Olivine δ18O Research Articles

Disequilibrium in historic volcanic rocks from Fogo, Cape Verde traces carbonatite metasomatism of recycled ocean crust

Fogo, Cape Verde, located upon thick oceanic lithosphere, provides a window into processes occurring in the mantle where recycled ocean crust in an upwelling mantle plume interacts with ambient mantle. Our objective is to investigate the nature of the lithologies of the mantle sources involved in the petrogenesis of historic volcanic rocks from Fogo. We observe enclaves and mingling textures in the lavas combined with oxygen isotope disequilibrium between olivine and clinopyroxene phenocrysts. Olivine δ18O values display positive correlations with Zr/Hf and Zr/Y and a negative correlation with U/Th, whereas clinopyroxene δ 18O values correlate positively with Ba/Nb. Heterogeneity between crystal populations and within the groundmass indicates that multiple magma batches are mixed beneath Fogo. In terms of mantle endmembers and source lithologies, a HIMU endmember was generated by melting of carbonated eclogite as indicated by low δ 18O values, Zr/Hf, Ba/Nb and high U/Th ratios. In contrast, we show the EM1 endmember has high δ 18O, Zr/Hf, Ba/Nb and low U/Th ratios, derived from melting of variably carbonated peridotite. Additionally, Ba/Th ratio are high, indicating that carbonatite melts have contributed to alkaline magma compositions at Fogo.

Petrogenesis of Lava from Christmas Island, Northeast Indian Ocean: Implications for the Nature of Recycled Components in Non-Plume Intraplate Settings

Lava samples from the Christmas Island Seamount Province (CHRISP) record an extreme range in enriched mantle (EM) type Sr-Nd-Pb-Hf isotope signatures. Here we report osmium isotope data obtained on four samples from the youngest, Pliocene petit-spot phase (Upper Volcanic Series, UVS; ~4.4 Ma), and four samples from the earlier, Eocene (Lower Volcanic Series, LVS; ~40 Ma) shield building phase of Christmas Island. Osmium concentrations are low (5–82 ppt) with initial Os isotopic values (187Os/188Osi) ranging from (0.1230–0.1679). Along with additional new geochemical data (major and trace elements, Sr-Nd-Pb isotopes, olivine δ18O values), we demonstrate the following: (1) The UVS is consistent with melting of shallow Indian mid-ocean ridge basalt (MORB) mantle enriched with both lower continental crust (LCC) and subcontinental lithospheric mantle (SCLM) components; and (2) The LVS is consistent with recycling of SCLM components related to Gondwana break-up. The SCLM component has FOZO or HIMU like characteristics. One of the LVS samples has less radiogenic Os (γOs –3.4) and provides evidence for the presence of ancient SCLM in the source. The geochemistry of the Christmas Island lava series supports the idea that continental breakup causes shallow recycling of lithospheric and lower crustal components into the ambient MORB mantle.

Origin of low oxygen isotopic compositions in alkalic lavas from Erebus volcano, Antarctica

Mount Erebus on Ross Island, Antarctica is an active volcano that has hosted a persistent phonolite lava lake for over 50 years. Oxygen isotope analysis of minerals in alkaline basanite to phonolite lavas from Erebus and surrounding volcanic centers show an anomalous trend of decreasing δ18O with magmatic evolution. The cold frozen climate and lack of radiogenic isotope evidence for assimilation of crustal materials makes this a unique location to investigate factors contributing to changes in oxygen isotope compositions. Olivine, feldspar, glass and whole rock samples from 29 lavas, 7 modern bombs and 2 xenoliths were analyzed for their oxygen isotopic compositions. Forward models indicate there should be an enrichment in δ18O of 0.4‰ in lavas due to Raleigh fractionation, but this is not observed. Olivine δ18O decreases from a typical mantle value of +5.3‰ in the parental basanite source to 4.3‰ in recently erupted phonolite lava bombs. The trend correlates with a decreasing forsterite content in the olivine. Feldspar and glasses exhibit δ18O values well below predicted values and are in fractionation disequilibrium with olivine compositions at measured magmatic temperatures of 1000 °C within the phonolite lava lake. Isotopic depletion of the melt may be accounted for by incorporation of 8–11% hydrothermally altered volcanic edifice over time assuming water–rock interaction from strongly isotopically depleted Antarctic precipitation. Enhanced disequilibrium with olivine phenocrysts, are seen during the late stages of Erebus volcano construction. This calls for differential fractionation of olivine and feldspar from their respective melts, and slow O-diffusion of olivine while in the shallow lava lake prior to eruption.

