Enrichment In Incompatible Elements Research Articles

The late Ediacaran (580–590 Ma) onset of anorogenic alkaline magmatism in the Arabian–Nubian Shield: Katherina A-type rhyolites of Gabal Ma’ain, Sinai, Egypt

The Katherina Volcanics of Gabal Ma’ain in the Sinai comprise an Ediacaran (580–590Ma) approximately 450m thick succession dominated by porphyritic rhyolite lava flows with subordinate related pyroclastics. These volcanics unconformably overlie the calc-alkaline Younger Granites (≥590Ma) and are intruded by alkaline granitoids (578±8Ma). The rhyolites have a potassic alkaline affinity and peraluminous to slightly metaluminous character. They exhibit many of the classic features of A-type magmas, including enrichment of incompatible elements, such as Zr, Nb, Y, Ga, Zn and Ce and total REE, as well as high FeO*/(FeO*+MgO) and 10,000*Ga/Al2O3 ratios. The A-type rhyolites have LREE-enriched patterns with pronounced negative Eu anomalies that are comparable with typical REE profiles for “hot-dry-reduced rhyolites”. Saturation thermometry has yielded zircon and apatite crystallization temperatures ranging between 913 and 925°C and 669 and 931°C, respectively. The investigated trace element patterns indicate that the Katherina A-type rhyolites were very likely to have evolved through simple fractional crystallization of a parental magma derived from an enriched (most probably asthenospheric) mantle source, supplemented by a crustal component inherited from pre-collision subduction events, or a ‘recycled component’ in the source. Katherina A-type rhyolites were likely erupted in a within-plate setting. The eruption of these rhyolites marks the onset of the anorogenic period during which the rigid massif (Arabian–Nubian Shield) was subjected to post-collisional tensional stresses and intra-plate rifting.

Crystallization conditions and controls on trace element residence in the main minerals from the Pedra Branca Syenite, Brazil: An electron microprobe and LA-ICPMS study

Major and trace-element microanalyses of the main minerals from the 610Ma Pedra Branca Syenite, southeast Brazil, allow inferences on intensive parameters of magmatic crystallization and on the partition of trace-elements among these minerals, with important implications for the petrogenetic evolution of the pluton. Two main syenite types make up the pluton, a quartz-free syenite with tabular alkali feldspar (laminated silica-saturated syenite, LSS, with Na-rich augite+phlogopite+hematite+magnetite+titanite+apatite) and a quartz-bearing syenite (laminated silica-oversaturated syenite, LSO, with scarce corroded plagioclase plus diopside+biotite±hornblende+ilmenite±magnetite+titanite+apatite). Both types share a remarkable enrichment in incompatible elements as K, Ba, Sr, P and LREE. Apatite saturation temperatures of ~1060–1090°C are the best estimates of liquidus, whereas the pressure of emplacement, based on Al-in-hornblende barometry, is estimated as 3.3 to 4.8kbar. Although both units crystallized under oxidizing conditions, oxygen fugacity was probably higher in LSS, as shown by higher mg# of the mafic minerals and higher hematite contents in Hem-Ilmss. In contrast with the Ca-bearing alkali-feldspar from LSO, which hosts most of the whole-rock Sr and Pb, virtually Ca-free alkali-feldspar from LSS hosts ~50% of whole-rock Sr and ~80% of Pb, the remainder of these elements being shared by apatite, pyroxene and titanite. This contrast reflects a strong crystal-chemical control, whereby a higher proportion of an element with similar ratio and charge (Ca2+) enhances the residence of Sr and Pb in the M-site of alkali feldspar. The more alkaline character of the LSS magma is inferred to have inhibited zircon saturation; Zr+Hf remained in solution until late in the crystallization, and were mostly accommodated in the structure of Ca–Na pyroxene and titanite, which are one order of magnitude richer in these elements compared to the same minerals in LSO, where most of Zr and Hf are inferred to reside in zircon. The REE, Th and U reside mostly in titanite and apatite; D(REE)Tit/Ap raises steadily from 1 to 6 from La to Tb then remains constant up to Lu in the LSO sample; these values are about half as much in the LSS sample, where lower contents of incompatible elements in titanite are attributed to its greater modal abundance and earlier crystallization.

