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     

Magma chamber processes in central volcanic systems of Iceland: constraints from layered gabbro of the Austurhorn intrusive complex

New field work and petrological investigations of the largest gabbro outcrop in Iceland, the Hvalnesfjall gabbro of the 6–7 Ma Austurhorn intrusive complex, have established a stratigraphic sequence exceeding 800 m composed of at least 8 macrorhythmic units. The bases of the macrorhythmic units are composed of 2–10 m thick melanocratic layers rich in clinopyroxene and sometimes olivine, relative to the thicker overlying leucocratic oxide gabbros. While the overall compositional variation is limited (Mg# clinopyroxene 72–84; An% plagioclase 56–85), the melanocratic bases display spikes in Mg# and Cr₂O₃ of clinopyroxene and magnetite indicative of magma replenishment. Some macrorhythmic units show mineral trends indicative of up-section fractional crystallisation over up to 100 m, whereas others show little variation. Two populations of plagioclase crystals (large, An-rich and small, less An-rich) indicate that the recharge magma carried plagioclase xenocrysts (high An-type). The lack of evolved gabbros suggests formation in a dynamic magma chamber with frequent recharge, tapping and fractionation. Modelling of these compositional trends shows that the parent magma was similar to known transitional olivine basalts from Iceland that had undergone about 20% crystallisation of olivine, plagioclase and clinopyroxene and that the macrorhythmic units formed from thin magma layers not exceeding 200–300 m. Such a “mushy” magma chamber is akin to volcanic plumbing systems in settings of high magma supply rate including the mid-ocean ridges and present-day magma chambers over the Iceland mantle plume. The Austurhorn central volcano likely formed in an off-rift flank zone proximal to the Iceland mantle plume during a major rift relocation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The magma plumbing system for the 1971 Teneguía eruption on La Palma,Canary Islands

The 1971 Teneguía eruption is the most recent volcanic event of the Cumbre Vieja rift zone on La Palma. The eruption produced basanite lavas that host xenoliths, which we investigate to provide insight into the processes of differentiation, assimilation and magma storage beneath La Palma. We compare our results to the older volcano magmatic systems of the island with the aim to reconstruct the temporal development of the magma plumbing system beneath La Palma. The 1971 lavas are clinopyroxene-olivine-phyric basanites that contain augite, sodic-augite and aluminium augite. Kaersutite cumulate xenoliths host olivine, clinopyroxene including sodic-diopside, and calcic-amphibole, whereas an analysed leucogabbro xenolith hosts plagioclase, sodic-augite-diopside, calcic-amphibole and hauyne. Mineral thermobarometry and mineral-melt thermobarometry indicate that clinopyroxene and plagioclase in the 1971 Teneguía lavas crystallised at 20–45 km depth, coinciding with clinopyroxene and calcic-amphibole crystallisation in the kaersutite cumulate xenoliths at 25–45 km and clinopyroxene, calcic-amphibole and plagioclase crystallisation in the leucogabbro xenolith at 30–50 km. Combined mineral chemistry and thermobarometry suggest that the magmas had already crystallised, differentiated and formed multiple crystal populations in the oceanic lithospheric mantle. Notably, the magmas that supplied the 1949 and 1971 events appear to have crystallised deeper than the earlier Cumbre Vieja magmas, which suggests progressive underplating beneath the Cumbre Vieja rift zone. In addition, the lavas and xenoliths of the 1971 event crystallised at a common depth, indicating a reused plumbing system and progressive recycling of Ocean Island plutonic complexes during subsequent magmatic activity.     

Multi-stage magma ascent beneath the Canary Islands: evidence from fluid inclusions

Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from Gran Canaria, La Palma, El Hierro, Lanzarote and La Gomera (Canary Islands) contain abundant CO₂-dominated fluid inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to one or more of the following pressures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) between 0.2 and 0.4 GPa (the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal density distribution, the lower-density maximum comprising both texturally early and late inclusions. This is taken as evidence for an incomplete resetting of inclusion densities, and simultaneous formation of young inclusions, at well-defined magma stagnation levels. For Gran Canaria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts in the host basanites overlap at 0.9 to 1.0 GPa, indicating that such magma reservoir depths coincide with levels of xenolith entrainment into the magmas. Magma chamber pressures within the mantle, inferred to represent levels of mantle xenolith entrainment, are 0.65–0.95 GPa for El Hierro, 0.60–0.68 GPa for La Palma, and 0.55–0.75 GPa for Lanzarote. The highest-density fluid inclusions in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment. Inclusion data from all islands indicate ponding of basaltic magmas at Moho or lower crustal depths, and possibly at an additional higher level, strongly suggestive of two main crustal accumulation levels beneath each island. We emphasize that repeated magmatic underplating of primitive magmas, and therefore intrusive accretion, are important growth mechanisms for the Canary Islands, and by analogy, for other ocean islands. Comparable fluid inclusion data from primitive rocks in other tectonic settings, including Iceland, Etna and continental rift systems (Hungary, South Norway), indicate that magma accumulation close to Moho depths shortly before eruption is not, however, restricted to oceanic intraplate volcanoes. Lower crustal ponding and crystallization prior to eruption may be the rule rather than the exception, independent of the tectonic setting. Received: 30 May 1997 / Accepted: 6 February 1998     

Magma storage and ascent during the 1995 eruption of Fogo,Cape Verde Archipelago

The 1995 eruption of Fogo (Cape Verde Islands) differed from previous eruptions by the occurrence of evolved lavas, the SW-orientation of vents, and pre-eruptive seismicity between Fogo and the adjacent (~20 km) island of Brava. We have conducted a thermobarometric and chemical study of this eruption in order to reconstruct its magma plumbing system and to test for possible connections to Brava. The bimodal eruption produced basanites (5.2–6.7 wt% MgO) and phonotephrites (2.4–2.8 wt% MgO) that are related by fractional crystallization. Clinopyroxene-melt-barometry of phenocrysts yields pressure ranges of 460–680 MPa for the basanites and 460–520 MPa for the phonotephrites. Microthermometry of CO₂-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yields systematically lower pressure ranges of 200–310 MPa for basanites and 270–470 MPa for phonotephrites. The combined data indicate pre-eruptive storage of the 1995 magmas within the lithospheric mantle between 16 and 24 km depth. During eruption, the ascending magmas stalled temporarily at 8–11 km depth, within the lower crust, before they ascended to the surface in a few hours as indicated by zonations of olivine phenocrysts. Our data provide no evidence for magma storage at shallow levels (<200 MPa) or lateral magma movements beneath the Fogo-Brava platform. Sr–Nd–Pb isotope ratios of samples from Brava differ significantly from those of the 1995 and older Fogo lavas, which rules out contamination of the 1995 magmas by Brava material and indicates different mantle sources and magma plumbing systems for both islands.     

Environments of Crystallization and Compositional Diversity of Mauna Loa Xenoliths

Two picrite flows from the SW rift zone of Mauna Loa containxenoliths of dunite, harzburgite, lherzolite, plagioclase-bearinglherzolite and harzburgite, troctolite, gabbro, olivine gabbro,and gabbronorite. Textures and olivine compositions precludea mantle source for the xenoliths, and rare earth element concentrationsof xenoliths and clinopyroxene indicate that the xenolith sourceis not old oceanic crust, but rather a Hawaiian, tholeiitic-stagemagma. Pyroxene compositions, phase assemblages and texturalrelationships in xenoliths indicate at least two different crystallizationsequences. Calculations using the pMELTS algorithm show thatthe two sequences result from crystallization of primitive MaunaLoa magmas at 6 kbar and 2 kbar. Independent calculations ofolivine Ni–Fo compositional variability in the plagioclase-bearingxenoliths over these crystallization sequences are consistentwith observed olivine compositional variability. Two parentsof similar bulk composition, but which vary in Ni content, arenecessary to explain the olivine compositional variability inthe dunite and plagioclase-free peridotitic xenoliths. Xenolithsprobably crystallized in a small magma storage area beneaththe rift zone, rather than the large sub-caldera magma reservoir.Primitive, picritic magmas are introduced to isolated rift zonestorage areas during periods of high magma flux. Subsequenteruptions reoccupy these areas, and entrain and transport xenolithsto the surface. KEY WORDS: xenolith; Hawaii; volcano plumbing; mineral composition; picrite     

Magma plumbing beneath Anak Krakatau volcano, Indonesia: evidence for multiple magma storage regions

