Neodymium and strontium isotope study of ophiolite and orogenic lherzolite petrogenesis

Neodymium isotopic analyses have been measured on nine ophiolites and four orogenic lherzolites. ε_Nd varies from +12 to +3 in the ophiolites and from +18 to +2 in the orogenic lherzolites. The majority of the analyses plot on a ε_Nd-ε_Sr correlation line as defined by Nd and Sr isotopic analyses of oceanic basalts. However, certain ophiolitic and lherzolitic samples exhibit high⁸⁷Sr/⁸⁶Sr ratios and as such lie to the right of the correlation line towards seawater compositions.From these data one can postulate several origins for ophiolites including that of mid-ocean ridges and ocean islands. If the orogenic lherzolites are interpreted as representative of the mantle occurring below active ridges a more complex model is required involving mantle heterogeneity and multi-episodic chemical fractionation starting prior to 2 Ga ago.     

Geodynamic significance of ultramafic xenoliths from Eastern Serbia: Relics of sub-arc oceanic mantle?

A suite of highly depleted peridotite xenoliths in East Serbian Palaeogene basanites represents the lithospheric mantle beneath the Balkan Peninsula. The xenoliths are harzburgites, clinopyroxene-poor lherzolites and rare dunites. They contain mostly <5 vol.% of modal clinopyroxene and are characterized by high Mg# in silicates (>91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al₂O₃ contents in orthopyroxene (mostly 1–2 wt.%). They have experienced some mantle metasomatism which has slightly obscured their original composition. Nevertheless, the general characteristics of the xenoliths imply a composition which is significantly more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peridotites. Geological and compositional evidence suggests that the xenoliths do not represent Archean mantle. The existence of Proterozoic mantle cannot be entirely excluded, although it is in disagreement with geological evidence. On the other hand, the studied xenoliths are compositionally very similar to peridotites of modern oceanic sub-arc settings. The existence of such a depleted lithospheric mantle segment is also inferred from the presence of rare orthopyroxene-rich xenoliths in the same suite. These are interpreted to have originated as lithospheric precipitates of high-Mg, SiO₂-saturated magmas that require a highly depleted mantle source. Such source is typically required by boninitic-like magmas of intraoceanic suprasubduction settings. A proposed geodynamic model to explain these observations involves accretion or underplating of the lower parts of the Tethyan oceanic lithosphere during the Upper Jurassic closure of the eastern branch of the Vardar ocean.     

Implications of melting for stabilisation of the lithosphere and heat loss in the Archaean

The increased depth and volume of melting induced in a higher temperature Archaean mantle controls the stability of the lithosphere, heat loss rates and the thickness of the oceanic crust. The relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions. The density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200°C or more hotter than modern upper mantle. If Archaean continental crust formed initially above oceanic lithosphere, the compositional density gradients may be sufficient to preserve a thick Archaean continental lithosphere within which the Archaean age diamonds are preserved. The amount of heat advected by melts at mid-ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases. Archaean tectonics could have been dominated by spreading rates 2–3 times greater than now and with mantle temperatures between ca. 1600°C and 1800°C at the depth of the solidus. Mid-ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form, protected from copious volcanism and high mantle temperatures.     

A petrologic model for the constitution of the upper mantle and crust of the Koolau shield,Oahu, Hawaii,and Hawaiian magmatism

A petrological model for the uppermost upper mantle and crust under the Koolau shield to a depth of about 60 km has been derived on the basis of petrology of the upper mantle and crustal xenoliths in nephelinites of the Honolulu Volcanic Series. Three main xenolith suites exist in the Koolau shield: dunites, spinel lherzolites, and garnet-bearing pyroxenites. On the basis of mineralogy, it is inferred that the dunites represent cumulates in shallow crustal tholeiitic magma chambers, the spinel lherzolites form a thick (～ 40 km) layer in the upper mantle, and the garnet pyroxenite suite occurs as veins and stringers in the spinel lherzolites at about 60 km depth.The eruption sequence in a Hawaiian volcano, i.e., tholeiite → transitional basalt → alkali basalt, is generated by partial melting of a volatile-bearing garnet-lherzolite part of a lithospheric plate as it rides over a hot spot. If the tholeiite, transitional, and alkali basalts of Hawaiian volcanoes are generated at the same depth, then the observed sequence of lavas requires replenishment of the source area with volatiles and gradual decrease of the degree of partial melting with time. Post-erosional olivine nephelinites are produced from isotopically distinct, deeper source area, which may be the asthenosphere.     

