The Generation and Compaction of Partially Molten Rock

The equations governing the movement of the melt and the matrixof a partially molten material are obtained from the conservationof mass, momentum, and energy using expressions from the theoryof mixtures. The equations define a length scale _c called thecompaction length, which depends only on the material propertiesof the melt and matrix. A number of simple solutions to theequations show that, if the porosity is initially constant,matrix compaction only occurs within a distance _c of an impermeableboundary. Elsewhere the gravitational forces are supported bythe viscous stresses resulting from the movement of melt, andno compaction occurs. The velocity necessary to prevent compactionis known as the minimum fluidization velocity. In all casesthe compaction rate is controlled by the properties of the matrix.These results can only be applied to geological problems ifthe values of the permeability, bulk and shear viscosity ofthe matrix can be estimated. All three depend on the microscopicgeometry of the melt, which is in turn controlled by the dihedralangle. The likely equilibrium network provides some guidancein estimating the order of magnitude of these constants, butis no substitute for good measurements, which are yet to becarried out. Partial melting by release of pressure at constantentropy is then examined as a means of produced melt withinthe earth. The principal results of geological interest are that a meanmantle temperature of 1350?C is capable of producing the oceaniccrustal thickness by partial melting. Local hot jets with temperaturesof 1550?C can produce aseismic ridges with crustal thicknessesof about 20 km on ridge axes, and can generate enough melt toproduce the Hawaiian Ridge. Higher mantle temperatures in theArchaean can produce komatiites if these are the result of modestamounts of melting at depths of greater than 100 km, and notshallow melting of most of the rock. The compaction rate ofthe partially molten rock is likely to be rapid, and melt-saturatedporosities in excess of perhaps 3 per cent are unlikely to persistanywhere over geological times. The movement of melt througha matrix does not transport major and trace elements with themean velocity of the melt, but with a slower velocity whosemagnitude depends on the distribution coefficient. This effectis particularly important when the melt fraction is small, andmay both explain some geochemical observations and provide ameans of investigating the compaction process within the earth.     

Steady-state Mantle-Melt Interactions in One Dimension: II. Thermal Interactions and Irreversible Terms

Progress in development of thermodynamically based models ofsilicate equilibria with explicit entropy budgets has motivateda reexamination of the conclusion of McKenzie (Journal of Petrology25, 713–765, 1984) that isentropic upwelling sufficesas a model of mantle melting. An entropy budget equation forfractional melting with melt migration in an upwelling two-phasecontinuum is presented. The energetically self-consistent meltproduction model predicted by MELTS is used to evaluate numericallythe magnitudes of differences between fractional melting (withmelt migration) and equilibrium melting (without relative movement)that can be bounded in one dimension: chemical advection byout-of-equilibrium melt; thermal disequilibrium between migratingliquid and residue; frictional dissipation of gravitationalpotential; dissipation as a result of solid compaction. Likethe familiar isobaric case in which fractional melting is significantlyless productive than equilibrium melting, chemical isolationof the escaping melts from the residue reduces the oceanic crustalthickness by     

