Compositional Spectrum Research Articles

The Origin and Evolution of Silica-saturated Alkalic Suites: an Experimental Study

Experimental simulation of incremental crystal fractionation of a hy-normative hawaiite indicates that the spectrum of compositions from mildly alkalic hawaiite to sodic rhyolite found in silica-saturated alkalic suites of the ocean islands and continental hotspots and rifts can be produced by fractionation at 9� 3kbar with bulk water contents4� 0� 5 wt % (in the hawaiite) at fO2 � 1� 5 log units below the fayalite–magnetite–quartz buffer (FMQ). Along this path, mildly alkalic basalt becomes increasingly alkalic because of the domination of clinopyroxene in the early fractionating assemblage and suppression of plagioclase. Kaersutite dominates at intermediate temperatures and results in stronger silica enrichment as the melt evolves to rhyolite. The fractionation assemblages are strongly pressuresensitive. At mid-crustal pressures, melts become potassic rather than sodic. At shallow conditions, the abundance of early olivine produces strong silica enrichment and subalkalic total alkalis to silica ratios. Natural mineral assemblages from silica-saturated alkalic suites show a polybaric history with fractionation at � 30km depth followed by decompression of liquids residual to this fractionation and crystallization, but not extensive fractionation, of lower-pressure assemblages. Equilibrium crystallization paths suggest that partial melting of hawaiite could produce the intermediate members of these suites provided that sufficient water was available in the source region.

Re-Os and Lu-Hf Isotope Constraints on the Origin and Age of Pyroxenites from the Beni Bousera Peridotite Massif: Implications for Mixed Peridotite-Pyroxenite Mantle Sources

A suite of pyroxenites from the Beni Bousera peridotite massif, northern Morocco, have been analysed for Re–Os and Lu–Hf isotopic compositions. Measured sections of the massif indicate that pyroxenite layers make up between 1 and 9% by volume of the total outcrop. Clinopyroxenes from two Cr-diopside pyroxenites have unradiogenic Hf isotope compositions (Hfi -7·7 to -8·5) whereas those of the Al-augite suite are more radiogenic (Hfi 9·4 to 25·6). In general, the Nd–Hf isotope compositions of the pyroxenites lie close to the mantle array. One garnet pyroxenite lies significantly below the mantle Hf–Nd isotope array such that it requires an ancient history characterized by high Lu/Hf and Sm/Nd but low Lu/Hf relative to Sm/Nd. As with the Sm–Nd and Rb–Sr systems, parent–daughter and isotopic ratios for the Lu–Hf system have been recently decoupled by a partial melting event associated with transfer of the massif from mantle to crust. This created highly fractionated Sm/Nd and Lu/Hf ratios in many rocks and the pyroxenites can be referred to as ‘residual’. The near-solidus extraction of a siliceous melt from the pyroxenites is also a possible explanation for the orthopyroxene-rich margins to numerous pyroxenite layers, via reaction with peridotite. Pyroxenite Os isotope compositions are much more radiogenic than their host peridotites. In contrast to the non-systematic Nd and Hf model ages, a large portion of the pyroxenite Re–Os model ages cluster between 1·2 and 1·4 Ga, within error of the model ages defined by many Ronda pyroxenites and close to the precise 1·43 ± 0·07 Ga Lu–Hf isochron defined by clinopyroxenes from the peridotites. The Re–Os system thus seems to have been more robust to late-stage melting events that decoupled Sm/Nd and Lu/Hf isotope systematics in the pyroxenites. In contrast to pyroxenites measured from Ronda, some Beni Bousera pyroxenites have relatively radiogenic Os isotope compositions at high Os concentrations (0·18 to >2 ppb), comparable with values reported for some cratonic pyroxene-rich xenoliths. In contrast to cratonic eclogites, most pyroxenites analysed here and those reported in the literature lie close to the mantle Nd–Hf isotope array. The Nd–Sr–Pb–Hf isotopic compositions and stable isotope characteristics of these pyroxenites reflect signatures from recycled oceanic crust and sediment. Hence, mixing of such material, if present within the convecting mantle, with peridotite, could account for some of the heterogeneity seen in oceanic basalts. Small amounts of pyroxenite incorporated into peridotite can also produce the radiogenic Os isotope signatures evident in the source of oceanic basalts. However, these observations alone do not require pyroxenite to be an integral part of the convecting upper-mantle magma source region. The spectrum of Nd, Hf and Os isotope compositions also makes them a suitable component to explain some of the isotopic characteristics of the source regions of ultrapotassic magmas.

