Melting Of Mantle Peridotite Research Articles

Slab window–related magmatism as a probe for pyroxenite heterogeneities in the upper mantle

Abstract New high-precision trace-element analyses of magmatic olivines point to a pyroxenite-dominated source for recent alkali basalts erupted above slab windows formed along the Antarctic Peninsula. Melting occurred at ambient mantle temperature, and basalts have geochemical compositions that are indistinguishable from ocean-island basalts (OIBs). We propose that the pyroxenite component originally resided in the upper mantle beneath the subducted slab; formation of a slab window allowed limited decompression and the generation of melts of garnet-pyroxenite, but little or no melting of mantle peridotite. The pyroxenite component in the mantle formed ca. 550 Ma, an age that does not require long-term recycling of subducting slabs to the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the adjacent extinct Phoenix Ridge owes its enriched trace-element compositions to mixing between small melt fractions of pyroxenite and peridotite during a period of decreased spreading rate prior to the death of the ridge ca. 3.3 Ma. It is likely that the variable trace-element enrichment seen in East Pacific Rise E-MORB distal from hotspots results from the same process of interactions between small-melt-fraction (<~5%) melts of pyroxenite and peridotite.

ANALYSIS OF MANTLE SOURCES FOR MAFIC ASSOCIATIONS ON THE BASIS OF GEOCHEMICAL AND Nd ISOTOPIC DATA

The (La/Nb) pm , (Th/La) pm , (La/Sm) pm , (NbY) pm ratios <1 in mafic rocks correspond to a depleted mantle source, while ≥1 corresponds to a undepleted/enriched source. (La/Nb) pm does not change during melting of mantle peridotites, so the Nd minimum is a characterictic feature of the lithospheric mantle metasomatized by fluid/melt. According to the isotopic Nd composition, there are two mantle sources: (1) depleted (+e Nd ), which corresponds to the asthenospheric and/or lithospheric mantle of restite origin, (2) enriched (-e Nd ) lithospheric, having decreased Sm/Nd due to metasomatism. Estimates of the character of the mantle sources of mafic rocks in terms of their trace element and isotopic composition can either correlate with each other or differ.

Geological setting and genesis of the Kurmansky gabbro-trondhjemite massif (Middle Urals)

This paper reports the results of petrogeochemical studies of the Kurmansky gabbro-trondhjemite massif (eastern slope of the Middle Urals), lying in the western part of the large Reftinsky allochthonous block within the accretion East Uralian megazone. The relevance of this study is determined by the uncertainty in geodynamic setting and formation conditions of the rock massif and its role in the evolution of the Ural Mobile belt. We specified the countours of the massif. It is shown that the rocks were resulted from spatiotemporal convergence of partial melting in the mantle and lower crust at the island-arc stage of the Ural Mobile belt evolution. Partial melting of mantle peridotite, under the influence of an aqueous fluid rising from the subduction zone, initiated the occurrence of basite melts. The separation of the melt and its subsequent evolution to the compositions of gabbrodiorite and diorite took place at Ptot=10 kbar. Trondhjemites were formed as a result of partial melting of amphibolites at Ptot≥8 kbar, PH2O=0.1–0.2 kbars. The crystallization of trondhjemites in the crust was accompanied by the wollastonite skarns on contact with carbonate rock and xenoliths culminated at mesoabyssal level, Ptot=PH2O=1 kbar. The comparison between the composition of Kurmansky gabbro-trondhjemite massif and the island-arc- and collision-related magmatic suites in the region allowed us to assume that the Kurmansky massif belongs to the independent Early Devonian (?) gabbro-trondhjemite complex of island arc origin. The rock metamorphism conditions were evaluated, with the transformations supposedly related to the accretion of early island arc complexes at the Murzinsky-Aduysky microcontinent, which took place in the Devonian.

