Crystallization Of Fe-Ti Oxides Research Articles

THE UPPER ZONE OF THE BUSHVELD COMPLEX AT ROOSSENEKAL, SOUTH AFRICA: GEOCHEMICAL STRATIGRAPHY AND EVIDENCE OF MULTIPLE EPISODES OF MAGMA REPLENISHMENT

Drill core from a fence of five boreholes located in the vicinity of Roossenekal, in the eastern limb of the Bushveld Complex, South Africa, provides complete coverage of what is probably one of the thickest and most well-developed sections of the Upper Zone. The lowermost portions of the Rooiberg Group felsites that overlie the Upper Zone, and the uppermost portion of the Upper Main Zone, which forms the footwall, were also intersected. Field relationships from floor-attached domes (eastern limb) and gap areas (western limb) reveal a marked angular unconformity at the base of the Upper Zone. The Upper Zone represents a discrete intrusive event and should not be interpreted as a continuum, or the fractionated residue of the underlying components of the Rustenburg Layered Suite (RLS). A new subdivision of the Upper Zone into five subunits (A through E) is proposed on the basis of the cumulus mineralogy and whole-rock geochemistry, the latter based on samples at an average vertical spacing of 20 m throughout the column at Roossenekal. The separation of subzones A and B is based on our geochemical data, as is the identification of subzone E, but recognition of subzones C (olivine) and D (apatite) is broadly consistent with the detailed mapping and stratigraphic column presented by Von Gruenewaldt (1973). Subunits A through D reveal macro-layering for which the intrusion is so well known. More than 40 individual Ti-magnetite layers were identified in the drill-core, which together with the numerous anorthosite layers, are the most prominent feature of this part of the column. The principal lithology is a somewhat monotonous sequence of ferrogabbro or magnetite-olivine gabbro. The absence of Ti-magnetite layers and anorthosite from subzone E, which is dominated by diorite with <5 modal % Ti-magnetite is significant. The whole-rock geochemical data presented here is not consistent with a closed system fractionation hypothesis for subunits A through D, despite this being widely accepted in recent contributions. An upward differentiation trend is preserved only in subunit E. The incremental build up of cumulates from successive pulse of magma is not only consistent with our views on the underlying layered cumulates (periods of closed system differentiation are rare) but is also consistent with field relationships, including each of the principal limbs representing individual sill-like features. The Upper Zone crystallized from batches of iron-rich tholeiitic magma derived from a deep staging chamber. Magma was injected as near-lateral flows, probably in a southward direction in the eastern limb. The introduction of evolved magma into a shallow crustal environment resulted in the early crystallization of Fe-Ti oxides. Ti-magnetite layers crystallized from oxide-rich slurries intruded near the floor of the chamber. Fe-Ti oxide pipes formed from slurries forced downward in areas of structural weakness. The regular, albeit step-like, upward change in composition of the Ti-magnetite layers identified by Molyneux (1974) and others is ascribed to mixing of crystal-rich magma slurries with increasingly differentiated resident magma: the lowermost layers (and pipes) contain the highest concentrations of V2O5. The spatial association of Ti-magnetite layers and anorthosite is emphasized and we suggest the latter formed as a consequence of disequilibrium partial melting of earlier formed silicate cumulates due to heat associated with new pulses of magma.

Mineral characteristics and metallogenesis of the Wajilitag layered mafic–ultramafic intrusion and associated Fe–Ti–V oxide deposit in the Tarim large igneous province, northwest China

The Wajilitag layered mafic–ultramafic intrusion, as an important lithological unit of the Permian Tarim large igneous province, has genetic link with a large scale V-bearing ilmenite–titanomagnetite series mineralization. It consists mainly of olivine pyroxenite, pyroxenite and gabbro with layered structures. Olivine from the mafic–ultramafic rocks has low Fo values [molar 100×Mg/(Mg+Fe)] of 67–76mol%, probably indicating that the Wajilitag magmas have experienced fractional crystallization with an early removal of olivine. Magmas in equilibrium with olivine have high FeO/MgO ratios ranging of 1.1–1.6. Ilmenite exsolution lamellae are present in clinopyroxene. The Wajilitag magmas therefore are considered to be enriched in iron and titanium, which may account for the large amounts of Fe–Ti oxides hosted therein. High Sm/Yb ratios indicate that the Fe–Ti-rich Wajilitag magmas may be produced by low degree of partial melting from the mantle source. Layered structures and variable mineral compositions of olivine and clinopyroxene of the intrusion suggest that the magmas become more Fe–Ti-rich under extensively fractional crystallization from the evolved magma. A few Fe–Ti oxide crystals enclosed in the olivine of the olivine pyroxenite with slightly higher Cr2O3 contents than other oxides indicate that minor oxides crystallized in the early stage of the magmatic process, whereas most oxides interstitial to the silicate minerals denote that they were accumulated in the late stage of the magmatic process. We propose that the Fe–Ti oxides of the Wajilitag intrusion were formed by fractionation processes with two-stages. The presence of ilmenite exsolution lamellae in magnetite suggests that the Wajilitag layered mafic–ultramafic intrusion and the massive Fe–Ti ore deposits hosted therein formed in a slowly cooling magmatic crystallization process from a deep magma chamber.

