Oxygen isotope composition of phenocrysts from Tristan da Cunha and Gough Island lavas: variation with fractional crystallization and evidence for assimilation

We have measured the δ¹⁸O values of the major phenocrysts (olivine, clinopyroxene and plagioclase) present in lavas from Tristan da Cunha and Gough Island. These islands, which result from the same mantle plume, have enriched radiogenic isotope ratios and are, therefore, prime candidates for an oxygen isotope signature that is distinct from that of MORB. Consistent differences between the δ¹⁸O values of olivine, pyroxene and feldspar in the Gough lavas show that the phenocrysts in the mafic Gough Island lavas are in oxygen isotope equilibrium. The olivines in lavas with SiO₂ <50 wt% have a mean δ¹⁸O value of 5.19‰, consistent with crystallization from a magma having the same oxygen isotope composition as MORB. Phenocrysts in all the Gough lavas show a systematic increase in δ¹⁸O value as silica content increases, which is consistent with closed-system fractional crystallization. The lack of enrichment in δ¹⁸O of the Gough magmas suggests that the mantle source contained <2% recycled sediment. In contrast, the Tristan lavas with SiO₂ >48 wt% contain phenocrysts which have δ¹⁸O values that are systematically ∼0.3‰ lower than their counterparts from Gough. We suggest that the parental mafic Tristan magmas were contaminated by material from the volcanic edifice that acquired low δ¹⁸O values by interaction with water at high temperatures. The highly porphyritic SiO₂-poor lavas show a negative correlation between olivine δ¹⁸O value and whole-rock silica content rather than the expected positive correlation. The minimum δ¹⁸O value occurs at an SiO₂ content of about 45 wt%. Below 45 wt% SiO₂, magmas evolved via a combination of assimilation, fractionational crystallization and crystal accumulation; above 45 wt% SiO₂, magmas appeared to have evolved via closed-system fractional crystallization. Received: 23 November 1998 / Accepted: 27 September 1999     

The origin of hydrous,high-δ<Superscript>18</Superscript>O voluminous volcanism: diverse oxygen isotope values and high magmatic water contents within the volcanic record of Klyuchevskoy volcano,Kamchatka, Russia

Klyuchevskoy volcano, in Kamchatka’s subduction zone, is one of the most active arc volcanoes in the world and contains some of the highest δ¹⁸O values for olivines and basalts. We present an oxygen isotope and melt inclusion study of olivine phenocrysts in conjunction with major and trace element analyses of ¹⁴C- and tephrochronologically-dated tephra layers and lavas spanning the eruptive history of Klyuchevskoy. Whole-rock and groundmass analyses of tephra layers and lava samples demonstrate that both high-Mg (7–12.5 wt% MgO) and high-Al (17–19 wt% Al₂O₃, 3–6.5 wt% MgO) basalt and basaltic andesite erupted coevally from the central vent and flank cones. Individual and bulk olivine δ¹⁸O range from normal MORB values of 5.1‰ to values as high as 7.6‰. Likewise, tephra and lava matrix glass have high-δ¹⁸O values of 5.8–8.1‰. High-Al basalts dominate volumetrically in Klyuchevskoy’s volcanic record and are mostly high in δ¹⁸O. High-δ¹⁸O olivines and more normal-δ¹⁸O olivines occur in both high-Mg and high-Al samples. Most olivines in either high-Al or high-Mg basalts are not in oxygen isotopic equilibrium with their host glasses, and Δ¹⁸O_{olivine–glass} values are out of equilibrium by up to 1.5‰. Olivines are also out of Fe–Mg equilibrium with the host glasses, but to a lesser extent. Water concentrations in olivine-hosted melt inclusions from five tephra samples range from 0.4 to 7.1 wt%. Melt inclusion CO₂ concentrations vary from below detection (<50 ppm) to 1,900 ppm. These values indicate depths of crystallization up to ~17 km (5 kbar). The variable H₂O and CO₂ concentrations likely reflect crystallization of olivine and entrapment of inclusions in ascending and degassing magma. Oxygen isotope and Fe–Mg disequilibria together with melt inclusion data indicate that olivine was mixed and recycled between high-Al and high-Mg basaltic melts and cumulates, and Fe–Mg and δ¹⁸O re-equilibration processes were incomplete. Major and trace elements in the variably high-δ¹⁸O olivines suggest a peridotite source for the parental magmas. Voluminous, highest in the world with respect to δ¹⁸O, and hydrous basic volcanism in Klyuchevskoy and other Central Kamchatka depression volcanoes is explained by a model in which the ascending primitive melts that resulted from the hydrous melt fluxing of mantle wedge peridotite, interacted with the shallow high-δ¹⁸O lithospheric mantle that had been extensively hydrated during earlier times when it was part of the Kamchatka forearc. Following accretion of the Eastern Peninsula terrains several million years ago, a trench jump eastward caused the old forearc mantle to be beneath the presently active arc. Variable interaction of ascending flux-melting-derived melts with this older, high-δ¹⁸O lithospheric mantle has produced mafic parental magmas with a spectrum of δ¹⁸O values. Differentiation of the higher δ¹⁸O parental magmas has created the volumetrically dominant high-Al basalt series. Both basalt types incessantly rise and mix between themselves and with variable in δ¹⁸O cumulates within dynamic Klyuchevskoy magma plumbing system, causing biannual eruptions and heterogeneous magma products. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts

In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ¹⁸O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ¹⁸O_Olivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H₂O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr₂O₃ and raising Al₂O₃ and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.     

Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends

The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H₂O contents. The experimental results are used to explore the influence of H₂O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H₂O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO₂ contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO₂. We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H₂O-rich (>4.5--6 wt% H₂O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H₂O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustal-level differentiation occurs under near-H₂O saturated conditions producing the distinctive high SiO₂ and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H₂O contents (>10 wt% H₂O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.Editorial responsibility: J. Hoefs     

Oxygen isotope heterogeneity of the mantle beneath the Canary Islands: a discussion of the paper of Gurenko et al.

Gurenko et al. (Contrib Mineral Petrol 162:349–363, 2011) report laser-assisted fluorination (LF) and secondary ionization mass spectrometry (SIMS) ¹⁸O/¹⁶O datasets for olivine grains from the Canary Islands of Gran Canaria, Tenerife, La Gomera, La Palma and El Hierro. As with prior studies of oxygen isotopes in Canary Island lavas (e.g. Thirlwall et al. Chem Geol 135:233–262, 1997; Day et al. Geology 37:555–558, 2009, Geochim Cosmochim Acta 74:6565–6589, 2010), these authors find variations in δ¹⁸O_ol (~4.6–6.0 ‰) beyond that measured for mantle peridotite olivine (Mattey et al. Earth Planet Sci Lett 128:231–241, 1994) and interpret this variation to reflect contributions from pyroxenite-peridotite mantle sources. Furthermore, Gurenko et al. (Contrib Mineral Petrol 162:349–363, 2011) speculate that δ¹⁸O_ol values for La Palma olivine grains measured by LF (Day et al. Geology 37:555–558, 2009, Geochim Cosmochim Acta 74:6565–6589, 2010) may be biased to low values due to the presence of altered silicate, possibly serpentine. The range in δ¹⁸O_ol values for Canary Island lavas are of importance for constraining their origin. Gurenko et al. (Contrib Mineral Petrol 162:349–363, 2011) took a subset (39 SIMS analyses from 13 grains from a single El Hierro lava; EH4) of a more extensive dataset (321 SIMS analyses from 110 grains from 16 Canary Island lavas) to suggest that δ¹⁸O_ol is weakly correlated (R ² = 0.291) with the parameter used by Gurenko et al. (Earth Planet Sci Lett 277:514–524, 2009) to describe the estimated weight fraction of pyroxenite-derived melt (Xpx). With this relationship, end-member δ¹⁸O values for HIMU-peridotite (δ¹⁸O = 5.3 ± 0.3 ‰) and depleted pyroxenite (δ¹⁸O = 5.9 ± 0.3 ‰) were defined. Although the model proposed by Gurenko et al. (Contrib Mineral Petrol 162:349–363, 2011) implicates similar pyroxenite-peridotite mantle sources to those proposed by Day et al. (Geology 37:555–558, 2009, Geochim Cosmochim Acta 74:6565–6589, 2010) and Day and Hilton (Earth Planet Sci Lett 305:226–234, 2011), there are significant differences in the predicted δ¹⁸O values of end member components in the two models. In particular, Day et al. (Geochim Cosmochim Acta 74:6565–6589, 2010) proposed a mantle source for La Palma lavas with low-δ¹⁸O (<5 ‰), rather than higher-δ¹⁸O (c.f. the HIMU-peridotite composition of Gurenko et al. in Contrib Mineral Petrol 162:349–363, 2011). Here we question the approach of using weakly correlated variations in δ¹⁸O_ol and the Xpx parameter to define mantle source oxygen isotope compositions, and provide examples of why this approach appears flawed. We also provide reasons why the LF datasets previously published for Canary Island lavas remain robust and discuss why LF and SIMS data may provide complementary information on oxygen isotope variations in ocean island basalts (OIB), despite unresolved small-scale uncertainties associated with both techniques.     

