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The mineralogy of the oceanic basalt source region is examined by testing whether a peridotite mineralogy can yield observed whole-rock and olivine compositions from (1) the Hawaiian Islands, our type example of a mantle plume, and (2) the Siqueiros Transform, which provides primitive samples of normal mid-ocean ridge basalt. New olivine compositional data from phase 2 of the Hawaii Scientific Drilling Project (HSDP2) show that higher Ni-in-olivine at the Hawaiian Islands is due to higher temperatures (T) of melt generation and processing (by c. 300°C) related to the Hawaiian mantle plume. DNi is low at high T, so parental Hawaiian basalts are enriched in NiO. When Hawaiian (picritic) parental magmas are transported to shallow depths, olivine precipitation occurs at lower temperatures, where DNi is high, leading to high Ni-in-olivine. Similarly, variations in Mn and Fe/Mn ratios in olivines are explained by contrasts in the temperatures of magma processing. Using the most mafic rocks to delimit Siqueiros and Hawaiian Co and Ni contents in parental magmas and mantle source compositions also shows that both suites can be derived from natural peridotites, but are inconsistent with partial melting of natural pyroxenites. Whole-rock compositions at Hawaii and Siqueiros are also matched by partial melting experiments conducted on peridotite bulk compositions. Hawaiian whole-rocks have elevated FeO contents compared with Siqueiros, which can be explained if Hawaiian parental magmas are generated from peridotite at 4–5 GPa, in contrast to pressures of slightly greater than 1 GPa for melt generation at Siqueiros; these pressures are consistent with olivine thermometry, as described in an earlier paper. SiO2-enriched Koolau compositions are reproduced if high-Fe Hawaiian parental magmas re-equilibrate at 1–1·5 GPa. Peridotite partial melts from experimental studies also reproduce the CaO and Al2O3 contents of Hawaiian (and Siqueiros) whole-rocks. Hawaiian magmas have TiO2 contents, however, that are enriched compared with melts from natural peridotites and magmas derived from the Siqueiros depleted mantle, and consequently may require an enriched source. TiO2 is not the only element that is enriched relative to melts of natural peridotites. Moderately incompatible elements, such as Ti, Zr, Hf, Y, and Eu, and compatible elements, such as Yb and Lu, are all enriched at the Hawaiian Islands. Such enrichments can be explained by adding 5–10% mid-ocean ridge basalt (crust) to depleted mantle; when the major element composition of such a mixture is recast into mineral components, the result is a fertile peridotite mineralogy. [ABSTRACT FROM AUTHOR]

Highly magnesian, olivine-phyric tholeiitic basaltic and picritic lavas with >5 wt% TiO2 from the Prinsen af Wales Bjerge (PAWB) region are chemically distinct from all other Paleogene East Greenland flood basalts and from basalts in the North Atlantic Igneous Province. The ~100-m-thick lava succession rests on the 61 Ma Urbjerget Formation, is intercalated with volcaniclastic sediments, and has 57 Ma 40Ar/39Ar stepwise degassing ages. It is part of the Milne Land Formation, the first of the major flood basalt formations in East Greenland, and the result of plume impingement of the Kangerlussuaq area in East Greenland during the initial stages of continental breakup. The Ti-rich picrites have relatively primitive compositions and contain Mn- and Ni-rich olivine up to Fo88. Intermediate to high 87Sr/86Sri (0.7034–0.7044) and low Pb isotopic compositions reflect 4–11% crustal contamination, whereas the initial εNd (+4 − +5) and 187Os/188Os ratios (0.121–0.129) overlap with recent Icelandic basalts and appear little affected by contamination processes. The mantle source of the Ti-rich picrites contained garnet and was pyroxene-rich and similar to that of later low-Si alkaline basalts. The Ti-rich picrites of the PAWB, similar to other Ti-rich melts of the Kangerlussuaq region, represent analogies of MgO-rich and variably TiO2-enriched melts from pyroxene rich sources of traditionally accepted mantle plumes like Hawaii. [ABSTRACT FROM AUTHOR]

Oceanic island basalts and related magmatic rocks from Hawaii are derived from a compositionally heterogeneous mantle plume. Here we describe how this heterogeneity results from the transport of filaments of a specific composition in the plume, representing a relatively small volume of rocks (~15 %) interbedded inside a dry peridotite mantle. Four types of filaments are considered: sub-primitive mantle, ultralow-velocity zone, fertilized-harzburgite and eclogite type filaments. We present a model that describes the flow within a plume and the stress field in the overriding viscoelastic lithosphere and that can determine, from depth to the surface, the melting rate, composition and trajectory of melts produced within each type of filament. Our model shows that (1) the filaments melt at a depth corresponding to >5 GPa, where the temperature gap between the solidus and liquidus is narrow (~40–80 °C), and (2) the volume of filaments is small relative to the total volume of mantle, which therefore allows the latent heat required for the partial melting to be provided via conduction inside the hot plume. The primitive melts produced inside the filaments, occasionally mixed with the melt derived from an eclogite filament, represent a volume comparable with that expected in a plume composed only of dry peridotite that partially melts to a degree of ~10 % at the interface between the spinel and garnet fields (60–70 km depth). In particular, in the centre of the plume, sub-primitive mantle filaments produce up to 30 % tholeiite–picrite melts, whereas in fertilized-harzburgite filaments, the mantle melts completely to produce a melt having a meimechite-like composition. A key finding is that the fractional crystallization of these melts probably forms the so-called 'primary mantle-derived alkaline magmas' along with dunites and olivine-rich cumulates. Our plume model shows that the mantle flow divides into two parts. The first corresponds to hot flowlines that originate at a depth of ~200 km and at a distance of less than 25 km from the plume axis. Along these flowlines, when the mantle reaches a pressure of 5 GPa, the partially molten horizon in filaments is sufficiently thick for the interstitial melt to be squeezed out via dykes. This melt eventually ponds as sills in a subrectangular zone that is located inside the overlying lithosphere, between 70 and 50 km depth and centred over a distance of less than 40 km on either side of the axis. This zone is designated as the shield magmatic reservoir. The volatile-rich melt inside the sills infiltrates the surrounding mantle lithosphere and partially melts it. After ~0·1 Myr, the melt resumes its vertical ascent via dykes and eventually ponds and differentiates within subcrustal magma chambers located below active shield volcanoes. This sequence of processes matches the expected volume, petrology and geochemistry recorded for shield volcanoes. The second part of the melt flow does not pond within the shield magmatic reservoir. Rather, the mantle cold flowlines, originating at ~200 km depth and at 25–35 km from the plume axis, discharge their interstitial melt through dykes that were initially generated deeper, at ~5 GPa. The melt reaches the Moho at 100–150 km from the plume axis, where it forms magmatic bodies within which the melt differentiates. This melt probably represents that observed in pre- and postshield volcanoes. Finally, at ~70 km from the plume axis and at a depth greater than 200 km, the flowlines are subvertical. They then deflect at ~180 km depth and rotate toward the horizontal and eventually transit at 10–20° to the horizontal across an ~200 km distance from the axis and reach ~140 km depth. The fertilized-harzburgite and sub-primitive mantle/ultralow-velocity zone filaments that flow along these elbows partially melt by a few to several per cent. The resulting interstitial melt has a kimberlite-like composition. Thereafter, the excess pressure at the top of the filament at ~200 km from the axis overcomes the threshold for dyking and thus allows the escape of the interstitial melt via dykes ponding in subcrustal magma chambers or emerging directly at the surface. These melts have a composition similar to that associated with rejuvenated volcanism. We use the nature and the composition of whole erupted magmas and the seismic structure along the Hawaiian chain to validate this model. [ABSTRACT FROM AUTHOR]

