The Chemistry and Petrogenesis of a Suite of Pantellerites from the Ethiopian Rift

Data on the major and trace element chemistry of a suite oftwenty pantelleritic pitchstone lavas from the Quaternary Ethiopianvolcano Fantale is presented. This reveals a contrast betweenthe composition of the pre-caldera flows and the more siliceous,less peralkaline post-caldera lavas. Comparison with experimental and theoretical studies suggeststhat nearly all of the major and trace element variation withinthe two suites can be explained by assuming fractional crystallizationof alkali-feldspar, the most abundant phenocryst phase. Fractionationof the mafic phases appears to have been less significant. The trace element data strongly indicate that the lavas allbelong to a single suite. However, it is suggested that themajor element chemistry of the post-caldera flows was modifiedby the loss of volatiles at the time of the formation of thecaldera, an event which coincided with the eruption of a 2 km³welded ash-flow tuff.     

Geochemical Evolution of Akagi Volcano, NE Japan: Implications for Interaction Between Island-arc Magma and Lower Crust, and Generation of Isotopically Various Magmas

Major and trace element, and Sr, Nd and Pb isotopic compositionswere determined for whole-rock samples from the ‘isotopicallyanomalous’ Akagi volcano in the volcanic front of theNE Japan arc. Sr and Nd isotopic compositions of phenocrystswere also analyzed together with their major and trace elementcompositions. Compared with the other volcanoes from the volcanicfront, the whole-rock isotope compositions of Akagi show highlyenriched characteristics; ⁸⁷Sr/⁸⁶Sr = 0·7060–0·7088,     

Magma Emplacement and Remobilization Timescales Beneath Montserrat: Insights from Sr and Ba Zonation in Plagioclase Phenocrysts

Since 1995, the Soufrière Hills volcano on Montserrat,Lesser Antilles island arc, has erupted crystal-rich andesitemagma that is geochemically very similar to the volcanic productsof at least the last 174 ± 3 kyr (1     

Primary Arc Basalts from Daisen Volcano, Japan: Equilibrium Crystal Fractionation versus Disequilibrium Fractionation during Supercooling

Daisen volcano, southwest Japan, has been thought to be an exclusivelydacitic volcano, lavas having trace element patterns with agarnet signature. We studied the basalts at the western footof the volcano and made two unexpected findings. (1) The homogeneityof ⁸⁷Sr/⁸⁶Sr in Daisen basalts (Sr isotopic variability     

Hydrothermal alteration in andesitic volcanoes: Trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)

The mineralogy and the trace element compositions of hydrothermally-altered volcanic materials collected from ash fall deposits and in four debris-avalanche deposits (DADs) at La Soufrière volcano in Guadeloupe have been determined. Phreatic explosions of the 1976 eruption and flank collapse events have sampled various parts of the active and ancient hydrothermal systems of the volcano. Hydrothermal mineral assemblages (smectite + silica polymorphs ± pyrite/jarosite ± gypsum) are typical of rock alteration by low-temperature acid-sulphate fluids. High-temperature mineral assemblages are rare, indicating that phreatic explosions and flank collapse events have sampled mainly the upper parts of the volcanic edifice.Andesitic eruptive products affected by shallow hydrothermal alteration are complex assemblages of volcanic materials (glass, phenocrysts and xenocrysts with complex magmatic histories) of different ages and compositions. The use of incompatible element ratios and REE compositions normalised to an unaltered reference material overcomes the interpretation difficulties related to mass balance effects of alteration processes and the petrologic heterogeneity of the initial material.REE and other incompatible elements (Th, U, Hf, Zr) are mainly concentrated in the glassy matrix of unaltered andesitic rocks. Secondary S-bearing mineral phases (e.g., gypsum, jarosite) that have precipitated from acid-sulphate fluids do not contain substantial incompatible elements (REE, U, Th, Hf, Zr). Compositional variations of incompatible elements in hydrothermally-altered andesitic materials reflect mainly volcanic glass–smectite transformation, which is characterised by (i) strong depletion of alkalis and alkaline earths (Ba, Sr) and first transition series elements (Zn, Cu, Cr, Co, Ni), (ii) immobility of highly incompatible elements (Th, Zr, Hf, LREE) and (iii) strong depletion of MREE and HREE. The sigmoid shape of normalised REE pattern is characteristic of glass–smectite transformation by low-temperature acid-sulphate fluids. This transformation also produces significant variations in U/Th values, which offer the opportunity to date the cessation of hydrothermal alteration and to reconstruct the evolution in space and time of hydrothermal activity in a volcanic edifice.     

