Fe-rich Orthopyroxene Research Articles

Pressure-induced phase transitions in Ni-bearing ferrosilite (Ni-En31Fs65)

Abstract Orthopyroxene is an abundant mineral in subducting slabs. Studying its phase transitions at high pressure is important to the understanding of mineralogy of subducting slabs in the deep Earth. Synchrotron-based single-crystal X-ray diffraction experiments were conducted on a synthetic Ni-bearing ferrosilite (Ni-En31Fs65) at pressures up to 33.8 GPa. Three phase transitions were observed at 12.1(6), 15.6(6), and 31.3(25) GPa. The first two phase transitions in Ni-En31Fs65 resemble the previously described phase transitions in Ni-free Fe-rich orthopyroxenes, i.e., the initial α-opx (Pbca) transforms to β-opx (P21/c), then the latter transforms to γ-opx (Pbca). This indicates that the incorporation of a few mol% NiSiO3 does not influence the phase transition path of Fe-rich orthopyroxene. After the third phase transition, the structure (P21ca) of Ni-En31Fs65 resembles the previously reported β-popx observed in En90 at high pressure, although the onset pressure of the phase transition in Ni-En31Fs65 is ~7 GPa lower than that in En90. β-popx has a post-pyroxene structure that contains fivefold- and sixfold-coordinated Si cations. Our results indicate that the post-pyroxene structure is β-popx (P21ca) for either Fe-poor or Fe-rich orthopyroxenes, although the phase transition path before the pyroxene → post-pyroxene is compositionally dependent. Additionally, unlike the second and third transitions, whose onset pressures are monotonously decreased by increasing Fe content, the Fe effect on shifting the first transition is much more significant for orthopyroxenes within En <50 mol% than that within En >50 mol%.

Pyroxene-bearing low- and high-HREE TTGs from the northeastern margin of the Kaapvaal Craton, southern Africa: Implications for Archean geodynamics

Archean tonalite-trondhjemite-granodiorite (TTG) rocks are generally amphibole- and/or biotite-bearing. Pyroxene-bearing granitoids of TTG affinity are less commonly reported. This study evaluates the range in composition and petrogenesis of pyroxene-bearing TTGs from a high-grade terrane along Archean cratonic margin. The Southern Marginal Zone high-grade terrane along the northeastern margin of the Kaapvaal Craton exposes prominent Mesoarchean pyroxene-bearing granitoids. The present study describes two groups of the granitoids in the Commissiedraai-Hugomond areas. The tonalite-trondhjemites with Fe-rich orthopyroxene (XMg = 0.46–0.49) dominate and occur as massive homogenous and banded varieties, the latter likely due to anatectic overprint. Enclaves of metabasites occur in both varieties. The darker layers of the banded variety are identical in mineralogy, texture and chemistry to the massive dark greenish-grey rocks. A less dominant group of clinopyroxene-rich massive tonalites occur in the Hugomond area. Clinopyroxene exhibit prominent orthopyroxene exsolution, with a pre-exsolution composition of XMg = 0.61–0.63. The spatially associated orthopyroxene-bearing and clinopyroxene-rich rocks are compositionally connected by a two-pyroxene-bearing variety with Mg-rich orthopyroxene (XMg = 0.56–0.59). No textures of pyroxene formation after earlier FeMg minerals occur in the studied rocks, whereas textures of pyroxene destabilization are indicative of late magmatic interaction with a residual melt. Pyroxenes in all rocks are primary magmatic. Biotite-quartz intergrowths and amphibole after pyroxenes are late magmatic in origin. Small garnet crystals occur locally in the orthopyroxene-bearing tonalite-trondhjemites and are related to metamorphic/anatectic overprint. The rocks are affected by late alkali metasomatism close to shear zones. Geochemical characteristics of the orthopyroxene-bearing tonalite-trondhjemites are comparable to low-HREE TTGs, while that of the clinopyroxene-rich tonalites are similar to high-HREE TTGs. The proposed petrogenetic model considers the formation of both groups of pyroxene-bearing granitoids via remelting of mafic sources in a thickened plume-related oceanic plateau. The high temperatures of melting, low water activity and oxidized conditions likely resulted in the formation of charnockitic assemblages, and is supported by phase equilibria modelling. Shallow level melting of a MORB-like source produced clinopyroxene-rich high-HREE TTG. Subsequent melting of a slightly enriched-MORB-like source occurred at relatively greater depths forming orthopyroxene-bearing TTG with low-HREE characteristics. The two-pyroxene-bearing rocks likely resulted from the mingling-mixing of the high- and low-HREE TTG end members. Implications of the results on Archean geodynamics are discussed together with a comparative exercise with similar rocks from other Archean terranes.

