Showing papers on "Incompatible element published in 1988"

Regional variations in bulk chemistry, mineralogy, and the compositions of mafic and accessory minerals in the batholiths of California

Abstract: We define regional variations in mafic and accessory mineral assemblages and compositions and expand the current understanding of spatial variations in whole-rock geochemistry in the batholiths of California. In so doing, we gain new insights into the nature of magmatic source rocks and mechanisms of magma generation in volcano-plutonic arcs of active continental margins. Little-studied metaluminous to strongly peraluminous granites containing Fe-rich biotite with log(X Mg /X Fe ) F/OH and Mn in biotite and amphibole increase on a regional scale from western I-WC types to eastern I-MC and I-SC types, parallel to eastward increases in incompatible elements and decreases in compatible elements in the plutons. In contrast, the belts of western I-SCR granites and eastern I-WC quartz diorites and granodiorites disrupt the regional west-to-east systematics in both mineral and whole-rock geochemistry. Spatial variations in the Al content of amphibole are regional in scale and reflect pressures of pluton crystallization. We conclude that significant, previously unrecognized complexity exists in regional geochemical systematics in the California batholiths.

Solidus curves, mantle plumes, and magma generation beneath Hawaii

Abstract: The eruption of nepheline-normative lavas in the early and late stages of formation of the large Hawaiian tholeiite shields is well established, as is the conclusion that volatile components are involved in the genesis of these alkaline lavas. For magmas to be generated, the source materials must be transported across their solidus curves. Solidus curves for volatile-free peridotite and for peridotite-C-H-O provide the depth-temperature framework for the sites of magma generation. The assumption (controversial) that garnet remains in the source material locates the major melting region in plume material at depths of about 80 km, with isotherms in plume center exceeding 1500°C. The plume carries traces of volatile components from depths greater than 300 km. These dissolve in a trace of interstitial melt as the plume crosses the solidus for peridotite-C-H-O at depths decreasing from 350 km to about 150 km with distance from the plume axis. The volatile-charged melt, enriched in incompatible elements, is swamped by the picrites generated in the major melting region. From the outer portions of the plume, the volatile-rich melt enters the lithosphere at 80–90 km depth, where the change in rheology retards its upward percolation. This magma (remaining in equilibrium with peridotite) is carried toward the solidus for peridotite-C-H-O, changing composition toward nephelinite; evolution of vapor as magma approaches the solidus may facilitate intermittent crack propagation, releasing the nephelinitic magmas for eruption from depths of 75–85 km. Movement of the lithosphere plate over the rising plume establishes asymmetry. Eruption of nephelinitic magmas on the upstream side of the plume (early volcanism) may be suppressed or very close in time and space to eruption of alkaline lavas and tholeiites. On the downstream side (late volcanism), eruption of nephelinites is delayed by lateral transport away from the main melting region.

The origin and evolution of magmas from the San Pedro-Pellado volcanic complex, S. Chile: multicomponent sources and open system evolution

Abstract: The San Pedro-Pellado volcanic complex is located at 36° S in the Chilean Andes. The eruptive rocks of the complex record the development and collapse of a caldera, followed by voluminous, largely basaltic andesite, volcanism. At each stage of evolution, crystal fractionation was accompanied by variable degrees of contamination and mixing. Large variations in incompatible element ratios cannot be produced by closed system evolution. Correlations between indices of differentiation and incompatible element ratios, together with high δ 18O values, indicate that basaltic andesites have assimilated crust to generate the evolved volcanic rocks at San Pedro-Pellado. Even in the most mafic rocks, however, incompatible element characteristics are variable as a result of source heterogeneity and deep level processes. The restricted ranges in isotope ratios of Sr, Nd and Pb among San Pedro-Pellado rocks are due to the small contrast in isotopic compositions between magma and wallrock. Three source components are recognized as contributing to parental magmas at San Pedro-Pellado. Although the relative contributions of each cannot be quantified, the volumetrically dominant source component is the sub-arc asthenospheric mantle (MORB source). The major source of LILE is thought to be slab-derived fluids which modified the sub-arc mantle. Other incompatible elements may also have been enriched by interaction with the continental lithosphere (mantle and/or lower crust) during ascent.

Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

Abstract: The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.