Lead isotope variability in olivine-hosted melt inclusions from Iceland

The lead isotope and trace element compositions of a suite of olivine-hosted melt inclusions in primitive lava flows from the Reykjanes Peninsula in Iceland show extreme variability. Much of this variability is present in the composition of inclusions from one hand specimen of Háleyjabunga, a depleted picrite lava shield that erupted 13 ka. ²⁰⁸Pb/²⁰⁶Pb compositions in this sample span 50-90% of the total range found in Atlantic MORB, indicating that high-amplitude compositional heterogeneity is present in the mantle source of melts that aggregated to form a single eruption. The trace element and isotopic trends in the melt inclusions are coincident with those in whole rock samples from young lava flows of the Reykjanes Peninsula, and extend the total range of variation towards more depleted compositions. The incompatible trace element and lead isotope compositions of the inclusions are strongly coupled and lie close to binary mixing trends between the extreme melt inclusion compositions. These relationships indicate that the trace element variation in the melt inclusions reflects heterogeneity in the composition of the mantle source entering the melting region under the Reykjanes Peninsula. Large positive Sr concentration anomalies are present in three of the inclusions, but do not correlate with indicators of mantle melting or source variations and are likely to arise by reaction with plagioclase during crustal storage. Fractional melting of heterogeneous mantle is predicted to generate melts with a wide range of compositions, filling a large volume in trace element-isotope space. However, the compositional variations observed in the melt inclusions lie close to binary mixing curves. These observations may be accounted for by a two-stage model of melt mixing. The first stage occurs in porous channels that transport melt in the mantle and takes place before inclusion entrapment. This mixing stage generates a bimodal distribution of melt compositions that is supplied from the channels to sub-Moho and lower crustal magma lenses. The second stage of mixing occurs in these chambers, producing the binary mixing trends recorded in the inclusion compositions. The distribution of isotopic compositions observed in the melt inclusions and whole rock samples from the Reykjanes Peninsula is therefore controlled by melt mixing. These results have important implications for the interpretation of basalt composition in terms of distinct compositional entities within the upwelling solid mantle under mid-ocean ridges and ocean islands.     

Petrogenesis of Hawaiian tholeiites: 2, aspects of dynamic melt segregation

To Hawaiian magma genesis, dynamic melt segregation offers a potential resolution of conflict arising between trace-element evidence and phase-equilibria evidence, for deep garnet-present melting versus shallow garnet-absent melting. In this study comprehensive dynamic melting models, which incorporate phase-equilibria constraints and variable partition coefficients, have been applied in efforts to simulate decompression melting of a mantle plume. These models specifically endeavour to reproduce Hawaiian REE (rare-earth-element) patterns from a peridotitic upper mantle source with chondritic relative abundances of middle and HREE (heavy REE). If the flow of both melt and solid mantle is vertical through the partially molten source region, and melting proceeds beyond the stability limit of garnet in peridotite, dynamic melting processes are unable to produce the fractionated REE patterns of Hawaiian tholeiites. Instead, three-dimensional dynamic melting modles need to be invoked, in which lateral migration of the melt relative to the residual matrix also takes place. This enables the derivation of small garnet-equilibrated melt fractions from a larger source volume than that supplying more extensive melt fractions from shallower garnet-absent levels of melting (i.e melting shapes with a mean degree of melting smaller than the maximum extent of melting). This can be achieved by either drawing small-degree melt fractions, formed in the presence of garnet at the plume peripheries, toward the plume centre, or by advecting the mantle residue away from the plume centre as it ascends. Fluid dynamic theory supports a plume model incorporating the latter, with melt flow occurring vertically through a matrix flow which is deflected by the lithosphere and diverges away from the plume centre. In this framework, the generation of melting shapes dominated by small-degree garnetpresent melt fractions, requires a decrease in the rate of melting with progressive melting and height along melt-flow paths within the plume. This is consistent with a decrease in vertical velocity of the matrix (and thus decompression melting rate) upwards through the plume and, with diminishing melting rates upon exhaustion of garnet and clinopyroxene as melting progresses. Providing melt segregation occurs by percolation, equilibrium between the segregating melt and residual peridotite matrix may be maintained throughout the plume. In this way, primary melts extracted from the Hawaiian plume have their bulk compositions determined by phase equilibrium with the extensively melted matrix residue (harzburgite) at the plume top and shallowest level of melting (2.0 GPa), and their incompatible-trace-element characteristics determined by smaller-degree melt fractions derived from deeper, garnet-present levels of melting (3.0 GPa). Simple unidimensional models for melt segregation by percolation or via channels are shown to produce incompatible-trace-element abundances and ratios which are similar to those generated by equivalent degrees of batch melting. Moreover, contrary to a common belief held for dynamic melting, the enrichment of more-incompatible elements over less-incompatible elements is not always greater than that produced by an equivalent amount of batch melting.     