Petrology of the Grays River volcanics, southwest Washington: Plume-influenced slab window magmatism in the Cascadia forearc

The Grays River volcanics are part of the Coast Range basalt province and consist of ∼3500 m of tholeiitic basalt flows and volcaniclastic rocks that erupted in the Cascadia forearc from 42 to 37 Ma. Chemical and isotopic data, combined with migration of the location of magmatism through time, indicate that Grays River volcanics magmatism was related to subduction of a plume-influenced spreading ridge that produced a northward-migrating slab window. Involvement of a mantle plume source is indicated by ocean-island basalt (OIB)-like incompatible element enrichments and radiogenic Pb isotopic compositions (206Pb/204Pb > 19.3). These Pb isotope data are distinct from most Cascade arc rocks and from Cascadia sediment, but they overlap with compositions of other Coast Range basalts. A slab window setting accounts for the northward-younging age progression of the Grays River volcanics as well as geochemical traits, including low B/Be, that indicate the Grays River magmas ascended through the mantle wedge and subducting slab without acquiring an arc signature. Differentiation of Grays River magmas was dominated by clinopyroxene fractionation, which resulted in evolved compositions (Mg# = 59–32), low Sc contents, and Sr contents that increase with fractionation. Geochemical differences between the Grays River volcanics and other Cascadia forearc volcanic units that range from ca. 55 Ma (Crescent Basalts) to <3 Ma (Boring Lavas) were mainly caused by transient changes in tectonic setting (i.e., arrival of a mantle plume, ridge subduction) and do not record progressive chemical modification of the mantle wedge.

Element partitioning during carbonated pelite melting at 8, 13 and 22 GPa and the sediment signature in the EM mantle components

Within the subducting oceanic crust, carbonated eclogitic pelites are the lithology with the lowest melting temperature at >5GPa, i.e. at depths beyond major subarc dehydration. 200–400°C below the oceanic mantle geotherm, carbonated pelites generate alkali-rich Ca-carbonate melts that constitute efficient metasomatic agents of the mantle. Partition coefficients between residual minerals and such melts were experimentally determined at 8, 13, and 22GPa at 1100–1500°C. Compared to previous studies, clinopyroxenes have higher jadeite contents (57–82mol%) resulting in a larger compatibility for LILE. In garnet, the compatibility of REE increases from incompatible LREE (DLa~0.005 at 8–22GPa) to slightly compatible Lu (DLu=0.96 to 3.5 at 8–22GPa), DHFSE's increase with pressure from slightly incompatible at 8GPa to highly compatible at 22GPa, always with DHf>DZr. K-hollandite/carbonate melt partition coefficients at 13GPa are all <0.3 except for K itself. At 22GPa, Rb, Sr, Ba, and Pb also become compatible in K-hollandite. Also at 22GPa, FeTi-perovskites appear and have high D-values for HFSEs (DHFSE 28–88), similar to other Ti-rich minerals. In the CAS phase, also occurring at 22GPa, Ti, Sr, La to Gd, and Pb, Th and U are compatible (DPb>DTh>DU>1.7 with a DPb/DU of 12 to 26) leading to a strong fractionation of these elements during melting just above the 660km discontinuity.Calculated bulk residue/carbonate melt partition coefficients increase with pressure for almost all elements. At 22GPa, i.e. for carbonated sediment melting in the transition zone, element fractionation strongly effects the Pb isotopic evolution. Carbonate melt trace element compositions normalized to primitive mantle show strong enrichments in incompatible elements including LILE and LREE and relative negative anomalies for Ti at 8 and 13GPa and for Hf, Zr and Ti at 22GPa at which pressure absolute values are close to mantle concentrations. Primitive mantle normalized patterns for 8GPa carbonate melts are similar to ultrapotassic rocks and many lamproites on one hand confirming the involvement of a sedimentary component in the source region of these rocks, on the other hand defining this component as a carbonated sediment melt. The melting of mantle domains re-enriched by ~0.4wt.% of our 8–13GPa carbonate melts produces the typical trace element signature observed in the group II kimberlites. Finally, the Pb, Nd, and Sr isotopic evolution of mantle domains contaminated by ≤1 wt.% carbonate melt derived from carbonated pelites yields reservoirs which cover most of the compositions identified as EM I and II flavors in OIBs.

Trace element geochemistry of primary mantle minerals in spinel‐peridotites from polygenetic MOR–SSZ suites of SW Turkey: constraints from an LA‐ICP‐MS study and implications for mantle metasomatism

