Melt-filled hybrid fractures in the oceanic mantle: Melt enhanced deformation during along-axis flow beneath a propagating spreading ridge axis

Mid-ocean ridges represent important locations for understanding the interactions between deformation and melt production, transport, and emplacement. Melt transport through the mantle beneath mid-ocean ridges is closely associated with deformation. Currently recognized transport and emplacement processes at ridges include: 1) dikes and sills filling stress-controlled fractures, 2) porous flow in a divergent flow field, 3) self-organizing porous dunite channels, and 4) shear zones. Our recent observations from the sub-oceanic mantle beneath a propagating ridge axis in the Oman ophiolite show that gabbronorite and olivine gabbro dikes fill hybrid fractures that show both shear and extensional components of strain. The magnitudes of shear strain recorded by the dikes are significant and comparable to the longitudinal extensions across the dikes. We suggest that the hybrid dikes form from the interactions between shear deformation and pressurized melt in regions of along-axis flow at mid-ocean ridges. The displacement across the dikes is kinematically compatible with high temperature flow recorded by plastic fabrics in host peridotites. Field observations and mechanical considerations indicate that the dikes record conditions of higher stress and lower temperature than those recorded by the plastic flow fabrics. The features of hybrid dikes suggest formation during progressive deformation as conditions changed from penetrative plastic flow to strain localization along melt-filled fractures. The combined dataset indicates that the dikes are formed during along-axis flow away from regions of diapiric upwelling at propagating ridge segments. Hybrid dikes provide a potentially powerful kinematic indicator and strain recorder and define a previously unrecognized mechanism of melt migration. Our calculations show that hybrid dikes require less melt pressure to form than purely tensile dikes and thus may provide a mechanism to tap melt reservoirs that are under-pressurized with respect to lithostatic pressure.     

The effect of sloped isotherms on melt migration in the shallow mantle: a physical and numerical model based on observations in the Oman ophiolite

Field observations in the Oman ophiolite and petrological data are used to constrain a model of melt segregation at the top of the mantle beneath an oceanic spreading centre. Foliations and lineations in outcrops of mantle-derived peridotites oriented at high angle relative to the crust–mantle boundary have been interpreted as the footprint of a former axial asthenospheric convective upwelling several kilometers in cross-section that reached Moho levels. Basaltic melts migrating through this upwelling reacted with their host harzburgites and suffered fractional crystallization. The mantle–crust transition zone at the top of the upwelling is characterized by an very thick (about 400 m) dunite layer whose detailed structure and composition point to the development by compaction of a former “mantle mush”. The more important structures (in terms of volume of crystallization products) found in the underlying harzburgites are dunitic–troctolitic horizons a few meters thick and of lateral extent reaching 1 km and more. They crystallized at high temperature (>1190 °C) from melts similar to mid-ocean ridge basalts (MORB). These are called “sills” because they are sub-parallel to the crust–mantle boundary, but they can present a moderate dip (15° to 20° at most) relative to this paleo-horizontal surface. These observations have motivated the modelling of melt segregation by compaction within the crystallization domain inside the top convective boundary layer of the mantle upwelling. Two original inputs to the modelling are considered here: (i) the slope of the iso-curves of melt concentration due to the progressive cooling of the mantle in the boundary layer away from the axis of the rising convective flow; (ii) the reduction in permeability caused by the crystallization of the inter-granular melt. Modelling shows that a unique condition is required to generate the troctolite sills and the thick dunite layer nested at the top of the Maqsad diapir: namely a dramatic drop of the interstitial melt concentration at the top of the mantle. Besides, the model developed here allows to scale the time, volume and velocity of the melt segregation.     

Chromite in the mantle section of the Oman ophiolite: A new genetic model

Abstract   This paper reviews the compositional data (major elements, platinum group element [PGE] concentrations, Os- and O-isotopes) for chromites from the mantle section of the Oman ophiolite. Chromites in chromitite from the Oman ophiolite lie on a compositional spectrum between high-Cr♯, boninite-like and low-Cr♯, mid-oceanic ridge basalt-like end-members. The high-Cr♯ end-member is low in Ti, has a fractionated PGE pattern and is enriched in iridium group-platinum group elements (IPGE). The low-Cr♯ end-member has higher Ti and an unfractionated PGE pattern. The compositional variation in the chromitites reflects their crystallization from a range of different melt compositions. It is proposed that this wide variation in melt compositions was produced by the process of a melt–rock reaction, whereby a basaltic melt has reacted with harzburgitic mantle to yield successively more Cr-rich melts. In contrast to previous models, this approach does not require a change in the tectonic environment to explain the different chromite types.     

