Alkalic Lavas Research Articles

Mantle plume and rift-related volcanism during the evolution of the Rio Grande Rise

The Rio Grande Rise in the western South Atlantic Ocean has been interpreted as either an oceanic plateau related to the Tristan-Gough mantle plume, or a fragment of detached continental crust. Here we present new major and trace element data for volcanic rocks from the western and eastern Rio Grande Rise and the adjacent Jean Charcot Seamount Chain. The eastern Rio Grande Rise and older parts of the western Rio Grande Rise are comprised of tholeiitic basalt with moderately enriched trace element compositions and likely formed above the Tristan-Gough mantle plume close to the southern Mid-Atlantic Ridge. Younger alkalic lavas from the western Rio Grande Rise and the Jean Charcot Seamount Chain were formed by lower degrees of melting beneath thicker lithosphere in an intraplate setting possibly during rifting of the plateau. There is no clear geochemical evidence that remnants of continental crust are present beneath the Rio Grande Rise.

Isotopic evolution of prehistoric magma sources of Mt. Etna, Sicily: Insights from the Valle Del Bove

Mount Etna in NE Sicily occupies an unusual tectonic position in the convergence zone between the African and Eurasian plates, near the Quaternary subduction-related Aeolian arc and above the down-going Ionian oceanic slab. Magmatic evolution broadly involves a transition from an early tholeiitic phase (~ 500 ka) to the current alkaline phase. Most geochemical investigations have focussed on either historic (> 130-years old) or recent (< 130-years old) eruptions of Mt. Etna or on the ancient basal lavas (ca. 500 ka). In this study, we have analysed and modelled the petrogenesis of alkalic lavas from the southern wall of the Valle del Bove, which represent a time span of Mt. Etna’s prehistoric magmatic activity from ~ 85 to ~ 4 ka. They exhibit geochemical variations that distinguish them as six separate lithostratigraphic and volcanic units. Isotopic data (143Nd/144Nd = 0.51283–0.51291; 87Sr/86Sr = 0.70332–0.70363; 176Hf/177Hf = 0.28288–0.28298; 206Pb/204Pb = 19.76–20.03) indicate changes in the magma source during the ~ 80 kyr of activity that do not follow the previously observed temporal trend. The oldest analysed Valle del Bove unit (Salifizio-1) erupted basaltic trachyandesites with variations in 143Nd/144Nd and 87Sr/86Sr ratios indicating a magma source remarkably similar to that of recent Etna eruptions, while four of the five subsequent units have isotopic compositions resembling those of historic Etna magmas. All five magma batches are considered to be derived from melting of a mixture of spinel lherzolite and pyroxenite (± garnet). In contrast, the sixth unit, the main Piano Provenzana formation (~ 42–30 ka), includes the most evolved trachyandesitic lavas (58–62 wt% SiO2) and exhibits notably lower 176Hf/177Hf, 143Nd/144Nd, and 206Pb/204Pb ratios than the other prehistoric Valle del Bove units. This isotopic signature has not yet been observed in any other samples from Mt. Etna and we suggest that the parental melts of the trachyandesites were derived predominantly from ancient pyroxenite in the mantle source of Etna.

Origin of low oxygen isotopic compositions in alkalic lavas from Erebus volcano, Antarctica

Mount Erebus on Ross Island, Antarctica is an active volcano that has hosted a persistent phonolite lava lake for over 50 years. Oxygen isotope analysis of minerals in alkaline basanite to phonolite lavas from Erebus and surrounding volcanic centers show an anomalous trend of decreasing δ18O with magmatic evolution. The cold frozen climate and lack of radiogenic isotope evidence for assimilation of crustal materials makes this a unique location to investigate factors contributing to changes in oxygen isotope compositions. Olivine, feldspar, glass and whole rock samples from 29 lavas, 7 modern bombs and 2 xenoliths were analyzed for their oxygen isotopic compositions. Forward models indicate there should be an enrichment in δ18O of 0.4‰ in lavas due to Raleigh fractionation, but this is not observed. Olivine δ18O decreases from a typical mantle value of +5.3‰ in the parental basanite source to 4.3‰ in recently erupted phonolite lava bombs. The trend correlates with a decreasing forsterite content in the olivine. Feldspar and glasses exhibit δ18O values well below predicted values and are in fractionation disequilibrium with olivine compositions at measured magmatic temperatures of 1000 °C within the phonolite lava lake. Isotopic depletion of the melt may be accounted for by incorporation of 8–11% hydrothermally altered volcanic edifice over time assuming water–rock interaction from strongly isotopically depleted Antarctic precipitation. Enhanced disequilibrium with olivine phenocrysts, are seen during the late stages of Erebus volcano construction. This calls for differential fractionation of olivine and feldspar from their respective melts, and slow O-diffusion of olivine while in the shallow lava lake prior to eruption.