Understanding magma plumbing is essential for predicting the behaviour of explosive volcanoes. We investigate magma plumbing at the highly active Anak Krakatau volcano (Indonesia), situated on the rim of the 1883 Krakatau caldera by employing a suite of thermobarometric models. These include clinopyroxene-melt thermobarometry, plagioclase-melt thermobarometry, clinopyroxene composition barometry and olivine-melt thermometry. Petrological studies have previously identified shallow magma storage in the region of 2–8 km beneath Krakatau, while existing seismic evidence points towards mid- to deep-crustal storage zone(s), at 9 and 22 km, respectively. Our results show that clinopyroxene in Anak Krakatau lavas crystallized at a depth of 7–12 km, while plagioclase records both shallow crustal (3–7 km) and sub-Moho (23–28 km) levels of crystallization. These magma storage regions coincide with well-constrained major lithological boundaries in the crust, implying that magma ascent and storage at Anak Krakatau is strongly controlled by crustal properties. A tandem seismic tomography survey independently identified a separate upper crustal (<7 km) and a lower to mid-crustal magma storage region (>7 km). Both petrological and seismic methods are sensitive in detecting magma bodies in the crust, but suffer from various limitations. Combined geophysical and petrological surveys, in turn, offer increased potential for a comprehensive characterization of magma plumbing at active volcanic complexes.     

A clinopyroxene–basalt geothermobarometry perspective of Columbia Plateau (NW-USA) Miocene magmatism

The origin of NW-USA Columbia River Basalt Group Miocene magmatism and its relation to tectonism has been widely debated and is still open to study. We investigated the pre-eruptive evolution of the magmas, to constrain pressures and temperatures of the ascending magmas, and plumbing conditions. We determined major element concentrations of 17–6 Ma tholeiites, and applied clinopyroxene – liquid geothermobarometry to calculate pre-eruptive pressures and temperatures. These ranged from 0 to 0.66 GPa and 1120 to 1222 °C, respectively, defining two age-related parallel trends in a P–T diagram. This indicates a consistent crustal evolution of the magmas, and records at least two distinct initial temperatures. Using clinopyroxene interdiffusion coefficients we estimated magma ascent speeds ≥ 0.6 km yr⁻¹. Possible geological explanations for the calculated parameters are: lower-crust magma chamber processes; magmatism and tectonism feed-back consistent with an extensional environment.     

从地壳上地幔构造看大陆岩石圈伸展与裂解

本篇讨论大陆岩石圈拆沉、伸展与裂解作用过程。由于大陆岩石圈厚度大而且很不均匀,产生裂谷的机制比较复杂。大陆碰撞远程效应的触发,岩石圈拆沉,以及板块运动的不规则性和地球应力场方向转折,都可能产生岩石圈断裂和大陆裂谷。岩石圈拆沉为在重力作用下"去陆根"的作用过程,演化过程可分为大陆根拆离、地壳伸展和岩石圈地幔整体破裂三个阶段。大陆碰撞带、俯冲的大陆和大洋板块、克拉通区域岩石圈,都可能产生岩石圈拆沉。大陆岩石圈调查表明,拉张区可见地壳伸展、岩石圈拆离、软流圈上拱和热沉降;它们是大陆岩石圈伸展与裂解早期的主要表现。从初始拉张的盆岭省到成熟的张裂省,拆离后地壳伸展成复式地堑,下地壳幔源玄武岩浆侵位,断裂带贯通并切穿整个岩石圈,表明地壳伸展进入成熟阶段。中国东北松辽盆地和西欧北海盆地曾处于成熟的张裂省。岩石圈破裂为岩浆侵位提供了阻力很小的通道网。岩浆侵位作用伴随岩石圈破裂和热流体上涌,成熟的张裂省可发展成大陆裂谷。多数的大陆裂谷带并没有发展成威尔逊裂谷带和洋中脊,普通的大陆裂谷要演化为威尔逊裂谷带,必须有来自软流圈的长期和持续的热流和玄武质岩浆的供应。威尔逊裂谷带岩石圈地幔和软流圈为地震低速带,其根源可能与来自地幔底部的地幔热羽流有关。     

Chemical transfer between mantle xenoliths and basic magmas: Evidence from oceanic magma chambers. The trinity ophiolite (northern California)

The Trinity ophiolite consists of small magma chambers inside a large mantle body. Xenoliths of mantle peridotite occur both in gabbroic cumulates along the walls and in the matrices of ultrabasic breccias on the floors of the magma chambers. Field relationships and petrographic data suggest that these fragments of original mantle peridotite were modified by contact with basic magmas by modal metasomatism. Quantitative elemental mass transfers determined from the composition, volume and density variations of reacting minerals demonstrate both closed and open system conditions for the major (Si, Al, Ti, Na, Ca, Fe and Mg) and trace elements (Cr, Ni). In the open system, material gains and losses provide information on the composition of the fluid taking part in the metasomatic reaction.

