Xenoliths In Basalts Research Articles

Mantle sulfide geobarometry

A method of oxygen and sulfur barometry is based on the equilibrium, 2 Fe 2 SiO 4 + S 2 = 2 FeS + Fe 2 Si 2 O 6 + O 2, as contained in the peridotite mineral assemblage olivine-orthopyroxene-MSS (monosulfide solid solution {Fe,Ni,Co,Cu} S-S 2). The method was applied to spinel peridotite xenoliths in basalts from the Massif Central and Montferrier and to the Ariege (Pyrenees) orogenic peridotite massifs. Monosulfide solid solution compositions were taken exclusively from sulfide inclusions within silicates and are interpreted to represent subsolidus MSS formed during high P- T recrystallization and deformation of the peridotites. Calculated ƒ O 2 's are consistent with spinel oxybarometry of shallow subcontinental mantle lithosphere but are more scattered. Scatter is due, at least in part, to nonrandom sectioning of multiphase sulfide grains. The ƒ S 2 's, which are coupled to ƒ O 2 through the equilibrium above, average 1–2 log units above the quartz-fayalite-magnetite-pyrrhotite reference buffer. At such conditions, in primitive mantle containing 200–250 ppm S, at least 95% of that S would be present in MSS and 5% or less (as minor H 2S) in CO 2-H 2O fluids.

Hawaiian magma-reservoir processes as inferred from the petrology of gabbro xenoliths in basalt, Kahoolawe Island

Gabbro xenoliths in a tholeiitic lava of Kahoolawe Island, Hawaii, a ∼1.3–1.4 Ma shield volcano, are 1–3 cm in size and comprised of plagioclase, clinopyroxene, and orthopyroxene. Gabbro textures — while intergranular and in part subophitic-are “open” due to 28–48 vol.% of vesicular basalt occupying xenolith space. Vesicles in and around the xenoliths are lined or filled with rhyolitic glass (segregation vesicles). The host is evolved tholeiite (MgO 6.1 wt%) with phenocrysts, microphenocrysts, and glomerocrysts of olivine, clinopyroxene, orthopyroxene, and plagioclase, and megacrysts (∼1 cm) of plagioclase. The Sr-isotope ratio of one xenolith is 0.70489; the host basalt ratio is 0.70460. Xenolith isotope composition, grain resorption, and clinopyroxene (Fs12.5–15Wo38–35.5), orthopyroxene (Fs19.5–24Wo4.1), and plagioclase (An68–65Or0.8–1.2) compositions suggest that these gabbros crystallized from Kahoolawe tholeiitic magma of essentially the same composition as the host basalt, but pre-dating the magma represented by the host. Based on the absence of intergranular Fe−Ti oxide phases from the pl+cpx+opx assemblages, and the open, vuggy textures, we envision crystallization on a reservoir roof at temperatures >1100°C. Entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush/suspension zone occurred during convection associated with replenishment of the magma reservoir. These open-textured gabbro xenoliths are therefore not fragments of preexisting coarse-grained bodies such as sills or segregation veins. Rhyolitic glass in vesicles represents a gas-effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths.