The nature of the Moho beneath fast‐spreading centers: Evidence from the Pacific plate and Oman ophiolite

AbstractIt is common knowledge that the Moho is the boundary between the crust and the Earth's mantle. Here we show along several seismic profiles through the Pacific Plate that a correlation exists between the strength of Moho reflections, crustal thickness, and water depth. Where the Moho can be detected clearly, the overlying oceanic crust is systematically thicker and the water depths are shallower. We suggest that two end‐members of oceanic crust exist in fast spreading environments: one thick, underlain by a clear Moho; the other thinner, without a Moho; with all intermediate situations. In the Oman ophiolite, the best‐preserved on‐land analogue of fossil oceanic lithosphere created by fast‐spreading, the boundary between the mantle peridotites and the lower crustal gabbros mainly consists of a dunitic transition zone (DTZ) ranging from a few meters to a few hundred meters in thickness. A sudden influx of seawater down to the base of the crust at the mid‐ocean ridge (MOR) results in the hydrous (re‐)melting of mantle peridotites, producing a dunitic residue at the crust–mantle boundary that represents the most reflective Moho. At the same time, the hydrous melting, in addition to the normal decompression melting, beneath the MOR, increases the thickness of the oceanic crust by enhancing magma production. In the absence of hydrous melting, the DTZ is thin or absent at the crust–mantle boundary, and instead the uppermost mantle harzburgite is intruded by gabbros, and/or the overlying crustal gabbro is intruded by numerous wehrlite bodies, which will be seismically gradational.

Depletion of the upper mantle by convergent tectonics in the Early Earth

Partial melting of mantle peridotites at spreading ridges is a continuous global process that forms the oceanic crust and refractory, positively buoyant residues (melt-depleted mantle peridotites). In the modern Earth, these rocks enter subduction zones as part of the oceanic lithosphere. However, in the early Earth, the melt-depleted peridotites were 2–3 times more voluminous and their role in controlling subduction regimes and the composition of the upper mantle remains poorly constrained. Here, we investigate styles of lithospheric tectonics, and related dynamics of the depleted mantle, using 2-D geodynamic models of converging oceanic plates over the range of mantle potential temperatures (Tp = 1300–1550 °C, ∆T = T − Tmodern = 0–250 °C) from the Archean to the present. Numerical modeling using prescribed plate convergence rates reveals that oceanic subduction can operate over this whole range of temperatures but changes from a two-sided regime at ∆T = 250 °C to one-sided at lower mantle temperatures. Two-sided subduction creates V-shaped accretionary terrains up to 180 km thick, composed mainly of highly hydrated metabasic rocks of the subducted oceanic crust, decoupled from the mantle. Partial melting of the metabasic rocks and related formation of sodic granitoids (Tonalite–Trondhjemite–Granodiorite suites, TTGs) does not occur until subduction ceases. In contrast, one sided-subduction leads to volcanic arcs with or without back-arc basins. Both subduction regimes produce over-thickened depleted upper mantle that cannot subduct and thus delaminates from the slab and accumulates under the oceanic lithosphere. The higher the mantle temperature, the larger the volume of depleted peridotites stored in the upper mantle. Extrapolation of the modeling results reveals that oceanic plate convergence at ∆T = 200–250 °C might create depleted peridotites (melt extraction of > 20%) constituting more than half of the upper mantle over relatively short geological times (~ 100–200 million years). This contrasts with the modeling results at modern mantle temperatures, where the amount of depleted peridotites in the upper mantle does not increase significantly with time. We therefore suggest that the bulk chemical composition of upper mantle in the Archean was much more depleted than the present mantle, which is consistent with the composition of the most ancient lithospheric mantle preserved in cratonic keels.

Origin and tectonic implications of Late Jurassic high-Mg diorites along the Bangong-Nujiang suture zone, Tibet

ABSTRACT The Shiquanhe-Num Co ophiolitic mélange belt (SNOB) is one of two such belts along the Bangong-Nujiang suture zone (BNSZ) separating the Lhasa and Qiantang terranes of central Tibet. This study reports new geochronological, petrological and geochemical data for high-Mg dioritic dikes intruding the Shiquanhe ophiolitic peridotite. Based on LA-ICP-MS zircon U–Pb dating, dioritic dikes yield a Late Jurassic age of 162.8 ± 1.3 Ma. These are high-Mg diorites (53.0%–56.8% SiO2, 7.07%–8.82% MgO) with moderate TiO2 (0.95%–1.01%), high K2O+Na2O (6.02%–8.01%), and high contents of Cr (150–182 ppm), Ni (91–145 ppm) and Th (2.8–5.9 ppm). They are very similar to sanukites of the Setouchi volcanic rock belt (SW Japan). Shiquanhe diorites have high Sr/Y ratios (up to 41.3), distinct positive Pb anomalies, negative Nb anomalies and continental crust-like trace element patterns without negative Eu anomalies (average Eu/Eu* = 0.96). These geochemical characteristics indicate that Shiquanhe diorities were derived by partial melting of mantle peridotite as a result of interaction with hydrous melts of subducted sediments. Similar high-Mg magmas are common in the early evolution of a new subduction zone. Integrated with field observations and results from other studies of Late Jurassic high-Mg andesites and diorites along the SNOB, we argue that the Shiquanhe high-Mg diorites formed at an early stage of northward subduction of the Bangong-Nujiang Tethys ocean (BNO) beneath the Qiangtang terrane. Therefore, these results provide critical constraints on the Jurassic tectonic evolution of the Shiquanhe-Num Co ophiolitic mélange belt and are significant for understanding the evolution of the BNO.

