Small Degrees Of Partial Melting Research Articles

Petrogenesis of the Largest Intraplate Volcanic Field on the Arabian Plate (Jordan): a Mixed Lithosphere-Asthenosphere Source Activated by Lithospheric Extension

Miocene to Recent volcanism in northwestern Arabia produced the largest intraplate volcanic field on the Arabian plate (Harrat Ash Shaam, Jordan). The chemically and isotopically diverse volcanic field comprises mafic alkali basalts and basanites. The magmas underwent limited fractional crystallization of ol cpx plag and rare samples have assimilated up to 20% of Late Proterozoic crust en route to the surface. However, there are subtle Sr±Nd±Pb isotopic variations (Sr/Sr ˆ 0 70305±0 70377, Nd/Nd ˆ 0 51297±0 51285, Pb/ Pb ˆ 18 8±19 2), which exhibit marked correlations with major elements, incompatible trace element ratios and abundances in relatively primitive basalts (MgO 48 5 wt %), and cannot be explained by fractional crystallization and crustal contamination alone. Instead, the data require polybaric melting of heterogeneous sources. Semi-quantitative melt modelling suggests that this heterogeneity is the result of small degree melts (2±5%) from spineland garnet-facies mantle, inferred to be shallowArabian lithosphere, thatmixedwith smaller degreemelts (51%) from a predominantly deep garnet-bearing asthenospheric(?) source with ocean island basalt characteristics. The latter may be a ubiquitous part of the asthenosphere but is preferentially tapped at small degrees of partial melting. Volcanism in Jordan appears to be the result of melting lithospheric mantle in response to lithospheric extension. With time, thinning of the lithosphere allowed progressively deeper mantle (asthenosphere?) to be activated and melts from this to mix with the shallower lithospheric mantle melts. Although Jordanian intraplate volcanism is isotopically similar to examples of Late Cenozoic volcanism throughout the Arabian peninsula (Israel, Saudi Arabia), subtle chemical and isotopic differences between Yemen and Jordan intraplate volcanism suggest that the Afar plume has not been channelled northwestwards beneath the Arabian plate and played no role in producing the northern Saudi Arabian and Jordan intraplate volcanic fields.

Petrogenesis of Basanitic to Tholeiitic Volcanic Rocks from the Miocene Vogelsberg, Central Germany

The Miocene Vogelsberg volcano in Central Germany produced mafic magmas ranging in composition from basanite to quartz tholeiite and limited amounts of evolved magmas. Trace element and Nd, Sr and Pb isotopic compositions reveal the presence of three distinct mantle sources: (1) a trace element enriched, asthenospheric plume-type source, similar to the European Asthenospheric Reservoir composition inferred for many other Tertiary volcanic provinces in Central Europe; (2) a depleted mantle source, located in the lithospheric mantle or uppermost asthenosphere; (3) a veined lithospheric mantle source. The oldest basanites of the Vogelsberg volcano have distinctly higher Ti, Al, Sc and V contents than younger basanites. These high-Ti basanites may have been produced by partial melting of a veined lithospheric mantle source, formed during the earliest stages of uplift of the Rhenish Shield, 70Myr ago. Younger basanites were generated by small degrees of partial melting of the European Asthenospheric Reservoir, whereas alkali basalts and tholeiites formed by mixing of variable proportions of melts derived from the European Asthenospheric Reservoir and depleted mantle sources, respectively. These magmas then interacted with metasomatized sub-continental lithospheric mantle, which explains the observed range in Sr, Nd and Pb isotopic compositions. Subsequently the most depleted tholeiites were contaminated by lower-crustal rocks. The distinct stratigraphic position of the various lava groups in the 656 5 m `Forschungsbohrung Vogelsberg 1996' borehole and the correlation of their chemical stratigraphy with palaeomagnetic reversals reflects an episodic temporal evolution of magmas and mantle sources. During Stage I, melts from the veined lithospheric mantle source were pooled in crustal magma chambers and evolved to erupt a range of differentiated lavas. In Stage II melts were formed in the depleted mantle source and up-section gradually mixed with melts from the asthenospheric mantle. In Stage III the depleted mantle source was exhausted and pure asthenospheric melts were erupted.

Tertiary volcanism during extension in the Andean foothills of central Chile (33°15′–33°45′S)