Redox states of lithospheric and asthenospheric upper mantle

The oxidation state of lithospheric upper mantle is heterogeneous on a scale of at least four log units. Oxygen fugacities (\(f_{O_2 } \)) relative to the FMQ buffer using the olivine-orthopyroxene-spinel equilibrium range from about FMQ-3 to FMQ+1. Isolated samples from cratonic Archaean lithosphere may plot as low as FMQ-5. In shallow Proterozoic and Phanerozoic lithosphere, the relative\(f_{O_2 } \) is predominantly controlled by sliding Fe3+-Fe2+ equilibria. Spinel peridotite xenoliths in continental basalts follow a trend of increasing\(f_{O_2 } \) with increasing refractoriness, to a relative\(f_{O_2 } \) well above graphite stability. This suggests that any relative reduction in lithospheric upper mantle that may occur as a result of stripping lithosphere of its basaltic component is overprinted by later metasomatism and relative oxidation. With increasing pressure and depth in lithosphere, elemental carbon becomes progressively refractory and carbon-bearing equilibria more important for\(f_{O_2 } \) control. The solubility of carbon in H2O-rich fluid (and presumably in H2O-rich small-degree melts) under the P,T conditions of Archaean lithosphere is about an order of magnitude lower than in shallow modern lithosphere, indicating that high-pressure metasomatism may take place under carbon-saturated conditions. The maximum\(f_{O_2 } \) in deep Archaen lithosphere must be constrained by equilibria such as EMOG/D. If the marked chemical depletion and the orthopyroxene-rich nature of Archaean lithospheric xenoliths is caused by carbonatite (as opposed to komatiite) melt segregation, as suggested here, then a realistic lower\(f_{O_2 } \) limit may be given by the H2O +C=CH4+O2 (C-H2O) equilibrium. Below C −H2O a fluid becomes CH4 rather than CO2-bearing and carbonatitic melt presumably unstable. The actual\(f_{O_2 } \) in deep Archaean lithosphere is then a function of the activities of CO2 and MgCO3. Basaltic melts are more oxidized than samples from lithospheric upper mantle. Mid-ocean ridge (MORB) and ocean-island basalts (OIB) range between FMQ-1 (N-MORB) and about FMQ +2 (OIB). The most oxidized basaltic melts are primitive island-arc basalts (IAB) that may fall above FMQ+3. If basalts are accurate\(f_{O_2 } \) probes of their mantle sources, then asthenospheric upper mantle is more oxidized than lithosphere. However, there is a wide range of processes that may alter melt\(f_{O_2 } \) relative to that of the mantle source. These include partial melting, melt segregation, shifts in Fe3+/Fe2+ melt ratios upon decompression, oxygen exchange with ambient mantle during ascent, and low-pressure volatile degassing. Degassing is not very effective in causing large-scale and uniform\(f_{O_2 } \) shifts, while the elimination of buffering equilibria during partial melting is. Upwelling graphite-bearing asthenosphere will decompress along\(f_{O_2 } \)-pressure paths approximately parallel to the graphite saturation surface, involving reduction relative to FMQ. The relative\(f_{O_2 } \) will be constrained to below the CCO equilibrium and will be a function of\(a_{CO_2 } \). Upwelling asthenosphere whose graphite content has been exhausted by partial melting, or melts that have segregated and chemically decoupled from a graphite-bearing residuum will decompress along\(f_{O_2 } \)-decompression paths controlled by continuous Fe3+-Fe2+ solid-melt equilibria. These equilibria will involve increases in\(f_{O_2 } \) relative to the graphite saturation surface and relative to FMQ. Melts that finally segregate from that source and erupt on the earth's surface may then be significantly more oxidized than their mantle sources at depth prior to partial melting. The extent of melt oxidation relative to the mantle source may be directly proportional to the depth of graphite exhaustion in the mantle source.

Seismic constraints on the nature of lower crustal reflectors beneath the extending Southern Transition Zone of the Colorado Plateau, Arizona

We determine the reflection polarity and exploit variations in P and S wave reflectivity and P wave amplitude versus offset (AVO) to constrain the origin of lower crustal reflectivity observed on new three‐component seismic data recorded across the structural transition of the Colorado Plateau. The near vertical incidence reflection data were collected by Stanford University in 1989 as part of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that traversed the Arizona Transition Zone of the Colorado Plateau. The results of independent waveform modeling methods are consistent with much of the lower crustal reflectivity resulting from thin, high‐impedance layers. The reflection polarity of the cleanest lower crustal events is positive, which implies that these reflections result from high‐velocity contrasts, and the waveform character indicates that the reflectors are probably layers less than or approximately equal to 200 m thick. The lower crustal events are generally less reflective to incident S waves than to P waves, which agrees with the predicted behavior of high‐velocity mafic layering. Analysis of the P wave AVO character of lower crustal reflections demonstrates that the events maintain a constant amplitude with offset, which is most consistent with a mafic‐layering model. One exception is a high‐amplitude (10 dB above background) event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets. The event has a pronounced S wave response, which along with its negative AVO trend is a possible indication of the presence of fluids in the lower crust. The Arizona Transition Zone is an active but weakly extended province, which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers. Instead, we favor horizontal basaltic intrusions as the primary origin of high‐impedance reflectors based on (1) The fact that most xenoliths in eruptive basalts of the Transition Zone are of mafic igneous composition, (2) indications that a pulse of magmatic activity crossed the Transition Zone in the late Tertiary period, and (3) the high regional heat flow observed in the Transition Zone. The apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion. We present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust.