The Late-Paleoarchean Ultra-Depleted Commondale Komatiites: Earth's Hottest Lavas and Consequences for Eruption

Abstract The c.3·3 Ga Commondale komatiites located south of the Barberton greenstone belt in the Kaapvaal Craton are different from other komatiites, possessing compositional and textural features unique to this occurrence. Unlike almost all other known komatiite occurrences, they are not associated with komatiitic basalts or basalts. The komatiite flows are 0·5–25 m thick and are made up of a marginal zone of spinifex-textured and fine-grained aphyric rocks (low-Mg group) and an inner zone of olivine cumulates (high-Mg group), arranged in such a way to give highly symmetrical compositional profiles for many flows. Olivine is the dominant phase in all rocks, but orthopyroxene occurs as spinifex and elongate laths in the marginal zone. Clinopyroxene and plagioclase are entirely absent. The olivine cumulates formed from Mg-rich magma (36·1% MgO, 6·8% FeO) which caused inflation of the thicker flows. The maximum observed olivine composition in cores (Fo 96·6) is the highest recorded for any komatiite worldwide. The high-Mg magma would have erupted at a temperature close to 1670°C, the highest inferred temperature for an anhydrous terrestrial lava. The marginal zone is enriched in incompatible elements compared with the inner zone and formed by fractionation of the parental melt. However, all rock-types in the marginal zone are depleted in FeO (some as low as 3·5%) which could not have been derived by any primary magmatic process. The marginal zone rocks were modified by assimilation and/or alteration by seawater (or brine) components causing migration of iron and strong enrichment of sodium (up to 1·6 wt % Na2O) and chlorine (up to 2400 ppm). Zirconium has an identical distribution to sodium, with both elements greatly enriched above what would result from fractional crystallization, and may result from speciation of these elements at high temperature followed by post-crystallization alteration. Rare earth elements, Y and Nb have contents commensurate with fractionation of the primitive parental magma. Dendritic-textured olivine-rich rocks with orthopyroxene spinifex spatially and compositionally transitional between the marginal zone and the olivine cumulates resulted from interaction of the high temperature parental magma in the centre of the flows with the fractionated melt at the flow margins. A further manifestation of this association is the development of highly regular fine-scale (5–15 cm) layering (up to 45 layers) of alternating olivine cumulate and spinifex near the base of thick flows. This is overlain by olivine cumulates in which the melt/crystal-mush became arranged into a 3-dimensional network controlled by re-distribution of the trapped melt manifest by a spectacular knobbly texture in outcrop. Over 200 flow units are recognized and detailed chemical and mineralogical studies were carried out on drill cores intersecting 375 m of stratigraphy. The parental magma was highly depleted (in ppm Nb 0·017, Zr 1·18, total REE 1·7 and Gd/YbN=0·3, La/YbN=0·038) and although generally regarded to fall into the rare category of Al-enriched komatiites (AEKs), it is considered that these lavas are a unique class of their own of ultra-depleted komatiites. Relative to other AEKs the Commondale komatiites are both enriched in Al as well as being markedly depleted in Ti (390 ppm), giving rise to the extremely high Al2O3/TiO2 (81). The high temperature and low viscosity of the magma resulted in emplacement processes previously unrecognized in komatiites. The primary melt was derived by melting of mantle peridotite in equilibrium with olivine and orthopyroxene. The initial source was depleted in incompatible elements by small degrees of melting (3–4%) followed by high degrees of partial melting (70%) of the subsequent refractory source at 5 GPa (∼150 km).

