Stefan Bernstein

Silica enrichment in the continental upper mantle via melt/rock reaction

Abstract Mantle peridotite xenoliths from Archean cratons generally have high molar Mg/(Mg + Fe), or Mg#. The best known suites, from the Kapvaal and Siberian cratons, have high modal orthopyroxene (Opx). These high Opx compositions are probably not residues of partial melting. Less well known cratonic xenolith suites from Greenland and North America include high Mg# peridotites with much lower modal Opx. Such low Opx compositions could be residual from high degrees of polybaric, decompression melting, ending in the spinel lherzolite stability field at pressures of 30 to 20 kbar. This paper presents additional evidence that the great majority of both spinel- and garnet-bearing xenoliths are also residues of polybaric melting that ended at pressures ≤30 kbar. Where xenoliths record equilibration pressures > 30 kbar, this must result from tectonic transport of peridotites to greater depth after melting. Proposed mechanisms for producing the high Mg#, high Opx compositions include metamorphic differentiation of high pressure residues, mixtures of residual peridotites and high pressure igneous cumulates from ultramafic magmas, and addition of SiO 2 to low Opx peridotites via melt/rock reaction. This paper focuses on a positive correlation between Ni contents of olivine and modal proportions of Opx in mantle xenoliths, and uses this correlation to constrain the processes that produced high Mg#, high Opx cratonic mantle compositions. The observed correlation is probably not produced by partial melting, metamorphic differentiation, or formation of igneous cumulates. It can be produced by reaction between SiO 2 -rich liquids (e.g., small degree melts of subducted eclogite) and previously depleted, low Opx peridotites. We propose a two step process. First, high Mg#, low Opx peridotites were created by large degrees of polybaric melting ending at pressures 2 -rich melts generated mainly by partial melting of eclogitic basalt and sediment in a subduction zone. Magmas modified by such a process could have formed a major component of the continental crust. Thus, this hypothesis provides a genetic link between cratonic upper mantle and continental crust.

40Ar39Ar geochronology of Tertiary mafic intrusions along the East Greenland rifted margin: Relation to flood basalts and the Iceland hotspot track

Abstract The East Greeland Tertiary Igneous Province includes the largest exposed continental flood basalt sequence within the North Atlantic borderlands. More than ten layered gabbro complexes, including the ∼55 Ma Skaergaard intrusion, and a large dolerite sill complex are the plutonic equivalents of flood basalts; both lavas and intrusions have been regarded as synchronous with continental breakup at 57-54 Ma. We report ten new ages of the mafic intrusions, determined by40Ar 39Ar incremental heating experiments, demonstrating that the mafic intrusions formed in two distinct time windows. Only Intrusion II of the Imilik Gabbro Complex, the Skaergaard intrusion, and the Sorgenfri Glestcher Sill Complex formed at 57-55 Ma coeval with the eruption of regional flood basalts and continental breakup. Other layered gabbro intrusions at Imilik (Intrusion III), Kruuse Fjord, Igtutarajik, Nordre Aputiteˆq, Kap Edvard Holm, and Lilloise are distinctly younger and formed between 50 and 47 Ma. Plate-kinematic models indicate the axis of the ancestral Iceland mantle plume was located under Central Greenland at ∼60 Ma and subsequently crossed the East Greenland rifted continental margin. We propose that tholeiitic magmatism along the East Greenland rifted margin largely occurred in three distinct pulses at 62-59 Ma (lavas and dykes), 57-54 Ma (lavas, dykes, sills, and some gabbros) and 50-47 Ma (gabbros, dykes and rare lavas), related to discrete mantle melting episodes triggered by plume impact, continental breakup, and passage of the plume axis, respectively. This model implies northwestward continental drift of Greenland relative to the plume axis by ∼3.9-5.0 cm/yr between ∼60 and ∼49 Ma, consistent with estimates from seismic studies of submerged flood basalts.

Depleted spinel harzburgite xenoliths in Tertiary dykes from East Greenland: Restites from high degree melting

