Geochemical and Isotopic Evolution of Loihi Volcano, Hawaii

A 680m thick section from the deeply dissected east flank ofLoihi Volcano was sampled using the Pisces V submersible toevaluate the volcano's geochemical evolution. Three types oflavas were recovered: tholeiitic, weakly alkalic and stronglyalkalic. The ratio of alkalic to tholeiitic lavas varies systematicallywith depth, from predominantly alkalic at the base of the sectionto tholeiitic at the top. Glasses from these rocks have similarratios of highly incompatible elements and Pb, Sr and Nd isotopes,but distinct ratios of highly to moderately incompatible elements.Partial melting modeling indicates that these tholeiitic andalkalic lavas could be derived by variable degrees of partialmelting of a slightly heterogeneous source. Many distinct parentalmagmas were generated for each rock type during the 100–150k.y. that the east flank section was formed. Crystal fractionationand olivine accumulation were the dominant processes controllingcompositional variation among lavas of the same rock type. Magmamixing features were observed in only a few of the lavas collected. Loihi typifies the preshield stage of Hawaiian volcanism whenthe volcano drifts closer to the focus of the hotspot. The compositionalvariation in Loihi's east flank section, which may represent40% of the volcano's extrusive history, is consistent with thepredicted increase in partial melting during this drift. Thetransition from dominantly alkalic to tholeiitic volcanism onLoihi was fitful but relatively rapid and is now nearly complete.This transition is the opposite of that which occurs duringthe post-shield stage of Hawaiian volcanism as the volcano migratesaway from the hotspot focus. Loihi's tholeiitic lavas overlap in ratios of incompatible traceelements and Pb, Sr and Nd isotopes with lavas from its moreactive neighbor, Kilauea. The small differences in major elementcontents between lavas from these adjacent volcanoes can beexplained by high-pressure orthopyroxene fractionation of Loihimagmas, which may be a consequence of a low magma-supply rate,or by slightly shallower depths of melt segregation for Kilaueamagmas. KEY WORDS: Loihi volcano; Hawaii; geochemistry; Sr-Nd-Pb isotopes     

Geochemical evolution of Kohala Volcano,Hawaii

Kohala Volcano, the oldest of five shield volcanoes comprising the island of Hawaii, consists of a basalt shield dominated by tholeiitic basalt, Pololu Volcanics, overlain by alkalic lavas, Hawi Volcanics. In the upper Pololu Volcanics the lavas become more enriched in incompatible elements, and there is a transition from tholeiitic to alkalic basalt. In contrast, the Hawi volcanics consist of hawaiites, mugearites, and trachytes. ⁸⁷Sr/⁸⁶Sr ratios of 14 Pololu basalts and 5 Hawi lavas range from 0.70366 to 0.70392 and 0.70350 to 0.70355, respectively. This small but distinct difference in Sr isotopic composition of different lava types, especially the lower ⁸⁷Sr/⁸⁶Sr in the younger lavas with higher Rb/Sr, has been found at other Hawaiian volcanoes. Our data do not confirm previous data indicating Sr isotopic homogeneity among lavas from Kohala Volcano. Also some abundance trends, such as MgO-P₂O₅, are not consistent with a simple genetic relationship between Pololu and Hawi lavas. We conclude that all Kohala lavas were not produced by equilibrium partial melting of a compositionally homogeneous source.     

Evolution of alkalic lavas at Haleakala Volcano,east Maui,Hawaii

The postshield and posterosional stages of Haleakala Volcano contain intercalated alkalic basalt and evolved alkalic lavas. Isotopic and incompatible element abundance ratios in the Haleakala postshield basalts changed systematically with time, providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources. Specifically, a depleted, i.e. low⁸⁷Sr/⁸⁶Sr and high¹⁴³Nd/¹⁴⁴Nd, mantle component is more abundant in younger lavas. However, as magma-production rates decreased during the postshield and posterosional stages, basaltic melts in magma reservoirs cooled and fractionated, leading to evolved residual melts such as hawaiite. Because primary basalt compositions changed with time, the evolved Haleakala lavas formed from a range of parental compositions. However, basalts and evolved lavas of similar age and isotopic ratios (Sr and Nd) have major and trace element contents that are consistent with a crystal-fractionation model. Although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both Haleakala and Mauna Kea volcanoes, there are important differences between their lavas. For example, compositional differences between the hawaiite suites at Haleakala and Mauna Kea indicate that, on average, the evolved lavas at Haleakala formed at lower pressures. Also, at Haleakala basalts are intercalated with hawaiites, whereas at Mauna Kea basalts and hawaiites are separated by a sharp boundary. These differences probably reflect a higher magma supply rate to the Haleakala volcano.     

Petrology of Ultramafic Xenoliths from Loihi Seamount, Hawaii

Ultramafic xenoliths were recovered in four alkalic lava flowsfrom Loihi Seamount at depths between 2200 and 1400m. No xenolithbearing flows were sampled near the summit despite a concentrateddredge program. The flows, three of alkalic basalt and one ofbasanite, contain common olivine megacrysts and small xenolithsof dunite, rarer harzburgite, and a single wehrlite. Olivinemegacrysts as large as 8 mm are Fo_{84–88 6} and containmagnesiochromite inclusions with 1?1–3?5 wt.% TiO₂ Dunitecontains Fo_{83 5–88?5} olivine, magnesiochromite with l?5–6?9wt.% TiO₂ (avg. 3?2 wt.%), and extremely rare chrome-rich diopside.The wehrlite contains euhedral Fo_{85 9} olivine and magnesiochromitewith 1?9–4?7 wt.% TiO₂ poikilitically enclosed in chrome-richdiopside (Wo_{45 4}En_{48 0}Fs_6?6).Most of the olivine megacrysts,dunite, and the wehrlite are cumulates of Loihi alkalic lavasthat accumulated in a magma storage zone located at least 16kmbelow sea level. The rarity of dunite related to tholeiiticmagmas supports the interpretation that the alkalic lavas atLoihi generally predate the tholeiitic lavas. The harzburgitexenoliths have cataclastic textures and contain Fo_{89 5–926} olivine, enstatite (Wo_{2 0–2?7}En_{90?0–88 7}Fe_8?0–8?6),Cr-rich endiopside (Wo_{43 4–44 5}En_{52 0–50 0}Fs_{4 6–45}), and translucent red-brown magnesiochromite. The harzburgitexenoliths, which have 2-pyroxene temperatures of 1066 ? 35?C,originated in the uppermost mantle in a region of high strainrate, probably near the boundary between the mantle and theoverlying ocean crust. The presence of upper mantle xenolithsindicates that the magma storage zone is located below the baseof the ocean crust within the uppermost mantle.     

Volatiles in pillow rim glasses from Loihi and Kilauea volcanoes,Hawaii

Volatiles and major elements in submarine glasses from Loihi seamount and Kilauea volcano. Hawaii were analyzed by high temperature mass spectrometry and the electron microprobe. Loihi glasses are subdivided into three groups: tholeiitic, transitional and alkali basalts. The glasses are evolved: Mg numbers range from 48–58. The alkalic lavas are the most evolved.Total volatiles range from 0.73 to 1.40 wt.%. H₂O shows a positive linear correlation with K₂O content [H₂O = 0.83 (± .09) K₂O + 0.08 (± .06)]. Concentrations of H₂O are higher in the alkalic lavas, but Cl and F abundances are highly variable. Variations in ratios of incompatible elements (K₂O, P₂O₅, H₂O) indicate that each group was derived from a distinct source. CO₂ contents range from 0.05 to 0.19 wt.% but show no systematic correlation with rock type or Mg #. A well-defined decrease in glass CO₂ content with increasing vesicularity is shown by the alkalic lavas. CO₂ may have been outgassed from the tholeiitic and transitional magmas prior to eruption during storage in a shallow magma chamber. Reduced carbon species (CO and CH₄) were found in small amounts in most of the alkalic samples. Although the redox histories of Hawaiian lavas are poorly known, these new data indicate the presence of a reduced source for Loihi magmas.The Kilauea tholeiitic glasses are evolved (Mg # 48.3 to 55) and have higher H₂O contents (av. 0.54 wt.%) than Loihi tholeiites (av. 0.42 wt.%) at the same Mg # (~55). Cl is distinctly lower in Kilauea glasses (0.01 wt.%) compared to Loihi glasses (0.09 wt.%). The data indicate significant source differences for the two volcanoes, consistent with results of other geochemical studies.Loihi tholeiites have distinctly higher ³He/⁴He ratios than Kilauea tholeiites and are the highest measured in submarine basalts (KURZ et al., 1983). These high ratios have been used to invoke a primitive source for Loihi basalts. The high Cl content of these basalts, the highest we have ever measured in submarine basalts, may be a fingerprint of this primitive source, as previously noted for Icelandic basalts (Schillinget al. 1980).     

五大连池、天池和腾冲火山岩Sr、Nd同位素地球化学特征与岩浆演化

在的年来对长白山天池火山、五大连池火山和腾冲火山三个火山区火山岩岩石学的主、微量元素研究的基础上，新做出18个火山岩Sr-Nd同位素数据，进一步讨论地幔源区特征与岩浆成因演化。五大连池富钾火山岩浆源区由原始地幔与EMI两个地幔端员混合而成，岩浆直接来自地幔，未受地壳物质明显混染和分离结晶作用影响；天池火山各阶段火山岩一致的似原始地幔特征，可能指示存在巨大的壳内岩浆房和持续的幔源岩浆的补给；腾冲火山岩的高钾钙碱性岩浆源区为由陆内壳－幔相互作用导致的原始地幔与EMⅡ两个地幔端员不同程度混合而成。     

Geochemical Evolution of the Soufriere Hills Volcano, Montserrat, Lesser Antilles Volcanic Arc

The geochemical evolution of Montserrat provides an importantbackground to understanding the current activity of this islandarc volcano. Here we present major and trace element, and U-,Th- and O-isotope data for rocks generated in the last 300 kyrthat provide constraints on the magmatic processes occurringbeneath the volcano. Samples range from low- to medium-K calc-alkalinebasalts to dacites. Three suites can be distinguished on thebasis of major and trace element compositions: the South SoufrièreHills suite; the Soufrière Hills suite, including thelava from the current eruption; and the mafic inclusions. Magmaticdifferentiation of the magma that crystallized to form the maficinclusions appears to have been governed by closed-system processes,modelled by fractional crystallization (F     

Geochemistry of the Hawi lavas,Kohala Volcano,Hawaii

Hawi lavas form the late stage alkalic cap on Kohala Volcano and range in composition from hawaiite to trachyte. New, detailed field mapping of Kohala and reinterpretation of previously published age data suggest that there was no significant eruption hiatus between the Hawi and underlying Pololu shield lavas as was previously suggested. Mineral and whole-rock chemical data are consistent with a crystal fractionation origin for the hawaiite to trachyte compositional variation observed within the Hawi lavas. Plagioclase, clinopyroxene, Ti-magnetite, olivine and apatite fractionation are needed to explain this variation. The clinopyroxene fractionation may have occurred at moderate pressure because it is virtually absent in these lavas and is not a near liquidus phase at pressures of less than 8 Kb. Plagioclase would be buoyant in the Hawi hawaiite magmas so a mechanism like dynamic flow crystallization is needed for its fractionation and to account for the virtual absence of phenocrysts in the lavas. Hawi lavas are distinct in Sr and Nd isotopic ratios and/or incompatible element ratios from the Pololu lavas. Thus they were derived from compositionally distinct sources. Compared to other suites of Hawaiian alkalic cap lavas, Hawi lavas have anomalously high concentrations of phosphorus and rare earth elements. These differences could be due to greater apatite content in the source for the Hawi lavas.     

The Turiy Massif, Kola Peninsula, Russia: Isotopic and Geochemical Evidence for Multi-source Evolution

The Turiy Massif, lying within the Kandalaksha Graben, and onthe southern coast of the Kola Peninsula, contains carbonatites,phoscorites, melilitolites, ijolites and pyroxenites withinone central and four surrounding satellite complexes. Sr–Ndisotopic data from the central complex phoscorites and carbonatites,and the nearby Terskii Coast kimberlites, combined with otherrecently published data on the Devonian Kola Alkaline Province,allow us to redefine the position of the Kola Carbonatite Line(KCL) of Kramm (European Journal of Mineralogy 5, 985–989,1993). We propose that the revised-KCL mantle sources includea lower-mantle plume, and a second enriched source, which alsocontributed to the Terskii Coast and Archangelsk kimberlites.The Turiy Massif silicate rocks and northern complex carbonatiteshave more enriched isotopic signatures than the distinct, anddepleted signatures of the central complex phoscorites and carbonatites,particularly with respect to     

Origin of the Differentiated and Hybrid Lavas of Kilauea Volcano, Hawaii

Kilauea Volcano has erupted lava from its summit caldera andfrom two rift zones that extend from the summit towards theeast and south-west. Lavas erupted from the summit of the volcanodiffer from each other principally in their content of olivineand define lines of ‘olivine control’ on magnesiavariation diagrams. Lavas erupted on the rift zones may be similarin composition to the summit lavas or may be differentiatedby processes that involve minerals other than olivine. All ofthe differentiated lavas have less than 6·8 per centMgO and plot off the extension of olivine control lines forthe summit lavas. Prehistoric vents (before A.D. 1750) fromwhich differentiated lavas have been erupted are found on theeast rift zone and in the western Koae fault zone adjacent tothe south-west rift zone; historic vents for differentiatedlavas are confined to the east rift zone. Twenty-one new analysesare presented for several of the east rift differentiates andfor the newly discovered differentiates adjacent to the south-westrift zone. The differentiates have MgO as low as 3·9per cent and SiO₂ as high as 56 per cent; both extremes arefound in the prehistoric lavas adjacent to the south-west rift. Detailed petrochemical studies suggest the following conclusions:

1. Thechemical composition of magma erupted at Kilauea summitvarieswith the date of eruption. Lavas erupted before 1750,duringthe eighteenth and nineteenth centuries, and in the twentiethcentury form groups that can be distinguished chemically. Ona lesser scale, each Kilauea summit eruption in the twentiethcentury has a chemistry that is distinctive with respect tothe chemistry of every other summit eruption.
2. During lateprehistoric time pockets of differentiated magmawere formedwithin the rift zones by separation of the liquidremainingafter partial crystallization of bodies of summitmagma. Thisprocess presumably is still going on within theeast rift zone,but the more recently separated liquids havenot yet been eruptedto the surface. The relative time at whichthese differentiatedmagmas were produced can be estimated fromcalculations basedon their chemical compositions, which showthat the differentiatescould lie on the liquid line of descentfor Kilauea summit magmaof prehistoric composition but noton any liquid line of descentfor younger summit magmas.
3. Lava from some eruptions, notablythe early part of the 1955eruption on the lower east rift,has the composition of theliquid fraction as it is generatedwithin the rift. Lava compositionsof other eruptions, includingthose of the later lavas of 1955,are best explained by mixingof magma supplied from a centralreservoir beneath Kilauea summitwith the differentiated liquidin the rift. Lava from each summiteruption is unique chemically,so it is possible to recognizeits presence or absence as componentsof mixing in such mixedlavas. It appears that summit magmaof composition characteristicof the 1952 and 1961 Halemaumaueruptions contributed to thecomposition of the mixed lavasproduced in the latter part ofthe 1955 eruption. Summit magmaof 1961 composition is alonesufficient to explain the compositionof mixed lavas eruptedin 1960 and 1961. In rift lavas eruptedfrom 1962 to 1965, thecomposition of lava erupted in Halemaumauin 1967, in additionto the 1961 composition, is a componentof mixing, and it isthe dominant summit component in the compositionof the two1965 eruptions. The proportion of summit magma todifferentiatedmagma needed to explain the composition of lavaserupted onthe upper east rift increases from 1961 to 1965;this increaseindicates that the differentiated magma was beingdiluted andused up by repeated flooding of this part of therift zone bymagma supplied from the central reservoir.
4. The fact that componentsof ‘summit composition’appear in rift eruptionsbefore they appear undiluted in Halemaumausuggests that thecentral reservoir is vertically zoned. Rifteruptions are fedfrom lower levels where younger magma is available,and summiteruptions are fed from the relatively older magmaabove. Thechemical distinction between lava of successive summiteruptionsimplies that significant convective mixing of magmadoes nottake place throughout the central reservoir.
5. The unique anduniform composition of lava of each successivesummit eruptionalso suggests that summit eruptions end whenall of the magmaof one composition has been erupted. The magmaerupted fromthe upper levels of the reservoir during one cycleis continuallyreplaced from below by younger magma of differentcomposition.In order for eruption to be renewed in Halemaumau,new magmafrom the mantle must be held in storage at intermediatelevelsbefore it attains an ‘eruptive state’.
6. The hypothesispresented in 2–4 above permits qualitativepredictionsconcerning future lava compositions. The compositionof thenext lava to be erupted in Halemaumau is expected tobe distinctfrom that of the 1967 eruption, and this compositionwill presumablybe identified in rift eruptions occurring between1967 and thetime of its appearance in Halemaumau.
7. Differentiates of prehistoricage also were apparently formedin the same way as those ofhistoric age, but the mixing cannotbe described quantitativelybecause of poor control on the stratigraphyand the compositionsof erupted lavas. One lava in the Koaegroup, that from YellowCone, appears to be a mixture of a picriticmagma (12 per centMgO) with a differentiated liquid with lessthan 2·5per cent MgO and nearly 60 per cent SiO₂.

     

Gas composition of the January 1983 eruption of Kilauea Volcano,Hawaii

Gas collections were made from a ～900°C vent both by conventional evacuated-bottle/wet-chemical techniques and by manual pumping of flowthrough bottles. The complete analyses suggest an equilibrium assemblage quenched at 1,010°C, about midway between fountain and vent temperatures. I suggest that the very low $\text{C}\text{S}$ ratio is due to degassing of CO₂ during storage of the magma in a shallow reservoir before eruption. The two sampling techniques yielded analytical data in mutual agreement.     

Modelling the Recent Geochemical Evolution of the Piton de la Fournaise Volcano, Reunion Island, 1931-1986

The Piton de la Fournaise is the only active volcano on theisland of R?union. Samples of the 1931–1986 eruptionshave been analysed for their major and trace element contentswith emphasis on high accuracy measurement of REE, Th, and Zrby isotope dilution. The lavas include transitional basalts and picrites with 40–50%olivine. Examination of the dislocation structures in the picriticolivines indicates a xenocrystic origin which is confirmed bysome mineral-melt Fe/Mg disequilibria and anomalous incompatibleelement ratios. The picrites are therefore interpreted as dueto pulses of basalt magmas which disrupted preexisting, geneticallyunrelated, olivine-rich cumulates. The combination of whole rock, mineral and groundmass analysesenabled precise calculation of the modal proportions. The phenocrystassemblages obtained thereby are consistent with the chemicalevolution of the lava series suggested by the mass balance equations,which is interpreted as reflecting little mineral sorting duringfractionation. The whole range of basaltic compositions canbe generated by up to 15% fractional crystallization of clinopyroxene-rich(20 Ol-80 Cpx) to plagioclase-bearing (20 Ol-50 Cpx-30 Plag)cumulates. By cqmparison with experimental liquid lines of descent,the order of crystallization (olivine-clinopyroxene-plagioclase)suggests a pressure of crystallization in excess of 500 MPa,i.e. much deeper than the c. 2km deep magma chamber inferredfrom magnetic measurements and seismic activity. The Ni-MgOsystem indicates that the Fe-rich primitive magma (9% MgO, 225ppm Ni) crystallized 12 ? 5% cumulate to yield the least differentiatedbasalt of the series. The historic basalts erupted prior to1931 and analyzed in this work were extracted from a more Mg-richsource. The inversion of batch melting equations for incompatible elementsis improved relative to previous attempts and a complete errorhandling is incorporated. It is suggested that the Fournaiselavas were derived by melting of a source enriched in compatibleelements leaving a harzburgitic residue without significantfractionation of the incompatible elements. Coupled variations of Ce/Yb and major element ratios over c.17-y cycles suggest that lower degrees of melting produce smallerbatches of magma which, upon ascent and cooling, undergo moresevere fractionation.     

Isotopic and Geochemical Constraints on the Origin and Evolution of the Columbia River Basalt

New major and trace element data on over 70 samples are combinedwith a wider knowledge of the regional stratigraphy, and ofthe tectonic evolution of the boundary between the ColumbiaPlateau and the northern margin of the Basin and Range province,to distinguish three subgroups within the Columbia River BasaltGroup (CRBG): the Picture Gorge Basalt; the main sequence ofColumbia River flood basalts, here named the Clarkston Basalt;and the Saddle Mountains Basalt. The subgroups are characterizedby different incompatible element and Sr-, Nd-, and Pb-isotoperatios, and they are interpreted in terms of different sourceregions mobilized under different tectonic conditions. The majordifferences between the subgroups are consistent with partialmelting processes in the upper mantle, and it is argued thatthey reflect previous partial melting episodes which resultedin source regions that were variably enriched and depleted inincompatible elements. The major variations within the PictureGorge and Clarkston Basalt subgroups include increases in theabundances of large ion lithophile elements (LILE) and increasesin the ratios of LILE/high field strength elements (HFSE) whichare interpreted as the addition of a lithospheric/subduction-relatedcomponent. The Picture Gorge Basalt has a depleted isotopic and chemicalsignature on which is superimposed an enrichment of LILE toproduce a trace element pattern similar to that of other 17–0-Mabasalts erupted south of the Olympic Wallowa Lineament. Thispattern is characteristic of volcanism associated with the Basinand Range extensional province, and others have attributed itto a source component derived from an enriched subcontinentallithospheric mantle (SCLM). Of the Clarkston Basalts, the Imnaha and Grande Ronde Basaltsform chemical and isotopic arrays which indicate mixing of componentsfrom two distinct source regions. One had high ratios of LILE/HFSEand light rare earth elements (LREE)/HFSE, and as these arenot common in oceanic basalts, this component is thought tohave been derived from the continental mantle lithosphere. Itsisotope ratios are more enriched (older?) than those of thePicture Gorge Basalt, and its Rb/Sr ratios are much higher thanthose in its source rocks, consistent with preferential mobilizationof LILE at the time of magmatism. The second component was derivedfrom an asthenospheric source similar to that of Hawaii basaltsand is most obviously attributed to mantle plume activity. Basaltsof the Eckler Mountain and Wanapum Formations (smaller, separateformations of the Clarkston Basalt as redefined in this paper)fit this mixing model less well and may represent mixing betweenmore than two components. Flows of the third CRBG subgroup,the Saddle Mountains Basalt, also carry a lithospheric geochemicalsignature and have long been recognized as having more radiogenicisotopic signatures than the other two subgroups. Thus, SaddleMountains flows appear to require a lithospheric source enrichedin LILE at an even earlier time, and we concur with other workersthat the isotopic and chemical evidence implies their derivationfrom subcontinental lithospheric mantle enriched at {small tilde}2000Ma. Within each subgroup, the chemical effects of partial melting,fractional crystallization, and magma mixing processes can allbe distinguished within particular flow sequences. In the ImnahaBasalt variable degrees of partial melting during the generationof the CRBG magmas, and gabbro fractionation within the lowercrust, played major roles in their evolution. In the GrandeRonde Basalt fractional crystallization appears to be restrictedto >10%. The chemical and isotopic data for each CRBG subgroup, and thedifferent sources which those data imply, can be accommodatedin a tectonic model which includes the passing of the Yellowstonehotspot south of the center of the CRBG eruption before significantBasin and Range extension had moved north of the Brothers Faultzone at 15 Ma.     

Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano,Hawaii

Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K₂O in plagioclase and the abundances of TiO₂ and Fe₂O₃in spinel of xenoliths and mid-ocean ridge basalt, and a single ⁸⁷Sr/ ⁸⁶Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10^–3–1.91×10^–3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel.     

The tholeiite to alkalic basalt transition at Haleakala Volcano,Maui, Hawaii

Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of 10% to 15%, while the intercalated alkalic lavas reflect a range of 6.5% to 8% melting. However, within the uppermost 50 m of section. ⁸⁷Sr/⁸⁶Sr decreases from 0.70371 to 0.70328 as eruption age decreased from 0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at 0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.     

Origin of xenoliths in the trachyte at Puu Waawaa,Hualalai Volcano,Hawaii

Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost CaO and plagioclase lost MgO and Fe₂O₃ during subsolidus reequilibration. The xenoliths also reacted with the host trachyte to form secondary mica, amphibole, and orthopyroxene, and to further modify the compositions of some olivine, clinopyroxene, and spinel grains. The reaction products indicate that the host trachyte melt was hydrous. Clinopyroxene in one dunite sample and olivine in most dunite samples have undergone partial melting, apparently in response to addition of water to the xenolith. These xenoliths do not contain CO₂ fluid inclusions, so common in xenoliths from other localities on Hualalai, which suggests that CO₂ was introduced from alkalic basalt magma between the time CO₂-inclusion-free xenoliths erupted at 106±6 ka and the time CO₂-inclusion-rich xenoliths erupted within the last 15 ka.     

Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow, Hualalai Volcano, Hawaii

The 1800 Kaupulehu flow on Hualalai Volcano, Hawaii, containsabundant xcnoliths of dunitc, wehrlite, and olivine clinopyroxenitewith minor gabbro, troctolite, anorthosite, and wcbstcrite.The petrography and mineral compositions of 41 dunite, wehrlite,and olivine clinopyroxenite xenoliths have been studied, andclinopyroxene separates from eight of these have been analyzedfor Ba, K, Rb, Sr, rare earth elements, ⁸⁷Sr/⁸⁶Sr, and ¹⁴³Nd/¹⁴⁴Nd.Temperatures of equilibration obtained by olivine-spinel andpyroxene geothermometry range from 1000 to 1200 C. Mineralogicaldata combined with published fluid inclusion data indicate depthsof origin in the range of 8–30 km. The rarity of orthopyroxene, the presence of Fe-rich olivine(Fo⁸¹–⁸⁹) and clinopyroxene (Fs⁵–¹²), and the occurrenceof high TiO² in spinel (0.9–2.8 wt.%) and clinopyroxene(035–1.33 wt%) all indicate that the xenoliths are cumulates,not residues from partial fusion. The separated clinopyrox-eneshave ⁸⁷Sr/⁸⁶Sr (0-70348.0-70367) and ¹⁴³Nd/¹⁴⁴Nd (0.51293–0.51299)values that are different from Sr and Nd isotope ratios of Pacificabyssal basalts (>0.7032 and >0-5130, respectively). Also,clinopyroxenes and spinels in the xenoliths have generally higherTiO₂ contents (>O.35 and >0.91 wt.%, respectively) thantheir counterparts in abyssal cumulates (<0.40 and <0.70wt%,respectively). These differences indicate that the xenolithsare not a normal component of oceanic crust. Because the xenoliths and alkalic to transitional Hualalai lavashave similar values for Cr/(Cr + Al) and Cr/(Cr + Al + Fe³⁺)of spinels, ⁸⁷Sr/⁸⁶Sr of clinopyroxenes, and whole-rock ³He/⁴He,we conclude that the xenoliths are cumulates from such magmas.Multiple parental magmas for the xenoliths are indicated byslightly heterogeneous ⁸⁷Sr/⁸⁶Sr of clinopyroxene separates.Depths of formation of the xenoliths are estimated to be {smalltilde}8–30 km. Extensive crystallization of olivine in the absence of pyroxenesand plagioclase is a characteristic and prominent feature ofHawaiian tholeiitic magmatism. Dunite xenoliths crystallizedfrom alkalic magmas have previously been reported from MaunaKea Volcano (Atwill & Garcia, 1985) and Loihi Seamount (Clague,1988). Our finding of an alkalic signature for dunite xenolithsfrom a third Hawaiian volcano, Hualalai, shows that early olivinecrystallization should be considered a characteristic not justof Hawaiian tholeiitic magmatism but also of Hawaiian alkalicmagmatism.     

Petrogenesis of Tholeiitic Lavas from the Submarine Hana Ridge, Haleakala Volcano, Hawaii

Hana Ridge, the longest submarine rift zone in the Hawaiianisland chain, extending from Maui 140 km to the ESE, has a complexmorphology compared with other Hawaiian rift zones. A totalof 108 rock specimens have been collected from the submarineHana Ridge by six submersible dives. All of the rocks (76 bulkrocks analyzed) are tholeiitic basalts or picrites. Their majorelement compositions, together with distinctively low Zr/Nb,Sr/Nb, and Ba/Nb, overlap those of Kilauea lavas. In contrast,the lavas forming the subaerial Honomanu shield are intermediatein composition between those of Kilauea and Mauna Loa. The compositionalcharacteristics of the lavas imply that clinopyroxene and garnetwere important residual phases during partial melting. The compositionsof olivine and glass (formerly melt) inclusions imply that regardlessof textural type (euhedral, subhedral–undeformed, deformed)olivine crystallized from host magmas. Using the most forsteriticolivine (Fo_90·6) and partition coefficients     

Magmatic Processes During the Prolonged Pu'u 'O'o Eruption of Kilauea Volcano, Hawaii

The Pu’u ’O’o eruption is exceptional amonghistorical eruptions of Kilauea Volcano for its long duration(