Mid-ocean Ridge Basalt Source Research Articles

Melt–Peridotite Reactions and Fluid Metasomatism in the Upper Mantle, Revealed from the Geochemistry of Peridotite and Gabbro from the Horoman Peridotite Massif, Japan

An investigation of the petrology and geochemistry of peridotites and gabbros in the Horoman massif, Hokkaido, Japan was undertaken to constrain geochemical processes in the upper mantle. Two types of sample were studied: one type comprises peridotites and gabbros forming thin layers varying from a few millimeters to centimeters in scale (thin-layer peridotites and gabbros); the other comprises thick layers (>1 m scale; massive peridotites and gabbros). There is no clear trace element evidence for metasomatism in the thin-layer peridotites. Instead, they have melt–rock reaction textures interpreted in terms of the formation of secondary pyroxene at the expense of primary porphyroclastic olivine and dissolution of primary porphyroclastic pyroxene to form secondary olivine. The thin-layer gabbros also exhibit no metasomatic effects; they have incompatible element depleted trace element characteristics and mid-ocean ridge basalt (MORB)-like isotopic signatures consistent with the presence of a new type of gabbro that previously has not been described from the Horoman Massif. The whole-rock chemistry of the thin-layer peridotites and thin-layer gabbros can be explained by melt–peridotite reactions between isotopically highly depleted MORB mantle (represented by the thin-layer peridotites) and melt with geochemical affinity to Pacific MORB (represented by the thin-layer gabbros). Sm–Nd and Lu–Hf isotope systematics suggest that these reactions might have occurred at ∼300 Ma. Some of the plagioclase lherzolites and all of the spinel lherzolites and harzburgites within the massive peridotites show enrichment in incompatible trace elements and more radiogenic Hf–Nd–Pb isotopic compositions than the incompatible-element depleted thin-layer peridotites. The analyzed massive gabbros are interpreted as subduction-related magmas formed in a MORB-source mantle wedge, which have subsequently interacted with a fluid or melt in the Hidaka subduction zone. Hf–Nd–Pb isotope systematics reveal that this interaction may have occurred at an age younger than ∼50 Ma. Melt- and fluid-related processes occurring in the upper mantle are systematically identified from the samples of the Horoman Massif based on petrography, major and trace element, and Sr–Nd–Pb–Hf isotope geochemistry. These processes occurred in different tectonic settings such as the convecting oceanic mantle and supra-subduction zone mantle wedge and have variably modified the original chemistry of residual mantle protolith, formed by partial melting of a depleted MORB source mantle at ∼1 Ga.

Noble gases in the dynamic mantle

It is proposed that noble gases in the mantle reside mainly in a “hybrid pyroxenite” component that is formed as melt from old subducted oceanic crust reacts with surrounding refractory peridotite under mid‐ocean ridges. The gases are presumed to have been acquired early in Earth history, and mixing under mid‐ocean ridges of melts from pyroxenites and old crust would pass the noble gases to successive generations of pyroxenites. Much of the complement of noble gases would not be erupted at mid‐ocean ridges and would recirculate in the mantle, so the hybrid pyroxenite would degas rather slowly over Earth history. The hybrid pyroxenite would also be denser than average mantle and would tend to settle into the D″ zone at the base of the mantle, in the same way as subducted oceanic crust. Because residence times in D″ are longer than in the rest of the mantle, the D″ pyroxenite would be less degassed and its noble gases would be less radiogenic than that in the mid‐ocean ridge basalt (MORB) source. Plumes could therefore tap a mixture of old, degassed oceanic crust and less degassed hybrid pyroxenite. This could resolve the long‐standing question of the source of unradiogenic helium in many ocean island basalts (OIBs). Abundances of noble gases are not very well constrained and are plausibly larger than conventional estimates, which would remove the need for a large, deep, “undegassed” reservoir. The 40Ar mass balance may be satisfied if account is taken of uncertainties in the potassium content of the continental crust and the bulk silicate Earth and in the Earth's total budget of 40Ar. A relatively simple quantitative theory for the evolution of the noble gases is presented that accounts for the present concentrations and isotopic compositions of helium, neon, and argon in the MORB and OIB source regions. In particular, it confirms that longer residence times in D″ can account for the less radiogenic noble gases in OIBs.

