M. J. Norry

THERMAL AND CHEMICAL STRUCTURE OF THE ICELAND PLUME

Basaltic lavas, forming thick offshore seaward-dipping reflector sequences (SDRS) and onshore igneous provinces around the North Atlantic margins, represent melting of anomalously hot mantle in the head of the ancestral Iceland plume. Some of these lavas are chemically and isotopically indistinguishable from recent Icelandic basalt, but others more closely resemble basalt erupted at normal segments of mid-ocean ridges (N-MORB). In this paper we show that Icelandic basalt and N-MORB define parallel tight arrays on a plot oflog(Nb/Y) against log(Zr/Y), with N-MORB relatively deficient in Nb. Deficiency or excess of Nb, relative to the lower bound of the Iceland array, may be expressed as ΔNb=1.74+log⁡(Nb/Y)−1.92log⁡(Zr/Y)such that Icelandic basalt has ΔNb > 0 and N-MORB has ΔNb < 0. ΔNb is a fundamental source characteristic and is insensitive to the effects of variable degrees of mantle melting, source depletion through melt extraction, crustal contamination of the magmas, or subsequent alteration. We use new and published Nb, Zr and Y data to identify the mantle sources for Palaeocene and Eocene basaltic lavas erupted around the Atlantic margins in order to deduce the thermal and compositional structure of the head of the ancestral Iceland plume. The results show that the head of the plume was zoned, with an axial zone of Icelandic mantle surrounded by a thick outer shell of anomalously hot but compositionally normal N-MORB-source mantle. The zoning is very similar in scale and character to that seen today along the Reykjanes Ridge and is difficult to reconcile with the initiation of rifting and SDRS formation through the impact of a large plume head originating solely from the lower mantle. The thick outer shell of hot, depleted upper mantle, which formed more than half the volume of the plume head, suggests that at least part of the plume originated in the thermal boundary layer at the base of the upper mantle.

Elemental and Sr isotope variations in basic lavas from Iceland and the surrounding ocean floor

AbstractMajor and trace element data are used to establish the nature and extent of spatial and temporal chemical variations in basalts erupted in the Iceland region of the North Atlantic Ocean. The ocean floor samples are those recovered by legs 38 and 49 of the Deep Sea Drilling Project. Within each of the active zones on Iceland there are small scale variations in the light rare earth elements and ratios such as K/Y: several central complexes and their associated fissure swarms erupt basalts with values of K/Y distinct from those erupted at adjacent centres; also basalts showing a wide range of immobile trace element ratios occur together within single vertical sections and ocean floor drill holes. Although such variations can be explained in terms of the magmatic processes operating on Iceland they make extrapolations from single basalt samples to mantle sources underlying the outcrop of the sample highly tenuous. 87Sr/86Sr ratios measured for 25 of the samples indicate a total range from 0.7028 in a tholeiite from the Reykjanes Ridge to 0.7034 in an alkali basalt from Iceland and are consistent with other published ratios from the region. A positive correlation between 87Sr/86Sr and Ce/Yb ratios indicates the existence of systematic isotopic and elemental variations in the mantle source region. An approximately fivefold variation in Ce/Yb ratio observed in basalts with the same 87Sr/86Sr ratio implies that different degrees and types of partial melting have been involved in magma genesis from a single mantle composition. 87Sr/86Sr ratios above 0.7028, Th/U ratios close to 4 and La/Ta ratios close to 10 distinguish most basalts erupted in this part of the North Atlantic Ocean from normal mid-ocean ridge basalt (N-type MORE) — although N-type MORB has been erupted at extinct spreading axes just to the north and northeast of Iceland as well as the presently active Iceland-Jan Mayen Ridge.Comparisons with the hygromagmatophile element and radiogenic isotope ratios of MORB and the estimated primordial mantle indicate that the mantle sources producing Iceland basalts have undergone previous depletion followed by more recent enrichment events. A veined mantle source region is proposed in preference to the mantle plume model to explain the chemical variations.

