is smaller, producing small volumes of nephelinite with low 87Sr/86Sr. There is also a geographic pattern to both the distribu- tion of mantle plumes and their isotopic compositions. Mantle plumes appear to be preferentially located within Figure 11.31. Cartoon of a model that regions of slow lower mantle seismic velocities, as may be explains why isotopic signatures of seen in Figure 11.32. There are two areas where isotopic magmas become more ÒdepletedÓ as 87 compositions are particularly extreme (e.g., high Sr/86Sr), volcanos evolve. During the Òshield- one in the southeastern Indian Ocean and South Atlantic, building stageÓ the time of most vigor- the other in the central South Pacific (Hart, 1984; Castillo, ous growth, the volcano is located di- 1989). The anomaly in the Indian Ocean is called the rectly over the plume, and magmas are DUPAL anomaly, while that in the South Pacific is called derived from the hot core of the the SOPITA anomaly. Interestingly enough, both anoma- plume, which has enriched isotopic lies close to regions where lower mantle seismic velocities 87 signatures (high Sr/86Sr, low µ ). Nd are particularly slow (Figure 11.32), which indicate low Because of lithopsheric plate motion, densities. The low density in turn implies that these are the volcano will be located over the regions of high temperatures in the lower mantle. While edge of the plume during later stages, the exact significance of this remains unclear, it does estab- such as in the post-erosional stage. lish a connection with oceanic island volcanism and lower During this stage magmas are derived mantle properties, strengthening the plume hypothesis, from the viscously entrained sheath, and favoring a lower mantle origin for plumes. which has more ÒdepletedÓ isotopic There is still much to understand about the nature and [ Pobierz caÅ‚ość w formacie PDF ] zanotowane.pl doc.pisz.pl pdf.pisz.pl blacksoulman.xlx.pl