The chemistry of zircon: Variations within and between large crystals from syenite and alkali basalt xenoliths

Single grains of zircon can contain zones indicating several generations of crystal growth, each of which should reflect the chemical and physical conditions occurring at the time of its formation. Trace element analyses have been made of large zircon crystals from rocks of alkaline affinities by ion microprobe. The chondrite-normalised rare earth element (REE) concentrations increase rapidly from La to Lu, as would be expected from the decrease in ionic radius and consequent easier substitution into the Zr site within the zircon lattice. Lanthanum, praseodymium, and neodymium are considerably lower than values observed in bulk analyses of zircon. The partition coefficients for the light rare earth elements (LREEs), between zircon and melt or whole rock, must therefore be significantly lower than those calculated using bulk analyses. Cerium is enriched relative to neighbouring REEs due to the presence of Ce 4+. Estimates of partition coefficients of Ce 3+ and Ce 4+ between zircon and melt demonstrate that although the Ce anomalies are large the Ce 4+ Ce 3+ ratio is very small (less than 3 × 10 −3). The size of the Ce anomaly is variable and should be capable of providing information on oxygen fugacity changes.

Chemical compositions and Sr, Nd isotope ratios of gabbroic xenoliths in calc-alkali andesites of Naeba and Torikabuto volcanoes, North Fossa Magna, central Japan.

Gabbroic, doleritic and basaltic xenoliths found in calc-alkali andesites of Naeba and Torikabuto volcanoes are geochemically divided into three groups. Gabbro A of the group 1 from Naeba is rich in MgO and Ni, poor in alkalis, and shows depleted REE pattern resembling those of Ichinomegata (Sp. No. 2232) and Hakone (HKG1, HKG2) volcanoes. On the basis the REE pattern and high Al ?? V contents in clinopyroxenes, gabbro A is interpreted to have been cumulate from a primary magma generated by partial melting of upper mantle. From REE pattern, gabbros of the group 2 from Ichinomegata (Sp. No. 2218) may have derived from low alkali tholeiite magma which have been formed by removal of material such as the group 1 gabbro at shallow depth. Doleritic and basaltic xenoliths of the group 3 from Naeba, gabbroic xenoliths from Torikabuto and Umikawa are poorer in MgO and richer in alkalis than those of the group 1 and show enriched pattern in REE resembling that of high alkali tholeiite and contain clinopyroxenes having low Al ?? V. Therefore, these rocks are considered to be differentiates of high alkali tholeiite magma at shallow depth. On εNd-87Sr/8 ?? Sr diagram, isotope data of gabbro A of the group 1 plot near those of andesites from Asama and Myoko volcanoes of the North Fossa Magna. It is interpreted that these rocks have derived from the same mantle source as the Asama and Myoko volcanoes which are richer in incompatible elements than those of MORB.

Spinel harzburgite xenoliths in alkali basalt and camptonite from North Westland and southeast Nelson, New Zealand

Abstract Spinel-bearing harzburgite xenoliths are described from three localities in the Western Province of New Zealand. All xenoliths have low Al2O3 contents (<0.5% wt%), orthopyroxenes are Al and Ca poor, and clinopyroxene is absent. Minor modal metasomatism evidenced by phlogopite/ amphibole veinlets is observed in a xenolith from one locality. All xenoliths are inferred to be derived from relatively depleted mantle close to the upper pressure limit for spinel lherzolite (18 kbar). Such depleted mantle could not be the source for the magmas that brought them to the surface.

Are spinel Iherzolite xenoliths representative of the abundance of sulfur in the upper mantle?

In order to address systematic discrepancies of sulfur contents between orogenic Iherzolites and ultramafic xenoliths in alkali lavas, a study combining whole-rock sulfur geochemistry and microscopic investigation in reflected light has been performed on 40 type-I spinel Iherzolite xenoliths from the Neogene alkali basalts in the French Massif Central. These measurements yield sulfur contents ranging between 0 and 90 ppm S, with most values being below 60 ppm S, irrespective of rock texture and/or chemistry. Spinel Iherzolite xenoliths are strongly depleted in S relative to similar rocks found in orogenic spinel Iherzolite bodies or to the average sulfur content of 300 ppm calculated for the MORB mantle source region. Microscopic studies indicate S loss during secondary oxihydration of mantle-derived solid sulfides. This alteration seems to be independent of the effect of the host basalts and probably is related to latestage circulation of meteoric water. It is concluded that, except for a few samples armoured in massive basaltic lava flows, almost all the spinel Iherzolite xenoliths in continental basalts analyzed so far do not provide representative analyses of the sulfur content in the upper mantle.