Ophiolites exposed across the western Tauride Belt in SW Turkey represent tectonically emplaced fragments of oceanic lithosphere incorporated into continental margin following the closure of the Neotethys Ocean during the Late Cretaceous. The mantle sections of the ophiolites contain peridotites with diverse suites of geochemical signatures indicative of residual origin by melt depletion in both mid‐ocean ridge (MOR) and supra‐subduction zone (SSZ) settings. This study uses a laser‐ablation inductively‐coupled plasma‐mass spectrometry (LA‐ICP‐MS) for in situ measurements of trace elements in primary mantle phases in order to identify the upper mantle petrogenetic processes effective during variable stage of melt extraction in these discrete tectonic settings and to discriminate between the effects of reaction with chemically distinct mantle melts migrating through the solid residues. Trace element signatures in pyroxenes suggest small‐length scales of compositional variations which may be interpreted to be a result of post‐melting petrogenetic processes. Relative distribution of rare earth elements and Li between coexisting orthopyroxene‐clinopyroxene pairs in the peridotites suggests compositional disequilibrium in sub‐solidus conditions, which possibly reflects differential effects of diffusive exchange during melting and melt transport or interaction with subduction melts/fluids. On the basis of Ga abundances and Ga–Ti–Fe+3# [Fe+3/(Fe+3 + Cr + Al)] relationships of chrome‐spinels it is documented that the peridotites have experienced the combined effects of partial melting and variable extent of melt‐solid interaction. The MOR peridotites have spinels with geochemical signatures indicative of melt‐depleted residual origin with subsequent incompatible element enrichment through melt impregnation, while the Ga–Ti–Fe+3# relationships of chrome‐spinels in SSZ peridotites indicate that these highly depleted peridotites are not simple melt residues, but have been subject to significant compositional modification by interaction with subduction related melts/fluids. The observed compositional variations, which are related to long‐term tectonic reorganisation of oceanic lithosphere, provide evidence for a time integrated evolution from a mid‐ocean ridge to a supra‐subduction zone setting and may be a possible analogue to explain the coexistence of geochemically diverse MOR–SSZ suites in other Tethyan ophiolites. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Tectonomagmatic Constraints on the Sources of Eastern Mediterranean K-rich Lavas

Potassium-enriched mafic lavas, and in particular basalts of the high-K series, are important end-members of subduction zone volcanism. Two petrogenetic models can explain the generation of mafic K-rich lavas, involving either melting of ancient, enriched lithospheric mantle sources (single-stage model) or melting triggered by recent refertilization by subduction-related components derived from subducted sediments or oceanic crust (multi-stage model). These two models are tested for post-collisional high-K rocks of Eocene-Oligocene age from the Rhodopes, SE Bulgaria, based on new major element, trace element and Sr-Nd-Hf-Pb isotope data. The single-stage model is evaluated by Sr-Nd isotope modelling assuming the presence of ancient enriched lithospheric domains whereas the multi-stage model is assessed by comparing the compositions of the Bulgarian lavas with those of lavas from Santorini. Santorini lavas are considered to sample the current trace element and isotope inventory of the long-lived Aegean subduction zone system. This northward facing system has been active most probably since late Jurassic or early Cretaceous times and was potentially involved in refertilizing the mantle sources of the Bulgarian lavas. In addition to the data for Bulgarian lavas, we present new major element, trace element and Sr-Nd-Hf-Pb isotope data for Santorini. These are evaluated together with previously published data to infer the mode of source enrichment in the Aegean realm. The Bulgarian lavas exhibit a broad range of compositions from medium-K to high-K and shoshonitic with radiogenic Sr-87/Sr-86 (0.706-0.709), and unradiogenic epsilon Nd (-5.7 to -1.9) and epsilon Hf (-3 to +3) isotope signatures. The trace element budget was apparently well buffered against shallow-level crustal assimilation as documented by the trace element and Sr-Nd isotope systematics. Modelling of the Sr-Nd isotope compositions of the Bulgarian lavas argues for recent (Mesozoic to Cenozoic) source enrichment. Therefore single-stage models involving melting of ancient (>1 Ga) lithospheric mantle can be confidently ruled out, in agreement with tectonic models for the region. The enriched isotope signatures, together with a pronounced enrichment of incompatible elements, rather indicate mantle refertilization by a subduction component similar to continent-derived sediments subducted at the Hellenic Trench at present. The Bulgarian lavas record a predominant influx of subduction fluid components in comparison with the Santorini lavas. Collectively, the data presented for the Bulgarian lavas are clearly in favour of a multi-stage model.

Petrogenesis and thermobarometry of the ∼50 Ma rapakivi granite-syenite Acapulco intrusive: Implications for post-Laramide magmatism in southern Mexico

The Acapulco intrusion is a composite pluton that belongs to the coastal batholithic belt of southern Mexico, intruding the Xolapa metamorphic complex and cropping out in the neighboring area of Acapulco city. The Acapulco intrusion has been considered as an anomaly based on its age, which contrasts with the surrounding plutons and the general age trend from the coastal batholithic belt and corresponds to an Eocene–Oligocene age. It ranges in composition from granite (sensu stricto) to syenite and diorite. The most distinctive characteristic of the Acapulco intrusion is the rapakivi texture developed in the granites, which are characterized by biotite, amphibole, allanite, and fl uorite as distinctive minerals, plus titanite, zircon, and apatite as accessory phases. Geochemically, the Acapulco intrusion varies from metaluminous to peraluminous, and displays the distinctive signatures of arc-related magmas. The studied rocks show strong negative Sr, Ba, and Eu anomalies, coupled with incompatible element enrichments and high Ga/Al ratios, which are typical characteristics of A-type granites that underwent strong plagioclase fractionation from a formerly metaluminous magma. Initial isotopic ratios ( 87 Sr/ 86 Sr from 0.7035 to 0.7100, and eNd from +5.50 to +1.78) indicate a range from depleted mantle compositions to compositions consistent with crustal contamination by continental crust, particularly from the surrounding Xolapa Complex. U-Pb geochronology in zircons by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) established crystallization ages of 49.40 ± 0.40 Ma, 50.20 ± 1.0 Ma, 50.42 ± 0.39 Ma, and 50.56 ± 0.39 Ma for different lithologies of the Acapulco intrusion. These geochronological data, together with previous published works, confi rm that post-Laramide plutonism between 50 and 60 Ma is widespread in the southern continental margin of Mexico as a major magmatic event. Finally, new thermobarometric determinations established emplacement conditions of ~700 °C at 8–10 km depth (2.08–2.8 kbar), indicating an exhumation rate of ~0.21 km/m.y. between 50 and 20 Ma, which is slower than the previous estimated rate of 0.44 km/m.y. These results call for a review on models suggesting fast and/or slow exhumation of the southern Mexico coastal batholitic belt.