An Overview of the Geochemical and Isotopic Characteristics of the Eastern Australian CainozoicVolcanic Provinces

Abstract: A broad zone of intra-plate volcanism occurs for some 3000 km along eastern Australia. Mafic lavas dominate, and include the following types (with frequency % occurrence, based on 1757 analyses): Leucitites (21), melilitites, nephelinites, and analcimites (5-4), basanites (12-7), alkali basalts (7-0), ne- hawaiites and hawaiites (44-4), transitional basalts and Ol-tholeiites (17-4), and Q-tholeiites (11-0). These lavas are erupted through a wide variety of crustal-tectonic environments, from Proterozoic to Mesozoic. Marked differences in chemistry exist between the lavas erupted from central volcano provinces (in which most ‘evolved’ lava types occur) and lava-field provinces, the former exhibiting greater isotopic variability and evidence for more extensive crystal fractionation (AFC). More evolved lava types include mugearites, benmoreites, icelandites, peralkaline and non-peralkaline trachytes and phonolites, comendites, low-silica and high-silica rhyolites. Marked regional differences exist with respect to distribution of various lava types; northern Queensland and Tasmania, for example, apparently have very few strongly evolved lavas, the latter region also containing a disproportionately high percentage of nephelinites. Trace element geochemistry of the mafic lavas is very variable, but typically continental; the lavas are enriched in incompatible elements, but enrichment varies greatly, being extreme in the leucitites, melilitites, and nephelinites, and slight (relative to MORB) in certain Q-tholeiites. It is shown that the patterns of the more extreme incompatible element enrichments are consistent with recent work on extraction of small melt fractions. Nevertheless, marked source inhomogeneities are indicated by the data, believed to be lithospheric; arc-modified lithosphere is suggested as a source for at least some lava field tholeiites. It is clear, however, that the majority of lavas have been modified by some degree of low-medium pressure crystal fractionation processes (olivine ±augite ± plagioclase ± Fe-Ti oxides). The critical role of fractional crystallization is even more apparent in the chemistry of the intermediate and silicic lavas, which exhibit dual patterns of progressive and ultimately extreme element enrichment (e.g., Pb, Th) and depletion (e.g., Mg, V, Ni, Cr, Sr, Ba, Eu). These patterns are readily modelled by Rayleigh fractionation, but require elevated K values, appropriate to silicic magmas; continually varying Ks are also indicated by some data sets. The mafic lavas exhibit a wide, but continuous variation of isotopic compositions, there being marked regional differences, but with the leucitite exception, no particular compositional ranges characterize particular compositional types. Correlations are observed between Sr, Nd, and to a less extent Pb isotopic compositions with, for example, Bh/Th, Ba/Nb, and mg-ratios. Much of the observed geochemical-isotope data, excepting the most undersaturated lavas, can be modelled in terms of AFC processes, utilizing upper and lower crustal models. The isotopic data of the alkaline Tasmanian lavas are distinctive and are interpreted as asthenospheric; these compositions, and those of rare magnesian alkaline lavas from elsewhere in the region, suggest a mixed mantle source containing a component approaching the ‘St. Helena-type’. The leucitites have a marked DUPAL isotopic signature, and it is noted that these occur above an interpreted Proterozoic rift system, suggesting a lithospheric source. Isotopic and geochemical data for the trachytes and low-silica rhyolites are consistent with AFC processes, with variable assimilation, modelled in terms of upper crustal components. The high-silica rhyolites are isotopically distinctive, and are interpreted as local upper crustal melts, but modified by subsequent crystal-liquid fractionation.

Petrological and geochemical relationships between pyroxene megacrysts and associated alkali-basalts from Massif Central (France)

Abstract: Major, trace element, and REE analyses, as well as Sr isotopic ratios, have been obtained on twelve clinopyene megacrysts and phenocrysts and their alkali-basalt hosts from the French Massif Central. Equilibrium between crystals and host was examined based on petrographic and geochemical data. Two types of pyroxenes are recognized: the acmite-bearing clinopyroxenes, rich in incompatible elements and the salitic clinopyroxenes, poor in incompatible elements. 87Sr/ 86Sr isotopic data reveal no significant difference between clinopyroxenes and host lavas: they are in apparent isotopic equilibrium. The Sr isotopic ratios of the two types of pyroxenes are also quite similar. However pyroxene crystals from the first group are not in equilibrium with their host; they have crystallized at high-pressure from differentiated alkali-lavas and have been incorporated in a more primitive magma. Pyroxene crystals from the second group are in apparent equilibrium with their host lava; they have crystallized at various pressures. For the latter, distribution coefficients are proposed for compatible elements, trace elements and REE.