Consequences of plume-lithosphere interactions

Abstract Splitting or thinning of lithosphere above a mantle plume can result in voluminous melt generation, leading to the formation of large igneous provinces, or LIPs. Examples of LIPs include continental flood basalt provinces and oceanic plateaus. Basaltic samples from the Ontong Java Plateau, Nauru Basin and Manihiki Plateau, which are among the largest of the LIPs, have isotopic compositions within the range of ocean island basalts. The majority of continental basalts, however, record a trace element and isotopic contribution from the lithosphere through which they have erupted. We are thus unable to reconcile the available compositional data with models which derive the isotopic and large-ion lithophile element-enriched character of continental flood basalts solely from sub-lithospheric mantle plume sources. A combination of mantle sources is indicated, with the thermal energy being supplied by voluminous melts from a plume, and the lithospheric components in continental flood basalts being inherited by contamination of plume-derived melts by low melting point hydrous and carbonated fractions in the lithosphere. Successive injection of plume-derived melts serves to heat the lithosphere, reducing its viscosity and making it susceptible to rupture if allowed by regional plate forces. Furthermore, the lithosphere, including the mechanical boundary layer, may be thinned by thermal stripping from below, allowing the plume mantle to ascend and decompress further. Such a system has the potential for positive feedback leading to rapid melt generation. While we do not exclude recent models of LIP formation which require the sudden impact of a new mantle plume, we favour a model whereby the thermal anomaly builds gradually, incubating beneath a steady-state lithospheric cap.

Fluid Influence on the Trace Element Compositions of Subduction Zone Magmas

Subduction zones represent major sites of chemical fractionation within the Earth. Element pairs which behave coherently during normal mantle melting may become strongly decoupled from one another during the slab dehydration processes and during hydrous melting conditions in the slab and in the mantle wedge. This results in the large ion lithophile elements (e.g. K, Rb, Th, U, Ba) and the light rare earth elements being transferred from the slab to the mantle wedge, and being concentrated within the mantle wedge by hydrous fluids, stabilized in hydrous phases such as hornblende and phlogopite, from where they are eventually extracted as magmas and contribute to growth of the continental crust. High-field strength elements (e.g. Nb, Ta, Ti, P, Zr) are insoluble in hydrous fluids and relatively insoluble in hydrous melts, and remain in the subducted slab and the adjacent parts of the mantle which are dragged down and contribute to the source for ocean island basalts. The required element fractionations result from interaction between specific mineral phases (hornblende, phlogopite, rutile, sphene, etc.) and hydrous fluids. In present day subduction magmatism the mantle wedge contributes dominantly to the chemical budget, and there is a requirement for significant convection to maintain the element flux. In the Precambrian, melting of subducted ocean crust may have been easier, providing an enhanced slab contribution to continental growth.

Preservation of Early Cambrian animals of the Chengjiang biota

The Chengjiang biota of Yunnan, China, documents the earliest extensive radiation of the Metazoa recorded in the fossil record. Gauging preservational bias is crucial in providing an assessment of the completeness of this biota and thereby elucidating whether it represents a comprehensive depiction of Early Cambrian life. We here present a model to explain the nature of the exceptional preservation of the Chengjiang biota and details of the decay process. This study indicates that Chengjiang fossils were preserved through two taphonomic pathways that may have captured tissues of distinct compositions, and this finding should provide a foundation for the interpretation of Chengjiang fossils. Many Chengjiang fossils are preserved by pyrite (later pseudomorphed by iron oxides); the clay-rich host sediment was deficient in organic carbon but replete in available Fe, and this composition ensured that a decaying carcass acted as a local substrate for Fe- and S-reducing bacteria. Pyrite morphology probably reflects contrasts in the decay rate, and hence the H 2 S production rate, of different tissues in a carcass. Reactive, rapidly decaying tissues would have quickly supplied H 2 S, producing many pyrite nuclei, resulting in framboidal habits. More recalcitrant tissues would have produced H 2 S more slowly, so that crystal growth operated on fewer nuclei, resulting in larger euhedral pyrite crystals. Reflective films, especially common on Chengjiang arthropods, represent the remains of degraded carbon.

Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints

Abstract The Sr-, Nd-, and Pb-isotopic compositions of Late Tertiary to Recent mafic alkaline rocks of the Cape Verdes archipelago vary from 143 Nd 144 Nd = 0.512606–0.513045 (ϵ Nd = −0.5 to +8.1) , 87 Sr 86 Sr = 0.702922–0.703934 , and 206 Pb 204 Pb = 18.743–19.881 . The variation of 207 Pb 204 Pb with 206 Pb 204 Pb in Cape Verdes lavas coincides with data for MORB, Hawaii, and Iceland, while 208 Pb 204 Pb vs. 206 Pb 204 Pb is an oblique, positive trend, i.e., 208Pb is higher in samples from the southern Cape Verdes islands which have lower 206 Pb 204 Pb . At least three isotopically distinct components, including depleted upper mantle, are required in Cape Verdes magma sources. Samples from the southern islands have less radiogenic Nd and Pb and more radiogenic Sr relative to northern islands. Isotopic and trace element characteristics of an enriched source component (EM) present only in the southern Cape Verdes suggest an origin as recycled subcontinental mantle or lithosphere, except that this component in the Cape Verdes had a significant long-term depletion of U relative to Th. The isotopic and trace element characteristics of a component predominant in northern Cape Verdes magma sources with relatively radiogenic Pb and Nd (HIMU) suggest an origin for this component as ancient recycled oceanic crust. The trace element and isotopic variations, the large scale isotopic heterogeneity, the intra-island stratigraphic variations, the apparent geographic age progression of magmatism, and the geophysical constraints are best explained by variable partial melting of a heterogeneous plume (HIMU + depleted mantle) in the northern Cape Verdes, and mixing of plume-derived melts with lithospheric (EM) melts in the southern Cape Verdes. Alternatively, an EM plume produces magma supplied to the southern Cape Verdes, and the northern Cape Verdes, located off the plume track, are supplied by relatively lower degrees of melting at the plume margin which has entrained pre-existing HIMU components from a heterogeneous (DM + HIMU) upper mantle.

A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots : The geodynamic implications

Summary The mafic lavas of the Azores and Cape Verde Islands are of a highly varied composition, reflecting a complex history of magma genesis and a variety of source compositions. The lavas of the Cape Verde Islands are characteristically highly silica undersaturated, with alkali-rich ankaramites, larnite-normative melilitites and carbonatites. In contrast, the lavas of the Azores vary from strongly nepheline- to hypersthene-normative types. Isotopic ratios and trace elements also show considerable variation, consistent with derivation from multicomponent mantle sources. Three distinct groupings of lava compositions can be seen in Pb-Sr-Nd isotope space, and each is characterized by its own trace-element enrichment. (i) The majority of the lavas from the Azores and northern Cape Verdes Islands have isotope and trace-element systematics similar to basalts recovered from elevated segments of the mid-Atlantic Ridge (eg during Deep Sea Drilling Project Leg 82) (Ba/La <12, 206Pb/204Pb=19.3–20.0). (ii) The islands of Sao Miguel and Faial in the Azores are characterized by radiogenic strontium and lead isotope ratios and marked fractionation of the most incompatible trace elements (eg 87Sr/86Sr=0.70522, 206Pb/204Pb=19.88, 207Pb/204Pb=15.75, Ba/Nb=12, La/Nb=0.84). (iii) In contrast, the southern Cape Verde Islands have relatively unradiogenic Pb-Sr-Nd isotope ratios (eg 87Sr/86Sr=0.70393, 206Pb/204Pb=18.74, 207Pb/204Pb=15.53, Ba/La=22, Ba/Nb=15, La/Nb=0.67). Mixing relationships between isotope and trace-element ratios indicate the involvement of at least four, and possibly five, chemically distinct source regions in the petrogenesis of mafic lavas. It is shown that these different source regions represent combinations of the depleted upper mantle (mid-ocean ridge basalt source), recycled oceanic lithosphere and two components of the subcontinental lithospheric mantle. No direct unequivocal evidence is found for a fifth component, namely undepleted primitive mantle reservoir, although helium isotope data suggest influxes of material and heat from the lower mantle. Similarly, it is suggested that continental crust has no direct contribution to the petrogenesis of these ocean island basalts and its role in the mantle is limited to the production/modification of the subcontinental lithosphere above subduction zones.

Possible origin of K-rich volcanic rocks from Virunga, East Africa, by metasomatism of continental crustal material: Pb, Nd and Sr isotopic evidence

Abstract The isotopic compositions of Sr, Nd and Pb together with the abundances of Rb, Sr, U and Pb have been determined for mafic and felsic potassic alkaline rocks from the young Virunga volcanic field in the western branch of the East African rift system. 87 Sr/ 86 Sr varies from 0.7055 to 0.7082 in the mafic rocks and from 0.7073 to 0.7103 in the felsic rocks. The latter all come from one volcano, Sabinyo. Sabinyo rocks have negative e Nd values ofe Nd = −10. Nd and Sr isotopic variations in the basic potassic rocks are correlated and plot between Sabinyo and previously reported [1] compositions (e Nd = +2.5; 87 Sr/ 86 Sr≈ 0.7047) for Nyiragongo nephelinites. The Pb isotopic compositions for Sabinyo rocks are nearly uniform and average 206 Pb/ 204 Pb≈ 19.4, 207 Pb/ 204 Pb= 15.79–15.84, 208 Pb/ 204 Pb≈ 41.2. The basic potassic rocks have similar 206 Pb/ 204 Pb values but range in 207 Pb/ 204 Pb and 208 Pb/ 204 Pb from the Sabinyo values to less radiogenic compositions. Excellent correlations of 87 Sr/ 86 Sr with Rb/Sr, 1/Sr and 207 Pb/ 206 Pb for Sabinyo rocks suggest these to be members of a hybrid magma series. However, the nearly uniform Pb compositions for this series points to radiogenic growth of 87 Sr in the magma source region following an event which homogenized the isotopic compositions but not Rb/Sr. The Rb-Sr age derived from the erupted Sabinyo isochron-mixing line is consistent with the ∼500 Myr Pb-Pb age from Nyiragongo [1], which suggests that this event affected all Virunga magma sources. The event can again be traced in the Pb-Pb, Pb-Sr and Nd-Sr isotopic correlations for all Virunga rocks, including Nyiragongo, when allowances are made for radiogenic growth subsequent to this mixing or incomplete homogenization event. Inferred parent/daughter element fractionations point to a metasomatic event during which a mantle fluid invaded two lithospheric reservoirs: a +e Nd reservoir sampled by the Nyiragongo nephelinites and suggested to be the subcontinental mantle and a −e Nd reservoir sampled by the mafic and felsic potasssic volcanism. Whether this −e Nd reservoir is the crust, continental crustal material in the mantle or anomalous mantle cannot be decided from the data. The simplest answer, that this reservoir is the continental crust, seems to be at variance with experimental evidence suggesting a subcrustal origin for basic potassic magmas. Partial melting of the ancient metasomatised lithospheric domains and ensuing volcanism seems to be entirely a response to decompression and rising geotherms during rifting and thinning of the lithosphere.