Mantle and Recycled Oceanic Crustal Components in Mantle Xenoliths From Northeastern China and their Mantle Sources

AbstractThe subduction of the Paleo‐Pacific plate is widely credited for the destruction of the eastern North China Craton. However, how the Pacific plate subduction has affected the off‐craton lithospheric mantle in northeastern China is less clear, as few studies have focused on the lithospheric mantle‐derived xenoliths in that region. We report petrography, mineral and rock chemistry, and in situ isotopic compositions of Sr in clinopyroxene and O in olivine for 20 peridotite and 11 pyroxenite xenoliths hosted by the Cenozoic volcanic rocks at Jiaohe and Shuangliao localities. The peridotites include lherzolites, harzburgites, and a wehrlite, many are metasomatized. Group 1 garnet pyroxenites have high Al2O3, Ni, and Mg#; positive Eu and Sr anomalies; and olivine δ18O values of 4.8–5.1‰, which we attribute to recycling of oceanic crustal components to the asthenosphere and their reactions with host peridotites. Groups 2 and 3 pyroxenites are garnet‐free and contain peridotite‐like pyroxenes; we argue that they were formed by reactions of lithospheric peridotites with evolved melts derived from recycled oceanic crust. In addition, one pyroxenite contains orthopyroxene surrounded by fine‐grained olivine and clinopyroxene, while another pyroxenite and the wehrlite contain zoned clinopyroxene with gradual increases in Na2O, (La/Yb)N, Sr, and 87Sr/86Sr ratios and decreases in Ti/Eu from core to rim. These characteristics record infiltration of a carbonatite melt shortly before the transport of the xenoliths. The inferred carbonatite melt has 87Sr/86Sr ≥ 0.70415 and mantle‐like olivine δ18O and originated from carbonated asthenosphere. Our results, together with previous studies on the host basalts, suggest that subducted crustal materials were initially transported to the asthenosphere beneath northeastern China to produce carbonate‐bearing domains. Upwelling of the carbonated asthenosphere produced carbonatite melts that migrated upward and metasomatized the overlying lithospheric mantle.

Olivine phenocryst origins and mantle magma sources for monogenetic basalt volcanoes in northern New Zealand from textural, geochemical and δ18O isotope data

Olivine phenocrysts in basalts erupted in the Kaikohe-Bay of Islands volcanic field (KBIVF), a cluster of Quaternary monogenetic edifices in northern New Zealand, provide an opportunity to investigate magma source and assembly. The phenocryst population has both cognate and non-cognate origins with respect to the carrier magma. Common large subhedral-euhedral phenocrysts with compositions of Fo80–86 are autocrysts, having nucleated in a melt equivalent of the host whole-rock (~Mg#56–63). A subordinate population of large subhedral-euhedral phenocrysts that are Mg-rich (>Fo86) nucleated in a melt more mafic (>Mg#65). These are antecrystic and are likely to have a deep cumulate origin, perhaps the product of fractionation of a primitive parental magma. Smaller anhedral phenocrysts have compositions (~Fo70–75) that would have crystallised in a more evolved melt (~Mg#42–48). These are xenocrysts from stalled and fractionated crystal mush zones or intrusions in the mid and upper crust. Other small phenocrysts (Fo66–79) are euhedral and reversely zoned with mafic mantle zones (Fo80–85). They also nucleated in a more evolved crustal magma, but subsequently experienced magma-mixing with a more mafic magma. The olivine assemblage requires a vertically extensive magma system through the crust and upper mantle involving deep cognate crystallisation, and recycling of non-cognate crystals from deep and shallow intrusive bodies. This in accord with varied origins for the accompanying plagioclase and clinopyroxene populations in the rocks demonstrated in previous studies. This study presents the first δ18O analyses of olivines from monogenetic volcanoes in New Zealand. SIMS analysis of cognate and cumulate olivine phenocrysts in KBIVF basalts, and those from other volcanoes in northern New Zealand basaltic fields, show that they are non-zoned (δ18O ± 0.4‰). The inter-crystal range at most volcanoes is narrow (0.5‰), although the olivine population at a few volcanoes are more varied (range up to 1.25‰), requiring more than one source. The isotopic data for the more northern fields KBIVF (δ18O mean = 5.60‰; range = 5.12–6.20‰) and Whangarei volcanic field (δ18O mean = 5.50‰; range = 4.98–6.12‰) include values higher than that of olivine from MORB-HIMU sources, and overlap those associated with EMI/EMII sources or subduction zone magmas. Whole-rock trace element and Pb-isotope data from these fields have been interpreted as the result of subduction-induced contamination of the mantle wedge, a relic signal from now extinct Miocene-Pliocene arc volcanism. The magma compositions are also consistent with source depths above the garnet facies zone. This could explain the higher olivine δ18O values. In contrast, olivines from more southern fields do not display this O-isotope signature, consistent with postulated greater source depths.