During a first stage of metasomatism the mantle xenoliths were affected by high-temperature reactions at 600 to 925°C. They resulted from the interaction between solid mantle lherzolites and basic melts. The reactions are:

1. (1)those forming orthopyroxene-magnetite simplectite

2. (2)those forming plagioclase-magnetite corona

3. (3)clinopyroxene+spinel I→pargasitic hornblende+spinel II.

Chemical interactions between the upper mantle and oceanic magma chambers occurred as soon as the basic magmas had ascended through the upper mantle. The chemically modified magmas, within oceanic magma chambers, were depleted in Ti, Fe and Na. This could partly explain regional variations of the chemical compositions of primary magmas produced beneath slow-spreading ridges. The breakdown of olivine to orthopyroxene and magnetite participates in the control of the partition of magnetic Fe---Ti oxides between oceanic crust and mantle.

During the second stage, the serpentinization of olivine and the production of talc were superimposed on the products of the first stage. These reactions require large amounts of H₂O. The hydrothermal fluid was probably seawater. It circulated in the brecciated area along the walls and floors of the magma chambers located at shallow depths. Such structural discontinuities thus played the role of penetration channels favoring seawater circulation in the oceanic crust.

All the chemical reactions examined suggest a significant open-system element transfer by infiltrating melts or circulating fluids. The results of this study suggest that caution is required in the interpretation of mineralogical and chemical information provided by mantle xenoliths carried to the surface by ascending magmas.     

Geochemical Study of Ultramafic Volcanic and Plutonic Rocks from Gorgona Island, Colombia: the Plumbing System of an Oceanic Plateau

The only known post-Archaean komatiites are found on Gorgona,a small island off the Colombian coast that forms part of theCaribbean oceanic plateau. Mafic and ultramafic intrusions arelocated in the interior of the island. To establish the relationshipbetween intrusive and extrusive phases of ultramafic magmatism,and to help understand how an oceanic plateau is constructed,we undertook the first petrological and geochemical study ofthe intrusive rocks. Rare earth element patterns in gabbrosrange from almost flat to moderately depleted; in dunites andwehrlites, the depletion is more pronounced. These patternsfall midway in the range measured in Gorgona volcanics, whosecompositions vary from slightly enriched to extremely depleted.Nd isotope compositions indicate two distinct mantle sources,one highly depleted, the other less depleted. MgO contents ofparental liquids are estimated from olivine compositions at20–25 % in ultramafic lavas, and 12–13% in the intrusives.Petrographic observations and similarities in trace-elementcontents indicate that the two magma types are comagmatic, relatedthrough olivine fractionation. Modelling of major and traceelements indicates that the primary ultramafic magmas formedby advanced critical melting at high pressure in a rising mantleplume. The plumbing system that fed the Gorgona plateau wascomplex, being characterized by a series of magma chambers atdifferent crustal levels. Mantle-derived ultramafic liquidseither travelled directly to the surface to erupt as komatiiteflows, or were trapped in magma chambers where they differentiatedinto basaltic liquid and mafic to ultramafic cumulates. Gorgonagabbros and peridotites formed in shallow-level examples ofthese intrusions. KEY WORDS: Gorgona Island, Colombia; komatiite; mantle melting; oceanic plateau; melt transport     

Evolution of the East Philippine Arc: experimental constraints on magmatic phase relations and adakitic melt formation

Piston-cylinder experiments on a Pleistocene adakite from Mindanao in the Philippines have been used to establish near-liquidus and sub-liquidus phase relationships relevant to conditions in the East Philippines subduction zone. The experimental starting material belongs to a consanguineous suite of adakitic andesites. Experiments were conducted at pressures from 0.5 to 2 GPa and temperatures from 950 to 1,150°C. With 5 wt. % of dissolved H₂O in the starting mix, garnet, clinopyroxene and orthopyroxene are liquidus phases at pressures above 1.5 GPa, whereas clinopyroxene and orthopyroxene are liquidus (or near-liquidus) phases at pressures <1.5 GPa. Although amphibole is not a liquidus phase under any of the conditions examined, it is stable under sub-liquidus conditions at temperature ≤1,050°C and pressures up to 1.5 GPa. When combined with petrographic observations and bulk rock chemical data for the Mindanao adakites, these findings are consistent with polybaric fractionation that initially involved garnet (at pressures >1.5 GPa) and subsequently involved the lower pressure fractionation of amphibole, plagioclase and subordinate clinopyroxene. Thus, the distinctive Y and HREE depletions of the andesitic adakites (which distinguish them from associated non-adakitic andesites) must be established relatively early in the fractionation process. Our experiments show that this early fractionation must have occurred at pressures >1.5 GPa and, thus, deeper than the Mindanao Moho. Published thermal models of the Philippine Sea Plate preclude a direct origin by melting of the subducting ocean crust. Thus, our results favour a model whereby basaltic arc melt underwent high-pressure crystal fractionation while stalled beneath immature arc lithosphere. This produced residual magma of adakitic character which underwent further fractionation at relatively low (i.e. crustal) pressures before being erupted.     

Geochemical and Sr–O isotopic constraints on magmatic differentiation at Gede Volcanic Complex, West Java, Indonesia

The Gede Volcanic Complex (GVC) of the Sunda island arc (West Java, Indonesia) consists of multiple volcanic centres and eruptive groups with complex magmatic histories. We present new petrological, mineralogical, whole-rock major and trace element and Sr–O isotopic data to provide constraints on the relative importance of fractional crystallisation and magma mixing in petrogenesis, as well as on the role and nature of the arc crust. Banded juvenile scoria from Young and Old Gede provide unequivocal evidence for the (late-stage) interaction of distinct magmas at Gede volcano. However, the relatively small-degree compositional zoning observed in plagioclase phenocrysts of all eruptive groups (up to ~20 mol% An) may be attributed to physical changes in magma properties (e.g. P, T, and PH₂O) rather than changes in melt composition. Major element and trace element variations within each eruptive series are inconsistent with magmatic evolution through simple mixing processes. Instead, mixing of variably fractionated magma batches is suggested to account for the significant scatter in some element variation diagrams. No correlation is observed between textural complexity and/or mineral disequilibrium and whole-rock geochemistry. REE data and geochemical modelling indicate that fractional crystallisation involving amphibole in the mid- to lower crust, and fractionation of plagioclase, clinopyroxene, Fe–Ti oxide ± olivine ± orthopyroxene provide strong control on the geochemical evolution of GVC rocks. Two-pyroxene geothermobarometry provides pre-eruption crystallisation temperatures of 891–1,046°C and pressures of 3.4–6.5 kbar, equivalent to ~13–24 km depth beneath the volcanoes (mid- to lower crust). Low, mantle-like clinopyroxene δ¹⁸O values of GVC lavas and poor correlation of Sr isotope ratios with indices of differentiation precludes significant assimilation of isotopically distinct crust during magmatic differentiation. Therefore, we suggest that the geochemical character of the moderately thick West Javan arc crust is relatively immature compared to typical continental crust. Trace element ratios and strontium isotopes show that the magmatic source composition of the older geographical units, Gegerbentang and Older Quaternary, is distinct from the other GVC groups.     

地壳内的岩浆动力学过程及其资源与环境效应SCIEI北大核心CSCD

岩浆是将地球内部物质传送到表层系统的主要载体,并造成显著的资源聚集和环境效应。岩浆动力学是研究岩浆的迁移、储存、演化、就位以及喷发过程,侧重物理机制。这些岩浆过程主要发生在岩浆通道系统中,包括岩浆储库和岩浆管道。本文对目前国际岩浆动力学领域一些热点和前沿进行了介绍,这包括从岩浆房到岩浆储库概念的转变、岩浆储库的生长和动力学演化过程、岩浆过程的时间尺度以及岩浆中晶体的生长。然后阐述了岩浆中挥发分的种类和溶解度、获取天然岩浆挥发分含量的方法、一些典型镁铁质岩浆中的挥发分含量、岩浆去气的化学和物理机制,并简要梳理了热液金属矿床的形成过程和岩浆挥发分进入地表圈层系统引发的环境气候效应。最后列举了一些岩浆动力学有关的重要科学问题并建议了进一步的研究方向。     