Ferric-ferrous ratios,H2O contents and D/H ratios of phlogopite and biotite from lavas of different tectonic regimes

 Complete chemical analyses, including ferric and ferrous iron, H₂O contents and δD values for 16 phlogopite and biotite and 2 hornblende separates are presented. Samples were obtained from volcanic rocks from four localities: (1) phlogopite phenocrysts from minette lavas from the western Mexico continental arc, (2) biotite and hornblende phenocrysts from andesite lavas from Mono Basin, California, (3) phlogopite and biotite from clinopyroxenite nodules entrained in potassic lavas from the East African Rift, Uganda, and (4) phlogopite phenocrysts from a wyomingite lava in the Leucite Hills, Wyoming. The Fe₂O₃ contents in the micas range from 0.8 to 10.5 wt%, corresponding to 0.09 to 1.15 Fe³⁺ per formula unit (pfu). Water contents vary from 1.6 to 3.0 wt%, corresponding to 1.58 to 3.04 OH pfu, significantly less than would be expected for a site fully occupied by hydroxyl. Cation- and anion-based normalization procedures provide accurate mineral formulae with respect to most cations and anions, but are unable to generate accurate estimates of Fe³⁺/Fe^T, and overestimate OH at the expense of O on the hydroxyl site. These inaccuracies are present despite acceptable adjusted totals and stoichiometric calculated site occupancies. The phlogopite and biotite phenocrysts in arc-related lavas from western Mexico and eastern California have the highest Fe³⁺/Fe^T ratios (56–87%), reflecting high magmatic oxygen fugacities (ΔNNO = +2 to +5), in contrast to those from Uganda (25–40%) and the Leucite Hills (23%). There is no correlation between the OH content and the Fe³⁺/Fe^T ratio in the micas. Values of KMg/Fe²⁺D (± 2σ errors) were calculated for three phlogopite-olivine pairs (0.12 ± 0.12, 0.26 ± 0.14, 0.09 ± 0.12), two biotite-hornblende pairs (0.73 ± 0.08 and 1.22 ± 0.10) and a single phlogopite-augite pair (1.15 ± 0.12). Values of KF/OHD for two biotite and hornblende pairs could not be determined without significant error because of the extremely low F contents (< 0.2 wt%) of the four phases. The δD values obtained in this study encompass a large range (−137 to −43‰). The phlogopite and biotite separates from Uganda have δD values of −70 to −49‰, which overlap those believed to represent “primary” mantle. There is a larger range in δD values (−137 to −43‰) for phlogopite phenocrysts from western Mexico minette lavas, although their range in δ¹⁸O values (5.2–6.2‰) is consistent with “normal” mantle. It is unlikely, therefore, that the variable δD values reflect heterogeneity in the mantle source region of the minette magmas. Nor can the extremely low δD values reflect degassing of H₂ or H₂O since almost 100% loss of dissolved water in the magma is required, an unrealistic scenario given the stability of the hydrous phenocrysts. The very low δD values of the Mascota minette phlogopites require that the hydrogen be introduced from an external source (e.g., meteoric water). Whatever the process responsible for the observed hydrogen isotope composition, it had no effect on the δ¹⁸O value, f _{O 2}, a _{H 2O} or bulk composition of the host magmas. Received: 5 January 1995 / Accepted: 19 March 1996     

Revisiting the compositions and volatile contents of olivine-hosted melt inclusions from the Mount Shasta region: implications for the formation of high-Mg andesites

