Mesozoic evolution of the upper mantle beneath the eastern Pyrenees: evidence from xenoliths in Triassic and Cretaceous alkaline volcanics of the eastern Corbières (France)

Upper mantle material can be sampled from two distinctive suites in the North Pyrenean Zone (NPZ) of the Pyrenees. These occur either as ultramafic tectonic slices in the central and western part of the NPZ, or as discrete xenoliths in alkaline magmas in its eastern part, know as the Corbières. In the eastern part of the PNZ, two ultramafic xenolith suites have been found. The first suite is enclosed within Triassic basalts and the second suite is enclosed within Cretaceous monchiquites. Both suites essentially comprise spinel peridotites showing varying degrees of depletion, but each clearly distinguishable by texture and mineral chemistry.

The Triassic suite of ultramafic xenoliths is characterized by coarse texture and homogeneous composition of mineral constituents. This records equilibrium temperature of around 950 ° C before inclusion in the host basalt. They represent fragments of an upper mantle type normally occurring beneath continental rift systems.

The Cretaceous suite of ultramafic xenoliths display porphyroclastic textures, which grade locally to ultramylonites. The pyroxene porphyroclasts are compositionally zoned, titanian pargasite is ubiquitous, and equilibrium temperatures of around 750–800 ° C are indicated. They appear to be similar to peridotites occurring in ultramafic tectonic massifs in the NPZ, and with a common texture, mineralogy and thermal history. This indicates therefore that shear deformation and alkaline magmatism, experienced during the Middle Cretaceous, affected the upper mantle along the entire length of the NPZ. This can then be related to the regional transcurrent movements that were produced by sinistral strike-slip of Iberia with respect to the rest of Europe.     

Petrological significance of high-pressure ultramafic xenoliths from ultrapotassic rocks of Central Italy

Two suites of ultramafic xenoliths have been found in ultrapotassic lavas from the 0.9 Ma old Torre Alfina volcano sited at the northern border of the Vulsinian district (Central Italy). One group of Xenoliths consists of spinel-bearing lherzolites, harzburgites, minor wherlites and dunites with a maximum size of 3–4 cm. Some samples contain discrete laths of phlogopite. A second class consists of phlogopite-rich, glass-bearing peridotites. The first suite displays textural characteristics such as triple points, deformed olivine with well developed kink banding and porphyroclastic textures indicating equilibration at high pressure. Pressure estimates give values in the range 1.3–2.5 GPa, corresponding to mantle depths in the area, where the present-day Moho is about 25 km deep. Equilibration temperatures have been estimated in the range between 950–1000°C. The chemical composition of some phases, such as the very high Fo contents of olivines (up to Fo₉₄ in harzburgites), Mg content of orthopyroxenes and Cr/Cr+Al ratios of clinopyroxenes and spinels, suggest that these xenoliths represent peridotites which suffered different degrees of partial melting before being incorporated into the Torre Alfina magma. On the other hand, the occurrence of phlogopite speaks for metasomatic events. The phlogopite-rich, glass-bearing xenoliths consist of phlogopite, olivine, clinopyroxene, rare orthopyroxene and glass. Apatite is the most common accessory. Olivine is present in both euhedral and strained crystals. A few relics of protogranular textures are also observed. Textural and chemical evidence suggests that these xenoliths represent mica-rich peridotites which have undergone phlogopite breakdown during rapid rise to the surface with the development of a K-rich liquid which reacted with mafic phases producing a rapid growth of olivine and, to a lower extent, pyroxene. Originally, these xenoliths may have represented intensively metasomatized upper mantle. However, a cumulitic origin from previous potassic magmatic events cannot be excluded. The host lavas have compositions intermediate between high-silica lamproite and Roman-type ultrapotassic rock. They have high abundances of incompatible elements and radiogenic Sr, coupled with high Mg content, MgO/CaO, Ni and Cr. These features support a genesis in a residual upper mantle which has suffered partial melting with the extraction of basaltic liquids, followed by metasomatic events which caused an enrichment in incompatible elements and radiogenic Sr. The presence of mantle-derived ultramafic xenoliths in the torre Alfina lavas testifies for a rapid uprise of the magma which reached the surface without suffering fractional crystallization and significant interaction with the upper crust. Accordingly, the Torre Alfina lavas represent an unique example of primitive potassic liquid in Central Italy.     