Alkalic to tholeiitic magmatism near a mid-ocean ridge: petrogenesis of the KR1 Seamount Trail adjacent to the Australian-Antarctic Ridge

ABSTRACT Coexisting alkalic and tholeiitic basalt lavas has been identified in a seamount chain located near the Australian–Antarctic spreading ridge. The KR1 Seamount Trail (KR1 ST) is a series of volcanic seamounts extending to the southeast in the spreading direction of the Australian–Antarctic Ridge (AAR). We herein report Sr, Nd and Pb isotopic compositions and (U–Th)/He and K–Ar geochronology for dredge samples from the KR1 ST in order to evaluate mantle processes and the role of enriched components for alkalic to tholeiitic magma generation in this region. The KR1 ST is a medium-sized seamount chain that extends for ~60 km, has a maximum height of ~1600 m above the seafloor, and consists of alkaline basalts and tholeiites with formation ages of ~0.4 Ma to ≤1.3 Ma. The isotopic characteristics of the alkaline basalts (206Pb/204Pb = 19.52–19.91; 87Sr/86Sr = 0.7030–0.7033; 143Nd/144Nd = 0.5128–0.5130) from the KR1 ST reflect a dominant ‘PREMA (or FOZO)’ mantle component represented by radiogenic Pb and mildly enriched Sr and Nd isotopic compositions. On the other hand, the weak PREMA (FOZO)-affinity (206Pb/204Pb = 18.89–18.93; 87Sr/86Sr = 0.7028–0.7029; 143Nd/144Nd = ~0.5130; 3He/4He = 7.64 ± 0.13 (R/RA)) coupled with their enriched mid-ocean ridge basalt (E-MORB) characteristics of tholeiites from the KR1 ST largely overlap with the KR1 MORB composition. The potential source materials for the alkaline basalts are considered to be ancient, recycled oceanic crust (i.e. eclogite) as well as sub-KR1 depleted MORB mantle (DMM). Whereas the main source materials for the KR1 ST tholeiites are presumed to be the DMM-dominant lithology with minor recycled material. We interpret the KR1 ST as a submarine hotspot chain that was formed by asthenospheric upwelling and spreading processes that delivered fertile blobs of recycled oceanic crust to the sub-KR1 region. The fundamental reason for sub-KR1 upper mantle enrichment might be attributed to a mantle plume event that possibly occurred prior to the formation of the KR1 ST.

Variation of melting processes and magma sources of the early Deccan flood basalts, Malwa Plateau, India

The Deccan flood basalts cover large parts of India and represent one of the largest volcanic provinces on Earth. We present geochemical and isotopic data on a magnetically defined stratigraphic section of lavas from the northern portion of the Deccan, the Malwa Plateau, which are dated at ∼67.5 Ma. These are therefore some of the oldest Deccan lavas, erupted before the Main Deccan units further to the south. Although the younger Malwa Plateau lavas have undergone significant crustal contamination by at least two distinct crustal components, the older, more alkalic lavas are less contaminated and have similar isotope and trace element compositions to Réunion oceanic intraplate basalts. Our data therefore provide geochemical evidence for a contribution of the Réunion hotspot to the generation of the early Deccan flood basalts. Most lavas reflect fractional crystallization and assimilation of less than 10% of a crustal melt but samples with the highest SiO2 and Sr isotope compositions underwent more extreme crustal contamination. The composition of the least contaminated basalts from the Malwa Plateau suggests that the degree of partial melting increased significantly from the base to the top of the section. Most Malwa Plateau basaltic magmas formed by <5% partial melting of garnet peridotite in the Réunion mantle plume whereas the degree of melting in the Main Deccan Volcanic Province increased threefold and included MORB sources. Together, these geochemical variations indicate increasing degrees of melting between 68.5 and 65.4 Ma and a variation of the mantle sources from Réunion plume mantle to an increasing contribution from depleted upper mantle. These changes probably reflect lithospheric erosion by the mantle plume within ∼2 million years that allows melting of entrained upper mantle material. The geochemical data thus agree with geophysical models of significant thinning of cratonic lithosphere during the passage across mantle plumes. The magma evolution from alkaline to tholeiitic basalts observed in the Deccan flood basalts resembles that observed in early oceanic plume-related volcanism like Hawaii.