Theistareykir revisited

Postglacial basalts from Theistareykir (younger than 10,000 years), northern Iceland, define the depleted end of the spectrum of chemical and isotopic compositions observed in Icelandic volcanics but extend to some of the most enriched chemical and isotopic compositions found in Icelandic tholeiites. A spectacular feature of these basalts is the impressive correlations observed between radiogenic isotope ratios (Sr, Nd, Hf, and Pb) and almost the entire spectrum of major and trace element concentrations and ratios. The radiogenic isotope and major and trace element compositions are little affected by crystal fractionation and are essentially unaffected by interaction with the preexisting crust. The Theistareykir basalts must therefore be relatively close in composition to primary melts from the mantle. Consequently, their chemical and isotopic compositions provide a unique opportunity to investigate the nature of melting beneath Iceland and the geochemical character and origin of the mantle source. Large variations in incompatible element abundances require source heterogeneity, as well as variable extents of melting, to be important factors in determining the final chemical composition of the melts. Melting integrates over a large pressure range and is dominated by melting a depleted peridotite similar to the ambient depleted North Atlantic mantle. The isotopically enriched component is of relatively minor abundance and probably has a lower solidus temperature compared to the depleted component. More than one isotopically enriched component must be involved, but it is difficult to identify the end‐member compositions using those of the lavas because of preeruptive averaging and damping of the enriched isotopic signals by mixing with the ambient depleted mantle or melts thereof, suggesting that the isotopic signals in Icelandic melts represent a somewhat muted isotopic signal of the enriched component(s) in the Icelandic source mantle. Comparison of the isotopic arrays of Icelandic basalts with those of global OIB suggests that the dominant enriched component may have a HIMU (high μ) affinity rather than representing a component similar to the enriched end of the Iceland isotopic arrays, and small amounts of an enriched component similar to enriched mantle (EMI)‐ type OIB sources are probably also involved.

Experimental constraints on volatile abundances in arc magmas and their implications for degassing processes

Abstract Recent phase equilibrium studies, combined with analytical and petrological data, provide rigorous constraints on the pre-eruptive P-T-f H 2 O- f O 2 - f S 2 - f CO 2 conditions of silicic to mafic arc magmas. Pre-eruptive melts show a broad negative correlation between temperature and melt H 2 O contents. Pre-eruptive melt S contents cluster around 100 ppm in residual rhyolitic liquids of silicic to andesitic magmas, and range up to 5000 ppm in more mafic ones. For the entire compositional spectrum, melt sulphur contents are almost independent of prevailing f O 2 . In contrast, they are positively correlated to f S 2 , in agreement with experimental observations. Using these intensive constraints, the composition of coexisting fluid phases has been modelled through a MRK equation of state. Pre-eruptive fluids in silicic to andesitic magmas have X H 2 O (mole fraction of H 2 O) in the range 0.65–0.95. X H 2 O decreases as pressure increases, whereas X CO 2 increases up to 0.2–0.3. Pre-eruptive fluids in hydrous mafic arc magmas, such as high-alumina basalts, generally have similar mole fractions of H 2 O and CO 2 at mid-crustal levels, with X H 2 O increasing only for magmas stored at shallow levels in the crust (<1 kbar). The sulphur content of the fluid phase ranges from 0.12 up to 6.4 wt% in both mafic and silicic magmas. For silicic magmas coexisting with 1–5 wt% fluid, this implies that more than 90% of the melt+fluid mass of sulphur is stored in the fluid. Calculated partition coefficients of S between fluid and melt range from 17 up to 467 in silicic to andesitic magmas, tending to be lower at low f O 2 , although exceptions to this trend exist. For mafic compositions, the sulphur partition coefficient is constant at around 20. The composition of both melt and coexisting fluid phases under pre-eruptive conditions shows marked differences. For all compositions, pre-eruptive fluids have higher C/S and lower H/C atomic ratios than coexisting melts. Comparison between volcanic gas and pre-eruptive fluid compositions shows good agreement in the high temperature range. However, to reproduce faithfully the compositional field delineated by volcanic gases, silicic to andesitic arc magmas must be fluid-saturated under pre-eruptive conditions, with fluid amounts of at least 1 wt%, whereas mafic compositions require lower amounts of fluid, in the range 0.1–1 wt%. Nevertheless, volcanic gases colder than 700 °C are generally too H 2 O-rich and S-poor to have been in equilibrium with silicic to andesitic magmas under pre-eruptive conditions, which suggests that such gases probably contain a substantial contribution from meteoric or hydrothermal water.

