Peridotite Matrix Research Articles

Thermodynamics of pressure release melting of a veined plum pudding mantle

This study explores the thermodynamics of adiabatic decompression melting of peridotitic mantle containing pyroxenite veins that have lower solidi than the peridotite. When a vein of lower solidus‐temperature material melts adjacent to more refractory material, additional heat will flow into the melting region to increase its melting productivity. If pyroxenite veins have a solidus‐depletion gradient ((∂Tm/∂F)P) like that of olivine or peridotite, then the melting of the veins is enhanced by up to a factor of 4 by this heat. However, the solidus‐depletion gradient of pyroxenites is apparently lower than that of peridotites; thus pyroxenite melting would be even more enhanced. If pyroxenite veins have a gentler solidus‐pressure (T‐P) dependence (i.e., lower (∂Tm/∂P)F) than that of peridotite solidi, then although these veins will experience enhanced melting while they are the only melting assemblage, they will stop melting soon after their peridotite matrix begins to melt. During large‐scale peridotite melting the material ascends along a T‐P path close to that of the peridotite solidus, so that the mixture's temperature remains lower than the solidus of the residual pyroxenite, and pyroxenite melting ceases throughout the shallower sections of the melting column. If pyroxenitic material makes up a large fraction of the mantle mixture (∼20%), then the heat consumed by deep pyroxenite melting cools the ascending mantle mixture enough so that peridotite melting begins ∼5–10 km shallower than it would in the absence of precurser pyroxenite melting. After recycling into the mantle, the melt extraction residue will again melt if it is reheated to ambient mantle temperatures and rises again to shallow depths.

Evolution of Heterogeneous Lithospheric Mantle in a Plume Environment Beneath the Kerguelen Archipelago

A combined petrographic, geochemical and Sr–Nd–Pb isotopic investigation of peridotite xenoliths from the Kerguelen Archipelago (southern Indian Ocean) provides new insights into melt migration mechanisms and the sources of heterogeneities in the mantle associated with the long-lived (∼115 my) Kerguelen mantle plume. Large variations of trace element concentrations in clinopyroxenes and their isotopic compositions reflect the strong imprint of complex, multi-stage metasomatic episodes during evolution of the lithospheric mantle under the Kerguelen Archipelago. Two metasomatic agents have been identified that have interacted with the mantle peridotite matrix: (1) a basaltic melt, and (2) a carbonatitic melt that produced extremely high and variable incompatible element abundances in clinopyroxenes, which are attributed to chromatographic effects associated with metasomatic melt transport by porous flow through the mantle. Isotopic compositions of 12 peridotite xenoliths indicate that both types of metasomatic melts are related to the alkaline magmatism produced by the Kerguelen plume. In contrast, isotopic data from a single dunite xenolith indicate the strong influence of a continental lithospheric component, probably derived from Gondwanaland, that either forms part of the Kerguelen Plateau or was incorporated into the mantle beneath Kerguelen and mixed with plume-derivedmaterial. Our geochemical study of Kerguelen xenoliths testifiesto the importance of plumes as mechanisms for producing metasomatic melts with highly variable compositions and for entraining different components that may act as contaminants for eruptedlavas.

Evolution of Heterogeneous Lithospheric Mantle in a Plume Environment Beneath the Kerguelen Archipelago