Mafic and silica-rich glasses in mantle xenoliths from Wau-en-Namus, Libya: Textural and geochemical evidence for peridotite–melt reactions

Anhydrous spinel peridotite xenoliths in Quaternary nepheline-basanite and melilite- or sodalite-bearing lavas of the Wau-en-Namus volcano in S Libya range from lherzolites to harzburgites recording melt extraction in a shallow setting (≤ 2 GPa). Primary clinopyroxenes have distinct trace element characteristics documenting LILE (large ion lithophile element) depletion or enrichment events predating the formation of glass pockets and veins in the xenoliths. These glasses are aluminous and alkali-rich, range in composition from ultrabasic to silicic (43–67 wt.% SiO 2) and may contain empty vugs and micro-phenocrysts of olivine, clinopyroxene, spinel, plagioclase, sodalite, apatite that are similar in composition to phenocrysts in the host lavas. Reactions of infiltrating melt and xenolith minerals are documented by diffuse Fe–Ca-rich rims of olivine in contact with glass, and by spongy-textured reaction domains caused by incongruent dissolution of primary pyroxenes and spinel. Some glasses have trace element characteristics similar to that of the host Ne-basanite, suggesting they were derived from the same source during entrainment and transport to the surface. Incompatible element enrichment and Sr–Nd isotopic compositions of the analyzed host lava are similar to HIMU (high μ; μ = 238Pb/ 204Pb)-type magmas, but the Pb isotopic composition is less radiogenic compared to other intra-plate Neogene magmatic rocks from N Africa.

Mesozoic accretion of juvenile sub-continental lithospheric mantle beneath South China and its implications: Geochemical and Re–Os isotopic results from Ningyuan mantle xenoliths

The mechanism for the widespread Mesozoic magmatism in South China has been ascribed to either the paleo-Pacific plate subduction or intra-continental lithospheric extension. Mantle xenoliths entrained in the Jurassic Ningyuan alkaline basalts from southern Hunan Province, including twelve lherzolites and one harzburgite, have been studied to constrain the composition and age of the Mesozoic sub-continental lithospheric mantle. The lherzolites contain 2.06–4.09 wt.% Al 2O 3 and 2.03–3.91 wt.% CaO, which are higher than the abundances of these species in the harbzurgite (1.72 wt.% and 1.17 wt.%, respectively). The Cr# of spinel varies from 0.07 to 0.24. Orthopyroxene contains 3.61–4.96 wt.% Al 2O 3 and 0.51–0.8 wt.% CaO, whereas clinopyroxene contains 4.78–7.32 wt.% Al 2O 3 and 1.04–2.01 wt.% Na 2O. Both whole rock and mineral compositions suggest that the Ningyuan mantle xenoliths have been subjected to low degrees of partial melting. Modeling of Y and Yb contents of clinopyroxene indicates that the lherzolites have been subjected to 3–5% degrees of partial melting, while the harzburgite has experienced about 12% melting. Clinopyroxene in most Ningyuan mantle xenoliths shows variable depletion in incompatible elements, suggesting that they have been weakly enriched after melting. Clinopyroxene in one sample (TYS01-3) displays strong enrichment in incompatible elements, reflecting the local occurrence of melt metasomatism. Equilibrium temperatures estimated by two-pyroxene geothermometry vary from 994 to 1081 °C, indicating that the Mesozoic mantle lithosphere has a hot geotherm. All lherzolites display consistent highly siderophile element (HSE) patterns and have suprachondritic Ru/Ir and Pd/Ir ratios. The harzburgite shows remarkable depletion in Pt, Pd and Re. The lherzolites have 187Os/ 188Os ratios of 0.12116–0.12929, which are higher than that of the harzburgite (0.11681). A correlation between 187Os/ 188Os and Al 2O 3 exists among the Ningyuan mantle xenoliths, which if interpreted as an isochron analog yields a model age of ~ 2.2 Ga. This age is older than the Re depletion age (T RD) of the harzburgite (~ 1.8 Ga), which represents a minimum age of melt depletion. The old ages either support the existence of ancient lithosphere relics beneath the Ningyuan region or reflect the accretion of ancient mantle materials from the asthenosphere during the Mesozoic. Although the tectonic implication of the old ages is ambiguous, compositions of the Ningyuan mantle xenoliths suggest that ancient mantle lithosphere beneath South China has been thinned and replaced by hotter, younger mantle during the Mesozoic, which has led to extensive lithospheric extension and abundant magmatism.