Fluid flux in the lithosphere beneath southern Tibet during Neo-Tethyan slab breakoff: Evidence from an appinite–granite suite

The mechanism of magma generation during continental collision remains enigmatic. Appinite–granite suites commonly occur within collisional orogenic belts, providing an opportunity to monitor lithospheric evolution and the geodynamic processes involved. This paper presents geochronological, mineralogical, and geochemical data for the Eocene (~50 Ma) Pengcuolin appinite–granite suite, which is the first such suite to have been found in the southern Lhasa Terrane, southern Tibet. Two distinct populations of plagioclase are observed in the appinitic rocks: high-An plagioclases that crystallized as an early phase from H2O-saturated mafic magmas, and low-An plagioclases that formed as a late phase at pressures of ~3 kbar and ~5 kbar, as calculated from equilibrium thermobarometry using low-Al and high-Al amphiboles, respectively. The amphibole-rich appinitic rocks have low SiO2 contents and moderate values of Mg#, and are enriched in large ion lithophile elements and depleted in high field strength elements. They have homogeneous initial 87Sr/86Sr ratios of 0.705174–0.705330, εNd(t) values of 0.85 to 1.47, and zircon εHf(t) values of 3.3 to 7.9. These features suggest the appinites were derived from the melting of mantle peridotites that had been permeated by slab-derived fluids. Whole-rock geochemical data, equilibrium melting calculations, and ABS5 modeling results indicate that the primary melts of the appinitic rocks were generated from garnet-stabilized mantle, and that the melts then ascended with crystallization taking place under H2O-saturated conditions and under variable temperatures and pressures. The associated granites have a high and narrow range of SiO2 contents, and they exhibit a high-K calc-alkaline character, and low A/CNK ratios and Mg# values. These geochemical characteristics, plus their widespread distribution and depleted isotopic compositions (87Sr/86Sri = ~0.705, εNd(t) = 1.12–1.59, and zircon εHf(t) = 6.9–9.2), are comparable to those of Cenozoic granitoids elsewhere in southern Tibet and consistent with the partial melting of juvenile mafic lower-crustal materials. This appinite–granite suite not only attests to a flux of slab fluids through the lithospheric mantle during breakoff of the Neo-Tethyan slab, it also sheds light on the vertical profile of magma evolution from the deep mantle to the shallow crust. The emplacement of the appinite–granite suite was facilitated by regional faults, but the rising hydrous basaltic magmas were often trapped by the thick overlying continental crust, and those underplating magmas provided the heat and water that caused the partial melting of the continental crust, finally leading to the widespread Cenozoic felsic (and rare mafic) magmatic rocks of southern Tibet.

Oceanic lavas sampling the high-3He/4He mantle reservoir: Primitive, depleted, or re-enriched?

Helium isotopes are used as a tracer for primitive reservoirs that have persisted in the Earth’s mantle. Basalts erupted at several intraplate oceanic islands, including Hawaii, Iceland, Galapagos, and Samoa, have hosted the highest 3He/4He ratios (>30 Ra, where Ra is atmospheric 3He/4He ratio) globally that are far in excess of the 3He/4He typical of the upper mantle sampled at mid-ocean ridges (8 Ra). These lavas have been suggested to be melts of a primitive, or possibly slightly depleted, mantle reservoir, i.e., either fertile or a depleted peridotite. Here we report evidence for geochemical enrichment in the high-3He/4He mantle sampled by lavas with the highest 3He/4He from Hawaii, Samoa, and possibly Galapagos. The titanium concentrations in high-3He/4He lavas from Samoa are too high to be explained by melts of a mantle peridotite, even at infinitesimally small degrees of melting, and the elevated Ti corresponds to elevated Pb-isotopic ratios. The highest 3He/4He lavas from Loihi, Hawaii, also have Ti concentrations that are too high to be melts of primitive mantle peridotite at the degrees of melt extraction proposed for this ocean island. Thus, Ti-rich material must have been added to the high-3He/4He mantle reservoir, and this material is likely to be recycled mafic crust similar to MORB-like eclogite, which is consistent with the elevated Pb-isotopic ratios. We show that fractionation corrected, major element compositions of high-3He/4He alkalic lavas can be satisfactorily modeled by melting and melt-rock interaction scenario in a fertile peridotite-MORB-eclogite hybrid system. Primitive peridotitic and recycled eclogitic reservoirs are suggested to be intimately associated in the deepest mantle and high-3He/4He lavas from several localities may sample a mantle source that hosts a component of recycled oceanic crust.