Evolution of lithospheric mantle in the north of Nain‐Baft oceanic crust (Neo‐Tethyan ophiolite of Ashin,Central Iran)

This study is focused on a plagioclase‐bearing spinel lherzolite from Chah Loqeh area in the Neo‐Tethyan Ashin ophiolite. It is exposed along the west of left‐lateral strike‐slip Dorouneh Fault in the northwest of Central‐East Iranian Microcontinent. Mineral chemistry (Mg#^olivine < ~ 90, Cr#^{clinopyroxene} < ~ 0.2, Cr#^spinel < ~ 0.5, Al₂O₃^{orthopyroxene} > ~ 2.5 wt%, Al₂O₃^{clinopyroxene} > ~ 4.5 wt%, Al₂O₃^spinel > ~ 41.5 wt%, Na₂O^{clinopyroxene} > ~ 0.11 wt%, and TiO₂^{clinopyroxene} > ~ 0.04 wt%) confirms Ashin lherzolite was originally a mid‐oceanic ridge peridotite with low degrees of partial melting at spinel‐peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase‐peridotite facies during oceanic extension and diapiric motion of mantle along Nain‐Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g. increase in TiO₂ and Na₂O of clinopyroxene, and TiO₂ and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase‐peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain‐Baft oceanic crust in the northwest of Central‐East Iranian Microcontinent.     

Petrology of the Yugu peridotites in the Gyeonggi Massif,South Korea: Implications for its origin and hydration process

Peridotites exposed in the Yugu area in the Gyeonggi Massif, South Korea, near the boundary with the Okcheon Belt, exhibit mylonitic to strongly porphyroclastic textures, and are mostly spinel lherzolites. Subordinate dunites, harzburgites, and websterites are associated with the lherzolites. Amphiboles, often zoned from hornblende in the core to tremolite in the rim, are found only as neoblasts. Porphyroclasts have recorded equilibrium temperatures of about 1000°C, whereas neoblasts denote lower temperatures, about 800°C. Olivines are Fo_90–91 in lherzolites and Fo₉₁ in a dunite and a harzburgite. The Cr# (= Cr/(Cr + Al) atomic ratio) of spinels varies together with the Fo of olivines, being from 0.1 to 0.3 in lherzolites and around 0.5 in the dunite and harzburgite. The Na₂O content of clinopyroxene porphyroclasts is relatively low, around 0.3 to 0.5 wt% in the most fertile lherzolite. The Yugu peridotites are similar in porphyroclast mineral chemistry not to continental spinel peridotites but to sub‐arc or abyssal peridotites. Textural and mineralogical characteristics indicate the successive cooling with hydration from the upper mantle to crustal conditions for the Yugu peridotites. Almost all clinopyroxenes and amphiboles show the same U‐shaped rare earth element (REE) patterns although the level is up to ten times higher for the latter. The hydration was associated with enrichment in light REE, resulting from either a slab‐derived fluid or a fluid circulating in the crust. The mantle‐wedge or abyssal peridotites were emplaced into the continental crust as the Yugu peridotite body during collision of continents to form a high‐pressure metamorphic belt in the Gyeonggi Massif. The peridotites from the Gyeonggi Massif exhibit lower‐pressure equilibration than peridotites, with or without garnets, from the Dabie–Sulu Collision Belt, China, which is possibly a westward extension of the Gyeonggi Massif.     

