Shield-building Stage Research Articles

Tracing the Origin and Magmatic Evolution of the Rejuvenated Volcanism in Santa Clara Island, Juan Fernández Ridge, SE Pacific

Oceanic intraplate volcanoes sometimes experience late-stage eruptive activity known as rejuvenated volcanism, and contrasting interpretations for its petrogenesis depend on the compositional characteristics. In the Juan Fernández Ridge (JFR), a volcanic chain approximately 800 km in length emplaced on the Nazca Plate, some subaerial occurrences of rejuvenated volcanism have been recognized on the Robinson Crusoe and Santa Clara Islands, both part of the same deeply eroded shield volcano complex. This study aims to understand the origin and magmatic evolution of rejuvenated volcanism on Santa Clara Island, emplaced after ~2.15 Ma of quiescence above the shield sequence, mainly via the analysis of unpublished geochemical and isotopic data. Field reconnaissance identified two nearly coeval rejuvenated sequences on Santa Clara Island: Bahía W (BW) and Morro Spartan (MS), both formed by basanitic and picro-basaltic lava flows with brecciated levels and local intercalations of sedimentary and pyroclastic deposits. In comparison to the chemical signature of the preceding shield-building stage (comprised mainly of basalts and picrites), the two rejuvenated sequences exhibit a notable enrichment in incompatible elements, but the Sr, Nd, and Pb isotopes are very similar to the FOZO mantle endmember, with an apparent additional contribution of HIMU and EM1 components. The geochemistry of lavas revealed the involvement of various processes, including contamination by ultramafic xenoliths, high-pressure fractional crystallization of olivine and clinopyroxene, and potential partial assimilation of oceanic lithospheric components. While the oceanic lithosphere has been considered as a potential source, the isotopic data from Santa Clara lies outside of the mixing curve between depleted mantle (DM, here represented by the North Chile Rise and the East Pacific Rise) and the previous shield stage, suggesting that a lithospheric mantle is not the primary source for the rejuvenated stage volcanism. Therefore, we favor an origin of the rejuvenated volcanism from the mantle plume forming the JFR, supported by similarities in isotopic signatures with the shield stage and high values of 208Pb/204Pb (only comparable to San Félix—San Ambrosio in the vicinity of JFR), implying the presence of a regional source with radiogenic 208Pb/204Pb isotope ratios. In addition, isotopic variations are subparallel to the mixing line between HIMU and EM1 components, whose participation in different proportions might explain the observed trends. In conclusion, we propose that the source of the rejuvenated volcanism on Santa Clara Island is a heterogeneous mantle plume, the same one that fed the shield stage. The rejuvenated volcanism is derived from a secondary melting zone away from the main axis of the plume.

Toroidal flow around the Tonga slab moved the Samoan plume during the Pliocene

Abstract Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ~2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields—generated by westward migration of the Tonga Trench and associated slab rollback—have deflected the conduit northward over the past 2–3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries.

The Cumbre Nueva collapse (La Palma, Canary Islands): New age determinations and evidence of an isotopic excursion

Episodic giant landslides characterize the history of intraplate oceanic volcanic islands, disrupting the gradual accumulation of eruptive products during shield-building stages. On La Palma (Canary Islands), the giant Cumbre Nueva collapse, which ended volcanic activity at the Paleo-Cumbre Nueva rift, formed the 11 km wide “Caldera de Taburiente” collapse embayment. Lavas erupted before and after this collapse have been studied, and 40Ar/39Ar ages of groundmass separates from key flows are presented, particularly from the small post-collapse Bejenado volcano that grew within the collapse scar. The new data constrain the age of the Cumbre Nueva collapse to between 519 ± 20 ka and 529 ± 12 ka (2s) and confirm that it occurred during a period of rapid re-surfacing of the island. This study has also constrained the duration of activity of the post-collapse Bejenado volcano, and the results indicate that this was also brief (529 ± 12 ka to 491 ± 16 ka). Starting just before the time of the collapse, the radiogenic isotope compositions of the lavas shifted temporarily to more depleted compositions, perhaps indicating that an isotopically distinct magma source was being tapped. However, an isotopic excursion of similar magnitude also occurred later in the post-collapse activity, suggesting that the changes in isotope composition of the magma source continued after the collapse.

