Peridotite Ridge Research Articles

Lead and osmium isotopic constraints on the oceanic mantle from single abyssal peridotite sulfides

Single sulfides from abyssal peridotites have been analyzed for Pb and Re-Os to constrain the evolution of oceanic mantle composition. These represent the first analyses of Pb and Os isotopic compositions in the same sulfide grain. The sulfides are from Gakkel and Southwest Indian ridge peridotites, occur at <0.1% modal abundances, and contain 0.001–0.4ppm Re, 0.003–5ppm Os, and 0.12–12ppm Pb. Sulfide Pb isotopic compositions extend from depleted (e.g., 206Pb/204Pb=17.0) to enriched (19.6), covering a larger range than associated basalts. The Os isotopic range of sulfides is more restricted, but extends from depleted (187Os/188Os=0.116) to enriched (0.150). Pb and Os concentrations and isotopic compositions co-vary, with correlation coefficients of 0.76–0.94. Both Pb and Re–Os isotopic data follow ∼2Ga isochrons, with isotopic compositions varying down to small (⪡1km) length-scales and some sulfides containing supra-chondritic 187Re/188Os and 187Os/188Os. These observations are best explained by long-term recycling of oceanic lithosphere combined with melt extraction and refertilization at ancient ocean ridges, rather than a specific event at 2Ga.The concentration of Pb in sulfides indicates that they host <4% of the mantle Pb budget. A re-evaluation of the mass balance of Pb in peridotites indicates that most mantle Pb is stored in silicate phases. The Pb partition coefficient between sulfide melt and silicate melt is estimated to be ∼3, based on the correlation of Pb–Os concentrations in this study and measured Os partition coefficients from the literature. These observations indicate that sulfides do not exert a strong control on the fractionation of Pb during mantle melting, but they can be used to constrain mantle Pb isotopic composition.Sulfides in this study, combined with literature data for Pb isotopes in peridotite whole rocks and pyroxene separates, provide evidence for ultra-depleted mantle, as 24% of peridotites are unradiogenic (i.e., 207Pb/204Pb <15.440 and 206Pb/204Pb <17.726), compared to only 3% of ridge basalts. This suggests that the mantle contains volumetrically significant reservoirs of ultra-depleted material, probably derived from recycled oceanic lithospheric mantle. These depleted reservoirs contribute in only small amounts to oceanic crust generation, both due to a limited ability to melt and to the dilution of any melt by more enriched melts during crust formation.

Transition from symmetry to asymmetry during continental rifting: an example from the Bight Basin–Terre Adélie (Australian and Antarctic conjugate margins)

Terra Nova, 24, 167–180, 2012AbstractThe rifting history of the magma‐poor conjugate margins of Australia and Antarctica is still a controversial issue. In this article, we present a model for lithosphere‐scale rifting and deformation history from initial Jurassic rifting to Late Cretaceous breakup for the conjugate Bight Basin–Terre Adélie section of the margin, based on the interpretation of two regional conjugate seismic profiles of the margins, and the construction of a lithosphere‐scale, balanced cross‐section, sequentially restored through time. The model scenario highlights the symmetric pattern of initial stretching resulting from pure shear at lithospheric‐scale accompanied by the development of four conjugate detachments and crustal half‐graben systems. This system progressively evolves to completely asymmetric shearing along a single south‐dipping detachment at the scale of the lithosphere. Antarctica plays the role of the upper plate and Australia, the lower plate. The detachment accounts for the exhumation of the mantle part of the Australian lithosphere, and the isolation of a crustal klippe separated from the margin by a serpentinized peridotite ridge. The total elongation amount of the Australian–Antarctic conjugate system reaches ∼473 km (178%). Elongation was partitioned through time: ∼189 and ∼284 km during symmetric and asymmetric stages respectively. During the symmetric stage, both margins underwent approximately the same degree of crustal stretching [∼105 km (75%) and ∼84 km (67%) for Australia and Antarctica respectively]. Again, both margins accommodated relatively the same elongation during the asymmetric stage: the Antarctic upper plate records an elongation amount of ∼284 km (88%) as crustal/mantle stretching, above the inferred low‐angle south‐dipping detachment zone, whereas the Australian lower plate underwent ∼270 km (206%) of elongation through mantle exhumation. Although the restoration process does not allow reconstruction of the precise geometry before deformation, we propose that the Jurassic early geometric evolution of the margins may have been controlled by the inherited structure or rheological heterogeneities of the continental crust; its later evolution is thought to relate to the mechanical evolution of the crustal and mantle material during exhumation, with a strong increase in localization of shear in the lower crust and mantle part of the Australian margin. The geometry of the rifted margins is comparable to other magma‐poor rifted margin such as the Newfoundland–Iberia margins or the exhumed Alpine Tethys margin exposed in the Central Alps.