Mantle process beneath Philippine Sea back-arc spreading ridges: A synthesis of peridotite petrology and tectonics

Abstract In order to obtain a general view of the mantle process beneath a back‐arc basin spreading ridge, the diversity of peridotite petrology and tectonic occurrences in two back‐arc basin spreading ridges from the Philippine Sea were examined: the Parece Vela Rift and the Mariana Trough. The Parece Vela Basin spreading ridge (Parece Vela Rift) was a physically fast/intermediate‐spreading ridge, although many tectono‐magmatic features resemble those of slow‐ to ultraslow‐spreading ridges. Two unusual features of the Parece Vela Rift further demonstrate the uniqueness of the ridge: full‐axial development of oceanic core complexes and exposure of mantle peridotite at segment midpoints. The Parece Vela Rift yields a lithological assemblage of residual but still fertile lherzolite/harzburgite, plagioclase‐bearing harzburgite and dunite; similar assemblages are reported from the equatorial Mid‐Atlantic Ridge at the Romanche Fracture Zone and the ultraslow‐spreading ridges from the Indian and Arctic Oceans. The tectono‐magmatic characteristics of the Parece Vela Rift suggest that diffuse porous melt flow and pervasive melt–mantle interaction were the important mantle processes there. Globally, this ‘porous melt flow‐type’ mantle process is likely to occur beneath a segment midpoint of the ridge having a thick lithosphere, typically an ultraslow‐spreading ridge. In contrast, the Mariana Trough is a typical slow‐spreading ridge, exposing mantle peridotite at segment ends. The Mariana Trough yields a lithological assemblage of residual harzburgite and veined harzburgite, a common assemblage among the global abyssal peridotite suite. The tectono‐magmatic characteristics of the Mariana Trough suggest that channeled melt/fluid flow and limited melt–mantle interaction are the important mantle processes there, because of the colder wall‐rock peridotite in the segment end. This ‘channeled melt flow‐type’ mantle process is likely to occur in the shallow lithospheric mantle at the segment ends of any spreading ridges.     

Simple 2-D models for melt extraction at mid-ocean ridges and island arcs

A general set of 2-D equations for the conservation of mass and momentum of a two-phase system of melt in a deformable matrix is used to derive analytic solutions for the corner flow of a constant porosity melt-saturated porous medium. This solution is used to model the melt extraction processes at mid-ocean ridges and island arcs. The models indicate that flow of melt is controlled by pressure gradients induced by the Laplacian of the matrix velocity field and by the dimensionless percolation velocity which measures the relative contributions of buoyancy-driven flow to advection by the matrix. The models can account for many features of ridge and arc volcanism. Matrix corner flow at ridges causes melt to be drawn to the ridge axis enabling the extraction of small melt fractions from a wide melting zone while showing a narrow zone of volcanism at the surface. At subduction zones melts do not percolate vertically but are drawn to the junction of the upper plate and subducting slab by corner flow in the mantle wedge. For subduction zones, if the dimensionless percolation velocity is below a critical value, slab-derived fluids will be carried down by the matrix and cannot interact with the mantle wedge. The geochemistry of island arcs will be controlled by the geometry of melt streamlines. This model is consistent with geophysical and geochemical data from the Aleutian arc.     