Evidence for transitional and mildly alkalic eruptions during Hawai'i's dominantly tholeiitic shield-building stage: Insights from the Kulanaokuaiki Tephra (≥1.0 ka) at Kīlauea Volcano, HI

Vitric clasts from a marker horizon in the Kulanaokuaiki Tephra, deposited on the summit and flanks of Kīlauea Volcano, HI, during a prolonged period of explosive eruptions and low magma supply >1.0 ka, show unusual enrichments in alkalis relative to silica and in incompatible elements, in contrast with the volcano's dominantly tholeiitic shield-building lavas. The clasts are transitional basalts, with compositions near the tholeiitic-alkalic boundary (Macdonald and Katsura, 1964). Nearly uniform in composition across ∼200 km2, the clasts are the most proximal to the summit among rare occurrences of transitional and mildly alkalic shield-stage eruptions at Kīlauea. In contrast with the volcano's effusive shield-building style, stratigraphic evidence suggests the clasts were deposited in one of Kīlauea's most vigorous explosive eruptions in the past 2.5 ka—an episode punctuated by high fountaining along with an ∼12 km-tall ash plume. The tephra's composition is consistent with those of five unusual shield-stage Kīlauea lavas. All contain <50.0 wt% SiO2 and 3.0–4.0 wt% Na2O + K2O. All but one contain ≥0.70 wt% K2O, ≥3.0 wt% TiO2, and > 0.30 wt% P2O5. The transitional Kulanaokuaiki clasts also display elevated abundances and ratios of incompatible trace elements (e.g., ∼19 ppm La, La/Yb ∼ 8.6). Compositionally, these early shield-stage clasts and lavas resemble those erupted at the end of shield building at older Hawaiian volcanoes, attributed to lower degrees of partial mantle melting. Chemical constraints and extrinsic circumstances suggest that similar episodes of transitional and mildly alkalic volcanism will likely recur throughout shield building.

Removal of continental lithosphere beneath the Canary archipelago revealed from a U[sbnd]Pb Age and Hf/O isotope study of modern sand detrital zircons

We present new geochemical, UPb age, HfO isotope compositions for more than 400 detrital zircons collected from modern sands located throughout western Fuerteventura, Canary Islands as a proxy for the timing and origin of its plutonic history. This approach provides a new perspective and more complete time-integrated record of the islands geologic evolution. Fuerteventura UPb detrital zircon age spectra document that magmatism was initiated at ~35 Ma, was most prolific during the main shield-building stage (30–15 Ma), youngs southward, corroborates the existence of a low magma fluxing period (15–6 Ma), and provides new evidence for the existence that Pliocene and younger (at least as young as 0.24 ± 0.02 Ma) plutons with a variety of magma compositions accompanied post-shield volcanism until recent times. Most Fuerteventura detrital zircons have sub-mantle SIMS oxygen isotope compositions (δ18O < 5.0‰), and overlap compositions reported previously for gabbro/basalt xenoliths entrained in nearby Gran Canaria lavas, interpreted to be fragments of high-temperature seawater-altered Layer 3 oceanic crust.The detrital zircon Hf isotope compositions (εHfi: +15.0 to −0.2) display a much larger variation than recorded in previous studies of volcanic rocks from the Canary or Cape Verde islands. This extreme isotopic variation demonstrates that Fuerteventura detrital zircons provide a more sensitive record of the involvement of multiple sources in magma genesis. The high-εHf (>+12) zircons document the involvement of a depleted reservoir that is isotopically most similar to Middle Atlas SCLM. The low-εHf zircons, specifically those <+1, document the involvement of a reservoir that is more enriched than HIMU and overlaps the composition of NW African lower crust. These extreme εHf compositions indicate the presence of NW African lithosphere beneath Fuerteventura during the shield-building stage of island growth. The absence of a highly depleted Hf isotopic signature in young detrital zircons is interpreted to document the removal of SCLM during a magmatic shutdown initiated at ~15 Ma. Lithosphere erosion could be linked to a pulsed edge-driven convection cell responsible for SCLM delamination along a NE-trending corridor in NW Africa. The detrital zircon Hf and O isotope compositions indicate that both oceanic and continental lithosphere have existed beneath the island. This lithosphere is heterogeneous, consisting of high-T seawater altered oceanic crust, continental lower crust, and subcontinental mantle lithosphere; consistent with formation above a complex lithospheric transition zone created during Triassic-Jurassic rifting and opening of the Central Atlantic Ocean. Importantly, this detrital zircon study demonstrates that a more complete time-integrated record of a long-lived ocean island's geological history is preserved in modern sand samples, in part because a significant portion of Fuerteventura has been removed by erosion.

