Oceanic Gabbros Research Articles

Hydrous partial melting within the lower oceanic crust

AbstractWe studied more than 60 oceanic gabbros from the recent oceanic crust and from ophiolites (East Pacific Rise, Mid‐Atlantic Ridge, Southwest Indian Ridge, Oman ophiolite) by scanning electron microscopy and found in nearly all samples microstructures suggesting that hydrous partial melting reactions proceeded. The characteristic paragenesis consists of orthopyroxene and pargasite rimming olivine and clinopyroxene primocrysts in intimate contact with neoblastic plagioclase strongly enriched in anorthite. This is in agreement with recent water‐saturated melting experiments on a variety of natural gabbros between 900 and 1000 °C. The observed microtextures in the natural gabbros imply the propagation of water‐rich fluids on grain boundaries in a ductile regime causing hydrous partial melting. Thus, this type of hydrothermal activity proceeds within the deep oceanic crust at very high temperatures (900–1000 °C) without a crack system, a prerequisite in current models for enabling hydrothermal circulation.

Late stage magmatic evolution of oceanic gabbros as a result of hydrous partial melting: Evidence from the Ocean Drilling Program (ODP) Leg 153 drilling at the Mid‐Atlantic Ridge

Petrographic observations within oceanic gabbros drilled by Ocean Drilling Program (ODP) at Leg 153 from the Mid‐Atlantic Ridge, Kane Fracture Zone (MARK) area (23°N) in combination with experimental findings suggest that the late stage magmatic evolution of these gabbros is characterized by pervasive hydrous partial melting. Water‐saturated melting experiments on a variety of natural gabbros between 900 and 1000°C at 200 MPa produced newly formed minerals forming a characteristic paragenesis consisting of plagioclase, orthopyroxene, and pargasitic amphibole ± clinopyroxene. The An content of the new plagioclases is uniformly higher than that of the protolith. Olivine and clinopyroxene primocrysts react to form neoblastic orthopyroxene and pargasitic amphibole. These features can also be observed in the gabbros from the MARK area. Here we found zones at plagioclase grain boundaries showing a strong enrichment in An component, with An contents up to 20 to 25 mol % higher than those of the host plagioclase. Primary olivines and clinopyroxenes in contact with such zones react to orthopyroxene and pargasitic amphibole. These phases rim olivine and clinopyroxene and grow “interstitially,” typical petrographic characteristics of a late stage magmatic phase. The observed late stage microstructures in the gabbros thus appear to be the results of partial melting processes triggered by water‐rich fluids and are not crystallization products of a percolating differentiated late melt. Here we demonstrate that hydrothermal circulation within the gabbroic layer starts at much higher temperatures (900°–1000°C) than up to now believed. Water‐rich fluids propagate on grain boundaries in a ductile regime, causing hydrous partial melting on a large scale. A cracks system, a prerequisite in current models for enabling hydrothermal circulation, is not necessary. The observed process has the potential for transfer of heat and mass between the upper and lower oceanic crust. Provided that the water‐rich fluids triggering the partial melting process are seawater derived, this process may have a significant influence on the cooling of the deep oceanic crust which is in concordance with new thermal models implying that high‐temperature hydrothermal circulation is regarded to play an important role in transport of heat in the deep oceanic crust.

Influence of pressure and mineralogy on seismic velocities in oceanic gabbros: Implications for the composition and state of the lower oceanic crust

We have analyzed the relationship between P wave velocities, measured at pressures of 40, 100, and 200 MPa, and modal mineralogy in oceanic gabbro samples from Ocean Drilling Program Holes 735B, 894G, and 923A. At all pressures, increasing velocities correspond to increasing pyroxene contents and decreasing alteration (phyllosilicate and amphibole content) but indicate little or no variation of olivine content, in part because the olivine compositions are in the range Fo65 to Fo73. A Voigt‐Reuss‐Hill inverse model reveals that the effective bulk density and elastic moduli of olivine in the gabbros are low, even relative to Fo73, possibly because the olivine grains contain ubiquitous networks of cracks. Even if the cracks are not present in situ, on average, gabbros with velocities typical of seismic layer 3 (6.7–7.0 km s−1) contain 5–15% alteration products, including 5–15% amphibole and 0–5% phyllosilicates. These results suggest that the lower crust is, on average, slightly to moderately altered.