Abstract A new collection of mantle xenoliths in Tertiary dykes from the Wiedemann Fjord area in Southeast Greenland shows that this part of the central Greenland craton is underlain by highly depleted peridotites. The samples are mostly spinel harzburgites with highly forsteritic olivines (Fo 87–94 , average Fo 92.7 ). This, together with unusually high modal olivine contents (70–>95%), places the Wiedemann harzburgites in a unique compositional field. Relative to depleted Kaapvaal harzburgites with comparable Fo in olivine, the Wiedemann samples have considerably lower bulk SiO 2 (average 42.6 wt% versus 44–49 wt%). Spinel compositions are similar to those in other sub-cratonic harzburgites. Pyroxene equilibrium temperatures average 850°C, which is above an Archaean cratonic geotherm at an inferred pressure of 1–2 GPa, but low enough so that it is unlikely that the xenoliths represent residual peridotites created during Tertiary magmatism. Among mantle samples, the Wiedemann harzburgites are, in terms of their bulk composition, most similar to harzburgites from the ophiolites of Papua New Guinea (PNG) and New Caledonia (NC). One hypothesis is that the Wiedemann harzburgites, along with PNG and NC harzburgites, formed via dissolution of pyroxene from previously depleted peridotites, possibly beneath a volcanic arc. If so, higher spinel Cr/Al in Wiedemann samples may reflect a deeper origin compared to PNG and NC peridotites. Alternatively, using proposed primitive mantle compositions as a protolith, the Wiedemann harzburgites can be modeled as the residue after extraction of some 40% melt. The composition of this calculated hypothetical melt in terms of CaO, Al 2 O 3 , FeO, MgO and SiO 2 is similar to published experimental data on high degree melts of peridotite at 2–3 GPa. Munro-type komatiites lie close to these calculated and experimental melts but are slightly displaced towards low degree experimental melts at higher pressure (e.g., 6 GPa). We conclude that the Wiedemann harzburgites formed as a residue after about 40% melting, and that they may represent shallow, refractory residues after polybaric melting initiated at pressures ≥7 GPa and continuing to relatively low pressures (2–3 GPa or less). Extraction and aggregation of polybaric melts would produce liquids similar to Munro-type komatiites.

Consistent olivine Mg# in cratonic mantle reflects Archean mantle melting to the exhaustion of orthopyroxene

Shallow (garnet-free), depleted cratonic mantle, occurring as xenoliths in kimberlites and alkaline basaltic lavas, has a high Mg# (100 × Mg/(Mg + Fe) > 92) and is poor in Al and Ca compared to off-cratonic mantle. Here we compile data for many suites of shallow cratonic mantle xenoliths worldwide, and demonstrate a remarkably small range in their olivine Mg#, with an average of ≈92.8. Via comparison with data for experimental melting of mantle peridotite compositions, we explain consistent olivine Mg# as the result of mantle melting and melt extraction to the point of orthopyroxene exhaustion, leaving a nearly monomineralic olivine residue.

Post-breakup basaltic magmatism along the East Greenland Tertiary rifted margin

Mafic and ultramafic intrusions in East Greenland adjacent to the offshore Greenland‐Iceland ridge were emplaced 5‐9 My after continental breakup at 55 Ma [1]. Rare earth element (REE) concentrations determined by secondary ion mass spectrometry are reported for cumulus clinopyroxene from these intrusions, and the data are used to estimate REE abundance in equilibrium melts using available partitioning data. Estimated equilibrium melts from intrusions have strongly fractionated REE patterns with Nd=Dy(N) in the range 2 to 5.6 and Yb=Dy(N) 0.55 to 0.92, similar to values for coeval basalts. These melts have markedly higher Nd=Dy(N) than earlier breakup related flood basalts. The moderately low Yb=Dy(N) for the post-breakup volcanism is indicative of residual garnet in the source, while their high Nd=Dy(N) ratios can best be explained by aggregating low degree melts from a light-REE-enriched garnet- and spinel-bearing mantle source. We also report He, Sr, and Nd isotopic data for the intrusions. The highest 3 He= 4 He ratios (>10 R=Ra) are found in the samples whose REE data reflect the largest proportion of melts from a garnet-bearing source, and having Sr and Nd isotopic compositions identical with the radiogenic Sr and unradiogenic Nd isotope end of the Iceland compositional field. There is no indication of a MORB-type mantle in the source of the intrusions. We postulate that post-breakup volcanism along the East Greenland coastline reflects the increasing proximity of the mantle plume to the East Greenland continental margin. The low degree of melting at high mean pressure inferred for the parental melts for the intrusions may reflect re-thickening of the lithosphere, which in turn was caused by the vigorous volcanism during breakup, with accompanying depletion of upper mantle and underplating of the crust at the continental margin.

High-pressure fractionation in rift-related basaltic magmatism: Faeroe plateau basalts

The lower 3000 m basalt stratigraphy on the Faeroe Islands shows strong positive correlation between degree of fractionation and Zr/Y ratio. This is ascribed to the presence of garnet in the cumulus assemblage during fractionation at the base of the continental crust or in the upper lithospheric mantle at 40-50 km depth. Later basaltic magmas fractionated at low pressures, which is a typical shift in fractionation regime among continental flood basalts. Seismic velocity ( V p ) for the garnet-bearing high-pressure fractionates is in excess of 7 km/s. This is comparable to velocities observed in high- V p bodies in the basal crust of Precambrian shields. High-pressure fractional crystallization of basaltic magmas may thus be an important process in generating the chemical complexity of continental flood basalts and has implications for crustal underplating.