Nature and cause of compositional variation among the alkalic cap lavas of Mauna Kea Volcano, Hawaii

Abstract: Most Hawaiian basaltic shield volcanoes are capped by moderately to strongly evolved alkalic lavas (MgO<4.5 wt.%). On Mauna Kea Volcano the cap is dominantly composed of hawaiite with minor mugearite. Although these lavas contain dunite and gabbroic xenoliths, they are nearly aphyric with rare olivine and plagioclase phenocrysts and xenocrysts. The hawaiites are nearly homogeneous in radiogenic isotope ratios (Sr, Nd, Pb) and they define coherent major and trace element abundance trends. These compositional trends are consistent with segregation of a plagioclase-rich cumulate containing significant clinopyroxene and Fe-Ti oxides plus minor olivine. Elements which are usually highly incompatible, e.g., Rb, Ba, Nb, are only moderately incompatible within the hawaiite suite because these elements are incorporated into feldspar (Rb, Ba) and oxides (Nb). However, in the most evolved lavas abundances of the most incompatible elements (P, La, Ce, Th) exceed (by ∼5–10%) the maximum enrichments expected from models based on major elements. Apparently, the crystal fractionation process was more complex than simple, closed system fractionation. The large amounts of clinopyroxene in the fractionating assemblage and the presence of dense dunite xenoliths with CO2 inclusions formed at minimum pressures of 2 kb are consistent with fractionation occurring at moderate depths. Crystal segregation along conduit or magma chamber walls is a possible mechanism for explaining compositional variations within these alkalic cap lavas.

Possible petrogenetic relations between low- and high-MgO Aleutian basalts

Abstract: Although the volumetrically dominant lava in the Aleutian arc is basaltic, its petrographic, geochemical, and isotopic character is variable, and the petrologic significances of the different basalts have been much debated. To evaluate possible petrogenetic relations, all available petrographic, major-, trace- and rare-earth-element and isotopic data from Aleutian basalts (n = 205) have been collected. On the basis of MgO content, basaltic ( 2 ) lavas have been divided into three major classes: high-magnesia basalts, transitional basalts, and low-MgO basalts. Low-MgO basalts have less than 6% MgO and constitute 67% of the analyzed basalts. Using Al 2 O 3 content, this basalt class has been further divided into two subclasses. Low-alumina basalts, the less abundant class (15% of the low-MgO basalts), have less than 18 wt % Al 2 O 3 and 450 ppm Sr and high TiO 2 (>1.0 wt %), FeO t (>9%) and incompatible element abundances. High-alumina basalts (≥18 wt % Al 2 O 3 ) are characterized by low TiO 2 (≤1 wt %) and ≤9% FeO t , low incompatible- and compatible- element contents and high Sr abundances (≥450 ppm). Transitional basalts are defined as basalts with between 6 and 9 wt % MgO. High-magnesia basalts (MgO >9 wt %) constitute 10% of the analyzed Aleutian basaltic lavas and occur at only a few volcanic centers. Fractionation of high-magnesia basaltic magma is unlikely to produce magma compositions similar to high-alumina basalts. The compositional data are, however, consistent with assimilation of lithospheric material by high-alumina basaltic liquid to produce high-magnesia basaltic magmas and generation of low-alumina basalts by crystal fractionation of these hybrid liquids. Within this context, high-alumina basaltic magmas are considered primary.

The origin of pristine KREEP: effects of mixing between urKREEP and the magmas parental to the Mg-rich cumulates.

Abstract: The fact that pristine KREEP basalts generally have bulk-rock molar MgO/(MgO+FeO) ratios lower than average for the lunar crust despite their extraordinarily high incompatible element contents is discussed. Pristine KREEP basalts also have primitive Ni contents compared to mare basalts with comparable REE contents. It is suggested that these facts can be explained through mixing between ancient KREEP precursor materials and primitive Mg-rich melts. Finite-difference models of this mixing, followed by anorthosite assimilation and fractional crystallization provide satisfactory fits to the composition of these basalts. It is suggested that this is a further confirmation of the magmasphere hypothesis.

