Melt Evolution Research Articles

Origin and evolution of primitive melts from the Debunscha Maar, Cameroon: Consequences for mantle source heterogeneity within the Cameroon Volcanic Line

Debunscha Maar is a monogenetic volcano forming part of the Mt. Cameroon volcanic field, located within the Cameroon Volcanic Line (CVL). Partly glassy cauliflower bombs have primitive basanite-picrobasalt compositions and contain abundant normally and reversely zoned olivine (Fo 77–87) and clinopyroxene phenocrysts. Naturally quenched melt inclusions in the most primitive olivine phenocrysts show compositions which, when corrected for post-entrapment modification, cover a wide range from basanite to alkali basalt (MgO 6.9–11.7wt%), and are generally more primitive than the matrix glasses (MgO 5.0–5.5wt%) and only partly fall on a common liquid line of descent with the bulk rock samples and matrix glasses. Melt inclusion trace element compositions lie on two distinct geochemical trends: one (towards high Ba/Nb) is thought to represent the effect of various proportions of anhydrous lherzolite and amphibole-bearing peridotite in the source, while the other (for example, high La/Y) reflects variable degrees of partial melting. Comparatively low fractionation-corrected CaO in the melt inclusions with the highest La/Y suggests minor involvement of a pyroxenite source component that is only visible at low degrees of melting. Most of the samples show elevated Gd/Yb, indicating up to 8% garnet in the source. The range of major and trace elements represented by the melt inclusions covers the complete geochemical range given by basalts from different volcanoes of the Cameroon volcanic line, indicating that geochemical signatures that were previously thought to be volcano-specific in fact are probably present under all volcanoes. Clinopyroxene-melt barometry strongly indicates repeated mixing of compositionally diverse melts within the upper mantle at 830±170MPa prior to eruption. Mantle potential temperatures estimated for the primitive melt inclusions suggest that the thermal influence of a mantle plume is not required to explain the magma petrogenesis.

Distinct 238U/235U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes

Angrites are differentiated meteorites that formed between 4 and 11Myr after Solar System formation, when several short-lived nuclides (e.g., 26Al-26Mg, 53Mn-53Cr, 182Hf-182W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an “assumed” constant 238U/235U ratio (i.e., Pb-Pb ages published before 2009–2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D’Orbigny, and Sahara 99555) using multi-collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from −0.17 to −1.20Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (53Mn/55Mn)0 ratio in the Early Solar System (ESS) is recalculated as being (7±1)×10−6 at the formation of the Solar System (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions – CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs.A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17‰. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The δ238U values are consistent with fractionation during magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction.

Monticellite in group-I kimberlites: Implications for evolution of parental melts and post-emplacement CO2 degassing

Monticellite is a magmatic and/or deuteric mineral that is often present, but widely varying in concentrations in Group-I (or archetypal) kimberlites. To provide new constraints on the petrogenesis of monticellite and its potential significance to kimberlite melt evolution, we examine the petrography and geochemistry of the minimally altered hypabyssal monticellite-rich Leslie (Canada) and Pipe 1 (Finland) kimberlites. In these kimberlites, monticellite (Mtc) is abundant (25–45vol%) and can be classified into two distinct morphological types: discrete and intergrown groundmass grains (Mtc-I), and replacement of olivine (Mtc-II).Primary multiphase melt inclusions in monticellite, perovskite and Mg-magnetite contain assemblages dominated by alkali (Na, K, Ba, Sr)-enriched Ca-Mg-carbonates, chlorides, phosphates, spinel, silicates (e.g. olivine, phlogopite) and sulphides. These melt inclusions probably represent snapshots of a variably differentiated kimberlite melt that evolved in-situ towards carbonatitic and silica-poor compositions. Although unconstrained in their concentration, the presence of alkali-carbonates and chlorides in melt inclusions suggests they are a more significant component of the kimberlite melt than commonly recorded by whole-rock analyses.We present petrographic and textural evidence showing that pseudomorphic Mtc-II resulted from an in-situ reaction between olivine and the carbonate component of the kimberlite melt in the decarbonation reaction:Forsterite+Carbonatemelt⇌Monticellite+Periclase+CO2.This reaction is supported by the preservation of abundant primary inclusions of periclase and to a lesser extent Fe-Mg-oxides in monticellite, perovskite and Mg-magnetite. Based on the preservation of primary periclase inclusions, we infer that periclase also existed in the groundmass, but was subsequently altered to brucite.We suggest that CO2 degassing in the latter stages of kimberlite emplacement into the crust is largely driven by the observed reaction between olivine and the carbonate melt. For this reaction to proceed, CO2 should be removed (i.e. degassed), which will cause further reaction and additional degassing in response to this chemical system change (Le Chatelier's principle). Our study demonstrates that these proposed decarbonation reactions may be a commonly overlooked process in the crystallisation of monticellite and exsolution of CO2, which may in turn contribute to the explosive eruption and brecciation processes that occur during kimberlite magma emplacement and pipe formation.

Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite

TS34 is a Type B1 Ca-Al-rich inclusion (CAI) from the Allende CV3 chondrite, consisting of spinel, melilite, Ti-Al-rich clinopyroxene (fassaite) and minor anorthite in an igneous texture. Oxygen and magnesium isotopic compositions were measured by secondary ion mass spectrometry in spots of known chemical composition in all major minerals in TS34. Using the sequence of formation from dynamic crystallization experiments and from chemical compositions of melilite and fassaite, the oxygen isotopic evolution of the CAI melt was established. Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are uniformly 16O-rich (Δ17O=−22.7±1.7‰, 2SD), while the melilite grains are uniformly 16O-poor (Δ17O=−2.8±1.8‰) irrespective of their åkermanite content and thus their relative time of crystallization. The fassaite crystals exhibit growth zoning overprinting poorly-developed sector zoning; they generally grow from Ti-rich to Ti-poor compositions. The fassaite crystals also show continuous variations in Δ17O along the inferred directions of crystal growth, from 16O-poor (Δ17O∼−3‰) to 16O-rich (Δ17O∼−23‰), covering the full range of oxygen isotopic compositions observed in TS34. The early-crystallized 16O-poor fassaite and the melilite are in oxygen isotope equilibrium and formed simultaneously. The correlation of oxygen isotopic compositions with Ti content in the fassaite imply that the oxygen isotopic composition of the CAI melt evolved from 16O-poor to 16O-rich during fassaite crystallization, presumably due to oxygen isotope exchange with a surrounding 16O-rich nebular gas. Formation of spinel, the liquidus phase in melts of this composition, predates crystallization of all other phases, so its 16O-rich composition is a relic of an earlier stage. Anorthite exhibits oxygen isotopic compositions between Δ17O∼−2‰ and −9‰, within the range of those of fassaite, indicating co-crystallization of these two minerals during the earliest to intermediate stage of fassaite growth. The melilite and fassaite yield an 26Al–26Mg mineral isochron with an initial value of (26Al/27Al)0=(5.003±0.075)×10−5, corresponding to a relative age of 0.05±0.02Myr from the canonical Al–Mg age of CAIs. These data demonstrate that both 16O-rich and 16O-poor reservoirs existed in the solar nebula at least ∼0.05Myr after the birth of the Solar System.

Petrological and Re-Os isotopic constraints on the origin and tectonic setting of the Cuobuzha peridotite, Yarlung Zangbo suture zone, southwest Tibet, China

The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the Yarlung Zangbo suture zone in southwest Tibet comprises mainly clinopyroxene (cpx)-rich and depleted harzburgites. Spinels in the cpx-harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx-harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx-harzburgites have suprachondritic 187 Os/ 188 Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb). Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx-harzburgites formed from low-degree partial melting of a primitive mantle source, and they were subsequently modified by melt-rock interactions in a mid-oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx-harzburgites, which later interacted with mid-oceanic ridge basalt– or island-arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo-Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

Mineralogy, geochemistry, and melt evolution of the Kalaymyo peridotite massif in the Indo-Myanmar Ranges (western Myanmar), and tectonic implications

