Melt Conduits Research Articles

Podiform chromitites in the lherzolite‐dominant mantle section of the Isabela ophiolite, the Philippines

Abstract The Isabela ophiolite, the Philippines, is characterized by a lherzolite‐dominant mantle section, which was probably formed beneath a slow‐spreading mid‐ocean ridge. Several podiform chromitites occur in the mantle section and grade into harzburgite to lherzolite. The chromitites show massive, nodular, layered and disseminated textures. Clinopyroxene (±orthopyroxene/amphibole) inclusions within chromian spinel (chromite hereafter) are commonly found in the massive‐type chromitites. Large chromitites are found in relatively depleted harzburgite hosts having high‐Cr♯ (Cr/(Cr + Al) atomic ratio = ∼0.5) chromite. Light rare earth element (LREE) contents of clinopyroxenes in harzburgites near the chromitites are higher than those in lherzolite with low‐Cr♯ chromite, whereas heavy REE (HREE) contents of clinopyroxenes are lower in harzburgite than in lherzolite. The harzburgite near the chromitites is not a residual peridotite after simple melt extraction from lherzolite but is formed by open‐system melting (partial melting associated with influx of primitive basaltic melt of deeper origin). Clinopyroxene inclusions within chromite in chromitites exhibit convex‐shaped REE patterns with low HREE and high LREE (+Sr) abundances compared to the host peridotites. The chromitites were formed from a hybridized melt enriched with Cr, Si and incompatible elements (Na, LREE, Sr and H2O). The melt was produced by mixing of secondary melts after melt–rock interaction and the primitive basaltic melts in large melt conduits, probably coupled with a zone‐refining effect. The Cr♯ of chromites in the chromitites ranges from 0.65 to 0.75 and is similar to those of arc‐related magmas. The upper mantle section of the Isabela ophiolite was initially formed beneath a slow‐spreading mid‐ocean ridge, later introduced by arc‐related magmatisms in response to a switch in tectonic setting during its obduction at a convergent margin.

Lower continental crust formation through focused flow in km-scale melt conduits: The zoned ultramafic bodies of the Chilas Complex in the Kohistan island arc (NW Pakistan)

Abstract Whole-rock and Sm–Nd isotopic data of the main units of the Chilas zoned ultramafic bodies (Kohistan paleo-island arc, NW Pakistan) indicate that ultramafic rocks and gabbronorite sequences stem from a common magma. However, field observations rule out formation of both ultramafic and mafic sequences in terms of gravitational crystal settling in a large magma chamber. Contacts between ultramafic and gabbronorite sequences show emplacement of the dunitic bodies into a semi-consolidated gabbronoritic crystal-mush, which in turn has intruded and reacted with the ultramafic rocks to produce concentric zoning. Field and petrological observations indicate a replacive origin of the dunite. Bulk Mg#'s of dunitic rocks range from 0.87–0.81 indicating that the dunite-forming melt underwent substantial fractionation–differentiation and that percolative fractional crystallization probably generated the dunitic core. The REE chemistry of clinopyroxene in primitive dunite samples and the Nd isotopic composition of ultramafic rocks are in equilibrium with the surrounding gabbronorite. Accordingly, liquids that formed the dunitic rocks and later the mafic sequence derived from a similar depleted source (eNd∼4.8). We propose a mechanism for the comagmatic emplacement, where km-scale ultramafic bodies represent continuous channels reaching down into the upper mantle. The melt-filled porosity in these melt channels diminishes the mean-depth-integrated density difference to the surrounding rocks. Due to buoyancy forces, melt channels raise into the overlying crustal sequence. In the light of such processes, the ultramafic bodies are interpreted as melt channels through which the Chilas gabbronorite sequence was fed. The estimated basaltic–andesitic, low Mg# (∼0.53) bulk composition of the Chilas gabbronorite sequence closely matches estimates of lower crustal compositions. Since the mafic sequence originated from a primary, high Mg# (> 0.7) basaltic arc magma, differentiation of such high Mg# magmas within km-scale isolated melt conduits may explain the “Mg#-gap” between bulk estimates of the continental crust and primary basaltic magmas, a major paradox in the andesite model of crust formation.

Observations of Li isotopic variations in the Trinity Ophiolite: Evidence for isotopic fractionation by diffusion during mantle melting

The Trinity peridotite (northern CA) contains numerous lithologic sequences consisting of dunite to harzburgite to spinel lherzolite to plagioclase lherzolite. Previous workers have documented geochemical gradients in these sequences consistent with melt-rock reaction processes occurring around dunites, interpreted to reflect conduits for melt ascent. We have undertaken a study of Li isotope compositions of clinopyroxene and some olivine within these sequences using ion probe techniques to test the hypothesis that the geochemical gradients are related to diffusive fluxing of alkali elements into or away from the melt conduit. Results show large variations in 7Li/ 6Li occurring in a consistent pattern across three transects from dunite to plagioclase lherzolite within the Trinity peridotite. Specifically, measurements of average δ 7Li for single thin sections along the traverse reveal a low in δ 7Li in the harzburgite adjacent to the dunite returning to higher values farther from the dunite with a typical offset of ∼10 per mil in the low δ 7Li trough. This pattern is consistent with a process whereby Li isotopes are fractionated during diffusion through a melt either from the dunite conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit. The patterns in 7Li/ 6Li occur over a length scale similar to the decrease in REE concentration in these same samples. Explaining both the trace element and Li isotopic gradients requires a combined process of alkali diffusion and melt extraction. We develop a numerical model and examine several scenarios of the combined diffusion-extraction process. Using experimentally constrained values for the change in Li diffusion coefficient with isotope mass, large changes in δ 7Li as a function of distance can be created in year to decade timescales. The addition of the melt extraction term allows larger Li concentration gradients to be developed and thus produces larger isotopic fractionations than diffusion only models. The extraction aspect of the model can also account for the observed decrease in rare earth element concentrations across the transects.

Nature and distribution of dykes and related melt migration structures in the mantle section of the Oman ophiolite

We conducted a comprehensive field, petrographic, and microprobe study of the dykes and porous flow channels cropping out in the Oman harzburgites. The 36 rock types we recognized among of about 1000 samples can be grouped in two main magma suites contrasted in terms of structural and textural characteristics, modal composition, order of crystallization, and phase chemistry. One suite (troctolites, olivine gabbros, opx‐poor gabbronorites, and rare oxyde gabbros) derives from MORB‐like melts. The other suite (pyroxenites, opx‐rich gabbronorites, diorites, and tonalite‐trondhjemites) derives from melts richer in silica and water than MORBs and ultradepleted in incompatible elements. Dykes and porous flow channels from the MORB suite are restricted to a few areas, covering only 25% of the mantle section. This is an unexpected result as the deep Oman crust is made essentially of cumulates from MORB‐like melts. Their composition, texture, and relations with the host harzburgites point to high mantle temperatures at the time of crystallization (likely above 1100°C, up to 1200°C for part of them), i.e., conditions close to the “asthenosphere/lithosphere” boundary. The largest outcrop of mantle harzburgites enclosing MORB like dykes is a 80 km long and 10 km wide corridor, parallel to the strike of the sheeted dyke complex and centered on an area where a former mantle upwelling has been unambiguously defined (the Maqsad “diapir”). A few other occurrences of mantle cumulates from the MORB suite are smaller than the Maqsad area and have a lesser abundance of troctolites (i.e., of high‐temperature cumulates). We interpret the troctolite zones of Oman as the witnesses of former diapirs frozen at various stages of their development. Dykes belonging to the depleted suite are the most common in Oman harzburgites. Their structural and textural characteristics show that they crystallized in a mantle colder than the melt (likely in the range 600°C to 1100°C). A possible origin for the parent melts of this suite is in situ partial melting of the shallow and partly hydrated lithosphere residual after MORB extraction. Our data support the view that feeding magma chambers with MORBs is a focused (and likely episodic) process involving the rise of hot mantle to the base of the crust through a lithospheric lid accreted during a previous diapiric event. They suggest also that the shallow mantle beneath spreading centers is a place of important petrologic processes, some of them predicted on the basis of MORB composition (e.g., fractionation inside melt conduits) and other ones unexpected (e.g., remelting of the depleted lithosphere).

Dunite distribution in the Oman Ophiolite: Implications for melt flux through porous dunite conduits

Dunites in the mantle section of the Oman ophiolite represent conduits for chemically isolated melt transport through the shallow mantle beneath oceanic spreading centers. These dunite melt conduits exhibit a scale‐invariant power law relationship between width and cumulative abundance, as measured over 4 orders of magnitude. We use this size/frequency distribution to assess several hypotheses for dunite formation and estimate the total melt flux that a dunite network can accommodate beneath an oceanic spreading center. Dunites, measured from one‐dimensional lithologic sections and digital image mosaics at a variety of length scales, range in width from ∼3 mm to ∼100 m and follow a power law with a slope of ∼1.1. Extrapolation of the power law predicts that dunites as wide as 3.5 km may exist in the melting region beneath a mid‐ocean ridge. Alternatively, perhaps the widest dunites we observe (∼100 m) represent a maximum size. Modeling of dunites as diffusive reaction zones around melt‐filled hydrofractures cannot explain the existence of dunites wider than ∼10 m in Oman. Instead, dunites may represent high porosity conduits formed by reactive porous flow. Using the observed size/frequency relationship, the assumption that dunites form a coalescing network and the requirement that flux is conserved where dunites merge, we estimate the total flux through a porous dunite network and the fraction of that flux that remains chemically isolated. Our flux model predicts that the porosity in a dunite scales with the width. For maximum porosities of ∼1–4% in the widest dunites, a network of porous dunite conduits with the abundances observed in Oman can supply a sufficient flux of melt (of which > 95% remains chemically unequilibrated with shallow residual peridotites) to satisfy the observed mid‐ocean ridge flux.

Morphological and chemical variations of chromian spinel in dunite-harzburgite complexes from the Sangun zone (SW Japan): implications for mantle/melt reaction and chromitite formation processes

¶Many ultramafic complexes, some of which have chromitite bodies, are exposed in the Sangun zone in central Chugoku district, Southwest Japan. Harzburgite is always dominant over dunite, but the dunite/harzburgite ratio varies from complex to complex. Large chromitite bodies are exclusively found in relatively dunite-dominant complexes or portions. The degree of roundness, DR#=[area/(round-length)2] (normalized by a circle’s value: 1/4π), of chromian spinel is variable, depending on lithology of the peridotites. Chromian spinel is mostly anhedral or even vermicular (less than 0.4 in DR#) in harzburgite, and is most frequently euhedral or rounded (within the range of 0.7 to 0.9 in DR#) in dunite. The morphology of spinel is correlated with chemistry: the DR# is positively correlated with Ti content and Fe3+#(=Fe3+/(Cr + Al + Fe3+)), but is not related to Cr#. When chromitite is present in dunite, the spinel is relatively anhedral (vermicular) and low in Ti and Fe3+# in the dunite whereas it is relatively euhedral and high in Ti and Fe3+# in surrounding harzburgite. We define these spinels as “extraordinary” spinels, which are commonly found in Wakamatsu mine area in the Tari-Misaka complex, which exploits the largest chromite body in Japan. The rocks with the “extraordinary” spinels show transitional lithologies (a gradual boundary, one meter to several tens of meters in width) between dunite and harzburgite with “ordinary” spinels. The formation of dunite and chromitite is interpreted as a result of the reaction of harzburgite with a relatively Ti-rich magma (back-arc basin or MORB-like magma) and related magma mixing, as discussed by Arai and Yurimoto (1994). The dike-like occurrence of the dunite and chromitite indicates that the reaction took place along melt conduits (=fractures) less than 200 m in width. Podiform chromitites were formed only when the reaction zone was relatively wide (several tens of meters in width), that is, only when the degree of interaction was relatively high. The magma modified by the reaction percolated, possibly by porous flow from the reaction zone outward, and changed the texture and chemistry of chromian spinel, on the scale of several tens of meters. This type of melt transport, or melt flow through fractures with a melt percolation aureole, may be prevalent in the uppermost mantle.

Quantitative field methods for estimating melt production and melt loss

Abstract Melt extraction in migmatites occurs via melt channels of increasing width from source areas (often recognizable by the location of incongruent phases) via concordant leucosomes to discordant melt conduits. To test the contribution of migmatites to granite genesis, melt production and melt loss need to be quantified at specimen and outcrop scale. Melt production during dehydration melting can be modelled by calculating the volume ratio of melt and incongruent phases from balanced melting reactions. Melt loss can be quantified by (i) comparing these predicted volume ratios with ratios derived in outcrop; and (ii) modelling strain patterns near melt loss structures. A field test in SW Finland shows a reasonable correspondence between melt loss estimates from leucosome/garnet volume ratios and from melt loss structures, if the original shape of the melt patch is assumed to have been linear.

Study of the Hydrodynamic Parameters of Nonstationary Isoentropic Flow of a Viscous Polymer in the Melt Conduits of Melting and Molding Machines

A method was developed for calculating the characteristics of the flow rate and pressures of a viscous polymer based on the numerical control volume method, which allows considering the volume change in the form of flow and the force effect in it. The method was tested for standard MSM (MF-600-KSh24, MF-1000-KK18). The results obtained are sufficiently close to the process parameters of the machines. The method can be recommended for design calculations for melt conduits in both ordinary and in high-speed MSM. The pressure change in nonstandard situations of blocking of the melt conduit channel is similar to the phenomenon of hydrohammering and is subject to consideration in determining the wall thickness and selecting the construction materials for melt conduits, especially for high-output MSM. The dynamic flow characteristics indicate that the traditional structure of the automatic regulation system should be replaced by a compensation system, which will allow increasing the quality of the fibre in a wide range of MSM outputs.

Petrology of dunite/harzburgite with decimeter-scale stratification in a drill core from the Tari-Misaka ultramafic complex, southwestern Japan.

A drill core from Tari-Misaka ultramafic complex, southwestern Japan, shows a one meter interval in which a decimeter scale stratification of dunite/harzburgite is well preserved. This part was petrologicaly examined in detail. Special attention was focused on the morphology and chemistry of chromian spinel. Chromian spinel is more abundant, more euhedral and higher in Ti and Fe3+ contents, and slightly lower in Cr/(Cr+Al) ratio in dunite than in harzburgite. Harzburgite becomes orthopyroxene-poor, and its chromian spinel tends to be similar to dunite spinel near (usually within 5 cm) the lithological between dunite. It was presumed that the dunite was formed as veins as a result of a reaction between harzburgite and an exotic melt: olivine crystals was formed by a reaction between hartzbergite and the melt. The petrological and chemical features of this part of the drill is basically similar to those in dunite with large chromitite pods, and harzburgite widely distributed in the Wakamatsu mine area. It is suggested that the dunites within harzburgite have the same origin irrespective of their dimension and the presence of chromite pods or not. The lower Cr# of spinel in the drill-core rocks than those in widely distributed dunite is possibly due to the difference in amount of orthopyroxene reacted with the melt. A large amount of orthopyroxene reacted and turbulency of melt current may be indispensable for the formation of podiform chromitite. The large melt conduit represented by dunite of semi-regional distribution in the Wakamatsu mine area possibly fulfilled the all of the conditions for the formation through this process of chromitite pods.

Models of U-series disequilibria generation in MORB: the effects of two scales of melt porosity

Conflicting evidence exists about the extent of melt–mantle reaction as melt ascends beneath a mid-ocean ridge. On the one hand, uranium series (U-series) disequilibria in mid-ocean ridge basalts suggest that melts reactively flow towards the surface and continuously re-equilibrate with lower pressure mantle. On the other hand, trace elements in abyssal peridotites suggest fractional melting and isolation of ascending melt from surrounding lower pressure peridotite. Following a suggestion by Kelemen et al. (1997) [Kelemen, P.B., Hirth, G., Shimizu, N., Spiegelman, M., Dick, H.J.B., Philos. Trans. R. Soc. 355, 283–318], I construct a model for melting beneath mid-ocean ridges which consists of two materials having different melt porosity distributions in order to quantitatively assess whether the U-series and abyssal peridotite observations could actually be compatible. The melting column consists of a volumetrically dominant lherzolite which produces melt by decompression and a volumetrically minor dunite which serves as a conduit for fast melt ascent. A suction parameter S (after Iwamori (1994) [Earth Planet. Sci. Lett. 125, 1–16]) controls the transfer of melt between the lherzolite and dunite media. S can vary from 0 where none of the ascending melt flows through the dunite melt conduit (reactive porous flow melting) to 1 where the melt produced in the lherzolite is immediately transferred to the dunite (near-fractional melting). Transport times of U-series nuclides are accounted for in both porosity regimes based on calculated flux balances. Results show that the trace element patterns observed in abyssal peridotites require S to range from 0.5 to 0.8 depending on assumptions about the composition of the original source material. U-series models are evaluated in terms of three quantities: S, χ (the volumetric fraction of dunite), and a 2/ ηC which lumps together several constants relating to permeability. 226 Ra and 231 Pa excesses are most sensitive to a 2/ ηC (which affects the melt porosity within the melting region). 226 Ra excesses match the observed levels of 226 Ra excess in mid-ocean ridge basalts (MORB) at all values of S. 231 Pa excesses favor lower values of S as models with high S produce significantly less ( 231 Pa)/( 235 U) than that observed. There remains some disparity between the values of S required by the abyssal peridotite data and the observed 231 Pa excesses although further refinement of partition coefficients could resolve the difference. Within the models presented, the two geochemical observations are close to being reconciled if S=0.5 and the mantle permeability is at least 1×10 −14 m 2. However, a factor of two higher bulk partition coefficient for U ( D U=0.003) would allow the observed ( ( 231 Pa)/( 235 U) ) to be matched at S=0.5 while dropping the required bulk permeability to less than 1×10 −15 m 2.

Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites

Ophiolites are on‐land exposures of igneous crust and residual upper mantle formed beneath submarine spreading ridges. Upper mantle outcrops in ophiolites provide insight into focusing of melt transport from a ∼100 km wide region of partial melting into an ∼5 km wide zone of igneous crustal accretion beneath the ridges. Dunite veins, composed of the minerals olivine and spinel, mark conduits for melt transport through at least the uppermost 30 km of the mantle. New data in this paper, on dunite veins in the Ingalls ophiolite, central Washington Cascades, show a power law relationship between frequency and width, in which frequency/m ≈0.02 width−3 over a size interval from ∼0.1 to 2 m. There may be several ways to generate this relationship, but we favor the hypothesis that the dunites represent a coalescing melt transport network. This conclusion is broadly consistent with the related hypothesis that mantle melt extraction occurs in a fractal, branching network, and with recent results on formation of a coalescing network of dissolution channels via flow of a solvent through a partially soluble, compacting porous medium.

Polybaric Petrogenesis of Mafic Layers in the Horoman Peridotite Complex, Japan

Two major types of mafic granulite layers occur within the Horoman host peridotite (e.g. Nicolas & Jackson, 1982). [In this paper we use ‘mafic layers’ to describe pyroxenites to peridotite, an 8 km× 10 km× 3 km orogenic lherzolite exposed ‘gabbroic’ rocks (mafic granulites) whose composition in the high-T and low-P Hidaka metamorphic belt of Hokkaido, and mineral assemblage differs substantially from the Japan. The mineral assemblages and textures of these layers reflect host peridotites.] Hypotheses for the origin of these subsolidus reactions occurring during uplift from the upper mantle layers in orogenic peridotites include: (1) solidification to the crust. Nevertheless, their whole-rock compositions can be used of melt (Niida, 1984; Shiotani & Niida, 1997); (2) to infer the primary mineralogy of these layers, and a genetic crystal cumulates in melt conduits or sills (Loubet & relationship to melts geochemically similar to mid-ocean ridge basalts Allegre, 1982; Bodinier et al., 1987; Suen & Frey, (MORB). The intralayer compositional variation of Type I layers 1987; Bodinier, 1988; Pearson et al., 1993; Kumar et (Al–Ti augite type mafic granulites) shows that the centers formed al., 1996); (3) recycling of ancient subducted oceanic as garnet clinopyroxenites in equilibrium with an incompatible crust that was dispersed and thinned during mantle element depleted melt that crystallized to form the margins. In convection (Polve & Allegre, 1980; Allegre & Turcotte, contrast, the Type II layers (Cr-diopside type mafic granulites) 1986). Subsequent to their formation these layers may formed at relatively shallow depths and are much older, ~830 Ma, have experienced partial melting (Loubet & Allegre, than the Type I garnet pyroxenites, which formed at ~80 Ma. 1982) and interacted with metasomatic fluids (Garrido The temporal sequence supports the hypothesis that the Horoman & Bodinier, 1999). Ultimately, some mafic layers may peridotite represents shallow MORB-related oceanic mantle that be important source components for basalts (e.g. had subsided to deeper mantle depths before crustal emplacement. Hirschmann & Stolper, 1996). In addition to layers within massive peridotites, pyroxenites also occur as discrete xenoliths and as portions of composite pyroxenite–peridotite xenoliths

Constraints on a buoyant model for the formation of the axial topographic high on the East Pacific Rise

Bathymetry and gravity data from four geophysical surveys along the 15°–19°S superfast spreading section of the East Pacific Rise are used to study formation of the axial topographic high. After removing a best fitting thermal model the averaged, residual, axial topography is ∼265 m high and ∼15 km wide. The residual gravity shows no evidence for shallow isostatic compensation of this topography suggesting deep‐rooted compensation or dynamic support. Previous models postulate that the axial high is compensated by low densities in a zone of partial melting within the upper mantle. We model the physical properties of the lithosphere and asthenosphere required to match the residual gravity and bathymetry using a nonlinear inversion, assuming the high is uplifted by buoyancy forces from low‐density mantle. Deep‐rooted buoyancy forces cannot create a narrow axial high unless they occur in a low‐viscosity conduit surrounded by high‐viscosity mantle. This high‐viscosity region could be created by depletion and extraction of all water during the melting process. We find that the viscosity of the asthenosphere beneath the melt conduit must also be lower than the viscosity of the residual mantle surrounding the conduit. Our most reasonable model has a viscosity of 1020 Pa s in the residual mantle surrounding a melt conduit which extends 50 km below the seafloor and a viscosity of 1016.6 Pa s in the asthenosphere below the melt conduit. The viscosity of the melt conduit must be about 1015 Pa s, or about 105 times lower than the surrounding residual mantle, with a density contrast equivalent to the retention of a small fraction of melt (5%). Because higher melt concentrations are probably required to create even small changes in viscosity and the viscosity of the undepleted mantle is probably higher than 1016.6 Pa s, we conclude that it is unlikely that the axial topographic high is formed by buoyancy forces in the crust and mantle.

Melt conduits in a viscous porous matrix

Although melting is inferred to start at depths of 120 km or more in the mantle, little is known about the style of melt transport from the source region to the surface. Geochemical observations require the transport of melt from depth to be fast, and that melt is not in chemical equilibrium with the matrix through which it passes. We present a model which allows fast transport by an open melt conduit in a viscous permeable matrix and discuss the possibility of chemical isolation in such a conduit. We show that disturbances on the conduit surface propagate slowly upwards as solitary waves and cause melt to flow between the conduit and the surrounding porous matrix.

Compositional evolution of high-temperature sheared lherzolite PHN 1611

The evolution of “fertile” mantle has been studied by proton microprobe (PIXE) analysis of minerals of a high-temperature sheared xenolith from the Thaba Putsoa kimberlite in Lesotho, southern Africa. Analyzed elements include Ni, Cu, Zn, Ga, Sr, Y, and Zr. Garnets are homogeneous in Ni and Zn but have rims enriched relative to cores in Zr and Y. Compositions of olivine neoblasts define intergranular gradients of Fe, Zn, and Ni; Fe-rich olivine is relatively Zn-rich but Ni-poor. Although individual clinopyroxene grains are nearly homogeneous, clinopyroxene associated with Fe-rich olivine is relatively Fe- and Zn-rich but Sr- and Cr-poor. The trace-element abundances and compositional gradients constrain the processes of peridotite enrichment and the thermal history. Enrichment of Zr, Y, and Fe in garnet rims documents infiltration of a silica-undersaturated melt. The Fe-rich olivine compositions and the Zn and Fe gradients establish that the xenolith was sampled from near a melt conduit. Mechanical mixing of inhomogeneous peridotite and melt infiltration may have been concurrent. Because garnets appear homogeneous in Ni, mantle temperature changes affecting PHN 1611 occurred before or over a longer period than the melt infiltration. Measured and calculated abundances of many incompatible trace elements in the rock are similar to those proposed for primitive mantle. Calculated chondrite-normalized abundances of Sr, Ti, Zr, and Y are like those of appropriate REE. Enrichment processes in PHN 1611 proceeded at unusually high recorded temperature and in the apparent absence of minor phases common in lower-temperature metasomatized rocks, but similar processes may be common. In particular, mechanical mixing near mantle dikes may frequently occur. These enrichment mechanisms may produce xenolith compositions that resemble some proposed for primitive mantle but that have different implications for mantle evolution.