Carbonate-silicate composition of diamond-forming media of fibrous diamonds from the Snap Lake area (Canada)

This study presents new data on the compositions of microinclusions in fibrous diamonds from the Snap Lake area in the eastern part of the Slave Craton (Canada). The compositional trends of diamond microinclusions are consistent with those of diamond-forming media ranging continuously between a highly carbonatitic endmember and a highly silicic endmember. The microinclusions exhibit general enrichment of most incompatible elements, which is probably indicative of their crystallization during partial melting of mantle peridotites and eclogites. Our results also suggest that the diamond analyzed in this study may have formed as a result of interaction between carbonate-silicate melts and peridotitic wall-rocks at the base of a thick lithospheric mantle at depths below 300 km. The trace element distributions in the studied diamond microinclusions show a general similarity to those previously found in the parental kimberlites and carbonatites. These data suggest that diamonds may have crystallized either directly from a kimberlitic/carbonatitic melt or from a proto-kimberlitic fluid/melt, which was derived from a source also common to kimberlites. This is supported by differences in the major element compositions of diamond-forming fluids/melts and kimberlites.

Petrology and geochemistry of the Tertiary Suez rift volcanism, Sinai, Egypt

The Tertiary basaltic rocks of Southwestern Sinai, situated along the Wadi Nukhul–Wadi Matullah–Wadi El Tayiba join, were selected to study the Gulf of Suez rift related-lavas and their geochemical and petrological relation with the rifting process. Whole rock samples were studied petrographically and analysed for major and trace elements. The samples from dykes, sills and flows from multiple magmatic events display a large variety in texture and in modal mineral compositions. They range from olivine dolerites and olivine-bearing basalts to vitrophyric, texturally heterogeneous basalts and crystal lithic tuffs. The transitional tholeiitic basalts display low compatible element concentrations and an enrichment of the whole spectrum of the incompatible elements. Major, trace and Rare Earth Element data suggest that the melts formed by 5% melting of mantle peridotite at the spinel–garnet transition zone (80–90km depth), in the presence of 2–4% residual garnet. During the melt ascent, the fractionating phases were olivine, clinopyroxene and, to a lesser extent, plagioclase. Thermobarometric calculations indicate the presence of two crystallization levels beneath the Gulf of Suez rift: a shallower stage at 15–20km and a deeper stage at depths of 25–30km. The mantle source consists of streaks and blobs of enriched mantle, preserved in the geochemical signatures of these rocks. The enriched mantle sources melted preferentially compared to the surrounding ambient mantle and thus led to a preferential enrichment of the sources of the Gulf of Suez rift.

Depths and temperatures of <10.5 Ma mantle melting and the lithosphere‐asthenosphere boundary below southern Oregon and northern California

AbstractPlagioclase and spinel lherzolite thermometry and barometry are applied to an extensive geochemical dataset of young (<10.5 Ma) primitive basaltic lavas from across Oregon's High Lava Plains, California's Modoc Plateau, and the central‐southern Cascades volcanic arc to calculate the depths and temperatures of mantle melting. This study focuses on basalts with low pre‐eruptive H2O contents that are little fractionated near‐primary melts of mantle peridotite (i.e., basalts thought to be products of anhydrous decompression mantle melting). Calculated minimum depths of nominally anhydrous melt extraction are 40–58 km below Oregon's High Lava Plains, 41–51 km below the Modoc Plateau, and 37–60 km below the central and southern Cascades arc. The calculated depths are very close to Moho depths as determined from a number of regional geophysical studies and suggest that the geophysical Moho and lithosphere‐asthenosphere boundary in this region are located in very close proximity to one another (within 5–10 km). The basalts originated at 1185–1383°C and point to a generally warm mantle beneath this area but not one hot enough to obviously require a plume contribution. Our results, combined with a range of other geologic, geophysical, and geochemical constraints, are consistent with a regional model whereby anhydrous mantle melting over the last 10.5 Ma in a modern convergent margin and back arc was driven by subduction‐induced corner flow in the mantle wedge, and to a lesser extent, toroidal flow around the southern edge of the subducting Juan de Fuca and Gorda plates, and crustal extension‐related upwelling of the shallow mantle.