Melting of the Shallow Upper Mantle: A New Perspective

Detailed examination of liquidus phase relationships in binaryand ternary joins of the CFMAS +Cr system has permitted a rigorousdetermination of the dry melting path of an initially fertilespinel peridotite composition resembling Bulk Silicate Earthor MORB-pyrolite. It is demonstrated that it is impossible tomodel mantle melting accurately using only one set of ratiosof phases entering the melt; this implies that the melting processis primarily controlled by solid solution rather than eutecticbehaviour. The proportions of phases entering a melt dependon whether a phase reacts and/or disappears from a system, andon the choice of the initial and final peridotite compositions.Four discrete domains in the melting regime of upper-mantleperidotites are distinguished, each characterized by differentphase melting coefficients, relating to the melting of: (1)lherzolites, (2) clinopyroxene-bearing harzburgites (i.e., free-clinopyroxene),(3) clinopyroxene-saturated harzburgites (i.e., clinopyroxenein solid solution in orthopyroxene), and (4) clinopyroxene-freeharzburgites (i.e., no clinopyroxene). The proposed non-linearfashion in which mantle lithologies melt explains the inadequacyof all previous models to reproduce the observed compositionsof upper-mantle peridotite melting residues. It is suggestedthat: (1) olivine and orthopyroxene will melt cotectically;(2) clinopyroxene and spinel will lose most of their aluminouscomponent after {small tilde}8% melting within the first 4 kb({smalltilde} 12 km) of ascent from the dry solidus; and that (3) clinopyroxenewill disappear completely from a MORB-pyrolite mantle after{small tilde}42% melting. Although such a number is significantlyhigher than that dictated by the position of the clinopyroxene-outcurves from peridotite isobaric equilibrium melting experiments({small tilde}22%), it is emphasized that the latter are a grossoversimplification of the natural melting process and are notequivalent to melting during adiabatic upwelling. It is concludedthat the commonly postulated disappearance of clinopyroxenefrom fertile peridotite compositions at {small tilde}22% meltingis greatly in error if melting in an adiabatically rising mantleis considered, thus providing an explanation for many unsuccessfulattempts by various authors to model the behaviour of transitionelements in sub-oceanic and supra-subduction-zone mantle andderivative magmas.     

TTG岩系Nb-Ta-La分馏特征的地球化学模拟:对太古宙板块俯冲与大陆地壳生长机制的约束

TTG的Nb/Ta比值以及Nb、Ta相对于La(代表LILE)的亏损取决于部分熔融体系中金红石、角闪石作为残留相矿物存在与否.本研究采用金红石和低Mg#角闪石的微量元素分配系数模拟部分熔融过程中Nb-Ta-La的分馏.模拟结果表明:如果与TTG熔体平衡的残留相是不含金红石的石榴角闪岩,熔体Nb/Ta比值低于源岩但Ta含...     

Geochemical aspects of permeability controlled partial melting and fractional crystallization

Magma accumulation in the mantle requires that the mantle be permeable. Experimental investigations show that the permeability threshold first will be attained after a certain degree of partial melting. The influence of the permeability threshold on the composition of partial melts is evaluated using the fayalite-forsterite system as an example. In addition the variation in trace element concentrations are calculated for different distribution coefficients. Primary magmas formed by accumulation when a minimal degree of partial melting is required for permeability display a remarkably small variation in composition up to 30% partial melting. It is suggested from REE abundances that primary tholeiitic magmas have been generated by permeability controlled partial melting. The compositions of the primary magmas generated by permeability controlled partial melting will not differ much from the compositions obtained by batch melting, but the degrees of partial melting will differ for similar compositions.     

Non-modal melting in an upwelling mantle column: Steady-state models with applications to REE depletion in abyssal peridotites and the dynamics of melt migration in the mantle

