Pyroxene Crystals Research Articles

Mineralogy of the P-12 K-Ti-richterite diopside olivine lamproite from Wajrakarur, Andhra Pradesh, India: implications for subduction-related magmatism in eastern India

The P-12 “para-kimberlite” from Wajrakarur consists of forsteritic olivine, Al-Na-poor diopside, Fe-Ti-rich, Al-poor phlogopite, K-Ti-richterite, spinel, perovskite, cymrite, apatite, barite, Ba-Sr- bearing calcite, gittinsite, witherite, strontianite, and hydrogrossular (hydrogarnet). The rock also contains small clasts consisting dominantly of calcite, with lesser Ba-Sr-bearing calcite, cymrite, barite, strontianite, witherite, apatite, and hydrogrossular. Two generations of forsteritic olivine (Fo80–93) crystals are present: common phenocrystal-to-microphenocrystal; and rare anhedral macrocrystic olivines. Phlogopite occurs as microphenocrysts and as groundmass poikilitic plates with inclusions of spinel, perovskite, apatite, and chlorite pseudomorphs (after pyroxene). Phlogopites also occur as reaction rims around olivine crystals. The phlogopites have extremely low Al2O3 (2.2–3.8 wt.%), moderate-to-high FeO (6.9–16 wt.%), TiO2(1.9–4.6 wt.%), and Na2O (0.4–2.7 wt.%) contents and are enriched in fluorine (up to 6.0 wt.%) and considered to be tetraferriphlogopite. The pyroxenes occur in five parageneses as: (1) phenocrysts and microphenocrysts; (2) small slender crystals(<30 μm) forming part of the groundmass; (3) the cores of richterite crystals; (4) reaction products replacing earlier-formed olivine; (5) acicular crystals mantling carbonate clasts. These pyroxenes do not differ significantly in composition and are all diopsides with minor variation in their TiO2, Al2O3, Na2O contents. Titanian-potassium richterite commonly occurs as: (1) groundmass poikilitic plates; (2) small prismatic crystals (<30 μm); (3) reaction rims on olivine and pyroxene crystals. Groundmass poikilitic richterites commonly enclose pyroxene and apatite. Perovskites have a bimodal size distribution. Small (<20 μm) euhedral perovskites are scattered throughout the groundmass, whereas larger (100–300 μm) subhedral-to-euhedral perovskites are patchily-zoned and commonly broken. Micro-clasts consisting of accumulations of perovskite with phlogopite and apatite are also present. Spinels occur as large atoll crystals and small (<20 μm), euhedral-to-subhedral crystals, scattered throughout the groundmass. Some small spinel crystals are also present in the rims of olivine and pyroxene crystals. Atoll spinels are up to 100 μm in size, commonly with single and double cores. Atoll spinels are typically associated with perovskites. The euhedral-to-subhedral small spinels are ulvospinels. The atoll spinels have cores of titanian aluminous magnesiochromite with rims of magnesian titaniferous magnetite. The spinels have compositions which evolve along the lamproite-spinel compositional trend. Zoned calcite crystals occur as residual phases. Late stage residual calcite and carbonate clasts host prismatic cymrite crystals which are interpreted as pseudomorphs after potassium feldspar and/or barite. Subhedral-to-euhedral gittinsite and its Sr-analog are reported for the first time from the groundmass carbonate-chlorite mesostasis of a lamproite. Square-to-rectangular crystals of cymrite and hydrogrossular occur in the carbonate clasts and groundmass material. Barite anhedra commonly occur in the carbonate clasts together with witherite, strontianite, and Ba-Sr-bearing calcite. The texture and compositions of olivine, phlogopite, spinel, and K-Ti-richterite, together with the presence of cymrite pseudomorphs, possibly after potassium feldspar, demonstrate that this intrusion is a bona fide olivine lamproite and not a kimberlite. It is postulated that this, and other lamproites, located adjacent to the Eastern Ghats Mobile Belt, are derived by extensional decompressional melting of ancient subduction zones underlying the cratonic regions.