Trace Element and Isotopic Variations in Scottish and Irish Dinantian Volcanism: Evidence for an OIB-like Mantle Source

Abstract: The Dinantian volcanic province of southern Scotland, northern England and parts of Ireland is represented by a large volume of transitional to mildly alkaline rocks. Many (although not all) of its basic members display inter-regional variations in incompatible element concentrations, particularly LREE, Zr, Nb, K, and Ba. Sr, Nd, and Pb isotopic data also show some minor inter-regional variations, although they are not as well-defined as those observed in the trace elements. Indeed, much of the variation is intra-regional. Modelling of the basic rocks (≥4% MgO) using Ce/Y and Zr/Nb ratios reveals that fractional crystallization and varying degrees of partial melting alone cannot explain the compositional diversity. Models involving variable-depth melting in the stability fields of both garnet and spinel, and garnet exhaustion through progressive melting are equally untenable. The best model to fit the ratios is one involving variable-degree melting superimposed on slight source region heterogeneity. Investigations of potential magma sources suggest that this heterogeneity is not related to crustal contamination or to incorporation of relict slab material derived from a pre-existing subduction zone (c. 50 Ma earlier) in the same region. Instead, the observed trace element and isotopic characteristics show marked similarities to many modern ocean island basalts (OIB): a feature which suggests that the mantle source region was sub-lithospheric. It is suggested that the variations observed in basic Dinantian volcanic rocks merely mirror on a local scale the variations observed in OIB globally, and that no other source is necessary to explain the chemical diversity of Dinantian magmas. If the speed of northward motion (15° of latitude between the Lower Carboniferous and Lower Permian) and the longevity of Dinantian volcanism (c. 40 Ma) are taken into account, it is hardly surprising that such distinct chemical variations are observed: the magmas may well have sampled an enormous volume of convecting sublithospheric mantle.

Boron and lithium in high-grade rocks and minerals from the Wawa–Kapuskasing region, Ontario

Abstract: There is no preferential partitioning of boron among the principal rock-forming minerals in high-grade rocks of the Kapuskasing Structural Zone (KSZ) and the Wawa Domal Gneiss region (WDG). Lithium is strongly concentrated in biotite and other ferromagnesian minerals but does not show consistent partitioning between these and the sialic minerals.The distribution of B and Li within a rock may be studied using an alpha-track image, which shows that the inconsistencies in partitioning may be largely attributed to disturbance of mineral equilibria by postmetamorphic low-grade alteration that deposited B and Li.Boron has similar concentrations in all the rock types studied, although it is an incompatible element that elsewhere accumulates in pegmatites. Lithium concentrations are low in the anorthositic rocks but are otherwise very variable. In some but not all rocks higher than usual B and Li can be attributed to introduction during alteration.Boron occurs at low concentrations (2–3 ppm) throughout both the K...

Major, trace element and Sr isotopic composition of lavas from Vico volcano (Central Italy) and their evolution in an open system

Abstract: Major, trace element and Sr isotopic compositions have been determined on 21 lava samples from Vico volcano, Roman Province, Central Italy. The rocks investigated range from leucite tephritic phonolites to leucite phonolites and trachytes. Trace element compositions are characterized by high enrichments of incompatible elements which display strong variations in rocks with a similar degree of evolution. Well-defined linear trends are observed between pairs of incompatible trace elements such as Th-Ta, Th-La, Th-Hf. A decrease of Large Ion Lithophile (LIL) elements abundance contemporaneously with the formation of a large central caldera is one of the most prominent characteristics of trace element distribution. Sr isotope ratios range from 0.71147 to 0.71037 in the pre-caldera lavas and decreases to values of 0.70974–0.70910 in the lavas erupted after the caldera collapse. Theoretical modelling of geochemical and Sr isotopic variations indicates that, while fractional crystallization was an important evolutionary process, AFC and mixing also played key roles during the evolution of Vico volcano. AFC appears to have dominated during the early stages of the volcanic history when evolved trachytes with the highest Sr isotope ratios were erupted. Mixing processes are particularly evident in volcanites emplaced during the late stages of Vico evolution. According to the model proposed, the evolution of potassic magmas emplaced in a shallow-level reservoir was dominated by crystal fractionation plus wall rock assimilation and mixing with ascending fresh mafic magma. This process generated a range of geochemical and isotopic compositions in the mafic magmas which evolved by both AFC and simple crystal liquid fractionation, producing evolved trachytes and phonolites with variable trace element and Sr isotopic compositions.

Alkalic to transitional ferrogabbro magma associated with paleohelikian anorthositic plutons in the Flowers River area, southeastern Nain igneous complex, Labrador

Abstract: Fine grained gabbroic chilled margins and crosscutting dikes are associated in space and time with three ca. 1400 Ma anorthositic plutons in the Flowers River area, southeastern Nain igneous complex. Both the anorthositic and gabbroic rocks have distinctive compositions compared to rocks of similar age and lithology elsewhere in northcentral Labrador. The anorthositic rocks contain olivine and augite rather than orthopyroxene, and Fe-Ti oxides, apatite and orthoclase are unusually abundant. Cumulus plagioclase is abnormally enriched in incompatible elements. Most of the gabbroic rocks are uniform in composition, although the effects of contamination and fractionation are evident in some places. They define a transitional to alkalic ferrogabbro magma that is strongly enriched in K, P and incompatible trace elements. The chemical characteristics of the ferrogabbro magma imply derivation from enriched mantle or involvement of a significant crustal component. A parent-daughter relationship between the ferrogabbro magma and anorthositic rocks is suggested by their compositional similarities and the fact that the gabbroic chilled margins and plagioclase-rich pluton interiors appear to be completely gradational in composition and texture.

The petrology and geochemistry of the Igaliko Dyke swarm, south Greenland.

Abstract: The dykes from the Igaliko Nepheline Syenite complex belong to at least 3 individual swarms (i) a Mid-Gardar swarm in the Ostfjordsdal valley, (ii) a Late-Gardar, Si-oversaturated swarm associated with the Younger Giant Dykes of Tugtutoq and (iii) a Si-undersaturated swarm intimately associated with the Late Gardar Igaliko Nepheline Syenite Central Complexes. In addition Early Gardar activity is recorded by the presence of some ultramafic lamprophyres which predate the Motzfeldt centre, sparse trachytes which are truncated by intrusions within the Motzfeldt centre and a possible BD(_0) dolerite which is also cut by the Motzfeldt centre. Most dykes however are bracketed between the Early and Late Igdlerfigssalik syenite intrusions. The main oversaturated and undersaturated suites can be separated on their Zr/Nb ratios (≈6.4 and 3.9 respectively). In addition, the undersaturated basic rocks have smooth chondrite normalised incompatible element spidergrams whereas the oversaturated basic rocks are characterised by negative Nb and positive P anomalies. Evolution of both suites can be modelled in terms of fractional crystallisation of feldspar, clinopyroxene, olivine, apatite and opaques from basaltic parents to either phonolitic or rhyolitic minimum compositions. In each instance these evolved composi tions are extremely rich in incompatible trace elements (REE, Nb, Zr, Rb). In some cases a high CO(_2) content in the undersaturated rocks may lead to the formation (by liquid immiscibility) of late stage carbonatite magmas. High CO(_2) also produces high ƒo(_2) in these magmas and it is argued that in some cases this can suppress the development of negative Eu anomalies on feldspar fractionation. The undersaturated swarm may have evolved from lamprophyric parental magmas, eg. camptonites, which are relatively abundant basic dykes. Ultramafic lamprophyres, often early, may have formed as extremely small degree partial melts at the onset of Gardar rifting. In the Late Gardar, magma genesis is related to the different extensional tectonic regimes which were operative at that time. Mineralogical evolution follows paths similar to several other Gardar suites and records a higher ƒo(_2) in the undersaturated rocks. Zr becomes concentrated in interstitial residual liquids in benmoreites and substitutes into amphibole as the newly proposed end-member zirconian-arfvedsonite.

Assimilation of continental crust by komatiites in the Precambrian basement of the Carswell structure (Saskatchewan, Canada)

Abstract: The isotope geochemistry (Sm-Nd, Pb-Pb and Rb-Sr) of mafic gneisses from the basement of the Carswell structure (Saskatchewan, Canada), rich both in Mg and incompatible elements (K, Rb, REE) has been investigated. A good Sm-Nd alignment gives a slope corresponding to an age of 3.7 Ga. However, comparison with major elements data strongly suggests that this alignment is a mixing line between Mg-rich, high CaO/Al2O3 magmas and the local felsic crust older than 2.9 Ga. The mafic magmas were probably of komatiitic affinity (MgO > 20 percent) but, nevertheless, were extracted from a source with nearly chondritic to slightly enriched light REE distribution. The age of the komatiite emplacement (1.9–2.9 Ga) is only loosely constrained by the oldest crustal residence age in the series and the subsequent metamorphic events. The granulite facies climax is dated at ca. 1.9 Ga by concordant whole rock Pb-Pb and Sm-Nd garnet-whole rock isochrons. The Rb-Sr systematics have been disturbed by later event(s) younger than 1.5–1.7 Ga, but do not permit a more precise assessment of the perturbation age.