Honolulu Volcanics Research Articles

A Multi‐Hydrogeophysical Study of a Watershed at Kaiwi Coast (Oʻahu, Hawaiʻi), using Seismic Ambient Noise Surface Wave Tomography and Self‐Potential Data

AbstractWe study a watershed containing an old stream valley at the Kaiwi Coast on Oʻahu, Hawaiʻi. The study site has undergone significant hydrogeological changes throughout its history, and at present displays complex interactions of a variety of geological formations. We highlight an innovative and efficient combination of two hydrogeophysical methodologies for studying groundwater systems: ambient noise surface wave tomography (ANSWT), and self‐potential (SP) measurements. We collected (1) 5 days of continuous seismic data, using a total of 54 autonomous seismic nodes, exploiting both ocean wave and traffic noise sources and (2) 386 SP measurements with an average 20 m spacing, in the same area. We showcase how these two data sets complement each other in a joint‐interpretation framework. The SP data display the distribution of groundwater in the region, as well as zones of higher infiltration and/or flow rates. Using ANSWT, we identify the structural contact between the older Koʻolau basalt formation and the younger post‐erosional Honolulu volcanics, as well as the distribution of sedimentary valley fill and coastal deposits. The joint interpretation of the SP and seismic data clearly illustrates that groundwater flow occurs in the identified paleo‐channels at the erosional surface of the basaltic bedrock. Furthermore, it highlights that the basaltic bedrock of the ridges likely forms an important groundwater flow unit. Some flow may occur in the shallower valley fill material. Complementing SP data with seismic data enables us to place groundwater flow inferences, as made from SP data, at depth.

Compositional heterogeneity of the Sugarloaf melilite nephelinite flow, Honolulu Volcanics, Hawai‘i

The Sugarloaf flow is a melilite nephelinite erupted from the Tantalus rift during rejuvenated-stage volcanism on O‘ahu, the Honolulu Volcanics. The flow ponded in Mānoa Valley forming a ∼15m thick flow which was cored and sampled in a quarry. Nepheline from a pegmatoid segregation in the flow yielded a 40Ar–39Ar age of 76ka. This age, combined with others, indicates that the Tantalus rift eruptions are some of the youngest on O‘ahu. Honolulu Volcanics erupt on average about every 35–40ka indicating that future eruptions are possible.We evaluated the compositional variability of 19 samples from the flow, including 14 from the core. Twelve samples are representative of the bulk flow, four are dark- or light-colored variants, one is a heavy rare earth element (REE)-enriched pegmatoid, and two visually resemble the bulk flow, but have chemical characteristics of the dark and light variants. Our objective was to determine intraflow heterogeneity in mineralogy and composition. Variable abundances of Na2O, K2O, Sr, Ba, Rb, Pb and U in the flow were caused by post-eruptive mobility in a vapor phase, most likely during or soon after flow emplacement, and heterogeneous deposition of secondary calcite and zeolites. Relative to fine-grained samples, a pegmatoid vein that crosscuts the flow is enriched in incompatible trace elements except Sr and TiO2. Element mobility after eruption introduced scatter in trace element ratios including light-REE/heavy-REE, and all ratios involving mobile elements K, Rb, Ba, Sr, Pb, and U. Lavas from some of the 37 Honolulu Volcanics vents have crosscutting REE patterns in a primitive mantle-normalized plot. Such patterns have been interpreted to reflect variable amounts of residual garnet during partial melting. Previous studies of lavas from different vents concluded that garnet, phlogopite, amphibole, and Fe–Ti oxides were residual phases of the partial melting processes that created the Honolulu Volcanics (Clague and Frey, 1982; Yang et al., 2003). However post-eruptive processes in the Sugarloaf flow also produced crossing REE patterns.Eruptions on the Tantalus rift, including the Sugarloaf flow, produced volatile- and crystal-rich ash with interstitial glass and melt inclusions in olivine containing 4.2–6.4wt% MgO compared to the flow average of 11.8wt%. This flow erupted as a partially crystallized viscous magma at least 100°C below its liquidus. The slow advance and cooling of the 15-m thick ‘a’ ā low promoted the segregation of pegmatoids, formation of light and dark bands with differing proportions of melilite and clinopyroxene, and induced volatile-enhanced mobility of incompatible elements.