Submarine Deep‐Water Lava Flows at the Base of the Western Galápagos Platform

AbstractTo investigate the initial phases of magmatism at the leading edge of the upwelling mantle plume, we mapped, photographed, and collected samples from two long, deep‐water lava flows located at the western base of the Galápagos Platform using the remotely operated vehicle Hercules. Lavas were recovered from four areas on the seafloor west of Fernandina volcano, including the western flow fronts of two deep‐water flows, heavily sedimented terrain between the two flows, and the eastern, shallower end of one flow. The sediment cover and morphologies are distinct between the western flow fronts and the eastern region based on seafloor imagery, suggesting that the long lava flows are not a single eruptive unit. Major and trace element concentrations reveal both tholeiitic and alkalic compositions and support the interpretation that multiple eruptive units comprise the deep‐water flows. Alkalic lavas have higher [La/Sm]N ratios (2.05–2.12) and total alkali contents (5.18–5.40) compared to tholeiitic lavas, which have [La/Sm]N ratios ranging from 1.64 to 1.68 and total alkali contents ranging from 3.07 to 4.08 wt%. Radiogenic isotope ratios are relatively homogeneous, suggesting a similar mantle source. We use petrologic models to assess three alternative mechanisms for the formation of the alkalic magmas: (1) high‐pressure crystallization of clinopyroxene, (2) mixing of high silica and mafic magmas, and (3) variable extents of melting of the same mantle source. Our modeling indicates that the alkalic samples form from lower extents of melting compared to the tholeiitic lavas and suggests that the deep‐water alkalic lavas are analogous to the initial, preshield building phase observed south of Hawaii and at the base of Loihi Seamount.

Geochemical sourcing of runoff from a young volcanic watershed to an impacted coral reef in Pelekane Bay, Hawaii

Runoff of sediment and other contaminants from developed watersheds threatens coastal ecosystems and services. A sediment geochemical sourcing study was undertaken on a sediment-impacted coral reef flat to identify terrestrial sediment sources and how these changed over time. Geochemical signatures were identified for watershed soils that formed on Hawaiian basaltic and alkalic lavas using relatively immobile compatible (Ni, Sc) and incompatible (Nb, REE, Th) elements quantified by ICP-MS in total decompositions of the fine fraction of surface soils. Some soils also contained ash from late-erupting cinder cones that added alkalic geochemical signatures, resulting in distinctive mixed signatures near these geologic features. Sediment was collected in Pelekane Bay using sediment traps during winter 2010–2011 and a sediment core in order to geochemically source runoff entering the bay. Geochemical signatures in trapped sediment showed that runoff predominantly originated from the lower watershed along a highway corridor rather than from the upper watershed or areas with alkalic lavas. Soil in the highway corridor also contained Zn above levels of concern for aquatic organisms and anthropogenic Pb, indications that runoff control measures could reduce the exposure of the reef community to potentially toxic metals and other road-associated contaminants. The upper portion of a 60-yr long sediment record from Pelekane Bay was disturbed by tsunamigenic waves from the Tohoku earthquake, and the remainder was likely subject to mixing by winter waves that averaged out geochemical variations. On average, watershed regions with basaltic soils were the predominant source of runoff to the bay over the past 60 years. By linking sediment runoff to geographic regions or features in watersheds, geochemical sourcing can provide insights that allow resource managers to direct runoff mitigation and soil conservation efforts to areas where they will be most effective.

Oceanic lavas sampling the high-3He/4He mantle reservoir: Primitive, depleted, or re-enriched?

Helium isotopes are used as a tracer for primitive reservoirs that have persisted in the Earth’s mantle. Basalts erupted at several intraplate oceanic islands, including Hawaii, Iceland, Galapagos, and Samoa, have hosted the highest 3He/4He ratios (>30 Ra, where Ra is atmospheric 3He/4He ratio) globally that are far in excess of the 3He/4He typical of the upper mantle sampled at mid-ocean ridges (8 Ra). These lavas have been suggested to be melts of a primitive, or possibly slightly depleted, mantle reservoir, i.e., either fertile or a depleted peridotite. Here we report evidence for geochemical enrichment in the high-3He/4He mantle sampled by lavas with the highest 3He/4He from Hawaii, Samoa, and possibly Galapagos. The titanium concentrations in high-3He/4He lavas from Samoa are too high to be explained by melts of a mantle peridotite, even at infinitesimally small degrees of melting, and the elevated Ti corresponds to elevated Pb-isotopic ratios. The highest 3He/4He lavas from Loihi, Hawaii, also have Ti concentrations that are too high to be melts of primitive mantle peridotite at the degrees of melt extraction proposed for this ocean island. Thus, Ti-rich material must have been added to the high-3He/4He mantle reservoir, and this material is likely to be recycled mafic crust similar to MORB-like eclogite, which is consistent with the elevated Pb-isotopic ratios. We show that fractionation corrected, major element compositions of high-3He/4He alkalic lavas can be satisfactorily modeled by melting and melt-rock interaction scenario in a fertile peridotite-MORB-eclogite hybrid system. Primitive peridotitic and recycled eclogitic reservoirs are suggested to be intimately associated in the deepest mantle and high-3He/4He lavas from several localities may sample a mantle source that hosts a component of recycled oceanic crust.