Trace element geochemistry of coesite-bearing eclogites from the Roberts Victor kimberlite, Kaapvaal craton

Trace element characteristics of seven coesite-bearing eclogitic xenoliths from the Roberts Victor kimberlite demonstrate that this suite of eclogites originated as gabbroic cumulates in oceanic crust that was subsequently subducted. All but one of the garnets show positive Eu anomalies, accompanied by a flat heavy rare earth pattern, which is atypical of garnet, but characteristic of plagioclase, arguing for a considerable amount of plagioclase in the protoliths. Forward modelling of the accumulation of liquidus minerals from primitive komatiitic, picritic, and basaltic liquids suggests that at least some of the eclogite protoliths were not derived from basaltic parental liquids, whereas derivation from either komatiitic or picritic liquids is possible. The reconstructed eclogite bulk rocks compare favourably with oceanic gabbros from ODP hole 735B (SW Indian Ridge), even to the extent that oxygen isotopic systematics show signs of low-temperature seawater alteration. However, the oxygen isotope trends are the reverse of what is expected for cumulates in the lower section of the oceanic crust. These new findings show that δ 18O values in eclogitic xenoliths, despite being sound indicators for their interaction with hydrothermal fluids at low pressure, do not necessarily bear a simple relationship with the inferred oceanic crustal stratigraphy of the protoliths.

Bound water content of the lower oceanic crust estimated from modal analyses and seismic velocities of oceanic diabase and gabbro

Hydrous minerals in the oceanic crust are known to carry bound water into subduction zones, but the water content of the lower oceanic crust is poorly constrained. Previous studies suggest that lower crustal diabases and gabbros contain 1–3 and 1–2 wt% H2O. This analysis, based on the modal mineralogy and seismic properties of oceanic diabase and gabbro samples, shows that the H2O content of diabase dike rocks ranges from 1–3%, with an average of 1.5%; in oceanic gabbros, the H2O content decreases with increasing seismic velocity, and gabbros that have velocities typical of the lower oceanic crust (6.7–7.0 km s−1) contain 0.2 to 0.7 wt% H2O, with a mean near 0.5 wt%.

Petrogenesis of oceanic plagiogranites by partial melting of gabbros: an experimental study

We performed hydrous partial melting experiments at shallow pressures (0.2 GPa) under slightly oxidizing conditions (NNO oxygen buffer) on oceanic cumulate gabbros drilled by ODP (Ocean Drilling Program) cruises to evaluate whether the partial melting of oceanic gabbro can generate SiO2-rich melts with compositions typical of oceanic plagiogranites. The experimental melts of the low-temperature runs broadly overlap those of natural plagiogranites. At 940 °C, the normalized SiO2 contents of the experimental melts of all systems range between 60 and 61 wt%, and at 900 °C between 63 and 68 wt%. These liquids are characterized by low TiO2 and FeOtot contents, similar to those of natural plagiogranites from the plutonic section of the oceanic crust, but in contrast to Fe and Ti-rich low-temperature experimental melts obtained in MORB systems at ~950 °C. The ~1,500-m-long drilled gabbroic section of ODP Hole 735B (Legs 118 and 176) at the Southwest Indian Ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high-temperature shearing. The compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins, while the compositions of the crystals of low-temperature runs correspond to those of minerals from high-temperature microscopic veins occurring in the gabbroic section of the Hole 735B. This suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water-rich fluid phase. If the temperature estimations for high-temperature shear zones are correct (up to 1,000 °C), and a water-rich fluid phase is present, the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust.

