Loa Trend Volcanoes Research Articles

Petrology and Geochemistry of Volcanic Rocks from the South Kaua`i Swell Volcano, Hawai`i: Implications for the Lithology and Composition of the Hawaiian Mantle Plume

The South Kauai Swell (SKS) volcano was sampled during four JASON dives and three dredge hauls recovering rocks that range from fresh pillow lavas to altered volcanic breccias. Two geo- chemical groups were identified: shield-stage tholeiites (5� 4-3� 9 Ma) and rejuvenation-stage alkalic lavas (1� 9-0� 1 Ma). The young SKS ages and the coeval rejuvenated volcanism along a 400 km segment of the Hawaiian Islands (Maui to Niihau) are inconsistent with the timing and duration predictions by the flexure and secondary plume melting models for renewed volcanism. The SKS tholeiites are geochemically heterogeneous but similar to lavas from nearby Kauai, Niihau and Waianae volcanoes, indicating that their source regions within the Hawaiian mantle plume sampled a well-mixed zone. Most SKS tholeiitic lavas exhibit radiogenic Pb isotope ratios ( 208 Pb*/ 206 Pb*) that are characteristic of Loa compositions (>0� 9475), consistent with the volcano's location on the west side of the Hawaiian Islands. These results document the existence of the Loa component within the Hawaiian mantle plume prior to 5 Ma. Loa trend volcanoes are thought to have a major pyroxenite component in their source. Calculations of the pyroxenitic component in the parental melts for SKS tholeiites using high-precision olivine analyses and modeling of trace element ratios indicate a large pyroxenite proportion (� 50%), which was predicted by recent nu- merical models. Rejuvenation-stage lavas were also found to have a significant pyroxenite compo- nent based on olivine analyses (40-60%). The abundance of pyroxenite in the source for SKS lavas may be the cause of this volcano's extended period of magmatism (>5 Myr). The broad distribution of the Loa component in the northern Hawaiian Island lavas coincides with the start of a dramatic magma flux increase (300%) along the Hawaiian Chain, which may reflect a major structural change in the source of the Hawaiian mantle plume.

The distribution of geochemical heterogeneities in the source of Hawaiian shield lavas as revealed by a transect across the strike of the Loa and Kea spatial trends: East Molokai to West Molokai to Penguin Bank

An important feature of <2Ma Hawaiian volcanoes is that they define two sub-parallel spatial trends known as the Loa- and Kea-trends. On the Island of Hawaii, the <1.5Ma shield lavas on the Loa and Kea spatial trends have distinctive geochemical characteristics that are designated as Loa-type and Kea-type. These geochemical differences are clearly expressed in Sr, Nd, Hf and Pb isotopic ratios, major element contents, and ratios of incompatible elements. They are interpreted to reflect varying proportions of sediment, basalt, gabbro and peridotite in subducted oceanic lithosphere. Pb isotopic ratios indicate that the Loa-type component reflects ancient subduction, >2.5Ga, whereas the Kea-type component reflects younger subduction, <1.5Ga. To evaluate the temporal persistence of these geochemical differences in the source of Hawaiian shield lavas, we analyzed lavas from the ∼1.5 to 2Ma Molokai Island volcanoes, East and West Molokai, and the adjacent submarine Penguin Bank. The three volcanoes form a nearly east–west trend that crosscuts the Loa and Kea spatial trends at a high angle; consequently we can determine if these older lavas are Kea-type in the east and Loa-type in the west. All lavas collected from the subaerial flanks of East Molokai, a Kea-trend volcano, have Kea-type geochemical characteristics; however, dive samples collected from Wailau landslide blocks, probably samples of the East Molokai shield that are older than those exposed on the subaerial flanks, include basalt with Loa-type geochemical features. Shield lavas from West Molokai and Penguin Bank, both on the Loa-trend, are dominantly Loa-type, but samples with Kea-type compositions also erupted at these Loa-trend volcanoes. The Loa-trend volcanoes, Mahukona, West Molokai, Penguin Bank, and Koolau, have also erupted lavas with Kea-type geochemical characteristics, and the Kea-trend volcanoes, Mauna Kea, Kohala, Haleakala, and East Molokai, have erupted lavas with Loa-type geochemical characteristics. The presence of both Loa- and Kea-type lavas in a volcano provides constraints on the distribution of geochemical heterogeneities in the source of Hawaiian shield basalts. Two plausible models are: (1) source components with Loa- and Kea-type geochemical characteristics are present in the sources of all <2Ma shields, but the Kea-to-Loa proportion is higher beneath Kea-trend than Loa-trend volcanoes, or (2) the magma source contains a uniform proportion of Loa- and Kea-type components, but these components have different solidi. Magmas derived from the low-temperature regions of the source preferentially sample the component with the lower solidus temperature and form Loa-type lavas. In contrast, magmas derived from the relatively high-temperature regions of the source sample both low and high solidus components in the source and form Kea-type lavas. This model is supported by the linear correlations between isotopic ratios and calculated temperatures of estimated primary magmas.

Sr, Nd, Hf and Pb isotope systematics of postshield-stage lavas at Kahoolawe, Hawaii

We report high-precision Sr, Nd, Hf and Pb isotope compositions for twenty-one postshield-stage and two post-caldera lavas from Kahoolawe, a Loa-trend Hawaiian volcano. Kahoolawe postshield- and shield-stage lavas have overlapping though highly heterogeneous Sr, Nd, Hf and Pb isotope compositions, implying that the shield- and postshield-stage volcanism at Kahoolawe sampled the same isotopically heterogeneous mantle source. This differs from that of most other Hawaiian volcanoes, such as Haleakala, Mauna Kea, and Hualalai, whose shield-to-postshield transitions are characterized by shifts to lower 87Sr/86Sr and higher 143Nd/144Nd. There are correlations between CaO, Sc and V contents and radiogenic isotope compositions within Kahoolawe postshield-stage lavas. For example, Sc abundance is negatively correlated with 87Sr/86Sr, and positively correlated with εNd and εHf; V abundance is positively correlated with εNd, εHf, and 206Pb/204Pb. Element-isotope correlations are also observed in Mauna Kea postshield-stage lavas: Sc and V abundances are negatively correlated with εHf and 206Pb/204Pb, and positively correlated with εNd. These trends may be due to magma–magma mixing. That is, in addition to clinopyroxene fractionation to account for the low CaO, Sc and V contents in some postshield-stage lavas, partial melts of eclogite/garnet pyroxenite, characterized by low CaO, Sc and V contents, may also be part of the petrogenesis of Kahoolawe postshield-stage lavas.It is well established that lavas erupted at the geographically defined Loa- and Kea-trend volcanoes have different isotopic and geochemical compositions. Specifically, compared to the Kea-trend lavas, Loa-trend lavas have higher 208Pb/204Pb at a given 206Pb/204Pb. However, cases exist of both shield- and postshield-stage volcanism where Kea-type isotopic signatures are present in Loa-trend volcanoes and the reverse. We propose that Loa- and Kea-type source components are present beneath both Loa- and Kea-trend volcanoes in such a way that the average source compositions of Loa-trend volcanoes have a Loa-type isotopic signature, and that of the Kea-trend volcanoes have a Kea-type isotopic signature. When the size of the magma capture zone is much larger than that of the source components, the erupted lavas have the average compositions of the source. If the size of the magma capture zone is comparable to that of the source components, the erupted lavas could have either Loa- or Kea-type isotopic signatures.

Modeling volcano growth on the Island of Hawaii: Deep-water perspectives

Recent ocean-bottom geophysical surveys, dredging, and dives, which complement surface data and scientific drilling at the Island of Hawaii, document that evolutionary stages during volcano growth are more diverse than previously described. Based on combining available composition, isotopic age, and geologically constrained volume data for each of the component volcanoes, this overview provides the first integrated models for overall growth of any Hawaiian island. In contrast to prior morphologic models for volcano evolution (preshield, shield, postshield), growth increasingly can be tracked by age and volume (magma supply), defining waxing alkalic, sustained tholeiitic, and waning alkalic stages. Data and estimates for individual volcanoes are used to model changing magma supply during successive compositional stages, to place limits on volcano life spans, and to interpret composite assembly of the island. Volcano volumes vary by an order of magnitude; peak magma supply also varies sizably among edifices but is challenging to quantify because of uncertainty about volcano life spans. Three alternative models are compared: (1) near-constant volcano propagation, (2) near-equal volcano durations, (3) high peak-tholeiite magma supply. These models define inconsistencies with prior geodynamic models, indicate that composite growth at Hawaii peaked ca. 800–400 ka, and demonstrate a lower current rate. Recent age determinations for Kilauea and Kohala define a volcano propagation rate of 8.6 cm/yr that yields plausible inception ages for other volcanoes of the Kea trend. In contrast, a similar propagation rate for the less-constrained Loa trend would require inception of Loihi Seamount in the future and ages that become implausibly large for the older volcanoes. An alternative rate of 10.6 cm/yr for Loa-trend volcanoes is reasonably consistent with ages and volcano spacing, but younger Loa volcanoes are offset from the Kea trend in age-distance plots. Variable magma flux at the Island of Hawaii, and longer-term growth of the Hawaiian chain as discrete islands rather than a continuous ridge, may record pulsed magma flow in the hotspot/plume source.

186Os– 187Os systematics of Hawaiian picrites revisited: New insights into Os isotopic variations in ocean island basalts

Eighteen picrites (MgO > 13 wt.%) and three related basalts from six Hawaiian volcanoes were analyzed for 187Os/ 188Os and 186Os/ 188Os. Variations in these ratios reflect long-term Re/Os and Pt/Os differences in the mantle source regions of these volcanoes. 187Os/ 188Os ratios vary from ∼0.129 to 0.136, consistent with the range defined by previous studies of Hawaiian picrites and basalts. Samples with lower 187Os/ 188Os are mainly from Kea trend volcanoes (Mauna Kea and Kilauea), and the more radiogenic samples are mainly from Loa trend volcanoes (Mauna Loa, Hualalai, Koolau and Loihi). As previously suggested, differences in 187Os/ 188Os between volcanic centers are most consistent with the presence of variable proportions of recycled materials and/or pyroxenitic components in the Hawaiian source. 186Os/ 188Os ratios vary from 0.1198332 ± 26 to 0.1198480 ± 20, with some samples having ratios that are significantly higher than current estimates for the ambient upper mantle. Although the range of 186Os/ 188Os for the Hawaiian suite is consistent with that reported by previous studies, the new data reveal significant heterogeneities among picrites from individual volcanoes. The linear correlation between 187Os/ 188Os and 186Os/ 188Os reported by a previous study is no longer apparent with the larger dataset. The postulated recycled materials and pyroxenites responsible for the dominant variations in 187Os/ 188Os are likely not responsible for the variations in 186Os/ 188Os. Such materials are typically characterized by both insufficiently high Os concentrations and Pt/Os to account for the 186Os/ 188Os heterogeneities. The lack of correspondence between 186Os/ 188Os variations and the Kea and Loa trends supports this conclusion. The primary cause of 186Os/ 188Os variations are evaluated within the framework of two mixing scenarios: (1) metasomatic transport of Pt and/or 186Os-rich Os into some portions of the Hawaiian source, and (2) interaction between an isotopically complex plume source with a common, Os- and 186Os-enriched reservoir (COs). Both scenarios require large scale, selective transport of Pt, Re and/or Os. Current estimates of HSE concentrations in the mantle source of these rocks, however, provide little evidence for either process, so the dominant cause of the 186Os/ 188Os variations remains uncertain.

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

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

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.

Source materials for inception stage Hawaiian magmas: Pb‐He isotope variations for early Kilauea

New noble gas and radiogenic isotopic compositions are presented for tholeiitic, transitional, and alkalic rocks from the submarine Hilina region on the south flank of Kilauea, Hawaii. The 3He/4He ratios for undegassed glass and olivine separates (11–26 Ra) contrast with those of postshield and rejuvenated alkalic lavas, consistent with the alkalic and transitional basalts at Hilina corresponding to early Kilauea magmas. Most early Kilauea samples contain highly radiogenic Pb isotopes compared with other Hawaiian rocks and therefore derive from a Hawaiian plume end‐member source (here referred to as the Hilina component) distinctive in that respect. Besides radiogenic Pb isotopes, the Hilina component has relatively low 3He/4He (&lt;12 Ra) among the Hawaiian magmas. Hawaiian inception stage magmas, including Hilina, Loihi, and deep Hana Ridge (east Maui), define a linear array in 206Pb/204Pb‐3He/4He isotope space, indicating that mixing between the Hilina and Loihi components (or their melts) dominates magmatism at the leading edge of the Hawaiian plume. The Hilina component's isotopic characteristics can be derived from young subduction‐recycled crust or metasomatised mantle. The isotopic differences between the geographically discriminated Kea and Loa trend volcanic chains, observed in shield stage lavas, are also seen in the inception stage magmas, suggesting that proportions of melts derived from the Hilina and Loihi components were different between the Kea and Loa trend volcanoes.

Geochemical Differences of the Hawaiian Shield Lavas: Implications for Melting Process in the Heterogeneous Hawaiian Plume

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

Tungsten in Hawaiian picrites: A compositional model for the sources of Hawaiian lavas

Concentrations of tungsten (W) and uranium (U), which represent two of the most highly incompatible elements during mantle melting, have been measured in a suite of Hawaiian picrites and primitive tholeiites from nine main-stage shield volcanoes. Tungsten abundances in the parental melts are estimated from correlations between sample W abundances and MgO contents, and/or by olivine correction calculations. From these parental melt determinations, along with independent estimates for the degree of partial melting at each volcanic center, we extrapolate the W content of the mantle sources for each shield volcano. The mantle sources of Hualalai, Mauna Loa, Kohala, Kilauea, Mauna Kea, Koolau and Loihi contain 9 ± 2 (2 σ), 11 ± 5, 10 ± 4, 12 ± 4, 10 ± 5, 8 ± 7 and 11 ± 5 ng/g, respectively. When combined, the mean Hawaiian source has an average of 10 ± 3 ng/g W, which is three-times as enriched as the Depleted MORB Mantle (DMM; 3.0 ± 2.3 ng/g). The relatively high abundances of W in the mantle sources that contribute to Hawaiian lavas may be explained as a consequence of the recycling of W-rich oceanic crust and sediment into a depleted mantle source, such as the depleted MORB mantle (DMM). However, this scenario requires varying proportions of recycled materials with different mean ages to account for the diversity of radiogenic isotope compositions observed between Kea- and Loa-trend volcanoes. Alternatively, the modeled W enrichments may also reflect a primary source component that is less depleted in incompatible trace elements than the DMM. Such a source would not necessarily require the addition of recycled materials, although the presence of some recycled crust is permitted within our model parameters and likely accounts for some of the isotopic variations between volcanic centers. The physical admixture of ⩽0.5 wt.% outer core material with the Hawaiian source region would not be resolvable via W source abundances or W/U ratios; however, W isotopes may provide a more sensitive to this mixing process. Recent W isotopic studies show no indication of core–mantle interaction, indicating that either such a process does not occur, or that mechanisms other than physical mixing may operate at the core–mantle boundary.

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.