Primary Mantle Melts Research Articles

Composition of the Marginal Rocks and Sills of the Rustenburg Layered Suite, Bushveld Complex, South Africa: Implications for the Formation of the Platinum-Group Element Deposits

The Bushveld Complex contains large ore deposits of platinum-group elements (PGE), V, and Cr. Understanding how these deposits formed is in part dependent on estimates of the compositions of the magmas that filled the Bushveld chamber. Over the past 20 years, estimates for the major oxides and some trace elements in the magmas have been made using the marginal rocks of the intrusion. However, data for most of the trace elements have not been available. This paper presents the results for a full range of trace elements, including the platinum-group elements. The marginal rocks of the Lower and lower Critical zones (B-1 magmas) are tholeiitic Mg-rich basaltic andesites with Mg# 71. It had been suggested that they are boninites but their mantle-normalized incompatible lithophile trace element patterns (spidergrams) resemble those of the upper continental crust and the concentrations of the elements are much higher than those of boninites. The patterns resemble siliceous high magnesium basalts. An unusual feature is that the Pt/Pd ratios are >1.5. The Pt contents of the B-1 rocks (15–25 ppb) are slightly higher than those observed in most primary mantle melts, suggesting that the high Pt/Pd ratio is due to Pt enrichment rather than Pd depletion. The crystallization order and composition of the minerals formed in equilibrium with the B-1 magma matches that of the Lower and lower Critical zones and thus this magma appears to be representative of the parental magma of these zones. The marginal rocks to the upper Critical zone (B-2) are tholeiitic basalts in terms of major element composition, with Mg# 55. The spidergrams show some similarities with E-MORB; however, the B-2 rocks have strong positive Ba and Pb anomalies and negative P, Ti, Hf, and Zr anomalies, and thus they more closely resemble lower continental crust. The B-2 rocks have lower and more variable Pt + Pd contents than the B-1 magma, suggesting that some of the samples have experienced sulfide saturation, but in common with the B-1 magmas, the Pt/Pd ratios are high, in excess of 1.5. The crystallization order of the Upper Critical zone cannot be modelled by the B-2 magma alone. However, mixtures of B-2 magma and B-1 magma satisfy the crystallization order and mineral composition of the upper Critical zone. The marginal rocks of the Main zone (B-3) are also tholeiitic basalts in terms of major element composition but have a higher Mg# (62) than the B-2 rocks. Trace element patterns in part resemble those of B-2 magmas but are depleted in most incompatible elements with large positive Ba, Pb, and Eu anomalies and negative Nb, Ta, Hf, and Zr anomalies, suggesting the rocks contain a plagioclase component. The PGE contents of the B-3 rocks are lower than those of the B-1 magma and less variable than those of the B-2 magma, but in common with both the other magmas, they have high Pt/Pd. The crystallization order and composition of the minerals in equilibrium with the B-3 magma matches that of the Main zone. Two processes have been suggested to explain the compositions of the Bushveld magmas: mixing of primitive mantle melts with partial melts of continental crust and mixing of primitive mantle melts with melts derived from the subcontinental lithospheric mantle (SCLM). The trace element concentrations of the magmas can be modelled by crustal contamination. This interpretation is supported by oxygen isotopes, initial 87Sr/86Sr ratios and ɛNd of the cumulate rocks. However, the high Pt/Pd ratios of all of the magmas and the overall higher than normal Pt concentrations of the B-1 magma are difficult to explain by mixing of primary mantle melt with crustal components. The SCLM has high Pt/Pd ratios and mixing of primary mantle magma with SCLM-derived magma could account for the high Pt concentrations and high Pt/Pd ratios. This interpretation is supported by recent work on Os isotopes of the Kaapvaal SCLM. It should be kept in mind that the two processes are not necessarily exclusive. A magma with a SCLM component could have been emplaced into the crust and subsequently have been contaminated by partial melts of the crust.

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.

