Open-system Melting Research Articles

The different responses of trace elements to equilibrium and disequilibrium melting: Implications for crustal differentiation and granite compositions

Equilibrium and disequilibrium meltings are two common mechanisms in the geochemical differentiation of the continental crust. However, responses of trace elements to the two melting processes are still not fully understood in granite petrogenesis. To address this, we conducted trace element modelling of open system melting using phase equilibrium calculations combined with solubility equations for accessory minerals and mineral–melt partition coefficients based on three melt-reintegrated felsic granulite compositions. The results indicate systematic differences between the compositions of disequilibrium (Type I) and equilibrium (Type II) melts involving water-present melting reaction, fluid-absent melting reaction involving biotite, and fluid-absent melting reaction in the absence of biotite. Type I melts generally have higher Rb, Ba, Co, Ni, V, and Cr contents, and Rb/Sr and Nb/Ta ratios, and lower Sr, P, Th, U, and light rare earth element contents than Type II melts. Significantly, disequilibrium melting involving biotite can produce melts with high Nb contents and super-chondritic Nb/Ta ratios, which may be a reservoir for the missing Nb on Earth. Compared with Type II melts, the trace element contents and ratios of Type I melts exhibit a larger range, more complex trends, and are more sensitive to changes in melting reaction types and melt loss. This suggests that, in addition to petrogenetic processes such as crystal entrainment, fractional crystallization and magma mixing, the disequilibrium melting would also contribute to the compositional diversity of granites. The modelling results also show that the trace element signatures of granites that are commonly used to determine their petrogenetic processes and tectonic setting may lead to erroneous conclusions if the granitic melts were produced by disequilibrium melting.

Contrasting melt percolation and melt-rock reactions in the Parece Vela back-arc oceanic lithosphere, Philippine Sea: A mineralogical perspective

The accretion and evolution of oceanic lithosphere beneath back-arc spreading ridges remain less understood compared with mid-oceanic ridges (MOR). Here, we investigate mantle peridotites and gabbros recovered from the southernmost side of the Parece Vela back-arc basin, Philippine Sea. The peridotites contain multiple types of clinopyroxenes (Cpx) and amphiboles (Amp) which record contrasting melt percolation and melt-rock reaction processes. Type-1 Cpx shows primary texture and light rare-earth elements (LREE)-depleted pattern, which can be reproduced by ~8–10% fractional melting of depleted MORB mantle. Type-2 Cpx occurs in the deformed peridotites (SY224-1280-2a & -2b) and shows less depleted LREE at similar middle to heavy REE (M-HREE) contents compared with type-1 Cpx. Such features reflect re-equilibration with MORB-type percolating melts, which is further evidenced by elevated TiO 2 and Cr# in coexisting spinels and crystallization of associated MORB-type gabbros. Type-3 Cpx occurs in the undeformed peridotites (SY225-1255-1 & -2) and is texturally replaced by type-1 Amp. They show variably enriched large ion lithophile elements (LILE) and LREE at similar M-HREE contents compared with type-1 Cpx, which reflect an open-system (flux) melting and reaction process with a melt flux enriched in LILE and LREE. Type-1 Amp is almost magnesio-hornblende and shows trace-element pattern characteristic of supra-subduction amphiboles, which supports the hydrous melt that formed type-3 Cpx probably originated from subducted slab. In contrast, type-2 Amp occurs in a leucocratic vein in sample SY225-1255-1 and is characterized by nearly flat M-HREE pattern at 600–200 × CI, LREE depletion and marked negative Eu anomaly. These features are consistent with crystallization from an amphibole-saturated melt formed by extreme differentiation of MORB-type melts. Our data, together with previous observations, suggest co-occurrence of hydrous MORB-type and slab-derived melt percolations beneath the Parece Vela spreading ridge, which may affect the chemical evolution of back-arc oceanic lithosphere different from MOR. • Multiple types of clinopyroxenes and amphiboles occur in the PVB peridotites. • They reflect open-system melting and reaction with hydrous MORB and slab melts. • Hydrous open-system mantle processes are features of back-arc oceanic lithosphere.

Melting and Evolution of Amphibole‐Rich Back‐Arc Abyssal Peridotites at the Mado Megamullion, Shikoku Basin

AbstractThe Mado Megamullion is an oceanic core complex (OCC) in the Shikoku back‐arc basin within the Philippine Sea Plate. Mantle peridotites (serpentinized) recovered by six dredge and submersible cruises exhibit signatures of extensive deformation. Amorphous pseudomorphs after plagioclase in many of the samples, as well as plagioclase‐spinel intergrowths, are clear evidence of melt stagnation and mantle reaction. Spinels show a wide range of compositions in terms of their Cr#, Mg#, and TiO2 content. The presence of apparently magmatic high‐temperature pargasitic amphibole in veins and as replacement of clinopyroxene suggests that it may be a primary or near‐primary mineral crystallized from a hydrous melt which is unusual for abyssal peridotites. Two trace‐element populations of clinopyroxenes are in equilibrium with depleted and enriched basaltic melts, respectively. Rare‐earth element (REE) in the most depleted clinopyroxenes are modeled by 10% fractional melting except for a ubiquitous La‐Ce “kick.” Multiple models of open system melting combined with subsequent mixing of an enriched melt can explain the REE data. Broadly it appears that the peridotites underwent variable degrees of partial melting with moderate influx of enriched melts, which agrees with the other textural and chemical evidence of melt‐rock reaction and re‐fertilization. The compositions of the accumulated melts simulated by the open system models reproduce the enrichments in fluid mobile elements (Ba, U, and Pb) observed in basalts dredged from the Shikoku basin. Back‐arc basin peridotites at Mado Megamullion appear to have a unique petrographic and geochemical character that is distinct from those of peridotites exposed at the seafloor after formation from mid‐ocean ridges.

Changes in the composition of anatectic melt and its complementary residue by forward-modelling using THERMOCALC

This paper presents results from forward modelling of graywacke and shale average bulk compositions assuming closed-system and open-system partial melting processes. For each process, several compositions of anatectic melt and corresponding solid residue are calculated, and the results compared in binary, ternary and normative diagrams to evaluate how their compositions evolve during an isobaric heating path. Melt compositions are peraluminous, with A/CNK increasing from 1.05 to 1.45, as P and T decrease. The mg# ranges from 0.22 to 0.39, increasing with consumption of biotite and other FeMg phases by melting reactions as T rises. On FeO + MgO, CaO + Na2O, K2O ternary, K2O vs. Na2O binary and chemical normative diagrams, compositions display trends with an increase in the K2O content of melt as T rises, and an increase in FeO + MgO content after the complete consumption of biotite. An increase in orthoclase component along the heating path, followed by an increase in the proportion of quartz component after K-feldspar/plagioclase breakdown is illustrated on mesonorm diagrams. Total melt volume produced by graywacke bulk compositions is in the range 33–53 vol% for closed-system and 21–33 vol% for open-system melting. For the shale bulk composition volumes are in the range 50–70 vol% during closed-system and 30–45 vol% during open-system melting. The rate of melt production varies with the type of melting reaction and is between 0.19 and 4.73 vol% °C−1 for water-present melting and 0.01–6.82 vol% °C−1 for hydrate-breakdown melting. Retrograde metamorphic assemblages can be formed even with the chemical depletion caused by melt extraction, but an external source of water and other components would make the consumption of the high-T assemblages more efficient. The correlation between data from this study and that from melt inclusions and natural leucosomes indicates that the melt compositions calculated via forward modelling are a useful guide in understanding how leucosome compositions and their mineral assemblages evolve.

Systematic LREE enrichment of mantle harzburgites: The petrogenesis of San Carlos xenoliths revisited

The dichotomy between partial melting and metasomatism is a paradigm of mantle geochemistry since the pioneering work of Frey and Prinz (1978) on the occurrence of LREE-enriched harzburgites. However, the thermo-chemical implications of such two-stage scenarios are often poorly considered, and the latter fail to explain why trace-element enrichment and major-element depletion are often proportional. We here re-envisage the petrogenesis of the famous San Carlos peridotites based on new petrographic observations and detailed modal, major- and trace-element compositions. The lherzolites (and pyroxenites) are characterized by homogeneously fertile mineral chemistry and LREE-depleted patterns consistent with low degrees of partial melting of the lherzolitic protolith. Bulk compositions and mineral zoning suggest that opx-rich pyroxenites formed by pressure-solution creep during melt-present deformation, locally accompanied by magmatic segregations of cpx. The harzburgites are characterized by stronger mineral zoning with low-Mg# and Na-, Al- and Cr-rich cpx rims, and can be discriminated in a low-Jd and high-Jd cpx groups. The high-Jd group is interpreted as the result of local elemental redistribution in the presence of a low-degree hydrous melt, in good agreement with their wide range of LREE enrichment. In contrast, the MREE-to-HREE fractionation and increasing Cr# in spinel of the low-Jd group indicate that they experienced higher degrees of melting. Open-system melting simulations of trace-element fractionation during hydrous flux melting suggests that the high-Jd harzburgites are the result of low fluid influx producing poorly extracted melt, while higher influx led to higher melting degrees and efficient melt extraction in the low-Jd harzburgites; the lherzolites mostly remained below or near solidus during that process. The lithological and chemical heterogeneity of San Carlos mantle is thus compatible with a single-stage evolution, which is also supported by the striking consistency between Fe-Mg exchange and REE thermometric estimates (1057 and 1074 °C on average, respectively), indicating that harzburgites and lherzolites probably followed a similar P-T path and relatively little sub-solidus re-equilibration. These interpretations suggest that the development of melt extraction pathways promoted by reactive channeling instability is able to produce complex lithological heterogeneities during hydrous flux melting. This process provides a self-consistent explanation for the systematic enrichment of harzburgites observed in many mantle terranes and xenoliths worldwide. We argue that San Carlos is one of such examples where a ca 1.5-Ga continental lithosphere experienced localized flux melting and deformation during the tectonic reactivation of a Proterozoic subduction zone, providing new constraints on the mantle sources of volcanic activity in the Jemez Lineament.

Mantle heterogeneities produced by open-system melting and melt/rock reactions in Patagonian extra-Andean backarc mantle (Paso de Indios, Argentina)

The Eocene basaltic extrusions in the Paso de Indios region (Chubut-Argentina) are one manifestation of the extensional tectonism of the active margin of South America during the Cenozoic. Ultramafic xenoliths embedded in these volcanics are mainly harzburgites and lherzolites with subordinate pyroxenites, estimated equilibrium temperatures ranging from 853 ± 15 to 1057 ± 32 °C and pressures in the spinel stability field.Geochemical and modal evidences point to a multistage magmatic history with record of a last reactional open-system episode associated to the influx of adakitic-like melts in a orthopyroxene-rich, clinopyroxene-poor mantle column. The great variability of clinopyroxene modal and geochemical composition in a ~20 km2 area suggests extreme variability of the physical parameters connected to melt infiltration and melt/rock reactions processes at a very small scale superimposed on a mantle with an inherited meter scale heterogeneity. Variations in the melt influx rate and residual porosity of the mantle column produced different melt/rock reactions which could be summarized in two entangled main reaction pathways: 1) opx + cpx + melt1-- > ol + melt2 and 2) opx + melt1-- > cpx + ol + melt2. These reactions deeply modified the trace elements content of clinopyroxenes producing variable enrichments in LREEs and LILEs related to both chromatographic and pure incremental open system processes.Petrological evidence suggests that the last reactional process occurred in the spinel stability field overprinting a strongly depleted mantle that, in a previous stage, had experienced extreme depletion in the garnet stability field, possibly under hydrous conditions.The adakitic-like nature of the influxing melt associates this episode to the subduction system along the western margin of South America, active at least since Late Triassic times.

Evidence for chemically heterogeneous Arctic mantle beneath the Gakkel Ridge

Ultraslow spreading at mid-ocean ridges limits melting due to on-axis conductive cooling, leading to the prediction that peridotites from these ridges are relatively fertile. To test this, we examined abyssal peridotites from the Gakkel Ridge, the slowest spreading ridge in the global ocean ridge system. Major and trace element concentrations in pyroxene and olivine minerals are reported for 14 dredged abyssal peridotite samples from the Sparsely Magmatic (SMZ) and Eastern Volcanic (EVZ) Zones. We observe large compositional variations among peridotites from the same dredge and among dredges in close proximity to each other. Modeling of lherzolite trace element compositions indicates varying degrees of non-modal fractional mantle melting, whereas most harzburgite samples require open-system melting involving interaction with a percolating melt. All peridotite chemistry suggests significant melting that would generate a thick crust, which is inconsistent with geophysical observations at Gakkel Ridge. The refractory harzburgites and thin overlying oceanic crust are best explained by low present-day melting of a previously melted heterogeneous mantle. Observed peridotite compositional variations and evidence for melt infiltration demonstrates that fertile mantle components are present and co-existing with infertile mantle components. Melt generated in the Gakkel mantle becomes trapped on short length-scales, which produces selective enrichments in very incompatible rare earth elements. Melt migration and extraction may be significantly controlled by the thick lithosphere induced by cooling at such slow spreading rates. We propose the heterogeneous mantle that exists beneath Gakkel Ridge is the consequence of ancient melting, combined with subsequent melt percolation and entrapment.

Behaviour of zircon and monazite during crustal melting

Ages retrieved from accessory minerals in high-grade metamorphic rocks place important constraints on the timing of events and the rates of tectonometamorphic processes operating in the deep crust. In suprasolidus rocks, the dissolution and growth of zircon and monazite are strongly dependent on the P–T conditions of metamorphism and the chemistry and quantity of anatectic melt present. Along a clockwise P–T path, prograde heating above the solidus leads to episodic melt loss and changes in melt chemistry that have important implications for the dissolution and growth of zircon and monazite. In this study, phase equilibria modelling of open-system melting is coupled with experimental data on zircon and monazite solubility to evaluate the stability of these minerals at suprasolidus conditions along several schematic clockwise P–T paths. In migmatite melanosomes and residual granulites, some zircon is expected to survive heating to peak temperature and subsequent isothermal decompression, whereas monazite may be completely consumed, consistent with the observation that inherited cores are less common in monazite than in zircon. After decompression, during cooling to the solidus, new zircon and monazite growth from melt trapped along grain boundaries in melanosomes and residual granulites is expected to be limited. By contrast, leucosomes in migmatites and anatectic granites are predicted to contain mostly newly formed zircon and monazite with minimal inherited components, unless significant entrainment of these minerals from the source occurs. The preservation of cores inside newly formed zircon, as observed in many anatectic granites, demonstrates that segregation, ascent and emplacement is commonly fast enough to limit dissolution of these inherited grains.

Percolation of enriched melts during incremental open-system melting in the spinel field: A REE approach to abyssal peridotites from the Southwest Indian Ridge

The effects of melting in an open-system scenario are here explored looking to the rare earth element (REE) distribution in mantle residues. We consider a peridotite matrix equilibrated in the spinel field accounting for melt inflow during partial melting. The fertility of the source, inflowing melt composition and melt addition rate as well as the effects of varying the critical mass porosity in an incremental scenario are tested. When a relatively enriched melt enters the system, residual clinopyroxene REE normalized patterns apparently rotate around a light to intermediate REE due to concomitant increase of the light REEs and decrease of the heavy REEs. This effect is enhanced when the critical mass porosity is large with respect to the degree of melting. In these cases the system approaches batch more than fractional melting behaviour because the liquid is preferentially retained in the matrix. This geometry is suggestive of melt accumulation at depth. Four sample suites from the Southwest Indian Ridge are considered. Spinel field equilibrated clinopyroxenes in lherzolites and harzburgites show dredge-scale REE compositional trends that crosscut model fractional melting trajectories. Observed local trends correspond to rotations of the REE patterns attesting for near-batch episodes in the subridge melting history and infiltration of enriched liquids whose composition resemble that of garnet field-generated melts.

Partial re-equilibration of highly siderophile elements and the chalcogens in the mantle: A case study on the Baldissero and Balmuccia peridotite massifs (Ivrea Zone, Italian Alps)

The conditions at which melt percolation and reactive infiltration of depleted mantle peridotites fractionate highly siderophile elements (HSE) and cause re-equilibration of 187Os/188Os in mantle rocks are still poorly constrained. In a comparative study of the Paleozoic Balmuccia (BM) and Baldissero (BD) peridotite tectonites (Ivrea-Verbano Zone, Northern Italy), the influence of partial melting and melt infiltration on abundances of HSE, chalcogens (S, Se and Te) and 187Os/188Os have been studied.At BM, Re depletion ages (TRD) of lherzolites and replacive dunites display a uniform distribution with a maximum near 400–500Ma. BD peridotites also display a Paleozoic distribution peak but a significant number of samples yielded Proterozoic TRD. The predominance of Paleozoic Re depletion ages in both bodies is consistent with Sm–Nd ages and the late Paleozoic magmatic and geodynamic evolution of the Ivrea-Verbano Zone. The different extents of preservation of ancient 187Os/188Os in BM and BD peridotites are interpreted to reflect different degrees of isotopic homogenization and chemical re-equilibration with incompatible element-depleted infiltrating melt during the Paleozoic. The differences between the two bodies are also reflected by differences in HSE and chalcogen abundances, with BD displaying large scatter among HSE patterns, slight re-enrichment of Re relative to Au, and linear trends of Pd, Se and Te with Al2O3. The differences in distributions of model ages and heterogeneity in HSE abundances support the view that the lithophile element, HSE and chalcogen variations of different suites of lherzolites likely reflect different extents of reactive melt infiltration in mantle peridotites, with partial re-equilibration and melt extraction in open system environments. However, the variable re-equilibration of BM and BD lherzolites apparently did not produce significant differences in HSE ratios such as Os/Ir, Ru/Ir, Rh/Ir, and Pd/Pt, which are in the range of primitive mantle values, whereas Au, Re and S are somewhat depleted.The good linear correlation of S with Al2O3 in both suites reflects sulfide removal controlled by sulfur solubility in silicate melt, or co-precipitation with pyroxenes and spinel, and indicates very similar bulk partition coefficients for S and Al. S/Se and Se/Te in the lherzolites change little with decreasing Al2O3. Results for BM lherzolites are consistent with sulfide–silicate melt partitioning as the dominant control on abundances of the HSE, S, Se and Te during low to moderate degrees of melt extraction (DPt>DPd>DTe⩾DSe⩾DS≈DRe).Replacive dunites at Balmuccia have low abundances of Re, Au, Pd and chalcogens, but variable and higher abundances of Os, Ir and Ru, high S/Se and Se/Te, yet 187Os/188Os similar to BM lherzolites. The residual HSE and chalcogen compositions differ from those in dunites of subduction-related ophiolites. The composition and contact relations of the BM dunites with the host rocks likely reflect focused flow of sulfur-undersaturated melt after open system melting and re-equilibration of the lherzolites. The compositional record of the Balmuccia massif thus reflects the composition of different types of melts and their interaction with the peridotites at different P–T conditions.

Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This “subduction factory” is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones.The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source:Cwr,add=[Ccpx-obs/[[Dcpx/(Dbulk-PF)]∗[1-(PF/Dbulk)](1/P)]]-[C0,wr]where Cwr,add=concentration of FME added to mantle wedge during a given melt increment, Ccpx-obs=concentration of observed pyroxene, Dcpx and Dbulk=mineral and bulk partition coefficients, P=melt proportion, and F=melt fraction required to model the observed MREE–HREE concentrations. Application of this algorithm to CRO peridotites shows that fluid influx must be continuous with open system melting, which allows us to calculate FME concentrations for small melt increments. Addition of the calculated FME concentrations to depleted MORB mantle (DMM) asthenosphere or refractory arc mantle (RAM) results in pooled magmas that match primitive arc tholeiites and boninites.

Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville and Vigarano (CV3) chondrites: A record of condensate evolution in the solar nebula

Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville, and Vigarano CV3 chondrites were studied using petrographic and microanalytical techniques to evaluate the origins of these inclusions and their relationships to other chondrite components. Our data support the idea that the inclusions formed by dust growth in the solar nebula as condensates and that they preserve a record of mineralogical and chemical evolution. Spinel-rich nodules and ribbons in AOAs often show Group II trace element patterns and formed either by fractional condensation or by condensation following partial vaporization. The dominant olivine component in AOAs can be explained as a condensate produced in the same way, but with fractionation occurring at a lower temperature. Other spinel-rich material in AOAs appears to be a vaporization residue. Ca-rich pyroxene+anorthite patches in AOAs show chemical signatures similar to those observed for spinel- or melilite-bearing regions but with lower refractory element and higher Mg and Si contents, supporting the idea that the patches formed by reaction between CAI-like material and Mg–Si-rich gas. Different trace element patterns (Group II and non-Group II refractory) are sometimes found for Al-rich regions (Ca-pyroxene+anorthite, or melilite-bearing) in a given AOA, suggesting that AOAs agglomerated from materials that formed under different conditions and that they did not subsequently homogenize. AOAs appear to have originated in a system with near-canonical solar composition and a low pressure gas (∼10−6–10−4bar) over a range of temperatures (mainly ∼1200–1384K) under somewhat non-equilibrium conditions. Relative to predicted equilibrium condensates, most AOAs show an apparent paucity of metal, possibly reflecting differential, density-dependent sorting between olivine and metal grains during aggregate formation. Trace element data are consistent with a simple model involving melting and igneous fractionation of AOA-like olivine to produce Type I chondrule olivine, but other data suggest that open-system melting of chondrules occurred in a different gaseous environment than that which produced AOAs.

Coexistence of abyssal and ultra-depleted SSZ type mantle peridotites in a Neo-Tethyan Ophiolite in SW Turkey: Constraints from mineral composition, whole-rock geochemistry (major–trace–REE–PGE), and Re–Os isotope systematics

We present new, whole-rock major and trace element chemistry, including rare earth elements (REE), platinum-group elements (PGE), and Re–Os isotope data from the upper mantle peridotites of a Cretaceous Neo-Tethyan ophiolite in the Muğla area in SW Turkey. We also report extensive mineral chemistry data for these peridotites in order to better constrain their petrogenesis and tectonic environment of formation. The Muğla peridotites consist mainly of cpx-harzburgite, depleted harzburgite, and dunite. Cpx-harzburgites are characterized by their higher average CaO (2.27 wt.%), Al 2O 3 (2.07 wt.%), REE (53 ppb), and 187Os/ 188Os ( i) ratios varying between 0.12497 and 0.12858. They contain Al-rich pyroxene with lower Cr content of coexisting spinel (Cr# = 13–22). In contrast, the depleted harzburgites and dunites are characterized by their lower average CaO (0.58 wt.%), Al 2O 3 (0.42 wt.%), and REE (1.24 ppb) values. Their clinopyroxenes are Al-poor and coexist with high-Cr spinel (Cr# = 33–83). The 187Os/ 188Os ( i) ratios are in the range of 0.12078–0.12588 and are more unradiogenic compared to those of the cpx-harzburgites. Mineral chemistry and whole rock trace and PGE data indicate that formation of the Muğla peridotites cannot be explained by a single stage melting event; at least two-stages of melting and refertilization processes are needed to explain their geochemical characteristics. Trace element compositions of the cpx-harzburgites can be modeled by up to ~ 10–16% closed-system dynamic melting of a primitive mantle source, whereas those of the depleted harzburgites and dunites can be reproduced by ~ 10–16% open-system melting of an already depleted (~ 16%) mantle. These models indicate that the cpx-harzburgites are the products of first-stage melting and low-degrees of melt–rock interaction that occurred in a mid-ocean ridge (MOR) environment. However, the depleted harzburgites and dunites are the product of second-stage melting and related refertilization which took place in a supra subduction zone (SSZ) environment. The Re–Os isotope systematics of the Muğla peridotites gives model age clusters of ~ 250 Ma, ~ 400 Ma and ~ 750 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto-Tethyan oceans, respectively. Furthermore, > 1000 Ma model ages can be interpreted as a result of an ancient melting event before the Proto-Tethys evolution.