Primitive chemical characteristics of high-Mg andesites (HMA) suggest equilibration with mantle wedge peridotite, and they may form through either shallow, wet partial melting of the mantle or re-equilibration of slab melts migrating through the wedge. We have re-examined a well-studied example of HMA from near Mt. Shasta, CA, because petrographic evidence for magma mixing has stimulated a recent debate over whether HMA magmas have a mantle origin. We examined naturally quenched, glassy, olivine-hosted (Fo_87–94) melt inclusions from this locality and analyzed the samples by FTIR, LA-ICPMS, and electron probe. Compositions (uncorrected for post-entrapment modification) are highly variable and can be divided into high-CaO (>10 wt%) melts only found in Fo > 91 olivines and low-CaO (<10 wt%) melts in Fo 87–94 olivine hosts. There is evidence for extensive post-entrapment modification in many inclusions. High-CaO inclusions experienced 1.4–3.5 wt% FeO^T loss through diffusive re-equilibration with the host olivine and 13–28 wt% post-entrapment olivine crystallization. Low-CaO inclusions experienced 1–16 wt% olivine crystallization with <2 wt% FeO^T loss experienced by inclusions in Fo > 90 olivines. Restored low-CaO melt inclusions are HMAs (57–61 wt% SiO₂; 4.9–10.9 wt% MgO), whereas high-CaO inclusions are primitive basaltic andesites (PBA) (51–56 wt% SiO₂; 9.8–15.1 wt% MgO). HMA and PBA inclusions have distinct trace element characteristics. Importantly, both types of inclusions are volatile-rich, with maximum values in HMA and PBA melt inclusions of 3.5 and 5.6 wt% H₂O, 830 and 2,900 ppm S, 1,590 and 2,580 ppm Cl, and 500 and 820 ppm CO₂, respectively. PBA melts are comparable to experimental hydrous melts in equilibrium with harzburgite. Two-component mixing between PBA and dacitic magma (59:41) is able to produce a primitive HMA composition, but the predicted mixture shows some small but significant major and trace element discrepancies from published whole-rock analyses from the Shasta locality. An alternative model that involves incorporation of xenocrysts (high-Mg olivine from PBA and pyroxenes from dacite) into a primary (mantle-derived) HMA magma can explain the phenocryst and melt inclusion compositions but is difficult to evaluate quantitatively because of the complex crystal populations. Our results suggest that a spectrum of mantle-derived melts, including both PBA and HMA, may be produced beneath the Shasta region. Compositional similarities between Shasta parental melts and boninites imply similar magma generation processes related to the presence of refractory harzburgite in the shallow mantle.     

Distribution of oxygen isotopes in rocks and minerals from the critical zone of the Ioko-Dovyren pluton

The oxygen isotopic composition was studied in minerals and rocks from the critical zone of the Ioko-Dovyren layered pluton. The δ¹⁸O values vary from +5.4 to +6.1‰ in rocks, from +4.8 to +5.8‰ in olivine, from +5.5 to 6.5‰ in pyroxene, and from +5.8 to +6.9‰ in plagioclase and fall into the interval of mantle values for continental mafic and ultramafic rocks. A decrease in δ¹⁸O could have been caused by penetration of meteoric water. Postmagmatic (retrograde) oxygen isotopic redistribution in the slowly cooling rocks is responsible for disturbance of oxygen isotope equilibria in the coexisting minerals, which were crystallized from the same magma at a high temperature. The nonequilibrium oxygen isotopic composition in the associated minerals and calculated temperature of the final isotopic equilibration do not contradict the model of “fluid” formation of low-sulfide PGE mineralization in the Ioko-Dovyren layered pluton.     

Oxygen isotope geochemistry of the Granite Harbour Intrusives, Wilson Terrane, Northern Victoria Land, Antarctica

Summary The oxygen and strontium isotope compositions of the Cambro-Ordovician granitoids cropping out in the Wilson Terrane (Granite Harbour Intrusives–GHI) constrain the petrological evolution of the magmatism in Antarctica, related to the Ross Orogeny. The measured δ¹⁸O_WR values of these intrusives define three different compositional groups: the metaluminous rocks (MAG), with δ¹⁸O_WR ranging from 6.9 (olivine gabbro) to 11.4‰ (monzogranite); the unaltered peraluminous granites (PAG), having δ¹⁸O_WR values ranging from 10.6 to 13.2‰, and the foliated peraluminous leucogranites (SKG), characterised by δ¹⁸O_WR values above 14‰. The analysis of equilibrium mineral assemblages indicates that the high δ¹⁸O_WR values are magmatic and unaffected by low-temperature processes. A few peraluminous granites sampled in the vicinity of Cenozoic intrusions show anomalously low δ¹⁸O_WR, due to meteoric-hydrothermal alteration. The isotopic data indicate that the coeval and spatially related metaluminous mafic and felsic intrusives forming the GHI were not comagmatic: the mafic and intermediate rocks were likely derived from lower crustal contamination of a pristine basaltic magma; their δ¹⁸O_WR values were also increased during emplacement, due to the interaction with the adjacent ¹⁸O-rich hydrous felsic magmas (mixing). Oxygen isotope data indicate that the crustal sources producing the Granite Harbour intrusives were not homogeneous: the felsic metaluminous intrusives were produced by partial melting of fertile rock with possible igneous origin, whereas partial melting of a metapelitic source rock is claimed for the genesis of the peraluminous granites. Received February 9, 2001; revised version accepted August 10, 2001     

Magnesian andesite and dacite lavas from Mt. Shasta,northern California: products of fractional crystallization of H<Subscript>2</Subscript>O-rich mantle melts

Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO₂. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H₂O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H₂O) to lower crustal depths with magmatic H₂O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H₂O-rich magmas containing 10–15 wt.% H₂O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H₂O-rich melts could also play an important role in mixing and eruption.     

Fluid history of UHP metamorphism in Dabie Shan, China: a fluid inclusion and oxygen isotope study on the coesite-bearing eclogite from Bixiling

This paper characterizes late Holocene basalts and basaltic andesites at Medicine Lake volcano that contain high pre-eruptive H₂O contents inherited from a subduction related hydrous component in the mantle. The basaltic andesite of Paint Pot Crater and the compositionally zoned basaltic to andesitic lavas of the Callahan flow erupted approximately 1000 ¹⁴C years Before Present (¹⁴C years b.p.). Petrologic, geochemical and isotopic evidence indicates that this late Holocene mafic magmatism was characterized by H₂O contents of 3 to 6 wt% H₂O and elevated abundances of large ion lithophile elements (LILE). These hydrous mafic inputs contrast with the preceding episodes of mafic magmatism (from 10,600 to ∼3000 ¹⁴C years b.p.) that was characterized by the eruption of primitive high alumina olivine tholeiite (HAOT) with low H₂O (<0.2 wt%), lower LILE abundance and different isotopic characteristics. Thus, the mantle-derived inputs into the Medicine Lake system have not always been low H₂O, primitive HAOT, but have alternated between HAOT and hydrous subduction related, calc-alkaline basalt. This influx of hydrous mafic magma coincides temporally and spatially with rhyolite eruption at Glass Mountain and Little Glass Mountain. The rhyolites contain quenched magmatic inclusions similar in character to the mafic lavas at Callahan and Paint Pot Crater. The influence of H₂O on fractional crystallization of hydrous mafic magma and melting of pre-existing granite crust beneath the volcano combined to produce the rhyolite. Fractionation under hydrous conditions at upper crustal pressures leads to the early crystallization of Fe-Mg silicates and the suppression of plagioclase as an early crystallizing phase. In addition, H₂O lowers the saturation temperature of Fe and Mg silicates, and brings the temperature of oxide crystallization closer to the liquidus. These combined effects generate SiO₂-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H₂O HAOT magmas at Medicine Lake differentiate to iron-rich basaltic liquids. When these Fe-enriched basalts mix with melted granitic crust, the result is an andesitic magma. Since mid-Holocene time, mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, during the late Holocene, H₂O-poor mafic magmas continued to be erupted along with hydrous mafic magmas, although in significantly smaller volumes. Received: 4 January 1999 / Accepted: 30 August 1999     

Oxygen isotope evidence for subduction and rift-related mantle metasomatism beneath the Colorado Plateau–Rio Grande rift transition

Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco Volcanic Field, New Mexico, were analyzed for oxygen isotope ratios by laser fluorination. In lherzolites, olivine δ¹⁸O values are high (+5.5‰), whereas δ¹⁸O values for pyroxenes are low (cpx=+5.1‰; opx=+5.4‰) compared to average mantle values. Pyroxenite δ¹⁸O values (cpx=+5.0‰; opx=+5.3‰) are similar to those of the lherzolites and are also lower than typical mantle oxygen isotope compositions. Texturally and chemically primary calcite in pyroxenite xenoliths is far from isotopic equilibrium with other phases, with δ¹⁸O values of +21‰. The isotopic characteristics of the pyroxenite xenoliths are consistent with a petrogenetic origin from mixing of lherzolitic mantle with slab-derived silicate and carbonatite melts. The anomalously low δ¹⁸O in the pyroxenes reflects metasomatism by a silicate melt from subducted altered oceanic crust, and high δ¹⁸O calcite is interpreted to have crystallized from a high δ¹⁸O carbonatitic melt derived from subducted ophicarbonate. Similar isotopic signatures of metasomatism are seen throughout the Rio Puerco xenolith suite and at Kilbourne Hole in the southern Rio Grande rift. The discrete metasomatic components likely originated from the subducted Farallon slab but were not mobilized until heating associated with Rio Grande rifting occurred. Oxygen diffusion modeling requires that metasomatism leading to the isotopic disequilibrium between calcite and pyroxene in the pyroxenites occurred immediately prior to entrainment. Melt infiltration into spinel-facies mantle (xenoliths) prior to eruption was thus likely connected to garnet-facies melting that resulted in eruption of the host alkali basalt.     