Rapid orthopyroxene growth induced by silica assimilation: constraints from sector-zoned orthopyroxene, olivine oxygen isotopes and trace element variations in the Huangshanxi Ni–Cu deposit, Northwest China

The Permian Huangshanxi Ni–Cu-hosted intrusion, located in the southern Central Asian Orogenic Belt in NW China, is dominated by cumulus olivine and orthopyroxene. Various types of compositional zonings in cumulate phases were studied using high-resolution synchrotron X-Ray fluorescence imaging, electron microprobe, laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectrometry, to shed light on the relationship between the orthopyroxene-rich cumulate and sulfide accumulation. Normal, oscillatory, and sector zonings of Cr in orthopyroxene from both lherzolites and olivine websterites were noted, likely due to the different cut orientation of orthopyroxene crystals composed of sector-zoned cores and reverse-zoned rims. Significant Fe–Mg–Al–Si variations found in the exterior regions of the grains do not correspond to the Cr zoning. However, Cr content is positively correlated with Ti–Al contents in interior regions of orthopyroxene, reflecting coupled charge substitution during the development of sector zoning, and indicating diffusion-modified Fe–Mg–Al–Si variation in the exterior regions. Primary Cr-sector zoning in the core is likely the result of rapid in situ growth in a boundary layer at the cumulus stage, probably induced by Si addition from wall rocks and crystallization under supersaturated conditions. In addition, some olivine shows Mg–Fe–Mn–Ca–Cr variations in the rim but not in the core, and both core and rim of olivine show a uniform O isotope composition, illustrating that they have not been modified by post-cumulus processes. Given the distinct difference in O isotopic signatures between mantle-derived melts and crustal materials, olivine O isotope composition could be used to decipher the relative degree of Si addition at crystallization. The Fo value, O isotopic signature and trace element content in the cores of olivine grains from different depths show systematic variations, dividing the sulfide-bearing cumulate into two intervals; the lower and upper lherzolite zones with harzburgitic rocks at the contact. In the lower zone, the olivine δ18O values increase upward from ~ 6 to ~ 7‰ and the oxygen fugacity decreases from QFM to QFM-2, suggesting increasing contamination by more reduced crustal materials. Over the same interval, the modal proportion of orthopyroxene and modal orthopyroxene/olivine ratio increase with increasing depth with little variation in whole-rock Mg#, clearly illustrating that the non-cotectic orthopyroxene proportions resulted from Si addition from the wall rock. In the upper zone, the decrease in δ18O values, and increase in olivine nucleation density, Fo value and oxygen fugacity suggest there was a pulse of sulfide- and olivine-charged magma into the magma chamber, that mixed with resident magma to form a hybrid bottom layer in contact with the early-crystallized orthopyroxene-rich cumulate pile. The sulfide content decreases up-section in the lower zone, and then increases again towards the top of the cumulate sequence. This, combined with the compositional variations with depth, suggests that sulfides in the Huangshanxi Ni–Cu deposit settled downward during at least two sulfide-loaded magma pulses. The fast-growing orthopyroxene-rich cumulate may have acted as a permeability barrier, preventing sulfide percolation into the lower cumulate formed during the first magma pulse. Overall, sector zoning in orthopyroxene and compositional variations in olivine suggest that Si and reduced material addition during contamination is of great importance in forming magmatic Ni–Cu deposits hosted by olivine- and orthopyroxene-dominated intrusions.

