Fresh Magma Research Articles

Detecting volcanic eruption precursors: a new method using gravity and deformation measurements

One of the fundamental questions in modern volcanology is the manner in which a volcanic eruption is triggered; the intrusion of fresh magma into a reservoir is thought to be a key component. The amount by which previously ponded reservoir magma interacts with a newly intruded magma will determine the nature and rate of eruption as well as the chemistry of erupted lavas and shallow dykes. The physics of this interaction can be investigated through a conventional monitoring procedure that incorporates the simple and much used Mogi model relating ground deformation (most simply represented by Δ h) to changes in volume of a magma reservoir. Gravity changes (Δ g) combined with ground deformation provide information on magma reservoir mass changes. Our models predict how, during inflation, the observed Δ g/Δ h gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity. Calderas in a state of unrest and large composite volcanoes are the targets for the methods proposed here and are exemplified by Campi Flegrei, Rabaul, Krafla, and Long Valley. We show here how the simultaneous measurement of deformation and gravity at only a few key stations can identify important precursory processes within a magma reservoir prior to the onset of more conventional eruption precursors.

Stable isotope geochemistry of fumaroles: an insight into volcanic surveillance

In active volcanic environments magmatic water may accumulate in the volcanic-hosted geothermal systems, or, more rarely may reach the surface along deep fractures inside the volcano crater. Knowledge of magmatic contribution to emerging fluids in volcanic active areas is critical to understanding the chemical evolution of the magma, the conditions in which it exists in the crust, and the mechanisms by which it erupts in the crust. The source of volatiles (especially water) is also of interest when eruptions are driven by the expansion of hydrothermal fluids against atmospheric pressure, without the involvement of fresh magma (‘hydrothermal’ or ‘phreatomagmatic’ eruptions). In both cases the occurrence of volcanic and/or phreatic activities is likely to be preceded by substantial isotopic and chemical changes in the crater fumarolic systems. H and O isotopic composition of condensed water from crater fumaroles appear to be able to give strong evidence for the existence of magmatic waters in the high-temperature manifestations of the volcanic systems. Isotopic data and specific hydrological models from seven different volcanic systems (Galeras Volcano, Colombia, Kilauea Volcano, Hawaii, Kudryvy Volcano, Kuril volcanic arc, Mt St Helens, USA; Guagua Pichincha, Ecuador; Vulcano island, Italy; the Aegean Volcanic Arc, Greece) are discussed in order to highlight the possibility to use those isotopic parameters in the assessment of the environmental risks of an active volcanic area.

A model of degassing and seismicity at Arenal Volcano, Costa Rica

Arenal volcano is the most active volcano in Costa Rica and has emitted at least 1.3Mt of SO2 since its reactivation in July 1968. Gas emissions from the volcano have been both by passive degassing and explosive eruptions, with passive degassing being dominant. Based on correlation spectrometry (COSPEC) measurements made during 1982, 1995 and 1996, the minimum average daily output is 130±60td−1 SO2 emitted from Arenal. Arenal is extremely active, with tremor fluctuations showing a distinct correlation with Earth tides; decreased explosive activity and increased tremor appear to coincide with the maximum rate of change in Earth tides. This suggests that a system such as Arenal may be extremely sensitive to small changes in the confining pressure or stress regime of the conduit. The cyclic nature of explosive activity also may be caused by corresponding fluctuations in the extrusion rate of lava. At high extrusion rates, lava from the non-explosive conduit may overflow into the explosive conduit, temporarily blocking it with a resulting pressurisation of the system. Arenal is likely tapping a deep to mid crustal magma chamber and, unlike many volcanoes, there is a comparatively small difference between petrological and COSPEC SO2 estimates (0.41 vs. 1.3Mt, respectively, since 1968), suggesting that Arenal is being continuously supplied by fresh magma.

Sulfur and chlorine degassing from primitive arc magmas: temporal changes during the 1982–1983 eruptions of Galunggung (West Java, Indonesia)