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Xenoliths from Late Cretaceous Seamounts in the Central Pacific: Cumulates of Fractionating Alkalic Basalt Magma Chambers: ABSTRACT

Abundant xenoliths in alkalic basalt were recovered from two Late Cretaceous seamounts in the Central Pacific. One seamount, located in the Phoenix archipelago (lat 0{degree}22'5, long 176{degree}05'W), is dated by {sup 40}Ar/{sup 39}Ar techniques as 65 Ma. The other seamount, located in the northern Line Islands (lat 15{degree}39'N, long 170{degree}23'W), is dated as 70 Ma. Host lavas are basanite and differentiated alkalic basalt. Mafic xenolith assemblages consist of clinopyroxene with variable amounts of amphibole and mica. Intermediate assemblages have abundant feldspar in addition to the mafic minerals. Rare felsic xenoliths consist of two or more feldspars. Variable amounts of apatite, titanite, and magnetite are poikilitically enclosed in mafic phases, and minor feldspathoids are present in some xenoliths. Most xenoliths are holocrystalline with fine- to medium-grained, equigranular cumulus texture, but two xenoliths have a seriate, interlocking crystal framework in a small amount of glassy to microcrystalline matrix. Clinopyroxene in the holocrystalline samples is partially replaced by amphibole. In a few samples, extensive replacement of clinopyroxene by rounded amphibole grains results in a nearly granoblastic texture. Clinopyroxene compositions range from diopside to ferrosalite and are essentially Cr-free but generally have high Ti and Al contents. Cr-rich diopside and Al-augite, characteristic of mantlemore » clinopyroxene, are absent. Feldspars include plagioclase, anorthoclase, and sanidine. Mineral compositions of xenoliths are similar to those of phenocrysts in the host lavas, indicating that these xenoliths are not metasomatized mantle material, but rather are cumulates from fractionating alkalic basalt magma chambers.« less

Exemples de mélange de magmas en contexte plutonique: les enclaves des tonalites–granodiorites du massif de Bono (Sardaigne septentrionale)

In calc-alkaline orogenic plutons, the dark xenoliths and their host rocks must be considered the expression of partial mixing of magma.Three associations of this type have been investigated and are illustrated by the Bono pluton (northern Sardinia)— a composite pluton including three intrusives of different nature (tonalitic to granodioritic) and containing a very large number of basaltic xenoliths of magmatic origin. Detailed mineralogical analysis of the two end members in each association, coupled with geochemical data, has determined the major petrogenetic mechanisms intervening in the mixing process in a plutonic setting: temperature equilibration, mechanical exchanges of crystals, chemical exchanges, etc. The most important result of this article, however, is to show that each intrusion is related to a specific group of xenoliths that is characterized by constant FeOt/MgO. The latter reflects the different composition of basaltic components, and it is concluded that each intrusive event is associated with a unique mixing episode. As in volcanic settings, the mixing process may have initiated the intrusion.The extreme compositional variations in the magmatic xenoliths, recognized in several series of orogenic plutons, is explained here by different initial basaltic end members and by variation in the intensity of the interaction mechanisms. [Journal Translation]

NATURE AND SIGNIFICANCE OF IGNEOUS ROCKS CORED IN THE STATE 2–14 RESEARCH BOREHOLE: SALTON SEA SCIENTIFIC DRILLING PROJECT, CALIFORNIA

The State 2–14 research borehole of the Salton Sea Scientific Drilling Project penetrated 3.22 km of Pleistocene to Recent sedimentary rocks in the Salton Sea geothermal system, located in the Salton Trough of southern California and northern Baja California, Mexico. In addition, three intervals of igneous rocks were recovered; a silicic tuff and two sills of altered diabase. The chemical composition of the silicic tuff at 1704 m depth suggests that it is correlative with the Durmid Hill tuff, cropping out 25 km NW of the geothermal system. In turn, both of these tuffs may be deposits of the Bishop Tuff, erupted from the Long Valley caldera of central California at 0.7 Ma. The diabases are similar to basaltic xenoliths found in the nearby Salton Buttes rhyolite domes. These diabase are interpreted as hypabyssal intrusions resulting from magmatism due to rifting of the Salton Trough as part of the East Pacific Rise/Gulf of California transtensional system. The sills apparently intruded an already developed geo‐thermal system and were in turn altered by it.

