Wilfred B. Bryan

Fractionation of pyroxene-phyric MORB at low pressure: An experimental study

One-atmosphere melting experiments are used to assess the role of clinopyroxene in producing the compositional variations observed in mid-ocean-ridge basalts (MORBs) from the North Atlantic. Analog models of natural glasses and associated phenocrysts show that several possible parental magmas may undergo low pressure fractional crystallization involving olivine and spinel, followed by plagioclase, and then by augite. The phenocryst phase assemblages in natural deep-sea basalts are closely correlated with the major element compositions of their associated quenched glasses, and the projections of these glasses on the Oliv-Cpx-Qtz pseudoternary correspond to the 1-atmosphere phase boundaries and reaction points defined by laboratory experiments. Comparison of natural phenocrysts with experimental phases indicates that the augites preserved in moderately fractionated MORB from the FAMOUS area may have formed at or near the ocean floor and need not be relics of high pressure processes.

Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N

Fifty acoustically positioned samples of fresh basalt were collected by the submersible Alvin from the median valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (FAMOUS) in the summer of 1974. The samples show regular compositional variations from the center of the rift valley (central lava flows) out to the rift valley walls (flank lava flows). The central lava samples show higher ratios of olivine relative to clinopyroxene and plagioclase phenocrysts and contain chrome spinel. Glasses of the flank lava samples are enriched in SiO 2 , TiO 2 , K 2 O, H 2 O, and FeO/MgO relative to central lava samples. Studies of the thickness of palagonite and manganese crusts indicate that the flank lava flows are considerably younger than the inferred spreading age of the crust on which they occur. Flank lavas are generally older than central lavas, but notable exceptions occur. The composition of the flank lava glass can be derived by the removal of approximately 29 wt percent of analyzed phenocrysts (in the ratio 5.7 plagioclase, 2.5 olivine, 1.8 clinopyroxene) from the central lava glass. In addition, other processes (possibly involving volatile transfer) must enrich the flank lavas in K 2 O, TiO 2 , and H 2 O. A model is proposed whereby this crystal fractionation occurs in a shallow, narrow (6-km-wide) magma chamber underlying the median valley. The chamber is compositionally zoned, and central lavas are fed from dikes tapping its hotter axial zone, whereas flank lavas are fed from the cooler, differentiated melt on the margins. The nature of the chemical variations in the lavas permits an estimate of the composition and thickness of the cumulates forming at the base of the chamber.

Experimental petrology of normal MORB near the Kane Fracture Zone: 22°–25° N, mid-Atlantic ridge

Melting experiments carried out at 1-atm and at 2 kbar on mid-ocean ridge basalts dredged from the mid-Atlantic ridge near the Kane Fracture Zone (KFZ, 22° to 25° N. latitude) provide a basis for evaluating the role of crystal fractionation in generating compositional variability observed in “normal” mid-ocean ridge basalt. The 1-atm olivine-plagioclase-clinopyroxene saturation boundary for KFZ lavas defines a path in mineral projection schemes and in oxide-oxide diagrams that is displaced from the same experimentally determined boundaries in FAMOUS (Grove and Bryan 1983) and Oceanographer Fracture Zone (Walker et al. 1979) basalts. The glass margins of sparsely phyric KFZ lavas record small amounts of near surface, low pressure fractional crystallization, and their glass and bulk rock compositions are similar. An important signature of low pressure differentiation is recorded in the quenched glass margins of moderately phyric KFZ lavas compared to their bulk rock compositions, and the glass has evolved along low-pressure fractionation paths that are similar to those produced in the 1-atm experiments. Many of the lavas have retained phenocrysts in equilibrium proportions, so that their bulk rock compositions represent liquid compositions. When the effects of near-surface differentiation and crystal accumulation are removed from the Kane data set, and only liquid compositions are considered, a suite of basalt magmas can be identified that forms a trend in mineral component projection schemes parallel to the 1-atm oliv-plag-cpx multiple saturation boundary, but displaced from it toward olivine. These basalts have only olivine and plagioclase as phenocrysts, and are well removed from clinopyroxene saturation at low pressure. The compositional variation can not be generated by mixing any primary liquid composition with a low pressure liquid that has evolved along the oliv-plag-cpx multiple saturation boundary. Major and trace element models of this trend using olivine, plagioclase and clinopyroxene as fractionating phases match the compositional variability. This compositional trend is generated by fractionation at pressures greater than 2 kbar, but within the plagioclase stability field. A review of the data for other normal MORB suites from this part of the mid-Atlantic ridge reveals a similar elevated pressure fractionation signature which persists when the effects of low pressure magma mixing are removed from the data set.

