Experimental investigations of the role of H2O in calc-alkaline differentiation and subduction zone magmatism

Phase relations of natural aphyric high-alumina basalts and their intrusive equivalents were determined through rock-melting experiments at 2 kb, H₂O-saturated with fO₂ buffered at NNO. Experimental liquids are low-MgO high-alumina basalt or basaltic andesite, and most are saturated with olivine, calcic plagioclase, and either high-calcium pyroxene or hornblende (±magnetite). Cr-spinel or magnetite appear near the liquidus of wet high-alumina basalts because H₂O lowers the appearance temperature of crystalline silicates but has a lesser effect on spinel. As a consequence, experimental liquids follow calcalkaline differentiation trends. Hornblende stability is sensitive to the Na₂O content of the bulk composition as well as to H₂O content, with the result that hornblende can form as a near liquidus mineral in wet sodic basalts, but does not appear until liquids reach andesitic compositions in moderate Na₂O basalts. Therefore, the absence of hornblende in basalts with low-to-moderate Na₂O contents is not evidence that those basalts are nearly dry. Very calcic plagioclase (>An₉₀) forms from basaltic melts with high H₂O contents but cannot form from dry melts with normal are Na₂O and CaO abundances. The presence of anorthite-rich plagioclase in high-alumina basalts indicates high magmatic H₂O contents. In sum, moderate pressure H₂O-saturated phase relations show that magmatic H₂O leads to the early crystallization of spinel, produces calcic plagioclase, and reduces the total proportion of plagioclase in the crystallizing assemblage, thereby promoting the development of the calc-alkaline differentiation trend.     

An experimental study of the effects of melt composition on plagioclase-melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H₂O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H₂O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H₂O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H₂O content can notably increase the An content of plagioclase, up to 10 mol % for H₂O-undersaturated melts, and 20 mol % for H₂O-saturated melts. Exceptionally calcic plagioclases (up to An₁₀₀) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H₂O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na₂O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in equilibrium with the most calcic plagioclase (An₉₄) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na₂O = 12–15, or from melts with exceptionally high Al₂O₃(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H₂O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An₉₅ in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na₂O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks. Received: 31 August 1993 / Accepted: 20 May 1994     

Pillow lavas of the Sierra Leone test site,Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics,geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the ⁸⁷Sr/⁸⁶Sr ratio between the basalts and their chilled glasses and less significant difference in ?_Nd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.     

Petrological and experimental evidence for differentiation of water-rich magmas beneath St. Kitts,Lesser Antilles

St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure–temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An_≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An₉₇ and An₉₅, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH₂O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950–1025 °C, and fO₂ = NNO ? 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt Kd_Ca–Na and dissolved H₂O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H₂O) basaltic andesites that crystallised over a wide pressure range (1.5–6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.     

Contrasting compositional trends of rocks and olivine-hosted melt inclusions from Cerro Negro volcano (Central America): implications for decompression-driven fractionation of hydrous magmas

Melt inclusions in olivine Fo_83–72 from tephras of 1867, 1971 and 1992 eruptions of Cerro Negro volcano represent a series of basaltic to andesitic melts of narrow range of MgO (5.6–8 wt %) formed by ~46 wt % fractional crystallization of olivine (~6 wt %), plagioclase (~27 wt %), pyroxene (~13 wt %) and magnetite (<1 wt %) from primitive basaltic melt (average SiO₂ = 49 wt %, MgO = 7.6 wt %, H₂O = 6 wt %) as it ascended to the surface from the depth of about 14 km. The crystallization occurred at increasing liquidus temperature from 1,050 to 1,090 °C in the pressure range from 400 to 50 MPa and was induced by release of mixed H₂O–CO₂ fluid from the melt at decreasing pressure. Matrix glass compositions fall at the high-Si end of the melt inclusion trend and represent the final stage of melt crystallization during and after eruption. The bulk compositions of erupted Cerro Negro magmas (tephras and lavas) range from high- to low-MgO (3–10 wt %) basalts, which form a compositional array crossing the trend of melt inclusions so that virtually no rock from Cerro Negro has composition akin to true melt represented by the inclusions. The variations of the bulk magma (rocks) and melt (melt inclusions) compositions can be generated in a dyke connecting a deep primitive magma reservoir with the Cerro Negro edifice. While the melt inclusions represent the compositional trend of instantaneous melts along the magma pathway at decreasing pressure and H₂O content, occurrence of low-Mg to high-Mg basalts reflects the process of phenocryst re-distribution in progressively evolving melt. The crystallization scenario is anticipated to operate everywhere in dykes feeding basaltic volcanoes and can explain the predominance of plagioclase-rich high-Al basalts in island arc as well as typical compositional variations of magmas during single eruptions.     

