Showing papers on "Incompatible element published in 1994"

Petrology and Geochemistry of Boninites from the North Termination of the Tonga Trench: Constraints on the Generation Conditions of Primary High-Ca Boninite Magmas

Abstract: We report here a detailed mineralogical, geochemical, and experimental study of a high-Ca boninite suite from the northern termination of the Tonga trench. Most samples are strongly olivine porphyritic and show a significant range of phenocryst compositions including a very refractory olivine-spinel assemblage Fo94-CrN = 87. They are also characterized by a wide range of incompatible-element contents, e.g., (La/Yb)y varies from 05 to 16, whereas compatible major-element concentrations (A12O3, FeO, CaO, SiO2, and MgO) remain essentially the same. Primary melt compositions for the suite were established on the basis of an experimental study of melt inclusions in phenocrysts and numerical modelling of the reverse of fractional crystallization. Tongan primary melts are characterized by high MgO contents (22-24 wt.%) and originated in the mantle wedge at pressures of 20-25 kbar and temperatures of 1450-1550 °C. H2O contents in primary melts were estimated from direct measurements of melt inclusions by ion probe, and range from 2-0 to 10 wt.%, and a strong correlation exists between H2O and other incompatible element contents. The primary melts crystallized in the presence of an H2O-rich fluid in the temperature range 1390-1150 °C and pressures of 1-7-O15 kbar. Continuous degassing of melts took place during crystallization. Trace-element concentrations in primary melts were estimated using proton- and ion-probe analyses of melt inclusions in olivine, and whole-rock analyses. Our data suggest that three independent components (D, El, and E2) were involved. Component D was a refractory mantle depleted in incompatible elements, likely to be hot 'dry' lherzolite produced by previous melting within a mantle plume. Component El was an H2O-rich fluid containing LILE and Th, and had an H2O/K2O value of 20; it was probably produced by dehydration of the subducted slab. Component E2 is thought to have been an incompatible- element-enriched silicate melt of plume origin. Formation of high-Ca boninites requires interaction of hot 'dry' residual mantle, associated with plumes, with a subduction-rel ated H2O-bearing component.

Cenozoic alkali basaltic magmas of western Germany and their products of differentiation

Abstract: Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhon, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr−Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of δD with 87Sr/86Sr in amphibole and phlogopite and a slightly larger δ18O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.

Geochemical effects of dynamic melting beneath ridges: Reconciling major and trace element variations in Kolbeinsey (and global) mid‐ocean ridge basalt

Abstract: Zero-age basalts dredged from the Kolbeinsey Ridge directly north of Iceland are mafic quartz tholeiites (MgO 6-10 wt. %), strongly depleted in incompatible elements. Fractionation-corrected Na2O contents ('Na(sub 8)') are amongst the lowest found on the global ridge system, implying that the degree of partial melting at Kolbeinsey is amongst the highest for all mid-ocean ridge basalt (MORB). In contrast, the basalts show large ranges of incompatible-element ratios (e.g., K2O/TiO2 of 0.01 to 0.12 and Nd/Sm of 2.1 to 2.9) not related to variations in radiogenic isotope ratios; this suggests recent enrichment/depletion events associated with small-degree partial melting as their cause, rather than long-lived source heterogeneity. Tholeiitic MORB from many regions globally show similar or more extreme variations in K2O/TiO2. Dynamic melting of an adiabatically upwelling source can reconcile these conflicting indications of the degree of melting. Through dynamic melting, the incompatible elements are partially separated into different melt fractions based on their bulk partition coefficients, more incompatible elements being concentrated in deeper, smaller-degree partial melts. The final erupted magma is a mix of melts from all depths in the melting column. The concentration of highly incompatible elements in the mix will be very sensitive to the physical processes allowing the deep melts to separate and migrate to the site of mixing, and small fluctuations in the efficiency of the separation process can account for the large range of trace element ratios seen at Kolbeinsey. The major element chemistry of the erupted mix (and Na(sub 8) is much more robust, depending mainly on the integrated total amount of melting. The large variations of incompatible element ratios seen at Kolbeinsey, and in MORB in general, therefore give no information about the total degree of melting occuring beneath the ridge, nor do they require a heterogeneous source.