Petrology and Geochemistry of Volcanic Rocks from the South Kaua`i Swell Volcano, Hawai`i: Implications for the Lithology and Composition of the Hawaiian Mantle Plume

The South Kauai Swell (SKS) volcano was sampled during four JASON dives and three dredge hauls recovering rocks that range from fresh pillow lavas to altered volcanic breccias. Two geo- chemical groups were identified: shield-stage tholeiites (5� 4-3� 9 Ma) and rejuvenation-stage alkalic lavas (1� 9-0� 1 Ma). The young SKS ages and the coeval rejuvenated volcanism along a 400 km segment of the Hawaiian Islands (Maui to Niihau) are inconsistent with the timing and duration predictions by the flexure and secondary plume melting models for renewed volcanism. The SKS tholeiites are geochemically heterogeneous but similar to lavas from nearby Kauai, Niihau and Waianae volcanoes, indicating that their source regions within the Hawaiian mantle plume sampled a well-mixed zone. Most SKS tholeiitic lavas exhibit radiogenic Pb isotope ratios ( 208 Pb*/ 206 Pb*) that are characteristic of Loa compositions (>0� 9475), consistent with the volcano's location on the west side of the Hawaiian Islands. These results document the existence of the Loa component within the Hawaiian mantle plume prior to 5 Ma. Loa trend volcanoes are thought to have a major pyroxenite component in their source. Calculations of the pyroxenitic component in the parental melts for SKS tholeiites using high-precision olivine analyses and modeling of trace element ratios indicate a large pyroxenite proportion (� 50%), which was predicted by recent nu- merical models. Rejuvenation-stage lavas were also found to have a significant pyroxenite compo- nent based on olivine analyses (40-60%). The abundance of pyroxenite in the source for SKS lavas may be the cause of this volcano's extended period of magmatism (>5 Myr). The broad distribution of the Loa component in the northern Hawaiian Island lavas coincides with the start of a dramatic magma flux increase (300%) along the Hawaiian Chain, which may reflect a major structural change in the source of the Hawaiian mantle plume.

Effect of variable CO2on eclogite-derived andesite and lherzolite reaction at 3 GPa-Implications for mantle source characteristics of alkalic ocean island basalts

We have performed reaction experiments between 1, 4, and 5 wt % CO2-bearing MORB-eclogite (recycled oceanic crust)-derived low-degree andesitic partial melt and fertile peridotite at 1375°C, 3 GPa for infiltrating melt fractions of 25% and 33% by weight. We observe that the reacted melts are alkalic with degree of alkalinity or Si undersaturation increasing with increasing CO2 content in reacting melt. Consequently, an andesite evolves through basanite to nephelinite owing to greater drawdown of SiO2 from melt and enhanced precipitation of orthopyroxene in residue. We have developed an empirical model to predict reacted melt composition as a function of reacting andesite fraction and source CO2 concentration. Using our model, we have quantified the mutual proportions of equilibrated melt from andesite-peridotite (+ CO2) hybridization and subsequent peridotite (± CO2)-derived melt required to produce the major element composition of various ocean island basalts. Our model can thus be applied to characterize the source of ocean islands from primary alkalic lava composition. Accordingly, we determined that average HIMU source requires 24 wt % of MORB-eclogite-derived melt relative to peridotite containing 2 wt % CO2 and subsequent contribution of 45% of volatile-free peridotite partial melt. We demonstrate that mantle hybridization by eclogite melt-peridotite (± CO2) reaction in the system can produce high MgO (>15 wt %) basaltic melts at mantle potential temperature (TP) of 1350°C. Therefore, currently used thermometers to estimate TP using MgO content of primary alkalic melts need to be revised, with corrections for melt-rock reaction in a heterogeneous mantle as well as presence of CO2.

Mid-Tertiary magmatism in western Big Bend National Park, Texas, U.S.A.: Evolution of basaltic source regions and generation of peralkaline rhyolite

Tertiary magmatism in the Big Bend region of southwestern Texas spanned 47 to 17Ma and included representatives of all three phases (Early, Main and Late) of the Trans-Pecos magmatic province. Early phase magmatism was manifested in the Alamo Creek Basalt, an alkalic lava series ranging from basalt to benmoreite, and silicic alkalic intrusions of the Christmas Mountains. Main phase magmatism in the late Eocene/early Oligocene produced Bee Mountain Basalt, a lava series ranging from hawaiite and potassic trachybasalt to latite, widespread trachytic lavas of Tule Mountain Trachyte and silicic rocks associated with the Pine Mountain Caldera in the Chisos Mountains. Late main phase magmatism produced trachyte lava and numerous dome complexes of peralkaline Burro Mesa Rhyolite (~29Ma) in western Big Bend National Park. Late stage basaltic magmatism is sparsely represented by a few lavas in the Big Bend Park area, the adjacent Black Gap area and, most notably, in the nearby Bofecillos Mountains, where alkalic basaltic rocks were emplaced as lava and dikes concurrent with active normal faulting.Trace element modeling, Nd isotope ratios and calculated depths of segregation for estimated ancestral basaltic magmas suggest that Alamo Creek basalts (εNdt ~6.15 to 2.33) were derived from depths (~120 to 90km) near the lithosphere/asthenosphere boundary at temperatures of ~1600 to1560°C, whereas primitive Bee Mountain basalts (εNdt ~0.285 to −1.20) may have been segregated at shallower depths (~80 to 50km) and lower temperatures (~1520 to 1430°C) within the continental lithosphere. Nb/La versus Ba/La plots suggest that all were derived from OIB-modified continental lithosphere. Late stage basaltic rocks from the Bofecillos Mountains may indicate a return to source depths and temperatures similar to those calculated for Alamo Creek Basalt primitive magmas. We suggest that a zone of melting ascended into the continental lithosphere during main-phase activity and then descended as magmatism died out.Variation within Burro Mesa Rhyolite is best explained by fractional crystallization of a mix of alkali feldspar, fayalite and Fe–Ti oxide. Comendite of the Burro Mesa Rhyolite evolved from trachyte as batches in relatively small independent magma systems, as suggested by widespread occurrence of trachytic magma enclaves within Burro Mesa lava and results of fractionation modeling. Trachyte may have been derived by fractional crystallization of intermediate magma similar to that erupted as part of Bee Mountain Basalt. εNdt values of trachyte lava (0.745) and two samples of Burro Mesa Rhyolite (−0.52 and 1.52) are consistent with the above models. In all, ~5wt.% comendite may be produced from 100 parts of parental trachybasalt.Negative Nb anomalies in some Bee Mountain, Tule Mountain Trachyte and Burro Mesa incompatible element plots may have been inherited from lithospheric mantle rather than from a descending plate associated with subduction. Late phase basalts lack such a Nb anomaly, as do all of our Alamo Creek analyses but one. Even if some slab fluids partially metasomatized lithospheric mantle, these igneous rocks are much more typical of continental rifts than continental arcs.We relate Big Bend magmatism to asthenospheric mantle upwelling accompanying foundering of the subducted Farallon slab as the convergence rate between the North American and the Farallon plates decreased beginning about 50Ma. Upwelling asthenosphere heated the base of the continental lithosphere, producing the Alamo Creek series; magmatism climaxed with main phase magmatism generated within middle continental lithosphere, and then, accompanying regional extension, gradually died out by 18Ma.

Petrology and Sr-Nd-Pb-He isotope geochemistry of postspreading lavas on fossil spreading axes off Baja California Sur, Mexico

Postspreading volcanism has built large seamounts and volcanic ridges along the short axes of a highly segmented part of the East Pacific Rise crest that ceased spreading at the end of the middle Miocene, offshore Baja California Sur, Mexico. Lava samples from Rosa Seamount, the largest volcano, are predominantly alkalic basalts, mugearites, and benmoreites. This lavas series was generated through fractional crystallization and is compositionally similar to the moderately alkalic lava series in many oceanic islands. Samples from volcanic ridges at three adjacent failed spreading axes include mildly alkalic, transitional, and tholeiitic basalts and differentiated trachyandesites and andesite. The subtle but distinct petrologic and isotopic differences among the four sites may be due to differences in the degree of partial melting of a common, heterogeneous source. Postspreading lavas from these four abandoned axes off Baja California Sur together with those from other fossil spreading axes and from seamount volcanoes that grew on the East Pacific Rise flanks define a compositional continuum ranging from normal mid-ocean ridge basalt (NMORB)-like to ocean island basalt (OIB)-like. We propose that the compositional spectrum of these intraplate volcanic lavas is due to different degrees of partial melting of the compositionally heterogeneous suboceanic mantle in the eastern Pacific. A large degree of partial melting of this heterogeneous mantle during vigorous mantle upwelling at an active spreading center produces NMORB melts, whereas a lesser degree of partial melting during weak mantle upwelling following cessation of spreading produces OIB-like melts. The latter melts have a low (<8 RA) 3He/4He signature indicating their formation is different from that of OIBs from major “hot spot” volcanoes in the Pacific with high 3He/4He ratios, such as Hawaii and Galapagos.

