Indirect crustal contamination: evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania

 Alkali basalts and nephelinites from the volcanic province of northern Tanzania contain pyroxene and nepheline that show evidence for chemical and/or isotopic disequilibria with their host magmas. Olivine, pyroxene, nepheline and plagioclase all appear to be partially xenocrystic in origin. Five whole rock/mineral separate pairs have been analyzed for Sr, Nd, and Pb isotopic compositions. The ²⁰⁶Pb/²⁰⁴Pb ratios are distinct by as much as 20.94 (whole rock) vs. 19.10 (clinopyroxene separate). The Sr and Nd isotopic disequilibria vary from insignificant in the case of nepheline, to Δ ⁸⁷Sr/⁸⁶Sr of 0.0002 and Δɛ_Nd of 0.7 in the case of clinopyroxene. The mineral chemistry of 25 samples indicates the ubiquitous presence of minerals that did not crystallize from a liquid represented by the host rock. The northern Tanzanian magmas are peralkaline and exhibit none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source variations. Rather the xenocrystic phases present appear to have been derived from earlier alkali basaltic rocks or magmas that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that subsequently erupted on the surface. Disequilibrium in volcanic rocks has potentially serious consequences for the use of whole rock data to identify source reservoirs. However, mass balance calculations reveal that the ²⁰⁶Pb/²⁰⁴Pb isotopic compositions of the erupted lavas were changed by less than 0.25% as a result of this indirect crustal contamination. Received: 15 February 1995 / Accepted: 4 May 1996     

Signatures of the source for the Emeishan flood basalts in the Ertan area： Pb isotope evidence

The Emeishan flood basalts can be divided into high-Ti (HT) basalt (Ti/Y>500) and low-Ti (LT) basalt (Ti/Y<500). Sr, Nd isotopic characteristics of the lavas indicate that the LT- and the HT-type magmas originated from distinct mantle sources and parental magmas. The LT-type magma was derived from a shallower lithospheric mantle, whereas the HT-type magma was derived from a deeper mantle source that may be possibly a mantle plume. However, few studies on the Emeishan flood basalts involved their Pb isotopes, especially the Ertan basalts. In this paper, the authors investigated basalt samples from the Ertan area in terms of Pb isotopes, in order to constrain the source of the Emeishan flood basalts. The ratios of 206Pb/204Pb (18.31–18.41), 207Pb/204Pb (15.55–15.56) and 208Pb/204Pb (38.81–38.94) are significantly higher than those of the depleted mantle, just lying between EM I and EM II. This indicates that the Emeishan HT basalts (in the Ertan area) are the result of mixing of EMI end-member and EMII end-member.     

Geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the ‘famous’ region

Bulk rock major and trace element variations in selected basalts from the Famous area, in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. For a given Mg/Mg+Fe²⁺ atomic ratio of the mineral, the NiO content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts. The Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and, in addition, distinguishes the picritic basalts from the other basalt types. These differences indicate that the different magma groups could not have been processed through the same magma chamber, and preclude any direct inter-relationship via open or closed system fractional crystallization.The Fe-Mg partitioning between olivine and host rock suggests that the picritic basalts represent olivine (±Cr-spinel) enriched magmas, derived from a less MgO rich parental magma. The partitioning of Fe and Mg between olivine, Cr-spinel and coexisting liquid is used to predict a primary magma composition parental to the picritic basalts. This magma is characterized by relatively high MgO (12.3%) and CaO (12.6%) and low FeO^* (7.96%) and TiO₂ (0.63%).Least squares calculations indicate that the plagioclase phyric basalts are related to the plagioclase-pyroxene basalts by plagioclase and minor clinopyroxene and olivine accumulation. The compositional variations within the olivine basalts can be accounted for by fractionation of plagioclase, clinopyroxene and olivine in an open system, steady state, magma chamber in the average proportions 453223. It is suggested that the most primitive olivine basalts can be derived from a pristine mantle composition by approximately 17% equilibrium partial melting. Although distinguished by its higher Zr/Nb ratio and lower NiO content of phenocryst phases, the magma parental to the picritic basalts can be derived from a similar source composition by approximately 27% equilibrium partial melting. It is suggested that the parental magma to the plagioclase-pyroxene and plagioclase phyric basalts might have been derived from greater depth resulting in the fractionation of the Zr/Nb ratio by equilibration with residual garnet.C.O.B. Contribution No. 722     