Refertilization of ancient lithospheric mantle beneath the central North China Craton: Evidence from petrology and geochemistry of peridotite xenoliths

The petrology and geochemistry of peridotite xenoliths in the Cenozoic basalts from Fanshi, the central North China Craton (NCC), provide constraints on the evolution of sub-continental lithospheric mantle. These peridotite xenoliths are mainly spinel-facies lherzolites with minor harzburgites. The lherzolites are characterized by low forsterite contents in olivines (Fo < 91) and light rare earth element (LREE) enrichments in clinopyroxenes. In contrast, the harzburgites are typified by high-Fo olivines (> 91), high-Cr# spinels and clinopyroxenes with low abundances of heavy REE (HREE). These features are similar to those from old refractory lithospheric mantle around the world, and thus interpreted to be relics of old lithospheric mantle. The old lithospheric mantle has been chemically modified by the influx of melts, as evidenced by the Sr–Nd isotopic compositions of clinopyroxenes and relatively lower Fo contents than typical Archean lithospheric mantle (Fo > 92.5). The Sr–Nd isotopic compositions of harzburgites are close to EM1-type mantle, and of the lherzolites are similar to bulk silicate earth. The latter could be the result of recent modification of old harzburgites by asthenospheric melt, which is strengthened by fertile compositions of minerals in the lherzolites. Therefore, the isotopic and chemical heterogeneities of the Fanshi peridotite xenoliths reflect the refertilization of ancient refractory lithospheric mantle by massive addition of asthenospheric melts. This may be an important mechanism for the lithospheric evolution beneath the Central NCC.     

Mesozoic thermal evolution of the southern African mantle lithosphere

The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure.

Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a “cratonic” geotherm (40 mW m⁻²), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at 1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U–Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592–618].

The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at 150 Ma to the southeast of the craton, propagating to the west by 108–74 Ma, the craton interior by 85–90 Ma and the far southwest and northwest by 65–70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30–100 Ma earlier and are probably connected.

Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow in southern Africa is related to this dynamic event and is not a direct reflection of the pre-existing lithospheric architecture.     

Continental crust and lithospheric mantle interaction beneath North China: isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean craton

Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.     

Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex

The Ordovician Thetford Mines ophiolite complex (TMOC) formed by boninite-fed seafloor-spreading, probably in a fore-arc environment. The mantle section is dominated by foliated harzburgite (≤ 5–6% clinopyroxene), cut by dunitic (± chromitite cores) and orthopyroxenitic veins and dykes. Contrasting structures, textures and mineral compositions allow us to subdivide the mantle. The granular-textured rocks of the Duck Lake Block (DLB) have two steeply-dipping foliations. The older foliation strikes NW, is sub-perpendicular to the Moho, and is interpreted to have resulted from upflow of the asthenosphere beneath the spreading ridge. This fabric is overprinted by a 2nd ductile foliation striking ENE, oriented sub-parallel to the Moho, which we interpreted as having formed by crust–mantle shear as the lithosphere migrated away from the spreading ridge. The DLB mantle has a limited range of spinel Cr# (100Cr / (Cr + Al) = 51–71). Comparison with experimentally determined residual spinel compositions (equilibrium melting) implies a maximum loss of 27–38% melt if the protolith had a fertile MORB mantle composition. However, interstitial-textured clinopyroxene may have high TiO₂ (< 0.04wt.%) and Na₂O (< 0.27wt.%), and some interstitial spinel has higher TiO₂ (< 0.09wt.%), suggesting interaction with (or crystallization from) an “impregnating” melt. Interstitial tremolitic amphibole also indicates the passage of late hydrous fluids. The harzburgite in the Caribou Mountain Block (CMB) has a porphyroclastic texture, with a strong, locally mylonitic foliation striking roughly N–S, parallel to the orientation of seafloor-spreading related paleo-normal faults in the crust. These fabrics and textures imply a colder, lithospheric deformation, possibly related to tectonic denudation (oceanic core complex). This would explain problematic lava/mantle contacts, favour infiltration of seawater, serpentinization, and reduced fO₂ conditions. The CMB mantle shows a wider range of mineral compositions than the DLB, with spinel Cr# (28–86) implying ≤ 15–45% of equilibrium melting. Locally higher TiO₂ in spinel (< 0.05wt.%) and clinopyroxene (< 0.11wt.%), a local rimward decrease in spinel Cr#, clinopyroxene Cr#, and olivine Fo-content, and traces of interstitial amphibole, are attributed to the circulation of an evolved hydrous melt during peridotite deformation. This suggests that the lower limit to the extent of melting inferred for the CMB (15%), established on the basis of Al-rich spinel rims and neoblasts, is probably too low. On the other hand, the higher inferred degree of depletion of the CMB is probably unaffected by the metasomatic overprint and is a more robust conclusion.     