Intrusion history of the Portrush Sill, County Antrim, Northern Ireland: evidence for rapid emplacement and high-temperature contact metamorphism

AbstractThe gabbroic Portrush Sill in Northern Ireland, part of the North Atlantic Igneous Province, intruded Lower Jurassic mudstones and siltstones about 55 Ma ago. We used petrologic observations and geochemical analyses to study how the sill interacted with the sedimentary rocks. Field relationships show that an Upper Sill and numerous associated Minor Intrusions were emplaced in the sedimentary host rocks before intrusion of the Main Sill, some 10 m above its upper contact. Geochemical analyses reveal two magma contamination processes: Nb and Ta anomalies, coupled with incompatible element enrichment, record contamination by deep crustal rocks, whereas Li, Pb and Ba anomalies reveal a superficial contamination through fluid circulation at the contact between magmatic and sedimentary rocks. Analysis of mineral assemblages and geochemical data from the contact aureole demonstrate uniform metamorphic conditions between the two main intrusions and an absence of a thermal gradient. The identification of pyrrhotite by magnetization analyses and of orthopyroxene by microprobe analyses indicates very high temperatures, up to 660°C. Thermal modelling explains these temperatures as the coupled effects of the Main Sill and the earlier intruded Upper Sill and Minor Intrusions. Even though the chemical composition of the Main Sill suggests another type of parental liquid, all three units were emplaced in a very short time, certainly less than five years.

Evidence for heterogeneous enriched shergottite mantle sources in Mars from olivine-hosted melt inclusions in Larkman Nunatak 06319

Larkman Nunatak (LAR) 06319 is an olivine-phyric shergottite whose olivine crystals contain abundant crystallized melt inclusions. In this study, three types of melt inclusion were distinguished, based on their occurrence and the composition of their olivine host: Type-I inclusions occur in phenocryst cores (Fo 77–73); Type-II inclusions occur in phenocryst mantles (Fo 71–66); Type-III inclusions occur in phenocryst rims (Fo 61–51) and within groundmass olivine. The sizes of the melt inclusions decrease significantly from Type-I (∼150–250 μm diameter) to Type-II (∼100 μm diameter) to Type-III (∼25–75 μm diameter). Present bulk compositions (PBC) of the crystallized melt inclusions were calculated for each of the three melt inclusion types based on average modal abundances and analyzed compositions of constituent phases. Primary trapped liquid compositions were then reconstructed by addition of olivine and adjustment of the Fe/Mg ratio to equilibrium with the host olivine (to account for crystallization of wall olivine and the effects of Fe/Mg re-equilibration). The present bulk composition of Type-I inclusions (PBC1) plots on a tie-line that passes through olivine and the LAR 06319 whole-rock composition. The parent magma composition can be reconstructed by addition of 29 mol% olivine to PBC1, and adjustment of Fe/Mg for equilibrium with olivine of Fo 77 composition. The resulting parent magma composition has a predicted crystallization sequence that is consistent with that determined from petrographic observations, and differs significantly from the whole-rock only in an accumulated olivine component (∼10 wt%). This is consistent with a calculation indicating that ∼10 wt% magnesian (Fo 77–73) olivine must be subtracted from the whole-rock to yield a melt in equilibrium with Fo 77. Thus, two independent estimates indicate that LAR 06319 contains ∼10 wt% cumulate olivine. The rare earth element (REE) patterns of Type-I melt inclusions are similar to that of the LAR 06319 whole-rock. The REE patterns of Type-II and Type-III melt inclusions are also broadly parallel to that of the whole-rock, but at higher absolute abundances. These results are consistent with an LAR 06319 parent magma that crystallized as a closed-system, with its incompatible-element enrichment being inherited from its mantle source region. However, fractional crystallization of the reconstructed LAR 06319 parent magma cannot reproduce the major and trace element characteristics of all enriched basaltic shergottites, indicating local-to-large scale major- and trace-element variations in the mantle source of enriched shergottites. Therefore, LAR 06319 cannot be parental to the enriched basaltic shergottites.