An ocean-ridge type magma chamber at a passive volcanic, continental margin; the Kap Edvard Holm layered gabbro complex, East Greenland

The gabbros of the Tertiary Kap Edvard Holm Layered Serieshave a stratigraphic thickness of more than 5000 m. Earlier work has shown that the range in cumulus mineral compositions is restricted (plagioclase An 81 —An 51 ; olivine Fo 85 —Fo 66 ; pyroxenes Ca 43 Mg 46 Fe 11 to Ca 43 Mg 37 Fe 20 ). Field evidence of magma injections is common, which together with the restricted range in mineral chemistry suggests that the magma chamber was frequently replenished by a less fractionated magma. A detailed study of a 600 m section (900–1500 m) in the Lower Layered Series reveals a period of crystallization when the magma chamber behaved as a closed system (900–1300 m). The rocks formed during this periodare well-laminated olivine–gabbros (900–110 m), which evolved to well-laminated oxide-gabbros (1100–1300 m). Compositional trends in the cumulusminerals are towards more evolved compositions (plagioclase An 64 —An 58 , pyroxene Mg# from 80 to 76) with stratigraphic height. From 1300 m to 1500 m, granular olivine-gabbros dominate, with moreprimitive mineral compositions (plagioclase An 67 —An 76 , pyroxene Mg# from 78 to 82). The transition olivine–gabbro to oxide-gabbro at 1100m is a consequence of fractional crystallization, and it is shown how changes in activities of FeO and Fe 2 0 3 in the magma are reflected in the total iron content of plagioclases.The transition from oxide-gabbro to olivine-gabbro at 1300 m results from replenishment by less evolved basaltic magma. On the basis of calcic pyroxene chemistry and the mineral crystallization sequence it is concluded that the Kap Edvard Holm Layered Series crystallized from a tholeiitic magma similar to MORB. Melanogabbroic units occur throughout the intrusion as discordant to subconcordant sill-like bodies 0.2–2.0 m thick. The melanogabbroic units consist of Cr-rich augite-olivine-plagioclase heteradcumulates and contain deformed mica crystals of pre-emplacement origin. These units crystallized from a wet, MgO-rich magma which was injected into the layered host gabbros after the formation of the cumulus pile, but before the magma was completely solidified. The Kap Edvard Holm Layered Series has several parallels with the plutonic part of ophiolite sequences. These include: cumulus mineral assemblage, compositions of the minerals and the restricted range in compositions with stratigraphic height; field evidence of repeated replenishment of basaltic magma; dyke swarms overlying the roof zone of the magma chamber; and the existence of a late intrusive suite of wet, MgO-rich magma. These parallels suggest that the processes involved in the formation of the Kap Edvard Holm Layered Series were similar to those involved in the formation of the crustalpart of many ophiolites and beneath present-day spreading ridges. The Kap Edvard Holm Layered Series is therefore believed to represent a shallow-level magma chamber which acted as a reservoir for basaltic flows at the continental margin during the opening of the North Atlantic Ocean.

Tertiary alkaline volcanics in the Nunatak Region, Northeast Greenland: new observations and comparison with Siberian maymechites

Abstract Highly alkaline ultrabasic lavas which occur in the remote Nunatak Region of Northeast Greenland between latitudes 73°45′N and 74°25′N are approximately contemporaneous with the better known, voluminous Tertiary tholeiitic plateau basalts of the coastal region (68°N–71°N, 74°N–76°N). Recent reconnaissance mapping extended the known outcrop of the alkaline ultrabasic lavas southwards and northwards, at the same time revealing several new volcanic plugs, assumed to be feeders to the lavas. Rounded dunitic xenoliths were recovered from one of these plugs. The xenoliths have olivine with forsterite contents too low, and chrome–spinel too titaniferous, to be of mantle origin. Rather, these features are consistent with crystallisation from the picritic host magmas, and the xenoliths are thus regarded as cognate. The alkaline igneous rocks are all ultramafic with high MgO contents (up to 17 wt.%), high TiO2 and FeO (up to 8.5 wt.%, and 18.1 wt.%, respectively) and very low silica and aluminium (generally 37–44 wt.% SiO2 and 5–10 wt.% Al2O3). The lavas have phenocrysts of olivine, clinopyroxene and iron–titanium oxides. Some have discernable perovskite in the groundmass. Common mica in the groundmass testifies to the hydrous nature of the parental magma. Chemically, the Nunatak Region volcanics are rich in incompatible elements as well as in Ni and Cr, and range from melilitites to basanites. Compared to melilitites from most other settings, those from the Nunatak Region have lower Al2O3, and higher FeO and TiO2 for a given MgO. There is, however, close chemical affinity with the strongly undersaturated, primitive lavas, so-called maymechites (formerly meimechites [Arndt, N., Chauvel, C., Czamanske, G., Fedorenko, V., 1998. Two mantle sources, two plumbing systems: tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province. Contrib. Mineral. Petrol. 133, 297–313]), described from the Maymecha region in the Siberian Traps. As recently proposed for the origin of the maymechites, it is suggested that volcanic rocks of the Nunatak Region formed from low degrees of mantle melting ( 5GPa) when a hydrated mantle transgressed its wet solidus during ascent beneath a thick cratonic lithosphere. Trace-element profiles of the nunatak lavas suggest recycled oceanic crust as an important component in the mantle source. The occurrences of both the Nunatak Region volcanic rocks and the Maymecha volcanics at the rims of ascending mantle plumes during continental rifting suggest that some mantle plumes may have a hydrous component.

