Stability Field Of Garnet Lherzolite Research Articles

Petrological features of volcanic scoriae from the southern part of the Cameroon Volcanic Line and their supplementary cementations application

Volcanic scoriae from the southern part of the Cameroon Volcanic Line (Limbé, Loum-Tombel, Yamba, Doupé, Njinkouo, Foumbot, Manjo-manengollé, Galim and Djoungo) were investigated in order to determine their chemical and mineralogical composition, to deduce their origin and to identify their natural characteristics which may be useful to the cement industry. The mineralogical composition was determined by X-ray Diffractometer (XRD); X-ray Fluorescence (XRF) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) instruments provided geochemical data. In order to establish the relationship between the natural characteristics of volcanic scoriae and the properties of cements, the amount of amorphous phases was determined by dissolution using sodium hydroxide solution and the pozzolanic activity by thermogravimetric analysis. Field observations show that these rocks are basalts, basanites, hawaiites and picrobasalts. Their mineralogical composition includes augite, olivine, plagioclase, enstatite, feldspars, ettringite, portlandite and Fe–Ti minerals. Overall, they are characterized by high MgO, Fe2O3, CaO, and TiO2 contents. The behavior of major and trace elements suggests that volcanic scoriae have an evolution dominated by partial melting. Besides, high chondrite normalized La/Yb (8–22), Tb/Yb (>1.9) and Dy/Yb (>2) values suggest that the melting corresponds to the garnet lherzolite stability field. The scoriae exhibit good pozzolanic reactivity after 28 days according to their considerable amount of amorphous phases, low CaO contents and their large specific surface area. According to ASTM C618 standard, the sum of SiO2, Al2O3 and Fe2O3 (SAI = 65.96–76.34 wt.%); LOI (-0.1–16.99 wt.%), and of CaO, Fe2O3 and MgO (CIM = 23.43–34.06 wt.%) suggest that those less weathered materials seem appropriate as an additive in cement manufacture. The suitable use of volcanic scoriae in the cement industry closely depends on the petrological features of amorphous phases.

Mafic dikes at Kahel Tabelbala (Daoura, Ougarta Range, south-western Algeria): New insights into the petrology, geochemistry and mantle source characteristics

New petrological, geochemical and Sr–Nd isotopic data of the Late Triassic and Early Jurassic Kahel Tabelbala (KT) mafic dikes (south-western Algeria) offer a unique opportunity to examine the nature of their mantle sources and their geodynamic significance. An alkaline potassic Group 1 of basaltic dikes displaying relatively high MgO, TiO2, Cr and Ni, La/YbN ∼ 15, coupled with low 87Sr/86Sri ∼ 0.7037 and relatively high ɛNd(t) ∼ +3, indicates minor olivine and clinopyroxene fractionation and the existence of a depleted mantle OIB source. Their parental magma was generated from partial melting in the garnet–lherzolite stability field. A tholeiitic Group 2 of doleritic dikes displaying low MgO, Cr and Ni contents, La/YbN ∼ 5, positive Ba, Sr and Pb anomalies, the absence of a negative Nb anomaly coupled with moderate 87Sr/86Sri ∼ 0.7044 and low ɛNd(t) ∼ 0 (BSE-like), indicates a contamination of a mantle-derived magma that experienced crystal fractionation of plagioclase and clinopyroxene. This second group, similar to the low-Ti tholeiitic basalts of the Central Atlantic Magmatic Province (CAMP), was derived from partial melting in the peridotite source within the spinel stability field. Lower Mesozoic continental rifting could have been initiated by a heterogeneous mantle plume that supplied source components beneath Daoura, in the Ougarta Range.

A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

The origin and petrogenesis of the Cameroon Volcanic Line (CVL), composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, heterogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between SiO2 58–64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability field. Melting models suggest small degree (<3%) partial melting of mantle bearing (6–10%) garnet for Mt. Etinde, the Ngaoundere Plateau and the Biu Plateau, and <5% of garnet for the oceanic sector of the CVL, Mt. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope systematics suggest that mixing in various proportions of Depleted MORB Mantle (DMM) with enriched mantle 1 and 2 (EM1 and EM2) could account for the complex isotopic characteristics of the CVL lavas. Low Mg number (Mg# = 100 × MgO/(MgO + FeO)) and Ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in Nb/Ta and Zr/Hf ratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle heterogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes.