Trace element partitioning between mica- and amphibole-bearing garnet lherzolite and hydrous basanitic melt: 2. Tasmanian Cainozoic basalts and the origins of intraplate basaltic magmas

Oligocene volcanics from Oatlands in Tasmania, Australia, include olivine tholeiites, alkali olivine basalts, nepheline basanites and olivine nephelinites. They have compositional characteristics that are typical of intraplate basalts worldwide. They are generally enriched in incom- patible elements relative to the primitive mantle and are strongly enriched in Nb, Ta and light rare earths, but not heavy rare earths. At the same time, they have Sr and Nd isotope compositions that are similar to those in some incompatible-element-depleted mid-ocean ridge basalts (E-type MORB). Experimentally obtained mineral/melt partition coefficients for an Oatlands basanite allow the relative concentrations of incompatible elements in the volcanics to be produced by small degrees of melting (B1%) of a source similar to the E-type MORB source of Workman and Hart (2005). However, the absolute concentrations that can be achieved in this way are much less than present in the most incompatible-element-enriched basanites and nephe- linites at Oatlands. This contradiction can be explained by open-system melting under the influence of a conductive geotherm. This would have involved upwardly migrating near-solidus melts from the asthenosphere cooling along a sub-adiabatic geotherm. Cooling of the melts would have caused them to re-crystallize and accumulate in the overlying mantle, thereby enriching both the new host rocks and any residual melts in incompatible elements. This would also have increased the buoyancy of the host rocks leading to upwelling and further (decompression) melting of incom- patible-element-enriched peridotite. We were able to use our partition coefficients to quantitatively model the develop- ment of incompatible-element enrichments in the Oatlands magmas by these processes. Our explanation is consistent with the characteristically scattered but widespread distri- butions and long time scales of intraplate volcanism in a broad variety of tectonic settings. This is because the con- ditions required to initiate volcanism (i.e. those of near-sol- idus melting of the asthenosphere) are relatively easy to produce and can therefore be caused by both near-surface tectonics and deeper mantle processes. Furthermore, the super-enrichments of incompatible elements in some intra- plate volcanics can be attributed to the influence of normal geothermal gradients on melting processes. Without the very strong fractionation imposed by this combination of factors, the Oatlands volcanics would more closely resemble mid- ocean-ridge basalts.

Trace element and Sr-Nd isotopic features of ultramafic xenoliths from Kurose, Southwest Japan

Ultramafic xenoliths from Kurose, Southwest Japan, are fragments of the lithospheric mantle beneath the back arc of the active Southwest Japan volcano-tectonic arc. Trace element and Sr-Nd isotopic compositions of the clinopyroxenes in the ultramafic xenoliths suggest that several enrichment processes are responsible for the geochemical variation. These xenoliths would have witnessed the following events: (1) open-system partial melting of the continental lithosphere with an influx of an asthenospheric melt with MORB-like isotopic composition during the formation of the back-arc basin that resulted from the Japan Sea opening, (2) open-system melting under an influx of a fluid/melt derived from a subducted slab, and (3) metasomatism after these partial melting events by an island arc tholeiitic basalt melt in the Neogene period. Moreover, variations in the depletion levels of Zr and Ti in the clinopyroxenes suggest that source depletion preceded the melting and metasomatic events.

Geochemistry of the highly depleted peridotites drilled at ODP Sites 1272 and 1274 (Fifteen-Twenty Fracture Zone, Mid-Atlantic Ridge): Implications for mantle dynamics beneath a slow spreading ridge

During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ) that offsets one of the slowest portions of the spreading ridge. We present here the results of a bulk rock multi-elemental study of 27 peridotites drilled at Sites 1272 and 1274 (to the south and the north of the FZ, respectively). The peridotites comprise mainly of harzburgites with minor dunites. Clinopyroxene (Cpx), which is interstitial and interpreted as secondary, is observed in Site 1274 peridotites. Sites 1272 and 1274 peridotites have low Al 2O 3 contents (≤ 1 anhydrous wt.%), high Mg# (> 91.5), and bulk rock trace elements compositions mostly below 0.1 × primitive mantle (PM). These peridotites, and in particular Site 1272 peridotites, represent the most depleted peridotites yet sampled at a slow spreading ridge. Their compositions indicate high degrees of partial melting and melt extraction. A single open-system melting event (melting plus percolation of melts produced within upwelling mantle) can explain their highly depleted yet linear chondrite-normalized REE patterns, characterized by a steady depletion from HREE to LREE. Late melt-rock reactions and precipitation of Cpx explains the slightly less depleted compositions of Site 1274 peridotites. Hence, the differences in composition between Sites 1272 and 1274 peridotites do not provide evidence for regional variations in the degrees of partial melting from the south to the north of the FZ. The occurrence of highly refractory peridotites in the Fifteen-Twenty area suggests we sampled a more actively convecting mantle than generally supposed below slow spreading centers.