Reconciling the geophysical and geochemical mantles: Plume flows, heterogeneities, and disequilibrium

Geophysical evidence and numerical models of mantle stirring imply the source of mid‐ocean ridge basalts (MORBs) comprises most of the mantle, excepting only the D″ region and the “superpile” anomalies deep under Africa and the Pacific. Geophysical evidence is also strong that the mantle is heated substantially from within. Geochemical inferences of a strongly depleted MORB source are inconsistent with this picture because they would require the MORB source to be heated mainly from below and because they cannot accommodate all of the Earth's incompatible elements. Lacking any other large mantle reservoir, the MORB source is required to balance the global uranium budget, which implies a U concentration of about 10 ng/g, more than double recent estimates. The MORB source would then have been depleted only by a factor of two in highly incompatible elements, rather than four or more, relative to is primitive composition. Both geophysical and geochemical evidence support a heterogeneous, multicomponent MORB source. Surprisingly, former plume material may comprise 25% of the MORB source, and this alone could add 50–100% to previous inventories of incompatible elements. Previous geochemical estimates may also be less secure because of a continuing focus on the more common, more depleted MORBs, because of long chains of geochemical inference, and because of a reliance on peridotites that may not have equilibrated with the mean composition of the heterogeneous source. Mean compositions are of most geophysical relevance, rather than putative end‐member compositions, but mean compositions will be difficult to estimate accurately because more enriched components are less common and more variable. Nevertheless, a reconciliation of geochemical and geophysical inferences seems possible.

Noble gas isotopic compositions of mantle xenoliths from northwestern Pacific lithosphere

We measured noble gas isotopic compositions of mantle xenoliths and xenocrystic olivines sampled from seamounts—so-called petit-spot volcanoes—on the 135-million-year-old northwestern Pacific Plate. The xenoliths are spinel lherzolites originating from suboceanic lithospheric mantle. The samples' 3He/ 4He ratios are 7.0–8.5 Ra, where Ra signifies atmospheric 3He/ 4He. The 40Ar/ 36Ar ratios are as high as 7000. These observations suggest that the noble gas isotopic compositions of suboceanic lithospheric mantle resemble those of mid-ocean ridge basalt (MORB). A mantle source with a He/U ratio as high as an assumed value for MORB source is necessary to maintain the MORB-like 3He/ 4He over 135 million years, implying that melt extraction at mid-ocean ridges only slightly alters the He/U ratio of the oceanic upper mantle. The 4He/ 40Ar⁎ ratios of the samples described herein are much lower than the theoretical radiogenic production ratio, where an asterisk denotes correction for atmospheric contribution. The low 4He/ 40Ar⁎ is inferred to result from kinetic fractionation in the mantle. When magma migrates through a mantle source, lighter noble gases in the mantle source diffuse selectively into magma channels. The MORB generation does not cause low 4He/ 40Ar⁎, however. If a mantle source is depleted in lighter noble gases during ancient MORB generation, then noble gas isotopic compositions of the mantle source are affected over time by accumulation of radiogenic nuclides. Thereby, the mantle source adopts a radiogenic or nucleogenic noble gas isotopic composition. Recent kinetic fractionation contributes to the low 4He/ 40Ar⁎ in the samples. Based on the diffusive fractionation model, more than 100 years are necessary to produce a residual mantle source with 4He/ 40Ar⁎, as observed in the samples. However, petit-spot volcanoes, found as small knolls, seem to erupt within a short period. The low 4He/ 40Ar⁎ of the samples implies prior volcanism in this region. No recent volcanism has occurred near this region aside from petit-spot volcanism, indicating that petit-spot volcanoes are polygenetic with a long active period. Petit-spot volcanoes are regarded as common magmatism on the subducting oceanic plate. Consequently, the thermal structure and temperature-dependent physical properties of the oceanic plate are, at least partly, affected by remnant heat of the magmatism.

