Hawaiian Volcanism Research Articles

CO2‐Rich Rejuvenated Stage Lavas on Hawaiian Islands

AbstractSedimentary carbonates are sent to the deep mantle if they are not completely destroyed at subduction zones, and subducted carbonates may contribute to plume volcanism. To better constrain the role of recycled carbonates in Hawaiian volcanism, we report high‐precision olivine and whole‐rock geochemical compositions of shield and rejuvenated stage lavas from Kauai, Oahu and Maui Islands. The studied rejuvenated stage whole‐rocks have low SiO2 and high CaO concentrations, and are depleted in high field strength elements, such as, Nb and Zr, consistent with a role of carbonated melt. Rejuvenated stage olivines have Ni abundance lower than and CaO and MnO contents similar to those of shield stage at a given Fo. The calculated partition coefficients of Ca () and Mn () between olivine and shield melts are consistent with those of a dry melt system. However, the low and for rejuvenated lavas can only be explained by a volatile‐rich melt system. Based on the observed and in rejuvenated lavas, and considering the effect of H2O, our modeling calculation shows that rejuvenated primary magmas contain up to ∼10 wt% CO2, indicating a CO2‐rich mantle source. Using olivine‐spinel aluminum exchange thermometry, we show that the rejuvenated primary magma temperatures are similar to those of shield stage. We posit that the CO2‐rich rejuvenated stage lavas with lower olivine Ni than those of shield stage lavas were originated from melting of carbonated peridotites in the plume. The carbonated melts that metasomatized the peridotites were likely derived from subducted ancient carbonate‐bearing lithospheric mantle.

Particle Induced X-ray Emission spectrometry (PIXE) of Hawaiian volcanics: An analogue study to evaluate the APXS field analysis of geologic materials on Mars

The Alpha Particle X-ray Spectrometer (APXS), a field instrument onboard four martian rovers, measures largely unprepared, in situ samples on Mars. The APXS has high precision that enables the determination of elemental concentrations in a wide range of geologic materials. However, lack of sample preparation can lead to heterogeneous matrix effects, and understanding the associated uncertainty is essential for interpreting APXS data. Here we use Particle Induced X-ray Emission spectrometry (PIXE) to analyze a suite of geologic samples from Hawai'i as an analogue study to better understand APXS analyses of martian samples. Wavelength-Dispersive X-ray Fluorescence (WDXRF) analyses of fused glass beads establish higher-accuracy standards for the Hawaiian samples. Sulfate-silicate mixtures were made to evaluate sulfur analysis by PIXE. Results show that the PIXE concentrations for most major elements have 2–6% accuracy, which is comparable to the APXS. However, the PIXE concentrations are systematically high in Al and low in Mg, resulting in lower accuracy (13% and 20%, respectively). Olivine-phyric lavas and most of their altered products have the largest discrepancies with Al concentrations up to 25% high and Mg up to 35% low. Sulfur is systematically high (up to 30% in a basalt matrix) compared to gravimetric S concentrations in the sulfate-silicate mixtures. These systematic deviations in Mg, Al, and S are linked to heterogeneous matrix effects, because PIXE and APXS analyses assume all atoms in a sample to be homogeneously mixed on the sub-micrometer scale, which is not the case. Two key implications for APXS results are: (1) Olivine-bearing samples likely have reported concentrations of Mg that is too low and Al that is too high. Thus, olivine-phyric basalts in Gusev crater and the basaltic sand and soil at three landing sites may have Mg and Al concentrations closer to those of the olivine-phyric shergottites and modelled martian crust than previously thought. (2) Sulfate-silicate mixtures may have overestimated S concentrations reported, resulting in greater uncertainty in the stoichiometry of Ca-sulfates, which is used to deduce the geochemical associations of sulfur in samples.

The multiple depleted mantle components in the Hawaiian-Emperor chain

Oceanic island basalts are targeted for geochemical study because they provide a direct window into mantle composition and a wealth of information on the dynamics and timescales associated with Earth mixing. Previous studies mainly focused on the shield volcanic stage of oceanic islands and the more fusible, enriched mantle components that are easily distinguished in those basalts. Mantle depleted compositions are typically more difficult to resolve unless large amounts of this material participated in mantle melting (e.g., mid-ocean ridges), or unique processes allow for their compositions to be erupted undiluted, such as very small degrees of melting of a source with minimal fusible enriched components (e.g., rejuvenated basalts) or as xenoliths (e.g., abyssal peridotites). Mantle depleted components, defined here as material with low time-integrated Rb/Sr (low 87Sr/86Sr) and high time-integrated Sm/Nd and Lu/Hf ratios (high 143Nd/144Nd and 176Hf/177Hf) relative to primitive mantle, derive from a potentially very large volume reservoir (up to 80% of the mantle), and therefore need adequate characterization in order estimate the composition of the Earth and mantle-derived melts. This review focuses on mantle depleted compositions in oceanic island basalts using the Hawaiian-Emperor chain as a case study. The Hawaiian-Emperor chain is the ∼6000km long geological record of the deeply sourced Hawaiian mantle plume, active for >81 Myr. Hawaiian volcanism evolves through four volcanic stages as a volcano traverses the Hawaiian plume: alkalic preshield, tholeiitic shield (80–90% volcano volume), alkalic postshield (∼1%), and silica undersaturated rejuvenated (<0.1%). We report Pb-Sr-Nd-Hf isotope compositions and trace element concentrations of three rejuvenated Northwest Hawaiian Ridge basalts and compare them to an exhaustive compiled dataset of basalts from the Hawaiian Islands to the Emperor Seamounts. The Northwest Hawaiian Ridge (NWHR) includes 51 volcanoes spanning ∼42m.y. between the bend in the Hawaiian-Emperor chain and the Hawaiian Islands where there is no high-precision isotopic data published on the rejuvenated-stage over ∼47% of the chain. NWHR and Hawaiian Island rejuvenated basalts are geochemically similar, indicating a consistent source for rejuvenated volcanism over ∼12.5 million years. In contrast, shield-stage basalts from the oldest Emperor Seamounts are more depleted in isotopic composition (i.e., higher 176Hf/177Hf, and 143Nd/144Nd with lower 87Sr/86Sr and 208Pb*/206Pb*) and trace element concentrations (i.e., much lower concentrations of highly incompatible elements) than all other depleted Hawaiian basalts younger than the bend, including NWHR rejuvenated basalts. The strongly depleted source for the oldest Emperor Seamounts (>70 Ma) was likely related to interaction with the Kula-Pacific-Izanagi mid-ocean ridge spreading system active near the Hawaiian plume in the Late Cretaceous. In contrast, the incompatible trace element ratios of NWHR rejuvenated basalts require a distinct source in the Hawaiian mantle plume that was imprinted by ancient (>1 Ga) partial melting, likely ancient recycled oceanic lithosphere. This review of the geochemistry of Hawaiian depleted components documents the need for the sampling of multiple distinctive depleted compositions, each preferentially melted during specific periods of Hawaiian plume activity. This suggests that the composition of depleted components can evolve during the lifetime of the mantle plume, as observed for enriched components in the Hawaiian mantle plume. Changes in the composition of depleted components are dominantly controlled by the upper mantle tectonic configurations at the time of eruption (i.e., proximity to a mid-ocean ridge), as this effect overwhelms the signal imparted by potentially sampling different lower mantle components through time.

Sources vs processes: Unraveling the compositional heterogeneity of rejuvenated-type Hawaiian magmas

Spatial isotope variations in oceanic island basalts hold clues to the geochemical structure of mantle plumes. This signal is however susceptible to contributions from non-plume sources and modifications by physical processing in the upper mantle, i.e. source hybridization and segregation through gradients in melting conditions. A comprehensive survey of lava isotope systematics is therefore necessary to reveal spatiotemporal patterns at the scale of a single hotspot and disentangle plume sources from upper mantle physical and chemical contributions. We present Hf and Pb isotope data on 72 previously characterized onland and submarine lavas from the islands of Ni‘ihau and Kaua‘i, and from the North Arch Volcanic Field. These lavas cover the entire ∼ 5 Ma eruptive history of Ni‘ihau and Kaua‘i, allowing for a detailed assessment of the temporal geochemical evolution of Hawaiian volcanism. Furthermore, the broad lateral coverage of volcanic activity offered by these lavas constrains the spatial variability of sources across the volcanic chain, allowing investigation of the role of an upper asthenosphere contribution to plume volcanism. Early, shield-building (tholeiitic) lavas from Kaua‘i and Ni‘ihau overlap in Nd–Hf–Sr–Pb isotopes, contrasting with the across-plume asymmetry seen in younger shield lavas (i.e. the LOA-KEA double chain), and their isotope systematics suggest sampling of both LOA and KEA components. In contrast, late Kaua‘i and Ni‘ihau rejuvenated lavas do not overlap in isotope composition. They share common isotope characteristics with rejuvenated lavas from North Arch, Ka‘ula and O‘ahu, and together form two isotopically distinct, southwestern and northeastern groups. The two groups have steep slopes (>1.7) in a εNd − εHf plot and require distinct depleted components. The high εHf for a given εNd, high 87Sr/86Sr, and trace element systematics of the southwestern group (Ni‘ihau, Ka‘ula and O‘ahu) indicate a depleted component intrinsic to the plume. In contrast, lavas from the northeastern group (Kaua‘i and North Arch) indicate a source component isotopically identical to Pacific MORB. The shift from a spatially broad and homogeneous plume source in the shield stage of Kaua‘i and Ni‘ihau (6–3.5 Ma) to a bilaterally heterogeneous source during the rejuvenated stage (<3.5 Ma) is synchronous with the emergence of the LOA and KEA geochemical double chain in younger Hawaiian shield lavas. Our data reveal a shift to Pacific asthenosphere contribution to Kaua‘i volcanism through time, while Ni‘ihau exclusively samples the plume. These observations are inconsistent with a scenario where plume geochemical structure is the primary control on spatial heterogeneities in lava chemistry, and instead point to a more complex and dynamic interaction between plume, Pacific asthenosphere and lithosphere. Therefore, deep mantle structure might not be the primary control on the spatial distribution of isotope systematics in oceanic island lavas, making a link between the latter and contrasts imaged in the lower mantle through seismic tomography tenuous.

Implications for the origin of Hawaiian volcanism from a converted wave analysis of the mantle transition zone

The debate over the origin of intraplate volcanism has been ongoing since the discovery of age-progression at oceanic “hotspots.” The causes of such anomalous volcanic activity have been attributed to either deep-seated thermal plumes in the convecting mantle or shallower causes such as lithospheric structure and deformation or localized mantle flow. Data from the Hawaiian Plume–Lithosphere Undersea Melt Experiment (PLUME) have made it possible to provide detailed images of upper mantle heterogeneity beneath the Hawaiian Islands with much greater resolution than previous experiments. Using receiver function analysis, we determine the depth to and topography along the mantle transition zone discontinuities. Our results indicate that the 410-km discontinuity deepens from northwest to southeast beneath the Hawaiian Islands, corresponding to an average thermal anomaly of ∼325K located beneath and southeast of the Big Island of Hawaii. In general, temperatures remain elevated by at least ∼150K directly beneath the Big Island, as far as 200km away from the hottest measurement inferred from the discontinuity structure. Heterogeneity at these length scales raises questions about rheology, convective circulation, and melt transport in the mantle. Our results also robustly indicate the presence of a low-velocity converter in the deep upper mantle, which carries additional implications for melt transport, volatile budget, and chemical composition of the hotspot source.

Application of 50 multi-dimensional discrimination diagrams and significance tests: deciphering compositional similarities and differences between Hawaiian and Icelandic volcanism

As an innovative approach, I use multi-dimensional diagrams presented by our group during 2004–2013 and respective probability estimates for the newer diagrams to evaluate similarities and differences between the same magma types (subalkaline basalt, basic magma other than subalkaline basalt, and intermediate magma) from the Hawaiian Islands and Iceland. The known ocean island setting of the Hawaiian Islands was reproduced from these 50 multi-dimensional diagrams for basic, intermediate, and acid magmas. The possible influence of the mid-ocean ridge system on Icelandic magmas was also confirmed from the diagrams for basic rocks. Evaluating magma compositional similarities and differences between the Hawaiian Islands and Iceland was complemented by the application of significance tests. Statistical inferences for the chemical elements and, more importantly, their log-ratios, successfully explained the similarities and differences in the tectonic settings for these two regions.

Distribution of Some Elements in the Egyptian Black Sands from Abu Khashaba Beach Area

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) technique was used to determine the concentrations of titanium (Ti), vanadium (V), Chromium (Cr), Manganese (Mn), iron (Fe), zinc (Zn), arsenic (As), zirconium (Zr), cadmium (Cd) and hafnium (Hf) in black sands samples. 18 black sands samples were collected from the Mediterranean coast-Abu Khashabou Rosetta area, Egypt. Alkaline fusion procedure using mixture of Li-meta borate and tetra borate was used for the digestion of black sands samples. The average concentrations of Ti, V, Cr, Mn, Fe, Zn, As, Zr, Cd and Hf were 435.06 ppm, 322.32 ppm, 2515.03 ppm, 596.45 ppm, 185894 ppm, 249.95 ppm,309.32 ppm, 1077.26 ppm, 33.56 ppm and 3520.32 ppm, respectively. The procedure was tested with Basalt Hawaiian Volcanic Observatory (BHVO-2) standard reference material.

A3-05 ハワイ火山初期ステージのマグマ源(火山の岩石学2,口頭発表)

A3-05 ハワイ火山初期ステージのマグマ源(火山の岩石学2,口頭発表)

Bent-shaped plumes and horizontal channel flow beneath the 660 km discontinuity

Recent high-resolution seismic imaging of the transition zone topography beneath the Hawaiian archipelago shows strong evidence for a 1000 to 2000 km wide hot thermal anomaly ponding beneath the 660 km boundary west of Hawaii islands [Q. Cao et al. Seismic imaging of transition zone discontinuities suggests hot mantle west of Hawaii. Science (2011), 332, 1068–1071]. This scenario suggests that Hawaiian volcanism may not be caused by a stationary narrow plume rising from the core–mantle boundary but by hot plume material first held back beneath the 660 km discontinuity and then entrained under the transition zone before coming up to the surface. Using a cylindrical model of iso-chemical mantle convection with multiple phase transitions, we investigate the dynamical conditions for obtaining this peculiar plume morphology. Focusing on the role exerted by pressure-dependent thermodynamic and transport parameters, we show that a strong reduction of the coefficient of thermal expansion in the lower mantle and a viscosity hill at a depth of around 1800 km allow plumes to have enough focused buoyancy to reach and pass the 660 km depth interface. The lateral spreading of plumes near the top of the lower mantle manifests itself as a channel flow whose length is controlled by the viscosity contrast due to temperature variations ∆η T. For small values of ∆η T, broad and highly viscous plumes are generated that tend to pass through the transition zone relatively unperturbed. For higher values (10 2 ≤ ∆η T ≤ 10 3), we obtain horizontal channel flows beneath the 660 km boundary as long as 1500 km within a timescale that resembles that of Hawaiian hotspot activity. This finding could help to explain the origin of the broad hot anomaly observed west of Hawaii. For a normal thermal anomaly of 450 K associated with a lower mantle plume, we obtain activation energies of about 400 kJ/mol and 670 kJ/mol for ∆ ηT = 10 2 and 10 3, respectively, in good agreement with values based on lower mantle mineral physics. If an increase of the thermal conductivity with depth is also included, our model can predict both long channel flows beneath the 660 km discontinuity and also values of the radial velocity of local blobs sinking in the lower mantle of around 1 cm/yr, in good agreement with those inferred for the sinking rate of cold remnants of the subducted lithosphere.

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’.

Geochemical zoning of volcanic chains associated with Pacific hotspots

Lavas erupted from volcanoes in the north of the Hawaiian volcanic island chain have a different geochemical signature from those in the south. Analysis of the geochemistry of lavas erupted in the Samoan and Marquesas volcanic chains reveals similar trends, implying that the lowermost mantle beneath the southern Pacific is isotopically enriched. Recent Hawaiian volcanism is manifest as two geographically and geochemically distinct groups of volcanoes1, the Loa trend in the south and the Kea trend in the north2,3. The differences between the Loa and Kea lavas are attributed to spatial variations in the geochemical structure of the underlying Hawaiian mantle plume4,5,6,7,8,9. In turn, the Hawaiian plume structure is thought to reflect heterogeneities in its mantle source7,8. Here we compile geochemical data10 from the Hawaiian and two other volcanic ocean island chains—the Samoan and Marquesas—that formed above mantle plumes upwelling beneath the Pacific plate. We find that the volcanoes at both Samoa11 and the Marquesas12 show geographic and geochemical trends similar to those observed at Hawaii. Specifically, two subparallel arrays of volcanoes exist at both locations. In each case, the southern trend of volcanoes has higher radiogenic lead isotope ratios, 208Pb*/206Pb*, and lower neodymium isotope ratios, ɛNd, than those of the corresponding northern trend. We suggest that geochemical zoning may be a common feature of mantle plumes beneath the Pacific plate. Furthermore, we find that the pattern repeats between island chains, with the highest 208Pb*/206Pb* and the lowest ɛNdfound at Samoa in the south and the lowest 208Pb*/206Pb* and the highest ɛNd observed at Hawaii in the north. We infer that isotopically enriched material is preferentially distributed in the lower mantle of the Southern Hemisphere, within the Pacific low seismic velocity zone.

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

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

North Arch volcanic fields near Hawaii are evidence favouring the restite‐root hypothesis for the origin of hotspot swells

AbstractA pronounced regional bathymetric swell is a common feature of oceanic hotspot volcanism. Recently, the hypothesis of a buoyant sublithospheric swell‐root has been favoured, the root being either a ‘refracted plume’ of hot, buoyant and hence low‐viscosity plume material embedded within surrounding higher viscosity asthenosphere, or a ‘restite‐root’ composed of hot and more viscous residues to hotspot melt‐extraction from the hottest central portion of the upwelling plume. In this article, we present numerical experiments that show that these two scenarios predict different flow and melting patterns whenever the plume passes beneath an obliquely oriented fracture zone where the base of the overriding lithosphere changes in thickness. We find that the restite‐root hypothesis predicts an asymmetric spatial pattern of persistent ‘arch volcanism’ strikingly similar to that found in the Hawaiian chain surrounding the Molokai Fracture Zone. In contrast, the ‘refracted plume’ hypothesis predicts more chaotic patterns quite different from the off‐chain pattern of Hawaiian volcanism.

Distribution of Atmospheric Methane Oxidation and Methanotrophic Communities on Hawaiian Volcanic Deposits and Soils

Hawaiian volcanic deposits offer ideal opportunities to assess methanotrophic bacterial colonization of new substrates, and to determine the relative significance of methanotrophy during ecosystem succession. Activity and molecular ecological surveys indicated that significant methanotrophic activity was restricted to vegetated ecosystems characterized by closed-canopy forests and significant soil accumulation. In these systems, atmospheric methane oxidation rates (0.7-1.8 mg CH(4) m(-2) d(-1)) were comparable to the lower end of values reported for continental soils. No trends in activity related to deposit age or type were evident at ambient or elevated methane levels. Analyses of clone libraries based on particulate methane monooxygenase and ammonia monooxygenase (pmoA/amoA) genes revealed largely novel sequences, with distinct assemblages for each of two sites. Remarkably, sequences from a 300-yr old forest soil were most closely related to sequences from Arctic soils. Collectively, the evidence indicates that methanotrophs colonize volcanic substrates slowly and likely depend on interactions with plant and other microbial communities.

High-pressure partial melting of gabbro and its role in the Hawaiian magma source

We have conducted high-pressure experiments on a natural oceanic gabbro composition (Gb108). Our aim was to test recent proposals that Sr-enrichment in rare primitive melt inclusions from Mauna Loa, Hawaii, may have resulted from melting of garnet pyroxenite formed in the magma source regions by reaction of peridotite with siliceous, Sr-enriched partial melts of eclogite of gabbroic composition. Gb108 is a natural, Sr-enriched olivine gabbro, which has a strong positive Sr anomaly superimposed on an overall depleted incompatible trace element pattern, reflecting its origin as a plagioclase-rich cumulate. At high pressures it crystallises as a coesite eclogite assemblage, with the solidus between 1,300 and 1,350°C at 3.5 GPa and 1,450 and 1,500°C at 4.5 GPa. Clinopyroxenes contain 4–9% Ca-eskolaite component, which varies systematically with pressure and temperature. Garnets are almandine and grossular-rich. Low degree partial melts are highly siliceous in composition, resembling dacites. Coesite is eliminated between 50 and 100°C above the solidus. The whole-rock Sr-enrichment is primarily hosted by clinopyroxene. This phase dominates the mode (>75 wt%) at all investigated PT conditions, and is the major contributor to partial melts of this eclogite composition. Hence the partial melts have trace element patterns sub-parallel to those of clinopyroxene with ≈10× greater overall abundances and with strong positive Sr anomalies. Recent studies of primitive Hawaiian volcanics have suggested the incorporation into their source regions of eclogite, formerly gabbroic material recycled through the mantle at subduction zones. The models suggest that formerly gabbroic material, present as eclogite in the Hawaiian plume, partially melted earlier than surrounding peridotite (i.e. at higher pressure) because of the lower solidus temperature of eclogite compared with peridotite. This produced highly siliceous melts which reacted with surrounding peridotite producing hybrid pyroxene + garnet lithologies. The Sr-enriched nature of the formerly plagioclase-rich gabbro was present in the siliceous partial melts, as demonstrated by these experiments, and was transferred to the reactive pyroxenite. These in turn partially melted, producing Sr-enriched picritic liquids which mixed with normal picritic partial melts of peridotite before eruption. On rare occasions these mixed, relatively Sr-rich melts were trapped as melt inclusions in primitive olivine phenocrysts.

Shield-stage alkalic volcanism on Mauna Loa Volcano, Hawaii

Alkalic lavas have been discovered at two young (∼15–20 ka) submarine radial vents on the western flank of Mauna Loa, a volcano that was thought to have produced only tholeiitic lavas for at least the last ∼240,000 years. Major and trace element data show that the lavas from only one of these eruptions have undergone lower degrees of partial melting, the assumed dominant process for creating alkalic lavas on Hawaiian volcanoes. However, major and trace element analyses also suggest that lavas from both of the alkalic eruptions are evolved and have experienced moderate-pressure (∼1.0 GPa) fractionation of pyroxene, an alternative mechanism for producing an alkalic magma from a tholeiitic parent. Pyroxene fractionation is also indicated by MELTS modeling and phase diagram projections. Although lavas with similar compositions have been erupted from neighboring Mauna Kea and Hualalai volcanoes, Pb isotope and trace element data indicate that the submarine alkalic lavas were produced from the same source generating Mauna Loa tholeiites. Thus, these are the first alkalic lavas to be identified from this volcano. Holocene eruptions of Mauna Loa have all produced tholeiitic lavas and eruption rates have remained relatively high. Thus, the volcano has probably not yet entered the postshield stage of Hawaiian volcanism, which is characterized by alkaline lavas and less frequent eruptions. The occurrence of the alkalic lavas only at radial vents may indicate that these magmas bypassed the primary conduit, which supplies the summit reservoir with predominantly tholeiitic magma.

Re-examination of the magnetic lineations of the Gascoyne and Cuvier Abyssal Plains, off NW Australia

This thesis addresses causes of two forms of secondary Hawaiian volcanism: rejuvenated onshore eruptions and offshore Hawaiian Arch flows. It is proposed that secondary volcanism is generated as a direct consequence of lithospheric flexural uplift that surrounds new volcanic shields as they grow. This uplift causes decompression of the underlying asthenosphere, which is assumed to be chemically and isotopically heterogeneous, near its solidus, and derived from the Hawaiian mantle plume. Uplift is modeled as the axisymmetric response of an elastic plate to a (volcanic) point load that grows linearly in time. To model flow in the asthenosphere, the rate of flexure of the lithosphere is taken as the upper boundary condition on an isoviscous, incompressible, fluid half-space. The first feature of secondary volcanism this model explains is the observed spatial gap between secondary volcanism and active shields. Best agreement is found with the majority of the observed spatial gaps with a lithosphere of effective elastic thickness Te=25-35 km. Secondly, this work demonstrates that the flexural model can produce observed crustal production if some magma focusing toward individual eruption sites occurs from the mantle over an area two to ten times the eruption area. The third feature this model addresses is that secondary lavas are isotopically distinct from shield lavas. In this model, melting the same two-component mantle forms the secondary and shield lavas, but the components are sampled by melting at rates that differ between the locations as predictable functions of depth. Flexural decompression produces melts that preferentially sample the mantle component that begins melting shallowest and which is associated with Sr and Nd isotope ratios most like those of secondary lavas. Melting in the center of a mantle plume is assumed to generate shield volcanism and is predicted to preferentially sample the component that begins melting deepest which is associated with Sr and Nd isotope ratios more similar to shield lavas. Models therefore successfully predict the observed mean difference in 87Sr/86Sr and 143Nd/144Nd compositions between the secondary and shield lavas. The fourth feature addressed is that secondary lavas are alkalic and shield lavas are dominantly tholeiitic. To explain this difference, the mean extent of partial melting is computed, and it is found that a model plume composed mostly of depleted peridotite (90%) and some pyroxenite (10%) will yield a lower extent of melting for secondary lavas than shield lavas. This particular model assumes lithospheric thicknesses (90-100 km) and plume potential temperatures (mean of 1550 °C) that are consistent with independent studies of the Hawaiian hotspot. Thus, asthenospheric melting by flexural decompression is a viable mechanism of intraplate volcanism, which can explain many general characteristics of secondary Hawaiian volcanism.

Secondary Hawaiian volcanism formed by flexural arch decompression

This paper explains two forms of secondary Hawaiian volcanism (rejuvenated onshore and Hawaiian Arch offshore) as a direct consequence of lithospheric flexural uplift that surrounds growing shield volcanoes. This uplift decompresses the underlying asthenosphere, which is assumed to be chemically and isotopically heterogeneous, near its solidus, and derived from the Hawaiian mantle plume. Lithospheric uplift is modeled as the axisymmetric response of an elastic plate to a (volcanic) point load that grows linearly in time. To model flow in the asthenosphere, the rate of flexure of the lithosphere is taken as the upper boundary condition on an isoviscous, incompressible, fluid half‐space. This model successfully explains the majority of spatial gaps between secondary and active shield volcanism due to the flexing of a lithospheric plate with an effective elastic thickness of 25–35 km. Second, this work demonstrates that the flexural model can produce realistic magmatic fluxes if magma is focused toward individual eruption sites from the mantle over an area two to ten times the eruption area. Next, this model addresses the isotopic distinction between secondary and shield lavas. In this model, the same heterogeneous mantle plume feeds the secondary and shield lavas, but the compositional components are sampled by melting at rates that differ between the two settings. Flexural decompression mostly melts the component that begins melting shallowest, which we assume to be depleted in incompatible elements with relatively low 87Sr/86Sr and high 143Nd/144Nd. Melting in the center of a mantle plume is assumed to generate shield volcanism and is predicted to mostly melt components that begin melting deepest, which we assume to be enriched in incompatible elements with higher 87Sr/86Sr and lower 143Nd/144Nd. Thus the models successfully predict the observed mean 87Sr/86Sr and 143Nd/144Nd isotopic compositions of secondary and shield lavas to arise out of the melting process alone. The fourth feature addressed is that secondary lavas are alkalic, having formed from relatively low extents of partial melting, and shield lavas are dominantly tholeiitic, consistent with more extensive partial melting. Indeed, models predict lower mean extent of melting for secondary lavas compared to shield lavas if the source material, which is mostly peridotite, contains at least some pyroxenite. Results show that model predictions are consistent with the geochemical constraints for a range of reasonable starting mantle compositions, lithospheric thicknesses, and plume temperatures.

Hf-Nd-Sr isotope systematics of garnet pyroxenites from Salt Lake Crater, Oahu, Hawaii: Evidence for a depleted component in Hawaiian volcanism

We present the first comprehensive major, trace element and Hf, Nd and Sr isotope investigation of clinopyroxene and garnet mineral separates from a set of garnet clinopyroxenite xenoliths from the Salt Lake Crater, Oahu, Hawaii. These xenoliths occur in the posterosional Honolulu Volcanics Series lavas and represent some of the deepest samples from the oceanic mantle lithosphere. Our study shows that the Salt Lake Crater pyroxenites represent high pressure (>20 kb) accumulates from melts similar (but not identical) to the erupted Honolulu Volcanics, and unlike MORB or E-MORB-type melts. All clinopyroxene-garnet mineral pairs in these xenoliths show, within error, zero-age Lu-Hf and Sm-Nd isotope systematics. These pyroxenites have relatively radiogenic Hf isotope compositions (for a given Nd) and define a distinct steep slope (3.3) in ε Hf-ε Nd isotope space, similar to the Honolulu Volcanics but unlike other ocean island basalts (OIB). These compositions require an end-member component that falls above the OIB array in Nd-Hf space. This component is different than present-day MORB-mantle and it is best explained by an old depleted oceanic lithosphere. We suggest that this depleted component most likely represents a recycled depleted lithosphere that is intrinsic to the Hawaiian plume. In this respect, the Hawaiian plume is sampling both the enriched portion of a subducted oceanic crust (basalt and sediments) as well as the depleted lithospheric portion of it. This suggests that, at least for Hawaii, the whole subducted oceanic slab package has retained its integrity during subduction and subsequent mixing and storage in the mantle, probably in the order of a billion years, and that the plume is sampling the full range of these compositions.

Basalt core paleomagnetic data from Ocean Drilling Program Site 883 on Detroit Seamount, northern Emperor Seamount chain, and implications for the paleolatitude of the Hawaiian hotspot

Paleomagnetic data were measured from basaltic flows cored by the Ocean Drilling Program (ODP) at Site 883 on the summit of Detroit Seamount, located in the northernmost Emperor seamounts. These data are important because they reflect the paleolatitude of Hawaiian volcanism for the Late Cretaceous and bear upon geodynamic models of hotspot drift. A total of 143 samples were measured, from cores acquired at two holes ∼20–30 m apart. Most samples gave apparently reliable magnetic directions that were analyzed in a tiered fashion to compute a composite inclination vs. depth curve. One hole gave 13 distinct inclination groups, the other 10, and the two were combined into nine groups thought to represent independent measurements of paleofield direction. These data indicate normal magnetic polarity and give a mean inclination of 61.5+10.6°/−6.4° and paleolatitude of 42.8+13.2°/−7.6° (95% confidence limits). This paleolatitude is 6.2° higher than results from another ODP site (884) drilled on the lower flank of the same seamount. The difference is thought to result partly from an age difference (1–3 Myr) and partly from incomplete averaging of paleosecular variation at both drill sites. Together, the data from the two sites reinforce the conclusion that the northern Emperor seamounts were formed far north of the present-day hotspot latitude (∼19.5°N) and suggest prior estimates of the amount and rate of southward drift may have been low. This analysis also illustrates uncertainties in determining paleolatitude from a small number of lava flow units from a single drill site.