Petrology and isotope-geochemistry of San Vincenzo rhyolites (Tuscany,Italy)

Two groups of rhyolites have been recognized at San Vincenzo (Tuscany, Italy). Group A rhyolites are characterized by plagioclase, quartz, biotite, sanidine and cordierite mineral assemblages. They show constant MgO and variable CaO and Na₂O contents. Initial⁸⁷Sr/⁸⁶Sr ratios in group A samples range between 0.71950 and 0.72535, whereas the Nd isotopic compositions are relatively constant (0.51215–0.51222). Group B rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals, and show textural, mineralogical and chemical evidence of interaction with more mafic magmas. The Sr and Nd isotopic ratios range between 0.71283–0.71542 and 0.51224–0.51227 respectively. Magmatic inclusions of variable size (1 mm to 10 cm) were found in groups B rhyolites. These inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite. They are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts. These magmatic inclusions have relatively high Sr contents (996–1529 ppm) and Sr and Nd isotope-ratios of 0.70807–0.70830 and 0.51245–0.51252 respectively.⁸⁷Sr/⁸⁷Sr data on minerals separated from both group A and B rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group A rhyolites and due to interaction of rhyolites with a mantle-de-rived magma in group B rhyolites. Isotopic data on whole rocks and minerals allow us to interpret the group A rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source; conversely, group B rhyolites underwent interactions with a mantle-derived magma. The crustal source as inferred from isotopic systematics would be characterized by⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios close to 0.7194 and 0.51216 respectively. The sub-crustal magma would have Sr isotopic composition close to 0.7077 and a¹⁴³Nd/¹⁴⁴Nd ratio greater than or equal to 0.51252. These isotopic features are different from those reported for the parental magmas postulated for Vulsini and Alban Hills in the nearby Roman Magmatic Province, and are similar to those of the Vesuvius and Ischia magmas.     

Petrogenesis of an 800 m lava sequence in eastern Uruguay: insights into magma chamber processes beneath the Paraná flood basalt province

Major and trace element along with representative Sr, Nd and Pb isotope data are presented for drill core samples which intersect an 800 m lava pile in eastern Uruguay. The lavas form part of the Paraná flood basalt province, are low-Ti in composition but distinct from the low-Ti Gramado magma type, and have been termed the Treinte Y Trés magma type. The lava pile overlies a large positive gravity anomaly inferred to reflect an east–west trending, mid-crustal mafic intrusive body with a calculated volume of 35,000 km³. Smooth up-section compositional variations in the basalts are interpreted to record magma evolution within this mid-crustal magma chamber. ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb increase throughout the sequence yet Mg remains relatively constant in the lower 200 m of the sequence, suggesting a role for magma chamber recharge. Above this the lavas show a regular, up-section decrease in Mg coupled with increasing ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb and this is interpreted to reflect crystal fractionation combined with crustal contamination. The data provide further evidence that contamination of flood basalt magmas in crustal magma chambers is a common phenomenon and calculations suggest that the amount of crustal addition may be as high as 60–70%. Nevertheless, the effects of this crustal contamination do not appear able to account for the discrepancy between key incompatible trace element ratios and isotope ratios of the lavas and those of any putative mantle plume. In fact, La/Ta decreases with decreasing Mg and increasing ⁸⁷Sr/⁸⁶Sr indicating that the effects of crustal contamination were actually to reduce La/Ta and implying that the parental magmas had very high La/Ta (90). These constraints are clearly inconsistent with an asthenospheric origin for the parental magmas and so, consistent with mass balance calculations, it is inferred that they were derived from the lithospheric mantle.     