A combined petrographic, geochemical and Sr‐Nd‐Pb isotopic graphy; metasomatized xenoliths investigation of peridotite xenoliths from the Kerguelen Archipelago (southern Indian Ocean) provides new insights into melt migration mechanisms and the sources of heterogeneities in the mantle associated with the long-lived (~115 my) Kerguelen mantle plume. Large variations of trace element concentrations in clinopyroxenes and their INTRODUCTION isotopic compositions reflect the strong imprint of complex, multi- It is now well accepted that the Earth’s mantle behaves stage metasomatic episodes during evolution of the lithospheric mantle as a viscous fluid over geologic time-scales. The variation under the Kerguelen Archipelago. Two metasomatic agents have been and amplitude of mantle geochemical heterogeneities identified that have interacted with the mantle peridotite matrix: (1) reflect the eYciency of heat and mass transfer processes a basaltic melt, and (2) a carbonatitic melt that produced extremely as well as the composition of entrained components (e.g. high and variable incompatible element abundances in clinopyroxenes, Hauri et al., 1994). The heterogeneous nature of the which are attributed to chromatographic eVects associated with convecting mantle has been delineated through the geometasomatic melt transport by porous flow through the mantle. chemical study of (1) mid-ocean ridge basalts (MORB) Isotopic compositions of 12 peridotite xenoliths indicate that both and oceanic island basalts (OIB) (e.g. Zindler & Hart, types of metasomatic melts are related to the alkaline magmatism 1986; Hofmann, 1997), and (2) mantle xenoliths, typically produced by the Kerguelen plume. In contrast, isotopic data from a characterized by a much greater geochemical variability single dunite xenolith indicate the strong influence of a continental than their host and related basalts (e.g. Hauri et al., lithospheric component, probably derived from Gondwanaland, that 1993). Mantle plumes represent the main mechanism for either forms part of the Kerguelen Plateau or was incorporated into entrainment in the mantle (Hart et al., 1992; Hauri et al., the mantle beneath Kerguelen and mixed with plume-derived 1994). material. Our geochemical study of Kerguelen xenoliths testifies The study of mantle xenoliths associated with ocean to the importance of plumes as mechanisms for producing meta- island hotspot volcanism can provide important insomatic melts with highly variable compositions and for entraining formation on melt extraction processes and sources of diVerent components that may act as contaminants for erupted heterogeneities in the mantle. The main purpose of this paper is to address the formation of compositional

Formation of diamond with mineral inclusions of “mixed” eclogite and peridotite paragenesis

Two unusual diamonds were studied from kimberlites from China, which contain both ultramafic and eclogitic mineral inclusions in the same diamond hosts. Diamond L32 contains seven Fe-rich garnets, four omphacites and one olivine inclusion. Four olivine, one sanidine and one coesite were recovered from diamond S32. Both garnet and omphacite inclusions have similar compositions as those from other localities of the world, and show basaltic bulk composition. All the garnet and omphacite inclusions in diamond L32 have positive Eu anomalies (Eu/Eu *1.64 ∼ 1.79). These observations support the proposal that mantle eclogite is the metamorphic product of subducted ancient oceanic crust. The Mg/(Mg + Fe) ratio of the olivine inclusions from the two diamonds (91-92) are evidently lower than the normal olivine inclusions in diamonds from the same kimberlite pipe (92-95). The following model is proposed for the formation of diamonds with “mixed” mineral inclusions. Ascending diamond-bearing eclogite (recycled oceanic crust) entrained in mantle plumes may experience extensive partial melting, whereas the ambient peridotite matrix remains subsolidus in the diamond stable field. This provides a mechanism for the transport of diamond from its original eclogitic host to an ultramafic one. Subsequent re-growth of diamond in the new environment makes it possible to capture mineral inclusions of different lithological suites. Partial melts of basaltic sources may interact with the surrounding peridotite, resulting in the relatively lower Mg/(Mg + Fe) ratios of the coexisting olivine inclusions from the studied diamonds. Diamonds with “mixed” mineral inclusions demonstrate that plume activity also occurred in the Archean cratons.

