Rosalind T. Helz

Geothermometry of Kilauea Iki lava lake, Hawaii

Data on the variation of temperature with time and in space are essential to a complete understanding of the crystallization history of basaltic magma in Kilauea Iki lava lake. Methods used to determine temperatures in the lake have included direct, downhole thermocouple measurements and Fe-Ti oxide geothermometry. In addition, the temperature variations of MgO and CaO contents of glasses, as determined in melting experiments on appropriate Kilauean samples, have been calibrated for use as purely empirical geothermometers and are directly applicable to interstitial glasses in olivine-bearing core from Kilauea Iki. The uncertainty in inferred quenching temperatures is ±8−10° C. Comparison of the three methods shows that (1) oxide and glass geothermometry give results that are consistent with each other and consistent with the petrography and relative position of samples, (2) downhole thermo-couple measurements are low in all but the earliest, shallowest holes because the deeper holes never completely recover to predrilling temperatures, (3) glass geothermometry provides the greatest detail on temperature profiles in the partially molten zone, much of which is otherwise inaccessible, and (4) all three methods are necessary to construct a complete temperature profile for any given drill hole. Application of glass-based geothermometry to partially molten drill core recovered in 1975–1981 reveals in great detail the variation of temperature, in both time and space, within the partially molten zone of Kilauea Iki lava lake. The geothermometers developed here are also potentially applicable to glassy samples from other Kilauea lava lakes and to rapidly quenched lava samples from eruptions of Kilauea and Mauna Loa.

The absence of lithium isotope fractionation during basalt differentiation: new measurements by multicollector sector ICP-MS

We report measurements of the isotopic composition of lithium in basalts using a multicollector magnetic sector plasma-source mass spectrometer (MC-ICP-MS). This is the first application of this analytical technique to Li isotope determination. External precision of multiple replicate and duplicate measurements for a variety of sample types averages ±1.1‰ (2σ population). The method allows for the rapid (∼8 min/sample) analysis of small samples (∼40 ng Li) relative to commonly used thermal ionization methods. The technique has been applied to a suite of samples from Kilauea Iki lava lake, Hawaii. The samples range from olivine-rich cumulitic lava to SiO2− and K2O-enriched differentiated liquids, and have δ7Li (per mil deviation of sample 7Li/6Li relative to the L-SVEC standard) of +3.0 to +4.8. The data indicate a lack of per mil-level Li isotope fractionation as a result of crystal–liquid fractionation at temperatures greater than 1050°C. This conclusion has been tacitly assumed but never demonstrated, and is important to the interpretation of Li isotope results from such geochemically complex environments as island arcs.

Iron Isotope Fractionation During Magmatic Differentiation in Kilauea Iki Lava Lake

Magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks. The extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements. We report analyses of iron isotopes in basalts from Kilauea Iki lava lake, Hawaii. The iron isotopic compositions (56Fe/54Fe) of late-stagemeltveins are 0.2 permil (‰) greater than values for olivine cumulates. Olivine phenocrysts are up to 1.2‰ lighter than those of whole rocks. These results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole-rock and crystal scales. This characteristic of iron relative to the characteristics of magnesium and lithium, for which no fractionation has been found, may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation.

Diapiric transfer of melt in Kilauea Iki lava lake, Hawaii: a quick, efficient process of igneous differentiation

