Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel,FRG)

The Quaternary foidites and basanites of the West Eifel (Germany) contain optically and chemically heterogeneous clinopyroxenes, some of which occur as discrete zones within individual crystals: Most clinopyroxene phenocrysts are made up of a core and a normally zoned comagmatic titanaugite mantle. Most cores are greenish pleochroic and moderately resorbed (fassaitic augite). Some are pale green and strongly resorbed (acmitic augite). Cores of Al-augite composition and of Cr-diopside derived from peridotite xenoliths are rare. The fassaitic augites are similar in trace element distribution pattern to the titanaugites, but are more enriched in incompatible elements. The acmitic augites, in contrast, are clearly different in their trace element composition and are enriched in Na, Mn, Fe and depleted in Al, Ti, Sr, Zr. A model for polybaric magma evolution in the West Eifel is proposed: Primitive alkali basaltic magma rises through the upper mantle precipitating Al-augite en route. It stagnates and differentiates near the crust/mantle boundary crystallizing Fe-rich fassaitic augites. The magma differentiated at high pressure is subsequently mixed with new pulses of primitive magma from which the rims of pyroxene are crystallized. Sporadic alkali pyroxenite xenoliths are interpreted to represent cumulates of cognate phases formed within the crust and not metasomatized upper mantle material (Lloyd and Bailey 1975).     

Origin of basaltic magmas in the Mojave Desert area,California

The basaltic lavas erupted throughout the Mojave Desert are basanites (SiO₂<46%, normative nepheline>5%, and K₂O>1.5%), alkali-olivine basalts (SiO₂=46–48%; ne=0–5%; and K₂O=1.0–1.5%), and low-alumina, sub-alkaline basalts (SiO₂=48–51%; ne=0; K₂O<1.0%). One volcano, Pisgah Crater, erupted five times, with lava from each successive phase containing more silica and less potash than the one proceeding it. This compositional trend is the reverse of that expected from differentiation of a single alkalic magma, and therefore, may represent a succession of magmas tapped from a zone of continuing partial melting in the mantle.These lava compositions suggest that first melting was under high water pressure and was followed by relatively dry partial melting of gamet-orthopyroxene-clinopyroxene-olivine assemblages. The successive increase in silica and alkali decrease also requires that the partial melting zone move to shallower levels.All lavas sampled in the Mojave Desert area have compositions that can best be explained by the extraction of magma from such a rising melting zone, analogous to the mantle diapirs suggested by Green and Ringwood.     

Limestone assimilation by basaltic magmas: an experimental re-assessment and application to Italian volcanoes

The results of an experimental study of limestone assimilation by hydrated basaltic magmas in the range 1,050–1,150°C, 0.1–500 MPa are reported. Alkali basalts doped with up to 19 wt% of Ca, Mg-carbonates were equilibrated in internally heated pressure vessels and the resulting phase relationships are described. The major effects of carbonate incorporation are: (1) generation of CO₂-rich fluid phases; (2) change in liquidus phase equilibria; the crystallization of Ca-rich clinopyroxene is favored and the other phases (e.g. olivine, plagioclase), present in the absence of carbonate assimilation, are consumed. As a consequence of the massive clinopyroxene crystallization, the residual melt is strongly silica-depleted and becomes nepheline-normative. Compositional and mineralogical evolutions observed in Mt. Vesuvius eruptive products match those documented in our experiments with added carbonates, suggesting the possibility that carbonate assimilation increased during the last 25 ka of activity. In Central-Southern Italy, carbonate assimilation at shallow levels probably superimposes on deeper source heterogeneities.     

