The Baffin Bay lavas and the value of picrites as analogues of primary magmas

The Baffin Bay picrites have been the focal point of a controversy concerning the MgO content of primary magmas derived from the upper mantle. A sample population of 48 lava chilled margins collected across the Baffin Bay volcanic succession at the northeastern tip of Padloping Island exhibits a prominent compositional mode between 14 and 16 weight percent MgO (19–22 Mg, cation units = Mg/100 cations). The petrography of these samples, however, requires that the Padloping magmas were mixtures of olivine crystals and liquid at their eruption. Olivine phenocrysts constituted 15 to 30 volume percent of these magmas and retain compositions requiring coexisting liquid compositions with only 10 to 13.5 weight percent MgO (14–18.5 Mg). However, highly magnesian, olivine xenocrysts (up to Fo 93) found in the most magnesian lavas require the former existence of liquids with at least 18 weight percent MgO (24 Mg). If these xenocrysts represent early cumulates, then the primary liquids of the Padloping suite must have been at least this MgO rich with temperatures greater than 1,425° C. Such primary liquids could have evolved by olivine crystallization to a steady state, equilibrated crystal — liquid mixtures in a shallow reservoir system prior to eruption. The compositions of the liquids of these mixtures appear to have been perched at the point of plagioclase saturation at approximately 1,275° C.Despite the complications of mechanical sorting of olivine crystals, the virtual compositional reciprocity of olivine addition and olivine fractionation requires that the bulk compositions of picritic lavas provide compositional analogues of their primary magmas. A comparison of Phanerozoic picrite suites indicates that the Fe contents of terrestrial primary magmas of tholeiitic affinity have a restricted range from 6–9 Fe. Primary magmas associated with intra-plate volcanism appear to be distinctly more Fe-rich than those associated with inter-plate volcanism. The Al/Si ratios of Phanerozoic picrite suites could suggest that the primary magmas of MORB volcanism have equilibrated with relatively Fe-poor source regions at deeper levels in the Earth's mantle than those of other tholeiitic primary magmas.     

The petrology of some picrites from Mauna Loa and Kilauea volcanoes,Hawaii

The mineralogy and chemistry of picrites from Mauna Loa and Kilauea have been investigated to evaluate, for Hawaiian tholeiitic picrites, the contrasting genetic models which have been proposed for these Mg-rich volcanics, namely products of direct crystallization of high-Mg melts (20–25% MgO) or the result of accumulation of olivine phenocrysts into less Mg-rich melts. Genetic interpretations rely heavily on Mg-Fe partitioning relations between olivines and their picrite hosts. Although the 100 Mg/(Mg + Fe²⁺) ratios (M) of picrites are wide-ranging (M=73.6–82.9 for Fe₂O₃/FeO=0.15), with MgO as high as 27.8%, the average 100 Mg/(Mg+Fe) ratios (mg) of the cores of olivine phenocrysts (megacrysts) show only restricted compositional variation (mg=87.2–89.0). Successive olivine generations are progressively more Fe-rich. Olivine/liquid Mg-Fe partitioning data and the Mn and Ni abundances in olivine phenocrysts collectively indicate that they were precipitated by Mg-rich basaltic melts with 12–14% MgO. Spinel compositions range from liquidus magnesiochromites, occurring mainly as inclusions in olivine phenocrysts, to groundmass titanomagnetites which crystallized at nearsolidus temperatures. The Cr₂O₃ contents and M values of liquidus magnesiochromites suggest that their parent melts were neither Mg-rich picritic (MgO>20%) nor relatively Mg-poor basaltic types.On MgO variation diagrams (extending from approximately 7% to more than 25% MgO), Mauna Loa and Kilauea picrites and their respective microcrystalline/glassy groundmasses (the major component of quickly-cooled picrites) plot on linear regression lines (olivine control lines). At a given MgO content, Kilauean picrites and tholeiites (M<75) generally contain more TiO₂ FeO ^t , CaO, K₂O and P₂O₅, and less SiO₂ and Na₂O than Mauna Loan types. The compositions of the groundmasses in picrites and Mg-rich ol-tholeiites equate closely with those of the Mg-poor tholeiites (7–9% MgO) which dominate the petrology of each shield.Low-pressure closed system differentiation of Hawaiian tholeiitic magmas (10–15% MgO) can yield picritic derivatives which differ, however, from the extrusive picrites by virtue of distinctly higher FeO ^t contents and correspondingly more Fe-rich olivines and Cr-spinels.The calculated Mg-Fe olivine megacryst-liquid partition coefficient K _D for individual picrites indicate that lowpressure equilibria (K _D =0.30–0.34) are defined only by melts with approximately 12–14% MgO (M 71–74). Assessed in conjunction with Ni-MgO modeling, these data indicate that the more Mg-rich picrites (MgO> 14–15%) are indeed olivine-enriched and do not represent melt compositions. Olivine enrichment resulted from post-eruptive mechanical (flow) differentiation of extruded mushes of intratelluric cognate olivine phenocrysts (mg88) and tholeiitic melts (M60), which are residua of the parental magmas (12–14% MgO), following the crystallization of the olivine phenocrysts. The parental magmas of both picrite suites were generated by 35–40% melting of relatively Fe-rich spinel lherzolites (mg84) containing kaersutitic amphibole as a major primary constituent.     