Kilauea Iki lava lake, formed in 1959, is a large pond of picritic basalt (average MgO content = 15.34% by weight), which has cooled and crystallized as a small, self-roofed magma chamber. Repeated drilling of the upper crust of the lake, down to its molten core, and more recent (1981) drilling, through the thermal maximum and part way through the lower crust, have made it possible to monitor the differentiation processes in the lake in detail. Differentiation processes recognized as active in the lake include rather inefficient settling of the larger (2-10 mm) olivine phenocrysts, formation of segregation veins, and formation of diapir-like vertical olivine-rich bodies, all processes which occur in one or more of the other Kilauean lava lakes as well. In addition, most of the central part of Kilauea Iki has been affected by diapiric melt transfer. In this process, relatively low-density liquid, present at 1145-1160 °C, rose from within the loose crystal mush at the base of the lens of melt and intruded the equivalent thermal horizon at the top of the lens of melt, passing through the hotter but denser melt in the core of the lake without mixing. The source volume from which the low-density liquid was extracted is depleted in TiO 2 and other incompatible elements and enriched in FeO and CaO. The upper part of the lake shows the opposite effects. The crystalline assemblage present was olivine + augite + minor plagioclase. The crystallinity of both the source and receiving layers was low enough that no obvious textural imprint was left by the transfer process; the principal evidence for its occurrence is the chemical zonation of the lake seen in core from depths of 13 to 80 m. Diapiric melt transfer was active from 1960 to 1971 and has affected most of the central part of the lake from 13 m to at least 80 m. The process ran simultaneously with the other three main differentia tion processes but started and stopped independently of the others. Calculations suggest that between 21 and 42 wt % liquid has been extracted from the depleted zone at 56-78 m in the center of the lake, making this a very efficient process of chemical differentiation.

Crystallization history of Kilauea Iki lava lake as seen in drill core recovered in 1967–1979

Kilauea Iki lava lake formed during the 1959 summit eruption, one of the most picritic eruptions of Kilauea Volcano in the twentieth century. Since 1959 the 110 to 122 m thick lake has cooled slowly, developing steadily thickening upper and lower crusts, with a lens of more molten lava in between. Recent coring dates, with maximum depths reached in the center of the lake, are: 1967 (26.5 m). 1975 (44.2 m), 1976 (46.0 m) and 1979 (52.7 m). These depths define the base of the upper crust at the time of drilling.The bulk of the core consists of a gray, olivine-phyric basalt matrix, which locally contains coarser-grained diabasic segregation veins. The most important megascopic variation in the matrix rock is its variation in olivine content. The upper 15 m of crust is very olivine-rich. Abundance and average size of olivine decrease irregularly downward to 23 m; between 23 and 40 m the rock contains 5–10% of small olivine phenocrysts. Below 40 m. olivine content and average grainsize rise sharply. Olivine contents remain high (20–45%, by volume) throughout the lower crust, except for a narrow (< 6 m) olivine depleted zone near the basalt contact.Petrographically the olivine phenocrysts in Kilauea Iki can be divided into two types. Type 1 phenocrysts are large (1–12 mm long), with irregular blocky outlines, and often contain kink bands. Type 2 crystals are relatively small (0.5–2 mm in length), euhedral and undeformed. The variations in olivine content of the matrix rock are almost entirely variations in the amount of type 1 olivines.Sharp mineral layering of any sort is rare in Kilauea Iki. However, the depth range 41–52 m is marked by the frequent occurrence of steeply dipping (70°–90°) bands or bodies of slightly vuggy olivine-rich rock locally capped with a small cupola of segregation-vein material. In thin section there is clear evidence for relative movement of melt and crystals within these structures.The segregation veins occur only in the upper crust. The most widely distributed (occurring from 4.5–59.4 m) are thin veins (most < 5 cm thick), which cut the core at moderate angles and appear to have been derived from the immediately adjacent wall-rock by filter pressing. There is also a series of thicker (0.1–1.5 m) segregation veins, which recur every 2–3 m, between 20 and 52 m. These have subhorizontal contacts and appear, from similarities in thickness and spacing, to correlate between drill holes as much as 100 m apart. These large veins are not derived from the adjacent wallrock: their mechanism of formation is still problematical. The total thickness of segregation veins in Kilauea Iki is 3–6 m in the central part of the lake, corresponding to 6–11% of the upper crust.Whole-rock compositions for Kilauea Iki fall into two groups: the matrix rock ranges from 20-7.5% MgO, while the segregation veins all contain between 6.0 and 4.5% MgO. There are no whole-rock compositions of intermediate MgO content. Samples from < 12 m show eruption-controlled chemistry. Below that depth, matrix rock compositions have higher Al2O3, TiO2 and alkalies, and lower CaO and FeO, at a given MgO content than do the eruption pumices. The probable causes of this are assimilation of low-melting components from foundered crust, plus removal of olivine, plus removal of minor augite, for rocks with MgO contents of < 8.0%.Given the observed rate of growth of the upper crust, one can infer that significant removal of the type 1 olivine phenocrysts from the upper part of the lake began in 1963 and ceased sometime prior to 1972. The process. probably gravitative settling, appears to have been inhibited earlier by gas streaming from the lower part of the lens of melt. The olivine cumulate zone, which extends into the upper crust, contains relatively few (25–40%) olivine crystals, few of which actually touch each other. The diffuseness of the cumulate zone raises the possibility that the crystals were coated with a relatively visous boundary layer of melt which moved with them. Calculations of the Stokes’ law settling rates of the largest olivine crystals found at the base of the crust in 1975–76 suggest that the «melt» had a viscosity of > 106 poises, and probably had the properties of a Bingham body, rather than a Newtonian fluid, by that date, which was several years after olivine removal ceased.

