William G. Melson

Volcanic glass compositions of the Troodos ophiolite, Cyprus

Fresh volcanic glass is preserved throughout the extrusive section of the Troodos ophiolite, which indicates that the lavas have not been pervasively metamorphosed. Glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals. The basal 400–500 m of the sequence consists of an andesite-dacite-rhyolite assemblage containing abundant hyaloclastites. The remainder of the section comprises a basalt–basaltic andesite assemblage with high MgO and low TiO 2 and total iron. The lower sequence is interpreted as an evolved arc-tholeiite suite; the upper has some similarities to boninitic lavas. The close association in time and space of these two suites is similar to that observed in the Mariana and Bonin arcs and suggests that all of the Troodos lavas were erupted in a subduction-zone environment, most probably in an incipient arc or fore-arc.

Metamorphism in the Mid-Atlantic Ridge, 22°N latitude

Abstract Greenstones derived from basalts, tuffs, and dolerites were dredged at two stations on the eastern slope of the median valley of the Mid-Atlantic Ridge at 22°N latitude. The rocks consist mainly of the typical greenschist assemblage albite, actinolite, chlorite, and epidote. The greenstones reflect increasing metamorphic grade and intensity of shearing with the depth at which they were recovered. Bulk chemical analyses of six greenstones give spilitic compositions characterized by high soda (maximum 5.4%) and low potash (minimum 0.05%) contents. Relics of calcic plagioclase, pseudomorphed olivine phenocrysts, and other textural features indicate derivation from abyssal basalts, which typically have soda around 2.7% and potash around 0.2%. The alkali contents of the greenstones are most likely a result of metasomatism during metamorphism, and do not indicate a primary spilitic magma, or reaction between magma and sea water. The significance of the greenstones in relation to the tectonics and structure of the Mid-Atlantic Ridge is discussed.

Petrology of a transform fault zone and adjacent ridge segments

The Verna Fracture Zone in the North Atlantic (9 to 11° N), which has been identified as a transform fault zone, contains exposures of serpentinized peridotites, while its adjacent ridge segments are floored mainly by typical abyssal ocean ridge basalts. This petrologic contrast correlates with the greater frequency of volcanic eruptions along the actively spreading ridge segments compared to the transform fault zone. Where rifting components occur across transform faults, exposures of the deeper zone of oceanic crust may result. The bathymetry of the Verna Fracture Zone suggests that some uplift parallel to the fracture zone as well as rifting led to exposures of deeper rocks. The basalts from the adjacent ridge axes contain ‘xenocrysts’ of plagioclase and olivine and more rarely of chromite. These appear to have a cognate origin, perhaps related to cooling and convection in near surface magma chambers. The basalts from the ridge axes, offset and on opposite sides of the transform fault, have similar features and compositions. The plagioclase peridotites have mineralogical features which indicate equilibration in the plagioclase pyrolite facies, suggesting maximum equilibration depths of around 30 km for a temperature of around 1200 °C. The chemical characteristics of the Vema F.Z. peridotites suggest that they may be undifferentiated mantle, emplaced as a subsolidus hot plastic intrusion or as a crystal mush. The abundance of peridotites and serpentinized peridotites is believed to reflect their abundance in seismic layer three of the oceanic crust.

Rhyodacites, andesites, ferro-basalts and ocean tholeiites from the galapagos spreading center

Abstract Volcanic rocks, dredged from depths greater than 1000 m on the Galapagos spreading center, show extreme chemical diversity, including rhyodacites, andesite, ferro-basalts, and low-K oceanic tholeiite. All samples have fresh glassy margins. The ferro-basalts contain up to 18.5% total iron as FeO and up to 3.75% TiO2, while the oceanic tholeiites are as low as 0.02% K2O. The ferro-basalts correlate with the previously proposed zone of high magnetic anomaly amplitudes which flank the Galapagos hot spot, and are consistent with a genesis by shallow fractional crystallization.