Simple models for trace element fractionation during concurrent melting and melt migration in an upwelling steady-state mantle were developed. Based on petrologic considerations, we divided the mantle column into two regions: a single-lithology lower region that consists of partially molten garnet and spinel lherzolites and a double-lithology upper region where high-porosity dunite channels or melt-filled fractures are embedded in a porous lherzolite/harzburgite matrix. Analytical solutions for the case of a constant and uniform relative melting suction rate and a linearly variable relative melt suction rate were obtained. Key parameters and the first order characteristics of melting and melt migration in a 1-D steady-state mantle column were examined through forward calculations and Monte Carlo simulations. Melting in the upwelling single-lithology column is equivalent to non-modal batch melting, whereas melting and melt migration in the double-lithology region can be viewed as a nonlinear combination of batch melting and fractional melting, depending on the amount of melt extracted to the channel. The degree of melting (F), the degree of melting at the depth of melt-channel initiation (F_d) and the relative rate of melt suction (R) are important in controlling the extent of depletion of the incompatible trace element in the matrix. Spatially variable R affects the abundance of an incompatible trace element in the melt and residual solid the most in near fractional melting. There is a strong nonlinear trade off among the three parameters. Given F_d, it is possible to constrain F and R from incompatible trace element abundances in residual peridotite.To explore the dynamics of melt migration in the mantle, we used the two melting models developed in this study and published REE and Y abundances in diopside in abyssal peridotites from the Central Indian Ridge to infer their melting and melt migration history. Overall, the degrees of melting inferred from the trace element data are not sensitive to the value of F_d used in the inversion and ranges from 10% to 15%. The relative rate of melt suction depends slightly on the choice of F_d and ranges from 0.85 to 1.0 for F_d = 0.05 and 0.75 to 0.97 for F_d = 0. Further, the estimated R is inversely correlated with F, a robust feature independent of the choice of F_d. The upward decrease of R in an upwelling mantle column can be understood in terms of melt focusing in the lower part of the double-lithology region. And finally, given F and R, we found that the permeability and porosity of the lherzolite/harzburgite matrix also increase as a function of F in the melting column, with melt fractions ranging from 0.2% to 0.7% for a grain size of 5 mm.     

Simple models for dynamic melting in an upwelling heterogeneous mantle column: Analytical solutions

Several lines of evidence suggest that the melt generation and segregation regions of the mantle are heterogeneous, consisting of chemically and lithologically distinct domains of variable size and dimension. Partial melting of such heterogeneous mantle source regions gives rise to a diverse range of basaltic magmas. In order to better assess the role of source heterogeneity during mantle melting, we have undertaken a theoretical study of trace element distribution and fractionation during concurrent melting and melt migration in an upwelling, chemically heterogeneous, two-porosity double lithology melting column. Analytical solutions for the abundance of a trace element in the matrix and channel were obtained under the assumptions that the porosity, melt and solid velocities, and solid-melt partition coefficients are constant and uniform. For simplicity, we neglected diffusion and dispersion in the melt. Chemical source heterogeneities of arbitrary size and shape were integrated into the simple melting models by allowing trace element abundance in the source region to vary as a function of time and space. Concurrent melting and melt migration in an upwelling heterogeneous mantle may be approximated as a quasi-steady state problem in which time-dependent concentration patterns produced by melting of heterogeneous source regions are superimposed on a reference steady-state concentration distribution established by melting of the ambient or background mantle. Chromatographic fractionation is especially important for the matrix melt and solid when chemical heterogeneities are involved during melting and melt migration in the mantle, giving rise to significant phase-shift between two incompatible trace elements in the matrix melt and scattered correlations among incompatible trace elements in residual peridotites. Mixing is the chief mass transfer process in the dunite channel where the chromatographic effect is negligible for most of the incompatible trace elements. The lack of chromatographic fractionation among incompatible trace elements and isotopic ratios in MORB suggests either most MORB are channel melts or mixing in magma conduit and chamber is very efficient such that the phase-shift is averaged out during magma transport and storage processes. Advection brings melt produced by smaller-degree of melting in the deeper part of the melting column to the overlying melting region, increasing the incompatible trace element abundance in the matrix and the channel. This advection-induced self-enrichment is especially important when heterogeneous sources are involved and may account for some of the enriched incompatible trace element patterns observed in residual peridotite that were previously interpreted to be a result of mantle metasomatism. Systematic studies of high-resolution spatially correlated mantle samples may help to constrain the melting history and the size and nature of chemical heterogeneities in the mantle.     

俯冲带壳-幔相互作用的高温高压实验:对地幔不均一性成因的启示

使用活塞-圆筒式高温高压装置进行一系列榴辉岩部分熔融熔体与橄榄岩反应实验,可以为深入了解俯冲带壳-幔相互作用的影响因素及地幔不均一性的成因提供重要信息.实验使用反应偶的方法,并在0.8~3.0 GPa和1 200~1 425℃条件下进行.实验结果表明,榴辉岩部分熔融熔体-橄榄岩反应的动力学和结果受控于熔体主量元素成分、熔体中的H₂O、温度、压力和橄榄岩的物理状态等因素.大陆俯冲带地幔岩石中斜方辉石的富集是再循环陆壳熔体与上覆地幔反应的结果,地幔岩石中斜方辉石岩脉的形成与含水熔体交代有关,地幔岩石中的石榴辉石岩和石榴石岩可能形成于高压、低温条件下的熔体-橄榄岩反应.      

Determining melt productivity of mantle sources from U-Th and U-Pa disequilibria; an example from Pico Island, Azores

We use coupled ²³⁸U-²³⁰Th and ²³⁵U-²³¹Pa disequilibria measurements from Pico Island, Azores to examine the melting behavior of the underlying mantle. U-series disequilibria in young, mafic lavas are dependent on the melting rate of their source, which in most cases is primarily controlled by its melt productivity. Mafic lithologies such as eclogite and pyroxenite have much higher melt productivities than peridotite and so U-series measurements may provide constraints on the mineralogy of the melting mantle. Recent Pico Mountain lavas show limited geochemical variations and a restricted range of U-series disequilibria with (²³⁰Th/²³⁸U) = 1.22-1.25 and (²³¹Pa/²³⁵U) = 1.46-1.50. Using a simple, dynamic melting model of a homogeneous source, these results can be reproduced with melting rates of <1 × 10⁻⁴ kg/m³/a and melt porosities of <0.7% near the onset of melting. For a plausible range of upwelling rates, this implies that the melt productivity is <6%/GPa. This value is consistent with a garnet peridotite source but not with more highly productive mafic lithologies. Given independent evidence for the involvement of mafic lithologies such as recycled oceanic crust in Pico magmagenesis, we suggest a scenario in which initial eclogitic melts are dispersed through melt-rock reaction into a larger volume of surrounding peridotite. Subsequent re-melting of the resultant incompatible element enriched peridotite carries a geochemical signature of the mafic lithologies but not necessarily a record of their high melt productivity.     

The importance of melt extraction for tracing mantle heterogeneity

Numerous isotope and trace element studies of mantle rocks and oceanic basalts show that the Earth’s mantle is heterogeneous. The isotopic variability in oceanic basalts indicates that most mantle sources consist of complex assemblages of two or more components with isolated long-term chemical evolution, on both global and local scales. The range in isotope and highly incompatible element ratios observed in oceanic basalts is commonly assumed to directly reflect that of their mantle sources. Accordingly, the end-points of isotope arrays are taken to represent the isotopic composition of the different components in the underlying mantle, which is then used to deduce the origin of mantle heterogeneity. Here, a melting model for heterogeneous mantle sources is presented that investigates how and to what extent isotope and trace element signatures are conveyed from source to melt. We model melting of a pyroxenite-bearing peridotite using recent experimental constrains for melting and partitioning of pyroxenite and peridotite. Identification of specific pyroxenite melting signatures allows finger-printing of pyroxenite melts and confirm the importance of lithological heterogeneity in the Earth’s mantle. The model results and the comparison of the calculated and observed trace element-isotope systematics in selected MORB and OIB suites (e.g. from the East Pacific Rise, Iceland, Tristan da Cunha, Gough and St.Helena) further show that factors such as the relative abundance of different source components, their difference in solidus temperature, and especially the extent, style and depth range of melt aggregation fundamentally influence the relationship between key trace element and isotope ratios (e.g. Ba/Th, La/Nb, Sr/Nd, La/Sm, Sm/Yb, ¹⁴³Nd/¹⁴⁴Nd). The reason for this is that any heterogeneity present in the mantle is averaged or, depending on the effectiveness of the melt mixing process, even homogenized during melting and melt extraction. Hence to what degree mantle heterogeneity is reflected in the erupted melts is not only a function of source and melting-induced variability. It also depends on the extent of mixing during melting and melt extraction and thus strongly on the relative incompatibility of the elements considered. The observed trace element variation in erupted melts can be greater or smaller than that of their mantle sources, depending on the incompatibility of the elements investigated. The isotopic variability in erupted melts, on the other hand, is generally smaller than that of their mantle source. Melt mixing during melt extraction consequently has an important influence on the relative extent of variation, and hence the degree of correlation between the isotope and trace element ratios. Overall fewer correlations between trace element and isotope ratios are expected whenever melts are extracted from a restricted depth range, as is the case for ocean island basalts, than for cases where melts are extracted over a larger depth interval (mid ocean ridges and especially ridge centered hotspots like Iceland). While the isotopic composition of the most enriched melts may correspond closely to those of the enriched source component, even the most depleted mid ocean ridge basalts are likely to underestimate the isotopic depletion of the depleted mantle component. These observations imply that using the chemical and isotopic range observed in oceanic basalts as directly representative of that in the corresponding mantle source can be misleading, since this assumption is strictly true only for homogeneous mantle sources. In addition to identifying source or partitioning-related differences in melts from different mantle sources, inferring the true composition, origin, and distribution of heterogeneous components in the Earth’s mantle therefore requires detailed knowledge about the mechanisms of melting and melt mixing during the melt extraction process. Only if these processes and their influence on the isotope-trace element relationship are understood, can the composition and origin of the different source components, and thus mantle heterogeneity, be accurately constrained.     

Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15° 20′N: ODP Hole 1274A

ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (∼0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt–rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equilibration with residual harzburgites. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

A Melt Extraction Model Based on Structural Studies in Mantle Peridotites

This study, largely based on field observations in various peridotitemassifs and on basalt xenoliths, traces the successive stagesof melt extraction from mantle diapirs. The first stage, initiatedin the garnet lherzolite field and pursued in the spinel lherzolitefield, creates melt pockets at the sites of the former garnets.During ascent, the melt fraction stable in spinel-lherzolitesis estimated to be 7 per cent. Above this fraction, but dependingupon plastic strain, permeability is obtained and melt extractionstarts. This occurs at 50 km depth. A network of connected meltveins and gashes, opened by fluid assisted shear fracturingin the deforming peridotites, is first created. When its verticalextension attains 10 km. the melt pressure fractures the overlyingperidotites (tensional hydrofracturing) creating a conduit formelt extraction. The buoyant forces generate a negative pressureat the base of the conduit. After communication with the earthsurface is achieved, the melt network surrounding the dyke rootis thus drained. This mechanism explains the remarkable efficiencyof melt extraction in residual harzburgites and dunites. Theconduit is a dyke, with a 20cm width at shallow depth. The meltvelocity through such dykes in shallow mantle is 5 cm s^–1.The rate of extraction of melt is so large that melt extractionis necessarily a discontinuous process even in the case of oceaniccrust generation. Each dyke of the dyke swarm in oceanic crustand ophiolites (and possibly each cumulate layer in the underlyingmafic cumulates) corresponds to a melt extraction event. Thuseach event creates 1 m of crust, during the time lapse of afew weeks. The periodicity of such events (5–50 yr) dependson the spreading rate (10–1 cm yr^–1). Each one corresponds,in the rising diapir, to a hydrofracture produced locally inthe area of the mantle melt network. For spreading rates > 1 cm yr^–1, a 6 km thick oceaniccrust is created with an underlying uppermost mantle composedof residual harzburgites. For smaller rates, the oceanic crustis thinner as a result of smaller degrees of melting, with plagioclaselherzolites as residue. For even smaller rates, no oceanic crustis created (continental rifting) and the residue is a comparativelyfertile spinel lherzolite. This is explained by a direct relationbetween spreading rate and ascent rate of the mantle diapir.For spreading rates < 1 cm yr^–1, the adiabatic meltingin the diapir stops at about 15 km depth in plagioclase lherzolites(except for a final melt extraction just below the Moho) andat > 30km in spinel lherzolites. This model has implications on melting processes (disequilibriummelting), the nature of primary melts and implies a straighterconnection than generally accepted between the physics and chemistryof volcanism and melting processes in the mantle.     