Plagioclase zoning as an indicator of magma processes at Bezymianny Volcano,Kamchatka

Back-scattered electron (BSE)-derived zoning patterns of plagioclase phenocrysts are used to identify magma processes at Bezymianny Volcano, Kamchatka, based on the 2000–2007 sequence of eruptive products. The erupted magmas are two-pyroxene andesites, which last equilibrated at ~915°C temperature, 77–87 MPa pressure, and a water content of ~1.4 wt%. Textural and compositional zoning of individual plagioclase phenocrysts typically includes a repeated core-to-rim sequence of oscillatory zoning (An_50–60) truncated by a dissolution surface followed by an abrupt increase in An content (up to An₈₅), which then gradually decreases rimward. This zoning pattern is interpreted to be the result of frequent replenishments of the magma chamber which cause both thermal and chemical interaction between resident and recharge magmas. The outermost 70- to 150-μm-wide zoning patterns of plagioclase phenocrysts are composed of dissolution surface with a subsequent increase in An and Fe contents. Zoning patterns of the rims exhibit correlation among plagioclase phenocrysts within one eruption. Rims are interpreted as a result of crystallization of a batch of magma in the conduit after recharge event.     

Quaternary Tholeiitic to Alkaline Volcanism in the Karasu Valley,Dead Sea Rift Zone,Southeast Turkey: Sr-Nd-Pb-O Isotopic and Trace-Element Approaches to Crust-Mantle Interaction

Rare-earth-element, radiogenic and oxygen isotope, and mineral chemical data are presented for tholeiitic and alkaline Quaternary volcanism from Karasu Valley (Hatay, southeastern Turkey). Karasu Valley is the northern segment of the Dead Sea transform fault and is filled with flood-basalt type volcanics of Quaternary age. This valley is an active fault zone that is known as “Karasu fault,” extending in a NE-SW direction. The Karasu Valley basaltic volcanics (KVBV) are subaphyric to porphyritic, with variable amounts of olivine, clinopyroxene, and plagioclase phenocrysts. Alkali basalts are generally characterized by high contents of olivine, clinopyroxene, and plagioclase phenocrysts. Their groundmass contains olivine, clinopyroxene, plagioclase, and Fe-Ti oxides. Tholeiitic basalts are subaphyric to porphyritic (high contents of olivine, clinopyroxene, and plagioclase). Their groundmass is similar to that of alkali basalts. The range of olivine phenocryst and microlite compositions for all analyzed samples is Fo₈₁ to Fo₄₃. Plagioclase compositions in both tholeiitic and alkali basalts range from andesine, An₃₈ to bytownite, An₇₂. Clinopyroxene compositions range from diopside to calcic augite. Most of the olivine, plagioclase, and clinopyroxene phenocrysts are normally zoned and/or unzoned. Fe-Ti oxides in both series are titanomagnetite and ilmenite.

Based on normative and geochemical data, the Karasu Valley basaltic volcanics are mostly olivine and quartz-tholeiites, and relatively lesser amount of alkali olivine-basalts. KVBV have low K₂O/Na₂O ratios, typically between 0.25 and 0.45. Olivine- and quartz-tholeiites are older than alkali olivine-basalts. Olivine tholeiites have Zr/Nb and Y/Nb ratios similar to alkaline rocks, but their Ba/Nb, Ba/La, and La/Nb ratios are slightly higher than alkali olivine-basalts. In contrast, quartz-tholeiites have the highest Ba/Nb, Ba/La, Zr/Nb, and Y/Nb and the lowest Nb/La ratios among the KVBV. Alkali basalts have ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios ranging from 0.703353 to 0.704410 and 0.512860 to 0.512910, respectively. In contrast, quartz-tholeiites have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd ratios, which vary from 0.704410 to 0.705490 and 0.512628 to 0.512640, respectively. Olivine tholeiites have intermediate isotopic compositions ranging from 0.703490 to 0.704780 and 0.512699 to 0.512780, respectively. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb isotopic ratios of KVBV range from 18.817 to 19.325, 15.640 to 15.718, and 39.054 to 39.223, respectively. The range of O isotope values is between +5.84 and +7.97‰. The higher O and Sr isotopes in olivine- and quartz-tholeiites relative to alkali olivine-basalts can be explained by contamination of magmas by crustal materials.

The KVBV have intraplate chemistry similar to that of other tholeiitic and alkaline basalts in other within-plate environments, and isotopes range from isotopically depleted mantle to enriched isotope compositions similar to some enriched ocean islands. Trace-element and isotope data indicate that the KVBV are derived from a common OIB-like asthenospheric mantle source, but they have experienced different degrees of crustal contamination during their ascent to the surface, contemporaneous with little fractional crystallization. Although quartz-tholeiites display significant effects of crustal contamination, alkali olivine-basalts appear to have negligible or no crustal contamination in their geesis.     

新疆百口泉闪长岩中高An值斜长石的成因及岩石学意义

岩浆成因的高An值斜长石(An80~100)是玄武质岩浆早期结晶的产物,由于其形成条件较为苛刻,它对于限定寄主岩浆条件和探讨岩石成因有重要的指示作用。在新疆西准噶尔百口泉地区发育含高An值斜长石(An80~90)的闪长岩,高An值斜长石呈不规则包裹体形式存在于闪长岩主体斜长石中(An40~60)。锆石SHRIMP定年显示百口泉闪长岩形成于316.9±2.9Ma(MSWD=1.5),闪长岩主、微量元素和Sr-Nd同位素与区域内同时期岩浆作用产物具有相似的演化趋势。根据闪长岩岩石学特征,结合高An值斜长石的实验研究成果,本文认为百口泉闪长岩中的高An值斜长石不是捕虏晶,也不是闪长岩的结晶产物,而是与闪长岩成岩过程相关的循环晶。依据角闪石铝压力计的估算,闪长岩结晶于1.2~2.9kbar,与实验条件下获得的高An值斜长石产出的压力范围相符。高An值斜长石的产出表明原始岩浆具有富水、亏损的特征,对于探讨整个岩浆系统的演化过程有重要指示意义。     

The genesis of mid-ocean ridge basalt

The tholeiitic volcanics erupted at mid-ocean ridges (mid-ocean ridge basalts or MORB) constitute the dominant volcanic lithology on Earth. Analyses of tachylites from Atlantic, Pacific and Indian Ocean spreading centres range widely in 100 Mg/(Mg + Fe²⁺) ratios (= M) and M varies from 70 to 30. Glasses with M = 55?65 are the most common variants and only a small percentage of glass analyses has M approaching 70. The latter defines the M -value of basaltic melts in equilibrium with residual upper-mantle source peridotites with M ～ 88. The frequency histogram of the M -values of average compositions of MORB glasses at 88 ocean floor localities is similar in analysis distribution to the frequency histograms depicting variation in the M -values of glasses from the various spreading centres.M -values and nickel contents of MORB and the nature and compositions of the near-liquidus phases crystallized experimentally from MORB melts at elevated pressures have been applied to identify primary (unfractionated) melts erupted in a mid-ocean ridge environment. However, Ni abundances and high-pressure phase relationships are not necessarily unique or definitive parameters of primary melts. The latter are generally linked genetically with Mg-rich lherzolitic source rocks of ‘pyrolite’ type (M ～ 90. The spectrum of M -values displayed by MORB glasses, with a definite bias towards relatively Fe-rich compositions (average M of approximately 600 MORB glasses is 58.6), suggests that the melts may have evolved either via ferromagnesian fractionation of relatively Mg-rich parental melts (M = 70?80), or by partial melting of a heterogeneous upper mantle with variable M values, or as a result of magma mixing of already fractionated melts and primitive magma batches.For a number of reasons fractonation models based on the extraction of olivine or one or more of olivine, plagioclase and clinopyroxene, either from picritic melts (M > 75 or ‘primitive’ basaltic melts with M ～ 70, are questionable as prime controls of MORB chemistry. These include: (1) the extreme rarity of ‘quenched’ picritic or Mg-basaltic melts in ocean ridge environments; (2) the lack of adequate evidence of the appropriate (of necessity voluminous) complementary cumulates (dunites, allivalites, troctolites, anorthosites) demanded by olivine, plagioclase, or olivine + plagioclase fractionation models; and (3) the aberrent frequencies of glass M -values whereby the assumed derivatives (M = 55?65 are much more abundant (and presumably much more voluminous) than the alleged parents or transitional derivatives (65 < M < 75). The nature of the trends of Na₂O, CaO and Al₂O₃ in Galapagos Spreading Centre tachylites of extended composition (M = 65?30) indicates the ‘gabbroic’ fractionation is also unlikely to exert important controls on MORB chemistry.As their M -values increase, mid-ocean ridge basalts increase in Al, Ca, Ni, Co, Cr and decrease in Ti, Mn, Na, K and P. Except for Al and Ca, these trends are similar to those displayed by upper-mantle peridotites increasing in M, i.e., becoming more refractory following one or more partial melting episodes. It is suggested that at least a majority of mid-ocean ridge basalts is intrinsically primary and generated by variable degrees of partial melting of heterogeneous lherzolitic upper mantle (80 < M < 90) with variable abundances of elements such as Ti, Al, Ca and Na and also depleted in large ion lithophile (LIL) elements. Negative europium anomalies in the rare-earth patterns of some oceanridge basalts (ferrobasalts with low M) are ascribed mainly to the persistence of residual plagioclase in relatively Fe-rich plagioclase lherzolite source rocks, following low degrees of partial melting. The partial melting events leading to the generation of mid-ocean ridge basalts took place over a relatively modes pressure range (approximately 8–15 kb) which encompassed the transition of plagioclase lherzolite to spinel lherzolite. This proposal appears consistent with the nature and occurrence of megacrysts (xenocrysts) of tschermakitic Cr-diopside (Ca₄₃Mg₅₂Fe₅), olivine (mg 89–91), plagioclase (An_92-85) and spinel (Fe₂Al₆₀Cr₃₈) in some MORB. The megacryst compositions suggest that these phases represent disaggregated plagioclase peridotite or spinel lherzolite acquired by melts during their passage through the oceanic upper mantle.     

Geochemistry,mineralogy, and zircon U–Pb–Hf isotopes in peraluminous A-type granite xenoliths in Pliocene–Pleistocene basalts of northern Pannonian Basin (Slovakia)

Anorogenic granite xenoliths occur in alkali basalts coeval with the Pliocene–Pleistocene continental rifting of the Pannonian Basin. Observed granite varieties include peraluminous, calcic to peralkalic, magnesian to ferroan types. Quartz and feldspars are dominant rock-forming minerals, accompanied by minor early ilmenite and late magnetite–ulvöspinel. Zircon and Nb–U–REE minerals (oxycalciopyrochlore, fergusonite, columbite) are locally abundant accessory phases in calc-alkalic types. Absence of OH-bearing Fe, Mg-silicates and presence of single homogeneous feldspars (plagioclase in calcic types, anorthoclase in calc-alkalic types, ferrian Na-sanidine to anorthoclase in alkalic types) indicate water-deficient, hypersolvus crystallization conditions. Variable volumes of interstitial glass, absence of exsolutions, and lacking deuteric hydrothermal alteration and/or metamorphic/metasomatic overprint are diagnostic of rapid quenching from hypersolidus temperatures. U–Pb zircon ages determined in calcic and calc-alkalic granite xenoliths correspond to a time interval between 5.7 and 5.2 Ma. Positive εHf values (14.2 ± 3.9) in zircons from a 5.2-Ma-old calc-alkalic granite xenolith indicate mantle-derived magmas largely unaffected by the assimilation of crustal material. This is in accordance with abundances of diagnostic trace elements (Rb, Y, Nb, Ta), indicating A₁-type, OIB-like source magmas. Increased accumulations of Nb–U–REE minerals in these granites indicate higher degree of the magmatic differentiation reflected in Rb-enrichment, contrasting with Ba-enrichment in barren xenoliths. Incipient charnockitization, i.e. orthopyroxene and ilmenite crystallization from interstitial silicate melt, was observed in many granite xenoliths. Thermodynamic modeling using pseudosections showed that the orthopyroxene growth may have been triggered by water exsolution from the melt during ascent of xenoliths in basaltic magma. Euhedral-to-skeletal orthopyroxene growth probably reflects contrasting ascent rates of basaltic magma with xenoliths, intermitted by the stagnation in various crustal levels at a <3 kbar pressure. The Tertiary suite of intra-plate, mantle-derived A₁-type granites and syenites is geochemically distinct from pre-Tertiary, post-orogenic A₂-type granites of the Carpatho–Pannonian region, which exhibit geochemical features diagnostic of crustal melting along continental margins.     

Experimental study into the petrogenesis of crystal-rich basaltic to andesitic magmas at Arenal volcano

Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) continuous, small-scale activity erupting compositionally monotonous basaltic andesites that also dominate the entire, ~7000 year long, eruptive history. Only mineral zoning records reveal that basaltic andesites are the result of complex, open-system processes deriving minerals from a variety of crystallization environments and including the episodic injections of basalt. The condition of the mafic input as well as the generation of crystal-rich basaltic andesites of the recent, 1968–2010, and earlier eruptions were addressed by an experimental study at 200 MPa, 900–1,050 °C, oxidizing and fluid-saturated conditions with various fluid compositions [H₂O/(H₂O + CO₂) = 0.3–1]. Phase equilibria were determined using a phenocryst-poor (~3 vol%) Arenal-like basalt (50.5?wt% SiO₂) from a nearby scoria cone containing olivine (Fo₉₂), plagioclase (An₈₆), clinopyroxene (Mg# = 82) and magnetite (X_ulvö = 0.13). Experimental melts generally reproduce observed compositional trends among Arenal samples. Small differences between experimental melts and natural rocks can be explained by open-system processes. At low pressure (200 MPa), the mineral assemblage as well as the mineral compositions of the natural basalt were reproduced at 1,000 °C and high water activity. The residual melt at these conditions is basaltic andesitic (55 wt% SiO₂) with 5 wt% H₂O. The evolution to more evolved magmas observed at Arenal occurred under fluid-saturated conditions but variable fluid compositions. At 1,000 °C and 200 MPa, a decrease of water content by approximately 1 wt% induces significant changes of the mineral assemblage from olivine + clinopyroxene + plagioclase (5 wt% H₂O in the melt) to clinopyroxene + plagioclase + orthopyroxene (4 wt% H₂O in the melt). Both assemblages are observed in crystal-rich basalt (15 vol%) and basaltic andesites. Experimental data indicate that the lack of orthopyroxene and the presence of amphibole, also observed in basaltic andesitic tephra units, is due to crystallization at nearly water-saturated conditions and temperatures lower than 950 °C. The enigmatic two compositional groups previously known as low- and high-Al₂O₃ samples at Arenal volcano may be explained by low- and high-pressure crystallization, respectively. Using high-Al as signal of deeper crystallization, first magmas of the 1968–2010 eruption evolved deep in the crust and ascent was relatively fast leaving little time for significant compositional overprint by shallower level crystallization.     

A petrological study of the younger Tongan andesites and dacites,and the olivine tholeiites of Niua Fo'ou Island,S. W. Pacific

Basaltic andesites are the dominant Tongan magma type, and are characterized by phenocrysts of augite, orthopyroxene (or rarely pigeonite), and calcic plagioclase (modally most abundant phase, and interpreted as the liquidus phase). The plagioclase phenocrysts exhibit slight oscillatory reverse zoning except for abrupt and thin more sodic rims, which are interpreted to develop during eruptive quenching. These rim compositions overlap those of the groundmass plagioclase. The pyroxene phenocrysts also exhibit only slight compositional zoning except for the outermost rim zones; the compositions of these rims, together with the groundmass pyroxenes, vary throughout the compositional range of subcalcic augite to ferroaugite through pigeonite to ferropigeonite, and are interpreted in terms of quench-controlled crystallization. This is supported, for example, by the random distribution of Al solid solution in the groundmass pyroxenes, compared to the more regular behaviour of Al in the phenocryst pyroxenes. The analysed Niua Fo'ou olivine tholeiites are aphyric; groundmass phases are plagioclase (An_17–88), olivine (Fa_18–63), titanomagnetite (usp. _59–73), and augite-ferroaugite which does not extend to subcalcic compositions; this is interpreted to be due to higher quenching temperatures and lower viscosities of these tholeiites compared to the basaltic andesites.Application of various geothermometers to the basaltic andesites suggest initial eruptive quenching temperatures of 1,008–1,124 ° C, plagioclase liquidus temperatures (1 bar) of 1,210–1,277 ° C, and orthopyroxene-clinopyroxene equilibration of 990–1,150 ° C. These calculated temperatures, together with supporting evidence (e.g. absence of olivine and amphibole, liquidus plagioclase, and plagioclase zoning patterns) are interpreted in terms of phenocryst crystallization from magmas that were either strongly water undersaturated, nearly anhydrous, or at best, water saturated at very low pressures (< 0.5 kb). This interpretation implies that these Tongan basaltic andesites did not originate by any of the currently proposed mechanisms involving hydrous melting within or above the Benioff zone.     

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     

Origin of Archean anorthosites: Evidence from the Bad Vermilion Lake anorthosite complex,Ontario

The Bad Vermilion Lake anorthosite complex (2,700 m.y.) is exposed over an area of about 100 km² near Rainy Lake, Ontario. As is typical of other Archean anorthosites, it is composed of coarse (1–30 cm across), equidimensional, euhedral to subhedral, calcic (An₈₀) plagioclase, in a finer grained mafic matrix. The amount of mafic matrix in individual samples ranges from none to about 70% by volume. The complex has been variably metamorphosed to greenschist facies. Zoisite, chlorite, and hornblende are abundant, but primary plagioclase is preserved in many places. The anorthosite complex is associated with gabbro and with mafic to felsic metavolcanic rocks, and is cut by tonalite plutons and by mafic dikes. Some gabbros contain local concentrations of Fe-Ti oxides and/or apatite, but no chromite. The mafic groundmass of the anorthositic rocks is similar in major and trace element chemistry, including rare earth elements, to the associated basaltic metavolcanics, suggesting that the anorthositic complex may have accumulated from a subvolcanic magma chamber which fed mafic lavas to the surface during its crystallization. Mafic flows and dikes chemically similar to the mafic metavolcanics contain plagioclase megacrysts akin to those of the anorthositic rocks, and thus may represent a link between the anorthosite complex and associated mafic lavas. Elongate pretectonic tonalite intrusions were comagmatic with the felsic metavolcanics, but not with the anorthosites or metabasalts. These silicic rocks may represent low-pressure partial melts of the mafic rocks. There is no direct or indirect evidence for significant volumes of ultramafic material at the present exposure level of the complex. An estimate of the bulk composition of all rocks presumed to be comagmatic with the anorthosites, including gabbros and mafic metavolcanics, is an aluminous basalt with about 20 wt.% Al₂O₃. This composition has REE abundances unlike those of typical Archean high-Al basalts and probably does not represent that of a primary or evolved melt. The possibility must be considered, therefore, that a substantial fraction of material comagmatic with the anorthosites has been separated from the complex, either by magmatic or tectonic processes.     

Geochemistry of the shawmere anorthosite complex,kapuskasing structural zone,Ontario

The Archean Shawmere Anorthosite Complex, at the southern end of the Kapuskasing Structural Zone, consists dominantly of anorthosite (An_{65 –85}) with minor gabbroic and ultramafic units, which are completely enclosed and cut by tonalites. Both the anorthosites and the tonalites are themselves cut by narrow dikes of gabbroic anorthosite. All of the rocks have undergone high grade metamorphism and are recrystallized so that few igneous textures remain.The anorthosites, gabbros and ultramafic rocks of this complex are cumulates which contain calcic plagioclase (An_65–95) and have atomic Mg/(Mg + Fe²⁺) ratios (Mg#) greater than 0.6; less than 3 ppm Rb; 150–210 ppm Sr; and less than 60 ppm Ba. REE abundanees range from 0.2 to 10 times chondritic and exhibit both light-enriched and light-depleted REE patterns. The lower Mg^# for the samples having more enriched light REE indicates substantial fractions of ferromagnesian minerals crystallized in addition to plagioclase during fractional crystallization, suggesting that the parent magma was basaltic, and not anorthositic. The ranges in Sr, Ba and REE abundances required for the magmas are typical of those for tholeiitic basalts from Archean greenstone belts. Thus the Shawmere Anorthosite Complex may represent cumulates of a crustal-level magma chamber which could have been the immediate source of basic Archean volcanics.One gabbroic anorthositic dike sample has a steeply fractionalted REE pattern with heavy REE abundances less than chondrites and a large positive Eu anomaly. The proposed interpretations is that this rock formed by partial melting of mafic cumulates, perhaps those of the Shawmere Anorthosite Complex itself.     

Primitive andesites from the Taupo Volcanic Zone formed by magma mixing

Andesites with Mg# >45 erupted at subduction zones form either by partial melting of metasomatized mantle or by mixing and assimilation processes during melt ascent. Primitive whole rock basaltic andesites from the Pukeonake vent in the Tongariro Volcanic Centre in New Zealand’s Taupo Volcanic Zone contain olivine, clino- and orthopyroxene, and plagioclase xeno- and antecrysts in a partly glassy matrix. Glass pools interstitial between minerals and glass inclusions in clinopyroxene, orthopyroxene and plagioclase as well as matrix glasses are rhyolitic to dacitic indicating that the melts were more evolved than their andesitic bulk host rock analyses indicate. Olivine xenocrysts have high Fo contents up to 94%, δ¹⁸O_(SMOW) of +5.1‰, and contain Cr-spinel inclusions, all of which imply an origin in equilibrium with primitive mantle-derived melts. Mineral zoning in olivine, clinopyroxene and plagioclase suggest that fractional crystallization occurred. Elevated O isotope ratios in clinopyroxene and glass indicate that the lavas assimilated sedimentary rocks during stagnation in the crust. Thus, the Pukeonake andesites formed by a combination of fractional crystallization, assimilation of crustal rocks, and mixing of dacite liquid with mantle-derived minerals in a complex crustal magma system. The disequilibrium textures and O isotope compositions of the minerals indicate mixing processes on timescales of less than a year prior to eruption. Similar processes may occur in other subduction zones and require careful study of the lavas to determine the origin of andesite magmas in arc volcanoes situated on continental crust.     

Complete preservation of ophiolite suite from south Andaman,India: A mineral-chemical perspective

Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair (11°39′N: 92°45′E) to Chiriyatapu (11°30′24″N: 92°42′30″E) stretch of South Andaman. The ophiolite suite reveals serpentinite at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo_80.1–86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An_60.7-An_35.3) whereas groundmass plagioclase are relatively sodic (An₃₃-An_23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite suite.     

Plagioclase buoyancy in basaltic liquids as determined with a centrifuge furnace

Plagioclase buoyancy experiments have been carried out in a high-temperature centrifuge furnace using seventeen basaltic liquids and plagioclase crystals of three compositions: An₈₉, An⁷⁶ and An₅₅. The results show that the floating tendency of plagioclase in basaltic liquids is at least 0.03 g/cm³ greater than indicated by the calculations. If this correction factor is applied to calculations of plagioclase buoyancy in the Skaergaard Intrusion, it is found that the plagioclase crystals in the lower and middle zones were less dense than the coexisting liquids.Other phenomena relevant to crystal transport in basaltic liquids were observed in the centrifuge experiments. These included crystal flotation by rising bubbles, plagioclase sinking because of the formation of plagioclase-magnetite composite grains, graded bedding of olivine and magnetite, and more than 60% intercumulus basaltic liquid between settled olivine crystals.     

Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite,and origin of dusty plagioclase in andesites

The textures and kinetics of reaction between plagioclase and melts have been investigated experimentally, and origin of dusty plagioclase in andesites has been discussed. In the experiments plagioclase of different compositions (An₉₆, An₆₁, An₅₄, An₂₃, and An₂₂) surrounded by glasses of six different compositions in the system diopside-albite-anorthite was heated at temperatures ranging from 1,200 to 1,410° C for 30 min to 88 h. Textures were closely related to temperature and chemical compositions. A crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt (glass) and remained its original euhedral shape below the liquidus. Whatever the temperature, the crystal-melt interface became rough and often more complicated (sieve-like texture composed of plagioclase-melt mixture in the scale of a few m was developed from the surface of the crystal inward; formation of mantled plagioclase) if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt, and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase. From these results the following two types of dissolution have been recognized; (1) a crystal simply dissolves in the melt which is undersaturated with respect to the phase (simple dissolution), and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt (partial dissolution). The amount of a crystal dissolved and reacted increased proportional to the square root of time. This suggests that these processes are controlled by diffusion, probably in the crystal.Mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so-called resorbed plagioclase in igneous rocks. Chemical compositions and textures of plagioclase phenocrysts in island-arc andesites of magma mixing origin have been examined. Cores of clear and dusty plagioclase were clacic (about An₉₀) and sodic (about An₅₀), respectively. This result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing.     

Petrology of the Younger Andesites and Dacites of Iztacc?huatl Volcano, Mexico: I. Disequilibrium Phenocryst Assemblages as Indicators of Magma Chamber Processes

Disequilibrium phenocryst assemblages in the Younger Andesitesand Dacites of Iztacc?huatl, a major Quaternary volcano in theTrans-Mexican Volcanic Belt, provide an excellent record ofepisodic replenishment, magma mixing, and crystallization processesin calc-alkaline magma chambers. Phenocryst compositions andtextures in ‘mixed’ lavas, produced by binary mixingof primitive olivine-phyric basalt and evolved hornblende dacitemagmas, are used to evaluate the mineralogical and thermal characteristicsof end-members and the physical and chemical interactions thatattend mixing. Basaltic end-members crystallized olivine (FO_90–88) andminor chrome spinel during ascent into crustal magma chambers.Resident dacite magma contained phenocrysts of andesine (An_45–35),hypersthene (En67–61), edenitic-pargasitic hornblende,biotite, quartz, .titanomagnetite, and ilmenite. On reachinghigh-level reservoirs, basaltic magmas were near their liquidiat temperatures of about 1250–1200?C according to theolivine-liquid geothermometer. Application of the Fe-Ti-oxidegeothermometer-oxygen barometer indicates that hornblende dacitemagma, comprising phenocrysts (<30 vol. per cent) and coexistingrhyolitic liquid, had an ambient temperature between 940 and820?C at f_O2s approximately 0?3 log units above the nickel-nickeloxide buffer assemblage. Mixing induced undercooling of hybridliquids and rapid crystallization of skeletal olivine (Fo_88–73),strongly-zoned clinopyroxene (endiopside-augite), calcic plagioclase(An_65–60); and orthopyroxene (bronzite), whereas low-temperaturephenocrysts derived from hornblende dacite were resorbed ordecomposed by hybrid melts. Quartz reacted to form coronas ofacicular augite and hydroxylated silicates were heated to temperaturesabove their thermal stability limit ({small tilde}940?C foramphibole, according to clinopyroxene-orthopyroxene geothermometry,and {small tilde}880?C for biotite). Calculations of phenocrystresidence times in hybrid liquids based on reaction rates suggestthat the time lapse between magma chamber recharge and eruptionwas extremely short (hours to days). It is inferred that mixing of magmas of diverse compositionis driven by convective turbulence generated by large differencesin temperature between end-members. The mixing mechanism involves:(1)rapid homogenization of contrasting residual liquid compositionsby thermal erosion and diffusive transfer (liquid blending);(2) assimilation of phenocrysts derived from the low-temperatureend-member; and (3) dynamic fractional crystallization of rapidlyevolving hybrid liquids in a turbulent boundary layer separatingbasaltic and dacitic magmas. The mixed lavas of lztacc?huatlrepresent samples of this boundary layer quenched by eruption.