Geochemistry of sulfides in Hawaiian garnet pyroxenite xenoliths: Implications for highly siderophile elements in the oceanic mantle

Two dominant petrographic types of sulfides occur in Hawaiian garnet pyroxenite xenoliths — Type I occurs as globular, poikilitic inclusions only in clinopyroxenes, and Type II occurs interstitially between silicate phases. Chemically, the two types are essentially identical and their compositions are consistent with Fe–Ni monosulfide solid solution (Fe = 55–57%, Ni = 3–10%, Cu = 0.5–2.0%, S = 35–37% and Zn = 0.01–0.5%). Both types of pyroxenitic sulfides have a factor of 10 to 1000 lower PGE contents than sulfides found in peridotites elsewhere (e.g. Os ∼ 1 to < 0.01 ppm), show fractionated PGE patterns (Pd ( n )/Ir ( n ) ∼ 1–35; n = chondrite-normalized) and very high Re ( n )/Os ( n ) ratios (∼ 10–400). The Ni and the total PGE (Os + Ir + Pt + Pd) contents of the sulfides correlate positively with the Mg# of clinopyroxene in the host rock, which points to a genetic link between the sulfides and silicates and a high-temperature mantle origin. The Hawaiian sulfides formed from an immiscible sulfide melt that separated from a silicate melt, which formed the host garnet pyroxenite. Experimental constraints place this sulfide–silicate melt immiscibility at 1530 ± 100 °C and 3.1 ± 0.6 GPa, i.e. near the base or slightly below the Pacific lithosphere. Melt/mineral partition calculations suggest that the parental silicate melts (prior to immiscibility) were similar to the Honolulu Volcanics (HV) alkali lavas that host the xenoliths, and that limited (0.1–0.3%) sulfide fractionation from a parental HV-type melt can account for the observed HSE variability in the sulfides. The relatively low Pt/Re ratios (< 10) suggest that, upon “aging”, such sulfides cannot generate the coupled 186Os– 187Os isotope enrichments observed in Hawaiian lavas and some komatiites. Therefore, recycling of mantle sulfides of pyroxenitic parentage is unlikely to explain the enriched Pt–Re–Os isotope systematics of plume-derived lavas.

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

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

A submarine perspective of the Honolulu Volcanics, Oahu

Lavas and volcaniclastic deposits were observed and collected from 4 submarine cones that are part of the Honolulu Volcanics on Oahu, Hawaii. The locations of these and a few additional, but unsampled, vents demonstrate that nearly all the vents are located on or very close to the shoreline of Oahu, with the most distal vent just 12 km offshore. The clastic samples and outcrops range from coarse breccias to cross-bedded ash deposits and show that explosive volcanism at depths between about 350 and 590 m depth played a part in forming these volcanic cones. The eruptive styles appear to be dominantly effusive to strombolian at greater depths, but apparently include violent phreatomagmatic explosive activity at the shallower sites along the submarine southwest extension of the Koko Rift. The compositions of the recovered samples are broadly similar to the strongly alkalic subaerial Honolulu Volcanics lavas, but the submarine lavas, erupted further from the Koolau caldera, have slightly more radiogenic Sr isotopic ratios, and trace element patterns that are distinct from either the subaerial Honolulu Volcanics or the submarine North Arch lavas. These patterns are characterized by moderate to strong positive Sr and P anomalies, and moderate to strong negative Cs, Rb, U, Th, Zr, and Hf anomalies. Most samples have strong negative K and moderate negative Ti anomalies, as do all subaerial Honolulu Volcanics and North Arch samples, but one group of samples from the Koko Rift lack this chemical signature. The data are consistent with more garnet in the source region for the off-shore samples than for either the on-shore Honolulu Volcanics lavas. New Ar–Ar ages show that eruptions at the submarine vents and Diamond Head occurred between about 0.5 Ma and 0.1 Ma, with the youngest ages from the Koko Rift. These ages are in general agreement with most published ages for the formation and suggest that some much younger ages reported previously from the Koko Rift are probably erroneously young.

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.

Hawaiian mantle xenoliths and magmas: Composition and thermal character of the lithosphere

Recent seismological investigations into the Hawaiian mantle and geophysical studies of the Hawaiian Swell have led to divergent models regarding the thermal structure and role of the lithosphere in Hawaiian magmatism. Whereas some models require that the Hawaiian plume erodes the lower lithosphere and replaces it with plume-derived residues and cumulates, others do not require any thinning of the lithosphere. In this paper, we develop a model for the physical and thermal characteristics of the oceanic lithosphere beneath Oahu, Hawaii, from the mantle xenoliths included in the rejuvenated stage lavas (Honolulu Volcanics, HV) that erupted through the Salt Lake Crater (SLC) and Kaau-Pali-Kalihi (KPK) vents. Three main xenolith suites are found at Oahu: spinel peridotite, garnet pyroxenite, and dunite. Dunites are generally shallow cumulates, whereas the peridotite and pyroxenite xenoliths are from the mantle. Plagioclase peridotite (harzburgite) is rare, and represents a ~15 km thick lithospheric layer below the ~15 km thick crust (MOR-generated oceanic crust + thickened crust formed by Koolau shield volcano activity). The harzburgite layer is underlain by a spinel peridotite layer. Spinel peridotites from SLC and KPK differ in chemical and isotopic composition. SLC spinel peridotites have unusually enriched 176 Hf/ 177 Hf but highly nonradiogenic 187 Os/ 188 Os isotopic compositions, likely indicating that they are metasomatized, recycled (previously subducted) oceanic lithospheric fragments. On the other hand, KPK spinel peridotites exhibit compositional characteristics of the 90 m.y. old lithosphere beneath Oahu that has been variably metasomatized by passing HV magmas. We suggest that the lower lithosphere beneath SLC and KPK are very different in their petrologic composition. The lower lithosphere (60–90 km) beneath SLC is extensively veined and fragmented by garnet clinopyroxenite intrusives and mixed with spinel peridotite blobs that are recycled subducted lithospheric fragments. For the KPK lithosphere, we favor a model in which the lower part is peridotitic. The thickness of our model lithosphere beneath Oahu is ~90 km. The seismic low-velocity zone beneath Oahu is likely due to the presence of small amounts of hydrous-alkalic and carbonatitic (+kimberlitic) melts. The isotopic data suggest little or no interaction between the basal lithosphere and Koolau shield magmas (mostly plume-derived, particularly those with near bulk-earth Nd- and Sr-isotope ratios), suggesting that during shield-stage volcanism, magmas used well-insulated (i.e., a reaction zone of 60 km) xenoliths are isotopically depleted and lack the typical Koolau-like signature. Therefore, the lower lithosphere was not significantly eroded by Koolau magmatism. KPK spinel peridotites give temperatures of 900–1040 °C for the intermediate lithosphere, which is about 200–400 °C hotter than what would be expected of a normal 90 Ma lithosphere. We suggest that such an anomalously high temperature is due to heating of the wall rocks (lithosphere) by ascending HV magmas and not due to simple basal heating of the lithosphere by the hot spot/plume followed by gradual (conductive) upward transport of heat. Thermobarometry of garnet-bearing xenoliths suggests that these rocks equilibrated at ~2–3 GPa and 1200–1350 °C. This temperature range is 50–200° less than that predicted by a recent model (Ribe and Christensen 1999). The plume that generated the Koolau shield magmas was a complex mixture composed predominantly of Koolau source materials at its center and blocks of old recycled lithosphere in its outer fringes that were later rafted into the lithosphere beneath SLC.

Unspiked K–Ar dating of the Honolulu rejuvenated and Ko‘olau shield volcanism on O‘ahu, Hawai‘i

Many mantle plume volcanoes undergo rejuvenated volcanism after a period of construction and erosion of their shield. The cause of this renewed volcanism has been enigmatic and various models have been proposed. However, the lack of geochronological data has hindered evaluation of these models. Unspiked K–Ar ages on groundmass in 41 samples from 32 vents of Honolulu Volcanics and eight samples of underlying Ko‘olau Volcanics were determined in order to reveal the temporal distribution of rejuvenated vents and the length of the hiatus between the end of shield and start of rejuvenated volcanism. The new geochronological results show that Ko‘olau shield volcanism ended at 2.1 Ma and that rejuvenated volcanism started at 0.8 Ma, resulting in a 1.3 million year hiatus in volcanic activity. Two distinct pulses were found for Honolulu volcanism at 0.80–0.35 and ∼0.1 Ma. During the first pulse, the eruption frequency increased with time and there was no spatial pattern in vent distribution, although three vents along a NNE–SSW trend produced similar compositions and may have been coeval. Volcanism apparently waned from 0.35–0.12 Ma, with only one eruption. The second pulse occurred along two rifts that trend N–S and NE–SW. Although the ages for the 10 dated flows are indistinguishable at around 0.1 Ma, lavas from the two rifts have distinct compositions: weakly alkalic vs. melilite nephelinite. The first, more widely distributed pulse of volcanism is probably related to secondary melting downstream from the Hawaiian plume stem, which may be related to lithospheric thinning. The second pulse, focused along two rifts, may be related to decompressional melting as the shield passed over the flexural arch.

Hf–Nd isotope decoupling in the oceanic lithosphere: constraints from spinel peridotites from Oahu, Hawaii

We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and major element contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii. These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of a MORB-related melting event some 80–100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from the Salt Lake Crater (SLC) show a large range of Hf isotopic compositions, from ϵ Hf=12.2 (similar to the Honolulu volcanics series) to extremely radiogenic, ϵ Hf=65, at nearly constant 143Nd/ 144Nd ratios ( ϵ Nd=7–8). None of these samples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is the only sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlate positively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REE contents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. The Lu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositions cannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere. Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparent ages and the Nd–Hf isotope decoupling, but requires that the most depleted samples were subject to the least amount of metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenes are best described by metasomatism of the 80–100 Ma depleted oceanic lithosphere by melts products of extensive mantle–melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere.

Koolau shield basalt as xenoliths entrained during rejuvenated-stage eruptions: perspectives on magma mixing

Rejuvenated-stage tuff cones (Honolulu Volcanics) on Koolau volcano, Oahu, Hawaii, contain xenoliths of Koolau shield basalt. Because Koolau subaerial shield lavas represent a Hawaiian geochemical 'end member', and submarine shield lavas have compositions with some affinities to Mauna Loa and Kilauea, we analyzed 28 xenolithic basalts from Salt Lake and Koko Head cones to determine how these seemingly random samplings of the Koolau profile compare to established Koolau geochemistry. Analyses reveal that 24 are shield tholeiitic basalt—the focus of this study—and 4 are rejuvenated-stage basaltic rocks. The tholeiitic xenoliths represent largely upper Koolau shield lavas, as these samples (8.3 to 5.8 wt% MgO) have, with one exception, overall major- and trace-element compositions that overlap those of Koolau subaerial shield lavas. Secondary processes, however, created some distinctions—namely, enrichments/depletions in K, Ba, Sr, SiO2, and FeO, and, due to zeolitization (chabazite with attending okenite and apophyllite), elevated CaO. One xenolithic basalt with 8.2 wt% MgO has higher Ti, Zr, Nb, and Sc, and lower Zr/Nb than subaerial lavas, and appears to represent relatively early, deeper shield—thereby reinforcing that the Koolau shield source varied temporally. Olivine, orthopyroxene, and plagioclase are the phenocrysts (clinopyroxene is rare), and their core compositions range widely across the suite—Fo87.8−72, orthopyroxene Mg#s 85–72, and An74–60. Several xenolithic basalts have both normally and reversely zoned orthopyroxene and plagioclase with a variety of core compositions (e.g., orthopyroxene-core Mg#s 82, 77, and 72, all in one sample). These compositions and zonations record evidence for wide compositional ranges of replenishment (MgO ~13–8 wt%) and reservoir (MgO ~7 to <5 wt%) magmas mixing in varying proportions; however, extreme MgO lavas (~13 and <5 wt%) are not observed as either subaerial or xenolithic basalt, but are indicated by phenocryst cores of Fo87.8 and orthopyroxene-Mg# 72. The Koolau magma-mixing history resembles that of Kilauea, and is unlike the 'steady-state' mixing known for Mauna Loa. Finally, these basalt samples show that any xenolithic occurrence of Koolau lava is subject to the zeolitization prevalent in the tuff-cone hosts.

Constraints on the Source Components of Lavas Forming the Hawaiian North Arch and Honolulu Volcanics

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 5Myr. Alkalic lavas with geochemical characteristics similar to rejuvenatedstage 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 Sr/Sr and Nd/Nd ratios in lavas forming the North Arch and Honolulu Volcanics are consistent with mixing between the Hawaiian plume and a depleted component related to midocean ridge basalts. Specifically, the enrichment of incompatible elements coupled with low Sr/Sr and high Nd/Nd relative to bulk Earth ratios is best explained by derivation from depleted lithosphere recently metasomatized by incipient melt (52% 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 Os/Os data (0 134±0 176) reflects heterogeneity caused by various proportions of pyroxenite veins residing in a depleted peridotite matrix.

Paleomagnetic secular variation of the Honolulu Volcanic Series (33–700 ka), O’ahu (Hawaii)

Abstract Thirty years after the precursory study, [J. Geophys. Res. 13 (1972) 2129] 19 dated sites with ages between 33 and 700 ka from the Honolulu Volcanic Series (HVS; southeastern part of the island of O’ahu in Hawaii) have been revisited. A minimum of 16 samples per site have been studied using alternating field and thermal demagnetization. Both techniques yielded similar results. Coeval flows could be identified by relying on their KAr ages, similar rock magnetic parameters and identical directions. Reliable paleomagnetic directions have been obtained for all sites. Two site mean directions which deviate significantly from the geocentric axial dipole (GAD) correspond to periods of low dipole intensity and may thus reflect a larger proportion of non-dipole components. The average value of the site mean declinations is 358.6° while the mean inclination of 34.5° is 3.5° lower than the value of the geocentric dipole at the site. The north geographic pole is largely contained within the uncertainties surrounding the position of the mean virtual geomagnetic poles (VGP) latitude. The deviation of 2.3° in latitude is further reduced by 1° if we consider the existence of a standing small quadrupole component that would represent 5% of the dipole. The angular dispersion of the virtual geomagnetic poles (12°) is only 1.7° smaller than expected at the site latitude and thus not a firm indication of low secular variation. Thus, there is no significant difference with the global time-averaged field for the same period. Other recent results from the same island for an older time period display similar characteristics, but with reverse and normal polarities that are not perfectly antipodal. It is suggested that this could result from changes in the mean dipole field intensity during these time intervals. This points out to the importance of scrutinizing the entire field vector in any analysis of paleosecular changes. These results do not bring any evidence for a persistent field anomaly under Hawaii.

Melt depletion and subsequent metasomatism in the shallow mantle beneath Koolau volcano, Oahu (Hawaii)

Xenoliths from Pali (Oahu, Hawaii) include samples of the mantle lithosphere underlying Koolau shield volcano. Most such xenoliths are spinel peridotites, the remainder being plagioclase-spinel peridotites, and garnet-free pyroxenites. Clinopyroxene separates from Pali peridotite xenoliths have relatively depleted 87Sr/86Sr (0.70309–0.70346) and 143Nd/144Nd (0.512967–0.513206). The mantle lithosphere beneath the Koolau volcano has a range of present-day epsilonNd = 6.9–11. On a 143Nd/144Nd versus 147Sm/144Nd diagram, they define a 61 ± 20 Ma errorchron, within error of the estimated age of ~80–85 Ma of the Pacific lithosphere at Oahu, and an intercept of epsilonNd = 6.8. We interpret the Pali spinel peridotites as samples of the Pacific lithosphere residual to melt extraction of the Pacific crust. These rocks were not metasomatized by melts having isotopic composition similar to the shield-building Koolau volcano, but they could have been metasomatized by melts related to the Honolulu Volcanic Series. Plagioclase mineral separates from two peridotites are in 87Sr/86Sr isotopic disequilibrium with coexisting clinopyroxenes. The plagioclases are light rare earth element depleted and have “enriched” 87Sr/86Sr ratios (0.70436 and 0.70443). The delta18O of olivine (5.09–5.12‰) and clinopyroxene (5.32–5.33‰) from spinel peridotites are typical for oceanic upper mantle rocks. In contrast, delta18O of olivine, pyroxene, and plagioclase in plagioclase-bearing peridotites are enriched by ~0.5‰ relative to the spinel peridotites (e.g., delta18O ratios of olivines from plagioclase peridotites are 5.4–5.5‰). It is here interpreted that the plagioclases represent phases that formed by reaction with or precipitation from a melt that intruded the Pacific lithospheric mantle. The Pacific lithospheric isotopic composition is reflected in the clinopyroxene separates. The high delta18O and 87Sr/86Sr of the plagioclase resemble Koolau lavas and suggest that the melt passing through the lithosphere was plume-related. The elevated delta18O of the plagioclase indicates that the melt from which it fractionated had a component of recycled crustal material not acquired from local crustal contamination. It is also estimated here that the duration of the porous melt flow of the Koolau-like melt though the mantle lithosphere was short-lived, ~10^2–10^4 years.