Newania carbonatites, Western India: example of mantle derived magnesium carbonatites

The key mineralogical features of the Newania carbonatites, that illustrate their derivation from primary mantle melts (Gruau et al. Terra Nova, Abstract Suppl 1:336, 1995; Viladkar Petrology 6(3):272–283, 1998; Basu and Murty Abstracts of Goldschmidt Conference A40, 2006), are the presence of magnesite, graphite and Cr-rich magnetite. Magnesite is an early crystallizing phase. Cr-rich magnetite and graphite coexist with carbonatite minerals and precipitated from carbonate magma. Graphite, as well as gaseous CO2 and carbonate minerals such as dolomite and magnesite, can be stable in peridotite mantle. Coexistence of these minerals is controlled by fO 2 and PT-conditions. Mineral geothermometers for the Newania carbonatite give temperatures from 463 to 950°C. The parental source for Newania carbonatites was characterized by a relatively high log (fHF/fH2O) level which increased during the crystallization history of Newania. The estimated oxygen fugacity (for ilmenite–magnetite pairs) varies from −1.5 to +3.5 (log-bar unit deviation from FMQ buffer), which is supported by the presence of Fe-columbite, and the composition of phlogopite, amphibole and pyroxene that have an elevated concentration of Fe3+. However, the oxygen fugacity range represented by co-existing early-crystallized graphite and magnesite is below that of the FMQ buffer and lies on the CCO buffer.

Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example from the Isle of Rum, NW Scotland

AbstractSr and Nd isotope ratios, together with lithophile trace elements, have been measured in a representative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the North Atlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amounts of melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component. Concentrations of the trace element Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rb loss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneity with respect to87Sr/86Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites, early gabbros) are mixtures of crustal melts and and primary mantle melts. Forward Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose trace element content varies from slightly depleted to slightly enriched. Crustal anatexis is a common process in the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Margins systems. The ‘early felsic–later mafic’ volcanic rock associations from Rum are compared to similar associations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenland and on the Vøring Plateau (Norwegian Sea). These three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place.

Experimental petrology of the basaltic shergottite Yamato-980459: Implications for the thermal structure of the Martian mantle

The Martian meteorite Yamato (Y-) 980459 is an olivine-phyric shergottite. It has a very primitive character and may be a primary melt of the Martian mantle. We have conducted crystallization experiments on a synthetic Y-980459 composition at Martian upper mantle conditions in order to test the primary mantle melt hypothesis. Results of these experiments indicate that the cores of the olivine megacrysts in Y-980459 are in equilibrium with a melt of bulk rock composition, suggesting that these megacrysts are in fact phenocrysts that grew from a magma of the bulk rock composition. Multiple saturation of the melt with olivine and a low-calcium pyroxene occurs at approximately 12 ± 0.5 kbar and 1540 ±10 °C, suggesting that the meteorite represents a primary melt that separated from its mantle source at a depth of ~100 km. Several lines of evidence suggest that the Y-980459 source underwent extensive melting prior to and/or during the magmatic event that produced the Y-980459 parent magma. When factored into convective models of the Martian interior, the high temperature indicated for the upper Martian mantle and possibly high melt fraction for the Y-980459 magmatic event suggests a significantly higher temperature at the core-mantle boundary than previously estimated.

RELATION BETWEEN MELT PERCOLATION AND LIGHT ELEMENT CONTENTS (LI, BE AND B) IN PERIDOTITES FROM THE PINDOS OPHIOLITE (GREECE)

