Incompatible element

About: Incompatible element is a research topic. Over the lifetime, 2420 publications have been published within this topic receiving 154052 citations.

Isotopic and trace element constraints on the origin and evolution of alkaline and calc-alkaline magmas in the Northwestern Mexican Volcanic Belt

Abstract: A wide variety of rock types has been produced by Pliocene to Recent volcanism in the northwestern portion of the Mexican Volcanic Belt. Composite cones erupted rocks typical of calc-alkaline volcanic belts associated with subduction. Monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks. One larger center, Las Navajas, produced basalts, trachytes, and peralkaline rhyolites. Such alkaline rocks are typically associated with crustal extension related to rifting. Both subduction and rifting appear to be taking place in western Mexico. The most basic calc-alkaline rocks, although somewhat fractionated, show depletion in high field strength elements (HFSE) and enrichment in alkaline earth elements relative to other incompatible elements, a characteristic typical of magmas in subduction-related volcanic arcs. The basic alkaline rocks show no depletion in HFSE and show higher concentrations of incompatible elements than the basic calc-alkaline rocks. The calc-alkaline rocks show a tight cluster of 87Sr/86Sr and 143Nd/144Nd ratios, both of which correlate weakly with SiO2. Basic alkaline rocks have lower Sr and higher Nd isotopic ratios, whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc-alkaline rocks. Trace element data indicate that calc-alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas. The evolution of both calc-alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation (AFC) of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite. Assuming a contaminant located in the lower or middle crust, AFC modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc-alkaline magmas. Calc-alkaline and alkaline rocks appear to have resulted from melting of two distinct sources: alkaline from an OIB-type source, and calc-alkaline from a mixed mantle and slab-derived source.

An enriched mantle source for potassic basanites: evidence from Karisimbi volcano, Virunga volcanic province, Rwanda

Abstract: Lavas from Karisimbi, the largest volcano in the Virunga province in the Western Branch of the African rift on the Zaire-Rwandan border, constitute a suite of mafic potassic basanites and more evolved potassic derivatives. All of the lavas are potassic with K2O/Na2O≥1, and enriched in incompatible elements, with chondrite normalised (La/Yb)n>18 and Nb/Zr>0.25. The 87Sr/86Sr and 143Nd/144Nd ratios reflect these enriched compositions, varying from 0.7052 and 0.51258 respectively in the K-basanites to 0.7132 and 0.51226 in the most evolved K-trachyte, although at MgO abundances >4% there is no systematic variation of isotope ratios with fractionation. At >4% MgO, lava compositions were controlled by assimilation and fractional crystallization in a sub-volcanic magma chamber. Trace-element and isotope variations in the more mafic lavas appear to reflect mixing between a “primitive” K-basanite (PKB) magma and a Sr-rich end-member, similar to melilite nephelinites from the neighbouring volcano, Nyiragongo. Both endmembers are mantle-derived and isotopically distinct, with the PKB being characterised by 87Sr/86Sr up to 0.707 and 143Nd/144Nd as low as 0.51236. Alternatively, isotope variations may be the time-integrated response to trace-element fractionations in a variably enriched mantle source. The Pb isotope variations within Karisimbi are complex. In the more evolved lavas all three ratios increase coherently with fractionation, whereas in the mafic varieties 206Pb/204Pb remains roughly constant at ∼19.2 while 207Pb/204Pb and 208Pb/204Pb vary from 15.67 to 15.78 and 39.49 to 40.80 respectively, defining sub-vertical trends, consistent with PKB-nephelinite magma mixing. The Nd and Sr isotopes indicate trace-element fractionation in the PKB source at ∼1 Ga, similar to ages derived from the overlying crust and suggesting a lithospheric origin. Elevated 208Pb/204Pb and 208Pb*/206Pb* values of the PKB are also consistent with Th/U fractionation at a similar time. However, this 1Ga age contrasts with that derived from the elevated 207Pb/204Pb ratios which indicate U/Pb fractionation during the Archaean. Crustal contamination can be excluded as the major control of Pb isotope variation in the PKB because their high Ce/Pb ratios (∼27) are similar to those typical of oceanic basalts. Parent/daughter trace-element fractionation and the high Ti, Nb and Ta abundances of the PKB lavas are all consistent with enrichment of a lithospheric source region by small-degree silicate melts at ∼1Ga. Comparison between measured and time-integrated trace-element ratios suggests that the degree of melting associated with recent magmatism was ≥5%. These data show that significant Th/U and Rb/Sr fractionation can be produced by intra-mantle melting processes and that high 208Pb/204Pb and 208Pb*/206Pb* values can evolve within the upper mantle and do not necessarily require the recycling of crustal material. Comparable isotope features in continental flood basalts and DUPAL ocean island basalts may be explained in a similar way.

Formation of juvenile continental crust in the Arabian-Nubian shield: Evidence from granitic rocks of the Nakasib suture, NE Sudan

Abstract: Major and trace element data, U–Pb zircon ages, and initial isotopic compositions of Sr, Nd, and Pb are reported for ten granitic and one rhyolitic rock sample from the neo-Proterozoic Nakasib suture in NE Sudan. Chemical data indicate that the samples are medium- to high-K, "I-type" granitic rocks that mostly plot as "volcanic arc granites" on discriminant diagrams. Geochronologic data indicate that rifting occurred 790±2 Ma and constrain the time of deformation associated with suturing of the Gebeit and Haya terranes to have ended by approximately 740 Ma. Isotopic data show a limited range, with initial 87Sr/86Sr=0.7021 to 0.7032 (mean=0.7025), eNd(t) =+5.5 to +7.0 (mean=+6.4), and 206Pb/204Pb = 17.50–17.62. Neodymium model ages (TDM; 0.69–0.85 Ga; mean = 0.76 Ga) are indistinguishable from crystallization ages (0.79–0.71 Ga; mean=0.76 Ga), and the isotopic data considered together indicate derivation from homogeneously depleted mantle. The geochronologic data indicate that the terrane accretion to form the Arabian–Nubian shield began just prior to 750 Ma. The isotopic data reinforces models for the generation of large volumes of juvenile continental crust during neo-Proterozoic time, probably at intra-oceanic convergent margins. The data also indicate that crust formation was associated with two cycles of incompatible element enrichment in granitic rocks, with an earlier cycle beginning approximately 870 Ma and culminating approximately 740 Ma, and the second cycle beginning after pervasive high-degree melts – possibly hot-spot related – were emplaced approximately 690–720 Ma.

Evidence from oceanic gabbros for porous melt migration within a crystal mush beneath the Mid‐Atlantic Ridge

Abstract: [1] The evolution of a gabbroic crystal mush beneath the Mid-Atlantic Ridge has been investigated using evidence from gabbros recovered from Ocean Drilling Program (ODP) Hole 923A (Leg 153) Lithological variations occur on a vertical scale of meters and correlate with mineral compositions This defines a detailed chemical stratigraphy in which variations in olivine, plagioclase, and clinopyroxene solid-solution component compositions correlate with each other The volumetrically dominant lithology (plagioclase + clinopyroxene ± olivine ± orthopyroxene gabbros) has variable grain size, grain shape, and mineral compositions These variations correlate, such that coarser grained samples have more granular textures and lower mafic phase Mg/Fe ratios than adjacent finer grained samples Clinopyroxene trace element systematics, determined by ion probe, cannot be explained by growth from a melt that evolved along either an equilibrium or a fractional crystallization trend Clinopyroxene crystals are strongly zoned and enriched in Zr with respect to rare earth elements (more to less incompatible elements) These textural and geochemical characteristics are not expected from simple crystal accumulation processes or the crystallization of trapped melt Instead, melt migration within a crystal mush is suggested as the most likely process to explain them The meter-scale mineral compositional variations, which correlate between phases (eg, olivine forsterite content and plagioclase anorthite content), suggest that the porous melt flux after the formation of this layering was insufficient to destroy this correlation