Cadomian Lower-crustal Contributions to Variscan Granite Petrogenesis (South Bohemian Batholith, Austria): a Comment

of opx, through partial melting of heterogeneous, highPREAMBLE K, biotite-bearing, lower-crustal source rocks. We inKlotzli et al. (2001) present a genetic model for a particular terpreted the orthopyroxenes in the SQMD rocks as granitic rock unit within the Variscan (Carboniferous) having crystallized early, and probably at somewhat South Bohemian Batholith. We studied and published higher P than the final emplacement of the magma. In on these same rocks some years previously (Finger & the course of protracted cooling and magmatic crysClemens, 1995). The interpretations of Klotzli et al. (2001) tallization, in a crustal section already heated by regional contrast so significantly with our own that we consider metamorphism, the orthopyroxenes were affected by it necessary to comment briefly on their paper. various exsolution and recrystallization processes. They The rock unit in question is the so-called Sarleinsbach quartz monzodiorite (SQMD), first described by Frasl & reacted with the residual melts and became marginally Finger (1988). This unit is an opx-bearing facies of the overgrown and replaced by clinopyroxene, amphibole Weinsberg granite—a very coarse-grained biotite granite and biotite–quartz symplectites (Haunschmid & Finger, and the most prominent rock type in the batholith. As 1994). Such magmatic overgrowths are to be expected with all granitic units assigned to the Weinsberg type, because aH2O and aKAlSi3O8 both increase in the the SQMD contains large K-feldspar phenocrysts. On residual liquids as crystallization proceeds toward the chemical variation diagrams, the data for the SQMD solidus. join smoothly with the other Weinsberg points, at the In contrast with this, Klotzli et al. (2001) hypothesize more mafic end of the compositional spectrum. Although that the pyroxenes are xenocrystal in origin, derived from such a variation is by no means definitive, this and the a Cadomian magmatic rock that crystallized some 200 field and textural evidence suggest that the SQMD is Myr earlier, and accidentally incorporated into the part of the Weinsberg granite. This has long been held Weinsberg magma. Klotzli et al. arrive at this conclusion to be the case. mainly through a geochronological study of the zircons of the SQMD. This work reveals the existence of a population of zircons with Variscan ages (interpreted as dating the formation and intrusion of the granite magma), ORIGIN OF PYROXENES IN THE and morphologically distinct (so-called J4-type) zircons SQMD with Cadomian ages (>520 Ma), clearly pre-dating the Variscan magmatic cycle. They observed the relationship We (Finger & Clemens, 1995) considered that the that pyroxene-rich variants of SQMD have a sysWeinsberg magma (as a whole) originated under granulite-facies conditions, i.e. within the P–T stability field tematically greater proportion of J4-type zircons than

Andesite petrogenesis in the Ordovician Borrowdale Volcanic Group of the English Lake District by fractionation, assimilation and mixing

A suite of composite basalt–andesite dykes in the Ordovician Borrowdale Volcanic Group of the English Lake District records magmatic evolution from basalt to andesite. Although altered, the rocks preserve textures that indicate magma mixing. Primitive basalts were derived from a fertile, subduction-modified mantle source. Evolution by fractional crystallization is suggested by rapid decrease of compatible trace element concentrations and similar incompatible trace element and REE profiles throughout the sample range. Recharge of the evolving magma with fresh batches of basaltic composition is indicated by some samples that have compatible trace element concentrations much higher than expected from their otherwise evolved compositions. Geochemical evidence of magma mixing is provided by linear trends between pairs of incompatible trace elements not at constant ratio. e Nd values indicate addition of a crustal component, but samples do not conform to a simple mixing equation. The combined evidence points to magma evolution by assimilation of wallrock during fractional crystallization (AFC) with periodic recharge. We have modelled an AFC process using a Skiddaw Group metasediment contaminant that can explain trace element and isotopic variation through variable amounts of assimilation, crystal fractionation and recharge, to produce a spectrum of compositions that was sampled periodically by the dykes. There is evidence from other parts of the Borrowdale Volcanic Group that similar processes operated throughout its history.

Compositional spectrum—revealing patterns for genomic sequence characterization and comparison

In this paper we propose a natural approach to characterizing genomic sequences, based on occurrences of fixed length words (strings over the alphabet { A, C, G, T}) from a sufficiently large set W of arbitrary (in general case) words. According to our approach, any genomic sequence can be characterized by a histogram of frequencies of imperfect matching of words from the set W that is called a compositional spectrum (CS). The specificity of CSs is manifest in a reasonable similarity of spectra obtained on different stretches of the same genome and, simultaneously, in a broad range of dissimilarities between spectral characteristics of different genomes. The proposed approach may have various applications in intra- and intergenomic sequence comparisons.