The Pindos ophiolite represents non-metamorphic oceanic lithosphere obducted during the Jurassic. The mantle part was hydrothermally altered to different degrees during oceanization, but fresh peridotites are still preserved. The Pindos mantle mainly consists of highly depleted spinel harzburgites, but some plagioclase lherzolites are present in the northwestern part of the body. Textures in thin sections clearly demonstrate a direct relation between plagioclase formation and melt infiltration into the oceanic mantle. Two main textures were observed: a “porous flow” texture where plagioclase is homogeneously distributed in the rock and a channelized texture where plagioclase crystallized in microveins. Melt infiltration also led to crystallization of orthopyroxene and clinopyroxene associated with plagioclase. Similar textures were observed in the Othris ophiolite (Greece), southeast of Pindos, by Dijkstra et al. (2001) and in other Tethyan ophiolites (Piccardo et al., 2004). A study on light element contents of minerals from Pindos harzburgite and serpentinite was conducted in order to evaluate the role of the oceanic mantle as repository for these elements. Results obtained on spinel harzburgites and plagioclase lherzolites by SIMS (Secondary Ion Mass Spectrometry) show no significant difference between Li, Be and B contents of primary mantle minerals and melt impregnation phases. Li contents of olivine (0.68-1.1 ppm) and orthopyroxene (0.2-1.5 ppm) seem to be consistent with values for “normal” mantle minerals (Eggins et al., 1998; Woodland et al., 2004). The Li contents of clinopyroxene (0.2-3.7 ppm) are within the upper range of values published for unmetasomatised mantle clinopyroxene (only xenolith data available; Seitz & Woodland, 2000; Woodland et al., 2004). The Li characteristics of spinel and plagioclase peridotites differ from published data for non-metasomatised mantle by higher values for clinopyroxene than for olivine. This inverse Li partitioning could be explained by a reaction with a mafic silicate melt (Seitz and Woodland, 2000). This would be in agreement with the textures observed in the plagioclase lherzolites. Be abundances are below detection limit and B is low in all minerals from below the detection limit to 0.4 ppm. Li, Be and B contents in plagioclase are extremely low, often below the detection limit. In conclusion, the percolating melt did not add considerable amounts of B, Li and Be, probably because it was depleted in these elements. Melt impregnation is also supported by major element chemistry. Clinopyroxenes (first generation cpx and cpx recrystallized from the melt) of plagioclase lherzolites are enriched in Na, Ti and their XMg is higher compared to those of the spinel harzburgites. The same observation was made on the Othris peridotites by Dijkstra et al. (2001 and 2003). In plagioclase lherzolites, serpentinization textures confirm that hydrothermalism occured after melt infiltration. In highly hydrothermalized peridotites, light element contents of primary mantle minerals are low and similar to those of fresh samples. B is mainly concentrated in serpentine (0.4- 24 ppm) and Li in chlorite (2-3.5 ppm). Values for Be were always below the detection limit. Low contents of Li and B in primary minerals such as olivine, orthopyroxene and clinopyroxene after hydrothermalism can be explained by HT serpentinization.

The brevity of carbonatite sources in the mantle: evidence from Hf isotopes

Hf, Zr and Ti in carbonatites primarily reside in their non-carbonate fraction while the carbonate fraction dominates the Nd and Sr elemental budget of the whole rock. A detailed investigation of the Hf, Nd and Sr isotopic compositions shows frequent isotopic disequilibrium between the carbonate and non-carbonate fractions. We suggest that the trace element and isotopic composition of the carbonate fraction better represents that of the carbonatite magma, which in turn better reflects the composition of the carbonatitic source. Experimental partitioning data between carbonatite melt and peridotitic mineralogy suggest that the Lu/Hf ratio of the carbonatite source will be equal to or greater than the Lu/Hf ratio of the carbonatite. This, combined with the Hf isotope systematics of carbonatites, suggests that, if carbonatites are primary mantle melts, then their sources must be short-lived features in the mantle (maximum age of 10–30 Ma), otherwise they would develop extremely radiogenic Hf compositions. Alternatively, if carbonatites are products of extreme crystal fractionation or liquid immiscibility then the lack of radiogenic initial Hf isotope compositions also suggests that their sources do not have long-lived Hf depletions. We present a model in which the carbonatite source is created in the sublithospheric mantle by the crystallization of earlier carbonatitic melts from a mantle plume. This new source melts shortly after its formation by the excess heat provided by the approaching hotter center of the plume and/or the subsequent ascending silicate melts. This model explains the HIMU-EMI isotope characteristics of the East African carbonatites, their high LREE/HREE ratios as well as the rarity of carbonatites in the oceanic lithosphere.