Hydrothermal alteration vs. ocean-floor metamorphism. A comparison between two case histories: the TAG hydrothermal mound (Mid-Atlantic Ridge) vs. DSDP/ODP Hole 504B (Equatorial East Pacific)

This article presents a review of hydrothermal alteration of the various basaltic rocks forming the oceanic crust, emphasizing especially on the water–rock interaction processes, the petrography and secondary mineralogy of hydrothermally altered rocks from the present-day ocean, hence excluding the ophiolitic complexes. A brief summary of the history of the first studies of hydrothermally altered oceanic rocks leads to a comparison between Miyashiro's concept of ‘ocean-floor metamorphism’ and that of hydrothermal alteration that warrant caution when applying Eskola's metamorphic facies of regional metamorphism to hydrothermally altered oceanic rocks. The functioning of mid-oceanic ridge axial hydrothermal systems, and the role of oceanic Layer-3 gabbros are discussed in detail. The mechanisms of the various alteration processes of the rocks forming the oceanic crust are presented. The case histories of the two examples more particularly studied by the author and his collaborators, are compared, i.e., the DSDP-ODP Hole 504B reference section South of the Costa Rica Ridge, and the TAG active mound at the Mid-Atlantic Ridge. The significance of the paragonitic phyllosilicate in highly altered rocks form the TAG Mound is compared to the other known occurrences of ‘white micas’ in the oceanic crust. The importance of metabasites as major components of the oceanic crust is emphasized. The elemental fluxes resulting from hydrothermal alteration of oceanic rocks in regulating seawater chemistry are only briefly alluded, because this topic is covered by the last article of this thematic issue by J.C. Alt, who assesses the chemical budgets resulting from ocean hydrothermal activity. To cite this article: J. Honnorez, C. R. Geoscience 335 (2003).

Microstructural and experimental constraints on the rheology of partially molten gabbro beneath oceanic spreading centers

Flow laws for high-temperature creep of olivine, plagioclase, and diabase are used to place constraints on the rheology of partially molten lower oceanic crust. This analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene. Extrapolation of experimental flow laws indicates that at temperatures above 1100°C and stresses less than 10 MPa, olivine may be the weakest phase in rocks with gabbroic composition. By accounting for variations in the melt fraction (0–10%) and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep. Calculated effective viscosities range from 10 15 to 10 19 Pa s; based on observations of the geometry of the partially molten zone beneath the East Pacific Rise and the microstructural and experimental constraints we favor a value of ∼10 18 Pa s. This value approaches estimates for the viscosity of the upper mantle beneath ridge axes, but is significantly higher than previously suggested for the partially molten lower crust. Such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution. However, the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2D, ‘sheet-like’ fashion.

Kistufell: Primitive Melt from the Iceland Mantle Plume

This paper presents new geochemical data from Kistufell (64°48′N, 17°13′W), a monogenetic table mountain situated directly above the inferred locus of the Iceland mantle plume. Kistufell is composed of the most primitive olivine tholeiitic glasses found in central Iceland (MgO 10·56 wt %, olivine Fo89·7). The glasses are interpreted as near-primary, high-degree plume melts derived from a heterogeneous mantle source. Mineral, glass and bulk-rock (glass + minerals) chemistry indicates a low average melting pressure (15 kbar), high initial crystallization pressures and temperatures (10–15 kbar and 1270°C), and eruption temperatures (1240°C) that are among the highest observed in Iceland. The glasses have trace element signatures (Lan/Ybn <1, Ban/Zrn 0·55–0·58) indicative of a trace element depleted source, and the Sr–Nd–Pb isotopic ratios (87Sr/86Sr 0·70304–0·70308, 143Nd/144Nd 0·513058–0·513099, 206Pb/204Pb 18·343–18·361) further suggest a long-term trace element depletion relative to primordial mantle. High He isotopic ratios (15·3–16·8 R/Ra) combined with low 207Pb/204Pb (15·42–15·43) suggest that the mantle source of the magma is different from that of North Atlantic mid-ocean ridge basalt. Negative Pb anomalies, and positive Nb and Ta anomalies indicate that the source includes a recycled, subducted oceanic crustal or mantle component. Positive Sr anomalies (Srn/Ndn = 1·39–1·50) further suggest that this recycled source component involves lower oceanic crustal gabbros. The δ18O values (4·2–4·7‰), which are lower than those observed in mantle peridotites but similar to those observed in ophiolites and in situ oceanic gabbros, are consistent with this interpretation. The elevated 3He/4He ratios are primarily attributed to a primitive, relatively undegassed component in the Iceland mantle plume, which dominates the He isotope signature as a result of long-term depletion of U, Th and He in the recycled gabbroic component.