Osmium isotope evidence for a heterogeneous 3He/4He mantle plume beneath the Juan Fernandez Islands

Mantle plume models have been widely applied to explain the formation of ocean island basalts (OIB), with high-3He/4He in their lavas being explained by sampling of a primitive deep mantle source. The Juan Fernandez Islands have 3He/4He (7.8–18 RA) similar to or higher than in mid-ocean ridge basalts (MORB; 8 ± 1 RA) and have been used to both support and refute the mantle plume hypothesis. Ambiguity regarding the origin of the Juan Fernandez Islands primarily originates from interpretation of mantle source signatures between the lava series from the two main islands, Robinson Crusoe and Alexander Selkirk. To examine this issue, we report new whole-rock and olivine separate 187Os/188Os ratios and major-, trace-, and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances. The HSE and trace element abundances in Juan Fernandez main shield lavas can be explained by up to 30% olivine removal, together with spinel crystallization at 1–5 kbar, whereas Robinson Crusoe rejuvenated lavas can be reproduced by higher-pressure fractional crystallization (up to 10 kbar). An assemblage of 30 modal % olivine and 1–5 modal % spinel, combined with the additional contribution of primary melt trapped in olivine inclusions reproduces the range of HSE compositions observed in Juan Fernandez Archipelago olivine grains. Ratios of 187Os/188Os for Juan Fernandez lavas are generally less radiogenic than global OIB and show no correlation with indices of fractionation, indicating that they reflect mantle source compositions. Younger basanite lavas from Robinson Crusoe represent rejuvenated volcanism dominantly from a depleted lithospheric mantle source (187Os/188Os < 0.13) mixed with a high-3He/4He component from the main shield building stage. These lavas are similar in origin and composition to other Pacific rejuvenated lavas (e.g., Samoa, Hawaii). Robinson Crusoe main shield lavas are from a high-3He/4He (>18 RA) and enriched mantle source (187Os/188Os = 0.1312) similar to the ‘C’ or ‘FOZO’ component, whereas Alexander Selkirk lavas are consistent with a dominant contribution from a depleted, low-3He/4He (<10 RA) mantle component. The mantle sources of the shield lavas yield subtle variations in Sr-Nd-Os-Pb isotope space and have no clear variations with relative or absolute abundances of the HSE or trace elements. These results are consistent with a heterogeneous mantle plume model, with initial eruption of lavas from a primitive high-3He/4He mantle source ∼4 million years ago (Ma) to form Robinson Crusoe Island, which also led to 3He enrichment of the oceanic lithosphere, followed by eruption of Alexander Selkirk lavas from a more depleted mantle source at ∼2 Ma. Juan Fernandez lavas show that mantle heterogeneity preserved in OIB can occur over short-timescales (<2 Ma) and can impact lithospheric compositions, leading to eruption of rejuvenated lavas with unusual isotopic characteristics.

A Late-Quaternary full-vector geomagnetic record from El Golfo section, El Hierro, Canary Islands

Regional full-vector palaeosecular variation (PSV) curves are essential to further our understanding of geodynamo operation. Such curves typically lack palaeointensity information (with palaeodirectional being much more common), particularly for periods before the Holocene. Notably low palaeointensity values have been reported for significant portions of the Late Quaternary. Here, we present full-vector palaeomagnetic from the El Golfo section on El Hierro (Canary Islands, Spain), spanning the age range between ∼120 and ∼450 ka, to substantiate these reported low palaeointensity values for the Canary Islands. We sampled 28 flows (mostly alkaline oceanic basalts of the shield building stage) along the Camino de Jinama, a few kilometres to the south of a section studied by Szeremeta et al. for PSV. In line with their results, we observe an easterly declination deviation of ∼14° for the middle and lower part of our section. We relate this to undersampling of the complete PSV spectrum; rotation of El Hierro since ∼500 ka is deemed unlikely. Attempts to date the flows with the 40Ar/39Ar method on a multicollector instrument were unsuccessful due to the presence of copious amounts of methane in the extracted gases. Stratigraphic correlation to Szeremeta et al.’s section lends support to ages (technically of very low quality) obtained for the sites reported on here. Importantly, we add palaeointensity information in the present study, for the first time on El Hierro. Three different palaeointensity protocols were utilized, IZZI-Thellier, multispecimen, and pseudo-Thellier, to provide an additional consistency check and to increase the success rate. After applying strict selection criteria, reliable palaeointensities could be obtained for 18 flows, a success rate of ∼64 per cent. If more than one protocol yielded results for the same flow, the obtained intensities were often within error of each other, testifying to their robustness. Flows with a common true mean direction tend to produce similar intensity values as well; this testifies to the presumed under-sampling of PSV within the section. Obtained palaeointensity values typically range between ∼20 and ∼35 μT, distinctly lower than the present-day field value of ∼39 μT and lower than the average intensity during the Brunhes Chron. The determinations are in line with other rather low values obtained elsewhere for this particular time span.