Dependence of clinopyroxene composition on cooling rate in basaltic magmas: Implications for thermobarometry

The compositional variation of clinopyroxene and the partitioning of major elements between clinopyroxene and melt are estimated as a function of the cooling rate. Clinopyroxenes were crystallized under variable cooling regimes (15, 9.4, 3, 2.1, and 0.5 °C/min from 1250 down to 1000 °C) and at isothermal conditions of 1000 °C from a basaltic composition at a pressure of 500 MPa under anhydrous and hydrous (H₂O = 1.3 wt.%) conditions. The clinopyroxene chemistry shows that, as the cooling rate increases, crystals are progressively depleted in Ca, Mg, Fe²⁺ and Si and enriched in Na, Fe³⁺, Al (mainly Al^IV), and Ti. Di and Hd versus CaTs and CaFeTs form a continuous binary solid solution characterized by higher amounts of tschermakitic components with increasing cooling rate. Two parameters (DH = Di + Hd and TE = CaTs + CaFeTs + En) are calculated to describe the effect of cooling rate on the clinopyroxene composition. The variation of DH/TE with increasing cooling rate evidences the kinetic process induced by rapid cooling in basic rocks under hydrous and anhydrous conditions.Dynamic crystallization conditions affect the partitioning of major elements between clinopyroxene and melt; with increasing cooling rate, the value of crystal–melt partition coefficient departs from that obtained at the isothermal condition. However, in spite of these variations, the values of ^cpx–meltKd_Fe–Mg remain almost constant. Therefore, the Fe²–Mg exchange between clinopyroxene and melt is not suitable to prove the (dis)equilibrium conditions in basaltic cooling magmas, giving rise to possible mismatches in the application of thermobarometers. The results of our study are consistent with that observed at the margin of dikes or in the exterior portions of lavas, where the cooling rate is maximized and disequilibrium compositions of clinopyroxene have been found.     

Major element evolution of basaltic magmas: a comparison of the information in CMAS and ALFE projections

Basalt petrologists disagree as to whether the commonly used projection, forsterite-diopside-silica, in the system CMAS (CaO-MgO-Al₂O₃-SiO₂), can adequately resolve differences in basaltic glass compositions for purposes of petrogenetic modelling. Here, we suggest than an analogous plot, the aluminium-factor diagram (ALFE) of Nesbitt and Cramer (1981), has greater diagnostic value than Fo-Di-Sil. A plot of molar (Al₂O₂-CaO-Na₂O-K₂O)/(FeO + MgO) vs FeO/(FeO + MgO), it produces more coherent patterns both for experimental basalt glasses, and for natural lavas. It is, like Fo-Di-Sil, a projection through plagioclase, but has the advantage that it monitors changes in Fe/(Fe + Mg) in melts and associated crystalline phases, and is particularly useful in assessing the timing of clinopyroxene crystallization in a suite of lavas. The diagram owes its greater resolving power to the fact that the computation of its coordinates is less sensitive to analytical uncertainty than for Fo-Di-Sil. In the latter diagram, normative quartz is calculated as a residual and thus manifests the uncertainties in all the major elements.     

The redox state of cerium in basaltic magmas: an experimental study of iron-cerium interactions in silicate melts

Ce(IV)-Ce(III) and Fe(III)-Fe(II) redox equilibria in Ca-Mg-Al-silicate melts have been individually measured with respect to the base composition, melt temperature, imposed oxygen fugacity, and multivalent element concentration (up to about 1.5 wt%). The mutual interaction of these two redox couples has been studied in analogous glasses which simultaneously contained iron and cerium. Analyses of Fe(III) concentrations in iron-cerium glasses by electron paramagnetic resonance and optical absorption spectroscopy indicate that Ce(IV) is stoichiometrically reduced by Fe(II) in the melt to produce Fe(III) and Ce(III) and that Ce(III)-O-Fe(III) complexes are formed in the melt. Consequently, it is concluded that cerium exists only as Ce(III) in basaltic magmas; cerium anomalies cannot be ascribed to the stabilization of Ce(IV) in magmas.     