Chemical and chronologic complexity in the convecting upper mantle: Evidence from the Taitao ophiolite, southern Chile

Exposure of the ca. 6 Ma Taitao ophiolite, Chile, located ∼50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ 18O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ 18O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by 87Sr/ 86Sr and 143Nd/ 144Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial 187Os/ 188Os ratios that range widely from 0.1168 to 0.1288 ( γ Os = −8.0 to +1.1), averaging 0.1239 ( γ Os = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg # of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions. In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ 18O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial 187Os/ 188Os ratios, calculated for 6 Ma, that range from 0.126 ( γ Os = −1) to as high as 0.561 ( γ Os = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in 187Os/ 188Os between MORB and abyssal peridotites. The collective results of this study suggest that this >500 km 3 block of the mantle underwent at least two stages of melting. The first stage occurred at ∼1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes.

Across‐arc geochemical variation of Quaternary lavas in West Java, Indonesia: Mass‐balance elucidation using arc basalt simulator model

AbstractThe Sunda Arc of Indonesia developed along the convergent margin between the Eurasian and the Australian Plates. More than 100 Quaternary volcanic centers occur along the arc. The West Java Arc is a segment of the Sunda Arc in which more than 10 volcanic centers are located, corresponding to the 120 to 200 km depth contours of the Wadati–Benioff zone. The geochemistry of 207 Quaternary lavas from six centers across the arc was investigated. The lavas range from basalt to dacite. Incompatible element abundances increase from the volcanic front to the rear‐arc in response to a change from low‐K to high‐K suites. Nd–Sr isotope compositions of the basalts scatter between mid‐ocean ridge basalt (MORB) source mantle and Indian Ocean sediment (SED) compositions, with volcanic front low‐K basalts having more radiogenic Nd than the rear‐arc basalts. It is suggested that mixing between slab‐derived fluids mainly from the SED and melt from MORB source mantle played a significant role in determining the geochemistry of the West Java basalts. Incompatible element patterns in primitive mantle normalized multi‐element plots are almost identical across the arc, except for greater inclination and weaker positive Sr spikes in the rear‐arc basalts. This suggests a lower degree of partial melting in the rear‐arc mantle, accompanied by change in SED fluid composition between the volcanic front and the rear‐arc. The latter is confirmed by fluid‐fluxed melting model calculations using multiple trace elements and Nd and Sr isotopes. All the West Java Arc lavas require deficit of Sr from the slab SED. This may occur due to selective breakdown of Sr‐rich hydrous silicate minerals, such as zoisite, at shallower depths before the SED component reaches the depth of dehydration effective for magma genesis. The rear‐arc basalts need further Sr deficits along with lesser fluid. These features are commonly observed in many arc basalts, and are likely attributable to the same mechanism.

Volatile composition of microinclusions in diamonds from the Panda kimberlite, Canada: Implications for chemical and isotopic heterogeneity in the mantle

In order to better investigate the compositions and the origins of fluids associated with diamond growth, we have carried-out combined noble gas (He and Ar), C and N isotope, K, Ca and halogen (Cl, Br, I) determinations on fragments of individual microinclusion-bearing diamonds from the Panda kimberlite, North West Territories, Canada. The fluid concentrations of halogens and noble gases in Panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances. However, noble gas, C and N isotopic ratios ( 3He/ 4He = 4–6 Ra, 40Ar/ 36Ar = 20,000–30,000, δ 13C = −4.5‰ to −6.9‰ and δ 15N = −1.2‰ to −8.8‰) are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt (MORB) source value. The high 36Ar content of the diamonds (>1 × 10 −9 cm 3/g) is at least an order of magnitude higher than any previously reported mantle sample and enables the 36Ar content of the subcontinental lithospheric mantle to be estimated at ∼0.6 × 10 −12 cm 3/g, again similar to estimates for the MORB source. Three fluid types distinguished on the basis of Ca–K–Cl compositions are consistent with carbonatitic, silicic and saline end-members identified in previous studies of diamonds from worldwide sources. These fluid end-members also have distinct halogen ratios (Br/Cl and I/Cl). The role of subducted seawater-derived halogens, originally invoked to explain some of the halogen ratio variations in diamonds, is not considered an essential component in the formation of the fluids. In contrast, it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid–melt partitioning in forming silicic fluids, and during separation of an immiscible saline fluid.