Two Primary Basalt Magma Types from Northwest Rota-1 Volcano, Mariana Arc and its Mantle Diapir or Mantle Wedge Plume

Primitive basalts are rarely found in arcs.The active NWRota-1 volcano in the Mariana arc has erupted near-primitive lavas, which we have sampled with ROV Hyper-Dolphin (HPD). Samples from the summit (HPD480) and eastern flank (HPD488) include 17 magnesian basalts (51^52 wt % SiO2) with 7·5^9·5 wt % MgO and Mg-number of 61^67, indicating little fractionation. Olivine phenocrysts are as magnesian as Fo93 and contain 0·4 wt % NiO; the Cr/(CrþAl) values of spinels are mostly 0·5^0·8, indicating equilibrium with depleted mantle.There are three petrographic groups, based on phenocryst populations: (1) cpx^olivine basalt (COB); (2) plagioclase^olivine basalt (POB); (3) porphyritic basalt. Zr/Y and Nb/Yb are higher in POB (3·1^3·2 and 1·2^1·5, respectively) than in COB (Zr/Y1⁄4 2·8^3·0 and Nb/ Yb1⁄4 0·7^0·9), suggesting that POB formed from lower degrees of mantle melting, or that the COB mantle source was more depleted. On the other hand, COB have Ba/Nb (70^80) and Th/Nb (0·4^ 0·5) that are higher than for POB (Ba/Nb1⁄4 30^35 and Th/ Nb1⁄4 0·1^0·2), and also have steeper light rare earth element (LREE)-enriched patterns. Moreover, COB have enriched Sr/Sr and Nd/Nd, and higher Pb isotope values, suggesting that COB has a greater subduction component than POB. Hf/Hf between COB and POB are similar and Hf behavior in COB and POB is similar to that of Zr,Yand HREE, suggesting that Hf is not included in the subduction component, which produced the differences between COB and POB.The calculated primary basaltic magmas of NW Rota-1 volcano (primary COB and POB magmas) indicate segregation pressures of 2^1·5 GPa (equivalent to 65^50 km depth).These magmas formed by 24^18% melting of mantle peridotite having Mg-number 89·5. Diapiric ascent of hydrous peridotite mixed heterogeneously with sediment melts may be responsible for the NW Rota-1 basalts. These two basalt magma types are similar to those found at Sumisu andTorishima volcanoes in the Izu^Bonin arc, with COB representing wetter and POB representing drier magmas, where subduction zone-derived melt components are coupled with the water contents.

The redox state of arc mantle using Zn/Fe systematics

Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle. As a consequence, the sub-arc mantle wedge is widely believed to be oxidized. The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source. Here we show that Zn/Fe(T) (where Fe(T) = Fe(2+) + Fe(3+)) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe(2+) and Zn behave similarly, but because Fe(3+) is more incompatible than Fe(2+), melts generated in oxidized environments have low Zn/Fe(T). Primitive arc magmas have identical Zn/Fe(T) to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/Fe(T) during early differentiation involving olivine requires that Fe(3+)/Fe(T) remains low in the magma. Only after progressive fractionation does Fe(3+)/Fe(T) increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood.

クロミタイト―不思議なマントル構成岩―

Various characteristics of podiform chromitites, an enigmatic mantle rock member, are reviewed in this article. Chromitites are composed of chromian spinel, with the general formula (Mg, Fe2+)(Cr, Al, Fe3+)2O4, and silicates (mainly olivine). The Fe3+ content is generally very low, being less than 0.1 to all trivalent cations, in mantle chromian spinels. The Mg/(Mg + Fe2+) ratio (= Mg#) changes inversely with the Cr/(Cr + Al) ratio (= Cr#), which increases with an increase of degree of partial melting of mantle peridotites. The Cr# of chromian spinel is generally higher than 0.4 (generally 0.6 to 0.8) in podiform chromitites, varying widely from 0.1 to 0.9 in the mantle peridotite. The podiform chromitite forms pod-like bodies (dimensions of up to 1.5 km × 150 m for an individual pod) with a dunite envelope, totally set within mantle harzburgite. In well-preserved ophiolites, they occur in the uppermost mantle, especially in and beneath the Moho transition zone, which is dominated by dunite. The Cr# of chromian spinel is relatively low (0.4 to 0.6) around the Moho transition zone, and high (>0.6) at deeper levels in the mantle section. Chromitites are denser and less anisotropic in Vp than peridotites, and the Vp is 8.5 to 9 km/sec depending on the proportion of chromian spinel, and higher in the former than in the latter. The podiform chromitite has been interpreted to be one of melt/rock interaction products within the uppermost mantle harzburgite; hybridization of relatively Si-rich melt formed by the breakdown of orthopyroxenes of the wall harzburgite and subsequently supplied primitive melt cause oversaturation in chromian spinel, giving rise to formation of chromitite with a dunite envelope. The fractionated melt leaving high-Cr# podiform chromitite is possibly of arc-magma affinity. Chromitites with low-Cr# (0.4 to 0.6) chromian spinel can be in equilibrium with MORB. Recently found ultra-high pressure minerals, such as diamond, moissanite, Fe-silicides and Ni-Fe-Cr-C alloys, within chromian spinel of podiform chromitites make the genetical history of chromitites highly enigmatic. A new story, which incorporates the genesis and involvement of these highly reducing, ultra-high pressure minerals, is required.

