Incompatible element

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

Petrology and SHRIMP age of the Hongliugou ophiolite at Milan,north AItun,at the northern margin of the Tibetan plateau.

Abstract: The Hongliugou ophiolite in Milan is the best-developed and best-preserved body in the North Altun ophiolite belt at the northern edge of the Tibetan plateau.The ophiolite consists mainly of mantle peridotites,mafic-ultramafic cumulates,sheeted dikes and basaltic lavas,which crop out as tectonic blocks in a melange zone about 100km long.Individual blocks are variable in size,with the largest over 10km long and 1km wide.The mantle peridotites consist chiefly of harzburgite accompanied by minor dunite.The harzburgite consists mainly of olivine ( Fo_(91.2-92.7) ),orthopyroxene ( En_(93-98) ),and sparse clinopyroxene ( En_(46) ).Spinels in these rocks have Cr~# ( 100Cr/( Cr + Al) ) of 43 to 69 ( average 55 ) and Mg~# ( 100Mg/( Mg+Fe) ) of 43 to 64 ( average 58 ),typical of both abyssal and suprasubduction zone (SSZ) peridotites.The cumulates are composed of wehrlite,Ol-websterite,(Ol)-clinopyroxenite, gabbro and anorthosite,an assemblage similar those formed in SSZ settings.Both the sheeted dikes and basaltic lavas have relatively uniform,MORB-like compositions with 1%～1.5wt% TiO_2 and very low contents of K_2O (0.3 wt% ) and P_2O_5.Their MORB character is confirmed by their incompatible element compositions and their flat,LREE-depleted,chondrite-normalized REE patterns. A few ocean island basalts (OIB) have also been identified in the ophiolite.The two types of basalt suggest that some of the ophiolitic units in the melange may have originated in different tectonic settings.The presence of blueschists and eclogites in the melange, coupled with the occurrence of various subduction-related granitic rocks in the region,suggest that the melange zone represents a complex plate suture.Zircon from an ophiolitic gabbro yielded a SHRIMP U-Pb age of 479.4±8.5Ma,which represents the formation age of the ophiolite.Thus,the North Altun suture zone is comparable in age and character to the North Qilian suture on the east side of the Altyn Tagh fault,and confirms that the two belts were offset about 400km by sinistral movement on the Altyn Tagh fault.

Final Magma Storage Depth Modulation of Explosivity and Trachyte–Phonolite Genesis at an Intraplate Volcano: a Case Study from Ulleung Island, South Korea

Abstract: Ulleung Island is the top of a 3000 m (from sea floor) intraplate alkalic volcanic edifice in the East Sea/Sea of Japan. The emergent 950 m consist of a basaltic lava and agglomerate succession (Stage 1, 137-097Ma), intruded and overlain by a sequence of trachytic lavas and domes, which erupted in two episodes (Stage 2, 083-077Ma; Stage 3, 073-024Ma). The youngest eruptions, post 20 ka BP, were explosive, generating thick tephra sequences of phonolitic composition (Stage 4), which also entrained phaneritic, porphyritic and cumulate accidental lithics. Major element chemistry of the evolved products shows a continuous spectrum of trachyte to phonolite compositions, but these have discordant trace element trends and distinct isotopic characteristics, excluding a direct genetic relationship between the two end-members. Despite this, the Stage 3 trachytes and some porphyritic accidental lithics have chemical characteristics transitional between Stage 2 trachytes and Stage 4 phonolites. Within the phonolitic Stage 4 tephras three subgroups can be distinguished. The oldest,Tephra 5, is considerably enriched in incompatible elements and chondrite-normalized rare earth element (REE) patterns display negative Eu anomalies.The later tephras, Tephras 4-2, have compositions intermediate between the early units and the trachyte samples, and their REE patterns do not have significant Eu anomalies.The last erupted,Tephra 1, from a small intra-caldera structure, has a distinct tephriphonolite composition. Trace element and isotopic chemistry as well as textural characteristics suggest a genetic relationship between the phaneritic lithics and their host phonolitic pumices.The Stage 4 tephras are not related to earlier phases of basaltic to trachytic magmatism (Stages 1-3). They have distinct isotopic compositions and cannot be reliably modelled by fractional crystallization processes.The differences between the explosive phonolitic (Stage 4) and effusive trachytic (Stage 2-3) eruptions are mainly due to different pre-eruptive pressures and temperatures, causing closed-versus open-system degassing. Based on thermodynamic and thermobarometric modelling, the phonolites were derived from deeper (subcrustal) magma storage and rose quickly, with volatiles trapped until eruption. By contrast, the trachytes were stored at shallower crustal levels for longer periods, allowing open-system volatile exsolution and degassing before eruption. © The Author 2014. Published by Oxford University Press. All rights reserved.

Noble Gas Diffusion in Silicate Glasses and Melts

Abstract: Studies of noble gas diffusion and solubility provide information on the internal structure of silicate glasses, i.e., the size distribution and interconnection of cavities within the framework (Doremus 1994). Additionally, such diffusion data are of interest for understanding degassing kinetics of glasses and melts. Water and carbon dioxide, the most important volatiles in magmas, can be present in silicate melts both as unreacted (molecular) species and as dissociated species (hydroxyl groups or carbonate groups, respectively) (e.g., Holloway and Blank 1994; Kohn 2000). The contributions of the different species to the overall transport of the volatiles are difficult to separate. In the case of rhyolitic melts it was found that the diffusivity of Ar is similar to that of molecular CO2 (Watson 1994; Behrens and Zhang 2001). Assuming that this similarity is a general property for silicate melts, Nowak et al. (2004) analyzed the diffusion behavior in synthetic rhyolitic to basaltic melts and estimated the relative abundance of carbonate and molecular CO2 in the melt. Knowledge of noble gas diffusivity is also of importance for interpretation of geochemical findings both on short and on large scales. For instance, the isotopic compositions of noble gases have been extensively used to identify long-lived heterogeneities within the Earth’s mantle (Allegre et al. 1986; Graham 2002). The noble gases He, Ne and Ar are highly incompatible elements, which will be preferentially incorporated into a melt during melting of the mantle, so at partial melt fractions relevant to mid-ocean ridges a primary melt will inherit the relative noble gas abundance pattern of the mantle source (Burnard et al. 2004). Helium, on the other hand, is a geochemical tracer for mantle signatures in rocks. Measurements of both helium abundance and the 3He/4He ratios in volcanic rocks, gases and waters …