Lithospheric control on geochemical composition along the Louisville Seamount Chain

Major and trace element and Sr, Nd, and Pb isotope data for lavas from 12 seamounts along the western (older) 1500 km section of the Louisville Seamount Chain in the southwest Pacific show remarkably uniform compositions over a ∼30–40 Myr period of volcanism. All 56 samples analyzed are alkalic to transitional in composition. Unlike Hawaiian volcanoes, Louisville volcanoes appear not to pass through a sequence of evolutionary stages characterized by older tholeiitic basalts overlain by incompatible element enriched alkalic and silica-undersaturated lavas. The youngest lavas from a given Louisville seamount tend to have the least enriched incompatible element compositions. This unusual chemical evolution may be the result of re-melting of heterogeneous hot spot mantle that was partially depleted during the earlier, age progressive stages. The oldest Louisville seamounts were constructed close to the extinct Osbourn Trough spreading center, located north of the chain, but age-progressive lavas from these older seamounts are not significantly different to lavas from younger seamounts. This may indicate that spreading at this fossil ridge ceased several tens of millions of years before the oldest Louisville seamounts were constructed. Large fracture zones apparently had no significant effects on the composition of Louisville magmatism. However, lavas from the central part of the Louisville Seamount Chain, where volcanoes are smaller and more widely spaced, tend to have more variable and more enriched compositions. We suggest this may reflect smaller degrees of melting resulting from greater lithosphere thickness, and hence a shorter melting column for this section of the Louisville Seamounts.

Geochemistry of rare high-Nb basalt lavas: Are they derived from a mantle wedge metasomatised by slab melts?

Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb > 20 ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas.

Enriched Basaltic Andesites from Mid-crustal Fractional Crystallization, Recharge, and Assimilation (Pilavo Volcano, Western Cordillera of Ecuador)

The origin of andesite is an important issue in petrology because andesite is the main eruptive product at convergent margins, corresponds to the average crustal composition and is often associated with major Cu^Au mineralization. In this study we present petro- graphic, mineralogical, geochemical and isotopic data for basaltic andesites of the latest Pleistocene Pilavo volcano, one of the most frontal volcanoes of the Ecuadorian Quaternary arc, situated upon thick (30^50km) mafic crust composed of accreted Cretaceous oceanic plateau rocks and overlying mafic to intermediate Late Cretaceous^Late Tertiary magmatic arcs. The Pilavo rocks are bas- altic andesites (54^57·5 wt % SiO2) with a tholeiitic affinity as opposed to the typical calc-alkaline high-silica andesites and dacites (SiO2 59^66wt %) of other frontal arc volcanoes of Ecuador (e.g. Pichincha, Pululahua). They have much higher incompatible element contents (e.g. Sr 650^1350 ppm, Ba 650^1800 ppm, Zr 100^225 ppm, Th 5^25 ppm, La 15^65 ppm) and Th/La ratios (0·28^0·36) than Pichincha and Pululahua, and more primitive Sr (87Sr/86Sr 􏰃0·7038^0·7039) and Nd (eNd 􏰃 þ5·5 to þ6·1) isotopic signatures. Pilavo andesites have geochemical affinities with modern and recent high-MgO andesites (e.g. low-silica ada- kites, Setouchi sanukites) and, especially, with Archean sanukitoids, for both of which incompatible element enrichments are believed to result from interactions of slab melts with peridotitic mantle. Petrographic, mineral chemistry, bulk-rock geochemical and isotopic data indicate that the Pilavo magmatic rocks have evolved through three main stages: (1) generation of a basaltic magma in the mantle wedge region by flux melting induced by slab-derived fluids (aque- ous, supercritical or melts); (2) high-pressure differentiation of the basaltic melt (at the mantle^crust boundary or at lower crustal levels) through sustained fractionation of olivine and clinopyroxene, leading to hydrous, high-alumina basaltic andesite melts with a tholeiitic affinity, enriched in incompatible elements and strongly impoverished in Ni and Cr; (3) establishment of one or more mid-crustal magma storage reservoirs in which the magmas evolved through dominant amphibole and clinopyroxene (but no plagioclase) fractionation accompanied by assimilation of the modified plutonic roots of the arc and recharge by incoming batches of more primitive magma from depth. The latter process has resulted in strongly increasing incompatible element concentrations in the Pilavo basaltic andesites, coupled with slightly increasing crustal isotopic signatures and a shift towards a more calc-alkaline affinity. Our data show that, although ultimately originating from the slab, incompatible element abundances in arc andesites with primitive isotopic signa- tures can be significantly enhanced by intra-crustal processes within a thick juvenile mafic crust, thus providing an additional process for the generation of enriched andesites.

