Mauna Kea Lavas Research Articles

Stable chromium isotope fractionation during magmatic differentiation: Insights from Hawaiian basalts and implications for planetary redox conditions

The stable isotope compositions of chromium (Cr) are fractionated during magmatic differentiation of lunar mare basalts, which might be attributed to redox-related mineral crystallization. It has yet to be demonstrated whether magmatic differentiation fractionates Cr isotope composition of terrestrial samples. Here, we present high-precision stable Cr isotope measurements, reported as δ53Cr relative to NIST SRM 979, of well-characterized Hawaiian tholeiitic basalts from Koolau, Mauna Kea and Kilauea. The studied Makapuu-stage Koolau lavas have MgO ranging from 6.58 to 21.54 wt.%, and they have homogeneous δ53Cr ranging from −0.21‰ to −0.17‰. Similarly, studied Mauna Kea lavas have MgO ranging from 9.11 to 17.90 wt.%, and they also have homogeneous δ53Cr ranging from −0.17‰ to −0.13‰. Some Makapuu-stage Koolau and Mauna Kea lavas experienced subaerial or submarine alteration. The homogenous δ53Cr within each sample suites implies that the post-magmatic alterations have not significantly changed the Cr isotope compositions of these samples. Conversely, nine Kilauea Iki basalts have MgO ranging from 7.77 to 26.87 wt.% reflecting varying degrees of magmatic differentiation, and they show resolvable Cr isotope variations with δ53Cr ranging from −0.18‰ to 0.00‰. The δ53Cr values of the Kilauea Iki samples are positively correlated with indicators of magmatic differentiation such as Cr and MgO contents, and Mg# values. The most evolved samples have the lightest isotope compositions, whereas the olivine-spinel cumulates display complementary heavy isotope compositions. This fractionation is most likely generated by the crystallization and accumulation of spinel, which is dominated by Cr3+ and, hence, enriched in heavier Cr isotopes relative to the residual melt. At a given MgO content, Kilauea and Mauna Kea lavas, both Kea-trend volcanoes, have higher δ53Cr than Makapuu-stage Koolau lavas, a Loa-trend volcano. This difference might reflect recycling of altered oceanic crusts or redox differences of their magmatic sources, with the mantle source of Makapuu-stage lavas being more reducing.To understand the different Cr isotope fractionation behaviors of terrestrial and extraterrestrial basalts, we present a quantitative model that relates the Cr isotope compositions of basalts from the Earth, the Moon and Vesta, to the crystallization assemblage, the degree of fractional crystallization, and the Cr2+/ΣCr ratios of minerals and melts, which are related to the oxygen fugacity during differentiation. The primitive Hawaiian basaltic magma for Kilauea Iki and Mauna Kea lavas is estimated to have δ53Cr of −0.15‰, which is close to the average value of the BSE (−0.14‰ to −0.12‰). We further speculate that the initial lunar mantle is relatively homogeneous with BSE-like isotope composition (−0.16‰ to −0.09‰). The observed low δ53Cr in lunar mafic rocks is the result of redox-dominated fractional crystallization and accumulation processes of lunar mafic magmas. These magmas might be derived from variable degrees of partial melting of the primitive lunar mantle.Combined with previous results on the variations in Cr valences and contents in silicate melts and minerals related to oxygen fugacity, Cr concentration and isotope composition can serve as a useful oxybarometer for understanding the redox conditions of planetary differentiation and magmatic evolution.

Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth’s oxygenation

The behavior of C, H, and S in the solid Earth depends on their oxidation states, which are related to oxygen fugacity (fO2). Volcanic degassing is a source of these elements to Earth's surface; therefore, variations in mantle fO2 may influence the fO2 at Earth's surface. However, degassing can impact magmatic fO2 before or during eruption, potentially obscuring relationships between the fO2 of the solid Earth and of emitted gases and their impact on surface fO2 We show that low-pressure degassing resulted in reduction of the fO2 of Mauna Kea magmas by more than an order of magnitude. The least degassed magmas from Mauna Kea are more oxidized than midocean ridge basalt (MORB) magmas, suggesting that the upper mantle sources of Hawaiian magmas have higher fO2 than MORB sources. One explanation for this difference is recycling of material from the oxidized surface to the deep mantle, which is then returned to the surface as a component of buoyant plumes. It has been proposed that a decreasing pressure of volcanic eruptions led to the oxygenation of the atmosphere. Extension of our findings via modeling of degassing trends suggests that a decrease in eruption pressure would not produce this effect. If degassing of basalts were responsible for the rise in oxygen, it requires that Archean magmas had at least two orders of magnitude lower fO2 than modern magmas. Estimates of fO2 of Archean magmas are not this low, arguing for alternative explanations for the oxygenation of the atmosphere.

Isotopic systematics of the early Mauna Kea shield phase and insight into the deep mantle beneath the Pacific Ocean

The 3500 m deep Hawai'i Scientific Drilling Project core provides a ~680 kyr record of the magmatic history and source components of Mauna Kea volcano. We report high‐precision Pb‐Sr‐Nd isotopic compositions of 40 basalts from the last 408 m of the final drilling phase (HSDP2‐B and HSDP2‐C) and show that these lowermost basalts represent the early shield stage of Mauna Kea's growth history. Two sample groups are distinguished based on their isotopic variability compared to the rest of the core. Over a depth interval of 210 m (3098.2–3308.2 mbsl), the basalts show very restricted isotopic variation and represent sampling of a relatively homogeneous source. Samples from the bottom 192 m record the largest range of 206Pb/204Pb and 208Pb/204Pb in the core, reflecting the greater isotopic variability of the earlier stages of volcanism compared to subsequent stages. The heterogeneity of Mauna Kea lavas is explained by mixing variable proportions of four distinct components intrinsic to the Hawaiian mantle plume. One of these components, Kea, is a prevalent and long‐lived composition within the Hawaiian plume, whereas the other three components are involved at different stages of the volcano's history and contribute to the short‐term isotopic variability of Mauna Kea. The compositional similarity of the Kea component to “C” and to the super‐chondritic bulk‐silicate Earth suggests that Kea may be part of the primitive mantle of a non‐chondritic Earth. Other Pacific oceanic island basalts share Kea‐like compositions, indicating that the Kea component is a common, widespread composition within the Pacific deep mantle.

Compositional diversity of Mauna Kea shield lavas recovered by the Hawaii Scientific Drilling Project: Inferences on source lithology, magma supply, and the role of multiple volcanoes

The final Stage (Phase‐2) of the Hawaii Scientific Drilling Project (HSDP) recovered 408 m of basaltic core (3098–3506 mbsl) attributed to Mauna Kea volcano. We determined the major and trace element composition of 40 samples from this core. Our results show that the incompatible element ratios, such as Zr/Nb, which are correlated with Pb isotopic ratios, are more variable in the lower 408 m of Mauna Kea shield lavas than in the overlying 2855 m (∼450 ka). We argue that this geochemical diversity was present in the mantle source of Mauna Kea shield lavas and does not require the inter‐fingering of lavas from adjacent volcanoes. Because of uncertainties in Ni partitioning between olivine and melt and the wide range of Ni contents in peridotites, we show that all Mauna Kea lavas may have been derived from a peridotite source. We also obtained major and trace element compositions for 24 whole‐rock clasts and hyaloclastites and 7 glasses from HSDP Phase‐1 core between 1767 and 1808 mbs. These enigmatic lavas, previously recognized by the distinctive high CaO and K2O contents of their glasses, are also relatively enriched in highly incompatible trace elements. We show that this group of lavas have affinities with post‐shield lavas and argue that they are a consequence of lower degrees of melting (∼a factor of two) than other Mauna Kea shield lavas, thereby providing evidence that magma supply varied significantly during the growth of the Mauna Kea shield.

Oxygen isotope constraints on the structure and evolution of the Hawaiian Plume

The oxygen isotope stratigraphy of Ko‘olau volcano, Hawaii, is constructed by analyzing olivine phenocrysts from the KSDP drill core and submarine land-slide deposits. Along with those of subaerial (Makapu‘u) Ko‘olau olivines (Eiler and others, 1996a), they span the full range of the δ^(18)OVSMOW variation previously observed in Loa-trend Hawaiian volcanoes (Lō‘ihi, Mauna Loa, Hualalai, and Ko‘olau), vary systematically with the stratigraphic position, and correlate with other geochemical properties of their host lavas (Tanaka and others, 2002; Haskins and Garcia, 2004; Huang and Frey, 2005; Salters and others, 2006; Fekiacova and others, 2007). These observations can be explained if the Loa-trend volcanoes (including Ko‘olau) are constructed of magmas made by mixing peridotite melt with variable proportions of eclogite melt derived from a mafic constituent of the Hawaiian plume having a composition resembling recent mid-ocean-ridge basalts. We present a model of this magma mixing process that simultaneously explains the correlations among oxygen isotopes, major elements, trace elements and radiogenic isotopes. Although a number of models of this kind, differing in several parameters, describe the data equally well, all statistically acceptable models require an component with a MORB-like HREE pattern and enriched oxygen isotope composition (δ^(18)OVSMOW = 7.8-9.7‰), consistent with this component being an upper crustal (layer 1 or 2) basalt or gabbro with a low-temperature alteration history, possibly containing a small amount of sediment. Abundances of some minor elements—Ni and Ti—are not well described by this model; we show that these shortcomings are derived from the compositional assumption and operational difficulties, that is, TiO_2 content is too high in our assumed eclogite end-member, and the inversion of NiO content in the melt by the olivine addition calculation is imprecise due to the sensitivity of D_(NiO)^(olivine/melt) to the melt composition and to crystallization process. Previous studies have advocated that Hawaiian lavas were derived from partial melts of an olivine-free pyroxenite formed by reaction of eclogite-derived melt with peridotite (for example, Sobolev and others, 2005). Our study shows that the peridotite-derived and eclogite-derived melt-mixing model can explain the geochemistry of Hawaiian lavas as well, including high-Ni Ko‘olau olivines. We find that an olivine-free mantle source for Hawaiian lavas is unnecessary, although melt-rock interaction could be important in modifying melt composition. Inverting for mixing proportions and degree of melting, we estimate that the amount of recycled crust in the Hawaiian plume is <24 weight percent. Comparison of the late shield-stage Loa-trend (particularly Ko‘olau lavas) and Kea-trend (particularly Mauna Kea lavas) suggests that the geochemical diversity of Hawaiian lavas is produced by a thermally and chemically-zoned plume.

Dynamics and internal structure of the Hawaiian plume

A thorough understanding of the internal structure of the Hawaiian plume conduit requires to link geochemical observations of surface lavas to fluid dynamic simulations able to quantify the flow trajectories of upwelling geochemical heterogeneities and their sampling by volcanoes. With the present work we fill a gap between the numerous geochemical studies of Hawaiian lavas and the paucity of dynamical models that relate the observed geochemical record to the internal plume structure. Our three-dimensional numerical simulation of a vigorous plume sheared by a fast moving oceanic plate shows that the dominant deformation in the conduit is vertical stretching, while horizontal spreading and vertical shortening prevail in the sublithospheric part of the plume (hereafter referred to as plume head). Flow trajectories indicate that a young volcano like Loihi samples the ‘upstream’ side of the plume, not its center, whereas volcanoes in the post-shield phase sample deep melts from the ‘downstream’ side of the plume. To constrain the internal conduit structure we focus on two geochemical observations: old (>350kyr) Mauna Kea lavas from the Hawaii Scientific Drilling Project are isotopically distinct from recent Mauna Kea lavas, but they are isotopically identical to present-day Kilauea lavas. By modelling a plume conduit with several long-lasting filaments of 10km radius, we find that the isotopic record of a volcano (e.g., Mauna Kea) is expected to change over time-scales of ∼400kyr. Furthermore, by requiring that two age progressive volcanoes (e.g., Mauna Kea and Kilauea) sample the same filament, we constrain the minimum filament length to be ∼600km. In this paper we adopt a ‘top-down’ approach: from geochemical observations of surface lavas, to dynamical models of the conduit structure, and further down to the ‘geochemical architecture’ of the thermal boundary layer feeding the plume. A conduit structure with filaments maps back into heterogeneous volumes with azimuthal and radial extents of several hundred kilometers in the source region of plumes.

Horizontal and vertical zoning of heterogeneities in the Hawaiian mantle plume from the geochemistry of consecutive postshield volcano pairs: Kohala‐Mahukona and Mauna Kea–Hualalai

Sr‐Nd‐Pb‐Hf isotopic compositions of postshield lavas from two pairs of Hawaiian volcanoes, Mauna Kea and Kohala (Kea trend) and Hualalai and Mahukona (Loa trend), allow for identification of small‐scale (tens of kilometers) heterogeneities in the Hawaiian mantle plume and provide constraints on their distribution. The postshield lavas range from transitional/alkalic basalt to trachyte and are enriched in incompatible trace elements (e.g., LaN/YbN = 6.0–16.2). These lavas are characterized by a limited range of Sr‐Nd‐Hf isotopic compositions (87Sr/86Sr = 0.70343–0.70365, 143Nd/144Nd = 0.51292–0.51301, and 176Hf/177Hf = 0.28311–0.28314) and have distinct Pb isotopic compositions (206Pb/204Pb = 17.89–18.44, 207Pb/204Pb = 15.44–15.49, and 208Pb/204Pb = 37.68–38.01) that correspond to their respective Kea or Loa side of the Pb‐Pb isotopic boundary. Mauna Kea lavas show a systematic shift to less radiogenic Pb isotopic compositions from the shield to postshield stage and they trend to low 87Sr/86Sr toward, but not as extreme as, compositions characteristic of rejuvenated stage lavas. Hualalai postshield lavas lie distinctly above the Hf‐Nd Hawaiian array and have much lower Pb isotopic ratios than shield lavas, including some of the least radiogenic values (e.g., 206Pb/204Pb = 17.89–18.01) of recent Hawaiian volcanoes. In contrast, comparison of Kohala with the adjacent Mahukona volcano shows that these older postshield lavas become more radiogenic in Pb during the late stages of volcanism. The isotope systematics of the postshield lavas cannot be explained by mixing between Hawaiian plume end‐members (e.g., Kea, Koolau, and Loihi) or by assimilation of Pacific lithosphere and are consistent with the presence of ancient recycled lower oceanic crust (±sediments) in their source. More than one depleted component is sampled by the postshield lavas and these components are long‐lived features of the Hawaiian plume that are present in both the Kea and Loa source regions. The depleted components in the postshield lavas, particularly as sampled at Hualalai, are different from the much more homogeneous component present in rejuvenated lavas. The geochemistry of the postshield lavas provides evidence for a bilateral symmetry to the plume where the compositional boundary between the Kea and Loa sources is complex and vertical components of heterogeneity are significant.

Ancient carbonate sedimentary signature in the Hawaiian plume: Evidence from Mahukona volcano, Hawaii

Lavas from Mahukona, a small Hawaiian volcano on the Loa trend, exhibit major and trace element abundance variations exceeding those in lavas from large Hawaiian shields, such as Mauna Loa and Mauna Kea. Mahukona lavas define three geochemically distinct groups of tholeiitic shield basalt and a transitional group of postsshield basalt. At 10% MgO the tholeiitic groups range from 9 to 12% CaO; such differences in CaO can reflect partial melts derived from garnet pyroxenite (low CaO) and peridotite (high CaO), but the negative CaO‐Yb (both at 10% MgO) trend formed by Mahukona lavas is inconsistent with this explanation. Within Mahukona lavas, radiogenic Nd‐Hf‐Pb isotopic ratios are highly correlated with each other; however, 87Sr/86Sr is decoupled from these radiogenic isotopic ratios. Rather, 87Sr/86Sr is correlated with trace element abundance ratios involving Sr, and importantly, Mahukona lavas define a negative Rb/Sr‐87Sr/86Sr trend, implying that a Sr‐rich source component characterized by high 87Sr/86Sr is important in the petrogenesis of Mahukona lavas. We infer that this Sr‐rich source component is recycled ancient carbonate‐rich sediments. Intershield heterogeneity among Hawaiian shields also shows a negative Rb/Sr‐87Sr/86Sr trend. For example, Makapuu‐stage Koolau lavas have higher 87Sr/86Sr but lower Rb/Sr than Mauna Kea lavas. Consequently, we infer that a recycled ancient carbonate‐rich sedimentary source component is important in the Hawaiian plume. Although most lavas from Loa and Kea trend volcanoes define distinct fields in isotopic ratios of Sr, Nd, Hf, and Pb, the majority of Mahukona lavas have isotopic ratios at the boundary between the fields defined by Loa and Kea trend lavas. However, a subgroup of Mahukona shield lavas have Kea‐like isotopic and trace element signatures, an observation that can be explained by vertical heterogeneity in a bilaterally asymmetrical plume.

Mixing of isotopic heterogeneities in the Mauna Kea plume conduit

We have measured high-precision Hf, Nd, and Pb isotope compositions for 40 samples from the final leg of the Hawaii Scientific Drilling Project (HSDP) core using solution chemistry and MC-ICP-MS. The new isotope data are indistinguishable from those higher up in the Mauna Kea part of the core. The Pb isotope data define three linear trends with a common intersection. Principal component analysis recognizes no more than three geochemical end-members among these trends. When all the Pb isotope data acquired so far for the HSDP core are pooled, two contrasting groups stand out. A first coherent, prevalent group represents the Kea main eruptive sequence. This group combines the Kea-mid8 and Kea-lo8 subgroups of Eisele et al. (Eisele, J., Abouchami, W., Galer, S.J.G., Hofmann, A.W., 2003. The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP-2 drill core. Geochem. Geophys. Geosyst. 4, doi: 10.1029/2002GC000339): although defining two separate trends in Pb–Pb isotope space, they follow each other smoothly in time. A second group of 14 flows below 2000 mbsl is characterized by distinctly higher source Th/U and high 3He/ 4He. These samples belong to the Kea-hi8 group and are about 0.4 (Hf) and 0.5 (Nd) ε units below the main sequence Kea values. The toggle between these two groups is so sharp and repetitive and the gap between the values so conspicuous that the alternation must reflect eruptions from distinct volcanic centers, those of Mauna Kea and an unknown volcano, the latter of which bears isotopic resemblance to Kilauea for Nd and Hf and to Loihi for He. This ‘lost’ volcano stopped erupting about 550 ka ago, i.e., 200 ka before the oldest known ages for Kilauea, and is presently buried under 2000–3000 m of Kea flows. Periodograms show no wavelengths exceeding the 95% significance level (white spectrum). Except for a few minor wiggles in Hf at shallow depth, the isotopic signals display overall smooth up-core variations with little short-distance variability. Analysis of the flow regime within the plume conduit shows that temperature dependence of viscosity makes the conduit narrow and sharply bound. The hot core is highly mobile with minimum shear rates and a regime of pipe (Poiseuille) flow. The core is rimmed by a sheath of colder, strongly stretched material (filaments). A broadly concentric zoning of the plume is expected. The white spectra suggest, however, that, at any given depth within the plume conduit, lateral mixing results from the rheological variability across the plume conduit.

Insights into the origin of low- δ18O basaltic magmas in Hawaii revealed from in situ measurements of oxygen isotope compositions of olivines

In situ measurements of oxygen isotope and elemental compositions of olivines from subaerial Mauna Kea lavas reveal that their δ 18O values correlate positively with their forsterite contents, consistent with addition of one or more low- δ 18O components into magmas from which they grew over the course of their crystallization–differentiation histories. This result supports previous suggestions that low- δ 18O components to Mauna Kea lavas are contaminants derived from hydrothermally-altered rocks in the volcanic edifice or lithosphere, rather than components of the underlying mantle sources of these lavas. The slope of the correlation between δ 18O values and forsterite contents of olivines is steeper for subaerial Mauna Kea lavas than for submarine Mauna Kea lavas, and olivines from Mauna Loa lavas exhibit negligible changes in δ 18O values over a similar range of forsterite contents. Models of assimilation–fractional crystallization (AFC) processes can explain our observations if the δ 18O values of crustal contaminants decrease sharply at the submarine–subaerial transition in Mauna Kea volcano, and if Mauna Loa lavas are either uncontaminated or contaminated only by rocks that have δ 18O values similar to that of primary Mauna Loa magmas. We suggest that the differences in oxygen isotope systematics among Mauna Loa, submarine Mauna Kea and subaerial Mauna Kea lavas principally reflect the sources and amounts of water available to hydrothermal systems in the volcanic edifice.

Stratigraphy of the Hawai‘i Scientific Drilling Project core (HSDP2): Anatomy of a Hawaiian shield volcano

The Hawai‘i Scientific Drilling Project (HSDP2) successfully drilled ∼3.1 km into the island of Hawai‘i. Drilling started on Mauna Loa volcano, drilling 247 m of subaerial lavas before encountering 832 m of subaerial Mauna Kea lavas, followed by 2019 m of submarine Mauna Kea volcanic and sedimentary units. The 2.85 km stratigraphic record of Mauna Kea volcano spans back to ∼650 ka. Mauna Kea subaerial lavas have high average olivine contents (13 vol.%) and low average vesicle abundances (10 vol.%). Most subaerial Mauna Kea flows are ‘a‘ā (∼63%), whereas the Mauna Loa section contains nearly equal amounts of pāhoehoe and ‘a‘ā (like its current surface). The submarine Mauna Kea section contains an upper, ∼900 m thick, hyaloclastite‐rich section and a lower, ∼1100 m thick, pillow‐lava‐dominated section. These results support a model that Hawaiian volcanoes are built on a pedestal of pillow lavas capped by rapidly quenched, fragmented lava debris. The HSDP2 section is compared here to a 1.7 km deep hole (SOH1) on Kilauea's lower east rift zone. Differences in the sections reflect the proximity to source vents and the lower magma supply to Kilauea's rift zone. Both drill core sections are cut by intrusions, but the higher abundance of intrusions in SOH1 reflects its location within a rift zone, causing more extensive alteration in the SOH1 core. The HSDP2 site recovered a relatively unaltered core well suited for geochemical analyses of the single deepest and most complete borehole ever drilled through a Hawaiian or any other oceanic island volcano.

Geochemical structure of the Hawaiian plume: Sr, Nd, and Os isotopes in the 2.8 km HSDP‐2 section of Mauna Kea volcano

Sr, Nd, and Os isotopic measurements were made on 110 Mauna Kea lava and hyaloclastite samples from the drillcore retrieved from the second phase of the Hawaii Scientific Drilling Project (HSDP‐2). The samples come from depths of 255 to 3098 meters below sea level, span an age range from 200 to about 550‐600 kyr, and represent an ordered record of the lava output from Mauna Kea volcano as it drifted a distance of about 40 km over the magma‐producing region of the Hawaiian hot spot. The deepest (oldest) samples represent the time when Mauna Kea was closest to the center of the melting region of the Hawaiian plume. The Sr and Os isotopic ratios in HSDP‐2 lavas show only subtle isotopic shifts over the ∼400 kyr history represented by the core. Neodymium isotopes (ɛNd values) increase systematically with decreasing age from an average value of nearly +6.5 to an average value of +7.5. This small change corresponds to subtle shifts in 87Sr/86Sr and 187Os/188Os isotope ratios, with small shifts of ɛHf, a large shift in 208Pb/204Pb and 208Pb/207Pb values, and with a very large shift in He isotope ratios from R/RA values of about 7–8 to values as high as 25. When Mauna Kea was closest to the plume core, the magma source did not have primitive characteristics for Nd, Sr, Pb, Hf, and Os isotopes but did have variable amounts of “primitive” helium. The systematic shifts in Nd, Hf, Pb, and He isotopes are consistent with radial isotopic zoning within the melting region of the plume. The melting region constitutes only the innermost, highest‐temperature part of the thermally anomalous plume mantle. The different ranges of values observed for each isotopic system, and comparison of Mauna Kea lavas with those of Mauna Loa, suggest that the axial region of the plume, which has a radius of ∼20 km, is a mixture of recycled subducted components and primitive lower mantle materials, recently combined during the formational stages of the plume at the base of the mantle. The proportions of recycled and primitive components are not constant, and this requires there be longitudinal (vertical) heterogeneity within the core of the plume. The remainder of the plume, outside this plume “core zone,” is less heterogeneous but distinct from upper mantle as represented by mid‐ocean ridge basalt (MORB). The plume structure may provide a detailed view of mantle isotopic composition near the core‐mantle boundary.

Lead isotopes reveal bilateral asymmetry and vertical continuity in the Hawaiian mantle plume

The two parallel chains of Hawaiian volcanoes ('Loa' and 'Kea') are known to have statistically different but overlapping radiogenic isotope characteristics. This has been explained by a model of a concentrically zoned mantle plume, where the Kea chain preferentially samples a more peripheral portion of the plume. Using high-precision lead isotope data for both centrally and peripherally located volcanoes, we show here that the two trends have very little compositional overlap and instead reveal bilateral, non-concentric plume zones, probably derived from the plume source in the mantle. On a smaller scale, along the Kea chain, there are isotopic differences between the youngest lavas from the Mauna Kea and Kilauea volcanoes, but the 550-thousand-year-old Mauna Kea lavas are isotopically identical to Kilauea lavas, consistent with Mauna Kea's position relative to the plume, which was then similar to that of present-day Kilauea. We therefore conclude that narrow (less than 50 kilometres wide) compositional streaks, as well as the larger-scale bilateral zonation, are vertically continuous over tens to hundreds of kilometres within the plume.

Composition of basaltic lavas sampled by phase‐2 of the Hawaii Scientific Drilling Project: Geochemical stratigraphy and magma types

This paper presents major and trace element compositions of lavas from the entire 3098 m stratigraphic section sampled by phase‐2 of the Hawaii Scientific Drilling Project. The upper 245 m are lavas from Mauna Loa volcano, and the lower 2853 m are lavas and volcanoclastic rocks from Mauna Kea volcano. These intervals are inferred to represent about 100 ka and 400 ka respectively of the eruptive history of the two volcanoes. The Mauna Loa tholeiites tend to be higher in SiO2 and lower in total iron, TiO2, alkalis, and incompatible elements at a given MgO content than Mauna Kea lavas. The transition from Mauna Loa to Mauna Kea lavas is all the more pronounced because the Mauna Loa tholeiites overlie a thin sequence of postshield Mauna Kea alkalic to transitional tholeiitic lavas. The Mauna Loa tholeiites display well‐developed coherent trends with MgO that are indistinguishable in most respects from modern lavas. With depth, however, there is a slight decline in incompatible element abundances, and small shifts to depleted isotopic ratios. These characteristics suggest small changes in melt production and source components over time, superimposed on shallow melt segregation. The Mauna Kea section is subdivided into a thin, upper 107 m sequence of postshield tholeiites, transitional tholeiites and alkali basalts of the Hamakua volcanics, overlying four tholeiitic magma types that are intercalated throughout the rest of the core. These four magma types are recognized on the basis of MgO‐normalized SiO2 and Zr/Nb values. Type‐1 lavas (high SiO2 and Zr/Nb) are ubiquitous below the postshield lavas and are the dominant magma type on Mauna Kea. They are inter‐layered with the other three lava types. Type‐2 lavas (low SiO2 but high Zr/Nb) are found only in the upper core, and especially above 850 m. Type‐3 lavas (low SiO2 and Zr/Nb) are very similar to tholeiites from Loihi volcano and are present only below 1974 m. There are only 3 discrete samples of type‐4 lavas (high SiO2 and low Zr/Nb), which are present in the upper and lower core. The differences between these magma types are inferred to reflect changes in melt production, depth of melt segregation, and differences in plume source components over about 400 ka of Mauna Kea's eruptive history. At the start of this record, eruption rates were high, and two distinct tholeiitic magmas (type‐1 and 3) were erupting concurrently. These two magmas require two distinct source components, one similar to that of modern Loihi tholeiites and the other close to that of Kilauea magmas. Subsequently, the Loihi‐like source of the type‐3 magmas was exhausted, and these lavas are absent from the remainder of the core. For the next 200 ka or so, the eruptive sequence consists of inter‐layered type‐1 and ‐2 lavas that are derived from a common Mauna Kea source, the major difference between the two being the depth at which the melts segregated from the source. At around 440 ka (corresponding with the transition in the core from submarine to subaerial lavas) eruption rates began to decline and low‐MgO lavas are suddenly much more abundant in the record. Continuing gradual decline in melting and eruption rates was accompanied by a decline in normalized SiO2 content of the type‐1 magmas, and the eventual onset of postshield magmatism.

Rhenium volatility in subaerial lavas: constraints from subaerial and submarine portions of the HSDP-2 Mauna Kea drillcore

High Re abundances in mid-ocean ridge basalts (MORB) relative to primitive mantle (ave. MORB[Re]≈930 ppt; PM[Re]≈260 ppt) indicate that Re behaves as an incompatible element during MORB generation. However, contrary to expectations for an incompatible element, average Re abundances in subaerial ocean island and arc lavas (∼330 ppt and ∼190 ppt, respectively) are much lower than in MORB. Previous studies have argued that the low Re abundances in ocean island and arc lavas reflect greater Re compatibility during melt generation, caused by higher modal abundances of garnet and sulfide. However, available partitioning data for Re suggest that reasonable variations in the modal abundances of these phases cannot fully account for the observed differences in Re abundance in different tectonic settings. Higher modal garnet in the sources of ocean island and arc lavas cannot produce the observed low Re/Yb ratios (relative to MORB), because available data suggest that Re is less compatible in garnet than Yb. High sulfide abundances can in principle produce the observed Re depletions, but would require extremely high sulfur contents in the sources of ocean island and arc lavas (>1000 ppm). Large differences in modal sulfide abundance in different tectonic settings are also contradicted by the similarity of MgO–Os trends in ocean island, arc, and mid-ocean ridge samples. An alternative explanation for the low Re abundances in ocean island and arc settings is that much of the Re initially contained in these (largely subaerial) lavas escaped as volatile Re-oxide or Re-chloride species during magma degassing prior to or during subaerial eruption. The ∼3 km deep HSDP-2 Mauna Kea drillcore on Hawaii provides a unique opportunity to examine the effects of magma degassing on Re abundances. The upper ∼800 m of the Mauna Kea portion of the drillcore is composed of subaerial lavas, whereas the lower 2 km is submarine. Os-isotopes are nearly constant throughout the core ( 187Os/ 188Os=0.128–0.130), so large variations in source composition (e.g. Re content) over the period sampled by the core are unlikely. Rhenium abundances in the subaerial lavas are consistently low (ave. Re≈180 ppt). In contrast, Re abundances in the submarine lavas increase systematically with increasing depth, ranging from an average of ∼300 ppt in samples emplaced at <1000 m below sea level (mbsl) up to ∼755 ppt in lavas from >1000 mbsl. Rhenium/ytterbium ratios also increase with core depth, ranging to values significantly exceeding those observed in MORB. These trends are best explained by progressive Re loss in the subaerial and shallow submarine lavas during melt degassing. Assuming that the deepest submarine lavas are unaffected by Re loss during degassing, the subaerial Mauna Kea lavas appear to have lost on average ∼80% of their initial Re. The systematic differences in Re abundance in ocean island, arc, and mid-ocean ridge lavas may therefore reflect the fact that most analyzed ocean island and arc samples are subaerial and therefore degassed, whereas MORB retain a greater fraction of their volatile inventory.

Oxygen isotope geochemistry of the second HSDP core

Oxygen isotope ratios were measured in olivine phenocrysts (∼1 mm diameter), olivine microphenocrysts (generally ∼100–200 μm diameter), glass, and/or matrix from 89 samples collected from depths down to 3079.7 m in the second, and main, HSDP core (HSDP‐2). Olivine phenocrysts from 11 samples from Mauna Loa and 34 samples from the submarine section of Mauna Kea volcano have δ18O values that are similar to one another (5.11 ± 0.10‰, 1σ, for Mauna Loa; 5.01 ± 0.07‰, for submarine Mauna Kea) and within the range of values typical of olivines from oceanic basalts (δ18O of ∼5.0 to 5.2‰). In contrast, δ18O values of olivine phenocrysts from 20 samples taken from the subaerial section of Mauna Kea volcano (278 to 1037 mbsl) average 4.79 ± 0.13‰. Microphenocrysts in both the subaerial (n = 2) and submarine (n = 24) sections of Mauna Kea are on average ∼0.2‰ lower in δ18O than phenocrysts within the same stratigraphic interval; those in submarine Mauna Kea lavas have an average δ18O of 4.83 ± 0.11‰. Microphenocrysts in submarine Mauna Kea lavas and phencrysts in Mauna Loa lavas are the only population of olivines considered in this study that are typically in oxygen isotope exchange equilibrium with coexisting glass or groundmass. These data confirm the previous observation that the stratigraphic boundary between Mauna Loa and Mauna Kea lavas defines a shift from “normal” to unusually low δ18O values. Significantly, they also document that the distinctive 18O‐depleted character of subaerial Mauna Kea lavas is absent in phenocrysts of submarine Mauna Kea lavas. Several lines of evidence suggest that little if any of the observed variations in δ18O can be attributed to subsolidus alteration or equilibrium fractionations accompanying partial melting or crystallization. Instead, they reflect variable proportions of an 18O‐depleted source component or contaminant from the lithosphere and/or volcanic edifice that is absent in or only a trace constituent of subaerial Mauna Loa lavas, a minor component of submarine Mauna Kea lavas, and a major component of subaerial Mauna Kea lavas. Relationships between the δ18O of phenocrysts, microphenocrysts, and glass or groundmass indicate that this component (when present) was added over the course of crystallization‐differentiation. This process must have taken place in the lithosphere and most likely at depths of between ∼5 and 15 km. We conclude that the low‐δ18O component is either a contaminant from the volcanic edifice that was sampled in increasingly greater proportions as the volcano drifted off the center of the Hawaiian plume or a partial melt of low‐δ18O, hydrothermally altered perdotites in the shallow Pacific lithosphere that increasingly contributed to Mauna Kea lavas near end of the volcano's shield building stage. The first of these alternatives is favored by the difference in δ18O between subaerial and submarine Mauna Kea lavas, whereas the second is favored by systematic differences in radiogenic and trace element composition between higher and lower δ18O lavas.

Trace element abundances of Mauna Kea basalt from phase 2 of the Hawaii Scientific Drilling Project: Petrogenetic implications of correlations with major element content and isotopic ratios

The temporal geochemical variations defined by lavas erupted throughout the growth of a single volcano provide important information for understanding how the Hawaiian plume works. The Hawaii Scientific Drilling Project (HSDP) sampled the shield of Mauna Kea volcano to a depth of 3100 meters below sea level during Phase 2 of the HSDP. Incompatible element abundance ratios, such as La/Yb, Sm/Yb, Nb/Zr, and Ti/Zr, in conjunction with SiO2 abundance and radiogenic isotopic ratios, especially He and Pb, in the reference sample suites of the Mauna Kea portion of cores from Phases 1 and 2 of the HSDP define three distinct geochemical groups. The upper 550 m of Mauna Kea lavas in the Phase 2 core include the Postshield Group with eruption ages of ∼200 ka to &lt;370 ka. These lavas have relatively low SiO2 content, 3He/4He and 206Pb/204Pb, and they define a trend to relatively high La/Yb, Sm/Yb, and Nb/Zr. The eruption of these lavas coincides with migration of the Mauna Kea shield off the hot spot. As a result, extent of melting decreased, melt segregation occurred at greater depth, within the garnet stability field, and a geochemically distinct component associated with the periphery of the plume was sampled. Deeper in the Phase 2 core two other geochemical groups of lava are intercalated. One group has relatively low SiO2 abundance and high Nb/Zr Ti/Zr, 3He/4He and high 208Pb/204Pb at a given 206Pb/204Pb. These are distinctive geochemical characteristics of lavas erupted at Loihi seamount. Variations in incompatible element abundance ratios (e.g., Sm/Yb versus Nb/Zr and La/Yb versus Ti/Zr) define mixing trends between these low SiO2 lavas (Loihi‐type) and lavas belonging to a high SiO2 group that are the dominant lava type in the shield part of the core (Kea‐type). These two groups are presumed to reflect components intrinsic to the plume. Correlations of incompatible element abundance ratios, such as La/Nb, with radiogenic isotope ratios show that Hawaiian shields contain different proportions of geochemically distinctive components. The Koolau shield contains a recycled sedimentary component that is not present in the Mauna Kea shield. The anomalously high Ba/Th in Hawaiian lavas is inferred to be a source characteristic. Ba/Th is correlated with some radiogenic isotope ratios in Kilauea and Mauna Loa lavas, but there is no correlation in Mauna Kea lavas which range in Ba/Th by a factor of 2.6.

The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP‐2 drill core

We analyzed Pb isotopic compositions of 50 samples from the HSDP‐2 drill hole, covering the time interval 180 to 550 kyr B.P. in the stratigraphic record of Mauna Kea. All analyses were corrected for instrumental bias using a triple‐spike technique. The aims of this study are to document temporal changes in sources contributing to Mauna Kea and to investigate how these may relate to the chemical structure of the Hawaiian plume. Lead isotopic compositions of the lavas have 206Pb/204Pb ratios ranging from 18.41 to 18.63, 207Pb/204Pb from 15.47 to 15.49, and 208Pb/204Pb from 37.97 to 38.22. In 207Pb/204Pb‐206Pb/204Pb space, the samples display a broad linear array, while three distinct arrays are found in 208Pb/204Pb‐206Pb/204Pb space. These arrays can clearly be distinguished by their 208Pb/204Pb ratios and are referred to as “Kea‐lo8,” “Kea‐mid8,” and “Kea‐hi8.” The 206Pb/204Pb isotope ratios exhibit rapid shifts by ∼0.2 over 100 m depth intervals, and jumps from one Pb isotope array to another and back in less than ∼100 m depth. Despite these rapid Pb isotope fluctuations, a particular Pb isotope array dominates over periods of several tens to hundreds of kiloyears. We interpret the Pb isotope arrays found in HSDP‐2 in terms of mixing of end‐members lying along the radiogenic and unradiogenic extensions of the arrays. At the radiogenic extension the three HSDP‐2 arrays converge to a common end‐member. The lower extensions of the arrays diverge in three directions, each with different 208Pb/204Pb ratios. This topology suggests that the HSDP‐2 arrays were produced by mixing of at least four end‐members. The origin of these end‐members was investigated using Monte Carlo simulations of a Pb isotope evolution model. The simulations suggest that the common radiogenic end‐member of the three Pb isotope arrays contains material with elevated μ values and has a relatively young age (&lt;1.5 Ga). Such a signature can be plausibly interpreted in terms of the presence of recycled oceanic crust in the source. The HSDP‐2 Kea‐lo8, Kea‐mid8, and Kea‐hi8 Pb isotope arrays dominate over different time periods and can be related to the displacement of Mauna Kea relative to the plume center over time. The Kea‐lo8 array is present between ∼180 and 370 ka and samples more peripheral parts of the plume, while the Kea‐mid8 and Kea‐hi8 arrays occur in the deeper parts of the core (∼370 to 550 kyr ago), when Mauna Kea was closer to the plume center. Over the time intervals when each array dominates, we derive corresponding “lengths” of materials in the source by integrating the estimated upwelling velocity across the plume. These calculations suggest Pb isotope heterogeneities of at least several tens of kilometers in vertical length within the Hawaiian plume. The Pb isotope arrays may correspond to relatively small‐scale heterogeneities derived from the D″ layer in the lower mantle.

Lithium isotope geochemistry of the Hawaiian plume: Results from the Hawaii Scientific Drilling Project and Koolau Volcano

We determined lithium isotopic compositions of Mauna Loa and Mauna Kea basalts from the 3.1 km drill hole of the Hawaiian Scientific Drilling Project (HSDP); for comparison Li isotopic ratios were also determined for basalts from Koolau volcano. These two suites of samples define geochemical extremes in the range of Hawaiian shield lavas. The 400 Ka record of Mauna Kea in the HSDP core shows temporal fluctuations between low δ7Li (∼4‰ relative to the L‐SVEC standard) and high δ7Li (5–6‰), suggesting that the source components in the Hawaiian plume are heterogeneous in Li isotopic composition. Based on SiO2 content and isotopic ratios of He, Li, Nd, Hf and Pb, three geochemical groups are identified in Mauna Kea lavas. Mauna Kea basalts between 1900 and 2500 mbsl have relatively low δ7Li of about 4‰. They are low SiO2 lavas distinguished by the highest 3He/4He and 208Pb/204Pb, and low 176Hf/177Hf and 143Nd/144Nd. Like basalt from Loihi seamount, this Mauna Kea group is considered to originate from the core of the plume. Above 1900 mbsl, high δ7Li lavas with high SiO2 contents appear in both the submarine and subaerial sections. They are marked by low 3He/4He and high 176Hf/177Hf. The 7Li‐rich signature of some samples (δ7Li up to 5.7) is indicative of recycled oceanic crust in the plume. This magma group defines the Kea component. The low SiO2 lavas in the subaerial section have low δ7Li (∼4‰), 3He/4He and 208Pb/204Pb. Their δ7Li values overlap the range of δ7Li in unaltered mid‐ocean ridge basalt (MORB) and are consistent with upper mantle material entrained by the plume or contamination of plume‐derived magmas by the Pacific lithosphere. The δ7Li of Koolau lavas mostly fall within the range of 4.5 ± 0.3‰. Exceptions are two samples that have δ7Li of 2–3‰. The lightest isotopic values may indicate subducted Li that was isotopically fractionated during slab dehydration. In contrast to other isotopic systems, most Koolau samples, however, resemble Mauna Kea samples in Li isotopic composition. Mauna Loa samples have δ7Li values of 3.5 to 4.9‰, within the range of the Koolau and Mauna Kea lavas. Based on these data, the Loa trend volcanoes and Kea trend volcanoes have largely overlapping Li isotopic compositions. In summary, the Hawaiian plume is not highly variable in Li isotopic composition; δ7Li is typically ∼4‰ with perturbations by subducted components to lower and higher ratios (2.5 to 5.7‰). The overlap of most Hawaiian basalt and MORB in their range of Li isotopic ratios suggests minor influence of recycled oceanic crust in the plume and perhaps similar Li isotopic ratios in the upper and lower mantle.

Characteristic magnetic behavior of subaerial and submarine lava units from the Hawaiian Scientific Drilling Project (HSDP‐2)

This study presents rock magnetic properties and the magnetic mineralogy of subaerial and submarine lava flows of Mauna Loa and Mauna Kea volcanoes collected from the 3109 m deep HSDP‐2 drill hole in Hawaii. Three different groups of magnetic behavior are recognized in the subaerial lava flows related to the degree of high temperature oxidation during extrusion. Group 1 shows homogenous titanomagnetite with low Xmt, low Curie temperatures (TC: 100°–200°C) and weak median demagnetizing fields (&lt; 20 mT). Further subdivision into 1a and 1b subgroups is based on the low temperature behavior of magnetic susceptibility (MS) and hysteresis loops, which indicate a contribution from ferrimagnetic Cr‐Al spinel below ca. −160°C in the 1b‐type samples. Group 2 samples, with exsolution lamellae of ilmenite in the titanomagnetites, have higher TC (480°–580°C) and higher coercive forces (20–40 mT). Group 3, the highest oxidation stage, is characterized by titanohematite‐bearing assemblages with enhanced median demagnetizing fields (35–85 mT) and a significantly different low‐temperature MS behavior. MS core logging shows a systematic variation occurs in the subaerial lava flows, directly related to the degree of high temperature oxidation and their flow morphology. Aa lava flows have higher mean MS than other lava flow types. Besides these factors, MS appears to be also affected by the magma composition of the various shield‐building stages. Mauna Loa subaerial lava flows generally show lower mean susceptibilities (4.6 ± 3 × 10−3 SI) than subaerial Mauna Kea lava flows (9.8 ± 5 × 10−3 SI). As submarine lava flows show no group 3 assemblages no high temperature oxidation influenced these rocks. Some hyaloclastites and pillow breccias show low MS (&lt; 1 × 10−3 SI), small amounts of nearly pure magnetite (TC = 580°C) and high coercive forces up to 110 mT suggesting single domain and/or superparamagnetic behavior. The controlling mechanism of the magnetic properties in the submarine lava units is the cooling and quenching rate of lava flows, which creates large grain size variations in titanomagnetites of varying compositions. Hydrothermal alteration, as described from ocean floor or Icelandic basalts, is not an important process that influences the magnetic properties in the ocean island basalts from the HSDP‐2 drill hole.