Experimental solidification of an andesitic melt by cooling

Solidification experiments at (a) five different cooling rates (25, 12.5, 3, 0.5 and 0.125 °C/min) between 1300 and 800 °C, and (b) variable quenching temperatures (1100, 1000, 900 and 800 °C) at a fixed cooling rate of 0.5 °C/min were performed on an andesitic melt (SiO 2 = 58.52 wt.% and Na 2O + K 2O = 4.43 wt.%) at air conditions from high superheating temperature. The results show that simultaneous and duplicated experiments with Pt-wire or Pt-capsule produce identical run-products. Preferential nucleation on Pt-containers or bubbles is lacking. Plagioclase and Fe–Ti oxide crystals nucleate firstly from the melt. Clinopyroxene crystals form only at lower cooling rates (0.5 and 0.125 °C/min) and quenching temperatures (900 and 800 °C). At higher cooling rates (25, 12.5 and 3 °C/min) and quenching temperature (1100 °C), plagioclase and Fe–Ti oxide crystals are embedded in a glassy matrix; by contrast, at lower cooling rates (0.5 and 0.125 °C/min) and below 1100 °C they form an intergrowth texture. The crystallization of plagioclase and Fe–Ti oxide starts homogeneously and then proceeds by heterogeneous nucleation. The crystal size distribution (CSD) analysis of plagioclase shows that crystal coarsening increases with decreasing cooling rate and quenching temperature. At the same time, the average growth rate of plagioclases decreases from 2.1 × 10 − 6 cm/s (25 °C/min) to 5.7 × 10 − 8 cm/s (0.125 °C/min) and crystals tend to be more equant in habit. Plagioclases and Fe–Ti oxides depart from their equilibrium compositions with increasing cooling rate; plagioclases shift from labradorite–andesine to anorthite–bytownite. Therefore, kinetic effects due to cooling significantly change the plagioclase composition with remarkable petrological implications for the solidification of andesitic lavas and dikes. The glass-forming ability (GFA) of the andesitic melt has been also quantified in a critical cooling rate ( R c ) of ~ 37 °C/min. This value is higher than those measured for latitic ( R c ~ 1 °C/min) and trachytic ( R c < 0.125 °C/min) liquids demonstrating that little changes of melt composition are able to significantly shift the initial nucleation behavior of magmas and the following solidification paths.

Correlation of basaltic tephra from Mt Wellington volcano: Implications for the penultimate eruption from the Auckland Volcanic Field

Mt Wellington volcano in the Auckland Volcanic Field is a basaltic complex comprising several vents, tuff rings, cones and lava flows. Interstitial glass in shard and lapilli groundmass displays considerable variability (∼41–48 wt% SiO 2, anhydrous), but within-sample variation is relatively narrow (mostly <2–3 wt% SiO 2) allowing two broad compositional affinities to be recognised that relate to the eruptive sequence. The tuff ring contains lower SiO 2 glasses, whereas the main cinder cone comprises higher SiO 2 glasses. Fe–Ti oxides also display a moderate compositional range (Ulvospinel, Usp = ∼40–55), although within-sample variation is generally narrower (∼5 Usp) allowing tuff ring deposits (Usp < 47) to be distinguished from those of the cinder cone (Usp > ∼50). Compositional outliers are evident amongst the Fe–Ti oxide crystals in many samples, and probably result from slow post-depositional cooling and/or variable oxidation conditions during syn-eruption recycling at the vent. Although abundant microlites restrict the deposits that can be geochemically fingerprinted, randomly selected analyses of glass shards show only limited variation within samples (±1 wt% SiO 2), suggesting that variable microlite formation had little effect on the residue melt. The larger compositional differences such as that between the tuff ring and cinder cone also do not appear to be related to microlite formation because both deposits display a similar range in microlite content. Correlations between the edifice and distal deposits in maar sediments provide insights to the eruption episode. They demonstrate that the main cinder cone was a late-stage event, there was no significant hiatus (months to decades) within the entire sequence, and that the cinder cone was responsible for the wide dispersal of ejecta in the region. By placing the Mt Wellington event within a stratigraphic framework of well-dated tephra from other volcanoes, it is possible to evaluate radiocarbon chronologies and constrain the eruption age to ∼10,000 BP.