The 1982–1983 eruptions of Galunggung represent a nine-month period of intermittent volcanic activity with significant changes in explosivity and emission of volatiles. Eruptions started with Vulcanian explosions but changed gradually to Strombolian activity. Compositions of juvenile material changed from basaltic andesite to high-Mg basalt, which are among the most primitive rock types known in the Indonesian arc system. Although bulk compositions suggest a single evolution trend, we infer from the compositions of melt inclusions in olivine phenocrysts that the magmas represent derivatives of a complex spectrum of primary melts. Primitive inclusions in olivine phenocrysts from magma erupted during the Strombolian phase contain up to ∼2000ppm sulfur, but concentrations decrease rapidly with increasing SiO2 down to matrix glass values (50–100ppm). ‘Vulcanian’ inclusions appear to be degassed before eruption (∼200ppm S). Chlorine concentrations increase from 750 to 2200ppm in Strombolian, and from 800 to 1500 in Vulcanian magmas, whereas matrix glass contains about 1000 ppm in both cases. Ash leachates show two cycles of decreasing S/Cl ratios: from 9.7 to 5.6 at the start of the activity, and from 12.2 to 2.0 after four months. As the second cycle follows upon increased seismic activity at shallow depth, it probably reflects degassing of fresh sulfur-rich magma arriving in the shallow Galunggung reservoir. In contrast to the degassed state of Vulcanian magma, the significant amounts of adsorbed sulfur on the ashes point to an excess source of sulfur, which was most likely derived from intruding Strombolian magma. Hence, the observed sulfur flux of 2Mt is not in accordance with a petrologic estimate of 0.09Mt. Using a published value of 550Mt of erupted material about 0.34km3 fresh undegassed magma is needed to account for the observed sulfur flux. This is close to the erupted volume of Vulcanian magma (0.26km3), which presumably was replaced completely by Strombolian magma during the eruption. Using the petrologic method, we calculate a total release of 0.3Mt chlorine, which agrees well with an output of 0.47Mt estimated independently from S/Cl ratios of the ash leachates and TOMS sulfur yields. Ash leachates show that about 35% of the sulfur and 30% of the chlorine was scavenged from the eruption plumes. Our results suggest that sulfur and chlorine were largely decoupled during degassing, which resulted in considerable variations in S/Cl ratios during the Galunggung eruptions. We infer that sulfur degassing reflects the arrival of fresh magma at shallow depth, whereas chlorine is largely derived from simultaneously erupted material. As a consequence, the petrologic estimates are more consistent with observed emissions for chlorine than for sulfur.

Geochemical evidence for the existence of high-temperature hydrothermal brines at Vesuvio volcano, Italy

A high-temperature hydrothermal system is present underneath the crater area of Vesuvio volcano. It is suggested that NaCl brines reside in the high-temperature reservoir and influence the chemical composition of the gases discharged by the fumaroles of the crater bottom (vents FC1, FC2, and FC5). These have typical hydrothermal compositions, with H 2O and CO 2 as major components, followed by H 2, H 2S, N 2, CH 4, and CO (in order of decreasing contents) and undetectable SO 2, HCl, and HF. Fumarolic H 2O is either meteoric water enriched in 18O through high-temperature water-rock oxygen isotope exchange or a mixture of meteoric and arc-type magmatic water. Fumarolic CO 2 is mainly generated by decarbonation reactions of marine carbonates, but the addition of small amounts of magmatic CO 2 is also possible. All investigated gas species (H 2O, CO 2, CO, CH 4, H 2, H 2S, N 2, and NH 3) equilibrate, probably in a saturated vapor phase, at temperatures of 360 to 370°C for vent FC1 and 430 to 445°C for vents FC2 and FC5. These temperatures are confirmed by the H 2-Ar geoindicator. The minimum salt content of the liquid phase coexisting with the vapor phase is ∼14.9 wt.% NaCl, whereas its maximum salinity corresponds to halite saturation (49.2–52.5 wt.% NaCl). These poorly constrained salinities of NaCl brines reflect in large uncertainties in total fluid pressures, which are estimated to be 260 to 480 bar for vents FC2 and FC5 and 130 to 220 bar for vent FC1. Pressurization in some parts of the hydrothermal system, and its subsequent discharge through hydrofracturing, could explain the relatively frequent seismic crises recorded in the Vesuvio area after the last eruption. An important heat source responsible for hydrothermal circulation is represented by the hot rocks of the eruptive conduits, which have been active from 1631 to 1944. Geochemical evidence suggests that no input of fresh magma at shallow depths took place after the end of the last eruptive period.

