Phase relations of a high-Mg basalt from the Aleutian Island Arc: Implications for primary island arc basalts and high-Al basalts

Many volcanic centers in the Aleutian Islands have erupted lavas that range in composition from high-Mg basalt (MgO>9 wt%) to more fractionated and voluminous high-Al basalts and basaltic andesites. The petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated. The phase relations of a typical high-Mg basalt from the Makushin volcanic field on Unalaska Island provide important constraints on petrogenetic models. Results of one-atmosphere and moderate-to high-pressure (5–20 kb) anhydrous experiments are similar to results obtained from primitive MORB. At low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures. Clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb. Above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize. Glasses in equilibrium with olivine and clinopyroxene at intermediate-pressure (5 to 10 kb) are similar in composition to high-Al basalt. Plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly. Although our studies do not confirm the primary nature of high-Mg basalts they do support a model in which high-Al basalts are generated by moderate amounts of crystal fractionation from more primitive (high Mg/Mg+Fe, lower Al₂O₃) basaltic magmas near the arc crust-mantle boundary.Abbreviations Ol olivine - Cpx Clinopyroxene - Pl plagioclase - L liquid - Sp spinel - Pig pigeonite - Opx Orthopyroxene     

Physicochemical model for the crystallization of rocks of the calc-alkaline series

The Al-rich region of the CaO-MgO-Al₂O₃-SiO₂ system was experimentally studied at pressures of 1.0–2.8 GPa and temperatures of 1300–1535°C. The slopes of the lines of the monovariant reactions An + Sp = Cpx + Cor + (Ga) and L = Cpx + Ga + Cor + Sp and the compositions of the phases involved in these reactions are determined. The results are utilized in the topological analysis of the aluminous region of the CaO-MgO-Al₂O₃-SiO₂ system. On this basis, the principal structure of the phase diagram is analyzed, and a phase diagram is constructed for the junction region of the quaternary system and the CaO-Al₂O₃-SiO₂ ternary system. A continuous series of the monovariant eutectic: L = Cpx + Opx + Fo + An, L = Cpx + Opx + An + Sp, L = Cpx + (Ga) + An + Sp, L = Cpx + Cor + (Ga) + An, L = An + Ga + Cpx + Ky and L = Ga + Cpx + Ky + Qz is examined within the pressure range from atmospheric to 3.0 GPa and higher. Analogous “telescoped” eutectic series are of fundamental character for interpreting the evolution of magmatic melts. A physicochemical model is suggested for the evolution of magmatic melts that produce rocks of the calc-alkaline series, with this model underlain by the fact that a change in the composition of magmatic melt at a pressure decrease should correspond to the minimum melting temperatures, i.e., to melts in the fundamental series of eutectic reactions. The comparison of our physicochemical model and rocks of the calc-alkaline series shows that the compositions of rocks of the calc-alkaline series are close to the compositions determined for the eutectic equilibria, and the mineralogical composition of xenoliths and megacrysts in volcanic and dike varieties of the rocks are similar to the subsolidus phases of the established fundamental eutectic system.     

Phase equilibria along a basalt-rhyolite mixing line: implications for the origin of calc-alkaline intermediate magmas

One-atmosphere, anhydrous phase equilibria determined for alkali basalt/high-silica rhyolite mixtures provide a model for crystallization of natural calc-alkaline mixed magmas. The compositional trend defined by these mixtures mimics the trends of many continental calc-alkaline volcanic suites. As with many naturally occurring suites, the mixtures studied straddle the low-pressure olivine-plagioclase-augite thermal divide. Magma mixing provides a convenient method for magmas to cross this thermal divide in the absence of magnetite crystallization. For the mixtures, Mg-rich olivine (Fo_82–87) coexists alone with liquid over an exceptionally large range of temperature and silica content (up to 63 wt% SiO₂). This indicates that the Mg-rich olivines found in many andesites and dacites are not necessarily out of equilibrium with the host magma, as is commonly assumed. Such crystals may be either primary phenocrysts, or inherited phenocrysts derived from a mafic magma that mixed with a silicic magma. For the bulk compositions studied, the distribution of Fe and Mg between olivine and liquid (K _D ) is equal to 0.3 and is independent of temperature and composition. This result extends to silicic andesites the applicability of K _D arguments for tests of equilibrium between olivine and groundmass and for modeling of fractional crystallization. In contrast, the distribution of calcium and sodium between plagioclase and liquid varies significantly with temperature and composition. Therefore, plagioclase-liquid K _D s cannot be used for fractional crystallization modeling or as a test of equilibrium. Calcic plagioclase from a basalt will be close to equilibrium with andesitic mixtures, but sodic plagioclase from a rhyolite will be greatly out of equilibrium. This explains the common observation that calcic plagioclase crystals in hybrid andesites are generally close to textural equilibrium with the surrounding groundmass, but sodic plagioclase crystals generally show remelting and armoring with calcic plagioclase.     

Siderophile and chalcophile metal variations in Tertiary picrites and basalts from West Greenland with implications for the sulphide saturation history of continental flood basalt magmas

Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nuussuaq Peninsula in West Greenland, and they include ～1,000 m of picritic lavas and discrete 10- to 50-m-thick members of highly contaminated basalts. On Qeqertarssuaq, the lavas are allocated to the Vaîgat and Maligât Formations of which the former includes the Naujánguit member, which consists of picrites with 7–29 wt% MgO, 80–1,400 ppm Ni, 5.7–9.4 ppb Pt and 4.2–12.9 ppb Pd. The Naujánguit member contains two horizons of contaminated basalts, the Asûk and Kûgánguaq, which have elevated SiO2 (52–58 wt%) and low to moderate MgO (7.5–12.8 wt%). These lavas are broadly characterized by low Cu and Ni abundances (average, 40 ppm Ni and 45 ppm Cu) and very low Pt (0.16–0.63 ppb) and Pd (0.13–0.68 ppb) abundances, and in the case of the Asûk, they contain shale xenoliths and droplets of native iron and troilite. The contaminated basalts from Nuussuaq, the B0 to B4 members, are also usually Ni-, Cu-, and platinum-group elements (PGE)-depleted. The geochemical signatures (especially the ratios of incompatible trace elements such as Th/Nb) of all of the contaminated basalts from Qeqertarssuaq and some of those from Nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas. This suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks, and hence, at a high crustal level. Nickel, Cu, and PGE depletion together with geochemical signatures produced by crustal contamination are also a feature of Siberian Trap basalts from the Noril’sk region. These basalts belong to the 0- to 500-m thick, ～5,000- to 10,000-km³ Nadezhdinsky Formation, which is centered in the Noril’sk Region. A major difference between Siberia and West Greenland is that PGE depletion in the Nadezhdinsky Formation samples with the lowest Cu and Ni contents is much more severe than that of the West Greenland contaminated basalts. Moreover, the volumes of the contaminated and metal-depleted volcanic rocks in West Greenland pale is significant when compared to the Nadezhdinsky Formation; local centers rarely contain more than 15 thin flows with a combined thickness of <50 m and more typically 10–20 m, so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the Nadezhdinsky edifice. The West Greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the Tertiary delta, and so emplacement along N–S structures appears to be a principal control on the distribution of lavas and feeder intrusions. This leads us to suggest that the Greenland system is small and segregation of sulphide took place at high levels in the crust, whereas at Noril’sk, the saturation event took place at depth with subsequent emplacement of sulphide-bearing magmas into high levels of the crust. As a consequence, it may be unreasonable to expect that the West Greenland flood basalts experienced mineralizing processes on the scale of the Noril’sk system.     

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.     

REE Abundaces in Megacrysts and Host Basalts：REE Behavior of Magma at High Pressures

Based on REE abundances in megacrysts and host basalts and their equilibrium conditions,it has proved that megacrysts may have been produced from the magma derived from the host rocks or of more basic composition.The REE ratios of megacrysts to host rocks may be taken as partition coefficients when both are equilibrium with each other.The crystal fractionation of megacrysts has caused the evolution of REE in the magma.It is obvious that some host basalts are the product of magma evolution after crystal fractionation.According to REE abundances in the host rocks and the partition coefficients between crystal and liquid,the history of crystal fractionation of magma can be traced.     

Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications

The dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes. In this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization. Removal of olivine and pyroxene, whose values of fractionation density are larger than the density of the magma, causes the density of residual liquid to decrease. Removal of plagioclase, with fractionation density less than the magma density, can cause the density of residual liquid to increase. During the progressive differentiation of basaltic magma, density decreases during fractionation of olivine, olivine-pyroxene, and pyroxene assemblages. When plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum. Calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects.In the closed-system evolution of basaltic magma, several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization, as well as on the geometry of the chamber. In an early stage of the evolution, where olivine and/or pyroxenes are the fractionating phases, compositional stratification can occur due to side-wall crystallization and replenishment by new magma, with the most differentiated magma tending to accumulate at the roof of the chamber. When plagioclase becomes a fractionating phase a zone of well-mixed magma with a composition close to the density minimum of the system can form in the chamber. The growth of a zone of constant composition destroys the stratification in the chamber. A chamber of well-mixed magma is maintained while further differentiation occurs, unless the walls of the chamber slope inwards, in which case dense boundary layer flows can lead to stable stratification of cool, differentiated magma at the floor of the chamber.In a basaltic magma chamber replenished by primitive magma, the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma. Successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber. In a chamber containing well-mixed, plagioclase — saturated magma, the primitive magma can be either denser or lighter than the resident magma. In the first case, the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma. Mixing then occurs between magmas of different temperatures and compositions. In the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma.     

From basalt to dacite: origin and evolution of the calc-alkaline series of Salina, Aeolian Arc, Italy

The island of Salina comprises one of the most distinct calc-alkaline series of the Aeolian arc (Italy), in which calc-alkaline, high-K calc-alkaline, shoshonitic and leucite-shoshonitic magma series are developed. Detailed petrological, geochemical and isotopic (Sr, Nd, Pb, O) data are reported for a stratigraphically well-established sequence of lavas and pyroclastic rocks from the Middle Pleistocene volcanic cycle (430–127 ka) of Salina, which is characterized by an early period of basaltic volcanism (Corvo; Capo; Rivi; Fossa delle Felci, group 1) and a sequence of basaltic andesites, and andesites and dacites in the final stages of activity (Fossa delle Felci, groups 2–8). Major and trace element compositional trends, rare earth element (REE) abundances and mineralogy reveal the importance of crystal fractionation of plagioclase + clinopyroxene + olivine/ orthopyroxene ± titanomagnetite ± amphibole ± apatite in generating the more evolved magma types from parental basaltic magmas, and plagioclase accumulation in producing the high Al₂O₃ contents of some of the more evolved basalts. Sr isotope ratios range from 0.70410 to 0.70463 throughout the suite and show a well-defined negative correlation with ¹⁴³Nd/¹⁴⁴Nd (0.51275–0.51279). Pb isotope compositions are distinctly radiogenic with relatively large variations in ²⁰⁶Pb/²⁰⁴Pb (19.30–19.66), fairly constant ²⁰⁷Pb/²⁰⁴Pb (15.68–15.76) and minor variations in ²⁰⁸Pb/²⁰⁴Pb ratios (39.15–39.51). Whole-rock δ¹⁸O values range from +6.4 to +8.5‰ and correlate positively with Sr isotope ratios. Overall, the isotopic variations are correlated with the degree of differentiation of the rocks, indicating that only small degrees of crustal assimilation are overprinting the dominant evolution by crystal–liquid fractionation (AFC-type processes). The radiogenic and oxygen isotope composition of the Salina basalts suggests derivation from primary magmas from a depleted mantle source contaminated by slab-derived fluids and subducted sediments with an isotopic signature of typical upper continental crust. These magmas then evolved further to andesitic and dacitic compositions through the prevailing process of low-pressure fractional crystallization in a shallow magma reservoir, accompanied by minor assimilation of crustal lithologies similar to those of the Calabrian lower crust. Received: 29 November 1999 / Accepted: 16 April 2000     

Geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the Gardar Province, South Greenland: implications for crystallisation processes in anorthositic magmas

Chemical and Sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the Gardar Province can be used to elucidate magma-chamber and emplacement processes. The megacrysts occur either as single crystals or assembled as anorthosite xenoliths. The size of the megacrysts varies from <1 cm to 1 m. They consist of a large core with variable zonation (An_58-39) and a relatively small (<600 µm), normally zoned rim (An_62-27). The contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol% An. This gap is interpreted as having formed during dyke emplacement due to a sudden pressure release. Some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy-oscillatory zoning which has not been reported elsewhere to our knowledge. The oscillatory zoning has wavelengths of up to 2,500 µm and a maximum amplitude of 7 mol% An. It is interpreted as reflecting movements of the crystals in the magma reservoir. The Sr isotopic composition of one crystal shows a radiogenic inner core ((⁸⁷Sr/⁸⁶Sr)_i=0.7044) and a less radiogenic outer core ((⁸⁷Sr/⁸⁶Sr)_i=0.7039-0.7036). The lack of a significant change between outer core and rim ((⁸⁷Sr/⁸⁶Sr)_i=0.7037) is consistent with formation of the more An-rich rim due to pressure release. Variations in the core may be related to movements of the crystal and/or magma mixing. A trace-element profile across a megacryst shows a small increase in Sr and small decreases in Ba and La contents of the recalculated melt composition across the core-rim boundary, whereas P, Ce, Nd and Eu remain constant. Melt compositional changes upon emplacement are therefore considered to be of minor importance. Constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace-element budgets.     

Water content of primitive low-K tholeiitic basalt magma from Iwate Volcano,NE Japan arc: implications for differentiation mechanism of frontal-arc basalt magmas

The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An_85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa.     

昌乐玄武岩内刚玉巨晶（蓝宝石）中发现富碳酸盐和硫酸盐熔融包裹体及其意义

山东昌乐新生代玄武岩内的刚玉巨晶(蓝宝石)中含有多种类型熔融包裹体,其成分对了解华北深部地幔交代过程中的流/熔体性质和刚玉母岩浆特点具有重要意义.详细的岩相学和激光拉曼分析鉴定出一类富碳酸盐和硫酸盐成分的原生熔融包裹体以及一类含硫酸盐和氯化物等成分的次生熔融包裹体,二者同时还含有CO2和H2O.碳酸盐和硫酸盐成分在世界范围玄武岩内刚玉巨晶中是首次发现,结合已有的包裹体稀有气体同位素和测温资料,反映两种成分可能来源于交代地幔的碳酸岩熔体,预示着华北深部地幔不仅经历了硅酸盐成分的交代还经历了富碳酸盐和硫酸盐成分(碳酸岩)的交代,同时也显示刚玉母岩浆成分复杂,至少有富这两类成分物质的参与,刚玉很可能是硅酸盐岩浆/岩石和幔源碳酸岩岩浆相互作用的产物,后被玄武岩喷发携带至地表.     

Differentiation and crystallization conditions of basalts from the Kerguelen large igneous province: an experimental study

Phase relations of basalts from the Kerguelen large igneous province have been investigated experimentally to understand the effect of temperature, fO₂, and fugacity of volatiles (e.g., H₂O and CO₂) on the differentiation path of LIP basalts. The starting rock samples were a tholeiitic basalt from the Northern Kerguelen Plateau (ODP Leg 183 Site 1140) and mildly alkalic basalt evolved from the Kerguelen Archipelago (Mt. Crozier on the Courbet Peninsula), representing different differentiation stages of basalts related to the Kerguelen mantle plume. The influence of temperature, water and oxygen fugacity on phase stability and composition was investigated at 500 MPa and all experiments were fluid-saturated. Crystallization experiments were performed at temperatures between 900 and 1,160°C under oxidizing (log fO₂ ~ ΔQFM + 4) and reducing conditions (log fO₂ ~ QFM) in an internally heated gas-pressure vessel equipped with a rapid quench device and a Pt-Membrane for monitoring the fH₂. In all experiments, a significant influence of the fO₂ on the composition and stability of the Mg/Fe-bearing mineral phases could be observed. Under reducing conditions, the residual melts follow a tholeiitic differentiation trend. In contrast, melts have high Mg# [Mg²⁺/(Mg²⁺ + Fe²⁺)] and follow a calk-alkalic differentiation trend at oxidizing conditions. The comparison of the natural phenocryst assemblages with the experimental products allows us to constrain the differentiation and pre-eruptive conditions of these magmas. The pre-eruptive temperature of the alkalic basalt was about 950–1,050°C. The water content of the melt was below 2.5 wt% H₂O and strongly oxidizing conditions (log fO₂ ~ ΔQFM + 2) were prevailing in the magma chamber prior to eruption. The temperature of the tholeiitic melt was above 1,060°C, with a water content below 2 wt% H₂O and a log fO₂ ~ ΔQFM + 1. Early fractionation of clinopyroxene is a crucial step resulting in the generation of silica-poor and alkali-rich residual melts (e.g., alkali basalt). The enrichment of alkalis in residual melts is enhanced at high fO₂ and low aH₂O.     

Mixed Caledonian appinite magmas: implications for lamprophyre fractionation and high Ba-Sr granite genesis

Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic ⁸⁷Sr/⁸⁶Sr and nonradiogenic ¹⁴³Nd/¹⁴⁴Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant juvenile additions to the crust. Received: 26 September 1995 / Accepted: 5 June 1996     

Crystallization conditions of leucite-bearing magmas and their implications on the magmatological evolution of ultrapotassic magmas: the Vico Volcano, Central Italy

Summary Mineral compositions in leucite-bearing and leucite-free rocks from Vico volcano are reported. FeO/MgO partitioning (Kd_ol/liq) between olivine and latite (0.14–0.22), and between olivine and trachyte (0.06–0.10) indicates a lack of equilibrium between mineral and host rock. This suggests that mingling and/or mixing between magmas was a leading process during magmatic differentiation. In addition, a phono-tephrite olivine population with high (0.84) and equilibrium (0.23–0.29) Kd_ol/liq values has been produced by the interaction of differently evolved magmas. Zoning in clinopyroxene and plagioclase from these rocks recorded the same processes. In addition, resorbed quartz xenocrysts with coronas of clinopyroxene microlites indicate that digestion of crustal rocks occurred during the residence of magma in a shallow level reservoir. Increasing Fe coupled with decreasing Ca in diopside crystals from some phonolites, together with the petrographic and trace element data, indicate that polybaric fractional crystallisation also may be involved in the genesis of magmas of the second period of Vico activity. Leucite-free trachybasalts erupted in a late stage contain highly forsteritic olivine phenocrysts (forsterite 84–88 mol.%) in-equilibrium (Kd_ol/liq = 0.24–0.35) with the host rock, which indicate that they did not suffer chemical modification at low pressure. Received November 28, 2000; revised version accepted September 27, 2001     

Oxygen fugacity and geochemical variations in the martian basalts: implications for martian basalt petrogenesis and the oxidation state of the upper mantle of Mars

The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.     

山东昌乐新近纪玄武岩中刚玉巨晶反应边的成因

通过对山东昌乐方山新近纪碱性玄武岩中原生刚玉巨晶的观察研究,发现在刚玉巨晶与玄武岩间存在反应边,可见新近纪碱性玄武岩中的刚玉巨晶为捕虏晶.电子探针下观察,反应边有两类：第1类由尖晶石带和钛磁铁矿＋熔体带构成,第2类由尖晶石带和长石＋钛磁铁矿带构成.在第1类反应边中,尖晶石带很窄（＜40μm）,钛磁铁矿＋熔体带较宽（100～120 μm）;而且尖晶石的成分变化很大,从靠近刚玉一侧（内侧）向外,Al2O3逐渐降低,从85.08%～58.94%,MgO和FeO逐渐升高,分别从5.14%～14.82%和8.08%～28.24%.在第2类反应边中,尖晶石带较宽且稳定（100μm左右）;长石＋钛磁铁矿带不稳定,特别是钛磁铁矿不但窄（＜40μm）,而且分布断断续续;尖晶石的成分变化较小,Al2O3 59.25%～62.82%、MgO12.34%～13.50%、FeO 23.25%～27.37%.本文对两类反应边的形成模式进行了探讨,认为刚玉巨晶与玄武岩浆反应持续时间的长短是导致形成两类反应边的主要因素,第1类反应边是反应没有达到平衡的产物,第2类反应边为反应平衡或者是接近平衡的产物.     

The effects of FeO on the system CMAS at low pressure and implications for basalt crystallization processes

The common occurrence of olivine, clinopyroxene and plagioclase in natural basalts may suggest that crystallization of these phases exerts important controls on the evolution of natural basalts at low pressure. In order to understand the effects of such an assemblage on the evolution of basalt, an experimental study of the divariant assemblage 1+an+cpx+ol in the system CMAS + FeO was undertaken at 1 bar and under very reducing conditions. Experiments have been performed along three isothernal sections. Detailed electron microprobe data have been collected on all the coexisting phases. Combining these data with available data in the system CMAS, the compositions of all the coexisting phases have been described as functions of temperature and CaO content in the melt by applying a multiple linear regression method. This allows a quantitative characterization of the divariant assemblage 1+an+epx+ol in the system CMAS + FeO, from 1275°C to 1160°C, with liquid compositions ranging from mg#=1 to mg#=0.28. Knowing the composition-temperature relationships, the basic T-X configuration of the assemblage 1+an+cpx+ol has been analysed, and mass balance calculations have been performed to examine the FeO effect on different crystallization processes. Addition of FeO to the system CMAS transforms the thermal divide in the assemblage 1+an+cpx+ol to a thermal ridge, and shifts the spine of the thermal ridge towards SiO₂-poor compositions with decreasing temperature. This indicates that tholeiitic basalts can be relatively silica-poor, and evolve towards slightly more silica-poor compositions. With increasing FeO content, pigeonite replaces ortheonstatite as a crystallizing phase along the silica-rich boundary of the assemblage 1+an+cpx+ol. In some iron, and silica rich composition, olivine possibly terminates its crystallization or even starts resorption prior to pigeonite precipitation; this suppresses silica enrichment in the melt. Crystallization paths of the assemblage 1+an+cpx+ol are determined by the detailed T-X relations of the thermal ridge with the melt evolving away from the spine. The liquid evolution trend depends strongly on the initial compositions. A tholeiitic liquid can follow a moderate silica enrichment path at the same very reducing condition of there is a high modal proportion of olivine. ‘A Fenner mechanism therefore is at work at the same place as a Bowen mechanism’.     

Mid-Pleistocene lavas from the Seguam volcanic center,central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO^*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at 1160°C between about 3–5 kb (±1–2% H₂O), andesites crystallized Plag+Cpx+Opx±Mt at 1000°C between 3–4 kb with 3–5% H₂O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H₂O. All lavas crystallized at f _{o 2} close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al₂O₃, Cr, Ni, and Sr abundances decrease and K₂O, Na₂O, Rb, Ba, Zr, and Pb increase with increasing SiO₂ (50–71%). LREE enrichment [(Ce/Yb)_n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO₂. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K₂O, Na₂O, TiO₂, Rb, Ba, Sr, Zr, Y, and LREE abundances. ⁸⁷Sr/⁸⁶Sr ratios (0.70361–0.70375) and ratios of ²⁰⁶Pb/²⁰⁴Pb (18.88–18.97), ²⁰⁷Pb/²⁰⁴Pb (15.58–15.62), ²⁰⁸Pb/²⁰⁴Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, Nd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of 80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.     

Highly depleted isotopic compositions evident in Iapetus and Rheic Ocean basalts: implications for crustal generation and preservation

Subduction of both the Iapetus and Rheic oceans began relatively soon after their opening. Vestiges of both the Iapetan and Rheic oceanic lithospheres are preserved as supra-subduction ophiolites and related mafic complexes in the Appalachian–Caledonian and Variscan orogens. However, available Sm–Nd isotopic data indicate that the mantle source of these complexes was highly depleted as a result of an earlier history of magmatism that occurred prior to initiation of the Iapetus and Rheic oceans. We propose two alternative models for this feature: either the highly depleted mantle was preserved in a long-lived oceanic plateau within the Paleopacific realm or the source for the basalt crust was been recycled from a previously depleted mantle and was brought to an ocean spreading centre during return flow, without significant re-enrichment en-route. Data from present-day oceans suggest that such return flow was more likely to have occurred in the Paleopacific than in new mid-ocean ridges produced in the opening of the Iapetus and Rheic oceans. Variation in crustal density produced by Fe partitioning rendered the lithosphere derived from previously depleted mantle more buoyant than the surrounding asthenosphere, facilitating its preservation. The buoyant oceanic lithosphere was captured from the adjacent Paleopacific, in a manner analogous to the Mesozoic–Cenozoic “capture” in the Atlantic realm of the Caribbean plate. This mechanism of “plate capture” may explain the premature closing of the oceans, and the distribution of collisional events and peri-Gondwanan terranes in the Appalachian–Caledonian and Variscan orogens.     

Fractional crystallization of high-K arc magmas: biotite- versus amphibole-dominated fractionation series in the Dariv Igneous Complex,Western Mongolia

Many studies have documented hydrous fractionation of calc-alkaline basalts producing tonalitic, granodioritic, and granitic melts, but the origin of more alkaline arc sequences dominated by high-K monzonitic suites has not been thoroughly investigated. This study presents results from a combined field, petrologic, and whole-rock geochemical study of a paleo-arc alkaline fractionation sequence from the Dariv Range of the Mongolian Altaids. The Dariv Igneous Complex of Western Mongolia is composed of a complete, moderately hydrous, alkaline fractionation sequence ranging from phlogopite-bearing ultramafic and mafic cumulates to quartz–monzonites to late-stage felsic (63–75 wt% SiO₂) dikes. A volumetrically subordinate more hydrous, amphibole-dominated fractionation sequence is also present and comprises amphibole (±phlogopite) clinopyroxenites, gabbros, and diorites. We present 168 whole-rock analyses for the biotite- and amphibole-dominated series. First, we constrain the liquid line of descent (LLD) of a primitive, alkaline arc melt characterized by biotite as the dominant hydrous phase through a fractionation model that incorporates the stepwise subtraction of cumulates of a fixed composition. The modeled LLD reproduces the geochemical trends observed in the “liquid-like” intrusives of the biotite series (quartz–monzonites and felsic dikes) and follows the water-undersaturated albite–orthoclase cotectic (at 0.2–0.5 GPa). Second, as distinct biotite- and amphibole-dominated fractionation series are observed, we investigate the controls on high-temperature biotite versus amphibole crystallization from hydrous arc melts. Analysis of a compilation of hydrous experimental starting materials and high-Mg basalts saturated in biotite and/or amphibole suggests that the degree of K enrichment controls whether biotite will crystallize as an early high-T phase, whereas the degree of water saturation is the dominant control of amphibole crystallization. Therefore, if a melt has the appropriate major-element composition for early biotite and amphibole crystallization, as is true of the high-Mg basalts from the Dariv Igneous Complex, the relative proximity of these two phases to the liquidus depends on the H₂O concentration in the melt. Third, we compare the modeled high-K LLD and whole-rock geochemistry of the Dariv Igneous Complex to the more common calc-alkaline trend. Biotite and K-feldspar fractionation in the alkaline arc series results in the moderation of K₂O/Na₂O values and LILE concentrations with increasing SiO₂ as compared to the more common calc-alkaline series characterized by amphibole and plagioclase crystallization and strong increases in K₂O/Na₂O values. Lastly, we suggest that common calc-alkaline parental melts involve addition of a moderate pressure, sodic, fluid-dominated slab component while more alkaline primitive melts characterized by early biotite saturation involve the addition of a high-pressure potassic sediment melt.