Petrology of the Cenozoic volcanism in the Upper Benue valley,northern Cameroon (Central Africa)

Thirty-one plugs of alkaline volcanic rocks of Cenozoic age (37 Ma in mean) occur in the Upper Benue valley, northern Cameroon (Central Africa). The complete alkaline series (alkaline basalts, hawaiites, mugearites, phonolites, trachytes and rhyolites) is represented. Basalts contain phenocrysts of olivine, Al-Ti-rich diopside, and Ti-magnetite, and hawaiites-abundant microphenocrysts of plagioclase. Mugearites have a trachytic texture and contain xenocrysts of K-feldspar, apatite, quartz and unstable biotite. Phonolites are peralkaline. Trachytes (peralkaline and non-peralkaline) and rhyolites are characterised by their sodic mineralogy with aegirine-augite, richterite, and arfvedsonite phenocrysts. There is a large compositional gap between basaltic and felsic lavas, except the mugearites. Despite this gap, major- and trace-element distributions are in favour of a co-magmatic origin for the basaltic and felsic lavas. The Upper Benue valley basalts are similar in their chemical and isotopic features to other basalts from both the continental and oceanic sectors of the Cameroon Line. The Upper Benue valley basaltic magmas (⁸⁷Sr/⁸⁶SrƸ.7035; k Nd=+3.9) originate from an infra-lithospheric reservoir. The Sr-Nd isotopic composition and high Sr contents of the mugearites suggest that they are related to mantle-derived magmas and that they result from the mixing, at shallow crustal levels, of a large fraction of trachytic magma with a minor amount of basaltic magma. Major-element modelling of the basalt-trachyte evolution (through hawaiite and mugearite compositions) does not support an evolution through fractional crystallization alone. The fluids have played a significant role in the felsic lavas genesis, as attested by the occurrence of F-rich minerals, calcite and analcite. An origin of the Upper Benue valley rhyolitic magmas by fractional crystallization of mantle-derived primitive magmas of basaltic composition, promoted or accompanied by volatile, halogen-rich fluid phases, may be the best hypothesis for the genesis of these lavas. These fluids also interact with the continental crust, resulting in the high Sr-isotope initial ratios (0.710) in the rhyolites, whereas the Nd isotopic composition has been less affected (k Nd=+0.4).     

Temporal evolution of the kerguelen plume: Geochemical evidence from 38 to 82 ma lavas forming the Ninetyeast ridge

Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have²⁰⁶pb/²⁰⁴Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher²⁰⁶Pb/²⁰⁴Pb, such as that expressed in lavas from St. Paul Island.     

Evidence of hydrous differentiation and crystal accumulation in the low-MgO,high-Al2O3 Lake Basalt from Medicine Lake volcano,California