日本海竹島海山 (仮称) のかんらん岩捕獲岩の成因とその意義

The Sea of Japan is one of the back-arc basins distributed around the western rim of the Pacific. Nature of the plume for back-arc basin (= BAB) opening is unknown partly because petrological characteristics of the BAB mantle are not well known compared with mid-ocean ridge (= MOR) mantle. Peridotite xenoliths in an alkali basalt dredged from the Takeshima seamount (tentative name), ca. 230 km to the north of Oki islands, are examined to clarify the petrological feature of the Japan-Sea mantle. They are spinel-bearing lherzolite to harzburgite with equigranular to porphyroclastic textures. They can be classified into two types (Type 1 and Type 2 peridotites) in terms of REE (= rare earth elements) patterns of clinopyroxene (= cpx). Type 1 peridotites (Cr# of spinels, 0.4-0.5) are similar to abyssal peridotites in terms of major element chemistry and middle- to heavy-REE concentrations of cpx. The cpx in the former shows slight enrichment of LREE (= light REE), which is apparently due to metasomatism by slab-derived fluids/melts. Type 2 peridotites have high-REE cpx with flat to slightly LREE-enriched patterns, and are quite similar to subcontinental peridotites obtained as xenoliths from China. The southern part of the Sea of Japan was formed by mainly thinning and rifting of the continental lithosphere. Type 2 peridotites are a remnant of the continental lithospheric mantle. Chemical characteristics of BAB basalts and Type 1 peridotites indicate that open-system melting of peridotite affected by a flux from the downgoing slab was involved in the Japan-Sea opening.

Rb–Sr and Sm–Nd isotopic systematics of the Hayachine–Miyamori ophiolitic complex: Melt generation process in the mantle wedge beneath an Ordovician island arc

The Hayachine–Miyamori (HM) ophiolitic complex in the Kitakami Mountains, northeastern Japan consists of ultramafic tectonite and cumulate members. The most fertile lherzolites have mineral and trace element compositions similar to those of abyssal peridotites. They show 350–430 Ma Nd depleted mantle model ages, which are within the range of the K–Ar emplacement ages obtained from intrusive gabbroic rocks, suggesting a partial melting event just before the emplacement. The measured 143Nd/ 144Nd ratio of clinopyroxene in the tectonite peridotites shows positive correlation with 147Sm/ 14 4Nd and decreases with increasing refractoriness, which cannot be explained by a simple melting and melt extraction to a various extent followed by radiogenic ingrowth. It clearly suggests influx of a melt/fluid enriched in highly incompatible trace elements during melting. Time corrected isotopic compositions of the HM complex exhibit a clear island arc signature with uniform initial isotopic ratio ( 87Sr/ 86Sr = 0.7035–0.7041, ε Nd = + 7.8–+ 5.0). Application of an open-system melting model to the observed trace element abundances in clinopyroxene suggests influx of three distinct agents to the HM mantle with the following characteristics: (1) moderate enrichment in highly incompatible elements with negative anomalies of Sr and Zr; (2) extensive enrichment of highly incompatible elements with positive Sr and negative Zr anomalies; and (3) extensive enrichment of highly incompatible elements with positive anomalies of Sr and Zr. These characteristics cover a variety of slab-derived components proposed in the literatures, suggesting the agents responsible for the open-system melting in the HM ophiolite might represent full spectrum of slab-derived components from back-arc to fore-arc regions of the Ordovician island arc system.

Vapor pressures and evaporation coefficients for melts of ferromagnesian chondrule-like compositions

To determine evaporation coefficients for the major gaseous species that evaporate from silicate melts, the Hertz–Knudsen equation was used to model the compositions of residues of chondrule analogs produced by evaporation in vacuum by Hashimoto [Hashimoto A. (1983) Evaporation metamorphism in the early solar nebula-evaporation experiments on the melt FeO–MgO–SiO 2–CaO–Al 2O 3 and chemical fractionations of primitive materials. Geochem. J. 17, 111–145] and Wang et al. [Wang J., Davis A. M., Clayton R. N., Mayeda T. K., Hashimoto A. (2001) Chemical and isotopic fractionation during the evaporation of the FeO–MgO–SiO 2–CaO–Al 2O 3–TiO 2 rare earth element melt system. Geochim. Cosmochim. Acta 65, 479–494], in vacuum and in H 2 by Yu et al. [Yu Y., Hewins R. H., Alexander C. M. O’D., Wang J. (2003) Experimental study of evaporation and isotopic mass fractionation of potassium in silicate melts. Geochim. Cosmochim. Acta 67, 773–786], and in H 2 by Cohen et al. [Cohen B. A., Hewins R. H., Alexander C. M. O’D. (2004) The formation of chondrules by open-system melting of nebular condensates. Geochim. Cosmochim. Acta 68, 1661–1675]. Vapor pressures were calculated using the thermodynamic model of Ghiorso and Sack [Ghiorso M. S., Sack R. O. (1995) Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid–solid equilibria in magmatic systems at elevated temperatures and pressures. Contrib. Mineral. Petrol. 119, 197–212], except for the late, FeO-free stages of the Wang et al. (2001) and Cohen et al. (2004) experiments, where the CMAS activity model of Berman [Berman R. G. (1983) A thermodynamic model for multicomponent melts, with application to the system CaO–MgO–Al 2O 3–SiO 2. Ph.D. thesis, University of British Columbia] was used. From these vapor pressures, evaporation coefficients ( α) were obtained that give the best fits to the time variation of the residue compositions. Evaporation coefficients derived for Fe (g), Mg (g), and SiO (g) from the Hashimoto (1983) experiments are similar to those found by Alexander [Alexander C. M. O’D. (2004) Erratum. Meteoritics Planet. Sci. 39, 163] in his EQR treatment of the same data and also adequately describe the FeO-bearing stages of the Wang et al. (2001) experiments. From the Yu et al. (2003) experiments at 1723 K, α Na = 0.26 ± 0.05, and α K = 0.13 ± 0.02 in vacuum, and α Na = 0.042 ± 0.020, and α K = 0.017 ± 0.002 in 9 × 10 −5 bar H 2. In the FeO-free stages of the Wang et al. (2001) experiments, α Mg and α SiO are significantly different from their respective values in the FeO-bearing portions of the same experiments and from the vacuum values obtained at the same temperature by Richter [Richter F. M., Davis A. M., Ebel D. S., Hashimoto A. (2002) Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions: experiments, theoretical considerations, and constraints on their thermal evolution. Geochim. Cosmochim. Acta 66, 521–540] for CMAS compositions much lower in MgO. When corrected for temperature, the values of α Mg and α SiO that best describe the FeO-free stages of the Wang et al. (2001) experiments also adequately describe the FeO-free stage of the Cohen et al. (2004) H 2 experiments, but α Fe that best describes the FeO-bearing stage of the latter experiment differs significantly from the temperature-corrected value derived from the Hashimoto (1983) vacuum data.

The formation of chondrules by open-system melting of nebular condensates

Experiments were conducted under canonical nebular conditions to see whether the chemical compositions of the various chondrule types can be derived from a single CI-like starting material by open-system melting and evaporation. Experimental charges, produced at 1580 °C and PH2 of 1.31×10−5 atm over 1 to 18 hours, consisted of only two phases, porphyritic olivine crystals in glass. Sulfur, metallic-iron and alkalis were completely evaporated in the first minutes of the experiments and subsequently the main evaporating liquid oxides were FeO and SiO2. Olivines from short runs (2–4 hours) have compositions of Fo83-Fo89, as in Type IIA chondrules, while longer experimental runs (12–18 hours) produce ∼Fo99 olivine, similar to Type IA chondrules. The concentration of CaO in both olivine (up to 0.6 wt.%) and glass, and their Mg#, increased with increasing heating duration. Natural chondrules also show increasing CaO with decreasing S, alkalis, FeO and SiO2. The similarities in bulk chemistry, mineralogy and textures between Type IIA and IA chondrules and the experimental charges demonstrate that these chondrules could have formed by the evaporation of CI precursors. The formation of silica-rich chondrules (IIB and IB) by evaporation requires a more pyroxene-rich precursor.Based on the FeO evaporation rates measured here, Type IIA and IA chondrules, were heated for at least ∼0.5 and ∼3.5 h, respectively, if formed at 1580 °C and PH2 of 1.31×10−5 atm. Type II chondrules may have experienced higher cooling-rates and less evaporation than Type I.The experimental charges experienced free evaporation and exhibited heavy isotopic enrichments in silicon, as well as zero concentrations of S, Na and K, which are not observed in natural chondrules. However, experiments on potassium-rich melts at the same pressure but in closed capsules showed less evaporation of K, and less K isotopic mass fractionation, than expected as a function of decreasing cooling rate. Thus the environment in which chondrules formed is as important as the kinetic processes they experienced. If chondrule formation occurred under conditions in which evaporated gases remained in the vicinity of the residual melts, the extent of evaporation would be reduced and back reaction between the gas and the melt could contribute to the suppression of isotopic mass fractionation. Hence chondrule formation could have involved evaporative loss without Rayleigh fractionation. Volatile-rich Type II and volatile-poor Type I chondrules may have formed in domains with high and low chondrule concentrations, and high partial pressures of lithophile elements, respectively.