Else-Ragnhild Neumann

Fluid inclusions in mantle xenoliths

Fluid inclusions in olivine and pyroxene in mantle-derived ultramafic xenoliths in volcanic rocks contain abundant CO2-rich fluid inclusions, as well as inclusions of silicate glass, solidified metal sulphide melt and carbonates. Such inclusions represent accidentally trapped samples of fluid- and melt phases present in the upper mantle, and are as such of unique importance for the understanding of mineral–fluid–melt interaction processes in the mantle. Minor volatile species in CO2-rich fluid inclusions include N2, CO, SO2, H2O and noble gases. In some xenoliths sampled from hydrated mantle-wedges above active subduction zones, water may actually be a dominant fluid species. The distribution of minor volatile species in inclusion fluids can provide information on the oxidation state of the upper mantle, on mantle degassing processes and on recycling of subducted material to the mantle. Melt inclusions in ultramafic xenoliths give information on silicate–sulphide–carbonatite immiscibility relationships within the upper mantle. Recent melt-inclusion studies have indicated that highly silicic melts can coexist with mantle peridotite mineral assemblages. Although trapping-pressures up to 1.4 GPa can be derived from fluid inclusion data, few CO2-rich fluid inclusions preserve a density representing their initial trapping in the upper mantle, because of leakage or stretching during transport to the surface. However, the distribution of fluid density in populations of modified inclusions may preserve information on volcanic plumbing systems not easily available from their host minerals. As fluid and melt inclusions are integral parts of the phase assemblages of their host xenoliths, and thus of the upper mantle itself, the authors of this review strongly recommend that their study is included in any research project relating to mantle xenoliths.

The Oslo Rift: a review

The tectonomagmatic history of the Oslo Rift may be subdivided into 5 main periods. Stage 1 (> 300 Ma): development of a shallow depression. Stage 2 (300-295 Ma): roughly simultaneous onset of widespread basalt volcanism (B1) and vertical movement along NNW-SSE- to N-S-trending faults. Stage 3 (295-275 Ma): main rifting period, accompanied by volcanism dominated by rhomb porphyry lavas (latites). Stage 4 (275-240 Ma): change of magmatic style from basaltic shield volcanism to central volcanoes of mixed compositions, many of which went through caldera collapses; intrusion of composite batholiths of intermediate to granitic compositions. Stage 5 (< 240 Ma): dike intrusions may have continued into Triassic time. Both the crust and total lithosphere under the rift are thinned relative to the Precambrian basement on both sides. A 12-km-thick high-density and high-velocity body occupies the lower crust along the rift axis and extends under the eastern and western rift-shoulder. Petrological and geochemical data imply that this massive layer represents dense cumulates and gabbroic rocks formed from mantle-derived magmas in deep crustal magma-chambers. Two mantle sources, one nearly undepleted (age corrected ϵNd of about + 1 and ϵSr of −10 to −15), the other mildly depleted (ϵNd of about + 4, ϵSr of −10), contributed melts to the Oslo Rift magmatism. The two sources are interpreted as different parts of a heterogeneous upper mantle which, before the rifting event, belonged to the subcontinental lithosphere. On their way to the surface, the mantle-derived magmas were retained in deep crustal magma chambers Where they underwent extensive fractional crystallization, some also suffered moderate contamination. Anatectic melts, and melts formed by mixing between mantle-derived and anatectic melts, gave rise to syenitic and granitic intrusions.