Geochemistry of the mantle source and magma feeding system beneath Turrialba volcano, Costa Rica

Turrialba volcano lies in the southern sector of the Central American Volcanic Front (CAVF) in Costa Rica. The geochemistry of major and trace elements, and Sr and Nd isotopes of a selected suite of volcanic rocks ranging in composition from basaltic andesite to dacite and belonging to the last 10ka of activity of Turrialba volcano is described, together with the He-, Ne-, and Ar-isotope compositions of fluid inclusions hosted in olivine and pyroxene crystals. Most of the variability in the rock chemistry is consistent with typical trends of fractional crystallization, but there is an outlying group of andesites that displays an adakite-like composition (with a consistent depletion in high-field-strength elements and a marked enrichment in Sr) and low 3He/4He ratios (7.0–7.2 Ra). The trace-element composition of these rocks is typical of subduction-related magmas influenced by an OIB-like component at the source associated with the subduction of the Galapagos seamounts. The 87Sr/86Sr (0.703612–0.703678) and 143Nd/144Nd (0.512960–0.512984) ratios of the bulk rocks vary within narrow ranges, and are among the least-radiogenic isotope signatures of the CAVF volcanoes. The 3He/4He ratios measured in fluid inclusions hosted in olivine crystals (up to 8.1 Ra) are among the highest for the CAVF, and indicate that radiogenic 4He from fluids derived from the subducting slab contribute negligibly to the mantle wedge. The difference in He isotopes between most of studied rocks and those showing adakite-like features reasonably reflects two distinct components in the local mantle: (1) a MORB-like component, characterized by the highest He-isotope ratios (7.8–8.1 Ra), and (2) an OIB-like component, characterized by lower He-isotope ratios (7.0–7.2 Ra), coming from the subduction of the Galapagos seamounts. An overview at the regional scale indicates that high He-isotope ratios are peculiar to the two extreme sectors of the CAVF (Costa Rica to the south and Guatemala to the north), whereas in the central sector (Nicaragua) the magma source is probably contaminated by slab fluids.For the past few years Turrialba volcano has been in a volcanic unrest phase that has included a series of explosions, the most recent of which occurred between October 2014 and May 2015. The volcano is subject to an ongoing safety alert due to the possibility of a magmatic eruption. One of the crucial questions to be addressed is the kind of eruption that can be expected, and hence what type of magma is likely to be involved. The high 3He/4He ratios (7.8–8.0 Ra) measured during 2011 at high-temperature fumaroles of Turrialba craters are comparable to those measured in fluid inclusions of basaltic andesites that erupted in 1864–1866, suggesting that the magma currently feeding the shallow plumbing system has similar geochemical characteristics to the most recently erupted magma.

Platinum-group element geochemistry of Cenozoic basalts from the North China Craton: Implications for mantle heterogeneity

Forty-two Cenozoic (mostly Miocene) basalt samples from Jining, Chifeng, Fansi, Xiyang, and Zuoquan areas of the North China Craton (the NCC basalts hereafter) were analyzed for platinum-group elements (PGE, including Os, Ir, Ru, Rh, Pt, and Pd). Most of them are alkaline basalts and tholeiites and all of them display little crustal contamination. The total PGE contents of the NCC basalts vary from 0.1 to 0.9 ppb, much lower than those of the primitive mantle values of 23.5 ppb. Primitive mantle-normalized PGE patterns of these basalts define positive slopes and Pd/Ir ratios vary from 1.2 to 25. In terms of both PGE contents and Pd/Ir ratios, they are quite similar to the mid-ocean ridge basalts. There are no obvious negative correlations between PGE vs. MgO, Ni, and Cu in the NCC basalts, indicating that fractional crystallization of olivine, pyroxene, and/or sulfides during magmatic process cannot be the controlling factor for the observed PGE variation. The observed Pd/Ir variations of the NCC basalts require involvement of non-chondritic heterogeneous mantle sources. Based on Sr-Nd-Pb-Hf isotopic systematics and incompatible-element signatures, a mixing of partial melts from both asthenospheric peridotites and enclosed mantle eclogites at the top of asthenosphere was proposed for the origin of these NCC basalts. The lenses of eclogites are derived from upwelling of recycled continental crust during the westward subduction of the Pacific plate from the ∼600 km discontinuity zone. The PGE geochemistry of these basalts provides independent evidence to support this conclusion and the observed Pd/Ir variations may reflect variations in proportions of tapped peridotitic and eclogitic melts.

Crystal and magnetic structures, phase transitions in quasi-one-dimensional pyroxenes Na0.5Li0.5FeGe2O6