Geochemistry and petrogenesis of Mesoproterozoic (~ 1.1 Ga) magmatic enclaves in granites of the eastern Llano Uplift, central Texas, USA

Mesoproterozoic (~1.1Ga) plutons of the eastern Llano Uplift, central Texas, USA contain two types of magmatic enclaves (<1% by vol.). Although volumetrically insignificant, the enclaves contain important petrogenetic information. Type I enclaves are felsic in composition (70–75wt.% SiO2), with mineral assemblages and chemical compositions comparable with the host granites, but typically display a finer grained texture. They are interpreted as partly chilled disrupted material from the margins and roof of the plutons. Type II enclaves are intermediate in composition (~56–69wt.% SiO2), with many elements defining trends continuous with the host granites. Both types of enclaves display sharp borders in contact with the host granite suggesting magma quenching with little or no physical exchange between host granite and enclave magma.Type II enclaves contained within the Marble Falls (MF) and Lone Grove (LG) plutons exhibit enrichments in Y, Nb, and Zr relative to their respective host granites. Enrichments in these incompatible trace elements at low SiO2, renders unlikely the possibility that the MF and LG Type II enclaves are the result of partial melting (anatexis) of mafic crustal rocks. Numerical modeling of fractional crystallization and simple mixing fails to explain the observed trace element trends.Because no coeval mafic to intermediate rocks are exposed in the uplift, characteristics of Type II enclave source magma(s) is uncertain. However, assuming source magmas similar to primitive continental arc basaltic andesite, trace-element trends (i.e., incompatible element enrichment and compatible element depletion) can be adequately replicated by a replenishment fractional crystallization (RFC) model. Chemistry of the MF and LG Type II enclaves suggest repeated replenishment of primitive magmas with only limited interaction with the host granitic magmas; the more primitive enclave magmas evolving in near chemical isolation by RFC processes. However, evidence from Type II enclaves in two other plutons in the Llano Uplift (Kingsland and Enchanted Rock) suggest that the isolation was non-ideal; i.e., some limited mixing may have occurred. Rapid quenching likely limited the potential for physical and chemical exchange between Type II enclaves and their host granite magmas.

Age and geochemistry of the oceanic Manihiki Plateau, SW Pacific: New evidence for a plume origin

We present 40Ar/ 39Ar age and geochemical (major and trace element and Sr–Nd–Hf–Pb isotope) data from submarine samples recovered from the basement of the Manihiki Plateau during the R/V Sonne research expedition SO193. The samples, predominately tholeiites, with minor occurrences of basaltic andesites and hawaiites, give a mean age of 124.6 ± 1.6 Ma from four different localities on the plateau. Based on TiO 2 content, we define two groups of volcanic rocks that differ in trace element and isotopic compositions. Partial melting modeling suggests that the low-Ti group lavas were derived through large degrees of melting (c. 30%) of a peridotitic source at mantle potential melting temperatures of c. T p = 1510 °C, more than 100 °C above the ambient mantle potential melting temperature. Since the primary water contents of both groups of lavas are low (0.1–0.3g wt.%) and the source is peridotitic, excess temperature is most likely the reason for the large degrees of melting producing the large volume of plateau basalts, consistent with the involvement of a mantle plume. The incompatible element contents of the low-Ti group lavas show a multistage history with enrichment in the most incompatible elements of a previously highly depleted source. They have isotopic compositions similar to enriched mid-ocean-ridge basalt (EMORB) and similar to the common focal zone (FOZO) component. The high-Ti group lavas have more enriched incompatible element compositions overall. Their isotopic compositions tend towards an enriched mantle (EMI)-type endmember, similar, although less extreme, than lavas from the Pitcairn Islands. The geochemistry of the Manihiki Plateau can best be explained by a plume containing three components: 1) a dominant peridotitic FOZO-type component, 2) delaminated EMI-type subcontinental lithospheric mantle (SCLM), and 3) a HIMU (recycled oceanic crustal)-type component possibly in the form of eclogite/pyroxenite. The similarity in age and geochemical composition of Manihiki, Hikurangi and Ontong Java basement lavas, including volcanism in some adjacent basins, suggests that the Greater Ontong Java Volcanic Event covered c. 1% of the Earth's surface with volcanism.

Thermal Evolution of the Lithosphere in a Rift Environment as Inferred from the Geochemistry of Mantle Cumulates, Northern Victoria Land, Antarctica

Among the abundant mantle xenoliths carried by the Cenozoic alkaline basalts of northern Victoria Land, Antarctica, we have studied a suite of clinopyroxene-rich cumulates collected at Browning Pass (Mt. Melbourne Volcanic Province), ranging in composition from wehrlites to clinopyroxenites. Clinopyroxenes belonging to both the Cr-diopside (wehrlites) and Al-augite series (ol-clinopyroxenites and clinopyroxenites) all show convex-upward REE patterns. Modal and cryptic metasomatism has variably affected the xenoliths, accounting for amphibole replacement of clinopyroxene and/or selective enrichment in incompatible elements. Chemical features, along with O–Sr–Nd isotopic data, indicate that both the parental magmas and the metasomatizing melts are related to the Cenozoic magmatic activity and imply the role of at least two mantle components with distinct isotopic fingerprints. The positive covariation between δ18Oolivine and the amount of modal olivine, and between δ18Oolivine and olivine Fo content, suggest that during the fractionation of olivine and pyroxene, the parent magma experienced a change in O-isotope composition; a low-δ18O melt component was not only added to the minerals during the metasomatic event but was also involved in the genesis of the parental melts. The Browning Pass cumulates are used to constrain the origin of the Antarctic Cenozoic magmatism from a heterogeneous mantle source whose depleted end-member is inferred to be the local lithospheric mantle, whereas the enriched end-member is represented by early metasomatic veins or domains emplaced into the depleted mantle during an amagmatic phase of rifting at the beginning of Ross Sea opening. Thermobarometric analysis of the process shows that the respective contribution to magma generation of the two end-members is related to the change of local thermal regime induced by an ‘edge effect’ in the mantle circulation at the lithospheric step between the thick East Antarctic craton and the thinned Ross Sea crust.