Petrogenesis of Hawaiian tholeiites: 1, phase equilibria constraints

The most magnesian olivine phenocrysts [Mg no.=100 Mg/(Mg+Fe)=90.5] in Hawaiian tholeiites provide evidence for the earliest stages of differentiation of Hawaiian magmas. Based on the correction of olivine fractionation effects, the primitive melt compositions which have crystallised these olivines are picritic with 16 wt% MgO. They are excellent primary-melt candidates. An experimental study on a new Hawaiian picritic primary-melt estimate demonstrates multiple saturation with peridotite (harzburgite) at 2.0 GPa and 1450° C. Garnet is not a liquidus phase at pressures below 3.5 GPa, and garnet peridotite is not a liquidus phase assemblage at any pressure or temperature. This result confirms previous experimental studies on Hawaiian primary-melt estimates and conflicts with trace-elementgeochemistry-based interpretations, which claim that melt generation occurs in the presence of residual garnet. If Hawaiian tholeiite primary magmas are picritic and have equilibrated with garnet-absent peridotite residues, the geochemical and isotopic characteristics of Hawaiian tholeiites (i.e. Sm/Nd chondrites and Nd>0) are consistent with their source recently having been enriched in incompatible elements. Previous modelling shows that such characteristics are consistent with source enrichment through the migration of small melt fractions generated at depth in the presence of garnet. This may be effected either at the time of Hawaiian magma genesis through dynamic melt segregation processes or, by melting of a previously enriched mantle source; possibly oceanic lithospheric mantle which has been infiltrated by melt fractions from the underlying asthenosphere prior to Hawaiian magmatism. Alternatively, if Hawaiian primary magmas are ultramafic in composition (20 wt% MgO) they may be generated in the presence of garnet peridotite at pressures 3.0 GPa.     

Origins of compositional heterogeneity in olivine-hosted melt inclusions from the Baffin Island picrites

The Baffin Island picrites are highly magnesian (22 wt% MgO) olivine tholeiites, erupted through felsic continental crust. Plots of most major and minor element oxides against MgO for the lavas define very tight trends consistent with modification of melts parental to the erupted suite by olivine fractionation or accumulation. However, melt inclusions trapped in primitive olivine phenocrysts in these lavas have much more diverse compositions. After correction for post-entrapment modification, the inclusions are systematically slightly lower in Al₂O₃, and significantly higher in SiO₂, K₂O and P₂O₅ than the lavas fractionation trends. CaO, Na₂O and TiO₂ contents lie within the lavas fractionation trends. Similarly, most inclusions are higher in Sr/Nd, K/Nb, Rb/Ba, Rb/Sr, U/Nb and Ba/Th than the lavas. These characteristics resulted from up to 15% contamination of evolving picritic-basaltic liquids by locally-derived, broadly granitic partial melts of the quartz + feldspar-rich crust through which the picrites erupted. Contamination was minor in the bulk lavas (<1%), suggesting that the inclusions compositions partly reflect a link between wall rock reaction and precipitation of liquidus olivine. Rapid crystallisation of liquidus olivine from the picrites, along with melting of felsic crustal wall rocks of magma chambers or conduits, were likely during emplacement of hot picritic magmas into cooler felsic crust. Inclusion compositions may thus reflect mixing trends or may be constrained to phase boundaries between olivine and a phase being resorbed, for example, an olivine-plagioclase cotectic. The extent of contamination was probably a complex function of diffusion rates of components in the magmas, and phenocryst growth rates and proximity to wall rock. These results bear on the common observation that melt inclusions compositions are frequently more heterogeneous than those of the lavas that host them.     