Bimodal volcanism at the Katla subglacial caldera,Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas

The Katla subglacial caldera is one of the most active and hazardous volcanic centres in Iceland as revealed by its historical volcanic activity and recent seismic unrest and magma accumulation. A petrologic and geochemical study was carried out on a suite of mid-Pleistocene to Recent lavas and pyroclastic rocks originated from the caldera. The whole series is characterised by a bimodal composition, including Fe-Ti transitional alkali basalts and mildly alkalic rhyolites. Variations in trace-element composition amongst the basalts and rhyolites show that their chemical differentiation was mainly controlled by fractional crystallisation and possible assimilation. The petrology and chemistry of the few intermediate extrusive rocks show that they were derived from magma mingling or hybridisation. The absence of extrusive rocks of true intermediate magmatic composition and the occurrence of amphibole-bearing felsic xenoliths support the hypothesis of partial melting of the hydrated basalt crust as the main process leading to the generation of rhyolites. The ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr values of Katla volcanic rocks fit the general isotopic array defined by late Quaternary to Recent lavas from Iceland. A few rock specimens are distinguished by low ¹⁴³Nd/¹⁴⁴Nd values suggesting assimilation and mixing of much older crustal material. Despite their similar whole-rock chemical compositions, the postglacial rhyolitic extrusives differ from the felsic xenoliths by their glass composition and the absence of amphibole. This, together with the general chemical trend of volcanic glasses, indicates that the postglacial rhyolitic extrusives were probably derived by a process involving late reheating and partial melting of crustal material by intrusion of basaltic magmas.     

Sr–Nd isotope ratios of gabbroic and dioritic rocks in a Cretaceous-Paleogene granite terrain, Southwest Japan

Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial ⁸⁷Sr/⁸⁶Sr (0.7063–0.7076) and low initial Nd isotope ratios (?_Nd, ?2.5 to ?5.3); whereas those from the North Zone have lower initial ⁸⁷Sr/⁸⁶Sr (usually less than 0.7060) and higher Nd isotope ratios (?_Nd, ?0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.     

Geochemical approach to magmatic evolution of Mt. Erciyes stratovolcano Central Anatolia, Turkey

Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite (48.4–70.5 wt.% SiO₂). Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic composition of the volcanics range between 0.703344–0.703964, 0.512920–0.512780 for alkaline basalts and change between 0.704322–0.705088, 0.512731–0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios (0.703434–0.705468, 0.512942–0.512600). Low Rb, Ba, Nb content with high Zr/Nb, low Ba/Nb, La/Yb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high La/Yb, Ba/Nb, low Zr/Nb and low ⁸⁷Sr/⁸⁶Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The Ba/Nb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments.     

Partial melting models for the petrogenesis of Reykjanes Peninsula basalts,Iceland Implications for the use of trace elements and strontium and neodymium isotope ratios to record inhomogeneities in the upper mantle

The mixing of magmas derived from two major compositional layers in a vertically stratified mantle has been favoured by Zindler et al. [1] in their interpretation of the REE and Sr and Nd isotope data for basalts from the Reykjanes Peninsula. However, a model involving the dynamic partial melting of a regionally homogeneous, veined mantle can also explain the major and trace element data and be reconciled with an alternative interpretation of the time relationships of the lavas to that presented by Jakobsson et al. [2]. Moreover, it is possible to explain the constant⁸⁷Sr/⁸⁶Sr but variable¹⁴³Nd/¹⁴⁴Nd ratios of the lavas by this model if the vein and wall rock components of the mantle source have equilibrated for Sr but not for Nd isotopes — a state that has been interpreted for some veined mantle nodules [13]. The model presented also involves more realistic degrees of partial melting than the alternative magma mixing models and satisfactorily explains the erupted volumes of the different magma types found in the area. Interpreting the basalt geochemistry in these terms suggests that Sr isotope ratios of the lavas monitor different scales of heterogeneity in the precursor mantle sources than Nd isotope ratios.     