The Publishers wish to apologise for the incorrect representation of figure 10 which accompanied this paper. The correct figure is reproduced below. [ABSTRACT FROM AUTHOR]

Based on an evaluation of major and trace element data for ocean island basalts (OIB), we demonstrate that oceanic lithosphere thickness variation, which we refer to as the lid effect, exerts the primary control on OIB geochemistry on a global scale. The lid effect caps the final depth (pressure) of melting or melt equilibration. OIB erupted on thick lithosphere have geochemical characteristics consistent with a low extent and high pressure of partial melting, whereas those erupted on thin lithosphere exhibit the reverse; that is, a high extent and low pressure of melting cessation. This observation requires that mantle melting beneath intra-plate volcanic islands takes place in the asthenosphere and results from dynamic upwelling and decompression. Melting beneath all ocean islands begins in the garnet peridotite facies, imparting the familiar ‘garnet signature’ to all OIB melts (e.g. [Sm/Yb]N > 1); however, the intensity of this signature decreases with increasing extent of melting beneath thinner lithospheric lids as a result of dilution. The dilution effect is also recorded in the radiogenic isotope composition of OIB, consistent with the notion that their mantle source regions are heterogeneous with an enriched component of lower solidus temperature dispersed in a more refractory matrix. High-quality data on the compositions of olivine phenocrysts from mid-ocean ridge basalt and global OIB sample suites are wholly consistent with the lid effect without the need to invoke olivine-free pyroxenite as a major source component for OIB. Caution is necessary when using basalt-based thermobarometry approaches to estimate mantle potential temperatures and solidus depth because OIB do not unequivocally record such information. For plate ages up to ∼80 Ma, we demonstrate that the geophysically defined base of the growing oceanic lithosphere corresponds to both an isotherm (∼1100°C) and the pargasite (amphibole) dehydration solidus of fertile mantle peridotite. As pargasite in H2O–CO2-bearing mantle peridotite is stable under conditions of T ≤ 1100°C and P ≤ 3 GPa (∼90 km), this solidus is essentially isothermal (i.e. dT/dP ∼ 0 in P–T space) with T ∼ 1100°C) at depths ≤90 km, but becomes isobaric (i.e. dP/dT ∼ 0 in P–T space) at the ∼90 km depth. The latter explains why older (>70 Ma) oceanic lithosphere cannot be thicker than ∼90 km without the need to invoke physically complex processes such as convective removal. [ABSTRACT FROM AUTHOR]

We conducted 1 atm experiments on a synthetic Hawaiian picrite at f O2 values ranging from the quartz–fayalite–magnetite (QFM) buffer to air and temperatures ranging from 1302 to 1600°C. Along the QFM buffer, olivine is the liquidus phase at ∼1540°C and small amounts of spinel (< 0·2 wt %) are present in experiments conducted at and below 1350°C. The olivine becomes progressively more ferrous with decreasing temperature [Fo92·3 to Fo87·3, where Fo = 100 × Mg/(Mg + Fe), atomic]; compositions of coexisting liquids reflect the mode and composition of the olivine with concentrations of SiO2, TiO2, Al2O3, and CaO increasing monotonically with decreasing temperature, those of NiO and MgO decreasing, and FeO* (all Fe as FeO) remaining roughly constant. An empirical relationship based on our data, T(°C) = 19·2 × (MgO in liquid, wt %) + 1048, provides a semi-quantitative geothermometer applicable to a range of Hawaiian magma compositions. The olivine–liquid exchange coefficient, = (FeO/MgO)ol/(FeO/MgO)liq, is 0·345 ± 0·009 (1σ ) for our 11 experiments. A literature database of 446 1 atm experiments conducted within 0·25 log units of the QFM buffer (QFM ± 0·25) yields a median of 0·34; values from single experiments range from 0·41 to 0·13 and are correlated with SiO2 and alkalis in the liquid, as well as the forsterite (Fo) content of the olivine. For 78 experiments with broadly tholeiitic liquid compositions (46–52 wt % SiO2 and &leq; 3 wt % Na2O + K2O) coexisting with Fo92–80 olivines, and run near QFM (QFM ± 0·25), is approximately independent of composition with a median value of 0·340 ± 0·012 (error is the mean absolute deviation of the 78 olivine–glass pairs from the database that meet these compositional criteria), a value close to the mean value of 0·343 ± 0·008 from our QFM experiments. Thus, over the composition range encompassed by Hawaiian tholeiitic lavas and their parental melts, ∼ 0·34 and, given the redox conditions and a Fo content for the most magnesian olivine phenocrysts, a parental melt composition can be reconstructed. The calculated compositions of the parental melts are sensitive to the input parameters, decreasing by ∼1 wt % MgO for every log unit increase in the selected f O2, every 0·5 decrease in the Fo-number of the target olivine, and every 0·015 decrease in . For plausible ranges in redox conditions and Fo-number of the most MgO-rich olivine phenocrysts, the parental liquids for Hawaiian tholeiites are highly magnesian, in the range of 19–21 wt % MgO for Kilauea, Mauna Loa and Mauna Kea. [ABSTRACT FROM AUTHOR]