Tahiti: Geochemical evolution of a French Polynesian Volcano

Abstract: The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks from this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K-Ar radiometric dating we have established the age of volcanic activity. The oldest lavas (--1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and a late- stage, valley-filling phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42-49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene-phyric alkali basalts (Mg# 60-64), while the later valley-filling lavas are basanitic (Mg# 64-68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+ clinopyroxene+ spinel). Isotopic compositions also change systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield- building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K-Ar ages and magnetostratigraphic boundaries allow high-resolution age assignments to this -0.7-km-thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0.9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield- building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25 to 50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognize a quasi-periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25 + 10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield-building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB-like. We propose that the Society plume is radially zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal and compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield-building stage are derived mainly from the hot, incompatible element-enriched central zone of the plume; late-stage magmas are derived from the cooler, incompatible element-depleted, viscously coupled sheath. A correlation between Pb/Ce and isotope ratios suggests that the Society plume contains deeply recycled continental material.

Mineralogical and geochemical study of granular xenoliths from the Alban Hills volcano, Central Italy: bearing on evolutionary processes in potassic magma chambers

Abstract: Granular xenoliths (ejecta) from pyroclastic deposits emplaced during the latest stages of activity of the Alban Hills volcano range from ultramafic to salic. Ultramafic types consist of various proportions of olivine, spinel, clinopyroxene and phlogopite. They show low SiO2, alkalies and incompatible element abundances and very high MgO. However, Cr, Co and Sc are anomalously low, at a few ppm level. Olivine is highly magnesian (up to Fo%=96) and has rather high CaO (1% Ca) and very low Ni (around a few tens ppm) contents. These characteristics indicate a genesis of ultramafic ejecta by thermal metamorphism of a siliceous dolomitic limestone, probably with input of chemical components from potassic magma. The other xenoliths have textures and compositional characteristics which indicate that they represent either intrusive equivalents of lavas or cumulates crystallized from variably evolved ultrapotassic magmas. One sample of the former group has major element composition resembling ultrapotassic rocks with kamafugitic affinity. Some cumulitic rocks have exceedingly high abundances of Th (81–84 ppm) and light rare-earth elements (LREE) (La+Ce=421–498 ppm) and extreme REE fractionation (La/Yb=288–1393), not justified by their modal mineralogy which is dominated by sanidine, leucite and nepheline. Finegrained phases are dispersed through the fractures and within the interstices of the main minerals. Semiquantitative EDS analyses show that Th and LREE occur at concentration levels of several tens of percent in these phases, indicating that their presence is responsible for the high concentration of incompatible trace elements in the whole rocks. The interstitial position of these phases and their association with fluorite support a secondary origin by deposition from fluorine-rich fluids separated from a highly evolved potassic liquid. The Nd isotopic ratios of the cjecta range from 0.51182 to 0.51217. 87Sr/86Sr ratios range from 0.70900 to 0.71036. With the exception of one sample, these values are lower than those of the outcropping lavas, which cluster around 0.7105±3. This indicates either the occurrence of several isotopically distinct potassic magmas or a variable interaction between magmas and wall rocks. However, this latter hypothesis requires selective assimilation of host rocks in order to explain isotopic and geochemical characteristics of lavas and xenoliths. The new data indicate that the evolutionary processes in the potassic magmas of the Alban Hills were much more complex than envisaged by previous studies. Interaction of magmas with wall rocks may be an important process during magmatic evolution. Element migration by gaseous transfer, often invoked but rarely constrained by sound data, is shown to have occurred during the latest stages of magmatic evolution. Such a process was able to produce selective enrichment of Th, U, LREE and, to a minor degree, Ta and Hf in the wall rocks of potassic magma chamber. Finally, the occurrence of xenoliths with kamafugitic composition points to the existence of this type of ultrapotassic magma at the Alban Hills.

