Incompatible element

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

Chemical Geodynamics in a Non-chondritic Earth

Abstract: Over the past 30 years, chemical geodynamic models held that terrestrial silicate reservoirs differentiated from a primitive mantle (PM) with chondritic abundances of refractory lithophile elements. This basic assumption has had major consequences for our understanding of the mantle’s structure and composition. It is well established, for example, that in a chondritic Earth, the present-day continental mass does not contain a large enough budget of trace elements to balance depletion of the whole mantle. This result was taken as evidence that a hidden reservoir, either primitive or enriched in incompatible elements, segregated early in the history of the Earth and remained since then isolated from the convective system. This common view, however, is now being reconsidered, as 146Sm–142Nd studies of planetary and meteoritic material show that the bulk silicate Earth may in fact not have perfectly chondritic abundances of refractory lithophile elements, as has been assumed. This observation represents a challenge to compositional models of the Earth, which are systematically anchored on chondritic reference parameters. Chemical geodynamic models, based on mass balance relationships between the depleted, primitive, and enriched silicate reservoirs, would also need to be reconsidered. Here, I present a new set of reference parameters for the Sm–Nd , Lu–Hf , and Rb–Sr systems consistent with the presence of an 18 ppm 142Nd excess in the bulk silicate Earth. The trace element pattern obtained for the PM using this super-chondritic Earth model (SCHEM ) suggests that the proto-Earth accreted from material depleted in incompatible elements compared to chondrites, most likely as a result of preferential impact erosion of shallow crustal reservoirs during the early stage of accretion. The isotopic and trace element signature of the (non-chondritic) PM matches that observed in most high 3He/4He oceanic island basalts, suggesting that pristine material may have been preserved in the interior of the Earth for the past 4.45 Ga and is now sampled by modern plume magmatism . These primordial heterogeneities, however, are unlikely to represent a large reservoir, as most of the mantle appears to have been thoroughly degassed and depleted by crustal extraction. In a non-chondritic Earth, the compositional evolution of the “accessible” mantle appears to be essentially controlled by the continuous growth of the continental crust over the past 3.8 Ga and does not require the presence of hidden mantle reservoirs, either enriched or primitive.

N-MORB crust beneath Fuerteventura in the easternmost part of the Canary Islands: evidence from gabbroic xenoliths

Abstract: Gabbro xenoliths reported in this paper were collected in northern Fuerteventura, the Canary Island located closest to the coast of Africa. The xenoliths are very fresh and consist of Ti–Al-poor clinopyroxene + plagioclase (An87–67) + olivine (Fo72–86) ± orthopyroxene. Clinopyroxene and orthopyroxene are constantly and markedly depleted in light rare earth elements (LREE) relative to heavy REE (HREE), as expected for cumulus minerals formed from highly refractory N-MORB-type melts. In contrast, whole-rock Primordial Mantle-normalized trace element patterns range from mildly S-shaped (mildly depleted in Pr–Sm relative to both the strongly incompatible elements Rb–La and the HREE) to enriched. Estimates show that the trace element compositions of the rocks and their minerals are compatible with formation as N-MORB gabbro cumulates, which have been infiltrated at various extents (≤1% to >5%) by enriched alkali basaltic melts. The enriched material is mainly concentrated along grain boundaries and cracks through mineral grains, suggesting that the infiltration is relatively recent, and is thus associated with the Canary Islands magmatism. Our data contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean. No evidence of continental material that might reflect attenuated continental crust in the area has been found. Gabbro xenoliths with REE and trace element compositions similar to those exhibited by the Fuerteventura gabbros are also found among gabbro xenoliths from the islands of La Palma (western Canary Islands) and Lanzarote. The compositions of the most depleted samples from these islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. The presence of N-MORB oceanic crust beneath Fuerteventura implies that the continent–ocean transition in the Canary Islands area must be relatively sharp, in contrast to the situation both further north along the coast of Morocco, and along the Iberian peninsula.

The Rudnik Mts. volcano-intrusive complex (central Serbia): An example of how magmatism controls metallogeny

Abstract: This study reports and discusses new radiometric ages, petrographical and volcanological observations and whole rock geochemical data of the rocks of the Rudnik Mts. volcano-intrusive complex. The complex hosts a Pb-Zn-Ag deposit and belongs to the Serbo-Macedonian metallogenetic belt. Two distinct igneous events are distignuished. The first occurred >30 Ma and was mainly characterized by extrusive and shallow intrusive dacites and andesites and was unrelated to mineralization. The second igneous event occurred <23 Ma and was highly heterogeneous in terms of volcanic products and petrographic varieties, but with predominance of quartzlatites. The dacite-andesites (first event) and the quartzlatites (second event) are geochemically similar and display a calc-alkaline affinity and highly incompatible element enriched patterns on spider diagrams, but the younger quartzlatites are richer in K2O, Rb and Ba and poorer in Sr. This is taken as evidence that mixing between an ultrapotassic lamprophyre/lamproite magma and an acid calc-alkaline (dacite-like) magma was essential petrogenetic processes during the second event. The proposed simplified volcanological model suggests that this mixing was responsible for triggering strongly explosive volcanic activity as well as for providing conditions for active hydrothermal and mineralization processes. The observed link between a specific magmatic phase and ore deposit formation can be a general phennomenon in the Balkans, and must be addressed by further and more advanced studies.

Geochemistry and origin of the Mirny field kimberlites, Siberia

Abstract: Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65–300). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t = 360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ɛNd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387–0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.