Re-Os geochronology, O isotopes and mineral geochemistry of the Neoproterozoic Songshugou ultramafic massif in the Qinling Orogenic Belt, China

The Qinling Orogenic Belt was formed by subduction and collision between the North and South China Blocks along the Shangdan suture. The Songshugou ultramafic massif located on the northern side of the Shangdan suture provides essential insights into the mantle origin and evolutionary processes during spreading and subduction of the Shangdan oceanic lithosphere. The ultramafic massif comprises harzburgite, coarse- and fine-grained dunites. The spinels from harzburgite exhibit low Cr# and high Mg# numbers, suggesting a mid-ocean ridge peridotite origin, whereas spinels from both coarse- and fine-grained dunites are indicated as resulted from melt-rock reaction due to their systematic higher Cr# and low Mg# numbers. This melt-rock reaction in the dunites is also indicated by the low TiO2 (mostly <0.4 wt%) in the spinel and high Fo (90–92) in olivines. Due to its relatively homogeneous nature in the mantle, oxygen isotopic composition is a sensitive indicator for the petrogenesis and tectonic setting of the Songshugou ultramafic rocks. Based on in-situ oxygen isotope analyses of olivines from twenty-six rock samples, most harzburgites from the Songshugou ultramafic massif show low δ18O values of 4.54–5.30‰, suggesting the olivines are equilibrium with N-MORB magmas and originally formed in a mid-ocean ridge setting. The coarse- and fine-grained dunites exhibit slightly higher olivine δ18O values of 4.69–6.00‰ and 5.00–6.11‰, respectively, suggesting they may have been modified by subduction-related boninitic melt-rock reaction. The δ18O values of olivines systematically increasing from the harzburgites, to coarse-grained dunites and fine-grained dunites may suggest enhancing of melt-rock reaction. The decreasing of Os concentration, 187Re/188Os and 187Os/188Os ratios from harzburgite to dunite suggest an 187Os-enriched, subduction zone melt was responsible for creating the melt channel for melt-rock reactions. Together with the high-temperature ductile deformation microstructures, these isotopic and mineral geochemical features suggest that the harzburgites represent mantle residues after partial melting at mid-ocean ridge or supra-subduction zone, while the dunites were probably resulted from reactions between boninitic melt and harzburgites in a supra-subduction zone. Re-Os geochronology yields a maximum Re depletion model age (TRD) of 805 Ma, constraining the minimum formation age of the harzburgites derived from oceanic mantle. Eight samples of whole rock and chromite yield a Re-Os isochron age of 500 ± 120 Ma, constraining the timing of melt-rock reactions. Combined with the regional geology and our previous investigations, the Songshugou ultramafic rocks favors a mantle origin at mid-ocean ridge before 805 Ma, and were modified by boninitic melt percolations in a SSZ setting at ca. 500 Ma. This long-term tectonic process from spreading to subduction might imply a huge Pan-Tethyan ocean between the Laurasia (e.g., North China Block) and Gondwana (e.g., South China Block) and/or a one-side subduction.

Olivine Oxygen Isotope Evidence for Intracontinental Recycling of Delaminated Continental Crust

AbstractThis paper reports the oxygen isotope compositions of olivine from dunite, wehrlite, harzburgite, and lherzolite xenoliths entrained by the Early Cretaceous high‐Mg diorites and Late Cretaceous basalts in the central and eastern North China Craton, with the aim of constraining processes that happened in the lithospheric mantle. Olivine oxygen isotope compositions were analyzed in situ using secondary ion mass spectrometry. Olivine δ18O of harzburgite and lherzolite xenoliths hosted in the Fushan and Tietonggou diorites with Archean and Paleoproterozoic rhenium‐depletion model ages ranges from 5.1‰ to 5.8‰, similar to olivine from spinel lherzolite xenoliths in the Late Cretaceous Fuxin basalts (olivine δ18O = 5.2–5.4‰) and typical mantle olivine. In contrast, olivine δ18O from dunite and wehrlite xenoliths ranges from 6.1‰ to 7.7‰, significantly higher than the typical mantle values. The olivine oxygen isotope data, combined with the petrologic observations, mineral compositions, and whole‐rock trace element and Sr‐Nd‐Os isotope compositions of the peridotite, suggest that the peridotites in Early Cretaceous high‐Mg diorites had been affected by interaction with melt derived from delaminated continental crust. These data provide compelling evidence for the recycling of continental crust in an intracontinental setting.