Contemporaneous eruption of calc-alkaline and alkaline lavas in a continental arc (Eastern Mexican Volcanic Belt): chemically heterogeneous but isotopically homogeneous source

Nearly contemporaneous eruption of alkaline and calc-alkaline lavas occurred about 900 years BP from El Volcancillo paired vent, located behind the volcanic front in the Mexican Volcanic Belt (MVB). Emission of hawaiite (Toxtlacuaya) was immediately followed by calc-alkaline basalt (Río Naolinco). Hawaiites contain olivine microphenocrysts (Fo_67–72), plagioclase (An_56–60) phenocrysts, have 4–5 wt% MgO and 49.6–50.9 wt% SiO₂. In contrast, calc-alkaline lavas contain plagioclase (An_64–72) and olivine phenocrysts (Fo_81–84) with spinel inclusions, and have 8–9 wt% MgO and 48.4–49.4 wt% SiO₂. The most primitive lavas in the region (Río Naolinco and Cerro Colorado) are not as primitive as parental melts in other arcs, and could represent either (a) variable degrees of melting of a subduction modified, garnet-bearing depleted mantle source, followed by AFC process, or (b) melting of two distinct mantle sources followed by AFC processes. These two hypotheses are evaluated using REE, HFSE, and Sr, Os and Pb isotopic data. The Toxtlacuaya flow and the Y & I lavas can be generated by combined fractional crystallization and assimilation of gabbroic granulite, starting with a parental liquid similar to the Cerro Colorado basalt. Although calc-alkaline and alkaline magmas commonly occur together in other areas of the MVB, evidence for subduction component in El Volcancillo magmas is minimal and limited to <1%, which is a unique feature in this region further from the trench. El Volcancillo lavas were produced from two different magma batches: we surmise that the injection of calc-alkaline magma into an alkaline magma chamber triggered the eruption of hawaiites. Our results suggest that the subalkaline and hawaiitic lavas were formed by different degrees of partial melting of a similar, largely depleted mantle source, followed by later AFC processes. This model is unusual for arcs, where such diversity is usually explained by melting of heterogeneous (enriched and depleted) and subduction-modified mantle.     

Chemical and mineralogical heterogeneity in the basal zone of the Partridge River Intrusion: implications for the origin of Cu–Ni sulfide mineralization in the Duluth Complex,midcontinent rift system

Copper–nickel sulfide mineralization in the Partridge River Intrusion of the 1.1 Ga Duluth Complex is restricted primarily to a 100 m thick zone near the base of the intrusion, which is heterogeneous at meter scales in terms of both sulfide contents and rock types, which include dunite, melatroctolite, troctolite, leucotroctolite, gabbro, olivine gabbro, gabbronorite, and rare norite. Olivine-rich troctolites and melatroctolites appear to have required mineral accumulation on a substrate, whereas augite troctolite and gabbros are thought to have formed via in situ crystallization of magmas ranging in composition from high-Al olivine tholeiite to high-Ti tholeiite. δ¹⁸O values of orthopyroxene-poor rocks in the Partridge River Intrusion range from 5.2 to 6.7‰. δ¹⁸O values of 6.7‰ are consistent with less than 20% contamination by high-¹⁸O metasedimentary country rock, either via devolatilization or local partial melting. Rocks with greater than ∼15% orthopyroxene, gabbronorites, and norites, are characterized by δ¹⁸O values in excess of 6.9‰, and required the assimilation of larger amounts of siliceous country rocks. Sulfur isotopic values in leucotroctolitic rocks that contain less than ∼400 ppm S and that overlie the basal zone range between −1.5 and 2‰, values that are consistent with those of mantle-derived sulfur. In contrast, δ³⁴S values in the basal zone range from −1.4 to 10.5‰, where the ³⁴S-enriched samples require an input of sulfur from metasedimentary country rocks. δ³⁴S values of the rocks in the basal zone correlate with variations in olivine Fo content but not with S abundance. The wide range in δ³⁴S values of rocks in the basal zone strongly suggests that magmas interacted with layers in the sedimentary country rocks that were themselves characterized by variable sulfide contents and δ³⁴S values. The S isotopic data suggest that the heterogeneity observed in the basal zone results from the emplacement of relatively thin sheets of compositionally distinct magma. All rock types present in the basal zone can be produced as a result of variable degrees of fractionation of a parental high-Al olivine tholeiite, followed by varying degrees of contamination of derivative liquids by country rocks. The S-contamination process was essential for the development of Cu–Ni mineralization, and was restricted to the earliest stages in the development of the Duluth Complex at a time when volatile species such as S and H₂O, and low-T partial melts of country rocks, were available to magmas. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Stable isotope characterisation of co-existing carbonates from the Holocene Gallocanta lake (NE Spain): palaeolimnological implications

The sedimentary succession of Gallocanta lake, a closed saline lake located in the Iberian Range (NE Spain), documents two successive lacustrine stages: (1) brackish lake stage and (2) shallow saline lake stage. The saline stage corresponds to the present-day situation in which the lake water properties are mainly controlled by a strongly negative annual water balance. The carbonates of the brackish lake stage have relatively constant δ¹⁸O values, however, they are rather high (δ¹⁸O_Do = 2.4‰ and δ¹⁸O_Cc = 4.5‰ mean values) suggesting a hydrologically closed lake with a long residence time of the waters. δ¹⁸O values of carbonates from the saline stage vary greatly, and are lighter than in the previous stage (δ¹⁸O_Do = 0.5‰, δ¹⁸O_Cc = −0.7‰, δ¹⁸O_Mgs = −2.3‰ mean values). These carbonates also precipitated in a hydrologically closed lake, but in equilibrium with a lake water of more variable isotopic composition. The δ¹³C values for carbonates of both stages reflect a mixing of different pools of carbon, but during saline stage δ¹³C values have been more controlled by the equilibrium of the lake waters with atmospheric CO₂. During the current stage, calcite and dolomite precipitate in Gallocanta lake mainly during spring and summer, although dolomite precipitation is more favoured towards the summer. Magnesite precipitates at the beginning of autumn, when the first rainfall re-dissolves the saline surface crust, producing saline waters with a high Mg²⁺ content. The isotopic composition of lake waters sampled in 2005 are far higher than those calculated from the carbonates. It is considered that this could be due to two factors: either because there have not been many extremely dry years (like the year 2005) during the development of the lake, or because the physical and chemical characteristics of the lake waters in such conditions are not appropriate for the development of these minerals.     

4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon

Analysis of δ¹⁸O in igneous zircons of known age traces the evolution of intracrustal recycling and crust-mantle interaction through time. This record is especially sensitive because oxygen isotope ratios of igneous rocks are strongly affected by incorporation of supracrustal materials into melts, which commonly have δ¹⁸O values higher than in primitive mantle magmas. This study summarizes data for δ¹⁸O in zircons that have been analyzed from 1,200 dated rocks ranging over 96% of the age of Earth. Uniformly primitive to mildly evolved magmatic δ¹⁸O values are found from the first half of Earth history, but much more varied values are seen for younger magmas. The similarity of values throughout the Archean, and comparison to the composition of the “modern” mantle indicate that δ¹⁸O of primitive mantle melts have remained constant (±0.2‰) for the past 4.4 billion years. The range and variability of δ¹⁸O in all Archean zircon samples is subdued (δ¹⁸O(Zrc)=5–7.5‰) ranging from values in high temperature equilibrium with the mantle (5.3± 0.3‰) to slightly higher, more evolved compositions (6.5–7.5‰) including samples from: the Jack Hills (4.4–3.3 Ga), the Beartooth Mountains (4.0–2.9 Ga), Barberton (3.5–2.7 Ga), the Superior and Slave Provinces (3.0 to 2.7 Ga), and the Lewisian (2.7 Ga). No zircons from the Archean have been analyzed with magmatic δ¹⁸O above 7.5‰. The mildly evolved, higher Archean values (6.5–7.5‰) are interpreted to result from exchange of protoliths with surface waters at low temperature followed by melting or contamination to create mildly elevated magmas that host the zircons. During the Proterozoic, the range of δ¹⁸O(Zrc) and the highest values gradually increased in a secular change that documents maturation of the crust. After ∼1.5 Ga, high δ¹⁸O zircons (8 to >10‰) became common in many Proterozoic and Phanerozoic terranes reflecting δ¹⁸O(whole rock) values from 9 to over 12‰. The appearance of high δ¹⁸O magmas on Earth reflects nonuniformitarian changes in the composition of sediments, and rate and style of recycling of surface-derived material into magmas within the crust. Electronic Supplementary Material Supplementary material is available for this article at     

Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform vary from 4.7 to 5.9‰ VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe differences not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich ilmenite. Several lines of evidence indicate that these ¹⁸O values are unaltered by kimberlite magmas during eruption and represent compositions preserved since crystallization in the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The average ¹⁸O of mantle zircons is 5.3‰, ∼0.1 higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from within 250 km of Kimberley, South Africa have average ¹⁸O=5.32±0.17 (n=28). Small, but significant, differences among other kimberlite pipes or groups of pipes may indicate isotopically distinct reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle values of 5.3±0.3. Precambrian zircons (2.1–2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle zircon, ¹⁸O=3.4 to 4.7‰. These zircon megacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism associated with kimberlite magmatism or during metamorphism. The low ¹⁸O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated seawater prior to underplating as eclogite and to associated metasomatism of the mantle wedge. Smaller differences among other pipes and districts may result from variable temperatures of equilibration, mafic versus ultramafic hosts, or variable underplating. The narrow range in zircon compositions found in most pipes suggests magmatic homogenization. If this is correct, these zircons document the existence of significant quantities of magma in the sub-continental mantle that was regionally variable in ¹⁸O and this information restricts theories about the nature of ancient subduction. Received: 8 August 1997 / Accepted: 6 May 1998     