Intergranular trace elements in mantle xenoliths from Russian Far East: Example for mantle metasomatism by hydrous melt

Based on both major and trace element chemistry, the occurrence of the intergranular component in mantle-derived xenoliths from far eastern Russia has been constrained. Whole-rock trace element measurements of one xenolith show apparent negative anomalies in Ce, Th, and high field strength elements on normalized trace element patterns. The trace element pattern of the whole rock differs from those of constituent minerals, indicating that the anomalies in the whole rock are attributable to the presence of an intergranular component. That assumption was confirmed using in situ analysis of trace elements in the intergranular substance and melt inclusion using laser ablation inductively coupled plasma–mass spectrometry. Both the intergranular component and the melt inclusions have identical trace element patterns, which mean that these materials are a cognate metasomatizing agent. The anomalies are regarded as mantle metasomatism related to an aqueous fluid. Hydrous minerals were observed on the wall of the melt inclusions using micro-Raman spectroscopy, indicating that the melt inclusions contained a large amount of water. Thus, this study reveals a trace element composition of a hydrous metasomatizing agent in the mantle.     

Dunite channels within a harzburgite layer from the Horoman peridotite complex,Japan: Possible pathway for magmas

Two discordant dunite channels in a polished slab were collected from a harzburgite layer in the Horoman orogenic lherzolite massif, Hokkaido, Japan. The dunite channels show extreme grain‐size variations, and include very large olivine megacrysts up to 1.6 m long. Spinel‐rich pyroxenite veins or veinlets occasionally cross‐cut the olivine megacrysts. Mineral compositions differ between channels, and the mineral compositions and textures of the dunite channels and the host harzburgite suggest a replacive origin for the dunites. The Sr and Nd isotopic ratios of clinopyroxene separates from the pyroxenite veins in two dunite channels are different, but these ratios are generally similar to those of mid‐ocean ridge basalts. Assuming the channels formed by the through‐flow of melts, then the trace element compositions of the melts estimated from the clinopyroxenes are broadly similar. The melts have similar features such as enriched in incompatible elements and depleted in heavy rare earth elements, but there are differences in the depletions in Ti and Ba, indicating melts that were different but of similar origins. A Rb–Sr crystallization age (~ 52 Ma) was obtained for clinopyroxene–orthopyroxene pairs in a dunite channel and its host harzburgite. The distinctive dunite channels with megacrysts formed as a result of the through‐flow of mafic magma(s), and those magmas had trace and minor element contents that are indicative of contributions of a melt derived from subducted oceanic lithosphere.     

Interaction of adakitic melt-peridotite: Implications for the high-Mg# signature of Mesozoic adakitic rocks in the eastern North China Craton

Abundant dunite and harzbugite xenoliths are preserved in Early Cretaceous high-Mg# [63–67, where Mg# = molar 100 × Mg/(Mg + Fe_tot)] diorite intrusions from western Shandong in the North China Craton (NCC). Dunite and some harzburgite xenoliths typically preserve areas of orthopyroxenite (sometimes accompanied by phlogopite) either as veins or as zones surrounding chromite grains. Harzburgite is chiefly composed of olivine, orthopyroxene, minor clinopyroxene and chromian-spinel. High Mg#'s (averaging 91.4) and depletions in Al₂O₃ and CaO (averaging 0.52 wt.% and 0.29 wt.%, respectively) in harzburgite and dunite xenoliths suggest that they are residues formed by large degrees of polybaric melting. However, olivines and orthopyroxenes from dunite xenoliths spatially associated with orthopyroxenite display lower Mg#'s (i.e., 82–87 and 83–89, respectively), suggesting that an adakitic melt–peridotite reaction has taken place. This is consistent with the production of veined orthopyroxene or orthopyroxene + phlogopite in dunite and some harzburgite xenoliths in response to the introduction of adakitic melt into the previously depleted lithospheric mantle (i.e., harzburgite and dunite xenoliths). The presence of orthopyroxene in veins or as a zones surrounding chromite in peridotite xenoliths is thought to be representative of adakitic melt metasomatism. The dunite and harzbugite xenoliths are relatively rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), poor in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), and lack Eu anomalies on chondrite normalized trace element diagrams. The initial ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values for the xenoliths range from 0.7058 to 0.7212 and + 0.18 to ? 19.59, respectively. Taken together, these features, combined with the strong depletion in HFSE and the existence of Archean inherited zircons in the host rocks, suggest that the adakitic melt was derived from the partial melting of early Mesozoic delaminated lower continental crust. The interaction of the adakitic melt with peridotite is responsible for the high-Mg# character of the early Cretaceous diorites in western Shandong.     

