Incompatible element

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

The transition from Calc-alkaline to Potassic Volcanism in the Aeolian Islands, Southern Italy

Abstract: The geochemical and petrological variations of a range of lavas from the Aeolian Islands volcanic arc, southern Italy are investigated and compared with potassic and ultra-potassic volcanism in the Roman region, and with subduction related basalts from throughout the world. Aeolian Islands lavas range from calc-alkaline basalts and andesites, typified by lavas from the island of Salina, to potassium rich shoshonites and tephrites found at Stromboli and Vulcano. Variations within individual lava series may be largely explained by fractional crystallisation, however the differences between the series are not due to this process and the various series are thought to originate from a range of parental magmas with quite different K2O content. Potassic lavas from Vulcano are characterised by incompatible element ratios typical of subduction related volcanism, and by relatively low 87Sr/86Sr ratios (0.70432-0.70494) similar to those of calc-alkaline lavas from Salina (0.70411-0.70466). In contrast at Stromboli, potassic lavas have relatively high Nb and Ta contents and elevated 87Sr/86Sr (0.7050-0.7075) A three component mixing model is proposed to explain the geochemistry of the Aeolian Islands lavas, this involves mantle wedge, a subduction related component thought to originate by dehydration of the subducting lithosphere, and a component derived from subducted sedimentary material. In addition some heterogeneity of the mantle wedge is indicated by within-plate style trace element enrichment in Stromboli lavas. A similar mixing model is applicable to Roman province lavas and also to subduction related basalts throughout the world. Evidence is presented to suggest that variations in certain incompatible element ratios may result from the involvement of small degree partial melting in the genesis of island arc basalts. However it is clear that such a process is not solely responsible for the anomalous trace element geochemistry of subduction related magmatism.

Trace element geochemistry of Raobazhai ultramafic complex, North Dabie Mountain

Abstract: The major and trace element compositions of 15 samples of 5 drill cores from Raobazhai ultramafic complex are reported here. The complex is composed of upper and lower parts. The ultramafic rocks are composted of predominantly harzburgite and subordinately dunite and lherzolite. It is deduced that the ultramafic complex is a part of sub-continental lithospheric mantle with different depletion of basaltic components by partial melting of fertile mantle in different degrees from the covariance among the major element compositions of the complex. Also, the complex experienced cryptical and modal mantle metasomatism that resulted in the enrichment of incompatible elements and formation of hydrated mineral phases such as amphibole and phlogopite after mantle partial melting. Comparing LREE and LILE enrichments, it is indicated that there exist two kinds of metasomatism by two different metamorphic agencies in the complex. The LREE enrichments were probably related with silicate melt from the asthenosphere by small degree partial melting, while the LILE (for example, Rb, Ba and K etc.) enrichments were probably related with the liquid from the subduction zone by devolatilization. The effect of metasomatism in lower part of the complex was stronger than that in upper part.

Geochemistry of eclogites of the Tso Morari complex, Ladakh, NW Himalayas: Insights into trace element behavior during subduction and exhumation

Abstract: Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure (UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs (Rb, Th, K except Ba) with LREE enrichments ((La/Lu)n = 1.28–5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of (La/Yb)n (2.1–9.4) and Ce/Yb (6.2–16.2) along with Ti/Y (435–735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams (viz. Ti–Zr–Y and Nb–Zr–Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts (WPB) to enriched MORB (E-MORB) indicating that the eclogite protoliths originated from basaltic magmas. Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB (E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities (viz. Zr, Nb, La/Yb, La/Gd, La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.

The origin of primitive ocean island and island arc basalts

Abstract: Fundamental aspects concerning the origin of ocean island basalts and primitive island arc magmas are addressed using examples from the Vanuatu Arc, Hawaii, and the Tasmantid Seamounts. A suite of alkali-olivine to tholefitic basalts, newly dredged from the Tasmantid Seamounts, are possible primary and near-primary compositions (e.g. Mg#'s 61-70, Ni = 221-322 ppm). Their bulk compositions correspond with those produced by experimental melting of peridotite between -1.0GPa (for tholeiites) and -2.5GPa (for alkali-olivine basalts), leaving residual mineralogies of (spine!) lherzolite and harzburgite. The inferred absence of residual garnet necessitates magma generation from sources with middle/heavy REE values >chondrites to account for the fractionated REE patterns of the Tasmantid basalts. An experimental liquidus study on a new Kilauea primary melt estimate (16wt% MgO), based on the most Mg-rich olivine phenocrysts occurring in Hawaiian lavas (i.e. Mg# 90.5), demonstrates equilibrium with mantle peridotite (harzburgite) at 2.0GPa and -14500C. Garnet is not a liquidus phase below -3.5GPa, reaffirming previous interpretations based on experimental studies, for shallow garnet-absent generation of Hawaiian olivine tholeiite and picrite primary magma estimates. In an effort to reconcile phase equilibria evidence for shallow melt segregation and trace element geochemistry arguments for deep garnet-present melting, geochemical models for dynamic melt segregation from an upwelling mantle plume have been assessed. These models are found to have little or no capacity to reproduce the geochemical characteristics of Hawaiian, or other ocean island tholeiites, if melting proceeds beyond the garnet peridotite stability field to shallower levels. Two possible models may account for the geochemical characteristics of ocean island tholeiites: (1) melting occurs entirely within the presence of residual garnet, requiring the generation of ultramagmesian primary melts (>20wt% MgO) that are capable of equilibrating at high temperature and pressure (>3.0GPa) with garnet peridotite; (2) melting of an incompatible element enriched source, bearing a "residual garnet" geochemical signature, occurs at relatively shallow levels (-1-2GPa) to produce olivine tholeiitepicritic primary melts. A suitable source enriched in incompatible elements is the oceanic lithosphere, fertilised by small melt fractions migrating from the underlying mantle, as is consistent with peridotite-C-H-0 phase equilibria and melt segregation considerations. Ambae is a site of voluminous eruptions of primitive olivine and clinopyroxene phenocryst-rich lavas in the Vanuatu Arc. Three distinct lava suites, all erupted in the previous 100Ka, can be identified on the basis of stratigraphy, phenocryst mineralogy, and geochemistry. The youngest suite, which mantles much of the island, ranges from highalumina basalt through to picritic compositions with up to -20 wt% MgO. Geochemical variation in this suite is controlled by fractional crystallisation (and accumulation) of magnesian olivine (to Mg# 93.4) and clinopyroxene (to Mg# 92), and accessory Cr-rich spinel. The liquid line of descent can be traced from an Mg-rich picritic parent (Mg#-81) to high-alumina basalts, in which crystallise calcic plagioclase (to An -90), relatively Ferich olivine (Mg#-80) and clinopyroxene (Mg#-80), and titanomagnetite. The dominant basaltic lava suites erupted throughout the Vanuatu Arc are notable for their primitive phenocryst assemblages, which comprise magnesian olivine and clinopyroxene, Cr-rich spinel, and calcic plagioclase. These assemblages enable identification of a range of Mg-rich parent magmas (Mg# 75-82), spanning low-K tholeiites to high-K alkali picrites, from which the spectrum of more evolved high-alumina basalts and andesites occurring in the arc are derived by fractionation processes. The primitive nature of the parent compositions provide unequivocal evidence for melt derivation from upper mantle peridotite. They require high temperature (>-13000C) melting of peridotite between -2 and -4 GPa, conditions which are likely to exist only within the core of the mantle wedge. Incompatible element geochemistry in the Vanuatu Arc parent magmas is dominated by the interaction of two components, which are separately responsible for LILE enrichment over LREE, and HFSE depletion relative to LREE. The enrichment of LILE over LREE, which dominates the low-K tholeiitic compositional end-member, is consistent with the introduction into the arc source of an LlLE (and Pb) -rich fluid, released from the dehydration of amphibole. The other component, which dominates highK alkaline end-member, is associated with strong enrichment of both 111 P. and LREE but not of HFSE, characteristics which suggest contamination of the source by a small melt fraction generated in the presence of a residual HFSE-bearing phase. A third source component, equivalent to the regional upper mantle (N-MORB source) peridotite, is recognised from the affinity of several lava suites to the N-MORB basalts of the North Fiji Basin. The majority of lava suites in the Vanuatu Arc, however, have very low HFSE and HREE (and sometimes also LREE) abundances (0.2-0.5x NMORB), which require their upper mantle protoliths to be considerably more refractory and depleted in incompatible elements than an N-MORB source.

The ubiquitous presence of silica‐rich glass inclusions in mafic minerals: Examples from Earth, Mars, Moon and the aubrite parent body

Abstract: — Highly silicic glass inclusions are commonly present in mafic minerals of xenolithic terrestrial upper mantle rocks (Schiano and Clocchiatti, 1994). They are believed to be the products of volatile-rich silicic melts for which several sources have been proposed (Francis, 1976; Frey and Green, 1974; Schiano et al, 1995), but their origin(s) and, consequently, that of the glasses, remains unknown. However, in situ formation by very low-degree partial melting seems to be possible as has been shown by experiments (e.g., Baker et al, 1995; Draper and Green, 1997). Glass inclusions of silicic chemical composition are also present in some mafic minerals of achondritic meteorites (e.g., Fuchs, 1974; Okada et al, 1988; Johnson et al, 1991). The enstatite achondrites (aubrites) Aubres and Norton County, which record early planetesimal and planet formation in the solar nebula, and the olivine achondrite (chassignite) Chassigny, a rock believed to originate from Mars, contain abundant glass inclusions in their main minerals enstatite and olivine, respectively. Glasses of glass-bearing inclusions have a highly silicic and volatile-rich chemical composition similar, but not identical, to that of glass inclusions in terrestrial upper mantle peridotite minerals. Furthermore, glass inclusions in olivines from the Moon (e.g., Roedder and Weiblen, 1977) are also silica-rich. Because different physicochemical conditions prevail in the source regions of these rocks, the process of melting is, perhaps, not generally applicable for the generation of silica-rich glasses. Alternatively, the glasses could have been formed via precipitation from silicate-loaded fluids (Schneider and Eggler, 1986) or vapors. Another possible mechanism, not previously identified, could be dehydrogenation of nominally nonhydrous mafic minerals by heating or depressurization that should be accompanied by expulsion of excess silica and incompatible elements. This process will mimic low-temperature, very low-degree partial melting. It could account also for the highly variable glass/bubble ratios observed in glass inclusions in aubrite enstatites. We suggest that such a process could have been operating in the solar nebula, the Moon and Mars, and could be operating still on Earth.