The petrology of lower crustal xenoliths from the Executive Committee Range, Marie Byrd Land Volcanic Province, West Antarctica

Alkaline magmas from the late-Cenozoic Marie Byrd Land Volcanic Province, West Antarctica, have entrained lithospheric xenoliths which vary from spinel lherzolites to supracrustal rocks. Lower crustal xenoliths have been collected from the Executive Committee Range (Mounts Hampton in the north and Mount Sidley in the south) in central Marie Byrd Land, and their petrological characteristics together with preliminary geochemical data are discussed here. Granulite xenoliths include metaigneous gabbros and norites with varying proportions of clinopyroxene, spinel and either olivine or orthopyroxene. Pyroxenites occur together with granulites, which on the basis of their similar mineral assemblage, texture and composition are considered to be related to the granulites. The composition of xenoliths from Mounts Hampton and Sidley differ (e.g. Mount Sidley xenoliths have Mg# 32–80, are relatively LREE enriched and have ⁸⁷Sr/⁸⁶Sr of 0.70286–0.70376 and ¹⁴³Nd/¹⁴⁴Nd of 0.512864–0.512870, whereas Mount Hampton xenoliths have Mg# 68–78, are LREE depleted and have ⁸⁷Sr/⁸⁶Sr of 0.70420–0.70458 and ¹⁴³Nd/¹⁴⁴Nd of 0.512771–0.512819), defining a major lateral lower crustal discontinuity beneath the Executive Committee Range. Relict igneous textures and low abundances of incompatible elements indicate that the xenoliths initially formed as cumulates rather than as trapped melts. The xenolith suite differs in composition to the host rocks (Mount Sidley VOLCANICS = ⁸⁷Sr/⁸⁶Sr of 0.70300–0.70312 and ¹⁴³Nd/¹⁴⁴Nd of 0.512814–0.512907) and cannot be co-genetic with them. They are interpreted here to represent the cumulates of mantle melts that evolved by crystal fractionation at lower crustal depths.     

Layered mantle at the Karelian Craton margin: P–T of mantle xenocrysts and xenoliths from the Kaavi–Kuopio kimberlites, Finland

Peridotitic clinopyroxene (cpx) and pyrope garnet xenocrysts from four kimberlite pipes in the Kaavi–Kuopio area of Eastern Finland have been studied using major and trace element geochemistry to obtain information on the vertical compositional variability of the underlying mantle. The xenocryst data, when combined with the petrological constraints provided by peridotite xenoliths, yield a relatively complete section through the lithospheric mantle. Single-grain cpx thermobarometry fits with a 36-mW/m² geotherm calculated using heat flow constraints and xenolith modes and geophysical properties. Ni thermometry on pyrope xenocrysts gives 700–1350 °C and, based on the cpx xenocryst/xenolith geotherm, indicates a wide sampling interval, ca. 80–230 km. Plotting pyrope major and trace element compositions as a function of temperature shows there are three distinct layers in the local lithospheric mantle:

(1) A low-temperature (<850 °C) harzburgite layer distinguished by Ca-rich but Ti-, Y- and Zr-depleted pyropes. The xenoliths originating from this layer are all fine-grained garnet-spinel harzburgites with secondary cpx.

(2) A variably depleted lherzolitic, harzburgitic and wehrlitic horizon from 950 to 1150 °C or 130 to 180 km.

(3) A deep layer from 180 to 240 km composed largely of fertile material.

The peridotitic diamond window at Kaavi–Kuopio stretches from the top of the diamond stability field at 140 km to the base of the harzburgite-bearing mantle at about 180 km, implying a roughly 40-km-wide prospective zone.     

Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence

This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1–5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the migrating MORB-type melts, acquired by interaction with the ambient plagioclase peridotite. These replacive spinel harzburgite and dunite bodies are interpreted as channels exploited for focused and reactive migration of silica-undersaturated melts with aggregate MORB compositions. Such melts were unrelated to the silica-saturated melts that refertilized the pre-existing plagioclase peridotites. Finally, MORB melt migration occurred along open fractures, now recorded as gabbroic dikes.

Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.     

Petrology and Geochemistry of Peridotite Xenoliths in Alkali Basalts from the Transdanubian Volcanic Region, West Hungary

Type I (Cr-diopside group) peridotite xenoliths occur abundantlyat four volcanic vents (Szentb?k?lla, Szigliget, Bondor?-hegy,G?rce) in the Transdanubian Volcanic Region of Hungary (TVR).They comprise four-phase spinel Iherzolites and spinel harzburgites.The only additional phase a brown amphibole in some of the nodulesfrom Szigliget. Protogranular and equigranular textures prevailexcept at G?rce where porphyroclastic textures predominate.Attention has been focused however, on a rarer textural typedisplaying spherical, subhedral, or rarely even euhedral spinelinclusions in silicate phases (special group xenoliths). Major and trace element data permit the reconstruction of severalmantle events and provide indications of the nature and dimensionsof heterogeneities in the lithospheric mantle beneath the TVR.The most important events in their supposed order of time aresummarized as follows: (A) An old depletion event evidencedby LREE abundances in diopside (Type IA xenoliths) (B) laterenrichment in LREE and other LIL elements (Type IB xenoliths);(C) hydrous metasomatism bringing about the formation of amphibole;(D) magmatic introduction of Type II (Al-augite group) materialand related chemical exchange reactions with cool mantle rocks;and (E) veining of the uppcr mantle by plagioclase-free amphibolite(probably slightly prior to the volcanic eruptions). In addition,in our interpretation special group xenoliths indicate an ancientmagmatic event. As evidence we can mention: (1) textural featureswhich may be connected with generation of cumulates and whichare associated with (2) anomalous bulk rock and phase chemistry(as compared with most published data) such as: (a) a generallow abundance of Al; (b) high Cr in the spinel phase; (c) unusuallyhigh contents of Fe³⁺ in spinel; (d) anomalous distributionof Al between enstatite and diopside; and (e) other disequilibriumfeatures such as the lack of the expected correlation of majorelements with mg-value or important departures from the correlationlines. These peculiarities suggest that the special group xenolithsprobably reflect crystal accumulation rather than an equilibriumpartial melting process.     

鲁西中生代辉长-闪长岩中辉石岩捕虏体的岩石成因

鲁西中生代辉长-闪长岩中包含有变晶结构和堆积结构两种类型辉石岩类捕虏体,它们的矿物化学和岩石地球化学特征可同中国东部新生代玄武岩中的辉石岩类包体相对比.它们代表了上地幔两次岩浆底侵事件的产物.辉石岩类捕虏体母岩浆来自于含有陆壳物质的软流圈及其上部岩石圈地幔的部分熔融.辉石岩类捕虏体是由该母岩浆高压分离结晶堆积的产物.辉石岩的母岩浆在上地幔的运移是引起鲁西中生代岩石圈地幔富硅质交代作用的主要因素.     