Isotopic evolution of lavas from Haleakala Crater, Hawaii

Pb and Sr isotopic ratios have been determined for tholeiitic shield-building, alkalic cap, and post-erosional stage lavas from Haleakala Crater. Pb isotopic compositions of the tholeiites overlap those of the alkalic cap lavas, although⁸⁷Sr/⁸⁶Sr ratios of these two suites are distinct. Alkalic cap and post-erosional lavas appear to be indistinguishable on the basis of Sr and Pb isotopic composition.Sr and Pb isotopic ratios of Haleakala post-shield-building lavas are positively correlated. Such a trend is previously undocumented for any suite of Hawaiian lavas and contrasts with the general negative correlation observed for data from Hawaiian tholeiites. These relations are consistent with a three-component petrogenetic mixing model. Specifically, it is proposed that magma batches at individual Hawaiian volcanoes formed by: (1) mixing of melts generated from mantle plumes containing two isotopically distinct mantle components (primitive vs. enriched), and (2) subsequent variable degrees of interaction between these plume melts and a third (MORB signature) mantle reservoir prior to their emplacement in a crustal magma chamber. These observations and inferences provide new constraints on physical models of Hawaiian magmatism. Based on observed temporal isotopic variations of Haleakala lavas, it is suggested that the ratio of enriched: primitive mantle components in the Hawaiian plume source decreases during the waning stages of alkalic volcanism. Over the same time interval, both decreasing melt production and protracted residence of ascending melts within the upper mantle contribute to a systematic increase in the ratio of depleted vs. plume component.     

Possible origin of K-rich volcanic rocks from Virunga,East Africa,by metasomatism of continental crustal material: Pb,Nd and Sr isotopic evidence

The isotopic compositions of Sr, Nd and Pb together with the abundances of Rb, Sr, U and Pb have been determined for mafic and felsic potassic alkaline rocks from the young Virunga volcanic field in the western branch of the East African rift system.⁸⁷Sr/⁸⁶Sr varies from 0.7055 to 0.7082 in the mafic rocks and from 0.7073 to 0.7103 in the felsic rocks. The latter all come from one volcano, Sabinyo. Sabinyo rocks have negative ε_Ndvalues ofε_Nd = ?10. Nd and Sr isotopic variations in the basic potassic rocks are correlated and plot between Sabinyo and previously reported [1] compositions (ε_Nd = +2.5;⁸⁷Sr/⁸⁶Sr≈ 0.7047) for Nyiragongo nephelinites. The Pb isotopic compositions for Sabinyo rocks are nearly uniform and average²⁰⁶Pb/²⁰⁴Pb≈ 19.4,²⁰⁷Pb/²⁰⁴Pb= 15.79–15.84,²⁰⁸Pb/²⁰⁴Pb≈ 41.2. The basic potassic rocks have similar²⁰⁶Pb/²⁰⁴Pb values but range in²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb from the Sabinyo values to less radiogenic compositions.Excellent correlations of⁸⁷Sr/⁸⁶Sr with Rb/Sr, 1/Sr and²⁰⁷Pb/²⁰⁶Pb for Sabinyo rocks suggest these to be members of a hybrid magma series. However, the nearly uniform Pb compositions for this series points to radiogenic growth of⁸⁷Sr in the magma source region following an event which homogenized the isotopic compositions but not Rb/Sr. The Rb-Sr age derived from the erupted Sabinyo isochron-mixing line is consistent with the ～500 Myr Pb-Pb age from Nyiragongo [1], which suggests that this event affected all Virunga magma sources. The event can again be traced in the Pb-Pb, Pb-Sr and Nd-Sr isotopic correlations for all Virunga rocks, including Nyiragongo, when allowances are made for radiogenic growth subsequent to this mixing or incomplete homogenization event. Inferred parent/daughter element fractionations point to a metasomatic event during which a mantle fluid invaded two lithospheric reservoirs: a +ε_Nd reservoir sampled by the Nyiragongo nephelinites and suggested to be the subcontinental mantle and a ?ε_Nd reservoir sampled by the mafic and felsic potasssic volcanism. Whether this ?ε_Nd reservoir is the crust, continental crustal material in the mantle or anomalous mantle cannot be decided from the data. The simplest answer, that this reservoir is the continental crust, seems to be at variance with experimental evidence suggesting a subcrustal origin for basic potassic magmas. Partial melting of the ancient metasomatised lithospheric domains and ensuing volcanism seems to be entirely a response to decompression and rising geotherms during rifting and thinning of the lithosphere.     

