Crystallization of Cr-poor and Cr-rich megacryst suites from the host kimberlite magma: implications for mantle structure and the generation of kimberlite magmas

Cr-poor and Cr-rich megacryst suites, both comprising of varying proportions of megacrysts of orthopyroxene, clinopyroxene, garnet, olivine, ilmenite and a number of subordinate phases, coexist in many kimberlites, with wide geographic distribution. In rare instances, the two suites occur together on the scale of individual megacryst hand specimens. Deformation textures are common to both suites, suggesting an origin related to the formation of the sheared peridotites that also occur in kimberlites. Textures and compositions of the latter are interpreted to reflect deformation and metasomatism within the thermal aureole surrounding the kimberlite magma in the mantle. The megacrysts crystallized in this thermal aureole in pegmatitic veins representing small volumes of liquids derived from the host kimberlite magma, which were injected into a surrounding fracture network prior to kimberlite eruption. Close similarities between compositions of Cr-rich megacryst phases and those in granular lherzolites are consistent with early crystallization from a primitive kimberlite liquid. The low-Cr megacryst suite subsequently crystallized from residual Cr-depleted liquids. However, the Cr-poor suite also reflects the imprint of contamination by liquids formed by melting of inhomogeneously distributed mantle phases with low melting temperatures, such as calcite and phlogopite, present within the thermal aureole surrounding the kimberlite magma reservoir. Such carbonate-rich melts migrated into, and mixed with some, but not all, of the kimberlite liquids injected into the mantle fracture network. Contamination by the carbonate-rich melts changed the Ca–Mg and Mg–Fe crystal–liquid distribution coefficient, resulting in the crystallization of relatively Fe-rich and Ca-poor phases. The implied higher crystal-melt Mg–Fe distribution coefficient for carbonate-rich magmas accounts for the generation of small volumes of Mg-rich liquids that are highly enriched in incompatible elements (i.e. primary kimberlite magmas). The inferred metasomatic origin for the sheared peridotites implies that this suite provides little or no information regarding vertical changes in the thermal, chemical and mechanical characteristics of the mantle.     

Thermal and redox state of the subcontinental lithospheric mantle of NE Spain from thermobarometric data on mantle xenoliths

Mantle xenoliths in within-plate Cenozoic alkaline mafic lavas from NE Spain are used to assess the local subcontinental lithospheric mantle geotherm and the influence of melting and metasomatism on its oxidation state. The xenoliths are mainly anhydrous spinel lherzolites and harzburgites and gradations between, with minor pyroxenites. Most types show protogranular textures, but transitional protogranular–porphyroclastic and equigranular lherzolites also exist. Different thermometers used in the estimates provide higher subsolidus equilibrium temperatures for harzburgites (1,062 ± 29°C) than for lherzolites (972 ± 89°C), although there is overlap; the lowest temperatures correspond to porphyroclastic lherzolites, whereas pyroxenites give the highest temperatures (up to 1,257°C). Maximum pressures for subsolidus equilibrium of peridotites are at 2.0–1.8 GPa. Later they followed adiabatic decompression and harzburgites registered lower pressures (1.02 ± 0.19 GPa) than lherzolites (1.41 ± 0.27 GPa). One pyroxenite gives values consistent with the spinel lherzolite field (1.08 GPa). The shallowest barometric data are in agreement with the highest local conductive geotherms, which implies that the lithosphere–asthenosphere boundary is at 70–60 km minimum depth. Higher equilibrium temperatures for the harzburgites could be explained by the existence of mafic magma bodies or dykes at the lower crust–mantle boundary. Paleo-fO₂ conditions during partial melting as inferred from the covariation between V and MgO concentrations are mainly between QFM−1 and QFM−2 in log units. However, most thermobarometric fO₂ estimates are between QFM−1 and QFM+1, suggesting oxidation caused by later metasomatism during uplift and cooling.     

Major element composition of the lithospheric mantle under the North Atlantic craton: Evidence from peridotite xenoliths of the Sarfartoq area,southwestern Greenland

The composition and thermal state of the lithospheric mantle under the North Atlantic craton was investigated using a suite of peridotite xenoliths from the diamond-bearing Sarfartoq kimberlite dike swarm of southwestern Greenland. Elevated olivine and whole-rock Mg# (>0.9) attest to the refractory nature of the Sarfartoq mantle showing comparable degrees of depletion to other cratonic roots. Modal analyses indicate that the Sarfartoq mantle is not typified by the orthopyroxene enrichment observed in the Kaapvaal root, but shows more affinity with the Canadian Arctic (Somerset Island), Tanzania, and East Greenland (Wiedemann Fjord) peridotites. The Sarfartoq peridotites have equilibrated at temperatures and pressures ranging from 660 to 1,280 °C and from 2.2 to 6.3 GPa, and define a relatively low mantle heat flow of 13.2±1 mW/m^2. In addition, the lithospheric mantle underneath the Sarfartoq area is compositionally layered as follows: (1) an internally stratified upper layer (70 to 180 km) consisting of coarse, un-deformed, refractory garnet-bearing and garnet-free peridotites and, (2) a lower layer (180 to 225 km) characterized by fertile, CPX-bearing, porphyroclastic garnet lherzolites. The stratification observed in the upper refractory harzburgite layer (70–180 km) is reflected by an increase in fertility (e.g., decrease in olivine abundance and forsterite content) with depth. The sharp nature of the boundary between the upper and lower layers may indicate multistage growth of the lithospheric mantle.Editorial responsibility: T.L. Grove     

Garnet lherzolites from the Hanaus-I and Louwrensia kimberlites of Namibia

The Gibeon cluster of Namibian kimberlites is emplaced into the Orange River Belt which has accreted to the Kaapvaal Craton. These offcraton kimberlites lack diamonds and are younger than the diamondiferous on-craton kimberlites. The Hanaus-I and Louwrensia kimberlites each contain a bimodal suite of upper-mantle-derived garnet lherzolite xenoliths characterized by a coarse granular or mosaic porphyroclastic texture. The Louwrensia pipe in addition contains garnet harzburgites. Deformed lherzolites are not iron-enriched relative to the coarse types. Conditions of equilibration calculated by the Wells-Wood method are 841–1,013° C at 25.6–36.3 kbars, and 869–1,195° C at 23.9–39.4 kbars, for coarse lherzolites from Louwrensia and Hanaus respectively, and from 1,080–1,112° C at 31.6–34.5 kbars, and 983–1,228° C at 24.7–35.2 kbars, for mosaic porphyroclastic types from Louwrensia and Hanaus respectively. The coarse varieties from both localities have similar equilibration conditions to coarse lherzolites from on-craton kimberlites and define the lower limb of a perturbed geotherm. The upper high temperature limb of the Namibian geotherm is considered to be an apparent geotherm generated by the deformation and metasomatism of the upper mantle by a rising diapir. Such geotherms, being the result of kimberlite-xenolith interactions, provide no stratigraphic or thermal information concerning the site of kimberlite or diamond formation.     

Peridotite xenoliths from Grenada,Lesser Antilles Island Arc

Ultramafic xenoliths comprising harzburgite, lherzolite (reacted harzburgite) and spinel-rich dunite, occur in alkali olivine basalts (M series) of Grenada in the Lesser Antilles island arc. Textures are protogranular, porphyroclastic and granular; the latter are restricted to dunites and areas of the harzburgites/lherzolites where interaction with host magma has occurred. Primary mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel. Harzburgites are residual from a fractional partial melting event totaling ~22%. Infiltration of harzburgite by (and reaction with) basalt has produced: a wehrlite, with partial dissolution of primary spinel, an increase in the oxygen fugacity (ƒ_O2) from primary values 1–2 log ƒ_O2 units above the fayalite-magnetite-quartz (FMQ) buffer, to 2–2.5 log units above the buffer; reaction of orthopyroxene to form patches of intergrown olivine and clinopyroxene, and bronzite andesite glass (60 wt%, SiO₂ 18–20 wt% Al₂O₃ and 3–4 wt% Na₂O) with flat to light rare earth element-depleted, chondrite-normalized abundances. Refertilisation of the mantle by reacting melts, producing a clinopyroxene-rich lithology, may form a source of ankaramitic (high-Ca) arc basalts.Editorial responsibility: T.L. Grove     

Garnet lherzolites from Somerset Island,Canada and aspects of the nature of perturbed geotherms

Garnet lherzolite xenoliths of similar petrography and mineralogy are found in the Elwin Bay, Nanorluk, and Amayersuk kimberlites. The xenoliths are either coarse equant to coarse tabular or porphyroclastic in texture. Compositions of coexisting pyroxenes indicates equilibration at 1000–1270° C at 34–41 kb (Wood-Banno/Wood method) or 865–1200° C at 29–36 kb (Wells/Wood method). No simple correlation exists between textural types and equilibration temperature. A primary spinel-bearing garnet lherzolite has equilibrated at 840° C at 21 kb (Wells/Wood) and provides the only known example of a xenolith with relatively high Cr/Cr+Al which has equilibrated at the spinel to garnet lherzolite transition along the continental geotherm. The pressure and temperature estimates for the xenoliths lie above those of the steady state geotherm and indicate that a perturbed geotherm existed in this region at the time of kimberlite intrusion. The formation of perturbed geotherms is discussed and it is considered that the upper high temperature limbs of inflected geotherms are transient pseudogeotherms generated in response to a thermal aureole about a rising mantle diapir and that the lherzolites which define such a geotherm represent a telescoped section of the mantle and include xenoliths derived from above and below the point of kimberlite liquid segregation. The lower temperature limbs of inflected geotherms are considered to be representative of the steady state geotherm and are sampled by the kimberlite which after segregation from the diapir rises at a much faster rate than the parent diapir and passes through material which is unaffected by the diapir thermal aureole.     

Thermal history of iherzolite xenoliths—I. Petrology of iherzolite xenoliths from the ichinomegata crater,oga peninsula,northeast Japan

The thermal history of four spinel lherzolites (Lhz-13. Lhz-28, Lhz-29 and Lhz-53) from tuff breccia of the Ichinomegata crater, northeast Japan, has been studied in detail. Lhz-13 and Lhz-53 showed nearly perfect chemical homogeneity of the constituent minerals, and increase of Ca near the rim of olivine is the only disequilibrium evidence observed. In addition to the Ca zoning in olivine, Lhz-28 and Lhz-29 revealed compositional zoning in the Mg/Mg + Fe ratio and Ca content in ortho- and clinopyroxenes. Lhz-13 and Lhz-53 equilibrated at about 800°C in the upper mantle, based on Fe/Mg partitioning between olivine/spinel and olivine/clinopyroxene, and on the mutual solubility of Ca between olivine and pyroxenes. Lhz-28 and Lhz-29 also equilibrated originally at about 800°C, but were preheated at about 1000°C prior to their entrapment by the ascending host magma. The Fe/Mg partitioning between olivine /spinel and olivine/clinopyroxene reequilibrated during the preheating event: however, the Ca solubility did not reequilibrate. Olivine alone has rehomogenized with a high-Ca content but pyroxenes were compositionally zoned with Ca. The preheating event, indicated by the high-Ca content in the core of olivine, is recognized from about a half of the Ichinomegata Iherzolites (50 xenoliths were studied).The duration of heating during the transport of the xenolith by the magma (estimated from the width of the Ca zoning in the rim of olivine) ranges between several hours to a year depending on the rock specimen. From the requirement to reset olivine core compositions, the duration of the preheating event was estimated as greater than 1000 yr.     

Thermal evolution of the mantle underneath the Mid-German crystalline rise: Evidence from mantle xenoliths from the Rhön area (Central Germany)

Summary A suite of ultramafic xenoliths (spinel peridotites, one olivine-clinopyroxene hornblendite, and one spinel pyroxenite) from Tertiary basalt vents and lava flows of the Rhön area (Central Germany) were investigated petrologically and geochemically. With regard to P-T estimates two distinct groups of peridotite xenoliths can be discriminated: (I) A low- to intermediate-temperature group of spinel lherzolites and wehrlites mainly displaying coarse equant textures yielded temperature estimates in the range of 840–1050 °C at rather variable pressures of 11–24 kbar. The strong variability of the P-T estimates is attributed to mineral chemical disequilibria and different diffusion rates of the elements used for geothermometry and geobarometry. Spinel-pyroxene symplectites within part of these xenoliths point to a former position in the stability field of garnet lherzolite. These xenoliths are variably depleted in the basaltic component by partial melt extraction. They often show an enrichment in LREE and MREE which is due to a later overprinting by cryptic metasomatism. (II) A high-temperature group of xenoliths, which mainly consists of porphyroclastic and subordinate coarse equant spinel lherzolites and harzburgites, experienced temperatures of 1190–1270 °C at 19–26 kbar. The P-T values for these xenoliths fall close to a geothermal gradient of about 90 mW/m² and illustrate intense heating processes in the mantle which were often coupled with ductile deformation caused by lithospheric stretching. The thermal disturbance which led to the reequilibration of these peridotites must have occurred during the Tertiary magmatic event as indicated by the absence of retrograde mineral zoning, missing textural reequilibration, and the presence of partial melting phenomena in clinopyroxene. Unlike sheared xenoliths from other locations, the porphyroclastic high-temperature peridotites from the Rhön are depleted in basaltic component, in HREE, Y, and Sc. An olivine-clinopyroxene hornblendite is classified as some kind of basaltic cumulate which - according to its P-T estimate of about 1150 °C at 9 kbar - originates from hte transition zone between the lower crust and the upper mantle.⁴⁰Ar-³⁹Ar dating of kaersutite from this sample indicates an age of about 25 Ma which is in accordance with the beginning of Tertiary volcanism in the Rhön area. These investigations show that part of the lithospheric mantle underneath the Rhön area experienced a thermal reequilibration during the Tertiary magmatic event while other parts give evidence of an older history, i.e. a cryptic metasomatism and a transition from the garnet- to the spinel-lherzolite field. A possible geotectonic scenario for the transition could be the post-Variscan crustal reequilibration.

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Thermische Entwicklung des Mantels unter der Mitteldeutschen Kristallinschwelle abgeleitet aus Mantelxenolithen der Rhön

Zusammenfassung Eine Reihe von ultramafischen Xenolithen (Spinell-Peridotite, ein Olivin-KlinopyroxenHornblendit und ein Spinell-Pyroxenit) aus tertiären Basalten der Rhön wurde petrologisch and geochemisch untersucht. Zwei Gruppen von Peridotiten können hinsichtlich ihrer Äquilibrierungstemperaturen unterschieden werden: (I) Spinell-Lherzolithe and -Wehrlite mit niedrigen und mittleren Temperaturen von 840–1050 °C bei Drucken von 11-24 kbar zeigen zumeist granulare Gefüge. Die große Schwankungsbreite dieser P-T-Abschätzungen ist durch mineralchemische Ungleichgewichte und unterschiedliche Schließungstemperaturen der Geothermometer und Geobarometer bedingt. Spinell-Pyroxen-Symplektite, die in einigen dieser Xenolithe auftreten, weisen auf eine vormalige Position im Stabilitätsfeld von GranatLherzolith hin. Diese Xenolithe sind durch partielle Schmelzextraktion unterschiedlich stark depletiert. Häufig zeigen she eine Anreicherung der leichten and mittleren Seltenerdelemente, was durch eine spätere kryptometasomatische Überprägung bedingt ist. (II) Eine Gruppe hochtemperierter, vorwiegend porphyroklastischer und untergeordnet granularer Spinell-Lherzolithe und -Harzburgite wurde unter Temperaturen von 1190–1270 °C bei Drucken von 19–26 kbar überprägt. Die P-T Werte für diese Xenolithe liegen auf einem geothermischen Gradienten von über 90 mW/m², was auf intensive Aufheizprozesse im Mantel hinweist. Oftmals wurde these Aufheizung von einer duktilen Deformation begleitet, deren Ursache eine Dehnung der Mantellithosphäre war. Aufgrund der fehlenden Gleichgewichtstexturen, der Abwesenheit von retrograden Mineralzonierungen und partieller Schmelzprozesse bei den Klinopyroxenen dieser Xenolithe müssen ihre Deformation and Hochtemperaturüberprägung während des tertiären Magmatismus stattgefunden haben. Die porphyroklastischen, hochtemperierten Xenolithe zeigen die stärkste Abreicherung an basaltischer Komponente, den schweren Seltenerdelementen sowie an Y and Sc. Bei dem Olivin-Klinopyroxen-Hornblendit handelt es sich um ein basaltisches Kumulat, welches aufgrund seiner P-T Abschätzung (um 1150 °C bei 9 kbar) aus dem Übergangsbereich von unterer Kruste zu oberem Mantel stammt. Eine⁴⁰Ar-³⁹Ar Datierung von Kaersutiten dieser Probe weist auf ein Alter von etwa 25 Ma, was in Übereinstimmung mit dem Beginn des tertiären Vulkanismus in dieser Region ist. Diese Untersuchungen verdeutlichen, daß ein Teil des lithosphärischen Mantels unter der Rhön eine thermische Äquilibrierung während des tertiären magmatischen Ereignisses erfuhr. Dagegen zeigen andere Teile noch Relikte einer älteren Geschichte, speziell eine kryptische Metasomatose and den Übergang vom Stabilitätsfeld des Granat-Lherzoliths zum Spinell-Lherzolith. Ein mögliches geotektonisches Szenario für diesen Transfer könnte die postvariscische Krustenreäquilibrierung sein.

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With 7 Figures     

Composition of the Siberian cratonic mantle: evidence from Udachnaya peridotite xenoliths

Bulk compositions and mineral analyses for forty-one, large, garnet- and spinel-facies peridotite xenoliths from the Udachnaya kimberlite in the central Siberian platform have many similarities to those of well-studied peridotites from the Kaapvaal craton in southern Africa. Coarse Mg-rich lherzolites and harzburgites with equilibration temperatures below 1000 °C are abundant and are believed to form the principal rock type in the Siberian lithosphere. The low-temperature Udachnaya peridotites have an average mg number [Mg/(Mg+Fe)] of 92.6 with a wide dispersion in modal enstatite, ranging to over 40 wt%. High-temperature peridotites are relatively richer in Fe and Ti and are commonly deformed, with porphyroclastic or mosaic-porphyroclastic textures, some of the latter having fluidized enstatite. The Udachnaya peridotites have experienced late-stage metasomatism before, during and after eruption. Garnets and pyroxenes in many of the high-temperature rocks are zoned, probably by reaction with melt prior to eruption. Virtually all the peridotites contain secondary diopside, inhomogeneous on a micron scale, that mantles primary orthopyroxene. It is believed to have crystallized along with lesser amounts of intergranular calcite and monticellite during eruption. Bulk analyses for total Fe in many specimens are higher than whole-rock Fe calculated from the electron probe analyses and the modes. The magnitude of the difference between the two measurements of total Fe correlates with loss-on-ignition, suggesting that Fe has been introduced during serpentinization following eruption. These late metasomatic processes have thus affected some major as well minor and trace element compositions. The similarities in bulk composition of peridotites from Udachnaya and the Kaapvaal are evidence of a common origin. Low-temperature cratonic peridotites differ from oceanic peridotites in having higher mg numbers (>92) and in having relatively high but wide-ranging modal enstatite (Mg/Si = 1.06–1.49 weight fraction). The Udachnaya low-temperature peridotites have an inverse correlation between FeO (calculated from the probe analyses and modes) and SiO₂. This correlation is also present in the Kaapvaal data but is complicated by a greater range in fertility that produces a positive variation of Fe with Si. A negative trend for Fe/Si can be seen within a portion of the Kaapvaal data, that for low-Ca harzburgites, in which the variation in fertility is restricted. The negative trends for Fe/Si can be interpreted as a consequence of either segregation of olivine and orthopyroxene by metamorphic differentiation or partial sorting during cumulate formation. Received: 18 June 1996  / Accepted: 11 February 1997     

Evidence from Antarctic mantle peridotite xenoliths for changes in mineralogy, geochemistry and geothermal gradients beneath a developing rift

Garnet and spinel peridotite xenoliths associated with the Phanerozoic Lambert-Amery Rift in eastern Antarctica contain evidence for several stages in the development of the mantle beneath the rift. Despite the fact that equilibria were only partly attained, a combination of petrography, whole-rock geochemistry, mineral chemistry and thermobarometry can be used to decipher four stages prior to entrainment of the xenoliths in the host magma during the initial stages of the breakup of Antarctica, India and Madagascar. The first chronological stage is represented by harzburgitic protoliths represented by rare occurrences of low-Ca olivines and orthopyroxenes in spinel lherzolites: these yield the lowest temperatures of 830-850 °C, and are also characterized by distinct trace element contents; lower Ti, Cr, V and Zn in olivine and orthopyroxene, and additionally lower Cu, Ni, Ga and Li in orthopyroxene. Some garnets are subcalcic, indicating that the spinel-garnet lherzolites also formed from harzburgitic protoliths. The second stage is the formation of garnet due to a pressure increase probably related to collision at 1.1 Ga. The third stage is marked by the growth of clinopyroxene, demonstrably in cpx-poor spinel lherzolites but probably in all xenolith groups: equilibrium of clinopyroxene with olivine and orthopyroxene was not attained in all samples, so that the non-judicious use of thermobarometers can produce bewildering results. The fourth stage is an enrichment episode that affected all spinel-garnet peridotites and about half of the spinel peridotites. During this stage, reaction rims were produced on the clinopyroxenes that formed during stage 3, the modal content of olivine and Mg/(Mg + Fe) in the rocks was reduced, CaO, Al₂O₃ and trace elements were enriched, and garnets were almost completely transformed to kelyphites. A later stage is documented by interstitial glasses and films around spinels related to infiltration of melt from the host magma. These post-date, and are more enriched in alkalies than, partially melted rims on clinopyroxenes, demonstrating that all the three earlier episodes were pre-entrainment events. Pressures indicated by the spinel + garnet lherzolites are restricted to 20-24 kbar at 1040-1180 °C. Early harzburgitic assemblages are interpreted to represent an earlier, cooler geotherm, whereas the kelyphite assemblages indicate temperatures 180-200 °C hotter than the main xenolith geotherm. This event also caused recrystallization of the clinopyroxene rims and is attributed to heating during rifting, but not due to the host magma itself. The preservation of evidence for three progressively hotter geotherms can be related to the upward movement of isotherms during the development of the sub-rift mantle.     

Geological implications and validity of calculated equilibration conditions for ultramafic xenoliths from the pipe 200 kimberlite,northern Lesotho

Current methods of geothermometry and geobarometry applicable to garnet lherzolite are reviewed with reference to recent experimental studies of the equilibration of natural garnet lherzolite and it is concluded that the Wells and Mori-Green formulations of the two pyroxene solvus provide the most reasonable temperature estimates. Pressures are best estimated by using these temperatures with Wood's formulation of the orthopyroxene-garnet geobarometer without chromium corrections. Pipe 200 garnet lherzolites are considered to have equilibrated at 907°–950° C at 30.0–34.5 kb.It is shown that the transport times of xenoliths from the mantle are sufficiently long (0.5–24 h) to allow thermal equilibration with kimberlite but are too short to allow chemical re-equilibration to occur. Xenolith suites therefore retain information regarding the pressure/temperature history of the upper mantle despite being heated to the temperature of the kimberlite magma during transport.The Pipe 200 xenolith suite indicates that the upper mantle beneath Lesotho has been perturbed to temperatures slightly above those defined by steady state geotherms. The Pipe 200 suite is derived from a narrow depth range (90–110 km) and derivation of chromite and garnet lherzolites from similar depths implies that the mantle is heterogeneous over short vertical distances. No simple stratigraphy, in which chromite lherzolites overlie garnet lherzolites is evident. Comparison with other suites of Lesotho garnet lherzolites shows that it is not possible to construct an upper mantle stratigraphy except in the most general terms because of the prevailing lateral and vertical heterogeneity and apparent limited depth range represented by the xenolith suites.     

A first insight into the constitution of the upper mantle under the hoggar area (Southern Algeria): The lherzolite xenoliths in the alkali-basalts

Quaternary alkali olivine basalts and nephelinites from the Hoggar area (Southern Algeria) contain numerous lherzolite inclusions. The investigated nodules have been classified into three textural categories: coarse, porphyroclastic and granuloblastic. Microprobe analyses have been carried out on the coexisting phases (olivine, clinopyroxene, orthopyroxene, spinel, pargasite, Al rich glass) from 10 selected samples, with a special attention to pyroxene porphyroclasts which are zoned in Al and Cr. Most of the porphyroclastic xenoliths contain secondary pargasite (or its breakdown products) which is thought to reflect a metasomatic event in mantle conditions. Equilibrium temperatures and pressures have been calculated for the three groups, using the single-pyroxene method: the coarse samples have been equilibrated at higher pressures (20–25 kb) and temperatures (1,000° C–1,100° C) than the granuloblastic samples (about 10 kb and 900° C); with regards to the porphyroclastic xenoliths, the estimated T and P have been related to two stages of crystallization (corresponding to porphyroclasts and neoblasts). Relationships between phase compositions, rock-textures and metasomatism are briefly discussed.     

Petrogenesis of ultramafic and mafic xenoliths from Mesozoic basanites in southern Sweden: constraints from mineral chemistry

Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites) and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region. The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fe_tot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C) than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within the Fennoscandian crust.     

Low-calcium garnet harzburgites from southern Africa: their relations to craton structure and diamond crystallization

Low-Ca garnet harzburgite xenoliths contain garnets that are deficient in Ca relative to those that have equilibrated with diopside in the iherzolite assemblage. Minor proportions of these harzburgites are of wide-spread occurrence in xenolith suites from the Kaapvaal craton and are of particular interest because of their relation to diamond host rocks. The harzburgite xenoliths are predominantly coarse but one specimen from Jagersfontein and another from Premier have deformed textures similar to those of high-temperature peridotites. Analyses for many elements in the harzburgites and associated iherzolites form concordant overlapping trends. On the average, however, the harzburgites are deficient in Si, Ca, Al and Fe but enriched in Mg and Ni relative to the lherzolites. Both the harzburgites and lherzolites are enstatite-rich with mg numbers [100.Mg/(Mg+Fe_total)] greater than 92 and in these respects differ markedly from residues generated by extraction of MORB. Equilibration temperatures and depths calculated for the harzburgites have the ranges 600–1,400°C and 50–200 km. Those of deepest origin overlap the interval between low-and high-temperature lherzolites that commonly is observed in temperature-depth plots for the Kaapvaal craton, suggesting that some harzburgites may be concentrated relative to lherzolites at the base of the lithosphere. The low-Ca harzburgites and lherzolite xenoliths have overlapping depths of origin, gradational bulk chemical characteristics and similar textures, and therefore both are believed to have formed as residues of Archaen melting events. The harzburgites differ from the lherzolites only in that they are more depleted. Garnets and associated minerals in harzburgite xenoliths differ from minerals of the same assemblage that are included in diamonds in that the latter are more Cr-rich, Mg-rich and Ca-poor. Coarse crystals of low-Ca garnet with the compositional characteristics of diamond inclusions commonly occur as disaggregated grains in diamondiferous kimberlites. Their host rocks are presumed to have been harzburgites and dunites. The differences in composition between the disaggregated grains that are similar to diamond inclusions and those comprising xenoliths imply some differences in origin. Possibly the disaggregated harzburgites with diamond-inclusion mineralogy have undergone repeated partial melting and depletion near the base of the lithosphere subsequent to their primary depletion and aggregation in the craton. Equilibration with magnesite may have reduced the Ca contents of their garnets and decomposition of the magnesite during eruption may have caused their disaggregation.     

Nature and timing of magma interactions before,during, and after the caldera-forming eruption of Volcán Ceboruco,Mexico

Volcán Ceboruco, Mexico, erupted ~1,000 years ago, producing the Jala pumice and forming a ~4-km-wide caldera. During that eruption, 2.8 to 3.5 km³ of rhyodacite (~70 wt% SiO₂) magma and 0.2 to 0.5 km³ of mixed dacite (~67 wt% SiO₂) magma were tapped and deposited as the Jala pumice. Subsequently, the caldera was partially filled by extrusion of the Dos Equis dome, a low-silica (~64 wt% SiO₂) dacite dome with a volume of ~1.3 km³. Petrographic evidence indicates that the Jala dacite and Dos Equis dacite originated largely through the mixing of three end-member magmas: (1) rhyodacite magma, (2) dacite magma, and (3) mafic magma. Linear least-squares modeling and detailed modal analysis indicate that the Jala dacite is predominantly a bimodal mixture of rhyodacite and dacite with a small additional mafic component, whereas the Dos Equis dacite is composed of mostly dacite mixed with subordinate amounts of rhyodacite and mafic magma. According to Fe–Ti oxide geothermometry, before the caldera-forming eruption the rhyodacite last equilibrated at ~865 °C, whereas the dacite was originally at ~890 °C but was heated to ~960 °C by intrusion of mafic magma as hot as ~1,030 °C. Zoning profiles in plagioclase and/or magnetite phenocrysts indicate that mixing between mafic and dacite magma occurred ~34–47 days prior to eruption, whereas subsequent mixing between rhyodacite and dacite magmas occurred only 1–4 days prior to eruption. Following the caldera-forming eruption, continued inputs of mafic magma led to effusion of the Dos Equis dome dacite. In this case, timing between mixing and eruption is estimated at ~93–185 days based on the thickness of plagioclase overgrowth rims.Editorial responsibility: T.L. Grove     