岚皋碱性镁铁-超镁铁质潜火山杂岩中金云角闪辉石岩类地幔捕虏体矿物学特征

金云角闪辉石岩类捕虏体产于扬子板块北缘岚皋县境的早古生代碱性镁铁质-超镁铁质潜火山杂岩中,它们的典型矿物组成为单斜辉石、角闪石、金云母、磷灰石和钛铁矿。捕虏体岩石具粗-细粒等粒结构、板状等粒结构、三联点结构、碎裂结构和肯克带结构,交代结构十分发育。岩石产状、结构和矿物成分特征研究表明,金云角闪辉石岩类捕虏体是地幔交代作用产物,可能代表了该区碱性镁铁质-超镁铁质潜火山杂岩的源区岩石。     

Pressure and temperature evolution of upper mantle under the Rio Grande Rift

Spinel peridotite xenoliths associated with the Rio Grande Rift axis (Potrillo and Elephant Butte volcanic fields) and the western rift shoulder (Adams Diggings) have been investigated to correlate pre-eruptive pressure and temperature conditions with xenolith deformation textures and rift location. Temperatures of xenolith equilibration at the rift shoulder are 100–250°C cooler for a given pressure than the temperatures at the rift axis. Undeformed xenoliths (protogranular texture) are derived from higher temperature and higher pressure conditions than deformed xenoliths (porphyroclastic and equigranular textures) in the rift axis. Exsolution lamellae in pyroxenes, small decreases in Al contents of orthopyroxenes from core to rim, and small differences in porphyroclastic orthopyroxene compositions versus neoblastic orthopyroxene compositions indicate high temperatures followed by cooling and a larger cooling interval in deformed rocks than in undeformed rocks. These features, along with thermal histories based on calcium zoning in olivine rims, indicate that the upper mantle under Adams Diggings and Elephant Butte has undergone cooling from an initial high temperature state followed by a late heating event, and the upper mantle under Potrillo has undergone cooling, reheating, and late heating events.     

Os, Nd, and Sr isotopic and chemical compositions of ultramafic xenoliths from Kurose, SW Japan: Implications for contribution of slab-derived material to wedge mantle

We examined seven ultramafic xenoliths from 1~3 Ma alkali olivine basalt reefs near the Eurasian continent and one sample of the host alkali basalt to identify the mantle wedge material and to constrain the origin and evolution of mantle beneath SW Japan. Six xenoliths are from Kurose and one xenolith is from Takashima, northern part of the Kyushu islands, SW Japan. The Sr and Nd isotopic ratios vary from 0.70416 to 0.70773 and from 0.51228 to 0.51283, respectively. The Kurose and Takashima xenoliths have higher Sr isotopic ratios and lower Nd isotopic ratios than those of the peridotite xenoliths from the other arc settings such as Simcoe and NE Japan.

The Kurose xenoliths have less radiogenic Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os = 0.123–0.129) than the primitive upper mantle (PUM) estimate and limited variation compared to the other arc xenoliths. Their Os isotope compositions are rather similar to the ultramafic xenoliths from NE and east China. In addition, the samples of the Kurose and Takashima xenoliths plot along a mixing line between ultramafic xenoliths from SE and NE China and a slab component in Sr–Nd–Os isotopic space. Our results suggest that fragments of continental lithospheric mantle from the China craton may exist beneath Kurose and Takashima after the Sea of Japan expansion when the Japanese islands were rifted away from the Eurasian continent during Miocene. Later magmatism due to subduction of the Philippine Sea Plate beneath the SW Japan arc around 15 Ma ago may have introduced fluids or melts derived from slab component, interpreted to be oceanic sediments rather than altered oceanic crust, that possibly modified the original composition of the lithospheric mantle sampled by the peridotite xenoliths from Kurose and Takashima.     

Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.     

Particle size distributions of fault rocks and fault transpression: are they related?