Eclogitic and websteritic diamond sources beneath the Limpopo Belt – is slab-melting the link?

Trace-element concentrations in eclogitic and websteritic inclusions in diamonds from Venetia (South Africa) were analysed using an ion microprobe (SIMS). Garnets of both parageneses show similar, positive LREEN/HREEN slopes, but eclogitic garnets have higher MREE and almost flat MREEN–HREEN, and are also different in having significantly higher Sr and Zr. The occurrence of negative and positive Eu anomalies in garnets and clinopyroxenes of both parageneses points towards feldspar fractionation and accumulation in a magmatic precursor, suggesting subducted oceanic crust as a common protolith. Assuming equilibrium between clinopyroxene and garnet included in the same diamond, a bulk eclogite was reconstructed from these inclusions plus (expected) accessory rutile. The whole rock has a trace-element pattern lying between oceanic gabbro and EMORB, but is depleted in highly incompatible elements relative to these possible precursors. Quantitative modelling shows that relative and absolute trace-element abundances of the reconstructed eclogite and the hypothetical oceanic precursor agree if the latter is subjected to a loss of partial melts after subduction into the eclogite stability field. Major- and trace-element characteristics of websteritic inclusions could imply a more mafic precursor, which may have been part of a heterogeneous oceanic crust. However, new experimental data show that major- and trace-element compositions of websteritic inclusions in diamond are consistent with a mixing model in which they result from the reaction of slab-derived melts with surrounding mantle peridotite. This reaction generates major element contents that are intermediate between those of eclogitic and peridotitic sources whereas trace-element characteristics, such as Eu anomalies, are inherited from the melt source.

Magnetic stability of oceanic gabbros from ODP Hole 735B

Ocean Drilling Program (ODP) Hole 735B was drilled to a depth of 1.5 km in a tectonic window of gabbroic lower oceanic crust created at the Southwest Indian Ridge. The gabbros have a very stable natural remanent magnetization (NRM) of reversed polarity with most unblocking temperatures slightly below the Curie temperature of magnetite. The NRM includes a drilling-induced overprint but its intensity decays strongly towards the interior of the drill core. The demagnetization data yield no or only a very small secondary magnetization component acquired during the present Brunhes chron or an earlier normal chron, suggesting cooling through most of the blocking temperature range during chron C5r and a strong resistance against the acquisition of thermoviscous magnetization. A novel furnace has been designed to measure magnetizations and their time dependences at high temperatures (up to 580°C) inside a commercial SQUID magnetometer. Magnetic viscosity experiments have been conducted on the gabbros at temperatures up to 550°C to determine the time and temperature stability of remanent magnetization. Viscosities are generally small and increase little with temperature below the main blocking temperature, where the increase becomes almost an order of magnitude. Extrapolations to geological times infer viscous acquisitions that would be 5–25% of a thermoremanence in 100 kyr and at temperatures of 200–500°C. At ocean bottom temperature the predicted magnetization of one sample acquired in the present Brunhes chron should be 10% of the NRM. However, this is not recognized during NRM demagnetization and partial thermoremanent magnetization (pTRM) acquisitions at 250°C are also much smaller than predicted. It thus appears that the NRMs are generally magnetically harder than magnetizations acquired after heating to 570°C in the laboratory. Susceptibility changes during heating are small (<5%) indicating a seemingly stable magneto-mineralogy, but conspicuous minima occur after heating to 520°C. Also, quasi paleointensity experiments reveal characteristic patterns in the NRM/pTRM ratios and also large increases in pTRM capacity after heating to 570°C. Moreover, anhysteretic remanent magnetization acquisition in the low field range (≤10 mT) is strongly enhanced after heating by factors up to three. The alteration of the magneto-mineralogy is interpreted to result from the annealing of defects in magnetite that originate from tectonically induced strain. The oceanic gabbros of Hole 735B are thus ideal source layer material for marine magnetic anomalies, and secondary thermoviscous acquisition, as a possible cause for anomalous skewness, is essentially absent.