New constraints on the HIMU mantle from neon and helium isotopic compositions of basalts from the Cook–Austral Islands

High 4He/ 3He ratios of 100 000 to 160 000 found at HIMU ocean islands (“high μ,” where μ is the U/Pb ratio) are usually attributed to the presence of recycled oceanic crust in the HIMU mantle source. However, significantly higher 4He/ 3He ratios are expected in recycled crust after residence in the mantle for periods greater than 1 Ga. In order to better understand the helium isotopic signatures in HIMU basalts, we have measured helium and neon isotopic compositions in a suite of geochemically well-characterized basalts from the Cook–Austral Islands. We observe 4He/ 3He ratios ranging from 56 000 to 141 000, suggesting the involvement of mantle reservoirs both more and less radiogenic than the mantle source for mid-ocean ridge basalts (MORBs). In addition, we find that the neon isotopic compositions of HIMU lavas extend from the MORB range to compositions less nucleogenic than MORBs. The Cook-Austral HIMU He–Ne isotopic compositions, along with Sr, Nd, Pb, and Os isotopic compositions, indicate that in addition to recycled crust, a relatively undegassed mantle end-member (e.g., FOZO) is involved in the genesis of these basalts. The association of relatively undegassed mantle material with recycled crust provides an explanation for the close geographical association between HIMU lavas and EM (enriched mantle)-type lavas from this island chain: EM-type signatures represent a higher mixing proportion of relatively undegassed mantle material. Mixing between recycled material and relatively undegassed mantle material may be a natural result of entrainment processes and convective stirring in deep mantle.

Composition of the Earth's interior: the importance of early events

The detection of excess 142Nd caused by the decay of 103Ma half-life 146Sm in all terrestrial rocks compared with chondrites shows that the chondrite analogue compositional model cannot be strictly correct, at least for the accessible portion of the Earth. Both the continental crust (CC) and the mantle source of mid-ocean ridge basalts (MORB) originate from the material characterized by superchondritic 142Nd/144Nd. Thus, the mass balance of CC plus mantle depleted by crust extraction (the MORB-source mantle) does not sum back to chondritic compositions, but instead to a composition with Sm/Nd ratio sufficiently high to explain the superchondritic 142Nd/144Nd. This requires that the mass of mantle depleted by CC extraction expand to 75-100 per cent of the mantle depending on the composition assumed for average CC. If the bulk silicate Earth has chondritic relative abundances of the refractory lithophile elements, then there must exist within the Earth's interior an incompatible-element-enriched reservoir that contains roughly 40 per cent of the Earth's 40Ar and heat-producing radioactive elements. The existence of this enriched reservoir is demonstrated by time-varying 142Nd/144Nd in Archaean crustal rocks. Calculations of the mass of the enriched reservoir along with seismically determined properties of the D'' layer at the base of the mantle allow the speculation that this enriched reservoir formed by the sinking of dense melts deep in a terrestrial magma ocean. The enriched reservoir may now be confined to the base of the mantle owing to a combination of compositionally induced high density and low viscosity, both of which allow only minimal entrainment into the overlying convecting mantle.

Cretaceous‐oligocene multiphase magmatism on three kings islands, northern New Zealand

The Three Kings Islands, New Zealand, contain some of the few on‐land autochthonous examples of Cretaceous‐Oligocene magmatism in the Southwest Pacific region. Analyses of primarily mafic lavas reveal two different geochemical signatures, both predominantly medium‐K series lavas with Mg# <40 and characteristically flat, high field strength element and high large‐ion lithophile element contents normalised against mid ocean ridge basalts (MORB). Group 1 lavas have continental arc signatures and were generated by partial melting of an N(normal)‐MORB source indicating formation during the final stages of arc volcanism as crustal thinning began, during spreading along the eastern Gondwana margin. The younger Group 2 lavas were generated from an enriched mantle source with high degrees of partial melting and have minor continental signatures; we propose they represent Oligocene arc volcanism. These results support the hypothesis that between the mid cretaceous and early Tertiary, the New Zealand sector of eastern Gondwanaland underwent long periods of compressional tectonics resulting in subduction‐related volcanism and associated magmatism.

Helium deficit in high‐3He/4He parent magmas: Predegassing fractionation, not a “helium paradox”

We evaluated He‐Ne‐Ar elemental compositions of mid‐ocean ridge basalt (MORB) glasses and basalt glasses from the Hawaiian hot spot (Loihi Seamount, Kilauea) to determine the (3He/22Ne)solar, 4He*/21Ne*, and 4He*/40Ar* MORB‐type and “plume”‐type end‐member compositions within a binary mixing model. Although the 3He/22Ne ratios of “plume” magma and MORB magma marginally agree with current estimates of solar compositions, the calculated 4He*/21Ne* ratios are lower than the theoretical production ratio and indicate that the “plume” magma component experienced a predegassing fractionation event. This fractionation event resulted in a deficit in both radiogenic 4He* and primordial 3He and accounts for the differences in 3He/22Ne, 4He*/21Ne*, and 4He*/40Ar* ratios and 3He concentrations for both mantle domains. This resolves an apparent contradiction previously termed “mantle He paradox,” though the derived 3He concentrations are also at odds with a primitive mantle concept that requires still higher concentrations. We propose that the primary fractionation of He/Ne and He/Ar ratios is related to a more compatible behavior of He relative to Ne, Ar during partial melting. One model is presented: at depths below a shallow MORB source region a rising mantle plume causes formation of low‐degree partial melts, which are deficient in He. During melt ascent, these “plume”‐derived melts mix with melts of the shallow (MORB‐type) mantle source that experienced a higher degree of melting and hence show no He deficit. Depending on the relative proportion of mixing the resulting oceanic basalts will encompass the whole observed range from Loihi‐type and MORB‐type noble gas isotope compositions.

Plume–ridge interaction revisited: Evidence for melt mixing from He, Ne and Ar isotope and abundance systematics

Submarine volcanic glass He, Ne and Ar data from the Pacific–Antarctic-Rise at its intersection with the Foundation Seamount Chain have been obtained to further elucidate one of the main outstanding problems in plume–ridge interaction studies: the physical state in which mantle flow and mixing occur in those settings. Mixing of plume mantle and mid-ocean ridge basalt source material may either be in the form of two solids, of solid-melt or of two melt phases only. The positive correlation of 4He/ 40Ar ⁎ with 206Pb/ 204Pb as well as the negative correlation of 4He/ 40Ar ⁎ with 40Ar ⁎ and 4He concentrations, combined with a lack of correlation between 4He/ 40Ar ⁎ and ridge depth or MgO concentration, indicate that the enriched mantle plume material has been extensively degassed prior to mixing with the mid-ocean ridge source material. Degassing of the plume source and mid-ocean ridge source material prior to mixing is also suggested by the He versus Pb isotope systematics. However, the occurrence of degassing processes requires the existence of melts, implying that mixing between the plume and the mid-ocean ridge material occurs in the physical form of melts.

Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism

Radioactive decay of uranium and thorium produces 4He, whereas 3He in the Earth's mantle is not produced by radioactive decay and was only incorporated during accretion-that is, it is primordial. 3He/4He ratios in many ocean-island basalts (OIBs) that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts (MORBs). This is inferred to be the result of outgassing by melt production at mid-ocean ridges in conjunction with radiogenic ingrowth of 4He, which has led to a volatile-depleted upper mantle (MORB source) with low 3He concentrations and low 3He/4He ratios. Consequently, high 3He/4He ratios in OIBs are conventionally viewed as evidence for an undegassed, primitive mantle source, which is sampled by hot, buoyantly upwelling deep-mantle plumes. However, this conventional model provides no viable explanation of why helium concentrations and elemental ratios of He/Ne and He/Ar in OIBs are an order of magnitude lower than in MORBs. This has been described as the 'helium concentration paradox' and has contributed to a long-standing controversy about the structure and dynamics of the Earth's mantle. Here we show that the helium concentration paradox, as well as the full range of noble-gas concentrations observed in MORB and OIB glasses, can self-consistently be explained by disequilibrium open-system degassing of the erupting magma. We show that a higher CO2 content in OIBs than in MORBs leads to more extensive degassing of helium in OIB magmas and that noble gases in OIB lavas can be derived from a largely undegassed primitive mantle source.