ABSTRACT Laboratory experiments on rock faulting show that processes of particle comminution in fault rocks are influenced by several parameters, including fault strike and normal stress across faults. In nature, normal stress across faults increases with increasing transpressional strike of faults. Accordingly, different structural fabrics and particle size distributions are expected for cataclastic rocks that have developed along faults with different transpressional orientations and comparable displacements within regional-scale strike-slip fault zones. Adjacent bands of cataclastic gouge and breccia were analysed from four small-scale fault zones. All have comparable displacements and very similar protolith (i.e. shallow-water limestone), structure, kinematics, size, and tectonic environment, but different transpressional strikes within the regional-scale left-lateral Mattinata strike-slip fault, Italy. An inverse linear relationship is found between fault transpressional angles and fractal dimensions of particle size distributions from cataclastic rock samples.     

Seismic properties of the upper mantle beneath Lanzarote (Canary Islands): Model predictions based on texture measurements by EBSD

We present a petrophysical analysis of upper mantle xenoliths, collected in the Quaternary alkali basalt fields (Series III and IV) from the island of Lanzarote. The samples consist of eight harzburgite and four dunite nodules, 5 to 15 cm in size, and exhibit a typical protogranular to porphyroclastic texture. An anomalous foliation resulting from strong recovery processes is observed in half of the specimens. The lattice preferred orientations (LPO) of olivine, orthopyroxene and clinopyroxene were measured using electron backscatter diffraction (EBSD). In most samples, olivine exhibits LPOs intermediate between the typical single crystal texture and the [100] fiber texture. Occasionally, the [010] fiber texture was also observed. Simultaneous occurrence of both types of fiber textures suggests the existence of more than one deformation regime, probably dominated by a simple shear component under low strain rate and moderate to high temperature. Orthopyroxene and clinopyroxene display a weaker but significant texture. The LPO data were used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and were compared to field geophysical data reported from the literature. The velocity of P-waves (7.9 km/s) obtained for a direction corresponding to the existing seismic transect is in good agreement with the most recent geophysical interpretation. Our results are consistent with a roughly W–E oriented fastest P-wave propagation direction in the uppermost mantle beneath the Canary Islands, and with the lithosphere structure proposed by previous authors involving a crust–mantle boundary at around 18 km in depth, overlaid by intermediate material between 11 and 18 km.     

The late Cretaceous lithospheric mantle beneath the Central Andes: Evidence from phase equilibria and composition of mantle xenoliths

Temperature estimates and chemical composition of mantle xenoliths from the Cretaceous rift system of NW Argentina (26°S) constrain the rift evolution and chemical and physical properties of the lithospheric mantle at the eastern edge of the Cenozoic Andean plateau. The xenolith suite comprises mainly spinel lherzolite and subordinate pyroxenite and carbonatized lherzolite. The spinel lherzolite xenoliths equilibrated at high-T (most samples >1000 °C) and P below garnet-in. The Sm–Nd systematics of compositionally unzoned clino- and orthopyroxene indicate a Cretaceous minimum age for the high-T regime, i.e., the asthenosphere/lithosphere thermal boundary was at ca. 70 km depth in the Cretaceous rift. Major elements and Cr, Ni, Co and V contents of the xenoliths range between values of primitive and depleted mantle. Calculated densities based on the bulk composition of the xenoliths are <3280 kg/m³ for the estimated P–T conditions and indicate a buoyant, stable upper mantle lithosphere. The well-equilibrated metamorphic fabric and mineral paragenesis with the general lack of high-T hydrous phases did not preserve traces of metasomatism in the mantle xenoliths. Late Mesozoic metasomatism, however, is obvious in the gradual enrichment of Sr, U, Th and light to medium REE and changes in the radiogenic isotope composition of an originally depleted mantle. These changes are independent of the degree of depletion evidenced by major element composition. ¹⁴³Nd/¹⁴⁴Nd_i ratios of clinopyroxene from the main group of xenoliths decrease with increasing Nd content from >0.5130 (depleted samples) to ca. 0.5127 (enriched samples). ⁸⁷Sr/⁸⁶Sr_i ratios (0.7127–0.7131, depleted samples; 0.7130–0.7134, enriched samples) show no variation with variable Sr contents. Pb_i isotope ratios of the enriched samples are rather radiogenic (²⁰⁶Pb/²⁰⁴Pb_i 18.8–20.6, ²⁰⁷Pb/²⁰⁴Pb_i 15.6–15.7, ²⁰⁸Pb/²⁰⁴Pb_i 38.6–47) compared with the Pb isotope signature of the depleted samples. The large scatter and high values of ²⁰⁸Pb/²⁰⁴Pb_i ratios of many xenoliths indicates at least two Pb sources that are characterized by similar U/Pb but by different Th/Pb ratios. The dominant mantle type in the investigated system is depleted mantle according to its Sr and Nd isotopic composition with relatively radiogenic Pb isotope ratios. This mantle is different from the Pacific MORB source and old subcontinental mantle from the adjacent Brazilian Shield. Its composition probably reflects material influx into the mantle wedge during various episodes of subduction that commenced in early Paleozoic or even earlier. Old subcontinental mantle was already replaced in the Paleozoic, but some inheritance from old mantle lithosphere is represented by rare xenoliths with isotope signatures indicating a Proterozoic origin.     