Oceanic and orogenic fluid–rock interaction in 18O/ 16O-enriched metagabbros of an ophiolite (Tinos, Cyclades)

Two spatial scales of fluid–rock interaction in an ophiolite suite are revealed by oxygen isotope and hydrogen isotope studies of metagabbros on the island of Tinos (Cyclades, Greece). Sequentially formed mineral generations in the metagabbros include relict igneous augite, hornblende of sub-seafloor hydrothermal origin, and actinolite and albite formed by regional greenschist-facies metamorphism during orogenesis. With the exception of augite (δ 18O=4.4–5.6‰), the metagabbros are characterized by unusually high δ 18O values: hornblende (5.8–7.4‰), actinolite (6.5–10.2‰), feldspar (14.6–14.9‰) and whole rocks (7.0–10.5‰). Hornblende δ D values range from −57 to −66‰. The high δ 18O values and the δ D range of the hornblendes are compatible with interaction of oceanic gabbro with seawater that had previously been enriched in 18O/ 16O (δ 18O=6.5–8‰) by isotopic exchange at moderate to high temperatures. The high degree of oceanic alteration in the layered gabbros, mass balance calculations of isotopic exchange, and field evidence for early oceanic thrusting suggest that seawater could have penetrated deeply into the ocean crust, becoming 18O/ 16O-enriched through isotopic exchange with gabbros at progressively increasing temperature. Upward, down-temperature flow of the high-δ 18O water would be very effective in elevating the δ 18O values of gabbros. The regional greenschist metamorphic overprint of the ophiolite, possibly the result of continued thrusting and piling up of nappes during obduction, is characterized by localized fluid–rock exchange. Actinolite in massive gabbroic layers has δ 18O values (6.5–7.2‰) close to those of the hornblende, whereas in deformed meter-sized gabbroic blocks the amphiboles have significantly higher values (8.4–10.2‰). Likewise, albite in the gabbroic blocks has high δ 18O values of ca. 15‰ that are ascribed to meter-scale exchange with 18O-rich fluids derived from dehydration reactions in low-temperature hydrothermally altered basaltic host rock enclosing the blocks. Deformation-enhanced permeability facilitated fluid infiltration in gabbroic blocks, whereas the relatively undeformed, and therefore less permeable, massive gabbros experienced minor interaction with fluids. The orogenic fluid–rock interaction thus represents local-scale redistribution of hydrous mineral components introduced during seafloor hydrothermal exchange.

Metasomatism of oceanic gabbros by late stage melts and hydrothermal fluids: Evidence from the rare earth element composition of amphiboles

We report new compositional and textural observations on amphiboles hosted in oceanic gabbros recovered from the slow spreading Mid‐Atlantic (MARK area) and Southwest Indian (Hole 735B) ridges and the fast spreading East Pacific Rise (Hess Deep) in an effort to find a set of reliable criteria that unequivocally distinguishes magmatic from hydrothermal amphibole. We find that there is no simple set of criteria that may be applied to distinguish between these parageneses. Rather, it is necessary to assess both a magmatic and hydrothermal origin for all amphibole grains in order to establish appropriate criteria for a particular sample suite. The rare earth element (REE) content of ∼75% of our amphibole grains may be accounted for by crystallization from residual melts; some of these melts were enriched in the light rare earth elements by the crystallization of apatite and/or amphibole. Of significance, only a small subset of these grains crystallized at magmatic temperatures (&gt;825°C). We conclude that the grains that equilibrated at lower temperatures were initially magmatic but that their major elements were reset during subsolidus, hydrothermal alteration, and deformation. This decoupling of the major and rare earth elements was most prevalent in the Hess Deep and MARK suites, both of which were tectonically exhumed at amphibolite to greenschist facies conditions. The remaining REE data identify hydrothermal amphibole that formed by mineral‐scale reactions involving clinopyroxene, fluids, and ± plagioclase at subsolidus conditions. Evaluation of fluids in equilibrium with amphibole shows that deep hydrothermal fluids are unlike those venting at the seafloor in terms of REE contents and/or patterns.