Rifting History of the Woodlark Basin in the Southwest Pacific

Marine geophysical evidence has been obtained for the rate and history of sea-floor spreading in the Woodlark Basin. The eastern part of this basin is presently separating from the Australian Plate at over 4 cm per yr in a northerly direction. The western part of the basin is not presently spreading. This spreading rift marks the southern boundary of the Solomons Plate which is bounded by subduction zones in the north and east (the New Britain and northern Solomons Trenches, respectively) and in the west by a combination strike-slip rifting (dip-slip) boundary in eastern Papua (New Guinea). A vector triangle solution near the Solomons Trench-Woodlark Rift triple point gives underthrusting of the Solomons Plate beneath the northern Solomon Trench in a northeasterly direction at about 11 cm per yr. The Woodlark Basin began opening as a sphenochasm, with a pole near the tip of eastern Papua about 20 m.y. B.P. This was caused by left-lateral shear in the region induced by a change in the relative motion pole of the Australia and Pacific Plates. The basin opened only a few degrees at this time, then stopped. Rifting in the entire basin resumed about 3 m.y. B.P., based on magnetic anomaly data. About 1 m.y. B. P., the spreading center in the western basin shifted to the Woodlark Rise.

Systematics of modal phenocryst assemblages in submarine basalts: Petrologic implications

Phenocryst assemblages in ocean-ridge basalts generally show an increasing proportion of plagioclase as the total amount of phenocrysts increases. The variations in phase assemblages, as well as most crystal-liquid Kds, are similar to variations (equimodal trends) predicted by low-pressure laboratory experiments, suggesting that many of these basalts have experienced varying degrees of low-pressure cyrstallization prior to quenching, with little sorting of crystals and liquid. Important exceptions include moderately to highly phyric basalts enriched either in plagioclase or olivine which lie well off the experimental trends. In these basalts, megacrysts and xenocrysts usually cited as evidence for magma mixing commonly represent a small proportion of the total crystalline phase assemblage. However, phase proportions for many of these basalts lie well outside the range that could be produced by simple mixing; selective gravitative sorting either prior or subsequent to mixing appears to be the likely explanation for these phyric basalts. A relation between spreading rate and phase proportions is neither supported nor refuted by the data, which as yet do not adequately represent fast-spreading ridges.Pyroxene-phyric varieties are especially common among LIL-element enriched (Group 2) basalts, and these basalts also show the greatest abundance of olivine-enriched (picritic) samples. Selective enrichment in plagioclase is more common among LIL-element depleted (Group 1) basalts, and pyroxene appears in Group 1 basalts only at relatively high degrees of crystallinity. These differences are consistent with expected compositional effects (including volatiles) on phase boundaries, as well as likely differences in depth (pressure) of mantle melting and magma fractionation.Sparsely to moderately phyric basalts tend to contain only olivine (±spinel) as phenocrysts, and lie in the olivine field in the projection from plagioclase in the CMAS tetrahedron. This is consistent with the concept that these magmas approach low-pressure equilibrium by olivine fractionation from a more picritic parent. The origin of these basalts, and relationships between them, remains an important fundamental problem.Phenocryst phase assemblages are consistent with the low-pressure phase saturation indicated by the projected positions of the associated glasses in CMAS. It is suggested that, in contrast to the classical practice of classifying basalts according to phase proportions, a classification based on presence and/or first appearance of each crystalline phase is both practical and petrogenetically significant for water-quenched submarine basalts.

Sr-isotope, K, Rb, Cs, Sr, Ba, and rare-earth geochemistry of basalts from the FAMOUS area

Ten basalt samples recovered from the FAMOUS area were selected so as to obtain representatives of a wide geographical and compositional range. The samples were analyzed for 87Sr/86Sr, K, Rb, Cs, Sr, Ba, and rare-earths. Sr-isotope ratios fall in the narrow range of 0.70288 to 0.70307, which implies that these samples were derived from an isotopically homogeneous source. The FAMOUS area lies in a geochemical transition zone between the Azores Plateau and “normal” ridge areas south of lat 33°N. The LIL (large-ion-lithophile) and Sr-isotope geochemistry of FAMOUS basalts is thus influenced by the Azores mantle plume; this results in higher Sr-isotope and LIL concentrations in these basalts than is typical of Mid-Atlantic Ridge basalts. Trace-element distributions in FAMOUS area basalts cannot be entirely accounted for by fractional crystallization models that are based on major-element chemistry. The LIL distribution in FAMOUS basalts could be due to variable extents of partial melting. Zonation within the magma chamber may result from incomplete mixing of successive batches of magma entering the chamber and could be further enhanced by fractional crystallization. The variation in partial melting would require significant increases in mantle temperature over a relatively short period of time. According to this model, the Mount Pluto magma represents the highest degree of partial melting and may mark the initiation of a new cycle of eruptive activity in the median valley.

Petrography and Geochemistry of the Igneous Rocks from Eua, Tongan Islands

A slightly metamorphosed igneous sequence is exposed beneath upper Eocene limestone on Eua, the southernmost of the eastern line of limestone-covered islands of the Tonga group. This igneous sequence occurs as lava, agglomerate, conglomerate, and tuff; compositions include high-alumina tholeiite, basaltic andesite, quartz gabbro, olivine basalt, and dacite tuff. Well-defined dikes of acid andesite (also pre-limestone) cut this sequence. Phenocryst mineralogy is typically highly calcic plagioclase, augite, and titanomagnetite, plus alteration products (mainly uralite, chlorite, epidote, calcite, and pyrite). Characteristic chemical features of the Eua igneous suite include relatively high alumina, very low K 2 O, P 2 O 5 , Rb, Ba, and REE, and relatively low TiO 2 , Ni, and Cr. The Na/K, V/Ni, and K/Rb ratios are high, and Rb/Sr ratios are very low. Measured Sr 87 /Sr 86 ratios (= initial) of a tholeiitic basalt and an acid andesite give 0.7034 and 0.7038, respectively. The Eua igneous suite evidently represents a very early stage of island arc evolution. Geochemically, these Eua rocks are decidedly less fractionated (or more “primitive”) than the recent Tongan basaltic andesite-dacite association which constitutes the western line of active volcanic islands of Tonga. This latter suite can be classified as one of the circum-Pacific “low potash andesite suites” (or “island arc tholeiitic series”). In terms of the trace and minor element data, the Eua igneous suite is similar in many respects to ocean-floor basaltic lavas, although perhaps characterized by lower TiO 2 , Ni, and Cr than is typical of most ocean floor lavas. It is concluded that the over-all chemistry and mineralogy of the Eua igneous suite suggest close similarities with the volcanic and mafic plutonic components of described ophiolite complexes; this is partly supported by the reported occurrence of peridotite and dunite from the nearshore flank of the Tonga trench. Thus, it is postulated that on Eua we have the topmost part of an underlying ophiolite complex exposed, possibly developed at the location of and at the inception of the site of initial subduction of lithosphere that has subsequently been responsible for the development of the Tonga arc system.

Compositional variation in a steady-state zoned magma chamber: Mid-Atlantic Ridge at 36°50′N

Abstract Basalt glasses collected in the FAMOUS dive area by the submersible “Alvin” have been analyzed for B, Li, Cr, Ni, Co, Cu, Ga, Sr, Ba, V, Y and Zr by optical emission spectrometry. Field relations, petrography, and other published trace element data are reviewed. Published partition coefficients are combined with least-squares estimates of mineral proportions to compute variations expected for postulated equilibrium crystallization in a high-level, zoned magma chamber. Variation in Cr, Ni, Co and Cu is consistent with such a process, involving crystallization of plagioclase, pyroxene, olivine, and sulphide. The other elements are enriched by factors from about 1.5 to 4.0 times the amounts predicted by this model; the excess enrichment is greatest for Ba and least for Y and V. These excess enrichments may in part reflect the progressive accumulation of incompatible elements in the margins of a steady-state magma chamber, and may also reflect variations in initial concentrations in successive batches of mantle-derived parent liquid. Concentration of excess incompatible elements in a steady-state chamber may be sensitive to spreading rates and may, in favorable cases, lead to localized concentrations having many of the characteristics popularly attributed to mantle plumes.

Axial processes along a segment of the East Pacific Rise, 10-12°N

Axial segments of the East Pacific Rise are made up of individual volcanoes. Each volcano has a distinct magma composition and shows a systematic variation in the fractional crystallization with distance along the axis from the central chamber. Hydrothermal venting, lava morphology and tectonics also vary along the axis. Lavas erupted near transform faults, from the tips of propagating rifts in overlapping spreading centres, and from near-axis seamounts have different and variable compositions.

Shallow Structure of the New Hebrides Island Arc

Marine geological and geophysical studies of the New Hebrides island arc have been made to study (a) the present development of lithospheric plate boundaries, (b) evidence for creation of oceanic crust behind the frontal arc in interarc basins, and (c) evidence for reversal of the arc from east-facing to the present-day west-facing orientation. The arc system is bisected between 13° and 15° S. by the east-west Hazel Holme Fracture Zone which connects the trench and a north-south—trending spreading center (Nova Rise) on the Fiji Plateau near 174° E. The crust on the plateau south of the fracture zone is very young. Narrow interarc basins are present but youthful, south of about 18° S. North of the Hazel Holme Fracture Zone, interarc basins are less well developed and apparently even younger. Most of the Fiji Plateau has apparently been formed by spreading from the Nova Rise rather than within interarc basins associated with the New Hebrides. The tectonics of the central region of the arc system, immediately south of 15° S., appears to be governed by the transform section of the Hazel Holme Fracture Zone and by subduction of the D9Entrecasteaux Fracture Zone into the trench rather than by interarc spreading. In this region, the western and eastern chains of the New Hebrides group have been recently uplifted and tilted toward one another, creating a sedimentary basin. Most data do not support the idea that the eastern island belt in this region, including Maewo and Pentecost islands, is an ancient remnant of an east-facing arc system. These islands have been uplifted only very recently and later than the western islands. Therefore, any east-facing subduction phase must have ceased recently and occurred after subduction beneath the western islands. We suggest instead that the eastern island belt represents an interarc basin floor or a frontal arc uplifted behind the volcanic line.