Incompatible element

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

Geochemical similarities between the Vesuvius, Phlegraean Fields and Stromboli Volcanoes: petrogenetic, geodynamic and volcanological implications

Abstract: Vesuvius and Stromboli are two active and extensively studied volcanoes that traditionally have been considered as having different styles of eruption, rock composition and tectonic setting. Data reveal close compositional affinities between these two volcanoes. The abundant 13–15 Ka old Stromboli leucite-tephritic rocks have radiogenic isotope signatures, and abundances and ratios of incompatible elements with the exception of Rb and K, which are identical to those of Vesuvius. The Phlegraean Fields also show close affinities to these volcanoes.

Melting in the lithospheric mantle: Inverse modelling of alkali-olivine basalts from the Big Pine Volcanic Field, California

Abstract: The variations in trace element abundances of a suite of alkali-olivine basalts from the Big Pine volcanic field, California, have been ‘inverted’ following the method of Hofmann and co-workers to obtain source concentration and distribution coefficient data. The high Mg-numbers and ne-normative mineralogy of these lavas allow a simple correction to be made for fractional crystallisation, and together with a limited range in 87Sr/86Sr (0.7056–0.7064), suggest derivation from a relatively homogeneous source region. Negative correlations between SiO2 and P2O5, and SiO2 and Rb in the calculated primary magmas imply that both major and trace elements vary in a coherent fashion as a function of the degree of partial melting. The Big Pine lavas are characterised by high ratios of large-ion lithophile to high-field strength elements (Ba/Nb>60), and the inverse procedure demonstrates that this reflects source concentrations, as opposed to a mineralogical control. The calculated mantle source is further characterised by generally high abundances of Sr, Ba, K, and Th relative to Nb and Ta which imply that incompatible element enrichment of the source occurred above a subduction zone. A model Sm/Nd age of 1.8 Ga for this enrichment coincides with the regional crustal formation age. Such features imply that both the major and trace element components of the Big Pine lavas are derived from within lithospheric mantle, perhaps mobilised by the high geothermal gradients which characterise the extensional environment of the Basin and Range Province. A comparison with other Cenozoic mafic lavas throughout the western United States suggests that a substantial proportion of the mantle lithosphere in this area has similar chemical characteristics to the source of the Big Pine lavas. If this is the case, then it implies that convergent margins represent an important tectonic environment for the formation of lithospheric mantle.

Formation of platiniferous sulfide horizons by crystal fractionation and magma mixing in the Munni Munni layered intrusion, West Pilbara Block, Western Australia

Abstract: The Munni Munni Complex (T Nd CHUR model age 2.85 Ga), located in the west Pilbara block of Western Australia, is one of the best preserved layered intrusions in Australia. Exposed over an area of 4 X 9 km, it is composed of a lower 1,850-m-thick ultramafic zone and an overlying gabbroic zone which has a minimum thickness of 3,630 m. The ultramafic zone contains rhythmically layered dunite, lherzolite, olivine websterite, clinopyroxenite, and websterite, with orthopyroxenite, norite, chromitite, and platiniferous websterite prominent near the top of the zone. The gabbroic zone consists of gabbronorite, anorthositic gabbro, and minor anorthosite which display a pronounced tholeiitic fractionation trend. The order of appearance of cumulus mineral assemblages in the complex is olivine, olivine + clinopyroxene, clinopyroxene + olivine, clinopyroxene, clinopyroxene + orthopyroxene, orthopyroxene + chromite, and plagioclase + clinopyroxene + ?orthopyroxene. This sequence is at variance with major platinum-group element-bearing intrusions in which crystallization of orthopyroxene generally precedes that of clinopyroxene.Trace-element data, obtained on samples collected across the entire intrusion to investigate the effects of crystal fractionation and S evolution on the distribution of the platinum-group elements, show that in the sulfide-undersaturated ultramafic zone, Pt, Pd, Au, Cu, S, Se, Cs, Rb, St, and Zr behaved incompatibly and were concentrated in the melt during fractionation. The S content of the melt began to increase above the 700-m stratigraphic level of the ultramafic zone, but Pt, Pd, and Au contents increased above background levels of approximately 3 ppb to 3 ppm Pt + Pd only with the attainment of sulfide saturation at approximately the 1,830-m stratigraphic level. The concentration trends of Zr, St, Cs, Rb, and Cu paralleled that of S, but Ir and Ni largely partitioned with early crystallizing olivine and decreased in concentration with increasing fractionation. In contrast to the ultramafic zone, Pt, Pd, It, and Au have depletion trends in the sulfide-saturated gabbroic zone. Hence, the evolution of S largely governed the behavior of the platinum-group elements during the fractionation of the Munni Munni magma(s).The platinum-group element mineralization occurs immediately below the ultramafic-gabbroic contact. It resulted from the combined magmatic processes of crystal fractionation (as evidenced by increasing Cu/(Cu + Ni) ratios and incompatible element trends with stratigraphic height), and magma mixing. Two models are presented. In model 1, a hot, buoyant sulfide-saturated tholeiitic magma (containing 1,700-2,600 ppm S) rose through the density stratified platinum-group element-enriched, sulfide-undersaturated resident ultramafic magma (containing 530 ppm S) until reaching its own density level near the top of the chamber, where it spread out laterally for a distance of at least 12 km. Due to crystallization of plagioclase and subsequent Fe-enrichment [of the melt], the density of the gabbroic melt increased until it overturned and mixed with the platinum-group element-enriched fractionated parts of the ultramafic magma. Model 2 is similar to model 1, except that it involves the fractionation and internal mixing of one magma. In both models, magma mixing triggered sulfide saturation in the hybrid magma and established a high R factor (the silicate/sulfide mass ratio). The chalcophile platinum-group elements, due to their high sulfide/silicate partition coefficients, were scavenged by immiscible sulfide droplets which precipitated slowly in a porphyritic plagioclase websterite in a zone up to 20 m below the ultramafic-gabbroic zone contact. The platinum-group minerals comprising arsenides, sulfarsenides, tellurides, native metals, and mercury-bearing phases form small (<10 mu m) grains now enclosed within silicate minerals or along silicate grain boundaries (78% of occurrences), or are associated with chalcopyrite-pyrrhotite-pentlandite blebs (22%).The Munni Munni mineralized websterite layer is defined by discontinuities in (Pt + Pd)/ Se, (Pt + Pd)/S, (Pt + Pd)/Cs, (Pt + Pd)/Zr, and Cu/Zr ratios and Cu, Cs, Zr, Sr, S, and Se concentrations. Hence, these parameters together with information on the intercumulus component to the cumulates are useful for indicating potentially mineralized units in layered intrusions. Sulfide saturation also occurred along the basal contact of the ultramafic zone, but platinum-group element concentrations (< or =92 ppb Pt + Pd) are low since sulfide precipitation was probably induced by rapid cooling and the sulfides equilibrated with a small volume of silicate melt, implying a low R value. The different settings of sulfide mineralization in the Munni Munni Complex illustrate that timing and physical characteristics of the sulfide-saturation event are critical for the development of platinum-group element-enriched sulfide units in layered intrusions.

Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)

Abstract: Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia-Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. A set of Macquarie Island basaltic glasses has been analysed by electron microprobe for major elements, S, Cl and F; by Fourier transform infrared spectroscopy for H2O; by laser ablation-inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. An outstanding compositional feature of the data set (47.4-51.1 wt % SiO2, 5.65-8.75 wt % MgO) is the broad range of K2O (0.1-1.8 wt %) and the strong positive covariation of K2O with other incompatible minor and trace elements (e.g TiO2 0.97-2.1%; Na2O 2.4-4.3%; P2O5 0.08-0.7%; H2O 0.25-1.5%; La 4.3-46.6 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic rations of Sr (Sr-87/Sr-86 = 0.70255-0.70275) and Pb (Pb-206/Pb-204 = 18.951-19.493; Pb-207/Pb-204 = 15.528-15.589; Pb-208/Pb-204 = 38.523-38.979), and negatively with Nd (Nd-143/Nd-144 = 0.51310-0.51304). Macquarie Island basaltic glasses are divided into two compositional groups according to their mg-number-K2O relationships. Near-primitive basaltic glasses (Group I) have the highest mg-number (63-69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primitive olivine (Fo(86-89.5)). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo(90.5)). Fractionated, Group II, basaltic glasses are saturated with olivine + plagioclase +/- clinopyroxene, and have lower mg-number (57-67), and relatively low Al2O3 and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that represent a new primitive enriched MORB end-member. Compared with N-MORB, this new end-member is characterized by relatively low contents of MgO, FeO, SiO2 and CaO, coupled with high contents of Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements, and has the most radiogenic Sr and Pb regional isotope composition. These unusual melt compositions could have been generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island region must have been enriched and heterogeneous on a very fine scale.We suggest that the mantle enrichment implicated in this study is more likely to be a regional signature that is shared by the Balleny Islands magmatism than directly related to the hypothetical Balleny plume itself.