Emplacement processes of the mafic Villa Senni Eruption Unit (VSEU) ignimbrite succession, Colli Albani volcano, Italy

The Villa Senni Eruption Unit (VSEU) represents the last large explosive event of the Tuscolano–Artemisio phase of volcanic activity in the Colli Albani (C.A.) (Alban Hills) Volcanic District, situated about 25 km south east of Rome, Italy. The VSEU succession consists of pyroclastic fallout, density current deposits, covering an area of 1500 km 2, reaching a distance of 35 km or more from vent, and having a volume of 30 km 3 DRE. The VSEU is unusual in that its juvenile products are K-foiditic and phono-tephritic in composition, with less than 50% SiO 2 content. The most important products of the VSEU eruption are two pyroclastic density current depositional units; the Tufo Lionato (TL) and Pozzolanelle (TP), with each deposit containing several recognisable internal facies types. A short time gap (hours to days) between the deposition of these two ignimbrites is inferred due to the lack of a palaeosoil or evidence of fluvial reworking at the contact between the two units. A new stratigraphic framework for the VSEU is presented based on field observations, component analyses and detailed lateral correlations, with several new stratigraphic subdivisions proposed. The eruptive sequence is subdivided into depositional facies types, each related to a discrete phase of eruption, and a reconstruction of the three-dimensional geometry and the internal facies association within the VSEU is presented. This reconstruction is used to assess the pyroclastic density current emplacement processes resulting from this ignimbrite forming explosive mafic eruption. The morphology of and internal structures preserved within the mafic VSEU are comparable to those seen in more silicic ignimbrite sequences, suggesting that similar processes of emplacement by pyroclastic density currents can be applied over the entire compositional spectrum. We interpret lateral and vertical facies changes in the VSEU ignimbrite deposits to be caused by changes in the palaeotopographic surface over which the depositing pyroclastic density currents were travelling, as well as spatial and temporal changes in the dynamics of the pyroclastic density currents responsible for their deposition. In particular, interaction between the emplacing pyroclastic density current and the underlying palaeotopography caused local changes in current dynamics, leading to a wide range of localised flow structures and internal facies complexity. These interpretations are made possible because the long axes of valleys into which the VSEU was emplaced, which occur as part of the drainage network surrounding the C.A. volcano, are aligned from 0 to 90° to the radial pyroclastic current direction. The erosive power of pyroclastic density currents is recorded by an erosive contact between the TL and TP ignimbrites in proximal exposures (<12 km) to the north west of the C.A. volcano.

Melt evolution beneath thick lithosphere: a magmatic inclusion study of La Palma, Canary Islands

Volcanism in the Canary Islands is notable for its highly alkalic character even in the shield building lavas of the currently most active island, La Palma. In order to understand better the processes responsible for this alkalic end of the compositional spectrum of ocean island basalts (OIB), we have studied primary magmatic inclusions in olivine and clinopyroxene from La Palma. The investigated primary melt and fluid inclusions are hosted by olivine and clinopyroxene from lavas of two ∼700-ka, continuous stratigraphic sections in the Fagundo and Izcagua barrancos of the main shield volcano Taburiente. Lavas from the two barrancos show contrasting whole-rock compositions: Izcaqua lavas are highly alkalic or “basanitic”, typical of La Palma lavas in general, whilst the Fagundo lavas are characterised by more silica-rich, “transitional”, compositions. Although whole-rock compositions of the lavas from the two barrancos show different degrees of silica undersaturation, they show a similar range of elemental abundances such as MgO. In contrast, the Fagundo melt inclusions show a wider range, to more mafic compositions, than those from the Izcagua samples. This is in keeping with more variable partial homogenisation temperatures and host mineral compositions in the Fagundo samples, which are also generally higher (1130–1260 °C) and more primitive ( F o 76–87) than in the Izcagua suite. Indeed, the Izcagua melt inclusions are hosted in relatively evolved liquidus mineral assemblages ( F o<82), and have (partial) homogenisation temperatures of 1080–1140 °C. The Izcagua samples lie at the differentiated end of a compositional array defined by the Fagundo melt inclusions suggesting that these basanites are derived by fractionation of more primary transitional melts. Fractionation of a clinopyroxene-rich assemblage can quantitatively account for production of an Izcagua-like melt from a mafic Fagundo parent. Modelling of phase equilibria suggests this process is consistent with differentiation at ∼12 kbar, which is also in keeping with clinopyroxene geobarometry in our samples. The basanitic lavas that largely comprise the shield building phase of La Palma have previously been interpreted by low degrees of melting as a result of the thick (Jurassic) lithospheric lid lying above the Canary mantle plume. Our study instead suggests deep fractionation as the major process leading to the highly alkalic nature of lavas in La Palma and possibly other ocean islands.

Natural fire regime: a guide for sustainable management of the Canadian boreal forest

The combination of certain features of fire disturbance, notably fire frequency, size and severity, may be used to characterize the disturbance regime in any region of the boreal forest. As some consequences of fire resemble the effects of industrial forest harvesting, conventional forest management is often considered as a disturbance that has effects similar to those of natural disturbances. Although the analogy between forest management and fire disturbance in boreal ecosystems has some merit, it is important to recognise that it also has its limitations. Short fire cycles generally described for boreal ecosystems do not appear to be universal; rather, important spatial and temporal variations have been observed in Canada. These variations in the fire cycle have an important influence on forest composition and structure at the landscape and regional levels. Size and severity of fires also show a large range of variability. In regions where the natural matrix of the boreal forest remains relatively intact, maintenance of this natural variability should be targeted by forest managers concerned with biodiversity conservation. Current forest management tends to reduce this variability: for example, fully regulated, even-aged management will tend to truncate the natural forest age distribution and eliminate over-mature and old-growth forests from the landscape. We suggest that the development of strategic-level forest management planning approaches and silvicultural techniques designed to maintain a spectrum of forest compositions and structures at different scales in the landscape is one avenue to maintain this variability. Although we use the boreal forest of Quebec for our examples, it is possible to apply the approach to those portions of the boreal forest where the fire regime favours the development of even-aged stands in burns.

Olivine‐poor sources for mantle‐derived magmas: Os and Hf isotopic evidence from potassic magmas of the Colorado Plateau

The contribution of olivine‐poor lithologies (pyroxenite, eclogite, etc.) in the mantle to the composition of mantle‐derived melts is a growing topic of discussion in light of results suggesting that some magmatic characteristics are not easily explained by melt derivation solely from peridotitic sources. To investigate this issue, we have examined the Hf and Os isotopic composition of potassic magmas from the Navajo Volcanic Field of the Colorado Plateau. Most of the magmas have Os isotopic compositions much more radiogenic than observed for any uncontaminated oceanic basalt. The Os isotope compositions of these samples overlap those measured for crustal/upper mantle eclogitic xenoliths carried by nearby intrusions. Compositional and Os isotope systematics, however, suggest that the characteristics of the magmas are not determined by crustal contamination, but rather are the result of melting of pyroxene‐ and mica‐rich veins/layers in the lithospheric mantle beneath the plateau. Once generated, the melts of olivine‐poor lithologies interacted with surrounding metasomatized peridotite to create the compositional spectrum from katungite to minette. One component of this compositional variation is the appearance of strong relative depletions of high field strength elements in the minette end‐member, suggesting that this chemical signature, common in convergent margin magmas, may be caused by melt‐wall rock interaction.

U/Pb and Pb/Pb zircon ages from granitoid rocks of Wallagga area: constraints on magmatic and tectonic evolution of Precambrian rocks of western Ethiopia

The Precambrian rocks in western Ethiopia consist of high- and low-grade terranes intruded by granitoids with a wide compositional spectrum. The formation ages of these granitoid rocks are, so far, poorly understood. Single-grain zircon Pb/Pb evaporation and conventional U/Pb dating conducted on four granitoids places time constraints on their emplacement and tectonothermal events. Three granitoid magmatic events were identified at 815 Ma, 700–730 Ma, and 620–625 Ma, which were marked by emplacement of the calc-alkaline Ujjukka granite and granodiorite, the anatectic Suqii-Wagga two-mica granite and the Guttin K-feldspar megacrystic granite, and the anorogenic Ganjii monzogranite, respectively. We interpret the 815 Ma age to mark a major magmatic episode in this part of Africa. A tectonothermal event at ∼ 630 Ma preceded the emplacement of the within-plate granitoids at 620–625 Ma. The decrease of ages from the calc-alkaline to anorogenic granitoids suggests a shift of magmatic styles and tectonic setting of the granitoids over a period of 200 million years. The Suqii-Wagga and Guttin granites, representing the granitoid population in the migmatitic terrane, formed as part of the successive evolution of the granitoid magmatism in the region. The presence of xenocrystic zircons of Mesoproterozoic ages in both granitoid populations emplaced into the low-grade volcanosedimentary sequence and the high-grade, often migmatitic, gneisses suggest contribution of pre-Pan-African crust to the origin and evolution of the granitoids. Conventional U/Pb studies of zircons from the Guttin K-feldspar megacrystic granite and the Ganjii monzogranite yielded upper intercept ages of ∼ 3 Ga and ∼ 2 Ga, respectively, possibly indicating the presence of reworked Archean-Proterozoic crustal material.