Abyssal peridotite Hf isotopes identify extreme mantle depletion

The isotopic composition of radiogenic systems is a powerful tool to investigate Earth's evolution through time. Most of our understanding of the processes that affect the isotopic composition of the oceanic mantle comes from the study of basalts. Far fewer isotope analyses of actual oceanic mantle rocks (i.e. abyssal peridotites) exist, owing to their scarcity and often altered state. Here we present new Hf and Nd isotope analyses of clinopyroxene (cpx) from abyssal peridotites from the Gakkel Ridge in the Arctic Ocean, the Southwest Indian ridge and the South Atlantic. The Hf and Nd isotope ratios in the Indian Ocean and the South Atlantic peridotite cpx mostly overlap those of MORB, whereas coupled depletions in Nd and Hf isotope ratios in the Gakkel Ridge samples (ε Hf and ε Nd values of 60.4 and 20.5, respectively) extend the MORB and ocean island basalt (OIB) Hf–Nd isotope array to considerably more depleted values. Some samples from the Gakkel Ridge range to extreme Hf isotope values up to ε Hf of 104, but lack the corresponding depletion in Nd isotopes (ε Nd of about 8). The Hf, rather than the Nd isotope ratios of the Gakkel Ridge peridotites correlate with major and trace element indices of depletion (e.g. Al 2O 3 and Yb content, spinel Cr#) and their previously determined Os isotope ratios. Hence the Hf and Os isotope compositions of these samples preserve a record of ancient mantle depletion, whereas their Nd isotope signatures often do not. The example of the Gakkel Ridge peridotites suggests that the Sm/Nd and Nd isotope ratios in abyssal peridotites in general are very susceptible to resetting by melt–rock interaction, and that Nd isotope ratios in abyssal peridotites rarely preserve ancient mantle depletion to a similar extent as the Os and Hf isotope ratios. The oceanic mantle could thus range to more depleted Hf isotope signatures, and is thus isotopically more variable than inferred from oceanic basalts alone. MORB may therefore underestimate the average Hf and Nd isotope composition of the depleted mantle (DM), which may present an, as of yet, unrecognized obstacle for global mass balance models of Earth's differentiation. If the average DM is more depleted than MORB suggest, these models generally overestimate its mass fraction and would permit prolonged timescales for early Earth differentiation (> 30 Ma).

Nonchondritic 142Nd in suboceanic mantle peridotites

The discovery that several solid Earth reservoirs have a superchondritic 142Nd/144Nd ratio led to the hypothesis that either the bulk silicate Earth is not chondritic or that a subchondritic reservoir lies hidden somewhere within the Earth's interior. One important reservoir, i.e., mid‐ocean ridge peridotites representing the main component of the upper oceanic mantle and the source of mid‐ocean ridge basalt, has never been tested for 142Nd/144Nd. We determined the 142Nd/144Nd ratio in clinopyroxene separated from two peridotites and a pyroxenite from the SW Indian Ridge and one peridotite from the northern Mid‐Atlantic Ridge. All samples analyzed have superchondritic 142Nd/144Nd ratios in line with mantle‐derived material measured to date, except for some ancient cratonic rocks.

Tectonotype of nonvolcanic passive margins in the Iberia-Newfoundland region

The tectonotype of nonvolcanic passive margins is discussed on the basis of data on the conjugate margins of West Iberia and Newfoundland. Magmatic, structural, and historical aspects are considered. The Late Mesozoic structural elements related to rifting and transition to spreading are considered, as well as the Early Mesozoic sedimentary basins that begin the history of oceanic opening. The problem is set to determine the tectonic conditions of the early opening of the ocean in the framework of the chosen tectonoptype. These conditions are compared with the setting at the volcanic margins. The formation of the conjugate Iberia-Newfoundland margins is reconstructed as an asymmetric rift system developing in an almost amagmatic regime. All three segments of the margins on both sides of the ocean reveal similar features of transverse zoning with zones of the tectonized continental, transitional, and oceanic crust oriented nearly parallel to the margin. Special attention is called to the old age of the continental crust and subcontinental mantle and the absence of newly formed crystalline crust; the stadial tectonic and rheological evolution of the crust and lithospheric mantle; the specific features of the transitional zone; the serpentinization and exhumation of mantle peridotites and their role in the development of detachment at the crust-mantle interface, related listric faults and the Peridotite Ridge, attenuation of the medium, further localization of continental breakup, and the eventual development of asymmetric conjugate margins. Two papers characterizing the tectonotypes of volcanic and nonvolcanic passive margins ([2] and this paper) determine the line of further comparative analysis necessary for insights into the geodynamics of ocean opening.