High-Pressure Melting Experiments on Basalt-Peridotite Layered Source (KLB-1/N-MORB): Implications for Magma Genesis in Hawaii

In order to understand the melting processes that occur within recycled oceanic crust and mantle in a heterogeneous plume (e.g., that beneath the Hawaiian Islands), a series of high-pressure-high-temperature layered experiments were performed at 2.9 GPa, 5 GPa, and 8 GPa, from 1300°C to 1650°C, using a fertile peridotite KLB-1 and N-MORB. Our experiments at conditions below the dry peridotite solidus produced melt compositions that ranged from basaltic andesite to tholeiite. An Opx reaction band formed between eclogite and peridotite layers, likely via chemical reaction between a silica-rich eclogite-derived partial melt and olivine in the peridotite matrix. At temperatures at or above the dry peridotite solidus, substantial melting occurred in both basalt and peridotite layers, and fully molten basalt melt and melt pockets from the peridotite layer combined. In our layered experiments, major and minor element contents in reacted melts closely matched those of Hawaiian tholeiite and picrite, except for Fe. Partial melts of anhydrous run products had ~55 - 57 wt% SiO2 at low temperature (i.e., were andesitic) and had ~50 - 53 wt% SiO2 at high temperatures, slightly below the dry peridotite solidus (i.e., were tholeiitic, and similar to those that occur during the Hawaii shield-building stage). Based on the Fe- and LREE-enriched signature in Hawaiian tholeiites, we propose that recycled components in the Hawaiian plume are not modern N-MORB, but are Fe-rich tholeiite; a lithology that was common in the Archaean and early Proterozoic. We have demonstrated that the entire compositional spectrum of Hawaiian tholeiites (basalt to picrite) can be formed by basalt-peridotite reactive melting near the dry solidus of peridotite. Based on these results, we propose that the potential temperature of the sub-Hawaiian plume may be much lower than previously estimated.

Contrasting magmatic cannibalism forms evolved phonolitic magmas in the Canary Islands

Volcanic sequences on ocean islands record the temporal evolution of underlying magmatic systems and provide insights into how silicic crust is produced away from convergent margins. Assimilation has often been suspected to contribute, but the detection of such a process and its evolving maturity during migration across a mantle plume is less well documented. Here we present new major and trace element and Sr-Nd-Pb-U-Th-Ra-Pa isotope data that facilitate comparison of basanite to phonolite evolution on Tenerife (Canary Islands) with that shown by published data from La Palma. On both islands, (230Th/238U) ratios decrease with differentiation from parental magmas with 230Th excess toward different, silicic contaminants in secular equilibrium. On La Palma, this is inferred to reflect assimilation of small amounts of mafic wall rock. On Tenerife, both (230Th/238U) and (231Pa/235U) ratios decrease toward 1 with increasing differentiation, and this is accompanied by a subtle increase in Pb isotope ratios. At the same time, (226Ra/230Th) ratios change from 1 (a hitherto unreported magnitude). The Tenerife assimilant is thus constrained to be a partial melt of syenite formed in equilibrium with residual feldspar. The differences reflect a primarily deeper, more mafic magma system beneath La Palma during its late shield-building stage, whereas recent magmatic evolution at Tenerife occurs primarily at lower temperatures in small, shallower magma systems formed during its post–basaltic shield stage. Differentiation takes millennia or less.

Volcanic successions in Marquesas eruptive centers: A departure from the Hawaiian model

The temporal evolution and geochemical evolution of Marquesas hotspot volcanoes have often been interpreted with reference to the Hawaiian model, where a tholeiitic shield-building stage is followed by an alkali basaltic post-shield stage, followed after a 0.4 to 2.5Myr long quiescence period, by a rejuvenated basanitic/nephelinitic stage. Here we discuss geochemical data on 110 Marquesas lavas also dated using the unspiked 40K–40Ar method on separated groundmass (including 45 new ages measured on the southern islands of Hiva Oa, Motane, Tahuata and Fatu Hiva). Sample locations were positioned on detailed geological maps to determine their shield or post-shield position with respect to the caldera collapse event(s), without taking into account their geochemical features. A rather regular decrease of the ages towards SE, consistent with the Pacific plate motion, is observed from Eiao (5.52Ma) to Fatu Hiva (1.11Ma), and rejuvenated basanitic volcanism occurs only in Ua Huka (1.15–0.76Ma). The occurrence of intermediate and evolved lavas is restricted to the post-caldera stage, with the exception of Eiao island. However, many other features of the Marquesas chain are rather atypical with respect to those of Hawaii. Although Marquesas shields are tholeiitic, several of them (Eiao, Tahuata) contain interbedded alkali basaltic and basanitic flows. Moreover, post-shield volcanoes are either alkali basalts (Ua Huka), tholeiites (Hiva Oa, Tahuata, Fatu Hiva) or both (Nuku Hiva). This feature is consistent with the temporal continuity of the two stages and the usually short length of the post-shield period (<0.2Myr). In a given island, the trace element and isotopic compositions of shield and post-shield lavas overlap, although both display large variations. The sources of alkali basalts and basanites are more enriched than those of the contemporaneous tholeiites. These specific features support the hypothesis of an extremely heterogeneous Marquesas plume. The “weak” character of this plume led to low partial melting degrees, which in turn resulted in the preservation in the basaltic magmas of geochemical features inherited from small-size source heterogeneities.