Low‐productivity Hawaiian volcanism between Kaua‘i and O‘ahu

The longest distance between subaerial shield volcanoes in the Hawaiian Islands is between the islands of Kaua‘i and O‘ahu, where a field of submarine volcanic cones formed astride the axis of the Hawaiian chain during a period of low magma productivity. The submarine volcanoes lie ∼25–30 km west of Ka‘ena Ridge that extends ∼80 km from western O‘ahu. These volcanoes were sampled by three Jason2 dives. The cones are flat topped, &lt;400 m high and 0.4–2 km in diameter at water depths between ∼2700 and 4300 m, and consist predominantly of pillowed flows. Ar‐Ar and K‐Ar ages of 11 tholeiitic lavas are between 4.9 and 3.6 Ma. These ages overlap with shield volcanism on Kaua‘i (5.1–4.0 Ma) and Wai‘anae shield basalts (3.9–3.1 Ma) on O‘ahu. Young alkalic lavas (circa 0.37 Ma) sampled southwest of Ka‘ena Ridge are a form of offshore secondary volcanism. Half of the volcanic cones contain high‐SiO2 basalts (51.0–53.5 wt % SiO2). The trends of isotopic compositions of West Ka‘ena tholeiitic lavas diverge from the main Ko‘olau‐Kea shield binary mixing trend in isotope diagrams and extend to lower 208Pb/204Pb and 206Pb/204Pb than any Hawaiian tholeiitic lava. West Ka‘ena tholeiitic lavas have geochemical and isotopic characteristics similar to volcanoes of the Loa trend. Hence, our results show that the Loa‐type volcanism has persisted for at least 4.9 Myr, beginning prior to the development of the dual, subparallel chain of volcanoes. Several West Ka‘ena samples are similar to higher SiO2, Loa trend lavas of Ko‘olau Makapu‘u stage, Lāna‘i, and Kaho‘olawe; these lavas may have been derived from a pyroxenite source in the mantle. The high Ni contents of olivines in West Ka‘ena lavas also indicate contribution from pyroxenite‐derived melting. Average compositions of Hawaiian shield volcanoes show a clear relation between 206Pb/204Pb and SiO2 within Loa trend volcanoes, which supports a prominent but variable influence of pyroxenite in the Hawaiian plume source. In addition, both Pb isotopes and volcano volume show a steady increase with time starting from a minimum west of Ka‘ena Ridge. The entrained mafic component in the Hawaiian plume is probably not controlling the increasing magma productivity in the Hawaiian Islands.

Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands

Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O–Sr–Nd–Os–Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (δ 18O olivine = 5.17 ± 0.08‰; 87Sr/ 86Sr = 0.7029 to 0.7031; ε Nd = +5.7 to +7.1; 187Os/ 188Os = 0.1481 to 0.1750; 206Pb/ 204Pb = 19.1 to 19.7; Pd/Ir = 6 ± 3) versus those from La Palma (δ 18O olivine = 4.87 ± 0.18‰; 87Sr/ 86Sr = 0.7031 to 0.7032; ε Nd = +5.0 to +6.4; 187Os/ 188Os = 0.1421 to 0.1460; 206Pb/ 204Pb = 19.5 to 20.2; Pd/Ir = 11 ± 4) are revealed from the dataset. Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal–liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2–6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (⩾110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai’ian tholeiites. The HIMU affinities of the lavas, in conjunction with the low δ 18O olivine and high 206Pb/ 204Pb for La Palma, and elevated 187Os/ 188Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic 206Pb/ 204Pb, 187Os/ 188Os, elevated Re/Os and Pt/Os, and low-δ 18O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (∼50 km).

Kahoolawe Island, Hawaii: The role of an ‘inaccessible’ shield volcano in the petrology of the Hawaiian islands and plume