The possibility of cation substitution in pyroxenes allows the investigation the gradual change of structural and magnetic properties under doping in these compounds. Here, Na0.5Li0.5FeGe2O6 was prepared by the standard solid-phase reaction method and has been investigated using X-ray and neutron powder diffraction, and further characterized by magnetic and calorimetric measurements. The crystal structure of Na0.5Li0.5FeGe2O6 at 300K is monoclinic C2/c (a=10.0333(1), b=8.8136(1), c=5.5295(9)Å, β=108.921(1)°). Calorimetric investigations indicate a displacive first order phase transition at T=271±1K which is accompanied by the appearance of superstructure reflections in the X-ray patterns. At this transition Na0.5Li0.5FeGe2O6 undergoes a space group change from C2/c to P21/c (a=9.9692(3), b=8.8545(3), c=5.4752(2)Å, β=108.494(1)°). Magnetic order has been found below the Néel temperature TN≈18K and has been refined from neutron diffraction. The quasi-low-dimensional magnetic spin system Na0.5Li0.5FeGe2O6 exhibits a collinear antiferromagnetic structure with the space group Pa21/c and the doubling of the unit cell along the crystallographic a-axis of the pyroxene crystal (propagation vector k=(1/2, 0, 0)). The critical modes which are responsible for the phase transition from C2/c to P21/c symmetry and for the magnetic transition from paramagnetic to antiferromagnetic state have been determined and their role in the transition is described. They are also the relevant mechanism driving the magnetic order in LiFeGe2O6.

Synthesis of steel slag ceramics: chemical composition and crystalline phases of raw materials

Abstract Two types of porcelain tiles with steel slag as the main raw material (steel slag ceramics) were synthesized based on the CaO-Al2O3-SiO2 and CaO-MgO-SiO2 systems, and their bending strengths up to 53.47 MPa and 99.84 MPa, respectively, were obtained. The presence of anorthite, α-quartz, magnetite, and pyroxene crystals (augite and diopside) in the steel slag ceramics were very different from the composition of traditional ceramics. X-ray diffraction (XRD) and electron probe X-ray microanalysis (EPMA) results illustrated that the addition of steel slag reduced the temperature of extensive liquid generation and further decreased the firing temperature. The considerable contents of glass-modifying oxide liquids with rather low viscosities at high temperature in the steel slag ceramic adobes promoted element diffusion and crystallization. The results of this study demonstrated a new approach for extensive and effective recycling of steel slag.

Bridging basalts and rhyolites in the Yellowstone–Snake River Plain volcanic province: The elusive intermediate step

Many magmatic provinces produce strongly bimodal volcanism with abundant mafic and silicic magmas yet a scarcity of intermediate (55–65 wt.% SiO2) compositions. In such bimodal settings, much debate revolves around whether the basaltic magmas act as heat sources to melt pre-existing crust, or whether they are the parents to the silicic magmas (a fractionation-dominated evolution). Until now, this lack of coeval intermediate compositions has commonly been used to support models involving large degrees of crustal melting. Detailed analysis of mineral cargoes of ignimbrites from the 6.6–4 Ma Heise volcanic field in the famously bimodal Yellowstone–Snake River Plain (YSRP) volcanic province has revealed the existence of intermediate liquids associated with main stage rhyolitic volcanism. Two closely spaced rhyolitic ignimbrites, the Wolverine Creek Tuff and the Conant Creek Tuff, contain pyroxene crystals with major and trace elemental compositions in equilibrium with intermediate melts prior to significant plagioclase fractionation. Hosted within these crystals are glassy melt inclusions that have compositions (57–67 wt.% SiO2) directly recording the intermediate liquids. The combined mineral and melt inclusion data provide the first evidence for the occurrence of intermediate melts, typically erased in the high temperature YSRP ignimbrites by crystal resorption or diffusive re-equilibration. The results suggest the existence of mostly unerupted mid-crustal reservoirs that drive magma compositions towards the erupted rhyolites via assimilation-fractional crystallisation (AFC).

Petrography and Geochemistry of the Mbip Granitic Massif, SW Tchollir&amp;#233; (Central North Cameroon): Petrogenetic and Geodynamic Implication

In this article, we discussed about the petrography and geochemistry of magmatic rocks of the Mbip massif located SW of Tcholliré, in Central North Cameroon (Central African Fold Belt). Petrographic study shows that this massif is made of granodiorite, amphibole-biotite granite, and biotite granite which often contain enclave of mafic rocks (gabbro). Granodiorites and granites show porphyritic texture and consist dominantly of plagioclase and alkaline feldspar phenocrysts, quartz, biotite and often few amphibole. The gabbro enclaves are characterized by a granular porphyroid texture constituted of amphibole, plagioclase and olivine phenocrysts, all in a matrix of small crystals of plagioclase, olivine, amphibole, pyroxene and opaque minerals. All the analyzed rocks provide geochemical features of sublkaline serie and have nothing to do with the alkaline nature previously signaled in the former works. Granodiorites and granites are shoshonitic to calc-alkaline strongly potassic affinity consistent with emplacement in the continental collision setting. They present positive anomalies in Rb, Th, U; negative anomalies in Sr, P, Ti, Nb, Ta and some facies contain hydrated minerals such as amphibole. These characters are consistent with crustal and mantle contribution in their genesis. The mafic rocks (gabbro) have geochemical characteristics conferring a mantle origin, as confirmed by the high Mg# value (49.9). On the whole, the fractional crystallization of the magma formed by melting of crustal material and the magmatic mixing are the main petrogenetic process in the Mbip massif. Petrographic studies and geochemical data shows that the Mbip massif was emplaced in an active continental margin, into transitional regime from the end of maximum compression to the beginning of relaxation.