Pleistocene to recent alkalic volcanism in the region of Sanganguey volcano,Nayarit, Mexico

Forty five cinder cones and associated lava flows have erupted within the last 300,000 years along five parallel lines through the calc-alkaline volcano, Sanganguey, in the northwestern segment of the Mexican Volcanic Belt. Lavas erupted from these cinder cones include ne- and hynormative alkali basalts, hawaiites, mugearites, and benmoreites. It is unusual that this suite has erupted in a calc-aikaline volcanic belt where volcanoes in the vicinity have been erupting calc-alkaline andesites, dacites and rhyodacites.Incompatible trace elements such Ba, Rb, Sr, and LREEs show little change with decreasing Mg, Ni, and Cr in the series alkali basalt to hawaiite, suggesting that simple crystal fractionation of observed phenocrysts has not been the dominant process in the derivation of the hawaiites from the alkali basalts. Petrographic evidence of magma mixing along with observed variation of trace element abundances suggests that the alkali basalts might represent mixtures of primitive magma with more evolved compositions.Crystal fractionation is capable of explaining major and most trace element trends in the series hawaiite — mugearite — benmoreite. However, such a process could only occur at pressure because of the requirement that clinopyroxene be a major crystallizing phase.The anomolous association of alkaline magmatism contemporaneously with calc-alkaline magmatism is probably related to the complex tectonic history associated with the rearrangement of plate boundaries in the vicinity of western Mexico.     

Geochemical evolution of historical lavas from Askja Volcano, Iceland: Implications for mechanisms and timescales of magmatic differentiation

The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjöll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as ²³⁸U-²³⁰Th-²²⁶Ra systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10 wt% MgO), consisting of lavas older than 1875 A.D., ⁸⁷Sr/⁸⁶Sr increases systematically with increasing SiO₂ content; this suite is suggested to have evolved in a magma chamber located at ∼600 MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5 wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, ⁸⁷Sr/⁸⁶Sr tends to decrease slightly with increasing SiO₂ content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at ∼100 MPa beneath the Öskjuvatn caldera, and that icelandite and rhyolite magmas were produced by extraction of the less and more evolved interstitial melt, respectively, from the mushy boundary layer along the margin of the ferrobasalt magma chamber, followed by accumulation of the melt to form separate magma bodies. Ferrobasalt and icelandite lavas in the ferrobasalt-rhyolite suite have a significant radioactive disequilibrium in terms of (²²⁶Ra/²³⁰Th), and its systematic decrease with magmatic evolution is considered to reflect aging, along with assimilation and fractional crystallization processes. Using a mass-balance model in which simultaneous fractional crystallization, crustal assimilation, and radioactive decay are taken into account, the timescale for the generation of icelandite magma from ferrobasalt was constrained to be <∼3 kyr which is largely dependent on Ra crystal-melt partition coefficients we used.     

Magma Storage Region Processes Inferred from Geochemistry of Fe-Ti Oxides in Andesitic Magma, Soufriere Hills Volcano, Montserrat, W.I.

Analyses of Fe–Ti oxides help constrain models of magmastorage region processes for the Soufrière Hills Volcano,Montserrat (W.I.), and provide clear evidence of the natureof transient heating events in the magma storage region. Toconstrain timescales of magma heating and remobilization, theTiO₂ zoning patterns in a time series of natural titanomagnetiteswere compared with those produced in controlled phase equilibriumexperiments on the andesite. Most samples of andesite eruptedfrom 1995 to 2002 contain titanomagnetite crystals with uniformcore compositions (TiO₂     

Geochemical characteristics of the La Réunion mantle plume source inferred from olivine-hosted melt inclusions from the adventive cones of Piton de la Fournaise volcano (La Réunion Island)

Major and trace element compositions were obtained for bulk rocks and melt inclusions hosted in olivine crystals (Fo > 85) from the adventive cones of the Piton de La Fournaise volcano (La Réunion Island). Ratios of highly incompatible trace elements for these magmas are used to identify the nature of the La Réunion mantle plume source. Although adventive cone lavas display unusual major element compositions compared to the historical lavas of the volcano (e.g., lower CaO/Al₂O₃), trace element data suggest that the magmas emitted by the adventive cones originate from a common chemical source. This source may correspond to either a homogeneous mixed source of different mantle components or a near-primitive less-differentiated mantle source. The melt inclusions display ratios of highly incompatible elements (e.g., Th/La, Nb/La) which are similar to primitive mantle values, and lower Nb/U ratios compared to most oceanic basalts. These results and previous isotopic and trace element data suggest that La Réunion plume samples a source which is intermediate between a primitive-like mantle domain and a slightly depleted one almost unaffected by the recycling processes. This source could have originated from early depletion of the primitive mantle. Assuming a depletion 4.45 Gyr ago, ~10% melting of this slightly depleted source could explain the enriched trace element concentrations of the melt inclusions.     