Flood basalt-related Fe–Ti oxide deposits in the Emeishan large igneous province, SW China

In the Panzhihua–Xichang region (Sichuan Province, SW China), there are a number of world-class magmatic Fe–Ti oxide deposits. They are hosted as conformable masses in lower parts of layered mafic–ultramafic intrusions that are part of the end-Guadalupian (∼ 260 Ma) Emeishan large igneous province. The ore-bearing Panzhihua, Hongge, Baima, Taihe and Xinjie intrusions are spatially and temporally associated with flood basalts and granitoids in the province. New mineralogical data for Fe–Ti oxide ores in the Hongge and Baima intrusions, combined with an overview of previously published data, provide a better understanding on the formation of these enigmatic deposits. Geochemical and Sr–Nd isotopic data confirm a genetic relation between the intrusions and high-Ti Emeishan flood basalts. Parental magma compositions, estimated using the most Mg-rich cumulus olivine in the intrusions, are characterized by slight to moderate degrees of fractionation (MgO = 7.2–11 wt.%). Occurrences of rare Cr-bearing titanomagnetite (Cr 2O 3 = 1.2–10.7 wt.%) in the Panzhihua, Hongge and Xinjie intrusions are consistent with early crystallization of Fe–Ti oxides. Oxide-silicate equilibria and low V concentration (< 4800 ppm) in magnetite are consistent with relatively high oxygen fugacity. Flood basalt-related origin, early Fe–Ti oxide crystallization and relatively high oxygen fugacity, are characteristic of Fe–Ti oxide deposits in the Panxi region but are fundamentally different from those hosted in highly-differentiated, upper parts of large layered intrusions (e.g. the Bushveld Complex). Similar Fe–Ti oxide deposits have not been documented in other well-studied large igneous provinces to date, but a number of Fe–Ti oxide ore occurrences worldwide are potentially similar to this type of flood basalt-related oxide deposits.

Differentiation and Compaction in the Skaergaard Intrusion

Igneous differentiation processes are constrained from bulk compositions, densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the Layered Series of the Skaergaard intrusion, East Greenland. The stratigraphic distribution of P, U and Rb in cumulates and residual magma, modeled by Rayleigh fractionation, constrains the final porosities or trapped liquid contents to 30-52% in LZa troctolites, decreasing to 4-12% at the top of LZb olivine-gabbros and remaining low (1-13%; 4{middle dot}6% on average) in the oxide-gabbros of LZc, MZ and UZ. Local variations in trapped liquid content are associated with modal layering: leucocratic, low-density rocks have higher proportions of trapped melt than adjacent melanocratic, high-density rocks. These observations are explained by varying degrees of compaction. Compaction was most important after the onset of Fe-Ti oxide crystallization because of the high densities of the crystal matrix. Here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick. In the basal section (LZa) the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor. In the overlying section (LZb) the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation. The modeling constrains the P2O5 content of the residual magma to [~]1{middle dot}7 wt % at the level of apatite-in, suggesting that the magma contained [~]49 wt % SiO2 and followed a trend of iron enrichment. Compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix.

The role of Fe–Ti oxide crystallization in the formation of A-type granitoids with implications for the Daly gap: An example from the Permian Baima igneous complex, SW China

The Baima igneous complex (BIC) is a member of the Permian Emeishan large igneous province, SW China, and is composed of a layered gabbroic unit and an isotropic syenitic unit. The gabbroic unit consists of four distinct zones, 1) lower cumulate zone, 2) massive Fe–Ti–V oxide ore zone, 3) olivine gabbro zone and 4) upper gabbro zone. SHRIMP zircon U–Pb dating results yield an age of 261 ± 2 Ma for the gabbroic unit, contemporaneous with the spatially associated syenitic unit. The gabbroic rocks show slightly LREE enriched patterns with (La/Yb) N values between 1.6 and 12.1 and positive Eu anomalies (Eu/Eu⁎ = 1.1–3.6). In contrast, the syenites display stronger enrichments of LREE (La/Yb N = 6.8–51.3) and negative Eu anomalies (Eu/Eu⁎ = 0.3–1.1). Rocks of the syenitic unit are chemically similar to the ferroan alkalic A-type granitoids (K 2O/Na 2O = 0.58–0.80, FeOt/(FeOt +MgO) = 0.80–0.91) and have a within-plate geochemical signature. Both units have similar trace element ratios (Zr/Nb, Th/U, Ni/Co, Cu/Ni, Hf/Ta and Zn/Ga) resembling an ocean–island basalt source. The εNd (T) values for the gabbroic unit (εNd (T) = + 1.6–+ 4.2) and syenitic unit (εNd (T) = + 2.5–+ 3.2) are within the range of the high-Ti Emeishan flood basalts. The geochemical and geochronological data indicate that the gabbroic and syenitic units are comagmatic and cogenetic. Geochemical modeling suggests that the two units originated by fractional crystallization of a common parental magma that resembles the high-Ti Emeishan flood basalts. The formation of the two units by fractional crystallization was likely responsible for the enrichment of Fe–Ti–V oxide minerals in the gabbroic unit and thus the development of the Daly gap. The results of this study indicate that silica saturated peralkaline A-type granitoids can be formed by fractionation of mantle derived mafic magmas.