Discriminating between pyroxenite and peridotite sources for continental flood basalts (CFB) in southern Africa using olivine chemistry

Continental Flood Basalts (CFB) result from voluminous outpourings of magma that often precede continental break-up. Notwithstanding the petrogenetic importance of CFBs, the nature of the mantle source for such magmas is contentious, particularly with regard to picrites with Ni-rich olivine phenocrysts. Previous studies have suggested that Ni-rich olivines associated with plume volcanism in regions of thickened (>90 km) lithosphere are related to either source mineralogy differences (peridotite versus pyroxenite) or change in olivine-melt partitioning due to pressure increase. In order to evaluate these two hypotheses, we present trace element data for olivines from the Karoo CFB Tuli and Mwenezi picrites and the Etendeka CFB Horingbaai/LTZ-L type picrites, all of which erupted in regions of thickened (>90 km) lithosphere in southern Africa. Karoo picrite olivines are Ni-rich, Ca- and Mn-poor, and have low (<1.4) 100*Mn/Fe. These compositions are consistent with a pyroxenitic source. Etendeka Horingbaai/LTZ-L picrite olivines do not show Ni-enrichment, but are characterized by high Al and Cr, and high (>1.4) 100*Mn/Fe, which is more consistent with high temperature melting of a dominantly peridotitic source. We also show that the Karoo and Etendeka olivines are characterized by distinct Mn/Zn ratios of <13 and >15, respectively.In addition, bulk rock geochemical data compilations and previously reported olivine δ18O for Karoo and Etendeka CFBs are discussed in order to further constrain source components based on previously described pyroxenite melt geochemical indices such as MgO–CaO systematics, FeO/MnO, Zn/Fe, and FC3MS (FeO/CaO–3*MgO/SiO2). These geochemical indices suggest a pyroxenite-dominated source for Karoo CFBs as well as for Etendeka ferropicrites whereas a peridotite-dominated source is indicated for Etendeka Horingbaai/LTZ-L type picrites analyzed in this study. Based on our data, Ni-enrichment of olivine in plume-related magmas in regions of thickened lithosphere in southern Africa is not ubiquitous. We therefore suggest that mineralogical variation of the source is a more likely major control of olivine chemistry and parent melt variations for Karoo and Etendeka CFBs. We also show that olivine Mn–Zn correlations are a useful discriminator for source variation and recommend the use of olivine Mn/Zn<13 for a pyroxenite-dominated source relative to olivine Mn/Zn>15 for a peridotite-dominated source.

Geochemistry and oxygen isotopic composition of olivine in kimberlites from the Arkhangelsk province: Contribution of mantle metasomatism