The composition and distribution of the rejuvenated component across the Hawaiian plume: Hf‐Nd‐Sr‐Pb isotope systematics of Kaula lavas and pyroxenite xenoliths

Rejuvenated volcanism refers to the reemergence of volcanism after a hiatus of 0.5–2 Ma following the voluminous shield building stage of Hawaiian volcanoes. The composition of the rejuvenated source and its distribution relative to the center of the plume provide important constraints on the origin of rejuvenated volcanism. Near‐contemporaneous lavas from the Kaula‐Niihau‐Kauai ridge and the North Arch volcanic field that are aligned approximately orthogonally to the plume track can constrain the lateral geochemical heterogeneity and distribution of the rejuvenated source across the volcanic chain. Nephelinites, phonolites and pyroxenite xenoliths from Kaula Island have radiogenic Hf, Nd and unradiogenic Sr isotope compositions consistent with a time‐integrated depleted mantle source. The pyroxenites and nephelinites extend to the lowest 208Pb/204Pb reported in Hawaiian rocks. These data, along with new Pb isotope data from pyroxenites from the Salt Lake Crater (Oahu) redefine the composition of the depleted end‐member of the Hawaiian rejuvenated source at 208Pb/204Pb=37.35±0.05, 206Pb/204Pb = 17.75±0.03, εNd = 9–10, εHf ∼16–17 and 87Sr/88Sr &lt;0.70305. The revised isotope composition also suggests that this depleted component may contribute to LOA and KEA trend shield stage Hawaiian lavas, consistent with the rejuvenated source being part of the Hawaiian plume and not entrained upper mantle. The isotope systematics of rejuvenated magmas along the Kaula‐Niihau‐Kauai‐North Arch transect are consistent with a larger proportion of the rejuvenated depleted component in the periphery of the plume track rather than along its axis.

A Palaeocene intracanyon-style lava emplaced during the early shield-building stage of the Cuillin Volcano, Isle of Skye, NW Scotland

ABSTRACTThe twin summits of Preshal More and Preshal Beg, near Talisker, Isle of Skye, comprise the erosional remnants of a thick (at least 120 m) compound olivine tholeiite lava, or flow field, that ponded in palaeo-valleys within the Palaeocene lava field of west-central Skye. This unique flow field constitutes the Talisker Formation and is the youngest preserved extrusive unit of the Skye Lava Field. The lava inundated a complex of palaeo-valleys incised into the higher stratigraphical levels of the existing lava field, and remnants of the original sedimentary fill of these valleys still exist, the Preshal Beg Conglomerate Formation. The lava displays spectacularly well-developed two-tier (colonnade-entablature) columnar joint sets that formed as a consequence of slow, uninterrupted cooling through its base and sidewalls, aided by groundwater circulation and water ingress (from displaced drainage) directed into the lava's interior by master-joint systems. Intrusive phenomena developed at both the base and the top of the lava and there is evidence for the existence of subsurface feeder tubes. The tholeiitic composition of the Talisker Formation lava contrasts with the transitional, mildly alkaline characteristics of the remainder of the (older) lavas of Skye Lava Field. In broad terms, the Talisker Formation lava is compositionally very similar to the suite of cone-sheets emplaced into the oldest of the four intrusive centres that comprise the Skye Central Complex – the Cuillin Intrusive Centre – together with a high proportion of the Skye regional dyke swarm. The stratigraphical position, field relationships and compositional characteristics of the lava indicate that it was erupted and emplaced as an intracanyon-style flow field during the early shield-building stage in the growth of the (tholeiitic) Cuillin Volcano, which post-dates the main Skye ‘plateau’ Lava Field. Although the remnant outcrops are detached from their likely source area through erosion, this tholeiitic lava provides the first direct evidence linking the central complexes of the British Palaeogene Igneous Province and their eruptive products.

Quantitative Size Classification of Scoria Cones: The Case of Tenerife (Canary Islands, Spain)