Norite from the Mahalapye granitoid complex, northern edge of Kaapvaal Craton: Implications for the extent of the Paleoproterozoic Bushveld LIP

The Paleoproterozoic (centred at ∼2.054 Ga) Bushveld LIP is one of the largest intracratonic LIPs in the world. Its components are primarily distributed within the Archean Kaapvaal Craton. This study reports dyke-like and sill-like norite intrusions from Lose quarry within the Mahalapye Complex, a Paleoproterozoic granitoid-migmatitic gneiss terrane along the northern edge of Kaapvaal Craton. Field relations indicate the norite postdate the ∼2.061 Ga diorite gneiss and predate the ∼2.039 Ga granodiorite. Significantly, it exhibit structures indicative of comingling with ∼2.053 Ga leucosome/leucogranite, a product of partial melting of diorite gneiss. The cross-cutting relations provide unequivocal evidence that the norite and the leucogranite were liquid at the same time, and should be of similar age. Fe-rich orthopyroxene (XMg = 0.43–0.46) constitutes the dominant mafic mineral in the norite. Geochemical characteristics of the Mahalapye Complex norites are similar to those of marginal facies norites from the Bushveld Complex (B1 sills) and Molopo Farms Complex layered intrusions, two of the prominent constituents of the Bushveld LIP. This is the first solid evidence that the original extent of the Bushveld LIP covered an area larger than the current known limits of the Archean Kaapvaal Craton – into the youngest and westernmost terrane of the high-grade Limpopo Complex. The potential of high-grade terranes along Archean cratonic margins as locales to look for LIP fragments is explored.

An experimental study of $$ ^{{{\text{Fe}}^{2 + } {-}{\text{Mg}}}} K_{\text{D}} $$ between orthopyroxene and rhyolite: a strong dependence on H2O in the melt

The effect of temperature, pressure, and dissolved H2O in the melt on the Fe2+–Mg exchange coefficient between orthopyroxene and rhyolite melt was investigated with a series of H2O fluid-saturated phase-equilibrium experiments. Experiments were conducted in a rapid-quench cold-seal pressure vessel over a temperature and pressure range of 785–850 °C and 80–185 MPa, respectively. Oxygen fugacity was buffered with the solid Ni–NiO assemblage in a double-capsule assembly. These experiments, when combined with H2O-undersaturated experiments in the literature, show that $$ ^{{{\text{Fe}}^{2 + } {-}{\text{Mg}}}} K_{\text{D}} $$ between orthopyroxene and rhyolite liquid increases strongly (from 0.23 to 0.54) as a function of dissolved water in the melt (from 2.7 to 5.6 wt%). There is no detectable effect of temperature or pressure over an interval of 65 °C and 100 MPa, respectively, on the Fe2+–Mg exchange coefficient values. The data show that Fe-rich orthopyroxene is favored at high water contents, whereas Mg-rich orthopyroxene crystallizes at low water contents. It is proposed that the effect of dissolved water in the melt on the composition of orthopyroxene is analogous to its effect on the composition of plagioclase. In the latter case, dissolved hydroxyl groups preferentially complex with Na+ relative to Ca2+, which reduces the activity of the albite component, leading to a more anorthite-rich (calcic) plagioclase. Similarly, it is proposed that dissolved hydroxyl groups preferentially complex with Mg2+ relative to Fe2+, thus lowering the activity of the enstatite component, leading to a more Fe-rich orthopyroxene at high water contents in the melt. The experimental results presented in this study show that reversely zoned pyroxene (i.e., Mg-rich rims) in silicic magmas may be a result of H2O degassing and not necessarily the result of mixing with a more mafic magma.