The Natash alkaline volcanic field, Egypt: geochemical and mineralogical inferences on the evolution of a basalt to rhyolite eruptive suite

Extensive lava flows were erupted during the Upper Cretaceous in the Wadi Natash of southern Egypt. The lavas are mainly of alkaline (sodium dominated) composition and include alkali olivine basalt (AOB), hawaiite, mugearite, and benmoreite that intruded with acidic volcanics of trachytic to rhyolitic composition. Abundances of major oxides and trace elements including the REE vary systematically through this compositional spectrum. The gradual decrease of CaO with decreasing MgO is consistent with the dominance of phenocrysts of labradoritic plagioclase (An 75–62) and Mg-rich olivine (Fo 84–80) in the AOB and hawaiite. Olivine phenocrysts are normally zoned with cores consistent with crystallization from a magma having the bulk-rock composition. The sharp decrease of alkalis at low MgO contents (∼0.4% MgO) indicates significant alkali feldspar fractionation during the evolution of trachytes and rhyolites. All Natash lavas show steep chondrite-normalized REE patterns with considerable LREE/HREE fractionation and a regular decrease in La/Lu ratios from the least to the most evolved lavas (La/Lu n =12.5−9.5). The low absolute abundances of HREE in basic members reflects residual garnet in the source. The basic lavas have experienced compositional modifications after they segregated from the source as evidenced by lower averages of Mg # (51), Ni (134) and Cr (229) in the AOB. Much of this variation can be explained by variable degrees of polybaric fractional crystallization. Petrographic and geochemical data supported by quantitative modelling suggest the evolution of the Natash Lavas from a common AOB parent in multiple, short-lived magma chambers. In agreement with the phenocryst mineralogy of the Natash lavas, the geochemical models suggest that with increasing degree of differentiation, Mg-rich olivine, calcic plagioclase, and augite are joined and progressively substituted by ferrohedenbergite, alkali feldspars and magnetite. The OIB (ocean island basalt)-like nature of the AOB and hawaiite lavas suggests that the volumetrically dominant source component is the asthenospheric mantle. A mantle-plume source is suggested for the Natash basaltic lavas, with the lavas being generated by partial melting of a garnet peridotite in the asthenosphere.

Geochemistry of near‐trench intrusives associated with ridge subduction, Seldovia Quadrangle, southern Alaska

An extensive belt of plutons and related dikes along the southern Alaska margin was emnplaced into the accretionary prism anomalously close to the paleotreneh and far seaward of a coeval magmatic arc. Numerous workers have attributed the near‐trench magmatism to early Tertiary subduction of the Kula‐Farallon Ridge beneath North America. We present new major, minor, and trace element data for a dike swarm in the Seldovia Quadrangle, southern Kenai Peninsula, that is part of the near‐trench Sanak‐Baranof belt. The dikes are generally calc‐alkaline and cover the entire compositional spectrum from basalts to rhyolites. The basalts and andesites of obvious mantle (I type) origin compositionally grade into dacites and rhyolites having a significant sedimentary (S type) component. Trace element modeling suggests that the dikes evolved from parental magmas, some possibly as refractory as island arc tholeiites, through assimilation of sedimentary material coupled with fractional crystallization. The dikes appear to be products of interaction between older sedimentary rocks at the base of the accretionary prism and basaltic magmas that were emplaced by the subducting ridge (slab window). I‐type basalts and basaltic andesites are products of mafic liquids that were only slightly contaminated by sedimentary material, whereas more extensive assimilation produced S‐type dacites and rhyolites. This idea is supported by the geographical distribution of dikes within the forearc. Basalts, andésites, and low‐silica dacites are found throughout the accretionary complex, whereas highsilica dacites and rhyolites only occur at some distance inboard from the paleotrench where the sedimentary column is thicker and more assimilation was likely to have taken place. We suggest that the subducting spreading center produced basaltic liquids that became more refractory closer to the paleotrench such that subducted portions of the ridge formed parental melts having the composition of island arc tholeiites. The basaltic liquids were injected into the overlying accretionary prism where variable assimilation of sedimentary material produced the wide range of dike compositions.