Origin of Basalts in a Hot Subduction Setting: Petrological and Geochemical Insights from Mt. Baker, Northern Cascade Arc

The northern Cascade arc is an end-member ‘hot’ subduction zone where slab dehydration may be essentially complete prior to reaching sub-arc depths, presenting a potential problem for a flux melting origin for arc basalts. Nevertheless, mafic lavas from the Mt. Baker volcanic field, the most magmatically productive volcano in the northern Cascade arc, record subduction input and compositions typical of calc-alkaline magmas. Relative to normal mid-ocean ridge basalt, the most primitive lavas at Mt. Baker show elevated abundances of large ion lithophile elements (LILE), Pb and light rare earth elements (LREE). High field strength elements (HFSE) and heavy REE (HREE) are depleted relative to LILE. Reconstructed primary magmas define three groups: Group I calcalkaline basalts (Sulphur Creek, Lake Shannon, Hogback) have lower SiO2, (La/Yb)N and LILE/HFSE, and are olivineand plagioclase-phyric; Group II high-Mg basaltic andesites (Tarn Plateau, Cathedral Crag) also contain clinopyroxene phenocrysts and have higher H2O, SiO2, (La/Yb)N and LILE/HFSE; Group III low-K olivine tholeiite (Park Butte) has the lowest (La/Yb)N but highest LILE/HFSE.The redox states of all groups lie between QFM (quartz^fayalite^magnetite) and QFM þ1·4. Pb, Sr and Nd isotope compositions are similar among all the analysed samples and are consistent with a depleted mantle source modified by input from the subducting slab. Trace element^isotope modeling indicates a subduction component composed predominantly of metabasaltderived fluid with lesser amounts of sediment melt and metabasalt melt. Group I basalts record the smallest melt fractions ( 5^7%), lowest water contents (1·5^2·1wt %), and highest temperatures and pressures of mantle segregation (up to 13548C, 1·5 GPa) from lherzolitic ( spinel) residues. Group II basaltic andesites show the greatest extents of mantle metasomatism, the highest water contents (2·7^3·7 wt %) and partial melt fractions (10^12%), and segregated from harzburgite at 12708C, 1GPa, consistent with pooling of melts at the Moho. Group III records P^Tconditions similar to Group I ( 1·4 GPa, 13268C) but melt fractions (12%) and mantle residues (harzburgite) are more similar to Group II, and H2O contents ( 2·1wt %) are intermediate. Melting beneath Mt. Baker was initiated by dehydration melting of amphibole peridotite at 95 km and 10208C, within the stability field of garnet lherzolite. Initial melt fractions were small ( 1^2%) and near water-saturation. Phase equilibria and trace element modeling show no evidence for garnet-bearing mantle residues, indicating that progressive melting during ascent of diapirs through the hot core of the convecting mantle wedge reduced H2O contents and erased any residual garnet signature. Because fluid release from the slab is restricted to the forearc, mantle hydrated at shallow depths in the serpentine and/or chlorite stability fields must be down-dragged to the region of amphibole stability to initiate dehydration melting.

Origin of submarine volcanism at the eastern margin of the central atlantic: Investigation of the alkaline volcanic rocks of the carter seamount (Grimaldi Seamounts)

This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.

Hydrous, Low-carbon Melting of Garnet Peridotite

The presence of volatile species in the Earth's upper mantle drives the formation of low-degree melts at pressures and temperatures at which volatile-free mantle rocks would be subsolidus. The two most abundant volatile species, given the oxidation state of the Earth's upper mantle, are carbon dioxide and water; each species has a distinct effect on the melting process. We present experimental melting results from 3 GPa and 1375°C on hydrous systems with controlled water contents and rigorously minimized carbon contamination that constrain the independent effects of these volatiles. The hydrous melts in these experiments are in equilibrium with garnet peridotite at pressures reasonable for hydrous melting under mid-ocean ridges. Compared with anhydrous experiments or carbon-rich silicate melting, the addition of water produces a melt with increased SiO_2 content relative to MgO and FeO, tantamount to an increase in the stability of olivine at the solidus relative to the other crystalline phases. We also report a substantial and unexpected change in the composition of clinopyroxene in equilibrium with the melt; the clinopyroxene stability field contracts when water is added to the system, producing clinopyroxenes with higher CaO and lower Al_2O_3 than found at the same pressure without water. The contraction of the clinopyroxene field decreases the bulk partition coefficients of TiO_2, Na_2O, heavy rare earth elements, U, and H_2O, with important implications for hydrous melting of the mantle; for example, initiating hydrous melting deeper in the garnet lherzolite stability field.