Mantle genesis and crustal evolution of primitive calc-alkaline basaltic magmas from the Lesser Antilles arc

Abstract Most eruptive rocks in the Lesser Antilles arc are compositionally evolved. However, lavas with primitive characteristics do occur including, in the central part of the arc, a suite of rocks from Soufriere, St Vincent, and the Ilet à Ramiers basalt from Martinique. High-pressure experiments performed on a Soufriere basalt point to a spinel lherzolite source. Glass inclusion data and phase equilibria analysis suggest extraction of the Soufriere melt under relatively dry conditions ( c. 2 wt% H 2 O in melt). Using estimates of the H 2 O content of mantle sources fluxed by an hydrous slab-derived component, H 2 O concentrations as high as 5 wt% are considered possible for primary mantle melts in the Lesser Antilles arc. Experiments at low pressures (4 kbar) simulate the evolution of primitive melts within the arc crust. For elevated melt H 2 O concentrations (6–8.5 wt%), derivative liquids ranging from low-MgO basalt to basaltic andesite are generated at 1050–1100°C. Their crystallization at 950–1000°C yield andesitic liquids similar to those erupting at active volcanic centres such as Mt Pelée, Martinique, and Soufriere Hills, Montserrat. Therefore, experimental data support the derivation of Lesser Antilles arc eruptives by different degrees of fractionation from primary mantle melts.

Coupled evolution of back-arc and island arc-like mafic crust in the late-Neoproterozoic Agardagh Tes-Chem ophiolite, Central Asia: evidence from trace element and Sr-Nd-Pb isotope data

We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5°N, 95°E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg#≥0.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine+spinel assemblage. The island-arc rocks have high abundances of incompatible trace elements (light rare-earth element abundances up to 100 times chondritic, chondrite-normalised (La/Yb)n=14.6–5.1) and negative Nb anomalies (Nb/La=0.37–0.62), but low Zr/Nb ratios (7–14). Initial eNd values are around +5.5, initial Pb isotopic compositions are 206Pb/204Pb=17.39–18.45, 207Pb/204Pb=15.49–15.61, 208Pb/204Pb=37.06–38.05. Enrichment of large-ion lithophile elements within this group is significant (Ba/La=11–130). Another group of samples consists of back-arc basin-related volcanic rocks. They are most likely derived from the same depleted mantle source as the island-arc rocks, but underwent higher degrees of melting (8–15%) and are not influenced by slab components. They have lower abundances of incompatible trace elements, flat rare-earth element patterns [(La/Yb)n=0.6–2.4] and higher eNd values (+7.8 to +8.5). Negative Nb anomalies are absent (Nb/La=0.81–1.30), but Zr/Nb is high (21–48). At least three components are necessary to explain the geochemical evolution of the volcanic rocks: (1) an enriched (ocean island-like) component characterised by a high Nb concentration (up to 30 ppm), an absent negative Nb anomaly, a low Zr/Nb ratio (~6.5), a low eNd value (around 0), and radiogenic 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb; (2) a back-arc basin component similar to N-MORB with a flat rare-earth element pattern and a high eNd value (around +8.5); and (3) an island-arc component from a mantle source which was modified by the downgoing slab. Crystal fractionation superimposed on mixing and source contamination by subducted sediments is suitable to explain the observed geochemical data. The most likely geodynamic environment to produce these characteristics is a young, intra-oceanic island-arc system and an associated back-arc basin.