THE STRUCTURE OF THE LUKKULAISVAARA INTRUSION, OULANKA GROUP, NORTHERN KARELIA: PETROLOGICAL IMPLICATIONS

The Lukkulaisvaara intrusion (U–Pb age: 2442 ± 1.9 Ma), in northern Karelia, Russia, belongs to the Oulanka plutonic group. The intrusion cuts rocks of the Archean granite – migmatite – gneiss basement and is disconformably overlain by Proterozoic metavolcanic rocks. The layered body does not exceed 4600 m in thickness; data obtained on the structural profile of the intrus ion suggest a complete section. Its weighted-mean composition is equivalent to a magma of the marianite–boninite series, but whethe r or not this composition corresponds to the parental magma is uncertain owing to possible multiphase features of the intrusion. It is quite possible that large and small lenticular bodies of fine-grained gabbronorite whose texture suggests rapid crystallizat ion are associated with injections of fresh magma. Crystallization to fine-grained gabbronorite in the process of magma chilling is related, in turn, to decompression. Chilling would be equally feasible in the case of the injection of residual melts, squeezed from lower horizons and already carrying cumulus minerals in the process of compaction. Structures in which fine-grained rocks occur do not differ from “potholes” in morphology and structural setting. Two genetic types of sulfides are distinguished: (a) sulfid es of magmatic stage are present in fresh rocks, commonly with magmatic quartz and biotite; (b) a metasomatic sulfide assemblage contains the richest sulfide and platinum mineralization and is related to potholes. Amounts of sulfide in the metasomatic rock s are very variable (1–30 vol.%). Forty platinum-group minerals have been documented in sulfide-bearing metasomatic rocks; the concentration of noble metals (Pt + Pd) covers a wide range (0.3–10 ppm Pt, 0.42–66 ppm Pd). Sulfides in the cumulates and iron bearing magmatic minerals altered by metasomatism are considered to be the source of the ore-bearing metasomatic assemblages. These assemblages were formed under the action of a reducing hydrothermal fluid. A high content of chlorine was observed in biotite (up to 0.55 wt%), amphibole (up to 2.5 wt%), chlorite (up to 0.1 wt%), and scapolite (up to 2.3 wt%). Examination of th e metasomatic quartz reveals the presence of different types of micro-inclusions. Some are filled with liquid only, others are ga s– liquid, aqueous – salt inclusions with a gas bubble, and hydrocarbon inclusions with various amounts of liquid. The highest T h recorded is 370°C at a pressure of 1.5 kbar, as estimated using aqueous – salt inclusions. These data agree well with thermobarometric results calculated using estimated equilibration states. Nd and Sr isotopic data suggest that the metasomatic assemblages formed simultaneously with the layered intrusion (2442 Ma) during an autometasomatic process due to reworking of intrusive rocks by a mantle-derived fluid with an Nd of +2.1 and an initial 87 Sr/ 86 Sr value of 0.7028.

Degassing of stratified magma by compositional convection

A new model is proposed for passive degassing from sub-volcanic magma chambers. The water content in stably stratified shallow magma chamber will be equated to its solubility at the upper boundary by convection. Water from a lower layer high in water content can enrich the contact zone of the upper layer and lead to further convective overturn of this boundary layer. A complete set of equations describing convection with bubble formation and dissolution is reduced to a simplified form by assuming a small bubble content. The development and pattern of flow driven by vesiculation is modeled numerically in a 2D magma chamber for relatively low Raleigh numbers (5×10 5). Bubbles rising from the magma will collect near the roof in a layer of 8–10 vol% and then escape upward to fumaroles. The Stokes flux of bubbles escaping from an andesitic magma with viscosity 10 4 P and a top surface of about 500×500 m corresponds with observed total magmatic water fluxes of 35 kg/s. Pressure within the chamber is buffered by elastic (and local visco-elastic) deformations in the solid rocks bounding the chamber to the range between ambient and close to lithostatic values. In a chamber closed to fresh magma inputs, the decrease in volume due to such gentle volatile escape lowers the reference pressure. Bubbles flux from the lower layer induced by variation of the saturation level around stratification boundary may be efficient mechanism for the water transport between layers.

Growth and Deformation of the Ladakh Batholith, Northwest Himalayas: Implications for Timing of Continental Collision and Origin of Calc-Alkaline Batholiths.