Os isotope geochemistry of mantle peridotites from Sal Island, Cape Verde Archipelago

Sulfide globules with Fe‐Ni alloys and separate grains of metallic phases were found in the intergranular glass from mantle nodules on Sal Island [1, 2]. The occurrence of native metals and the high temperature of mineral equilibria in lherzolites from Sal Island [2] allow us to suggest the contribution of the lower mantle material to the ascending mantle plume that controls magmatism of Cape Verde Archipelago. This suggestion is substantiated by experimental data, which indicate that disproportionation of the ferrous component with formation of the metallic phase and excess of Fe 3+ incorporated into Al-bearing silicate perovskite must occur under the lower mantle conditions [3‐5]. The equilibrium with participation of the metallic phase is clearly expressed in the behavior of strongly siderophile elements, in particular PGE, and exerts a substantial influence on the Re‐Os isotopic system that serves as a sensitive indicator of the processes with participation of sulfides and native metals [6]. We determined PGE contents and measured the Os isotopic composition for a number of mantle nodules from Sal Island in order to provide possible evidence for participation of the lower mantle processes in the formation of plume material pertaining to Cape Verde Archipelago. Since both Os and Re have prominent siderophile and chalcophile properties, sulfide and metallic phases must be their main species in the mantle rocks. The partition of these elements between immiscible silicate and sulfide melts in the course of partial melting of mantle peridotites and crystallization of sulfide‐metallic melts results in an appreciable shift of Re/Os ratios. In these processes, Os behaves as a compatible element; Re, as a moderately incompatible component. In other isotopic systems (Sm‐Nd, Rb‐Sr, Lu‐Hf, U‐Th‐Pb) used as petrogenetic indicators, both parental and daughter radioactive elements are incompatible. As a result, the difference in the Os isotopic composition in various geological objects becomes much greater with time than, for example, for Nd or Sr. The Nd isotopic composition deviates from the meteoritic standard by a fraction of a percent, whereas the Os deviation is measured by several percent. In particular, the Re/Os ratio in basalts turns out to be several times higher than in mantle restite. Thus, Os in the Earth’s crust acquires a substantially radiogenic character with time, while the 187 Os/ 188 Os ratio in the residual mantle becomes subchondritic with time. These features make the Re‐Os isotopic system much more sensitive to participation of the crustal and mantle sources in geological processes in comparison with other systems of radiogenic isotopes.

The geology and petrochemistry of the Mashikiri Formation along the Olifants River Section, Kruger National Park, South Africa.

The volcaniclastic sediments and nephelinite lavas of the Mashikiri Formation, Lebombo Group, have been well documented in the Pafuri region and in the Sabi region, Zimbabwe. The well-exposed section along the Olifants River in the Kruger National Park provides cogent additional information. The Shishwayini Beds at the base of the succession represents volcaniclastic deposits on the Tshipise Sandstone Member of the Clarens Formation. The rest of the Mashikiri Formation in the Olifants River section is nephelinite in which nepheline is accompanied by olivine, clinopyroxene, devitrified glass, katophorite and opaque oxides. Chemical modelling shows that the nephelinites were derived by partial melting of mantle peridotite enriched in incompatible trace elements and by subsequent fractionation of pyroxene and olivine. Crustal contamination did not play a significant role in derived magma composition. The Olifants River section defines a third source of undersaturated parent magma distinct from the two sources previously identified from outcrops to the north by other authors.

Nb/Ta variations of mafic volcanics on the Archean-Proterozoic boundary: Implications for the Nb/Ta imbalance

The HFSE and REE of the Precambrian mafic volcanics from the North China craton demonstrate obvious A(Archean)-P(Proterozoic) boundary. The Neoarchean mafic volcanics show weak correlation between HFSE and TiO2. Their superchondritic Nb/Ta ratio (18.8±1.2) could be attributed to partial melting of mantle peridotite in the presence of garnet. Compared with Neoarchean mafic volcanics, the Paleoproterozoic ones have higher HFSE contents and lower Nb/Ta ratio (15.6±2.9). The significantly elevated HFSE and REE contents of Paleoproterozoic mafic volcanics imply metasomatic enrichment of mantle source, in which Ti-rich silicates could be present as suggested by significant positive correlations between TiO2 and HFSE. The global database of Precambrian mafic volcanics shows a similar A-P boundary. 23 Archean mafic volcanic suites yield an average Nb/Ta ratio of 17.8±1.9 higher than or close to the PM value; Proterozoic mafic volcanics from 28 suites yield an average Nb/Ta ratio of 14.7±4.1 close to the bulk continental crust (BCC) value (11–17.5). Thus, we suggest that the Nb/Ta deficit could be mainly formed in post-Archean time. Archean mafic volcanics could be one of the geochemical reservoirs complementing the low Nb/Ta of the post-Archean continental crust and DM.

Origin of Recent alkaline lavas by lithospheric thinning beneath the northern Canadian Cordillera

Recent alkaline lavas that have erupted across the disparate terranes of the northern Canadian Cordillera provide natural probes with which to interrogate the underlying lithosphere. The lavas range between two compositional end members, olivine nephelinite (NEPH) and hypersthene-normative olivine (Hy-NORM) basalt. The chemical signature of amphibole in the incompatible element enriched NEPH end member indicates that it is derived in the lithospheric mantle. The Hy-NORM end member is characterized by lower incompatible trace element contents but is still relatively enriched relative to primitive mantle. Although the Hy-NORM end member is always more radiogenic in Pb and Sr isotopes and less radiogenic in Nd isotopes than the NEPH end member, its isotopic signature varies with tectonic belt. In particular, Hy-NORM basalts in the Omineca Belt are strikingly more radiogenic in Sr and Pb isotopes and less radiogenic in Nd isotopes than otherwise equivalent Hy-NORM basalts in the adjacent Intermontane Belt, indicating the existence of a major lithospheric boundary between the two belts. Cordilleran and other continental Hy-NORM basalts have distinctly low Ca and high Na contents compared with their equivalents in oceanic hot spots or at mid-ocean ridges. A comparison with experimental melts of mantle peridotite indicates that these characteristics reflect smaller degrees of partial melting (<10%) in the stability field of garnet in the lower lithospheric mantle beneath the northern Cordillera. Contrary to the conclusion commonly drawn from experimental results, the Cordilleran NEPH lavas may be derived from similar or shallower depths than coeval Hy-NORM basalts.

The role of an H2O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California

This paper presents analyses of the trace element abundances and isotopic compositions in primitive lavas from the Mt. Shasta region, N California. These data are combined with estimates of pre-eruptive H2O contents and constraints from experimental petrology to develop a model of subduction zone magmatism. These lavas share geochemical characteristics of high-Mg andesites from the Setouchi volcanic belt in SW Japan and Adak-type high-Mg andesites of the western Aleutian arc. Estimates of the pre-eruptive water contents of the Shasta region lavas range from 8 wt % H2O. The pre-eruptive H2O content and an inferred melt of a harzburgitic residue are used to carry out a mass balance for the relative contributions from a mantle-derived melt and slab-derived fluid-rich component. We assume that elements are contributed either from melting of mantle peridotite or from a subduction-related fluid-rich component. Estimated fluid-rich component compositions are characterized by strong light rare earth element (LREE) enrichments ([La/Gd]N=3 to 7) and variable heavy rare earth element (HREE) depletions ([Dy/Yb]N=1 to 3). Sr and Ba abundances vary by approximately a factor of 2.5 in the fluid compositions calculated for the Mt. Shasta region lavas and large ion lithophile element (LILE) abundances are similar to those calculated by Stolper and Newman (1994) and Eiler et al. (2000). The major elements in the fluid-rich component are H2O (~55–68 wt%), Na2O (~25–33 wt%) and K2O (~5–13 wt%). This composition may be that of a supercritical fluid or a low-degree melt of the slab that has reacted with the overlying mantle wedge. Although the slab beneath Mt. Shasta is inferred to be hot (~600 – 650 °C), the calculated fluid-rich components do not resemble a pure slab melt. The calculated isotopic composition of the fluid-rich component is bimodal. One component has 87Sr/86Sr=0.7028 and eNd=+8, and is most similar to a MORB source. The second component has more radiogenic 87Sr/86Sr=0.7038 and eNd=+1 and is most similar to a sediment. These fluid-rich components probably represent a mixture of fluids and melts from the slab (serpentinized mantle, altered basalt, and sediment).