Redox evolution of differentiating hydrous basaltic magmas recorded by zircon and apatites in mafic cumulates: The case of the Malayer Plutonic Complex,Western Iran

Mafic-ultramafic cumulates can provide records of basaltic magma chambers' conditions and processes, which are often difficult to determine in areas dominated by crustal-derived felsic intrusions, such as the Malayer Plutonic Complex (MPC), Western Iran. New U-Pb zircon ages for mafic cumulates in the MPC confirm the presence of isolated magma chambers of contrasting compositions during Middle Jurassic. Mafic cumulates found in seven separate zones across the MPC vary from olivine gabbro to anorthosite. While the mineralogical, textural, and geochemical lines of evidence recorded in mafic cumulates indicate pH₂O controls on the liquidus phases, the estimated oxygen fugacity (logfO₂) using zircon and apatite chemistry suggests a smoothly rising redox state during the fractionation process, consistent with the trend expected for late-stages differentiation of hydrous arc magmas. This trend is further confirmed by sulfur speciation in apatites determined from microbeam sulfur K-edge X-ray absorption near edge structure (μ-XANES) spectra (S⁶⁺/∑S = 0.93–0.98 ~ FMQ + 2 to 0.99 ~ FMQ + 3, where ∑S = S⁶⁺+S⁴⁺+S²⁻). The low S content and increasing redox state of the fractionating basaltic melts most likely resulted from preferential removal of sulfur en-route to the magma chambers along with effective assimilation of oxidizing crustal components. The reduced condition in the early basaltic melt is also evidenced by the presence of pyrite and magnetite inclusions in olivines in mafic cumulates. The shift in the prevailing fO₂ from sulfide-saturated to sulfate-bearing recorded by MPC mafic cumulates, similar to that in other magmatic arcs, is accompanied by changes in the differentiation path from transitional tholeiitic to calc-alkaline.     

Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand

Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption ( 0.01 km³ magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of  0.3 km³. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) ( 0.3 km³), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) ( 4.5 km³). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km³ of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of < 8 km. Each of the magmas display quartz crystals containing melt inclusions that are compositionally highly evolved relative to the accompanying matrix glass, and thus point to a stage of more complete crystallisation. The matrix glass, enriched in Sr and Ti, represents a re-melting event of underlying the crystal pile induced by basaltic intrusion, presumably part of the same event that erupted scoria at the start of the eruption. This recharge rhyolite melt percolated upward and hybridised with the resident melts in each of the three magma pods. The Okareka episode rhyolites contrast with other well-documented rhyolites that are either continuously or discontinuously zoned, or have been homogenised during re-activation to a uniform composition. Rapid basalt dike intrusion to shallow levels appears to have (prematurely?) triggered the Okareka rhyolites into eruption, so that their early ponding in separate melt pods has been recorded before it could be masked by mixing or stratification had amalgamation into a larger body occurred.     

Commingling and mixing of S-type peraluminous, ultrapotassic and basaltic magmas in the Cayconi volcanic field, Cordillera de Carabaya, SE Peru

The Cayconi district of the Cordillera de Carabaya, SE Peru, exposes a remnant of an upper Oligocene–Lower Miocene (22.2–24.4 Ma) volcanic field, comprising a diverse assemblage of S-type silicic and calc-alkaline basaltic to andesitic flows, members of the Picotani Group of the Central Andean Inner Arc. Basaltic flows containing olivine, plagioclase, clinopyroxene, ilmenite and glass, and glassy rhyolitic agglutinates with phenocrystic quartz, cordierite, plagioclase, sanidine, ilmenite and apatite, respectively exhibit mineralogical and geochemical features characteristic of medium-K mafic and Lachlan S-type silicic lavas. Cordierite-bearing dacitic agglomerates and lavas, however, are characterized by dispersed, melanocratic micro-enclaves and phenocrysts set in a fine-grained quartzo-feldspathic matrix. They contain a bimodal mica population, comprising phlogopite and biotite, as well as complexly zoned, sieve-textured plagioclase grains, sector-zoned cordierite, sanidine, quartz, irregular patches of replaced olivine, clinopyroxene and orthopyroxene and accessory phases including zircon, monazite, ilmenite and chromite. The coexistence of minerals not in mutual equilibrium and the growth/dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing. Trachytic-textured andesite flows interlayered with olivine+plagioclase–glomerophyric, calc-alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite, Ca-rich sanidine, ilmenite and aluminous spinel. The mineralogical and mineral chemical relationships in both the dacites and the trachytic-textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts, not represented by exposed rock types, and both the S-type silicic and calc-alkaline mafic magmas. Such mixing relationships are commonly observed in the Oligo-Miocene rocks of the Cordillera de Carabaya, suggesting that the S-type rocks in this area and, by extension, elsewhere derive their unusually high K₂O, Ba, Sr, Cr and Ni concentrations from commingling and mixing with diverse, mantle-derived potassic mafic magmas.     

Volatiles in basaltic magmas of ocean islands and their mantle sources: II. Estimation of contents in mantle reservoirs

In this paper, we address the average compositions (including the contents of H₂O, Cl, F, and S) and the compositional structure of oceanic mantle plumes on the basis of element contents and ratios in ocean island magmas. The average contents of incompatible volatile and nonvolatile elements were calculated for the material of mantle plumes using a thermal and a more plausible moderately enriched model. The following average contents were estimated for the plume mantle: 510 ppm K₂O, 520 ppm H₂O, 21 ppm Cl, 55 ppm F, and 83 ppm S. These values are significantly higher than those of the depleted mantle (except for S). The primitive mantle normalized average content of water in mantle plumes is similar to those of La and Ce but lower than those of K, Cl, and Sr. This is at odds with the hypothesis of “wet” mantle plumes. Three types of basaltic magmas distinguished in our previous study (Part I) characterize three types of plume sources (MI, MII, and MIII). Using the favored moderately enriched model, the average contents of H₂O, Cl, F, and S were estimated for the three sources (ppm): 130, 33, 11, and 110 for MI; 110, 12, 65, and 45 for MII; and 530, 29, 49, and 110 for MIII, respectively. The plume mantle is heterogeneous and its heterogeneity can be described by the presence of three main types of compositions, one of which (MI) is similar to the composition of the mid-ocean ridge mantle and the other two types (MII and MIII) are moderately enriched in K, Ti, P, F, and incompatible trace elements but depleted in Cl, H₂O, and sometimes S. The compositions of MII and MIII have different H₂O, Cl, and S contents: MII is significantly depleted in these components compared with MIII. The MII component is probably similar to the enriched mantle (EM). In addition to the aforementioned three main components, the plume mantle probably contains high-Cl and low-F materials, which are related to the recycling of the oceanic and continental crust. All the observed characteristics of the mantle plumes are in adequate agreement with the model of a zonal mantle plume including a central part hot and depleted in H₂O, Cl, and S; a periphery enriched in volatile components; and the enclosing mantle interacting with the plume material.     

Some constraints on the origin of phonolites from the Gregory Rift,Kenya, and inferences concerning basaltic magmas in the Rift System

Plateau-type phonolites of the Gregory Rift represent magmas with densities of about 2.3 g/cm³. These magmas must have erupted soon after their formation. The average depth of the bases of the approximately 100 to 300 magma chambers from which they erupted was between about 10 and about 24 km. The average vertical extent of the magma chambers was between about 3 and about 8 km. The aggregate volume of phonolitic magma formed beneath the Rift in Miocene times probably lies in the range of 0.5 to 1×10⁵ km³. Both the crystal fractionation model and the anatectic model for formation of this volume of phonolitic magma require the presence of a large reservoir of basaltic magma, probably picritic in character, with a volume of at least 5×10⁵ km³ and perhaps as much as 20×10⁵ km³. This reservoir presumably is now part of the dense basic intrusive complex along the Rift axis. The Miocene and Pliocene episodes of basaltic volcanism in this region may be related to eruption of evolved liquids from this reservoir.     

Evolution of the trachydacite and pantellerite magmas of the bimodal volcanic association of Dzarta-Khuduk, Central Mongolia: Investigation of inclusions in minerals

Using various methods of melt inclusion investigation, including electron and ion microprobe techniques, we estimated the composition, evolution, and formation conditions of melts producing the trachydacites and pantellerites of the Late Paleozoic bimodal volcanic association of Dzarta-Khuduk, Central Mongolia. Primary crystalline and melt inclusions were detected in anorthoclase from trachydacites and quartz from pantellerites and pantelleritic tuffs. Among the crystalline inclusions, we identified hedenbergite, fluorapatite, and pyrrhotite in the trachydacites and F-arfvedsonite, fluorite, ilmenite, and the rare REE diorthosilicate chevkinite in the pantellerites. Melt inclusions in anorthoclase from the trachydacites are composed of glass, a gas phase, and daughter minerals (F-arfvedsonite, fluorite, villiaumite, and anorthoclase rim on the inclusion wall). Melt inclusions in quartz from the pantellerites are composed of glass, a gas phase, and a fine-grained salt aggregate consisting of Li, Na, and Ca fluorides (griceite, villiaumite, and fluorite). Melt inclusions in quartz crystalloclasts from the pantelleritic tuffs are composed of homogeneous silicate glasses. The phenocrysts of the trachydacites and pantellerites crystallized at temperatures of 1060–1000°C. During thermometric experiments with quartz-hosted melt inclusions from the pantellerites, the formation of immiscible silicate and salt (fluoride) melts was observed at a temperature of 800°C. Homogeneous melt inclusions in anorthoclase from the trachydacites have both trachydacite and rhyolite compositions (wt %): 68–70 SiO₂, 12–13 Al₂O₃, 0.34–0.74 TiO₂, 5–7 FeO, 0.4–0.9 CaO, and 9–12 Na₂O + K₂O. The agpaitic index ranges from 0.92 to 1.24. The glasses of homogenized melt inclusions in quartz from the pantellerites and pantelleritic tuffs have rhyolitic compositions. Compared with the homogeneous glasses trapped in anorthoclase of the trachydacites, quartz-hosted inclusions from the pantellerites show higher SiO₂ (72–78 wt %) and lower Al₂O₃ contents (7.8–10.0 wt %). They also contain 0.14–0.26 wt % TiO₂, 2.5–4.9 wt % FeO, 9–11 wt % Na₂O + K₂O, and 0.9–0.15 wt % CaO and show an agpaitic index of 1.2–2.05. Homogeneous melt inclusions in quartz from the pantelleritic tuffs contain 69–72 wt % SiO₂. The contents of other major components, including TiO₂, Al₂O₃, FeO, and CaO, are close to those in the homogeneous glasses of quartzhosted melt inclusions in the pantellerites. The contents of Na₂O + K₂O are 4–10 wt %, and the agpaitic index is 1.0–1.6. The glasses of melt inclusions from each rock group show distinctive volatile compositions. The H₂O content is up to 0.08 wt % in anorthoclase of the trachydacites, 0.4–1.4 wt % in quartz of the pantellerites, and up to 5 wt % in quartz of the pantelleritic tuffs. The content of F in the glasses of melt inclusions in the phenocrysts of the trachydacites is no higher than 0.67 wt %, and up to 1.4–2.8 wt % in quartz from the pantellerites. The Cl content is up to 0.2 wt % in the glasses of melt inclusions in the minerals of the trachydacites and up to 0.5 wt % in the glasses of quartz-hosted melt inclusions from the pantellerites. The investigation of trace elements in the homogenized glasses of melt inclusions in minerals showed that the trachydacites and pantellerites were formed from strongly evolved rare-metal alkaline silicate melts with high contents of Li, Zr, Rb, Y, Hf, Th, U, and REE. The analysis of the composition of homogeneous melt inclusions in the minerals of the above rocks allowed us to distinguish magmatic processes resulting in the enrichment of these rocks in trace and rare earth elements. The most important processes are the crystallization differentiation and immiscible separation of silicate and fluoride salt melts. It was also shown that all the melts studied evolved in spatially separated magma chambers. This caused the differences in the character of melt evolution between the trachydacites and pantellerites. During the final stages of differentiation, when the magmatic system was saturated with respect to ore elements, Na-Ca fluoride melts were separated and extracted considerable amounts of Li.     

Experimental study of interaction between fluid-bearing basaltic melts and peridotite: A mantle-crustal source of trap magmas in the Norilsk area

The paper reports data on the geology and tectono-magmatic reactivation of the Norilsk area and on the stratigraphy and geochemistry of its volcanic sequence, with the discussion of the sources and genesis of the ore magmas and the scale of the ore-forming process. According to the geochemistry of the lavas and intrusive rocks (Ti concentration and the La/Sm and Gd/Yb ratios), two types of the parental magmas are recognized: high-Ti magmas of the OIB type (from bottom to top, suites iv, sv, and gd of phase 1) and low-Ti magmas (suites hk, tk, and nd of phase 2 and suites mr-mk of phase 3), which were derived from the lithospheric mantle. The magmatic differentiation of the parental low-Ti magma of the tk type into a magma of the nd type was associated with the derivation of an evolved magma of the nd type, which was depleted in ore elements, and an ore magma, which was a mixture of silicate and sulfide melts, protocrysts of silicate minerals, and chromite. Judging from their geochemical parameters, the intrusions of the lower Norilsk type were comagmatic with the lavas of the upper part of the nd suite, and the ore-bearing intrusions of the upper Norilsk type were comagmatic with the lavas of the mr-mk suites. When the ore-bearing intrusions were emplaced, their magmas entrained droplets of sulfide melt and protocrysts of olivine and chromite and brought them to the modern magmatic chamber. These protocrysts are xenogenic with respect to the magma that formed the intrusions. In certain instances (Talnakh and Kharaelakh intrusions), the moving magma entrained single portions of sulfide magma, which were emplaced as individual subphases. The experimental study of the peridotite-basalt-fluid system shows that mantle reservoirs with protoliths of subducted oceanic crustal material could serve as sources of relatively low-temperature (1250–1350°C) high-Ti magnesian magmas of the rifting stage from an olivine-free source.     

Crystallization of primitive basaltic magmas at crustal pressures and genesis of the calc-alkaline igneous suite: experimental evidence from St Vincent,Lesser Antilles arc

Near-liquidus crystallization experiments have been carried out on two basalts (12.5 and 7.8 wt% MgO) from Soufriere, St Vincent (Lesser Antilles arc) to document the early stages of differentiation in calc-alkaline magmas. The water-undersaturated experiments were performed mostly at 4 kbar, with 1.6 to 7.7 wt% H₂O in the melt, and under oxidizing conditions (ΔNNO = −0.8 to +2.4). A few 10 kbar experiments were also performed. Early differentiation of primitive, hydrous, high-magnesia basalts (HMB) is controlled by ol + cpx + sp fractionation. Residual melts of typical high-alumina basalt (HAB) composition are obtained after 30–40% crystallization. The role of H₂O in depressing plagioclase crystallization leads to a direct relation between the Al₂O₃ content of the residual melt and its H₂O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo_89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H₂O) and very oxidized (ΔNNO = +1.5–2.0) HMB liquids at middle crustal pressures and temperatures from ∼1,160 to ∼1,060°C. Hornblende played no role in the early petrogenetic evolution. Derivative HAB melts may contain up to 7–8 wt% dissolved H₂O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H₂O concentrations (2–5 wt%).     

Perspectives on basaltic magma crystallization and differentiation: Lava-lake blocks erupted at Mauna Loa volcano summit, Hawaii

Explosive eruptions at Mauna Loa summit ejected coarse-grained blocks (free of lava coatings) from Moku'aweoweo caldera. Most are gabbronorites and gabbros that have 0–26 vol.% olivine and 1–29 vol.% oikocrystic orthopyroxene. Some blocks are ferrogabbros and diorites with micrographic matrices, and diorite veins (≤ 2 cm) cross-cut some gabbronorites and gabbros. One block is an open-textured dunite.

The MgO of the gabbronorites and gabbros ranges  7–21 wt.%. Those with MgO > 10 wt.% have some incompatible-element abundances (Zr, Y, REE; positive Eu anomalies) lower than those in Mauna Loa lavas of comparable MgO; gabbros (MgO < 10 wt.%) generally overlap lava compositions. Olivines range Fo_83–58, clinopyroxenes have Mg#s  83–62, and orthopyroxene Mg#s are 84–63 — all evolved beyond the mineral-Mg#s of Mauna Loa lavas. Plagioclase is An_75–50. Ferrogabbro and diorite blocks have  3–5 wt.% MgO (TiO₂ 3.2–5.4%; K₂O 0.8–1.3%; La 16–27 ppm), and a diorite vein is the most evolved (SiO₂ 59%, K₂O 1.5%, La 38 ppm). They have clinopyroxene Mg#s 67–46, and plagioclase An_57–40. The open-textured dunite has olivine  Fo_83.5. Seven isotope ratios are ⁸⁷Sr/⁸⁶Sr 0.70394–0.70374 and ¹⁴³Nd/¹⁴⁴Nd 0.51293–0.51286, and identify the suite as belonging to the Mauna Loa system.

Gabbronorites and gabbros originated in solidification zones of Moku'aweoweo lava lakes where they acquired orthocumulate textures and incompatible-element depletions. These features suggest deeper and slower cooling lakes than the lava lake paradigm, Kilauea Iki, which is basalt and picrite. Clinopyroxene geobarometry suggests crystallization at < 1 kbar P. Highly evolved mineral Mg#s, < 75, are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional ‘shifts.’ Ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes. Clinopyroxene geobarometry suggests < 300 bar P. Open-textured dunite represents olivine-melt mush, precursor to vertical olivine-rich bodies (as in Kilauea Iki). Its Fo_83.5 identifies the most primitive lake magma as  8.3 wt.% MgO. Mass balancing and MELTS show that such a magma could have yielded both ferrogabbro and diorite by ≥ 50% fractional crystallization, but under different fO₂: < FMQ (250 bar) led to diorite, and FMQ (250 bar) yielded ferrogabbro. These segregation veins, documented as similar to those of Kilauea, testify to appreciable volumes of ‘rhyolitic’ liquid forming in oceanic environments. Namely, SiO₂-rich veins are intrinsic to all shields that reached caldera stage to accommodate various-sized cooling, differentiating lava lakes.     

Iron in plagioclase in the Bushveld and Skaergaard intrusions: implications for iron contents in evolving basic magmas

The evolved, iron-rich rocks of the tholeiitic Bushveld and Skaergaard intrusions are similar in containing cumulus magnetite, ilmenite, plagioclase, clinopyroxene, apatite and olivine, and also orthopyroxenes/pigeonite in Bushveld. Here, we evaluate their liquid evolution trends using the total iron content in plagioclase determined by electron microprobe analyses. To aid this analysis a revised mass balance model for the liquid evolution of Skaergaard is presented. For plagioclase in the Upper Zone of Skaergaard it was previously demonstrated that total FeO increases from ~0.25 to ~0.45 wt% with differentiation and correlates inversely with An% [100 × Ca/(Na + Ca)]. The reverse trend is observed in two recently published datasets for Bushveld, showing that total FeO in plagioclase decreases upward through the magnetite-bearing Upper Zone from ~0.30 to ~0.15% and from ~0.40 to ~0.25% in the western and northern limbs, respectively, and correlates positively with An%. The partition coefficient of total iron between plagioclase and magma increases with oxidation and polymerisation in the liquid. Although Bushveld formed under slightly more oxidizing conditions than Skaergaard, differences in the partition coefficients cannot explain the two observed trends. We therefore conclude that the differentiation trends of the liquids subsequent to magnetite saturation were fundamentally different. The inferred liquid composition for Bushveld contained about 15 wt% total FeO at the level of magnetite-in, which is slightly less than the total FeO content of the subsequent cumulates. In contrast, the Skaergaard liquid contained more total FeO than the ensuing cumulates. As a result, in Bushveld residual liquids total FeO decreased after magnetite saturation, whereas in Skaergaard the residual liquids continued to become enriched in iron. This conclusion is corroborated by simple mass balance calculations between modelled residual liquids and extracted cumulate rocks. Despite the mineralogical similarities of evolved iron-rich rocks of Skaergaard and Bushveld, their liquid evolution trends were very different, and generalizations about the extent of iron enrichment in tholeiitic magmas should be avoided.     