Geochemistry and origin of the Proterozoic ultrapotassic rocks of the Churchill Province, Canada

Abstract: Early Proterozoic ultrapotassic dikes, lava flows, and pyroclastic rocks of the Christopher Island Formation (CIF) erupted throughout an area 600 × 300 km within the Churchill Province of the Canadian Shield at 1.84 Ga. The rocks range from mafic lamprophyres (mg # ⩾ 60; SiO2 47–54%, mean K2O/Na2O > 4) with phenocrysts of phlogopite + diopside + apatite ± olivine ± magnetite, to phenocryst-poor felsic rocks and sanidine porphyries (SiO255–69%). All samples have high incompatible element contents and display large depletions of high field strength elements relative to K, Rb, Sr, Ba, and Th. The CIF has geochemical and petrographic characteristics of both minettes and lamproites, but overall most closely resembles young Mediterranean lamproites. Felsic rocks of the CIF were produced by crystal fractionation and crustal contamination of mafic ultrapotassic magma, and include both high-silica lamproites strongly enriched in Zr, U, and Th, and weakly potassic to sodic rocks of trachytic composition. Flows and feeder dikes have relatively homogeneous ɛNd, 1840 Ma (−6 to −11) but highly variable ES., 1840 Ma (−40 to + 100); samples classified as lamproites have higher average ɛSr. Dike samples have highly variable present-day Pb isotope compositions, ranging from moderately to strongly nonradiogenic. Geochemical and isotopic data are consistent with contributions from depleted Archean lithospheric mantle, and OIB-type convecting mantle, both metasomatized by subduction-related processes during the Early Proterozoic. The lithospheric mantle probably contained Archean enriched domains as well. Proterozoic enrichment may have accompanied shallow underplating of subducted oceanic lithosphere beneath the Churchill Province during amalgamation of the Laurentian supercontinent. There are strong analogies in isotopic composition, and interpreted source region history, between the CIF and lamproites and minettes of the Wyoming Province and western Greenland, which suggest the existence of a Laurentian ultrapotassic “superprovince”.

The mesothermal gold-lamprophyre association: significance for an accretionary geodynamic setting, supercontinent cycles, and metallogenic processes

Abstract: Archean shoshonitic lamprophyres are cotemporal and cospatial with gold mineralization in the Superior Province of Canada, both being emplaced along translithospheric structures that demark subprovince boundaries By analogy with geochemically similar Phanerozoic counterparts, the dikes are a product of specific plate interactions rather than a deep asthenosphere plume-initiated event, and their onset in the late-Archean at ∼ 27 Ga signifies that Phanerozoic style plate-tectonics was operating at this time Fresh shonshonitic dikes are characterized by normal background gold contents of 39 ± 81 ppb (lσ), close to the value of 30 ppb for the bulk continental crust, and average abundances of As, Sb, Bi, W, TI, B, Cu, Pb, Zn, and Mo are also close to their values in bulk continental crust Thus, fresh lamprophyres are not intrinsically enriched either in Au or elements affiliated with gold in mesothermal deposits, and accordingly do not constitute a special source rock Platinum group element contents (Ir = 04 ± 058 ppb; Pt = 59 ± 265, Pd = 55 ± 18), in conjunction with Cu, Au, and Ni abundances, define approximately flat patterns on primitive mantle-normalized diagrams, consistent with derivation of the alkaline magmas from a depleted mantle source variably enriched by incompatible elements Comparable abundances and ratios of Pd/Au, Os/Ir, and Ru/Ir in Archean lamprophyres, Archean komatiites, and Gorgona komatiites signify that the Archean and Phanerozoic upper mantle had similar noble metal contents, such that the prolific greenstone belt Au-Ag vein deposits cannot be explained by secular variations in upper mantle Au abundance alone The lack of covariation between Au and light rare earth elements in lamprophyres rules out mantle metasomatism as a process generating intrinsically Au-rich magmas Emplacement of the lamprophyres was diachronous from north (2710 Ma) to south (2670 Ma) in the Superior Province, as was the gold mineralization Both were related to late transpressional tectonics during successive accretions of individual subprovinces Alkaline magmatism and gold mineralization are temporally and spatially related because they share a common geodynamic setting, but they are otherwise the products of distinct processes Much of Archean time was devoid of shoshonites and mesothermal gold deposits The first widespread inception of this duality at 271–265 Ga in the Superior and Slave Provinces, Canada, and in India and Australia, may reflect one of the first supercontinent aggregations involving accretionary, “Cordilleran style” tectonics Giant mesothermal gold provinces and shoshonites recur through time in the Palaeozoic and Mesozoic in this geodynamic setting

Geochemical and isotopic (Nd, O, and Pb) constraints on granite sources in the Humber and Dunnage zones, Gaspésie, Quebec, and New Brunswick: implications for tectonics and crustal structure