Isotope Compositions of Submarine Hana Ridge Lavas, Haleakala Volcano, Hawaii: Implications for Source Compositions, Melting Process and the Structure of the Hawaiian Plume

We report Sr, Nd, and Pb isotope compositions for 17 bulk-rocksamples from the submarine Hana Ridge, Haleakala volcano, Hawaii,collected by three dives by ROV Kaiko during a joint Japan–USHawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios forthe submarine Hana Ridge lavas are similar to those of Kilauealavas. This contrasts with the isotope ratios from the subaerialHonomanu lavas of the Haleakala shield, which are similar toMauna Loa lavas or intermediate between the Kilauea and MaunaLoa fields. The observation that both the Kea and Loa componentscoexist in individual shields is inconsistent with the interpretationthat the location of volcanoes within the Hawaiian chain controlsthe geographical distribution of the Loa and Kea trend geochemicalcharacteristics. Isotopic and trace element ratios in Haleakalashield lavas suggest that a recycled oceanic crustal gabbroiccomponent is present in the mantle source. The geochemical characteristicsof the lavas combined with petrological modeling calculationsusing trace element inversion and pMELTS suggest that the meltingdepth progressively decreases in the mantle source during shieldgrowth, and that the proportion of the recycled oceanic gabbroiccomponent sampled by the melt is higher in the later stagesof Hawaiian shields as the volcanoes migrate away from the centralaxis of the plume. KEY WORDS: submarine Hana Ridge; isotope composition; melting depth; Hawaiian mantle plume     

Fractional crystallisation and the petrology of Dunedin volcano

Major and trace element data and mineral chemical data indicate that the range in rock types making up the Dunedin volcano has developed by crystal fractionation processes acting upon mantle derived basaltic magmas at various levels in the crust and upper mantle. A diversity among parental materials and the operation of the fractionation process at varying levels in the crust and mantle under varying conditions of pH₂O have resulted in a diverse series of overlapping fractionation trends. ‘End member’ series are: basalt-hawaiite-mugearite-benmoreite; basanite-nepheline hawaiitenepheline mugearite-nepheline benmoreite; moderately potassic variants on these series. The phonolitic rocks of the volcano are low pressure differentiates derived by fractional crystallization, involving feldspar, as end member products in all the series outlined above. Quartz normative trachytes of the volcano appear to be differentiates from a distinct saturated or oversaturated magma series of different strontium isotopic and trace element characteristics from the undersaturated magma series.     

Magmatic Differentiation in the Uwekahuna Laccolith, Kilauea Caldera, Hawaii

Petrographic and chemicoal studies of a suite of rocks fromthe Uwekahuna laccolith of Kilauea Volcano show that the originalmafic tholeiitic magma differentiated into tholeiitic picrite,tholeiitic olivine gabbro, and an aphanitic rock approachingquartz-basalt in composition. Mechanisms involved were an initialgravity settling of olivine and a final filter pressing of theresidual liquid. The range of composition represented by therocks of the laccolith is as great as that found among all hithertoanalysed lavas of the volcano. ²Present address: U.S. Geological Survey, Menlo Park, California     

Petrogenesis and Implications of Calc-Alkaline Cryptic Hybrid Magmas from Washburn Volcano, Absaroka Volcanic Province, USA