The effect of the Fernando de Noronha plume on the mantle lithosphere in north-eastern Brazil

New xenolith occurrences in the Cenozoic alkali basalts of north-eastern Brazil have been studied in order to constrain the possible imprint on the continental mantle lithosphere of its passage over the Fernando de Noronha plume and the regional mantle processes. Texturally, the lherzolite and harzburgite xenoliths define three groups: group 1, porphyroclastic; group 2, protogranular; group 3, transitional between groups 1 and 2. Equilibrium temperatures are highest for group 1 and lowest for group 2. Clinopyroxenes from group 1 peridotites have Primitive Mantle (PM)-normalised REE patterns varying from L-MREE-enriched convex-upward, typical of phases in equilibrium with alkaline melts, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. Group 2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. The M-HREE are at 3–5 ×PM concentration level, as typical in fertile lithospheric lherzolites. Most of group 3 clinopyroxenes show LREE-depleted patterns similar to the group 2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and EMI-like components, both of them are also present in the host basalts. Melts estimated to be in equilibrium with the group 1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The groups 2 and 3 clinopyroxenes define two distinct compositional fields at higher ¹⁴³Nd/¹⁴⁴Nd values, correlated with their LREE composition. The isotopes of the groups 2 and 3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes.

The LREE-enriched spoon-shaped group 3 clinopyroxenes are characterised by the highest ¹⁴³Nd/¹⁴⁴Nd values at any given ⁸⁷Sr/⁸⁶Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, group 3 clinopyroxenes. Group 1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the group 2 documents the shallower lithospheric section, with group 3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.     

Flow in upper-mantle rocks: Some geophysical and geodynamic consequences

Flow mechanisms effective in the upper mantle and some of the parameters of the creep equation are determined from the study of peridotites from basalt and kimberlite xenoliths and alpine-type massifs. Creep controlled by dislocation climb, as inferred by Weertman, is the dominant mechanism. Evidence for superplastic flow is found in the deepest kimberlite xenoliths. Flow in the alpine-type massifs is ascribed either to intrusion in the crust when continental plates collide (lherzolite massifs) or to sea-floor spreading (harzburgite massifs included in ophiolites). The consideration of textures, crystal substructures and preferred orientations connected with P,T equilibrium conditions derived from pyroxenes, helps in deciphering the large-scale structure and flow of peridotites in the crust and in the mantle down to 200 km. For the first 150 km, the representative structures are those of the basalt xenoliths and the kimberlite xenoliths with a coarsegrained texture. They have many features in common and probably represent a static lithosphere with, in basalt xenoliths, possible evidence for the transition to the shear flowing asthenosphere. The porphyroclastic and mosaic-textured xenoliths, in kimberlites equilibrated at depth between 150 and 200 km and a few more superficial basalt xenoliths, reflect a much larger strain rate and applied stress and might be connected to vertical instabilities also responsible for magma genesis.