The paper presents data on the composition of olivine macrocrysts from two Devonian kimberlite pipes in the Arkhangelsk diamond province: the Grib pipe (whose kimberlite belongs to type I) and Pionerskaya pipe (whose kimberlite is of type II, i.e., orangeite). The dominant olivine macrocrysts in kimberlites from the two pipes significantly differ in geochemical and isotopic parameters. Olivine macrocrysts in kimberlite from the Grib pipe are dominated by magnesian (Mg# = 0.92–0.93), Ti-poor (Ti < 70 ppm) olivine possessing low Ti/Na (0.05–0.23), Zr/Nb (0.28–0.80), and Zn/Cu (3–20) ratios and low Li concentrations (1.2–2.0 ppm), and the oxygen isotopic composition of this olivine δ18O = 5.64‰ is higher than that of olivine in mantle peridotites (δ18O = 5.18 ± 0.28‰). Olivine macrocrysts in kimberlite from the Pionerskaya pipe are dominated by varieties with broadly varying Mg# = 0.90–0.93, high Ti concentrations (100–300 ppm), high ratios Ti/Na (0.90–2.39), Zr/Nb (0.31–1.96), and Zn/Cu (12–56), elevated Li concentrations (1.9–3.4 ppm), and oxygen isotopic composition δ18O = 5.34‰ corresponding to that of olivine in mantle peridotites. The geochemical and isotopic traits of low-Ti olivine macrocrysts from the Grib pipe are interpreted as evidence that the olivine interacted with carbonate-rich melts/fluids. This conclusion is consistent with the geochemical parameters of model melt in equilibrium with the low-Ti olivine that are similar to those of deep carbonatite melts. Our calculations indicate that the variations in the δ18O of the olivine relative the “mantle range” (toward both higher and lower values) can be fairly significant: from 4 to 7‰ depending on the composition of the carbonate fluid. These variations were formed at interaction with carbonate fluid, whose δ18O values do not extend outside the range typical of mantle carbonates. The geochemical parameters of high-Ti olivine macrocrysts from the Grib pipe suggest that their origin was controlled by the silicate (water–silicate) component. This olivine is characterized by a zoned Ti distribution, with the configuration of this distribution between the cores of the crystals and their outer zones showing that the zoning of the cores and outer zones is independent and was produced during two episodes of reaction interaction between the olivine and melt/fluid. The younger episode (when the outer zone was formed) likely involved interaction with kimberlite melt. The transformation of the composition of the cores during the older episode may have been of metasomatic nature, as follows from the fact that the composition varies from grain to grain. The metasomatic episode most likely occurred shortly before the kimberlite melt was emplaced and was related to the partial melting of pyroxenite source material.

The origin of hydrous, high-δ18O 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 Δ18Oolivine–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.

Oxygen Isotope Evidence for Chemical Interaction of Ki lauea Historical Magmas with Basement Rocks

Kīlauea historical summit lavas have a wide range in matrix δ18OVSMOW values (4·9–5·6‰) with lower values in rocks erupted following a major summit collapse or eruptive hiatus. In contrast, δ18O values for olivines in most of these lavas are nearly constant (5·1 ± 0·1‰). The disequilibrium between matrix and olivine δ18O values in many samples indicates that the lower matrix values were acquired by the magma after olivine growth, probably just before or during eruption. Both Mauna Loa and Kīlauea basement rocks are the likely sources of the contamination, based on O, Pb and Sr isotope data. However, the extent of crustal contamination of Kīlauea historical magmas is probably minor (< 12%, depending on the assumed contaminant) and it is superimposed on a longer-term, cyclic geochemical variation that reflects source heterogeneity. Kīlauea's heterogeneous source, which is well represented by the historical summit lavas, probably has magma δ18O values within the normal mid-ocean ridge basalt mantle range (5·4–5·8‰) based on the new olivine δ18O values.

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.

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 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.

Crustal Contamination of Kilauea Volcano Magmas Revealed by Oxygen Isotope Analyses of Glass and Olivine fromPuu Oo Eruption Lavas

Oceanic island basalts have a large range in δ18O values (4.5−7.5‰) compared with the assumed primordial mantle values (5.5–6.0‰) and with mid-ocean ridge basalts (5.7 ± 0.2‰). Some Hawaiian tholeiitic basalts have low 18O values (4.6–5.2‰), which have been interpreted to be either a primary source feature or caused by crustal contamination. This study was undertaken to evaluate the cause of low δ18O values in Hawaiian tholeiitic basalts. We determined the δ18O values of glassy matrix material and coexisting olivines from pristine basalts produced during the current, 14-year-old Puu Oo eruption of Kilauea Volcano. Our results show that the Puu Oo eruption lavas have significant ranges in matrix (0.7‰) and olivine δ18O values (0.5‰) which do not correlate consistently with other geochemical parameters and that many of the lavas are out of oxygen isotopic equilibrium. These features probably reflect partial assimilation of and oxygen exchange with metamophosed Kilauea rocks during the magma's 19 km transit through the volcano's east rift zone. The parental magmas for Puu Oo lavas had a δ18O value of at least 5.2‰ and perhaps as high as 5.6‰. Thus, Puu Oo lavas do not give a clear indication of the δ18O value of Kilauea's mantle source but they do indicate that the oxygen in these otherwise pristine basalts has undergone significant modification by interaction with crustal rocks.