Howard G. Wilshire

Magma transport and metasomatism in the mantle: a critical review of current geochemical models

Abstract Navon et al. (1996) demonstrated that the Navon and Stolper (1987) model can be formulated to reproduce a pattern of light-ion lithophile trace element (LIL) enrichments produced by a single, small-scale metasomatic process recorded in a composite xenolith from Dish Hill, California (Nielson et al. 1993). The Navon and Stolper model has failed repeatedly to reproduce the shape and lateral positions of LIL enrichment patterns for samples from peridotite massifs, which are of appropriate scale to test the assumption that LIL fractionation takes place in percolating melts over distances > 100 m. The model results also produce unreasonably long times for solidification of thin dikes, which imply untenable thermal conditions for lithospheric mantle. Using parameters drawn from sample compositions, Nielson et al. (1993) demonstrated, and the calculations of Navon et al. (1996) have shown again, that fractionated trace element patterns of a melt are imprinted upon relatively refractory peridotite matrix in zones closest to a melt source. The observed process sequentially extracts LIL into matrix, analogous to the ion-exchange chromatography of water-purification columns. We have never contended that this process is mathematically distinct from the percolation model of Navon and Stolper (1987), which assumes concentration ofLIL elements in melt. The choice of parameters defines the result, and one would notice a major difference in the taste of water from an ion-exchange column that traps target ions in matrix compared with one that concentrates those ions in the liquid. The difference between the models is in the selection of parameters and values: The model ofNavon and Stolper (1987) assumes the reaction mechanism, uses theoretical melt compositions, and contains as many as nine unmeasurable parameters. We used the simplified model calculation to avoid reliance on theoretical parameters and to test our assumptions about the process. When the compositions of actual samples are taken as end-members of mantle reactions, the successful results imply that a fractionation-bypercolation process is not applicable to lithospheric mantle. Repetition of the observed small-scale reaction in refractory peridotite must extend the zone of reactions and relative enrichment, centimeter by centimeter, as long as melt aliquots percolate beyond peridotite matrix that had previously reacted to equilibrium with the melt composition. This process satisfactorily explains the wide variations of LIL fractionation patterns over short distances that characterize mantle rocks in xenoliths and massifs, all of which contain complex systems of mafic intrusions with varied LIL fractionation patterns.

Mantle metasomatism: The REE story

Refractory rocks with light REE/heavy REE ratios greater than chondrite are common as xenoliths in basalts and kimberlites and are found in some oceanic peridotite massifs. This has led to the supposition that large parts of the upper mantle have been metasomatically altered by addition of light REE and other hyperfusible constituents. Structural and major-element geochemical evidence from xenoliths and alpine peridotites, however, suggest that the metasomatic effects are local and are related to emplacement of partial melts. The melts are represented by dikes of pyroxenite, hydrous minerals, and gabbro that occur in mantle peridotites of all origins and were emplaced in them in the same sequence as indicated by crosscutting relations. REE distributions in both the peridotite and the dikes may be explained as a result of metasomatic interaction between dikes and peridotite wall rock in which the peridotite is enriched in light REE and the dikes depleted in light REE relative to the original liquid. Differentiation of the intrusions and separation of residual liquids may further reduce the light REE/heavy REE ratio in pyroxenite dikes; these residual liquids (hydrous mineral veins) enriched in light REE extend the volume of metasomatized peridotite as they too interact with their wall rocks. Differences in the relative abundances of altered peridotite (reacted wall rock) in xenoliths and massifs are seen as a sampling problem rather than a difference in process.

