Magma Supply Rate Research Articles

Peridotites from the Mariana Trough: first look at the mantle beneath an active back-arc basin

Two dives of the DSV Shinkai 6500 in the Mariana Trough back-arc basin in the western Pacific sampled back-arc basin mantle exposures. Reports of peridotite exposures in back-arc basin setting are very limited and the lack of samples has hindered our understanding of this important aspect of lithospheric evolution. The Mariana Trough is a slow-spreading ridge, and ultramafic exposures with associated gabbro dykes or sills are located within a segment boundary. Petrological data suggest that the Mariana Trough peridotites are moderately depleted residues after partial melting of the upper mantle. Although some peridotite samples are affected by small-scale metasomatism, there is no evidence of pervasive post-melting metasomatism or melt-mantle interaction. Spinel compositions plot in the field for abyssal peridotites. Clinopyroxenes show depletions in Ti, Zr, and REE that are intermediate between those documented for peridotites from the Vulcan and Bouvet fracture zones (the American- Antarctic and Southwest Indian ridges, respectively). The open-system melting model indicates that the Ma- riana Trough peridotite compositions roughly corre- spond to theoretical residual compositions after 7% near-fractional melting of a depleted MORB-type upper mantle with only little melt or fluid/mantle interactions. The low degree of melting is consistent with alow magma budget, resulting in ultramafic exposure. We infer that the mantle flow beneath the Mariana Trough Central Graben is episodic, resulting in varying magma supply rate at spreading segments.

Volcanic SO2 emissions and the abundance and distribution of exsolved gas in magma bodies

The mass of sulfur dioxide (SO2) released by most explosive volcanic eruptions greatly exceeds the amount originally dissolved in the erupted volume of silicate melt. Several lines of evidence suggest that this discrepancy is due to the presence of SO2-bearing gas bubbles in the magma before the time of eruption. Comparison of remote sensing data for SO2 emissions with conventional petrologic estimates suggests that the discrepancy can be resolved if andesitic and dacitic magmas contain about 1–6wt% exsolved gas, equivalent to 5–30vol%, prior to eruption. Such large mass fractions of exsolved gas in pre-eruptive magma are consistent with previously published physical models in which crystallization-induced gas exsolution gradually increases overpressuring of a magma reservoir, eventually triggering an eruption. Simple mass balance models for SO2 emissions from magma bodies in which there is an upwards increasing gradient in exsolved gas mass fraction (i.e. gas-rich at the top) yield SO2 vs. eruptive volume trends that are similar to those observed for eruptions ranging in size from 0.01 to 10km3 of magma. Despite the many uncertainties involved, these patterns are consistent with the hypothesis that subvolcanic magma reservoirs are separated from volcanic vents by cupolas of gas-rich magma that supply much of the gas released in explosive eruptions. The volume fraction of exsolved gas inferred from SO2 data for the upper regions of many pre-eruptive magma bodies (∼30vol%) is similar to the percolation threshold at which gas bubbles become sufficiently interconnected to allow permeable gas flow through a bubble network. Thus this value may reflect a physical limitation on the maximum exsolved gas volume fraction that can occur at the roof zone of a magma reservoir because any additional gas would be lost by advective flow through the permeable bubble network. Andesitic and dacitic magma in crustal reservoirs are probably gas saturated due to recharge and underplating by basaltic magma that is saturated with H2O–CO2–S gas. Comparison of repose times, eruptive volumes, and basaltic magma supply rates for a spectrum of volcanic systems suggests a relatively steady state flux of S from Earth's mantle to atmosphere through many andesitic and dacitic magma systems.

On origin of near-axis volcanism and faulting at fast spreading mid-ocean ridges

At fast and superfast spreading mid-ocean ridges, such as the East Pacific Rise, a plate boundary is defined by a narrow (tens to hundreds of meters wide) neovolcanic zone within which the bulk of the upper oceanic crust is created. However, detailed near-bottom observations indicate that the volcanic construction may occasionally persist several kilometers off of the ridge axis. It has been proposed that off-axis volcanism manifests tapping of a wide melting region that supplies magma to the ridge axis, or spatial migration of magmatic sources in the crust and upper mantle. We demonstrate that off-axis eruptions may be a natural consequence of variations in magma supply rate even if the ridge axis is stationary in space, and the magma delivery is perfectly focussed at the ridge axis. Theoretical modeling and field observations indicate that off-axis volcanism may result from magma emplacement in sills that propagate toward the surface after their characteristic horizontal size exceeds their emplacement depth. Volcanic construction and faulting due to sill intrusions may contribute to the formation of abyssal hills, arguably the most abundant relief form on Earth.

Development of Large Volcanoes on Venus: Constraints from Sif, Gula, and Kunapipi Montes

Sif, Gula, and Kunapipi Montes are large volcanoes on Venus. They have differing morphologies reflecting their different eruptive histories and plumbing systems. All have a steeper upper cone surrounded by more shallowly sloping flanks, interpreted as resulting from histories of more frequent, shorter, lower effusion rate summit eruptions accompanied by infrequent, more voluminous flank eruptions. All three volcanoes have multiple flow field types (digitate, sheet, and fan), which probably reflect variations in effusion rate, eruption duration, and vent geometry. Sif Mons' summit has a large caldera, Gula Mons has summit calderas at either end of a summit rift, and the summit of Kunapipi Mons is composed of two volcanic centers on a summit plateau. The summit morphologies of Sif, Gula, and Kunapipi Montes are interpreted to reflect differences in magma chamber location over time, and differing internal plumbing. We find no positive evidence that neutral buoyancy has governed magma chamber position. Instead, magma chambers at the three volcanoes appear to have changed position over time, with lithospheric stresses as well as neutral buoyancy affecting chamber location. All three volcanoes have had episodes of diking at their summits, followed by episodes of extrusion. We interpret this to be caused by variations in either magma supply rate or magma volatile content, or both, over time. Fractures on the flows of the volcano indicate that dike intrusion has occurred throughout the visible history of the volcanoes, indicating that the local stress field within the volcano favored intrusion as well as extrusion over much of its history. The overall shape and history of the venusian volcanoes is similar to the histories of terrestrial volcanoes. However, some aspects of these volcanoes, including the rift at Gula, the large size and very long flows, differ significantly from terrestrial volcanoes.

Evidence for episodicity in the magma supply to the large Tharsis volcanoes

The volumes and estimated total lifetimes of the Tharsis shield volcanoes on Mars imply that they were built with an average magma supply rate within a factor of 2–3 of ∼0.05 m3 s−1. The morphologies and positions of the summit calderas of these volcanoes, specifically the overlapping relationships between the calderas, are evidence that multiple magma reservoirs existed within them, centered on distinctly different locations at various times. The requirement that the magma in an older reservoir must have cooled below its solidus temperature in order to change the local stresses in such a way that a new reservoir grew at a different location argues for long time gaps (probably tens of Myr) during which the supply rate of magma from the mantle was much smaller than at other times. Conversely, the need to offset conductive cooling to the surface once an active magma reservoir was established implies that the magma supply rate was large, ∼1–10 m3 s−1, for times of order 0.1 Myr. An initial pulse involving rates of more than 150 m3 s−1 acting for at least a few weeks was needed to initiate each new reservoir. The lengths of numerous long lava flows on the flanks of the Tharsis Montes imply that lava must have been erupted at rates of ∼100–300 m3 s−1 for periods of a few years to a few tens of years. The volumes of many of these flows exceed the amounts of magma that could be released from overpressured magma reservoirs of the sizes implied by the dimensions of the summit calderas with the surrounding rocks behaving elastically. A maintained high magma supply rate from the mantle (leading to buffered eruption conditions) is almost certainly required for such flows, whereas inelastic caldera collapse events (unbuffered eruption conditions) can explain the emplacement of other flows. Taken together, the various lines of evidence suggest that the Tharsis volcanoes were built episodically with active phases lasting less than 1 Myr alternating with ∼100 Myr quiet phases. We comment on the implications of these issues for the episodicity of the generation of magma in the mantle and for the mode of transport of the magma in dikes or diapirs.

Constraints on the energy and chemical balances of the modern TAG and ancient Cyprus seafloor sulfide deposits

The size, chemical composition, energy flux, and fluid composition of the TAG hydrothermal sulfide deposit at the Mid‐Atlantic Ridge and the size, chemical composition and reaction zone characteristics of the Skouriotissa volcanogenic massive sulfide deposit of the Troodos ophiolite in Cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit. We conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years, with episodes of activity as short as a few tens of years separated by thousands of years of inactivity. About 2×1019 J of energy supplied at high temperature is required to form a deposit the size of the TAG mound. Metals (up to 4 times the mass of any element present in the sulfide deposit) are leached out of relatively small reaction zones most likely at the base of the sheeted dikes. Chemical balance can be struck for all elements except sulfur with a reaction zone 1–2 km3 in volume from which a small proportion of iron and large proportions of copper, zinc, and manganese are removed. A sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate, as suggested by stable isotope analyses. We argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section. Furthermore, we speculate that activity of the TAG hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in Hawaii and Iceland.

Cyclical summit collapse events at Ascraeus Mons, Mars

The sequential development of multiple summit calderas of shield volcanoes gives clues about how the spatial location of shallow magma chambers evolves with time and how the magma supply into these chambers evolves. We re-interpret the sequence in which caldera collapse events took place at the Martian shield volcano Ascraeus Mons and argue that the formation of the central caldera, usually accepted to be the most recent, was followed by two further collapse events. We also present structural evidence for an early, large central caldera and, therefore, also an early large magma reservoir. This challenges the current hypothesis for Ascraeus Mons that successive magma reservoirs have increased in size as this volcano matured and that a reservoir occupied a central location within the volcano only at the end of its active lifespan. We propose that there has been a cyclical oscillation in the site of shallow storage of magma at Ascraeus Mons between central and peripheral magma reservoirs. The earliest reservoir formed centrally, but magma was subsequently redirected into peripheral reservoirs, over which the peripheral calderas formed. Later, the central location was utilized again, only to be followed once more by a development of peripheral storage reservoirs. This alternating pattern implies substantial variations with position and time in both the magma supply rate from the Martian mantle and the distribution of stresses within the volcanic edifice.

Magmatic history of the East Rift Zone of Kilauea Volcano, Hawaii based on drill core from SOH 1

Deep drilling has allowed for the first time an examination of most of the shield stage of a Hawaiian volcano when it is centered over the hotspot and most of its volume is produced. We determined the lithologies, ages, geochemical characteristics and accumulation rates of rocks from the continuously cored, ∼1.7km deep Scientific Observation Hole (SOH) 1, which was drilled into Kilauea's East Rift Zone. The uppermost ∼750m of this hole contain relatively unaltered subaerially quenched lavas; the lower portion is a mixture of weakly to moderately altered hyaloclastites and massive basalts intruded by dikes. Unspiked K–Ar dating was attempted on 14 lavas but only four yielded geologically reasonable ages. The oldest age is 351±12ka for a sample from 1541m. Eighty XRF major and trace element analyses of lavas from throughout the section demonstrate that these lavas have the same compositional range as historical Kilauea lavas and that there was no systematic geochemical variation for the volcano over the last 350ka. Kilauea's average yearly magma supply during this period was probably ∼0.05±.01km3, which is identical to its average historical magma supply rate. Thus, Kilauea's overall source and melting conditions have remained remarkably constant for the last 350ka despite its having drifted ∼42km over the Hawaiian plume. These results require that the portion of the plume being melted to produce Kilauea magmas be relatively large and well mixed, which is inconsistent with models that place this source on the margin of a radially zoned plume.

Magma Supply Rates at Kilauea

Magma Supply Rates at Kilauea

Extrinsic controls on the evolution of Hawaiian ocean island volcanoes

Extrinsic parameters that affect the evolution of magmatic systems within and beneath ocean island volcanoes include physical variables such as confining pressure, which controls magma degassing, and temperature of the underlying lithosphere and crust, which controls magma crystallization during ascent. Other extrinsic parameters are environmental variables coupled to the hydrosphere and atmosphere such as hydrothermal circulation systems and even rainfall. All these extrinsic factors interact with intrinsic parameters, such as magma supply rates or composition, to modulate the evolution of magma chambers and the petrologic processes that take place within them.

Variations in magma supply and magma partitioning: the role of tectonic settings

Magma supply rates for 200 years at Krafla and Lakagigar, Iceland, and those for 150 years at Kilauea and Mauna Loa, Hawaii, are estimated roughly, based on their geophysical and geological observations. A diagram that relates erupted volumes to eruption intervals at volcanoes under various tectonic settings is represented. These results lead to a new model that a large volume (1–10 km 3) of magma is supplied intermittently at a long interval (10 2–10 4 years) beneath volcanoes in rift zones, while magma is supplied continuously with oscillations or fluctuations beneath intraplate volcanoes. Chemical data such as the MgO wt.% of lava may be one indicator in evaluating the magma supply rates of Hawaiian volcanoes. Systematic variation with time in magma partitioning within a volcano or to the surface is obtained in comparisons between among migration patterns of eruption sites, cumulative supplied volumes, and the volume ratios of erupted to supplied magma at Krafla and Kilauea. The variations suggest that a magma plumbing system may act under self-control (regulating) system through stress as one system. In response to a change in magma supply rate, the system partitions magma horizontally into dikes or vertically toward the surface. A large magma supply rate promotes the vertical extent of a crack to result in an eruption with a large volume ratio of erupted to supplied magma. This tendency is supported by field observations of flood basalts. The partitioned magma as dike intrusions suppresses magma supply partially in the shallow crust. Using analog experiments on liquid-filled cracks in gelatin, this paper demonstrates fundamental processes for magma partitioning on the effect of magma supply and stress change by the partitioned magma. A dynamical system of two differential equations on magma supply rate and stress around a magma plumbing system is proposed, to understand the qualitative variations in magma supply rate imposed by tectonic settings.

The behaviour of the fronts of komatiite lavas in medial to distal settings

Field characteristics of komatiites and observations of flow behaviour and propagation processes for subaerial basalt lavas are used to reconsider the flow behaviour of Archean komatiite lavas. Planar, conformable bases at contacts with pelitic sedimentary strata, the existence of coherent to quench fragmented tops, but the absence of autobreccias, indicate that many komatiite lavas were emplaced passively under laminar flow conditions. To determine how widely this may apply to active komatiite lavas, we model flow front thicknesses. Noting that the preserved flow thickness is thus significantly greater than the thickness of the propagating flow front, we apply the concept of inflation of basalt lava flows to komatiites. Application of the Jeffreys equation allows lava thickness to be determined as a function of flow velocity, viscosity, density and slope of the terrain. Using the results together with estimates of flow front velocities and viscosity in the Reynolds number equation indicates that at expected low flow front velocities in medial to distal settings, most komatiites would have propagated in a laminar flow state. We therefore envisage that komatiites were turbulent in near vent settings and capable of physical erosion, and at times, as previously proposed, channel-forming thermal erosion. As flow area increased downstream, and magma supply rate to the flow front and the flow front velocity decreased significantly, the flow state would have transformed to laminar flow. Preserved flow thicknesses are often considerably greater than the calculated flow front thicknesses for such low viscosity lavas indicating that the final preserved thicknesses are either due to flow inflation, ponding in topography, or that some komatiites were intrusions.

A Rapid Fluctuation in the Mantle Source and Melting History of Kilauea Volcano Inferred from the Geochemistry of its Historical Summit Lavas (1790-1982)

The geochemical variations of Kilauea's historical summit lavas (1790–1982) document the short-term magmatic evolution of one of the Earth's most active volcanoes. Most of these lavas are thought to have erupted directly from the shallow (2–4 km deep) magma reservoir that underlies the volcano's summit region. This paper details a remarkable variation of lava chemistry that spans nearly the entire known compositional range of the volcano in only 200 years. The Pb, Sr, and Nd isotope and incompatible trace element ratios (e.g. La/Yb or Nb/Y) of the lavas vary systematically over time with an abrupt reversal after 1924. This rapid geochemical fluctuation records the temporal changes in the parental magma composition delivered to Kilauea's summit reservoir since 1790. The isotope and incompatible trace element ratio systematics suggest that the source region of historical Kilauea magma is both isotopically and chemically heterogeneous. These source variations can be explained by the melting of small-scale heterogeneities within the Hawaiian mantle plume. Model calculations suggest that the degree of partial melting decreased from the early 19th century until the mid-20th century, which correlates with a lower eruption rate (and presumably a lower magma supply rate) for the volcano between 1840 and 1959. This interval of declining output from the Hawaiian plume culminated with an explosive summit eruption in 1924 and the longest quiescent period in Kilauea's historical record (1934–1952). Lavas erupted just after 1924 are geochemically anomalous and may have been contaminated by the assimilation of country rock into the volcano's magma reservoir during the explosions. Subsequently, the inferred degree of partial melting and the volcano's eruption rate have increased, with the highest values since the early 19th century observed during the current Puu Oo rift zone eruption.

Tectonic stress controls on ascent and emplacement of magmas

The tectonic stresses can significantly affect the propagation of a magma-filled crack. It has been pointed out that the rheological boundaries control the emplacement of magmas through the effect of stress. However, it has not been clarified how the role of rheological boundaries depends on the regional tectonic and thermal states. We have evaluated the role of rheological boundaries under various tectonic and thermal conditions and found that the level of magma emplacement may jump according to the changes in the tectonic force or the surface heat flow. The stress profiles were estimated by a simple model of lithospheric deformation. We employed a three-layer model of the lithosphere; the upper crust, the lower crust and the upper mantle have different rheological properties. A constant horizontal force is applied to the lithosphere, and the horizontal strain is assumed to be independent of depth. When realistic tectonic forces (>10 11 N/m) are applied, the rheological boundaries mainly control the emplacement of magma. The emplacement is expected at the MOHO, the upper–lower crust boundary, and the brittle–ductile boundary. For lower tectonic forces (<10 11 N/m), the tectonic stress no longer plays an important role in the emplacement of magmas. When the tectonic stress controls the emplacement, the roles of rheological boundaries strongly depend on the surface heat flow. When the surface heat flow is relatively high (>80 mW/m 2), the stress in the mantle is quite low and the MOHO cannot trap ascending magmas. For relatively low heat flow (<80 mW/m 2), on the other hand, the MOHO acts as a magma trap, and the upper–lower crust boundary acts as a magma trap only when the magma supply rate is sufficiently high. Our results suggest that the emplacement depth can change responding to the change in the tectonic force and/or that in the surface heat flow. This may provide us a key to understand the relation between the evolution of a volcanic region and its tectonic and/or thermal history.

Shallow-seated controls on styles of explosive basaltic volcanism: a case study from New Zealand

The pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles, e.g., Hawaiian versus Strombolian, or `dry' magmatic versus `wet' phreatomagmatic. These transitions are controlled dominantly by variations in degassing patterns, magma ascent rates and degrees of interaction with external water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre in the Auckland volcanic field, New Zealand, as a case study of the transitions between these end-member eruptive styles. The Crater Hill eruption took place from at least 4 vents spaced along a NNE-trending, 600-m-long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases. Early explosive phases at Crater Hill were characterised by eruption from multiple unstable and short-lived vents; later, dominantly extrusive, volcanism took place from a more stable point source. Most of the Crater Hill pyroclastic deposits were formed in 3 phreatomagmatic (P) and 4 `dry' magmatic (M) episodes, forming in turn the outer tuff ring and maar crater (P1, M1, P2) and scoria cone 1 (M2–M4). This activity was followed by formation of a lava shield and scoria cone 2. Purely `wet' activity is represented by the bulk of P1 and P2, and purely `dry' activity by much of M2–M4. However, M1 and parts of M2 and M4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma:water interaction at each vent. The nature of the wall-rock lithics, and these rapid variations in inferred water/magma ratios imply interaction was occurring mostly at depths of ≤80 m, and the vesicularity patterns in juvenile clasts from these and the P beds imply that rapid degassing occurred at these shallow levels. We suggest that abrupt transitions between eruptive styles, in time and space, at Crater Hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent.

Crustal contamination in Palaeogene East Greenland flood basalts: plumbing system evolution during continental rifting

Palaeogene central East Greenland flood basalts have Sr, Nd and Pb isotope characteristics consistent with crustal contamination of Icelandic-type basalts. In the Lower Basalts of central East Greenland, amphibolite facies gneiss is the dominant contaminant at low stratigraphic levels, whereas contamination with granulite facies gneiss prevails in the upper part of the lava succession. The contamination probably occurred by partial melting of the crust, and can be modelled by assimilation–fractionation–crystallisation processes. Similar contaminants have been detected in lavas from ODP Leg 152 off the SE Greenland coast, although in a reverse sequence. Contamination in Lower Basalts and ODP Leg 152 lavas, which erupted through an Archaean/Proterozoic substrate, differs from lavas in NE Greenland, which overly the Caledonian fold–belt and large Mesozoic sediment deposits. In the NE Greenland lavas, the dominant contaminant appears to be a dacitic component formed by partial melting of Mesozoic sediments. Changes in magma supply rates from the mantle may be the determining factor for the sequence of contaminants and may also control the depth of ponding in crustal chambers. The compositional variation of the least contaminated East Greenland basalts suggests that melts were produced from mantle sources similar to present-day Iceland and North Atlantic MORB at the time of continental breakup but distinction between mantle sources is very complicated. REE systematics indicate a maximum in initial depth of mantle melting at Kangerlussuaq during formation of the East Greenland flood basalts, consistent with a plume being centered in that region.

A multidisciplinary approach combining geochemical, gravity and structural data: implications for pluton emplacement and zonation

Abstract Granitic pluton emplacement and zonation are controlled, among others, by regional deformation and the rate of magma supply. The latter has consequences for the disposition of successively emplaced, more chemically evolved, batches of magma. Our general interpretation is based on a multidisciplinary approach combining field observations, gravity data, internal structures and geochemical variations. Magma feeders are identified in the plutons as the deepest zones, inferred from gravity measurements, when they also correspond to vertical lineations. Correlation of the root zone location with compositional zoning indicate how the magma evolved during emplacement. Two case studies of Hercynian granite plutons illustrate the interpretations: the normally zoned Cabeza de Araya pluton (Spain), and the multiphase Fichtelgebirge pluton (Germany) which displays both normal and reverse zoning. It is proposed that reverse zoning reflects discontinuous magma injection due to a tectonic rate slower than the rate of magma supply. Conversely, normal zoning can occur when magma injection is continuous in time, with successive magma batches entering within not yet crystallized magma. The two case studies illustrate how the understanding of compositional zoning and emplacement of granitic plutons can be improved by multidisciplinary approaches combining classical and modern techniques.

Relationship between the Sr and O isotope compositions of hydrothermal fluids and the spreading and magma-supply rates at oceanic spreading centers

We find a first-order correlation between the Sr and O isotope compositions of hydrothermal fluids and spreading rate at oceanic spreading centers. The isotope signatures of hydrothermal fluids at slow-spreading ridges are more rock dominated than those at fast-spreading ridges, implying that they have undergone a greater extent of Sr and O exchange with the crust. The local spatial and temporal variability in Sr and O isotope signatures within individual hydrothermal sites can be attributed to short-term variations in the hydrothermal system in response to magmatic or tectonic events, phase separation at depth, and mixing with seawater in the upwelling zone. However, these processes cannot explain the systematic global relationship of an increasing rock signature in hydrothermal fluids with decreasing spreading rate. A greater extent of Sr and O exchange between rocks and the circulating fluids can be achieved by (1) a longer fluid-flow pathway through the oceanic lithosphere, (2) more fractured rock that exposes a greater reaction area, (3) an increased residence time of the fluid due to slower hydrothermal circulation, or (4) a combination of all three. Our preferred interpretation is that fluid-flow paths are longer, and depths of penetration are likely greater, at slow-spreading ridges.

Remote monitoring of Mount Erebus Volcano, Antarctica, using Polar Orbiters: Progress and Prospects

Mount Erebus (Antarctica) is a remote and inhospitable volcano, where field campaigns are possible only during the austral summer. In addition to continuously monitoring seismic instruments and video cameras, data from scanners flown aboard polar orbiting space-craft, such as the Thematic Mapper (TM) and Advanced Very High Resolution Radiometer (AVHRR), can contribute to continuous, year-round monitoring of this volcano. Together these data allow measurement of the temperature of, thermal and gas flux from, and mass flux to a persistently active lava lake at Erebus' summit. The monitoring potential of such polar-orbiting instruments is enhanced by the poleward convergence of sub-spacecraft ground-tracks at the Erebus latitudes, permitting more frequent return periods than at the equator. Ground-based observations show that the Erebus lava lake was active with an area of ∼2800m2 and sulphur dioxide (SO2) flux of (230 +/- 90)td-1 prior to September 1984. AVHRR-based lake area and SO2 flux estimates are in good agreement with these measurements, giving (2320 +/- 1200)m2 and (190 +/- 100)td-1, respectively, during 1980. However during late-1984 the lava lake became buried, with TM data showing re-establishment of the lake, with a TM-derived surface temperature of 578-903 C, by January 1985. Following these events, ground-based lake area and SO2 flux measurements show that the lake area and SO2 flux was lower (180-630m2 and 9-91td-1, respectively). This is matched by a decline in the AVHRR- and TM-derived rate of magma supply to the lake from 330 167kgs-1 prior to 1984 to 30-76kgs-1 thereafter. Clearly, a reduction in magma supply to, and activity at, the lava lake occurred during 1984. We look forward to using data from such future polar-orbiting sensors as the Moderate Resolution Imaging Spectrometer (MODIS), Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER), Enhanced Thematic Mapper (ETM+) and Advanced Along Track Scanning Radiometer (AASTR) to contribute to high (once a day) temporal resolution measurement and monitoring of activity at this volcano. Such analyses will in turn contribute to a more complete understanding of how this volcano works.

EVOLUTION OF COMPOSITION OF MAJOR MINERAL PHASES IN LAYERED COMPLEX OF OPHIOLITE ASSEMBLAGE: EVIDENCE FOR THE VOYKAR OPHIOLITES (POLAR URALS, RUSSIA)

We present a detailed study of compositional variation of major minerals through a cross section of the layered complex of the Late Devonian Voykar ophiolite assemblage (Polar Urals). The principal characteristics of this layered complex suggest crystallization from a periodically replenished open magma system in a tectonically dynamic, oceanic environment. The complex may be described in terms of two sequences of cumulus rocks, or megarhythms, that each display an upward progression from ultramafic to gabbroic composition. A transitional zone between the megarhythms is characterized by an upwardly reverse lithologic progression from gabbroic to ultramafic composition. Broad cryptic variation in mineral composition over intervals >100 m parallel changes in the lithologic abundances and suggest changes in the rate of magma supply relative to crystallization and(or) tapping of different mantle sources that had been previously depleted to different degrees. The mineralogy, mineral compositions and isotopic composition of the layered complex coupled with the association of the Voykar ophiolite with island-arc complexes suggest that it most likely formed in a back-arc basin.