Enriched component of the proto-Icelandic mantle plume revealed in alkaline Tertiary lavas from East Greenland

Alkaline ultramafic lavas in north East Greenland were contemporaneous with Tertiary tholeiitic flood basalts in East Greenland. The alkaline lavas are enriched in light rare earth elements (REEs) and Ti and Nb, but depleted in Rb, Ba, K, Th, and P. The flood basalts have trace element profiles similar to those of the alkaline lavas. These features are not caused by fractional crystallization or crustal contamination and are thus regarded as inherited from the mantle source. The trace element profiles and Sr and Nd isotopic compositions of the flood basalts are shown to be consistent with a simple model, in which 5%–10% alkaline melt is mixed into normal-mid-ocean ridge basalt. This alkaline melt has a composition reflecting low-degree melting at high pressures of incompatible element– enriched mantle. In areas where only deep melting occurred, this enriched melt component reached the surface in an essentially undiluted form. Elsewhere, under the rift axis, it mixed with asthenospheric melts, resulting in the typical enriched characteristics of East Greenland flood basalts. The presence of pronounced negative P anomalies in modern Iceland lavas suggests this to be a long-term feature of the Iceland mantle plume.

Petrology and geochemistry of the Kruuse Fjord gabbro complex, East Greenland

The Kruuse Fjord Gabbro Complex is a composite intrusion of layered gabbro and troctolite with subordinate ultramafic rocks and minor trondhjemitic bodies. It was emplaced into Archaean continental crust of East Greenland during early Tertiary rifting of Greenland from Eurasia. The work to date has identified an outer gabbro series and an inner troctolite series, and these are separated by a narrow zone of trondhjemitic intrusions. In the southeast, the partially crystallized cumulates of the gabbro series were intruded by a lenticular, ultramafic pluton 800 m in thickness. Volumetrically minor, syenite–trachyandesite net-veined dykes and later, diabase dykes cross-cut the plutonic rocks. Structural and topographic features suggest that the layered rocks were affected by synmagmatic subsidence and deformation but not by monoclinal coastal flexure. The gabbro series is composed of a marginal gabbro unit, about 20 m wide, bordering more than a 2 km thickness of layered olivine and magnetite gabbro cumulates. The marginal gabbro is interpreted to be chilled magma. The layered cumulates are the product of repeated injections of magma that fractionated in an open-system magma chamber. Anorthositic and troctolitic layers in the lower part of the sequence may represent inputs of magma and suggest that the order of cumulus mineral crystallization was (1) plagioclase (An 39–85 ), (2) olivine (Fo 46–82 ), (3) augite (Wo 28–47 En 39–58 Fs 8–18 ) and (4) magnetite. The disappearance of cumulus magnetite and a reversal in mineral compositions at 1.5 km from the base of the succession suggests a major input of magma occurred at this height. In the troctolite series, the composition of cumulus minerals, mineral crystallization sequence and style of emplacement are similar to those in the gabbro series. The ultramafic pluton is composed of coarse-grained wehrlite, olivine melagabbro and troctolite that were formed by at least three injections of magma. The typical mineral crystallization sequence was (1) cumulus chromite and olivine (Fo 84–88 ); (2) poikilitic chrome diopside (Wo 29–51 En 43–63 Fs 3–13 ); and (3) intercumulus plagioclase (An 75–90 ), phlogopite, apatite and localized disseminated sulphides containing Au and platinum-group elements. Comparison of crystallization sequences and the major and trace element compositions of clinopyroxene suggests that the gabbroic and troctolitic rocks formed from a magma represented by the chilled marginal gabbro, a tholeiitic basalt magma similar to E-MORB, whereas the ultramafic rocks formed from a magma that was relatively enriched in incompatible trace elements and volatiles. The association of these two magma types is an example of bimodal mafic–ultramafic magmatism in a rifting environment.