Kahoolawe volcano (∼10×17 km) forms one of the eight major Hawaiian islands. Access for geologic sampling has long been restricted due to military and preservation policies. However, limited visits to Kahoolawe in the 1980s yielded >200 samples, many of which have since been used to study the volcano within the framework of Hawaiian shield and mantle source geochemistry, petrology, mineralogy, and igneous processes. Kahoolawe is a tholeiitic shield with tholeiitic caldera-filling lavas, and at least seven postshield vents that erupted tholeiitic and (sparse) alkalic lavas. On smaller scales are a gabbro intrusion and ultramafic and gabbroic xenoliths in some postshield lavas. There is no evidence for rejuvenated volcanism. In its structural setting, Kahoolawe lies along the “Loa” trend of Hawaiian shields. Major element compositions of shield and caldera-filling lavas are similar and cluster at ∼6–7 wt% MgO, range from ∼5.5 to 16 wt% MgO, and include ∼9 wt% MgO examples that can be modeled as parental to the evolved lavas. For example, least squares mass balancing demonstrates that from ∼15% to 30% crystallization of olivine (±orthopyroxene), clinopyroxene, and plagioclase accounts for the ∼5.5–6 wt% MgO range of tholeiitic lavas. Greater differentiation occurred in the gabbro (diabasic) intrusive body as a segregation vein with ∼2.9 wt% MgO, and extreme differentiation produced local, small-volume rhyolitic melts that segregated into lava vesicles. Postshield lavas are mainly tholeiitic, have ∼5–7 wt% MgO, and overlap shield compositions. Some are alkalic, as low as ∼3.9 wt% MgO (hawaiite), and can be modeled as liquids after a ∼9 wt% MgO alkalic magma crystallized ∼30% olivine, clinopyroxene, plagioclase, and magnetite. Important aspects of Sr, Nd, Hf, and Pb isotopic ratios in Kahoolawe shield and caldera-filling lavas are slightly higher 87Sr/ 86Sr than in Koolau shield lavas (Oahu island; Makapuu-stage; e.g., Koolau mantle ‘endmember’) when compared at a given 143Nd/ 144Nd (e.g., ∼0.7042 at 0.5128), 206Pb/ 204Pb largely at the low end of the range for Hawaiian shields (e.g., ∼18), and ε Hf generally comparable to the values of other Hawaiian shields and ocean islands (e.g., ε Hf 8 at ε Nd 4). The isotopic ratios overall suggest small-scale source heterogeneity, considering the island size, and that Kahoolawe shield and caldera lavas were derived from a Hawaiian plume source containing recycled oceanic crust of gabbro and sediments. Based on certain geochemical indicators, however, such as Ce/Sr and La/Nb vs. 87Sr/ 86Sr, the source contained slightly less gabbro component than other shield sources (e.g., Koolau). Isotopic data for Kahoolawe postshield lavas are scarce, but those available generally overlap the shield data. However, ratios among certain alteration-resistant incompatible trace elements (e.g., Zr/Nb) discriminate some postshield alkalic from shield lavas. But because the different ratios for those postshield lavas can be explained by smaller partial-melting percentages of the shield source and by differentiation, neither isotopes nor trace elements identify postshield magmas as originating in a source unlike that for the shield lavas.

Source materials for inception stage Hawaiian magmas: Pb‐He isotope variations for early Kilauea

New noble gas and radiogenic isotopic compositions are presented for tholeiitic, transitional, and alkalic rocks from the submarine Hilina region on the south flank of Kilauea, Hawaii. The 3He/4He ratios for undegassed glass and olivine separates (11–26 Ra) contrast with those of postshield and rejuvenated alkalic lavas, consistent with the alkalic and transitional basalts at Hilina corresponding to early Kilauea magmas. Most early Kilauea samples contain highly radiogenic Pb isotopes compared with other Hawaiian rocks and therefore derive from a Hawaiian plume end‐member source (here referred to as the Hilina component) distinctive in that respect. Besides radiogenic Pb isotopes, the Hilina component has relatively low 3He/4He (&lt;12 Ra) among the Hawaiian magmas. Hawaiian inception stage magmas, including Hilina, Loihi, and deep Hana Ridge (east Maui), define a linear array in 206Pb/204Pb‐3He/4He isotope space, indicating that mixing between the Hilina and Loihi components (or their melts) dominates magmatism at the leading edge of the Hawaiian plume. The Hilina component's isotopic characteristics can be derived from young subduction‐recycled crust or metasomatised mantle. The isotopic differences between the geographically discriminated Kea and Loa trend volcanic chains, observed in shield stage lavas, are also seen in the inception stage magmas, suggesting that proportions of melts derived from the Hilina and Loihi components were different between the Kea and Loa trend volcanoes.

Petrogenesis of Davidson Seamount lavas and its implications for fossil spreading center and intraplate magmatism in the eastern Pacific

Seafloor spreading causes abundant magmatism along active ocean spreading centers, but the cause of magmatism along fossil spreading centers is enigmatic. Samples collected from Davidson Seamount, a typical volcanic ridge along an abandoned spreading center in the eastern Pacific, consist of an alkalic basalt to trachyte lava series; transitional basalts were sampled from another part of the abandoned axis, 20 km from the seamount. All samples experienced complex fractional crystallization prior to eruption, but they all share a common, compositionally heterogeneous mantle source. The parental magmas of the transitional basalts were produced from this source at higher degree of melting than those of the alkalic lava series. The composition of Davidson lavas overlaps with those of ridges along other fossil spreading centers and isolated near‐ and off‐ridge seamounts in the eastern Pacific. Together they define a compositional continuum ranging from tholeiitic, normal mid‐ocean ridge basalt (MORB)‐like to alkalic, ocean island basalt (OIB)‐like, similar to lavas that form linear island chains and ridges. We propose that this entire compositional spectrum of intraplate lavas that do not form linear volcanic chains in the eastern Pacific results from variations in the degree of partial melting of a common, compositionally heterogeneous mantle source. This source consists of more easily melted, geochemically enriched components of varying sizes and amounts embedded in a depleted lherzolitic matrix. Large degree of partial melting produces normal MORB–like melts represented by some near‐ridge seamount lavas, whereas small degree of melting produces OIB‐like fossil spreading center lavas. The small degree of partial melting beneath recently abandoned spreading centers results from either buoyancy‐driven decompression melting of the hot lithospheric and asthenospheric mantle material beneath active spreading centers or rapid motion, with respect to the underlying asthenosphere, of abandoned spreading axes that are thickening over a fertile mantle. Mid‐Tertiary volcanic rocks in coastal California, which are compositionally akin to intraplate lavas, are interpreted to be small degree partial melts of the same compositionally heterogeneous sub‐Pacific mantle that has upwelled through windows in a subducted slab.

K-Ar geochronology of basalt petrogenesis, Newer Volcanic Province, Victoria

The largest component of the basaltic Newer Volcanic Province of Victoria, Australia, the Western Plains subprovince, comprises extensive tholeiitic to alkalic lava flows and pyroclastics. The subprovince is subdivided into eastern and western sectors with distinctive Sr isotopic signatures; the isotopic discontinuity corresponds to an underlying Paleozoic tectonic boundary. Fifty-one new K–Ar ages plus previously published results encompass the geographic, lithological and isotopic variants of the subprovince, and are used to date extensive flow units, provide time calibration of regolith mapping units, and to compile eruptive and petrogenetic histories. The onset of volcanism at 4.6 Ma, the peak of activity at 3.0–1.8 Ma, and the progression from tholeiitic to alkalic compositions towards the present day are subprovince-wide, indicating that the basis of activity is independent of the lithospheric substrate. Local activity appears to have little pattern: some subareas have a conservative aspect, erupting similar tholeiites over much of the time span of the field; others show the complete range of lithological and isotopic composition. Peak activity (3.0–1.8 Ma) produced the maximum range of rock and isotopic compositions erupted subprovince-wide: the western sector has alkalic basalt to tholeiite (87Sr/86Sr 0.7041–0.7044), and the eastern sector transitional basalt to basaltic icelandite with much larger isotopic variation (87Sr/86Sr 0.7042–0.7055). In contrast to this apparently erratic behaviour, plots of 87Sr/86Sr vs time reveal a coherent system with the distinct eastern and western sectors expressing parallel isotopic change, a progressive decline in 87Sr/86Sr as magma compositions change from tholeiitic to more alkalic. Potential explanations, mantle chemical heterogeneity and crustal contamination, are examined in the light of chronological constraints. The tholeiitic to alkalic progression is analogous to that of oceanic islands and considered characteristic of intraplate volcanism; it implies that the most recent phase of Cenozoic volcanic activity in Victoria is coming to a close.

Evidence for recycled plate material in Pacific upper mantle unrelated to plumes

We report Sr, Nd, and Pb isotopic data of young alkaline basalt lava from a new type of volcano (petit-spot) on the northwestern Pacific Plate. Petit-spot lavas show Dupal, or extremely EM-1-like, Sr–Nd–Pb isotopic compositions. The data cannot be explained by contamination of pelagic sediment, in spite of the prediction on the basis of geological observation. We thus consider that the geochemistry of petit-spot lava indicates the existence of recycled fertile plate materials, not only the Dupal isotopic signature, in the northern hemisphere Pacific upper mantle unrelated to one or more active plumes. In consideration of published experimental results for fertile plate materials, selective melting of recycled material is a process critical in generating petit-spot lava. Moreover, the small volume of the volcano and low degree of melting in the mantle source needed to form strongly alkalic lavas suggest that petit-spot volcanism is originated from small-scale heterogeneities of recycled material. This idea consistently explains the geochemistry and noble gas isotopic composition of petit-spot lava, and also suggests small-scale heterogeneity widespread in the upper mantle of the Pacific Ocean. Together with a revised view of upper mantle heterogeneity, we propose that gross upper mantle composition is controlled by abundances and scales of regions of recycled material that correspond to differences in the relative position to the Pangea supercontinent, suggesting the link to the tectonic origin of the global scale heterogeneity.

Deccan plume, lithosphere rifting, and volcanism in Kutch, India

Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO 2-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO 2-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.