Experimental study of a biotite-bearing granitic system under water-saturated and water-undersaturated conditions

Phase relationships in haplogranitic melts containing ferromagnesian minerals were investigated in the temperature range 760–850°C, atP _total=2 kbar, =0.25 1.00, and at NNO buffer conditions. The compositions of coexisting biotite, orthopyroxene, magnetite and granitic melt were determined by electron microprobe. Biotite was the only Fe–Mg mineral in the starting material. It disappeared in water-undersaturated melts at temperatures above 830°C and in melts saturated with water above 810°C. Magnetite was present in all runs. Orthopyroxene became stable between 780 and 800°C. The Mg/(Mg+Fe) ratios of biotite (0,63–0,72) and orthopyroxene (0,63–0,71) are similar in water-saturated and undersaturated melts at comparable temperatures and increase slightly with increasing temperature. The solubility of Mg in haplogranitic melts is very low (0,07–0,24 wt% MgO). It increases slightly with temperature and is independent of the prevailing water activity. The solubility of Fe is low (0,91–1,37 wt% FeO); it also increases with increasing temperature, and it is higher in water-saturated than in water-undersaturated melts.     

Revisiting the compositions and volatile contents of olivine-hosted melt inclusions from the Mount Shasta region: implications for the formation of high-Mg andesites

Primitive chemical characteristics of high-Mg andesites (HMA) suggest equilibration with mantle wedge peridotite, and they may form through either shallow, wet partial melting of the mantle or re-equilibration of slab melts migrating through the wedge. We have re-examined a well-studied example of HMA from near Mt. Shasta, CA, because petrographic evidence for magma mixing has stimulated a recent debate over whether HMA magmas have a mantle origin. We examined naturally quenched, glassy, olivine-hosted (Fo_87–94) melt inclusions from this locality and analyzed the samples by FTIR, LA-ICPMS, and electron probe. Compositions (uncorrected for post-entrapment modification) are highly variable and can be divided into high-CaO (>10 wt%) melts only found in Fo > 91 olivines and low-CaO (<10 wt%) melts in Fo 87–94 olivine hosts. There is evidence for extensive post-entrapment modification in many inclusions. High-CaO inclusions experienced 1.4–3.5 wt% FeO^T loss through diffusive re-equilibration with the host olivine and 13–28 wt% post-entrapment olivine crystallization. Low-CaO inclusions experienced 1–16 wt% olivine crystallization with <2 wt% FeO^T loss experienced by inclusions in Fo > 90 olivines. Restored low-CaO melt inclusions are HMAs (57–61 wt% SiO₂; 4.9–10.9 wt% MgO), whereas high-CaO inclusions are primitive basaltic andesites (PBA) (51–56 wt% SiO₂; 9.8–15.1 wt% MgO). HMA and PBA inclusions have distinct trace element characteristics. Importantly, both types of inclusions are volatile-rich, with maximum values in HMA and PBA melt inclusions of 3.5 and 5.6 wt% H₂O, 830 and 2,900 ppm S, 1,590 and 2,580 ppm Cl, and 500 and 820 ppm CO₂, respectively. PBA melts are comparable to experimental hydrous melts in equilibrium with harzburgite. Two-component mixing between PBA and dacitic magma (59:41) is able to produce a primitive HMA composition, but the predicted mixture shows some small but significant major and trace element discrepancies from published whole-rock analyses from the Shasta locality. An alternative model that involves incorporation of xenocrysts (high-Mg olivine from PBA and pyroxenes from dacite) into a primary (mantle-derived) HMA magma can explain the phenocryst and melt inclusion compositions but is difficult to evaluate quantitatively because of the complex crystal populations. Our results suggest that a spectrum of mantle-derived melts, including both PBA and HMA, may be produced beneath the Shasta region. Compositional similarities between Shasta parental melts and boninites imply similar magma generation processes related to the presence of refractory harzburgite in the shallow mantle.     

Anhydrous PT phase relations of an Aleutian high-MgO basalt: an investigation of the role of olivine-liquid reaction in the generation of arc high-alumina basalts

We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnston's (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H₂O, which will affect the influence of bulk CaO, MgO, and Al₂O₃ contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than 12 kbar.     

Phase relations of a natural MARID composition and implications for MARID genesis,lithospheric melting and mantle metasomatism

Melting experiments were performed on a natural mica-amphibole-rutile-ilmenite-clinopyroxene (MARID) sample from the Kaapvaal mantle lithosphere (AJE137) at 20 to 35 kbar and 800 to 1450°C. A solidus was determined at 1260°C and 30 kbar above which phlogopite, clinopyroxene and olivine were stable with an alkali-rich silicate melt. Olivine is the only crystallizing phase just below the liquidus of the AJE137 bulk composition and K-richterite was only stable in the subsolidus region ( 1100°C at 30 kbar). These results are consistent with previous studies in more simple systems. In experiments with 10 wt% added water the solidus was depressed by ca. 300°C and K-richterite was stabilized above this solidus. MARIDs represent a potential lowtemperature component in the lithospheric mantle beneath the Kaapvaal Craton of southern Africa. The addition of > 10 wt% water (with less than a 120°C rise of temperature above the geotherm) to this mantle region would create conditions for the melting of this component. This may then be incorporated in any continental flood basalt parent magma that traverse this lithospheric mantle. The derivation of MARIDs from a silicate melt of their bulk composition, even if water saturated, is considered unlikely as such small degree melts could not sustain the elevated liquidus temperatures required (> 1200°C at 30 kbar) in a cold (< 800°C at 30 kbar) mantle lithosphere. MARID xenoliths may be produced by the interaction of an alkali-rich fluid with a peridotite or as the residue to a group II kimberlitic parent magma that has undergone fractionation of olivine and the exsolution of a carbonatite component.     

Field setting,mineralogy, chemistry,and genesis of arc picrites,New Georgia,Solomon Islands

The field setting, petrography, mineralogy, and geochemistry of a suite of picrite basalts and related magnesian olivine tholeiites (New Georgia arc picrites) from the New Georgia Volcanics, Kolo caldera in the active ensimatic Solomon Islands arc are presented. These lavas, with an areal extent in the order of 100² km and almost 1 km thick in places, are located close to the intersection of the Woodlark spreading zone with the Pacific plate margin. They contain abundant olivine (Fo_94-75) and diopside (Cr₂O₃ 1.1-0.4%, Al₂O₃ 1–3%), and spinels characterised by a large range in Cr/(Cr+Al) (0.85–0.46) and Mg/(Mg+ Fe⁺⁺) (0.65–0.1). The spinels are Fe⁺⁺⁺ rich, with Fe⁺⁺⁺/ (Fe⁺⁺⁺+Cr+Al) varying from 0.06 to 1.0. A discrete group of spinels with the highest Cr/(Cr+Al) (0.83–0.86) and lowest Fe⁺⁺⁺ contents are included in the most Mg-rich olivine (Fo_91–94) and both may be xenocrystal in origin.The lavas, which range between 10–28% MgO, define linear trends on oxide (element) — MgO diagrams and these trends are interpreted as olivine (0.9) clinopyroxene (0.1) control lines. For the reconstructed parent magma composition of these arc picrites, ratios involving CaO, Al₂O₃, TiO₂, Zr, V and Sc are very close to chondritic. REE patterns are slightly LREE — enriched ((La/Sm)^N 1.3–1.43) and HREE are flat. All lavas show marked enrichments in K, Rb, Sr, Ba, and LREE relative to MORB with similar MgO contents, but the TiO₂ content of the proposed parent magma is close to those of postulated primary MORB liquids. It is proposed that the arc parent magma was produced by partial melting of sub-oceanic upper mantle induced by the introduction of LILE — enriched hydrous fluids derived by dehydration and/or partial melting of subducted ocean crust and possibly minor sediments.     

Late Palaeozoic Ultramafic Lavas in Yunnan, SW China, and their Geodynamic Significance

We report petrological and geochemical data on ultramafic pillowlavas from Late Palaeozoic marine sequences in Yunnan, SW China.These lavas have >26 wt % MgO, euhedral to subhedral olivinephenocrysts and acicular or quench clinopyroxene crystals withor without microlitic plagioclase in a devitrified and alteredglassy matrix. These ultramafic lavas are compositionally komatiitic(>18 wt % MgO), but we term them high-Mg picrites becausethey lack spinifex-textured olivine. Although olivines in thesepicrites are cumulate crystals, causing the high MgO contentsof the bulk rocks, the high forsterite content of these olivines,Fo = 0·902 ± 0·011, suggests that the primitivemagmas parental to the picrites would have had     