Pseudotachylite from the Vredefort Ring, South Africa, and the origins of some lunar breccias

Pseudotachylite veins in the Vredefort impact structure occur in rocks of widely varying composition, and in stratigraphic units as thin as 10 m. The compositions of the pseudotachylites, however, are the same as those of the rocks in which they occur, with some systematic variations, thus indicating in situ formation. The textures of the pseudotachylites indicate that they formed predominantly by mechanical processes, but temperatures were locally high enough to cause thermal metamorphism and possibly fusion. The host rocks of the pseudotachylites are extensively mylonitized and show selective crushing of different mineral species. Greater resistance of quartz and feldspar to crushing partly, but not entirely, explains the systematic differences in composition between host rock and pseudotachylite. Shock features in the host rocks are generally of a low order but indicate pressures far in excess of those reasonably to be expected in crustal rocks deformed by terrestrial processes. Characteristics of the pseudotachylites, breccias formed by one impact, provide simple models for lunar breccias that may have undergone more than one cycle of brecciation.

Upper-mantle diapirism: Evidence from analogous features in alpine peridotite and ultramafic inclusions in basalt

Ultramafic xenoliths in basalt from the western United States are dominantly spinel Iherzolite in which at least four types of gabbroic and pyroxene-rich bands have formed. These bands closely resemble compositional bands in some alpine peridotite, and we infer a similar mode of origin: partial fusion over a range of T-P conditions during diapiric rise of a part of the upper mantle. The sequence of formation of bands, established by crosscutting relationships, is appropriate to crystallization of the products of melting in spinel Iherzolite that has risen from a level of spinel- and garnet-pyroxenite stability in the upper mantle to the lower crust where gabbroids are stable. Both the textures and compositions of the bands have been modified by repeated deformation and partial melting.

Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridotite and anhydrous and hydrous pyroxenite, some cut by igneous-textured pyroxenite-gabbro veins and dikes and some by veins of amphibole ± plagioclase. Igneous-textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma-mantle wall rock reactions, episodic intrusion and infiltration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (eNd = −5.7 to +3.4; 87Sr/86Sr = 0.7051 – 0.7073; 206Pb/204Pb = 19.045 – 19.195) and the igneous-textured xenoliths (eNd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 – 0.7036 with one carbonate-affected outlier at 0.7054; and 206Pb/204Pb = 18.751 – 19.068), so that they cannot be related. The igneous-textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have eNd of +5.1 to +9.3; 87Sr/86Sr of 0.7028 – 0.7050, and 206Pb/204Pb of 18.685 – 21.050. The igneous-textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous-textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm-Nd two-mineral “isochrons” yield apparent ages for petrographically identical rocks believed to be coeval ranging from ∼0 to 113±26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr-diopside group isotopic compositions do not overlap with those of the host basalts.

Garnet clinopyroxenite xenolith from Dish Hill, California

Abstract A single garnet clinopyroxenite xenolith found at the Dish Hill basanite cone near Ludlow, California, has well developed unmixing and reaction textures like those found in garnet pyroxenite inclusions in Hawaiian, African and Australian basalts and like those of pyroxenites in some European alpine peridotites. Reconstructed pyroxene compositions suggest that before unmixing the rock consisted of clinopyroxene and about 10% garnet plus spinel, but all of the garnet may have been dissolved in clinopyroxene. Most or all of the garnet formed by exsolution from clinopyroxene and by reaction between clinopyroxene and spinel in an open system. Following exsolution, the rock was deformed and partly recrystallized in the solid state. Similarity of compositions of exsolved and recrystallized minerals suggests recrystallization at P-T conditions similar to those of exsolution. The rock is not the chemical equivalent of the host basanite and cannot represent magma of basanitic composition crystallized in the mantle. Its history of deformation and recrystallization, like that of accompanying spinel lherzolite inclusions, supports the idea that the garnet clinopyroxenite is an accidental inclusion derived from the upper mantle.

Origin of Mojave Desert dust plumes photographed from space

Six dust plumes, arising from a Santa Ana wind and covering an area of 1,700 km 2 of the western Mojave Desert, were photographed by the National Aeronautics and Space Administration LANDSAT/ERTS-1 satellite on January 1,1973. The cause of the erosion was identified as man9s destabilization of the natural surface through road-building, agriculture, urbanization, stream-channel modification, and off-road vehicle recreation. The extensive, and growing, destabilization of the California desert surface provides for ever-increasing dust yield in storms of the future.