Rare earth geochemistry of fused ophiolitic and alpine lherzolites

Partial fusion hypotheses have been proposed for the origin of lherzolite-harzburgite alpine peridotite associations. Analyzed lherzolites from Othris, Ronda, Lanzo and Beni Bouchera, have light REE depleted to chondritic REE abundances, and clinopyroxenes contain most of the REE relative to depleted olivine and orthopyroxene. Variation in the level of REE enrichment within these lherzolites indicates mantle heterogeneity probably caused by partial melting processes. The Beni Bouchera spinel lherzolite and the Othris plagioclase lherzolite are the best candidates for relatively undepleted mantle based on REE studies. Fractional fusion calculations (15–25%) reveal that partial melts have REE characteristics somewhat similar to oceanic tholeiites. Conversely, computed source peridotites from oceanic tholeiites (Schilling, 1975) are similar to the alpine lherzolites reported here. Alpine lherzolites are, however, depleted in trace elements (K, Rb, Sr and Ba, Menzies and Murthy 1976). Since the lherzolites have an undepleted major, minor and REE chemistry close to that of pyrolite, the lost trace element-rich fraction must represent a small degree of melting. It is proposed that alpine lherzolites are residue left after the loss of a nephelinitic/alkalic fraction, ([Ce/Yb]_N=2.0–4.01) representing a small degree of partial fusion. This labile fraction may have existed as an intergranular phase or hydrous mineral prior to melting.     

Peridotites from the Mid-Atlantic Ridge at 43° N and their petrogenetic relation to abyssal tholeiites

Whole-rock, major and trace element analyses and microprobe mineral analyses were conducted on serpentinized peridotites recovered from the walls of a MAR (Mid-Atlantic Ridge) 43° N fracture zone. These peridotites are extensively serpentinized; serpentine usually makes up 30–100 vol. percent of the bulk rocks. The relict minerals observed consist mainly of olivine and orthopyroxene with subordinate amounts of clinopyroxene and brown spinel. The range in olivine composition is very limited (Fo_91–92). Orthopyroxene forms large, anhedral crystals with clinopyroxene exsolution lamellae and shows undulose extinction with bent cleavages and lamellae. Broad beam microprobe analyses indicate that the composition range of orthopyroxene is also limited (En_89.1–87.6Fs_8.2-8.0Wo_2.7–4.4; Al₂O₃=1.82–2.64 wt%; Cr₂O₃=0.63–0.88 wt%). Clinopyroxene tends to fringe large orthopyroxene crystals or fills the interstices between them. The Mg/Fe ratios of clinopyroxene are practically constant; however, the Ca/(Ca + Mg + Fe) ratios range from 0.48 to 0.45. The Cr/(Cr+Al) and Mg/(Mg+ Fe²⁺) ratios of brown spinel range from 0.57 to 0.36 and 0.69 to 0.56, respectively. The geothermometers utilizing coexisting spinel lherzolite mineral assemblages suggest that the MAR 43° N peridotites attained equilibrium at temperatures from 1100° to 1250° C.Peridotites recovered from the ocean floor are generally considered to have been subjected to partial melting processes and are regarded as residues left after primary magma was removed. Major element chemistry of the MAR 43° N peridotites are compared with those of the ocean-floor ultramafic tectonites reported previously and used together with those published data to demonstrate that the major element abundances of the oceanfloor peridotites define an average trend which is compatible with removal of primary magma from these peridotites at moderate pressures (10–15 kb). Then, the most primitive abyssal tholeiite glasses could be produced by ca. 10% olivine fractionation of such primary magma. Extensive fractionation of olivine and/or orthopyroxene from picritic liquids which are in equilibrium with the lherzolitic or harzburgitic mantle sources at higher pressures (>20 kb) could not yield the majority of the most primitive abyssal tholeiite glasses.     

Lithospheric mantle beneath the south-eastern Siberian craton: petrology of peridotite xenoliths in basalts from the Tokinsky Stanovik

We provide petrographic, major and trace element data for over 30 spinel peridotite xenoliths from the Tokinsky Stanovik (Tok) volcanic field on the Aldan shield to characterize the lithospheric mantle beneath the south-eastern margin of the Siberian craton, which formed in the Mesoproterozoic. High equilibration temperatures (870–1,010°C) of the xenoliths and the absence of garnet-bearing peridotites indicate a much thinner lithosphere than in the central craton. Most common among the xenoliths are clinopyroxene-poor lherzolites and harzburgites with Al₂O₃ and CaO contents nearly as low as in refractory xenoliths from kimberlite pipes (Mir, Udachnaya) in the central and northern Siberian craton. By contrast, the Tok peridotites have higher FeO, lower Mg-numbers and lower modal orthopyroxene and are apparently formed by shallow partial melting (3 GPa). Nearly all Tok xenoliths yield petrographic and chemical evidence for metasomatism: accessory phlogopite, amphibole, phosphates, feldspar and Ti-rich oxides, very high Na₂O (2–3.1%) in clinopyroxene, LREE enrichments in whole-rocks.Electronic Supplementary Material Supplementary material is available for this article at     

Stages in the P-T path of ascending basalt magma: An example from San Quintin,Baja California

Late Pleistocene or Recent lavas from San Quintin, Baja California are basanitoids and alkali basalts. The surface quench temperatures of the lavas average 1 005° C with log =–11.4, as deduced from the groundmass Fe-Ti oxides. Spinel lherzolite xenoliths and megacrysts of augite and andesine have been found in lava flows and cinder deposits. Using analytical data on the rocks and minerals and simple thermodynamic expressions, the pressures and temperatures of equilibration of lavas and xenoliths, megacrysts and phenocrysts have been calculated. The lavas could have been in equilibrium with lherzolite at 1 330–1 410° C and 27.5–31.6 kb, the more silica-poor liquid having the higher values. The basanitoid could have equilibrated with the megacrysts at about 10.5 kb and with phenocrysts at about 1.4 kb and 1130° C. The variation in composition of the lavas may be explained by a rising zone of melting within the mantle, the most silica-poor liquid having the deepest source. The source of the San Quintin basalts is probably related to spreading of the ocean floor in the Gulf of California.     

Composition and evolution of lithosphere beneath the Carpathian–Pannonian Region: a review

Our knowledge of the lithosphere beneath the Carpathian–Pannonian Region (CPR) has been greatly improved through petrologic, geochemical and isotopic studies of upper mantle xenoliths hosted by Neogene–Quaternary alkali basalts. These basalts occur at the edge of the Intra-Carpathian Basin System (Styrian Basin, Nógrád-Gömör and Eastern Transylvanian Basin) and its central portion (Little Hungarian Plain, Bakony-Balaton Highland).The xenoliths are mostly spinel lherzolites, accompanied by subordinate pyroxenites, websterites, wehrlites, harzburgites and dunites. The peridotites represent residual mantle material showing textural and geochemical evidence for a complex history of melting and recrystallization, irrespective of location within the region. The lithospheric mantle is more deformed in the center of the studied area than towards the edges. The deformation may be attributed to a combination of extension and asthenospheric upwelling in the late Tertiary, which strongly affected the central part of CPR subcontinental lithosphere.The peridotite xenoliths studied show bulk compositions in the following range: 35–48 wt.% MgO, 0.5–4.0 wt.% CaO and 0.2–4.5 wt.% Al₂O₃ with no significant differences in regard to their geographical location. On the other hand, mineral compositions, particularly of clinopyroxene, vary according to xenolith texture. Clinopyroxenes from less deformed xenoliths show higher contents of ‘basaltic’ major elements compared to the more deformed xenoliths. However, clinopyroxenes in more deformed xenoliths are relatively enriched in strongly incompatible trace elements such as light rare earth elements (LREE).Modal metasomatic products occur as both hydrous phases, including pargasitic and kearsutitic amphiboles and minor phlogopitic micas, and anhydrous phases — mostly clinopyroxene and orthopyroxene. Vein material is dominated by the two latter phases but may also include amphibole. Amphibole mostly occurs as interstitial phases, however, and is more common than phlogopite. Most metasomatized peridotites show chemical and (sometimes) textural evidence for re-equilibration between metasomatic and non-metasomatic phases. However, amphiboles in pyroxenites are sometimes enriched in K, Fe and LREE. The presence of partially crystallized melt pockets (related to amphiboles and clinopyroxenes) in both peridotites and pyroxenites is an indication of decompression melting and, rarely, incipient partial melting triggered by migrating hydrous melts or fluids. Metasomatic contaminants may be ascribed to contemporaneous subduction beneath the Carpathian–Pannonian Region between the Eocene and Miocene.Sulfide inclusions are more abundant in protogranular and porphyroclastic xenoliths relative to equigranular types. In mantle lithologies, sulfide bleb compositions vary between pentlandite and pyrrhotite correlating with the chemistry and texture of the host xenoliths. While sulfides in peridotites are relatively rich in Ni, those in clinopyroxene-rich xenoliths are notably Fe-rich.