Sr, Nd and Pb isotopic and REE geochemistry of St. Paul's Rocks: the metamorphic and metasomatic development of an alkali basalt mantle source

Surface samples of peridotites and hornblendite mylonites from St. Pauls Rocks, and dredge samples from the flanks of the massif, have been analyzed for Sr, Nd and Pb isotopic ratios and Rb, Sr, and REE concentrations. This data, coupled with previous K and REE data, are used to develop a self-consistent model for the genesis of these ultramafic rocks. This model involves metasomatism of an ocean island-type mantle about 155 m.y. ago by a strongly light-REE-enriched metasomatic fluid, probably derived from the same mantle. This metasomatism produced light-REE-enriched materials which were isotopically homogeneous on a small scale (100 m), and isotopically heterogeneous on a large (km) scale. The geochemical relationships between the peridotites and the hornblendites were established by metamorphic equilibration on a relatively small scale (<10 m). The “average” mantle produced by these events is characterized by87Sr/86Sr=0.7034,143Nd/144Nd=0.51291,206Pb/204Pb=19.33 and 207/204=15.63. An alkali basalt which postdates the mylon-itization of the ultramafic massif has an isotopic character which is identical to the “average” ultramafic massif; it also lies on the five-dimensional isotopic mantle plane of Zindler et al. (1982). With respect to major elements, trace elements, and Sr, Nd and Pb isotopes, the average ultramafic rock of the St. Pauls massif is an ideal candidate for a mantle source from which alkali basalts can be derived by partial melting; the St. Pauls massif is in fact the first such example of an ultramafic rock which meets all the requirements to be an alkali basalt source.

Nuées Ardentes of the 1968 Eruption of Mayon Volcano, Philippines

Mayon Volcano, southeastern Luzon, began a series of explosive eruptions at 0900 April 21, 1968, and by May 15 more than 100 explosions had occurred, at least 6 people had been killed, and roughly 100 square km had been covered by more than 5 cm of airfall ash, blocky ash flows, and a lava flow. All material crupted was porphyritic augite-hypersthene andesite.Explosions from the summit crater (elevation 2460 m) ejected large quantities of ash and incandescent blocks to a height exceeding 600 m and produced ash-laden clouds which rose to heights of 3 to 10 km. Backfall of the coarser material fed nuées ardentes which repeatedly swept down ravines on all sides of the volcanic cone. The velocity of one nuée ardente ranged from 9 to 63 m per sec. The largest nuées descended to the southwest and reached as far as 7 km from the summit. An aa lava flow also descended 3 1/2 km down this flank.The nuées ardentes deposited pyroclastic flows that contained large breadcrust-surfaced blocks averaging about 30 cm across, but occasionally reaching 25 m in greatest dimension. These blocks were still very hot in their interiors several days later. Surrounding the pyroclastic flows is a seared zone as much as 2 km wide, but averaging a few hundred meters, in which vegetation is charred and splintered, but over which only a thin layer of airfall ash was deposited.

Boron contents of serpentinites and metabasalts in the oceanic crust: Implications for the boron cycle in the oceans

Abstract Serpentinization of peridotite rocks in the oceanic crust is accompanied by boron enrichment. Hydrothermally altered basalts in the oceanic crust show no comparable increase in boron. In high-temperature reactions of basaltic rocks and sea-water, boron is preferentially partitioned into the liquid phase; basaltic rocks exposed on the ocean floor undergo low-temperature weathering by sea-water resulting in the addition of boron. Juvenile boron-containing solutions may be responsible for serpentinization. Such solutions may also be important in maintaining the boron concentration of the oceans, since boron is removed from sea-water both by clay minerals entering the oceans and by alteration of submarine igneous rocks.

Explosive activity associated with the growth of volcanic domes

Abstract Domes offer unique opportunities to measure or infer the characteristics of magmas that, at domes and elsewhere, control explosive activity. A review of explosive activity associated with historical dome growth shows that: 1. (1) explosive activity has occurred in close association with nearly all historical dome growth; 2. (2) whole-rock SiO 2 content, a crude but widely reported indicator of magma viscosity, shows no systematic relationship to the timing and character of explosions; 3. (3) the average rate of dome growth, a crude indicator of the rate of supply of magma and volatiles to the near-surface enviornment, shows no systematic relationship to the timing or character of explosions; and 4. (4) new studies at Arenal and Mount St. Helens suggest that water content is the dominant control on explosions from water-rich magmas, whereas the crystal content and composition of the interstitial melt (and hence magma viscosity) are equally or more important controls on explosions from water-poor magmas. New efforts should be made to improve current, rather limited techniques for monitoring pre-eruption volatile content and magma viscosity, and thus the explosive potential of magmas.

St. Peter and St. Paul Rocks: A High-Temperature, Mantle-Derived Intrusion

St. Pauls Rocks, often postulated to be an exposure of the suboceanic mantle, consists of a wider variety of rocks than previously recognized. These perhaps crystallized at different mantle levels, and were subsequently incorporated and mylonitized in a hot but solid intrusion.

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.