Correction for volatile fractionation in ascending magmas: noble gas abundances in primary mantle melts

Accurate relative noble gas abundances of mantle-derived melts are required in order to further understand the distribution of noble gases in the mantle and fractionation of noble gases during the melting process. Noble gas relative abundances in the majority of oceanic basalts are highly fractionated, at least in part due to late stage, solubility controlled fractionation. Noble gas concentrations in the volatile phase (≡ noble gas:CO2 ratio) will vary systematically during solubility controlled degassing of a magma. This contribution models the noble gas concentrations in the volatile phase during degassing at different pressures and vesicularities in order to develop a method for correcting fractionation resulting from magmatic degassing, and thereby estimate the “initial” (pre-degassing) noble gas compositions.Correcting for fractionation during magmatic degassing requires: a) a method for determining the volatile fractionation trajectory during degassing; and b) one well constrained mantle volatile composition with which to “fix” the extrapolation.The trajectory of volatile fractionation can be estimated by sequential crushing of basaltic glasses. Vesicles grow during ascent, therefore large vesicles trap early (less fractionated) volatiles while small vesicles trap late (fractionated) volatiles. Sequential crushing of basaltic glasses releases volatiles from progressively smaller vesicles, thereby allowing the fractionation trajectory resulting from degassing to be determined on individual samples.The production rate of both 21Ne and 4He in the mantle is a function of U concentration only, resulting in a constant 21Ne/4He production ratio in the mantle which can be used to “fix” the degassing fractionation trajectory determined by sequential crushing. This correction then allows fractionation of 4He from 40Ar prior to degassing to be assessed. This method is illustrated using multiple crushes of a single basaltic glass from the mid-Atlantic Ridge that shows that 4He appears to have been fractionated from 40Ar before degassing.

The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift

The southernmost exposed rocks of the North American Midcontinent rift system (1100 Ma) consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the St. Croix River in Minnesota and Wisconsin. The flows are mostly high-Fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene. Abundant secondary chlorite, epidote, and actinolite indicate the group was metamorphosed to greenschist facies (~350 °C). Low sodium (M4 site) and tetrahedral aluminum (AlIV) contents of actinolite indicate low-pressure metamorphism (0.25 GPa) and imply a geothermal gradient of 45 – 50 °C/km. Low magnesium (Mg# = 0.37–0.58) and Ni contents (36–185 ppm) indicate the basalts have undergone significant fractionation and are not primary mantle melts. Incompatible element abundances are inversely correlated with Mg#, and most samples plot within either high or low trace element groups (e.g., Ti, P, Zr). The basalts are enriched in the light rare earth elements and Th, and are variably depleted in Ta and Nb relative to La and Th. Initial 143Nd/144Nd compositions of the group range from 0.51099 to 0.51122 (initial εNd = −4.5 to +0.1). Most flows have isotopic compositions within a relatively limited range (initial εNd = −2.5 to −1.6), but exhibit variable trace element abundances. Flows with the highest and lowest initial 143Nd/144Nd ratios have isotopic compositions that are inversely correlated with trace element abundances and ratios (e.g., La/Yb, Th/La, Th/Ta). The combined geochemical data suggest that the Chengwatana basalts originated from plume-derived melts and underwent variable fractional crystallization and crustal contamination. These melts may have interacted with lithospheric mantle enriched during Penokean subduction.

Lithospheric Contributions to Arc Magmatism: Isotope Variations Along Strike in Volcanoes of Honshu, Japan

Major chemical exchange between the crust and mantle occurs in subduction zone environments, profoundly affecting the chemical evolution of Earth. The relative contributions of the subducting slab, mantle wedge, and arc lithosphere to the generation of island arc magmas, and ultimately new continental crust, are controversial. Isotopic data for lavas from a transect of volcanoes in a single arc segment of northern Honshu, Japan, have distinct variations coincident with changes in crustal lithology. These data imply that the relatively thin crustal lithosphere is an active geochemical filter for all traversing magmas and is responsible for significant modification of primary mantle melts.