Geological and geochemical evidence for variable magmatism and tectonics in the southern Canadian Cordillera: Paleozoic to Jurassic suites, Greenwood, southern British Columbia

The Paleozoic and early Mesozoic rocks of the Greenwood mining camp in southern British Columbia are a part of the Quesnel terrane in the eastern part of the Intermontane Belt of the Canadian Cordillera. Upper Paleozoic rocks include the Knob Hill Group composed of oceanic tholeiitic basalts (with (La/Yb)n [Formula: see text] 0.4–1.2), associated with deep ocean sedimentary rocks and serpentinites; the Attwood Group that comprises island-arc tholeiites (with (La/Yb)n [Formula: see text] 1–4 and positive εNd values), clastic sedimentary rocks and limestones; and a unit of oceanic gabbros with (La/Yb)n &lt; 0.5. These lithologically defined units occur as tectonically emplaced slivers of oceanic crust probably produced during the closure of the Slide Mountain basin during the Permian. They are unconformably overlain by Middle Triassic calc-alkaline volcanic and sedimentary rocks of the Brooklyn Group. The Brooklyn Group volcanic rocks have characteristics of mature island-arc rocks, including (La/Yb)n [Formula: see text] 2.5–4.5 and positive εNd values. The Paleozoic rocks are crosscut by a 200 million years old granodioritic intrusion containing zircon with an Early Proterozoic inheritance age (~2.4 Ga). By inference, southern Quesnellia may have been well offshore from the ancestral North American margin in the Mississippian, in close proximity to the margin by the Middle Triassic, and contiguous with it by the Early Jurassic. It is suggested that the complex tectonic history of extension and contraction of the southern Canadian Cordillera during the post Middle Jurassic can be extended in south-central British Columbia as far back as the upper Paleozoic.

Geological and geochemical evidence for variable magmatism and tectonics in the southern Canadian Cordillera: Paleozoic to Jurassic suites, Greenwood, southern British Columbia

The Paleozoic and early Mesozoic rocks of the Greenwood mining camp in southern British Columbia are a part of the Quesnel terrane in the eastern part of the Intermontane Belt of the Canadian Cordillera. Upper Paleozoic rocks include the Knob Hill Group composed of oceanic tholeiitic basalts (with (La/Yb)n 0.4–1.2), associated with deep ocean sedimentary rocks and serpentinites; the Attwood Group that comprises island-arc tholeiites (with (La/Yb)n 1–4 and positive εNd values), clastic sedimentary rocks and limestones; and a unit of oceanic gabbros with (La/Yb)n < 0.5. These lithologically defined units occur as tectonically emplaced slivers of oceanic crust probably produced during the closure of the Slide Mountain basin during the Permian. They are unconformably overlain by Middle Triassic calc-alkaline volcanic and sedimentary rocks of the Brooklyn Group. The Brooklyn Group volcanic rocks have characteristics of mature island-arc rocks, including (La/Yb)n 2.5–4.5 and positive εNd values. The Paleozoic rocks are crosscut by a 200 million years old granodioritic intrusion containing zircon with an Early Proterozoic inheritance age (~2.4 Ga). By inference, southern Quesnellia may have been well offshore from the ancestral North American margin in the Mississippian, in close proximity to the margin by the Middle Triassic, and contiguous with it by the Early Jurassic. It is suggested that the complex tectonic history of extension and contraction of the southern Canadian Cordillera during the post Middle Jurassic can be extended in south-central British Columbia as far back as the upper Paleozoic.

Gabbroic xenoliths from La Palma, Tenerife and Lanzarote, Canary Islands: evidence for reactions between mafic alkaline Canary Islands melts and old oceanic crust

Gabbroic and hornblendite xenoliths from La Palma, Tenerife and Lanzarote fall into three main groups based on petrography and chemistry. One group (comprising all xenoliths from Lanzarote and some from La Palma) consists of highly deformed orthopyroxene-bearing gabbroic rocks that show a strong affinity to N-MORB and oceanic gabbro cumulates in terms of mineral chemistry and REE relations. However, they show mild enrichment in the most incompatible elements (particularly Rb+Ba±K) relative to intermediate and heavy REE, and their Sr–Nd isotope ratios fall within or close to the N-MORB field. The second group (60% of the xenoliths from La Palma) are gabbroic cumulates with zoned clinopyroxenes (Ti–Al-poor cores, Ti–Al-rich rims) and reaction rims of hornblende, biotite and clinopyroxene on other phases. Their trace-element and Sr–Nd isotope relations are in general transitional between N-MORB cumulates and Canary Islands alkali basalts, but they show strong enrichment in Rb, Ba and K relative to other strongly incompatible elements. The third group (comprising some xenoliths from La Palma and all those from Tenerife) are undeformed gabbroic and hornblendite rocks in which hornblende and biotite appear to belong to the primary assemblage. These rocks show strong affinities to Canary Islands alkali basaltic magmas with respect to mineral, trace-element, and Sr–Nd isotope chemistry. The first two groups are interpreted as fragments of old oceanic crust which have been mildly to strongly metasomatized through reactions with Canary Islands alkaline magmas. The reaction process is a combination of enrichment in elements compatible with biotite (and hornblende), and simple mixing between N-MORB cumulates and trapped alkaline magmas. The third group represents intrusions/cumulates formed from mafic alkaline Canary Islands magmas. Modeling indicates that locally up to 50% new material has been added to the old oceanic crust through reactions with ocean island basalts. Reactions and formation of cumulates do not represent simple underplating at the mantle/crust boundary, but have taken place within the pre-existing oceanic crust, and are likely to have significantly thickened the old oceanic crust.

Evidence from oceanic gabbros for porous melt migration within a crystal mush beneath the Mid‐Atlantic Ridge

The evolution of a gabbroic crystal mush beneath the Mid‐Atlantic Ridge has been investigated using evidence from gabbros recovered from Ocean Drilling Program (ODP) Hole 923A (Leg 153). Lithological variations occur on a vertical scale of meters and correlate with mineral compositions. This defines a detailed chemical stratigraphy in which variations in olivine, plagioclase, and clinopyroxene solid‐solution component compositions correlate with each other. The volumetrically dominant lithology (plagioclase + clinopyroxene ± olivine ± orthopyroxene gabbros) has variable grain size, grain shape, and mineral compositions. These variations correlate, such that coarser grained samples have more granular textures and lower mafic phase Mg/Fe ratios than adjacent finer grained samples. Clinopyroxene trace element systematics, determined by ion probe, cannot be explained by growth from a melt that evolved along either an equilibrium or a fractional crystallization trend. Clinopyroxene crystals are strongly zoned and enriched in Zr with respect to rare earth elements (more to less incompatible elements). These textural and geochemical characteristics are not expected from simple crystal accumulation processes or the crystallization of trapped melt. Instead, melt migration within a crystal mush is suggested as the most likely process to explain them. The meter‐scale mineral compositional variations, which correlate between phases (e.g., olivine forsterite content and plagioclase anorthite content), suggest that the porous melt flux after the formation of this layering was insufficient to destroy this correlation.

Rare earth and trace elements in igneous and high-temperature metamorphic minerals of oceanic gabbros (MARK area, Mid-Atlantic Ridge)