The calc-alkaline Ladakh batholith (NW Himalayas) was dated to constrain the timing of continental collision and subsequent deformation. Batholith growth ended when collision disrupted subduction of the Tethyan oceanic lithosphere, and thus the youngest magmatic pulse indirectly dates the collision. Both U-Pb ages on zircons from three samples of the Ladakh batholith and K-Ar from one subvolcanic dike sample were determined. Magmatic activity near Leh (the capital of Ladakh) occurred between 70 and 50 Ma, with the last major magmatic pulse crystallizing at ca. 49.8+/-0.8 Ma (2sigma). This was followed by rapid and generalized cooling to lower greenschist facies temperatures within a few million years, and minor dike intrusion took place at 46+/-1 Ma. Field observations, the lack of inherited prebatholith zircons, and other isotopic evidence suggest that the batholith is mantle derived with negligible crustal influence, that it evolved through input of fresh magma from the mantle and remelting of previously emplaced mantle magmatic rocks. The sedmimentary record indicates that collision in NW Himalaya occurred around 52-50 Ma. If this is so, the magmatic system driven by subduction of Tethys ended immediately on collision. The thermal history of one sample from within the Thanglasgo Shear Zone (TSZ) was determined by Ar-Ar method to constrain timing of batholith internal deformation. This is a wide dextral shear zone within the batholith, parallel to the dextral, N 30 degrees W-striking crustal-scale Karakoram Fault. Internal deformation of the batholith, taken up partly by this shear zone, has caused it to deviate from it regional WNW-ESE trend to parallel the Karakoram Fault. Microstructures and cooling history of a sample from the TSZ indicate that shearing took place before 22 Ma, implying that (1) the history of dextral shearing on NW-striking planes in northern Ladakh started at least 7 m.yr. before the <15 Ma Karakoram Fault, (2) shearing was responsible for deviation of the regional trend of the Ladakh batholith, and (3) dextral shearing occured within a zone apporximately 100 km wide that includes the Ladakh batholith and portions of the younger Karakoram batholith.

Seismic scaling exponents as a tool in detecting stress corrosion crack growth leading to the September–October 1989 flank eruption at Mt. Etna Volcano

The September–October 1989 flank eruption at Mt. Etna marked a change in the recent eruptive style of the volcano. Seismicity, at mid (order of months) and short (order of days) time, preceding the eruption onset has been analysed by means of D (scaling exponent), inferred from the seismic b value, that represents the scaling exponent of fracture length distribution. Time evolution of D preceding the eruption onset revealed: a general increase of D (9 to 4 months prior the eruption onset), intermittent rates of D decreases and increases (4 to 1 month), steady values of D (1 month) and sharp decreases of D marking the eruption onset. Integration of these results with ground deformation data reveals that at Mt. Etna stress corrosion crack growth, enhanced by fresh magma arrival, is a plausible mechanism for weakening the country rock, by promoting crack growth and the coalescence of major faults.

A fifty year perspective of magmatic evolution on Ruapehu Volcano, New Zealand: verification of open system behaviour in an arc volcano

Geochemical and petrological data for samples from well documented eruptions that occurred at Ruapehu volcano over the period 1945–1996 can be used to illustrate the complexity of short term geochemical variation in an arc-type volcano. Collectively, data from Ruapehu Volcano show trends with time of increasing SiO 2 abundance and rising 87Sr/ 86Sr ratios, consistent with broad control by assimilation and crystal fractionation processes (AFC). However, the magmas emplaced during the past fifty years show geochemical variability that spans most of the range shown by lavas erupted over the entire history of the volcano. Magma compositions fluctuate through wide ranges over relatively short time intervals reflecting the effects of processes associated with magma recharge events within the volcano. These complex trends are also manifested when the geochemistry and petrography of sequences of prehistoric lavas are examined in detail and they arise from short-term effects that are imposed during recharge on the overall AFC trend. We show that the temporal geochemical and petrographic variations among erupted magmas are modulated by processes of mixing and mingling between fresh magma from below and stagnant melt and entrained crystals from earlier events remaining in the volcanic edifice, probably in dikes and sills. These processes are probably replicated over longer time periods (hundreds to thousands of years) as melts arrested at different levels in the near surface conduit system are progressively displaced by new magma batches. Arc type volcanoes such as Ruapehu are characterised by pulsatory growth in which bursts of high magma production are superimposed on a background of subdued but more or less continuous activity. This style of activity is difficult to predict through the usual (seismicity, ground deformation, lake water geochemistry) volcano monitoring techniques, and petrology and geochemistry may provide the basis for an alternative strategy.

Geology and petrology of the Voisey's Bay intrusion: reaction of olivine with sulfide and silicate liquids