Petrography and Chemical Composition of a Suite of Ultramafic Xenoliths from Lashaine, Tanzania

Mafic and ultramafic xenoliths from the Lashaine volcano, Tanzania are composed of garnet and spinel peridotites and pyroxenites and rocks enriched in hydrous phases. Composite xenoliths, composed of two or more lithologies, include peridotite with thin phlogopite bands, and peridotite-pyroxenite assemblages. The dominant metamorphic textures of all lithologies are overprinted by several generations of partial melt textures; the youngest is interstitial glass quenched upon eruption. Earlier partial melting took place under conditions that permitted relatively coarse crystallization of the melts that now are represented by the pyroxenites and hydrous mineral veins and dikes, emplaced in the peridotite long before eruption of the Lashaine volcano. Metasomatic reaction between peridotite and a pyroxenite dike yielded compositional changes in the peridotite that are like the chemical trends separating garnetiferous from nongarnetiferous peridotites in the suite as a whole. We infer that the chemical heterogeneity of the suite resulted from episodic partial melting, concentration of fluids in veins, and metasomatic reactions between veins and garnet-peridotite wall rock. Evidence from Lashaine and other localities indicates that local melting-vein formation-metasomatism has occurred repeatedly in the mantle, probably in diapiric masses. Alkalic components become concentrated by these events until ultimately enough melt is produced to erupt as alkalic basalt.

Structure and Petrology of a Cumulus Norite Boulder Sampled by Apollo 17 in Taurus-Littrow Valley, the Moon

A glass-coated half-meter-size boulder was sampled by the Apollo 17 crew at station 8 near the foot of the Sculptured Hills. The rock proved to be a coarse-grained (0.5-cm) plagioclase-orthopyroxene cumulate, and the samples are the only true norites returned from the lunar surface. Photographs of the boulder showed it to contain at least nine structural surfaces and four glass veins. Orientation and inspection of three of the returned samples resulted in the identification of six surfaces and one vein. One of the structural surfaces visible in the boulder was identified as primary cumulus planar lamination, which was folded through an angle of at least 35° between two oriented samples, whereas fracture sets representing the other surfaces were coincident. The boulder is believed to be a sample of the deeper highlands or submare lunar crust, derived from a depth of 8 to 30 km and somewhat shock-metamorphosed during at least two excavation events. The chemical composition of the norites, when determined, should be of special interest in view of the large amount of literature concerning glass, cataclasite, hornfels, and “basalt” of noritic composition returned by other Apollo missions. However, the cumulus texture of the boulder precludes its being representative of any magmatic liquid composition, suggests that the lunar crust is heterogeneously layered, and that plagioclase sank, not floated, in magmatic liquids that formed the lunar crust.

Velocities of southern Basin and Range xenoliths: Insights on the nature of lower crustal reflectivity and composition

To reconcile differences between the assessments of crustal compositioninthesouthernBasinandRangeprovinceonthebasis of seismic refraction and reflection data and lower-crustal xenoliths, we measured velocities of xenoliths from the Cima volcanic field in southern California. Lower-crustal samples studied includedgabbro,microgabbro,andpyroxenite.Wefindthatthemafic xenolith velocities are compatible with regional in situ measurements from seismic refraction studies, provided that a mixture of gabbro and pyroxenite is present in the lower crust. Supporting thismodelareobservationsthatmanyofthelower-crustalxenoliths from the Cima volcanic field are composites of these rock types, with igneous contacts. Vertical incidence synthetic seismograms show that a gabbroic lower crust with occasional pyroxenite layering can produce a reflective lower crust that is similar in texture to that shown by seismic reflection data recorded nearby.