Partitioning of REE, Y, Sr, Zr and Ti between clinopyroxene and silicate melts in the mantle under La Palma (Canary Islands): implications for the nature of the metasomatic agents

Abstract Ion microprobe analyses of clinopyroxenes in equilibrium with glasses (SiO2 from 42 to 69 wt%) formed as the result of AFC-type reactions between infiltrating basaltic melt and peridotite wall-rock in the upper mantle under La Palma (Canary Islands) reveal trace element signatures usually ascribed to carbonatite metasomatism (i.e. strong REE-enrichment and Zr,Ti-depletion). Cpx/glass partition coefficients for REE, Y and Sr progressively increase with increasing SiO2 in response to liquid composition/structure effects, approaching unity for La, Ce and Sr, and exceeding unity for the other REE in the most silicic glasses. Partition coefficients for Zr and Ti remain constant or decrease, probably as the combined effect of melt composition/structure and difficulty of charge-balancing 4+ cations in the crystal. Incorporation of the higher-charged HFSE in the cpx lattice requires complex coupled substitutions (e.g. M1Zr+1TAl+2M1R2+−1TSi−2 or M2Na+1M1Zr+1TAl+1M2Ca−1M1R2+−1TSi−1). Our results demonstrate that REE-enrichment and REE/HFSE fractionation of clinopyroxene do not necessarily reflect these characteristics in the metasomatic agent and that cpx/basaltD cannot confidently be used to infer the geochemical nature of equilibrium melts under upper mantle conditions. Moreover, we provide evidence that the strong REE-enrichment and Zr,Ti-depletion of some clinopyroxene from La Palma xenoliths is a feature commonly observed in clinopyroxenes from various mantle occurrences often interpreted as reflecting chemical interaction with metasomatic fluids and melts of different composition (either alkaline or carbonatitic) and provenance (from either the enriched asthenospheric mantle or the subducted slab). Therefore, claims of carbonatite metasomatism as opposed to silicate melt infiltration based on trace element signatures of investigated clinopyroxene should be regarded with caution unless the role of liquid composition/structure and of crystal chemical control is investigated.

Age relations among Oslo Rift magmatic rocks: implications for tectonic and magmatic modelling

Abstract This paper presents a compilation of 67 published and unpublished radiometric age determinations by the Rb-Sr method on magmatic rocks in the continental Oslo Rift in southeast Norway. The data include most major rock types and structural units in the subaerial part of the rift. The implications of these data with respect to the tectonomagmatic evolution of the rift, and processes in the mantle and crust, are discussed. The Oslo Rift was active for about 60 million years. The earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre-rift Lower Paleozoic sediments at about 300 Ma. Extrusion of plateau lavas of basaltic and intermediate composition (rhomb-porphyries) started about 295 Ma and lasted to about 275 Ma, with largest magma volumes and highest extrusion rate from 295 to 285 Ma in the Vestfold Graben Segment (GS) (one flow per 250,000 years and 0.30 km3 per 103 years). The main faulting activity and graben formation occurred during this period. The plateau volcanism was succeeded by a stage of bimodal central volcanoes, most of which later underwent caldera collapses. Ring-dykes and central intrusions in these calderas give ages in the range 268-266 Ma in the Vestfold and 266-243 Ma in the Akershus GS. Large composite intrusive complexes were emplaced in the periods 27–268 and 273-241 Ma in the Vestfold and Akershus GS, respectively. A rate of propagation of the rift from SSW towards the NNE has been estimated to 1–2 cm y−1. On the basis of 87 Sr 86 Sr initial ratios and type of magmatic products, we estimate magmas to have been emplaced into the crust only during the first 20–25 Ma of the magmatic period. Some of the magmatism appears to be initiated by movement along Precambrian fault zones which were reactivated during the Oslo rifting event.

Petrogenesis of spinel harzburgite and dunite suite xenoliths from Lanzarote, eastern Canary Islands: Implications for the upper mantle

Abstract We present data on petrography, mineral and whole rock major element relations and fluid inclusions on ultramafic xenoliths from Quaternary to Recent alkaline basalts in Lanzarote, eastern Canary Islands. The xenoliths have been divided into two main suites: the spinel harzburgite suite (harzburgites and rare lherzolites) and the spinel dunite suite (spinel dunites and rare spinel-plagioclase dunites) The spinel-harzburgite suite xenoliths from Lanzarote represent fragments of highly refractory, old suboceanic lithospheric mantle similar to that found beneath Hierro in the western part of the Canary Island chain. This mantle has been somewhat modified through a combination of melt extraction and metasomatism caused by infiltration of Fe-Ti-rich silicate melts and CO 2 fluids, probably in association with the formation of the Canary Islands. Also the spinel-dunite suite xenoliths show oceanic affinities, but are not directly related to the harzburgites through partial melting. Temperature estimates combined with isochores representing the densest CO 2 inclusions ( T h of − 12 ° C) in these nodules indicate a high geothermal gradient in the upper mantle under Lanzarote, 1100 ° C at ⩾ 26 km depth, and a correspondingly thinned lithosphere ( ⩾ 27 km). This implies much hotter conditions than those expected in “normal” suboceanic lithospheric mantle of an age corresponding to that off West Africa, and hotter conditions under Lanzarote than under the western Canary Islands. A possible explanation for this is the presence of a mantle plume under the Canary Islands, which causes thermal erosion at the base of the lithosphere, whereas ascending plume melts are responsible for heating, partial melting and metasomatism in the overlying mantle. Edge effects such as small-scale convection caused by interaction between hot plume material flowing eastwards underneath the lithosphere and the continental margin of West Africa, may account for enhanced thermal erosion under the easternmost Canary Islands and recurrent volcanism.

Compositional relations among pyroxenes, amphiboles and other mafic phases in the Oslo Region plutonic rocks: Publication No. 115 in the Norwegian Geotraverse project

Abstract Mafic minerals in samples from older and younger intrusions in the Permian Oslo rift were analysed to determine temperatures and oxygen fugacities of crystallization and possible genetic relations between different rock types. Applications of various geothermometers and oxygen barometers to plutonic rocks have been assessed in connection with this work. Pyroxenes in the peralkaline rocks define a continuous trend from augites to pure aegirines. This trend was found to be the result of fractional crystallization of plagioclase (and augite), suggesting that the peralkaline rocks formed from alkaline initial liquids.

Carboniferous-Permian rifting and magmatism in southern Scandinavia, the North Sea and northern Germany: a review

Abstract During the Late Carboniferous and Early Permian an extensive magmatic province developed within northern Europe, intimately associated with extensional tectonics, in an area stretching from southern Scandinavia, through the North Sea, into northern Germany. Within this area magmatism was unevenly distributed, concentrated mainly in the Oslo Graben and its offshore continuation in the Skagerrak, Scania in southern Sweden, the island of Bornholm, the North Sea and northern Germany. Available geochemical (major- and trace-element, and Sr-Nd isotope, data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. Peak magmatic activity was concentrated in a narrow time-span from c. 300 to 280 Ma. The magmatic provinces developed within a collage of basement terranes of different ages and lithospheric characteristics (including thicknesses), brought together during the preceding Variscan orogeny. This suggests that the magmatism in this area may represent the local expression of a common tectono-magmatic event with a common causal mechanism. Available geochemical (major and trace element and Sr-Nd isotope data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. The magmatism covers a wide range in rock types both on a regional and a local scale (from highly alkaline to tholeiitic basalts, to trachytes and rhyolites). The most intensive magmatism took place in the Oslo Graben (ca. 120 000 km3) and in the NE German Basin (ca. 48 000 km3). In both these areas a large proportion of the magmatic rocks are highly evolved (trachytes-rhyolites). The dominant mantle source component for the mildly alkali basalts to subalkaline magmatism in the Oslo Graben and Scania (probably also Bornholm and the North Sea) is geochemically similar to the Prevalent Mantle (PREMA) component. Rifting and magmatism in the area is likely to be due to local decompression and thinning of highly asymmetric lithosphere in responses to regional stretching north of the Variscan Front, implying that the PREMA source is located in the lithospheric mantle. However, as PREMA sources are widely accepted to be plume-related, the possibility of a plume located beneath the area cannot be disregarded. Locally, there is also evidence of other sources. The oldest, highly alkaline basaltic lavas in the southernmost part of the Oslo Graben show HIMU trace element affinity, and initial Sr-Nd isotopic compositions different from that of the PREMA-type magmatism. These magmas are interpreted as the results of partial melting of enriched, metasomatised domains within the mantle lithosphere beneath the southern Olso Graben; this source enrichment can be linked to migration of carbonatite magmas in the earliest Paleozoic (ca. 580 Ma). Within northern Germany, mantle lithosphere modified by subduction-related fluids from Variscan subduction systems have provided an important magma source components.

Fluid and silicate glass inclusions in ultramafic and mafic xenoliths from Hierro, Canary Islands: implications for mantle metasomatism

Fluid and solid inclusions have been studied in selected samples from a series of spinel-bearing Crdiopside-and Al-augite-series ultramafic (harzburgites, lherzolites, and olivine-clinopyroxene-rich rocks), and gabbroic xenoliths from Hierro, Canary Islands. In these samples several generations of fluid inclusions and ultramafic-and mafic-glass inclusions may be texturally related to different stages of crystal growth. The fluid inclusions consist of pure, or almost pure, CO2. The solid inclusions in the ultramafic xenoliths comprise early inclusions of devitrified ultramafic glass, sulphide inclusions, as well as polyphase inclusions (spinel+clinopyroxene±glass±other silicates) believed to have formed from trapped basaltic melts. Vitreous basaltic glass±CO2±sulphide±silicates are common as secondary inclusions in the ultramafic xenoliths, and as primary inclusions in the gabbroic xenoliths. Microthermometry gives minimum trapping temperatures of 1110° C for the early ultramafic-and mafic-glass inclusions, and a maximum of 1260–1280° C for late inclusions of host basaltic glass. In most samples the CO2 inclusions show a wide range in homogenization temperatures (-40 to +31° C) as a result of decrepitation during ascent. The lowest homogenization temperatures of about-40° C, recorded in some of the smallest CO2 inclusions, indicate a minimum depth of origin of 35 km (12 kbar) for both the Cr-diopside-and Al-augite-series xenoliths. The gabbroic xenoliths originate from a former magma chamber at a depth of 6–12 km.

Petrology of basalts from the Mohns-Knipovich Ridge; the Norwegian-Greenland Sea

Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30′N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K2O, TiO2, and P2O5 contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe2+) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe2+) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C.Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30′ N is also briefly considered.It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform.

Permo-Carboniferous magmatism and rifting in Europe: introduction

An extensive rift system developed within the northern foreland of the Variscan orogenic belt during Late Carboniferous-Early Permian times, post-dating the Devonian-Early Carboniferous accretion of various Neoproterozoic Gondwana-derived terranes on to the southern margin of Laurussia (Laurentia-Baltica; Fig. 1). Rifting was associated with widespread magmatism and with a fundamental change, at the Westphalian-Stephanian boundary, in the regional stress field affecting western and central Europe (Ziegler 1990; Ziegler & Cloetingh 2003). The change in regional stress patterns was coincident with the termination of orogenic activity in the Variscan fold belt, followed by major dextral translation between North Africa and Europe. Rifting propagated across a collage of basement terranes with different ages and thermal histories. Whilst most of the Carboniferous-Permian rift basins of NW Europe developed on relatively thin lithosphere, the highly magmatic Oslo Graben in southern Norway initiated within the thick, stable and, presumably, strong (cold) lithosphere of the Fennoscandian craton. The rift basins in the North Sea, in contrast, developed in younger Caledonian age lithosphere, which was both thinner and warmer than the lithosphere of the craton to the east. A regional hiatus, corresponding to the Early Stephanian, is evident in much of the Variscan foreland, with Stephanian and Early Permian red beds unconformably overlying truncated Westphalian series (e.g. McCann 1996) (Fig. 2). Regional uplift coincides with the onset of voluminous magmatism across the region, raising the possibility that uplift could have been related to the presence of a widespread thermal anomaly within the upper mantle (i.e. a mantle plume or, possibly, several plumes). In