The Voisey's Bay Ni–Cu–Co sulfide deposit is located in Eastern Labrador, and transects the 1.85 Ga collisional boundary between interbanded garnet–sillimanite and quartzo-feldspathic paragneisses (Tasuiyak gneisses) of the Proterozoic Churchill Province to the west and granitic, intermediate and mafic orthogneisses of the Archean Nain Province to the east. The deposit is associated with an upper troctolitic chamber (the Eastern Deeps chamber) which is connected to a lower troctolitic chamber (the Reid Brook chamber) by a gabbroic/troctolitic feeder sheet approximately 1 km in vertical extent. Rock sequences that have been defined to date include Leuco-troctolite (LUT) in the Reid Brook chamber, Feeder Breccia (FB) in the Reid Brook Zone, Feeder Olivine Gabbro (FOG), Leopard Troctolite (LT) and Basal Breccia Sequence (BBS) in the feeder sheet, and Olivine Gabbro (OG), Normal Troctolite (NT) and Varied-textured Troctolite (VT) in the Eastern Deeps chamber. Mineralisation has been found in the feeder sheet, as massive stringers in gneisses flanking the feeder, in the `Ovoid', a 600×300×110 m 3 basin of massive sulfide at surface, and as a zone of massive sulfide that has developed close to the entry of the feeder to the Eastern Deeps chamber. The mineralisation comprises pyrrhotite (both hexagonal and troilite in varying proportions), pentlandite, chalcopyrite, magnetite and minor cubanite. An important component of all mineralized zones is a breccia of gneissic fragments in a troctolitic to noritic matrix referred to as BBS or FB. Considerable reaction has occurred between the troctolite magma and the gneiss inclusions. Olivines in the LUT, OG and FOG have low Ni contents relative to their forsterite (Fo) contents. Olivines in the NT and VT have higher Ni contents but show a great scatter in both their Ni and Fo contents. The Ni content of olivine in the LT increases systematically with decreasing Fo content. All of these variations are explained in terms of fractional crystallisation of magma, sulfide segregation, trapped liquid shift and re-equilibration between sulfide and olivine. Our current understanding of the development of the deposit is that an initial pulse of magma rose to a lower chamber, fractionated to form mafic-ultramafic cumulates and at the same time became sulfide-saturated as a result of interacting with the enclosing, sulfide- and graphite-bearing Tasiuyak gneisses. The magma lost much of its Ni and Cu as it fractionated olivine and segregated sulfides. A new wave of fresh magma entered the lower chamber, forcing the chalcophile-depleted magma, along with some of its contained sulfide, up the feeder to spread out in the upper chamber. Remnants of this chalcophile-depleted magma are preserved as olivine gabbro which forms upper layers in the upper chamber, and which in places has frozen to the walls of the feeder. Sulfides that were swept up in this event were forced out into fractures in the feeder walls, are preserved within thickened zones within the feeder, and were deposited as the magma flow decreased at the entry of the feeder to the upper chamber. Continued influx of the later, undepleted magma resulted in sulfides being picked up, transported farther up the system, upgraded during transportation and then redeposited.

Magma replenishment, and the significance of poikilitic texlures, in the Lower Main Zone of the western Bushveld Complex, South Africa

AbstractPetrographic and compositional variations in the Lower Main Zone (LMZ) of the western Bushveld Complex indicate changing regimes of magma replenishment. The lowermost unit of the LMZ, designated N-I, is an enigmatic sequence of leuconoritic cumulates, characterized primarily by up-sequence increases in both orthopyroxene Mg# and whole-rock Sr isotope initial ratio. The Sr isotope profile of N-I is ascribed to injection and progressive integration of small influxes of fresh magma with high (Main Zone-type) Sr isotope initial ratios. The basal Fe-enrichment in N-I, on the other hand, is ascribed to a separate, later mechanism involving the downward migration of late-stage Fe-rich liquids. The overlying two units, N-II and G-I, delineated chiefly in terms of basal Mg-enrichment of orthopyroxene, are ascribed to injections of fresh magma into the chamber. Poikilitic orthopyroxene grains in the basal parts of both N-II and G-I suggest entrainment and partial resorption of plagioclase grains from the semi-crystalline resident material into which the fresh magma was intruded.

Long magma residence times at an island arc volcano (Soufriere, St. Vincent) in the Lesser Antilles: evidence from 238U– 230Th isochron dating

High-precision (TIMS) measurements of U and Th isotope concentrations have been determined on whole rocks and mineral separates from Soufriere volcano on St. Vincent in the Lesser Antilles island arc. The whole rocks display relatively constant U/Th ratios (∼0.5), and are characterised by excess 238U relative to 230Th which is attributed to the addition of U-rich fluids from the subducting slab to the mantle source of the magmas. Mineral isochrons for four recently erupted (<4 ka) rocks yield ages of 46–77 ka and the combined data have an age of 58±7 ka with an initial ( 230Th/ 230Th) ratio of 1.05. Mixing of young magmas, or magma–cumulate mixing, are unlikely to be responsible for these isochronous relationships. Contamination of young, phenocrystic magmas with previously crystallised magma could produce the isochrons, in which case the ages provide maximum and minimum ages, respectively, for these two components. However, no lavas with ( 230Th/ 230Th) ratios of 1.054 have been found and there is no petrographic evidence for mixing or phenocryst–magma disequilibrium. Our preferred interpretation is that the isochrons reflect long magma residence times within the arc crust. Following initial crystallisation, due to heat loss to the wall rocks, the magmas were maintained at a temperature close to their liquidus while 230Th-ingrowth occurred in both the magma and the crystals. Maintenance of a relatively constant temperature requires good insulation by cumulate layers and probably heating from below by influxes of fresh magma which either did not mix with the resident magma or else had an essentially indistinguishable composition. Subtraction of the residence times from the ∼90 ka inferred to have elapsed since fluid addition to the mantle wedge leaves ∼30 ka for transfer through the wedge. Long residence times may typify more evolved lavas that develop in thicker arc crust whereas tholeiitic arc lavas are inferred to have trivial crustal residence times but similar transfer times through the mantle wedge.

Pit crater structure and processes governing persistent activity at Masaya Volcano, Nicaragua

Persistent activity at Masaya Volcano, Nica- ragua, is characterised by cycles of intense degassing, lava lake development and pit crater formation. It pro- vides a useful site to study the processes which govern such activity, because of its easy accessibility and rela- tively short cycles (years to decades). An understanding of the present activity is important because Masaya is visited by large numbers of tourists, is located close to major cities and has produced voluminous lavas, plin- ian eruptions and ignimbrites in the recent past. We provide structural and geophysical data that character- ise the "normal" present state of activity. These indi- cate that the ongoing degassing phase (1993 to present) was not caused by fresh magma intrusion. It was asso- ciated with shallow density changes within the active Santiago pit crater. The activity appears to be asso- ciated predominantly with shallow changes in the pit crater structure. More hazardous activity will occur only if there are significant departures from the present gravity, deformation and seismic signatures.

The volcanic history of Ruapehu during the past 2 millennia based on the record of Tufa Trig tephras

Tufa Trig Formation comprises a sequence of at least 19 andesitic tephras erupted from Mt. Ruapehu (Tongariro Volcanic Centre, New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Ruapehu, dated between ca. 1850 years B.P. and the present. Members of the Formation show restricted dispersals, principally to the east of Mt. Ruapehu. Volumes calculated for the most widespread members are all less than 0.1 km3. Compared with other Mt. Ruapehu eruptives, Tufa Trig Formation tephras represent small eruptions that have contributed little tephra to the ring plain. They do, however, show a greater frequency of eruption with one event occurring on average every 100 years. Tufa Trig Formation members Tf3–Tf18 are black to dark grey, vitric, coarse-ash and lapilli-grade tephras which mantle the relief. They contain juvenile vitric particles which exhibit varying degrees of vesicularity, together with free crystals of pyroxene and feldspar, and few lithic fragments. Several morphological types of vitric pyroclasts are recognised in these tephras, the dominant type being of equant blocky morphology with fracture-bound surfaces (type-1 morphology). Field characteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members (Tf3–Tf18) are the products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric water. We propose that a source of this water was an ancestral crater lake which formed within the late Holocene ca. 3000 years B.P. The morphological, compositional, and chemical (major-element) characteristics of three Tufa Trig Formation Tephras are compared with those of two new tephras erupted from Ruapehu Volcano during the October 1995 eruptions which comprise part of a newly defined member (Tf19) of Tufa Trig Formation. The comparisons support our interpretation that the majority of the Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other members of the Formation (Tf1 and Tf2) are coarse-grained scoriaceous tephras and are interpreted to be the products of strombolian events.

Petrology and geochemistry of the ultrapotassic rocks from the Sabatini Volcanic District, central Italy: the role of evolutionary processes in the genesis of variably enriched alkaline magmas

The Sabatini Volcanic District (SVD) is a large volcanic field characterised by the lack of any major volcanic center. Its activity, spread over a wide area, started at 0.6 Ma and developed through five main phases, during which several calderas and the Bracciano lake volcano-tectonic depression were formed. All the volcanic rocks belong to the Roman-type ultrapotassic series (HKS), ranging from leucite tephrites to leucite and haüyne phonolites. Although the major- and compatible-element contents indicate a single series of evolution, there are differences in the incompatible trace-element abundances. A high-Ba series (HBaS) has been distinguished from a low-Ba series (LBaS), with the former also enriched in all other incompatible elements (e.g., REE, Nb, Zr, Th) except Rb. The HBaS rocks are plagioclase-free, leucite-bearing lavas and were abundantly outpoured from the Bracciano volcanoes during the third and fifth phase of activity. Plagioclase- and phlogopite-bearing rocks constitute the LBaS and were erupted during the other phases generally from smaller and eccentric volcanic centers. The initial 87Sr/ 86Sr values are higher in the HBaS rocks and do not vary significantly with magma evolution (0.71047–0.71080), but cover a wider range in the LBaS rocks (0.70944–0.71038), with the lowest Sr isotope ratios occurring in the least evolved lavas. The higher Ca content in the olivine and Ti and Al IV in the clinopyroxene, and the lower ulvöspinel content of the Ti-magnetites of the HBaS rocks suggest an evolution at lower pressure and higher temperature for this magma. The observed petrologic characteristics suggest that the HBaS magma evolved at lower depths by processes of refilling, tapping, fractionation and probably assimilation (RTFA), where the crystallisation rate of clinopyroxene+leucite±olivine dominates over the input rate of the fresh magma. The LBaS magma evolved at slightly higher pressure, in separate and small magma bodies, by fractional crystallisation of clinopyroxene+plagioclase±phlogopite±olivine that was often associated with crustal assimilation (AFC). It has been suggested that RTFA processes with high input rate/crystallisation rate ratios could also be responsible for the differentiation between the HBaS and LBaS. The different processes of evolution undergone by the HBaS and LBaS could have been related to the different volumes of magma rising from the source.

Mineralogic and Oxygen Isotopic Studies of Open System Magmatie Processes in the South Kawishiwi Intrusion, Spruce Road Area, Duluth Complex, Minnesota

The South Kawishiwi intrusion, located along the western margin of the Duluth Complex, Minnesota, is one of several composite intrusions that are found in the Complex. The Duluth Complex is the principal exposed plutonic portion of the 1-1 Ga Midcontineni Rift system. In the Spruce Road area the South Kawishiwi intrusion is divided into seven distinct units that are part of the broader South Kawishiwi Troctolite Series defined by Severson (Tech. Rep. NRRI/TR-91/13a, Natural Resources Research Institute, University of Minnesota, Duluth, 1994). Units may be characterized as follows: Unit I—basal accumulation of heterogeneous gabbro, troctolite, and norite; Unit II—norite with abundant inverted pigeonite; Unit III—troctolite and olivine gabbro with local oxide-rich layers; Unit IV—melatroctolite, troctolite, olivine gabbro; Unit V— increased plagioclase abundance in troctolites and leucocratic troctolites; Unit VI—strongly altered troctolite; Unit VII— similar to Unit V, troctolite and leucocratic troctolite. Country rocks in the Spruce Road area are granodiorite to quartz monzonite of the Archean Giants Range Batholith. Sutfide mineralization, consisting of 1-5 vol. % of disseminated pyrrhotite, cubanite, chalcopyrite andpentlandite, occurs in Units I, II, III, and VI. Oxygen isotopic analyses indicate that Unit II has experienced extensive crustal contamination. 5O values of Unit II range from 69 to 7-l%o and are 0 enriched compared with values of 5-1-6-8%o found in other units. Silica contamination is indicated based not only on 5' 0 values, but also by the predominance of orthopyroxene in the unit. Possible high-' 0 contaminant rocks include the Giants Range Batholith and pelitic rocks of the Lower Proterozoic Virginia Formation or Biwabik Iron Formation. Mass balance computations suggest that units in the Spruce Road area may be related through varying degrees offractionation of a high-Al, olivine tholeiite magma. Modeling of trace element concentrations and variations in mineral chemistry suggest that discontinuities within the major units developed by in situ boundaryAayer equilibrium crystallization of solidification zones ~ 20—50 m in thickness, followed by recharge of fresh magma. Upward enrichment of incompatible elements, olivine Fa content, and plagioclase Ab content may be effectively explained by this process. 5 0 values of uncontaminated rock types are strongly correlative with modal mineralogy, and can also be modeled by boundary-layer fractionation. A parental magma 5 0 value of ~6-3%o is calculated for Unit VII based on olivine and plagioclase values, and is similar to that of several other large, layered mafic intrusives.

The size‐isotopic evolution connection among layered mafic intrusions: Clues from a Sr‐Nd isotopic study of a small complex

Several theoretical and experimental works have focused on the processes occurring in continental mafic magma chambers. In contrast, systematic isotopic studies of natural remnants of these latter remain scarce, although they can give fundamental constraints for theoretical studies. This is especially true if different layered complex with contrasting characteristics (e.g., different size) are compared. For this reason, we present the results of a Sr‐Nd isotopic profile across a small layered mafic intrusion of Permian age exposed near Fozzano (SW Corsica). In the main zone of the layered section, decreasing 87Sr/86Sri and increasing 143Nd/144Ndi are observed from less evolved (bottom) to more evolved (top) rocks. This peculiar pattern precludes assimilation and fractional crystallization (AFC) as a dominant mechanism in the petrogenesis of this body. Instead, we interpret this trend as reflecting the dilution of an early stage contaminated magma by several reinjections of fresh basalt in the chamber. In agreement with mineralogical and structural data, every cyclic unit is interpreted as a new magmatic input. On the basis of rough refill and fractional crystallization (RFC) calculations, the average volume for each reinjection is estimated to have been about 0.04 km3. The cumulative volume of these injections would amount to about 75% of the total volume of the layered complex. This implies that reinjections were accompanied by an important increase of the volume of the chamber or by magma withdrawal by surface eruptions. The RFC mechanism documented within this small layered body contrasts with the isotopic pattern observed between several intrusions at the regional scale in SW Corsica, and within large continental mafic magma chambers elsewhere. In these cases the isotopic evolution is dominated by AFC processes, and there is no clear isotopic evidence for reinjections, unless major influx of fresh magma occurred. It is suggested that there is a close relationship between the assimilation rate and the magma chamber volume. Small magma chambers are quickly isolated from their country rocks and better preserve the subtle isotopic signature of reinjection processes.

Helium isotope and gas discharge variations associated with crustal unrest in Long Valley Caldera, California, 1989–1992

The onset of anomalous seismic activity in 1989 beneath Mammoth Mountain on the southwestern rim of the Long Valley caldera, California, was followed within ∼4 months by a large increase in 3He/4He in vapor discharged from a fumarole on the north side of the mountain. The helium isotopic ratio at this vent rose to a maximum of 6.7 RA in July 1990 and subsequently declined to values near 5 RA. Potential sources of the 3He‐rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous rocks, and volatile release from a 3He‐rich gas chamber situated above previously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7 RA), the persistence of relatively high values (&gt;5 RA) over a period of 3 years, the increase in the flux of total He relative to gases in air‐saturated water, and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion may be as shallow as 2 km, consistent with the prompt appearance of increased 3He/4He ratios in the fumarolic gas, and that the intrusive process may have persisted for ∼1 year. In contrast, a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome‐south moat region during the 1989–1991 period resulted in at most relatively small changes in 3He/4 He in fumarolic discharge at the southern edge of the resurgent dome. The more subdued response may result from a combination of greater intrusive depths and greater dilution of 3He‐rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with Mammoth Mountain.

Radiative temperature measurements at Kupaianaha Lava Lake, Kilauea Volcano, Hawaii

Field spectroradiometer data in the wavelength range of 0.4–2.5 μm and spectral resolution of 1–5 nm have been used to compute the radiative temperature of the surface of Kupaianaha lava lake, Kilauea Volcano, Hawaii. Two sets of observations (a total of 120 spectra) were made on October 12, 1987, and January 23, 1988, when the lava lake was in a period of active overturning. The area of the surface for which temperatures were measured was ∼0.23–0.55 m2. Two numerical models of two and three components have been used to match the measured radiant flux ratios and to describe the surface of the lava pond in terms of radiant area and temperature. Three stages of activity on the lake surface are identified: Stage 1, characterized by magma fountaining and overturning events exhibited the hottest crustal temperatures (180–572°C) and the largest fractional hot areas (&gt; 10−3). Stage 1 average flux densities were ∼2.2 × 104 W/m2, the highest recorded for the three stages of activity on either day. The largest radiative area of fresh magma was 29% at 1100°C, while cooling from magmatic temperatures to newly formed crust at 790°C took place in a matter of seconds. Stage 2, marked by rifting events between plates of crust, exhibited crustal temperatures between 100 and 340°C with fractional hot areas at least an order of magnitude lower than those found for stage 1. Average flux densities calculated for three examples of stage 2 activity were 5.3 × 103 W/m2. Stage 3, which was quiescent periods when the lake was covered by a thick crust, dominated the activity of the lake both temporally and spatially over 90% of the time. The characteristic crustal temperature of stage 3 was 80–345°C with most solutions near 200–300°C and fractional hot areas of ≤ 10−5 of the viewing area. Average flux densities for stage 3 were 4.9 × 103 W/m2. For many stage 3 examples, a two‐component model was sufficient to describe the spectral data; however, for almost all of the stage 1 and 2 examples and the remainder of the stage 3 examples a three‐component model was required. These determinations of lava temperature and radiant area have relevance for satellite and airborne measurements of the thermal characteristics of active volcanoes and indicate that temporal variability of the thermal output of lava lakes occurs on the time scale of seconds to minutes.