The mode of emplacement of the Honningsvåg Intrusive Suite, Magerøya, northern Norway

The Honningsvåg Intrusive Suite consists of several layered mafic/ultramafic intrusions and a transgressive body of igneous breccia that appears to represent a magma conduit. It is emplaced into a Silurian, flysch-type sedimentary sequence that is thermally metamorphosed to spotted slate, cordierite–andalusite or pyroxene hornfels and agmatitic migmatite. Folds and flattened reduction spots in the hornfelses suggest that emplacement took place after Caledonian deformation and development of a slaty cleavage. Tectonic rotation subsequent to emplacement has led to exposure of the Honningsvåg Intrusive Suite in a natural cross-section corresponding to ∼10 km of crustal depth. Basaltic magma was initially emplaced as a several-kilometre-tall pipe that crystallized to form Intrusion 1. A second magma chamber was initiated alongside this pipe and subsequently expanded laterally into a sill-like magma body as batches of olivine-saturated basalt were added. A later magma chamber, represented by Intrusion 4, developed largely within the cumulates forming the upper part of Intrusion 2 and appears to have been accompanied by opening of a broad inclined feeder into which blocks and slabs of older cumulates collapsed. The resulting igneous breccias of Intrusion 3 are chaotic and largely clast-dominated in the lower part of the conduit, but enclosed slabs are matrix supported and orientated parallel to an originally subhorizontal banding in the feldspathic peridotite matrix in the upper part. The core of the breccia body has a troctolite matrix and contains blocks of older breccia, suggesting re-opening of the conduit, either during the crystallization of Intrusion 4 or possibly during the development of chambers represented by the younger layered intrusions. The cumulates in Intrusion 4 subsided sufficiently to invert marginal parts of the Layered Series before a further magma chamber was initiated in its roof rocks. The last major magma chamber opened alongside Intrusion 5 and extended upwards as a pipe or broad dyke to the highest structural levels exposed. Cross-cutting relationships show that the Honningsvåg magma chambers were not active simultaneously but were emplaced sequentially, generally at successively higher structural levels. Olivine tholeiite magma initially pooled in a crustal zone where it had neutral buoyancy. Subsequent chambers are suggested to have been initiated by emplacement of magma along the density discontinuities that existed above and around crystallized intrusions and their associated hornfelses. Chambers evolved by fractional crystallization, assimilation of country rocks and periodic replenishment. The abandonment of magma chambers may have resulted from the expulsion of low-density residual melts.

Melt-Peridotite Interactions in Shallow Mantle at the East Pacific Rise: Evidence from ODP Site 895 (Hess Deep)

AbstractOcean Drilling Program (ODP) Leg 147 recently drilled at Site 895 in Hess Deep (eastern Equatorial Pacific), where a structurally dissected section of the East Pacific Rise (EPR) is preserved, and intersected the mantle-crust transition zone of a fast-spreading centre for the first time. The core from Hole 895D (latitude 2°16.635′N, longitude 101°26.777′W) revealed that harzburgite is predominant over dunite in the top section of the Hole, but the reverse relationship is found lower in the section where dunite is closely associated with gabbroic rocks (gabbro and troctolite). Texture, mineralogy and mineral chemistry suggest a two-stage evolution for harzburgite preserved at the transition zone. Harzburgite with a porphyroclastic texture was produced by partial melting of peridotite to, or beyond the clinopyroxene-out phase boundary before or during asthenospheric (>1000°C) flow, which suggests a higher degree of mantle melting than normally expected below mid-ocean ridges. Subsequently, basaltic melt(s) interacted with this refractory harzburgite (olivine + orthopyroxene + spinel), which resulted in dissolution of orthopyroxene, re-equilibration and formation of olivine and spinel, and formation of clinopyroxene ± plagioclase, this is manifested as a progressive conversion of harzburgite to gabbroic rock through an intermediate dunite. At low melt/peridotite ratios, harzburgite was refertilised as the plagioclase component of the melt completely reacted with the peridotite matrix to produce clinopyroxene-spinel intergrowths and Al enrichment in ferromagnesian minerals. At high ratios, orthopyroxene completely dissolved incongruently, plagioclase appeared, and spinel was partially to completely resorbed; this produced olivine-bearing and olivine-free gabbroic rocks. Residual minerals in peridotites adjacent to gabbroic zones were enriched in Fe and Ti and depleted in Al.

The relationship between websterite and peridotite in the Balmuccia peridotite massif (NW Italy) as revealed by trace element variations in clinopyroxene

Clinopyroxenes from peridotite, Cr-diopside and Al-augite websterites of the Balmuccia massif have been analyzed by ion-probe for REE, Sr, Zr, Ti, V, Y, Sc and Cr. In the peridotites, the REE pattern of clinopyroxene varies from markedly LREE depleted [(Ce/Yb)n=0.03] to moderately LREE enriched [(Ce/Yb)n=1.96]. Clinopyroxenes of the Cr-Diopside suite vary from moderately LREE depleted to enriched [(Ce/Yb)n=0.53–1.36], but HREE are always depleted with respect to MREE [(Sm/Yb)n=1.19–2.06]. The clinopyroxenes of the Al-Augite dikes have higher trace element concentrations and are relatively depeleted in LREE. Two types of peridotite to websterite transitions have been documented. One type characterizes the transitions from peridotite to Cr-diopside websterite. The depletion of peridotite increases, and cpx “fertility” decreases, towards the contact; i.e. REE from Sm-Eu to Yb and Ti, V, Y, Sc and Cr in cpx define abundance trends consistent with melt extraction. LREE, Sr, and perhaps Zr, on the contrary, increase in the contact region over a distance of 0–40 cm. The variation of the latter elements and of CaO and Al2O3 from peridotite to websterite is smooth, whereas it is steep for the other elements. Alumina, HREE, Y and Sc are lower, Ca is higher, and the MREE/HREE pattern is steeper in the clinopyroxene of the websterite with respect to peridotite. In the second type of transition, as the contact is approached, there is an increase in modal cpx and decrease of the opx/cpx ratio. Cpx trace element patterns are similar in peridotite and websterite. This transition is generally at the Al-augite websterite contacts, but also was found at two Cr-diopside contacts. The wall rock depletion of the first transition type is interpreted as caused by melt extraction induced by intrusion of melts derived from deeper mantle (garnet-facies), whereas the LREE and Sr variation is interpreted in terms of diffusive redistribution of trace elements after emplacement of the dikes and after the modal clinopyroxene has been removed from the peridotite. Diffusive exchange occurred under stagnant conditions but in the presence of a partial melt or a “metamorphic fluid”, rather than under dry subsolidus conditions. There is no conclusive evidence favouring either a partial melt or an aqueous fluid as a medium for diffusive transport. However, we favour transport via a fluid phase because the presence of such a fluid is indicated by ubiquitous traces of amphibole in the peridotite. The second transition type is interpreted in terms of melt infiltration and re-equilibration of the melt with the peridotite matrix.

Differences between oceanic basalts by multitrace element ratio topology

Trace element multidimensional analysis of ocean island basalts defines a structure delimited by the same four end members (Hawaiian Islands, St. Helena-Tubuai-Mangaia Islands, Kerguelen-Gough-Tristan da Cunha and the Society Islands) as determined by isotope ratios. In contrast to the results obtained for the distribution of isotopic ratios, the dispersions of trace element concentrations in mid-ocean ridge basalts are greater than those for ocean island basalts. This can be accounted for by a two-component mantle source composed of pyroxenite layers embedded in a peridotitic matrix, which melts to varying degrees; ocean island basalts are produced by a relatively uniform low degree of melting of the pyroxenite and limited isotopic exchange with the surrounding matrix, whereas mid-ocean ridge basalts are melts of both components with higher and more variable extents of melting and complete isotopic exchange between the pyroxenite strips and the peridotitic matrix.