Monogenetic volcanoes are the most abundant volcanic structures on the Earth's surface. Thousands of them exist in different tectonic contexts (e.g., subduction zones, intraplate areas, and divergent boundaries), appearing in groups forming volcanic fields. Tenerife, the largest and highest island of the Canarian archipelago, started to form about 14 million years ago (Ma), as a result of a complex eruptive history and a varied geomorphological evolution, including shield-building stages, volcanic rifts, stratovolcanoes, and monogenetic cones. Tenerife is an active, volcanic, oceanic island, where the last eruption occurred in 1909. In this paper, we propose a methodology for calculating the size of monogenetic basaltic volcanoes based on morphometric parameters, as well as a classification of the size of the volcanoes through the study of 297 scoria cones of Tenerife. Morphometric techniques, based mainly on height (Hco), volume (Vco), and area (Aco), were used together with correlations between different morphological parameters. The result is a simple quantitative classification, easy to use for estimating the size of monogenetic volcanoes. Of the monogenetic basaltic volcanoes of Tenerife, 98.98% fit into this classification, which distinguishes between large (Hco >200; Vco >0.1; Aco >0.5), medium (Hco >100 to <200; Vco >0.01 to <0.1; Aco >0.2 to <0.5), and small (Hco <100; Vco <0.01; Aco <0.2) volcanoes. There is a strong correlation between the size of cinder cones and their age: the more recent scoria cones are smaller in size than the older ones. This relationship supports suggestions by other researchers that eruptions have decreased in magnitude and the volume of available magma has become smaller. The results obtained by calculating the size of the scoria cones of Tenerife make possible the extrapolation of this technique to other insular or continental monogenetic volcanic fields.

A basic radial dike swarm of Boa Vista (Cape Verde Archipelago); its significance in the evolution of the island

A basic radial dike swarm unrelated to other basic units of Boa Vista (Cape Verde Archipelago) has been localized and characterized in the central sector of the island.According to new radiometric data three main stages in the evolution of Boa Vista are distinguished: the earlier (the Old Volcanic Complex: 17–16Ma) is equivalent to the shield building stage of Hawaii and the later (the Recent Volcanics (8–4Ma) is in some aspects comparable to the post-erosional stage. An important intermediate essentially felsic stage (the Trachytic–Phonolitic Complex: 14.3–12.8Ma) followed the basaltic shield stage. This felsic stage has equivalents in some other oceanic islands as the Canary Islands, specially Gran Canaria, Tenerife, and La Gomera. The central sector of Boa Vista is also occupied by the Felsic Subvolcanic Complex, a unit consisting of phonolitic breccias, syenites and monzonites that represent the Trachytic–Phonolitic Complex hypabyssal roots. The felsic rocks as a whole constitute half of the total amount of igneous rocks on the island making up Boa Vista, the island with the highest percentage of felsic rocks in the Central Atlantic Ocean.More than 200 dikes of the basic radial swarm intruding the Felsic Subvolcanic Complex have been measured. The intensity of the multiple dike injection is sometimes rather high, roughly a dike every 5m. The individual dikes have an observable mean length of about 300m. The composition of these dikes is always foiditic (nephelinites, melilitites, and limburgites), slightly different in composition (more alkaline and richer in incompatible elements) to the other basic units of the island (the Old Volcanic Complex and the Recent Volcanics).The radial dikes converge in an area located NW of the geometrical center of Boa Vista, a zone where the hypothetical center of the Old Volcanic Complex and the Trachytic–Phonolitic Complex edifices must also have been situated. The ages obtained from the dikes (between 14.8 and 11.5Ma) indicate that the radial injections are contemporary with the phonolites and the trachytes of the Trachytic–Phonolitic Complex and yet are part of the intermediate evolutionary stage of Boa Vista.

Sourcing the Megalithic Stones of Nan Madol: an XRF Study of Architectural Basalt Stone from Pohnpei, Federated States of Micronesia

Nan Madol is a massive 81 hectare prehistoric administrative and ceremonial complex made up of 93 constructed islets on the high volcanic island of Pohnpei. Built between A.D. 900 and 1650 over earlier settlement remains along the southern coast of Temwen Island within the fringing reef, the site is noted for its distinctive use of columnar basalt and large boulders. XRF analysis presented here suggests the site’s builders favoured columnar basalt from the island’s main shield building stage (7-8 mya) over post-shield material. Boulders incorporated in the architecture are primarily from post-shield stage Temwen Island but supplemented by some main shield boulders from mainland Pohnpei. Preliminary findings further suggest that there were shifts in the relative frequency of quarries used to construct different structures. These shifts could relate to exhaustion of accessible stone from specific sources or perhaps changing preferences for stone from different sources due to social or political imperatives.