The 1977 eruption of Kilauea volcano, Hawaii

Abstract Kilauea volcano began to erupt on September 13, 1977, after a 21.5-month period of quiescence. Harmonic tremor in the upper and central east rift zone and rapid deflation of the summit area occurred for 22 hours before the outbreak of surface activity. On the first night, spatter ramparts formed along a discontinuous, en-echelon, 5.5-km-long fissure system that trends N70°E between two prehistoric cones, Kalalua and Puu Kauka. Activity soon became concentrated at a central vent that erupted sporadically until September 23 and extruded flows that moved a maximum distance of 2.5 km to the east. On September 18, new spatter ramparts began forming west of Kalalua, extending to 7 km the length of the new vent system. A vent near the center of this latest fissure became the locus of sustained fountaining and continued to extrude spatter and short flows intermittently until September 20. The most voluminous phase of the eruption began late on September 25. A discontinuous spatter rampart formed along a 700-m segment near the center of the new, 7-km-long fissure system; within 24 hours activity became concentrated at the east end of this segment. One flow from the 35-m-high cone that formed at this site moved rapidly southeast and eventually reached an area 10 km from the vent and 700 m from the nearest house in the evacuated village of Kalapana. We estimate the total volume of material produced during this 18-day eruption to be 35 × 10 6 m 3 . Samples from active vents and flows are differentiated quartz-normative tholeiitic basalt, similar in composition to lavas erupted from Kilauea in 1955 and 1962. Plagioclase is the only significant phenocryst; augite, minor olivine, and rare orthopyroxene and opaque oxides accompany it as microphenocrysts. Sulfide globules occur in fresh glass and as inclusions in phenocrysts in early 1977 lavas; their absence in chemically-similar basalt from the later phases of the eruption suggests that more extensive intratelluric degassing occurred as the eruption proceeded. Bulk composition of lavas varied somewhat during the eruption, but the last basalt produced also is differentiated, suggesting that the magma withdrawn from the summit reservoir during the rapid deflation has not yet been erupted.

Tungsten-182 heterogeneity in modern ocean island basalts

A mantle story told with metal and gas Differences in isotopic compositions of trace elements can help identify how regions of Earths mantle have evolved over time. Mundl et al. identified several ancient domains that have been isolated from mantle homogenization and thus contain signatures of primordial material. Tungsten and helium isotope values indicate fractionation and isolation of these mantle domains just after Earths formation. The findings help constrain ancient processes such as core formation, but also provide insight into unexplained structures in the lower mantle today. Science, this issue p. 66 Tungsten and helium isotopes from the deep mantle require primordial sequestration of metal. New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts per million lower. The tungsten isotopic data negatively correlate with 3He/4He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low 3He/4He.

Differentiation and magma mixing on Kilauea's east rift zone

The lavas of the 1955 east rift eruption of Kilauea Volcano have been the object of considerable petrologic interest for two reasons. First, the early 1955 lavas are among the most differentiated ever erupted at Kilauea, and second, the petrographic character and chemical composition of the lava being erupted changed significantly during the eruption. This shift, from more differentiated (MgO=5.0–5.7%) to more magnesian (MgO=6.2–6.8%) lava, has been variously interpreted, as either due to systematic excavation of a zoned, differentiated magma body, or to invasion of the differentiated magma by more primitive magma, followed by rapid mixing and eruption of the resulting hybrid magmas. Petrologic examination of several nearvent spatter samples of the late 1955 lavas shows abundant evidence for magma mixing, including resorbed and/or reversely zoned crystals of olivine, augite and plagioclase. In addition, the compositional ranges of olivine, plagioclase and groundmass sulfide are very large, implying that the assemblages are hybrid. Core compositions of olivine phenocrysts range from Fo85 to Fo77. The most magnesian olivines in these samples must have originally crystallized from a melt containing 8.0–8.5% MgO, which is distinctly more magnesian than the bulk composition of the late 1955 lavas. The majorelement and trace-element data are either permissive or supportive of a hybrid origin for the late 1955 lavas. In particular, the compositional trends of the 1955 lavas on plots of CaO vs MgO, and the virtual invariance of Al2O3 and Sr in these plagioclase-phyric lavas are more easily explained by magma mixing than by fractionation. The pattern of internal disequilibrium/re-equilibration in the late 1955 spatter samples is consistent with reintrusion and mixing having occurred at least twice, during the latter part of the 1955 eruption. Plagioclase zonation preserves possible evidence for additional, earlier reintrusion events. Least-squares modelling the mixing of early 1955 bulk compositions with various summit lavas±olivine pick the 1952 summit lava as most like the primitive component. The results also indicate the primitive component had MgO=7.5–8.0%, corresponding to liquidus temperatures of 1165–1175°C. The absence of Fe-Ti oxide phenocrysts in the late 1955 lavas implies that the cooler component of the hybrid had T>1110°C. Thus the thermal contrast between the two components may have been as much as 55–65°C, sufficient to produce the conspicuous disequilibrium effects visible in the spatter samples.

Toward an understanding of disequilibrium dihedral angles in mafic rocks

The median dihedral angle at clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, Θcpp, is generally lower than equilibrium (109˚ {plus minus} 2˚). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θcpp is incorrect in several important respects. Firstly, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three-grain junctions, and hence Θcpp, is formed during solidification (the igneous process): sub-solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θcpp, is insignificant. Θcpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt-filled, resulting in an initial value of Θcpp of 78˚, rather than 60˚ which would be expected if all melt-filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θcpp due to changes in relative growth rates of the two minerals at the nascent three-grain junction. Low Θcpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt-free rocks by late-stage evolved liquids (the metasomatic process). Modification of Θcpp by sub-solidus textural equilibration (the metamorphic process) is only important for fine-grained mafic rocks such as chilled margins and intra-plutonic chill zones. In coarse-grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centres of oikocrysts, associated with rounding of chadacrysts.

Disequilibrium dihedral angles in dolerite sills: A new proxy for cooling rate

The geometry of clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, measured by the median dihedral angle Θcpp, is created during solidification. In the solidifying Kilauea Iki (Hawaii) lava lake, the wider junctions between plagioclase grains are the first to be filled by pyroxene, followed by the narrower junctions. The final Θcpp, attained when all clinopyroxene-plagioclase-plagioclase junctions are formed, is 78° in the upper crust of the lake, and 85° in the lower solidification front. Θcpp in the 3.5-m-thick Traigh Bhan na Sgurra sill (Inner Hebrides) is everywhere 78°. In the Whin Sill (northern England, 38 m thick) and the Portal Peak sill (Antarctica, 129 m thick), Θcpp varies symmetrically, with the lowest values at the margins. The 266-m-thick Basement Sill (Antarctica) has asymmetric variation of Θcpp, attributed to a complex filling history. The chilled margins of the Basement Sill are partially texturally equilibrated, with high Θcpp. The plagioclase grain size in the two widest sills varies asymmetrically, with the coarsest rocks found in the upper third. Both Θcpp and average grain size are functions of model crystallization times. Θcpp increases from 78° to a maximum of ∼100° as the crystallization time increases from 1 to 500 yr. Because the use of grain size as a measure of crystallization time is dependent on an estimate of crystal growth rates, dihedral angles provide a more direct proxy for cooling rates in dolerites.