SIMS analyses have been carried out on clinopyroxenes, plagioclases and amphiboles of six gabbroic samples from Holes 921–924 of the Ocean Drilling Program Leg 153 sited in the MARK area of the Mid-Atlantic Ridge at the ridge–transform intersection, to investigate the rare earth, trace and volatile element distribution in the lower ocean crust during igneous crystallization and higher grade metamorphic conditions. The metagabbros underwent granulite to subgreenschist facies conditions through three main tectono-metamorphic phases: (1) ductile regime (750 < T < 1000 °C and P ≈ 0.3 GPa); (2) transitional regime (600 < T < 700 °C and P ≈ 0.2 GPa); (3) brittle regime (350 ≤ T < 600 °C and P < 0.2 GPa). Igneous clinopyroxenes show C1-chondrite normalized patterns depleted in LREE, and nearly flat for HREE. The rare earth and trace element distributions in igneous clinopyroxenes and plagioclases indicate that these minerals act as REE reservoirs, and comprise the main contribution to the overall rock content. The abundances in igneous minerals reflect the degree of fractionation of the parent liquids. In metamorphic clinopyroxenes recrystallized in anhydrous assemblages, the REE and trace elements patterns mimic those of the primary ones. Conversely, clinopyroxene re-equilibrated in amphibole-bearing assemblages shows a significant increase in REE, Ti, Zr, Y and V, a negative Eu anomaly, and slight decreases in Sr and Ba. An overall increase of REE and some trace elements is evident in hydrous assemblages, with preferential partitioning in the amphibole. It shows high Ti (18196–22844 ppm), LREE depleted patterns and LaN/SmN=0.10–0.33, LaN/YbN=0.10–0.30. Amphiboles from granoblastic assemblages show homogeneous patterns with no or a positive anomaly for TiN and negative anomalies for SrN and ZrN. Volatiles in amphibole are low, with Cl/F < 1; H2O% is significantly lower than the stoichiometric ratio (1.33–1.53%). The composition of the clinopyroxene and amphibole recrystallized in low-strain domains records evidence of incomplete re-equilibration, and element diffusion and partitioning is in part controlled by the textural site. The possible origins of the fluids involved in the metamorphic recrystallization are discussed: (1) remobilization from igneous amphibole; (2) exsolution from evolved melts; (3) introduction of seawater-derived fluids modified in rock-dominated systems; (4) injection of highly evolved hydrous melts during the metamorphic process.

Recycled oceanic crust observed in 'ghost plagioclase' within the source of Mauna Loa lavas

The hypothesis that mantle plumes contain recycled oceanic crust is now widely accepted. Some specific source components of the Hawaiian plume have been inferred to represent recycled oceanic basalts, pelagic sediments or oceanic gabbros. Bulk lava compositions, however, retain the specific trace-element fingerprint of the original crustal component in only a highly attenuated form. Here we report the discovery of exotic, strontium-enriched melt inclusions in Mauna Loa olivines. Their complete trace-element patterns strongly resemble those of layered gabbros found in ophiolites, which are characterized by cumulus plagioclase with very high strontium abundances. The major-element compositions of these melts indicate that their composition cannot be the result of the assimilation of present-day oceanic crust through which the melts have travelled. Instead, the gabbro has been transformed into a (high-pressure) eclogite by subduction and recycling, and this eclogite has then been incorporated into the Hawaiian mantle plume. The trace-element signature of the original plagioclase is present only as a 'ghost' signature, which permits specific identification of the recycled rock type. The 'ghost plagioclase' trace-element signature demonstrates that the former gabbro can retain much of its original chemical identity through the convective cycle without completely mixing with other portions of the former oceanic crust.

Petrology, isotope geochemistry and chemical budgets of oceanic gabbros-seawater interactions in the Equatorial Atlantic

Petrological and chemical variations, as well as oxygen and strontium isotopic data are presented for metagabbros from the Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alterations ranging from slight hydrothermal alteration to complete amphibolitization. Five major kinds of alteration processes ranging from late-magmatic deuteric alteration (stage I) to low temperature (<150 °C) alteration (stage V) were identified. Water-rock interactions between 300 and 650 °C are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which characterize the amphibolite and/or greenschist facies (amphibole ± plagioclase ± epidote ± titanite ± chlorite ± prehnite). Hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater-derived fluids. These exchanges lead to decrease of gabbroic δ18O toward values as low as +3.9‰, and larger Sr isotopic variations than other oceanic gabbroic rocks (87Sr/86Sr ratios shift to 0.7029–0.7051). Calculation of a chemical budget indicates that metagabbros are hydrated and enriched in Fe and probably in Mg and Cl, while Si, Ca and Ti are released to the hydrothermal fluids. In addition to metamorphic recrystallization and geochemical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry.