Spinel Garnet Lherzolite Research Articles

Petrogenesis of newly identified mantle-plume-related ca. 3.3 Ga tholeiitic-komatiitic rocks in the Anshan area, North China Craton

The geodynamic regime that governed the crustal evolution of Earth during the early Archean remains intensely debated. The North China Craton (NCC) preserves a geological history spanning 3.8–2.5 Ga, particularly in the Anshan area, making it an ideal natural laboratory for probing the formation and evolution of the NCC and its associated geodynamic regime. Here, we document new geochronology, geochemistry, and zircon Hf-O isotope data from newly discovered Paleoarchean meta-mafic rocks in the Anshan area. Based on their whole-rock geochemistry, these unit can be divided into tholeiitic and komatiitic series. Of which, the komatiitic rocks are characteristic by high MgO contents (up to 20.6 wt%). Zircon U-Pb dating shows that all meta-mafic rocks formed contemporaneously, with crystallization ages of ca. 3.3 Ga. Most of the studied meta-mafic rocks display radiogenic zircon Hf isotopes, with TDM1 ages ranging from 3592 to 3346 Ma and εHf(t) values from −1.94 to + 3.49. They also exhibit mantle-like zircon δ18O values (+4.92 ‰ to + 5.98 ‰). These zircon Hf-O isotope data indicate that the protoliths of the ca. 3.3 Ga meta-mafic rocks originated from the partial melting of a depleted mantle without significant crustal contamination. Furthermore, the parental magmas of the komatiitic rocks and some of tholeiitic rocks were generated from 20 % and 5–10 % partial melting of spinel-lherzolite, respectively, whereas other tholeiitic rocks were the result of a low degree (1–5 %) of partial melting of spinel-garnet lherzolite. The ca. 3.3 Ga tholeiitic-komatiitic rocks were most likely formed in an intraplate rifting setting, associated with a mantle plume.

Geochemistry, Sr-Nd isotopes and zircon U-Pb dating of magmatic rocks from the Talesh range, western Alborz: New insights into Late Cretaceous evolution of the southern Eurasian margin

The eastern flank of the Talesh range of western Alborz, northwestern Iran, exposes volumetrically significant Late Cretaceous volcanic and volcaniclastic associations as well as cross-cutting dykes and minor subvolcanic equivalents. The volcanic units appear as massive to pillowed flows interlayered with volcaniclastic beds and subordinate limestones. We present new field data, zircon UPb ages, bulk-rock major and trace element geochemistry, and SrNd isotopes on the igneous rock suite. Geochemical data show that the lava units and dykes are mainly basaltic in composition with minor basaltic andesites and trachytes. They form two contrasting series of arc-related calc-alkaline (Group 1) and OIB-like (Group 2) rocks. The zircon UPb age of Group 1 rocks is 95.6 ± 1.8 Ma (2σ), whereas for Group 2 rocks, cross-cutting relationships and stratigraphic constraints indicate a somewhat younger age. Group 1 rocks have 87Sr/86Sr(i) varying from 0.7038 to 0.7070 and εNd(t) values ranging from +0.5 to +5.0. Group 2 rocks have 87Sr/86Sr(i) = 0.7040–0.7065 and εNd(t) from −3.0 to +3.7. Trace element and isotopic modelling shows that the primitive Group 1 magmas were derived from a metasomatized mantle source enriched by the addition of ∼6–10 % sediment melt component, while Group 2 rocks are consistent with melts of an asthenospheric mantle source enriched by ∼1–5 % EMII component. Trace element modelling indicates that Group 1 rocks formed from ∼6–13 % partial melts of a spinel-garnet lherzolite with garnet:spinel ratios of 45:55 to 10:90, whereas primitive Group 2 melts were generated through ∼1–3 % partial melting of a garnet-spinel-lherzolite with garnet:spinel ratios of 90:10 to 80:20. We propose a geodynamic model in which a north-dipping Neotethyan slab beneath the western Alborz (Talesh area) continental margin produced arc assemblages of Group 1 rocks and subsequent slab rollback led to the upwelling of an asthenospheric mantle to generate OIB-like magmatism of Group 2 rocks in an extensional intra-arc to back-arc setting.

Petrogenesis and geochemistry of WNW-ESE to NW-SE trending doleritic dykes of the Paleoproterozoic Liptako basement (West African Craton, West Niger)

Doleritic dykes with regional orientations WNW-ESE to NW-SE, cut the Paleoproterozoic Birimian formations of the Liptako basement (Ouest-Niger). Mineralogical compositions studied from these doleritic dykes consist of augite (Wo21-40-En24-45-Fs13-54), pigeonite (Wo8-12-En45-59-Fs31-41), plagioclase, alkali feldspars, titano-magnetite, ilmenite, edenite, apatite, biotite and chlorite. The geochemical affinity of the doleritic dykes in the two trends is similar to oceanic plateau basalts (OPB), with high iron content relative to Mg. Negative Nb, Ti and Ta anomalies and the Y/Nb ratio (1.59–3.15) indicate a continental tholeiitic character for while light rare earth element enrichment, low to moderate Ti contents and the Th/Yb (0.28–0.59) and Nb/Yb (3.25–7.46) ratios suggest sources similar to E-MORB. Their Th/Nb (0.06–0.10), Th/Yb (0.28–0.59), TiO2/Yb (0.54–0.82) ratios suggest that the magma was generated by partial melting (5–17%) of a residual spinel-garnet lherzolite, that could have been part of a mantle plume. Relatively low Mg# (36–51) reflect some degrees of fractional crystallization, with some of this differentiation recorded between the margins and interiors of dykes. This differentiation was accompanied by unusually low degrees of crustal contamination, as indicated by relatively low Th/Ta ratios (1.18–2.03). This petrological and geochemical information provides new constraints on the origin of these WNW to NE trending mafic dykes of the West African Craton.

A Jurassic volcanic passive margin in Iran and Turkey

AbstractBroadly similar Early to Middle Jurassic stratigraphic sequences including bimodal igneous rocks of the Sanandaj–Sirjan Zone of Iran and the Sakarya Zone of Turkey suggest that these formed in a common tectonic setting in an extensional basin that evolved from a terrestrial magmatic rift to a marine shelf and passive continental margin. Whole‐rock chemistry and Sr–Nd isotope signatures indicate derivation of mafic melts from partial melting of the subcontinental lithosphere. Decompression associated with extension led to 5%–30% partial melting of spinel–garnet lherzolite with minor involvement of continental crust, producing tholeiitic to transitional basaltic magma. Extensional basins inverted during the Mid‐Late Jurassic. These relationships suggest the Early to Middle Jurassic formation of a volcanic rifted margin on the SW Eurasian margin, similar to that of offshore Norway.

New U[sbnd]Pb zircon and geochemical constraints on Late Devonian Back-arc basin origin of eclogite protoliths from northeastern Hainan Island, South China

Geochronological, geochemical, and SrNd isotopic data are used to constrain the age, origin, and tectonic setting of the protoliths of newly discovered eclogites from Chaotanbi, northeastern Hainan Island, South China. New sensitive high-resolution ion microprobe (SHRIMP) UPb zircon dating reveals that the protoliths of the Chaotanbi eclogites formed at 364 ± 2 Ma and were metamorphosed at 297 ± 1 Ma. All samples collected from a coherent area of ~1.8 km2 in the intertidal and subtidal zones on the island exhibit tholeiitic compositions, with initial 87Sr/86Sr and εNd values of 0.705–0.707 and − 1.1 to +7.4, respectively. The samples can be further categorized into three groups based on incompatible trace elements. Group #1 samples have a normal mid-ocean ridge basalt (N-MORB) affinity and were originated from relatively high-degree partial melting of a depleted spinel lherzolite mantle source slightly modified by slab-derived fluids. Group #2 samples have an enriched mid-ocean ridge basalt (E-MORB) affinity and were derived from an enriched spinel and spinel–garnet lherzolite transition mantle source at depths greater than that of Group #1. Group #3 samples have an island arc basalt (IAB) affinity and were generated by partial melting of an enriched spinel–garnet lherzolite mantle source modified by fluid- and sediment-related recycled components. The association of these diverse basalt types, with MORB-type samples accounting for ~90% of all samples, suggest a Late Devonian mature back-arc basin existed on northeastern Hainan Island, as a part of the Jinshajiang–Ailaoshan–Song Ma–Hainan back-arc basin system. Therefore, the Chaotanbi eclogites are a fingerprint of the opening and closure of the eastern Paleo-Tethyan Ocean.