Cooling-induced fractionation of mantle Li isotopes from the ultraslow-spreading Gakkel Ridge

Li isotopic compositions of magmatic rocks have gained considerable attention recently as probes of mantle-scale processes. However, the concentrations and isotopic composition of Li in mantle minerals from mid-ocean ridges remain relatively unconstrained. This is largely because of the general presence of seawater alteration in abyssal peridotites. Lithium elemental and isotopic compositions for mineral separates of coexisting olivine, clinopyroxene, orthopyroxene and bulk rocks of serpentine-free Gakkel Ridge peridotites were investigated. Bulk rocks have Li contents of 1.6 to 2.7ppm and δ7Li values of 3 to 5‰, which fall within the range of reported normal pristine “MORB mantle” values. Lithium concentrations vary in the order cpx (2.1–4.7ppm)>opx (0.9–1.7ppm)≥olivine (0.4–0.9ppm), the opposite found in “equilibrated” mantle peridotite xenoliths (Seitz and Woodland, 2000). The Li isotopic compositions indicate a systematic mineral variation with δ7Liolivine (7.14‰–15.09‰)>δ7Liopx (1.81‰–3.66‰)>δ7Licpx (−2.43‰−−0.39‰). The δ7Li values of cpx are negatively correlated with their Li concentrations with the lightest value for the most enriched cpx grains. There is a first order negative linear correlation between Δolivine–cpx (δ7Liolivine−δ7Licpx) and ol/cpxD (Liolivine/Licpx).Numerical simulations indicate that the observed systematic inter-mineral variations of Li concentrations and isotopic compositions could be explained by a cooling driven diffusive redistribution between minerals in a closed system if there is a temperature dependent partitioning of Li between olivine and clinopyroxene. The studied Gakkel Ridge abyssal peridotites may alternatively have cooled under a variable cooling rate with a rapid cooling before the Li system was closed, which is less likely given the tectonic setting. Our calculations confirm that Li systematics in minerals, especially in coexisting mineral phases could potentially be used as a mantle geospeedometer, even for slowly cooled mantle rocks.

Slope and fan deposition in deep-water turbidite systems, East Antarctica

Detailed seismic stratigraphic analysis of 2D reflection seismic data from the East Antarctic rifted continental margin provides an ancient analogue for turbidity fan systems at high-latitude margins. Reflector sequences characteristic of turbidity flow channel migration and aggradation are identified, along with other depositional elements integral to the understanding of deep-water deposition, such as channel levees, splays and wavy bedforms. Wavy bedforms are contrasted with slump-related features, which although present do not explain the morphological features interpreted here as sediment waves. Slump-related features suggest, qualitatively, a relationship between a basal detachment layer and a widespread bottom-simulating reflector. The influence of the initial morphology on slope deposition and distributary patterns for turbidite deposits is clear in these data. Healed-slope accommodation on an above-grade slope is evident in seismic profiles. Ponded and/or healed-slope accommodation is associated with pre-existing bathymetric highs, interpreted as rift-related, serpentinised, peridotite ridges.

Structure and rifting evolution of the northern Newfoundland Basin from Erable multichannel seismic reflection profiles across the southeastern margin of Flemish Cap

We present the results from processing and interpreting five lines from the 1992 Erable multichannel seismic reflection experiment extending from the southeastern margin of Flemish Cap into the northern Newfoundland Basin. These profiles reveal significant along strike variations in the rifting styles experienced by Flemish Cap. In the southwest, a 100-km-wide transition zone is identified between thinned continental crust and thin oceanic crust. Similar to the conjugate Galicia Bank and Iberian margins, this transition zone contains a section of deep basement adjacent to a series of shallower ridges and is interpreted as exhumed serpentinized mantle. Along strike towards the northeast, this transition zone pinches out completely within 100 km and is replaced by thin oceanic crust directly adjacent to thinned continental crust. By interpreting nearby seismic profiles and profiles on the conjugate margins using the same classification criteria, we construct regional maps of the distribution of crustal domains on both sides of the North Atlantic. These maps reveal significant variations in rifting style on the conjugate margins and along strike of each margin and also highlight the role of ancient transfer zones in compartmentalizing these rifting variations into four distinct regions. We propose that the limited localization of shallow topographically high serpentinized peridotite ridges on the Newfoundland-Iberia and Flemish Cap-Galicia Bank conjugate margins, was directly related to an increase in the rate of extension following the separation of Flemish Cap and Galicia Bank which exhumed deeper, less serpentinized mantle.

Composition and Genesis of Depleted Mantle Peridotites from the Wadi Tayin Massif, Oman Ophiolite; Major and Trace Element Geochemistry, and Os Isotope and PGE Systematics

The Oman ophiolite consists of several massifs cropping out along a 500 km long band trending NW-SE along the coast of Oman; it is one of the best exposed sections of oceanic crust and mantle in the world. There is a gradient in igneous processes and composition in the ophiolite, with the northern massifs recording a polygenetic igneous history involving an increasingly important subduction component, whereas the southern massifs were formed primarily via a mid-ocean ridge basalt (MORB)-like, single-stage process at a submarine spreading ridge. In this study we use geochemical data from Wadi Tayin, which is one of the southern massifs of the Oman ophiolite, to constrain the composition and genesis of oceanic crust and upper mantle. The Wadi Tayin harzburgites are residues of partial melting that are as depleted as the most depleted mid-ocean ridge peridotites. They have low middle- to heavy rare earth element ratios, most probably reflecting melting close to and beyond the exhaustion of clinopyroxene. Like many abyssal peridotites, the Wadi Tayin samples show enrichment in highly incompatible elements, giving rise to U-shaped MORB-normalized trace element patterns. We favor the idea that this is caused by enrichment of highly incompatible elements along grain boundaries in peridotite, which may be the result of near-equilibrium partitioning between grain boundaries and grain interiors, rather than disequilibrium processes. Equilibrium partitioning between crystals and grain boundaries during melting and melt extraction is also our preferred explanation for ubiquitous high Pb contents relative to Ce and La in our samples as well as in most abyssal peridotites. The Wadi Tayin samples record substantial variability in terms of osmium isotopic composition and platinum group element (PGE) concentrations. The more radiogenic nature of the dunites and impregnated peridotites compared with the residual harzburgites may be due to relatively high Os-187/Os-188 in melt transported through the dunites. The presence of residual peridotites with whole-rock osmium isotopic compositions less radiogenic than MORB may be explained by melting of a veined (upwelling) mantle forming mixed melts in which much of the Os derives from veins with high Re/Os in a matrix of previously depleted peridotite. An important result of this study is that the shallowest samples are refertilized (i.e. anomalously enriched in incompatible elements), and record relatively high metamorphic closure temperatures. These observations suggest that migrating melt underwent crystallization during rapid cooling in the uppermost mantle during and immediately after ridge magmatism. A variety of geothermometers all yield the result that the stratigraphically highest harzburgites equilibrated at higher temperature than the deeper ones. The systematic decrease in closure temperature with increasing depth below the Moho transition zone probably reflects systematic variation in cooling rate as a function of depth in the mantle section. We hypothesize that the refertilization and higher closure temperatures recorded by the uppermost mantle samples are linked. More rapid cooling led to higher closure temperatures, and to partial crystallization of migrating melts in the shallowest part of the mantle section, yielding slightly elevated abundances of elements such as Ca and Na, and incompatible trace elements.

Taxa de soerguimento atual do arquipélago de São Pedro e São Paulo, Oceano Atlântico Equatorial

O arquipélago de São Pedro e São Paulo situa-se no Oceano Atlântico Equatorial, no topo de uma elevação morfológica submarina de 90 km de comprimento, 25 km de largura e 3.800 m de altura, composta de rochas peridotíticas do manto, denominada Cadeia Peridotítica de São Pedro e São Paulo. A morfologia submarina é altamente acidentada com escarpas subverticais de centenas de metros de altura, sugerindo ocorrência de tectonismo recente, provavelmente ativo. O mapa de seppômen do arquipélago mostra duas plataformas de abrasão marinha, com altitude respectiva de 4~5 m e de 7~9 m. As plataformas são observadas nas três ilhas maiores nas mesmas faixas de altitude, indicando que não houve basculamento ou soerguimento diferencial entre as ilhas. Considerando a plataforma superior formada durante a Transgressão Flandriana e a plataforma inferior desenvolvida por erosão recente, a taxa média de soerguimento nos últimos 6 mil anos é calculada como 1.2~1.5 mm/ano. As datações 14C para os fósseis coralígenos da ilha Belmonte indicam a taxa de soerguimento nos últimos 6.600 anos como 1.5~1.8 mm/ano. Portanto uma taxa de 1.5 mm/ano é razoável para o soerguimento atual. Interpretando a cadeia peridotítica como um megamullion tectonicamente deformado, o soerguimento total seria 1500~3000 m e o início do tectonismo seria 1~2 Ma. Se a cadeia peridotítica é originada do manto oceânico subcrustal, o soerguimento total seria, aproximadamente, 9000 m e o início do tectonismo seria cerca de 6 Ma.

Geochemical constraints on the provenance and depositional setting of sedimentary rocks from the islands of Chios, Inousses and Psara, Aegean Sea, Greece: implications for the evolution of Palaeotethys

The provenance and depositional setting of Late Palaeozoic and Early Mesozoic clastic sediments from the eastern Aegean archipelago are examined here for the first time using whole-rock geochemistry and composition of detrital chrome spinel. Major- and trace-element data for Late Palaeozoic and Permo-Triassic clastic sediments from the Lower and Upper Units of Chios are compatible with an acidic to intermediate source, minor input of (ultra)mafic detritus and recycling of older sedimentary components. Chondrite-normalized REE profiles are uniform with light REE enrichments (La N /Yb N c . 7.7), negative Eu anomalies (Eu/Eu* c . 0.67) and flat heavy REE patterns (Gd N /Yb N c . 1.5), indicating an upper continental crustal source and/or young differentiated arc material. Detrital chrome spinel from the clastic sediments of Chios has Cr-number (Cr/(Cr + Al)) values between 0.29 and 0.89 and Mg-number (Mg/(Mg + Fe 2+ )) values between 0.24 and 0.70, suggesting a probably mixed (ultra)mafic source involving ridge peridotites (mid-ocean ridge type), fore-arc peridotites and island-arc basalts. The metasediments from the islands of Inousses and Psara have similar whole-rock chemical signatures to those of Chios, although no evidence was found for an (ultra)mafic source. We conclude that both the Late Palaeozoic sediments from the Lower and Upper Units of Chios and the metasediments from Inousses and Psara were deposited in a continental island-arc setting until at least Late Permian times, probably at a single Palaeotethyan margin. They are interpreted to be allochthonous, tectonically transported to their present position by Late Mesozoic to Cenozoic orogenic processes.

A deep seismic investigation of the Flemish Cap margin: implications for the origin of deep reflectivity and evidence for asymmetric break-up between Newfoundland and Iberia

SUMMARY Seismic reflection and refraction data were acquired along the southeast margin of Flemish Cap at a position conjugate to drilling and geophysical surveys across the Galicia Bank margin. The data document first-order asymmetry during final break-up between Newfoundland and Iberia. An abrupt necking profile of continental crust observed off Flemish Cap contrasts strongly with gradual tapering on the conjugate margin. There is no evidence beneath Flemish Cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong ‘S’-like reflection, which indicates that this mode of extension occurred only on the Galicia Bank margin. Compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the Flemish Cap margin. Instead, anomalously thin, 3‐4-km-thick oceanic crust is observed. This crust is highly tectonized and broken up by high-angle normal faulting. The thin crust and rift structures that resemble the abandoned spreading centre in the Labrador sea suggest that initial seafloor spreading was affected by processes observed in present-day ultra-slow spreading environments. Landwards, Flemish Cap is underlain by a highly reflective lower crust. The reflectivity most likely originates from older Palaeozoic orogenic structures that are unrelated to extension and break-up tectonics.

Exhumed upper mantle within the ocean‐continent transition on the northern West Iberia margin: Evidence from prestack depth migration and total tectonic subsidence analyses

The West Iberia continental margin contains an enigmatic north‐south ridge of serpentinized peridotite located within the ocean‐continent transition. We used prestack depth migration and total tectonic subsidence analysis to constrain the basement type adjacent to the ridge in order to learn more about its emplacement. Prestack depth migration was performed on segments of the nine east‐west lines from the Iberia Seismic Experiment (ISE 97), and total tectonic subsidence analyses were performed on the depth sections. The seismic data and subsidence analyses are consistent with exhumed upper mantle occurring immediately landward of the ridge, suggesting that the ridge is located within a broad zone of exhumed mantle that has been serpentinized. The subsidence analyses also provide minimum bounds on the thickness of the serpentinized layer, which extends from the zone of exhumed mantle landward under thinned continental crust. Exhumed upper mantle has previously been identified in the wider ocean‐continent transition to the south in the southern Iberia Abyssal Plain, and we conclude that similar but narrower exposures occur to the north in the Galicia Bank area. While upper mantle appears to have been tectonically exhumed along all the ISE 97 lines, it does not always form a distinct basement ridge. There is no well‐defined ridge on three seismic profiles in the middle of the ISE 97 survey. Where it is well‐developed, the peridotite ridge parallels a deeply‐penetrating, west‐dipping normal fault, and the three middle lines appear to represent a transfer zone where this normal fault is not well‐developed.

Late Paleozoic and Triassic plume-derived magmas in the Canadian Cordillera played a key role in continental crust growth

Two major Late Paleozoic–Triassic oceanic terranes are exposed in the North America Cordillera. Slide Mountain Terrane is made up of dolerites, pillow basalts associated with cumulate gabbros and peridotites ranging in age from Carboniferous to Permian. Cache Creek Terrane consists of tectonic slices of Paleozoic platform carbonates, undated cumulate gabbros intruded by dolerites, foliated ultramafic rocks and mafic volcanic rocks interbedded within Upper Triassic siliceous and volcaniclastic sediments. Among the Permian (Slide Mountain) and Upper Triassic (Cache Creek) volcanic rocks, four types have been distinguished. Type 1 is geochemically similar to N-MORB. Type 2 is mildly to highly enriched in LREE, Ti, Zr, Hf, Nb, Ta and Th, and differs from Type 1 by lower εNd and higher Pb isotopic ratios. It displays alkaline affinities. Types 3 and 4 occur only in the Cache Creek Terrane. Type 3 has flat REE patterns similar to oceanic plateau basalts and isotope compositions intermediate between Types 1 and 2. Type 4 is distinguished from Type 3 by convex REE patterns and higher Pb isotopic ratios. Cumulate peridotites and gabbros are LREE-depleted and their high εNd suggest that they derived from the melting of a depleted MORB-type mantle source. The foliated ultramafic rocks are either serpentinized harzburgites (Trembleur ultramafics) or dunites, harburgites and minor lherzolites (Murray Ridge), which are intruded by undeformed pyroxenite veins. The ultramafic rocks have very low contents of rare earth elements (REE) and incompatible elements, and U-shaped REE patterns. The pyroxene-bearing peridotites are distinguished from the Trembleur harzburgites by an absence of Nb negative anomalies and lower ( 87Sr/ 86Sr) i ratios. Slide Mountain and Cache Creek Terranes formed in different tectonic settings. Slide Mountain basin, fringing the North American margin, was probably floored by oceanic crust locally thickened by oceanic island magmas. Indeed, in the Slide Mountain Terrane, N-MORB type basalts predominate and are associated with minor alkaline volcanic rocks within the same thrust sheet. Correlations between incompatible elements and values of the εNd of the Slide Mountain rocks suggest that the N-MORB-type basalts and alkaline rocks are genetically linked. N-MORB type basalts and alkaline volcanic rocks could derive from the mixing of depleted MORB-type and enriched Oceanic island Basalt-type mantle sources. The Cache Creek Terrane may represent remnants of a Late Triassic oceanic plateau because plume-related volcanic rocks predominate while N-MORB type rocks are represented solely by cumulate gabbros and sheeted dykes. The Murray Ridge peridotites could represent the roots of the oceanic plateau. We conclude that the Slide Mountain and Cache Creek Terranes contributed to the growth of the North American continent as when they were accreted to the craton during the Early Triassic and Jurassic times.

Magmatism at the west Iberia non-volcanic rifted continental margin: evidence from analyses of magnetic anomalies

We discuss the magmatic development of the west Iberia non-volcanic rifted continental margin in the North Atlantic Ocean. So-called ‘non-volcanic’ rifted continental margins are characterized by a lack of syn-rift magmatism and are considered to be largely amagmatic. However, this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously. We concentrate our attention on the recently described zone of exhumed continental mantle (ZECM) to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the ZECM leading to the onset of seafloor spreading. We use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide-angle seismic experiments presented elsewhere. Forward and inverse modelling of a sea-surface magnetic anomaly chart and of surface and deep-towed magnetometer profiles shows that anomalies within the ZECM differ in trend, amplitude and source type from those in the adjacent oceanic crust and thinned continental crust. The ZECM anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin. We interpret such bodies as syn-extensional intrusions that increase in volume oceanward. They eventually merge in the vicinity of a margin-parallel, basement peridotite ridge to give rise to a continuous crust that records reversals in the Earth's magnetic field from the time of anomaly M4(N)–M5(R), i.e. to mark the onset of seafloor spreading. We find no evidence for anomalies formed by seafloor spreading, at either slow or ultraslow rates, before M5(R) (128 Ma).

Deep structure of the ocean‐continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: The IAM‐9 transect at 40°20′N

We present a crust and mantle velocity structure for the West Iberia passive continental margin derived from a 320‐km‐long wide‐angle seismic profile acquired in the southern Iberia Abyssal Plain. We observe a 170‐km‐wide ocean‐continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault‐bounded blocks of continental crust. The profile lies ∼40 km south of the transect sampled by Ocean Drilling Program (ODP) Legs 149 and 173. The transition zone structure can be divided into an upper layer, 2–4 km thick with velocities of between 4.5 and 7.0 km s−1 and generally a high‐velocity‐gradient (1 s−1), and a lower layer up to 4 km thick with a velocity of ∼7.6 km s−1 and a low‐velocity‐gradient. A weak Moho reflection in this zone was seen only on wide‐angle profiles at an offset of ∼30 km. The upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope. Conversely, the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust. On the coincident seismic reflection profile, fault‐bounded crustal blocks, identified in unequivocal extended continental crust, are not observed in the transition zone. The upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges, but no oceanic layer 3 velocity structure is present. While magnetic anomalies have been identified within the transition zone, they have not been modeled successfully as seafloor spreading magnetic anomalies, nor do they generally form long linear margin‐parallel features. Finally, ODP boreholes, ∼40 km north of our profile and within the interpreted transition zone, have recovered up to 140‐m‐thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material. We conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust. Although current melting models predict a considerably thicker crust of decompression melt products, we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite.

Gabbroic intrusions and magmatic metasomatism in harzburgites from the Garrett transform fault: implications for the nature of the mantle–crust transition at fast-spreading ridges

Mafic and ultramafic rocks sampled in the Garrett transform fault at 13°28′S on the East Pacific Rise (EPR) provide insight on magmatic processes occurring under a fast-spreading ridge system. Serpentinized harzburgite from Garrett have modal, mineral and bulk chemical compositions consistent with being mantle residue of a high degree of partial melting. Along with other EPR localities (Terevaka transform fault and Hess Deep), these harzburgites are among the most residual and depleted in magmatophile elements of the entire mid-ocean ridge system. Geothermometric calculations using olivine-spinel pairs indicate a mean temperature of 759 ± 25 °C for Garrett residual harzburgite similar to the average of 755 °C for tectonite peridotites from slow-spreading ridges. Results of this study show that mid-ocean ridge peridotites are subject to both fractional melting and metasomatic processes. Evidence for mantle metasomatism is ubiquitous in harzburgite and is likely widespread in the entire Garrett peridotite massif. Magma-harzburgite interactions are very well preserved as pyroxenite lenses, plagioclase dunite pockets or dunitic wall rock to intrusive gabbros. Abundant gabbroic rocks are found as intrusive pockets and dikes in harzburgite and have been injected in the following sequence: olivine-gabbro, gabbro, gabbronorite, and ferrogabbro. The wide variety of magmas that crystallized into gabbros contrast sharply with present-day intratransform basalts, which have a highly primitive composition. Ferrogabbro dikes have been intruded at the ridge-transform intersection and as they represent the last event of a succession of gabbros intrusive into the peridotite, they likely constrain the origin of the entire peridotite massif to the same location. In peridotite massifs from Pacific transform faults (Garrett and Terevaka), primitive to fractionated basaltic magmas have flowed and crystallized variable amounts of dunite (±plagioclase) and minor pyroxenite, followed by a succession of cumulate gabbroic dikes which have extensively intruded and modified the host harzburgitic rocks. The lithosphere and style of magmatic activity within a fast-slipping transform fault (outcrops of ultramafic massif, discontinuous gabbro pockets intrusive in peridotite, magnesian and phyric basalts) are more analogous to slow-spreading Mid-Atlantic Ridge type than the East Pacific Rise.

Deep structure of the ocean‐continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: Ocean Drilling Program (Legs 149 and 173) transect

We present a wide‐angle seismic refraction study of an 80×40 km region of the southern Iberia Abyssal Plain, south of Galicia Bank. An intersecting grid of two E‐W and four N‐S wide‐angle reflection/refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean‐continent transition (OCT). These structures can be systematically linked to variations in acoustic basement morphology and to results from Ocean Drilling Program (ODP) boreholes. Lateral changes in the velocity structure of the basement occur abruptly over distances of ∼20 km where complex variations may be found. Thinned upper continental crust, 2–5 km thick with velocities of 5.0–6.6 km/s, is limited to a series of N‐S fault blocks immediately south of Galicia Bank. This crust is underlain by a high‐velocity layer (7.3–7.9 km/s) of weakly serpentinized (i.e., 0–25%) peridotite, which exists throughout the eastern part of the survey area. Basement within the OCT appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally. In the southeast sector of our survey where basement topography deepens and becomes subdued, continental fault blocks are absent; instead, basement contains an upper layer of more pervasively serpentinized (i.e., 25–45%) peridotite that is ∼2 km thick. This layer is characterized by low velocity at the top of basement (4.2 km/s) that increases rapidly with depth, and it probably corresponds to a seismically unreflective layer, previously identified in reflection profiles to the south of our survey. In the western section of our survey, beneath a series of elevated basement ridges, velocities are reduced within both the upper basement layer (3.5–6.0 km/s) and lower layer (6.4–7.5 km/s). These changes suggest that both upper and lower layers have become more highly serpentinized (with values of 60–100% in the upper layer and 25–45% in the lower layer) probably during the last stages of rifting and immediately before formation of oceanic crust. A normal or slow spreading oceanic crustal structure is not found within the survey region. Thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at ODP Site 897 and east of the J magnetic anomaly.

SPATIAL AND TEMPORAL HETEROGENEITY OF THE OCEANIC MANTLE IN THE CENTRAL ATLANTIC

Studies of Mid Ocean Ridge Peridotites (MORP) sampled from the Kane (24°N) to the Chain (2°S) transforms in the Central Atlantic reveal upper mantle domains with different composition and subjected to different thermal conditions and degrees of melting along this stretch of mid ocean ridge. These different domains include thermal minima (“cold spots”), such as observed in the equatorial area, and thermal maxima (mini-“hot spots”). Zero-age topography, gravimetry and seismic tomography support the concept of along axis mantle thermal and/or compositional heterogeneity. Close spaced sampling profiles of mantle-derived peridotites were obtained along seafloor spreading flow lines at two Latitude (0° and 11°N). These profiles indicate significant temporal variations in the composition and thermal structure of the oceanic mantle, and of processes of lithosphere creation at ridge axis.

PHLOGOPITE AND HORNBLENDE IN SPINEL HARZBURGITES FROM THE MID-ATLANTIC RIDGE: MINERAL ASSEMBLAGES AND ORIGIN

Phlogopite in mid-oceanic ridge peridotites was detected mainly as inclusions in spinel and rarely as discrete grains from ultramafic cumulates (Arai et al., 1997). Phlogopite and biotite were also found in oceanic gabbro (Cannat et al., 1992; Cannat and Casey, 1995; Silantyev, 1998). In most cases, hornblende is associated with phlogopite. Origin of these minerals in oceanic peridotites and gabbro is now under discussion. In our investigations, phlogopites were detected in spinel harzburgites from a number of localities within the Mid-Atlantic Ridge - at 15oN and 35oN areas. The compositional variety of phlogopites is maximal in harzburgites containing thin gabbroic veins from dredge 68 (16-th cruise of r/v “Akademic Boris Petrov”) located in the west slope of the rift valley at 14o49’N. Since associated minerals are often preserved in these rocks, it is possible to address the problem of the origin of this association. In gabbro veins and of their contact with peridotite, phlogopite forms tables with clear pleochroism. Phlogopite is characterized by high TiO2 (1.5-3.0 wt%), low Mg# (65-85) and low Cr2O3 (0.0-0.65 wt%). Rare small grains of high-Ti spinel (1.1 wt% TiO2) are present at its rims. In peridotites, phlogopite occurs as small (< 0.1 mm) plates in narrow shear zones near the contact with gabbros. Phlogopite is usually colorless or with hardly recognizable pleochroism; it is Ti-rich (2.0-2.3 wt% TiO2), relatively Crpoor (0.1-0.7 wt% Cr2O3), and Mg-rich (Mg# 93.2-93.7). Orthopyroxene and hornblende are associated with phlogopite, both having elevated TiO2 contents (1.3-2.1 and 0.14- 0.23 wt%, respectively). Ti-poor phlogopite (0.3-0.6 wt% TiO2) has been found in a similar textural position. Ti-poor phlogopite has the same ranges of Cr2O3 contents and Mg#, and it is associated with Ti-poor hornblende and orthopyroxene (TiO2 = 0.4-0.7 and 0.02-0.05 wt%, respectively), and sometimes with plagioclase. Ti-rich spinel (TiO2 up to 0.37 wt%) seems to take part in these associations. Phlogopite has also been found in harzburgites that do not show relations with gabbros. Phlogopite occurs either as discrete grains (0.1-0.4 mm) along shear zones, or single grains in olivine, or in contact with spinel, or as plates replacing orthopyroxene along cleavage. It shows low TiO2 (0.04-0.4 wt%), high Mg# (90.1-94.7) and high Cr2O3 (0.65-1.6 wt%) content. No silicates associated with phlogopite are present, and associated spinels are enriched in iron and sometimes in titanium (up to 0.27 wt% TiO2). The orthopyroxenes associated with phlogopite and hornblende are commonly characterized by low Al2O3, Cr2O3 and CaO (1.0-1.8 wt%, 0.18-0.70 wt% and 0.35-0.80 wt%, respectively). Hornblende has high K2O (0.22-0.50 wt%) and is represented by edenitic and pargasitic hornblendes according to Leake’s nomenclature. In harzburgites, clinopyroxene usually is not in contact with phlogopite and rarely with hornblende. According to Wells’s geothermometer, the crystallization of the phlogopite-hornblende mineral association took place at 870-1000oC. Distribution of Ti between coexisting phases is consistent with evaluated coefficients (Ionov et al., 1997) and indicates TiO2 liquid contents in the range of 0.56-1.40 wt% for the Ti-rich mineral association. Therefore, its crystallization possibly took place from fractionated melts. Origin of the low-Ti association is disputable. On the basis of mineral compositions, the low-Ti association is inferred to have crystallized at the same temperature range. The two minerals associations can be detected at one millimeter scale. The preferred hypothesis is crystallization of the low-Ti mineral association from a fluid phase, which was exsolved from the liquid. Alternatively, crystallization derived from extremely low-Ti melts (below 0.24 wt% TiO2). However, some phlogopites show TiO2 contents as low as 0.10 wt%, which lead to unrealistically low calculated TiO2 contents in the liquid (0.04 wt%). Crystallization of the phlogopite-hornblende mineral association in spinel harzburgites from Capo Verde FZ area took place after solidification of peridotites, mainly along shear zones, which was a place for segregation of residual differentiated melts and related fluids. In other places of Mid-Ocean Ridges, a shearing event under the same temperature conditions is associated with the development of plagioclase-bearing mineral associations, where only in rare cases edenitic-pargasitic hornblende occurs (Cannat and Seyler, 1995). This difference is related to the compositions of fractionated melts that interacted with the mantle peridotites, so that generation of phlogopite is restricted to areas of geochemical anomalies along Mid-Atlantic Ridge. Some gap between two areas of water silicate generation in mantle peridotites under MOR seems to exist. In a first one, outlined above, the source of water should be of juvenile origin. The second area is connected with deep penetration of seawater into the lithosphere. In the latter case, the temperature hardly exceeds 800-820oC, phlogopite does not crystallize, hornblende is represented mostly by tremolite, rarely by edenite or edenitic hornblende, and never by pargasitic hornblende. Recrystallized orthopyroxene in this process is characterized by extra-low calcium contents (CaO = 0.15-0.20 wt%). This work was supported by the Russian Foundation for Basic Research grant 97-05-64842.