The genesis of refractory melts in the formation of oceanic crust

Refractory, primary liquids arising in various oceanic plate tectonic settings are characterized by high MgO, SiO₂, Ca/Na, low TiO₂ and generally low incompatible element abundances relative to primary liquids parental to MORB. We propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to MORB. A compositional range in the second-stage melts is expected, depending on the extent of previous depletion of the peridotite, the temperature and pressure of melt segregation, and the possible influence of volatile phases (C-H-O) present during melting.An example of a second stage melt is of magnesian quartz tholeiite composition, identified from among the Upper Pillow Lavas, Troodos ophiolite, Cyprus. Experimental studies determine that this composition has appropriate liquidus phases to have segregated from depleted upper mantle peridotite at about 25 km, 1360° C leaving a harzburgite residue. The experimental studies are applied to interpretation of cooling histories and water contents of specific Upper Pillow Lavas. Magma batches are estimated to have contained 0.5–1.0% H₂O. Picritic lavas quenched from olivine +liquid at <5 kb. Magnesian, pyroxene-phyric lavas exhibit intratelluric crystallization at 5 kb, 1270° C (Mg₈₈ pigeonite and Mg₈₉ orthopyroxene).These and other second-stage melts will crystallize extremely refractory minerals identical to many found in cumulate sequences in ophiolites, in plutonic rocks dredged and drilled from ocean basins, and occurring as xenocrysts in ocean floor basalts. Multistage melting of upper mantle peridotite, with and without presence of water, reconciles some of the present difficulties in relating ophiolite and ocean floor basalt compositions, and is an important process in ocean crust formation in a variety of different oceanic settings (mid-ocean ridges, marginal basins, and island arcs).     

Cellular plagioclase intergrowths as a result of crystal-magma mixing in the Proterozoic Åland rapakivi batholith,SW Finland

Finely cellular plagioclase intergrowths have been studied in xenocrystic andesine (An₃₂) and andesine mantled K-feldspars within mafic magmatic enclaves in a quartz-feldspar porphyry from the Proterozoic subvolcanic Hammarudda complex, Åland rapakivi batholith, SW Finland. The cellular intergrowths usually occur as 0.2–2.0 mm mantles around xenocrysts but also as entirely cellular grains, and are built up of a network of two distinct phases: one relatively Na-rich (An₃₁) and one relatively Ca-rich (An₅₀). The grains are also covered by a thin (0.08–0.12 mm), continuous, normally zoned rim outside the cellular mantle. Small inclusions (0.01–0.05 mm) of Fe–Mg minerals are concentrated in the Ca-rich part of the network. Compositionally, the Na-rich phase of the network is close to the inner non-cellular andesine of the xenocrysts. However, it has a lower Or- and a slightly lower An-content. The Ca-rich phase has the same composition as the inner part of the normally zoned rim, which outwards grades into lower An-contents that overlap the An-content of the matrix plagioclases. The cellular network was developed after the andesine xenocrysts (or andesine mantled K-feldspars) were engulfed in mafic magmatic enclaves during a mixing event. The xenocrysts became heated to a temperature just below the liquidus of the mafic magma. Dissolution of the xenocrysts developed a spongy cellular texture which was penetrated by enclave magma. Ca-rich plagioclase crystallized in the cells in equilibrium with the enclave magma, trapping Fe–Mg-rich melt. As the enclaves cooled the outermost thin rim and matrix plagioclases crystallized from the mafic melt. These processes operated in fairly large enclaves, as the one studied here, which has a diameter of 70cm. Smaller enclaves, on the other hand, were cooled more rapidly to temperatures close to the solidus of the enclave magma, and consequently had no time to dissolve the xenoxrysts.     

Geochemical constraints on the petrogenesis of arc picrites and basalts,New Georgia Group,Solomon Islands

Island arc picrites are restricted to a few localities including the Lesser Antilles, Japan, Vanuatu and the Solomon Islands. The picrite occurrences appear to be linked to the subduction of young, hot oceanic crust and anomalous geotherms. At the Solomon arc, the Australian plate is presently subducted beneath the Pacific plate. A particular feature of the Solomon arc is the subduction of a spreading center (Woodlark Ridge). In the Solomon Islands, picrites only occur in the New Georgia archipelago, located above or close to the subducting Woodlark Ridge. These picrites contain between 12 and 30 wt% MgO, the associated primitive basalts show MgO contents from 11.5 to 13.6 wt%. Linear trends defined by Cr, Ni and other trace elements vs. MgO indicate that the picritic bulk compositions originate from mixing between a basaltic-picritic melt and a Mg- and Cr-rich endmember, rather than from fractional crystallization of extremely Mg-rich magmas. Major and trace element modeling identify mantle wedge peridotite as the most likely mixing endmember. Trace element abundances in the Solomon arc picrites indicate a mantle source enrichment by subduction components and a large depletion of Nb and Ta that is typical for island arc volcanic rocks. Most incompatible trace element patterns of the New Georgia picrites and basalts are parallel, supporting a cogenetic evolution of these rocks by mixing processes. ⁸⁷Sr/⁸⁶Sr and Nd values in the basalts and picrites range from 0.7033 to 0.7043 and +5.8 to +8.0, respectively. These values partially overlap with compositions of the Indian MORB field. Alternatively, subducted sediment and fluids from altered MORB may have displaced the Sr isotope composition to more radiogenic ⁸⁷Sr/⁸⁶Sr. Hf values range from +12.2 to +14.6 and show in combination with Nd that the picrites were most likely generated within the Indian mantle domain.This revised version was published online September 2004 with a correction to Table 2.     

Potassic primary melts of vulsini (Roman Province): evidence from mineralogy and melt inclusions

The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo_92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO=13.5 wt%, NiO=340 ppm; Cr=1275 ppm) in the northern part of the Roman Comagmatic Province. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo<90.5, fractional crystallization (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The olivine (Fo_92-90.5) and the Cr-spinel (Cr#=100. Cr/(Cr+Al)=63-73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H₂O-rich ( 1 wt%) primary melts (MgO=8.4-9.7 wt%,FeO^total=6-7.5 wt%). They evidence a wide compositional range (in wt%: SiO₂=46.5-50, K₂O=5.3-2.8, P₂O₅=0.4-0.2, S=0.26-0.12; Cl=0.05-0.03, and CaO/Al₂O₃= 0.8-1.15), with negative correlations between SiO₂ and K₂O, Al₂O₃ and CaO, as well as positive correlations between K₂O, and P₂O₅, S, Cl, with nearly constant ratios between these elements. These results are discussed in terms of segregation of various mantle-derived melts. The high and constant Mg# [100.Mg/(Mg+Fe²⁺)] 73-75 of studied melts and their variable Si, K, P, Ca, Al, S contents could be explained by the melting of a refractory lithospheric mantle source, heterogeneously enriched in phlogopite and clinopyroxene (veined mantle source).     

Petrochemistry of eclogites from the Koidu Kimberlite Complex,Sierra Leone

Petrography, mineral and bulk chemistry of upper mantle-derived eclogites (garnet and clinopyroxene) from the Koidu Kimberlite Complex, Sierra Leone, are presented in the first comprehensive study of these xenoliths from West Africa. Although peridotite-suite xenoliths are generally more common in kimberlites, the upper mantle sample preserved in Pipe Number 1 at Koidu is exclusively eclogitic, making this the fifth locality in which eclogite is the sole polymineralic xenolith in kimberlite. Over 2000 xenoliths were collected, of which 47 are described in detail that include diamond, graphite, kyanite, corundum, quartz after coesite, and amphibole eclogites. Grossular-pyrope-almandine garnets are chromium-poor (<0.72 wt% Cr₂O₃) and fall into two distinct groups based on magnesium content. High-MgO garnets have an average composition of Pyr₆₇Alm₂₂Gross₁₁, low-MgO garnets are grossular- and almandine-rich with an average composition of Gross₃₄Pyr₃₃Alm₃₃. Clinopyroxenes are omphacitic with a range in jadeite contents from 7.7 to 70.1 mol%. Three eclogites contain zoned and mantled garnets with almandine-rich cores and pyrope-rich rims, and zoned clinopyroxenes with diopside-rich cores and jadeite-rich rims, and are among a very rare group of eclogites reported on a world-wide basis. The bulk compositions of eclogites have ranges comparable to that of basalts. High-MgO eclogites (16–20 wt% MgO) have close chemical affinities to picrites, whereas low-MgO eclogites (6–13 wt% MgO) are similar to alkali basalts. High-MgO eclogites contain high-MgO garnets and jadeiterich clinopyroxenes. Low-MgO eclogites contain low-MgO garnets, diopside and omphacite, and the group of primary accessory phases (diamond, graphite, quartz after coesite, kyanite, and corundum); grospydites are peraluminous. Estimated temperatures and pressures of equilibration of diamond-bearing eclogites, using the diamond-graphite stability curve and the Ellis and Green (1979) geothermometer, are 1031°–1363° C at 45–50 kb.K _D values of Fe-Mg in garnet and clinopyroxene range from 2.3 to 12.2. Diamonds in eclogites are green, yellow, and clear, and range from cube to octahedral morphologies; the entire spectrum in color and morphology is present in a single metasomatized eclogite with zoned garnet and clinopyroxene. Ages estimated from Sm-Nd mineral isochrons range from 92–247 Ma. _Nd values range from +4.05 to 5.23. Values of specific gravity range from 3.06–3.60 g/cc, with calculated seismic Vp of 7.4–8.7 km/s. Petrographie, mineral, and bulk chemical data demonstrate an overall close similarity between the Koidu xenolith suite and upper mantle eclogites from other districts in Africa, Siberia and the United States. At least two origins are implied byP-T, bulk chemistry and mineral compositions: low-MgO eclogites, with diamond and other accessory minerals, are considered to have formed from melts trapped and metamorphically equilibrated in the lithosphere; high-MgO eclogites are picritic and are the products of large degrees of partial melting, with equilibration in the asthenosphere. Fluid or diluted melt metasomatism is pervasive and contributed here and elsewhere to the LIL and refractory silicate incompatible element signature in kimberlites and lamproites, and to secondary diamond growth.     

Mafic magma batches at Vesuvius: a glass inclusion approach to the modalities of feeding stratovolcanoes

Glass inclusions in olivine and diopside phenocrysts from pyroclasts of various eruptions of Vesuvius are representative of the magmas that supplied the volcano in the last 4–5000 years. During this interval the volcano alternated between open conduit activity (e.g. 1944 and 1906 eruptions) with long pauses interupted by Plinian and sub-Plinian eruptions (e.g. 3360 B.P. Avellino, A.D. 79 Pompei, A.D. 472 Pollena). The eruptive behaviour was conditioned in all cases by the presence of shallow reservoirs: two cases are distinguished: (1) small and very shallow, 1906-type; (2) large and deeper Plinian-sub-Plinian magma chamber. Lapilli of 1906 lava fountains contain olivine (Fo_89.5–90.4) including Cr-spinel [Cr/(Cr+Al)] (Cr#>75) and volatile-K-rich tephritic glasses, which represent the first recognized Vesuvius primary magmas. Mg-poorer olivine (Fo_83–89) also occurs in 1906 and 1944 products; it formed within the shallow reservoir, together with pyroxene and leucite, between 1200 and 1130°C, from K-tephritic melts (MgO=6–8 wt%). The Plinian and sub-Plinian pumices contain diopside, phlogopite and minor olivine (Fo_85–87) representing adcumulates wrenched from the chamber walls. Glass inclusions in diopside (and some olivine) range from K-basalt to K-tephrite (MgO=6–8 wt%), with homogenization temperature of 1130–1170°C. They have been regarded as representative of the magmas supplying the Plinian-sub-Plinian chamber(s). The Avellino glass inclusions have K-basaltic compositions, contrasting with the mostly K-tephritic Pompei and Pollena inclusions. They display lower C1 and P contents with respect to the younger tephritic melts, and these variations should reflect primary features of the mantle-derived magmas. The primary and the near-primary Vesuvius magmas, as illustrated by melt inclusions, emphasize high K, P and volatile (H₂O, Cl, F, S) contents, with high K₂O/H₂O (2–2.5), Cl/F (2.5) and Cl/S (2–3) ratios, consistent with a metasomatized mantle source, and distinguishing the Vesuvius potassic primary magmas from those of the northern part of the Roman Province.     

Ti-content of high-Ca pyroxenes as a petrogenetic indicator: an experimental study of Mafic Alkaline Rocks from the Mt. Erebus volcanic region,Antarctica

We report chemical and mineralogical data for one atmosphere melting experiments conducted on alkalic rocks from the Mt. Erebus volcanic region: DVDP2 basanite, two hawaiites (DVDP2 and a nepheline-bearing variety), and an anorthoclase phonolite. Temperatures between 1,224 and 1,049°C were investigated at f_O2~QFM. DVDP2 basanite appears to be an intermediate pressure liquid or a cumulate, because only olivine coexists with melt from above 1,224–1,160°C. High-Ca pyroxene joins olivine in the crystallization sequence at 1,138°C. These minerals are joined by plagioclase at a temperature between 1,120 and 1,104°C. In contrast, DVDP2 hawaiite appears to be relatively evolved, because it is multiply saturated with olivine, plagioclase, and high-Ca pyroxene near its liquidus (between 1,120 and 1,104°C). Plagioclase crystallizes in the Ne-hawaiite by 1,160°C followed by olivine below 1,120°C. The liquidus of anorthoclase phonolite is between the lowest temperatures investigated, 1,089 and 1,049°C, and plagioclase is the liquidus mineral. Our results indicate that DVDP2 hawaiite can be derived from a DVDP2 basanitic parental magma by crystal fractionation at low pressures, that the nepheline hawaiite is an olivine cumulate, and that the liquids parental to the anorthoclase phonolite represent the end products of crystal fractionation. They also allow us to illustrate how the Ti-content of pyroxene may be used as a petrogenetic indicator of processes and events in the evolution of the Erebus volcanic system.     