Fractionation of the noble metals by physical processes

During partial melting in the earth’s mantle, the noble metals become fractionated. Os, Ir, Ru, and Rh tend to remain in the mantle residue whereas Pt, Pd, and Re behave mildly incompatible and are sequestered to the silicate melt. There is consensus that sulfide plays a role in the fractionation process; the major noble metal repository in the mantle is sulfide, and most primitive mantle melts are sulfide-saturated when they leave their mantle sources. However, with sulfide–silicate partitioning, the fractionation cannot be modeled properly. All sulfide–silicate partition coefficients are so extremely high that a silicate melt segregating from a mantle source with residual sulfide should be largely platinum-group elements free. We offer a physical alternative to sulfide–silicate chemical partitioning and provide a mechanism of generating a noble metal-rich melt from a sulfide-saturated source: Because sulfide is at least partially molten at asthenospheric temperature, it will behave physically incompatible during melt segregation, and a silicate melt segregating from a mantle residue will entrain molten residual sulfide in suspension and incorporate it in the basaltic pool melt. The noble metal abundances of a basalt then become independent of sulfide–silicate chemical partitioning. They reflect the noble metal abundances in the drained sulfide fraction as well as the total amount of sulfide entrained. Contrary to convention, we suggest that a fertile, sulfide-rich mantle source has more potential to generate a noble metal-enriched basaltic melt than a refractory mantle source depleted by previous partial melting events.     

Experimental peridotite–melt reaction at one atmosphere: a textural and chemical study

Sieve-textured clinopyroxene and spinel are common in mantle xenoliths and have been interpreted to be the result of partial melting, mantle metasomatism and host magma–xenolith reaction during transport. In this paper, we test the latter hypothesis with a series of reduced and oxidized experiments at 1,200 and 1,156°C at one atmosphere using a synthetic leucitite melt and discs of natural peridotite. Our results show that sieve texture development on clinopyroxene and spinel in mantle xenoliths is the result of a multistage reaction process. In the first step, orthopyroxene undergoes incongruent dissolution to produce a silica and alkali-rich melt together with olivine. As this melt migrates along grain boundaries it causes incongruent dissolution of clinopyroxene and spinel. The incongruent dissolution mechanism involves complete dissolution of the clinopyroxene or spinel followed by nucleation and growth of a secondary clinopyroxene or spinel once the reacting melt is saturated. The reaction of orthopyroxene, clinopyroxene and spinel with infiltrated host magma results in a range of melt compositions that are very similar to those interpreted to be due to very small degrees of partial melting. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Harrat Al-Birk basalts in southwest Saudi Arabia: characteristic alkali mafic magmatism related to Red Sea rifting

Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~5 Ma. This volcanic field is almost monotonous and is dominated by basalts that include mafic–ultramafic mantle xenoliths (gabbro, websterite, and garnet-clinopyroxenite). The present work presents the first detailed petrographic and geochemical notes about the basalts. They comprise vesicular basalt, porphyritic basalt, and flow-textured basalt, in addition to red and black scoria. Geochemically, the volcanic rock varieties of the Harrat Al-Birk are low- to medium-Ti, sodic-alkaline olivine basalts with an enriched oceanic island signature but extruded in a within-plate environment. There is evidence of formation by partial melting with a sort of crystal fractionation dominated by clinopyroxene and Fe–Ti oxides. The latter have abundant titanomagnetite and lesser ilmenite. There is a remarkable enrichment of light rare earth elements and depletion in Ba, Th and K, Ta, and Ti. The geochemical data in this work suggest Harrat Al-Birk basalts represent products of water-saturated melt that was silica undersaturated. This melt was brought to the surface through partial melting of asthenospheric upper mantle that produced enriched oceanic island basalts. Such partial melting is the result of subducted continental mantle lithosphere with considerable mantle metasomatism of subducted oceanic lithosphere that might contain hydrous phases in its peridotites. The fractional crystallization process was controlled by significant separation of clinopyroxene followed by amphiboles and Fe–Ti oxides, particularly ilmenite. Accordingly, the Harrat Al-Birk alkali basalts underwent crystal fractionation that is completely absent in the exotic mantle xenoliths (e.g. Nemeth et al. in The Pleistocene Jabal Akwa Al Yamaniah maar/tuff ring-scoria cone complex as an analogy for future phreatomagmatic to magmatic explosive eruption scenarios in the Jizan Region, SW Saudi Arabia 2014).     