Geochemistry of late Caledonian minettes from Northern Britain: implications for the Caledonian sub-continental lithospheric mantle

AbstractThe geochemistry of late Caledonian minettes from across the orogenic belt is compared in order to constrain the composition of the Caledonian sub-continental lithospheric mantle (SCLM). All the minettes are similar petrographically and chemically and several samples have characteristics typical of near primary mantle melts. Samples from the Northern Highlands and the Caledonian foreland show enrichment in many trace elements (notably LILE and LREE) relative to those from the Grampians, the Southern Uplands and northern England, coupled with distinct Nd and Sr isotope characteristics. Processes such as fractional crystallization, crustal assimilation, and partial melting played a negligible role in creating the differences between the two groups which reflect long-term, time-integrated differences in the compositions of their SCLM sources. The Great Glen Fault appears to represent the boundary between these two lithospheric mantle domains. Other currently exposed Caledonian tectonic dislocations cannot be correlated directly with compositional changes within the SCLM. The chemical provinciality displayed by the minettes shows some resemblance to that within other late Caledonian igneous suites, including the newer granites, suggesting that the minettes may represent the lithospheric mantle contributions to these rocks.

Petrogenesis of the Flood-Basalt Sequence at Noril'sk, North Central Siberia

The 3500-m-thick sequence of volcanic rocks at Noril'sk, formed during a brief interval (∼1 m.y.) at the Permian/Triassic time boundary (∼251 Ma), represents the earliest part of the ∼6500-m-thick sequence presently ascribed to the Siberian flood-basalt province. It is composed of picritic and basaltic lavas of both low-Ti and high-Ti parentage. Extensive geological, geochemical, and isotopic study of the lava sequence and related intrusions allows detailed reconstruction of its petrogenesis. Various crustal-related processes-fractionation, crustal contamination, sulfide separation, and magma mixing-participated in the formation of the lavas. The geochemical and isotopic characteristics indicative of these processes, as well as mantle-related signatures of lava compositions, are discussed. Based on these characteristics, detailed interpretations of lava genesis and evolution throughout the Noril'sk sequence are presented. Eight varieties of lavas are recognized to be primitive, similar in composition to primary mantle melts; they varied from low-Mg basalts to olivine tholeiites or picrites, with normal tholeiites predominating. The primitive lavas are subdivided into four groups (magma types) on the basis of trace-element ratios (principally, Gd/Yb, Th/U, La/Yb, Ta/La, Ti/Sc, and V/Yb) and isotopic data. Three of the groups include both basaltic and picritic primitive lavas (with low-Mg basalts present in one of them), whereas the fourth group is represented exclusively by tholeiites. Distinctions among the groups cannot be related to degree of melting, and isotopic data indicate that none of the magma types could have formed by mixing or contamination of other types. Apparently, only differences in source composition and/or depth of melting can explain the magmatic diversity. This multitude of primitive magma types may be explained by melting in different layers of the upper mantle, which is complexly layered beneath Siberia to depths of 270 km. Moreover, no clear boundary between lithosphere and asthenosphere is evident in the deep seismic profile. A large-scale event is necessary to account for melting in different parts of the upper mantle and formation of the great volume of the Siberian flood basalts in ∼1 m.y. Extension, caused by ascent of a mantle plume, would provide a reasonable explanation, but no plume-related uplift is documented in north-central Siberia prior to, or during, eruption of the volcanic sequence.

Mantle plume origin for the Bushveld and Ventersdorp magmatic provinces

Geochemical and isotopic data demonstrate that the parental magma of the lower zone of the Bushveld complex has been contaminated by the crust. Magmas similar in composition are identified in the Ventersdorp Supergroup, but these magmas are part of a series which includes members that are free of crustal contamination. Primary mantle melts can therefore be identified and their conditions of origin have been determined. The depth of origin is between 18 and 40 km. The presence of mantle, at such shallow depths in an intracratonic setting, is only explicable if a mantle diapir temporarily intruded the crust up to the base of the strong middle and upper crustal layer. Crustal melting and contamination occurred at the top of the diapir. In the Ventersdorp, crustal melts failed to impede the passage of mantle melts to the surface. In the Bushveld, mantle melts were impeded and only intruded outside the central ellipse of the crustal melts.

High-Al gabbros in the Laramie Anorthosite Complex, Wyoming: implications for the composition of melts parental to Proterozoic anorthosite

High-Al gabbro represents one of the latest phases of magmatism in the 1.43 Ga Laramie anorthosite complex (LAC) in southeastern Wyoming. This lithology, which is mineralogically and geochemically the most primitive in the LAC, forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks. High-Al gabbro is characterized by high Al2O3 (15–19 wt%), REE patterns with positive europium anomalies (Eu/Eu*=1.2–3.8), and the lowest initial 87Sr/86Sr (as low as 0.7033) and highest initial ɛNd (up to +2) in the LAC. Their Sr and Nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent. Intermediate plagioclase (An50–60) and mafic silicate (Fo54–63) compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement. High-Al gabbros of the LAC are similar compositionally to gabbros from several other Proterozoic anorthosite complexes, including rocks from the Harp Lake complex and the Hettasch intrusion in Labrador and the Adirondack Mountains of New York. These gabbros are considered to be parental to their associated anorthositic rocks, a theory that is supported by recent experimental work. We interpret LAC high-Al gabbros to represent mantle-derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber. Continued evolution of this magma eventually resulted in the formation of plagioclase-rich diapirs which ascended to mid-crustal levels and formed the anorthositic rocks of the LAC. Because these gabbros intrude the anorthositic rocks, they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes.

Petrology and geochemistry of basaltic rocks from the Balleny Is, Antarctica

Rock samples obtained from Sabrina and Sturge Is in the Balleny Group, Antarctica during the 1978 Lewis Expedition are ocean island basanite, verging on hawaiite. A noteworthy geochemical feature is a consistently low Zr/Nb ratio (∼ 3.5) compared with a typical value of 5.8 (range 4.5–6.6) for other ocean island alkaline rocks of basanitic to alkaline basaltic composition. Sturge samples are partly cumulative, with conspicuous clinopyroxene and olivine phenocrysts. All rocks are derivative from primary mantle melts, and evidence for fractionation at moderate pressure (probably corresponding to mantle depths of 15–20 km) is provided by the presence of aluminous orthopyroxene, aluminous (high AlVI) clinopyroxene, kaersutitic amphibole and olivine with complex minor element zoning patterns. Oxide phases present are varied, from chrome spinel included in olivine and pleonaste included in plagioclase, to microphenocrysts of ilmenite and titanomagnetite. The Balleny volcanism appears to represent typical hot sp...

Pb isotopic systematics of alkaline volcanic rocks and carbonatites from the Kaiserstuhl, Upper Rhine rift valley, F.R.G.

Lead isotopic data of alkaline volcanic rocks and carbonatites from the Kaiserstuhl are presented in combination with Sr isotopic ratios. As a whole the initial lead ratios cover the range typical for other Tertiary to Recent alkaline rock provinces in Europe: 206 Pb 204 Pb = 18.9–19.7 , 207 Pb 204 Pb = 15.61–15.70 and 208 Pb 204 Pb = 38.7–39.7 . Especially in the 208 Pb 204 Pb vs. 206 Pb 204 Pb diagram the data reveal a linear array which is interpreted in terms of mixing of two mantle components. In detail the data comprise three distinct groups which are correlated with different magmatic rock types: unfractionated primary mantle melts (olivine nephelinites, olivine melilitites), fractionated alkaline rocks (tephrites, phonolites), and a group with carbonatites, bergalites and hauynophyres. These results are interpreted to reflect either derivation from different mantle sources, or the data trace a mixing line between two mantle reservoirs, one of them being more enriched. Contamination with crustal lead is unlikely: it may have affected the tephritic magma series, but not the carbonatitic rocks. The uppermost part of the updomed mantle beneath the Kaiserstuhl, which is known by petrographic evidence to be partially metasomatized, probably is the site of the enriched reservoir. A “plume”-like structure within the deeper mantle is suggested as the initial source of this mechanism.

Petrogenetic relationships between melilitite and lamproite

A detailed petrological study of the S. Venanzo olivine melilitite and Cupaello kalsilitite, located at the NE margin of the Roman Comagmatic Region of Italy has provided new data to evaluate their genetic relationships and related tectonic/magmatic conditions. Early crystallized olivine (Fo92) from S. Venanzo is compatible with crystallization from near primary mantle melts, while late-crystallized olivine from both rocks (Fo88–79) reflects primarily the high Ca of the host liquids, attained under the volcanic crystallization regime. Magnesiochromite inclusions in the early-crystallized olivine are consistent with near-primary melts close to lamproite in composition. Nepheline and kalsilite from both lavas contain high alkalis (+Ca), relative to Al, and thus correspond more closely with nepheline compositions from carbonatite-related assemblages, than with those from a wide compositional range of Alban Hills lavas. Coexisting melilite has high (Na+K)/Al, reflecting the Peralkalinity Index of the bulk rocks. Diopside and phlogopite from both lavas are characteristic of lamproites and groundmass kimberlites in their high Mg/(Mg+Fe2+) ratio (0.86–0.95; 0.80–0.90, respectively) and T-site (Si+Al) deficiencies. Gotzenite, Ca2Na[Zr, Ti]Si2O7(O, OH, F)2, identified in both lavas, is typical of nephelinite-ijolite assemblages. On the other hand, khibinskite, K4Zr2Si4O14, found in the Cupaello lava, may be regarded as a Si-poor variety of wadeite, a mineral characteristic of lamproites. Clinopyroxene and monticellite, coexisting as late-crystallized phases in both lavas, suggest a common P-T liquid path of thermal descent in the system CaMgSi2O7−CO2 in the presence of excess CO2, but with different intersections with the akermanite stability field. Substantial differences in SiO2 saturation combined with high Mg number and liquidus temperatures experimentally determined at atmospheric pressure in both lavas (1276° C and 1260° C, respectively) indicate that a parent-daughter relationship is unlikely under the volcanic P-T regime. In La Roche's “Rm-Ri-Rs” diagram, the S. Venanzo composition fall close to the Oldoinyo Lengai alkalic pyroxenite trend of Donaldson and Dawson, while the Cupaello compositions follow a lamproitic trend, consistent with that indicated by the quartz-normative glass of the Gaussberg lava. Mantle compositions corresponding to wehrlite-clinopyroxenite and enriched in H2O, CO2, F, and LILE, are favoured as potential sources for the lavas. Their origin is probably related to subcrustal fluid transfer promoted by the Tyrrhenian mantle doming.

Isotope studies on alkaline volcanics and carbonatites from the Kaiserstuhl, Federal Republic of Germany

Sr, Nd and Pb isotopic data on alkaline volcanic rocks and carbonatites from the Kaiserstuhl Miocene alkaline volcano in the southern part of the Rhinegraben rift valley are presented. Isotopically, three distinct groups of volcanic rocks can be distinguished which are correlated to different magmatic series: 1. (1) Unfractionated primary mantle melts (olivine nephelinites, olivine melilitites) and nepheline basanites: 87 Sr 86 Sr =0.7032-0.7040 ; ϵ Nd=4.18-5.03; 206 Pb 204 Pb =18.95-18.98 ). Geochemically, these rocks correspond to a Na-dominated magmatic series. 2. (2) Fractionated alkaline rocks (tephrites, phonolites: 87 Sr 86 Sr =0.7039-0.7051 ; ϵ Nd=2.07-3.96; 206 Pb 204 Pb =19.10−19.42 ), corresponding to a K-dominated magmatic series. 3. (3) A group with carbonatites, bergalites, hauynophyres ( 87 Sr 86 Sr =0.7036-0.7040 ; ϵ Nd=2.81-4.04; 206 Pb 204 Pb =19.24-19.66 ) These results are discussed in terms of derivation from different mantle sources and interaction of asthenospheric melts with subcontinental lithospheric mantle during their ascent. Contamination with crustal material is important in the tephritic magma series, also in some phonolites. The isotopic data point to a metasomatic overprinting of the upper mantle below the Kaiserstuhl. This metasomatism is thought to be directly connected with the updomed mantle structure in this region. For the carbonatitic rocks a genetic linkage to olivine nephelinites is most likely. This implies an asthenospheric origin of the carbonatitic melts.