Fractionation of zirconium in pyroxenes of alkaline magmas

Pyroxenes of the Lovozero Massif (Kola Peninsula) intensely accumulate zirconium. The Zr content in the pyroxenes varies from 0.01 to 2.3%. In the pyroxenes from the Eudialyte Complex, Zr content increases with increasing the stratigraphic height in the sequence of the Lovozero intrusion. In contrast, Zr concentration in pyroxenes from the Differentiated Complex show very uneven distribution. This is related to the crystallization of pyroxenes at different stages of melt evolution and to the variations in their alkalinity. The zirconium distribution in pyroxenes of the Lovozero Massif is determined by the fractional crystallization differentiation. The partition coefficient of zirconium between pyroxene and ultraalkaline magmatic system was determined for the first time on the basis of alkaline pyroxene—melt equilibrium in hypabyssal porphyritic lujavrites. The model of generation of zirconium (eudialyte) ores in the Lovozero Massif was developed using this value and equations of equilibrium and fractional crystallization.

Application of non-extensive statistical physics on Martian nakhlites: A first-order approach on the crystal size distribution of pyroxene using Tsallis entropy

In this paper, we present a Non-Extensive Statistical Physics (NESP) approach in order to investigate the crystal size distribution of pyroxene crystals from Martian meteorites, nakhlites MIL090030 and MIL090032, which reflect igneous processes on Mars 1.3 Ga ago. The formation of pyroxene crystals is a complex process in which fractional crystallization of an igneous melt is predominant in the evolution process. It is exactly this type of complex process, such as that of crystal-melt interaction, for which NESP could be applied. The results of the analysis indicate that a model based on the Tsallis entropy is an appropriate framework for the statistical-physics interpretation of crystal size distribution of pyroxene grains in these rocks. The similarity of the estimated q values supports the previous conclusions on the pairing between the studied nakhlites.

Crystal chemistry of merrillite from Martian meteorites: Mineralogical recorders of magmatic processes and planetary differentiation

AbstractMerrillite is a ubiquitous accessory phase in a variety of Martian meteorite lithologies. The Martian merrillites exhibit a positive correlation between Mg# and Na and a negative correlation between Mg# and both Mn and vacancies in the octahedral Na‐site. Their REE patterns are varied and range from LREE‐depleted to LREE‐enriched. The dominant cation substitutions in the Martian merrillites are Fe2+VI Mg‐site⇔Mg2+VI Mg‐site and Ca2+VI Na‐site + □VI Na‐site⇔2Na+VI Na‐site. The REE substitution into the 8‐fold coordinated Ca‐site is accommodated by the coupled substitution CaVIII Ca‐site + (Na)VI Na‐site ⇔(Y3+ + REE3+)VIII Ca‐site + □VI Na‐site. The REE substitution is significantly more prevalent in lunar merrillite and can be used as a “fingerprint” to distinguish lunar from Martian meteorites. The substitution of OH− (whitlockite) and/or F− (bobdownsite) for O2− on one of the phosphate tetrahedrons appears to be rather insignificant. The correlations among Na, Mg#, Mn, and Na‐site vacancies are linked to the premerrillite crystallization history of the melt and the crystal chemical behavior of the Mg‐ and Na‐sites. The former reflects the sequence and extent of plagioclase and pyroxene crystallization. The differences in REE pattern shapes among the merrillites reflect source regions for the Martian basalts and the shapes are not greatly perturbed by the crystallization history. The occurrence of merrillite does not imply low‐volatile component in the Martian magmas. However, the low whitlockite and bobdownsite contents suggest that these samples were not altered by hydrothermal fluids and therefore not reset owing to aqueous fluid interactions. Consequently, the young ages of the shergottites are probably true igneous crystallization ages.

Timescales of magmatic processes at Ruapehu volcano from diffusion chronometry and their comparison to monitoring data

Recent eruptions from Mt. Ruapehu have been difficult to predict, despite the presence of a multi-parametric monitoring network. As a result, it is necessary to assess precursory signals prior to an eruption and align those to magmatic processes at depth. Fortuitously, scoria from all historical Ruapehu eruptions contains pyroxene crystals that are strongly reversely zoned in the form of a thin (2 to 3μm), outermost rim. These crystals therefore preserved changes in the magmatic system soon before their eruption. We used experimentally determined diffusion coefficients to assess the timescales of magma–magma interaction, and compared those to the monitoring record. Four of the five eruptions analysed (1969, 1971, 1977, 1995) gave diffusion timescales ~3 to 5months before their eruption, with an increased number of crystals recording timescales within 1month of eruption. Pyroxene crystals from the 1996 eruption record events that occurred prior to and during the 1995 eruption suggesting that the bulk of the 1996 crystals was derived from the 1995 magma. These diffusion timescales do not compare well to a change in any monitoring signal before historical eruptions. However, an examination of recent seismicity (2005–2013) since a significant upgrade (both in number of stations and type of seismometers) showed that two phreatic eruptions in 2006 and 2007 were preceded by a seismic swarm from ~5 to 15km depth, ~3 to 5months before each eruption — consistent with the diffusion timescales. Based on this correlation, deep seismic swarms likely indicate a period of pressurisation in the magmatic system, which may lead to gas-rich, phreatic eruptions.

Platinum-group element geochemistry of komatiite-derived 3.1 Ga ultramafic–mafic rocks and chromitites from the Nuggihalli greenstone belt, Western Dharwar craton (India)

The 3.1Ga Nuggihalli greenstone belt in the Western Dharwar craton (southern India) comprises a sill-like layered ultramafic–mafic igneous complex with associated metasedimentary and metavolcanic (komatiitic to komatiitic basalt) schists that are enclosed by the tonalite–trondhjemite granodiorite suite of rocks (TTG). The sill-like layered complex is represented by a succession of chromitite-bearing serpentinite (after dunite) and peridotite, anorthosite, pyroxenite, and gabbro hosting magnetite bands. Extensive bulk-rock trace element and platinum-group element (PGE) geochemical study of the plutonic sill-like layered complex and the metavolcanic schists, suggest immobility of most trace elements (except La and Cu) and the PGEs, despite greenschist facies metamorphism and hydrothermal alteration experienced by the rocks. Their immobile nature is understood from good correlation of the trace elements and PGE with MgO and Cr. Other than chromitites and serpentinites all plutonic rocks show PPGE (Pd, Pt, Rh) enriched primitive-mantle normalized PGE patterns (Pd/IrN=3.9–81.1) that are suggestive of fractionation of IPGEs (Ir, Os, Ru) by the early crystallizing chromite mineral, and the incompatible nature of PPGEs in the same. The chromitites show high PGE abundances (∑PGE=96–296ppb), especially IPGEs (∑IPGE=63–223ppb), due to the presence of inclusions of IPGE-bearing minerals. In the primitive-mantle normalized PGE plot the chromitites show an IPGE enriched pattern. The PPGE enriched pattern (Pd/IrN=7.7–26) of the komatiitic to komatiitic basalt schists in a primitive-mantle normalized PGE plot indicates retention of IPGEs in the mantle or IPGE-bearing alloy saturation in the melt, while incompatible behavior of the PPGEs implies the sulfide undersaturated nature of the mantle source.The PGE pattern of the metavolcanic schists resembles the pattern of early Archean (3.5Ga) Barberton komatiites (Pd/IrNBarberton=1–40.7; Pd/IrNNuggihalli=6.3–21.3), which corroborates our previous results based on REE study, and also resembles the pattern of komatiites from the 2.9Ga Sandstone greenstone belt in the Youanmi Terrane of Western Australia (Pd/IrNSandstone=6). The metavolcanic schists exhibit the typical PGE depleted character observed in early Archean komatiites (∑PGEschist=0.4–27.2ppb; ∑PGEBarberton=15.0–20.8ppb; ∑PGEYouanmi Terrane, Western Australia=4.2–7.0ppb) which is explained to be a result of progressive mixing of late veneer matter in the Earth's mantle with time. Pt fractionation in the Nuggihalli metavolcanic schists and in early or late Archean komatiites indicates Pt alloy dispersal in the lower mantle during crystallization of the primary magma ocean and a consequent formation of Pt-enriched and Pt-depleted isolated upper mantle domains that did not homogenize and mix away by 2.7Ga.In the plutonic layered sequence, pyroxenite represents a change from sulfide-undersaturation to sulfide-saturation. The pyroxenite represents a break in trend from the negative correlation of Pt and Pd with MgO displayed by the serpentinites and peridotites due to incompatible behavior of the PPGEs during lava differentiation, to the positive pattern displayed by the gabbro and metavolcanic schists due to attainment of sulfide saturation. Sulfide-saturation was probably triggered by fractional crystallization of olivine, chromite and pyroxenes. Chondrite-normalized REE patterns and a plot of incompatible elements negate the role of crustal contamination of the parental komatiitic magma. In addition, the absence of ambient sulfidic sediments rules out assimilation of crustal sulfur in the Nuggihalli rocks. The immiscible sulfides segregated from the Al-depleted komatiitic parental magma concentrating the PGEs during crystallization of the pyroxenes that accumulated to form pyroxenite.

Crystallization kinetics of olivine‐phyric shergottites

AbstractCrystal size distribution (CSD) and spatial distribution pattern (SDP) analyses are applied to the early crystallizing phases, olivine and pyroxene, in olivine‐phyric shergottites (Elephant moraine [EET] 79001A, Dar al Gani [DaG] 476, and dhofar [Dho] 019) from each sampling locality inferred from Mars ejection ages. Trace element zonation patterns (P and Cr) in olivine are also used to characterize the crystallization history of these Martian basalts. Previously reported 2‐D CSDs for these meteorites are re‐evaluated using a newer stereographically corrected methodology. Kinks in the olivine CSD plots suggest several populations that crystallized under different conditions. CSDs for pyroxene in DaG 476 and EET 79001A reveal single populations that grew under steady‐state conditions; pyroxenes in Dho 019 were too intergrown for CSD analysis. Magma chamber residence times of several days for small grains to several months for olivine megacrysts are calculated using the CSD slopes and growth rates inferred from previous experimental data. Phosphorus imaging in olivines in DaG 476 and Dho 019 indicate rapid growth of skeletal, sector‐zoned, or patchy cores, probably in response to delayed nucleation, followed by slow growth, and finally rapid dendritic growth with back‐filling to form oscillatory zoning in rims. SPD analyses indicate that olivine and pyroxene crystals grew or accumulated in clusters rather than as randomly distributed grains. These data reveal complex solidification histories for Martian basalts, and are generally consistent with the formation at depth of olivine megacryst cores, which were entrained in ascending magmas that crystallized pyroxenes, small olivines, and oscillatory rims on megacrysts.

Geochemical and isotopic constraints on the evolution of Late Paleozoic dyke swarms in West Junggar, Xinjiang, China

There are two sets (NW and NE trending) of basic to intermediate dykes emplaced into both the Carboniferous volcanic-sedimentary strata and undeformed plutons in the West Junggar, northern Xinjiang, China. The Early to Middle Permian NW trending dykes are mainly diorite and diorite porphyrite characterized by moderate Mg# (55–66.1) belonging to calc-alkaline series. The Late Permian NE trending dykes are diabase and diabase porphyritic belonging to tholeiitic series with low Mg# (41.4–58.3). Both sets of dykes exhibit enriched LILEs, depleted Nb, Ta, HREEs and highly depleted isotopic signatures (εNd=5.1–8.1, ISr=0.70379–0.70675). Trace element and isotopic data indicate that the NW trending dykes were generated by partial melting of a recently metasomatized subducted cold lithospheric mantle (SCLM) coupled with variable assimilation of juvenile crustal materials, whereas the NE trending dykes were derived from the partial melting of a similar SCLM with lower degree of metasomatism than the NW trending dykes which were subsequently subjected to magma modification by the fractional crystallization of olivine, pyroxene and/or plagioclase. The arc signatures exhibited by the trace element patterns of both sets of dykes in North Xinjiang imply a recently metasomated mantle in this region formed during subduction prior to Late Paleozoic. The mafic dyke swarms in this region are the products of Phanerozoic continental growth in North Xinjiang indicating widespread lithospheric extension.

An experimental study of pyroxene crystallization during rapid cooling in a thermal gradient: application to komatiites

Abstract. To investigate the crystallization of pyroxene in spinifex-textured komatiites, we undertook a series of experiments in which compositions in the CaO-MgO-Al2O3-SiO2 CMAS system were cooled rapidly in a thermal gradient. Cooling rates were generally between 5 and 10 °C h−1, but some runs were made at 100–200 °C h−1; thermal gradients were between 10 and 20 °C cm−1. These conditions reproduced those at various depths in the crust of komatiite lava flow. The starting composition was chosen to have pigeonite on the liquidus, and most of the experimental charges crystallized zoned pigeonite–diopside crystals like those in komatiite lavas. An intriguing aspect of the experimental results was their lack of reproducibility. Some experiments crystallized forsterite, whereas others that were run under similar conditions crystallized two pyroxenes and no forsterite; some experiments were totally glassy, but others crystallized entirely to pyroxene. The degree of supercooling at the onset of pyroxene crystallization was variable, from less than 25 °C to more than 110 °C. We attribute these results to the difficulty of nucleation of pyroxene under the conditions of the experiments. In some cases forsterite crystallized metastably and modified the liquid composition to inhibit pyroxene crystallization; in others no nucleation took place until a large degree of supercooling was achieved, and then pyroxene crystallized rapidly. Pigeonite crystallized under a wide range of conditions, at cooling rates from 3 to 100 °C h−1. The notion that this mineral only forms at low cooling rates is not correct.

The Relevance of Crystal Transfer to Magma Mixing: a Case Study in Composite Dykes from the Central Pyrenees

Two composite dykes containing abundant mafic enclaves within a felsic host crop out in the Maladeta Plutonic Complex, Pyrenees, Spain. Field, petrographic and geochemical criteria reveal mixing between gabbroic and aplitic magmas, giving rise to a variety of hybrid compositions. The rocks contain spongy plagioclase, quartz ocelli and amphibole-biotite clots that are interpreted as early crystals destabilized by reaction with the hybrid melts. Spongy plagioclase and quartz ocelli were mechanically transferred from the felsic to the mafic magma, whereas amphibole-biotite clots are former pyroxene crystals from the mafic magma.Multivariate statistics (principal component analysis) have been used to examine variations in the mineral trace element compositions, which are best explained by the crystal transfer process. This study shows that crystal transfer represents a mixing mechanism that can overcome some of the physical limitations of interaction between rheologically contrasting magmas and explain deviations of the hybrid compositions from the theoretical mixed chemical composition. In particular, hybrid wholerock compositions show non-linear correlations in inter-element variation diagrams for elements that are enriched or depleted in preferentially transferred crystals. This effect has been quantified by extending magma mixing modelling to include crystal transfer.The composite dykes studied could be regarded as scale models of the behaviour of larger-scale magmatic systems, so this investigation has important implications for interpreting the petrogenesis of igneous suites by magma mixing.

The Mount Manengouba, a complex volcano of the Cameroon Line: Volcanic history, petrological and geochemical features

The volcanic story of Mount Manengouba is related to four chronological stages: (1) forming of the early Manengouba shield volcano between 1.55 and 0.94Ma, (2) building of the Eboga strato-cone between 0.94 and 0.89Ma, (3) caldera collapse and silicic extrusions of the Elengoum Complex between 0.89 and 0.70Ma, and (4) intra-caldera and flank activity between 0.45 and 0.11Ma. The volume of the volcano is calculated at 320km3±5%. The volcanic rocks are attributed to two magmatic outputs. The first and main magma generation produced the shield volcano, the strato-cone, and the syn- to post-caldera extrusions, displaying a complete series from basanites to trachytes (magmatic Group 1). The second magma generation is limited to the late and flank activity evolving from basanites to trachy-phonolite (magmatic Group 2). Both magmatic groups belong to the under-saturated alkaline sodic series. Petrological calculations locate the magmatic reservoir between 37 and 39km in the upper mantle for the Group 1 lavas, and between 42 and 44km for the Group 2 lavas. Trachytes were generated in a secondary crustal reservoir. Magmatic series evolve with medium to low pressure fractional crystallization of olivine, pyroxene, oxides, feldspar, and apatite. Significant crustal assimilation is evidenced in trachytes. The magma of Group 1 was generated with 3–6% of partial melting of a moderately enriched source containing 3–7% of garnet. Melting took place in the spinel to garnet transition zone located at 70–90km and around 25kb. The magma of Group 2 resulted from a slightly higher partial melting from a less garnet-rich source that indicates uprising of the melting column in the upper part of transition zone. Sr, Nd, and Pb isotope data of the Manengouba rocks and neighboring lavas are analyzed and compared with those of the mafic lavas of the CVL. Three source components are distinguished: a depleted component originated from the asthenospheric swell, a radiogenic component linked to the contaminated lithosphere of the Neoproterozoic mobile belt, and an enriched component or the lithosphere possibly related to pre-rifting magmatic processes.

Cumulate fragments in silicic ignimbrites: The case of the Snake River Plain

Large silicic ignimbrites commonly erupt from compositionally variable reservoirs. Although ignimbrite compositional architecture is often consistent with evacuation of a single zoned magma body, other examples are better interpreted as the products of amalgamation of multiple discrete subvolcanic melt-rich lenses. For example, multiple populations of pyroxene crystals and glass fragments within single ignimbrites from the central Snake River Plain (Idaho, USA) support the multibatch model. This presents a conundrum in terms of magma generation and storage; if the crystal-poor silicic magma batches are not generated nearly in situ in the upper crust, they must traverse, and reside within, a thermally hostile environment with large temperature gradients, resulting in low survivability in their shallow magmatic hearths. Ubiquitous crystal aggregates in central Snake River Plain rhyolites hint at another model. These aggregates contain the same plagioclase, pyroxene, and oxide mineral compositions as single phenocrysts of the same minerals in their host rocks, but they have significantly less silicic bulk compositions and lack quartz and sanidine, which occur as single phenocrysts in the deposits. These observations imply significant crystallization followed by melt extraction from mushy margins of the magma reservoirs. The extracted melt then pools and continues to evolve (crystallizing sanidine and quartz) while the melt-depleted walls and/or floors provide an increasingly rigid and refractory network segregating the crystal-poor batches of magma. Such hot refractory margins insulate the crystal-poor lenses, allowing (1) extended residence in the upper crust, and (2) preservation of chemical heterogeneities among batches. In contrast, systems that produce cumulates richer in low-temperature phases (quartz, K-feldspars, and/or biotite) can melt extensively upon recharge, leading to less segregation of eruptible melt pockets and the formation of gradationally zoned ignimbrites.

New data on lunar magmatic processes

New data based on a detailed analysis of pyroxene zoning strongly suggests that convection is an important process in lunar magmas. Elardo and Shearer (2014) carefully document irregular oscillatory zoning that is best explained by movement of pyroxene crystals in a convecting magma. Lunar samples that contain such data are rare, but this study should inspire more extensive efforts to further document magmatic processes.

Case study of magmatic differentiation trends on the Moon based on lunar meteorite Northwest Africa 773 and comparison with Apollo 15 quartz monzodiorite

Pyroxene and feldspar compositions indicate that most clasts from the Northwest Africa 773 (NWA 773) lunar meteorite breccia crystallized from a common very low-Ti (VLT) mare basalt parental magma on the Moon. An olivine cumulate (OC), with low-Ca and high-Ca pyroxenes and plagioclase feldspar formed during early stages of crystallization, followed by pyroxene gabbro, which is characterized by zoned pyroxene (Fe#=molar Fe/(Fe+Mg)×100 from ∼35 to 90; Ti#=molar Ti/(Ti+Cr)×100 from ∼20 to 99) and feldspar (∼An90–95Ab05–10 to An80–85Ab10–16). Late stage lithologies include alkali-poor symplectite consisting of fayalite, hedenbergitic pyroxene and silica, and alkaline-phase-ferroan clasts characterized by K-rich glass and/or K,Ba-feldspar with fayalite and/or pyroxene. Igneous silica only occurs with the alkaline-phase-ferroan clasts. This sequence of clasts represents stages of magmatic evolution along a ferroan–titanian trend characterized by correlated Fe# and Ti# in pyroxene, and a wide range of increase in Fe# and Ti# prior to crystallization of igneous silica.Clasts of Apollo 15 quartz monzodiorite (QMD) also have pyroxene co-existing with silica, but the QMD pyroxene has more moderate Fe# (∼70). Thus, in AFM components (A=Na2O+K2O, M=MgO, F=FeO), the QMD clasts are similar to the terrestrial calc-alkaline trend (silica-enrichment at moderate Fe#), whereas the ferroan–titanian trend is similar to the terrestrial tholeiitic trend (silica-enrichment only after strong increase in Fe#). However, the variations in SiO2-contents of QMD clasts are due to variable mixing of SiO2-rich and FeO-rich immiscible liquids (i.e., not a progressive increase in SiO2). Immiscibility occurred after fractionation of a KREEP-rich parent liquid.A third trend is based on zoning relations within the NWA 773 OC, where pyroxene Ti# increases at constant Fe# with proximity to intercumulus, incompatible element-rich pockets rich in K,Ba-feldspar and Ca-phosphates. This type of fractionation (increasing refractory trace elements at constant Fe#) in a cumulate parent rock may have been important for generating lunar rocks that combine low Fe# with high incompatible trace element concentrations, such as KREEP basalts and the magnesian suite.MELTS (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) models of one VLT, one low-Ti and two high-Ti mare basalts and one KREEP basalt all show evolution from low to high Fe# residual liquids during fractional crystallization; however strong enrichments in FeO-concentrations are limited to the VLT and low-Ti liquids. In the high-Ti liquids, crystallization of Fe–Ti-oxides prevents enrichment in FeO, and the increases in Fe# are due to depletion of MgO. Fe–Ti-oxide fractionation results in steady silica-enrichment in the high-Ti mare compositions. Intervals of FeO-enrichment on the VLT and low-Ti mare liquid lines of descent are linked to shifts from olivine to pyroxene crystallization. The onset of plagioclase feldspar crystallization limits the depletion of FeO during crystallization of one high-Ti mare basalt and of the KREEP basalt composition modeled.