The relationship between E- and N-type magmas in the Baffin Bay Lavas

The picritic lavas of Baffin Island represent one of the most primitive Phanerozoic volcanic suites in the world with MgO contents ranging from 22 wt% (29 Mg, cation unit = Mg/100 cations) for olivine-rich lavas to 11 wt% (16 Mg) for olivine-poor lavas. Two magma types can be recognized on the basis of trace element and isotopic geochemistry. N-type magma, which dominates the high-MgO lavas, has depleted LREE patterns [(La/Sm)_N0.6–0.7] typical of N-MORB, K/Ti<0.05, and ⁸⁷Sr/⁸⁶Sr <0.7032. E-type magma, which dominates the lower MgO lavas, has flat to slightly enriched LREE patterns [(La/Sm)_N1.1–1.2] typical of E-MORB, K/Ti>0.5 and ⁸⁷Sr/⁸⁶Sr ranging between 0.7032–0.7039. These two magma types are, however, virtually indistinguishable in terms of major clements and many other trace elements. The E and N-type samples are intermixed throughout the volcanic succession, indicating that both types of magma erupted contemporaneously. Although the compositional spectrum observed for major and highly incompatible elements is consistent with olivine fractionation, crystal fractionation cannot account for the difference in the LREE between E-type and N-type lavas. Crustal contamination involving a lower crust composition cannot reproduce the more magnesian E-type lavas.and can only repoduce the lowest (La/Sm)_N ratios of the E-type lavas if high degrees of assimilation (50%) have occurred. Partial melting models can reasonably account for the distinct (La/Sm)_N ratios and the similar Zr/Y values of the two magma types, but fail to reproduce the observed abundances or REE, Sr, Y and Zr. Compositionally different mantle sources are required to explain the two distinct magma types observed in Baffin Island. A model in which the mantle source is a mixture of enriched plume material and depleted entrained mantle in the head of a mantle plume may explain the contemporaneous eruption of N and E-type magmas in the Baffin Bay picritic suite. The Baffin Island E-type lavas are less enriched in Sr, Y and Zr contents and have lower Zr/Y, for similar (La/Sm)_N than the other E-type lavas of the northern North Atlantic region.     

Isotope geochemistry of ore fluids for the Dongsheng sandstone-type uranium deposit, China

The Dongsheng sandstone-type uranium deposit is one of the large-sized sandstone-type uranium deposits discovered in the northern part of the Ordos Basin of China in recent years. Geochemical characteristics of the Dongsheng uranium deposit are significantly different from those of the typical interlayered oxidized sandstone-type uranium ore deposits in the region of Middle Asia. Fluid inclusion studies of the uranium deposit showed that the uranium ore-forming temperatures are within the range of 150–160℃. Their 3He/4He ratios are within the range of 0.02–1.00 R/Ra, about 5–40 times those of the crust. Their 40Ar/36Ar ratios vary from 584 to 1243, much higher than the values of atmospheric argon. The δ18OH2O and δD values of fluid inclusions from the uranium deposit are -3.0‰– -8.75‰ and -55.8‰– -71.3‰, respectively, reflecting the characteristics of mixed fluid of meteoric water and magmatic water. The δ18OH2O and δD values of kaolinite layer at the bottom of the uranium ore deposit are 6.1‰ and -77‰, respectively, showing the characteristics of magmatic water. The δ13CV-PDB and δ18OH2O values of calcite veins in uranium ores are -8.0‰ and 5.76‰, respectively, showing the characteristics of mantle source. Geochemical characteristics of fluid inclusions indicated that the ore-formation fluid for the Dongsheng uranium deposit was a mixed fluid of meteoric water and deep-source fluid from the crust. It was proposed that the Jurassic-Cretaceous U-rich metamorphic rocks and granites widespread in the northern uplift area of the Ordos Basin had been weathered and denudated and the ore-forming elements, mainly uranium, were transported by meteoric waters to the Dongsheng region, where uranium ores were formed. Tectonothermal events and magmatic activities in the Ordos Basin during the Mesozoic made fluids in the deep interior and oil/gas at shallow levels upwarp along the fault zone and activated fractures, filling into U-bearing clastic sandstones, thus providing necessary energy for the formation of uranium ores.     

Oxygen isotopes composition of sapphires from the French Massif Central: implications for the origin of gem corundum in basaltic fields

Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ (n = 22; mean δ¹⁸O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < δ¹⁸O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ¹⁸O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ¹⁸O values of 4.4–4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ¹⁸O values obtained for anorthoclase (7.7–7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ¹⁸O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.