Stirring in three‐dimensional mantle convection models and implications for geochemistry: 2. Heavy tracers

Three‐dimensional, spherical models of subduction and stirring of oceanic crust, taking account of the excess density of subducted oceanic crust in the mantle, confirm the main results from previous two‐dimensional models. Thus we find that the excess density of subducted crust significantly increases the ages of ocean island basalt (OIB) samples, assumed to come from the bottom 100 km of the mantle, but has little influence on the age of mid‐ocean ridge basalt (MORB) samples from the top of the mantle. The degree of processing of the mantle through melting zones is also little affected, and the Earth's mantle is still about 98% processed after 4.5 Ga. The latter results indicate that the processing time is still the main parameter controlling mantle processing and apparent MORB source ages, even with heavy tracers. However, the mean age of model OIB samples is significantly larger than observed apparent isotopic ages. Simplified estimates indicate that this discrepancy can be resolved by assuming that a population of ancient tracers in the OIB sampling zone, evident in the numerical models, is only marginally sampled by plumes because they are concentrated in denser accumulations at the bottom of the compositional boundary layer.

Cenozoic behind-arc volcanism in the Bolivian Andes, South America: implications for mantle melt generation and lithospheric structure

We develop a model of arc and behind-arc volcanism, as well as constraining the evolution of the lithospheric mantle structure beneath the Bolivian Andes during the last 25 Ma, using new geochronological and geochemical data on behind-arc mafic volcanic rocks, together with a regional isotopic study of geothermal helium emissions, and a comparison with seismic tomographic images of the same region. Helium isotopes measured in natural gas emissions in geothermal and mineral-water springs suggest recent mantle melting in a wide zone extending at c . 20°S from a few tens of kilometres west of the volcanic arc to over 300 km behind the arc, where the subducted slab is at depths increasing from c . 100 to c . 250 km. During the Neogene, there has been behind-arc mafic–felsic magmatism right across the high Andes between 17° and 22°S, in the Altiplano and western margin of the Eastern Cordillera. Modelling of Plio-Pleistocene basalts and basaltic andesites (&lt;5.2 Ma), using REE, suggests that the main melt source region is in spinel and spinel–garnet peridotite, at depths &lt;100 km. In detail, the best-fit REE inversions suggest two spatially distinct melt zones: (1) a c . 15% depleted mid-ocean ridge basalt (MORB) source at depths between c . 75 km and c . 110 km for mafic volcanic rocks that crop out &lt;75 km behind the arc; (2) a slightly enriched MORB source at depths between c . 65 km and c . 90 km for those that crop out &gt;100 km behind the arc. REE inversions for Oligo-Miocene shoshonites and basalts ( c . 25–21 Ma) suggest that they originated at significantly shallower depths ( c . 45 km to c . 100 km) from a slightly enriched MORB source. Eruption of shoshonitic lavas also occurred in the Altiplano between 13 and 11 Ma, and widespread behind-arc intermediate–felsic volcanism has been more or less continuous since the early Miocene. All this suggests that since c . 25 Ma there has been a thin (&lt;100 km) lithosphere beneath the high part of the Bolivian Andes. Long-lived mantle melting is best explained by wet adiabatic decompression in the upwelling part of the corner flow in the mantle wedge, controlled by the lithospheric thickness variations, with depleted melt advected into the melt zone beneath the arc. Widespread arc and behind-arc magmatism may have been triggered at c . 25 Ma by the opening up of a mantle wedge and inflow of hot asthenosphere, as a consequence of the steepening of the subducted slab and detachment of hydrated and weakened mantle lithosphere and possibly mafic lower crust.

Origin of CFB Magmatism: Multi-tiered Intracrustal Picrite-Rhyolite Magmatic Plumbing at Spitzkoppe, Western Namibia, during Early Cretaceous Etendeka Magmatism

Early Cretaceous tholeiitic picrite-to-rhyolite dykes around Spitzkoppe, western Namibia, are part of the extensive Henties Bay–Outjo swarm, penecontemporaneous with ∼132 Ma Etendeka lavas ∼100 km to the NW. Although only intermediate to rhyolitic dykes contain clinopyroxene phenocrysts, the behaviour of Ca, Al and Sc in the dyke suite shows that liquidus clinopyroxene—together with olivine—was a fractionating phase when MgO fell to ∼9 wt %. Both a plot of CIPW normative di–hy–ol–ne–Q and modelling using (p)MELTS show that a mid-crustal pressure of ∼ 0·6 GPa is consistent with this early clinopyroxene saturation. Sr, Nd, Hf and Pb isotope variations all show trends consistent with AFC contamination (assimilation linked to fractional crystallization), involving Pan-African Damara belt continental crust. The geochemical variation, including isenthalpic AFC modelling using (p)MELTS, suggests that the picrites (olivine-rich cumulate suspensions) were interacting with granulite-facies metamorphic lower crust, the intermediate compositions with amphibolite-facies middle crust, and the rhyolitic dykes (and a few of the basalts) with the Pan-African granites of the upper crust. The calculated densities of the magmas fall systematically from picrite to rhyolite and suggest a magmatic system resembling a stack of sills throughout the crust beneath Spitzkoppe, with the storage and fractionation depth of each magma fraction controlled by its density. Elemental and isotopic features of the ∼20 wt % MgO picrites (including Os isotopes) suggest that their parental melts probably originated by fusion of mid-ocean ridge basalt (MORB) source convecting mantle, followed by limited reaction with sub-continental lithospheric mantle metasomatized just prior to the formation of the parental magmas. Many of the distinctive features of large-volume picritic–basaltic magmas may not be derived from their ultimate mantle sources, but may instead be the results of complex polybaric fractional crystallization and multi-component crustal contamination.

Xenon isotope systematics, giant impacts, and mantle degassing on the early Earth

The relationships between the major terrestrial volatile reservoirs are explored by resolving the different components in the Xe isotope signatures displayed by Harding County and Caroline CO 2 well gases and mid-ocean ridge basalts (MORB). For the nonradiogenic isotopes, there is evidence for the presence of components enhanced in the light 124–128Xe/ 130Xe isotope ratios with respect to the terrestrial atmosphere. The observation of small but significant elevations of these ratios in the MORB and well gas reservoirs means that the nonradiogenic Xe in the atmosphere cannot be the primordial base composition in the mantle. The presence of solar-like components, for example U–Xe, solar wind Xe, or both, is required. For radiogenic Xe generated by decay of short-lived 129I and 244Pu, the 129Xe rad/ 136Xe 244 ratios are indistinguishable in MORB and the present atmosphere, but differ by approximately an order of magnitude between the MORB and well gas sources. Correspondence of these ratios in MORB and the atmosphere within the relatively small uncertainties found here significantly constrains possible mantle degassing scenarios. The widely held view that substantial early degassing of 129Xe rad and 136Xe 244 from the MORB reservoir to the atmosphere occurred and then ended while 129I was still alive is incompatible with equal ratios, and so is not a possible explanation for observed elevations of 129Xe/ 130Xe in MORB compared to the atmosphere. Detailed degassing chronologies constructed from the isotopic composition of MORB Xe are therefore questionable. If the present estimate for the uranium/iodine ratio in the bulk silicate Earth (BSE) is taken to apply to all interior volatile reservoirs, the differing 129Xe rad/ 136Xe 244 ratios in MORB and the well gases point to two episodes of major mantle degassing, presumably driven by giant impacts, respectively ∼ 20–50 Ma and ∼ 95–100 Ma after solar system origin assuming current values for initial 129I/ 127I and 244Pu/ 238U. The earlier time range, for degassing of the well gas source, spans Hf–W calculations for the timing of a moon-forming impact. The second, later impact further outgassed the upper mantle and MORB source. A single event that degassed both the MORB and gas well reservoirs at the time of the moon-forming collision would be compatible with their distinct 129Xe rad/ 136Xe 244 ratios only if the post-impact iodine abundance in the MORB reservoir was about an order of magnitude lower than current estimates. In either case, such late dates require large early losses of noble gases, so that initial inventories acquired throughout the Earth must have been substantially higher. The much larger 129Xe rad/ 136Xe 244 ratio in the well gases compared to MORB requires that these two Xe components evolve from separate interior reservoirs that have been effectively isolated from each other for most of the age of the planet, but are now seen within the upper mantle. These reservoirs have maintained distinct Xe isotope signatures despite having similar Ne isotope compositions that reflect similar degassing histories. This suggests that the light noble gas and radiogenic Xe isotopes are decoupled, with separate long-term storage of the latter. However, without data on the extent of heterogeneities within the upper mantle, this conclusion cannot be easily reconciled with geophysical observations without significant re-evaluation of present noble gas models. Nevertheless the analytic evidence that two different values of 129Xe rad/ 136Xe 244 exist in the Earth appears firm. If the uranium/iodine ratio is approximately uniform throughout the BSE, it follows that degassing events from separate reservoirs at different times are recorded in the currently available terrestrial Xe data.

Deep mantle origin of kimberlite magmas revealed by neon isotopes

We measured noble gases in olivines extracted from alteration‐free samples of the Udachnaya‐East kimberlite pipe, eastern Siberia. Neon isotope ratios indicate that the kimberlite has a less nucleogenic Ne signature than a typical mid‐ocean ridge basalt (MORB) source, which is consistent with the less radiogenic 40Ar/36Ar ratio compared to MORB and subcontinental lithospheric mantle (SCLM) sources. Based on He‐Ne systematics, the 3He/4He ratio of the plume source of the Udachnaya kimberlite pipe would be higher than 15 RA with the dominant He signature of the magma acquired by assimilation of SCLM and/or crustal material during magma ascent. Our results indicate that the source of the Udachnaya kimberlite has similar noble gas characteristics to those of ocean island basalts (OIBs). Its origin is therefore deeper than that of the MORB source mantle, and probably is the same as the OIB‐source mantle.

Deep mantle origin of kimberlite magmas revealed by neon isotopes

[1] We measured noble gases in olivines extracted from alteration-free samples of the Udachnaya-East kimberlite pipe, eastern Siberia. Neon isotope ratios indicate that the kimberlite has a less nucleogenic Ne signature than a typical mid-ocean ridge basalt (MORB) source, which is consistent with the less radiogenic 40Ar/36Ar ratio compared to MORB and subcontinental lithospheric mantle (SCLM) sources. Based on He-Ne systematics, the 3He/4He ratio of the plume source of the Udachnaya kimberlite pipe would be higher than 15 RA with the dominant He signature of the magma acquired by assimilation of SCLM and/or crustal material during magma ascent. Our results indicate that the source of the Udachnaya kimberlite has similar noble gas characteristics to those of ocean island basalts (OIBs). Its origin is therefore deeper than that of the MORB source mantle, and probably is the same as the OIB-source mantle.

Th/U and other geochemical evidence for the Réunion plume sampling a less differentiated mantle domain

Like Mid-Ocean Ridge Basalts (MORB), most Ocean Island Basalts (OIB) record a Th/U decrease during the history of their source. Notable exceptions are lavas sampling EM1 mantle end-member, which have super-chondritic time-integrated Th/U ([Th/U] Pb). An intermediate situation is observed for the Réunion plume, where both present-day [Th/U] and [Th/U] Pb are similar to the bulk Earth value (weight ratio ∼ 3.9). If [Th/U] measured in basalts reliably reflects the source ratio, then the Réunion plume source could have evolved in closed system with respect to Th/U. Th/U decrease in the mantle has been ascribed either to early continental crust extraction, or to preferential recycling of U over Th since oxidizing conditions appeared at the surface of the planet (∼ 2.2 Ga). The primitive-like Th/U signature of Réunion is best explained by the absence of subduction influence since at least the Archean. This possibility is consistent with the 187Os/ 186Os signature, which is the less radiogenic of ocean island shield basalts. In addition, the difference in 3He/ 4He between Réunion (R/R A ∼ 12.5) and high- 3He/ 4He plumes (Hawaii, Iceland) most likely reflects a Réunion source less depleted in U and Th, rather than more degassed. In Sr–Nd–Pb–Os–He isotope space, Réunion signature plots in the region where OIB arrays converge, suggesting that the Réunion plume samples a component common to OIB, which is neither the source of MORB, nor the region where subducted plates are stored. It is suggested that the Réunion plume taps an early-depleted mantle domain subsequently influenced little, or not at all by recycling processes.