Spectral characteristics and petrology of Sinai Mesozoic basaltic rocks: Gebel Farsh El Azraq-Gebel Himayir area, Um Bogma, southwestern Sinai, Egypt

Sinai Mesozoic basalts are restricted in its southwestern portion at Um Bogma area, where they occur as intra-continental fissure-fed sills, dykes, plugs and lava flows, following mainly the older structural and crustal weaknesses planes. During Late Paleozoic - Middle to Late Triassic, Egypt had witnessed sporadic tectonic disturbances and faulting activities accompanied by volcanisms. Faulting activity continued during Early to Middle Jurassic and was associated with crustal stretching and basaltic volcanism. At Um Bogma area, southwestern Sinai Peninsula, Middle Triassic, and Early Jurassic basaltic exposures intruded/extruded the Carboniferous sandstones. The main Mesozoic basalts outcrop at Gebel Farsh El Azraq - Gebel Himayir area, where they were successfully discriminated from their surrounding country rocks using Landsat-8 dataset. The Mesozoic basaltic rocks are the least reflective rocks and exhibit a characteristic spectral curve, compared to their country rocks. Landsat-8 bands 7, 5, 3 in RGB and Principal Component Analyses (PCA) technique were used in lithological discrimination of the Mesozoic basalts and their surrounding Late Neoproterozoic basement rocks and Phanerozoic sedimentary succession. Accordingly, a new geological map was produced for the study area. Petrography and geochemistry of these basalts were addressed. Characteristically they contain substantial MgO contents (22.24 to 7.09 wt%) and exhibit low TiO2 (1.56–0.77 wt%) and P2O5 (0.30–0.15 wt%) contents. Geochemically, they pertain to the low P2O5 –TiO2 continental tholeiitic to transitional basalts and point out to enriched MORB magma sources. They suggest derivation from spinel-garnet lherzolite source by moderate non-modal partial melting (ca 20–30%). Their petrogenesis was discussed and fractional crystallization modeling was performed as a plausible mechanism to be accounted for their evolutions.

Геохимическая характеристика позднедевонских базальтоидов полуострова Канин и Среднего Тимана

The petrochemistry and geochemistry of dolerites and basalts of the Late Devonian Kanin-Timan complex of the Kanin Peninsula and the Middle Timan are considered. Petrochemically, the rocks of the Kanin-Timan complex of the Kanin Peninsula and the Tsilma river area of the Middle Timan correspond to basaltoids of the normal range of alkalinity and partially to moderately alkaline varieties, and belong to the tholeiitic series. The least differentiated varieties are dolerites of the southeastern Kanin Peninsula, the most differentiated are the basalts of the river Tsilma of the Middle Timan. The lowest REE concentrations were found in the rocks of the central part of the Kanin Peninsula (36.5-56.8 g/t); in the same samples, the lowest LaN/YbN values were recorded (1.85 and 2.4, respectively), which indicates an increased degree of melting of the source. The highest REE concentrations were found in basalts from the river Tsilma (77.13-88.33 g/t), LaN/YbN values (2.49-2.7, respectively). The influence of the crustal component in the formation of melts from which rocks of the Kanin-Timan complex were formed, was established. The source of the melt was spinel-garnet lherzolite, the degree of melting varied from 10 to 30%. The maximum degree of melting was 30%, at which melts were formed, that gave rise to the least differentiated rocks of the Northern Timan and the central part of the Kanin Peninsula. The mantle source, that gave rise to the melts from which the rocks of the Kanin-Timan complex were formed, was enriched with subduction and crustal components, a similar type of source is characteristic of the basaltoids of the No-rilsk trough.

Temporal, geochemical and geodynamic evolution of the Late Cretaceous subduction zone volcanism in the eastern Sakarya Zone, NE Turkey: Implications for mantle-crust interaction in an arc setting

The Late Cretaceous Artvin volcanic rocks (LCAVs) from the eastern Sakarya zone (ESZ) of NE Turkey are composed of mafic/basaltic (S1-Çatak and S2-Çağlayan) and felsic/acidic (S1-Kızılkaya and S2-Tirebolu) rock types that occurred in two successive stages: (i) first stage (S1: Turonian–Early Santonian) and (ii) second stage (S2: Late Santonian–Campanian). Clinopyroxene thermobarometric results point that the S2-Çağlayan basaltic rocks have crystallised at higher temperatures and under deeper crustal conditions than those of the S1-Çatak basaltic rocks.The LCAVs show a wide compositional spectrum, ranging from tholeiite to calc-alkaline/shoshonite and are typically represented by a geochemical composition resembling subduction-related arc rocks although the 87Sr/86Sr(i) (0.7044–0.7071) and ɛNd(i) values (−0.63 to + 3.47) as well as 206Pb/204Pb(i) (18.07–18.56), 207Pb/204Pb(i) (15.57–15.62) and 208Pb/204Pb(i) (37.12–38.55) ratios show very limited variation. The parent magmas of the S1-Çatak and S2-Çağlayan mafic volcanic rocks were derived from underplated basaltic melts that originated by partial melting of metasomatised spinel lherzolite and spinel-garnet lherzolite, respectively. It is proposed that the compositions of the S1-Kızılkaya (mainly dacitic) and S2-Tirebolu (rhyolitic to trachytic) felsic rocks were particularly controlled by metasomatised mantle–crust interaction and MASH zone + shallow crustal fractionation processes. Our data, together with data from previous studies, suggest that the S1- and S2-mafic and felsic rocks of the LCAVs (~95–75 Ma) are the products of two-stage volcanic event that took place during the northward subduction of the northern Neotethys Ocean (NNO).

Composition of the lithospheric mantle in the northern part of Siberian craton: Constraints from peridotites in the Obnazhennaya kimberlite

The character of the lithospheric mantle of the northern Siberian craton is not well established; nearly all published data are for mantle xenoliths from a single kimberlite in the center of the craton (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160Ma) in the northern part of Siberian craton. The Obnazhennaya mantle xenoliths include spinel harzburgites, spinel dunites, spinel lherzolites and spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23–1.35wt% Al2O3, 0.41–3.11wt% CaO and 0.00–0.09wt% TiO2, whereas the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16–6.55wt% Al2O3, 2.91–7.55wt% CaO and 0.04–0.15wt% TiO2. The trace element compositions and mineralogical textures of the Obnazhennaya xenoliths indicate the occurrence of metasomatic enrichments, including carbonatite melts, basaltic melts from Siberian Trap and kimberlitic melts. The spinel harzburgites and dunites have 187Os/188Os of 0.11227–0.11637, giving a TRD age of 1.6–2.2Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, both spinel and spinel-garnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931–0.17627), enriched P-PGEs. But the higher Al2O3 and Os character of these lherzolites suggest that they were not juvenile mantle but the refertilized ancient mantle. Therefore, our results suggest that the cratonic mantle beneath the northern part of Siberian craton contain both ancient and reworked lithospheric mantle, and the metasomatism may not be effective at overprinting/eroding the pre-existing lithosphere.

Two types of Neoarchean basalts from Qingyuan greenstone belt, North China Craton: Petrogenesis and tectonic implications

Geochronological, geochemical and isotopic studies were carried out on the basalts from the Qingyuan greenstone belt to understand the petrogenesis of the basalts and the mantle source characteristics, providing insights into Neoarchean geodynamic evolution of the North China Craton. The Qingyuan greenstone belt is analogous to the typical Barberton-type greenstone belt in lithological units. U–Pb isotopic dating using the LA–ICP–MS method on zircons from the basalts and the associated andesite reveals that they erupted at 2510–2545Ma and were subjected to subsequent 2486–2491Ma metamorphism. The basalts are high-Fe tholeiitic, showing coherently positive εNd(t) and εHf(t) values. The geochemical and isotopic studies strongly imply that they experienced significant fractional crystallization and minor/no crustal contamination during magma evolution. On the basis of chondrite normalized REE patterns, the basalts can be further divided into two types, viz. the enriched and the depleted. The enriched basalts are abundant in LREE and REEs contents, depleted in HFSEs (Nb, Zr, Ti), endowed with arc-like geochemical peculiarities. Modeling reveals that the parent magma of the enriched basalts has been derived from <10% partial melting of spinel–garnet lherzolites. The depleted basalts are characterized by somewhat left-inclined to slightly right-inclined REE patterns and relatively low REEs contents, interpreted to have been formed when the rising plume head mixed with the upper mantle. We suggest the plume head contribution of >70% to the depleted basalts. The basalts from the Neoarchean Qingyuan greenstone belt witness the episode of the vertical growth of continental crust at ca. 2.5Ga. Combined with other geological considerations, a large Archean mantle plume model is favored to account for the generation of the late Neoarchean basalts from Qingyuan greenstone belt in the Eastern Block, North China Craton.

Mantle evolution in the Variscides of SW England: Geochemical and isotopic constraints from mafic rocks

The geology of SW England has long been interpreted to reflect Variscan collisional processes associated with the closure of the Rhenohercynian Ocean and the formation of Pangea. The Cornish peninsula is composed largely of Early Devonian to Late Carboniferous volcanosedimentary successions that were deposited in pre- and syn-collisional basins and were subsequently metamorphosed and deformed during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is broadly coeval with the emplacement of ca. 280–295Ma lamprophyric dykes and flows. Although these lamprophyres are well mapped and documented, the processes responsible for their genesis and their relationship with regional Variscan tectonic events are less understood.Pre- to syn-collisional basalts have intra-continental alkalic affinities, and have REE profiles consistent with derivation from the spinel–garnet lherzolite boundary. εNd values for the basalts range from +0.37 to +5.2 and TDM ages from 595Ma to 705Ma. The lamprophyres are extremely enriched in light rare earth elements, large iron lithophile elements, and are depleted in heavy rare earth elements, suggesting a deep, garnet lherzolite source that was previously metasomatised. They display εNd values ranging from −1.4 to +1.4, initial Sr values of ca. 0.706, and TDM ages from 671Ma to 1031Ma, suggesting that metasomatism occurred in the Neoproterozoic.Lamprophyres and coeval granite batholiths of similar chemistry to those in Cornwall occur in other regions of the Variscan orogen, including Iberia and Bohemia. By using new geochemical and isotopic data to constrain the evolution of the mantle beneath SW England and the processes associated with the formation of these post-collisional rocks, we may be able to gain a more complete understanding of mantle processes during the waning stages of supercontinent formation.

Geochronology and geochemistry of the Paleoproterozoic meta-basalts from the Jiao-Liao-Ji Belt, North China Craton: Implications for petrogenesis and tectonic setting

Geochronology, geochemical and isotopic studies were carried out on the Paleoproterozoic meta-basalts and related rocks from the Jiao-Liao-Ji Belt, North China Craton, to understand the mantle source characteristics and the geodynamic setting. U-Pb isotopic dating using the LA-ICPMS method on zircons from the metamorphosed volcanic rocks reveals that they formed at ca. 2204–2158Ma and were metamorphosed at ca. 1895–1919Ma, respectively. The meta-basalts are of calc-alkali affinity and have arc-like geochemical compositions. They are enriched in LREEs ((La/Yb)N=3.5–5.1) and LILEs (Sr, Ba, Rb), depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), and show variable high ɛNd(t) values (+2.4 to +4.5). The basaltic rocks experienced significant fractional crystallization and minor crustal contamination during magma evolution. Modeling suggests that the parent magma of the meta-basalts was derived from 20% partial melting of spinel-garnet lherzolites that were previously metasomatized by subduction zone fluids/melts in the Late Archean. Petrological, geochronological and geochemical data suggest that the Paleoproterozoic volcanic rocks formed in an active continental margin setting and were subsequently deformed and metamorphosed to amphibolite facies due to the arc-continental collision at ca. 1.9Ga, rather than in a continental rifting setting as previously thought by most workers.

Geochemistry and U–Pb zircon ages of metamorphic volcanic rocks of the Paleoproterozoic Lüliang Complex and constraints on the evolution of the Trans-North China Orogen, North China Craton

The Paleoproterozoic Lüliang Complex is situated in the central part of the western margin of the Trans-North China Orogen and consists of volcanic rocks, sedimentary rocks and Paleoproterozoic granitoid intrusions. The volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist- to amphibolite-facies. These metamorphosed volcanic rocks are dominated by basalts to basaltic andesites. The parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5% to 30% partial melting of spinel lherzolites to spinel-garnet lherzolites which had been enriched by the subduction melts. Mafic magma experienced fractional crystallization and crustal assimilation.U–Pb zircon dating on two metamorphosed volcanic rocks from the Yejishan and Lüliang groups reveals that they formed at 2210±13Ma and 2213±47Ma, respectively, and were metamorphosed at ∼1832Ma. This suggests that the metamorphosed volcanic rocks in the Yejishan and Lüliang groups formed synchronously in the Paleoproterozoic. These new ages, integrated with recently reported U–Pb zircon ages for the Jiehekou Group and Paleoproterozoic granitoids, suggest that all of the lithological assemblages of the Lüliang Complex formed and were metamorphosed in the Paleoproterozoic, not in the Neoarchean.Petrological, geochronological and geochemical data suggest that the geodynamic evolution of the Paleoproterozoic Lüliang Complex was involved in the development of a magmatic arc system at an active continental margin, generating widespread arc-related magmatism at ∼2.2Ga. The Lüliang Complex then underwent intense deformation and metamorphism, and was incorporated into the Trans-North China Orogen during the 1.88–1.83Ga collisional event which was followed by post-collision extension at ∼1.80Ga.

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, Al 2O 3 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.

Spinel–garnet lherzolite transition in the system CaO-MgO-Al 2O 3-SiO 2 revisited: an in situ X-ray study

Large discrepancies are reported for the near-solidus, pressure–temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al 2O 3-SiO 2 (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz–coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.

Unique spinel-garnet lherzolite inclusion in kimberlite from Australia

An upper-mantle nodule found in kimberlite of Cenozoic age in southeastern Australia contains the assemblage aluminous spinel + garnet + orthopyroxene + clinopyroxene + olivine. This inclusion is the first recorded occurrence of an equilibrium mineral assemblage lying on the quasi-univariant boundary separating the spinel and garnet lherzolite fields inferred in the upper mantle. The temperature and pressure estimate of 1240 ± 20 °C and 22 ± 1 kb for the source of the inclusion indicates an abnormally high geothermal gradient, actually exceeding die oceanic geotherm. Heat-flow measurements also indicate a similarly high geothermal gradient for this part of Australia. It is inferred that this nodule is derived from a region of perturbed steady-state geotherm, generated during Tertiary volcanism and continuing to the present day.