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.     

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.     

Feldspar crystallization trends in leucite-bearing and related assemblages

Feldspar chemical variations in representative leucite-bearing and related rocks from well-known localities in Italy, Germany, Uganda and Australia demonstrate that phenocrystal core to rim variations may not represent the feldspar crystallization trend in the host lava and only the groundmass feldspar zoning trend is a reliable indicator of crystal-liquid relationships. Textural relationships indicate that coexisting plagioclase and alkali feldspar crystallized sequentially, the latter after the former, rather than cotectically.Groundmass alkali feldspar show Ca-, Na-depletion and K-enrichment zoning trends. Plagioclase crystallization follows Ca-depletion, Na and K-enrichment trends. Typically, Sr and Ba solid solubility is significant, particularly in groundmass feldspar.The alkali feldspar variation trend from groundmass assemblages is not consistent with the theoretical phase relationships in the system NaAlSiO₄-KAlSiO₄ CaAl₂Si₂O₈-SiO₂ (The phonolite pentahedron) proposed by Carmichael et al. (1974).Factors believed to be important in controlling feldspar crystallization trends are the Sr-Ba feldspar components, the role of the coexisting pyroxene and the presence of F, Cl and/or their alkali compounds.     

Melt inclusions in olivine from the boninites of New Caledonia: Postentrapment melt modification and estimation of primary magma compositions

Melt inclusions were investigated in olivine phenocrysts from the New Caledonia boninites depleted in CaO and TiO₂ and enriched in SiO₂ and MgO. The rocks are composed of olivine and pyroxene phenocrysts in a glassy groundmass. The olivine phenocrysts contain melt inclusions consisting of glass, a fluid vesicle, and daughter olivine and orthopyroxene crystals. The daughter minerals are completely resorbed in the melt at 1200?C1300°C, whereas the complete dissolution of the fluid phase was not attained in our heating experiments. The compositions of reheated and naturally quenched melt inclusions, as well as groundmass glasses were determined by electron microprobe analysis and secondary ion mass spectrometry. Partly homogenized melts (with gas) contain 12?C16 wt % MgO. The glasses of inclusions and groundmass are significantly different in H₂O content: up to 2 wt % in the glasses of reheated inclusions, up to 4 wt % in naturally quenched inclusions, and 6?C8 wt % in groundmass glasses. A detailed investigation revealed a peculiar zoning in olivine: its Mg/(Mg + Fe) ratio increased in a zone directly adjacent to the glass of inclusions. This effect is probably related to partial water (hydrogen) loss and Fe oxidation after inclusion entrapment. The numerical modeling of such a process showed that the water loss was no higher than a few tenths of percent and could not be responsible for the considerable difference between the compositions of inclusions and groundmass glasses. It is suggested that the latter were enriched in H₂O after the complete solidification of the rock owing to interaction with seawater. Based on the obtained data, the compositions of primary boninite magmas were estimated, and it was supposed that variations in melt composition were related not only to olivine and pyroxene fractionation from a single primary melt but also to different degrees and (or) depths of magma derivation.     

Pyroxene-selective impact smelting in ureilites

A characteristic feature of ureilite meteorites is reduction of FeO. But the reduction is usually confined to the rims of olivine. In the LAR 04315, LAP 03587 and Almahata Sitta ureilites, pyroxene was extensively reduced by impact smelting. In LAR 04315, the impact caused nearly all of the original pigeonite to melt or otherwise become sufficiently structurally compromised to allow smelting, and yet a minor proportion of the pyroxene escaped smelting and survived with its original composition (En_74.1Wo_10.2). Olivine mosaicism confirms that LAR 04315 experienced a major shock event. The smelted pyroxenes also show a distinctive patchiness in their interference colors (although each grain’s basic optical continuity, often including twinning, is still discernible). They also have reduced compositions, are ubiquitously porous (∼15%), and contain sprinklings of Fe-metal and felsic glass. For the most part the olivine underwent only very slight reduction. Much of the (small) pyroxene component of LAP 03587 shows the same oddly porous texture. LAR 04315 also contains large traces of silica and felsic glass (with a typical composition of, in wt%, 61 SiO₂, 23 Al₂O₃, 11 CaO, 3.7 Na₂O) glass; these two phases together form selvages that line the walls of many of the largest voids in the rock. Silica is a by-product of pyroxene smelting. The felsic glass probably derives largely from interstitial basaltic melt that predated the impact. However, the comparatively stiff surrounding/included silica may have promoted unusually high melt retention within LAR 04315 through the smelting episode (one aspect of which was a major stream-out, through the same large voids, of CO_x gas). The impact-smelted pyroxene of LAP 03587 is enigmatic because this ureilite also features little-shocked euhedral graphite laths and no olivine mosaicism. The fine-grained ureilitic component of Almahata Sitta appears to have likewise formed by impact smelting, but with more extensive melting of pyroxene (especially a Ca-rich pyroxene component), more pulverization and melting of olivine, and more displacement of both. However, in places the original coarse-equant ureilite texture is still discernible in relict form. Ordinarily, an impact shock melts olivine before, or at least no later than, pyroxene. But in the case of LAR 04315 and LAP 03587, the great shock event evidently occurred when the material was already anatectic or very nearly so; and thus the difference in melting temperature between pyroxene and olivine, ∼300 degrees lower for pyroxene, was decisive. If literature inferences of extremely fast cooling rates, implying shallow burial depths, are accurate, the proportion of CO_x gas generated by ureilite smelting exceeded by a very large factor (of order 10³ but possibly much greater) the volume represented as porosity in the final ureilites. The outflow of so much gas may have, by near-surface explosive expansion and jetting, enhanced the thoroughness of the impact-triggered catastrophic impact disruption of the parent asteroid.     

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.     

Experimental investigations of low-Ca pyroxene stability and olivine-pyroxene-liquid equilibria at 1-atm in natural basaltic and andesitic liquids

This paper reports experiments carried out at 1-atm under conditions of controlled oxygen fugacity, using natural andesites and andesite mixed with augite+synthetic pigeonite or augite+orthopyroxene. The experimental results are used (1) to investigate the controls of Mg^# (Mg/[Mg+Fe²⁺]) and temperature on low-Ca pyroxene stability (pigeonite vs orthopyroxene), (2) to quantify the effects of variations in bulk composition on the position of multiple saturation boundaries in mineral component projection schemes and (3) to develop a thermodynamic model for silica activity for melts saturated with olivine and pyroxene. Over the Mg^# range of 0.80–0.30 the minimum temperature of pigeonite stability in natural compositions is equivalent to the Lindsley (1983) boundary determined for pure Ca-Mg-Fe pigeonites. For the low variance, 5-phase assemblage oliv-aug-low-Ca pyroxene-plag-liquid, expressions involving liquid (Na₂O+K₂O)/(Na₂O+K₂O+CaO),Mg^# and TiO₂ content predict temperature and the movement of multiple saturation boundaries in pseudoternary projections in response to changing melt composition. The equilibrium for the low pressure melting of low-Ca pyroxene to olivine+liquid is formulated as a geothermometer and monitor of silica activity. Equilibrium constants estimated from thermochemical data and activities calculated for experimentally produced olivine and pyroxenes are used to develop a model for silica activity in liquid.     

Contributions to the mineral chemistry of Hawaiian rocks

Feldspar phenocrysts, microphenocrysts, groundmass feldspar, interstitial material of feldspar composition, and residual SiO₂-K₂O-rich glass in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively with the electron microprobe. Rocks studied include tholeiite, olivine tholeiite, oceanite, alkalic olivine basalt, alkalic basalt, hawaiite, mugearite, trachyte, basanite, and basanitoid. Results and conclusions: i) In all rocks studied, An decreases and Or increases from phenocrysts to microphenocrysts to groundmass feldspar to interstitial material of feldspar composition. ii) Phenocrysts occur in rocks of the tholeiitic and alkalic suites and, in spite of differences in bulk rock compositions, overlap in composition. iii) Groundmass feldspar in rocks of the tholeiitic suite are nearly identical in composition; the same is true for rocks of the nephelinic suite. However, in the highly differentiated alkalic suite, groundmass feldspar composition ranges from labradorite to sanidine; i.e. the higher the bulk rock CaO, the higher is the An content, and the higher the bulk K₂O, the higher is the Or content. iv) In general, rocks with phenocrysts have groundmass feldspar less An-rich than those without phenocrysts. v) In rocks of the tholeiitic suite, normative feldspar approaches modal feldspar. However, in rocks of the alkalic and nephelinic suites, normative feldspar, because of the presence of highly alkalic interstitial material and the absence of nepheline in the mode but its presence in the norm, is drastically different from modal feldspar. vi) Hawaiites contain labradorite and not andesine, as per definition, and mugearite contains andesine and not oligoclase, as groundmass feldspar. In fact, when considering phenocrysts and interstitial material of feldspar composition, hawaiites range from bytownite to sanidine and mugearite from andesine to sodic sanidine, but normative feldspar plots in the andesine field for hawaiites and the oligoclase field for mugearite. vii) Rocks of the three suites can be distinguished on the basis of Or and An in groundmass feldspar, the presence of thin rims of groundmass composition of phenocrysts of rocks of the alkalic suite, and the presence of interstitial material of anorthoclase to sanidine composition in rocks of the alkalic and nephelinic suites. iix) Rocks transitional between the tholeiitic and alkalic suites are observed and are characterized by transitional mineral compositions.This paper was first presented as a talk before the 68. Annual Meeting of the Cordilleran Section of the Geological Society of America, Honolulu, Hawaii, March 29–April 1, 1972.     

Tholeiitic andesite of high-pressure origin from the tweed shield volcano,northeastern New South Wales

A high-alumina tholeiitic andesite from the southern portion of the Tweed Shield Volcano in northeastern New South Wales contains abundant megacrysts of plagioclase (Ab₅₀An₄₆Or₄) and megacrysts of aluminian bronzite (Ca₄Mg₇₁Fe₂₅) and relatively Ca-poor aluminian augite (Ca₃₆Mg₄₁Fe₂₀). The pyroxenes commonly occur as inclusions in the plagioclase megacrysts. Electron probe microanalyses of the pyroxene megacrysts indicate that they differ in character and composition from the groundmass ferromagnesian phases, namely a more Al- and Na-poor augite (Ca₄₁Mg₄₂Fe₁₇) and olivine (Fa₅₃). The bulk composition of the plagioclase megacrysts is slightly more Ab-rich than that of the groundmass plagioclase, but differences in the two compositions are extended by microanalyses of groundmass plagioclases. Evaluation of the megacryst compositions in the light of experimental data and analogous occurrences in alkaline volcanics leads to the interpretation that the megacrysts represent cognate precipitates formed at pressures broadly equivalent to the crust-mantle boundary. More important, they provide strong evidence for the high pressure origin of tholeiitic andesites, customarily interpreted as the products of low pressure fractional crystallization of tholeiitic magma.     

Skeletal crystallization and residual glass compositions in a cellular alkalic pyroxenite nodule from Oldoinyo Lengai

The alkalic pyroxenite nodule consists of megacrysts of diopside, apatite, perovskite and titanomagnetite in a groundmass consisting of diopside, apatite, titanomagnetite, nepheline, melilite, garnet and vishnevite crystals of various shapes, including previously undescribed skeletal and dendritic shapes, together with vesicles and residual glass. The residual glass is poor in SiO₂ (38–40 wt%), and extraordinarily rich in Na₂O (12.8–15 wt%), SO₃ (1–1.5 wt%), and Cl (0.25–0.7 wt%), as a result of rapid, non-equilibrium crystallization of groundmass phases from a CO₂-rich nephelinite melt.The Oldoinyo Lengai alkalic carbonatite lavas do not represent extreme products of the fractional crystallization of pyroxene, wollastonite, nepheline and alkali feldspar from the carbonated nephelinite melt. The most likely connection between the carbonatite and silicate magma types is one of liquid immiscibility, probably involving phonolite melt.     

Compositional relations of coexisting orthopyroxene,pigeonite and augite in a tholeiitic andesite from Hakone Volcano

The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite and augite of phenocryst in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser. It is shown that there is a compositional gap of about 25 mole per cent CaSiO₃ between groundmass pigeonite and augite, compared with 35 per cent CaSiO₃ between phenocrystic augite and bronzite. Subcalcic augite or pigeonitic augite was not found. The groundmass augite, which occurs only as thin rims of pigeonite and hypersthene, is less calcic and more iron-rich than the phenocryst augite. It is also shown that the groundmass pigeonite is 3–4 mole per cent more CaSiO₃-rich than the coexisting groundmass hypersthene. The Fe/(Mg + Fe) ratios of these coexisting hypersthene and pigeonite are about 0.31 and 0.33, respectively. It is suggested from these results that a continuous solid solution does not exist between augite and pigeonite of the Fe/(Mg + Fe) ratio at least near 0.3 under the conditions of crystallization of groundmass of the tholeiitic andesite. It is suggested from the Mg-Fe partition and the textural relation that the groundmass augite crystallized from a liquid more iron-rich than that from which groundmass hypersthene and pigeonite crystallized.     

High-Ca,low-alkali carbonatite volcanism at Fort Portal,Uganda

A Quaternary volcanic field at Fort Portal, SW Uganda, contains approximately 50 vents that erupted only carbonatite. The vents are marked by monogenetic tuff cones defining two ENE-trending belts. Lava from a fissure at the west end of the northern belt formed a flow 0.3 km² in area and 1–5 m thick. The lava is vesicular throughout with a scoriaceous top, and probably formed by agglutination of spatter from lava fountains. Phenocrysts are olivine, clinopyroxene, phlogopite, and titanomagnetite enclosing blebs of pyrrhotite. Rims of monticellite, gehlenite, and reinhardbraunsite surround olivine, clinopyroxene and phlogopite, and magnetite is rimmed by spinel. The reaction relations suggest that these phenocryst phases are actually xenocrysts, perhaps from a source similar to that which supplied phlogopite clinopyroxenite xenoliths in the Katwe-Kikorongo volcanic field 75 km SW of Fort Portal. The groundmass of fresh carbonatite lava consists of tabular calcite, spurrite, periclase, hydroxylapatite, perovskite, spinel, pyrrhotite, and barite. The lava was readily altered; where meteoric water had access, spurrite and periclase are lacking, and some calcite is recrystallized. Vesicles in lava and rare dike rocks are partly filled with calcite, followed by jennite and thaumasite. Pyroclastic deposits cover 142 km² and are far more voluminous than lava. Carbonatite ejecta were identical to lava in primary mineralogy, but are much more contaminated by crustal rock fragments and xenocrysts. At Fort Portal, eruption of a CaO-MgO-CO₂-SiO₂-P₂O₅-SO₂-H₂O-F liquid was unaccompanied by that of a more silica-rich or alkali-rich liquid. Alkali-rich carbonatite lavas and pyroclastic deposits have been documented elsewhere in East Africa, and calcite-rich volcanic carbonatites have been attributed to replacement of magmatic alkali carbonates by calcite. However, the alkali-poor volcanic carbonatites at Fort Portal were not formed by leaching of alkalis in meteoric water; tabular calcite is not pseudomorphous after alkali carbonates such as nyerereite. The Fort Portal magma was low in alkalis at the time of eruption.     

Contributions to the mineral chemistry of Hawaiian rocks

Phenocryst and groundmass pyroxenes in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively by electron microprobe. Results and conclusions: i) Tholeiites contain augite, pigeonite, and bronzite; alkalic rocks contain salite, augite, and ferroaugite; and nephelinic rocks have salite, sometimes of Wo>50 mole %. ii) The three suites can be distinguished by Ca contents of pyroxenes: High-Ca pyroxenes of tholeiitic rocks have Wo_30–40; those of alkalic rocks have Wo_38–48; and those of the nephelinic rocks have Wo_47–51; i.e. Wo in clinopyroxene increases from tholeiitic, to alkalic, to nephelinic suites, iii). In the alkalic suite, rock types can be distinguished on the basis of clinopyroxene composition: Alkalic olivine and alkalic basalts have Wo_38–45, hawaiites and mugearites have Wo_45–48. Trachytes can be distinguished from both groups by higher Fe (Fs_22–30) and Ca contents (Wo_43–47). iv) Pyroxenes in tholeiitic rocks show higher intrarock variability (e.g. Fs₁₂Wo₄₀-Fs₃₇Wo₃₀) than those of the alkalic and nephelinic suites, v) Na₂O bulk-rock content affects Na₂O content of the precipitating high-Ca pyroxene; e.g. Na₂O in groundmass pyroxene increases from tholeiitic, to alkalic (mafic members only), to nephelinic suites; a similar relationship is present within the differentiated alkalic suite, vi) In tholeiites, changes in groundmass high-Ca pyroxene compositions are related to changes in bulk rock compositions, e.g. FeO/FeO+MgO+CaO in clinopyroxene increases as this ratio increases in the bulk rock; this is not true for alkalic and nephelinic rocks, vii) In groundmass high-Ca pyroxene, Al₂O₃, Na₂0, and TiO₂ contents increase and MnO content decreases with increasing Wo content from tholeiitic, to alkalic (mafic members only), to nephelinic suites, viii) Groundmass high-Ca pyroxenes are richer in MnO and Na₂O and poorer in Cr₂O₃ compared to coexisting phenocrysts. High-Ca pyroxene phenocrysts in nephelinic rocks and in one mugearite are depleted in SiO₂ and enriched in Al₂O₃ relative to coexisting groundmass clinopyroxene, indicating increased SiO₂ activity during crystallization. Some tholeiites show the reverse; this Si—Al relationship is not clear in other samples.     

Mineral chemistry and petrogenesis of an ultrapotassic-ultramafic volcanic rock

Compositions of the major phenocryst minerals (olivine, phlogopite) and groundmass minerals (olivine, phlogopite, kalsilite), and a glass phase have been determined from a biotite mafurite occurring as an ejected block in the highly K-rich ultramafic rocks of south west Uganda. Comparison of the phenocryst mineral compositions with those determined from recent high pressure experiments on biotite mafurite composition suggests this rock may have formed by partial melting of a K-enriched mantle source containing both H₂O and CO₂ at approximately 1,250 ° C and 30 kb. The absence of crystalline leucite but its presence as a major component of the glass phase and textural relations in the groundmass indicate that the final consolidation of the biotite mafurite took place at pressures greater than atmospheric. The presence of phlogopite, olivine, kalsilite, and glass mainly of leucite composition may suggest that consolidation took place under the conditions where these phases were in equilibrium. Based on the experimentally determined conditions for the reaction of phlogopite break down to olivine+kalsilite +liquid+vapor, a crude estimation of the consolidation conditions for ejected blocks of biotite mafurite are 1,150 °–1,180 ° C at a of 1–2 kb.     

Elevation of potassium content of basaltic magma by fractional crystallization: the effect of pressure

A wide range of natural quartz-normative liquids crystallizes olivine at low pressure. Addition of K₂O to the system results in expansion of the olivine primary phase field and replacement of pigeonite (stable in the K-free system) by hypersthene. Some variation in phase relations results from depression of crystallization temperature towards the temperature at which pigeonite reacts to form augite and hypersthene because of addition of K₂O. Another important influence on phase relations results from cation interactions in the liquid related to addition of K₂O. Studies of crystallization behavior of materials similar in most elements except K₂O show that K₂O content markedly alters crystallization behavior for more siliceous liquids but appears to have less effect on liquids with lower SiO₂ contents. Low-Ca pyroxenes melt congruently at P>5 kbar, so anhydrous liquids coprecipitate olivine, plagioclase, and two pyroxenes. Addition of K₂O to the liquid has the same effect as at 1 atm. Hypersthene replaces pigeonite as the Low-Ca pyroxene crystallization from liquids with >1.5% K₂O and the olivine primary phase field grows at the expense of those of pyroxenes and plagioclase. At 10 kbar, olivine may develop a reaction relationship with liquids containing >6% K₂O. At 15 kbar, however, liquids evolve to a pseudoeutectic involving alkali feldspar. The systematic variation in phase relations has important consequences for magmatic evolution in different environments. Dry mafic liquids at shallow levels in oceanic areas can crystallize olivine until the liquid is very evolved, resulting in extreme SiO₂-enrichment besides enrichment in K₂O, and producing potassic dacites. Olivine coexists with liquids with up to 54% SiO₂ if K₂O=0.6% (Grove and Baker 1984) but as much as 63% SiO₂ if K₂O3.5% (Ussler and Glazner 1989). Magmas rising beneath light continental crust may pond at the Moho and evolve to low-density liquids that can rise to the surface. Coprecipitation of olivine, plagioclase, augite, and a low-Ca pyroxene, produces enrichment in K₂O with only slight enrichment in SiO₂. This is terminated, at pressures of 6 to, possibly, 12 kbar, by development of a reaction relationship of olivine and liquid that progresses to higher K₂O contents with pressure. At pressures as high as 15 kbar, the reaction relation may not develop and only crystallization of alkali feldspar suppresses K₂O-enrichment. Any magmatic H₂O or crustal contamination may modify phase relations. The phase relations do, however, suggest that variation in K₂O:SiO₂ of evolved volcanic rocks is related to crustal thickness rather than to variation in the chemical compositions of primary magmas.     

Origin of biotite-hornblende-garnet coronas between oxides and plagioclase in olivine metagabbros,Adirondack region,New York

Complex multivariant reactions involving Fe-Ti oxide minerals, plagioclase and olivine have produced coronas of biotite, hornblende and garnet between ilmenite and plagioclase in Adirondack olivine metagabbros. Both the biotite (6–10% TiO₂) and the hornblende (3–6% TiO₂) are exceptionally Titanium-rich. The garnet is nearly identical in composition to the garnet in coronas around olivine in the same rocks. The coronas form in two stages:

1. Plagioclase+Fe-Ti Oxides+Olivine+water =Hornblende+Spinel+Orthopyroxene±Biotite +more-sodic Plagioclase
2. Hornblende+Orthopyroxene±Spinel+Plagioclase =Garnet+Clinopyroxene+more-sodic Plagioclase

The Orthopyroxene and part of the clinopyroxene form adjacent to olivine. Both reactions are linked by exchange of Mg²⁺ and Fe²⁺ with the reactions forming pyroxene and garnet coronas around olivine in the same rocks. The reactions occur under granulite fades metamorphic conditions, either during isobaric cooling or with increasing pressure at high temperature.     

Solidus and subsolidus compositional relationships of some coexisting Skaergaard pyroxenes

Electron-microprobe analyses of coexisting Ca-rich and Ca-poor pyroxenes from rocks of the Skaergaard intrusion indicate that their compositional relationships are controlled by two types of tie-line in the pyroxene quadrilateral. Solidus tie-lines join bulk compositions of pairs of pyroxenes that crystallized contemporaneously from a melt at equilibrium. Subsolidus tie-lines join the compositions of lamellae and host materials in pyroxene exsolution intergrowths. The solidus tie-line for a pair of pyroxenes in a specimen and their subsolidus tie-lines do not coincide and the subsolidus tie-line for inverted pigeonite is further from the hedenbergite-ferrosilite join of the quadrilateral than that for augite.The orientation of solidus tie-lines within the pyroxene quadrilateral indicates that during the simultaneous crystallization of two pyroxenes from the Skaergaard magma there was similar partitioning of Mg and Fe in the two phases relative to the melt. The relationship of the subsolidus tie-lines of a pair of coexisting pyroxenes to their solidus tie-line indicates that during the formation of exsolution intergrowths, changes in the composition of the pyroxene matrix involved primarily a change in its CaMg+Fe ratio while those of the lamellae involved both a change in their CaMg+Fe ratio and in their MgFe ratio. The MgFe ratio of the augite lamellae in inverted pigeonite progressively increased with cooling while that of the Ca-poor lamellae in augite progressively decreased with cooling.     

Fluid inclusions as petrogenetic indicators in granulite xenoliths,Pali-Aike volcanic field,Chile

Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO₂ fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO₂ during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm³. Higher density (=0.90–1.02 g/cm³) IIb inclusions occur only in symplectite phases. Secondary Type III CO₂+glass inclusions with =0.47–0.78 g/cm³ occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO₂ from the host basalt during its ascent to the surface. These data suggest that CO₂ is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO₂ from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth.     

Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459: Evidence for a two-stage cooling and a single-stage ejection history

The basaltic Martian meteorite Yamato 980459 consists of large olivine phenocrysts and often prismatic pyroxenes set into a fine-grained groundmass of smaller more Fe-rich olivine, chromite, and an interstitial residual material displaying quenching textures of dendritic olivine, chain-like augite and sulfide droplets in a glassy matrix. Yamato 980459 is, thus, the only Martian meteorite without plagioclase/maskelynite. Olivine is compositionally zoned from a Mg-rich core to a Fe-rich rim with the outer few micrometers being especially rich in iron. With Fo₈₄ the cores are the most magnesian olivines found in Martian meteorites so far. Pyroxenes are also mostly composite crystals of large orthopyroxene cores and thin Ca-rich overgrowths. Separate pigeonite and augites are rare. On basis of the mineral compositions, the cooling rates determined from crystal morphologies, and crystal grain size distributions it is deduced that the parent magma of Yamato 980459 initially cooled under near equilibrium conditions e.g., in a magma chamber allowing chromite and the Mg-rich silicates to form as cumulus phases. Fractional crystallization at higher cooling rates and a low degree of undercooling let to the formation of the Ca-, Al-, and Fe-rich overgrowths on olivine and orthopyroxene while the magma was ascending towards the Martian surface. Finally and before plagioclase and also phosphates could precipitate, the magma was very quickly erupted quenching the remaining melt to glass, dendritic silicates and sulfide droplets. The shape preferred orientation of olivine and pyroxene suggests a quick, thin outflow of lava. According to the shock effects found in the minerals of Yamato 980459, the meteorite experienced an equilibration shock pressure of about 20-25 GPa. Its near surface position allowed the ejection from the planet’s surface already by a single impact event and at relatively low shock pressures.     

Multi-stage evolution of the picritic Mælifell rocks,SW Iceland: constraints from mineralogy and inclusions of glass and fluid in olivine

The picritic Mælifell pillow lava series contains olivine macrocrysts (Fo 83.0–91.7) and microphenocrysts (Fo 86.8–88.5), resorbed Cr–Al endiopside, ± plagioclase, and microphenocrysts of Cr-spinel. The most primitive olivine cores (Fo 90–91.7) are probably derived from a peridotitic mantle. Gabbroic adcumulus xenoliths in the lavas contain plagioclase, Cr–Al endiopside and olivine (Fo 85.5–87.5) which overlap compositionally with lava minerals, ± Cr-spinel. This suggests that all pyroxene and much of the olivine ± feldspar in the lavas are xenocrysts. Olivines from the pillow lavas and from the gabbroic xenoliths contain inclusions of Cr-spinel, silicate glass and pure or nearly pure CO₂. Early (type 1) silicate melt inclusions which occur in lava-olivine only, have crystalized 0.1 to 4 vol.% daughter spinel and unknown amounts of olivine during pre-eruptive cooling. Later (type 2) glass inclusions in olivine from the lavas do not contain daughter minerals; similar type 2 inclusions also occur in the xenoliths. High-temperature microthermometry at buffered oxygen fugacity (f _{O 2}) gives a plagioclaseout temperature of about 1230°C for both types of silicate melt inclusions; this was interpreted as the liquidus temperature for type 2 inclusions. Molar volumes of undisturbed CO₂ inclusions in olivine from both lavas and xenoliths correspond to a depth of trapping of 7–10 km (2.2–3.0 kbar) at 1230°C. This is interpreted as a minimum depth to a partially molten layer near the crust/mantle boundary in the rift zone. The xenoliths are thus probably derived from a layered olivine-gabbro complex similar to those occurring in ophiolite complexes.