Incompatible element

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

The Pd deposits of the Lac des Iles Complex, Northwestern Ontario

Abstract: The Lac des Iles Complex contains the Roby, Twilight, and High-grade zones, which make up Canada’s only primary platinum-group element (PGE) ore deposit with a grade of ~3 ppm Pd + Pt. The ores have remarkably high Pd/Pt ratios, averaging 7 in the Roby and Twilight zones and even higher, 14, in the High-grade zone. In contrast most PGE-dominated deposits have Pd/Pt ratios of 0.5 to 3. The Lac des Iles ore zones occur within a small (~2 ×y 3.5 km) concentrically zoned mafic intrusion and are approximately 0.5 km wide by 1 km long at surface. There are three major rock types present, gabbronorite, metagabbronorite, and chlorite-actinolite schist. The Roby and Twilight zones consist of magmatic breccia of gabbronorite or metagabbronorite, which contains pegmatoidal and varitextured patches. The gabbronorites have adcumulate textures and consist of deformed plagioclase and orthopyroxene with minor interstitial clinopyroxene, biotite, and hornblende. In the metagabbronorites the pyroxenes and hornblende have been replaced by actinolite and the plagioclase has been partly altered to sericite. In the most altered metagabbronorites the plagioclase has been replaced by chlorite. The High-grade zone occurs between the breccia of the Roby zone and the homogeneous East Gabbro. The main rock type is actinolite-chlorite ± talc schist. Three sulfide assemblages are present: (1) pyrrhotite, pentlandite, chalcopyrite ± pyrite; (2) pentlandite, chalcopyrite, and pyrite; and (3) chalcopyrite, pyrite, and millerite. Assemblage (1) is present in all rock types and shows equilibrium textures in the fresh rocks. Assemblages (2) and (3) are present only in the metagabbronorite and chlorite-actinolite schist and show disequilibrium textures. Pentlandite is an important host for Pd in assemblages (1) and (2), the other important hosts for Pd in these assemblages are Pd tellurides. In assemblage (3) the Pd is found in a wide variety of platinum-group minerals (PGM); Pd tellurides, Pd sulfides, Pd antinomides, and Pd arsenides. The PGM in assemblages (1) and (2) are found in association with the sulfides, while in assemblage (3) they are found as isolated grains. Whole-rock geochemistry shows that the most of the rocks no matter what their texture or degree of alteration have similar compositions. Most compositions fall on plagioclase-orthopyroxene tie lines. Mantle-normalized patterns show that the rare earth element (REE) and high field strength element (HFSE) concentrations are low (0.8−2 times mantle) and similar. In this flat pattern there are positive Sr, Eu, Pb, and Sc anomalies. These observations are consistent with the rocks being plagioclase-orthopyroxene adcumulates. A small group of metagabbronorite and schist samples show MgO, CaO, and Cr enrichment, indicating the presence of some cumulate olivine, clinopyroxene, and/or chromite. The rocks no matter what their texture or degree of alteration have similar incompatible element ratios, indicating that they all are comagmatic. The low normative clinopyroxene concentrations and the low HFSE content of these rocks suggest that there is very little trapped liquid component present. This observation appears to be in contradiction to the field appearance of the magmatic breccia which indicates the matrix represents a frozen magma. We suggest that the magma chamber was being deformed at the time of intrusion and the fractionated liquid was squeezed out of both the matrix and fragments during this process. The formation of pegmatite and varitextured rocks could have occurred when the magma became fluid saturated and this fluid infiltrated the partially consolidated gabbronorite causing recrystallization. The composition of the varitextured and pegmatiodal rocks is similar to that of the other rocks and thus the fluid did not appreciably change the composition of the recrystallized rocks. Processes that have been considered for forming the ores include: collection by a sulfide liquid from a silicate magma; zone refining of the sulfides during repeated injections of magmas; and collection of the metals by deuteric or hydrothermal fluids. For samples from the Twilight and Roby zones there is a strong correlation between S, Cu, and PGE, indicating that sulfide minerals control the PGE and thus collection by a sulfide liquid could have occurred. However the high Pd/Pt ratio of the ores suggests that the sulfide liquid did not segregate from a primary mafic magma. Possibly, there was a feeder chamber to the Lac des Iles intrusion. The magma in the feeder system became saturated in sulfide liquid and this collected and crystallized in a structural trap between chambers. A fresh injection of S-undersaturated magma from the lower chamber partially melted the sulfides, enriching the magma in S, Cu, Pd, and Au. The Pd-enriched magma was then injected into the Lac des Iles chamber, mixed with the partially consolidated resident magma, and Pd-rich sulfides segregated from it. In the High-grade zone there appears to have been an additionally low temperature process that added Pd, Au, As, and Sb to these rocks. Possibly these elements were added by a fluid that exsolved from the magma in underlying magma chamber and which scavenged the elements from the sulfides formed at depth. The fluid was focussed in the shear zone between the East Gabbro and the Roby zone because most of the Lac des Iles intrusion had solidified at this point.

Origin and evolution of the Pleistocene magmatism of Linosa Island (Sicily Channel, Italy)

Abstract: Major, trace element and Sr-Nd isotope data are reported for volcanic rocks from Linosa Island (Sicily Channel) with the aim of discussing the genesis and evolution of magmatism at the northern margin of the African plate. The volcanic rocks exposed at Linosa exhibit a transitional to mildly Na-alkaline affinity and are mainly mafic in composition (alkali basalt to hawaiite); benmoreitic to trachytic lithic clasts occur in the lowest exposed pyroclastic deposits. Magmas have been erupted between 1.06 ± 0.10 and 0.53 ± 0.07 Ma during three main cycles of activity: Paleolinosa, Arena Bianca and Monte Bandiera. Major and trace element data indicate a magma evolution by dominant fractional crystallization. However, compatible vs. incompatible element diagrams highlight distinct variation trends, which are interpreted to suggest fractional crystallisation starting from slightly different parental magmas, and separation of distinct mineral assemblages during polybaric evolution. Small variation of Sr and Nd isotope ratios indicate modest interaction with the crust. As other mafic magmas in Eastern Sicily and Sicily Channel (Etna, Iblei, Pantelleria, Sicily Channel seamounts), the most primitive magmas at Linosa are characterised by enrichments in high-field-strength elements (Nb, Ta) and depletion in Rb, Cs and other large ion lithophile elements. Their isotopic signatures fall close to the field of the so-called EAR (European Asthenospheric Reservoir) and FOZO (Focus Zone) mantle compositions. However, there are many significant geochemical and isotopic differences among various volcanoes in Eastern Sicily and Sicily Channel, which suggest variable degrees of melting at different depths of a heterogeneous mantle source. Overall, radiogenic isotope signatures reflect mixtures between EAR-FOZO and DMM (Depleted MORB Mantle) and may be related to mixing of asthenosphere-lithosphere or to variably metasomatised lithospheric mantle.

The Caldwell Group lavas of southern Quebec: MORB-like tholeiites associated with the opening of Iapetus Ocean

Abstract: The Caldwell Group belongs to the Internal Nappe Domain of the Humber Zone and consists of basaltic lavas, quartzo-feldspathic sandstones, and mudslates. The lavas are clinopyroxene ± plagioclase ± olivine-phyric tholeiites, and are typically altered to epidote-, chlorite-, carbonate-, and (or) hematite-rich secondary assemblages. In most cases, the high field strength elements do not appear to have been perturbed by the alteration, and preserve magmatic signatures. Most Caldwell basalts exhibit coupled major and trace element variations compatible with low- to medium-pressure ( 10 kbar, where 1 kbar = 100 MPa) fractional crystallization. Paleotectonic discriminants imply an ocean-floor or normal mid-ocean ridge basalt (N-MORB) affinity. Most basalts have flat N-MORB-normalized profiles, except for the highly incompatible elements (Ba, Th, Nb), which show slight relative enrichment. Melting models suggest that most of these lavas formed by about 20% melting from a mantle slightly less depleted than fertil...

Geochemistry of Mesozoic volcanicr ocks in Da Hinggan Ling:The bimodal volcanic rocks

Abstract: The Mesozoic volcanic rocks in Da Hin ggan Ling,mainly formed from late Ju rassic to early Cretaceous,consist of alkaline series basalts,subalkaline series(high-K and low-K)basalts and associated intermediate -acid volcanics.Combining with the volcanic rocks of the same age in Russia and Mongolia,they constitute a mega-sc ale area-volcanic province.The alkali ne series basalts,similar to the int raplate alkaline basalts,are highl y enriched in incompatible elements.However,th ey are depleted in Nb and Ta,which is n evertheless similar to the calc-alk aline basalts from active continental margin and island arc.The sub-alkaline series basalts show intensive deple tion in HFSE and mildly enrichment in incomp atible elements,a characteristic o f basalts from active continental ma rgin and island arc.Amongst them the low - K basalts are similar to island arc th oleiite.Mesozoic rhyolites can be geochemically grouped into high-Sr and low-Sr subgoups.The high-Sr rhyolites,enriched in Ti,Ba,Sr,Co,Ni and depleted in highly incompatible element,such as Rb,Zr and Th,are sim ilar to the those in the continental flood basalt province formed by fractional crystallization.The low-Sr rhyolites are obviously enriched in Rb,Zr and Th and depleted in Ti,Ba,Sr and Co.They form,together with alkaline series basalts,typical bimodal pattern s similar to those in the continental r ift.The genetic association between sub-alkaline basalts and high-Sr r hyolites are comparable to the bimodal volcanic r ocks of continental extension setting.However,they constitute a successive geochemical variation without Daly gap,which suggests that the volcani c rocks in Da Hinggan Ling are much different from those in the continen tal flood basalt provinces in the world.It is therefore suggested that th e bimodality of continental flood basalts province is originated from dry magmatic systems where intermediate -composition magmas are more viscous than erupting basalts and contain le ss volatile than erupting rhyolites.In Da Hinggan Ling,the water -rich primary magma contributes to the oversatur ation of fractionated magma with volatile components,which result in the expl osive eruption of intermediate magm a.