Geothermal profile and crust-mantle transition beneath east-central Queensland: Volcanology, xenolith petrology and seismic data

Geothermobarometry of garnet granulite and garnet websterite xenoliths in basalts from numerous localities in east-central Queensland gives P-T points that fall along the geotherm previously defined for southeastern Australia. This elevated geotherm is ascribed to the advective transport of heat by Tertiary-Recent magmas ponded at the crust-mantle boundary. The lower crust in this region consists dominantly of mafic granulites, representing frozen basaltic melts and cumulates. Spinel lherzolite becomes a dominant rock type at depths of ca. 30 km, and persists, interlayered with pyroxenites, to depths of ca. 55 km. Seismic reflection profiles show a “layered lower crust” between depths of 20 and 36 km depth. The lithologically defined crust-mantle boundary lies within this zone, at least 6 km above the seismically defined Moho. This interpretation is consistent with the observed velocity ( V p) gradient downward through the layered zone. The constructed geotherm implies that the bottom of the lithosphere beneath eastern Australia is shallower than ca. 100 km. This makes it unlikely that the diamonds of eastern Australia are derived from local intrusions, unless these are > 200 Ma old.

Life-time of a stratified magma chamber recorded in ultramafic xenoliths from Ichinomegata volcano, northeastern Japan

Variations in major-element chemistry and modal compositions of the mantle xenolith-bearing calc-alkalic ejecta from Ichinomegata volcano are inferred to be due to mixing of three magmatic end members: Basalt I (SiO2 51 wt% , MgO 8.5 wt%), Basalt II (SiO2 54 wt%, MgO 5 wt%), and Dacite (SiO2 65 wt%, MgO2 wt%). Ultramafic xenoliths are found in mafic mixtures of Dacite-Basalt I and Dacite-Basalt II. The thermal histories of the xenoliths in both mixtures are compared with each other. Chemical compositions of olivine and orthopyroxene in xenoliths suggest that xenoliths in Basalt I were equilibrated at about 800 °C, while those in Basalt II were also equilibrated originally at about 800 °C but were subsequently annealed at about 1000 °C for more than 102–3 years prior to the eruption.

Characteristic mineralogy of arc-related cumulate gabbros: Implications for the tectonic setting of gabbroic plutons and for andesite genesis

Several mineralogically distinct suites of gabbroic cumulates occur as xenoliths in basalt and andesite from active arc volcanos and in plutons exposed in deeply eroded arcs. Calcic (An85–100) plagioclase and moderately Fe-rich (Fo60–80) olivine commonly occur together in arc cumulate gabbros but not in cumulate gabbros from mid-ocean ridges, ocean islands, or tholeiitic layered intrusions. Compositions of coexisting olivine and plagioclase in gabbro cumulates can establish the tectonic setting of ancient volcano-plutonic complexes. In deeply eroded arcs, gabbroic cumulates typically occur in zoned plutons that also contain gabbros, diorites, and quartz diorites of basaltic to andesitic composition. In many cases, it is clear that the cumulate gabbros are chemically and physically related to the gabbro-diorite (= basalt-andesite) fractionation trends seen in the zoned plutons. Zoned plutons in deeply eroded arcs provide physical evidence that crystal fractionation plays an important role in andesite petrogenesis.

49B. Sr・Nd isotopic study of volcanics and xenoliths in alkali basalts from Oki-Dogo Island.(Abstracts of Papers Presented at the Spring Meeting of the Society)

49B. Sr・Nd isotopic study of volcanics and xenoliths in alkali basalts from Oki-Dogo Island.(Abstracts of Papers Presented at the Spring Meeting of the Society)

Lherzolite xenoliths in kimberlites and basalts: petrogenetic and crystallochemical significance of some minor and trace elements in olivine, pyroxenes, garnet and spinel

ABSTRACTElectron and ion microprobe analyses for P, Si, Ti, Al, Cr, V, Sc, Fe, Mn, Mg, Ni, Co, Ca, Sr, Na, K and Li in olivine, pyroxenes and garnet in forty-two cold and twelve hot garnet lherzolites from kimberlites, nine spinel lherzolites from kimberlites and eighteen from alkali basalts, and one cold garnet lherzolite from the Malaita alnöite, are compared with published data for minerals occurring in lherzolite, harzburgite and eclogite xenoliths, for silicate megacrysts in kimberlites, and for silicate inclusions in diamonds. Despite wide ranges in the chemistry of minerals from garnet and spinel lherzolites, there are distinct regions in composition space that would enable determination of the parent lithology of disaggregated minerals in kimberlites and alkali basalts. Titanium correlates with Fe3+ in garnets. Chromium, Al, V and Sc are distributed similarly between silicates in lherzolites. Sodium correlates with trivalent ions in olivine, and increases with temperature. The distribution of Na, but not of K and Li, between olivine and clinopyroxene correlates with temperature. The regular partitioning of Ti, Mn and Ni places constraints on crystal-liquid partition coefficients. Above the stability temperature of mica, ‘metasomatising fluids’ may scavenge Cr and other trivalent ions as they increase Ti in silicates.

Geologic field studies of the Miki Fjord Area, East Greenland

Field relations between Tertiary intrusions and the lithologic units near Miki Fjord provide a record of physical and chemical processes associated with crustal extension and the formation of the East Greenland continental margin. At several localities the Precambrian basement has been remobilized and partly incorporated in the large macrodikes northeast of the Skaergaard intrusion. Blocks of vesicular basalts sank into the macro­dike magma and were metamorphosed to equigranular aggregates of olivine and ortho- and clinopyrox­enes with concentrations of plagioclase filling amygdale-like structures. We suggest that contact metamor­phic dehydration of hydrous alteration minerals in the basaltic xenoliths and diffusion of this water into the macrodike magma were responsible for the abundant rhythmic layering that occurs near swarms of the xenoliths in a manner similar to that proposed by Taylor &amp; Forester (1979) and Mc Birney &amp; Noyes (1979) for the Skaergaard intrusion. Concentrations of xenoliths of leucogabbro and migmatitic basement gneis­ses are found near the top of one macrodike and in six smaller mafic dikes and sills that intrude the lower lavas. In contrast to the basalt blocks, the leucocratic xenoliths appear to have floated to the top of these mafic intrusions, thus providing an effective mechanism for transport of basement material during the early stages of crustal extension. Basement rocks and Tertiary basalts of the area have undergone hydrothermal alteration that is con­centrated in fracture systems near the intrusive bodies. Alteration mineralogy in the basalts consists of calc-silicate assemblages similar to those found in active geothermal systems in Iceland. Extensive calcium metasomatism is evident in a number of coast parallel dolerite dikes that are mineralized by iron-rich prehnite. Thermodynamic analysis of the dehydration of prehnite to form epidote and garnet indicates that the fluids responsible for the formation of prehnite mineralized dikes were at temperatures less than about 250°C. Mineralized shear zones and mafic dikes of presumed Proterozoic age define a pronounced structural trend in the basement of the area. This east - west trend is parallel with the coastal dike swarm north of 68°N and with topographical trends within the basalts. We therefore suggest that the pre-Tertiary struc­tures may in part control the deflection of the coastline at 68°N and other aspects of the Tertiary structural and intrusive development.

The Ronda high temperature peridotite: Geochemistry and petrogenesis

The Ronda peridotite in southern Spain is a large (~300 km 2) exposure of upper mantle which provides direct information about mantle processes on a scale much larger than that provided by mantle xenoliths in basalt. Ronda peridotites range from harzburgite to lherzolite, and vary considerably in major element content, e.g., Al 2O 3 from 0.9 to 4.8%, and trace element abundances, e.g., Sr, Zr and La abundances vary by factors of 20 to 40. These compositional variations are systematic and correlate with (pyroxene + garnet)/olivine ratios and olivine compositions. The data are consistent with formation of residual peridotites by variable degrees of melting (~0 to 30%) of a compositionally homogeneous peridotite. None of the peridotites have geochemical characteristics of residues formed by extensive (⪢5%) fractional melting and the data can be explained by equilibrium (batch) melting, possibly with incomplete melt segregation in some samples. Based on compositional differences between Ronda peridotites, the segregated melts were picritic (12–22% MgO) with relative rare earth element abundances similar to mid-ocean ridge basalt (MORB). Prior to the melting event the Ronda peridotite body was a suitable source for MORB. The compositional characteristics of Ronda peridotites are consistent with diapiric rise of a fertile mantle peridotite with relatively small degrees of melting near the diapir-wall rock interface yielding residues of garnet iherzolite, and larger degrees of melting in the diapir interior yielding residues of garnet-free peridotite. Subsequently these residual rocks were recrystallized at sub-solidus conditions ( Obata, 1980), and emplaced in the crust by thrusting (Lundeen, 1978).

Orthopyroxene-bearing gabbroic xenoliths in basalts from the East Pacific rise axis near 12° 50' N

Orthopyroxene-bearing gabbroic xenoliths have been found within the ocean-floor tholeiites recovered from the active volcanic zone of the East Pacific Rise near 12° 50' N. These xenoliths, made up of olivine-orthopyroxene-clinopyroxene-plagioclase assemblages, are believed to represent differentiated plutonic cumulates from a magma chamber located beneath the Rise axis.