This lithologic and geochemical study treats two Tertiary volcanic formations in the Andean foothills of central Chile deposited during and after an inferred culmination of crustal attenuation. The Abanico and Farellones Formations, which are described in their type localities just east of Santiago, formed from volcanic arcs in continental basins during the Oligocene and Early Miocene, respectively. Aphyric basic lavas of tholeiitic affinity, acid pyroclastic rocks, and lacustrine deposits constitute the >3100-m-thick Abanico Formation. The overlying >2100-m-thick Farellones Formation consists of calc-alkaline lavas (basalts absent) with thick pyroclastic deposits at the base. Both formations have Nd-Sr isotope signatures within the mantle array; the Abanico rocks (eNd ≈ +5.7) plot closer to N-MORB (normal mid-oceanic-ridge basalt) than the Farellones rocks (eNd = +3.9 to +5.1). The REE (rare earth element) patterns indicate greater depth to the mantle source and a smaller degree of partial melting with time. The Abanico lavas segregated within the stability field of spinel, whereas the lavas of the upper Farellones member show residual garnet in their source. Geochemical changes with time are systematic: the greatest contrast is between the middle and upper Farellones members in 1–2 m.y., e.g., for basaltic andesites, La/Yb increases from 4.3 (Abanico) to 5.6, 6.0, and 11.6 (lower, middle, and upper Farellones members). The bimodal composition of the Abanico Formation and the lower Farellones member indicates that volcanism took place during episodes of extensional conditions. Extension with subsidence is independently shown by the burial metamorphic pattern. Voluminous pyroclastic flows, structural relationships, and other evidence suggest recurrent caldera collapse. The first extensional episode ended with contraction and folding of the Abanico rocks, and the second episode resulted in uplift of the lower and middle Farellones members, followed by a more passive tectonic regime. Sequences showing many similarities with the Abanico and Farellones Formations occur along the Andean foothills of Chile. They decrease in age from Late Cretaceous– Paleocene at 23°S to Early Miocene–late Miocene at 35°S and might be explained by oblique subduction of oceanic ridge.

Further Considerations of the Ce/Yb vs. Ba/Ce Plot in Volcanology and Tectonics

A plot of Ce/Yb vs. Ba/Ce, for locality averages, effectively separates mid-ocean ridge basalts (MORB) (Ce/Yb 10, Ba/Ce 4.2). The conventional interpretation is that these three types of volcanic environments involve oceanic rift-related, large-volume partial melts (˜20-30%) of a depleted source (MORB), small volume melts (˜5% for alkalic volcanics) of enriched sources related to plumes (OIV), and melts of hydrous-enriched sources during subduction, especially for Ba (IAV). Three OIV sites, however, have average ratios that fall in the MORB field (e.g., Krafla Volcano, Iceland), and these localities also tend to have other geochemical data similar to MORB. Average ratios of Hawaiian tholeiitic shield basalts of Mauna Kea and Koolau volcanoes occupy a restricted field on a plot of Ce/Yb vs. Ba/Ce of 10-18 for Ce/Yb and 2.8-3.1 for Ba/Ce, a field toward which ...

Geochemical trends across an arc-continent collision zone: magma sources and slab-wedge transfer processes below the Pantar Strait volcanoes, Indonesia

Four volcanoes in the Pantar Strait, the westernmost part of the extinct sector of the east Sunda arc, show remarkable across-arc variation in elemental abundances (K 2O: 1.2 to 4.3%), trace element ratios (Pb/Ce: 0.4 to 0.18; Ce/Yb: 20 to 55) and isotope ratios ( 143Nd/ 144Nd: 0.51263 to 0.51245; 87Sr/ 86Sr: 0.7053 to 0.7068; 206Pb/ 204Pb: 19.29 to 19.15). Pb isotopes are decoupled from Sr and Nd isotopes, with the frontal volcanoes showing the higher Nd and Pb and lower Sr isotopic ratios. The isotopic and trace element ratios of the volcanic samples are best explained by modification of a MORB-type source (with Indian Ocean island basalt–type Pb isotopic characteristics) by a fluid and a partial melt of subducted continental material (SCM). The frontal volcano contains the highest proportion of the fluid component, with a small contribution of partial melt. The source of the rear-arc volcano is strongly influenced by a partial melt of SCM that had undergone a previous dehydration event, by which it lost most of its fluid-mobile elements such as Pb. The SCM partial melt was in equilibrium with both rutile and garnet, whereas mantle melting took place in the presence of residual mica. The relatively large across-arc increase in incompatible elements can be explained by a combination of increasing addition of SCM partial melt, changing mantle wedge fertility and smaller degrees of partial melting toward the rear of the arc. Comparison with a more westerly across-arc transect shows that the relatively low 143Nd/ 144Nd ratios of the frontal volcano, and the decoupling of Pb from Sr and Nd isotopes are unique to the Pantar Strait volcanoes. This is likely to reflect magma generation in a collisional environment, where the leading edge of the Australian continent, rather than subducted sediment, contributes to the magma source.

The effect of crystal orientation on the wetting behaviour of silicate melts on the surfaces of spinel peridotite minerals

The effect of crystal anisotropy on wetting angles of equilibrium silicate melts on crystal faces of spinel, diopside, enstatite and olivine has been determined experimentally by the sessile melt drop technique. The anisotropy, \({\rm \~A}\gamma _{{\rm SL}} \) , of solid-liquid interfacial energies (γSL(max)–γSL(min)) can be related to the wetting angles, ψ, by \({\rm \~A}\gamma _{{\rm SL}} \propto \left| {\cos \psi (\max )\; - \cos \psi (\min )} \right| = Pw_{\left( {{\rm \tilde A}\gamma _{{\rm SL}} } \right)} \) . Normalising to the smallest wetting angle gives values of Pw for diopside=0.0728, olivine=0.0574, orthopyroxene=0.0152, and spinel=0.0075. Crystal anisotropy influences grain-scale morphology of small-degree partial melts, permeability and the melt connectivity threshold, φC. Results show that, at sufficient melt fractions, diopside should increase permeability in a peridotitic matrix, whereas enstatite should lower it. Despite its low anisotropy, spinel contributes positively to permeability and φC because of its high surface energies. These results suggest that harzburgitic mineral matrices typical of the subcratonic mantle should impede the movement of low-degree partial melts, whereas melts should flow more easily through spinel lherzolites.

Volatiles in basaltic glasses from a subglacial volcano in northern British Columbia (Canada): implications for ice sheet thickness and mantle volatiles

Abstract Dissolved H 2 O, CO 2 , S and Cl concentrations were measured in glasses from Tanzilla Mountain, a 500 m-high, exposed subglacial volcano from the Tuya-Teslin region, north central British Columbia, Canada. The absence of a flat-topped subaerial lava cap and the dominance of pillows and pillow breccias imply that the Tanzilla Mountain volcanic edifice did not reach a subaerial eruptive phase. Lavas are dominantly tholeiitic basalt with minor amounts of alkalic basalt erupted at the summit and near the base. Tholeiites have roughly constant H 2 O ( c. 0.56 ± 0.07 wt%), CO 2 (<30 ppm), S (980 ± 30 ppm) and Cl (200 ± 20 ppm) concentrations. Alkalic basalts have higher and more variable volatile concentrations that decrease with increasing elevation (0.62–0.92 wt% H 2 O, <30 ppm CO 2 , 870–1110 ppm S and 280–410 ppm Cl) consistent with eruptive degassing. Calculated vapour saturation pressures for the alkalic basalts are 36 to 81 bars corresponding to ice thicknesses of 400 to 900 m. Maximum calculated ice thickness ( c. 1 km) is at the lower end of the range of predicted maximum Fraser glaciation ( c. 1–2 km), and may indicate initiation of volcanism during the waning stages of glaciation. Temporal evolution from tholeiitic to alkalic compositions may reflect compositional gradients within a melting column, instead of convective processes within a stratified magma chamber. The mantle source region for the subglacial volcanoes is enriched in incompatible elements similar to that for enriched mid-oceanic ridge basalt (e.g. Endeavour Ridge) and does not contain residual amphibole. Thus, metasomatic enrichment most likely reflects small degree partial melts rather than hydrous fluids.

Evolution of the Ligurian Tethys in the Western Alps: Sm/Nd and U/Pb geochronology and rare-earth element geochemistry of the Montgenèvre ophiolite (France)

We provide geochemical and geochronological data for gabbro, diorite and albitite samples from the Montgenèvre ophiolite in the Western Alps. This well-preserved remnant of the Piemont–Ligurian oceanic basin shows evidence of intra-oceanic deformation and metamorphism, but has suffered minor ductile deformation and metamorphism during the Alpine orogeny. The gabbros have geochemical features and initial Nd isotopic signatures similar to that of Mid-Oceanic Ridge gabbros, indicating that they were extracted from a depleted mantle source with no evidence of continental contamination ( ε Nd( T)>+8). Cumulation played an important role in the genesis of these gabbros, and their rare-earth element (REE) patterns are controlled by the major cumulus phases. Modelling the REE data for the gabbros and diorites provides support for the hypothesis that the dioritic magmas have been derived from small-degree (≤5%) partial melting of the surrounding gabbros during shearing at high-temperature (high-T). Zircons from a leucodioritic vein within sheared gabbros are poorly discordant and cluster the Concordia at 156±3 Ma, while zircons from an albitite lens within the mantle-rocks display concordant ages at 148±2 Ma. In both cases, the zircons show no evidence of inheritance, and these values are interpreted as crystallisation ages. The ∼160–150 Ma age bracket very likely records a late stage in the magmatic history of the Montgenèvre ophiolite, since the emplacement of diorite clearly post-dates the layered gabbros. These radiometric ages correlate well with the Late Bathonian to Early Kimmeridgian stratigraphic age (∼160–140 Ma) of the earliest post-ophiolitic radiolarian sediments. The whole-rock Sm–Nd system of the gabbros yielded an isochron age of 198±22 Ma ( ε Nd( T) of +8.8). This value is significantly older than the ∼165 Ma age of Western Alps ophiolitic gabbros published elsewhere, but is similar within error to the ∼180 Ma age of eclogitised gabbros from the Ligurides and Corsica. This early Jurassic age is interpreted as the emplacement age of the gabbroic melts into the mantle lherzolites and could be associated with the early stage of rifting between the European and Austro-Alpine continental margins. These ages and their interpretation are consistent with the model of asymmetric mantle-denudation by an oblique detachment fault, proposed by Lemoine et al. (1987). This model implies that the newly formed lherzolite–gabbro oceanic domain probably remained close to the spreading centre and therefore experienced slow cooling and low spreading rates. This appears to be correlated by our radiometric results that suggest a life span of at least 30 Ma for the formation of the oceanic crust in this part of the Piemont–Ligurian ocean.

Geochemistry of Late Cenozoic basalts from the Crary Mountains: characterization of mantle sources in Marie Byrd Land, Antarctica

Late Cenozoic (9.34 to 0.04 Ma) alkaline basalts from the Crary Mountains, eastern Marie Byrd Land, Antarctica have low 87 Sr/ 86 Sr (0.70267–0.70283), moderately high 143 Nd/ 144 Nd (0.51286–0.51291) and high 206 Pb/ 204 Pb (19.9–20.9) ratios. The Crary Mountains basalts together with Hobbs Coast basalts from western Marie Byrd Land have the strongest HIMU signature ( 206 Pb/ 204 Pb>20.6 ) in the West Antarctic Rift System (WARS). There is no evidence of crustal contamination and their trace-element patterns are similar to HIMU basalts from oceanic islands. Higher concentrations of some large-ion lithophile (LIL) elements, particularly K, relative to oceanic HIMU, suggest melting of an enriched mantle. Models for small degrees of partial melting between 2% and 3% of a mantle peridotite containing 2.5% amphibole are consistent with trace-element variations in the basalts. The Crary data confirm the presence of a moderately depleted, lower-μ ( μ= 238 U/ 204 Pb ) mantle component with a 206 Pb/ 204 Pb signature between 19.3 and 19.8 in the source regions of West Antarctic basalts. The lower-μ component, defined by the focus of West Antarctic data arrays on 2-D and 3-D isotope plots, is prevalent as a mixing end-member throughout the WARS, whereas the HIMU component, defined by 206 Pb/ 204 Pb ratios greater than 20.5, is present at only a few localities. The positive correlation of many highly incompatible trace-elements with 206 Pb/ 204 Pb ratios indicate that smaller-degree melts preferentially sample the HIMU component, whereas larger degree melts sample the lower-μ component. We also suggest, based on variations in elements moderately incompatible to highly compatible in the mantle phases phlogopite and amphibole, that the lower-μ component is more hydrous than the HIMU source. The lower-μ and HIMU components are also found in alkaline basalts from Tasmania and New Zealand which were adjacent to the Antarctica coast prior to the mid-Cretaceous fragmentation of the proto-Pacific margin of Gondwanaland. We propose that before continental breakup, a HIMU-type mantle plume was trapped and stored beneath a much more extensive pre-existing metasomatised layer within the Gondwanaland lithosphere. Source regions for all known extreme HIMU basalts in the continental borderlands of the southwest Pacific would be encompassed by a modest plume head, 600–800 km in diameter, emplaced around 100 Ma. We suggest that an earlier metasomatic event, possibly related to the Jurassic Bouvet-plume, enriched the originally depleted upper mantle in highly incompatible elements. A fertile lithosphere formed by plume-driven melts and fluids during the Jurassic would allow for the widespread development of the present-day isotopic signature of the lower-μ source.

Fundamental leaking mode (PL) propagation along the Tonga--Kermadec--Hikurangi--Macquarie margin

High-quality digital data from the temporary Leeds Tararua broad-band array, North Island, New Zealand and the station SNZO at Wellington often record long-period oscillations within the body wave train from large, shallow events in the Tonga–Kermadec and Macquarie seismic zones at regional distances (8°–25°) along the Australasian/Pacific plate boundary. These arrivals are dispersed typically from 40 s period to 25 s and exhibit prograde elliptical polarization, which is diagnostic of the regional leaking mode PL. Theoretical dispersion curves are generated with the simplest structure of a slow layer overlying a faster half-space. We have analysed the group velocity dispersion characteristics of the recorded waveforms and successfully modelled it as purely fundamental mode propagation in a low-velocity waveguide. Our best-fitting structure north of New Zealand consists of a low-velocity layer within the mantle (β=4.1–4.3 km s-1) with a thickness of 32–34 km overlying a typical mantle structure. A seismicity study for the period 1976–1992 at SNZO shows that those events generating PL are shallow (10–60 km), with the highest concentration along the Kermadec arc, suggesting that the low-velocity layer is connected with this feature. We suggest that a small degree of partial melt within the uppermost mantle is responsible for creating a low-velocity channel within the region of backarc spreading in the Havre Trough. Dispersion curves derived from waveforms travelling northwards from events on the Macquarie Ridge give a similar structure with a slow layer 31 km thick with β=3.8 km s-1. This is consistent with the continental crust of South Island, which lies along most of the event–station paths.

Geochemical and Nd, Pb, and Sr Isotope Data from Deccan Alkaline Complexes-Inferences for Mantle Sources and Plume-Lithosphere Interaction

Previous chemical and isotopic studies based on alkaline rocks and carbonatites associated with large, continental flood basaltic provinces indicate their important role in monitoring plume–lithosphere interaction. We report new major and trace element data, and Nd, Pb, and Sr isotope ratios for various alkaline silica-undersaturated rocks and carbonatites from several Decca alkaline complexes in an attempt to evaluate the relative contributions of Réunion plume and Indian sub-continental mantles in their source regions. Major and trace element abundances for the most primitive silicate samples are consistent with an origin via small-degree partial melting of metasomatized mantle. Initial 87Sr/86Sr, 143Nd/144Nd and Pb isotope ratios for the most primitive alkaline silicate samples and associated carbonatites exhibit a large variation, and are attributed to mixing of three distinct mantle components—Réunion plume, continental lithosphere and asthenosphere (Indian MORB-like). For the silicate rocks, isotope ratios correlate with major and trace element composition and support derivation from distinct mantle sources. The data obtained here are consistent with previous models invoking Réunion plume–continental lithosphere interaction to explain the origin of Deccan alkaline complexes, which suggest a more prominent role of Réunion mantle during the early stages of Deccan volcanism involving small-degree melting of plume-modified lithosphere. With time, the isotope systematics of both alkaline and tholeiitic magmatism record a larger lithospheric imprint.

Seismic structure and crustal magmatism at the Mid‐Atlantic Ridge, 35°N

We have determined the three‐dimensional P wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the Mid‐Atlantic Ridge (MAR) near 35°N. The best fitting one‐dimensional model is ∼0.8 km/s lower in velocity at a given depth than for fast spreading ridges, while the isotropic three‐dimensional velocity structure shows a strong lateral heterogeneity (up to 1.4 km/s peak‐to‐peak variation at 1 km depth) in both the along‐ and cross‐axis directions. An anomalous region, with velocities as much as 0.8 km/s lower than average and 3–5 km in lateral extent, is located beneath a near‐axis seamount. We interpret this feature as a region of near‐solidus temperatures with possibly a small degree of partial melt. From the variation of travel time residuals with azimuth we determine a best fitting P wave anisotropy model that has 4% anisotropy from 500 m to 1 km depth, 2% anisotropy in the depth range 1–1.5 km, and no anisotropy at greater depth. This finding is consistent with vertical cracks aligned parallel to the axial valley. We find no evidence for the shallow, high‐velocity, ridge‐parallel feature commonly observed at the axis of fast spreading ridges and attributed to high‐velocity dikes beneath a thin extrusive layer. The absence of this anomaly at the MAR implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25,000 years. An unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes, which may account for the generally lower velocities beneath the MAR compared with the East Pacific Rise, and a lateral variation in upper crustal lithology.

The Effect of Alkalis on the Silica Content of Mantle-Derived Melts

A large body of experimental evidence shows that at low and moderate pressure (<1.5 GPa), alkali-rich silicate liquids coexisting with Mg-rich olivine and orthopyroxene are richer in silica than typical basalts. This phenomenon is caused by the tendency of alkali ions to reduce the number of Si-O-Si linkages in the melt, which translates to negative deviations from ideality for mixing between alkalis and silica and which requires increases in alkalis to be accompanied by increases in silica for liquids in equilibrium with mantle peridotite. P 2O 5 and TiO 2 have an effect opposite to alkalis, and when these elements are also enriched in the liquid, the high silica contents caused by alkali-enrichment may be reduced or eliminated. The effect of alkalis on the silica content of melts equilibrated with magnesian olivine and orthopyroxene is reduced at higher pressure, such that silica enrichments in alkali-rich melts will be small if the equilibration pressure is greater than ∼1.5 GPa. This pressure effect is largely the result of decreases with pressure in the extent of polymerization for all olivine + orthopyroxene-saturated liquids. As pressure increases and liquids in equilibrium with olivine and orthopyroxene become less polymerized, proportionally fewer alkalis break up highly polymerized (Q 4) silica tetrahedra, and, therefore, alkalis have less effect on the activity coefficient of silica. Secondarily, the observed changes with pressure may also be related to changes in the energetics of alkali-silica interactions. Because equilibration of alkali-rich melts with mantle peridotite only produces high silica at low and moderate pressures, small degree partial melts of anhydrous peridotite formed during adiabatic upwelling will not typically be silica-rich. However, if liquids rich in alkalis, perhaps formed by selective leaching of Na 2O and K 2O from peridotite during upward percolation, equilibrate with the mantle at depths <1.5 GPa, they will become silica-rich. Such silica-rich liquids, now preserved as glass inclusions in spinel peridotite xenoliths, are probably restricted to the shallowest part of the mantle (<45 km).

Tectonic Activity Recorded by Multiple Episodes of Zircon Growth: an Ion Microprobe (SHRIMP) Study of the Geological Evolution of the Cyclades, Greece

To interpret the significance of multiple episodes of zircon growth, the processes controlling zircon growth must be distinguished. There is currently little agreement concerning the origin and significance of metamorphic zircon and how it can be related to pressure-temperature-time ( P T t ) paths (cf. Fraser et al., 1997; Roberts and Finger, 1997). New zircon growth has been attributed to a number of different processes: (1) melt crystallisation; (2) net-transfer reactions; (3) hydrothermal precipitation; or (4) solid-state recrystallisation/replacement processes. Each of these mechanisms has different implications for the interpretation of zircon ages. As zircon remains closed to isotopic exchange at temperatures in excess of those commonly experienced during either metamorphism or magmatism, zircon ages will generally record the time of growth rather than the time of cooling below its closure temperature. While dating of different layers of zircon can constrain the time of zircon growth it will not shed light on the zircon formation mechanism. To help interpret zircon ages in terms of tectonic processes it is necessary to study zircon morphology, internal structure and chemistry and to try and relate this to the textural relations and chemistry of the zircon host rock. Regional setting. The tectonic evolution of the Cyclades during the Alpine orogeny includes polyphase metamorphism, deformation, fluid infiltration, anatexis and shearing. Hence the Cyclades provides a natural laboratory to assess the behaviour of zircon developed in response to different mechanisms. The Cyclades consist of a Hercynian orthogneiss basement, in unconformable contact with a variegated series of Mesozoic marbles, schists and metavolcanics (Drift et al., 1978). During the Alpine orogeny these rocks experienced an Eocene high pressure metamorphism (M1) overprinted by OligoMiocene greenschist-amphibolite facies medium pressure metamorphism (M2). They form the lower plate to a sequence of metamorphic core complexes exposed in the Aegean (Lister et al., 1984). The upper plate is comprised of ophiolites, Upper Cretaceous/Permo-Triassic limestones and granitoids that have experienced Cretaceous high temperature, low pressure metamorphism but have not experienced any Alpine metamorphism (Reinecke et al., 1982), along with unmetamorphosed sediments and volcanics. Methods. Zircons were separated from rock units that had experienced Alpine orogenesis from the islands of Naxos, Ios and Sifnos. They were imaged using cathodoluminescence techniques to identify internal zircon structures and choose potential target sites for analysis. Layers that could clearly be identified as new growth, potentially related to metamorphism, were preferentially analysed. U, Th and Pb/U in the sample zircons relative to those in the SL13 reference zircon were measured using the sensitive high resolution ion microprobe (SHRIMP). The main advantage in using SHRIMP for U-Pb dating, as opposed to conventional isotope dilution techniques, is that it allows the analysis of = 30 pm areas within mineral grains and thus the measurement of ages for different growth zones. To ensure no 'mixed ages' occurred due to spatial overlap of more than one growth zone by the probe, all mounts were imaged after analysis to check the actual pit location. Assessment of the age data-sets for the presence of discrete age populations was carried out both by construction of age probability distributions and by a mixture-modelling procedure. Zircon characteristics. The identification of multiple layers of metamorphic zircon growth is made according to zircon morphology, internal structure and chemistry. In general, magmatic zircon growth is characterised by the development of oscillatory zoning while metamorphic zircon is generally unzoned, displays low luminescence and has variable, but generally low Th/U ratios. However, zircons produced by small degrees of partial melting can appear as unzoned, low luminescent overgrowths

Evidence for Melting of Garnet Pyroxenite in the Generation of Hawaiian Post-Erosional Lavas: Effects of a Marble-Cake Mantle During Melt Generation

Isotopic variations in mantle-derived melts are a primary source of information on the age, origin and magnitude of chemical heterogeneities in the convecting mantle. Understanding the relationship between the melting process and the generation and preservation of chemical heterogeneity in basaltic magmas is therefore of the utmost importance. Preferential melting of mafic veins in predominantly peridotitic mantle is a potential source of chemical and isotopic heterogeneity in mantle-derived melts. Previous studies have proposed a role for melting of garnet pyroxenite in the origin of the garnet signature in MORB [e.g. Hirschmann and Stolper, 1996]. At high degrees of mantle melting, such as that responsible for the generation of MORB, the signature of pyroxenitic melting may be difficult to detect unambiguously. However, because pyroxenites have a lower Tsolidus than peridotites, small degree partial melts should contain a more pronounced signature from pyroxenite melting. We have examined the Os-isotope systematics of Hawaiian post-erosional lavas from the Koloa Volcanic Suite, Kauai and the Honolulu Volcanic Series, Oahu to evaluate the role of pyroxenite melting in the generation of these highly enriched small-degree partial melts.

MORB-type neon in an enriched mantle beneath Etna, Sicily

Noble gas elemental and isotopic compositions were determined for five CO 2–CH 4 samples collected around Etna, Sicily, to investigate the geochemical features of the mantle beneath the volcano. The samples contain mantle-derived noble gases. The measured helium isotopic ratios ( 3He/ 4He) vary between 5.9 and 6.4 times atmospheric ratio ( R a=1.4×10 −6), which are comparable to the ratios of olivines (6.1–8.2 R a) in the lavas of the same volcano [1]. Neon in the samples is enriched in both 20Ne and 21Ne ( 20Ne/ 22Ne 9.95–10.7, 21Ne/ 22Ne 0.030–0.037), indicating derivation from the mantle. The δ( 20Ne/ 22Ne)/ δ( 21Ne/ 22Ne) values are identical with that of mid-ocean ridge basalts (MORB), indicating a similarity in the time-integrated (U/Ne) ratio between the Etnean magma source and the depleted upper mantle (MORB source). Argon in the samples has 40Ar/ 36Ar ratios up to of 1800, which are higher than in atmosphere. These ratios are positively correlated with the 20Ne/ 22Ne ratios, indicating a mantle origin of the radiogenic argon. Compared with olivines from the Etnean lavas [1], the argon in our natural gas samples is less contaminated by atmospheric argon. Two samples from a CO 2 well show small but resolvable excess of 129Xe and 134,136Xe. The volcanic rocks of Etna, ranging from tholeiites to alkaline, are enriched in incompatible elements. Nd, Sr and Pb isotopes of the volcanic rocks indicate that the magma source is isotopically heterogeneous and contain a component with high Sr, Pb and low Nd isotopic ratios derived from a mantle region which has been enriched in incompatible elements for a few million years [2,3,4] relative to the depleted upper mantle. The helium isotopic ratios of the gas samples are lower than those of MORB and are consistent with geochemical signatures of the solid elements in the Etnean volcanic rocks. However, the observed MORB-type neon is on a MORB correlation array without accumulation of nucleogenic 21Ne. This apparent decoupling may be explained by a recent mixing of the depleted upper mantle (MORB source) with fluid enriched in both incompatible elements and radiogenic 4He. When the fluid was formed with a small degree of partial melting, fractionation of radiogenic 4He from nucleogenic 21Ne could have occurred because of smaller partition coefficient of He than Ne.

Hafnium isotope evidence for the origin of Cenozoic basaltic lavas from the southwestern United States

Hafnium isotope Hafnium analyses of 52 Cenozoic basalts from the southwestern United States document the differences and similarities of the mantle beneath the continents, as compared to the suboceanic mantle. One of the major conclusions of this work is documentation of a widespread Nd‐Hf isotope array that is oblique to that of the oceanic island basalt (OIB) array, indicating that the subcontinental mantle forms an important component to the Nd‐Hf isotope balance of the Earth. Basalts from the Basin and Range province have Sr, Nd, and Pb isotope compositions that overlap those of OIB, and have been interpreted by many workers to have been derived from an OIB‐like mantle plume [e.g., Fitton et al, 1991; Kempton et al., 1991]. However, the Hf isotope compositions of Basin and Range basalts do not overlap those of OIB, and are instead more similar to mid‐ocean ridge basalt (MORB) mantle. Recognition of a MORB source for high‐εNd Basin and Range lavas is possible only with addition of Hf isotope data. The Pb, Sr, Nd, and Hf isotope compositions and trace element contents of Basin and Range basalts can be matched by mixing melts that were derived from small degrees of partial melting of a depleted MORB mantle, with partial melts of incompatible element enriched pyroxenite veins. In order to match the low Lu/Hf ratios measured in these basalts, one of these components (pyroxenite veins or depleted peridotite) must have been garnet bearing. Moreover, to match the positive εNd and εHf values of Basin and Range basalts, these lithophile element‐enriched pyroxenite veins must be relatively young; we suggest they reflect mantle veining during earlier widespread Mesozoic or Cenozoic magmatism. Basalts from the Rocky Mountains and western Great Basin have similar trace element contents, high lithophile element contents and high large‐ion lithophile element (LILE) to high‐field‐strength element (HFSE) ratios. The Sr, Nd, and Pb isotope compositions of most western Great Basin samples plot along the enriched mantle (EM) II array of Zindler and Hart [1986], but basalts from the Rocky Mountains plot along the EM I array. The Hf isotope compositions of western Great Basin and Rocky Mountain samples overlap. However, these Hf isotope compositions are anomalous relative to the OIB array; most samples have higher 8nf values at a given εNd value as compared to OIB. The lead isotope compositions of both the Rocky Mountain and western Great Basin samples plot significantly above the northern hemisphere reference line in terms of 207Pb/204Pb ratios, supporting models for input of crustal material into the mantle. Crustal recycling into the mantle can produce a mantle source region that has high LILE/HFSE ratios, which are a characteristic of the source of basalts from the western Great Basin and Rocky Mountains. To produce the negative εNd and εHf values, but high εHf values at a given εNd value (as compared to the OIB mantle), the crustal material input into the mantle must be a blend of pelagic and turbidite sediments. Ancient subduction and storage of pelagic sediments in the mantle will produce a source region with negative εNd values but positive εHf values. In contrast, subduction of turbidite sediments will produce a mantle that has negative εHf and εNd values, but low εHf values at a given εNd value as compared to the OIB mantle. A mixture of pelagic to turbidite sediments that has a ratio greater than 1:1.2 can produce the negative εHf and εNd values, and high εHf values at a given εNd value (relative to OIB).