The MgO contents of primitive tholeiitic magmas are estimated using the olivine geothermometer. This application assumes that olivine phenocrysts form by equilibrium crystallization. It is shown here that tholeiitic magmas crystallize by supercooled crystallization, and a representative degree of supercooling is estimated to be about 40°C. A degree of supercooling of 40°C results in an underestimate of the MgO content of a tholeiitic magma by 2 wt % MgO. When supercooling occurs, the assumption of equilibrium crystallization excludes true primary tholeiitic compositions from being considered. [ABSTRACT FROM AUTHOR]

Recent high-resolution seismic observations and geodynamic calculations suggest that mid-plate swells and volcanoes are plausibly controlled by processes and materials entirely in the upper boundary layer (<220 km depth) of the mantle rather than by deep-seated thermal instabilities. The upper boundary layer (BL) of the mantle is fertile enough, hot enough and variable enough to provide the observed range of temperatures and compositions of mid-plate magmas, plus it is conveniently located to easily supply these. Seismic data show that the outer ∼220 km of the mantle is heterogeneous, anisotropic and has a substantially superadiabatic vertical temperature gradient. This is the shear, and thermal, BL of the upper mantle. It is usually referred to as the ‘asthenosphere’ and erroneously thought of as simply part of the well-mixed ‘convecting mantle’. Because it supports both a shear and a thermal gradient, the lower portions are hot and move slowly with respect to the surface and can be levitated and exposed by normal plate tectonic processes, even if not buoyant. The nature of BL anisotropy is consistent with a shear-induced laminated structure with aligned melt-rich lenses. The two polarizations of shear waves travel at different velocities, VSV and VSH, and they vary differently with depth. VSH is mainly sensitive to the temperature gradient and indicates a high thermal gradient to 220 km depth. VSV is mainly sensitive to melt content. The depth of the minimum isotropic shear velocity, Vs, under young plates occurs near 60 km and this rapidly increases to 150 km under older oceanic plates, including Hawaii; 150 km may represent the depth of isostatic compensation for swells and the source of tholeiiic basalt magmas. The high-velocity seismic lid thickens as the square-root of age across the entire Pacific, but the underlying mantle is not isothermal; average sub-ridge mantle is colder, by various measures, than mid-plate mantle. Ambient mantle potential temperature at depth under the central Pacific may be ∼200°C higher, without deep mantle plume input, than near spreading ridges. This is consistent with bathymetry and seismic velocities and the temperature range of non-ridge magmas. Some of the thinnest and, in terms of traditional interpretations, hottest transition zones (TZ; ∼410–650 km depth) are under hotspot-free areas of western North America, Greenland, Europe, Russia, Brazil and India. The lowest seismic velocity regions in the upper mantle BL are under young oceanic plates, back-arc basins and hotspot-free areas of California and the Pacific and Indian oceans. Cold slabs may displace hotter material out of the TZ but geophysical data, and geodynamic simulations, do not require deeper sources. Magmas extracted from deep in a thick conduction layer are expected to be hotter than shallower oceanic ridge magmas and more variable in temperature. Mid-plate magmas appear to represent normal ambient mantle at depths of ∼150 km, rather than very localized very deep upwellings. Shear-driven upwellings from the base of the BL explain mid-plate magmatism and its association with fracture zones and anomalous anisotropy, and the persistence of some volcanic chains and the short duration of others. The hotter deeper part of the surface BL is moving at a fraction of the plate velocity and is sampled only where sheared or displaced upwards by tectonic structures and processes that upset the usual stable laminar flow. If mid-plate volcanoes are sourced in the lower half of the BL, between 100 and 220 km depth, or below, then they will appear to define a relatively fixed reference system and the associated temperatures will increase with depth of magma extraction. Lithospheric architecture and stress control the locations of volcanoes, ... [ABSTRACT FROM AUTHOR]

Kaua‘i lavas provide a unique opportunity to examine over 4·5 Myr of magmatic history at one location along the Hawaiian chain. New field, geochronological, petrological and geochemical results for a large suite of shield, post-shield and rejuvenated lavas are used to examine models for the origin of rejuvenated volcanism, and to evaluate the composition and structure of the Hawaiian plume. Kaua‘i has the most voluminous (∼58 km3 based on new field and water well interpretations) and longest-lived suite of rejuvenated lavas (∼2·5 Myr) in Hawai‘i. New K–Ar ages and field work reveal an ∼1 Myr gap (3·6–2·6 Ma) in volcanism between post-shield and rejuvenated volcanism. Isotopic and trace element ratios, and modeling of major elements of Kaua‘i’s rejuvenated lavas require low-degree melting (0·02–2·6%) at ∼1525 ± 10°C and 3·5–4·0 GPa of a heterogeneous, peridotitic plume source. High-precision Pb, Sr, Nd and Hf isotopic, and inductively coupled plasma mass spectrometry trace element data show substantial source variations with a dramatic increase in the depleted component in younger lavas. Some shield, post-shield and rejuvenated lavas (4·3–0·7 Ma) have high 208Pb*/206Pb* (radiogenic Pb produced since the formation of the Earth) values (>0·947) indicative of Loa-type compositions, the first reported Loa values in rocks >3 Ma, questioning previous models for the emergence of the Loa component in Hawaiian lavas. The timing, long duration, temporal variation in rock types and voluminous pulse of rejuvenated volcanism (58 km3), and the synchronous eruption of compositionally similar rejuvenated lavas, indicating tapping of common components along 350 km of the Hawaiian chain, are inconsistent with current models for this volcanism. Combining the lithospheric flexure and secondary zone of melting models provides a physical mechanism to initiate and focus the melting at shallower levels within the plume (flexural uplift) with a means to extend the duration of Kōloa volcanism at higher degrees of partial melting. [ABSTRACT FROM PUBLISHER]

Kilauea volcano is a very intensively studied, active basaltic magmatic system and thus represents an ideal location to study magma solidification processes in a natural environment. Understanding solidification is important in refining models of magma chamber dynamics and its detailed study may improve our knowledge of magma system evolution. In this study magma solidification processes are examined and quantified using samples from the 1969–1974 Mauna Ulu (MU) rift eruption. We have collected major and trace element whole-rock data plus in situ olivine compositions, along with crystal size distribution data on 11 lava samples. The observed whole-rock chemical variation was partly produced by olivine addition within the Kilauea edifice. At least two distinct olivine populations are inferred from quantitative textural analysis: (1) a 3–40-year-old population characterized by a low crystal density, greater crystal length and flatter slopes of the crystal size distributions (CSDs); (2) a 1·5–15-year-old population marked by a high density of smaller crystals and steep CSD slopes. The range in olivine composition suggests that all these crystals grew from a range of different magmas, probably closely related by crystal fractionation. The ubiquitous presence of deformed olivine crystals shows that population 1 reflects a component that must have mostly originated by disruption of a deformed cumulate. This antecrystic olivine population represents an earlier-coarsened and aggregated, cumulate-forming magma component. In contrast, the phenocrystic population 2 represents a late magma component formed in the summit magma storage region. Our results are consistent with the hypothesis that the components of the MU magmas followed two different routes. The deformed-olivine-bearing magma moved along the deep basal décollement then rose through vertical pipe-like conduits under the MU rift. The undeformed-olivine-bearing magma rose via the main conduit to the summit reservoir and then moved out along the rift zone, where the magmas mixed in small chambers. The presence of narrow, reversely zoned olivine rims suggests that the mixing occurred just prior to eruption. [ABSTRACT FROM PUBLISHER]

Late shield-stage silicic (icelandite and rhyodacite) lavas and dikes are exposed within the caldera region of Wai‘anae Volcano, O’ahu, and comprise the most continuous silicic, subalkaline and transitional suites in Hawai‘i. The silicic rocks can be divided into two groups. Group I consists of transitional icelandite dikes and icelandite to basaltic icelandite lava flows that follow a tholeiitic liquid line of descent. Group II includes the Mauna Kūwale Rhyodacite and several intermediate-silica lavas that follow a K-rich, calc-alkaline evolutionary trend. Group II lavas are highly porphyritic, with biotite and hornblende in the rhyodacites, and are selectively enriched in Rb, U and Pb. Pb, Sr and Nd isotope ratios of a Group I dike are similar to those of other Wai‘anae basaltic samples, whereas Group II lavas have lower 206Pb/ 204Pb, 207Pb/ 204Pb, and ɛNd, and high ratios of radiogenic 208Pb*/206Pb*, similar to some lavas from Ko’olau, Lāna‘i, and Kaho’olawe volcanoes. Most Group I dikes define a trend consistent with fractional crystallization at ∼0·5 kbar with low initial water (0·3 wt %) contents, whereas the chemical trends for Group II samples cannot be reproduced by simple crystallization models. These results, together with trace element modeling and isotopic characteristics, suggest derivation of the Group II silicic flows by hydrous melting of lower Hawaiian crust that had been altered to amphibolite. Crustal melting toward the end of the shield stage may be related to an increase in lithostatic pressure following rapid accumulation of lavas in the shield stage and ponding of basaltic magmas at depths below the level of crack closure (∼9–10 km). Contemporaneously, Group I silicic magmas formed by shallow-level crystallization as a consequence of decreased magma supply to the upper crust as a result of waning activity at the end of the shield stage. Geochemical evidence indicates significant mixing between the different types of silicic magma and between basaltic and silicic magmas at this stage of volcano evolution. Although melting of recycled crustal material entrained in the upwelling Hawaiian plume has previously been proposed, this is the first documentation of a role for melting of lower Hawaiian crust in the genesis of Hawaiian volcanoes. [ABSTRACT FROM PUBLISHER]

A diverse range of crustal xenoliths is hosted in young alkali basalt lavas and scoria deposits (erupted ∼3–5 ka) at the summit of Hualālai. Leucocratic xenoliths, including monzodiorites, diorites and syenogabbros, are distinctive among Hawaiian plutonic rocks in having alkali feldspar, apatite, zircon and biotite, and evolved mineral compositions (e.g. albitic feldspar, clinopyroxene Mg-number 67–78). Fine-grained diorites and monzodiorites are plutonic equivalents of mugearite lavas, which are unknown at Hualālai. These xenoliths appear to represent melt compositions falling along a liquid line of descent leading to trachyte—a magma type which erupted from Hualālai as a prodigious lava flow and scoria cone at ∼114 ka. Inferred fractionating assemblages, MELTS modeling, pyroxene geobarometry and whole-rock norms all point to formation of the parent rocks of the leucocratic xenoliths at ∼3–7 kbar pressure. This depth constraint on xenolith formation, coupled with a demonstrated affinity to hypersthene-normative basalt and petrologic links between the xenoliths and the trachyte, suggests that the shift from shield to post-shield magmatism at Hualālai was accompanied by significant deepening of the active magma reservoir and a gradual transition from tholeiitic to alkalic magmas. Subsequent differentiation of transitional basalts by fractional crystallization was apparently both extreme—culminating in >5·5 km3 of trachyte—and rapid, at ≥2·75 × 106 m3 magma crystallized/year. [ABSTRACT FROM AUTHOR]

We report Sr, Nd, and Pb isotope compositions for 17 bulk-rock samples from the submarine Hana Ridge, Haleakala volcano, Hawaii, collected by three dives by ROV Kaiko during a joint Japan–US Hawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios for the submarine Hana Ridge lavas are similar to those of Kilauea lavas. This contrasts with the isotope ratios from the subaerial Honomanu lavas of the Haleakala shield, which are similar to Mauna Loa lavas or intermediate between the Kilauea and Mauna Loa fields. The observation that both the Kea and Loa components coexist in individual shields is inconsistent with the interpretation that the location of volcanoes within the Hawaiian chain controls the geographical distribution of the Loa and Kea trend geochemical characteristics. Isotopic and trace element ratios in Haleakala shield lavas suggest that a recycled oceanic crustal gabbroic component is present in the mantle source. The geochemical characteristics of the lavas combined with petrological modeling calculations using trace element inversion and pMELTS suggest that the melting depth progressively decreases in the mantle source during shield growth, and that the proportion of the recycled oceanic gabbroic component sampled by the melt is higher in the later stages of Hawaiian shields as the volcanoes migrate away from the central axis of the plume. [ABSTRACT FROM AUTHOR]

Hana Ridge, the longest submarine rift zone in the Hawaiian island chain, extending from Maui 140 km to the ESE, has a complex morphology compared with other Hawaiian rift zones. A total of 108 rock specimens have been collected from the submarine Hana Ridge by six submersible dives. All of the rocks (76 bulk rocks analyzed) are tholeiitic basalts or picrites. Their major element compositions, together with distinctively low Zr/Nb, Sr/Nb, and Ba/Nb, overlap those of Kilauea lavas. In contrast, the lavas forming the subaerial Honomanu shield are intermediate in composition between those of Kilauea and Mauna Loa. The compositional characteristics of the lavas imply that clinopyroxene and garnet were important residual phases during partial melting. The compositions of olivine and glass (formerly melt) inclusions imply that regardless of textural type (euhedral, subhedral–undeformed, deformed) olivine crystallized from host magmas. Using the most forsteritic olivine (Fo90·6) and partition coefficients KDFe−Mgol−melt and DCaO*ol−melt, the primary magma composition is constrained to have ∼16·7% MgO and ∼8·4 wt % CaO. Modeling calculations using MELTS show that olivine first crystallized at 1380–1390°C and 0·1–0·3 GPa, under slightly hydrous conditions (0·5–1 wt % water). [ABSTRACT FROM AUTHOR]

Stratigraphically well-constrained sequences of late shield-building stage lavas from West Maui volcano, Hawaii, show age-dependent compositional variability distinct from that seen in shield-stage lavas from any other Hawaiian volcano. These distinctions are defined by 206Pb/204Pb–208Pb/204Pb variation as well as 87Sr/86Sr correlation with 206Pb/204Pb and trace element compositions. The West Maui lavas from stratigraphically higher in the sequence have major and trace element and Sr–Pb–Hf–Nd isotopic compositions similar to Kea-type lavas sampled at the younger Mauna Kea and Kilauea volcanoes, indicating that the Kea compositional end-member of Hawaiian lavas has remained homogeneous over ∼1·5 Myr. The 87Sr/86Sr–206Pb/204Pb variation in the stratigraphically lowest lavas is orthogonal to the all-Hawaii variation, indicating that it is not the result of mixing between components normally sampled by Hawaiian shield-stage magmas. We compare our West Maui data with observed compositions of Pacific oceanic basaltic and gabbroic crust, and predicted compositions for 2 Ga basaltic and gabbroic oceanic crust. The observed fine-scale compositional variability in the stratigraphically lowest West Maui lavas is consistent with 10–15% mixing of small-degree (2%) partial melts of the Pacific gabbroic oceanic crust with plume-derived, Kea-type magmas. Mass balance considerations indicate that this geochemical signal can be present in no more than 5% of the total volume of lavas erupted at West Maui. [ABSTRACT FROM AUTHOR]

On January 29-30, 1997, prolonged steady-state effusion of lava from Pu'u'O'o was briefly disrupted by shallow extension beneath Napau Crater, 1-4 km uprift of the active Kilauea vent. A 23-h-long eruption (episode 54) ensued from fissures that were overlapping or en echelon with eruptive fissures formed during episode 1 in 1983 and those of earlier rift zone eruptions in 1963 and 1968. Combined geophysical and petrologic data for the 1994-1999 eruptive interval, including episode 54, reveal a variety of shallow magmatic conditions that persist in association with prolonged rift zone eruption. Near-vent lava samples document a significant range in composition, temperature and crystallinity of pre-eruptive magma. As supported by phenocryst-liquid relations and Kilauea mineral thermometers established herein, the rift zone extension that led to episode 54 resulted in mixture of near-cotectic magma with discrete magma bodies cooled to ≤1100°C. Mixing models indicate that magmas isolated beneath Napau Crater since 1963 and 1968 constituted 32-65% of the hybrid mixtures erupted during episode 54. Geophysical measurements support passive displacement of open-system magma along the active east rift conduit into closed-system rift-reservoirs along a shallow zone of extension. Geophysical and petrologic data for early episode 55 document the gradual flushing of episode 54 related magma during magmatic recharge of the edifice. [ABSTRACT FROM AUTHOR]

Hawaiian volcanoes, dominantly shields of tholeiitic basalt, form as the Pacific Plate migrates over a hotspot in the mantle. As these shields migrate away from the hotspot, highly alkalic lavas, forming the rejuvenated stage of volcanism, may erupt after an interval of erosion lasting for 0·25-2·5 Myr. Alkalic lavas with geochemical characteristics similar to rejuvenated- stage lavas erupted on the sea floor north of Oahu along the Hawaiian Arch. The variable Tb/Yb, Sr/Ce, K/Ce, Rb/La, Ba/La, Ti/Eu and Zr/Sm ratios in lavas forming the North Arch and the rejuvenated-stage Honolulu Volcanics were controlled during partial melting by residual garnet, clinopyroxene, Fe-Ti oxides and phlogopite. However, the distinctively high Ba/Th and Sr/Nd ratios of lava forming the North Arch and Honolulu Volcanics reflect source characteristics. These characteristics are also associated with shield tholeiitic basalt; hence they arise from the Hawaiian hotspot, which is interpreted to be a mantle plume. Inversion of the batch melting equation using abundances of highly incompatible elements, such as Th and La, requires enriched sources with 10-55% clinopyroxene and 5-25% garnet for North Arch lavas. The 87Sr/86Sr and 143Nd/144Nd ratios in lavas forming the North Arch and Honolulu Volcanics are consistent with mixing between the Hawaiian plume and a depleted component related to mid-ocean ridge basalts. Specifically, the enrichment of incompatible elements coupled with low 87Sr/86Sr and high 143Nd/144Nd relative to bulk Earth ratios is best explained by derivation from depleted lithosphere recently metasomatized by incipient melt (<2% melting) from the Hawaiian plume. In this metasomatized source, the incompatible element abundances, as well as Sr and Nd isotopic ratios, are controlled by incipient melts. In contrast, the large range of published 187Os/188Os data (0·134-0·176) reflects heterogeneity caused by various proportions of pyroxenite veins residing in a depleted peridotite matrix. [ABSTRACT FROM AUTHOR]

Two picrite flows from the SW rift zone of Mauna Loa contain xenoliths of dunite, harzburgite, lherzolite, plagioclase-bearing lherzolite and harzburgite, troctolite, gabbro, olivine gabbro, and gabbronorite. Textures and olivine compositions preclude a mantle source for the xenoliths, and rare earth element concentrations of xenoliths and clinopyroxene indicate that the xenolith source is not old oceanic crust, but rather a Hawaiian, tholeiitic-stage magma. Pyroxene compositions, phase assemblages and textural relationships in xenoliths indicate at least two different crystallization sequences. Calculations using the pMELTS algorithm show that the two sequences result from crystallization of primitive Mauna Loa magmas at 6 kbar and 2 kbar. Independent calculations of olivine Ni–Fo compositional variability in the plagioclase-bearing xenoliths over these crystallization sequences are consistent with observed olivine compositional variability. Two parents of similar bulk composition, but which vary in Ni content, are necessary to explain the olivine compositional variability in the dunite and plagioclase-free peridotitic xenoliths. Xenoliths probably crystallized in a small magma storage area beneath the rift zone, rather than the large sub-caldera magma reservoir. Primitive, picritic magmas are introduced to isolated rift zone storage areas during periods of high magma flux. Subsequent eruptions reoccupy these areas, and entrain and transport xenoliths to the surface. [ABSTRACT FROM PUBLISHER]

New H2O, CO2 and S concentration data for basaltic glasses from Loihi seamount, Hawaii, allow us to model degassing, assimilation, and the distribution of major volatiles within and around the Hawaiian plume. Degassing and assimilation have affected CO2 and Cl but not H2O concentrations in most Loihi glasses. Water concentrations relative to similarly incompatible elements in Hawaiian submarine magmas are depleted (Loihi), equivalent (Kilauea, North Arch, Kauai–Oahu), or enriched (South Arch). H2O/Ce ratios are uncorrelated with major element composition or extent or depth of melting, but are related to position relative to the Hawaiian plume and mantle source region composition, consistent with a zoned plume model. In front of the plume core, overlying mantle is metasomatized by hydrous partial melts derived from the Hawaiian plume. Downstream from the plume core, lavas tap a depleted source region with H2O/Ce similar to enriched Pacific mid-ocean ridge basalt. Within the plume core, mantle components, thought to represent subducted oceanic lithosphere, have water enrichments equivalent to (KEA) or less than (KOO) that of Ce. Lower H2O/Ce in the KOO component may reflect efficient dehydration of the subducting oceanic crust and sediments during recycling into the deep mantle. [ABSTRACT FROM PUBLISHER]

The Puu Oo eruption is exceptional among historical eruptions of Kilauea Volcano for its long duration (∼17 years and continuing), large volume (∼2 km3), wide compositional range (5·6–10·1 wt % MgO) and the detailed monitoring of its activity. The prolonged period of vigorous effusion (∼300 000 m3/day) and the simple phenocryst mineralogy of the lavas (essentially only olivine) has allowed us to examine the volcano’s crustal and mantle magmatic processes. Here we present new petrologic data for lavas erupted from 1992 to 1998 and a geochemical synthesis for the overall eruption. The dominant crustal magmatic processes are fractionation and accumulation of olivine, which caused short-term (days to weeks) compositional variations. Magma mixing was important only during the early part of the eruption and during episode 54. The overall systematic decrease in MgO-normalized CaO content and abundances of highly incompatible elements, without significant Pb, Sr and Nd isotope compositional variation, is interpreted to be caused by mantle melting processes. Experimental results and modeling of trace element variations indicate that neither batch melting nor simple progressive melting can explain these compositional variations. Instead, a more complex progressive melting model is needed. This model involves two source components with the same isotopic composition, but one was melted ∼3% in the Hawaiian plume. The model results indicate that the amount of this depleted source component progressively increased during the eruption from 0 to ∼25%. Given the isotopic similarity of Puu Oo lavas to many lavas from Loihi Volcano and the small extent of prior melting to form the depleted source component, the melting region for Puu Oo magmas may partially overlap with that of the adjacent, younger volcano, Loihi. [ABSTRACT FROM PUBLISHER]

Submarine lavas erupted onto the Hawaiian arch 200–400 km north of Oahu show that the areal extent of Hawaiian volcanism is much larger than previously recognized. The North Arch volcanic field comprises 25 000 km2 of ∼0·5–1·15 Ma, volatile-rich, olivine-phyric alkalic lavas (alkalic basalt to nephelinite). These lavas are similar in composition to rejuvenated-stage lavas such as the Koloa Volcanics (Kauai) and Honolulu Volcanics (Oahu). North Arch lavas that encompass the compositional extremes have similar Sr, Nd and Pb isotopic ratios. Olivine accumulation and fractionation was the major post-melting process that affected the compositions of North Arch lavas. After correction for these processes, the inferred primary magma compositions show that they were derived by variable, factor of four, and relatively low extents of melting of garnet peridotite. Garnet and olivine were important residual phases during partial melting; in contrast to the Honolulu Volcanics, there is little evidence for residual hydrous phases, sulfides or Fe–Ti oxides. The mantle source for the North Arch lavas had Sr, Nd and Pb isotopic ratios intermediate between those of Pacific Ocean lithosphere and the inferred range for Hawaiian plume components. These data are consistent with a mixed lithosphere–plume source. Although the plume-derived component was probably from the Hawaiian plume, an alternative hypothesis is that during the middle Cretaceous, South Pacific lithosphere was contaminated by plumes that formed large oceanic plateaux (e.g. Ontong Java). This mixed source was subsequently partially melted as it passed near the Hawaiian plume. [ABSTRACT FROM PUBLISHER]

Oceanic island basalts have a large range in δ18O values (4.5−7.5‰) compared with the assumed primordial mantle values (5.5–6.0‰) and with mid-ocean ridge basalts (5.7 ± 0.2‰). Some Hawaiian tholeiitic basalts have low 18O values (4.6–5.2‰), which have been interpreted to be either a primary source feature or caused by crustal contamination. This study was undertaken to evaluate the cause of low δ18O values in Hawaiian tholeiitic basalts. We determined the δ18O values of glassy matrix material and coexisting olivines from pristine basalts produced during the current, 14-year-old Puu Oo eruption of Kilauea Volcano. Our results show that the Puu Oo eruption lavas have significant ranges in matrix (0.7‰) and olivine δ18O values (0.5‰) which do not correlate consistently with other geochemical parameters and that many of the lavas are out of oxygen isotopic equilibrium. These features probably reflect partial assimilation of and oxygen exchange with metamophosed Kilauea rocks during the magma's 19 km transit through the volcano's east rift zone. The parental magmas for Puu Oo lavas had a δ18O value of at least 5.2‰ and perhaps as high as 5.6‰. Thus, Puu Oo lavas do not give a clear indication of the δ18O value of Kilauea's mantle source but they do indicate that the oxygen in these otherwise pristine basalts has undergone significant modification by interaction with crustal rocks. [ABSTRACT FROM AUTHOR]

One plagioclase–spinel lherzolite and four spinel lherzolite xenoliths from Oahu, Hawaii, contain clinopyroxene grains that show homogeneous rare earth element (REE) abundances and smooth REE patterns with systematic depletion of light REE (LREE). These five xenoliths are mid-ocean ridge basalt (MORB) magma-depleted residues with compositions that were not modified by later metasomatism. Trace element systematics of these xenoliths were used to investigate the melt production rate (dF/dP) within a 90-my-old residual mantle column (RMC). Such rates were calculated as ratios of the difference in extents of depletion (dF) to the difference in equilibrium pressures (dP) between two xenolith samples. The extents of melting were modeled from REE, Sr and Zr abundances in clinopyroxene; and equilibrium pressures were inferred from the two-pyroxene geobarometer of Mercier et al. (1984, Contributions to Mineralogy and Petrology, 85, 391–403). Equilibrium pressures range from 21 to 7 kbar and extents of melting vary from 2 to 8%. Together, these data constrain the maximum extent of melting in the garnet lherzolite stability field to be <2% and a dF/dP of 0.43%/kbar within the stability field of spinel lherzolite. Uncertainties in the estimates of equilibrium pressure and extent of depletion lead to a slightly broader range of dF/dP values (0.26-0.78%/kbar). These values are significantly lower than that of ∼1.2%/kbar suggested by most previous studies. With the best estimated mean dF/dP of 0.43%/kbar, only 3.9 km of crust could have been generated by melting lherzolite in the pressure range of 26–7 kbar. The thickness and composition of the crust that overlies the 90 Ma Oahu RMC require a higher extent of melting. Based on the REE abundances, most samples from the 90 Ma crust and East Pacific Rise can be explained as pooled melts derived from the lherzolitic source in a passive melting regime with a maximum extent of melting (Fmax) of 30%. This model produces a dF/dP of 4.4%/kbar in the pressure range of 7–2 kbar and generates an additional 1.8 km of crust. In detail, the high [Sm/Yb]DM ratios in most East Pacific Rise samples cannot be explained by melting a lherzolitic source. Instead, they reflect mixing between two components: (1) the pooled melt derived from lherzolitic source in a passive melting regime with an Fmax of 30% (∼80% of the mixture) and (2) melt derived from garnet pyroxenite by 40% fractional melting (∼20% of the mixture). This model produces 7.1 km of crust. [ABSTRACT FROM AUTHOR]

The North Arch volcanic field is a submarine suite of alkali basaltic to nephelinitic lavas of the seafloor north of Oahu at water depths of 3900–4380 m. Glasses from these lavas were analyzed for H2O, CO2, Cl, S, Fe3+/∑Fe, and noble gases to investigate the role of volatiles in the generation, evolution, and degassing of these alkalic series lavas. In contrast to the systemic negative correlation between concentrations of SiO2 and nonvolatile incompatible elements (e.g. P2O5), the behavior of the volatile components is much more irregular. Concentrations of H2O in glasses vary by a factor of two ( 0.69–1.42 wt %) and show a poor correlation with melt composition, whereas concentrations of dissolved CO2 in glasses (260–800 p.p.m) increase with increasing alkalinity of the glasses. The H2O2 concentrations in the glasses are in equilibrium with an H2O–CO2 vapor at the depth of eruption (400 bar pressure). Samples collected directly from vent structures are highly vesicular, suggesting that these samples were gas rich upon eruption. Estimated bulk volatile contents of the two most vesicular vent samples are high (1.9±0.1 wt % H2O and 5.4±0.4 CO2) and are interpreted to have formed by closed system degassing. Estimated bulk volatile contents in four other vesicular vent samples are lower 1.3±0.2 wt % H2O and 2.0±0.4 wt % CO2), and these samples are interpreted to have lost some gas during eruption. Glass samples from inflated, flat lava flows are nonvesicular and interpreted to have lost essentially all exsolved gas during eruption and flow. Forward degassing models can predict the observed range in dissolved H2O and CO2 contents, calculated vapor compositions, and vesicularity as a function of SiO2. The models involve open to closed system degassing of an H2O–CO2 vapor phase from melts initially having H2O/P2O5=3 and CO2H2O=1–4 mass. Cl concentrations (400–1360 p.p.m) in glasses correlate with concentrations of nonvolatile, incompatible elements. Concentrations of noble gases measured on bulk glass samples are low compared with mid-oceanic ridge basalt (MORB). The low concentrations result mainly from extensive vapor exsolution from the magma. The helium isotopic ratios for gases released from vesicles are similar to MORB values [6.8–8.5 times the air ratio (RA)], whereas those released from glasses are lower than MORB values as a result of in situ decay of U and Th. The S contents (0.11–0.22 wt %) of most of the alkali olivine basaltic and basanitic glasses are sufficient to saturate the silicate melt with immiscible Fe–S–O liquid at the T and P of eruption and quenching. However, two vesicular samples appear to hae lost some dissolved S owing to eruptive degassing. Magmatic oxygen fugacitites estimated from Fe3+/∑Fe range from ΔFMQ=-0.8 to +0.7, with the nephelinitic glasses being more oxidizing than the less alkalic glasses. We infer that the mantle source region for the North Arch magmas was homogeneous with respect to Fe3+/∑Fe and that melting occurred in the absence of graphite or CH4- rich fluid. The effect of variable partial melting on magmatic oxygen fugacity may be a common feature of Hawaiian volcanism. These complex data pint to a simple result, namely that parental magma compositions can be derived by variable extents of melting of a homogeneous source followed by olivine crystallization and degassing at 400 bar. If the parental liquids are produced by 1.6–9.0% partial melting (±20% relative), then mantle volatile contents are estimated to be 525±75 p.p.m. H2O, 1300±800 p.p.m. CO2 and 30± p.p.m. Cl. [ABSTRACT FROM AUTHOR]

Xenoliths from the southern flank of Mauna Kea volcano form two broad categories. (1) Ultramafic: porphyroclastic dunite, wehrlite, and olivine clinopyroxenite (Fo89.4–83.6, clinopyroxene mg-number 90.3–86.3, spinel mg-number 57–42, spinel cr-number 7–52, no palgioclase); and granular wehrlite and olivine clinopyroxenite (Fo83–76) with plagioclase (An84–69) ± orthopyroxene, and Cr-magnetite. (2) Gabbroic: granular gabbro, gabbronorite, and troctolite composed of olivine + clinopyroxene frameworks (Fo82–74, mg-number 85–79) enclosing plagioclase (∼An79–69) ±orthopyroxene, and Fe–Ti oxides; and plagioclase (4 wt %). These Mauna Kea xenoliths are plutonic complements to postshield lavas (Hamakua volcanics), and they identify that stage of volcano development with 15–5 wt % MgO magmas that underwent processes intrinsic to mafic-layered intrusions; e.g. in situ and gravity-settled crystallization, extensive differentiation, varieties of layering, mobilizations of late-stage, evolved liquids, compaction and connective disturbances in reservoirs. [ABSTRACT FROM AUTHOR]

Numerous geochemical studies have indicated that the Hawaiian mantle plume consists of several distinct components. However, their origin remains controversial, with a number of different interpretations having been proposed. We present new major element, trace element and high-precision Sr–Nd–Pb–He isotope data for a suite of fresh submarine lavas erupted by the Koolau, Kilauea and Loihi volcanoes, which are widely believed to have sampled three distinct Hawaiian plume components. The Sr and Nd isotope compositions of the Loihi lavas are similar to those of Kilauea lavas. However, our double-spike Pb isotopic data show that Loihi lavas have both Kilauea-like and Loihi-like compositions. This discovery implies that the Loihi source region contains a Kilauea-like (‘Kea’) mantle component. Our new data support the existence of three major types of intrinsic plume component: a Loihi component, an ‘enriched’ (Koolau) component and a ‘depleted’ (Kea) component. We propose that the Loihi component is a common component, forming the matrix in the Hawaiian mantle plume, and that the isotopic differences between the various shield lavas reflect different mixing proportions of the Loihi component and recycled oceanic crust components (EM-1-like and HIMU-like). The Koolau component contains a higher proportion of EM-1, whereas the Kea component contains a higher proportion of HIMU. EM-1- and HIMU-like recycled oceanic crust components are distributed on a fine scale throughout the peridotitic matrix within the Hawaiian plume. Both components are present in the sources beneath Kea- and Loa-trend volcanoes. We infer that the thermal structure and spatially distributed compositional heterogeneity of the plume are important in controlling the isotopic composition of lavas from a given Hawaiian volcano. [ABSTRACT FROM AUTHOR]

Many peridotite xenoliths sampled at ocean islands appear to have strongly refractory major element and modal compositions. To better constrain the chemistry, abundance and origin of these ultra-refractory rocks we compiled a large number of data for xenoliths from nine groups of ocean islands. The xenoliths were filtered petrographically for signs of melt infiltration and modal metasomatism, and the samples affected by these processes were excluded. The xenolith suites from most ocean islands are dominated by ultra-refractory harzburgites. Exceptions are the Hawaii and Tahiti peridotites, which are more fertile and contain primary clinopyroxene, and the Cape Verde suite, which contains both ultra-refractory and more fertile xenoliths. Ultra-refractory harzburgites are characterized by the absence of primary clinopyroxene, low whole-rock Al2O3, CaO, FeO/MgO and heavy rare earth element (HREE) concentrations, low Al2O3 in orthopyroxene (generally < 3 wt %), high Cr-number in spinel (0·3–0·8) and high forsterite contents in olivine (averages > 91·5). They are therefore on average significantly more refractory than peridotites dredged and drilled from mid-ocean ridges and fracture zones. Moreover, their compositions resemble those of oceanic forearc peridotites. The formation of ultra-refractory ocean island harzburgites requires potential temperatures above those normally observed at modern mid-ocean ridges, and/or fluid fluxed conditions. Some ultra-refractory ocean island harzburgites give high Os model ages (up to 3300 Ma), showing that their formation significantly pre-dates the oceanic crust in the area. A genetic relationship with the host plume is considered unlikely based on textural observations, equilibration temperatures and pressures, inferred physical properties, and the long-term depleted Os and Sr isotope compositions of some of the harzburgites. Although we do not exclude the possibility that some ultra-refractory ocean island harzburgites have formed at mid-ocean ridges, we favor a model in which they formed in a process spatially and temporally unrelated to the formation of the oceanic plate and the host plume. As a result of their whole-rock compositions, ultra-refractory harzburgites have a very high solidus temperature at a given pressure, low densities and very high viscosities, and will tend to accumulate at the top of the convecting mantle. They may be preserved as fragments in the convecting mantle over long periods of time and be preferentially incorporated into newly formed lithosphere. [ABSTRACT FROM AUTHOR]