The Shabogamo Intrusive Suite,Labrador: Sr and Nd isotopic evidence for contaminated mafic magmas in the Proterozoic

The Shabogamo Intrusive Suite comprises numerous bodies of variably metamorphosed gabbro which intrude Archean and Proterozoic sequences at the junction of the Superior, Churchill, and Grenville structural provinces in western Labrador. Combined Sm-Nd and Rb-Sr systematics in two bodies, ranging from unmetamorphosed to lightly metamorphosed, document a crystallization age of about 1375 m.y., and suggest that both bodies crystallized from magmas with similar Nd and Sr isotopic compositions. This age is in accordance with the existence of a regional magmatic event in the Churchill Province at approximately 1400 m.y.Rb-Sr systematics in two bodies of amphibolite-grade gabbro suggest a regional metamorphic event at about 950 m.y., corresponding to the waning stages of Grenville activity. Sm-Nd systematics in these high-grade bodies are affected to a much lesser degree than Rb-Sr.Initial ratios for¹⁴³Nd/¹⁴⁴Nd and⁸⁷Sr/⁸⁶Sr are lower and higher, respectively, than bulk earth values at 1375 m.y. Both these displacements are in the direction of older crustal material at 1375 m.y., and a model is proposed to produce the Shabogamo magma by mixing a mantle-derived magma with a partial melt of crustal rocks (approximately 4: 1 by volume). Young volcanic rocks with anomalous Nd and Sr isotopic ratios, which have previously been taken as evidence for “enriched” mantle, may be interpreted similarly.     

Isotope and trace element evidence for late-stage intra-crustal melting in the High Andes

Purico-Chascon is an acid igneous complex less than 1.5 Ma old rising to 5800 m in the North Chilean Andes, and consisting of andesite-dacite cones and dacite domes on an ignimbrite shield. The rocks are subdivided into two groups: those from Chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths, whereas among those at Purico no xenoliths have been found.⁸⁷Sr/⁸⁶Sr=0.7095?0.7081 at Purico, 0.7079?0.7069 at Chascon, and 0.7061-0.7057 in the xenoliths from the Chascon lavas:¹⁴³Nd/¹⁴⁴Nd=0.51222?0.51236 overall. The Purico lavas are characterised by higher SiO₂, Rb/Sr,⁸⁷Sr/⁸⁶Sr, and REE abundances, and lower Sr/Nd, Sr/Ba and¹⁴³Nd/¹⁴⁴Nd than most Andean igneous suites. There is no indication ofselective crustal contamination of Sr, or any systematic change in isotope ratios during differentiation. Nonetheless the trend of, for example, high Sr/Nd and Sr contents in rocks with low⁸⁷Sr/⁸⁶Sr (0.704, Ecuador) to low Sr/Nd and Sr and high SiO₂ in rocks with⁸⁷Sr/⁸⁶Sr=0.7081?0.7095 at Purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity. The formation of the least evolved Purico lavas (～60%SiO₂) is discussed in terms of bulk assimilation of crustal material, mixing between crustal- and mantle-derived magmas, and partial melting of pre-existing crust. Although such models are still extremely primitive, the simplest explanation of the observed chemical variations is that the Purico rocks evolved from parental magmas derived by crustal anatexies. Thermal considerations suggest that such late-stage crustal anatexis is a predictable response to crustal thickening which in the Andes is thought to have taken place during the Cenozoic.     

Geochemistry and Sr–Nd–Pb isotopic constraints on the petrogenesis of Cenozoic lavas from the Pali Aike and Morro Chico area (52°S), southern Patagonia,South America

Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO₂; 4.3–5.9 wt.% Na₂O+K₂O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO₂; 4.0–4.4 wt.% Na₂O+K₂O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.     

Oxygen isotopes and the origin of high-87Sr/86Sr andesites

Andesites from the Peruvian Andes and the Banda arc of Indonesia are characterized by unusually high and variable ⁸⁷Sr/⁸⁶Sr ratios. The Banda arc samples, including two cordierite-bearing lavas from Ambon, show a clear positive correlation between ⁸⁷Sr/⁸⁶Sr and δ¹⁸O. The andesitic rocks have δ¹⁸O values that range from 5.6 to 9.2‰. Over that range in δ¹⁸O, ⁸⁷Sr/⁸⁶Sr increases from 0.7044 to 0.7095. The cordierite-bearing lavas have δ¹⁸O values of approximately 15‰ and ⁸⁷Sr/⁸⁶Sr ratios of approximately 0.717. The similarity between δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios in total rocks and separated plagioclase phenocrysts of the Banda arc samples indicates that the measured isotope ratios are primary and have not been affected by secondary, low-temperature post-eruptive alteration. The observed variation between O and Sr isotopic ratios can be modeled by two-component mixing in which one component is of mantle isotopic composition. As the crust beneath the Banda arc is probably oceanic, contamination of the manle component may have resulted from the subduction of either continentally-derived sediments or continental crust. Mixing calculations indicate that the contaminant could have an isotopic composition similar to that observed in the cordierite-bearing lavas.The Andean samples, despite petrographic evidence of freshness, exhibit whole-rock δ¹⁸O values significantly higher than those of corresponding plagioclase phenocryst separates, indicating extensive low-temperature post-eruptive alteration. The plagioclase mineral separates show a range of δ¹⁸O values between 6.9 and 7.9‰. The ⁸⁷Sr/⁸⁶Sr ratios of these same samples are, in most instances, not significantly different from those measured for the whole rock, thus signifying that the phenocrysts and groundmass were in isotopic equilibrium at the time of eruption. Unlike the lavas of the Banda arc, the Andean lavas show no strong positive correlation between ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values, but instead lower ⁸⁷Sr/⁸⁶Sr ratios appear to be associated with higher δ¹⁸O values. The δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values of the Peruvian samples are both slightly higher than those of “normal” island arc volcanics.The small proportions of contaminant implied by the O isotope results seem to preclude continental crustal contamination as a primary cause of high ⁸⁷Sr/⁸⁶Sr ratios. The most plausible process that can explain both O and Sr isotope results is one in which sediments of continental origin are partially melted in the subduction zone. These melts rise into overlying mantle material and subsequently participate in the formation of calc-alkaline magmas.If the involvement of a sialic component in the genesis of andesitic magma occurs in the subduction zone, melting of that sialic material signifies temperatures of at least 750–800°C at the top of the subducted lithospheric slab at depths of approximately 150 km. The fact that contamination has apparently occurred in the Banda arc samples without producing any simple widespread correlations between Sr and O isotopic compositions on the one hand and major or trace element abundances on the other, shows that isotopic correlations, possibly including pseudo-isochrons, can be produced by mixing without producing trace element mixing correlations. Because O versus Sr isotope correlations are little affected by processes of partial melting of differentiation, they provide a direct means of testing whether Sr isotopic variations in volcanic rocks are of mantle origin or are due instead to mixing with sialic material.     

Assessment of a shallow magmatic system: the 1888–90 eruption,Vulcano Island,Italy

The magmatic system feeding the last eruption of the volcano La Fossa, Vulcano Island, Italy was studied. The petrogenetic mechanisms controlling the differentiation of erupted rocks were investigated through petrography, mineral chemistry, major, trace and rare earth element and Sr, Nd and Pb isotopic geochemistry. In addition, melt inclusion and fluid inclusion data were collected on both juvenile material and xenolithic partially melted metamorphic clasts to quantify the P-T conditions of the magma chamber feeding the eruption. A regular and continuous chemical zoning has been highlighted: rhyolites are the first erupted products, followed by trachytes and latites, whereas rhyolitic compositions were also found in the upper part of the sequence. The chemical and isotopic composition of the rhyolites indicates that they originated by fractional crystallization from latitic magmas plus the assimilation of crustal material; the trachytes represent hybrid magmas resulting from the mixing of latites and rhyolites, contaminated in the shallow magmatic system. The erupted products, primarily compositionally zoned from latites to rhyolites, are heterogeneous due to syn-eruptive mingling. The occurrence of magmacrust interaction processes, evidenced by isotopic variations (⁸⁷Sr/⁸⁶Sr=0.70474±3 to 0.70511±3; ¹⁴³Nd/¹⁴⁴Nd=0.512550±6 to 0.512614±8; ²⁰⁶Pb/²⁰⁴Pb=19.318–19.489; ²⁰⁷Pb/²⁰⁴Pb=15.642–15.782; ²⁰⁸Pb/²⁰⁴Pb=39.175–39.613), is confirmed by the presence of partially melted metamorphic xenoliths, with ⁸⁷Sr/⁸⁶Sr=0.71633±6 to 0.72505±2 and ¹⁴³Nd/¹⁴⁴Nd=0.51229±7, in rhyolites and trachytes. AFC calculations indicate a few percentage contribution of crustal material to the differentiating magmas. Thermometric measurements on melt inclusions indicate that the crystallization temperatures of the latites and trachytes were in the range of 1050–1100° C, whereas the temperature of the rhyolites appears to have been around 1000°C at the time of the eruption. Compositional data on melt inclusions reveal that the magmas involved in the eruption contained about 1–1.5 wt.% dissolved H₂O in pre-eruptive conditions. Secondary fluid inclusions found in metamorphic xenoliths give low equilibration pressure data (30–60 MPa), giving the location of the higher portions of the chamber at around 1500–2000 m of depth.     

Origin and evolution of mid-Cretaceous, garnet-bearing, intermediate and silicic volcanics from Canterbury, New Zealand

The Mt Somers Volcanics are part of a suite of mid-Cretaceous (89 ± 2 Ma) intermediate to silicic volcanics, erupted onto an eroded surface of Torlesse sediments. Rock types vary from basaltic andesite to high-silica rhyolite. Andesites are medium- to high-K with phenocrysts of plagioclase, orthopyroxene and pigeonite. Dacites are peraluminous and commonly contain granulite facies xenoliths and garnet xenocrysts. Equilibrium mineral assemblages indicate metamorphic pressures of close to 6 kbar at 800°C. Rhyolites are peraluminous with phenocrysts of quartz, sanidine, plagioclase, biotite, garnet and orthopyroxene. The ferromagnesian phases show textural evidence of magmatic crystallization and are chemically distinct from xenocryst phases in dacites. Equilibrium assemblages indicate that early magmatic crystallization occurred at close to 7 kbar (20 km depth) at above 850°C, with melt-water contents of less than 3.5%. Major-element contents, trace-element contents and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7085 indicate that the rhyolites formed by partial melting of dominantly quartzo-feldspathic Torlesse sediments, leaving a granulite-facies residue. The chemical variation displayed by the rhyolites is best explained by fractional crystallization of the observed high-pressure phenocryst assemblage. Most elements show a compositional gap between rhyolite and dacite. The major-element, trace-element and Sr isotope compositions of the intermediate lavas are best explained by assimilation of lower crustal material combined with fractional crystallization in mantle-derived tholeiitic magmas. Magmatism was the result of heat and magma flux from the mantle, during the change from compressive to extensional tectonics after the culmination of the Rangitata Orogeny.     

Nd-Sr systematics of the Setouchi volcanic rocks,southwest Japan: a clue to the origin of orogenic andesite

Twenty-three volcanic rocks from the Setouchi volcanic belt, southwest Japan, were analyzed for Nd and Sr isotopic compositions for the purpose of examining the genetic relationships among the basalt, high-magnesium andesite (HMA) and evolved porphyritic andesite. The andesites have higher⁸⁷Sr/⁸⁶Sr (0.70487–0.70537) and lower¹⁴³Nd/¹⁴⁴Nd (0.512509–0.512731) than the basalts, i.e., 0.70408–0.70468 and 0.512691–0.512830, respectively. This result confirms earlier conclusions obtained from petrologic study that the andesites cannot be fractionation products of basaltic magma but that the andesitic and basaltic magmas were generated independently. On the basis of melting experiments for HMA and basalt, it is inferred that there is an isotopically stratified mantle beneath southwest Japan. Evolved porphyritic andesites have essentially identical Sr and Nd isotopic ratios to HMA and can be derived by fractionation of primary andesitic magma. A model to produce orogenic andesite is proposed on petrologic, experimental and isotopic bases.     

Aeolian island arc (South Tyrrhenian Sea) magma heterogeneites in historical lavas: Sr and Pb isotopic evidence

Historical volcanic rocks of the Aeolian islands range in composition from shoshonitic basalts to rhyolites, which might reflect fractional crystallization of a shoshonitic parent magma. However Sr and Pb isotopic data indicate a more complex history. The shoshonitic basalts at present erupted at Stromboli, although chemically similar to the postulated parent magma, are genetically unrelated to the other studied rocks. Sr isotopes indicate that Vulcano, Vulcanello and Lipari had independent magma sources. It is proposed that crustal contamination raised the Sr isotopic composition of the Lipari rhyolites. The rocks of these island are related by a common very steep trend of²⁰⁷Pb/²⁰⁴Pbvs. ²⁰⁶Pb/²⁰⁴Pb. Such a trend is a common feature of orogenic magmas and shows that Pb was derived by mixing of at least two components. Presently it is impossible to constrain precisely either the timing or the physical meaning of the Pb end members. The Pb isotopic trend in the Eolian island is very distinct from those recorded in volcanic rocks both from behind the arc (Etna, Iblean Mts.) and from Central and Southern Italy.     

87Rb-87Sr constraints on the genesis and evolution of the Cantal continental volcanic system (France)

We have investigated 24 whole rocks and mineral separates of five different rock types from the Cantal shield volcano in France, applying high-precision Rb-Sr techniques. The chemical and isotopic systematics suggest the distinction of two series throughout the different rock classes, one practically uncontaminated, the other seriously influenced by wall rock assimilation. The first group comprises basalts and intermediate rocks with⁸⁷Sr/⁸⁶Sr= 0.70340–0.70382. The second group in addition includes rhyolites and the corresponding⁸⁷Sr/⁸⁶Sr ratios vary between 0.70421 and 0.71270. The data of mineral separates support the hybridization hypothesis and possibly suggest an original⁸⁷Sr/⁸⁶Sr ratio of 0.7028 for the magma source region. Moreover they provide internal isochron ages which place a period of extensive volcanic activity at 8.1–8.8 m.y. ago in accord with K-Ar ages of volcanic rocks from the center of the Cantal volcano.     

CO2- and LREE-rich mantle below eastern Australia: a REE and isotopic study of alkaline magmas and apatite-rich mantle xenoliths from the Southern Highlands Province,Australia

Alkaline magmatism in the Southern Highlands Province, New South Wales, Australia is associated with continental rifting. Near-primary liquids have a wide range in Nd and Sr isotope composition that indicates gross isotopic and chemical heterogeneities in a mantle source region depleted in light rare earth elements (LREE) for much of Earth's history. The large-ion lithophile element and LREE-enriched nature of the primary lavas ((Ce)_N = 95–182 and (Yb)_N = 8.5–13.3) is consistent with an enriched mantle source region. This elemental enrichment may be accomplished by veining of the subcontinental mantle with volatile-rich phases like amphibole, apatite and carbonate which provide the volatile flux necessary to trigger anatexis.Degassing of mantle CO₂ has led to migration of LREE-enriched fluids and local transformation of the lherzolitic mantle to pyroxenite veined by apatite ± kaersutite ± mica ± diopside. The mantle veining event may be related to upwelling of silica-undersaturated incompatible element-enriched magmas similar to the host magma of the Kiama xenoliths. In a relatively short period of time (100 m.y.), the Sr and Nd isotopes in essentially LREE-depleted mantle have evolved in response to low Sm/Nd and low Rb/Sr ratios, and now define a near-vertical vector on a isotope-isotope plot. From this rather unique signature we can infer that CO₂- and LREE-rich, Rb-poor mantle is a potentially suitable mantle source region for the genesis of alkali-potassic volcanic rocks characterized by a narrow range in⁸⁷Sr/⁸⁶Sr ratio and a wide range in¹⁴³Nd/¹⁴⁴Nd ratio (e.g. Leucite Hills).     

The Zealandia Volcanic Complex: Further evidence of a lower crustal “hot zone” beneath the Mariana Intra‐oceanic Arc,Western Pacific

This paper addresses formation of felsic magmas in an intra‐oceanic magmatic arc. New bathymetric, petrologic, geochemical, and isotopic data for Zealandia Bank and two related volcanoes in the south‐central Mariana arc is presented and interpreted. These three volcanoes are remnants of an older andesitic volcano that evolved for some time and became dormant long enough for a carbonate platform to grow on its summit before reawakening as a rhyodacitic volcano. Zealandia lavas are transitional between low‐ and medium‐K and tholeiitic and calc‐alkaline suites. They define a bimodal suite with a gap of 56–58 wt% SiO₂; this suggests that mafic and felsic magmas have different origins. The magmatic system is powered by mantle‐derived basalts having low Zr/Y and flat rare earth element patterns. Two‐pyroxene thermometry yields equilibration temperatures of 1000–1100 °C for andesites and 900–1000 °C for dacites. Porphyritic basalts and andesites show textures expected for fractionating magmas but mostly fine‐grained felsic lavas do not. All lavas show trace element signatures expected for mantle and crustal sources that were strongly melt‐depleted and enriched by subduction‐related fluids and sediment melts. Sr and Nd isotopic compositions fall in the normal range of Mariana arc lavas. Felsic lavas show petrographic evidence of mixing with mafic magma. Zealandia Bank felsic magmatism supports the idea that a large mid‐ to lower‐crustal felsic magma body exists beneath the south‐central Mariana arc, indicating that MASH (mixing, assimilation, storage, and homogenization) zones can form beneath intra‐oceanic as well as continental arcs.     

The origin of Samoa: new evidence from Sr, Nd, and Pb isotopes

We report Sr, Nd and Pb isotope ratios and parent and daughter element concentrations in 34 volcanic rocks from Samoa. The highly undersaturated post-erosional volcanics, which have erupted in Recent to Historic time along a 250-km-long fissure, have isotopic compositions that define fields distinct from those of the tholeiitic to alkalic lavas of the older Samoan shield volcanoes. Most shield lavas have²⁰⁶Pb/²⁰⁴Pb of 18.9–19.4,⁸⁷Sr/⁸⁶Sr of 0.7045–0.7055 and⁸⁷Sr/⁸⁶Sr (to 0.7075). In general, isotopic compositions of the shield lavas are similar to those of the Marquesas and Society Islands. Post-erosional samples have lower²⁰⁶Pb/²⁰⁴Pb and¹⁴³Nd/¹⁴⁴Nd and higher⁸⁷Sr/⁸⁶Sr than most shield lavas. The most striking feature of the post-erosional data is a negative correlation between²⁰⁷Pb/²⁰⁴Pb and²⁰⁶Pb/²⁰⁴Pb. This suggests that post-erosional lavas are derived from mixtures of the shield source and a high-²⁰⁷Pb/²⁰⁴Pb,⁸⁷Sr/⁸⁶Sr, low-²⁰⁶Pb/²⁰⁴Pb and¹⁴³Nd/¹⁴⁴Nd post-erosional source which may contain recycled ancient sediment. This enriched mantle domain may also underlie the Ontong-Java and Manihiki Plateaus to the north and west. Although both the Samoan shield and post-erosional lavas show chemical characteristics often associated with mantle plumes, only the shield volcanism can plausibly be related to a plume. The post-erosional eruptions appear to be the result of flexure and rifting due to plate bending at the northern termination of the Tonga Trench.