The late Pleistocene Lake Basalt of Medicine Lake volcano, California is comprised of variably porphyritic basalt and basaltic andesite flows and scoria. These eruptives are similar in composition and phenocryst abundance to the low-MgO, high-Al²O³ mafic magmas common in convergent margin settings. The petrogenesis of the magmas that produced the Lake Basalt has been inferred from field relations, melting experiments and subsequent major and trace element modeling. Their formation involved both hydrous differentiation and plagioclase accumulation and thus the Lake Basalt can be used to constrain the relative contributions of these processes to the production of high-Al²O³ arc basalt. Phenocryst-poor lavas of the Lake Basalt formed by hydrous differentiation; their compositions and observed phenocrysts were reproduced in 1 kbar, H²O-saturated melting experiments. Anorthite-rich plagioclase compositions of the lavas of the Lake Basalt necessitate crystallization from melts with between 4 and 6 wt% dissolved H²O. Phenocryst-rich lavas of the Lake Basalt, with 18 modal% phenocrysts and greater, formed by plagioclase accumulation in magmas similar to the phenocryst-poor lavas. This interpretation is supported by the depleted incompatible element abundances and enriched Sr/Zr ratio of the more porphyritic lavas relative to the phenocryst-poor lavas. We model the formation of the Lake Basalt as a two-stage process that combines a differentiation model and a plagioclase accumulation model. Stage one involved hydrous fractionation, granitic assimilation and mixing with undifferentiated parent magma. This process generated lavas with up to 19.2 wt% A1²O³ and 7 modal% phenocrysts. In stage two, plagioclase accumulated in these liquids and produced more aluminous and porphyritic lavas with up to 21.8 wt% A1²O³ and 33 modal% phenocrysts.     

Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios

High precision Sr-Nd isotope ratios together with Pb isotope ratios corrected for mass fractionation using a double spike are reported for an extensive suite of late Quaternary to Recent lavas of Iceland, the Kolbeinsey and Reykjanes Ridges, and a small number of basalts from further south on the Mid-Atlantic Ridge. Compared with global MORB, the Icelandic region is distinguished by having low ²⁰⁷Pb/²⁰⁴Pb for any given ²⁰⁶Pb/²⁰⁴Pb, expressed by negative Δ²⁰⁷Pb (−0.8 to −3.5) in all but four Icelandic samples. Most samples also have elevated ²⁰⁸Pb/²⁰⁴Pb (strongly positive Δ²⁰⁸Pb), which combined with their negative Δ²⁰⁷Pb is very unusual in MORB worldwide. The negative Δ²⁰⁷Pb is interpreted as a consequence of evolution in high-μ mantle sources for the last few hundred Ma. The region of negative Δ²⁰⁷Pb appears to correspond with the region of elevated ³He/⁴He, suggesting that both lithophile and volatile elements in melts from the whole region between 56 and 70°N are dominantly sourced in a plume that has incorporated recycled Palaeozoic ocean crust and unradiogenic He, probably from the deep mantle. At least four mantle components are recognized on Iceland, two with an enriched character, one depleted and one that shows some isotopic affinities to EM1 but is only sampled by highly incompatible-element-depleted lavas in this study. Within restricted areas of Iceland, these components contribute to local intermediate enriched and depleted components that display near binary mixing systematics. The major depleted Icelandic component is clearly distinct in Pb isotopes from worldwide MORB, but resembles the depleted mantle source supplying the bulk of the melt to the Kolbeinsey and southern Reykjanes Ridges. However, an additional depleted mantle source is tapped by the northern Reykjanes Ridge, which with very negative Δ²⁰⁷Pb and less positive Δ²⁰⁸Pb is distinct from all Icelandic compositions. These components must mostly mix at mantle depths because a uniform mixture of three Icelandic components is advected southward along the Reykjanes Ridge.Despite strong covariation with isotope ratios, incompatible trace element ratios of Icelandic magmas cannot be representative of old mantle sources. The observed parent-daughter ratios in depleted and enriched Icelandic lavas would yield homogeneous Sr, Nd, Hf and ²⁰⁶Pb isotope signatures ∼170 Ma ago if present in their sources. The heterogeneity in ²⁰⁷Pb/²⁰⁴Pb is not however significantly reduced at 170 Ma, and the negative present day Δ²⁰⁷Pb cannot be supported by the low μ observed in depleted lavas from Iceland or the adjacent ridges. Since μ is higher in melts than in their sources, it follows that all the depleted sources must be residues from <170 Ma partial melting events. These are thought to have strongly affected most incompatible trace element ratios.     

Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt Inclusions

Mid-ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO–Al2O3 and negative MgO–Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium-bearing MORB sample E13-3B (MgO=9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure >4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ~1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.     

Experimental petrology of normal MORB near the Kane Fracture Zone: 22°–25° N,mid-Atlantic ridge

Melting experiments carried out at 1-atm and at 2 kbar on mid-ocean ridge basalts dredged from the mid-Atlantic ridge near the Kane Fracture Zone (KFZ, 22° to 25° N. latitude) provide a basis for evaluating the role of crystal fractionation in generating compositional variability observed in normal mid-ocean ridge basalt. The 1-atm olivine-plagioclase-clinopyroxene saturation boundary for KFZ lavas defines a path in mineral projection schemes and in oxide-oxide diagrams that is displaced from the same experimentally determined boundaries in FAMOUS (Grove and Bryan 1983) and Oceanographer Fracture Zone (Walker et al. 1979) basalts. The glass margins of sparsely phyric KFZ lavas record small amounts of near surface, low pressure fractional crystallization, and their glass and bulk rock compositions are similar. An important signature of low pressure differentiation is recorded in the quenched glass margins of moderately phyric KFZ lavas compared to their bulk rock compositions, and the glass has evolved along low-pressure fractionation paths that are similar to those produced in the 1-atm experiments. Many of the lavas have retained phenocrysts in equilibrium proportions, so that their bulk rock compositions represent liquid compositions. When the effects of near-surface differentiation and crystal accumulation are removed from the Kane data set, and only liquid compositions are considered, a suite of basalt magmas can be identified that forms a trend in mineral component projection schemes parallel to the 1-atm oliv-plag-cpx multiple saturation boundary, but displaced from it toward olivine. These basalts have only olivine and plagioclase as phenocrysts, and are well removed from clinopyroxene saturation at low pressure. The compositional variation can not be generated by mixing any primary liquid composition with a low pressure liquid that has evolved along the oliv-plag-cpx multiple saturation boundary. Major and trace element models of this trend using olivine, plagioclase and clinopyroxene as fractionating phases match the compositional variability. This compositional trend is generated by fractionation at pressures greater than 2 kbar, but within the plagioclase stability field. A review of the data for other normal MORB suites from this part of the mid-Atlantic ridge reveals a similar elevated pressure fractionation signature which persists when the effects of low pressure magma mixing are removed from the data set.     

Mineralogical and isotopic evidence for phenocryst-matrix disequilibrium in the Garner Mountain andesite

Petrographic, trace element and isotopic evidence demonstrates that magma mixing preceded the eruption of the Garner Mountain andesite. The flow contains reversely zoned plagioclase phenocrysts and amphibole pseudomorphs composed of plagioclase, clinopyroxene, orthopyroxene and opaque oxides. Partially resorbed quartz grains are also present. In contrast to the isotopically uniform matrix, plagioclase phenocrysts have ⁸⁷Sr/⁸⁶Sr ratios that correlate negatively with matrix Sr and positively with matrix Rb abundances. These observations demonstrate increasing isotopic disequilibrium between the plagioclase and matrix in the more evolved varieties of the flow.Plagioclase phenocrysts and matrix are assumed to record fractionation-assimilation events in different parts of the magma chamber. Early formed plagioclase phenocrysts crystallized under AFC conditions close to the roof of the chamber and were subsequently entrained in a liquid mixture composed of evolved interstitial liquid held in the partly crystallized roof zone and newly injected parental magma.     