Displaced helium and carbon in the Hawaiian plume

High relative abundances of primordial 3He are commonly found in ocean island basalts (OIB) thought to be derived from mantle plumes, and high 3He/ 4He ratios have been used to distinguish plume-type from non-plume OIBs. In simple plume models, one expects to find the highest 3He/ 4He ratios in the axial part of the plume conduit, which is sampled during the shield building stage of the volcanoes. However, the actual locus of the highest 3He/ 4He ratios is sometimes significantly displaced. This is best documented for the Hawaiian plume, where the highest- 3He/ 4He basalts are found on Loihi, a volcano tens of kilometers ahead of the inferred plume center, and 3He/ 4He ratios decrease systematically toward MORB-type values during the main and late phases of eruption. We propose that this effect is caused by small amounts of carbonatite melt formed in plumes as they rise through the transition zone. If the plume conduit is tilted by plate-driven upper mantle flow, the carbonatite melt infiltrates more vertically due to its low density and viscosity and is thus displaced from the plume center. Helium, if partitioned into the carbonatite melt, will also be displaced from the plume center. To test this model we use a numerical simulation of the Hawaiian plume interacting with the fast-moving Pacific lithosphere. We obtain vertical separation velocities of the carbonatite melt on the order of a meter/year. Consequently, helium and carbon, initially located in the plume center at > 450 km depth, are laterally displaced by 50 to 80 km in the shallow mantle, depending on grain size, porosity and melt production rate. This can explain why the highest 3He/ 4HE ratios (R/Ra up to 39; R/Ra ≡ ( 3He/ 4He) sample/( 3He/ 4He) atmosphere) occur on pre-shield Loihi, why they decline during the shield phases of Mauna Loa, Mauna Kea and Haleakala, and why post-shield and rejuvenated Hawaiian volcanism delivers only low 3He/ 4He ratios (R/Ra = 8–10). Our results quantify the potential role of carbonatite liquids in transporting helium in the Hawaiian conduit, and they appear to apply also to other plumes tilted by upper-mantle ‘wind’.

Evolution of shield-building and rejuvenescent volcanism of Mauritius

We report chemical and isotopic analyses of 68 samples and 40Ar/ 39Ar ages of 47 samples from Mauritius undertaken to understand the compositional evolution of the volcano and its causes through time. New 40Ar/ 39Ar ages show that construction of the Mauritius shield was well underway by 8.9 Ma, 1.1 m.y. earlier than previously thought and that the hiatus between the Intermediate and Younger Series was shorter than previously thought, as eruption of the rejuvenescent Intermediate Series continued through at least 1.66 Ma and Younger Series volcanism began by at least 1.0 Ma. Eruption frequency over the last 50 ka has been rather typical of Younger Series volcanism over the last 400 ka and future eruptions are possible. Although outcrops of the Intermediate Series lavas are confined to the Southwest, Intermediate Series are present beneath Younger Series flows in drill cores throughout the island. We estimate the total volume of rejuvenescent lavas at ~ 35 km 3 or about 0.05% of the volume of the volcano, similar to rejuvenescent volume fractions on Hawaiian volcanoes. As earlier studies found, Older Series lavas, which on average are slightly normatively silica-saturated, are somewhat more incompatible-element enriched than are the Intermediate and Younger Series, which are both slightly silica-undersaturated on average. Mean Sr and Nd isotope ratios of the Intermediate and Younger Series are nearly identical, but mean Pb isotope ratios, La/Sm, Nb/Y, and Nb/Zr of the Intermediate Series are higher than in the Younger Series. New high precision Pb isotope data, which shows considerably less scatter than previously published data, plot between the Older Series and basalts of the Central Indian Ridge, allowing the possibility that the source of the rejuvenescent lavas is a mixture of this plume and depleted mantle. We propose two possible explanations for the composition of rejuvenescent lavas. The first is that plume-derived melts reacted with deep lithosphere to form pyroxenite veins during the early shield-building stage. Later, these veins melted as a consequence of conductive heating of the lithosphere by the plume to produce the rejuvenescent lavas. Alternatively, rejuvenescent lavas may be derived from a sheath of thermally entrained mantle that surrounds the plume and is a mixture of plume material and depleted upper mantle.

Origin and temporal evolution of Koʻolau Volcano, Hawaiʻi: Inferences from isotope data on the Koʻolau Scientific Drilling Project (KSDP), the Honolulu Volcanics and ODP Site 843