Abstract: Siluro–Devonian granitoids span a wide compositional range (~50–76% SiO2) and can be subdivided into two groups: (i) monzonitic or incompatible element enriched with affinities to within-plate magmatism (WPG); and (ii) calc-alkalic or incompatible element depleted with supra-subduction zone affinities (VAG). Granitoid eNd(T = 0.4 Ga) values range from −1 to +5.5; most lie between +3 and +5.5. 207Pb/204Pb isotopic compositions range from 15.52 to 15.61; most fall between ~15.55 and 15.59. Most δ18O values lie between +5.5 and +8‰. No well-established trends exist between SiO2 and isotopic composition, and isotopic compositions do not differ between the two trace element defined granitoid groups.Though Pb isotopic data are consistent with a major contribution to the granitoids from Proterozoic-aged Laurentian plate rocks (i.e., Grenville basement), Nd and O isotopic data are not. These isotopic data are consistent with major source components derived from early Paleozoic depleted or supra-subduction zone af...

Petrogenesis of the highly potassic 1.42 Ga Barrel Spring pluton, southeastern California, with implications for mid-Proterozoic magma genesis in the southwestern USA

Abstract: Syenites from the Barrel Spring pluton were emplaced in the Early Proterozoic Mojave crustal provine of southeastern California at 142 Ga All rocks, even the most mafic, are highly enriched in incompatible elements (eg K2O 4–12 wt%, Rb 170–370 ppm, Th 12–120 ppm, La 350–1500xchondrite, La/Ybn 35–100) Elemental compositions require an incompatible element-rich but mafic (or ultramafic) source Trace element models establish two plausible sources for Barrel Spring magmas: (1) LREE enriched garnet websterite with accessory apatite±rutile (enriched lithospheric mantle), and (2) garnet amphibolite or garnet-hornblende granulite with enriched alkali basalt composition, also with accessory apatite±rutile (mafic lower crust) Nd and Pb isotopic ratios do not distinguish a crust vs mantle source, but eliminate local Mojave province crust as the principal one, and indicate that generation of the enriched source occurred several hundred million years before emplacement of the Barrel Spring pluton 140–144 Ga potassic granites are common in southeastern California, suggesting a genetic link between the Barrel Spring pluton and the granites; however, although the same thermal regime was probably responsible for producing both the granitic and syentic magmas, elemental and isotopic compositions preclude a close relationship Isotopic similarity of the Barrel Spring pluton to 140–144 Ga granites emplaced in the Central Arizona crustal province to the east may imply that a common component was present in the lithosphere of these generally distinct regions

Ultra-Depleted Melts from Iceland: Data from Melt Inclusion Studies

Abstract: Melts which are ultra-depleted for highly incompatible elements (UDM) are expected to be important members of mantle derived melts if melting process approaches fractional melting, and melt fractions could avoid mixing to each other and reacting with mantle on their way to the surface. However, among basaltic lavas and glasses, the UDM is not present or very rare. The only reported UDM were found in mid-ocean ridge basalts (MORB) as inclusions in high-Mg olivines (Sobolev and Shimizu, 1993; Sobolev et al., 1994). Here we present data on the UDM inclusions found in high-Mg olivines from tholeiitic picrites and olivine basalts from Iceland, which represents a mantle plume centered on the Mid-Atlantic Ridge (Schilling, 1973). Discussion

Géochimie des granitoïdes miocènes de Bejaia-Amizour (Algérie du Nord)

Abstract: The Bejaia-Amizour volcanoplutonic complex includes seven zoned plutonic bodies intruding the Cretaceous and Tertiary flysch nappes of the northern Algerian margin. They range in composition from diorites to granodiorites, quartz-monzonites and granites. They display K-rich calc-alkaline petrographic and chemical characteristics, and have been affected by late- to post-magmatic hydrothermal alteration processes following their crystallization under low total pressures. The study of their trace element compositions suggests their derivation from a basaltic parent magma through combined crystal fractionation, magma mixing and crustal contamination processes. However, the lack of correlation between SiOz contents, 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, δ 18 O ratios and Rb/Nb, Ba/Nb, La/Nb ratios indicates that crustal contamination coupled with differentiation occurred within small individual magma chambers. This process is not chiefly responsible for their calc-alkaline trace element fingerprints, e.g. negative Nb anomalies with respect to neighbouring incompatible elements