Dacite Melt at the Puna Geothermal Venture Wellfield, Big Island of Hawaii
A dacite melt was encountered during routine commercial drilling operations of injection well KS-13 at the Puna Geothermal Venture wellfield, Big Island of Hawaii. The KS-13 drill hole, drilled in 2005, is located along a segment of the Kilauea Lower East Rift Zone which erupted basalt flows from rift-parallel fissures in 1955. During the drilling of KS-13 a 75-meter interval of microdiorite containing brown glass inclusions was penetrated at a depth of 2415 m. At a depth of 2488 m a melt of dacitic composition was encountered. The melt flowed up the wellbore and was repeatedly redrilled over a depth interval of ~8 m, producing several kilograms of clear, colorless vitric cuttings at the surface. The drill bit, when recovered at the surface, was missing several carbide insert teeth. Presumably the inserts were plucked cleanly from their sintered cone sockets due to differential thermal expansion under extreme heat conditions. The dacitic glass cuttings have a perlitic texture, a silica content of 67 wgt.%, are enriched in alkalis and nearly devoid of mafic minerals with the exception of rare pyroxene phenocrysts and minor euhedral to amorphous magnetite. The melt zone is overlain by an interval of strong greenschist facies metamorphism in basaltic and dioritic dike rock. The occurrence of an anhydrous dacite melt indicates a rock temperature of approximately 1050° (1922°F) and sufficient residence time of underlying basaltic magma to generate a significant volume of differentiated material. The dacite, with an inferred temperature of 1050 °C, is separated by 526 m of rock from the deepest overlying permeable zone in KS-13 at a temperature of 356 °C. The thermal gradient through this impermeable rock section is ~700°C/526 m = 1.331 °C/m. The calculated conductive heat flux from the magma upward into the deepest zone of hydrothermal circulation is given by k×(dT/dZ)=2.9 × 1.33 = 3.83 W/m2 = 3830 mW/m2 (thermal conductivity k=2.9 W m-1 °C-1 for basalt). This heat flux is an order of magnitude greater than the average of 270-290 mW/ m2 typical for the mid-ocean ridges. The high heat flux is sufficient to power the overlying commercial geothermal wellfield which has been producing 28 MW of net electrical power continuously since 1993.
Puna Dacite: Likely Temperature, Viscosity, Origin, Size, and Parent Body Nature
This is very likely the first accidental encounter of an in situ live magma within Earth. The importance of this occurrence to the possible ongoing interrogation of an active, docile magma cannot be overemphasized. Here we report on inferences on the nature of the magma and its relation to a parent basaltic body. The Glass: In oil the glass is colorless with 5-8 % euhedral, nonquench crystals of plagioclase, Fe-Ti oxide, orthopyroxene, and apatite. There is no vesiculation and the glass is unstructured except for patches of perhaps incipient spherulite and swirls, which may reflect drilling shear and quenching. Temperature: The temperature is inferred first using the bulk glass composition and matching the visually estimated crystallinity to that computed by MELTS, giving a temperature of 1050 C. Second, from a likely basaltic parent composition (1955 basalt) and matching the glass composition to the residual melt from protracted crystallization in MELTS, also gives a temperature of 1050 C. Comparing the dacite to the observed compositions of interstitial melts from the lava lakes, suggests a slightly higher temperature of 1065 C, reflecting the different parent basalt. One atm melting experiments confirm the former T. Magma Viscosity: The flow up the drill hole (25.88 cm diameter) can be used to estimate viscosity by calculating the time necessary for melt of a given viscosity to flow under a given pressure gradient a given distance up the drill hole. The melt flowed upward approximately 5.5.m in a few minutes. The most elusive part of the calculation involves estimating the pressure gradient driving the flow. The lithostatic load based on the depth (~2.54 km) is about 0.65 kb, which is assumed to act over a characteristic distance of about 2 m (lens size) to give a characteristic pressure gradient. Pipe flow yields a characteristic viscosity of 3.8 x 107 p. An independent calculation from MELTS using only melt composition, temperature, water content (zero), and crystal content gives an almost identical estimate of 3.16 x 107 p. The thermal residence time or time of solidification of the dacite as it flows up the drill hole was also calculated using this flow. This time of ~1 hr is far longer than the 'few minutes' observed before the magma came to rest, suggesting the up flowing magma caps itself not by cooling through the lateral walls but by cooling and solidification of the leading front, quenching against the stream of cool drilling mud. Origin and Size: Silicic lens like segregations are found in most basaltic sill like bodies and can reach over a meter in thickness and many tens of meters long. They often form from a gravitational instability that promotes internal tearing within the upper solidification front of intrusions undergoing progressive solidification. At high crystallinities the front tears and fills with local residual melt. One or more lenses may have flowed into a common region due to a strongly dipping, fissure like parent body. Size and Age of Parent Body: If the parent is the 1955 basalt, a minimum characteristic thickness of the parent body is ~100 meters. (U-series geochronologic data will be available by meeting time.) The lateral extent of sheet like bodies, which is the likely form of the parent body, is on the order of 1 km or more and will be roughly circular in plan form. This is a golden opportunity to study magma in its natural habitat.
Geodetic Evidence of Magma Beneath the Puna Geothermal Ventures Power Plant, Lower East Rift Zone, Kilauea Volcano, Hawaii.
Precise level surveys of the Puna Geothermal Ventures power plant site have been conducted at 2 to 3 year intervals over the past 16 years following an initial pre-production base-line survey in 1992. Pre-1992 USGS studies near the plant showed slow general subsidence and this pattern has continued since then. The average rate of subsidence for the first 11 years of the present survey series was 0.71 cm per year (1992- 2003). It was against this background of subsidence that small but significant upward movements were detected in 2005 in an area approximately 500 m wide directly under the power plant. This positive anomaly had an amplitude of only 0.5 cm but was clearly discernable because of the part-per-million resolution possible with traditional precise leveling. The 13-year (at that time) data set made it possible to interpret this event with confidence. The cause of the deformation was reported in 2005 to be shallow and localized in comparison to factors contributing to the subsidence of the surrounding area. Subsequent drilling activity penetrated magma beneath the anomaly, providing strong physical evidence that fluid pressure was the probable cause of the anomaly.
Results From a Borehole Seismometer Array I: Microseismicity at a Productive Geothermal Field, Kilauea Lower East Rift Zone, Puna, Hawaii
Borehole seismometer arrays have proven successful in both the exploration and monitoring of geothermal fields. Because the seismometers are located at depth, they are isolated from human noise and record microearthquakes with clearly identifiable seismic phases that can be used for event location. Further analysis of these events can be used to resolve earthquake clouds into identifiable faults. The local fault and dike structures in Puna, in southeastern Hawaii, are of interest both in terms of electricity production and volcanic hazard monitoring. The geothermal power plant at Puna has a 30MW capacity and is built on a section of the Kilauea Lower East Rift Zone where lava flows erupted as recently as 1955. In order to improve seismic monitoring in this area, we installed eight 3-component borehole seismometers. The instrument depths range from 24 to 210 m (80 to 690 ft); the shallower instruments have 2 Hz geophones and the deepest have 4.5 Hz geophones. The seismometers are located at the vertices of two rhombs, 2 km wide x 4 km long and 4 km wide x 8 km long, both centered at the power plant. Since June 2006, we have located >4500 earthquakes; P- and S-wave arrivals were hand picked and events located using Hypoinverse-2000. Most of the earthquakes occurred at depths between 2.5 and 3 km. The large majority of events were M-0.5 to M0.5; the Gutenberg-Richter b-value is 1.4, which is consistent with microearthquake swarms. Frequency analysis indicates a 7-day periodicity; a Schuster diagram confirms increased seismicity on a weekly cycle. The location, depth, and period of the microearthquakes suggest that power plant activity affects local seismicity. Southwest of the geothermal facility, up-rift towards the Kilauea summit, earthquakes were progressively deeper at greater distances. Depths also increased towards the south, which is consistent with the eastern extension of the south-dipping, east-striking Hilina fault system. To the northeast, down-rift of the array, there is a sudden cessation of seismicity not accounted for by known geologic structures. This borehole seismometer network is providing essential data for the detailed characterization of the Kilauea Lower East Rift Zone and the Puna geothermal field.
Results From a Borehole Seismometer Array II: 3-D Mapping of an Active Geothermal Field at the Kilauea Lower Rift Zone
The geothermal power plant in Puna, in southeastern Hawaii, is located in a section of the Kilauea Lower East Rift Zone that was resurfaced by lava flows as recently as 1955, 1960, and 1972. In 2006 a seismic array consisting of eight 3-component stations was installed around the geothermal field in Puna. The instrument depths range from 24 to 210 m. The shallower instruments have 2 Hz geophones and the deeper have 4.5 Hz geophones. 3-D tomographic analyses of P-wave velocity, S-wave velocity, and the Vp/Vs ratio show an area of very fast P-wave velocity at the relatively shallow depth of 2.5 km in the southern section of the field. The same area shows moderate S-wave velocity. This high P-wave velocity anomaly at the southern part of the geothermal field may indicate the presence of dense rock material usually found at greater depths.
Occurrences and Origins of Hawaiian Intermediate- to Silicic-composition Magmas
Magmas and liquids of intermediate- to high-SiO2 compositions –- namely, dioritic, dacitic, and rhyolitic rock types of SiO2 ~54 to 76 wt% –- are represented in various occurrences on the Hawaiian Islands. In view of the Puna dacite, it is meaningful to examine these occurrences and evaluate their significance and origins in the context of basaltic parentages. The manifestations of intermediate to silicic melts amidst Hawaiian basalts can be assessed as having small, medium, or large scale presence. (i) At small scales are rhyolitic melts, ~71-76 wt% SiO2 that formed interstitially in groundmasses and as vesicle linings. These represent last liquids from crystallization of basaltic magmas as lavas, lava lakes, and gabbros. MgO in these 'rhyolites' is generally depleted to <0.5 wt%, demonstrating the extreme differentiation that basalt can achieve. However, these occurrences, even though common, are at microscopic scales and have not meaningfully contributed to evolving oceanic crust. (ii) Medium-scale evolved melts are diorite (~54-59% SiO&2) in lava lakes and as quartz-bearing cumulate tonalite (67% SiO2; Mauna Kea xenolith). The diorites have segregation vein origins, and one example models by mass balance and by MELTS to represent 67% crystallization of largely ol, cpx, and pl from parental basalt (MgO 8.3%) at slightly below FMQ buffer over 1210-1060°C at 250 bars P. The tonalite may represent SiO2-rich interstitial liquids in a shield solidification zone that accumulated as pools or sills in fractures resulting from roof collapse. These medium-scale differentiates, while seemingly local, may be indications of how ocean-island bimodality (basalt-rhyolite) can be achieved by segregation and accumulative processes. (iii) Large-scale evolved magma is represented by Waianae Range rhyodacite lava, ~66.5 wt% SiO2, 0.01 km3, 0.1 km2, and avg. 100 m thick. This manifestation of intermediate magma is most appropriate to compare with Puna dacite. Mass balancing the rhyodacite with 'parental' Waianae tholeiitic basalt suggests that it represents liquid after ~67% crystallization of ol (4%), cpx (20%), pl (32%), FeTi ox (11%). MELTS modeling only generally confirms this origin, suggesting rather high P (~7 kb) equilibrium crystallization, but perhaps eclogite melting had a role.
How to Produce Dacitic Melt at Kilauea: Evidence from Historic Kilauea Lava Lakes
Kilauea Volcano, perhaps the most thoroughly studied shield volcano in the world, has erupted only basaltic lava so far as is known. Summit lavas are olivine tholeiite, with MgO contents are typically 7 to 10 weight percent but can range up to 20 percent MgO. Lavas that have undergone fractionation of augite + plagioclase, in addition to olivine have erupted only along the rift zones, away from the summit reservoir. The most differentiated Kilauea lavas known are the earliest products of the 1955 eruption (MgO = 5.0-5.1%) and some lavas of the 1977 eruption (MgO = 5.3-5.4%). These eruptions occurred on the lower and middle east rift of the volcano, respectively. Recent drilling operations in Puna encountered a body of dacitic magma (Teplow and others, 2008), a composition far more differentiated than any known lava at Kilauea. Re-examination of the 1955 and 1960 lavas has established that the Puna area contains pockets of differentiated magma, but nothing as differentiated as dacite. However, the existence of such melts is an expected consequence of long-term storage of differentiated magma in the lower east rift zone. Results from three historic Kilauea lava lakes (the 1959 Kilauea Iki, 1963 Alae, and 1965 Makaopuhi lava lakes) show that such melt compositions are attainable by differentiation of Kilauean basalt. The composition of the Puna dacite lies closer to the liquid line of descent observed for Kilauea Iki than that of Alae or Makaopuhi. On MgO variation diagrams of Kilauea Iki differentiates, the new dacite plots close to a small group of samples formed by internal differentiation of the ferrodiabasic segregation veins (or sills) that make up 5-10% of the upper crust (cumulative thickness 3.3-5.7 m). These sills are similar in bulk composition to the early 1955 lava, a plausible immediate parent for the dacite, given its presence in the lower east rift in 1955-1960. The dacitic bodies form by melt separation when the host segregation vein is about 60% crystallized. If this second stage of differentiation operated at maximum efficiency in a magma body, it would be possible to generate about 1.3-2.3 m of dacitic melt from a body of picritic magma having the thickness of Kilauea Iki (135 m). This corresponds to 1-2% dacitic melt by volume. The temperature at which such melts are generated is 1050-1060oC. The efficiency of this second-stage segregation in Kilauea Iki is less, but the occurrence of any small bodies of dacitic composition in the lava lake shows that such melts can be segregated into discrete bodies at Kilauea.
Evidence for Multiple Magma Bodies Beneath Kilauea Volcano, Hawaii
The volcanoes of the Hawaiian Islands are some of the most active volcanoes in the world. Kilauea Volcano on the Big Island has been continuously erupting since 1983. We have identified multiple sources of deformation in and around the summit caldera using interferometric synthetic aperture radar (InSAR) and the global positioning system (GPS) for the time period 2004 to 2007. The magmatic system beneath Kilauea volcano is not a simple magma chamber geometry but rather a complex series of reservoirs. Movement of magma in and out of these reservoirs is seen at the surface as the ground moves up and down. The InSAR technique provides a way to measure centimeter scale deformation over a large area from satellite radar data while GPS stations measure deformation at only one location but have the advantage of providing daily measurements. An intrusion in the East Rift Zone occurred on June 17, 2007, at which time the deformation sources identified around the summit area switched from inflation to deflation. The spatial and temporal coverage of the InSAR and GPS data provide the means to locate the deformation sources, identify the timing of the inflation and deflation events, and help track the movement of magma beneath the surface to better understand the magmatic system.