Spinels and Mg Ilmenites from the Noril'sk 1 and TalnakhIntrusions and Other Mafic Rocks of the Siberian Flood Basalt Province

Spinels from the mineralized Noril’sk 1 and Talnakh intrusions show a broad and distinctive compositional spectrum, from normal chromite to Ti-enriched aluminous magnetite. In the Talnakh intrusion, in particular, the range of spinel compositions extends across a compositional space where spinels are rare in other layered intrusions. Spinel compositions vary widely on a scale of individual thin sections and are influenced by the nature of the enclosing silicate phase. Spinels enclosed in olivines are less Ti enriched than those outside olivine grains. Compositional variance in the spinels is attributed to variable but generally advanced reaction between cumulus chromite grains and solidifying trapped intercumulus liquid. The advanced nature of the process is attributed to the open nature of the intrusions. Continuous, prolonged flow-through of hot magma enabled cumulates within the intrusions to remain above their solidus temperature for longer than would be normal for sills of comparable thickness. The barren Chibacheka River intrusion shows some of the same features but differs from Talnakh in that spinel-olivine pairs record equilibration temperatures higher than those at Talnakh. Temperatures recorded by spinel-olivine pairs at Talnakh are anomalously low for such a small intrusion, again indicating prolonged slow cooling consistent with a flow-through model for the sill. Spinel compositions support a hypothesized comagmatic relationship between the Chibacheka River intrusion and the Ti-, Cr-rich Gudchichinsky picritic basalts. Spinels in the Noril’sk and Talnakh intrusions imply a parent magma with a lower Cr/Al ratio than the Gudchichinsky picritic basalts. Spinels from the Siberian intrusions show a similar range in compositions to the feeder intrusions of the Karoo flood basalt province. Jinchuan spinels share the features of high Ti contents and anomalously low equilibration temperatures with the Noril’sk-Talnakh spinels. Mg ilmenites are a feature of the Siberian and Jinchuan intrusions. They are attributed to crystallization of ilmenite from evolved trapped intercumulus liquid buffered by surrounding Mg-rich cumulus phases. Their existence implies moderately but not unusually Ti-rich parent magmas, consistent with compositions of the Siberian Traps lavas.

Multiple pyroxene and amphibole assemblages in the amphibolite facies: Bulk compositional controls

This paper presents studies of samples from different localities of the Garnet-Amphibole-Magnetite (GAM)-Quartzite Member of the Ammonoosuc Volcanics from southern New England, USA. Based on stratigraphic and geochemical evidence, earlier workers interpreted the Ammonoosuc Volcanics as part of a volcanic island-arc complex. As such, these rocks have a highly varied stratigraphy with a wide spectrum of chemical compositions. During subsequent metamorphism, these rocks reached amphibolite-facies conditions (6 kbar and 580–640 °C). Within the Ammonoosuc Volcanics, a fine-grained, garnet-, magnetite-, and amphibole-bearing quartzite forms a distinct stratigraphic horizon. The quartzite commonly shows delicate millimeter- to centimeter-thick layering, which is interpreted as an original depositional feature that could have formed by precipitation from solutions enriched in silica, iron, and manganese. Volcanic detritus that was rich in Al, Ca, Mg, and Na could account for the variation in concentration of these elements, which are present along with Fe, Mn, and Si in some laminae of these quartzites. The mineralogy of layers within these quartzites can vary greatly on the scale of several centimeter. The most mineralogically complex layers contain, in addition to quartz, garnet, and magnetite, and one or more of the amphiboles cummingtonite, anthophyllite, or hornblende. In the most recent investigations of these rocks, both orthopyroxene and clinopyroxene have been identified in a few samples. A comparison of Mn contents of minerals that are common to different assemblages indicates that the pyroxenes are present in bulk compositions rich in Mn. The Mn content of the bulk composition is the most important factor controlling the mineralogy, and increasing the Mn content causes changes in the mineralogy that mimic the same progression of assemblages that is seen in mafic rocks undergoing the amphibolite- to granulite-facies transition.

The origin of composition gaps at South Sister volcano, central Oregon: implications for fractional crystallization processes beneath active calc-alkaline volcanoes

South Sister is an active, calc-alkaline strato-volcano located in west-central Oregon in the high Cascades. South Sister contains lavas that range from basalt (50% SiO2) to rhyolite (75% SiO2). Within this compositional spectrum the lavas are restricted to three distinct populations: basalt/basaltic-andesite, andesite and rhyolite. Compositional, textural and mineralogic criteria indicate that the basaltic-andesites are hybrid mixtures of basalt and andesite magma. This implies that the system is characterized by basalt, andesite and rhyolite magma populations that are separated from one another by composition gaps. Major, minor and trace element modeling suggests that the three magma populations are related to one another primarily through fractional crystallization. We propose a standard fractionation model for generating compositional diversity amongst erupted lavas wherein in situ fractional crystallization occurs within a downward descending solidification front. Extraction and eruption of the interstitial fractionated liquid, now separated from the parental liquid by a composition gap, occurs only after the percentage of crystallization within the solidification front has exceeded the appropriate critical crystallinity value for the given parental magma. We envision a magmatic plumbing system characterized by multiple, independently solidifying bodies of magma. Thus, at any given time, discrete bodies of basalt, andesite and rhyolite magma may exist. Interaction and mixing among these bodies could generate the continuous range of lava compositions that characterize most calc-alkaline volcanic complexes.