Phase transitions in orthopyroxene (En90) to 49 GPa from single-crystal X-ray diffraction

Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on ∼Mg0.9Fe0.1SiO3 (En90) orthopyroxene crystals at room temperature to a maximum pressure of 48.5GPa. The sample was compressed in a diamond anvil cell with a neon pressure-transmitting medium and a gold pressure calibrant. In addition to the previously described orthopyroxene to β-opx transition (designated HPCEN2 in previous studies), we observe two further phase transitions at 29.9GPa and 40.3GPa. However, we do not observe the γ-opx phase recently described in an Fe-rich orthopyroxene composition. The structures of both of the new phases were solved in space group Pca21. While their Mg–O layers remain pyroxene-like, their Si–O layers transform in a stepwise fashion to akimotoite-like sheets, with sites in 4-, 4+1-, or 6-fold coordination, depending on the specific structure and layer. Due to the increased Si–O coordination number, we designate the new structures α- and β-post-orthopyroxene (α-popx and β-popx). α-popx has one Si–O layer that is entirely tetrahedral, and one layer that contains both tetrahedra and 4+1-coordinated Si in distorted square pyramids. β-popx retains the mixed 4- and 4+1-coordinated Si layer found in α-popx, while the other Si layer adopts fully octahedral coordination. The α- and β-popx structures show a progressive transformation towards the arrangement of Si layers found in akimotoite, a potentially important phase in the earth’s transition zone. Metastable transformations in pyroxenes are of interest for understanding possible metastability in geological environments such as subducting slabs and meteorite impacts.

Crystal-poor, multiply saturated rhyolites (obsidians) from the Cascade and Mexican arcs: evidence of degassing-induced crystallization of phenocrysts

A detailed petrological study is presented for six phenocryst-poor obsidian samples (73–75 wt% SiO2) erupted as small volume, monogenetic domes in the Mexican and Cascade arcs. Despite low phenocryst (+microphenocryst) abundances (2–6 %), these rhyolites are each multiply saturated with five to eight mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± zircon ± hornblende ± clinopyroxene ± sanidine ± pyrrhotite). Plagioclase and orthopyroxene phenocrysts (identified using phase-equilibrium constraints) span ≤30 mol % An and ≤15 % Mg#, respectively. Eruptive temperatures (±1σ), on the basis of Fe–Ti two oxide thermometry, range from 779 (±25) to 940 (±18) °C. Oxygen fugacities (±1σ) range from −0.4 to 1.4 (±0.1) log units relative to those along the Ni–NiO buffer. With temperature known, the plagioclase-liquid hygrometer was applied; maximum water concentrations calculated for the most calcic plagioclase phenocryst in each sample range from 2.6 to 6.5 wt%. This requires that the rhyolites were fluid-saturated at depths ≥2–7 km. It is proposed that the wide compositional range in plagioclase and orthopyroxene phenocrysts, despite their low abundance, can be attributed to changing melt water concentrations owing to degassing during magma ascent. Phase-equilibrium experiments from the literature show that higher dissolved water concentrations lead to more Fe-rich orthopyroxene, as well as more calcic plagioclase. Loss of dissolved water leads to a progressive increase in melt viscosity, and phenocrysts often display diffusion-limited growth textures (e.g., dendritic and vermiform), consistent with large undercoolings caused by degassing. A kinetic barrier to microlite crystallization occurred at viscosities from 4.5 to 5.0 log10 Pa s for these rhyolites, presumably because the rate at which melt viscosity changed was high owing to rapid loss of dissolved water during magma ascent.

Thermal equation of state of synthetic orthoferrosilite at lunar pressures and temperatures

Iron-rich orthopyroxene plays an important role in models of the thermal and magmatic evolution of the Moon, but its density at high pressure and high temperature is not well-constrained. We present in situ measurements of the unit-cell volume of a synthetic polycrystalline end-member orthoferrosilite (FeSiO3, fs) at simultaneous high pressures (3.4–4.8 GPa) and high temperatures (1,148–1,448 K), to improve constraints on the density of orthopyroxene in the lunar interior. Unit-cell volumes were determined through in situ energy-dispersive synchrotron X-ray diffraction in a multi-anvil press, using MgO as a pressure marker. Our volume data were fitted to a high-temperature Birch–Murnaghan equation of state (EoS). Experimental data are reproduced accurately, with a \(\varDelta P\) standard deviation of 0.20 GPa. The resulting thermoelastic parameters of fs are: V0 = 875.8 ± 1.4 A3, K0 = 74.4 ± 5.3 GPa, and \(\frac{{\text d}K}{{\text d}T} = -0.032 \pm 0.005\,\hbox{GPa K}^{-1}\), assuming \({K}^{\prime}_{0} = 10 \). We also determined the thermal equation of state of a natural Fe-rich orthopyroxene from Hidra (Norway) to assess the effect of magnesium on the EoS of iron-rich orthopyroxene. Comparison between our two data sets and literature studies shows good agreement for room-temperature, room-pressure unit-cell volumes. Preliminary thermodynamic analyses of orthoferrosilite, FeSiO3, and orthopyroxene solid solutions, (Mg1−xFex) SiO3, using vibrational models show that our volume measurements in pressure–temperature space are consistent with previous heat capacity and one-bar volume–temperature measurements. The isothermal bulk modulus at ambient conditions derived from our measurements is smaller than values presented in the literature. This new simultaneous high-pressure, high-temperature data are specifically useful for calculations of the orthopyroxene density in the Moon.

Temporal Evolution of a High-K Andesitic Magmatic System: Taranaki Volcano, New Zealand

Taranaki (Mt. Egmont) in the western North Island of New Zealand is a high-K andesite volcano with an eruptive history extending over more than 200 kyr. In general, petrological research has concentrated on the post-10 ka record of the modern edifice. This study focuses on the earlier history, which is recorded in 11 major pre-7 ka debris avalanche deposits. Each of these formed as a result of a catastrophic collapse of the edifice of the time. The clast assemblages of these deposits provide insights into the chemical compositions of magmas erupted during the earlier stages of activity of the volcano and form the basis for a new chemo-stratigraphic analysis of the pre-10 ka volcanic succession. Sample suites from the studied debris avalanche deposits show a progressive enrichment in K2O and large ion lithophile elements (LILE), reflecting a gradual evolution to high-K andesite. The early magmatic system (pre-100 ka) produced a wide range of compositions including relatively primitive basalts and basaltic andesites.These rocks contain phenocryst assemblages that indicate crystallization within the lower crust or mantle, including a broad range of clinopyroxene compositions, high-Al2O3 hornblende, olivine and phlogopite. A higher proportion of high-silica compositions in the younger sample suites and the appearance of late-stage, low-pressure mineral phases, such as high-TiO2 hornblende, biotite and Fe-rich orthopyroxene, reflect a gradual shift to more evolved magmas with time.These new data are interpreted to reflect a multi-stage origin for Taranaki andesites. Parental magmas were generated within a lower crustal ‘hot zone’, which formed as a result of repeated intrusions of primitive melts into the lower crust. The geochemical and mineralogical evidence indicates that prior to 100 ka this zone was relatively thin and cold, so that primitive magmas were able to rise rapidly through the crust without significant interaction and modification. As the hot zone evolved, larger proportions of intruded and underplated mafic material were partially remelted, and interaction of these melts with fractionating mantle-derived magmas generated progressively more K- and LILE-enriched compositions. A complex and dispersed magma assembly and storage system developed in the upper crust where the hot-zone melts were further modified by fractional crystallization and magma mixing and mingling.

Complex Phenocryst Textures and Zoning Patterns in Andesites and Dacites: Evidence of Degassing-Induced Rapid Crystallization?

A detailed petrological study is presented of seven phenocryst-poor andesites and dacites erupted from short-lived, monogenetic, periph- eral vents in the Mexican volcanic arc. Despite low phenocryst (þ microphenocryst) abundances (55%), these magmas were each multiply saturated with five to seven mineral phases (plagio- clase þ orthopyroxene þ titanomagnetite þ apatite þ ilmeniteclino- pyroxenehornblendebiotite). Groundmass textures range from nearly complete glass to nearly complete microlite crystallization. Eruptive temperatures, on the basis of Fe^Ti two-oxide thermom- etry, range from 872 (� 13) to 1085 (� 16) 8C for the andesites and 760 (� 18) to 957 (� 19)8C for the dacites. Corresponding oxygen fugacities range from � 0· 9t oþ0·6 log units relative to the Ni^NiO buffer.With temperature known, the plagioclase^liquid hy- grometer was applied to the sparse plagioclase phenocrysts in each sample; the highest calculated water concentrations range from 6·4 to 2· 8w t %H 2O for the andesites and from 8· 3t o 4·1wt % H2O for the dacites. These results require that the magmas were fluid-saturated at depths 4 9a nd412 km, respectively, for the most hydrous andesite and dacite. The plagioclase population in each sample spans a wide range in composition (� 34 mol % An), which can be attributed to the effects of changing melt water concen- tration owing to degassing during magma ascent.The orthopyroxene population in each sample also spans a wide range in composition (� 17% Mg-number), and it is proposed on the basis of phase equi- librium experiments from the literature and data from the JANAF thermochemical tables that this also reflects variable melt water con- centrations, with higher water leading to more Fe-rich orthopyroxene. Degassing leads to a progressive increase in melt viscosity, and the phenocrysts crystallized over an interval of � 2·4^4· 3l og 10 Pa s in these samples. The phenocrysts often display diffusion-limited growth textures (e.g. skeletal and hopper crystals, large interior melt hollows) consistent with large undercoolings caused by degassing, which has produced a complicated petrography in these crystal-poor hydrous magmas, including reverse and patchy zoning in plagioclase and pyroxene.

The Petrology and Geochemistry of Calc-Alkaline Andesites on Shodo-Shima Island, SW Japan

Petrographical and geochemical characteristics of calc-alkaline McLennan, 1995) that are produced at convergent plate boundaries and are characterized by lower FeO∗/MgO andesites on Shodo-Shima Island, SW Japan, having bulk comratios than tholeiitic andesites (Miyashiro, 1974). An positions largely identical to the continental crust, are presented. understanding of the origin of such orogenic, calc-alkaline The following petrographic observations suggest a role for magma andesites is thus essential for comprehending not only mixing in producing such andesite magmas: (1) two types of olivine the magma flux in the modern Earth but also the role phenocrysts and spinel inclusions, one with compositions identical of subduction-related magmatism in the evolution of the to those in high-Mg andesites and the other identical to those in solid Earth. Since the pioneering work of Eichelberger basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe (1975, 1978), Anderson (1976), Sakuyama (1979, 1981, relations, respectively; (2) the presence of orthopyroxene phenocrysts 1984), and Luhr & Carmichael (1980), the majority of with mg-number >90 suggests the contribution of an orthopyroxenepetrologists have considered mixing of mafic and felsic bearing high-Mg andesite magma to production of calc-alkaline magmas to be one of the major mechanisms of generation andesites; (3) reversely zoned pyroxene phenocrysts may not be in of calc-alkaline andesites. Continental crust formation, equilibrium with Mg-rich olivine, suggesting the involvement of a however, may not be elucidated solely by such magma differentiated andesite magma as an endmember component; (4) the mixing processes. The reasons for this are twofold. First, presence of very Fe-rich orthopyroxene phenocrysts indicates the calc-alkaline andesite magmatism typifies continental arcs association of an orthopyroxene-bearing rhyolitic magma. Conat least on the modern Earth, suggesting that the contributions from the above at least five endmember magmas to the calctinental crust itself plays an important role in such andesite alkaline andesite genesis can also provide a reasonable explanation of formation, possibly as the source of the felsic endmember the Pb–Sr–Nd isotope compositions of such andesites. component. Second, there is a slight but significant difference in MgO contents between the bulk continental crust and the typical calc-alkaline andesite (e.g. Kelemen,

Na-Poor Ferro-Augites and Ferro-Hedenbergites from Ferrosilite-Bearing Ferrosyenites of the Prakasam District in Andhra Pradesh, India: Further Evidences for a Tholeiitic Parentage

Abstract The clinopyroxenes from the ferrosyenites in the Prakasam district of Andhra Pradesh are characterized by their subcalcic nature, enrichment in Fe with concomitant impoverishment in Mg, and low amounts of (calculated) Fe3+, Na, Al and Ti - features which specifically typify their subalkaline (tholeiitic) parentage. The acmite component is insignificantly low in the analyzed clino-pyroxenes (Na3.60-5.74 Mg10.01-27.09 Fe67.17-85.16) and bears no relationship whatsoever either with the hedenbergite component or with the calculated Fe3+ content. The results of the Fe3+ calculations were suspect, especially when the mineral analyses show low SiO2, low totals, Si+Al<2, and total cations more than 4 per formula unit of 6 oxygens. These strongly coloured green clinopyroxenes, commonly mistaken as sodium-rich acmitic pyroxenes, are indeed sodium-poorferro-augites and ferro-hedenbergites. The high Fe/Mg ratios in the clinopyroxenes (2.4-8.2; Ca43.3-45.6Mg5.8-16.0Fe39.6-50.5), coupled with the extremely high Fe/Mg ratios in the coexisting olivines (35.5-61.8; Fa97.3-98.5) and orthopyroxenes (3.6-11.8; FS78.2-91.3) are an indication of the very highly fractionated nature of the magma. Clinopyroxenes of comparable chemical characteristics are reported from ferrosyenitic rocks elsewhere in the world, which in contrast to these of the Prakasam district, are not cospatial with (feldspathoidal) alkaline rocks. The occurrence of ferrosyenites, containing Fe-rich orthopyroxene (eulite-ferrosilite) and inverted pigeonite, in the vicinity of alkaline rocks in the Prakasam district, is interpreted as purely fortuitous and merely coincidental, not having any genetic or causative relationship.

An experimental study of Fe-Mg partitioning between olivine and orthopyroxene at 1173, 1273 and 1423 K and 1.6 GPa

The partitioning of Mg and Fe2+ between coexisting olivines and orthopyroxenes in the system MgO-FeO-SiO2 has been investigated experimentally at 1173, 1273, 1423 K and 1.6 GPa over the whole range of Mg/Fe ratios. The use of barium borosilicate as a flux to promote grain growth, and the identification by back-scattered electron imaging of resulting growth rims suitable for analysis by electron microprobe, results in coexisting olivine and orthopyroxenene compositions determined to a precision of±0.003 to 0.004 in molar Fe/(Mg+Fe). Quasi-reversal experiments were performed starting with Mg-rich olivine and Fe-rich orthopyroxene (low KD) and vice versa (high KD), which produced indistinguishable results. The distribution coefficient, KD, depends on composition and on temperature, but near Fe/(Mg+Fe)=0.1 (i.e. mantle compositions) these effects cancel out, and KD is insensitive to temperature. The results agree well with previous experimental investigations, and constrain the thermodynamic mixing properties of Mg-Fe olivine solid solutions to show small near-symmetric deviations from ideality, with \(W_{G_{Mg - Fe}^{ol} }\) between 2000 and 8000 J/mol. Multiple non-linear least squares regression of all data gave a best fit with \(W_{G_{Mg - Fe}^{ol} } = 5625 \pm 574 J/mol\) (implying 5450 J/mol at 1 bar) and \(W_{G_{Mg - Fe}^{opx} } = 2145 \pm 614 J/mol\), but the two WG parameters are so highly correlated with each other that our data are almost equally well fit with \(W_{G_{Mg - Fe}^{ol} } = 3700 \pm 800 J/mol\), as obtained by Wiser and Wood. This value implies \(W_{G_{Mg - Fe}^{opx} } = 280 \pm 900 J/mol\), apparently independent of temperature. Our experimental results are not compatible with the assessment of olivine-orthopyroxene equilibria of Sack and Ghiorso.

Mineralogical relations and magma mixing in calc-alkaline andesites from lake Atitl�n, Guatemala

Quaternary calc-alkaline andesites erupted form three neighboring volcanoes along the Guatemalan volcanic front have mineralogic compositions and textures which show varying degrees of disequilibrium. Basaltic andesites and andesites (SiO2 % = 50−59), erupted from Atitlan volcano located nearer to the trench, have the lowest degree of disequilibrium. These lavas contain an anhydrous phenocryst assemblage of mildly bimodal plagioclase, olivine, augite opx, and magnetite. Orthopyroxene occurs at the expense of olivine with increasing whole rock SiO2. Most pyroxene phenocrysts show a trend of Fe enrichment. Andesites from Toliman (SiO2% = 53−62) and San Pedro (Si02% = 54−67) volcanoes, located further away from the trench, show comparatively high and moderate degrees of disequilibrium, respectively. Toliman andesites have bimodal plagioclase compositions and textures. Olivine persists with increasing whole rock Si02 and lacks clear modal relations with coexisting orthopyroxene and hornblende phenocrysts. When compared to Atitlan andesites, Toliman olivines are more forsteritic and pyroxenes contain higher proportions of Mg-rich rims, though normal zoned phenocrysts occur within the same rock. Toliman andesite also have lower proportions of phenocrysts to microphenocrysts, more calcic plagioclase groundmass compositions, and higher modal phenocrystic magnetite. San Pedro andesites have disequilibrium assemblages similar to Toliman andesites but are not as striking. Magma mixing is proposed as the dominant cause for observed disequilibrium. Disequilibrium features are preserved best in Toliman and San Pedro andesites because inferred durations between mixing and eruption are shortest, and consequently, these mixed andesites more clearly record mafic and silicic endmember compositions. The mafic component is a relatively high temperature, high-Al basalt containing phenocrysts of Mg-rich olivine (Fo = 78−80), calcic plagioclase (An 70−80), augite and titanomagnetite. The silicic component contains quartz, sodic plagioclase (An 40−50), Fe-rich orthopyroxene and titanomagnetite. Short durations between mixing and eruption produce petrographic features which, in part, mimic the effects of increasing PH20 in a fractionating magma. Inferred mixing durations for Atitlann andesites are longer and involve a less-silicic composition. The intervolcano disequilibrium relations suggest that as Si02 in a silicic endmember increases, the duration and efficiency of mixing decreases.

Petrology of Medicine Lake Highland volcanics: Characterization of endmembers of magma mixing

Lavas from Medicine Lake volcano, Northern California have been examined for evidence of magma mixing. Mixing of magmas has produced basaltic andesite, andesite, dacite and rhyolite lavas at the volcano. We are able to identify the compositional characteristics of the components that were mixed and to estimate the time lag between the mixing event and eruption of the mixed magma. Compositional data from pairs of phenocrysts identify a high alumina basalt (HAB) and a silicic rhyolite as endmembers of mixing. Mg-rich olivine or augite and Ca-rich plagioclase are associated with the HAB component, and Fe-rich orthopyroxene and Na-rich plagioclase are associated with the rhyolitic component. Some lavas contain multiple phenocryst assemblages suggesting the incorporation of several magmas intermediate between the HAB and silicic components. Glass inclusions trapped in Mg-rich olivine and Na-rich plagioclase are similar in composition to the proposed HAB and rhyolite end members and provide supportive evidence for mixing. Textural criteria are also consistent with magma mixing. Thermal curvature of the liquidus surfaces in the basalt-andesite-rhyolite system allows magmas produced by mixing to be either supercooled or superheated. Intergranular textures of basaltic andesites and andesites result from cooling initiated below the liquidus. The trachytic textures of silicic andesites form from cooling initiated above the liquidus. Reversed compositional zoning profiles in olivine crystals were produced by the mixing event, and the homogenization of the compositional zoning has been used to estimate the time interval between magma mixing and eruption. Time estimates are on the order of 80 to 90 h, suggesting that the mixing event triggered eruption.

An origin of small-scale fluctuations in orthopyroxene composition in the lower and critical zones of the Bushveld Complex, South Africa

Numerous, small and apparently haphazard reversals in Mg/Fe ratio of orthopyroxene grains have been reported by E.N. Cameron in the lower and lower critical zones of the Bushveld Complex. A hypothesis for their origin is presented by extending the suggestion of Cameron that modal variation in layered complexes may be attributed to pressure fluctuation. An increase in pressure will cause the magma to be supercooled; thus speeding up crystallization rates. This results in a greater proportion of interstitial liquid being trapped in the cumulate pile. On crystallization, this liquid adds relatively Fe-rich orthopyroxene to the cumulus of Mg-rich phase. Homogenization produces minerals whose compositions are displaced from the cumulus composition towards the low-temperature end-member by an amount proportional to the percent of interstitial liquid. Decreases in pressure produce the opposite effect, and may explain the origin of pure adcumulate rocks. Quantitative data are presented from the lower zone-critical zone contact and from the Duluth Complex to demonstrate this effect.