Tectonic implications of garnet-bearing mantle xenoliths exhumed by Quaternary magmatism in the Hangay dome, central Mongolia

Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed in the spinel-lherzolite upper mantle at pressures of 17–18 kbars and temperatures of 1,070–1,090°C, whereas garnet lherzolites were derived from greater depths (18–20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed at 18–22 kbars under significantly cooler conditions (960–1,000°C). These xenoliths reveal reaction coronas of (1) orthopyroxene, clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na, low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from both sites were derived from depths of 60–70 km and entrained in magma at 1,200–1,300°C. The average rate of ascent, as determined by olivine zoning, lies in the range 0.2–0.3 m s⁻¹. The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene geotherm beneath the Baikal rift.     

Mechanisms driving polymagmatic activity at a monogenetic volcano, Udo, Jeju Island, South Korea

High-resolution, stratigraphically ordered samples of the Udo tuff cone and lava shield offshore of Jeju Island, South Korea, show complex geochemical variation in the basaltic magmas that fed the eruption sequence. The eruption began explosively, producing phreatomagmatic deposits with relatively evolved alkali magma. The magma became more primitive over the course of the eruption, but the last magma to be explosively erupted had shifted back to a relatively evolved composition. A separate sub-alkali magma batch was subsequently effusively erupted to form a lava shield. Absence of weathering and only minor reworking between the tuff and overlying lava implies that there was no significant time break between the eruptions of the two magma batches. Modelling of the alkali magma suggests that it was generated from a parent melt in garnet peridotite at c. 3 to 3.5 GPa and underwent mainly clinopyroxene + olivine ± spinel fractionation at c. 1.5 to 2 GPa. The sub-alkali magma was, by contrast, generated from a chemically different peridotite with residual garnet at c. 2.5 GPa and evolved through olivine fractionation at a shallower level compared to its alkali contemporary. The continuous chemostratigraphic trend in the tuff cone, from relatively evolved to primitive and back to evolved, is interpreted to have resulted from a magma batch having risen through a single dyke and erupted the batch’s head, core and margins, respectively. The alkali magma acted as a path-opener for the sub-alkali magma. The occurrence of the two distinct batches suggests that different magmatic systems in the Jeju Island Volcanic Field have interacted throughout its history. The polymagmatic nature of this monogenetic eruption has important implications for hazard forecasting and for our understanding of basaltic field volcanism.     

Geochemistry and petrogenesis of a basalt-benmoreite-trachyte suite from the southern part of the Gregory Rift,Kenya

In the southern Gregory Rift valley a series of transitional basalt, ferrobasalt, and benmoreite flows (1.65–1.4 Myr) is overlain by flood trachyte lavas (1.3–0.9 Myr). Mass balance calculations for major element compositions of rocks of this suite and their phenocrysts and microphenocrysts suggest that the ferrobasalts and benmoreites formed from magma resembling the most primitive basalt by closed system fractionation of plagioclase, clinopyroxene, olivine, titanomagnetite, and apatite. The trachytes formed from evolved magmas largely by alkali feldspar fractionation. Estimates of phenocryst and liquid densities and Rayleigh-law modelling of trace element contents support these conclusions. From Rayleigh-law modelling, we derived a set of effective distribution coefficients. Partial melting of crustal rocks or volatile transfer processes had no significant effect on the petrogenesis of this suite. The duration of the eruptive cycle, cooling time calculations, and mass balance calculations suggest that fractionation occurred in a magma reservoir with volume of at least 3 × 10⁴ km³ during an interval of about 0.8 Myr. Temperatures during fractionation probably ranged from about 1200 °C to 900 °C, and pressures may have been roughly 5 to 8 Kb. We suggest that rift development was accompanied by large-scale injection of basaltic magma and dilation of the crust, extensive fractionation, preferential eruption of low-density and fluid trachytic flood lavas, and by several episodes of normal faulting.     

Plagioclase buoyancy in oceanic basalts: Chemical effects

Certain petrological features of oceanic volcanic and plutonic rocks are not completely consistent with previously proposed models of crystal fractionation or magma mixing. For example, Sr is often higher in the differentiated basalts of a suite of aphyric rocks than in the relatively primitive basalts even though the differentiated basalts have apparently been produced by crystallization of large amounts of plagioclase with olivine and clinopyroxene. Additionally, oceanic basalts and gabbroic rocks often contain plagioclase crystals in excess of the appropriate cotectic proportions. Certain differentiated oceanic basaltic glasses and aphyric rocks crystallize plagioclase as the liquidus mineral, which would seem inconsistent with the strongly cotectic nature of the olivine + plagioclase + liquid surface.It is proposed here that plagioclase in mid-ocean ridge magma chambers separates from the basaltic liquid that it crystallizes in at a slower rate than does co-crystallizing olivine or pyroxene. Magma mixing in which a portion of the plagioclase remains suspended in the liquid during crystallization results in much more complex liquid lines of descent in mixed magmas and appears to resolve the apparent discrepancies noted above.     

Orthopyroxene minettes and lamproites: their status and genetic significance

Orthopyroxene is a normal if rare, constituent of some minettes and lamproites. It is sometimes partly xenocrystic, but mostly authigenic, i.e. separated directly from lamprophyric magmas, as can be shown when orthopyroxene lamprophyres are compared with the experimental phase diagrams of appropriate systems. The orthopyroxene-bearing lamprophyres represent modified mantle-derived magmas characterized by a high MgO, high SiO₂ speciation which was acquired in different ways in individual regions. The crystallization of orthopyroxene lamprophyres was not controlled by particular pressure — temperature conditions. Compared with clinopyroxene potassic lamprophyres the orthopyroxene lamprophyres represent more primitive magmas. Orthopyroxene minettes can also be altered by a post-magmatic hydrothermal inflow, which occasionally increases their CaO content to the level normal in clinopyroxene minettes. If this had happened before magma consolidation, clinopyroxene instead of orthopyroxene would have crystallized. Orthopyroxene minettes are well defined mineralogically and chemically and may be related to other types in the minette series.     

Mineral chemical constraints on the petrogenesis of mafic and intermediate volcanic rocks from the Erciyes and Hasandağ volcanoes,Central Turkey

Hasandağ and Erciyes stratovolcanoes, which produced both calc-alkaline and alkaline eruptive products, are the two important volcanic complexes in Central Anatolia. There are three geochemical evolution stages in the history of the Hasandağ strato volcanic complex: (1) Keçikalesi tholeiitic, (2) Hasandağ calc-alkaline and (3) Hasandağ alkaline. Volcanologic and petrologic characteristics of the Hasandağ and Erciyes calc-alkaline series show that water played an important role on the genesis of these rocks. These rocks are phenocryst-rich with vesicular texture, and contain hydrous mineral phases. The approximate pressure and temperature estimates obtained from the mineral chemistry studies of the Hasandağ strato volcanic complex indicate crystallization temperature of 1100 °C with 2.5–3.4 kbar pressure interval for the first stage of Keçikalesi tholeiitic volcanism, and about 850 °C temperatures with 4.3–9.6 kbar pressure intervals for the second stage of Hasandağ calc-alkaline volcanism.The geochemical evolution of Erciyes volcanic complex also exhibits three distinct evolutionary stages: (1) Koçdağ alkaline, (2) Koçdağ calc-alkaline and (3) Erciyes calc-alkaline. The temperature of Koçdağ alkaline volcanism is 1097–1181 °C and in a range of 5.1–6.7 kbar pressure, for Koçdağ calc-alkaline volcanism 850–1050 °C temperature to 2.0–6.6 kbar pressure interval, and for Erciyes calc-alkaline volcanism about 950 °C temperature, to 3.2–7.9 kbar pressure intervals were calculated. Polybaric origin of magma chambers for calc-alkaline and alkaline rocks and disequilibrium parameters observed in phenocrysts indicate that the rocks were affected by magma mixing processes in crustal magma chambers. The disequilibrium features of amphibole and plagioclase phenocrysts in these rocks point the latent heat in magma chambers and periodic recharging with mafic magma chambers and also show that magmas reequilibrate before the eruption.