Melt aggregation within the crust beneath the Mid-Atlantic Ridge: evidence from plagioclase and clinopyroxene major and trace element compositions

Ocean Drilling Program Hole 923A, located on the western flank of the Mid-Atlantic Ridge south of the Kane Fracture Zone, recovered primitive gabbros that have mineral trace element compositions inconsistent with growth from a single parental melt. Plagioclase crystals commonly show embayed anorthitic cores overgrown by more albitic rims. Ion probe analyses of plagioclase cores and rims show consistent differences in trace element ratios, indicating variation in the trace element characteristics of their respective parental melts. This requires the existence of at least two distinct melt compositions within the crust during the generation of these gabbros. Melt compositions calculated to be parental to plagioclase cores are depleted in light rare earth elements, but enriched in yttrium, compared to basalts from this region of the Mid-Atlantic Ridge, which are normal mid-ocean ridge basalt (N-MORB). Clinopyroxene trace element compositions are similar to those predicted to be in equilibrium with N-MORB. However, primitive clinopyroxene crystals are much more magnesian than those produced in one-atmosphere experiments on N-MORB, suggesting that the major element composition of the melt was unlike N-MORB. These data require that the diverse array of melt compositions generated within the mantle beneath mid-ocean ridges are not always fully homogenised during melt extraction from the mantle and that the final stage of mixing can occur efficiently within crustal magma chambers. This has implications for the process of melt extraction from the mantle and the liquid line of descent of MORB. fl 2000 Elsevier Science B.V. All rights reserved.

Complex magma mixing, mingling, and withdrawal associated with an intra-Plinian ignimbrite eruption at a large silicic caldera volcano: Los Humeros of central Mexico

Los Humeros is the largest caldera volcano in the Mexican volcanic belt. Its second largest caldera-forming eruption, the ca. 0.1 Ma Zaragoza eruption, is recorded by two Plinian pumice-fall layers and a zoned intra-Plinian ignimbrite. Diverse pumice types within the ignimbrite provide insights about the way that different magmas within a single magmatic system interact, and the way in which this can give rise to a major explosive ignimbrite-forming eruption. Normal-and-reverse compositional zoning in the ignimbrite is defined by vertical variations in the relative abundance of rhyodacitic (69–71 wt%% SiO 2 ) and andesitic (54–63 wt% SiO 2 ) pumice lapilli: Lower parts are dominated by rhyodacite and pass gradationally up into a central part with andesitic and rhyodacite pumice, and this passes up into a rhyodacitic uppermost part, with no andesite. Petrographic and microprobe analyses of coexisting glass and phenocrysts provide mixed evidence of equilibrium and disequilibrium conditions in the magmas at the time of eruption. The Fe-Ti oxides record magma temperatures of ∼850 °C (andesite) and 780 °C (rhyodacite). The andesitic pumice contains euhedral labradorite (∼An 60 ), and orthopyroxene and clinopyroxene, in a dacitic glass groundmass, which yield equilibrium Na-Ca K d pl/liq and Fe-Mg K d pl/liq ratios. It also contains highly calcic plagioclase (to An 82 ) that in some cases is highly resorbed and mantled by the more sodic plagioclase, which may record early mixing between andesitic and plagioclase-bearing basaltic magmas, followed by equilibrium crystallization within the hybrid magma. The rhyodacite contains euhedral crystals of more-evolved plagioclase (∼An 30-40 ) and euhedral pyroxenes in a rhyolitic glass groundmass (74–75 wt% SiO 2 ). The pyroxenes yield disequilibrium Fe-Mg K d pl/liq ratios and indicate formation from a liquid that was more mafic than the liquid that formed the glass groundmass of the dacitic pumice. Subordinate pumices with interbanded rhyodacite and a scarcity of intermediate-composition pumices indicate that the magmas remained separate for most of the time, and mingled only immediately prior to, and during eruptive quenching. Rather than a simple density-stratified magma chamber, the Zaragoza eruption may have occurred in response to intrusion of a hybridized andesitic magma into a rhyodacitic magma reservoir, possibly arranged as semiconnected high-melt lenses or zones within a partially consolidated crystal mush. However, contrary to assumptions of simple replenishment, tapping, and fractionation-type systems, the Zaragoza magmas contain no record of previously erupted highly evolved rhyolites that developed when zircon joined the fractionating assemblage. This absence indicates that the highly evolved rhyolites had either been completely tapped or solidified prior to the Zaragoza eruption eruption, or that interaction was prevented by contrasting magma densities and viscosities.