Deep melting and sodic metasomatism underneath the highly oblique-spreading Lena Trough (Arctic Ocean)

Abyssal peridotites collected along the highly oblique-spreading Lena Trough north of Greenland and Spitsbergen have mineral compositions that are similar to residual abyssal peridotites, except for high sodium concentrations in clinopyroxene (cpx). Most samples are lherzolites with light rare earth element (REE)-depleted cpx trace element patterns, but significantly fractionated middle to heavy REE ratios at relatively high heavy REE concentrations. Such characteristics can only be explained by initial melting of a garnet peridotite followed by low degrees of melting in the stability field of spinel peridotite. The residual garnet signature requires either a high potential temperature of the upwelling mantle, or elevated solidus-lowering water contents. The limited spinel field melting suggests a deep cessation of melt extraction, possibly because of the presence of a thick lithospheric cap. This is consistent with the extremely low effective spreading rate and the vicinity to a passive continental margin, which allow conductive cooling to reach deeper levels than commonly estimated for faster mid-ocean ridges. High sodium concentrations in cpx are neither explainable by melt refertilization, nor by a simple diffusion mechanism. The efficient fractionation of sodium from the light REE requires post-melting metasomatism, which is typically restricted to the subcontinental lithosphere. This might imply that the Lena Trough peridotites represent unroofed subcontinental mantle, from which no melt was extracted during the opening of the Lena Trough. It is more likely that sodic metasomatism occurred after partial melting underneath the Lena Trough, and that such an enrichment process is responsible for elevated sodium concentrations in abyssal peridotites elsewhere. Sodium in cpx of residual peridotites can therefore not serve as an indicator of partial melting or melt refertilization.     

Petrology of the Green Knobs diatreme and implications for the upper mantle below the Colorado Plateau

Peridotite inclusions, crystal fragments, and kimberlite breccia at Green Knobs, New Mexico, have been studied to evaluate compositions and processes in the upper mantle below the Colorado Plateau. Most peridotite inclusions are spinel lherzolites and harzburgites, or their partly hydrated equivalents, in the Cr-diopside group. Orthopyroxene-rich websterites and olivine websterites comprise 3% of the peridotites and formed as cumulates. Typical anhydrous or slightly hydrated peridotites contain aluminous, calcic diopside (5–7% Al₂O₃), aluminous orthopyroxene (3–6% Al₂O₃), spinel, and olivine (near Fa₉). Geothermometers based on different mineral pairs yield temperatures from above 1100°C to below 700°C in single rocks. High values, derived from pyroxenes with included exsolution lamellae, may approximate temperatures of primary crystallization. Low values, based on olivine-spinel and olivine-clinopyroxene pairs, approach upper mantle temperatures before eruption. In rare samples, some spinel grains are rimmed by garnet while others are not rimmed; garnet formation was controlled by nucleation kinetics. About one-third of the peridotites were deformed shortly before eruption, with effects ranging from mild cataclasis to the production of ultramylonites.Discrete crystals of garnet, olivine (near Fa₈), and Cr-diopside represent garnet peridotite. Eclogites were not found. The garnet peridotite is more depleted than overlying spinel peridotite, and it is not a likely source for the minettes associated with the kimberlites.The mantle below Green Knobs consists of spinel peridotite from 45 to perhaps 60 km depth immediately underlain by more-depleted garnet peridotite. The position of the spinel-garnet transition may be fixed by kinetics. The kimberlite may have been produced when heat from ascending minette magma released volatiles from otherwise depleted garnet peridotite. Resulting gas-solid mixtures erupted along zones of deformation associated with Colorado Plateau monoclines. Sheared lherzolites formed during renewed movement along these zones.     

Diapirism of depleted peridotite — a model for the origin of hot spots

A model is proposed for the origin of hot spots that depends on the existence of major-element heterogeneities in the mantle. Generation of basaltic crust at spreading centers produces a layer of residual peridotite ～20–25 km thick directly beneath the crust which is depleted in Fe/Mg, TiO₂, CaO, Al₂O₃, Na₂O and K₂O, and which has a slightly lower density than undepleted peridotite beneath it. Upon recycling of this depleted peridotite back into the deep mantle at subduction zones, it becomes gravitationally unstable, and tends to rise as diapirs through undepleted peridotite. For a density contrast of 0.05 g cm^?3, a diapir 60 km in diameter would rise at roughly 8 cm y^?1, and could transport enough heat to the base of the lithosphere to cause melting and volcanism at the surface. Hot spots are thus viewed as a passive consequence of mantle convection and fractionation at spreading centers rather than a plate-driving force.It is suggested that depleted diapirs exist with varying amounts of depletion, diameters, upward velocities and source volumes. Such variations could explain the occurrence of hot spots with widely varying lifetimes and rates of lava production. For highly depleted diapirs with very low Fe/Mg, the diapir would act as a heat source and the asthenosphere and lower lithosphere drifting across the diapir would serve as the source region of magmas erupted at the surface. For mildly depleted diapirs with Fe/Mg only slightly less than in normal undepleted mantle, the diapir could provide not only the source of heat but also most or all of the source material for the erupted magmas. The model is consistent with isotopic data that require two separate and ancient source regions for mid-ocean ridge and oceanic island basalts. The source for mid-ocean ridge basalts is considered to be material upwelling at spreading centers from the deep mantle. This material forms the oceanic lithosphere. Oceanic island basalts are considered to be derived from varying mixtures of sublithospheric and lower lithospheric material and the rising diapir itself.     

Morphology and composition of spinel in Pu’u ’O’o lava (1996–1998), Kilauea volcano, Hawaii

The morphology and composition of spinel in rapidly quenched Pu’u ’O’o vent and lava tube samples are described. These samples contain glass, olivine phenocrysts (3–5 vol.%) and microphenocrysts of spinel (0.05 vol.%). The spinel surrounded by glass occurs as idiomorphic octahedra 5–50 μm in diameter and as chains of octahedra that are oriented with respect to each other. Spinel enclosed by olivine phenocrysts is sometimes rounded and does not generally form chains. The temperature before quenching was calculated from the MgO content of the glass and ranges from 1150°C to 1180°C. The oxygen fugacity before quenching was calculated by two independent methods and the log f_O2 ranged from −9.2 to −9.9 (delta QFM=−1). The spinel in the Pu’u ’O’o samples has a narrow range in composition with Cr/(Cr+Al)=0.61 to 0.73 and Fe²⁺/(Fe²⁺+Mg)=0.46 to 0.56. The lower the calculated temperature for the samples, the higher the average Fe²⁺/(Fe²⁺+Mg), Fe³⁺ and Ti in the spinel. Most zoned spinel crystals decrease in Cr/(Cr+Al) from core to rim and, in the chains, the Cr/(Cr+Al) is greater in the core of larger crystals than in the core of smaller crystals. The occurrence of chains and hopper crystals and the presence of Cr/(Cr+Al) zoning from core to rim of the spinel suggest diffusion-controlled growth of the crystals. Some of the spinel crystals may have grown rapidly under the turbulent conditions of the summit reservoir and in the flowing lava, and the crystals may have remained in suspension for a considerable period. The rapid growth may have caused very local (μm) gradients of Cr in the melt ahead of the spinel crystal faces. The crystals seem to have retained the Cr/(Cr+Al) ratio that developed during the original growth of the crystal, but the Fe²⁺/(Fe²⁺+Mg) ratio may have equilibrated fairly rapidly with the changing melt composition due to olivine crystallization. Six of the samples were collected on the same day at various locations along a 10-km lava tube and the calculated pre-collection temperatures of the samples show a 5°C drop with distance from the vent. The average Fe²⁺/(Fe²⁺+Mg) of the spinel in these samples shows a weak positive correlation with decreasing MgO in the glass of these samples. The range in Cr₂O₃ (0.041–0.045 wt.%) of the glass for these six samples is too small to distinguish a consistent change along the lava tube. The spinel in the Pu’u ’O’o samples shows a zoning trend in a Cr–Al–Fe³⁺ diagram almost directly away from the Cr apex. This compares with a zoning trend in rapidly quenched MORB samples away from Cr coupled with decreasing Fe³⁺. The trend away from Cr displayed by spinel in rapidly quenched samples is in marked contrast to the trend of increasing Fe³⁺ shown by spinel in slowly cooled lava.     

Chemical characteristics of chromian spinel in plutonic rocks: Implications for deep magma processes and discrimination of tectonic setting

We summarize chemical characteristics of chromian spinels from ultramafic to mafic plutonic rocks (lherzolites, harzburgites, dunites, wehrlites, troctolites, olivine gabbros) with regard to three tectonic settings (mid‐ocean ridge, arc, oceanic hotspot). The chemical range of spinels is distinguishable between the three settings in terms of Cr# (= Cr/(Cr + Al) atomic ratio) and Ti content. The relationships are almost parallel with those of chromian spinels in volcanic rocks, but the Ti content is slightly lower in plutonics than in volcanics at a given tectonic environment. The Cr# of spinels in plutonic rocks is highly diverse; its ranges overlap between the three settings, but extend to higher values (up to 0.8) in arc and oceanic hotspot environments. The Ti content of spinels in plutonics increases, for a given lithology, from the arc to oceanic hotspot settings by mid‐ocean ridge on average. This chemical diversity is consistent with that of erupted magmas from the three settings. If we systematically know the chemistry of chromian spinels from a series of plutonic rocks, we can estimate their tectonic environments of formation. The spinel chemistry is especially useful in dunitic rocks, in which chromian spinel is the only discriminating mineral. Applying this, discordant dunites cutting mantle peridotites were possibly precipitated from arc‐related magmas in the Oman ophiolite, and from an intraplate tholeiite in the Lizard ophiolite, Cornwall.     

Evidence of mantle metasomatism and heterogeneity from peridotite inclusions of northeastern Brazil and Paraguay

Textural-petrographic features, major and trace elements and mineral chemistry are reported for spinel-peridotite xenoliths from northeastern Brazil (NE-suite) and Paraguay (PY-suite).Variation trends defined by bulk rock and mineral chemistry are consistent with the effects of variable degree of high-pressure melting for both suites.The PY-suite strongly differs from the NE-suite by its very high contents of incompatible elements. This different whole-rock chemistry is related to the presence of glassy-microcrystalline blebs which are interpreted as remains of pre-existing hydrous phases of metasomatic origin. The difference between the two suites is also recorded in the chemistry of pyroxenes and spinels, which develop along different variation patterns.Although the suites partially overlap in their ranges ofmg_opx (Mg/(Fe²⁺ + Fe³⁺) ratio in orthopyroxenes), the PY-suite is, on the average, moremg_opx rich (residual) than the NE-suite. Within the PY-suite a rough positive correlation exists between degree of residuality and degree of metasomatic effects.The sharp differences between the NE-suite and the PY-suite imply mantle heterogeneity on regional scale, whereas the variability within each suite is essentially related to different degrees of melting and/or metasomatism and imply mantle heterogeneity on local scale.     

Fertile and barren AlCr-spinel harzburgites from the upper mantle: Ion and electron probe analyses of trace elements in olivine and orthopyroxene: Relation to lherzolites

Ion and electron microprobe analyses of twenty-one CrAl-spinel harzburgite xenoliths from southern African kimberlites show two chemical groups. Orthopyroxenes from “fertile” harzburgites have higher CaO (mean of 11, 0.95 wt.%), Al₂O₃ (3.05 wt.%), Cr₂O₃ (0.85 wt.%) and Li (0.8 ppmw) than those from “barren” harzburgites (mean of 10, CaO 0.24 wt.%, Al₂O₃ 1.10 wt.%, Cr₂O₃ 0.35 wt.%, Li 0.3 ppmw). Olivines from all harzburgites have similar chemistry except that mean values of Li and Na are higher for barren than fertile harzburgites (Li 0.9 vs. 0.4 ppmw; Na₂O 16 vs. 7 ppmw). Orthopyroxenes from fertile harzburgites are chemically distinct from those in garne lherzolites from southern Africa and spinel lherzolites from southwest U.S.A., but orthopyroxenes from barren harzburgites are indistinguishable from those in many coarse garnet lherzolites.Chromium, Ca, Ni, Na and Li in coexisting olivines and orthopyroxenes from the above rock types show complex patterns, which for Ca, Cr and Ni can be related to pressure and temperature. Temperatures from an empirically calibrated thermometer based on Ni-Mg exchange between olivine and orthopyroxene, measured modes of harzburgites (fertile, mean of 10: ol 68, opx 31, spinel-silicate intergrowth <0.5; barren, mean of 8: ol 76, opx 23, spinel and spinel-silicate intergrowth 1), and high-pressure experimental studies suggest (a) that harzburgites are residues of partial melting, (b) that barren harzburgites were melted to a greater extent at a higher temperature (though probably at a similar depth) than fertile harzburgites, and (c) that incomplete reequilibration during retrograde metamorphism has led to development of complex inter- and intragranular textures, probably in the range ～700–900°C.     

Complete mantle section of a slow-spreading ridge-derived ophiolite: An example from the Isabela ophiolite in the Philippines

Abstract   The Isabela ophiolite shows a complete ophiolite sequence exposed along the eastern coast of northern Luzon, the Philippines. It forms the Cretaceous basement complex for the northeastern Luzon block. This ophiolite is located at the northern end of a trail of ophiolites and ophiolitic bodies along the eastern margin of the Philippine Mobile Belt. This paper presents new findings regarding the nature and characteristics of the Isabela ophiolite. Peridotites from the Isabela ophiolite are relatively fresh and are composed of spinel lherzolites, clinopyroxene-rich harzburgites, depleted harzburgites and dunites. The modal composition, especially the pyroxene content, defines a northward depletion trend from fertile lherzolite to clinopyroxene-rich harzburgites and more refractory harzburgites. Variation in modal composition is accompanied by petrographic textural variations. The chromium number of spinel, an indicator of the degree of partial melting, concurs with petrographic observations. Furthermore, the Isabela ophiolite peridotites are similar in spinel and olivine major-element geochemistry and clinopyroxene rare earth-element composition to abyssal peridotites from modern mid-oceanic ridges. Petrological and mineral compositions suggest that the Isabela ophiolite is a transitional ophiolite subtype, with the fertile lherzolites representing lower sections of the mantle column that are usually absent in most ophiolitic massifs. The occurrence of the fertile peridotite presents a rare opportunity to document the lower sections of the ophiolitic mantle. The variability in composition of the peridotites in one continuous mantle section may also represent a good analogy of the melting column in the present-day mid-oceanic ridges.     

Temperature and pressure condition of garnet lherzolite and websterite from west Qinling,China

The mineral thermobarometry proposed in literature is used to calculate the equilibrium temperature and pressure of garnet lherzolite and websterite xenoliths within the Cenozoic kamafugite from west Qinling, Gansu Province, China. The results show that the equilibrium temperature and pressure of garnet lherzolites and websterite and 1127–1266°C, 2.9–3.6 Gpa and 1169–1248°C, 2.8–3.2 Gpa respectively. The equilibrium peressures reach or exceed the equilibrium peressure of spinel lherzolites (2.0–3.0 GPa), and fall into the stability range of garnet peridotite. The equilibrium temperature of the xenoliths reach or exceed the ocean geotherm, identical with the melting temperature of kamafugite magma determined by experiments under the conditions of post-orogenic lithosphere extension. So the thermal state of Cenozoic mantle of the west Qinling may be fit to generate the kamafugite magmatism. The research on petrology-mineralogy and geobarothermometry of the xenoliths shows that both garnet lherzolite and websterite are mantle components of the west Qinling, and may be considered as source rocks of the Cenozoic kamafugite magma.

     

Chrome spinel in normal MORB‐type greenstones from the Paleozoic–Mesozoic Mino terrane,East Takayama area,central Japan: Crystallization course with a U‐turn

Phenocrystic chrome spinel crystallized in normal MORB‐type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe²⁺) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe³⁺ content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe²⁺ and Fe³⁺. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe²⁺‐ and Fe³⁺‐rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe²⁺‐ and Fe³⁺‐rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr‐ and Fe²⁺‐rich to Al‐ and Mg‐rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe²⁺) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe²⁺)–Cr/(Cr+Al) diagram, exhibits a U‐turn, and appears to be set on a double‐lane route. The U‐turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.     

The petrochemistry of ophiolite gabbroic complexes. A key for the classification of ophiolites into low-Ti and high-Ti types

Ophiolites have been divided into two groups: high-Ti and low-Ti types. These can be discriminated by studying the fractionation trends of both gabbroic complexes (this work) and lavas and dykes [16], particularly in the TiO₂/M.I. diagram. The first type typically shows MORB-like magmas whereas in the second the magma types have a spectrum of composition from mid-ocean ridge basalts to island arc tholeiites and boninite-like magmas often occur.High-Ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back-arc basin crusts as well as oceanic crust generated during the intermediate and late-stage opening of intraoceanic back-arc basins.Parental magmas and fractionation processes of low-Ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back-arc basins.     

由包体推导的河北汉诺坝下地壳-上地幔地温线及其地质意义

通过对采自河北汉诺坝玄武岩中的下地壳和上地幔包体的详细研究 ,建立了本区下地壳—上地幔地温线。该地温线高于大洋地温线和古老地盾地温线 ,接近克拉通边缘的地温线 ,符合该区的大地构造环境。由该地温线建立的下地壳—上地幔地质结构剖面表明 ,该区下地壳主要由不同类型的麻粒岩相岩石组成 ,其化学成分以镁铁质为主 ,深度范围为 2 5～ 4 2km。上地幔由超镁铁质的二辉橄榄岩组成 ,在尖晶石二辉橄榄岩和石榴石二辉橄榄岩之间有一过渡层。由地温线确定的壳幔边界位于 4 2km附近 ,与地震资料确定的莫霍面一致 ,但在壳幔边界之上的下地壳底部有下地壳麻粒岩和超镁铁质岩的互层。这一现象可以解释在下地壳底部常见的层状反射层。该区岩石圈底界大约在 95km ,其下的软流层仍由石榴石二辉橄榄岩组成     

Lead isotope study of orogenic lherzolite massifs

Orogenic lherzolites allow for almost “in-situ” observation of mantle isotopic heterogeneities on a restricted geographical scale, in contrast to basalts for which melting processes have averaged original mantle compositions over uncertain scales. Pb isotopes from whole rocks and clinopyroxenes from the massifs of Lherz (Pyrenees), Lanzo (Alps), Beni Bousera (Morocco) and Zabargad (Red Sea) show internal heterogeneities that encompass the entire range of variation observed in oceanic basalts. Some depleted lherzolites have a very unradiogenic composition similar to that of the most depleted ridge tholeiites. Pyroxenites from mafic layers generally have more radiogenic compositions, some of them comparable to the most radiogenic oceanic island results. The isotopic differences between lherzolites and pyroxenites vanish where layers are very closely spaced ( < 2 cm). In this case, the lherzolites may have equilibrated with the more Pb-rich pyroxenites through solid-state diffusion under mantle conditions. These results directly illustrate the smallest scales at which Pb isotopic heterogeneity may survive within the mantle.The genesis of these heterogeneities are discussed within the framework of the “marble cake” mantle model [1], where lherzolites are residues left over after oceanic crust extraction, whereas pyroxenites represent either basaltic or cumulate portions of the oceanic crust, reinjected by subduction and stretched by solid-state mixing during mantle convection. The Pb isotope data suggest that each massif was involved in several cycles of convective overturn, segregation and reinjection of the oceanic crust, during periods well over 1 Ga.If the upper mantle is made of interlayered radiogenic and unradiogenic layers, basalt heterogeneities may result from preferential melt-extraction from different layers depending on the degree of melting, as well as from large-scale, plume-related mantle heterogeneities. Orogenic lherzolites therefore allow direct observation of disseminated small-scale heterogeneities previously inferred from observations of oceanic basalts from seamounts and ridges.     

Modelling the composition of melts formed during continental breakup of the Southeast Greenland margin

We have developed a generic dynamic model of extension of the lithosphere, which predicts major element composition and volume of melt generated from initial extension to steady state seafloor spreading. Stokes equations for non-Newtonian flow are solved and the mantle melts by decompression. Strengthening of the mantle due to dehydration as melting progresses is included. The composition is then empirically related to depletion. Using a crystallisation algorithm, the predicted primary melt composition was compared with mean North Atlantic mid-ocean ridge basalt (MORB). At steady state, using half spreading rates from 10 to 20 mm yr^− 1 and mantle potential temperatures of 1300 to 1325 °C we predict a major element composition that is within the variation in the mean of North Atlantic MORB.

This model is applied to the Southeast Greenland margin, which has extensive coverage of seismic and ODP core data. These data have been interpreted to indicate an initial pulse of magmatism on rifting that rapidly decayed to leave oceanic crustal thickness of 8 to 11 km. This pattern of melt production can be recreated by introducing an initial hot layer of asthenosphere beneath the continental lithosphere and by having a period of fast spreading during early opening. The hot layer was convected through the melt region giving a pulse of high magnesian and low silica melt during the early rifting process. The predicted major element composition of primary melts generated are in close agreement with primary melts from the Southeast Greenland margin. The observed variations in major element composition are reproduced without a mantle source composition anomaly.