Experimental data at 3 kbars pressure on parental magma to the Bushveld Complex

Experimental studies, mainly under 3 kbars pressure, have been undertaken on representative samples to determine if any of these compositions could be parental magma to the Bushveld Complex. One such composition, with 12.5% MgO, Mg/(Mg + Fe) of 0.72 and quartz-normative, crystallizes olivine, Fo₈₈, as liquidus mineral, at about 1,300° C, followed at only slightly lower temperature by orthopyroxene at 3 kbars pressure. There is a temperature drop of over 100° C before the appearance of plagioclase and finally clinopyroxene. This crystallization sequence is in excellent agreement with the observed sequence in the lower part of the Bushveld Complex.Results at higher pressures show that this composition cannot be a partial melt from mantle peridotite because olivine is replaced by orthopyroxene as the liquidus mineral at lower crustal pressures. A combination of olivine fractionation and contamination was probably involved in the early evolution of this magma.Experimental data on the other compositions show that they are not suitable as parental magma to the lowest portion of the complex. However, the data are used to construct phase diagrams within the basalt tetrahedron at 3 kbars pressure, which are of relevance to the crystallization of basic magmas in the upper crust.Research undertaken at the Grant Institute of Geology, University of Edinburgh, Scotland     

Petrogenesis of picrites from the Caribbean Plateau and the North Atlantic magmatic province

We studied the petrography, mineralogy and geochemistry of picrites from three different regions: the island of Curaçao which forms part of a Cretaceous oceanic plateau; Iceland, an active hot spot on the mid-Atlantic ridge; and the early Tertiary volcanic margin off the coast of Greenland, which formed during the rifting that created the Atlantic ocean. Using the compositions of olivine phenocrysts and relations between MgO and FeO, Al₂O₃ and Ni, we estimated compositions of parental liquids and the proportion of accumulated olivine in each rock. Picrites from Curaçao formed mainly from a liquid with 12 wt.% MgO and they contain up to 55 wt.% excess olivine in the form of phenocrysts. A small proportion of more forsterite-rich olivine grains are xenocrysts from a more magnesian source. Picrites from Iceland formed from a slightly less magnesian liquid but one with also about 12 wt.% MgO and they contain both olivine and plagioclase in the accumulated assemblage. Picrites from the Greenland volcanic margin formed from a liquid that was distinctly more magnesian, with up to 20 wt.% MgO. In some of these rocks the proportion of accumulated olivine was minimal and in these the whole-rock composition is roughly equivalent to the liquid composition. The picrites from the three areas formed under different conditions and through contrasting melting processes. The Curaçao picrites derive from pooled liquids formed through moderate degrees of melting at moderate depths beneath a relatively old and thick oceanic lithosphere. Iceland picrites, on the other hand, formed through advanced fractional melting of mantle that ascended almost to the base of the crust at the mid-ocean spreading center. An unusual combination of relatively high concentrations of incompatible trace elements and high MgO indicates that the Greenland picrites formed by relatively low degrees of melting at greater depths in the mantle.     

Multi-stage evolution of the picritic Mælifell rocks,SW Iceland: constraints from mineralogy and inclusions of glass and fluid in olivine

The picritic Mælifell pillow lava series contains olivine macrocrysts (Fo 83.0–91.7) and microphenocrysts (Fo 86.8–88.5), resorbed Cr–Al endiopside, ± plagioclase, and microphenocrysts of Cr-spinel. The most primitive olivine cores (Fo 90–91.7) are probably derived from a peridotitic mantle. Gabbroic adcumulus xenoliths in the lavas contain plagioclase, Cr–Al endiopside and olivine (Fo 85.5–87.5) which overlap compositionally with lava minerals, ± Cr-spinel. This suggests that all pyroxene and much of the olivine ± feldspar in the lavas are xenocrysts. Olivines from the pillow lavas and from the gabbroic xenoliths contain inclusions of Cr-spinel, silicate glass and pure or nearly pure CO₂. Early (type 1) silicate melt inclusions which occur in lava-olivine only, have crystalized 0.1 to 4 vol.% daughter spinel and unknown amounts of olivine during pre-eruptive cooling. Later (type 2) glass inclusions in olivine from the lavas do not contain daughter minerals; similar type 2 inclusions also occur in the xenoliths. High-temperature microthermometry at buffered oxygen fugacity (f _{O 2}) gives a plagioclaseout temperature of about 1230°C for both types of silicate melt inclusions; this was interpreted as the liquidus temperature for type 2 inclusions. Molar volumes of undisturbed CO₂ inclusions in olivine from both lavas and xenoliths correspond to a depth of trapping of 7–10 km (2.2–3.0 kbar) at 1230°C. This is interpreted as a minimum depth to a partially molten layer near the crust/mantle boundary in the rift zone. The xenoliths are thus probably derived from a layered olivine-gabbro complex similar to those occurring in ophiolite complexes.