Hawaiian Plume Research Articles

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 (<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.

Kahoolawe Island, Hawaii: The role of an ‘inaccessible’ shield volcano in the petrology of the Hawaiian islands and plume

Kahoolawe volcano (∼10×17 km) forms one of the eight major Hawaiian islands. Access for geologic sampling has long been restricted due to military and preservation policies. However, limited visits to Kahoolawe in the 1980s yielded >200 samples, many of which have since been used to study the volcano within the framework of Hawaiian shield and mantle source geochemistry, petrology, mineralogy, and igneous processes. Kahoolawe is a tholeiitic shield with tholeiitic caldera-filling lavas, and at least seven postshield vents that erupted tholeiitic and (sparse) alkalic lavas. On smaller scales are a gabbro intrusion and ultramafic and gabbroic xenoliths in some postshield lavas. There is no evidence for rejuvenated volcanism. In its structural setting, Kahoolawe lies along the “Loa” trend of Hawaiian shields. Major element compositions of shield and caldera-filling lavas are similar and cluster at ∼6–7 wt% MgO, range from ∼5.5 to 16 wt% MgO, and include ∼9 wt% MgO examples that can be modeled as parental to the evolved lavas. For example, least squares mass balancing demonstrates that from ∼15% to 30% crystallization of olivine (±orthopyroxene), clinopyroxene, and plagioclase accounts for the ∼5.5–6 wt% MgO range of tholeiitic lavas. Greater differentiation occurred in the gabbro (diabasic) intrusive body as a segregation vein with ∼2.9 wt% MgO, and extreme differentiation produced local, small-volume rhyolitic melts that segregated into lava vesicles. Postshield lavas are mainly tholeiitic, have ∼5–7 wt% MgO, and overlap shield compositions. Some are alkalic, as low as ∼3.9 wt% MgO (hawaiite), and can be modeled as liquids after a ∼9 wt% MgO alkalic magma crystallized ∼30% olivine, clinopyroxene, plagioclase, and magnetite. Important aspects of Sr, Nd, Hf, and Pb isotopic ratios in Kahoolawe shield and caldera-filling lavas are slightly higher 87Sr/ 86Sr than in Koolau shield lavas (Oahu island; Makapuu-stage; e.g., Koolau mantle ‘endmember’) when compared at a given 143Nd/ 144Nd (e.g., ∼0.7042 at 0.5128), 206Pb/ 204Pb largely at the low end of the range for Hawaiian shields (e.g., ∼18), and ε Hf generally comparable to the values of other Hawaiian shields and ocean islands (e.g., ε Hf 8 at ε Nd 4). The isotopic ratios overall suggest small-scale source heterogeneity, considering the island size, and that Kahoolawe shield and caldera lavas were derived from a Hawaiian plume source containing recycled oceanic crust of gabbro and sediments. Based on certain geochemical indicators, however, such as Ce/Sr and La/Nb vs. 87Sr/ 86Sr, the source contained slightly less gabbro component than other shield sources (e.g., Koolau). Isotopic data for Kahoolawe postshield lavas are scarce, but those available generally overlap the shield data. However, ratios among certain alteration-resistant incompatible trace elements (e.g., Zr/Nb) discriminate some postshield alkalic from shield lavas. But because the different ratios for those postshield lavas can be explained by smaller partial-melting percentages of the shield source and by differentiation, neither isotopes nor trace elements identify postshield magmas as originating in a source unlike that for the shield lavas.

Late Shield-Stage Silicic Magmatism at Wai'anae Volcano: Evidence for Hydrous Crustal Melting in Hawaiian Volcanoes

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

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.

Sea-Floor Study Gives Plumes From the Deep Mantle a Boost

On page 1388 of this week's issue of Science , eight researchers offer the most detailed seismic imaging yet as evidence for a so-called mantle plume beneath the world's most iconic hot spot, the island of Hawaii. The quality of the data and the apparent Hawaiian plume's resemblance to theorists' expectations have won some cautious support for the work.

Ultra-depleted melts from Kamchatkan ophiolites: Evidence for the interaction of the Hawaiian plume with an oceanic spreading center in the Cretaceous?

We report new data on the major and trace element composition of melt inclusions in spinel phenocrysts (Mg# = 0.7–0.8, Cr/(Cr + Al) = 0.32–0.52, TiO 2 = 0.06–0.60 wt.%) from Cretaceous MORB-like basalt (La/Yb = 0.94, Th/Nb = 0.055, Th/La = 0.041) in the Kamchatsky Mys ophiolites (Eastern Kamchatka). The melt inclusions preserved primitive melts (Mg# up to 0.72), which are remarkably depleted in incompatible trace elements compared to common MORBs. Numerous ultra-depleted inclusions from the studied sample have extraordinarily low Na 2O (0.20–0.67 wt.%), TiO 2 (0.16–0.5 wt.%), K (1.5–25 ppm), La (0.015–0.040 ppm), Zr (0.9–2 ppm), B (0.01–0.03 ppm), Ti/Zr = 300–1074, La/Yb = 0.008–0.075 and represent the most depleted melts known until now. The ultra-depleted melts from the Kamchatkan ophiolites are only comparable to a single melt inclusion from MORB of 9°N Mid-Atlantic Ridge [Sobolev and Shimizu, Nature 363 (1993) 151–154] yet have higher FeO, CaO, heavy rare-earth element (Dy, Er, Yb) contents and lower Na 2O and SiO 2. These melts, possibly the last melt fractions produced in an upwelling mantle column, could represent the highest degrees (up to ~ 20%) of near-fractional melting of mantle with T p ≥ 1400 °C, which started melting at ~ 75 km depth and continued to shallow depths of ~ 20 km. The presence of melts ranging in composition from ultra-depleted to compositions similar to Mauna Loa Volcano, Hawaii, high potential mantle temperature and association with rocks akin the Cretaceous Hawaiian tholeiites suggest that the trace element depleted melts preserved in spinel phenocrysts could have originated from extensive melting of a depleted component intrinsic to the Hawaiian plume or ambient upper mantle entrained and heated up at the plume margins.

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.

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.

Thermal and hydrodynamic relationships between the melt in the thermochemical plume conduit and the horizontal mantle flow

When modeling the formation and dynamics of thermochemical plumes, geologists and geophysicists of the world use equations of free convection in the Boussinesq approximation with addition of the mass transfer equation [1]. It is assumed that the base of the mantle corresponds to a dense chemical layer that was formed owing primarily to accretion of the oceanic crust in the lower mantle. The plume is forming in the thermal boundary zone in the presence of this dense chemical layer at the base of the mantle. The ascending migration of the plume under thermal and concentration gravitational convection is stimulated by the high temperature gradient between the core‐mantle interface and the surrounding mantle (750 K). The ascending movement of the plume is accompanied by the capture of material from the dense chemical layer. Our model of the thermochemical plume [2] differs from that accepted in [1]. Its difference from the latter is discussed in detail elsewhere [3]. According to our model, thermochemical plumes form at the core‐mantle interface under the influence of the heat flow from the outer core and local influx of a chemical dope that reduces the melting point of the lower mantle substance, which results in mantle substrate melting and upward plume migration [2, 3]. The heat and mass transfer due to interaction between horizontal free convection mantle flows and the conduit of the thermochemical plume located under the oceanic plate away from the mid-oceanic ridge (MAR) is considered in [4] with presentation of the main equations for calculating the heat and mass transfer by the thermochemical plume interacting with the horizontal convective mantle flow and quantitative correlation between the dope concentration at the plume base and the Lewis number. The purpose of this study is calculation of the heat exchange between the conduit of the thermochemical plume ascending from the core‐mantle interface and the horizontal mantle flow. We present the model of the plume conduit interacting with horizontal free convection flows in the upper and lower mantle and determine local coefficients of the heat transfer at the interface between the melt and the surrounding mantle (plume conduit boundary), as well as the diameter of the Hawaiian plume interacting with horizontal mantle flows. Based on the calculated heat exchange between the plume and the horizontal lower mantle flow, we also estimate the main parameters of the lower mantle. Let us consider the possible structure of the conduit and head part of the Hawaiian plume based on experimental modeling and seismic studies of the upper mantle beneath the Island of Hawaii. The thermal power of the Hawaiian plume is estimated to be 3 · 10 11 W, and the Raleigh number (Ra) for the free convection flow in the conduit of the Hawaiian plume is

Variation of Icelandic and Hawaiian magmatism: evidence for co-pulsation of mantle plumes?

Based on published estimates of areal extent, thickness and dating of igneous rocks related to formation of the North Atlantic Volcanic Province, we calculate the magmatic production since the first reported magmatism at 70 Ma until present. We relate the magmatism to the Icelandic Plume and estimate the total volumetric production to 22.1 × 106 km3. The magmatic production varied significantly with time, with a clear maximum of 55.5 m3/s around continental break-up at ca. 54 Ma. The lowest production is estimated at 4 m3/s, increasing to 7 m3/s at ca. 23 Ma. Two other pulses with increased activity are found around 40 Ma and in the late Miocene (10–5 Ma). The variations in productivity are consistent with a plume pulsing with a periodicity of ca. 15 million years. The same periodicity and relative timing is found for the Hawaiian Plume. If confirmed, these observations suggest that both plumes originate within the thin boundary layer near the Core-Mantle Boundary. This hypothesis may imply periodic heating of the earth’s core with subsequent heat-release to the mantle and increased global plume activity.

Carbonatite and silicate melt metasomatism of the mantle surrounding the Hawaiian plume: Evidence from volatiles, trace elements, and radiogenic isotopes in rejuvenated‐stage lavas from Niihau, Hawaii

We present new volatile, trace element, and radiogenic isotopic compositions for rejuvenated‐stage lavas erupted on Niihau and its submarine northwest flank. Niihau rejuvenated‐stage Kiekie Basalt lavas are mildly alkalic and are isotopically similar to, though shifted to higher 87Sr/86Sr and lower 206Pb/204Pb than, rejuvenated‐stage lavas erupted on other islands and marginal seafloor settings. Kiekie lavas display trace element heterogeneity greater than that of other rejuvenated‐stage lavas, with enrichments in Ba, Sr, and light‐rare earth elements resulting in high and highly variable Ba/Th and Sr/Ce. The high Ba/Th lavas are among the least silica‐undersaturated of the rejuvenated‐stage suite, implying that the greatest enrichments are associated with the largest extents of melting. Kiekie lavas also have high and variable H2O/Ce and Cl/La, up to 620 and 39, respectively. We model the trace element concentrations of most rejuvenated‐stage lavas by small degrees (∼1% to 9%) of melting of depleted peridotite recently metasomatized by a few percent of an enriched incipient melt (0.5% melting) of the Hawaiian plume. Kiekie lavas are best explained by 4% to 13% partial melting of a peridotite source metasomatized by up to 0.2% carbonatite, similar in composition to oceanic carbonatites from the Canary and Cape Verde Islands, with lower proportion of incipient melt than that for other rejuvenated‐stage lavas. Primary H2O and Cl of the carbonatite component must be high, but variability in the volatile data may be caused by heterogeneity in the carbonatite composition and/or interaction with seawater. Our model is consistent with predictions based on carbonated eclogite and peridotite melting experiments in which (1) carbonated eclogite and peridotite within the Hawaiian plume are the first to melt during plume ascent; (2) carbonatite melt metasomatizes plume and surrounding depleted peridotite; (3) as the plume rises, silica‐undersaturated silicate melts are also produced and contribute to the metasomatic signature. The metasomatic component is best preserved at the margins of the plume, where low extents of melting of the metasomatized depleted mantle surrounding the plume are sampled during flexural uplift. Formation of carbonatite melts may provide a mechanism to transfer plume He to the margins of the plume.

Geological implications of the thermochemical plume model

This paper reviews available the mantle plume models and formulates seven questions, which can be solved using geological and geochemical data in terms of the thermochemical plume model. The main features of thermochemical plumes, including calculated plume parameters, are presented. It has been estimated how the power of a plume depends on water concentration in melt at variable melt portions and diffusion coefficients (Lewis numbers). A correlation between the latter and temperature drop at the base of a plume has been shown. The obtained plume parameters are compared with model plume localities, such as the Hawaiian and East African plumes, Siberian and Emeishan flood basalts, etc. Based on the oceanic plate movement pattern and He, Os, and Sr isotope data, the Hawaiian plume was proved to be generated within the D 2 lower mantle layer; the diameter of the plume channel is about 100 km, the plume channel rotated and migrated to 100–200 km every 2 Ma; the plume power is about 3⋅10 8 kW, changing each 15 Ma. The reconstruction of complex core material-plume melts and Layer C-asthenosphere interactions is possible only by isotopic data and primary olivine composition. The plume model was applied to large igneous provinces (Siberian, Emeishan, and Ontong Java LIPs) using detailed mechanisms of plume head interaction with the lithosphere. Taking into account the similar age and evolution of the Tunguska syncline (Siberian LIP) and West Siberia, the plume-lithosphere interactions are deduced from the relationships between sedimentary and magmatic units, different modes of rifting, and variations of Sr and Nd isotopes.

Partitioning of Ni between olivine and siliceous eclogite partial melt: experimental constraints on the mantle source of Hawaiian basalts

Olivine is abundant in Earth’s upper mantle and ubiquitous in basaltic lavas, but rarely occurs in eclogite. Partial melts of eclogite are, therefore, not in equilibrium with olivine, and will react with peridotite as they migrate through the upper mantle. If such melts erupt at Earth’s surface, their compositions will be highly modified and they may be olivine-saturated. We investigated experimentally the reaction between olivine and siliceous eclogite partial melt, and determined element partitioning between olivine and the melt produced by this reaction. Our results demonstrate that mixing of reacted eclogite partial melt with primitive basalt is capable of producing the positive correlation between melt SiO2 content and olivine Ni content observed in some Hawaiian lavas. Experiments were carried out by equilibrating eclogite partial melt or basalt with San Carlos olivine at 1 bar and 1,201–1,350°C. Our results show that eclogite partial melts equilibrated with mantle olivine retain their high SiO2, low FeO and MgO characteristics. Further, olivine-melt partition coefficients for Ni measured in these experiments are significantly larger than for basalt. Mixing of these melts with primitive Hawaiian tholeiitic lavas results in crystallization of high-Ni olivines similar to those in Makapuu-stage Koolau lavas, even though the mixed magmas have only moderate Ni contents. This results from a hyperbolic increase of the Ni partition coefficient with increasing polymerization of the mixed melt. Note that while eclogite partial melt in contact with peridotite will equilibrate with pyroxene as well as olivine, this will have the effect of buffering the activity of SiO2 in the reacted melt at a higher level. Therefore, an eclogite partial melt equilibrated with harzburgite will have higher SiO2 than one equilibrated with dunite, enhancing the effects observed in our experiments. Our results demonstrate that an olivine-free “hybrid” pyroxenite source is not required to explain the presence of high-Ni olivines in Hawaiian lavas and, therefore, indicate that the proportion of eclogite in the Hawaiian plume is less than has been estimated in recent studies.

Geochemical Variations during Kilauea's Pu'u 'O'o Eruption Reveal a Fine-scale Mixture of Mantle Heterogeneities within the Hawaiian Plume

Geochemical Variations during Kilauea's Pu'u 'O'o Eruption Reveal a Fine-scale Mixture of Mantle Heterogeneities within the Hawaiian Plume

Weak thermal convection within tilted plume conduits

Shear from flow in the mantle tilts the conduits of mantle plumes. The density stratification at the top of the plume conduit is then unstable in cross section with cold material over hot. This stratification drives cross‐stream convection. Such convection including the Stokes' law rise of the plume conduit through the ambient mantle readily mixes isoviscous thermal plumes with their surroundings. Laboratory experiments by Kerr and Mériaux (2004) indicate that cross‐stream convection is weak within plumes with strongly temperature‐dependent viscosity and significant ∼56 viscosity contrasts. Analogy with stagnant‐lid convection provides scaling relationships with a Rayleigh number for cross‐stream convection within tilted plume conduits. Only material within a factor of a few e of the central viscosity of the plume convects significantly. The scaling analysis shows that the laboratory experiments by Kerr and Mériaux (2004) had the proper parameter range for the Hawaiian plume. The petrological conclusion that the plume conduit has been only mildly stirred by cross‐stream convection is reasonable. Cross‐stream convection is less likely for low‐flux plumes, like Galapagos, than high‐flux plumes and plumes in the deep mantle compared with those in the upper mantle.

Hawaiian double volcanic chain triggered by an episodic involvement of recycled material: Constraints from temporal Sr–Nd–Hf–Pb isotopic trend of the Loa-type volcanoes

The two parallel loci of recent Hawaiian volcanoes, Kea and Loa, have been regarded as the best targets to interpret the chemical structure of an upwelling mantle plume derived from the lower mantle. Here we show that the Sr–Nd–Hf–Pb isotopic data of the shield-building lavas along the Loa locus form a systematic trend from the main shield stage of Koolau (> 2.9 Ma) to the active Loihi volcanoes. During the growth of the Koolau volcano, the dominant material in the melting region successively changed from the proposed KEA, DMK (depleted Makapuu), to EMK (enriched Makapuu) components. The proportion of EMK, dominated by a recycled mafic component, is typified by some Koolau Makapuu-stage and some Lanai lavas. Subsequently, the EMK component decreased and LOIHI component increased toward the Loihi lavas. The temporal coincidence between the episodically elevated magma production rate and the abrupt appearance of the typical Loa-type lavas that is restricted to the last 3 Myr should be linked to magma genesis. We suggest that the abrupt appearance of Loa-type magmatism should be attributed to the transient incorporation of the relatively dense recycled material and surrounding less degassed lower mantle material that accumulated near the core–mantle boundary into the upwelling plume. This episodic involvement could have been trigged by episodic thermal pulses and buoyancy increases in the plume. The continuous appearance of Kea-type lavas during the long history of Hawaiian-chain magmatism and the larger magma volume of Kea-type lavas relative to that of the Loa-type lavas in the last 3 Myr indicate that the Kea locus is closer to the thermal centre of the Hawaiian plume relative to that of the Loa locus.

Geochemical characteristics of West Molokai shield‐ and postshield‐stage lavas: Constraints on Hawaiian plume models

There are systematic geochemical differences between the <2 Myr Hawaiian shields forming the subparallel spatial trends, known as Loa and Kea. These spatial and temporal geochemical changes provide insight into the spatial distribution of geochemical heterogeneities within the source of Hawaiian lavas, and the processes that create the Hawaiian plume. Lavas forming the ∼1.9 Ma West Molokai volcano are important for evaluating alternative models proposed for the spatial distribution of geochemical heterogeneities because (1) the geochemical distinction between Loa and Kea trends may end at the Molokai Fracture Zone and (2) West Molokai is a Loa‐trend volcano that has exposures of shield and postshield lavas. This geochemical study (major and trace element abundances and isotopic ratios of Sr, Nd, Hf, and Pb) shows that the West Molokai shield includes lavas with Loa‐ and Kea‐like geochemical characteristics; a mixed Loa‐Kea source is required. In contrast, West Molokai postshield lavas are exclusively Kea‐like. This change in source geochemistry can be explained by the observed change in strike of the Pacific plate near Molokai Island so that as West Molokai volcano moved away from a mixed Loa‐Kea source it sampled only the Kea side of a bilaterally zoned plume.

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.

Rapid passage of a small-scale mantle heterogeneity through the melting regions of Kilauea and Mauna Loa Volcanoes

Recent Kilauea and Mauna Loa lavas provide a snapshot of the size, shape, and distribution of compositional heterogeneities within the Hawaiian mantle plume. Here we present a study of the Pb, Sr, and Nd isotope ratios of two suites of young prehistoric lavas from these volcanoes: (1) Kilauea summit lavas erupted from AD 900 to 1400, and (2) 14C-dated Mauna Loa flows erupted from ∼ 2580–140 yr before present (relative to AD 1950). These lavas display systematic isotopic fluctuations, and the Kilauea lavas span the Pb isotopic divide that was previously thought to exist between these two volcanoes. For a brief period from AD 250 to 1400, the 206Pb/ 204Pb and 87Sr/ 86Sr isotope ratios and ε Nd values of Kilauea and Mauna Loa lavas departed from values typical for each volcano (based on historical and other young prehistoric lavas), moved towards an intermediate composition, and subsequently returned to typical values. This is the only known period in the eruptive history of these volcanoes when such a simultaneous convergence of Pb, Sr, and Nd isotope ratios has occurred. The common isotopic composition of lavas erupted from both Kilauea and Mauna Loa during this transient magmatic event was probably caused by the rapid passage of a small-scale compositional heterogeneity through the melting regions of both volcanoes. This heterogeneity is thought to have been either a single body (∼ 35 km long based on the distance between the summits of these volcanoes) or the plume matrix itself (which would be expected to be present beneath both volcanoes). The time scale of this event (centuries) is much shorter than previously noted for variations in the isotopic composition of Hawaiian lavas due to the upwelling of heterogeneities within the plume (thousands to tens of thousands of years). Calculations based on the timing of the isotopic convergence suggest a maximum thickness for the melting region (and thus, the heterogeneity) of ∼ 5–10 km. The small size of the heterogeneity indicates that melt can be extracted from small regions within the Hawaiian plume with minimal subsequent chemical modification (beyond the effects of crystal fractionation). This would be most effective if melt transport in the mantle beneath Hawaiian shield volcanoes occurs mostly in chemically isolated channels.

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

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