Apatite in the mantle: implications for metasomatic processes and high heat production in Phanerozoic mantle

The abundance of apatite in Phanerozoic mantle may be greatly underestimated. This study shows that apatite has a widespread occurrence in Phanerozoic lithospheric mantle and can be divided into two geochemically distinct types using halogen content, presence or absence of structural CO 2, Sr and trace element (especially U, Th, and light rare earth) ratios and abundances, and association with either metasomatised mantle wall-rock peridotites (Apatite A) or high-pressure magmatic crystallisation products (Apatite B). Apatite A is inferred to result from metasomatism by CO 2- and H 2O-rich fluids derived from a primitive mantle source region, while Apatite B compositions are consistent with crystallisation from magmas within the carbonate–silicate compositional spectrum. The presence of significant apatite in the lithospheric mantle is important not only for the geochemical budget but also for assessing heat production and heat flow in the mantle. The measured U and Th contents of mantle apatite average 60 and 200 ppm, respectively and 0.5% apatite would dominate heat production. Metasomatised mantle may also contain amphibole and mica with K 2O and clinopyroxene with detectable U and Th. In lithospheric mantle with a thickness of 70 km, this abundance of apatite would result in mantle heat flow contribution of about 12 mW/m 2, a significant proportion of the total “normal” mantle heat flow of about 18 mW/m 2.

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

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.

Hydration of lipoplexes commonly used in gene delivery: follow-up by laurdan fluorescence changes and quantification by differential scanning calorimetry

Lipoplexes, which are formed spontaneously between cationic liposomes and negatively charged nucleic acids, are commonly used for gene and oligonucleotide delivery in vitro and in vivo. Being assemblies, lipoplexes can be characterized by various physicochemical parameters, including size distribution, shape, physical state (lamellar, hexagonal type II and/or other phases), sign and magnitude of electrical surface potential, and level of hydration at the lipid–DNA interface. Only after all these variables will be characterized for lipoplexes with a broad spectrum of lipid compositions and DNA/cationic lipid (L +) mole (or charge) ratios can their relevance to transfection efficiency be understood. Of all these physicochemical parameters, hydration is the most neglected, and therefore the focus of this study. Cationic liposomes composed of DOTAP without and with helper lipids (DOPC, DOPE, or cholesterol) or of DC-Chol/DOPE were complexed with pDNA (S16 human growth hormone) at various DNA −/L + charge ratios (0.1–3.2). (DOTAP= N-(1-(2,3-dioleoyloxy)propyl)- N,N,N-trimethylammonium chloride; DC-Chol=(3β-[ N-( N′, N′-dimethylaminoethane)-carbamoyl]-cholesterol; DOPC=1,2-dioleoyl- sn-glycero-3-phosphocholine; DOPE=1,2-dioleoyl- sn-glycero-3-phosphoethanolamine). The hydration levels of the different cationic liposomes and the DNA separately are compared with the hydration levels of the lipoplexes. Two independent approaches were applied to study hydration. First, we used a semi-quantitative approach of determining changes in the ‘generalized polarization’ (GP) of laurdan (6-dodecanoyl-2-dimethylaminonaphthalene). This method was recently used extensively and successfully to characterize changes of hydration at lipid–water interfaces. Laurdan excitation GP at 340 nm (GP 340) was found to be the most informative parameter in our studies, suggesting that changes in hydration upon lipoplex formation do not involve a major liquid-crystalline to gel transition. DC-Chol/DOPE liposomes were the ‘driest’ of all liposome compositions. Among DOTAP-based cationic liposomes, the order of the ‘driest’ to the ‘wettest’ composition is DOTAP/Cholesterol>DOTAP/DOPE>DOTAP/DOPC>DOTAP. The GP 340 of lipoplexes of all lipid compositions (except those based on DC-Chol/DOPE) was higher than the GP 340 of the cationic liposomes alone and increased with increasing DNA −/L + charge ratio, reaching a plateau at a charge ratio of 1.0, suggesting an increase in dehydration at the lipid–water interface with increasing DNA −/L + charge ratio. Confirmation was obtained from the second method, differential scanning calorimetry (DSC). DOTAP/DOPE lipoplexes with charge ratio 0.44 had 16.5% dehydration and with charge ratio 1.5, 46.4% dehydration. For DOTAP/Chol lipoplexes with these charge ratios, there was 17.9% and 49% dehydration, respectively. These data are in good agreement with the laurdan data described above. They suggest that the dehydration occurs during lipoplex formation and that this is a prerequisite for the intimate contact between cationic lipids and DNA.