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.     

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.     

Isotope characteristics of submarine lavas from the Philippine Sea: implications for the origin of arc and basin magmas of the Philippine tectonic plate

Igneous rocks from the Philippine tectonic plate recovered on Deep Sea Drilling Project Legs 31, 58 and 59 have been analyzed for Sr, Nd and Pb isotope ratios. Samples include rocks from the West Philippine Basin, Daito Basin and Benham Rise (40–60 m.y.), the Palau-Kyushu Ridge (29–44 m.y.) and the Parece Vela and Shikoku basins (17–30 m.y.). Samples from the West Philippine, Parece Vela and Shikoku basins are MORB (mid-ocean ridge basalt)-like with ⁸⁷Sr/⁸⁶Sr= 0.7026−0.7032, ¹⁴³Nd/¹⁴⁴Nd= 0.51300−0.51315, and ²⁰⁶Pb/²⁰⁴Pb= 17.8−18.1. Samples from the Daito Basin and Benham Rise are OIB (oceanic island basalt)-like with ⁸⁷Sr/⁸⁶Sr= 0.7038−0.7040, ¹⁴³Nd/¹⁴⁴Nd= 0.51285−0.51291 and ²⁰⁶Pb/²⁰⁴Pb= 18.8−19.2. All of these rocks have elevated ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb compared to the Northern Hemisphere Regression Line (NHRL) and have δ²⁰⁷Pb values of 0 to +6 and δ²⁰⁸Pb values of +32 to +65. Lavas from the Palau-Kyushu Ridge, a remnant island arc, have ⁸⁷Sr/⁸⁶Sr= 7032−0.7035, ¹⁴³Nd/¹⁴⁴Nd= 0.51308−0.51310 and ²⁰⁶Pb/²⁰⁴Pb= 18.4−18.5. Unlike the basin magmas erupted before and after them, these lavas plot along the NHRL and have Pb-isotope ratios similar to modern Pacific plate MORB's. This characteristic is shared by other Palau-Kyushu Arc volcanic rocks that have been sampled from submerged and subaerial portions of the Mariana fore-arc.At least four geochemically distinct magma sources are required for these Philippine plate magmas. The basin magmas tap Source 1, a MORB-mantle source that was contaminated by EMI (enriched mantle component 1 [31]) and Source 2, an OIB-like mantle source with some characteristics of EMII (enriched mantle component 2 [31]). The arc lavas are derived from Source 3, a MORB-source or residue mantle including Sr and Pb from the subducted oceanic crust, and Source 4, MORB-source or residue mantle including a component with characteristics of HIMU (mantle component with high U/Pb [31]). These same sources can account for many of the isotopic characteristics of recent Philippine plate arc and basin lavas. The enriched components in these sources which are associated with the DUPAL anomaly were probably introduced into the asthenosphere from the deep mantle when the Philippine plate was located in the Southern Hemisphere 60 m.y.b.p.     

Petrology of Peter I Øy (Peter I Island), West Antarctica

Peter I Øy is located in the Bellinghausen Sea, 400 km NE of Thurston Island, West Antarctica. It is a Pleistocene volcanic island situated adjacent to a former tranform fault on the continental rise of the presently passive margin between the Pacific and Antarctica. New K-Ar age determinations ranging from 0.1 to 0.35 Ma show that the volcanism responsible for this island took place at the same time as post-subduction, rift-related volcanism occurred in the nearby Marie Byrd Land and the Antarctic Peninsula. The rocks of the island are alkalic basalt and hawaiite, benmoreite and trachyte. The basic tocks typically contain phenocrysts of olivine (Fo_61–84), diopsidic augite, and plagioclase (ca. An₆₀). Small xenoliths are present and consist of mantle-type spinel lherzolite, cumulate clinopyroxenite and gabbro and felsic inclusions that consist of medium-grained strained quartz, plagioclase, and abundant colorless glass. Chemically, the basic rocks are characterized by rather high MgO (7.8–10.2 wt.%) and TiO₂ (3.1–3.7 wt.%) and relatively low CaO (8.4–9.5 wt.%) contents. They have steep REE patterns, [(La/Yb)_N = 20] with HREE only 5 x chrondrite. Y and Sc are almost constant at relatively low levels. Compatible trace elements such as Ni and Cr show considerable variation (190–300 and 150–470 ppm, respectively.), whereas V shows only little variation. Sr and Nd isotope ratios vary slightly with ⁸⁷Sr/⁸⁶Sr averaging 0.70388 and ¹⁴³Nd/¹⁴⁴Nd 0.512782, both typical for ocean island volcanism. Lead isotope ratios are consistently high in basalts; ²⁰⁶Pb/²⁰⁴Pb = 19.194, ²⁰⁷Pb/²⁰⁴Pb = 15.728 and ²⁰⁸Pb/²⁰⁴Pb = 39.290, whereas benmoreïte is somewhat less radiogenic. Oxygen isotope analyses average δ¹⁸O = +6.0‰. Incompatible trace elements vary by a factor of 1.5–2.0 within the range of the basic rocks. It is proposed that the incompatible trace-element variations represent different degrees (<10%) of partial melting, and that these melts were later modified by minor (<15‰) olivine and spinel fractionation. The very small variation in Y (and Sc) and the very fractionated REE pattern indicate that the source had an Y- and HREE-rich residual phase, most probably garnet. Furthermore, it is suggested that the source was slightly hydrous and that melting took place at 18–20 kbar. Trachyte was derived by multiphase fractionation of ne-normative basalts, and benmoreite from hy-normative parental liquids. The rocks of Peter I Øy are generally of the same type and age as those outcropping in extensional regimes on the nearby continent, and therefore, these occurrences may be related to each other in some way. However, the Peter I Øy rocks are considerably more radiogenic in strontium and less radiogenic in neodymium than the rocks of the Antarctic Peninsula and Marie Byrd Land. Possible explanations are that Peter I Øy represent asthenospheric hot spot activity, or transtensional rifting as subduction ceased.     

Geochemical and Sr–Nd–Pb isotopic investigation of Triassic granitoids and basement rocks in the northern Gyeongsang Basin, Korea: Implications for the young basement in the East Asian continental margin

Abstract We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (⁸⁷Sr/⁸⁶Sr_initial = 0.7041 ~ 0.7050, ?_Nd(t) = 2.3 ~ 4.0, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr_initial = 0.7047 ~ 0.7065, ?_Nd(t) = 3.9 ~ 2.8, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.     

Isotope geochemistry and origin of calc-alkaline lavas from a caledonian continental margin volcanic arc

Analyses for major and trace elements, including REE, and Sr, Nd and Pb isotopes are reported from a suite of Siluro-Devonian lavas from Fife, Scotland. The rocks form part of a major calc-alkaline igneous province developed on the Scottish continental margin above a WNW-dipping subduction zone. Within the small area (ca. 15 km²) considered, rock types range from primitive basalts and andesites (high Mg, Ni and Cr) to lavas more typical of modern calc-alkaline suites with less than 30 ppm Ni and Cr. There is a marked silica gap between these rocks (< 62%) and the rare rhyolites (> 74%), yet the latter can be generated by fractional crystallization from the more mafic lavas. In contrast, variation in incompatible element concentrations and ratios in the mafic lavas can not be generated by fractional crystallization processes. Increasing SiO₂ is accompanied by increasing Rb, K, Pb, U and Ba relative to Sr and high field strength elements, increasing LREE enrichment and increasing _Sr calculated at 410 Ma, and by decreasing HREE, Eu/Eu*, Sm/Nd and _Nd (410). _Nd and _Sr are roughly anticorrelated and have more radiogenic compositions than the mantle array, in common with data reported elsewhere from this part of the arc. The correlation extrapolates up to cross the mantle array within the composition field of the contemporary MORB source, and extrapolates down towards the probable compositional range of Lower Palaeozoic greywackes, which may form the uppermost 8 km of the crust, or may be supplied to the source by subduction. One sample, however, lies within the mantle array, and closely resembles lavas from northwestern parts of the arc, where a mantle source with mild time-integrated Rb/Sr and LREE enrichment has been inferred. The lavas have relatively high initial ²⁰⁷Pb/²⁰⁴Pb for their ²⁰⁶Pb/²⁰⁴Pb, a feature which has been interpreted elsewhere as the result of incorporation of a sediment component into arc magmas. The systematic changes with increasing SiO₂ in isotopic and chemical parameters can be explained by mixing of a greywacke-derived component with depleted mantle. The various possible mixing mechanisms are discussed, and it is considered most likely that mixing occurred in the mantle source through greywacke subduction. The bulk of the Rb, K, Ba and Pb in the lavas is probably recycled from the crust, whereas less than some 40% of the Sr and Nd is recycled. The calc-alkaline chemical trends are solely a function of mixing with the sediment component.     

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.     

Strontium and neodymium isotopic evidence for the heterogeneous nature and development of the mantle beneath Afar (Ethiopia)

Neodymium isotope and REE analyses of recent volcanic rocks and spinel lherzolite nodules from the Afar area are reported. The¹⁴³Nd/¹⁴⁴Nd ratios of the volcanic rocks range from 0.51286 to 0.51304, similar to the range recorded from Iceland. However, the⁸⁷Sr/⁸⁶Sr ratios display a distinctly greater range (0.70328–0.70410) than those reported from the primitive rocks of Iceland. Whole rock samples and mineral separates from the spinel lherzolite nodules exhibit uniform¹⁴³Nd/¹⁴⁴Nd ratios (ca. 0.5129) but varied⁸⁷Sr/⁸⁶Sr ratios in the range 0.70427–0.70528.The SrNd isotope variations suggest that the volcanic rocks may have been produced by mixing between two reservoirs with distinct isotopic compositions. Two possible magma reservoirs in this area are the source which produced the “MORB-type” volcanics in the Red Sea and Gulf of Aden and the anomalous source represented by the nodule suite. The isotopic composition of the volcanics is compatible with mixing between these two reservoirs.It is shown that the anomalous source with a high⁸⁷Sr/⁸⁶Sr ratio cannot have been produced by simple processes of partial melting and mixing within normal mantle. Instead the high⁸⁷Sr/⁸⁶Sr is equated with a fluid phase. A primitive cognate fluid, subducted seawater or altered oceanic lithosphere may have been responsible for the generation of the source with a high⁸⁷Sr/⁸⁶Sr ratio.     

The relationship between calc-alkaline volcanism and within-plate continental rift volcanism: evidence from Scottish Palaeozoic lavas

Lower Carboniferous lavas from the Midland Valley and adjacent regions of Scotland are mildly alkaline and intraplate in nature. The sequence is dominated by basalt and hawaiite, although mugearite, benmoreite, trachyte and rhyolite are also present. Basic volcanic rocks display the LIL element and LREE enrichment typical of intraplate alkali basalt terrains. Low initial⁸⁷Sr/⁸⁶Sr (0.7029–0.7046), high ε_Nd (−0.4 to +5.6) and moderately radiogenic²⁰⁶Pb/²⁰⁴Pb (17.77–18.89) ratios are also comparable with alkali basalts from other continental rifts and oceanic islands.When the Carboniferous lavas are compared with subduction-related lavas of Old Red Sandstone age, erupted in and around the Midland Valley ca. 50 Ma earlier (at 410 Ma) remarkable similarities are apparent. Significant overlap occurs in Nd and Pb isotopic compositions. Sr isotopic compositions are, however, more radiogenic in the older subduction-related lavas. This, combined with high K and Rb concentrations in ORS lavas may be explained by the incorporation of a sediment component derived from the subducted slab, which by Lower Carboniferous times had been lost from the mantle source region by convection. A pronounced negative Nb anomaly in the ORS subduction-related lavas may be explained by the retention of a Nb-bearing phase in the mantle during hydrous melting of the mantle wedge above the subduction zone.Allowing for the effects of the added component from the subducted slab, there appears to be no necessity to invoke separate mantle source regions for the two suites of lavas: both may have been derived from chemically similar portions of mantle. If volcanic arc lavas are derived from the mantle wedge, the implication is that such a source lies at relatively shallow depth within the upper mantle: the same may therefore apply to the Carboniferous continental rift basalts. This evidence, combined with the fact that there is no evident hot-spot trail across the Midland Valley despite a long period of within-plate volcanism and rapid plate movements during the Carboniferous, suggests that the alkali basalt magmatism is not the product of a deep-seated mantle plume. Rather, the volcanism appears to owe more to passive rifting and to diapiric upwelling from a source region within the uppermost mantle.     

Geochemical and Pb-Sr-Nd isotopic compositions of Indosinian granitoids from the Bikou block,northwest of the Yangtze plate:Constraints on petrogenesis,nature of deep crust and geodynamics

This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma),Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These granitoids are enriched in Al (Al2O3:14.56%―16.48%) and Sr (352 μg/g―1047 μg/g),and depleted in Y (<16 μg/g) and HREE (e.g. Yb<1.61 μg/g),resulting in high Sr/Y (36.3―150) and (La/Yb)N (7.8―36.3) ratios and strongly fractionationed REE patterns. The Indosinian granotoids show initial Sr isotopic ratios (ISr) from 0.70419 to 70752,εNd(t) values from-3.1 to -8.5,and initial Pb isotopic ratios 206Pb/204Pb=17.891-18.250,207Pb/204Pb=15.494-15.575,and 208Pb/204Pb=37.788-38.335. Their geochemi-cal signatures indicate that the granitoids are adakitic. However,they are distinct from some adakites,generated by partial melting of subducted oceanic slab and/or underplated basaltic lower crust,be-cause they have high K (K2O: 1.49%―3.84%) and evolved Nd isotopic compositions,with older Nd iso-topic model ages (TDM=1.06―1.83 Ga). Geochemical and Sr-Nd isotopic compositions suggest that the magmas of the Insoninian adakitic rocks in the Bikou block were derived from partial melting of thick-ened basaltic lower crust. Combined with regional analyses,a lithospheric delamination model after collision between the North China and South China plates can account for the Indosinian adakitic magma generation. On the other hand,based on the Pb-Sr-Nd isotopic probing to the magma sources of the adakitic rocks,it is suggested that there is an unexposed continent-type basement under the exposed Bikou Group volcanic rocks. This can constrain on the Bikou Group volcanic rocks not to be MORB-or OIB-type.     

Lau Basin basalts (LBB): trace element and SrNd isotopic evidence for heterogeneity in backarc basin mantle

Diverse⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd isotopic compositions among basalts from the Lau Basin (LBB), an active backarc basin in the southwest Pacific, indicate heterogeneity in the underlying mantle. Isotopic compositions display bimodal distributions which are related to geographic location. Type I LBB (⁸⁷/Sr⁸⁶Sr 0.70366;¹⁴³Nd/¹⁴⁴Nd 0.51297) include tholeiites from the central basin, Peggy Ridge, and Rochambeau Bank, while Type II basaltic and andesitic glasses from the northeastern portion of the basin, near Niua Fo'ou island, have higher⁸⁷Sr/⁸⁶Sr ( 0.7038) and lower ¹⁴³Nd/¹⁴⁴Nd ( 0.51288). Both depleted (e.g. N-MORB) and enriched (e.g. E-MORB) trace element abundances occur among Type I and Type II LBB.Covariation between trace element and isotopic ratios among Type I LBB is consistent with mixing between depleted mantle similar to the source for MORB and relatively enriched peridotite similar to the source for E-MORB. Relative to MORB, uniformly high⁸⁷Sr/⁸⁶Sr ( +0.0005) among all Type I LBB for given Nd isotopic compositions ( ε_Nd = +8 to +12) may reflect a lithospheric component, such as ancient recycled altered ocean crust. Type II LBB have SrNd isotopic compositions which are gradational between enriched mantle similar to the source of OIB and a component with distinct Sr isotopic composition such as that observed in Samoan post-erosional basalts. Isotopic and geographic discontinuity between Type I and Type II LBB, and isotopic affinity of Type II and Niua Fo`ou island basalts with those from Samoa suggests that volcanism in the northeastern portion of the basin is tapping deeper mantle beneath the adjoining Pacific plate, as well as Indo-Australian mantle overlying the Pacific lithosphere that is subducted into the Tonga Trench.     

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.     

The La Breña — El Jagüey Maar Complex,Durango, Mexico: II. Petrology and geochemistry

A suite of 16 basanitic volcanic rocks, representing all stages in the evolution of the La Breña — El Jagüey (LBEJ) Maar Complex, has been studied petrographically and analyzed for mineral compositions and whole-rock major element, trace element, and Sr–Nd–Pb isotopic compositions. Two feldspathic granulite xenoliths were also studied as possible lower-crustal contaminants to the LBEJ magmas. The volcanic rocks contain the stable minerals olivine, plagioclase, augite, and titanomagnetite±ilmenite, plus a diverse suite of xenocrusts derived from disaggregation of mantle xenoliths of spinel lherzolite (olivine, orthopyroxene, spinel) and lower-crustal granulite xenoliths (plagioclase, quartz, augite, ilmenite). Late-stage interstitial melts rich in Fe and Ti migrated into vesicles in several samples, forming coarse-grained segregation vesicles that are dominated by ilmenite blades up to 2 mm long. The whole-rock elemental data are typical of intra-plate basanitic rocks, with strong enrichments in large ion lithophile elements (i.e. K, Th, U) as well as high field strength elements (i.e. Nb, Ta) relative to mid-ocean ridge basalts (MORB) and estimates of primordial mantle abundances. Mg# increased systematically with time during the evolution of the LBEJ Maar Complex, from 57.0–58.2 in the pre-maar lavas to 59.1–63.8 in the post-maar lavas. Compatible elements (Ca, Sc, Cr, Co, Ni) correlate positively with Mg#, whereas a large group of incompatible elements (Al, Na, K, P, Rb, Sr, Zr, Nb, Ba, La, Ce, Sm, Hf, Ta, Th, U) correlate negatively with Mg#. These trends can be closely reproduced by simple models of fractional crystallization, provided that the incompatible element abundances of the parental, high-Mg# magmas are allowed minor variability. All successful fractionation models demand an important role for augite, despite its presence in the LBEJ volcanic rocks as only a late-stage microphenocrystic and groundmass mineral. Minor garnet fractionation is necessary to produce depletion of heavy rare earth element (REE) abundances in the pre-maar lavas, whose REE patterns cross those for the rest of the suite. The importance of augite and garnet fractionation indicate that the differentiation of the LBEJ magmas took place within the upper mantle, a conclusion that is supported by the presence of spinel lherzolite xenoliths in magmas from all stages in the evolution of the maar complex. Isotopic data for seven LBEJ volcanic rocks show the following ranges: ⁸⁷Sr/⁸⁶Sr 0.70327–0.70347, ^Nd 4.2–5.0, ²⁰⁶Pb/²⁰⁴Pb 18.60–18.81, ²⁰⁷Pb/²⁰⁴Pb 15.58–15.65, ²⁰⁸Pb/²⁰⁴Pb 38.19–38.58. Sr-Nd values are negatively correlated and form a trend parallel to the mantle array, overlapping the field for ocean island basalts (OIB). The LBEJ rocks have similar ⁸⁷Sr/⁸⁶Sr values but lower ^Nd compared to basanitic rocks from the US Basin and Range Province (BRP). Pb isotopic ratios are positively correlated and overlap the braod fields for MORB and OIB and the small fields for Mexican ore deposits and volcanic rocks from the active subduction-related Mexican Volcanic Belt. The LBEJ rocks have slightly more radiogenic Pb than basanitic rocks from the US BRP. Despite correlations among the isotopic ratios of the LBEJ suite, none of these ratios correlate with position in the eruption sequence, Mg#, or any other compositional parameter. The two lower-crustal xenoliths have high ⁸⁷Sr/⁸⁶Sr values (0.707, 0.710) and low ^Nd (-1.5,-8.0) compared to the LBEJ volcanic rocks, but their Pb isotopic compositions are only slightly more radiogenic than the volcanic rocks. These data do not support the widely held view that the lower crust is a major reservoir of unradiogenic Pb. In order to further constrain the role played by crustal contamination in generating the isotopic diversity in the LBEJ suite, we conducted an extensive investigation of Sr–Nd–Pb isotopic ratios for scoria clasts from different levels of a single scoria-fall horizon in the pyroclastic sequence related to the formation of La Breña Maar. Our results do not support an important role for crustal contamination in the LBEJ magmas. Rather, we conclude that minor isotopic variability exists in the mantle source regions beneath the maar complex.     

The southeast Australian lithospheric mantle: isotopic and geochemical constraints on its growth and evolution

Trace elements and isotopic compositions of whole rocks and mineral separates are reported for 15 spinel-bearing harzburgite and lherzolite xenoliths from southeastern Australia. These samples have an exceedingly large range in isotopic compositions, with⁸⁷Sr/⁸⁶Sr ranging from 0.70248 to 0.70834 and ε_Nd values ranging from +12.7 to −6.3. This range in isotopic compositions can be found in xenoliths from a single locality. The isotopic compositions of clinopyroxene separates and their whole rocks were found to be different in some xenoliths. Samples containing small glass pockets, which replace pre-existing hydrous minerals, generally show only small differences in isotopic composition between clinopyroxene and whole rock. In a modally metasomatized peridotite, significant differences in the Sr and Nd isotopic compositions of a coexisting phlogopite-clinopyroxene pair are present. Coexisting clinopyroxenes and orthopyroxenes from an anhydrous lherzolite have Sr isotopic compositions that are significantly different (0.70248 versus 0.70314), and yield an apparent age of 625 Ma, similar to that found previously by Dasch and Green [1]. However, the Nd isotopic compositions of the clinopyroxene and orthopyroxene are identical indicating recent (within 40 Ma) re-equilibration of Nd.Sr and Nd concentrations in the whole rocks and clinopyroxenes show an excellent positive correlation, and have an average Sr/Nd ratio of 15. This ratio is similar to the primitive mantle value, as well as that found in primitive MORBs and OIBs, but is much lower than that measured in island arc basalts and what might be predicted for a subduction zone-derived fluid. This indicates that a significant proportion of the Sr and Nd in these peridotites is introduced as a basaltic melt with intraplate chemical characteristics.The isotopic compositions of the peridotites reflect long-term, small-scale heterogeneities in the continental lithospheric mantle, and are in marked contrast to the near uniform isotopic compositions of the host alkali basalts (⁸⁷Sr/⁸⁶Sr= 0.7038–0.7041andε_Nd = +3.6 to +2.9). A minimum of three evolutionary stages are identified in the growth of the continental lithospheric mantle: an early basalt depletion event, recording the initial development and stabilization of the lithospheric mantle, followed by at least two enrichment episodes. These observations are consistent with continental lithospheric mantle growth involving the underplating of refractory peridotite diapirs.     

Geochemical and Pb-Sr-Nd isotopic compositions of granitoids from western Qinling belt: Constraints on basement nature and tectonic affinity

Geochemical and Pb-Sr-Nd isotopic compositions of five Indosinian granitoid intrusions from the western Qinling belt provide insights into basement nature and tectonic affinity. The results show that the western Qinling granitoids incline towards basic in their bulk chemical composition. The granitoids belong to high-K to shoshonitic series with K2O/Na2O=1.04-1.86 and are dominantly metaluminous with A/CNK=0.90-1.05 (most samples have A/CNK of <1.0). They have similar trace elemental compo- sitional patterns. In Sr-Nd isotopic compositions, they display some extent heterogeneity with Isr=0.70682-0.70845, εNd(t)=?4.85 to ?9.17 and TDM=1.26-1.66 Ga. They are characterized by high ra- diogenic Pb isotopic compositions. Their initial Pb isotopic ratios are 206Pb/204Pb=17.996-18.468, 207Pb/204Pb=15.565-15.677 and 208Pb/204Pb=38.082-38.587. Geochemical and Sr-Nd isotopic composi- tions reveal that magma for the granitoids was derived from partial melting of high-K (Rb) basaltic rocks, which might be formed in 900-1400 Ma. It is suggested that a large amount of the Proterozoic high-K (Rb) basaltic rocks, which underlie the Phanerozoic sedimentary cover, constitute the crustal basement of the western Qinling belt. Pb-Sr-Nd isotopic compositional comparison between the east- ern Qinling and the western Qinling Indosinian granitoids indicates that the crustal basement of the western Qinling is distinct from that of the eastern Qinling. The Baoji-Chengdu railway close to south-north orientation can be taken as an approximate boundary between both basements. The Pb-Nd isotopic compositional characteristics of the western Qinling granitoids suggest that the basement of the western Qinling belt has an affinity with the Yangtze block.     

Sr, Nd, O and H isotopic ratios in Ascension Island lavas and plutonic inclusions; cogenetic origin

Eighteen basic rocks from Ascension Island (South Atlantic) give a mean⁸⁷Sr/⁸⁶Sr ratio of 0.70311 ± 17 for both volcanics and plutonic inclusions. The late-stage differentiated rocks (rhyolites and granitic inclusions) have much higher⁸⁷Sr/⁸⁶Sr ratios, up to 0.712. All these rocks display the same range of Nd isotopic compositions (ε_Ndvalues from6.9to11.1with a mean on12samples of8.4 ± 0.6) implying a cogenetic relation between the two sequences. The D/H systematics lead to the same conclusion.In the NdSr diagram, the data plot close to the mantle array and show a positive correlation. This suggests a mixing between a depleted MORB-type mantle, i.e. the upper mantle, and a hot-spot with less depleted geochemical characteristics, i.e. the OIB mantle source.The total range of δ¹⁸O values lies between 4.8‰ for plagioclase cumulates and 6.7‰ for the most evolved rocks (peralkaline granites and comendites). The basic rocks have values around 5.3‰, typical of mantle-derived material. These oxygen data indicate that the high⁸⁷Sr/⁸⁶Sr ratios in the most evolved rocks (both volcanic and plutonic terms) result from the combination of two different processes: incorporation of slight amounts (< 1%) of high-temperature altered oceanic crust by the magma in the late stages of the differentiation process and then in-situ Rb decay since the time of formation of these rocks. Both processes were very effective because of the high Rb and low Sr contents of these evolved rocks.Oxygen isotope systematics in the Ascension Island granites and rhyolites indicate that a fractional crystallization process alone does not produce δ¹⁸O values higher than 6.7‰, i.e. that the ultimate δ¹⁸O enrichment, relative to the initial basic magma, is not greater than 1.5‰.     

Development of continental lithospheric mantle as reflected in the chemistry of the Mesozoic Appalachian Tholeiites, U.S.A.

Geochemical analyses of dikes, sills, and volcanic rocks of the Mesozoic Appalachian Tholeiite (MAT) Province of the easternmost United States provide evidence that continental tholeiites are derived from continental lithospheric mantle sources that are genetically and geochronologically related to the overlying continental crust. Nineteen olivine tholeiites and sixteen quartz tholeiites from the length of this province, associated in space and time with the last opening of the Atlantic, display significant isotopic heterogeneity: initial ε_Nd = +3.8 to −5.7; initial ⁸⁷Sr/⁸⁶Sr= 0.7044−0.7072; ²⁰⁶Pb/²⁰⁴Pb= 17.49−19.14; ²⁰⁷Pb/²⁰⁴Pb= 15.55−15.65; ²⁰⁸Pb/²⁰⁴Pb= 37.24−39.11. In PbPb space, the MAT define a linear array displaced above the field for MORB and thus resemble oceanic basalts with DUPAL Pb isotopic traits. A regression of this array yields a secondary PbPb isochron age of ≈ 1000 Ma (μ₁ = 8.26), similar to Sm/Nd isochrons from the southern half of the province and to the radiometric age of the Grenville crust underlying easternmost North America. The MAT exhibit significant trace element ratio heterogeneity (e.g., Sm/Nd= 0.226−0.327) and have trace element traits similar to convergent margin magmas [e.g., depletions of Nb and Ti relative to the rare earth elements on normalized trace element incompatibility diagrams, Ba/Nb ratios (19–75) that are significantly greater than those of MORB, and low TiO₂ (0.39–0.69%)].Geochemical and geological considerations very strongly suggest that the MAT were not significantly contaminated during ascent through the continental crust. Further, isotope and trace element variations are not consistent with the involvement of contemporaneous MORB or OIB components. Rather, the materials that control the MAT incompatible element chemistry were derived from subcontinental lithospheric mantle. Thus: (1) the MAT/arc magma trace element similarities; (2) the PbPb and Sm/Nd isochron ages; and (3) the need for a method of introducing an ancient (> 2−3 Ga) Pb component into subcontinental mantle that cannot be much older than 1 Ga leads to a model whereby the MAT were generated by the melting of sediment-contaminated arc mantle that was incorporated into the continental lithosphere during arc activity preceding the Grenville Orogeny (≈ 1000 Ma).     

Implications of correlated Nd and Sr isotopic variations for the chemical evolution of the crust and mantle

Published data showing a linear correlation between initial Nd and Sr isotope compositions in young basalts indicate the existence of a spectrum of isotopically distinct reservoirs in the mantle which represent either (1) mixtures of two homogeneous endmember reservoirs, one of which may be undifferentiated material or (2) fractionated reservoirs all derived from a homogeneous initial reservoir with the same ratio of enrichment factors for Sm/Nd and Rb/Sr. The slope of the correlation, which can be described approximately by (⁸⁷Sr/⁸⁶Sr) = ?3.74114 (¹⁴³Nd/¹⁴⁴Nd) + 2.61935orε_Nd = ?2.7 ε_Sr, places constraints on the origin of these reservoirs and hence on the chemical evolution of the crust-mantle system. The reservoirs could be residual regions of the mantle left after ancient partial melting events. If so, the requirement of constant relative fractionation of Sm/Nd and Rb/Sr in refractory residues is a strong constraint on partial melting models. Calculations suggest that batch melting models are more compatible with this constraint than are fractional melting models, but models incorporating currently accepted distribution coefficients and residual phase assemblages cannot reproduce the observed isotope effects except under highly specific conditions. The slope of the correlation is not consistent with the hypotheses that chemical structure in the mantle is due to accretional heterogeneity or variable loss of elements to the core. If the mantle reservoirs are complementary in composition to the continental crust, and if the crust + mantle has ε_Nd = 0andε_Sr = 0 and chondritic Sr/Nd, then Rb/Sr in the crust is calculated to be less than 0.10, suggesting that the crust may be more mafic in composition and contain a smaller proportion of the earth's Rb and heat-producing elements than previously estimated.     

The isotopic and chemical evolution of Mount St. Helens

Isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the Mt. St. Helens volcano suggest three different episodes; (1) 40,000–2500 years ago: eruptions of dacite with ε_Nd = +5, ε_Sr = ?10, variable δ¹⁸O,²⁰⁶Pb/²⁰⁴Pb ～ 18.76, Ca/Sr ～ 60, Rb/Ba ～ 0.1, La/Yb ～ 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with ε_Nd = +4 to +8, ε_Sr = ?7 to ?22, variable δ¹⁸O (thought to represent melting of differing mantle-crust reservoirs), ²⁰⁶Pb/²⁰⁴Pb= 18.81?18.87, variable Ca/Sr, Rb/Ba, La/Yb and high Zr, (3) 1000 years ago to present day: eruptions of andesite and dacite with ε_Nd = +6, ε_Sr = ?13, δ¹⁸O～6‰, variable²⁰⁶Pb/²⁰⁴Pb, Ca/Sr ～ 77, Rb/Ba= 0.1, La/Yb ～ 11. None of the products exhibit Eu anomalies and all are LREE enriched. There is a strong correlation between⁸⁷Sr/⁸⁶Sr and differentiation indices. These data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet, but not feldspar, followed by intrusion of this crustal reservoir by mantle-derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components. Since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re-equilibration of Pb isotopic compositions than other components suggesting that Pb is being transported via a fluid phase. The Nd and Sr isotopic compositions lie along the mantle array and suggest that the mantle underneath Mt. St. Helens is not as depleted as MORB sources. There is no indication of seawater involvement in the source region.     

Petrology and geochemistry of potassic rocks in the Gölcük area (Isparta, SW Turkey): genesis of enriched alkaline magmas

The Lower Pliocene volcanic rocks occurring in the Gölcük area of SW Turkey exhibit alkaline major element trends with a general potassic character. The development of volcanism can be divided into 2 major stages such as trachytic ancient lavas/domes and tephriphonolitic, trachyandesitic to trachytic Gölcük eruptions (ignimbrites, lava/dome extrusions, phreatomagmatic deposits, and finally, young domes). Volcanic rocks consist primarily of plagioclase, clinopyroxene (which ranges in composition from diopside to augite and are commonly zoned), biotite, and phlogopite. Amphibole phenocrysts are restricted to the pyroclastic deposits. Pseudoleucites are also seen only in the lava/dome extrusions. Oxides and apatites are common accessory phenocryst phases. As would be expected from their potassic–alkaline nature, the volcanic rocks of the Gölcük area contain high amounts of LILE (Ba, Sr, Rb and K), LREE, and Zr. Concentrations of compatible elements such as Cr, Ni and V are very low, possibly indicating fractionation of olivine and clinopyroxene. Correlation of SiO₂, Rb/Sr and MgO with ⁸⁷Sr/⁸⁶Sr (0.703506–0.704142) exhibit an increasing trend in the direction of crustal contamination. However, the isotopic compositions of Sr are not as high to indicate a high level of crustal contamination. Geochemical data are consistent with the derivation of Gölcük volcanic rocks from a metasomatized and/or enriched lithospheric mantle source during crustal extension in the area. This metasomatism was probably occurred by fluids released from the northward subduction between African and Eurasian plates during Tertiary, as the Gölcük volcanic rocks display features of island-arc magmas with having high Ba/Nb (>28) ratios, and Nb and Ti depletions. Lower Pliocene volcanism in the Gölcük was response to extensional tectonics.