Abundant Fe–Ti oxide inclusions in olivine from the Panzhihua and Hongge layered intrusions, SW China: evidence for early saturation of Fe–Ti oxides in ferrobasaltic magma

Abundant Fe–Ti oxide inclusions in cumulus olivine (Fo77–81) from the Panzhihua and Hongge intrusions, Emeishan large igneous province, SW China, document the first evidence for early crystallization of Fe–Ti oxides in ferrobasaltic systems in nature. The intrusions also contain significant stratiform Fe–Ti–V oxide ores. The oxide inclusions are sub-rounded or irregular, range from ∼5 to 50 μm in diameter, and are dominated by either titanomagnetite or ilmenite. The fact that the inclusions are either titanomagnetite- or ilmenite-dominant suggests that they are trapped crystals, instead of immiscible oxide melt, formed during growth of the host olivine. The absence of other silicate phases in the inclusion-bearing olivine is difficult to reconcile with a possible xenocrystic origin of the oxide inclusions. These oxide inclusions are thus interpreted to be cumulus minerals crystallized together and trapped in olivine from the same parental magma. In addition to Fe–Ti oxides, some inclusions contain amphibole + biotite ± fluorapatite that might have formed by reaction of trapped hydrous liquid with the host olivine. Numerical modeling of high-Ti Emeishan basalts using the MELTS program successfully simulates early crystallization of olivine (∼Fo81) and Fe–Ti spinel in the presence of a moderate amount of H2O (∼1.5 wt%) under pressure and fO2 conditions generally pertinent to the Panzhihua and Hongge intrusions. The modal mineralogy of the oxide inclusions is in good agreement with the bulk compositions of the ore, as inferred from whole-rock data, in a given intrusion. This is consistent with the interpretation that the stratiform oxide ores in the intrusions formed by accumulation of Fe–Ti oxide crystals that appeared on the liquidus with olivine and clinopyroxene.

Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in gener- ating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830-1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low f H2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe-Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend.

Heterogeneous bubble nucleation on Fe-Ti oxide crystals in high-silica rhyolitic melts

The nucleation of H 2O bubbles in magmas has been proposed as a trigger for volcanic eruptions. To determine how bubbles nucleate heterogeneously in silicate melts, experiments were carried out in which high-silica rhyolitic melts were hydrated at 740–800°C and 50–175 MPa, decompressed by 20–70 MPa, and held at the lower pressures for ≥10 s before being quenched. The hydration conditions were subliquidus, and all samples contain blocky magnetite + needle-shaped hematite ± plagioclase. Magnetite is abundant at 800°C and high pressures, whereas hematite becomes more abundant at lower temperatures and pressures. Bubbles nucleated in a single event in all samples, with the number density ( N T ) of bubbles varying between 2 × 10 7 and 1 × 10 9 cm −3. At low degrees of supersaturation, one to a few bubbles nucleate on faces of magnetite, but at medium to high degrees of supersaturation, multiple bubbles nucleate on single magnetite grains. On hematite, one to a few bubbles nucleated at the ends of the needle-shaped crystals at medium supersaturations, but formed along their entire lengths at high supersaturations. N T increases as water diffusivity decreases, indicating that the number of bubbles nucleated is influenced by their growth, which depletes the melt with respect to H 2O and lowers supersaturation. If volcanic eruptions are triggered by bubble formation in magmas stored in shallow-level magma chambers, then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed, whereas homogeneous nucleation is unlikely to trigger eruptions.

Origin of two differentiation trends in the Emeishan flood basalts

Both the Bowen and Fenner differentiation trends have been recognized in the Emeishan flood basalts. While the Longzhoushan lavas in Panxi paleorift evolved on a trend of silica enrichment (Bowen trend), the lavas from Guizhou evolved along the Fenner trend leading to the magmas with high Fe2O3 (23%) and low SiO2 (44%) contents. This provides evidence for the existence of Fe-rich and Si-poor magmas in nature. Such contrasting differentiation trends, marked by different timing of crystallization of Fe-Ti oxides, are correlated with the extent of crustal contamination of the magmas. Limited crustal contamination in the Guizhou lavas is coupled with the low oxygen fugacity which delayed the onset of magnetite crystallization. In contrast, significant involvement of crustal components in the Longzhoushan lavas increased ƒO2 which in turn triggered early crystallization of magnetite. The close spatial association between the Longzhoushan lavas and the synchronous V-Fe-Ti deposit-bearing layered intrusions suggests a potential relationship between them.

Genesis of the Mars Pathfinder “sulfur-free” rock from SNC parental liquids

Combined rock and soil measurements from the Mars Pathfinder APXS established the composition of the “sulfur-free” rock whose chemical characteristics suggest it is an igneous andesite. We experimentally investigated the formation of the sulfur-free rock through equilibrium crystallization of a primitive SNC basalt under hydrous (1.0–1.5 wt% H 2O) and dry conditions at the QFM oxygen buffer. The experiments determined crystallization sequences and liquid lines of descent for a basalt with a high-FeO, low-Al 2O 3 (relative to tholeiitic magma) composition characteristic of SNC magmas. In the hydrous experiments, the crystallization sequence between ∼1090°C and 950°C is pigeonite → sub-calcic augite → Ti-magnetite → plagioclase → ilmenite + fayalite. The crystallization sequence of dry experiments between ∼1130°C and 980°C resembles that of the hydrous experiments except for the appearance of plagioclase and fayalite before Ti-magnetite. The liquid lines of descent reveal that fractionation of a hydrous, primitive SNC basalt can produce a melt equivalent to the sulfur-free rock composition whereas fractionation of a dry SNC basalt fails to produce such a melt. The effect of water on the timing and amount of phenocryst growth, particularly Fe-Ti oxide crystallization, is critical in achieving the observed SiO 2 enrichment in the hydrous experiments. The hydrous basalt reaches andesitic residual melt compositions at ∼40% crystallization and the dry basalt eventually produces a minimum andesitic composition melt at ∼90% crystallization. The low degree of crystallization necessary to reach andesitic residual melt compositions in the hydrous experiments would greatly facilitate the extraction of this andesitic melt (and more evolved melts) from associated crystals. Further experiments at other oxygen fugacities reveal that oxidizing fO 2’s (MnO-Mn 3O 4) encourage silicic melt formation by enhancing oxide crystallization; conversely, reducing fO 2’s (GCH) hinder silicic melt formation. By analogy with terrestrial magmatic processes, water could enter basaltic melt on Mars through one or more pathways: partial melting of water-bearing mantle and concentration of water by fractional crystallization; stoping or assimilation of hydrothermally altered crust; direct introduction of hydrothermal fluids into a magma during caldera collapse.

Genesis of a low-grade apatite-ilmenite-magnetite deposit in the Kauhajärvi gabbro, western Finland

The Paleoproterozoic Kauhajarvi gabbro is one of several Fe-, Ti-, and P-rich mafic intrusions associated with granitoids in the Fennoscandian shield in western Finland. The gabbro is cut by the late-orogenic Lauhanvuori granite (ca.1870 Ma), whereas the surrounding area is composed of synorogenic, collision-related granitoids and calc-alkaline volcanic rocks (ca. 1890 Ma) belonging to the Mid Finland Granitoid Complex. The mafic intrusions were probably emplaced into a Svecofennian rift zone. They are characterized by a high phosphorus content; the common occurrence of ilmenite as separate grains; and the coeval crystallization of apatite, Fe-Ti oxides, and Fe-Mg silicates. The Kauhajarvi gabbro is composed of two geochemically and structurally distinct zones. The basal zone is composed of poorly-layered, fine- to medium-grained gabbro, which represents an early intrusion of tholeiitic magma, and has rather high concentrations of chromium, magnesium and silica. Typically, the concentrations of iron, titanium and phosphorus are low, except for the top that is enriched in apatite and ilmenite. During most of the crystal-liquid fractionation of the basal zone magma, low f O2 limited the crystallization of Fe-Ti-oxides. Instead, titanium became enriched in the uppermost layer of the basal zone. The main zone represents a later injection of more evolved tholeiitic magma and makes up 80 to 90%of the total intrusion volume. Peridotite is common, along with gabbro and gabbronorite, in the lower and middle parts of the main zone, and anorthosite is common near the top of the main zone. The Mg:Fe ratio in mafic minerals and vanadium concentrations in magnetite decrease upwards. The variation within the main zone can be explained by crystal-liquid fractionation of a single batch of a parental magma under conditions of relatively high f O2. Titanium is not progressively enriched. The ratio of titanium to iron (TiO2/Fe2O3 = 0.16 to 0.20; Fe total as Fe2O3) is constant in the main zone and normal for mafic intrusions. Titanium is sited in separate ilmenite grains and in lamella within ilmenomagnetite (Ti-bearing magnetite). The high phosphorus content in the main zone is interpreted to result in crystallization of ilmenite and ilmenomagnetite instead of Ti-rich magnetite under relatively high f O2 conditions. High concentrations of titanium, iron and phosphorus in rocks of the main zone can be explained by pre-emplacement crystal-melt fractionation in a deep magma reservoir and/or contamination of mantle-derived mafic magmas by granitic magmas from partial melting of crustal rocks. A low-grade Fe-Ti-P resource at Kauhajarvi consists of layers with as much as 20 wt. % combined ilmenite (usually 8 to 11 wt. %), apatite (1 to 8 wt. %) and magnetite (1 to 9 wt. %). Mineralized layers are of variable thickness (2 m to 30 m) and occur in variable host rocks (peridotite or gabbro). The Fe-Ti oxides are most abundant in peridotite and pyroxene- or olivine-rich gabbronorite within the main zone. The contact between mineralized rocks (4%TiO2) and non- or slightly-mineralized rocks is gradual. The deposit as a whole consists of three to five mineralized layers with maximum combined thickness of 70 m. Apatite is most abundant in the oxide-rich layers, but is locally also concentrated in anorthosite with low Fe-Ti oxide contents. The weight ratio of ilmenite to magnetite is 3:2. The ratio of total Ti-Fe-oxides to apatite averages 4.0, with the range of 1.5 to >15.

Understanding Vesuvius magmatic processes: evidence from primitive silicate-melt inclusions in medieval scoria clinopyroxenes (Terzigno Formation)

Microthermometric investigations of silicate-melt inclusions and electron microprobe analyses were conducted on experimentally homogenized silicate-melt inclusions and on the host clinopyroxenes from 4 scoria samples of different layers from the Mt. Somma-Vesuvius medieval eruption (Formazione di Terzigno, 893 A.D.). The temperature of homogenization, considered the minimum trapping temperature, ranges from 1190 to 1260 ± 5 °C for all clinopyroxene-hosted silicate melt inclusions. The major and minor-element compositional trends shown by Terzigno scoria and matrix glass chemical analysis are largely compatible with fractional crystallization of clinopyroxene and Fe-Ti oxides. Sulfur contents of the homogenized silicate-melt inclusions in clinopyroxene phenocrysts compared with that in the host scoria show that S has been significantly degassed in the erupted products; whereas, Cl has about the same abundance in the inclusions and in host scoria. Fluorine is low (infrequently up to 800 ppm) in the silicate-melt inclusions compared to 2400 ppm in the bulk scoria. Electron microprobe analyses of silicate-melt inclusions show that they have primitive magma compositions (Mg# = 75-91). The composition of the host clinopyroxene phenocrysts varies from typical plinian-related (Mg#≥85) to non plinian related (Mg#≤85). The mixed source of the host clinopyroxenes and primitive nature of the silicate-melt inclusions implies that these phenocrysts, in part, may be residual and/or have a polygenetic origin. The similar variation trends of major and minor-elements between homogenized silicate-melt inclusions from the Terzigno scoria, and silicate-melt inclusions in olivine and diopside phenocrysts from plinian eruptions (Marianelli et al., 1995) suggest that the trapped inclusions represent melts similar to those that supplied the plinian and sub-plinian magma chambers. These geochemical characteristics suggest that the Vesuvius magmatic system retained a vestige of the most recent plinian event.

Magma mixing, fractional crystallization and volatile degassing during the 1883 eruption of Krakatau volcano, Indonesia

The 1883 eruption of Krakatau volcano, Indonesia produced approximately 12.5 km 3 (DRE) of magma, 90% rhyodacite, 4% mafic dacite, and 1% andesite (5% lithic material assumed). Melt compositions in the erupted products based on the study of glass inclusions and matrix glasses span a minimum range of 13 wt.% SiO 2. Well defined linear trends of major elements have been successfully modelled by incremental fractional crystallization of plagioclase, pyroxene, and Fe-Ti oxides from andesitic parental magma to produce rhyodacite. This fractional crystallization model is supported by trace-element and isotopic data. The 1883 magma chamber was compositionally and thermally zoned with an upper portion of homogenous rhyodacite at a temperature of 880–890 °C overlying more mafic dacite at 890–913 °C, and andesite at 980–1000 °C. Mafic dacite represents a hybrid magma formed by mixing small amounts of andesite with larger amounts of evolved rhyodacite. Evidence for magma mixing consists of banded pumices, glass inclusions that are more mafic than whole rock and matrix glass compositions, heterogeneous matrix glass compositions, and disequilibrium phenocrysts of anorthite-rich plagioclase in rhyodacite and mafic dacite. The 1883 rhyodacitic magma was probably not water saturated until it reached a shallow depth in the crust of 4–5 km. Glass inclusions indicate pre-eruption dissolved volatile content in rhyodacite and mafic dacite of 4.0 ± 0.5 wt.%. Estimated sulfur discharge from erupted magma is 2.8 × 10 12 g S, and estimated Cl discharge is 9.7 × 10 12 g Cl. Two potential sources of additional sulfur from this eruption may be vaporization of seawater during entrance of pyroclastic flows into the sea, and degassing of nonerupted andesitic parental magma lying beneath evolved rhyodacitic magma in the zoned 1883 chamber.

An empirical model for the calculation of spinel-melt equilibria in mafic igneous systems at atmospheric pressure: 1. Chromian spinels

In order to develop models simulating the crystallization of Fe-Ti oxides in natural lavas, we have processed published experimental data on magnetite-melt and ilmenite-melt equilibria. These data include 62 Mt-melt and 75 Ilm-melt pairs at temperatures 1040–1150 °C, oxygen fugacities from IW to NNO+2, and bulk compositions ranging from ferrobasalts to andesites and dacites. Five major cations (Fe3+, Fe2+, Ti4+, Mg2+ and Al3+) were considered for the purpose of describing Fe-Ti oxide saturation as a function of melt composition, temperature and oxygen fugacity at 1 atmosphere pressure. The empirically calibrated mineral-melt expression based on multiple linear regressions is: ln Di = a/T + blog fO2 + c + d1XNa + d2XK + d3XP, where Di represents molar distribution coefficients of the given cations between Mt/Ilm and melt; XNa, XK, and XP are the molar fractions of Na, K, and P in the melt. The empirically calibrated Mt-melt and Ilm-melt equilibria equations allowed us to develop two models for calculating crystallization temperatures of the Fe-Ti oxides in the melts with an accuracy of 10–15 °C, and compositions with an accuracy of 0.5–2 mol%. These models have been integrated into the COMAGMAT-3.5 program, improving our ability to study numerically the effects of temperature and oxygen fugacity on the stability and phase equilibria of Fe-Ti oxides. Application of this approach to the tholeiitic series of Chazhma Sill from Eastern Kamchatka (Russia) indicates oxygen fugacity conditions near NNO + 0.5. Numerical simulation of fractional crystallization of an iron-enriched basaltic andesite parent at these oxidizing conditions accurately reproduces the FeO-SiO2 relations observed in the Chazhma suite.

Experimental constraints on the low-pressure evolution of transitional and mildly alkalic basalts: the effect of Fe-Ti oxide minerals and the origin of basaltic andesites

One-atmosphere, melting experiments, controlled at the fayalite-magnetite-quartz oxygen buffer, on mildly alkalic and transitional basalts from Iceland show that these begin to crystallize Fe-Ti oxide minerals (magnetite and/or ilmenite) at 1105±5°C, apparently independently of bulk composition and the order of silicate and oxide mineral crystallization. Most samples crystalline plagioclase and olivine as the first two crystalline phases, augite as the third phase, and an Fe-Ti oxide mineral as the fourth phase. The main effects of Fe-Ti oxide crystallization are a marked decrease in FeO and TiO2 in the liquid, and a notable increase in SiO2 and Al2O3, and the minor oxides K2O and P2O5, with decreasing temperature. The most silicic glasses are compositionally mugearitic and shoshonitic basaltic andesites. Because the smallest amount of glass that could be analyzed with the microprobe represents 20–55 percent liquid remaining, it can be expected that more silicic liquids will occur at lower temperatures. On normative, pseudoternary projections, the general effect of Fe-Ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in normative quartz. This is caused by a strong systematic convergence, with the appearance of Fe-Ti oxides, of the bulk solid precipitates toward the liquid compositions, as projected on the triangle plagioclase-diopside-olivine. For alkalic basalts, the bulk solid precipitate shows an increase in normative diopside with falling temperature and Fe-Ti oxide crystallization. This causes the liquids to move toward decreasing normative diopside and relatively little variation in nepheline. The experimental observations imply that mildly alkalic and transitional magmas, without stabilizing a Fe-Ti oxide mineral, will not evolve toward early silica saturation.

Pyroxene in enclaves and syenites of the Red Hill complex, New Hampshire: an ion and electron microprobe study

The Red Hill complex, New Hampshire, contains both silica-undersaturated and silica-saturated to oversaturated syenites. Ion microprobe analyses of pyroxene from the Nepheline Sodalite Syenite (NSS) and its enclaves reveal that the REE abundances increase in a systematic manner from low values in the enclave pyroxenes to higher values in the NSS host rock pyroxenes. This variation is interpreted to have resulted from differentiation and coupled with previously published mineral and bulk-rock compositions, suggests that the enclaves represent samples of NSS parental liquids that intruded into residual, syenitic liquids in a zoned magma chamber. Pyroxene analyses of the Garland Peak Syenite (GPS) and its enclaves indicate that the enclaves are of several populations: some may be related to the GPS, others are not. The GPS itself is heterogeneous and pyroxene trace element zoning is difficult to explain by fractionation processes. The silica-oversaturated Outer Coarse Syenite (OCS) contains pyroxenes with trace element characteristics that are distinct from the NSS. The low V concentrations suggest that the OCS exprienced magnetite fractionation prior to pyroxene growth. It is proposed that high f O2in the OCS magma caused Fe-Ti oxide crystallization, which in turn, influenced magmatic silica activity. The chondrite normalized REE patterns of OCS pyroxenes are also suggestive of titanite crystallization, another indication of high f O2. In contrast to the influence of magnetite crystallization, the high and rimwardly increasing Y and Yb concentrations in both the NSS and the OCS pyroxenes suggest that amphibole fractionation was not the major influence on silica activity. Therefore, it is unlikely that the OCS was derived by amphibole fractionation from a NSS precursor magma.

Jotunites from the Grenville Province, Quebec: petrological characteristics and implications for massif anorthosite petrogenesis

We report new results on the geology, mineralogy and chemical compositions of jotunites (mainly pyroxene-monzodiorites) from the Morin, St. Urbain and Labrieville anorthosite complexes, all in the Grenville Province of Quebec. Jotunites occur between anorthosite and mangerite, as marginal units to anorthosite or mangerite plutons, or as fine-grained dikes in either rock type. Some occurrences are probably cumulates, but the dikes indicate the existence of jotunitic liquids. Jotunites consist predominantly of sodic plagioclase (∼An 25–An 45), clinopyroxene and orthopyroxene having intermediate X Mg (e.g., ∼En 60–En 40), along with significant ilmenite, magnetite and apatite; most samples contain small amounts of K-feldspar (locally mesoperthite) and quartz. It is noteworthy that plagioclase and pyroxene compositions in jotunites lie between those in associated anorthosite and mangerite. Jotunites from all areas span a similar range in SiO 2-content (40–55 wt.%) and are characterized by high levels of Fe 2O 3-T (⩽30 wt.%), TiO 2 (⩽6 wt.%) and P 2O 5 (⩽3.25 wt.%). All samples are alkalic, with K 2O (⩽3.6 wt.%) increasing in the sequence Morin → St. Urbain → Labrieville. They contain high Sr (up to 1400 ppm) and Ba (up to 4600 ppm) and concentrations correlate well with those in associated anorthosite in each case. Jotunites also are high in Ga (high Ga Al ), Zr, Zn, Y and REE, but low in Rb (high K Rb ), U, Th, Ni, Cr, Co, Sc and V (high Ti V . Jotunites can be linked spatially, mineralogically and chemically with associated anorthosite and mangerite, and we interpret them as transitional rocks in a largely comagmatic sequence from anorthosite to mangerite. We postulate a model for andesine- anorthosite petrogenesis involving early crystallization and accumulation of plagioclase from parental magmas of intermediate SiO 2-content—broadly “dioritic” in composition, resulting in Fe-, Ti- and P-enrichment in the remaining magma, leading to jotunite. High |(O 2) Table causes early crystallization of FeTi oxides, which suppresses Fe Mg enrichment in pyroxene. Fractionation of FeTi oxides, pyroxene and apatite ultimately results in Si- and K-enrichment in residual liquids, leading to quartz-mangerite. Significantly, liquids follow an initial trend of Si-depletion + Fe-enrichment followed by a reversal of this trend. This view contrasts strongly with current models that regard anorthosite, jotunite and mangerite as unrelated products of bimodal magmatism.

Caractérisation pétrologique et géochimique du magmatisme ubendien du secteur de Pepa-Lubumba, sur le plateau des Marungu (Nord-Est du Shaba, Zaire). Signification géodynamique dans l'évolution de la chaîne ubendienne

Volcanic and plutonic rocks of Pepa-Lubumba area in the Marungu plateau (Zaire) are mainly represented by intermediate and acid members which contain low abundances in TiO 2 (0.3 – 1.3 wt%). All these rocks exhibit a high-K calc-alkaline affinity and belong to a calc-alkaline continental-margin series. The large proportion of frequently zoned (normal and reverse zoning) plagioclase, the presence of hydrous phases (tschermakitic and Mg-hornblende, Mg-biotite), the early crystallization of FeTi oxides indicate crystallization under high water pressure conditions, consistent with a calc-alkaline affinity. High abundances in K, Rb, Sr and Ba; lack of correlation between Rb and Ba, low values of Rb/Zr (0.5 – 0.9) in granitoids, high contents of LREE and HFSE (Th/Ta : 6 – 14), are characteristic of “Andinotype” magmas. A whole rock RbSr isochron gives an emplacement age for granitoids of 1861 ± 28 Ma (MSWD : 0.82), Sr initial ratio of 0.7026 and epsilon Sr i of + 4.7. PbPb isotopes systematics for the same rocks give the following ratio ranges : 206Pb/ 204Pb : 17.26 – 20.10; 207Pb/ 204Pb 15.52 – 15.85, 208Pb/ 204Pb : 37.2 – 41.80. The low Sr initial ratio indicates an upper mantle source; while the PBPb isotope ratios suggest that the mantellic liquids have been contaminated by crustal material en route to the upper levels. In the regional geologic context, this magmatism forms a part of an important plutovolcanic Ubendian (Early Proterozoic) complex which crops out in Western Tanzania, North-Eastern Zambia and Marungu plateau (Zaire) and contains two magmatic cycles which define a spatial and temporal zonation suggesting a geodynamic evolution model for the Ubendian belt comprising subduction-obduction-collision processes. This latter episode has been relayed by a relaxation phase marked by emplacement of a third tholeiitic basic plutonic cycle.