Geochemistry and petrology of the Galeras Volcanic Complex, Colombia

The Galeras Volcanic Complex (GVC) has erupted lavas and pyroclastic flows, ranging from basaltic andesites to dacites, during the last million years. The chemical composition of the volcanic products has been fairly constant, characterized by calc-alkaline, high-silica, medium-potassium andesites. Variations from basaltic andesites to dacites have occurred during individual stages, but overall, composition and petrographic characteristics are quite constant. The Guaca cinder cone is the exception, being more alkaline, rather than calc-alkaline. The trace-element and REE contents of the GVC samples suggest that sediments or fluids from the subducted sediments are involved in the generation of the magmas at the source region. Although high Sr and Ba contents are also present in the upper continental crust, the high B levels are consistent with the contribution of sediment-related elements. Small degrees of partial melting of a hydrated mantle wedge are consistent with the observed distribution of REE, plus the addition of highly soluble elements from the subducted sediments. Crystal fractionation at depths of about 36 km (approximately 12 kbar) of a hydrated tholeiitic basalt melt can produce andesites and basaltic andesites having a phase assemblage of amphibole, plagioclase, clinopyroxene, magnetite, ilmenite and apatite. The magma then re-equilibrates at higher levels, reaching levels as shallow as 10 km (about 2.5 to 3 kbar) and crystallizing orthopyroxene and large amounts of plagioclase. At these shallow levels, amphibole is no longer stable and re-equilibrates to oxides, pyroxene and plagioclase. The melts become saturated with respect to most volatiles by depressurization and crystallization. In the GVC, we observe that it is not necessary to have large volumes of highly differentiated magma to cause caldera-forming eruptions.

Relationship between deformation, equilibration temperatures, REE and radiogenic isotopes in mantle xenoliths (Ray Pic, Massif Central, France): an example of plume-lithosphere interaction?

Anhydrous spinel peridotite xenoliths from the Ray Pic Quaternary alkali basalt volcano (French Massif Central) show a wide range of mineralogical and geochemical compositions, reflecting significant heterogeneities in the shallow sub-continental lithospheric mantle. Variations in modal mineralogy, mineral chem istry, REE patterns and radiogenic isotope data suggest that depletion by partial melting and enrichment by cryptic metasomatism were the major mantle processes which caused the heterogeneity. The lithospheric mantle beneath Ray Pic contains two contrasting types of peridotite: (i) lherzolites with LREE-depleted compositions, high 143Nd/144Nd, low 87Sr/86Sr and unradiogenic Pb isotope ratios; (ii) lherzolites, harzburgites and a wehrlite with LREE-enriched patterns, low 143Nd/144Nd, high 87Sr/86Sr and radiogenic Pb isotope ratios. The former closely resemble depleted MORB-source mantle. The latter are related to enrichment by recent infiltration of small degree partial melts or fluids from the asthenospheric mantle, possibly related to the “low velocity component” observed by Hoernle et al. (1995) in European Neogene alkaline magmas. Thus, the Ray Pic peridotite xenoliths represent interaction between asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle. This is probably linked to the upwelling mantle plume imaged beneath the Massif Central (Granet et al. 1995). A relationship between textural deformation, equilibration temperature and geochemistry of the xenoliths suggests that the hotter (> 900 °C) undeformed regions are LREE-enriched and tend to have more enriched isotope ratios, whereas the cooler (< 900 °C) regions have undergone more deformation and are more depleted both in LREE and in isotope compositions.

Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development

Magmatism that accompanied the 1.1 Ga Midcontinent rift system (MRS) is attributed to the upwelling and decompression melting of a mantle plume beneath North America. Five distinctive flood-basalt compositions are recognized in the rift-related basalt succession along the south shore of western Lake Superior, based on stratigraphically correlated major element, trace element, and Nd isotopic analyses. These distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other MRS localities around western Lake Superior. Four of these compositions are also recognized at Mamainse Point more than 200 km away in eastern Lake Superior. These regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood-basalt magmatism during rift development, extending a model originally developed for eastern Lake Superior. In this refined model, the earliest basalts were derived from small degrees of partial melting at great depth of an enriched, ocean-island-type plume mantle source (εNd(1100) value of about 0), followed by magmas representing melts from this plume source and interaction with another mantle source, most likely continental lithospheric mantle (εNd(1100) &lt; 0). The relative contribution of this second mantle source diminished with time as larger degree partial melts of the plume became the dominant source for the voluminous younger basalts (εNd(1100) value of about 0). Towards the end of magmatism, mixtures of melts from the plume and a depleted asthenospheric mantle source became dominant (εNd(1100) = 0 to +3).