Evidence for Melting of Garnet Pyroxenite in the Generation of Hawaiian Post-Erosional Lavas: Effects of a Marble-Cake Mantle During Melt Generation

Isotopic variations in mantle-derived melts are a primary source of information on the age, origin and magnitude of chemical heterogeneities in the convecting mantle. Understanding the relationship between the melting process and the generation and preservation of chemical heterogeneity in basaltic magmas is therefore of the utmost importance. Preferential melting of mafic veins in predominantly peridotitic mantle is a potential source of chemical and isotopic heterogeneity in mantle-derived melts. Previous studies have proposed a role for melting of garnet pyroxenite in the origin of the garnet signature in MORB [e.g. Hirschmann and Stolper, 1996]. At high degrees of mantle melting, such as that responsible for the generation of MORB, the signature of pyroxenitic melting may be difficult to detect unambiguously. However, because pyroxenites have a lower Tsolidus than peridotites, small degree partial melts should contain a more pronounced signature from pyroxenite melting. We have examined the Os-isotope systematics of Hawaiian post-erosional lavas from the Koloa Volcanic Suite, Kauai and the Honolulu Volcanic Series, Oahu to evaluate the role of pyroxenite melting in the generation of these highly enriched small-degree partial melts.

Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy

Trace element relationships of near-primary alkalic lavas from La Grille volcano, Grande Comore, in the Indian Ocean, as well as those of the Honolulu volcanic series, Oahu, Hawaii, show that their sources contain amphibole and/or phlogopite. Small amounts of each mineral (2% amphibole in the source of La Grille and 0.5% phlogopite plus some amphibole in the source of the Honolulu volcanics) and a range of absolute degrees of partial melting from ∼ 1 to ∼ 5% for both series are consistent with the observed trace element variation. Amphibole and phlogopite are not stable at the temperatures of convecting upper mantle or upwelling thermal plumes from the deep mantle; however, they are stable at pressure-temperature conditions of the oceanic lithospheric mantle. Therefore, the presence of amphibole and/or phlogopite in the magma source region of volcanics is strong evidence for lithospheric melting, and we conclude that the La Grille and the Honolulu series formed by melting of the oceanic lithospheric mantle.The identification of amphibole ± phlogopite in the source region of both series implies that the metasomatism by fluids or volatile-rich melts occurred prior to melting. The presence of hydrous phases results in a lower solidus temperature of the lithospheric mantle, which can be reached by conductive heating by the thermal plumes. Isotope ratios of the La Grille and the Honolulu series display a restricted range in composition and represent compositional end-members for each island. Larger isotopic variations in shield lavas, represented by the contemporaneous Karthala volcano on Grande Comore and the older Koolau series on Oahu, reflect interaction of the upwelling thermal plumes with the lithospheric mantle rather than the heterogeneity of deep-seated mantle plume sources or entrainment of mantle material in the rising plume. Literature OsSr isotope ratio covariations constrain the process of plume-lithosphere interaction as occurring through mixing of plume melts and low-degree melts from the metasomatized oceanic lithospheric mantle.The characterization of the lithospheric mantle signature allows the isotopic composition of the deep mantle plume components to be identified, and mixing relationships show that the Karthala and Koolau plume end-members have nearly uniform isotopic compositions. Based on independent arguments, isotopic variations on Heard and Easter islands have been shown to be a result of mixing between deep plume sources having distinct isotopic compositions with lithosphere or shallow asthenospheric mantle. To the extent that these case studies are representative of oceanic island volcanism, they indicate that interaction with oceanic lithospheric mantle plays an important role in the compositions of lavas erupted during the shield-building stage of plume magmatism, and that isotopic compositions of deep mantle plume sources are nearly uniform on the scale that they are sampled by melting.

A noble gas profile across a Hawaiian mantle xenolith: Coexisting accidental and cognate noble gases derived from the lithospheric and asthenospheric mantle beneath Oahu

A noble gas profile across a garnet pyroxenite xenolith from Salt Lake Crater, Oahu, Hawaii, provides information about the scale and origin of noble gas heterogeneities within such rocks. Variations in both absolute and relative noble gas concentrations are large and comparable to those observed between individual Salt Lake Crater pyroxenite xenoliths. 3He/4He varies from 7.7 to 9.4 times the atmospheric value (Ra) and correlates inversely with 40Ar/36Ar, which ranges between 4100 and 9700. Neon krypton and xenon isotopes are uniform and indistinguishable from air, with the exception of excess 129Xe. Overall, the observed noble gas compositions reflect a derivation from depleted MORB-type mantle sources.The spatial distribution of noble gas signatures within the xenolith and the observed correlation between helium and argon isotopes suggest the presence of two different noble gas components which are trapped in different phases and are unevenly distributed within the xenolith. 40Ar/36Ar and 1/36Ar correlate inversely, indicating that atmospheric contamination is insignificant. Hence, the observed isotopic variations reflect mixing of two mantle-derived noble gas components. Correlations between HeAr isotopes and CO2/H2O in different pyroxenites from Salt Lake Crater, including our sample, reveal that the first component is characterized by highly radiogenic helium and argon isotopes and related to abundant secondary CO2-rich fluid inclusions. Given the high diffusivity of He at mantle temperatures (Hart, 1984), the observed helium isotope heterogeneities on a sub-mm scale require that the fluids were introduced concurrently with eruption. This interpretation is supported by the low entrapment depths of fluid inclusions in Salt Lake Crater pyroxenites (<30 km; Murck et al., 1978). This implies that the fluids are genetically related to the host magma itself and reflect its composition. The second noble gas component is interpreted as being magmatic, i.e., cognate to the basaltic magma from which the pyroxenite precipitated within the mantle. It is proposed that this component resides inside the mineral lattices and was trapped during magmatic crystallization.Our results indicate that the depleted lithospheric mantle source of the post-erosional host magma (Honolulu Volcanic Series) is characterized by 40Ar/36Ar ≥ 10000, R/Ra ≤ 7.5, and excesses in129Xe. Noble gas signatures of the asthenospheric mantle source parental to the pyroxenite-producing magma are less radiogenic with respect to these isotopes. Combining noble gas and strontium, neodymium, and lead isotope evidence, we propose that the asthenosphere beneath Oahu was originally similar to depleted MORB-type mantle, but became slightly modified by noble gases and other incompatible trace elements derived from the Hawaiian mantle plume prior to partial melting.This study provides evidence that noble gas isotopes and solid radiogenic isotopes are coupled in the mantle sources of basalts (e.g., Allègre et al., 1983), but decoupled in xenoliths (e.g., Vance et al., 1989).

Constraints on the degree of dynamic partial melting and source composition using concentration ratios in magmas

We present a dynamic melting inversion (DMI) method that permits calculation of the degree of partial melting for cogenetic primary magmas in the context of the dynamic melting model of Langmuir et al. (1977). The method uses variations of between-magma concentration ratios for two incompatible trace elements that have different bulk distribution coefficients, and does not require assumptions regarding source incompatible element concentrations or ratios. Source concentrations can be calculated after obtaining the partial melting degree. The method is applied to the nephelinitic Koloa suite of Kauai, the Honolulu volcanics of Oahu, and alkalic basalts from southeastern Australia. Results, which display remarkable intersuite and interelement consistency, suggest that melilitites and alkali olivine basalts are formed by 2–4% and 8–11% partial melting, respectively, of a LREE-enriched mantle source. This result for basalts that are characterized by positive ε Nd argues persuasively that the often-invoked “recent source enrichment event” is actually an integral part of the total melting process.

Evolution of the lithosphere beneath Oahu, Hawaii: rare earth element abundances in mantle xenoliths

Rare earth element contents of clinopyroxenes in Hawaiian mantle xenoliths from Oahu were determined with an ion microprobe. The analyzed xenoliths are from four vents of the alkali Honolulu Volcanics (HV). Three (Kaau, Pali and Kalihi—KPK) are located close to the caldera of the extinct Koolau shield volcano, and the fourth, Salt Lake Crater (SLC), is on the periphery of the shield volcano. Systematic differences exist in REE contents between clinopyroxenes of the KPK and SLC xenoliths: (1) KPK pyroxenes are typically zoned in REE contents whereas SLC pyroxenes are homogeneous, (2) the LREE-depleted (chondrite-normalized) patterns that characterize many of the KPK xenoliths are not found in SLC xenoliths, and (3) the convex-upward REE patterns that are characteristic of SLC xenoliths are not found in KPK xenoliths. Relative to abyssal peridotites, the LREE-depleted Hawaiian lherzolite pyroxenes (interpreted to be residual oceanic lithosphere) have higher contents of REE, Na 2O, TiO 2 and FeO, and more modal clinopyroxene. These LREE-depleted Hawaiian xenoliths represent deeper, less-depleted parts of the melting column, whereas the abyssal peridotites represent the uppermost, more strongly depleted part of the mantle. The spoon-shaped, LREE-enriched and convex-upward REE patterns in the xenoliths have resulted from metasomatic enrichment of the lithosphere caused by reaction with magmas that formed the Honolulu Volcanics. A model for the evolution of the oceanic lithosphere is presented in which fractures were the main mode of transport of the Honolulu Volcanics. Metasomatic enrichment resulted from interaction between percolating Honolulu Volcanics magmas and wallrock. The differences between SLC and KPK xenoliths are attributed to chromatographic fractionation effects: SLC xenoliths are postulated to have come from a greater depth where they equilibrated to a larger extent with the percolating magmas than the KPK rocks.

Pliocene and Pleistocene alkalic flood basalts on the seafloor north of the Hawaiian islands

The North Arch volcanic field is located north of Oahu on the Hawaiian Arch, a 200-m high flexural arch formed by loading of the Hawaiian Islands. These flood basalt flows cover an area of about 25, 000 km 2; the nearly flat-lying sheet-like flows extend about 100 km both north and south from the axis of the flexural arch. Samples from 26 locations in the volcanic field range in composition from nephelinite to alkalic basalt. Ages estimated from stratigraphy, thickness of sediment on top of the flows, and thickness of palagonite alteration rinds on the recovered lavas, range from about 0.75–0.9 Ma for the youngest lavas to somewhat older than 2.7 Ma for the oldest lavas. Most of the flow field consists of extensive sheetflows of dense basanite and alkalic basalt. Small hills consisting of pillow basalt and hyaloclastite of mainly nephelinite and alkalic basalt occur within the flow field but were not the source vents for the extensive flows. Many of the vent lavas are highly vesicular, apparently because of degassing of CO 2. The lavas are geochemically similar to the rejuvenated-stage lavas of the Koloa and Honolulu Volcanics and were generated by partial melting of sources similar to those of the Koloa Volcanics. Prior to eruption, these magmas may have accumulated at or near the base of the lithosphere in a structural trap created by upbowing of the lithosphere.

Age and petrology of alkalic postshield and rejuvenated-stage lava from Kauai, Hawaii

At the top of the Waimea Canyon Basalt on the island of Kauai, rare flows of alkalic postshield-stage hawaiite and mugearite overlie tholeiitic flows of the shield stage. These postshield-stage flows are 3.92 Ma and provide a younger limit for the age of the tholeiitic shield stage. The younger Koloa Volcanics consist of widespread alkalic rejuvenated-stage flows and vents of alkalic basalt, basanite, nephelinite, and nepheline melilitite that erupted between 3.65 and 0.52 Ma. All the flows older than 1.7 Ma occur in the west-northwestern half of the island and all the flows younger than 1.5 Ma occur in the east-southeastern half. The lithologies have no spatial or chronological pattern. The flows of the Koloa Volcanics are near-primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet-bearing source that has about two-thirds the concentration of P2O5, rare-earth elements, and Sr of the source of the Honolulu Volcanics on the island of Oahu. The same lithology in the Koloa and Honolulu Volcanics is generated by similar degrees of partial melting of distinct source compositions. The lavas of the Koloa Volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt, respectively. Phases that remain in the residue of the Honolulu Volcanics, such as rutile and phlogopite, are exhausted during formation of the Koloa Volcanics at all but the smallest degrees of partial melting. The mantle source for Kauai lava becomes systematically more depleted in 87Sr/86Sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage: at the same time, the lavas become systematically more enriched in incompatible trace elements. On a shorter timescale, the lavas of the Koloa Volcanics display the same compositional trends, but at a lower rate of change. The source characteristics of the Koloa Volcanics, considered along with those of the Honolulu Volcanics, support a mixing model in which the source of rejuvenated-stage lava represents large-percent melts of a plume source mixed with small amounts of small-percent melts of a heterogeneous mid-ocean-ridge source.