Compression melting in subduction zones

Magmatism at convergent plate boundaries is related to processes associated with subduction, where H₂O brought into the mantle is thought to play an essential role in producing the melts. One of the important aspects of the melting of hydrous systems is that the rock packet can melt during compression corresponding to a gentle or negative dT/dP slope of the solidus curve at relatively low temperatures. Here, the effects of compression on batch and fractional melting of hydrous peridotites in the mantle wedge of subduction zones are estimated, taking account of the flow pattern, energy balance and phase equilibrium constraints. The results show that a significant amount of melt, comparable to the eruption rate on volcanic arcs, can be produced in the downward flow along the plate. Further studies on distribution and transportation of H₂O in the mantle wedge and melt segregation processes are required to estimate the importance of compression melting.     

Basic phase relations of polybaric batch, percolative and critical melting

Summary ?Partial melting of the mantle is polybaric which implies that the phase relations change during partial melting. In addition to the pressure the composition of the melt depends on the melting mode. Various melting models have been suggested. Here the basic phase relations of polybaric batch, percolative, and critical melting are considered, using a simple ternary system. The percolative melts are in equilibrium with their residua, but differ somewhat in composition from those of batch melting. Critical melting is a fractional type of melting where the residuum contain interstitial melt. The critical melts differ in composition from batch melts. The linear trends of peridotites from ophiolites show that the extracted melts had nearly constant compositions, and therefore were extracted within a small pressure interval. A comparison between the trends of mantle peridotite and experimental batch melts suggests strongly that the melt extracted from the peridotites are in equilibrium with their residua. This could suggest that either batch or percolative melting are relevant melting modes for the mantle. However, isotopic disequilibria favor instead a critical mode of melting. This inconsistency can be avoided if the ascending melts are accumulated within a source region and equilibrate with the residuum before the melt is extracted from the source region. The evidence for equilibrium suggests that multisaturation of tholeiitic compositions in PT-diagrams is relevant for estimating pressure and temperature of generation of primary tholeiitic magmas. Received September 2, 2001; revised version accepted March 20, 2002     

Major and trace elements of abyssal peridotites: evidence for melt refertilization beneath the ultraslow-spreading Southwest Indian Ridge (53° E segment)

This study examines the geochemistry of major and trace elements of abyssal peridotites from the Southwest Indian Ridge (SWIR) (53° E amagmatic segment), to determine the influence of mafic melts on mantle peridotites during melt extraction. The results show a great geochemical variability in the ~90 km-long ridge segment, with a degree of mantle melting ranging from 4% to 24%. An ancient melting event may explain the presence of highly depleted peridotites at the ultraslow-spreading ridge. The 53° E segment peridotites show enrichment of light rare earth elements (LREEs) (average La_N/Sm_N = 1.87) and significant positive anomaly of U and Pb normalized to primitive mantle (PM). The positive correlations between LREEs (La, Ce, Pr, Nd) and high field strength elements (HFSEs; e.g. Nb and Zr) suggest that the enrichment of LREEs is caused by melt refertilization, which is also supported by prevalent magmatic microstructures in the peridotites. The melt refertilization model shows that the addition of 0.02–2.7% basaltic melts to peridotites can be responsible for the LREE enrichment. We suggest that the positive anomaly of U is probably attributed to fluid alteration whereas the enrichment of Pb is probably attributed to both melt refertilization and fluid alteration. Melt refertilization in the 53° E segment peridotites may be a result of melt–rock reaction and crystallization of melts trapped in peridotites. These processes may be enhanced by increased melt permeability in the mantle owing to the refractory peridotites produced by ancient melting and the decreasing efficiency of melt extraction in the cold and thick lithosphere at the 53° E ridge segment. The presence of melt refertilization implies that melt extraction is incomplete in the ridge mantle, which may be one of the reasons for the extremely thin and irregular variation of the crustal thickness at ultraslow-spreading ridges.