The “Koʻolau” component of the Hawaiian mantle plume represents an extreme (EM1-type) end member of Hawaiian shield lavas in radiogenic isotope space, and was defined on the basis of the composition of subaerial lavas exposed in the Makapuʻu section of Koʻolau Volcano. The 679 m-deep Koʻolau Scientific Drilling Project (KSDP) allows the long-term evolution of Koʻolau Volcano to be reconstructed and the longevity of the “Koʻolau” component in the Hawaiian plume to be tested. Here, we report triple spike Pb isotope and Sr and Nd isotope data on KSDP core samples, and rejuvenation stage Honolulu Volcanics (HV) (together spanning ∼ 2.8 m.y.), and from ∼ 110 Ma basalts from ODP Site 843, thought to be representative of the Pacific lithosphere under Hawaiʻi. Despite overlapping ranges in Pb isotope ratios, KSDP and HV lavas form two distinct linear arrays in 208Pb/ 204Pb– 206Pb/ 204Pb isotope space. These arrays intersect at the radiogenic end indicating they share a common component. This “Kalihi” component has more radiogenic Pb, Nd, Hf, but less radiogenic Sr isotope ratios than the “Makapuʻu” component. The mixing proportions of these two components in the lavas oscillated through time with a net increase in the “Makapuʻu” component upsection. Thus, the “Makapuʻu” enriched component is a long-lived feature of the Hawaiian plume, since it is present in the main shield-building stage KSDP lavas. We interpret the changes in mixing proportions of the Makapuʻu and Kalihi components as related to changes in both the extent of melting as well as the lithology (eclogite vs. peridotite) of the material melting as the volcano moves away from the plume center. The long-term Nd isotope trend and short-term Pb isotope fluctuations seen in the KSDP record cannot be ascribed to a radial zonation of the Hawaiian plume: rather, they reflect the short length-scale heterogeneities in the Hawaiian mantle plume. Linear Pb isotope regressions through the HV, recent East Pacific Rise MORB and ODP Site 843 datasets are clearly distinct, implying that no simple genetic relationship exists between the HV and the Pacific lithosphere. This observation provides strong evidence against generation of HV as melts derived from the Pacific lithosphere, whether this be recent or old (100 Ma). The depleted component present in the HV is unlike any MORB-type mantle and most likely represents material thermally entrained by the upwelling Hawaiian plume and sampled only during the rejuvenated stage. The “Kalihi” component is predominant in the main shield building stage lavas but is also present in the rejuvenated HV. Thus this material is sampled throughout the evolution of the volcano as it moves from the center (main shield-building stage) to the periphery (rejuvenated stage) of the plume. The presence of a plume-derived material in the rejuvenated stage has significant implications for Hawaiian mantle plume melting models.

Microfossil assemblages on Tuscaloosa Seamount and their constraints on the age of the Nuʻuanu landslide, north of Oahu, HI

The Nuʻuanu submarine landslide, north of the Island of Oahu, is the largest of many debris avalanches along the flanks of the Hawaiian Islands. Several attempts have been made to date the Nuʻuanu landslide and to assess its relationship to the shield-building stage of Koʻolau Volcano as an indication of hazards that might be anticipated during the evolution of oceanic volcanoes. Previous age determinations based on magnetostratigraphy and the temporal distribution of turbidites have yielded a range of possible ages. We describe a distinctive fine-grained silty mudstone representing mantling sediment on the largest landslide block within the debris field, Tuscaloosa Seamount. This mudstone contains foraminifera and nannofossil assemblages that yield a micropaleontological age and paleoenvironmental determination for the interval immediately following landslide emplacement. The microfossils constrain the mantling sediment to have been deposited between 2.5 and 3.3 Ma, in offshore, mid- to lower-bathyal (∼1500–3000 m) water depth, consistent with a position on the upper flanks of Tuscaloosa Seamount. These findings indicate that landsliding occurred prior to or near the end of shield building at Koʻolau Volcano (2.9 Ma), and may have precipitated dramatic changes in volcano growth and deformation behavior, as well as changes in basalt geochemistry.

Oxygen–osmium isotope systematics of West Maui lavas: A record of shallow-level magmatic processes

New δ^(18)O and ^(187)Os/^(188)Os data from late shield-building stage lavas from West Maui volcano, Hawai'i, indicate that a range of distinct shallow-level processes can affect compositions of plume-generated ocean island magmas. The relative ages of the samples studied were determined either on the basis of field relationships, or from collection depth in a water well. The stratigraphically lower lavas have [Os] = 61–399 ppt, initial ^(187)Os/^(188)Os = 0.1311–0.1324 and δ^(18)O_(olivine) = 4.53–4.88‰. The stratigraphically higher lavas have [Os] = 20–273 ppt, initial ^(187)Os/^(188)Os = 0.1323–0.1390 (with the exception of one low [Os] sample with ^(187)Os/^(188)Os = 0.1517), and δ^(18)O_(olivine) = 4.73–5.21‰. These compositions are generally similar to those of Kea-type Hawaiian lavas from Mauna Kea and Kilauea volcanoes, but in detail, there are some significant differences between the West Maui lavas and other Kea-type lavas. The stratigraphically lower lavas have ^(187)Os/^(188)Os similar to those previously observed for Mauna Kea and Kilauea lavas, and define a nearly flat ^(87)Sr/^(86)Sr vs. ^(187)Os/^(188)Os trend that is consistent with assimilation by these magmas of small degree melts of Pacific oceanic crustal gabbros. The stratigraphically higher West Maui lavas extend to ^(187)Os/^(188)Os higher than observed at Mauna Kea or Kilauea. This probably results from interaction of these magmas with the hydrothermally altered volcanic edifice during the waning phases of the shield building stage. This process is likely to affect the isotope systematics of Os and O, but will have little effect on those of Hf, Nd, Pb or Sr.