The petrogenesis of calc-alkaline magmatism in the Eocene AbsarokaVolcanic Province (AVP) is investigated at Washburn volcano,a major eruptive center in the low-K western belt of the AVP.New ⁴⁰Ar/³⁹Ar age determinations indicate that magmatism atthe volcano commenced as early as 55 Ma and continued untilat least 52 Ma. Although mineral and whole-rock compositionaldata reflect near equilibrium crystallization of modal phenocrysts,petrogenetic modeling demonstrates that intermediate compositionmagmas are hybrids formed by mixing variably fractionated andcontaminated mantle-derived melts and heterogeneous siliciccrustal melts. Nd and Sr isotopic compositions along with traceelement data indicate that silicic melts in the Washburn systemare derived from deep-crustal rocks broadly similar in compositionto granulite-facies xenoliths in the Wyoming Province. Our preferredexplanation for these features is that mantle-derived basalticmagma intruded repeatedly in the deep continental crust leadingto fractional crystallization, silicic melt production, andhomogenization of magmas, followed by ascent to shallow reservoirsand crystallization of new plagioclase-rich mineral assemblagesin equilibrium with the intermediate hybrid liquids. The implicationsof this process are that (1) some calc-alkaline magmas may onlybe recognized as hybrids on purely chemical grounds, particularlyin systems where mixing precedes and is widely separated fromcrystallization in space and time, and (2) given the role ascribedto crustal processes at Washburn volcano, the variation betweenrocks that follow calc-alkaline trends in the western AVP andthose that follow shoshonitic trends in the east cannot simplyreflect higher pressures of fractionation to the east in Moho-levelmagma chambers in the absence of crustal interaction. KEY WORDS: petrogenesis; magma mixing; calc-alkaline; Absaroka Volcanic Province; ⁴⁰Ar/³⁹Ar dates     

Mercury gas emissions from La Soufrière Volcano, Guadeloupe Island (Lesser Antilles)

Quantifying mercury (Hg) emissions from active volcanoes is of particular interest for better constraining the global cycle and environmental impact of this highly toxic element. Here we report on the abundance of total gaseous (TGM = Hg⁰_(g) + Hg^II_(g)) and particulate (Hg_(p)) mercury in the summit gas emissions of La Soufrière andesitic volcano (Guadeloupe island, Lesser Antilles), where enhanced degassing of mixed hydrothermal-magmatic volatiles has been occurring since 1992 from the Southern summit crater. We demonstrate that Hg in volcanic plume occurs predominantly as gaseous mercury, with a mean TGM/Hg_(p) mass ratio of ~ 63. Combining the mean TGM/H₂S mass ratio of the volcanic plume (~ 3.2 × 10^− 6), measured close to the source vent, with the H₂S plume flux (~ 0.7 t d^− 1), determined simultaneously, allows us to estimate a gaseous mercury emission rate of 0.8 kg yr^− 1 from La Soufrière summit dome. Somewhat lower TGM/S_tot mass ratio in fumarolic gases from the source vent (4.4 × 10^− 7) suggests that plume chemical composition is not well represented by the emission source (fumaroles) due to chemical processes prior to (or upon) discharge. Current mercury emission from La Soufrìere volcano represents a very small contribution to the estimated global volcanic budget for this element.     

Partial Melting of Aluminous Metagreywackes in the Northern Sierra de Comechingones, Central Argentina

We describe a suite of metamorphic and migmatitic rocks fromthe Sierra de Comechingones (Sierras Pampeanas of Central Argentina)that include unmelted gneisses, migmatites and refractory granulites.The gneisses are aluminous greywackes metamorphosed in the amphibolitegrade and are likely to have been the protoliths for the higher-grademigmatites and granulites. Mineralogical characteristics andmajor and trace element compositions show that metatexite migmatites,diatexite migmatites and granulites are all melt-depleted rocks.The migmatites (both metatexites and diatexites) have undergoneH₂O-fluxed melting and lost     

Geochemical Constraints on the Role of Oceanic Lithosphere in Intra-Volcano Heterogeneity at West Maui, Hawaii

Stratigraphically well-constrained sequences of late shield-buildingstage lavas from West Maui volcano, Hawaii, show age-dependentcompositional variability distinct from that seen in shield-stagelavas from any other Hawaiian volcano. These distinctions aredefined by ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb variation as well as⁸⁷Sr/⁸⁶Sr correlation with ²⁰⁶Pb/²⁰⁴Pb and trace element compositions.The West Maui lavas from stratigraphically higher in the sequencehave major and trace element and Sr–Pb–Hf–Ndisotopic compositions similar to Kea-type lavas sampled at theyounger Mauna Kea and Kilauea volcanoes, indicating that theKea compositional end-member of Hawaiian lavas has remainedhomogeneous over     