Nd-isotopic mapping of the Archaean–Proterozoic boundary in southwestern Tanzania: Implication for the size of the Archaean Tanzania Craton

Nd-isotopic data are presented for the granitoids that straddle the Archaean–Proterozoic (A–P) boundary in southwestern Tanzania along the Itigi–Makongolosi road traverse. On the basis of their Nd depleted mantle (T DM) ages, two groups of granitoids can be identified: those which show T DM ages of 2541–2894 Ma and are therefore belonging to the Archaean and those which have Proterozoic T DM ages of 2031–2430 Ma. Archaean granitoids are deformed, grayish in colour and consist of plagioclase, biotite and quartz and include a few microcline-rich, rare biotite pink alkali granites whereas the Proterozoic ones include mainly undeformed, microcline-rich, pink alkali granites. Although there are several compositional overlaps between the two, Proterozoic granitoids differ from those of the Archaean in showing enrichment in incompatible elements and overall higher abundances of the rare earth elements. Both Archaean and Proterozoic granitoids show negative to slightly positive Eu anomalies. These geochemical characteristics are attributed to the involvement of plagioclase in their magmagenesis indicating their generation at low pressures and shallow depths with the Proterozoic granitoids being derived from a more felsic protolith. Nd-isotopic data, coupled with petrography and lithological field relationships as well as major and trace elements geochemistry of the granitoids, places the A–P boundary approximately 150 km inside from the southern traditionally accepted boundary near Lake Rukwa in southwestern Tanzania. This implies that the size of the Archaean Tanzania Craton is smaller than hitherto understood. Small vestiges/domains showing Archaean ages within the Proterozoic regions could be explained as being slivers of tectonically interleaved Archaean material found within the Proterozoic terrane in southwestern Tanzania.

Stratigraphy and geochemistry of the Early Carboniferous Clyde Plateau Lavas in south Bute, Midland Valley of Scotland

AbstractA succession of Viséan (mid- to late Holkerian) volcanic rocks up to 340 m thick is preserved in three fault-blocks at the south end of the Isle of Bute in the Firth of Clyde, Scotland. These rocks form part of the Clyde Plateau Volcanic Formation, which, in this area, disconformably overlies sandstones of the lower Millport Member of the Clyde Sandstone Formation. The lower part of the volcanic succession in south Bute,c. 140 m thick, corresponds to the lower Strathgryfe lavas of the Renfrewshire Hills. This part of the succession is composed dominantly of feldspar-macrophyric and feldspar-microphyric basaltic rocks and mugearites. It is present in all three fault-blocks, whereas the succeeding volcanic rocks (middle and upper divisions) are only preserved in the median St Blane's block where they have a combined thickness of about 200 m. The two younger subdivisions are respectively correlative to the Misty Law Trachytic Centre, which forms a lens between the lower and upper Strathgryfe Members, and the upper Strathgryfe Member of the North Ayrshire section. Lavas of the lower division are feldspar-macrophyric and feldspar-microphyric basaltic rocks and mugearites, but those of the middle and upper divisions display a wider compositional spectrum, including feldspar-macro- and microphyric rocks but ranging from olivine-augite-macrophyric and olivine-augite-feldspar-macrophyric basalts to trachytes. The mafic lavas of south Bute have chondrite-normalized multi-element plots similar to those of ocean island basalts, with enrichment in incompatible elements. The trachytic lavas have similar patterns but are strongly depleted in Sr, P and Ti, reflecting fractionation of such minerals as plagioclase, apatite and magnetite/ilmenite during evolution of the parent magmas. Distribution of high field strength elements favours a within-plate origin for the south Bute lavas and supports derivation from a relatively deep (&gt;50 km) mantle source (garnet lherzolite). Chondrite-normalized REE plots for basaltic lavas of the lower division show enrichment in LREEs and lack strong Eu anomalies. Strong positive Eu anomalies in both felsic and mafic lavas of the middle and upper divisions may be attributable to high oxygen fugacities, but hydrothermal activity or feldspar fractionation may also have played a role. Fe-rich weathering profiles attest to intermittent extrusion and intense weathering processes.

Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (‘melt layer’) is overlain by a ‘peridotite layer’ and a layer of vitreous carbon spheres (‘melt trap’). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170–1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T s ~ 1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr2O3 at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si ‘low-pressure’ signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros.