Crystallization conditions of Los Angeles, a basaltic Martian meteorite

The texture of Los Angeles (stone 1) is dominated by relatively large (0.5−2.0 mm) anhedral to subhedral grains of pyroxene, and generally subhedral to euhedral shocked plagioclase feldspar (maskelynite). Minor phases include subhedral titanomagnetite and ilmenite, Fe-rich olivine, olivine+augite-dominated symplectites [some of which include a Si-rich phase and some which do not], pyrrhotite, phosphate(s), and an impact shock-related alkali- and silica-rich glass closely associated with anhedral to euhedral silica grains. Observations and model calculations indicate that the initial crystallization of Mg-rich pigeonitic pyroxenes at ≤1150 °C, probably concomitantly with plagioclase, was followed by pigeonitic and augitic compositions between 1100 and 1050 °C whereas between 1050 and 920 to 905 °C pyroxene of single composition crystallized. Below 920 to 905 °C, single composition Fe-rich clinopyroxene exsolved to augite and pigeonite. Initial appearance of titanomagnetite probably occurred near 990 °C and FMQ-1.5 whereas at and below 990 °C and ≥FMQ-1.5 titanomagnetite and single composition Fe-rich clinopyroxene may have started to react, producing ilmenite and olivine. However, judging from the most common titanomagnetite compositions, we infer that most of this reaction likely occurred between 950 and 900 °C at FMQ-1.0±0.2 and nearly simultaneously with pyroxene exsolution, thus producing assemblages of pigeonite, titanomagnetite, olivine, ilmenite, and augite. We deem this reaction as the most plausible explanation for the formation of the olivine+augite-dominated symplectites in Los Angeles. But we cannot preclude possible contributions to the symplectites from the shock-related alkali- and silica-rich glass or shocked plagioclase, and the breakdown of Fe-rich pigeonite compositions to olivine+augite+silica below 900 °C. Reactions between Fe-Ti oxides and silicate minerals in Los Angeles and other similar basaltic Martian meteorites can control the T-fO₂ equilibration path during cooling, which may better explain the relative differences in fO₂ among the basaltic Martian meteorites.     

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.     

Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts

In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ¹⁸O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ¹⁸O_Olivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H₂O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr₂O₃ and raising Al₂O₃ and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.     

Chemical evolution of basalts from 23°N along the mid-Atlantic ridge: evidence from melt inclusions

The chemical compositions of melt inclusions in a primitive and an evolved basalt recovered from the mid-Atlantic ridge south of the Kane Fracture Zone (23°–24°N) are determined. The melt inclusions are primitive in composition (0.633–0.747 molar Mg/(Mg+Fe²⁺), 1.01–0.68 wt% TiO₂) and are comparable to other proposed parental magmas except in having higher Al₂O₃ and lower CaO. The primitive melt inclusion compositions indicate that the most primitive magmas erupted in this region are not near primary magma compositions. Olivine and plagioclase microphenocrysts are close to exchange equilibrium with their respective basalt glasses, whose compositions are displaced toward olivine from 1 atm three phase saturation. The most primitive melt inclusion compositions are close to exchange equilibrium with the anorthitic cores of zoned plagioclases (An_78.3-An_83.1; the hosts for the melt inclusions in plagioclase) and with olivines more forsteritic (Fo₈₉-Fo₉₁) than the olivine microphenocrysts (the hosts for the melt inclusions in olivine). Xenocrystic olivine analyzed is Fo₈₉ but contains no melt inclusions. These observations indicate that olivines have exchanged components with the melt after melt inclusion entrapment, whereas plagioclase compositions have remained the same since melt inclusion entrapment. Common denominator element ratio diagrams and oxide versus oxide variation diagrams show that the melt inclusion compositions, which represent liquids higher along the liquid line of descent, are related to the glass compositions by the fractionation of olivine, plagioclase and clinopyroxene (absent from the mincral assemblage), probably occurring at elevated pressures. A model is proposed whereby clinopyroxene segregates from the melt at elevated pressures (to account for its absence in the erupted lavas that have the chemical imprint of clinopyroxene fractionation). Zoned plagioclases in the erupted lavas are thought to be survivors of decompressional melting during magma ascent. Since similar primitive melt inclusions occur in olivine microphenocrysts and in the cores of zoned plagioclases, any model must account for all phases present.     

Simulation of primary phase relations and mineral compositions in the Partridge River intrusion,Duluth Complex,Minnesota: implications for the parent magma composition

In order to describe the composition and crystallinity of the initial (parental) magma of the Partridge River intrusion of the Keweenawan Duluth Complex, and thereby understand the mode of emplacement and solidification of the intrusion, we have applied a numerical simulation technique called geochemical thermometry (Frenkel et al. 1988). The parental magma was a low-alumina, high-Ti-P olivine tholeiite similar to typical Keweenawan low-alumina, high-Ti-P basalts associated with the Duluth Complex and from the nearby Portage Lake area of the Lake Superior region. The parental magma was emplaced as a crystal-liquid suspension, followed by chilling of an evolved, leading edge ferrodioritic liquid in the basal zone of the intrusion. The conditions of emplacement at the present crustal location were 1,150°C, 2 kbar, and f _{O 2} slightly above the wustite-magnetite (WM) buffer. The main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine. Calculated crystallization sequence for the parental magma is olivine+plagioclase (1,240°C)olivine+plagioclase+magnetite (1,146°C, WM+0.5)olivine+plagioclase+magnetite+augite (1,140°C, WM+0.5). The calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1,150°C are Fo_66.7±1.1 and An_64.5±2.5, respectively, and are similar to the estimated average composition of primary olivine (Fo_69.1±2.8) and the average composition of plagioclase core (An_66.3±2.8) measured in drill core samples through the intrusion (Chalokwu and Grant 1987).     

Anhydrous PT phase relations of an Aleutian high-MgO basalt: an investigation of the role of olivine-liquid reaction in the generation of arc high-alumina basalts

We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnston's (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H₂O, which will affect the influence of bulk CaO, MgO, and Al₂O₃ contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than 12 kbar.     

A regular solution model for met-aluminous silicate liquids: Applications to geothermometry,immiscibility, and the source regions of basic magmas

Adopting a set of multioxide components and using published compositional data on olivineand plagioclase-liquid equilibria we have developed a 17 component regular solution model for met-aluminous silicate liquids. The partial molar excess free energies predicted from this model can be used together with phenocryst compositions as an effective geothermometer, with an approximate error of 20 °C (30 °C for olivine, 12 °C for plagioclase). The regular solution formulation is also successful in predicting liquid immiscibility at (1) high mole fractions of silica commonly observed in phase diagrams, and at (2) lower temperatures in lunar basalts and intermediate lavas. The model yields activities of silica which are consistent with those obtained from solid-liquid silica buffers in rocks which contain olivine and enstatite or quartz. From predicted activities of KAlSi₃O₈ in liquids coexisting with plagioclase a value is obtained for the limiting Henry's law activity coefficient of KAlSi₃O₈ in the solid. This coefficient agrees well with that inferred from plagioclase-sanidine equilibrium phenocryst assemblages in rhyolites. The activities of silica obtained from this model are used to place constraints on the pressure-temperature regions where various types of basic magmas are generated. In conjunction with plagioclase geothermometry an application is given where the pressure, temperature, and water content of an olivine andesite is predicted from the activity of silica.     

Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano,Hawaii

Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K₂O in plagioclase and the abundances of TiO₂ and Fe₂O₃in spinel of xenoliths and mid-ocean ridge basalt, and a single ⁸⁷Sr/ ⁸⁶Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10^–3–1.91×10^–3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel.     

Ti-content of high-Ca pyroxenes as a petrogenetic indicator: an experimental study of Mafic Alkaline Rocks from the Mt. Erebus volcanic region,Antarctica

We report chemical and mineralogical data for one atmosphere melting experiments conducted on alkalic rocks from the Mt. Erebus volcanic region: DVDP2 basanite, two hawaiites (DVDP2 and a nepheline-bearing variety), and an anorthoclase phonolite. Temperatures between 1,224 and 1,049°C were investigated at f_O2~QFM. DVDP2 basanite appears to be an intermediate pressure liquid or a cumulate, because only olivine coexists with melt from above 1,224–1,160°C. High-Ca pyroxene joins olivine in the crystallization sequence at 1,138°C. These minerals are joined by plagioclase at a temperature between 1,120 and 1,104°C. In contrast, DVDP2 hawaiite appears to be relatively evolved, because it is multiply saturated with olivine, plagioclase, and high-Ca pyroxene near its liquidus (between 1,120 and 1,104°C). Plagioclase crystallizes in the Ne-hawaiite by 1,160°C followed by olivine below 1,120°C. The liquidus of anorthoclase phonolite is between the lowest temperatures investigated, 1,089 and 1,049°C, and plagioclase is the liquidus mineral. Our results indicate that DVDP2 hawaiite can be derived from a DVDP2 basanitic parental magma by crystal fractionation at low pressures, that the nepheline hawaiite is an olivine cumulate, and that the liquids parental to the anorthoclase phonolite represent the end products of crystal fractionation. They also allow us to illustrate how the Ti-content of pyroxene may be used as a petrogenetic indicator of processes and events in the evolution of the Erebus volcanic system.     

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.     

Molybdenum and tungsten in volcanic rocks and in surface and <100 °C ground waters in Iceland

The concentrations of Mo and W in tholeiite series basaltic to silicic volcanics in Iceland increase with the concentrations of incompatible magmatic elements. In normal Mid Ocean Ridge Basalt (MORB), Mo and W levels are as low as 0.03 and 0.01 ppm but in rhyolites they are up to 4 and 1.3 ppm, respectively. In the slightly evolved Tertiary tholeiites of the main study area of Skagafjördur, northern Iceland, Mo and W concentrations are 1.01 and 0.32 ppm, respectively, on average. Surface waters in this area typically contain <0.5 ppb Mo and <0.1 ppb W. Concentrations are higher in ground waters and, on the whole, they increase with increasing temperature and increasing age of these waters, being as high as ∼100 ppb for Mo and ∼10 ppb for W in the hottest (∼90 °C) and oldest (>10,000 years) waters. In river and ground waters, the average Mo/W ratio is the same as that of the basalt host rock. On the other hand, in peat soil waters, the average Mo/W ratio is higher due to adsorption processes involving W. In river and ground waters, both Mo and W display conservative behavior. Progressive water-rock interaction leads to successively lower aquatic B/Mo and B/W ratios. However, the B/Mo and B/W ratios of the most reacted waters are considerably higher than those of the basalt due to non-stoichiometric rock dissolution: B is dissolved preferentially to both Mo and W because B is largely present in a soluble form, but Mo and W are concentrated in the Fe-Ti-oxides. These minerals are stable, under both surface and sub-surface conditions, i.e. the waters are over-saturated with respect to these minerals. The main supply of Mo and W to surface waters is from dissolution of plagioclase and pyroxene, as well as volcanic glass. Olivine is not an important source due to its scarcity in the basalts. In <30 °C ground waters, the dominant supply of Mo and W is plagioclase. At higher temperatures pyroxene also contributes, with its contribution increasing with increasing temperature, as deduced from its decreasing stability at higher temperature. Dissolution of basaltic glass produces lower B/Mo and B/W aqueous ratios than dissolution of crystalline basalt.     

High pressure phase relations of primitive high-alumina basalts from Medicine Lake volcano,northern California

Anhydrous phase relations were determined at 1 atm and 10 to 15 kbar for primitive high-alumina basalts (79–35g and 82–72f) from Giant Crater at Medicine Lake volcano. These compositions are multiply saturated with olivine+augite+plagioclase+spinel+/-orthopyroxene near the liquidus at about 11 kbar. Experiments on mixtures of sample 79–35g with orthopyroxene and olivine determined the location of the multiple saturation boundaries where liquid coexists with the assemblage olivine+augite+orthopyroxene+plagioclase at 10 kbar and olivine+augite+orthopyroxene+spinel at 15 kbar. The mix experiments showed that primitive Medicine Lake high alumina basalts (HABs) are close in composition to liquids in equilibrium with a mantle lherzolite source containing olivine+augite+ orthopyroxene+spinel+plagioclase at 11 kbar. Orthopyroxene observed as a near liquidus phase in an 11 kbar experiment on sample 82–72f supports this conclusion. The most primitive HABs from Medicine Lake are low in K₂O (0.07 wt.%), high in MgO (>10 wt.%) and Ni (231 ppm), and have light-rare earth element depletions and large ion lithophile element enrichments. A model for the origin of these near-primary high-alumina basalts is that they are partial melts of a MORB-like mantle lherzolite source that has been enriched by a fluid component derived from the subducted slab. The HAB magma segregated from its mantle residue just below the base of the crust near the crust-mantle boundary.     

Fe-rich tholeiitic liquids and their cumulate products in the Pleasant Bay layered intrusion, coastal Maine

Fe-rich tholeiitic liquids are preserved as chilled pillows and as the chilled base of a 27 meter thick macrorhythmic layer in the Pleasant Bay mafic-silicic layered intrusion. The compositions of olivine (Fo₁) and plagioclase (An₁₃₋₈) in these extremely fine grained rocks suggest that they represent nearly end stage liquids that formed by fractionation of tholeiitic basalt. Their major element compositions (∼17.5 wt% FeO_T and 54 wt%SiO₂) closely resemble highly evolved glasses in the Loch Ba ring dike and some recent estimates of end-stage liquids related to the Skaergaard layered intrusion, and are consistent with recent experimental studies of tholeiite fractionation. Their trace element compositions are consistent with extensive earlier fractionation of plagioclase, olivine, clinopyroxene, ilmenite, magnetite and apatite. The mineral assemblage of the chilled rocks (olivine, clinopyroxene, quartz, ilmenite and magnetite), apatite saturation temperatures, and very low Fe³⁺/Fe²⁺indicate conditions of crystallization at temperatures of about 950 °C and f _{O 2} about two log units below FMQ. Cumulates that lie about 3 meters above the chilled base of the macrorhythmic layer contain cumulus plagioclase, olivine, clinopyroxene, ilmenite, apatite and zircon. This mineral assemblage and the Fe-Mg ratio in clinopyroxene cores suggest that this cumulate was in equilibrium with a liquid having a composition identical to that of the chilled margin which lies directly beneath it. The high FeO_T and low SiO₂ concentrations of this cumulate (23.3 and 45.8 wt%, respectively) are comparable to those in late stage cumulates of the Skaergaard and Kiglapait intrusions. This association of a chilled liquid and cumulate in the Pleasant Bay intrusion suggests that late stage liquids in tholeiitic layered intrusions may have been more SiO₂-rich than field-based models suggest and lends support to recent experimental studies of tholeiite fractionation at low f _O2 which indicate that saturation of an Fe-Ti oxide phase should cause FeO_T to decrease in the remaining liquid. Received: 17 January 1997 / Accepted: 10 June 1997     

Early crystallization history of alkali olivine basalts,Diamond Craters,Oregon

Basaltic lavas form the isolated Quaternary volcanic centers of Diamond Craters and occur at Cow Lakes in S.E. Oregon. The exposed rocks are olivine and plagioclase phyric alkali olivine basalts. The holocrystalline groundmasses contain plagioclase, titanaugite, olivine, Fe-Ti oxides and minor apatite, and occur in the high-alumina (greater than 17%) petrographic province of the northwestern Great Basin.The whole rock and phenocryst compositions provide enough information to define the equilibrium state of the magmas. The solutions to the thermodynamic equations are valid for systems of known bulk composition and can be used to evaluate whether rock compositions are close to magma compositions. Two of the lavas from Diamond Craters have whole rock and phenocryst compositions consistent with their being natural melts. Rock compositions that reflect the effects of crystal separation (open systems) are recognized by significant differences in predicted versus observed phenocryst modes. The P-T conditions of phenocryst saturation and the crystallization history of the magmas can be inferred from petrologic data combined with thermodynamic modelling and mass balance constraints. The initial melts are inferred to have equilibrated at 0.8–1.0 GPa and 1210–1240°C. The majority of the lavas from Diamond Craters have compositions that reflect olivine and plagioclase crystal separation. These lavas can be related to two possible parent rock compositions by small amounts (2–8% by weight) of olivine and plagioclase separation.     

Melting of plagioclase-quartz assemblages at 2 kbar water pressure

The melting of plagioclase and quartz has been investigated at P _H2O =2kb. A single crystal of plagioclase was surrounded by quartz powder and water. A reaction rim consisting of glass and of An-rich plagioclase developed around unchanged starting plagioclase. Microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase. For example at 850° C the approximate X _An of melt is 0.3 and that of plagioclase is 0.8. The temperature interval between beginning of melting and complete melting of cotectic compositions is 100° C or more for quartz-plagioclase mixtures with plagioclases between An 40 and An 75. In comparison to the system Ab-An-H₂O the plagioclase melting loop is somewhat wider in the investigated system Qz-Ab-An-H₂O but the temperature interval is much smaller (100° C) than in the system Ab-An-H₂O (200° C). The solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments. The results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi-component rocks than in the pure plagioclase system. This finding is important for the development of albite rich rocks from more basic compositions.     

The role of olivine in the crystallization of the prehistoric Makaopuhi tholeiitic lava lake,Hawaii

On eruption, the tholeiitic basalt lava of the prehistoric Makaopuhi lake contained nearly seven percent euhedral olivine phenocrysts of approximately Fa₁₄ composition. In the center of the 225 foot vertical section of the lake, the lava became more than 90 percent solid at 1000° C after about 30 years. At the surface the lava was quenched to air temperature, whereas, at the bottom, quenching to 800° C was followed by a 40 year period before the temperature reached 700° C. The olivine phenocrysts settled at an average rate of about 4 × 10^–6 cm Sec^–1 to form a zone that contains 21 percent olivine 75 feet above the base. Sinking of olivines continued until some time after the beginning of the crystallization of augite and plagioclase. Thin rims of iron-rich olivine (up to Fa₅₅) surrounding the phenocrysts, and a second generation of fine-grained olivines (Fa₂₀ Fa₄₈) restricted to the uppermost 20 feet indicate local extensions of the period of crystallization of olivine. During crystallization of the groundmass and later subsolidus cooling in the range 1000° C to at least as low as 800° C, the olivine phenocrysts were converted to Fa_30–40 by interdiffusion of Fe, Mg, Ni, and Mn. Homogenization of Mg-rich cores and Fe-rich margins and equilibration of olivine composition with the groundmass phases was progressively less well achieved toward the top of the lake. Reaction rims around the olivines are composed primarily of Ca-rich pyroxene. Pigeonite crystallized alongside augite except in the uppermost 5 feet where there is abundant ground mass olivine. Poikilitic hypersthene grew at the expense of pre-existing ferromagnesian minerals in the cumulate zone.Publication authorized by the Director, U.S. Geological Survey.     

An experimental investigation of Iceland and Reykjanes Ridge tholeiites: I. Phase relations

Tholeiite basalts from 60° N to 65° N on the Mid-Atlantic Ridge were melted and recrystallized at atmospheric pressure in a CO₂-H₂ gas mixture. Seven basalts are from the Langjokull-Thingvellir volcanic zone and the Reykjanes Peninsula of Iceland and nine are from the Reykjanes Ridge. The crystallization sequence in both Iceland and Reykjanes Ridge basalts with (Total Fe as FeO)/(Total Fe as FeO+ MgO) [F/F + M] less than 0.6 is olivine, plagioclase, clinopyroxene. Chromian spinel crystallizes before plagioclase in one Iceland and one Reykjanes Ridge basalt with F/F+M less than 0.57. Chemical differences of the two groups of basalts (lower SiO₂ and higher alkalis in Iceland basalts) can not simply be a result of low pressure fractional crystallization. Liquidus temperatures of the seven Iceland basalts decreases from 1,230° C to 1,170° C as the F/F+M of the rock increases from 0.52 to 0.70. The liquidus temperatures of the Reykjanes Ridge basalts are about 10° C lower than those of the Iceland basalts for the same F/F+M value. The profile of measured liquidus temperatures from 65° N on Iceland to 60° N on the Reykjanes Ridge has a minimum value at 63.2° N on the Reykjanes Ridge just south of Iceland. Model calculations of the pressure of phenocryst crystallization indicate that olivine and plagioclase in Langjokull basalts could have equilibrated between 2.0 and 6.2 kb (200 to 620 MPa). Phenocryst assemblages in Reykjanes Ridge basalts at 60° N could have crystallized together at greater than 2 kb (200 MPa) and probably less than 8 kb (800 MPa). A minimum in the equilibrium pressure of phenocryst crystallization occurs between 62.9° and 64° N and coincides with the minimum in the experimentally determined liquidus temperatures. The more extensive fractionation at low pressure in this area could be related to the shift of the Mid-Atlantic Ridge axis along the leaky transform fault from the Reykjanes Ridge to the Thingvellir volcanic zone.     

Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids

Experimental study of natural alkalic lava compositions at low pressures (pO₂QFM) reveals that crystallization of primitive lavas often occurs in the sequence olivine, plagioclase, clinopyroxene, nepheline without obvious reaction relation. Pseudoternary liquidus projections of multiply saturated liquids coexisting with plagioclase (±olivine±clinopyroxene±nepheline) have been prepared to facilitate graphical analysis of the evolution of lava compositions during hypabyssal cooling. Use of (TiAl₂)(MgSi₂)_–1 and Fe³⁺ (Al)_–1 exchange components is a key aspect of the projection procedure which is succesful in reducing a wide range of compositions to a systematic graphical representation. These projections, and the experiments on which they are based, show that low pressure fractionation plays a significant role in the petrogenesis of many alkalic lava suites from both continental and oceanic settings. However, the role of polybaric fractionation is more evident in the major element chemistry of these lava suites than in many tholeiitic suites of comparable extent. For example, the lavas of Karisimbi, East Africa, show a range of compositions reflecting a polybaric petrogenesis from primitive picrites at 1360° C/18 kb and leading to advanced low pressure differentiates. Evolved leucite-bearing potassic members of this and other suites may be treated in a nepheline-diopside-kspar (+olivine+leucite) projection. Compositional curvature on the plagioclase+clinopyroxene+olivine+leucite cotectic offers a mechanism to explain resorption of plagioclase in alkalic groundmass assemblages and the incompatibility of albite and leucite. This projection is useful for evaluating the extent of assimilation of the alkalic portions of crustal granulites. Assimilation appears to have played some role in the advanced differentiates from Karisimbi.     

Experimental data at 3 kbars pressure on parental magma to the Bushveld Complex

Experimental studies, mainly under 3 kbars pressure, have been undertaken on representative samples to determine if any of these compositions could be parental magma to the Bushveld Complex. One such composition, with 12.5% MgO, Mg/(Mg + Fe) of 0.72 and quartz-normative, crystallizes olivine, Fo₈₈, as liquidus mineral, at about 1,300° C, followed at only slightly lower temperature by orthopyroxene at 3 kbars pressure. There is a temperature drop of over 100° C before the appearance of plagioclase and finally clinopyroxene. This crystallization sequence is in excellent agreement with the observed sequence in the lower part of the Bushveld Complex.Results at higher pressures show that this composition cannot be a partial melt from mantle peridotite because olivine is replaced by orthopyroxene as the liquidus mineral at lower crustal pressures. A combination of olivine fractionation and contamination was probably involved in the early evolution of this magma.Experimental data on the other compositions show that they are not suitable as parental magma to the lowest portion of the complex. However, the data are used to construct phase diagrams within the basalt tetrahedron at 3 kbars pressure, which are of relevance to the crystallization of basic magmas in the upper crust.Research undertaken at the Grant Institute of Geology, University of Edinburgh, Scotland     

Petrogenesis of picrite and associated basalts from the southern Mid-Atlantic Ridge

Basalts dredged from the south wall of a fracture zone transecting the southern Mid-Atlantic Ridge (SMAR) at 54° S are unusual in that they include a suite of highly olivine phyric basalts, sampled along with more normal sparsely plagioclase phyric basalts, and a highly plagioclase phyric basalt. Four basalt types (olivine phyric, sparsely plagioclase phyric, evolved sparsely plagioclase phyric and highly plagioclase phyric) are readily distinguished on the basis of petrography, mineralogy and bulk composition. They range from primitive to evolved, with the olivine phyric basalts having elevated MgO (up to 15.5%) and the plagioclase phyric basalt having elevated Al₂O₃ (19.3%) and CaO (13.1%) contents. Compositional variations are extremely consistant, with the olivine phyric basalts and the sparsely plagioclase phyric basalts defining coherent linear trends. On the basis of the ratios and covariation of the incompatible trace elements Zr, Nb, Y and Ba, distinct parental magmas for each basalt type are required. An investigation of Fe-Mg and Mg-Ni distribution coefficients between olivine and magma indicates that olivines from the olivine phyric basalts are on average too forsteritic and too Ni poor to have crystallized in a magma corresponding to the host bulk rock composition. This implies that these basalts are enriched in xenocrystic olivine. Olivines from the other basalt types are mostly of equilibrium composition, although there are some exceptions. Petrogenetic models for the formation of the different basalt types are quantitatively evaluated in terms of fractional crystallization/crystal accumulation processes. These indicate that (1) the olivine phyric basalts are the products of olivine and minor Cr-spinel accumulation and do not represent analogues of primary magma, or a liquid fractionation trend; (2) that the sparsely plagioclase phyric basalts were formed by polybaric fractional crystallization of olivine, plagioclase and clinopyroxene; and (3) that the evolved sparsely plagioclase phyric basalts are not readily related to one another. The single highly plagioclase phyric basalt is unrelated to the other basalt types and is cumulus enriched in plagioclase.The different basalt types are unrelated to one another and document the presence of at least four distinct magma types erupted in close proximity at this ridge/transform intersection on the southern end of the Mid-Atlantic Ridge.