The Colima Volcanic complex,Mexico

Volcán Colima is Mexico's most historically active andesitic composite volcano. It lies 150 km north of the Middle America Trench at the western end of the Mexican Volcanic Belt, closer to the trench than any other composite volcano in Mexico. Since its earliest reported eruption in 1576, V. Colima has evolved through three cycles of activity. Each cycle culminated in a major ashflow eruption, halting activity for 50 or more years. The last major ashflow eruption occurred in 1913. Andesitic block lava eruptions in 1961–1962 and 1975–1976 marked the inception of activity in a fourth historical cycle which may also terminate with a major ashflow eruption in the early part of the next century.Major and trace element analyses of whole rock samples and all constituent phases are presented for a suite of nine post-caldera hornblende and olivine-andesites. The suite includes samples from Colima's four major eruptions since 1869, spanning the last two eruptive cycles. Colima's post-caldera andesites are poor in K and other incompatible elements (Ti, P, Zn, Rb, Y, Zr, Ba, La, Yb, Hf, Th, and U) as may be characteristic of near trench andesites. From the 1913 ashflow eruption through the fourth cycle andesites, there have been increases in whole rock abundances of Si, Ba, and Cs, and decreases in Ti, Fe, Mg, Ni, Cr, and Sc. Crystal fractionation models can closely reproduce major element variations in the post-caldera suite, but systematically fail to predict sufficient concentrations of the compatible trace elements Cr, Ni, and Zn. Anomalous enrichments of compatible trace elements in Colima's andesites probably reflect simultaneous crystal fractionation and magma mixing in the subvolcanic system.Estimated pre-eruptive temperatures range from 940 °–1,000 ° C in the hornblende-andesites and 1,030 °–1,060 ° C in the olivine-andesites. Pre-eruptive magmatic water contents of 1.0–3.6 wt.% are calculated for the hornblende-andesites; the phenocryst assemblage of the olivine-andesite is calculated to equilibrate at 1,000 bars with 0.8% H₂O.Orthopyroxenes and certain clinopyroxenes in all pre-1961 samples are reversely zoned, with relatively Mg-rich rims. The most pronounced Mg-rich rims occur in the olivine-andesites and are thought to reflect pre-eruptive magma mixing, involving a basic, olivine+/-clinopyroxene-bearing magma. In addition to their normally zoned pyroxenes, the post-1961, fourth cycle andesites display a number of other features which distinguish them from earlier post-caldera hornblende-andesites of similar bulk composition. These include: (1) higher total crystal contents, (2) lower modal hornblende contents, (3) higher calculated pre-eruptive silica activities, and (4) lower calculated pre-eruptive water contents. These features are all consistent with the interpretation that the fourth cycle andesites were less hydrous prior to eruption. The slight Mg-rich pyroxene rims in pre-1961 hornblende-andesites may record late-stage, pre-eruptive increases in magmatic water content, which act to raise magmatic f _{O 2} and Mg/Fe⁺² ratios in the melt and in all crystalline phases. The fourth cycle andesites apparently did not experience a strong, pre-eruptive influx of water, resulting in lower magmatic water contents and normally zoned pyroxenes.     

Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe,Tanzania

 This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K₂O/Na₂O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An_82–90), clinopyroxene (4.5–9.5 wt% Al₂O₃), and olivine (Fo_79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P₂O₅) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P₂O₅/Al₂O₃, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO₂-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle. Received: 30 May 1994 / Accepted: 5 April 1995     

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.     

Geochemistry of eucrites: genesis of basaltic eucrites, and Hf and Ta as petrogenetic indicators for altered antarctic eucrites

We performed instrumental neutron activation analysis on a large suite of antarctic and nonantarctic eucrites, including unbrecciated, brecciated, and polymict eucrites and cumulate and noncumulate eucrites. We evaluate the use of Hf and Ta, two highly incompatible elements, as sensitive indicators of partial melting or fractional crystallization processes. Comparison with rare earth element (REE) data from nonantarctic and antarctic eucrites shows that Hf and Ta are unaffected by the terrestrial alteration that has modified the REE contents and patterns of some antarctic eucrites. The major host phases for Hf and Ta—zircon, baddeleyite, ilmenite, and titanite—are much less susceptible to terrestrial alteration than the phosphate hosts of REEs. The host phases for Hf and Ta are minor or trace phases, so sample heterogeneity is a serious concern for obtaining representative compositions. The trace lithophile and siderophile element contents of noncumulate eucrites do not allow for a single, simple model for the petrogenesis of the howardite-eucrite-diogenite suite. Fractional crystallization models cannot reproduce the compositional relationship between eucrites of the main group-Nuevo Laredo trend and those of the Stannern trend. Equilibrium crystallization models cannot explain the trace element diversity observed among diogenites. Partial melting models cannot explain the W variations among eucrites, unless source regions had different metal contents. We suggest that slight variations in oxygen fugacity of eucrite source regions during partial melting can explain the W variations without requiring different metal contents. This hypothesis may fail to account for eucrite Co contents, however.