We present new whole-rock major, trace, and platinum group element (PGE) and mineral chemistry data from the Kalaymyo peridotite massif in the central part of the Indo-Myanmar Ranges (western Myanmar) and discuss its mantle melt evolution. The Kalaymyo peridotites consist mainly of harzburgites, which show typical porphyroclastic or coarse-grained equigranular textures. They are composed of olivine (forsterite, Fo = 89.8–90.5), orthopyroxene (enstatite, En 86–91 , wollastonite, Wo 1–4 , ferrosilite, Fs 8–10 ; Mg# = 89.6–91.9), clinopyroxene (En 46–49 Wo 47–50 Fs 3–5 ; Mg# = 90.9–93.6), and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole-rock compositions with Mg#s of 90.1–90.8 and SiO 2 (41.5–43.65 wt%), Al 2 O 3 (1.66–2.66 wt%), and CaO (1.45–2.67 wt%) contents. They display light rare earth element (LREE)–depleted chondrite-normalized (CN) REE patterns with (La/Yb) CN = 0.04–0.21 and (Gd/Yb) CN = 0.40–0.84, and show a slight enrichment from Pr to La with (La/Pr) CN in the range of 0.98–2.36. The Kalaymyo peridotites are characterized by Pd-enriched chondrite-normalized PGE patterns with superchondritic (Pd/Ir) CN ratios (1.15–2.36). Their calculated oxygen fugacities range between the quartz-fayalite-magnetite (QFM) oxygen buffers, QFM–0.57 and QFM+0.90. These mineralogical and geochemical features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5%–15%) of partial melting at a mid-oceanic ridge environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched mid-oceanic ridge basalt–like melts percolating through these already depleted residual peridotites. The Kalaymyo and other ophiolites in the Indo-Myanmar Ranges therefore represent mid-oceanic ridge–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward-migrating subduction-accretion system along the eastern margin of India.

Petrological and Re‐Os Isotopic Constraints on the Origin and Tectonic Setting of the Cuobuzha Peridotite, Yarlung Zangbo Suture Zone, SW Tibet, China

The Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the north (Dupuis et al., 2005; Yang et al., 2011). In the western part of the YZSZ, the Northern and the Southern sub‐belts form two sub‐parallel zones of mafic – ultramafic rock assemblages with overlapping crystallization ages (Xiong et al., 2011; Hebért et al., 2012; Liu et al., 2015). The upper mantle section of the Cuobuzha ophiolite in the northern sub‐belt of the Yarlung–Zangbo Suture Zone (YZSZ) in SW Tibet comprises mainly clinopyroxene (cpx)–rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx‐harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb). The cpx‐harzburgites plot in a Re vs. Al2O3 diagram as a result of subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes (Uysal et al., 2015).Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐ocean ridge type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting (∼5%) of a primitive mantle source, and they were subsequently modified by melt–rock interactions in a mid‐ocean ridge environment. The depleted harzburgites, on the other hand, were produced by re‐melting of the cpx‐harzburgites, which later interacted with MORB‐ or island arc tholeiite (IAT)‐like melts (Fig. 1) possibly in a trench‐distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neotethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

Geochemical Characterization of Intermediate to Silicic Rocks in the Global Ophiolite Record

Ophiolites consist predominantly of mafic‐ultramafic rocks but also contain in subordinate amounts intermediate to silicic intrusive – extrusive rocks in them. These rocks, although not in large volumes in comparison to mafic–ultramafic units, reveal significant clues about melt evolution, magmatic accretion processes, and partial melting of pre‐existing oceanic crust during ophiolite evolution. They also yield ample accessory minerals, such as zircon and monazite, which are widely used to date the timing of oceanic crust generation. We present a global synthesis of the occurrence of such leucocratic, intrusive and volcanic rocks from 150 Phanerozoic to Archean ophiolites, and evaluate models for their genesis during the development of oceanic crust in different tectonic environments.In the ophiolite complexes we have investigated, intrusive and extrusive rocks show a wide range in composition. Intermediate rocks (with SiO2 between 52 and 63 wt.%) include andesite and diorite, whereas the silicic rocks (with SiO2 more than 63 wt.%) include dacite, trondhjemite, tonalite and granite. For subduction‐related and subduction‐unrelated MORB‐type ophiolites in the Phanerozoic orogenic belts, the most commonly reported intermediate to silicic intrusive rocks, independent of their original tectonic setting, are trondhjemite, plagiogranite and tonalite, and less commonly quartz‐diorite and diorite. These rock types have been documented from 80 percent of the 104 ophiolites included in this study. Intermediate to silicic lavas and/or pyroclastic rocks (basaltic andesite, andesite, dacite and rhyolite) are less abundant, and have been reported from only 35 percent of the 104 ophiolites. Trachyte occurs in some of the Rift/Continental Margin type, subduction‐unrelated ophiolitesEvolved rocks in subduction‐unrelated, Rift/Continental Margin ophiolites are predominantly basaltic andesite and andesite, whereas MOR type (mid‐ocean ridge) ophiolites exhibit nearly equal proportions of basaltic andesite/andesite and rhyodacite and Plume/MOR type ophiolites are characterized by rhyolites. Intermediate to silicic volcanic uints in the Backarc sub‐group of subduction‐related ophiolites are characterized by similar amounts of basaltic andesite/andesite and rhyodacite, whereas in the Backarc to Forearc, Forearc, and Volcanic Arc sub‐groups they are mainly basaltic andesite/andesite. Intermediate to silicic rocks in Rift/Continental Margin and Plume/MOR type ophiolites are generally LREE‐enriched, whereas those in the MOR type vary from LREE‐depleted to LREE‐enriched. The Backarc and Backarc to Forearc types are similar to the MOR type; silicic rocks of the Forearc and Volcanic Arc types are generally LREE‐enriched. The main process in the formation of the majority of the intermediate to silicic rocks in both subduction‐unrelated and subduction‐related ophiolites is partial melting of basaltic and/or gabbroic rocks beneath the spreading centers, whereas a minor volume in subduction‐related ophiolites are adakites that were produced from partial melting of a subducting slab. Silcic to intermediate rocks in Plume/MOR type ophiolites are generated by fractional crystallization of basaltic melt. The incompatible, non‐conservative elements, such as Ba and Th, are weakly to strongly enriched in subduction‐related ophiolites as a result of shallow to deep enrichment associated with subduction zone processes. The field occurrence and the geochemical character of leucocratic rocks in ophiolites show considerable variations, providing additional constraints on the petrogenesis of ophiolites in different tectonic settings.Fractional crystallization appears to have been an important process in the formation of the intermediate to silicic rocks in Rift/CM and P/MOR ophiolites in the subduction‐unrelated class, as well as VA ophiolites in the subduction‐related class. In these types of ophiolites magma chambers might have operated as closed‐systems for a sufficient amount of time for fractionation and compositional zoning of the magma to take place (Fig. 1C1).Fractional crystallization was of subordinate importance during the igneous construction of the MOR, BA, FA, and BA‐FA ophiolites. Instead, partial melting of hydrothermally altered crust was the most common process resulting in the majority of the intermediate to felsic rocks of the Rift/CM (estimated to 60%), MOR (estimated to 95%), and the BA, FA and BA‐FA (estimated to 75%) ophiolites, as well as a high proportion of the VA ophiolites (∼ 40%) (Fig. 1C2).Slab‐generated melts that result in the production of intermediate to silicic rocks of adakitic character represent a relatively small fraction (∼15% in Phanerozoic ophiolites) of subduction‐related ophiolites (Fig. 1C3). Some adakites are very low in incompatible elements (e.g., La), suggesting that these melts were generated from highly depleted MORB. Precambrian ophiolites contain a higher percentage of adakites (ca. 35%), indicating that physical conditions were more favorable for slab melting during the Precambrian than in the Phanerozoic. This phenomenon may have resulted from higher geothermal gradient along relatively shallow‐dipping subduction zones during the Precambrian.

Geochemical Features of a Subduction Initiation Ophiolite Type in Habana‐Matanzas Region (Western Cuba)

We present new geochemical data for the upper mantle and crustal sections (whole‐rock major and trace element compositions) as well as mineral chemical data, from the Northern Carbibbean ophiolites in the Habana‐Matanzas region in Western Cuba. These ophiolites are part of the Northern Cuban Ophiolitic Belt (NCOB), extending for more than 1000 km along the island. The upper mantle peridotites are composed mainly of refractory harzburgite with tectonite textures, and show convex‐downward patterns depleted in MREE normalized to chondrite values (McDonough and Sun, 1995). These geochemical trends are characteristic for depleted mantle wedge peridotites metasomatized by slab‐derived, LREE enriched melts. The NCOB also includes abyssal peridotites having lower LREE/HREE ratios and displaying relatively homogeneous and flat patterns from MREE to HREE. These peridotites represent fragments accreted into the continental margin from a subducted oceanic lithosphere. Gabbro and dolerite units in the NCOB are systematically depleted in High Field Strength Elements (HFSE: Nb, Ta, Hf, Ti) and REE with respect to N‐MORB (<1 X N‐MORB). Their melt evolution was affected by subduction input. Spatially associated granitic rocks have a volcanic arc geochemical affinity. Some mafic extrusive rocks within the NCOB exhibit boninitic signatures, and may represent the products of subduction initiation magmatism, whereas other extrusive rock occurrences display N‐MORB to E‐MORB geochemical fingerprints, slightly modified by subduction derived fluids. Using these geochemical data and constraints, we present a tectonomagmatic model for the evolution of the NCOB within the framework of the Caribbean geology.

The stishovite paradox in the evolution of lower mantle magmas and diamond-forming melts (experiment at 24 and 26 GPa)

Experimental studies of phase relations in the oxide–silicate system MgO–FeO–SiO2 at 24 GPa show that the peritectic reaction of bridgmanite controls the formation of stishovite as a primary in situ mineral of the lower mantle and as an effect of the stishovite paradox. The stishovite paradox is registered in the diamond-forming system MgO–FeO–SiO2–(Mg–Fe–Ca–Na carbonate)–carbon in experiments at 26 GPa as well. The physicochemical mechanisms of the ultrabasic–basic evolution of deep magmas and diamondforming media, as well as their role in the origin of the lower mantle minerals and genesis of ultradeep diamonds, are studied.

TheREE-and HFSE-bearing phases in the Itatiaia alkaline complex (Brazil) and geochemical evolution of feldspar-rich felsic melts

AbstractThe Late Cretaceous Itatiaia complex is made up of nepheline syenite grading to peralkaline varieties, quartz syenite and granite, emplaced in the metamorphic rocks of the Serra do Mar, SE Brazil. The nepheline syenites are characterized by assemblages with alkali feldspar, nepheline, Fe-Ti oxides, clinopyroxene, amphibole, apatite and titanite, while the peralkaline nepheline syenites have F-disilicates (rinkite, wöhlerite, hiortdahlite, låvenite), britholite and pyrophanite as the accessory phases. The silica-oversaturated rocks have alkali feldspar, plagioclase, quartz, amphibole, clinopyroxene and Fe-Ti oxides; the chevkinite-group minerals are the featured accessory phases and are found with allanite, fluorapatite, fluorite, zircon, thorite, yttrialite, zirconolite, pyrochlore and yttrocolumbite. The major- and trace-element composition of the Itatiaia rocks have variations linked to the amount of accessory phases, have smooth, enriched chondritenormalized rare-earth element (REE) distribution patterns in the least-evolved nepheline syenites and convex patterns in the most-evolved nepheline syenites. TheREEdistribution patterns of the quartz syenites and granites show a typical pattern caused by fractional crystallization of feldspar and amphibole, in an environment characterized by relatively high oxygen fugacity (>NiNiO buffer) and high concentrations of H2O and F, supporting the crystallization of hydrous phases, fluorite and F-disilicates. The removal of small amounts of titanite in the transition from the least-evolved to the most-evolved nepheline syenites stems from petrogenetic models involvingREE, and is shown to be a common feature of the magmatic evolution of many other syenitic/ trachytic/ phonolitic complexes of the Serra do Mar and elsewhere.

Evidence for Residual Melt Extraction in the Takidani Pluton, Central Japan

The Takidani pluton represents one of a few locations where melt extraction from a crystal mush is preserved in the natural rock record, making it an extremely good case study for investigating the generation of evolved melt reservoirs in the upper crust. Located in the Japan Alps, the Takidani pluton shows a clear vertical zonation consisting of granite and granodiorite in the lower and mid- dle section, a fine-grained porphyritic granitic unit in the upper section and a marginal granodiorite at the roof contact with the host-rock. We present a detailed petrographic and geochemical study using samples collected along a section that traverses the entire vertical section of the pluton. No sharp contacts are found between units. Instead, gradual changes in rock fabric and mineralogy are observed between the lower granodiorite and overlying porphyritic unit. Major and trace elem- ent bulk-rock compositions show sigmoidal variations from the bottom to top of the pluton. Incompatible elements and silica contents increase roofwards within the porphyritic unit. Plagioclase chemistry reveals three main crystal populations (P1, P2 and P3) with Fe contents increasing towards the base of the pluton. Comparison with existing crystallization experiments, thermobarometry and hygrometry indicate that the magmas were emplaced at around 200 MPa, 850–900 C and bulk water contents of 3–4wt %. Whole-rock major and trace element analyses to- gether with mineral chemistry and textural observations suggest that the fine-grained porphyritic unit was extracted from the underlying granodiorite at temperatures between 800 and 740 C and crystallinities of 45–65 wt %. Radiogenic isotopes indicate only minor assimilation (2–6 wt %) and support melt evolution through crystal fractionation. The fine-grained matrix of the porphyritic unit may have been the result of pressure quenching associated with a volcanic eruption.

Sulphide melt evolution in the Volspruit project: the most primitive, yet most contaminated platinum-group element (PGE) deposit in the bushveld complex?

The Volspruit Ni-PGE Project is the only PGE resource in the Bushveld Complex hosted within ultramafic rocks (i.e. in the absence of cumulus plagioclase). It is one of only a few known PGE prospect...

Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation

Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr–Nd–Hf–Pb–Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ΔεHf coupled with >+5‰δ7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05–3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (−3.6 to −2.4‰δ13C for carbonatites versus −5.7 to −3.6‰δ13C for kimberlites) require open-system fractionation at magmatic temperatures.Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of ‘pure’ carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16–33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated ‘cratonic carbonatites’ have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiation – as exemplified by the Mesozoic–Cenozoic breakup of the North Atlantic craton.

High Sr/Y Magma Petrogenesis and the Link to Porphyry Mineralization as Revealed by Garnet-Bearing I-Type Granodiorite Porphyries of the Middle Cauca Au-Cu Belt, Colombia

Porphyry-style Cu (Au, Mo) mineralization is widely associated with igneous suites with Sr/Y > 40 and low heavy rare earth element (HREE) content. This geochemical signature is commonly attributed to garnet or amphibole fractionation in the lower crust where mantle derived melts evolve into intermediate composition arc suites. However, the presence of a residuum related to mineralized porphyry systems where amphibole and garnet are stable is generally inferred from geochemistry and has nowhere been documented directly. We here report on two occurrences of late Miocene garnet-bearing granodiorite porphyry at the Tesorito Au-Cu prospect in the Quinchia district and El Poma Au-Cu prospect in the Middle Cauca Au-Cu porphyry belt in Colombia. These rocks provide proof that garnet was stable in the lower crust at the time of porphyry mineralization. At both studied sites, garnet-bearing I-type granodiorite porphyries were intruded by intramineralization garnet-free plagioclase phyric granodiorite porphyries that were cut by quartz and K-feldspar veins and affected by biotite alteration. Garnet-bearing porphyries predate intramineralization porphyries by 0.4 to 2 m.y. Garnet occurs as unzoned grains, commonly included in large plagioclase phenocrysts, that in some cases also contain inclusions of igneous epidote; the latter indicating pressures of >1 GPa for early crystallized phases. Large plagioclase phenocrysts exhibit two growth stages separated by a resorption boundary. High-Al amphibole phenocrysts as well as titanite and magnetite formed after the first stage but prior to the second stage of plagioclase growth at pressures of ca. 0.9 GPa. Whole-rock geochemical compositions of garnet-bearing porphyries are consistent with only limited garnet fractionation whereas some of the intramineralization porphyries have Y 40, which is, together with other trace element indicators, taken as evidence for significant garnet fraction at depth during porphyry mineralization. HREE content of zircon in garnet-bearing porphyries are higher than that of intramineralization porphyries, which supports the interpretation that garnet fractionation in the lower crust was significant during the formation of the intramineralization porphyries. The phase relationships and geochemical data reflect a geodynamic change from an environment that permitted rapid ascent of oxidized garnet-bearing porphyries prior to Au-Cu mineralization to a regime that favored melt evolution in the lower crust where garnet was a residual phase and at the same time allowed the formation of large mid- to upper-crustal magma chambers. The latter are widely inferred to be essential for porphyry mineralization to occur.

Evolution of melts and fluids during the crust and mantle formation in Neo-Archean—Paleo-Proterozoic. Stratigraphic effects

Evolution of melts and fluids during the crust and mantle formation in Neo-Archean—Paleo-Proterozoic. Stratigraphic effects

Variscan metagranitoids in the central Tauern Window (Eastern Alps, Austria) and their role in the formation of the Felbertal scheelite deposit

The W mineralised Early Carboniferous orthogneisses (K1 and K3 orthogneiss) in the Felbertal scheelite deposit represent a chemically evolved metagranitoid series. Some of its characteristics are high concentrations of F (<4438ppm), Nb (<86ppm), Ta (<13ppm), and U (<74ppm) and REE patterns with distinct negative Eu-anomalies (Eu/Eu*=0.24–0.48) and increasing HREE concentrations (LuN/HoN=1.93–2.81). The systematic chemical trends documented for a multitude of elements (e.g., SiO2, TiO2, P2O5, Ba, Nb, Ta) and their respective ratios (e.g., 1/TiO2, Nb/Ta, Zr/Hf) indicate that crystal-melt fractionation controlled the evolution of the granitic melts. The higher differentiated, peraluminous light-coloured K1-K3 variety (ASI=0.99–1.08, Nb/Ta=5–7, Zr/Hf=13–18) evolved from the less differentiated, metaluminous dark-coloured variety (ASI=0.93–1.03; Nb/Ta=6–10, Zr/Hf=18–24). Peraluminous holo-leucocratic aplite gneiss represents the most evolved member of the series (ASI=1.11–1.12, Nb/Ta=4, Zr/Hf=9–10). Modelling of magmatic differentiation assuming Rayleigh fractionation shows that c. 70–90% of the residual granitic magma had crystallised at the time of the emplacement of the aplites. When compared to barren metagranitoids in the central Tauern Window (“Zentralgneis”), the metaluminous dark-coloured K1-K3 orthogneiss shows some geochemical similarities with the peraluminous Felbertauern augengneiss, one of the regional orthogneisses exposed near the W deposit. Elevated concentrations of Nb (<36ppm), Ta (<5.3ppm) and U (<30ppm) distinguish it from other regional Zentralgneis types and illustrate its genetic relation with the K1-K3 orthogneiss. We propose that the Felbertauern augengneiss represents a peraluminous granitic melt, generated by melting of a source assemblage containing hydrous F-bearing minerals (i.e., biotite). Progressive dehydration melting of the same (or a similar source) at higher temperature involving a Ca-phase (i.e., hornblende) produced melt batches of metaluminous composition; i.e., dark-coloured K1-K3 orthogneiss. An alternative model for explaining the unusual chemical characteristics of the latter would be entrainment and separation of a peritectic and restitic accessory mineral assemblage co-existing with the granitic melt. Higher contents of fluxing elements such as F in the melt may have been responsible for lowering the solidus temperature of the granitic melts allowing concentration of W and other elements in the residual melts and later on in the exsolved magmatic hydrothermal fluids. These processes were a pre-requisite for the development of highly specialised W-rich granitic melts and formation of the world-class Felbertal scheelite deposit.

Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, South-West Indian Ocean)

Felsic veins (plagiogranites) are distributed throughout the whole oceanic crust section and offer insight into late-magmatic/high temperature hydrothermal processes within the oceanic crust. Despite constituting only 0.5% of the oceanic crust section drilled in IODP Site 735B, they carry a significant budget of incompatible elements, which they redistribute within the crust. Such melts are saturated in accessory minerals, such as zircon, titanite and apatite, and often zircon is the only remaining phase that preserves magmatic composition and records processes of felsic melt formation and evolution. In this study, we analysed zircon from four depths in IODP Site 735B; they come from the oxide gabbro (depth approximately 250 m below sea floor) and plagiogranite (depths c. 500, 860, 940 m below sea floor). All zircons have similar εHf composition of c. 15 units indicating an isotopically homogenous source for the mafic magmas forming IODP Site 735B gabbro. Zircons from oxide gabbro are scarce and variable in composition consistent with their crystallization from melts formed by both fractionation of mafic magmas and hydrous remelting of gabbro cumulate. On the other hand, zircon from plagiogranite is abundant and each sample is characterized by compositional trends consistent with crystallization of zircon in an evolving melt. However, the trends are different between the plagiogranite at 500 m bsf and the deeper sections, which are interpreted as the record of plagiogranite formation by two processes: remelting of gabbro cumulate at 500 m bsf and fractionation at deeper sections. Zircon from both oxide gabbro and plagiogranite has δ18O from 3.5 to 6.0‰. Values of δ18O are best explained by redistribution of δ18O in a thermal gradient and not by remelting of hydrothermally altered crust. Tentatively, it is suggested that fractionation could be an older episode contemporaneous with gabbro crystallization and remelting could be a younger one, triggered by deformation and uplift of the crustal pile.

Petrogenesis and provenance of ungrouped achondrite Northwest Africa 7325 from petrology, trace elements, oxygen, chromium and titanium isotopes, and mid-IR spectroscopy

Northwest Africa (NWA) 7325 is an ungrouped achondrite that has recently been recognized as a sample of ancient differentiated crust from either Mercury or a previously unknown asteroid. In this work we augment data from previous investigations on petrography and mineral compositions, mid-IR spectroscopy, and oxygen isotope compositions of NWA 7325, and add constraints from Cr and Ti isotope compositions on the provenance of its parent body. In addition, we identify and discuss notable similarities between NWA 7325 and clasts of a rare xenolithic lithology found in polymict ureilites.NWA 7325 has a medium grained, protogranular to poikilitic texture, and consists of 10–15vol.% Mg-rich olivine (Fo 98), 25–30vol.% diopside (Wo 45, Mg# 98), 55–60vol.% Ca-rich plagioclase (An 90), and trace Cr-rich sulfide and Fe,Ni metal. We interpret this meteorite to be a cumulate that crystallized at ⩾1200°C and very low oxygen fugacity (similar to the most reduced ureilites) from a refractory, incompatible element-depleted melt. Modeling of trace elements in plagioclase suggests that this melt formed by fractional melting or multi-stage igneous evolution. A subsequent event (likely impact) resulted in plagioclase being substantially remelted, reacting with a small amount of pyroxene, and recrystallizing with a distinctive texture.The bulk oxygen isotope composition of NWA 7325 plots in the range of ureilites on the CCAM line, and also on a mass-dependent fractionation line extended from acapulcoites. The ε54Cr and ε50Ti values of NWA 7325 exhibit deficits relative to terrestrial composition, as do ordinary chondrites and most achondrites. Its ε54Cr value is distinct from that of any analyzed ureilite, but is not resolved from that of acapulcoites (as represented by Acapulco).In terms of all these properties, NWA 7325 is unlike any known achondrite. However, a rare population of clasts found in polymict ureilites (“the magnesian anorthitic lithology”) are strikingly similar to NWA 7325 in mineralogy and mineral compositions, oxygen isotope compositions, and internal textures in plagioclase. These clasts are probably xenolithic in polymict ureilites, and could be pieces of NWA 7325-like meteorites.Using constraints from chromium, titanium and oxygen isotopes, we discuss two possible models for the provenance of the NWA 7325 parent body: (1) accretion in the inner solar system from a reservoir similar to that of acapulcoites in Δ17O, ε54Cr and ε50Ti; or (2) early (<1Ma after CAI formation) accretion in the outer solar system (beyond the snow line), before 54Cr and 50Ti anomalies were introduced to this region of the solar system. The mid-IR emission spectrum of NWA 7325 obtained in this work matches its modal mineralogy, and so can be compared with spectra of new meteorites or asteroids/planets to help identify similar materials and/or the parent body of NWA 7325.

A Preconditioner for the Ohta--Kawasaki Equation

We propose a new preconditioner for the Ohta--Kawasaki equation, a nonlocal Cahn--Hilliard equation that describes the evolution of diblock copolymer melts. We devise a computable approximation to the inverse of the Schur complement of the coupled second-order formulation via a matching strategy. The preconditioner achieves mesh independence: as the mesh is refined, the number of Krylov iterations required for its solution remains approximately constant. In addition, the preconditioner is robust with respect to the interfacial thickness parameter if a timestep criterion is satisfied. This enables the highly resolved finite element simulation of three-dimensional diblock copolymer melts with over 1 billion degrees of freedom.