Makran ophiolitic basalts (SE Iran) record Late Cretaceous Neotethys plume-ridge interaction

ABSTRACT

The Makran complex in southeast Iran provides a spectacular subduction-related accretionary complex to understand the mechanism of oceanic accretion and the evolution of subduction zones. In this paper, we present new major and trace element data as well as isotopic compositions of mafic volcanic blocks from the Makran ophiolitic mélange complex (OMC). Our aim is to assess the genesis of these rocks and discuss their implications on the evolution of Neotethys Ocean. These volcanic blocks are composed mainly of basalts with minor trachytes. The Makran lavas are occasionally interlayered with tuff layers. Zircons from these tuffs give U-Pb ages of 95 Ma, which is well in accordance with the reposted microfossil data for the interlayered pelagic limestones with pillow lavas. Makran basalts can be geochemically subdivided into four groups; normal to transitional MORB, enriched-MORB, Plume-type MORB and alkaline (-OIB-like) basalts. The OIB-like pillow lavas are represented by high values of Th/Tb (6.3–7.4) which are higher than other basalts (group 1 = 0.3–0.8; groups 2 = 0.7–1.6; group 3 = 1.58–1.36).¹⁴³Nd/¹⁴⁴Nd_(t) ratios for basalts ranges from 0.51247 to 0.51292, whereas ⁸⁷Sr/⁸⁶Sr_(t) isotopic composition of the OMC lavas varies from 0.704433 to 0.709466. The Pb isotopic composition of the lavas are quite high, ranging from 15.49–15.66 for ²⁰⁷Pb/²⁰⁴Pb_(t), 18.09–19.12 for ²⁰⁶Pb/²⁰⁴Pb_(t) and 37.80–39.23 for ²⁰⁸Pb/²⁰⁴Pb_(t). The chemistry of these rocks suggests that they were formed most likely in an oceanic setting with clear plume-ridge interaction. These rocks can form from partial melting of a highly heterogeneous mantle source, which is extensively metasomatized with deep mantle OIB-type components. We suggest these rocks have been generated in an oceanic ridge with plume-ridge interaction, similar to the Iceland-Reykjanes Ridge, before being fragmented and accreted into the Makran accretionary complex.     

Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field,northern Rio Grande rift,New Mexico

Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO₂ rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO₂) to quartz latite (67% SiO₂). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO₂ peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km³ high-SiO₂ peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.     

Open System Magmatic Evolution of the Taos Plateau Volcanic Field, Northern New Mexico: II. The Genesis of Cryptic Hybrids

Tholeiitic lavas of the Servilleta Basalt exhibit only subtletextural and mineralogical evidence for a hybrid origin, butelemental and isotopic analyses of these basalts are best modeledin terms of mixing Servilleta parent magma with a range of contemporaneousandesite and dacite magmas. Cryptic compositional heterogeneitiesin some flows interpreted as hybrids apparently reflect incompletehomogenization following pre-emptive magma mingling. The generalscarcity of mixing-related textural disequilibrium is ascribedin part to mixing of mineralogically similar end-members. Eradicationof some phenocrysts during post-mixing residence and evolutionin a convecting magma body may be an even more important factor. Eruptions of hybrid magmas may frequently be triggered by magmamixing events (i.e. injection or replenishment), and minglingof compositionally diverse magmas may ensue as a consequenceof tapping a compositionally graded or layered magma chamber.These hybrids are instantly recognizable by the preservationof disequilibrium textures and mineral assemblages, and by discontinuouscompositional heterogeneities. Cryptic hybrids, which have notpreserved this record, will be recognizable as mixed magmasprimarily by geochemical evidence for open system evolution.     

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

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

Geochemical evolution of Indian Ocean basaltic magmatism

A comparison of new and published geochemical characteristics of magmatism in the western and eastern Indian Ocean at the initial and recent stages of its evolution revealed several important differences between the mantle sources of basaltic melts from this ocean.

1. The sources of basalts, from ancient rises and from flanks of the modern Central Indian Ridge within the western Indian Ocean contain an enriched component similar in composition to the source of the Réunion basalts (with radiogenic Pb and Sr and unradiogenic Nd), except for basalts from the Comores Islands, which exhibit a contribution from an enriched HIMU-like component.
2. The modern rift lavas of spreading ridges display generally similar geochemical compositions. Several local isotopic anomalies are characterized by the presence of an EM2-like component. However, two anomalous areas with distinctly different enriched mantle sources were recognized in the westernmost part of the Southwestern Indian Ridge (SWIR). The enriched mantle source of the western SWIR tholeiites in the vicinity of the Bouvet Triple Junction has the isotopic ratios indicating a mixture of HIMU + EM2 in the source. The rift anomaly distinguished at 40° E displays the EM1 signature in the mantle source, which is characterized by relatively low ²⁰⁶Pb/²⁰⁴Pb (up to 17.0) and high ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr. This source may be due to mixing with material from the continental lithosphere of the ancient continent Gondwana. The material from this source can be distinguished in magmas related to the Mesozoic plume activity in Antarctica, as well as in basalts from the eastern Indian Ocean rises, which were formed by the Kerguelen plume at 100–90 Ma.
3. The geochemical heterogeneities identified in the ancient and present-day magmatic products from the western and eastern Indian Ocean are thought to reflect the geodynamic evolution of the region. In the eastern part of the ocean, the interaction of the evolving Kerguelen plume with the rift zones produced magmas with specific geochemical characteristics during the early opening of the ocean; such a dispersion of magma composition was not recognized in the western part of the ocean.

     

Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones

The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo₈₁ to Fo₉₁. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe⁺²)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999     

Water content of primitive low-K tholeiitic basalt magma from Iwate Volcano,NE Japan arc: implications for differentiation mechanism of frontal-arc basalt magmas

The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An_85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa.     

From basalt to dacite: origin and evolution of the calc-alkaline series of Salina, Aeolian Arc, Italy

The island of Salina comprises one of the most distinct calc-alkaline series of the Aeolian arc (Italy), in which calc-alkaline, high-K calc-alkaline, shoshonitic and leucite-shoshonitic magma series are developed. Detailed petrological, geochemical and isotopic (Sr, Nd, Pb, O) data are reported for a stratigraphically well-established sequence of lavas and pyroclastic rocks from the Middle Pleistocene volcanic cycle (430–127 ka) of Salina, which is characterized by an early period of basaltic volcanism (Corvo; Capo; Rivi; Fossa delle Felci, group 1) and a sequence of basaltic andesites, and andesites and dacites in the final stages of activity (Fossa delle Felci, groups 2–8). Major and trace element compositional trends, rare earth element (REE) abundances and mineralogy reveal the importance of crystal fractionation of plagioclase + clinopyroxene + olivine/ orthopyroxene ± titanomagnetite ± amphibole ± apatite in generating the more evolved magma types from parental basaltic magmas, and plagioclase accumulation in producing the high Al₂O₃ contents of some of the more evolved basalts. Sr isotope ratios range from 0.70410 to 0.70463 throughout the suite and show a well-defined negative correlation with ¹⁴³Nd/¹⁴⁴Nd (0.51275–0.51279). Pb isotope compositions are distinctly radiogenic with relatively large variations in ²⁰⁶Pb/²⁰⁴Pb (19.30–19.66), fairly constant ²⁰⁷Pb/²⁰⁴Pb (15.68–15.76) and minor variations in ²⁰⁸Pb/²⁰⁴Pb ratios (39.15–39.51). Whole-rock δ¹⁸O values range from +6.4 to +8.5‰ and correlate positively with Sr isotope ratios. Overall, the isotopic variations are correlated with the degree of differentiation of the rocks, indicating that only small degrees of crustal assimilation are overprinting the dominant evolution by crystal–liquid fractionation (AFC-type processes). The radiogenic and oxygen isotope composition of the Salina basalts suggests derivation from primary magmas from a depleted mantle source contaminated by slab-derived fluids and subducted sediments with an isotopic signature of typical upper continental crust. These magmas then evolved further to andesitic and dacitic compositions through the prevailing process of low-pressure fractional crystallization in a shallow magma reservoir, accompanied by minor assimilation of crustal lithologies similar to those of the Calabrian lower crust. Received: 29 November 1999 / Accepted: 16 April 2000     

长白山区二道白河流域早更新世玄武质熔岩的成因

采用岩石化学和同位素分析方法,研究了二道白河流域早更新世玄武质熔岩的成因。玄武质熔岩由钠质拉斑玄武岩和钾质粗面玄武岩、玄武质粗面安山岩组成。它们的REE分配形式比较相近,表明它们来自共同的源区。Sr、Nd、Pb同位素示踪表明,二道白河流域早更新世玄武质熔岩岩浆源区接近于似原始地幔。它们的Mg#=100Mg O/(Mg O+Fe O)低于中国东部新生代玄武岩原始岩浆的Mg#(60~68),Ni(27.76×10-6~200.6×10-6)低于原始地幔,Rb/Sr(0.05~0.09)、Ba/Rb(15.64~264)高于原始地幔,说明这些岩石不是源自原始地幔。玄武质熔岩的DI变化于42~67,具有高Ca、高Sr、Eu正异常,微量元素图解显示玄武岩保留部分熔融趋势,粗面玄武岩、玄武质粗安岩具有结晶分异趋势,岩浆上升过程中发生了不同程度的地壳混染作用。玄武质熔岩的Nb/Ta之比为14.8~15.8,与勘察加半岛深俯冲带火山类似。Nb/Ta-(Na2O-K2O)关系图解显示研究区玄武质岩浆的形成与俯冲板片的部分熔融有关。     

Compositional and Dynamic Controls on Mafic--Silicic Magma Interactions at Continental Arc Volcanoes: Evidence from Cordn El Guadal, Tatara-San Pedro Complex, Chile

Heterogeneous andesitic and dacitic lavas on Cordn El Guadalbear on the general problem of how magmas of differing compositionsand physical properties interact in shallow reservoirs beneathcontinental arc volcanoes. Some of the lavas contain an exceptionallylarge proportion (<40%) of undercooled basaltic andesiticmagma in various states of disaggregation. Under-cooled maficmagma occurs in the silicic lavas as large (<40 cm) basalticandesitic magmatic inclusions, as millimeter-sized crystal-clotsof Mg-rich olivine phenocrysts plus adhering Carich plagioclasemicrophenocrysts (An_50–70), and as uniformly distributed,isolated phenocrysts and microphenocrysts. Compositions andtextures of plagioclase phenocrysts indicate that inclusion-formingmagmas are hybrids formed by mixing basaltic and dacitic melts,whereas textural features and compositions of groundmass phasesindicate that the andesitic and dacitic lavas are largely mechanicalmixtures of dacitic magma and crystallized basaltic andesiticmagma. This latter observation is significant because it indicatesthat mechanical blending of undercooled mafic magma and partiallycrystallized silicic magma is a possible mechanism for producingthe common porphyritic texture of many calc-alkaline volcanicrocks. The style of mafic-silicic magma interaction at CordonEl Guadal was strongly dependent upon the relative proportionsof the endmembers. Equally important in the Guadal system, however,was the manner in which the contrasting magmas were juxtaposed.Textural evidence preserved in the plagioclase phenocrysts indicatesthat the transition from liquid-liquid to solid-liquid mixingwas not continuous, but was partitioned into periods of magmachamber recharge and eruption, respectively. Evidently, duringperiods of recharge, basaltic magmas rapidly entrained smallamounts of dacitic magma along the margins of a turbulent injectionfountain. Conversely, during periods of eruption, dacitic magmagradually incorporated small parcels of basaltic andesitic magma.Thus, the coupled physical-chemical transition from mixed inclusionsto commingled lavas is presumably not coincidental. More likely,it probably provides a partial record of the dynamic processesoccurring in shallow magma chambers beneath continental arevolcanoes. KEY WORDS: Chile; commingling; magma mixing; magmatic inclusions ^*Present address: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA