Basalt Tetrahedron Research Articles

High-pressure melting experiments on garnet clinopyroxenite and the alkalic to tholeiitic transition in ocean-island basalts

It has been suggested, on the basis of recent high-pressure melting experiments, that high-Mg garnet clinopyroxenite is the most important lithology controlling the major-element budget of ocean-island lavas [Geology 31 (2003) 481–484]. To clarify these claims and further our understanding of the petrogenesis of ocean-island basaltic (OIB) lavas, we present the results of a high-pressure (2.0–2.5 GPa) melting study on a high-Mg garnet-clinopyroxenite mantle nodule (77SL-582) from Hawaii. Major-element compositions of the partial melts as a function of pressure ( P), temperature ( T), and degree of melting ( F), mineral chemistry of the coexisting crystalline phases, and the solidus/liquidus brackets of this particular garnet clinopyroxenite are reported. The solidus of 77SL-582, which resembles a tholeiitic picrite in terms of its bulk composition, is bracketed at 1295±15 and 1335±15 °C at 2.0 and 2.5 GPa, respectively, which is ∼60–70 °C lower than the solidus of mantle lherzolite at identical pressures [Geochem. Geophys. Geosystems 1 (2000) 2000GC000070]. The solidus of 77SL-582 is also lower by ∼30–40 °C than reported for a slightly alkalic, high-Mg garnet clinopyroxenite [Geology 31 (2003) 481]. At both pressures, the moderate degree (∼18–20%) partial melts of 77SL-582 are strongly alkalic with ∼8–12 wt.% nepheline in the norm. Even at a degree of melting as high as ∼60%, moderately alkalic basaltic liquids are produced. With further melting, however, partial melts become hypersthene-normative. In the CaO–MgO–Al 2O 3–SiO 2 (CMAS) system, the eclogite surface is restricted to the tholeiitic part of the basalt tetrahedron [D.C. Presnall, Effect of pressure on fractional crystallization of basaltic magma, in: Y. Fei, C. Bertka, and B. Mysen (eds.) Mantle Petrology: Field observations and High Pressure Experimentation: A Tribute to Francis (Joe) R. Boyd, The Geochemical Society Special Publication no. 6, 1999, pp. 209–224.]. A comparison with high-pressure melting experiments in the CMAS system at 2.0–3.0 GPa indicates that the alkalic to tholeiitic transition in our experiments can be explained by a rapid expansion of the eclogite “surface” from the tholeiitic part to well into the alkalic part of the basalt tetrahedron in natural systems. Importantly, the partial melting trends of 77SL-582 are transverse to those seen in ocean-island basalts. In addition, although the alkalic to tholeiitic basalt transition observed in ocean-island basalts is well reproduced in our experiments, there is very little overlap between the partial melts of 77SL-582 and ocean-island basalts. It appears that most of the major-element systematics of the ocean-island basalts considered here can be explained by combined contributions of melts from anhydrous and carbonated lherzolite [Geophys. Res. Lett. 24 (1997) 2837; G.H. Gudfinnsson, D.C. Presnall, Continuous gradations among primary kimberlitic, carbonatitic, melilititic, and komatiitic melts in equilibrium with garnet lherzolite at 3–8 GPa, Proc. 8th Int. Kimber. Conf. Extended abstracts.] at high pressures with possible contributions from melts from garnet clinopyroxenite of bulk composition similar to 77SL-582.

Melting Behaviour of Model Lherzolite in the System CaO–MgO–Al2O3–SiO2–FeO at 0·7–2·8 GPa

We have determined the isobarically univariant melting relations of the lherzolite phase assemblage in the CMASF system in the only at small degrees of melting. 0·7–2·8 GPa pressure range. Isobarically, for every 1 wt % increase in the FeO content of the melt in equilibrium with the lherzolite phase assemblage, the equilibrium temperature is lower KEY WORDS: CMASF; lherzolite solidus; mantle melting by about 3–5°C. Relative to the solidus of model lherzolite in the CaO–MgO–Al2O3–SiO2 system, melt compositions in the CMASF system are displaced slightly towards the alkalic side of the basalt tetrahedron. The transition on the solidus from spinel to plagioclase

Melting of Refractory Mantle at 1middle dot5, 2 and 2middle dot5 GPa under Anhydrous and H2O-undersaturated Conditions: Implications for the Petrogenesis of High-Ca Boninites and the Influence of Subduction Components on Mantle Melting

Boninites are an important ‘end-member’ supra-subduction zone magmatic suite as they have the highest H2O contents and require the most refractory of mantle wedge sources. The pressure–temperature conditions of boninite origins in the mantle wedge are important to understanding subduction zone initiation and subsequent evolution. Reaction experiments at 1·5 GPa (1350–1530°C), 2 GPa (1400–1600°C) and 2·5 GPa (1450–1530°C) between a model primary high-Ca boninite magma composition and a refractory harzburgite under anhydrous and H2O-undersaturated conditions (2–3 wt % H2O in the melt) have been completed. The boninite composition was modelled on melt inclusions occurring in the most magnesian olivine phenocrysts in high-Ca boninites from the Northern Tongan forearc and the Upper Pillow Lavas of the Troodos ophiolite. Direct melting experiments on a model refractory lherzolite and a harzburgite composition at 1·5 GPa under anhydrous conditions (1400–1600°C) have also been completed. Experiments establish a P, T ‘melting grid’ for refractory harzburgite at 1·5, 2 and 2·5 GPa and in the presence of 2–3 wt % H2O. The effect of 2–3 wt % dissolved H2O produces a liquidus depression in primary boninite of ∼112 ± 19°C at a given temperature. The H2O-bearing melts, recalculated to 100 wt % anhydrous, are ∼2–6 wt % higher in MgO, ∼1–2 wt % higher in SiO2 and ∼1–1·5 wt % lower in FeO, compared with nominally anhydrous melts at the same P and T. These differences are consistent with a change in the melting reaction, resulting in a higher contribution of orthopyroxene to the melt phase, compared with anhydrous conditions. We conclude that high-Ca boninite petrogenesis requires temperatures as high as ∼1480°C at depths of ∼45 km in the mantle wedge; these are constraints for any proposed model of intra-oceanic subduction zones. A comparison of the results from the boninite–harzburgite reaction experiments with the direct melting experiments on refractory lherzolite and harzburgite indicates that the influence of subduction components (included in the composition of the added model boninite) is to cause high-pressure melting cotectics to move towards the olivine apex (i.e. to relatively higher pressures) of the molecular normative projection from diopside onto the base of the ‘basalt tetrahedron’ [Jd + CaTs + Lc–Qz–Ol] compared with anhydrous melting of normal mantle in the absence of a subduction component. The subduction component involved in high-Ca boninite petrogenesis in addition to H2O has relatively high Al2O3 and Na2O contents. The experimental data from this and other studies empirically quantify the absolute effect of dissolved H2O (0·2–21 wt %) on the liquidus depression of olivine-saturated basaltic melts with ∼<3 wt % total alkalis as follows: \[\ olivine\ liquidus\ depression\ ({^\circ}C)\ =\ 74.403\ {\times}\ (H_{2}O\ wt\ \%)^{0{\cdot}352}.\ \] The equation that describes this empirical relationship is non-linear with an error of ∼9 relative percent.

Liquidus Phase Relations in the CaO-MgO-Al2O3-SiO2 System at 3.0 GPa: the Aluminous Pyroxene Thermal Divide and High-Pressure Fractionation of Picritic and Komatiitic Magmas

We present liquidus phase equilibrium data at 3.0 GPa for the model tholeiitic basalt tetrahedron, diopside–anorthite–fosterite–quartz, in the CaO–MgO–Al2O3–SiO2 system. This pressure coincides with the invariant point (1568°C) on the simplified model lherzolite solidus that marks the transition between spinel lherzolite and garnet lherzolite (fo + en + di + sp + gt + liq). The composition of the liquid at the invariant point (46.4 An, 16.0 Di, 33.5 Fo, 4.2 Qz, wt %) is a model olivine-rich basalt that lies slightly (0.2% excess fo) to the SiO2-poor side of the aluminous pyroxene plane, MgSiO3–CaSiO3–Al2O3. A large garnet primary phase volume is bordered by primary phase volumes for forsterite, spinel, sapphirine, corundum, enstatite, diopside, quartz, and kyanite. The observed absence of enstatite at the solidus of lherzolite at pressures above ∼3.3 GPa is readily understood from the phase relations in this system. During melting at these high pressures, enstatite first forms at a temperature somewhat above the solidus and then dissolves before complete melting. As pressure increases above 3.0 GPa, the aluminous pyroxene plane, MgSiO3–CaSiO3–Al2O3, becomes increasingly effective as a thermal divide that causes picritic and komatiitic melts lying on the silica-poor side of the plane to fractionate toward alkalic picritic compositions. However, if the rate of ascent of these melts is sufficiently rapid, expansion of the olivine primary phase volume as pressure decreases produces a fractionation trend dominated by olivine crystallization and the thermal divide is ignored.

Liquidus Phase Relations in the CaO-MgO-Al2O3-SiO2 System at 3&amp;middot;0 GPa: the Aluminous Pyroxene Thermal Divide and High-pressure Fractionation of Picritic and Komatiitic Magmas

We present liquidus phase equilibrium data at 3.0 GPa for the model tholeiitic basalt tetrahedron, diopside–anorthite–fosterite–quartz, in the CaO–MgO–Al2O3–SiO2 system. This pressure coincides with the invariant point (1568°C) on the simplified model lherzolite solidus that marks the transition between spinel lherzolite and garnet lherzolite (fo + en + di + sp + gt + liq). The composition of the liquid at the invariant point (46.4 An, 16.0 Di, 33.5 Fo, 4.2 Qz, wt %) is a model olivine-rich basalt that lies slightly (0.2% excess fo) to the SiO2-poor side of the aluminous pyroxene plane, MgSiO3–CaSiO3–Al2O3. A large garnet primary phase volume is bordered by primary phase volumes for forsterite, spinel, sapphirine, corundum, enstatite, diopside, quartz, and kyanite. The observed absence of enstatite at the solidus of lherzolite at pressures above ∼3.3 GPa is readily understood from the phase relations in this system. During melting at these high pressures, enstatite first forms at a temperature somewhat above the solidus and then dissolves before complete melting. As pressure increases above 3.0 GPa, the aluminous pyroxene plane, MgSiO3–CaSiO3–Al2O3, becomes increasingly effective as a thermal divide that causes picritic and komatiitic melts lying on the silica-poor side of the plane to fractionate toward alkalic picritic compositions. However, if the rate of ascent of these melts is sufficiently rapid, expansion of the olivine primary phase volume as pressure decreases produces a fractionation trend dominated by olivine crystallization and the thermal divide is ignored.

Origin of the continental crust in the Colorado Plateau: Geochemical evidence from mafic xenoliths from the Navajo Volcanic Field, southwestern USA

Mafic xenoliths from the Navajo Volcanic Field (NVF) in the Colorado Plateau are represented by four lithologies, in order of decreasing abundance: garnet granulite, amphibolite, gabbro, and pyroxene granulite, Eight geochemical groups are defined based chiefly on REE (rare earth element) and HFSE (high field strength element) distributions. Mg number exhibits a broad negative correlation with SiO 2, and most xenoliths plot in the hy-di-ol-plg volume of the CIPW normative basalt tetrahedron. REE distributions include groups with LREE enrichment (Gps I, IV, VII), LREE depletion (Gp II), HREE depletion (Gps III, V), flat REE patterns (Gp VI), and u-shaped patterns (Gp VIII). With the exception of Groups VI and VII, all of the NVF mafic xenoliths show negative Ta Nb anomalies on primitive-mantle-normalized diagrams; also, all groups show negative Th anomalies, and all but Gp IV show positive Sr anomalies. Thorium and Sr anomalies appear to reflect introduction of Rb, Ba, Sr, and K during xenolith alteration. Nickel changes rapidly in concentration with only a small change in Zr, suggesting the rocks are dominantly mafic cumulates. Geochemical results also indicate that the mafic xenoliths are not genetically related to the host minettes. Associated spinel lherzolite xenoliths are depleted in incompatible elements, and cannot serve as sources for any of the mafic xenoliths, but they may represent restite left during extraction of the xenolith protolith liquids. Garnet lherzolite xenoliths, which are enriched in incompatible elements, could have served as sources for the LREE-enriched mafic xenolith groups. If there is no sampling bias in the NVF mafic xenoliths, mafic xenolith Groups IV, V, VI, and VII dominate in the lower crust of this region. A component of enriched lower crust, as represented by xenolith Group IV and by xenoliths from the San Francisco Volcanic Field, occurs only northwest of a boundary extending from near Flagstaff, Arizona, into northwestern New Mexico and may be coincident with a buried Early Proterozoic crustal boundary. Published geochemical and Nd, Sr, and Pb isotopic data from Proterozoic surface rocks and from NVF mafic xenoliths indicate that the crust and mantle lithosphere beneath the Colorado Plateau is dominantly Early proterozoic (2.0–1.75 Ga) in age. With the possible exception of gabbros from Moses Rock, there is no evidence among the mafic crustal xenoliths for later magmatic underplating. Incompatible element distributions are consistent with an origin for the crust by Early Proterozic arc accretion, and they do not reflect a contribution from submarine plateaus. Also, a negative Eu anomaly must exist in both the upper and lower crust in this region.

Forsterite-diopside-spinel-liquid equilibria in the system CaO?MgO?Al2O3?SiO2 at 20 kbar and petrological applications

Liquidus phase relationships in the CaAlAl−SiO6−Mg2SiO4−CaMgSi2O6−CaAlSi2O8 portion of the simplified basalt tetrahedron in the CaO−MgO−SiO2−Al2O3 system have been experimentally determined at 20 kbar pressure. The fo+diss+sp+li univariant curve, that pierces the fo-di-an join and meets the fo+diss+ enss+sp+li invariant point in the basalt tetrahedron, extends all the way to and pierces the di-fo-CaTs join, the limit of the simplified basalt tetrahedron toward the silica undersaturated portion.

Petrology of Olivine Basalt Dyke of Lamprophyre Affinity at Uppalapadu, Prakasam District, Andhra Pradesh

Abstract The basic dyke combines the mineralogical assemblage of an olivine basalt and textural characteristics of a lamprophyre. This porphyritic rock contains few euhedral phenocrysts of olivine set in a fine-grained groundmass, constituted by plagioclase and clinopyroxene. The olivine crystals are invariably mantled by a thin reaction rim of Ca-poor pyroxene and in spite of this, the rock has developed nepheline in the norm. The composition of the basaltic liquid can be expected to lie very close to the Fo-Di-Ab plane (or the Ol-Cpx-Pl thermal divide) within the basalt tetrahedron. Petrogenetic considerations regarding (a) the crystallization of olivine basalt magma and (b) the role played by volatiles in modifying its composition (corresponding to that of olivine lamprophyre) elsewhere in the Prakasam district are briefly discussed.

The petrology, phase relations and tectonic setting of basalts from the taupo volcanic zone, New Zealand and the Kermadec Island arc - havre trough, SW Pacific

Volcanism in the Taupo Volcanic Zone (TVZ) and the Kermadec arc-Havre Trough (KAHT) is related to westward subduction of the Pacific Plate beneath the Indo-Australian Plate. The tectonic setting of the TVZ is continental whereas in KAHT it is oceanic and in these two settings the relative volumes of basalt differ markedly. In TVZ, basalts form a minor proportion (< 1%) of a dominant rhyolite (97%)-andesite association while in KAHT, basalts and basaltic andesites are the major rock types. Neither the convergence rate between the Pacific and Indo-Australian Plates nor the extension rates in the back-arc region or the dip of the Pacific Plate Wadati-Benioff zone differ appreciably between the oceanic and continental segments. The distance between the volcanic front and the axis of the back-arc basin decreases from the Kermadec arc to TVZ and the distance between trench and volcanic front increases from around 200 km in the Kermadec arc to 280 km in TVZ. These factors may prove significant in determining the extent to which arc and backarc volcanism in subduction settings are coupled. All basalts from the Kermadec arc are porphyritic (up to 60% phenocrysts) with assemblages generally dominated by plagioclase but with olivine, clinopyroxene and orthopyroxene. A single dredge sample from the Havre Trough back arc contains olivine and plagioclase microphenocrysts in glassy pillow rind and is mildly alkaline (< 1% normative nepheline) contrasting with the tholeiitic nature of the other basalts. Basalts from the TVZ contain phenocryst assemblages of olivine + plagioclase ± clinopyroxene; orthopyroxene phenocrysts occur only in the most evolved basalts and basaltic andesites from both TVZ and the Kermadec Arc. Sparsely porphyritic primitive compositions (Mg/(Mg+Fe2) > 70) are high in Al2O3 (>16.5%), and project in the olivine volume of the basalt tetrahedron. They contain olivine (Fo87) phenocrysts and plagioclase (> An60) microphenocrysts. These magmas have ratios of CaO/Al2O3, A12O3/TiO2 and CaO/TiO2 in the range of MORB and MORB picrites and can evolve to the low-pressure MORB cotectic by crystallisation of olivine±plagiociase. Such rocks may be the parents of other magmas whose evolutionary pathways are complicated by interaction of crystal fractionation, crystal accumulation and mixing processes and the filtering action of crust of variable density and thickness. The interplay of these processes likely accounts for the scatter of data about the cotectic. More evolved rocks from both TVZ and KAHT contain clinopyroxene and orthopyroxene phenocrysts and their compositions merge with basaltic andesites and andesites. Stepwise least-squares modelling using phenocryst assemblages in proportions observed in the rocks suggest that crystal fractionation and accumulation processes can account for much of the diversity observed in the major-element compositions of all lavas. We conclude that the parental basaltic magmas for volcanism in the TVZ and KAHT segments are similar thereby implying grossly similar source mineralogy. We attribute the diversity to secondary processes influencing liquids as they ascended through complex plumbing systems in the sub arc mantle and cross.

Preliminary phase equilibria of the nepheline-diopside system under variable pressures (up to 28 kb) and temperatures

The nepheline-diopside join defines the ultra-alkaline portion of the basalt tetrahedron and the bulk composition of nephelinitic rocks lie in this join. Schairer and others established that under atmospheric pressure, the join cuts through the primary phase volumes of oliviness, carnegieitess and nepheliness. Melilite coexists with nepheliness, oliviness and diopsidess below 1160±10°C and olivine reacts out at low temperature.Experimental studies on seven compositions show the presence of a pseudoeutectic at Ne70Di30 and 1420°C, where diopsidess, nepheliness and liquid are in equilibrium. Olivine and melilite do not appear in the system and the assemblage below 1225±20°C is diopsidess+nepheliness.Four compositions studied at 1000°C show the appearance of the assemblage diopsidess+nepheliness+melilite at 15kb, whereas diopsidess and nepheliness are the stable phases at 20 and 25kb. The appearance of melilite is therefore restricted to a pressure of 18±3kb. Diopsidess and nepheliness coexist without olivine and melilite in albite-nepheline-diopside and sanidine-nepheline-diopside system. However, this study shows that feldspar-free nephelinitic rocks, which are devoid of melilite, may have crystallized under mantle conditions, whereas their melilite-bearing counterparts equilibrated at a shallower depth within the crust.

Anhydrous Partial Melting of Peridotite from 8 to 35 kb and the Petrogenesis of MORB

Equilibrium partial melts under anhydrous conditions have been determined for a MORB pyrolite composition (considered suitable for the production of primary MORB melts) from 8 to 35 kb and a relatively more depleted peridotite composition, Tinaquillo lherzolite, at 15 and 20 kb. The equilibrium partial melts were determined using ‘sandwich’ experiments. The primitive MORB glass DSDP3-18-7-1 was used in experiments using MORB pyrolite while a calculated liquid composition from Jacques & Green (1980) was used in experiments with Tinaquillo lherzolite. The equilibrium partial melts in conjunction with previous studies are used to establish a melting grid from 5 to 35 kb in the CIPW normative basalt tetrahedron. Equilibrium partial melts from MORB pyrolite combined with constraints imposed by the composition of primitive MORB glasses, MORB picrites and primitive trapped glass inclusions are subjected to olivine fractionation calculations to establish the range in composition of primary MORB melts. The results suggest primary MORB melts segregate from MORB source diapirs at pressures of 8 to 25kb in equilibrium with either lherzolite or harzburgite residues. MgO contents of primary MORB melts range from 10–17 wt.% while primary melts with > 17 wt.% MgO are of minor importance. Primary MORB melts range in composition from slightly ne-normative and more hy-normative picrites to olivine and quartz normative olivine tholeiite. Although some primitive MORB glass compositions are possible primary melts from 8–20 kb the majority of primitive MORB glass compositions are derived by olivine fractionation (11–25 wt.%) from more picritic parents from pressures of 15–25kb. The results of this study are in agreement with previous workers suggesting that there is an array of primary MORB melt compositions which evolve to multiple saturation in olivine+ plagioclase + clinopyroxene at different points along a latm. cotectic via differing degrees of olivine fractionation.

Anhydrous partial melting of MORB pyrolite and other peridotite compositions at 10 kbar: Implications for the origin of primitive MORB glasses

Anhydrous partial melting experiments on four peridotite compositions have been conducted at 10 kbar providing a relatively internally consistent set of data on the character of primary melts expected from the oceanic upper mantle in the mid-ocean ridge setting. The four peridotite compositions are: “MORB pyrolite” (considered to be suitable for the production of primitive (Mg#⩾0.68) MORB glasses at 10 kbar), “Hawaiian pyrolite” (representative of “enriched” upper mantle), Tinaquillo lherzolite (representative of more “depleted” upper mantle), and the spinel lherzolite KLB-1 which is a suitable composition for the production of primitive MORB glasses. The equilibrium liquids were determined by “sandwich” experiments. The primitive MORB glass DSDP 3-18-7-1 was used in experiments using MORB pyrolite and KLB-1, while a calculated 10 kbar liquid composition fromJaques andGreen (1980) was used in experiments with Hawaiian pyrolite and Tinaquillo lherzolite. The results of the experiments are used to test a 10 kbar melt model for the generation of primitive MORB glasses, which are parental magmas to typical MORB compositions. The melt compositions from the four peridotites studied are significantly different from primitive MORB glasses in major element chemistry and plot away from the field of primitive MORB glasses in the CIPW molecular normative “Basalt tetrahedron”. The results indicate that primitive MORB glasses are derivative compositions lying on olivine fractionation lines from picritic parents, which themselves are primary magmas at pressures greater than 10 kbar. The results of this study are integrated with previous 10 kbar experimental studies.

Mineralogical data on a multiple intrusion in the Rustenburg Layered Suite of the Bushveld Complex

AbstractMineralogical and chemical data are presented on an intrusion of a hypersthene gabbro body into coarse-grained noritic cumulates of the critical zone, which occurs at the margin of the Rustenburg Layered Suite of the Bushveld Complex. Two types of contact relations are found between the above two rock suites: (1) a clearly cross-cutting contact is observed on the southern margin of the hypersthene gabbro body close to the interface between the layered suite and the floor rocks; (2) a diffuse heterogeneous contact is found on the north-western limb of the body which is further into the complex. Field observations and chemical data suggest that, at the time of the hypersthene gabbro injection, the surrounding cumulates were above ambient temperatures. The difference in the contacts described above is probably related to the prevailing temperature gradient at the time of emplacement. The cumulate rocks further into the complex were at a higher temperature than those closer to the margin.Trace element chemistry and phase relations in the basalt tetrahedron indicate that the hypersthene gabbro is genetically unrelated to the cumulate rocks which it intrudes. The body probably represents an offshoot of the magma from which the upper zone of the Rustenburg Layered Suite crystallized.

Effect of chromium on phase relations in the join forsterite-anorthite-diopside in air at 1 Atm

The effect of Cr on the silicate system has been studied in air at 1 atm by adding a small amount of MgCr2O4 (0.2–0.5 wt.%) to the join Mg2SiO4 (forsterite) — CaAl2Si2O8 (anorthite) — CaMgSi2O6 (diopside), which has been considered to form a thermal divide in the system CaO-MgO-Al2O3-SiO2. The spinel primary field is enlarged compared with that in the Cr-free join at the expense of the anorthite primary field. The piercing points forsterite+anorthite+diopside+liquid and forsterite+anorthite+spinel+liquid approach each other with increasing MgCr2O4, meet at the join with 0.25 wt.% MgCr2O4 (0.20 wt.% Cr2O3) to form the ‘isobaric quaternary invariant point’ forsterite+anorthite+diopside+spinel+liquid, and then separate again as new ‘piercing points’ of diopside+spinel+anorthite+liquid and forsterite+diopside+ spinel+liquid. This process indicates that the join Mg2SiO4-CaAl2Si2O8-CaMgSi2O6 containing more than 0.2 wt.% Cr2O3 cannot be a thermal divide in the basalt tetrahedron. The results of the present study show that the presence of a minor amount of Cr causes a significant effect on the phase relations and therefore, the role of Cr must be taken into account in the formulation of a petrologic model.

Phase relations in transitional and alkali basaltic glasses from Iceland

Basaltic glasses from the three alkalic areas of Iceland (Snaefellsnes Volcanic Zone, Sudurland Volcanic Zone and Vestmannaeyjar Volcanic Area) contain plagioclase, olivine, clinopyroxene, chromian spinel and titanomagnetite as phenocryst phases. The glasses are hypersthene to nepheline normative alkali basaltic with FeO/ MgO ratios between 1.4–4.7. Olivine ranges in composition from Fo90 to Fo55, plagioclase from An90 to An50 and clinopyroxene from En45Fs10Wo45 to En40Fs17Wo43. Clinopyroxene reveals systematic Ti:Al metastable crystallization trends related to the composition of the enclosing glass. Two types of phenocryst are present in most glasses and show a bimodality in size and composition. Microphenocryst phases are those most likely to have crystallized from the enclosing glass, while macrophenocrysts may have crystallized from a liquid of slightly less evolved composition. The glasses show complex phenocryst-glass relations which can be related to a polybaric effect. The normative glass compositions are related to 2-phase cotectic surfaces in the basalt tetrahedron and define the position of the 3-phase cotectic line. In general with increasing FeO/MgO in the glass the phenocryst assemblages vary from clinopyroxene, olivine and plagioclase along a clinopyroxene-olivine surface to olivine and plagioclase along an olivine-plagioclase surface. The normative glass compositions show a deflection from clinopyroxene-bearing to clinopyroxene-free glasses. The appearance of plagioclase together with clinopyroxene and olivine can be explained in the light of experimental investigations of the effect of pressure on phase relations. The major element variation of the glasses is interpreted as representing mantle derived magma batches of primary liquids, modified to some degree by high (6 kbar) and intermediate to low pressure (below 3 kbar) crystal fractionation towards equilibrium phase relations during ascent and residence in crustal magma chambers. The observed deflection in normative compositions of the glasses marks the position of the high pressure 3-phase cotectic line. The bimodality in size and composition of plagioclase and olivine phenocrysts can be related to high pressure crystal fractionation in the melt. The Fe-Ti basalt glasses from Sudurland are believed to be quenched high pressure compositions.

The join forsterite-anorthite-diopside with 0.3% MgCr2O4 at 1 atm and its petrologic significance. A preliminary report.

Phase relations in the join forsterite (Mg2SiO4, Fo)-anorthite (CaAl2Si2O8, An)-diopside (CaMgSi2O6, Di) with 0.3 wt% MgCr2O4 was determined in air at 1 atm. Spinel (sp) primary field is enlarged compared with that in the Cr-free limiting join Fo-An-Di at the expense of anorthite (an). The piercing points, fo (forsterite)+an+di (diopside)+liquid and fo+an+sp+liquid in the Cr-free join disappear, and two new “piercing points” are present; one is diss (ss: solid solution)+spss+fo+liquid at 1279°±3°C and the other at 1277°±3°C diss+spss+an+liquid. The “univariant lines” passing through these points meet at 1270°±5°C where spinel is dissolved and another “univariant line” di+fo+an+liquid is generated, indicating that the presence of a minor amount of Cr gives significant effect on the phase relations in the silicate system, and that the join Fo-An-Di with 0.3% MgCr2O4 (0.24% Cr2O3) cannot be thermal divide in the basalt tetrahedron. Therefore, alkali basalt magma can produce tholeiitic basalt by fractional crystallization at lower pressure if a minor amount of Cr2O3 (more than 0.2%) is present.

First‐order major element variation in basalt glasses from the Mid‐Atlantic Ridge: 29°N to 73°N

Basaltic glasses from 29°N to 73°N on the Mid‐Atlantic Ridge define two geographic and compositional groups that are characterized by different major element concentrations, phase assemblages and partition coefficients. Group A glasses occur on the ridge segment north of Gibbs Fracture Zone, from 54°N, along the Reykjanes Ridge, the western volcanic zone of Iceland and Kolbeinsey Ridge, up to 70°N. Similar glasses occur also on the ridge from 29°N to 34°N. Group A is characterized by pronounced alumina depletion and iron‐enrichment trends, lower silica and alkalis and higher iron and high CaO/Al2O3. Clinopyroxene is only present in the more evolved group A glasses (Mg‐value &lt; 58). Olivine‐glass partition coefficients for Mg and Fe are systematically lower in group A than group B (Schilling and Sigurdsson, 1979), reflecting effects of melt composition on structure of the liquids. Group B glasses define the ridge segment north of 35°N over the Azores platform and up to Gibbs F. Z., at 53°N. They are also present on the Mohns Ridge north of the Jan Mayen F. Z. at 71°N and at least as far north as 73°. Group B glasses are characterized by higher silica and alkalis, low iron and lack of alumina depletion and iron‐enrichment trends. They contain calcic clinopyroxene (Cpx) throughout the compositional range, whereas the sub‐calcic augite is absent. The major‐element variation within each group can be modeled quantitatively by fractional crystallization involving the phenocryst phases. In group A, the model involves only olivine (Ol) and plagioclase (Pl) extraction in the early stages, but a total of 60% crystallization of the most primitive magma is required, yielding a final solid extract of Ol‐Pl‐Cpx in the proportions 1:3.3:1.5. Group B fractionation models require clinopyroxene extraction throughout the compositional range, and 45% crystallization of the primitive magma is required to account for the most evolved glasses giving a total solid extract of Ol‐Pl‐Cpx in proportions 1:5:3.9. The range of glass compositions in the two groups forms two distinct but parallel cotectic trends within the basalt tetrahedron, which are believed to define the quaternary univariant line Ol‐Pl‐Cpx‐Liq along which the magmas evolved during fractionation at low to intermediate pressure. It is proposed that the shift in phase boundaries and univariant lines reflects differences in melt and source compositions of the two groups. The distribution of the two groups is thus taken to indicate two major petrographic provinces of the Mid‐Atlantic Ridge.

Samail Ophiolite plutonic suite: Field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge magma chamber

Geologic mapping of an intact plutonic sequence within the Samail ophiolite in the Ibra area, southeastern Oman Mountains, reveals stratigraphic, structural, and petrologic details of oceanic layer 3. Four measured stratigraphic sections, each spaced about 5 km apart across the southern flank of Jabal Dimh, define a time‐transgressive progression within the ophiolite and reveal geometric and petrologic features of a spreading‐ridge magma chamber. The sections show the following vertical sequence: (1) dunite (chr‐ol cumulates ± harzburgite xenoliths) 0–200 m thick, grading up from a transition zone with harzburgite tectonite, (2) interlayered wehrlite‐melagabbro‐gabbro (cpx‐ol and ol‐cpx‐pl cumulates) 0–100 m thick, (3) layered gabbro (chiefly ol‐cpx‐pl cumulates but including recurrent intervals of cumulus wehrlite and melagabbro) 2.6–5.5 km thick, (4) planar laminated nonlayered gabbro (chiefly ol‐cpx‐pl cumulates) 100–400 m thick, (5) hypidiomorphic (ol)‐hb‐cpx gabbro (high‐level gabbro) 200–800 m thick, (6) small, discontinous diorite to plagiogranite bodies at or near the top of the gabbro. Cumulus textures (adcumulus &gt; mesocumulus), planar lamination, and cumulus layering (phase, ratio, and grain size layers at mm to 10‐m scale, commonly graded) within this sequence show that crystals accumulated from the base of the magma chamber upward to within a few hundred meters of the top; downward solidification from the roof was minor. Cyclicity within the cumulus sequence is represented by the recurrence of olivine‐rich intervals (melagabbo and wehrlite) up to high stratigraphic levels and by hundreds of phase‐graded layers (ol‐rich at the bases to pl‐rich at the tops), individually up to 5 m thick. Limited cryptic variation relative to closed‐system layered intrusions and the limited range in solid‐solution components of olivine (Fo69–90), plagioclase (An62–95) and clinopyroxene (En40–54, Fs4–16, Wo37–49) from the cumulus suite require replenishment of the magma during its crystallization history. Zig‐zag normal and reverse cryptic variation differentiation trends (both in major solid‐solution components and in minor element concentration) indicate that the replenishment took place in pulses followed by periods of magma mixing and crystal fractionation. Recurrent olivine‐rich intervals commonly (but not always) coincide with reverse cryptic variation trends. They are considered to be the products of fresh draughts of primitive magma, with olivine and chromite as the only liquidus phases prior to extensive mixing with the more fractionated resident magma within the chamber. The sequence of crystallization is explained using the simplified tholeiitic basalt tetrahedron of Presnall et al. (1979). The plutonic sequence is roofed by sheeted dikes that are overlain by submarine basalt, indicating that the magma crystallized beneath a spreading ocean ridge. Major and trace element geochemistry of the dike complex is similar in many respects to that of mid‐ocean ridge basalt (MORB) and yield Mg/Mg + Fe+2 ratios that overlap with ratios predicted for parent liquids of the cumulus suite based on crystal/liquid equilibria. Field evidence, including continuity of layering over large areas, distinctive layer sets that are mapped for several km along strike, and lack of chamber edge contacts indicates that crystallization of Jabal Dimh plutonics took place in a single, large, long‐lived magma chamber rather than in small transitory chambers. Similarity of mineral compositions and tentative correlation of cryptic variation trends between stratigraphic sections favors a large, single‐chamber model. The chamber shape, controlled by floor growth upward greatly exceeding roof growth downward as the chamber halves diverged from the paleo‐spreading axis, must have been funnel‐shaped in cross section; i.e., the floor sloped inward to the center from a sandwich horizon that developed just beneath the roof at both sides. This conclusion is based on the occurrence of cumulates up to a very high level in the stratigraphic sections and on geologic mapping which shows that cumulus layering locally approaches the base of the cumulus section at an angle. This vergence angle is &lt;20°, well below inferred angles of repose for cumulates. Tentative correlation of cryptic variation between sections yields similar vergence angles. The scarcity of slump structures and of layer disruption within the main layered sequence verifies that the floor sloped rather gently, surely much less than 45°. Thus, the half‐width of the chamber must have exceeded its vertical dimension, which averages ∼5 km. A half‐width of ∼18 km is calculated from a preferred vergence angle of 15°. The cumulates crystallized within this spreading chamber from a magma body that was repeatedly replenished by primitive magma from a central feeder zone at the spreading axis. High‐level gabbro and diorite through plagiogranite crystallized from stagnant residual magma at the outer edges of the chamber, beyond reach of replenishment from the center.

Liquidus phase relations on the join diopside-forsterite-anorthite from 1 atm to 20 kbar: Their bearing on the generation and crystallization of basaltic magma

In the system CaO-MgO-Al2O3-SiO2, the tetrahedron CaMgSi2O6(di)-Mg2SiO4(fo)-SiO2-CaAl2 SiO6(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl2Si2O8(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO2-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.

Some chemical controls on igneous amphibole compositions

The composition of igneous amphiboles is extremely sensitive to magma composition and the nature of the co-existing phases. The extent of solid solution in amphiboles may be depicted in the basalt tetrahedron. They are more silica-undersaturated than the liquid from which they have crystallized for liquids containing about 11% normative nepheline. For liquids containing more than 11% normative nepheline the reverse is true. The existence of this compositional divide may be demonstrated for both intrusive and extrusive suites. Amphiboles are generally nepheline-normative, except when crystallizing from granodioritic to granitic magmas. Fractionation factors between amphibole and magma composition for MgO and FeO appear to increase with decreasing temperature; those for TiO 2 and Na 2O decrease with increasing pressure. The CaO content of amphiboles appears to be insensitive to the CaO content of the magma.