Melting and continent generation

The fractionated rare earth distribution in the continents requires that a small melt fraction ( 1%) separates at some stage in their development, either in the crust or in the mantle, and carries with it the continental budget of those elements. It is proposed that the very small melt fractions which control the continent's fractionated trace element budget are generated in response to lithospheric extension, plumes or aqueous fluids beneath island arcs, and emplaced into the deeper parts of the continental lithosphere. This budget is subsequently inherited by granite melts which only separate on relevant geological time scales when melt fractions reach 10% or more. The residence times of trace incompatible and major elements in the continents might be quite different.     

Komatiites and the structure of the Archaean mantle

The existence of Archaean komatiites with eruption temperatures greater than 1650°C requires that the mantle be vertically differentiated by the time of komatiite eruption. If in the unlikely event that undifferentiated mantle had survived primordial planetary differentiation and had been hot enough to deliver 1650°C komatiite, it would have been extensively molten to depths of ～250 km, resulting in rapid, profound, vertical differentiation anyway. During primordial differentiation (or Archaean komatiite petrogenesis) the high density and compressibility of ultrabasic melt allowed storage of a global melt layer beneath a buoyant residue of dunite and/or harzburgite. This refractory cap segregated by extraction of melt both upwards and downwards from the depth at which the density contrast between crystals and liquid vanishes. Eruption of komatiite from the melt layer by corrosion of the cap was the Archaean earth's principal means of dissipating excess heat. This subterranean magma ocean precluded vertical homogenization of the Archaean mantle by convection but effectively absorbed lateral mantle heterogeneities and imposed the relative uniformity of maximum eruption temperature and MgO contents (～32%) seen in primitive Archaean komatiites on all continents.Verification of the postulated density relations of liquids and crystals to 100 kbar becomes a pressing concern in view of the expected consequences these relations may have had.     

Lithospheric contributions to high-MgO basanites from the Cumbre Vieja Volcano,La Palma,Canary Islands and evidence for temporal variation in plume influence

New geochemical and isotopic data are presented from the oldest part of the Cumbre Vieja volcano, La Palma (Canary Islands), located near the assumed emergence of the Canary mantle plume. The volcanics comprise a suite dominated by basanite flows with subordinate amounts of phono-tephrite, tephri-phonolite and phonolite flows and intrusives. Two compositionally different basanite groups have been identified, both with HIMU (high-μ)-type incompatible trace element characteristics: Primitive high-MgO basanites (10.7–12.1% MgO), found only at the base of a stratigraphic profile near Fuencaliente on the south coast, and intermediate-MgO basanites (6.0–7.3% MgO), exposed in the upper part of the profile and widespread on the east coast of La Palma. The high-MgO basanites are interpreted as near-primary mantle melts (primary composition 14–15% MgO) derived by progressive melting (2.9% to 4.5%) of a common lithospheric mantle source. Model calculations indicate that it is not possible to generate the intermediate-MgO basanites from the high-MgO group by crystal fractionation of observed phenocrysts. Relative to intermediate-MgO basanites, the high-MgO flows have lower concentrations of LIL and HFS elements, except for Ti, which is markedly enriched in the primitive rocks (3.7–4.7% TiO₂ vs 3.4–3.9% TiO₂). Fuencaliente volcanics display limited temporal isotopic variations suggested to be a result of mixing of melts originating from the rising plume and the metazomatized lithospheric mantle. ⁸⁷Sr / ⁸⁶Sr and ¹⁴³Nd / ¹⁴⁴Nd ratios range 0.70305–0.70311 and 0.51285–0.51291, respectively, while the corresponding ranges in Pb-isotope ratios are ²⁰⁶Pb / ²⁰⁴Pb = 19.46–19.64, ²⁰⁷Pb / ²⁰⁴Pb = 15.55–15.61, and ²⁰⁸Pb / ²⁰⁴Pb = 39.16–39.53. The overall variation of the Cumbre Vieja isotopic data can be accounted for by mixtures of three mantle components in the proportions 72–79% plume source (LVC = low velocity component), 9–16% depleted mantle (DM) and up to 12% enriched mantle (EMI). Negative Δ7 / 4 Pb (− 0.6 to − 5.4) in the Cumbre Vieja volcanics suggest derivation from a young HIMU mantle source. The relative abundance of plume source material increase in younger rocks in the Fuencaliente section, suggesting waning plume–lithosphere interaction during the emplacement of this part of the Cumbre Vieja volcano. The high-MgO volcanics define regular and systematic geochemical trends, interpreted as partial melting trends, when plotted against abundances of highly incompatible elements (P, Ce). Evaluation of minor and trace element variation in consecutive melts suggests control by residual amphibole, phlogopite, garnet and a Ti-bearing phase, possibly ilmenite. The melting mode changed gradually, allowing increasing input from residual phlogopite during partial melting. The residual mineralogy constrains the source region of the high-MgO basanites to the lowermost oceanic lithospheric mantle, presumably around 100 km depths.     

Subduction of mantle wedge peridotites: Evidence from the Higashi-akaishi ultramafic body in the Sanbagawa metamorphic belt

The Higashi-akaishi garnet-bearing ultramafic body in the Sanbagawa metamorphic belt, Southwest Japan, represents a rare example of oceanic-type ultrahigh-pressure metamorphism. The body of 2 km × 5 km is composed mostly of anhydrous dunite with volumetrically minor lenses of clinopyroxene-rich rocks. Dunite samples contain high Ir-type platinum group elements (PGE) and Cr in bulk rocks, high Mg and Ni in olivine, and high Cr in spinel. On the other hand, clinopyroxene-rich rocks contain low concentrations of Ir-type PGE and Cr, high concentrations of fluid-mobile elements in bulk rocks, and low Ni and Mg in olivine. Clinopyroxene is diopsidic with low Al₂O₃. The compositions of bulk rocks and mineral chemistry of spinel, olivine, and clinopyroxene suggest that the olivine-dominated rocks are residual mantle peridotites after high degrees of influx partial melting, and that the clinopyroxene-rich rocks are cumulates of subduction-related melts. Thus, the Higashi-akaishi ultramafic body originated from the interior of the mantle wedge, most likely the forearc upper mantle. It was then incorporated into the Sanbagawa subduction channel by a mantle flow, and underwent high pressure metamorphism to a depth greater than 100 km. Such a strong active flow in the mantle wedge is likely facilitated by the lack of serpentinites along the interface between the slab and the overlying mantle, as it was too hot for serpentine. These unusually hot conditions and strong active mantle flow may reflect conditions in the earliest stage of development of subduction, and may have been maintained by massive upwelling and subsequent eastward flow of asthenospheric mantle in the northeastern Asian continent in Cretaceous time when the Sanbagawa belt began to form.     

Relative depletion of niobium in some arc magmas and the continental crust: partitioning of K, Nb, La and Ce during melt/rock reaction in the upper mantle

Depletion of Nb relative to K and La is characteristic of lavas in subduction-related magmatic arcs, as distinct from mid-ocean ridge basalts. Nb depletion is also characteristic of the continental crust. This and other geochemical similarities between the continental crust and high-Mg# andesite magmas found in arcs suggests that the continental crust may have formed by accretion of andesites. Previous studies have shown that the major element characteristics of high-Mg# andesites may be produced by melt/rock reaction in the upper mantle. In this paper, new data on partitioning of K, Nb, La and Ce between garnet, orthopyroxene and clinopyroxene in mantle xenoliths, and on partitioning of Nb and La between orthopyroxene and liquid, show that garnet and orthopyroxene have Nb crystal/liquid distribution coefficients which are much larger than those of K and La. Similar fractionations of Nb from K and La are expected in spinel and olivine. For this reason, reactions between migrating melt and large masses of mantle peridotite can produce substantial depletion of Nb in derivative liquids. Modeling shows that reaction between ascending, mantle-derived melts and mantle peridotite is a viable mechanism for producing the trace element characteristics of high-Mg# andesite magmas and the continental crust.

Alternatively, small-degree melts of metabasalt and/or metasediment in the subducting slab may leave rutile in their residue, and will thus have large Nb depletions relative to K and La [1]. Slab melts are too rich in light rare earth elements and other incompatible elements, and too poor in compatible elements, to be parental to arc magmas. However, ascending slab melts may be modified by reaction with the mantle. Our new data permit modeling of the trace element effects of reaction between small-degree melts of the slab and mantle peridotite. Modeling shows that this type of reaction is also a viable mechanism for producing the trace element characteristics of high-Mg# andesites and the continental crust. These findings, in combination with previous results, suggest that melt/rock reaction in the upper mantle has been an important process in forming the continental crust and mantle lithosphere.     

Melt–wall rock interaction in the mantle shown by silicate melt inclusions in peridotite xenoliths from the central Pannonian Basin (western Hungary)

In this paper we present a detailed textural and geochemical study of two equigranular textured amphibole-bearing spinel lherzolite xenoliths from Szigliget, Bakony–Balaton Highland Volcanic Field (BBHVF, western Hungary) containing abundant primary silicate melt inclusions (SMIs) in clinopyroxene rims and secondary SMIs in orthopyroxene (and rarely spinel) along healed fractures. The SMIs are dominantly composed of silicate glass and CO₂-rich bubbles. Clinopyroxene and orthopyroxene are zoned in both studied xenoliths, especially with respect to Fe, Mg, Na, and Al contents. Cores of clinopyroxenes in both xenoliths show trace element distribution close to primitive mantle. Rims of clinopyroxenes are enriched in Th, U, light rare earth elements (LREEs) and medium REEs (MREEs). Amphiboles in the Szg08 xenolith exhibit elevated Rb, Ba, Nb, Ta, LREE, and MREE contents. The composition of silicate glass in the SMIs covers a wide range from the basaltic trachyandesite and andesite to phonolitic compositions. The glasses are particularly rich in P₂O₅. Both primary and secondary SMIs are strongly enriched in incompatible trace elements (mostly U, Th, La, Zr) and display a slight negative Hf anomaly. The development of zoned pyroxenes, as well as the entrapment of primary SMIs in the clinopyroxene rims, happened after partial melting and subsequent crystallization of clinopyroxenes, most probably due to an interaction between hot volatile-bearing evolved melt and mantle wall-rocks. This silicate melt filled microfractures in orthopyroxenes (and rarely spinels) resulting in secondary SMIs.     

New insights on the origin of troctolites from the breakaway area of the Godzilla Megamullion (Parece Vela back‐arc basin): The role of melt‐mantle interaction on the composition of the lower crust

The troctolites and olivine‐gabbros from the Dive 6 K‐1147 represent the most primitive gabbroic rocks collected at the Godzilla Megamullion, a giant oceanic core complex formed at an extinct spreading segment of the Parece Vela back‐arc basin (Philippine Sea). Previous investigations have shown that these rocks have textural and major elements mineral compositions consistent with a formation through multistage interaction between mantle‐derived melts and a pre‐existing ultramafic matrix. New investigations on trace element mineral compositions basically agree with this hypothesis. Clinopyroxenes and plagioclase have incompatible element signatures similar to that of typical‐MORB. However, the clinopyroxenes show very high Cr contents (similar to those of mantle clinopyroxene) and rim having sharply higher Zr/REE ratios with respect to the core. These features are in contrast with an evolution constrained by fractional crystallization processes, and suggest that the clinopyroxene compositions are controlled by melt‐rock interaction processes. The plagioclase anorthite versus clinopyroxene Mg#[Mg/(Mg + Fe^Tot)] correlation of the Dive 6 K‐1147 rocks shows a trend much steeper than those depicted by other oceanic gabbroic sections. Using a thermodynamic model, we show that this trend is reproducible by fractionation of melts assimilating 1 g of mantle peridotite per 1 °C of cooling. This model predicts the early crystallization of high Mg# clinopyroxene, consistent with our petrological observation. The melt‐peridotite interaction process produces Na‐rich melts causing the crystallization of plagioclase with low anorthite component, typically characterizing the evolved gabbros from Godzilla Megamullion.     

Possible sub-arc origin of podiform chromitites

Abstract The sub-arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr#) of their chromian spinel frequently is higher than 0.7, which is comparable with the range for arc-related primitive magmas. This almost excludes the possibility of their sub-oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr# < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of high-field strength elements (HFSE) to large-ion lithophile elements (LILE), one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc-type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites.     

Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust

The average chemical compositions of the continental crust and the oceanic crust (represented by MORB), normalized to primitive mantle values and plotted as functions of the apparent bulk partition coefficient of each element, form surprisingly simple, complementary concentration patterns. In the continental crust, the maximum concentrations are on the order of 50 to 100 times the primitive-mantle values, and these are attained by the most highly incompatible elements Cs, Rb, Ba, and Th. In the average oceanic crust, the maximum concentrations are only about 10 times the primitive mantle values, and they are attained by the moderately incompatible elements Na, Ti, Zr, Hf, Y and the intermediate to heavy REE.This relationship is explained by a simple, two-stage model of extracting first continental and then oceanic crust from the initially primitive mantle. This model reproduces the characteristic concentration maximum in MORB. It yields quantitative constraints about the effective aggregate melt fractions extracted during both stages. These amount to about 1.5% for the continental crust and about 8–10% for the oceanic crust.The comparatively low degrees of melting inferred for average MORB are consistent with the correlation of Na₂O concentration with depth of extrusion [1], and with the normalized concentrations of Ca, Sc, and Al ( 3) in MORB, which are much lower than those of Zr, Hf, and the HREE ( 10). Ca, Al and Sc are compatible with clinopyroxene and are preferentially retained in the residual mantle by this mineral. This is possible only if the aggregate melt fraction is low enough for the clinopyroxene not to be consumed.A sequence of increasing compatibility of lithophile elements may be defined in two independent ways: (1) the order of decreasing normalized concentrations in the continental crust; or (2) by concentration correlations in oceanic basalts. The results are surprisingly similar except for Nb, Ta, and Pb, which yield inconsistent bulk partition coefficients as well as anomalous concentrations and standard deviations.The anomalies can be explained if Nb and Ta have relatively large partition coefficients during continental crust production and smaller coefficients during oceanic crust production. In contrast, Pb has a very small coefficient during continental crust production and a larger coefficient during oceanic crust production. This is the reason why these elements are useful in geochemical discrimination diagrams for distinguishing MORB and OIB on the one hand from island arc and most intracontinental volcanics on the other.The results are consistent with the crust-mantle differentiation model proposed previously [2]. Nb and Ta are preferentially retained and enriched in the residual mantle during formation of continental crust. After separation of the bulk of the continental crust, the residual portion of the mantle was rehomogenized, and the present-day internal heterogeneities between MORB and OIB sources were generated subsequently by processes involving only oceanic crust and mantle. During this second stage, Nb and Ta are highly incompatible, and their abundances are anomalously high in both OIB and MORB.The anomalous behavior of Pb causes the so-called “lead paradox”, namely the elevated U/Pb and Th/Pb ratios (inferred from Pb isotopes) in the present-day, depleted mantle, even though U and Th are more incompatible than Pb in oceanic basalts. This is explained if Pb is in fact more incompatible than U and Th during formation of the continental crust, and less incompatible than U and Th during formation of oceanic crust.     

Trace element composition in tuite decomposed from natural apatite in high-pressure and high-temperature experiments

Tuite has been suggested as a potential reservoir for trace elements in the deep mantle,but no evidence confirms this supposition.By using a natural apatite as starting material,the trace-element-bearing tuite large crystals were obtained under highpressure and high-temperature conditions(15 GPa and 1800 K).X-ray diffraction pattern and Micro-Raman spectrum of the run product confirm that tuite was synthesized.The concentrations of trace elements in tuite crystals were analyzed by laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS).The rare earth element patterns of tuite show enrichment of light rare earth elements relative to heavy rare earth elements.Tuite shows high concentrations of Th and Sr,and negative anomalies of Rb,Nb,and Hf.The results show that tuite can accommodate a large amount of trace elements.Tuite might be an important host to accommodate trace elements if there is much apatite subducted into the deep mantle.     

Geochemistry of peridotite xenoliths in alkali basalts from Jeju Island, Korea

Abstract   Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)_N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history.