Highly evolved hypabyssal kimberlite sills from Wemindji,Quebec, Canada: insights into the process of flow differentiation in kimberlite magmas

Kimberlite sills emplaced in granite located near the town of Wemindji (Quebec, Canada) range from 2 cm to 1.2 m in thickness. The sills exhibit a wide variation in macroscopic appearance from fine-grained aphanitic dolomitic hypabyssal kimberlite to ilmenite/garnet macrocrystal hypabyssal kimberlite. Diatreme or crater facies rocks are not present. Multiple intrusions are present within the sills, and graded bedding and erosional features such as cross-bedding are common. The sills exhibit a wide range in their modal mineralogy with respect to the abundances of spinel, apatite, phlogopite and dolomite. Olivine is the dominant macrocryst, with an average composition of Fo₉₀. Garnet macrocrysts are low chrome (2–3 wt. %) pyrope (G1/G9 garnet). Ilmenite occurs as rounded macrocrysts (7–13 wt. % MgO). Phlogopite microphenocrysts are Ti-poor and represent a solid solution between phlogopite and kinoshitalite end members. Spinel compositions mainly represent the Cr-poor members of the qandilite–ulvöspinel–magnetite series. The principle carbonate comprising the groundmass is dolomite, with lesser later-forming calcite. Accessory minerals include apatite, Sr-rich calcite, Nb-rich rutile, baddeleyite, monazite-(Ce) and barite. While some of these accessory minerals are atypical of kimberlites in general, it is expected that differentiation products of an evolved carbonate-rich kimberlite magma will crystallize these phases. The Wemindji kimberlites offer insight into the process of crystal fractionation and differentiation in evolved kimberlite magmas. The macroscopic textural features observed in the Wemindji sills are interpreted to represent flow differentiation of a mantle-derived, very fluid, low viscosity carbonate-rich kimberlite. The diverse modes and textural features result entirely from flow differentiation and multiple intrusions of different batches of genetically related kimberlite magma. The mineralogy of the Wemindji kimberlites has some similarities to that of the Wesselton and Benfontein calcite kimberlite sills but differs in detail with respect to dominant carbonate (i.e. dolomite versus calcite), and the character of the rare earth-bearing accessory minerals (i.e. monazite-(Ce) versus rare earth fluorocarbonates).     

A regular solution model for met-aluminous silicate liquids: Applications to geothermometry,immiscibility, and the source regions of basic magmas

Adopting a set of multioxide components and using published compositional data on olivineand plagioclase-liquid equilibria we have developed a 17 component regular solution model for met-aluminous silicate liquids. The partial molar excess free energies predicted from this model can be used together with phenocryst compositions as an effective geothermometer, with an approximate error of 20 °C (30 °C for olivine, 12 °C for plagioclase). The regular solution formulation is also successful in predicting liquid immiscibility at (1) high mole fractions of silica commonly observed in phase diagrams, and at (2) lower temperatures in lunar basalts and intermediate lavas. The model yields activities of silica which are consistent with those obtained from solid-liquid silica buffers in rocks which contain olivine and enstatite or quartz. From predicted activities of KAlSi₃O₈ in liquids coexisting with plagioclase a value is obtained for the limiting Henry's law activity coefficient of KAlSi₃O₈ in the solid. This coefficient agrees well with that inferred from plagioclase-sanidine equilibrium phenocryst assemblages in rhyolites. The activities of silica obtained from this model are used to place constraints on the pressure-temperature regions where various types of basic magmas are generated. In conjunction with plagioclase geothermometry an application is given where the pressure, temperature, and water content of an olivine andesite is predicted from the activity of silica.