Metasomatism-induced Melting in Mantle Xenoliths from Mongolia

Mantle xenoliths from two locations in Mongolia contain patches of glass-phenocryst aggregates (‘melt pockets’) up to 1 cm in diameter, including one ‘composite’ xenolith, which shows a complete transition from unaltered spinel Iherzolite to a zone containing melt pockets surrounded by a cpx and spinel-free peridotite matrix. We have analyzed major elements by wet chemistry, X-ray fluorescence (XRF), and electron microprobe, trace elements by ion microprobe and inductively coupled plasma mass spectrometry (ICP-MS) techniques, and Sr and Nd isotopes by mass spectrometry, to elucidate the origin of these melt pockets. Petrographic and chemical evidence shows that the melt pockets were formed neither by infiltration of the host basalt nor by dehydration melting of hydrous phases, such as amphibole. Instead, melting was induced by the interaction of a metasomatic fluid with clinopyroxene and spinel. The reaction produced melts of variable composition, with SiO2 ranging from 52 to 68% and MgO from 4.5 to 0.5%. The melts contain euhedral grains of olivine, clinopyroxene, and spinel, and a large number of (now empty) vugs. The melt shows no sign of having invaded the Iherzolite matrix surrounding the pockets. There is some evidence for fractional crystallization, but some of the major element chemical trends, such as the negative correlation between Na2O and SiO2, cannot be accounted for by such a mechanism. The glasses and clinopyroxene phenocrysts are very rich in light rare earth elements (LREE) and Sr, and completely dominate the bulk contents of these and some other incompatible elements in the rocks with melt pockets. The invading fluid introduced high concentrations of LREE, Th, U, Pb, and Sr, but was relatively depleted in Ba, Rb, Nb, Ta, Zr, Hf, and Ti, and had unusually high Zr/Hf and Nb/Ta ratios. Ion microprobe analyses of fresh glass directly adjacent to clinopyroxene microphenocrysts yield a series of cpx-melt partition coefficients for REE and several other trace elements. DYb (cpx-melt) varies between 0–3 and 1.6 and is positively correlated with the A12O3+SiO2 and Na2O contents of the glass, and negatively correlated with MgO, FeO, and CaO contents. These correlations are consistent with qualitative predictions from considerations of silicate melt structure. The clinopyroxenes in the unaltered zones of the composite xenolith show evidence of an earlier phase of metasomatism which enriched Ce, La, and Sr, but did not affect the other REE. Clinopyroxenes from these zones have high εNd values of + 14 and +19, indicating a history of low Nd/Sm ratios. At the same time, 87Sr/86Sr ratios are high (>0.704), indicating infiltration of relatively radiogenic Sr during the early stage of metasomatism. Ion microprobe traverses show no zoning of La/Nd ratios. Therefore, there was enough time to equilibrate the metasomatic effects in the grain interiors, and we estimate the time required for this equilibration to be of the order of 105 years. In sharp contrast, the second, or main, metasomatic event that caused the formation of the melt pockets must have been extremely short-lived and probably lasted only hours or days before the xenolith was captured by the magma and erupted at the surface. This short duration is required by the preservation of fresh glass and by the lack of equilibration of the melt pockets with their surrounding matrix. The isotopic compositions of Sr and Nd are identical between melt pockets and host basalts in both localities. Therefore, we conclude that the metasomatic fluids were probably derived from the same source rocks as the host basalts. We speculate that the xenoliths originally resided in an upper-mantle region which was intruded by a partially molten diapir. Volatiles were expelled from the unmelted margin of the diapir and invaded the adjacent upper-mantle peridotites. The fluid infiltration triggered formation of the melt pockets, whereupon the material was picked up by rapidly ascending magma and erupted at the surface. The fluids appear to have been poor in water, as no hydrous minerals are present among phenocryst or quench phases in melt pockets. The major component of the fluid may have been CO2 or liquid carbonate.

Carbonatite Metasomatism and Melting of the Arabian Lithosphere: Evidence from Oxygen Isotopes and Trace Element Composition of Spinel Lherzolites

The mineralogy and chemistry of many mantle xenoliths provide evidence for fluid-peridotite interaction leading to modification of the composition of primary minerals or crystallization of new metasomatic minerals. The isotopic and chemical composition of the different peridotite minerals can be used to study the nature and composition of the fluid circulating in the mantle and the nature of the interactions between these fluids and the peridotitic matrix. We have undertaken an integrated study of mantle xenoliths from Yemen, Arabian Peninsula, in order to constrain fluid-peridotite interaction using the oxygen isotope composition as well as the major and trace element composition of the constituent minerals.

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.

Modelling of Trace Element Transfer between Mantle Melt and Heterogranular Peridotite Matrix

Modelling of Trace Element Transfer between Mantle Melt and Heterogranular Peridotite Matrix

Mechanisms of Mantle Metasomatism: Geochemical Evidence from the Lherz Orogenic Peridotite

Detailed study of a 65 cm harzburgite section perpendicular to an amphibole pyroxenite vein from the Lherz massif reveals a strong mineralogical and chemical zonation with distance from the vein-host boundary. At less than ∽ 20 cm, the host peridotite is modally metasomatized and displays patterns of increasing Fe, Ti, Mn, Al, Ca, Na, and HREE, and decreasing Mg and Ni toward the vein contact This zone is also relatively impoverished in Cr but is enriched in K and Sr. It is characterized by relatively unfractionated, mainly convex-upward, chondrite-normalized REE patterns. At a distance over ∽ 20 cm, the host peridotite displays the typical feature of cryptic metasomatism, i.e., selective LREE enrichment in otherwise anhydrous mineralogy. The chondrite-normalized REE patterns vary from U-shaped in the range 15–25 cm to strongly fractionated in the range 25–65 cm. These variations encompass the whole range reported from metasomatized peridotite nodules in alkali basalts. They may be accounted for by a single, silicate-melt, metasomatic event associated with infiltration of the Pyrenean alkali basalts into the most refractory peridotites, during their ascent through the subcontinental lithosphere, ∽ 100 Ma ago. The proposed model involves a chemical evolution of the infiltrated melt with increasing distance in the host. At < 15–20 cm, the melt composition would be strongly influenced by the proximity of the vein conduit, because of the existence of advective chemical fluxes through grain boundaries (short-range porous flow; distance of percolation < 1 m) and into small branching cracks, and a possibly dominant diffusive flux within the infiltrated melt. This may explain the reactivity of the melt towards the anhydrous peridotite mineralogy, the existence of chemical gradients for most elements, and the lack of REE chromato-graphic fractionation. At a distance of > 20–25 cm, chemical exchange with the conduit would be negligible and the melt composition would be mainly controlled by re-equilibration of the peridotite matrix during long-range (> 1 m) porous flow percolation. Thus the melt would be buffered by the amphibole peridotite mineralogy, except for LREE. This may explain the lack of mineralogical reaction and chemical enrichments (except for REE) in this zone, and the chromatographic fractionation of REE. We propose a quantitative model of diffusion and percolation-controlled metasomatism associated with infiltration of alkali basalts into peridotites hosting vein-conduits. We also suggest that silicate-melt percolation may explain mineral disequilibrium features observed in mantle xenoliths.

Mixing and the distribution of heterogeneities in a chaotically convecting mantle

Chemical heterogeneities are introduced into the mantle by subduction of oceanic lithosphere and possibly by subduction of sediments and delamination of the continental lithosphere. These heterogeneities are blended into the mantle by convective mixing. Chemically distinct heterogeneities are stirred into the surrounding mantle matrix by convective shear, which deforms them, increasing the area of contact between an heterogeneity and the matrix. Final homogenization is accomplished by diffusion, which brings centimetersized heterogeneities into chemical equilibrium with the matrix. Numerical models of mantle convection suggest that it is chaotic. To study the effect of a chaotic flow on mixing, we have generated a space‐filling, two‐dimensional flow using the Lorenz equations in the chaotic regime as a driver. We specify a two‐mode expansion of the stream function, resulting in a flow which oscillates smoothly between one and two cells. Particle paths are chaotic in time and space. Mixing is very rapid; the mixing time for heterogeneities with an initial size of 6 km is 240 m.y. for layered mantle convection; the corresponding mixing time for whole mantle convection is 960 m.y. The source region for mid‐ocean ridge basalts has a “marble cake” structure; it is made up of material which has been processed through ridges, subducted, stretched, and thinned and is either partially or wholly homogenized. The incompletely mixed subducted oceanic crust forms bands of eclogite in the peridotite matrix. These bands range in scale from 6 km to a few centimeters.

Further Sr and Nd isotopic results from peridotites of the Ronda Ultramafic Complex

Clinopyroxenes derived from peridotites of the spinel and garnet facies of the Ronda Ultramafic Complex yield Sr and Nd isotopic ratios which extend the range of compositions found in the massif to values as depleted as 0.70205 for Sr and 0.51363 for Nd. Large-amplitude, short-wavelength isotopic variations are found to be ubiquitous throughout the massif. In the garnet facies, some of these variations are shown to be produced by the tectonic disaggregation of mafic layers in an isotopically depleted peridotite matrix. Ages obtained from garnet-clinopyroxene Sm Nd isochrons (about 22 m.y.) agree with previous determinations of the time of crustal emplacement. In the plagioclase facies, where the Sr and Nd isotopic compositions have been very strongly affected by recent cryptic metasomatism, detailed study of one sample reveals that intermineral Nd isotopic equilibrium exists between clinopyroxene, orthopyroxene, and plagioclase. This indicates that the metasomatism occurred at high temperatures, and thus probably within the mantle. A rough correlation between 143Nd/ 144Nd and 147Sm/ 144N , with an apparent “age” of 1.3 b.y. and an initial ε Nd (0) value of +6.0, is observed among clinopyroxenes derived from river sediments from throughout the massif. This age is interpreted as the time that the massif left the convecting mantle and became incorporated into the sub-continental lithosphere.

Disequilibrium mantle melting

The attainment of equilibrium between basalt and residual peridotite depends on the relative magnitudes of melt segregation rate and solid residue re-equilibration rate. Garnets and pyroxenes (and perhaps spinels) require at least 10 3 years to equilibrate with melt by volume inter-diffusion. Percolation velocities of melt in porous (1–10%) mantle rocks are of the order of 10 −4 to 10 −8 cm/s. If the melt moves by homogeneous percolation, then a column of peridotite matrix several kilometers in height must interact with the melt for an equilibrium partitioning of elements to be achieved. If melt segregation also involves flow through dykes, as seems likely, then the time available for equilibration is even shorter, and disequilibration melting is to be expected. By varying the time melt and residues remain in contact, variable equilibration will ensue. This can produce crossing REE profiles in basalts derived by identical degrees of melting from identical sources. Basalt-depleted residues from disequilibrium melting have fractionated REE profiles. This provides as alternative to “cryptic” metasomatism for the enigmatic decoupling of major and trace elements that is observed in many mantle peridotite nodules.

Metasomatism and partial melting in a subduction complex Catalina Schist, southern California

Partial melting accompanied high-pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the Catalina Schist, a subduction zone metamorphic terrane. Migmatitic blocks contain leucocratic zones, veins, and pods with albitic plagioclase ? quartz ? muscovite. Mineralogically similar dikes and veins cut the ultra-mafic matrix; a few veins can be traced back into migmatitic blocks. Uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts. Field relations, mineral and bulk compositions, and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites. Element-partitioning, phase-equilibrium, and fluid-inclusion data are compatible with partial melting of the amphibolites at P = ∼8–11 kbar and T = ∼640–750 °C, in the presence of a low-salinity aqueous fluid. Although the Catalina metamorphism occurred at much shallower levels than those inferred for subduction-related magmatism, the process envisioned is one of multistage, slab-derived metasomatic contamination of the mantle in the “hanging wall” of a subduction zone.