Origin and significance of spinel–pyroxene symplectite in lherzolite xenoliths from Tallante, SE Spain

We found spinel–pyroxene symplectites in lherzolite xenoliths from Tallante, SE Spain, and investigated their petrographical and geochemical signatures. The spinel–pyroxene symplectites are divided into two types, a spinel-type (= opx + cpx + sp) and a plagioclase-type (= opx + cpx + sp + pl) symplectites. The symplectites are always surrounded by lenticular aggregates of coarser-grained spinel–pyroxene. The petrography and major-element chemistry of bulk symplectites indicate an origin through subsolidus reaction between olivine and garnet like at Horoman (Japan; Morishita and Arai, Contrib Mineral Petrol 144:509–522, 2003). The spinel–pyroxene symplectites at Tallante were of garnet origin. However, the bulk Tallante spinel–pyroxene symplectites show a relatively flat rare earth element (REE) distribution with slight light REE (LREE) enrichment, i.e. there was no trace-element signature typical of mantle garnet. They also differ from the Horoman symplectites that occasionally preserve a garnet trace-element signature, i.e. depletion of LREE and enrichment of heavy REE. These conflicting results indicate that the symplectites record slight enrichment in pyroxene compositions during or after depletion by melt extraction and breakdown of garnet by decompression, and all the minerals including symplectite constituents have been homogenized in the stability field of spinel to plagioclase lherzolite, with the assistance of some melt (possibly an alkaline silicate melt; Downes, J Petrol 42:233–250, 2001). Moreover, some of the spinel-type symplectites experienced heating by injection of Si-rich melt, and consequently have been transformed to the plagioclase-type symplectite. The Tallante spinel–pyroxene symplectites developed from garnet + olivine and were carried from the garnet–lherzolite stability field to the spinel– and to the plagioclase–lherzolite stability fields. Our data indicates mantle upwelling (mantle diapirism) beneath the Betic–Rif zone in southern Spain.

Partitioning of water during melting of the Earth's upper mantle at H 2O-undersaturated conditions

We present the results of an experimental study of the partitioning of water between common upper mantle minerals (olivine, orthopyroxene, clinopyroxene, garnet) and silicate melt, consisting of 352 measurements in 23 hydrous melting experiments conducted over a broad range of melt water contents (1.7–25 wt.% H 2O) at upper mantle conditions (1–4 GPa; 1000–1380 °C). Our data show that, at water-undersaturated conditions, incorporation of H 2O into olivine and pyroxenes is accomplished through two independent mechanisms: a coupled substitution in which H + and Al 3+ replace Si 4+ in the mineral structure, and the substitution of 2H + for Mg 2+ previously identified in minerals hydrated at water-saturated conditions. At upper mantle temperatures and pressures < 2 GPa, these two substitution mechanisms appear to contribute approximately equal amounts of water to olivine; at higher pressures, the fugacity-dependent 2H +–Mg 2+ substitution dominates. For orthopyroxene, coupled substitution of H + and tetrahedral Al 3+ dominates over the 2H +–Mg 2+ substitution at pressures < 8 GPa, while the Al-coupled substitution dominates in clinopyroxene at all pressures. Our data permit a new evaluation of the maximum storage capacity of water in nominally anhydrous upper mantle peridotite and eclogite. The water storage capacity of peridotite increases gradually with pressure to a maximum of 0.6 wt.% H 2O at 410 km depth; the storage capacity of eclogite is 0.4 to 0.5 wt.% H 2O from 2–5 GPa, dropping gradually to ∼0.2 wt.% just above the transition zone as majorite is formed at the expense of pyroxene. We show that the water abundances inferred for mid-ocean ridge and hotspot magma sources are not consistent with the composition of water-rich mantle residues emerging from a hydrous melt layer at the top of the transition zone. Regional variations observed in the H 2O–LREE systematics of oceanic basalts can result from derivation of these magmas from depleted mantle sources having polybaric melting histories, with high-H 2O/Ce sources being residues of shallow (garnet-absent) melting, and low-H 2O/Ce sources being the residues of deep melting in the stability field of garnet lherzolite.

Mesozoic volcanism in the Middle East: geochemical, isotopic and petrogenetic evolution of extension-related alkali basalts from central Lebanon

Mesozoic picritic and alkali basalts from central Lebanon represent a significant part of an extension-related Upper Jurassic to Upper Cretaceous discontinuous volcanic belt which occurs throughout the Middle East. Volcanism was associated with an episode of intraplate extension that followed a period of continental break-up, where Mesozoic micro-continental blocks separated from Gondwana as the Neotethys ocean opened in Jurassic times. This volcanic episode produced mafic lava flows ranging in thickness from 5 to 20 m, along with some minor pyroclastic flows. These flows are stratigraphically intercalated with thick carbonate platform deposits. The basalts are made up of about 15–20% olivine (Fo78–91), 30–35% clinopyroxene (salite), 40–50% plagioclase (An56–71) and opaque Fe–Ti oxides (∼5%). Geochemically, the rocks exhibit a relatively wide range of SiO2 (40.4 to 50.5 wt%) and MgO (5.1 to 15.5 wt%) contents, are relatively enriched in TiO2 (1.7 to 3.7 wt%) and vary in composition from alkali picrite and basanite to alkali basalt. The Mg numbers range from 0.56 to 0.70, with an average of 0.63. The rocks are enriched in incompatible trace elements such as Zr (86–247 ppm), Nb (16–66 ppm) and Y (19–30 ppm). Such compositions are typical of those of HIMU-OIB and plume-related magmas. The REE patterns are fractionated ((La/Yb)N = 11), LREE enriched, and are generally parallel to subparallel. Elemental ratios such as K/P (1.1–4.7), La/Ta (11–13), La/Nb (0.57–0.70), Nb/Y (0.68–1.55) and Th/Nb (0.20–0.36) suggest that crustal contamination was minor or absent. This may be related to a rapid ascent of the magma, in agreement with the nature (mafic, oceanic-like) and the small thickness (about 12 km) of the Mesozoic crust of the Eastern Mediterranean region. The 143Nd/144Nd isotopic compositions of the lavas range from 0.512826 to 0.512886, and 87Sr/86Sr from 0.702971 to 0.703669, suggesting a HIMU-like mantle source. Trace element compositions indicate a melt segregation depth of 100–110 km, well within the garnet lherzolite stability field. The geochemical characteristics of the rocks are typical of within-plate alkali basalts, and suggest that the magmas were derived from a fertile, possibly plume-related, enriched mantle source. Petrogenetic modelling indicates that the magmas were produced by very small degrees of batch partial melting (F = 1.5%) of a primitive garnet-bearing mantle source (garnet lherzolite).

Trindade and Martı́n Vaz Islands, South Atlantic: Isotopic (Sr, Nd, Pb) and trace element constraints on plume related magmatism

Highly alkaline silica undersaturated lavas erupted at Trindade Island over its 5 Ma geologic history and comprise primitive nephelinites-basanites and more evolved nepheline-bearing phonolitic rocks. Nephelinites-basanites and phonolitic rocks are thought to be genetically related via crystal fractionation, as indicated by the very limited range in Sr, Nd and Pb isotope ratios, systematically increasing contents of incompatible trace elements from primitive to evolved rock types, and similar variation in chemical composition of the major phenocryst phases (clinopyroxene, amphibole, feldspar) in all rock types. Tb/Yb ratios of the primitive lavas are high (2.6–4.1) and silica contents are low (39.8–42.9 wt.% SiO 2), indicating that the melts were generated at deep mantle depths (∼150 km), within the garnet lherzolite stability field. Non-radiogenic 87Sr/ 86Sr (0.70377–0.70421) and radiogenic 143Nd/ 144Nd (0.512752–0.512837) values show that the Trindade and Martı́n Vaz rocks are derived from moderately depleted sources relative to bulk-earth. The lavas have moderate radiogenic 206Pb/ 204Pb ratios of 19.00–19.33, 207Pb/ 204Pb of 15.56–15.60, and 208Pb/ 204Pb of 38.89–39.34; they plot close to the Northern Hemisphere Reference Lines (NHRL). The narrow range of Sr, Nd, and Pb isotopic compositions in the Trindade and Martı́n Vaz lavas suggests either that the source region was homogeneous (similar to the common mantle components FOZO and “C”), or that melts from a heterogeneous three-component mantle source, involving HIMU, enriched mantle EM I, and depleted N-type MORB, were well mixed before eruption. Late Cretaceous to Present volcanism ranging from interior Brazil towards Trindade is thought to record the passage of the South American plate over the Trindade mantle plume (e.g., O’Connor and Duncan, 1990). Comparison with published data from other mafic rocks along the suggested plume track shows that Trindade isotopic compositions match those of transitional basalts from the Late Cretaceous Poxoreu igneous province (inland Brazil) and the Eocene Abrolhos platform (Brazilian Atlantic margin). Simple batch-melting considerations indicate that the degree of melting has decreased from Poxoreu through Abrolhos to Trindade. These estimates might suggest a diminishing thermal input of the Trindade mantle plume through time.

Mid-Ocean Ridge Melting: Constraints from Lithospheric Xenoliths at Oahu, Hawaii

One plagioclase–spinel lherzolite and four spinel lherzolite xenoliths from Oahu, Hawaii, contain clinopyroxene grains that show homogeneous rare earth element (REE) abundances and smooth REE patterns with systematic depletion of light REE (LREE). These five xenoliths are mid-ocean ridge basalt (MORB) magma-depleted residues with compositions that were not modified by later metasomatism. Trace element systematics of these xenoliths were used to investigate the melt production rate (dF/dP) within a 90-my-old residual mantle column (RMC). Such rates were calculated as ratios of the difference in extents of depletion (dF) to the difference in equilibrium pressures (dP) between two xenolith samples. The extents of melting were modeled from REE, Sr and Zr abundances in clinopyroxene; and equilibrium pressures were inferred from the two-pyroxene geobarometer of Mercier et al. (1984, Contributions to Mineralogy and Petrology, 85, 391–403). Equilibrium pressures range from 21 to 7 kbar and extents of melting vary from 2 to 8%. Together, these data constrain the maximum extent of melting in the garnet lherzolite stability field to be <2% and a dF/dP of 0.43%/kbar within the stability field of spinel lherzolite. Uncertainties in the estimates of equilibrium pressure and extent of depletion lead to a slightly broader range of dF/dP values (0.26-0.78%/kbar). These values are significantly lower than that of ∼1.2%/kbar suggested by most previous studies. With the best estimated mean dF/dP of 0.43%/kbar, only 3.9 km of crust could have been generated by melting lherzolite in the pressure range of 26–7 kbar. The thickness and composition of the crust that overlies the 90 Ma Oahu RMC require a higher extent of melting. Based on the REE abundances, most samples from the 90 Ma crust and East Pacific Rise can be explained as pooled melts derived from the lherzolitic source in a passive melting regime with a maximum extent of melting (Fmax) of 30%. This model produces a dF/dP of 4.4%/kbar in the pressure range of 7–2 kbar and generates an additional 1.8 km of crust. In detail, the high [Sm/Yb]DM ratios in most East Pacific Rise samples cannot be explained by melting a lherzolitic source. Instead, they reflect mixing between two components: (1) the pooled melt derived from lherzolitic source in a passive melting regime with an Fmax of 30% (∼80% of the mixture) and (2) melt derived from garnet pyroxenite by 40% fractional melting (∼20% of the mixture). This model produces 7.1 km of crust.

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

A suite of ultramafic xenoliths (spinel peridotites, one olivine-clinopyroxene hornblendite, and one spinel pyroxenite) from Tertiary basalt vents and lava flows of the Rhon 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 Rhon 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 Rhon area. These investigations show that part of the lithospheric mantle underneath the Rhon 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.

A possible role for garnet pyroxenite in the origin of the "garnet signature" in MORB

Geochemical data have been interpreted as requiring that a significant fraction of the melting in MORB source regions takes place in the garnet peridotite field, an inference that places the onset of melting at ≥80 km. However, if melting begins at such great depths, most models for melting of the suboceanic mantle predict substantially more melting than that required to produce the 7 ± 1 km thickness of crust at normal ridges. One possible resolution of this conflict is that MORBs are produced by melting of mixed garnet pyroxenite/spinel peridotite sources and that some or all of the “garnet signature” in MORB is contributed by partial melting of garnet pyroxenite layers or veins, rather than from partial melting of garnet peridotite. Pyroxenite layers or veins in peridotite will contribute disproportionately to melt production relative to their abundance, because partial melts of pyroxenite will be extracted from a larger part of the source region than peridotite partial melts (because the solidus of pyroxenite is at lower temperature than that of peridotite and is encountered along an adiabat 15–25 km deeper than the solidus of peridotite), and because melt productivity from pyroxenite during upwelling is expected to be greater than that from peridotite (pyroxenite melt productivity will be particularly high in the region before peridotite begins melting, owing to heating from the enclosing peridotite). For reasonable estimates of pyroxenite and peridotite melt productivities, 15–20% of the melt derived from a source region composed of 5% pyroxenite and 95% peridotite will come from the pyroxenite. Most significantly, garnet persists on the solidus of pyroxenite to much lower pressures than those at which it is present on the solidus of peridotite, so if pyroxenite is present in MORB source regions, it will probably contribute a garnet signature to MORB even if melting only occurs at pressures at which the peridotite is in the spinel stability field. Partial melting of a mixed spinel peridotite/garnet pyroxenite mantle containing a few to several percent pyroxenite can explain quantitatively many of the geochemical features of MORB that have been attributed to the onset of melting in the stability field of garnet lherzolite, provided that the pyroxenite compositions are similar to the average composition of mantle-derived pyroxene-rich rocks worldwide or to reasonable estimates of the composition of subducted oceanic crust. Sm/Yb ratios of average MORB from regions of typical crustal thickness are difficult to reconcile with derivation by melting of spinel peridotite only, but can be explained if MORB sources contain ∼5% garnet pyroxenite. Relative to melting of spinel peridotite alone, participation of model pyroxenite in melting lowers aggregate melt Lu/Hf without changing Sm/Nd ratios appreciably. Lu/Hf-Sm/Nd systematics of most MORB can be accounted for by melting of a spinel peridotite/garnet pyroxenite mantle provided that the source region contains 3–6% pyroxenite with ≥20% modal garnet. However, Lu/Hf-Sm/Nd systematics of some MORB appear to require more complex melting regimes and/or significant isotopic heterogeneity in the source. Another feature of the MORB garnet signature, (^(230)Th)/(^(238)U)>1, can also be produced under these conditions, although the magnitude of (^(230)Th)/(^(238)U) enrichment will depend on the rate of melt production when the pyroxenite first encounters the solidus, which is not well-constrained. Preservation of high (^(230)Th)/(^(238)U) in aggregated melts of mixed spinel peridotite/garnet pyroxenite MORB sources is most likely if the pyroxenites have U concentrations similar to that expected in subducted oceanic crust or to pyroxenite from alpine massifs and xenoliths. The abundances of pyroxenite in a mixed source that are required to explain MORB Sm/Yb, Lu/Hf, and (^(230)Th)/(^(238)U) are all similar. If pyroxenite is an important source of garnet signatures in MORB, then geochemical indicators of pyroxenite in MORB source regions, such as increased trace element and isotopic variability or more radiogenic Pb or Os, should correlate with the strength of the garnet signature. Garnet signatures originating from melts of the garnet pyroxenite components of mixed spinel peridotite/garnet pyroxenite sources would also be expected to be stronger in regions of thin crust.

High 143Nd/ 144Nd in extremely depleted mantle rocks

Using an iron hydroxide precipitation to concentrate the rare earth elements it was possible to obtain precise Nd isotopic data on ultramafic rocks. Four samples of unserpentinized harzburgite from the Voykar ophiolite, Polar Urals, are characterized by extreme depletion in fertile components including light rare earth elements down to ∼ 1.7 ppb Nd. These samples have f Sm/Nd values > +1 and ε Nd(O) values ranging from +21 to +47, the highest ever measured in terrestrial whole rock samples. Combined ε Nd and ε Sr values of the ultramafic and mafic rocks and a leach indicate that alteration from seawater and/or groundwater is orders of magnitude less than required to produce a detectable change in the ε Nd values even in samples with > 50% serpentine. Three of the harzburgites plot on a Sm-Nd whole rock array along with samples of dunite, websterite, gabbro and diabase from the ophiolite. The slope of the array corresponds to an age of 387 ± 34 Ma and an initial ε Nd = +8.6 ± 1.8. These results indicate that the oceanic crustal rocks and the harzburgites are complementary and come from a MORB source. While the linear 147Sm/ 144Nd- 143Nd/ 144Nd correlation is consistent with this model, one sample of harzburgite is far removed from the array, indicating the presence of older, depleted mantle material that is unrelated to the crustal section. This must be locally available in the upper mantle for tectonic occlusion during obduction of the ophiolite. Alternatively, this “aberrant” sample may represent the original ultrabasic section from older basalt extraction that was permeated by basalts derived from much deeper layers that provided the Voykar ophiolite crust. The major-element and low-REE-compositions of the Voykar harzburgites can be formed by progressive extraction of melt from an undepleted mantle protolith with the requirement that melt separation began in the garnet lherzolite stability field. The extreme chemical and isotopic characteristics of the Voykar harzburgites demonstrate that extraction of partial melt can generate an upper mantle with highly variable ε Nd and very low Nd concentrations. The potential presence of large amounts of such material in the upper mantle poses an intriguing geochemical problem in that its bulk isotopic signature may not be detectably expressed in basalts, which are dominated by sources with higher average Nd concentrations and much lower ε Nd and f Sm/Nd.

Experimental and natural partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene and basaltic melts

Partition coefficients for Th, U, Pb, rare-earth elements (REE), high field strength elements (HFSE), alkaline-earth elements, Sc, Cr, V and K were measured by ion microprobe techniques in two experiments on a natural high-alumina basalt composition from Medicine Lake, California. All elements were measured at natural abundance levels except Th, U and Pb, which were each present in the starting mix at 1-wt% levels. The results show that garnet retains U preferentially over Th ( D gt melt U = 0.0059 , D gt melt Th = 0.0014 ), while clinopyroxene shows the opposite sense of partitioning ( D cpx melt U = 0.0127 , D cpx melt Th = 0.014 ). The experimental Th, U and Pb partition coefficients for garnet-melt and cpx-melt are consistent with garnet-cpx pairs from garnet-bearing ultramafic rocks which exhibit U-Pb isochrons, thus demonstrating equilibrium ( D gt cpx U = 0.30 , D gt cpx Th = 0.072 , D gt cpx Pb = 0.016 ). The partition coefficients for Th and U between clinopyroxene and basaltic melt vary systematically as a function of the tetrahedral Al content of clinopyroxene. Garnet/melt values for Th, U and Pb agree with previous determinations, indicating that mid-ocean ridge basalt (MORB) generation begins in the stability field of garnet lherzolite. However, high 226 Ra 230 Th ratios in MORB require very small porosities near the region where the melts lose chemical equilibrium with the mantle. Partitioning data for HFSE and REE suggest that this region of melt segregation is not in the spinel lherzolite field. This requires either rapid transport of MORB magmas from ϵ 70 km, or some degree of disequilibrium during melt generation and/or transport.

Evolution of the Upper Mantle beneath the Pyrenees: Evidence from Orogenic Spinel Lherzolite Massifs

About 40 spinel lherzolite massifs occur as small isolated bodies throughout the North Pyrenean Metamorphic Zone. Peridotites are foliated and range in composition from harzburgite to lherzolite. Pyroxenites (spinel websterites to garnet pyroxenites) define a layering concordant to the foliation plane. Less commonly, the layering is cross-cut by dykes of amphibole pyroxenites and hornblendites (e.g., Lherz and Freychinède). The oldest event of the complex pre-emplacement evolution is a lowdegree partial melting (< 10% melt extracted) which was initiated in the garnet lherzolite stability field. This event may have been associated with asthenospheric upwelling of Palaeozoic age. Two magmatic episodes are superimposed on partial melting features. The first gave rise to the layered pyroxenites, interpreted as crystal segregates which separated within feeder dykes of tholeiites. These melts intruded the peridotites when the massifs were located at the top of mantle diapirs (i.e., 45–50 km). Subsequently, both peridotites and pyroxenites underwent deformation with tectonic rotation of the dykes and recrystallized within the spinel lherzolite stability field during an isobaric cooling stage from 1200 to 900 °C. Peridotites and pyroxenites contain trace amounts of Ti-pargasite which occur as interstitial grains, suggesting the arrival of volatile-rich, strongly evolved melt before the end of the isobaric cooling stage. The amphibole-rich veins correspond to crystal segregates which crystallized within feeder dykes of Mid-Cretaceous alkali basaltic magmas. The injection of alkali basalts was contemporaneous with high-stress, low-temperature plastic deformation which produced mylonitic textures in the peridotites and tectonic rotation of the amphibole-rich veins. These intra-lithospheric shear zones are related to the Mid-Cretaceous transcurrent movements of the Iberian Peninsula relative to the European Plate, which were also responsible for the tectonic introduction of the massifs into the lower crust. Several arguments suggest a slightly older age of crustal emplacement for the Western Pyrenean massifs compared with the eastern Pyrenean massifs.