David A. Clague

Helium isotopic variations in volcanic rocks from Loihi Seamount and the Island of Hawaii

Helium isotopic ratios ranging from 20 to 32 times the atmospheric 3He/4He(RA) have been observed in a suite of 15 basaltic glasses from the Loihi Seamount. These ratios, which are up to four times higher than those of MORB glasses and more than twice those of nearby Kilauea, are strongly suggestive of a primitive source of volatiles supplying this volcanism. The Loihi glasses measured span a broad compositional range, and the 3He/4He ratios were found to be generally lower for the alkali basalts than for the tholeiites. The component with a lower 3He/4He ratio appears to be associated with olivine xenocrysts, within which fluid inclusions are probably the carrier of contaminant helium. One Loihi sample has a much lower isotopic ratio (<5 RA), but a combination of low He concentration, high vesicularity, and presence of cracks lined with clay minerals suggests that the low ratio is due to gas loss and contamination by atmospheric helium. n nCrushing and melting experiments show that for modest vesicularities (<5% by volume) the Loihi glasses obey a MORB-type partitioning trend, but at higher vesicularities the data show considerably more scatter due to volatile mobilization. The high vesicularities, low extrusion pressure and generally low helium concentrations are consistent with a considerable degree of degassing. Analyses of dunites, plus a correlation between total helium concentrations with xenocryst abundances also suggest that xenocrysts are a significant carrier of contaminating (low 3He/4He) helium. n n3He/4He ratios from samples of other Hawaiian volcanoes (Kilauea, Mauna Loa, Hualalai, and Mauna Kea) show a smooth decrease in 3He/4He with increasing volcano age and volume. We interpret this to be a synoptic picture of the time evolution of a hot-spot diapir: the earliest stage is characterized by primitive (> 30 RA) helium with some (variable) component of lithospheric contamination added during “breakthrough”, while the later stages are characterized by a relaxation toward lithospheric 3He/4He ratios (∼ 8 RA) due to isolation of the diapir from the mantle below (as the plate moves on), and subsequent mining of the inherited helium and contamination from the surrounding lithosphere. The abrupt contrast in 3He/4He ratios between Kilauea and Loihi, despite their close proximity, is indicative of the small lateral extent of the plume.

Volcano growth and evolution of the island of Hawaii

The seven volcanoes comprising the island of Hawaii and its submarine base are, in order of growth, Mahukona, Kohala, Mauna Kea, Hualalai, Mauna Loa, Kilauea, and Loihi. The first four have completed their shield-building stage, and the timing of this event can be determined from the depth of the slope break associated with the end of shield building, calibrated using the ages and depths of a series of dated submerged coral reefs off northwest Hawaii. The composition of lavas collected adjacent to these reefs helps to define the eruptive history of the various volcanic centers. The island of Hawaii has grown at an average rate of about 0.02 km 2 /yr for the past 600 k.y. and presently is close to its maximum size. Mahukona completed shield building about 465 ka; Kohala, 245 ka; Mauna Kea, 130 ka; and Hualalai, 130 ka. On each volcano, the transition from eruption of tholeiitic to alkalic lava occurs near the end of shield building. The larger volcanic systems (which stood more than 4 km above their shoreline at the end of shield building) change in composition before the end of shield building, and the smaller volcanoes change near or after the end of shield building, or never make the change to eruption of alkalic lava. The rate of southeastern (south 40° east) progression of the end of shield building (and hence the postulated movement rate of the Pacific plate over the Hawaiian hotspot) in the interval from Haleakala to Hualalai is about 13 cm/yr. Based on this rate and an average spacing of volcanoes on each loci line of 40-60 km, the volcanoes require about 600 thousand years to grow from the ocean floor (generally from a point on the southeastern submarine flank of the next older volcano of the same loci line) to the time of the end of shield building. They arrive at the ocean surface about midway through this period.

The isotope systematics of a juvenile intraplate volcano" Pb, Nd, and Sr isotope ratios of basalts from Loihi Seamount, Hawaii

Sr, Nd, and Pb isotope ratios for a representative suite of 15 basanites, alkali basalts, transitional basalts and tholeiites from Loihi Seamount, Hawaii, display unusually large variations for a single volcano, but lie within known ranges for Hawaiian basalts. Nd isotope ratios in alkali basalts show the largest relative variation (0.51291–0.51305), and include the nearly constant tholeiite value ( ∼ 0.51297). Pb isotope ratios show similarly large ranges for tholeiites and alkali basalts and continue Tatsumotos [31] “Loa” trend towards higher 206Pb/204Pb ratios, resulting in a substantial overlap with the “Kea” trend. 206Pb/204Pb ratios for Loihi and other volcanoes along the Loa and Kea trends [31] are observed to correlate with the age of the underlying lithosphere suggesting lithosphere involvement in the formation of Hawaiian tholeiites. Loihi lavas display no correlation of Nd, Sr, or Pb isotope ratios with major element compositions or eruptive age, in contrast with observations of some other Hawaiian volcanoes [38]. Isotope data for Loihi, as well as average values for Hawaiian volcanoes, are not adequately explained by previously proposed two-end-member models; new models for the origin and the development of Hawaiian volcanoes must include mixing of at least three geochemically distinct source regions and allow for the involvement of heterogeneous oceanic lithosphere.

Noble gases in submarine pillow basalt glasses from Loihi and Kilauea, Hawaii: A solar component in the Earth

Noble gas elemental and isotopic abundances have been analysed in twenty-two samples of basaltic glass dredged from the submarine flanks of two currently active Hawaiian volcanoes, Loihi Seamount and Kilauea. Neon isotopic ratios are enriched in 20Ne and 21Ne by as much as 16% with respect to atmospheric ratios. All the Hawaiian basalt glass samples show relatively high 3He4He ratios. The high 20Ne22Ne values in some of the Hawaiian samples, together with correlations between neon and helium systematics, suggest the presence of a solar component in the source regions of the Hawaiian mantle plume. The solar hypothesis for the Earths primordial noble gas composition can account for helium and neon isotopic ratios observed in basaltic glasses from both plume and spreading systems, in fluids in continental hydrothermal systems, in CO2 well gases, and in ancient diamonds. These results provide new insights into the origin and evolution of the Earths atmosphere.

Pacific Plate Motion Recorded by Linear Volcanic Chains

Seamounts and islands are quite common features of the deep-ocean basins, particularly the Pacific Basin. A conspicuous but relatively small number of these volcanoes are arranged in long linear chains (Figure 1). The vast majority, however, occur as isolated edifices. Recent work on a small number of these isolated volcanoes shows that many were formed at or very close to mid-ocean spreading axes (Clague and Dalrymple, 1975; Batiza, 1980) but that others are significantly younger than the underlying oceanic crust (G. B. Dalrymple, D. A. Clague, H. G. Greene, unpublished data). These isolated volcanoes require much additional study before their origin is understood. The volcanoes arranged in linear chains have been the focus of a great deal of work, and much has been learned about their origin and evolution. Most of the linear volcanic chains in the Pacific Basin are oriented roughly west-northwesterly and apparently formed sequentially above stationary hot spots during the past 42 m. y. as the Pacific plate rotated clockxadwise about a pole located near 69°N, 68°W (Clague and Jarrard, 1973a). Another group of linear chains exhibit roughly north-trending orientations and apparently were formed by the same mechanism between at least 80 and 42 m. y. ago as the Pacific plate rotated about a pole of rotation located near 17°N, 107°W (Clague and Jarrard, 1973b). Earlier age data available from linear island and seamount chains were summarized by Jackson (1976), Jarrard and Clague (1977), and McDougall and Duncan (1980); we will briefly review the information presented there and update it with more recent data.

Geochemistry of tholeiitic and alkalic lavas from the Koolau Range, Oahu, Hawaii: implications for Hawaiian volcanism

Lavas of the post-erosional, alkalic Honolulu Volcanics have significantly lower 87Sr/86Sr and higher 143Nd/144Nd than the older and underlying Koolau tholeiites which form the Koolau shield of eastern Oahu, Hawaii. Despite significant compositional variation within lavas forming the Honolulu Volcanics, these lavas are isotopically (Sr, Nd, Pb) very similar which contrasts with the isotopic heterogeneity of the Koolau tholeiites. Among Hawaiian tholeiitic suites, the Koolau lavas are geochemically distinct because of their lower iron contents and Sr and Nd isotopic ratios which range to bulk earth values. These geochemical data preclude simple models such as derivation of the Honolulu Volcanics and Koolau tholeiites from a common source by different degrees of melting or by mixing of two geochemically distinct sources. There may be no genetic relationship between the origin and evolution of these two lava suites; however, the trend shown by Koolau Range lavas of increasing 143Nd/144Nd and decreasing 87Sr/86Sr with decreasing eruption age and increasing alkalinity also occurs at Haleakala, East Molokai and Kauai volcanoes. A complex mixing model proposed for Haleakala lavas can account for the variations in Sr and Nd isotopic ratios and incompatible element abundances found in lavas from the Koolau Range. This model may reflect mixing and melting processes occurring during ascent of relatively enriched mantle through relatively depleted MORB-related lithosphere. Although two isotopically distinct components may be sufficient to explain Sr and Nd isotopic variations at individual Hawaiian volcanoes, more than two isotopically distinct materials are required to explain variations of Sr, Nd and Pb isotopic ratios in all Hawaiian lavas.

Role of olivine cumulates in destabilizing the flanks of Hawaiian volcanoes

The south flank of Kilauea Volcano is unstable and has the structure of a huge landslide; it is one of at least 17 enormous catastrophic landslides shed from the Hawaiian Islands. Mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments. Slip on a low friction décollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest, or why this section contains a 300 km3 aseismic volume above the seismically defined décollement. It is proposed that this aseismic volume, adjacent to the caldera in the direction of flank slip, consists of olivine cumulates that creep outward, pushing the south flank seawards. Average primary Kilauea tholeiitic magma contains about 16.5 wt.% MgO compared with an average 10 wt.% MgO for erupted subaerial and submarine basalts. This difference requires fractionation of 17 wt.% (14 vol.%) olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates. Submarine-erupted Kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates. Deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas. The deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed, probably with as little as 5–10 vol.% intercumulus liquid, before entrainment of the xenocrysts. The olivine cumulates were at magmatic temperatures (>1100°C) when the xenocrysts were entrained. Olivine at 1100°C has a rheology similar to ice, and the olivine cumulates should flow down and away from the summit of the volcano. Flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank, leading to an extensional region that localizes deep intrusions behind the flank; these intrusions add to the seaward push. This mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and, therefore, limits catastrophic sector failure of Hawaiian volcanoes to their active growth phase, when the core of olivine cumulates is still hot enough to flow.

Geochemistry of diverse basalt types from Loihi Seamount, Hawaii: petrogenetic implications

The wide variety of basalt types, tholeiitic to basanite, dredged from Loihi Seamount have minor and trace element abundances that are characteristic of subaerial Hawaiian basalts, thereby confirming that Loihi Seamount is a manifestation of the Hawaiian “hot spot”. Within the Loihi sample suite there are well-defined positive correlations among abundances of highly incompatible elements (P, K, Rb, Ba, Nb, light REE and Ta) and moderately incompatible elements (Sr, Ti, Zr and Hf) and between MgO, Ni and Cr. However, within the Loihi suite abundance ratios of geochemically similar elements (Zr/Hf, Nb/Ta and La/Ce) vary by factors of 1.2–1.5 and abundance ratios of highly incompatible elements such as P/Ce, P/Th, K/Rb, Ba/Th and La/Nb vary by factors of 1.2–2.5. These abundance ratios are not readily changed by different degrees of fractionation and melting. Therefore, we conclude that these samples are not genetically related by different degrees of melting of a compositionally homogeneous source. n nOn the basis of K/P, K/Ti, P/Ce, Zr/Nb, Th/P and La/Sm abundance ratios, the twelve samples studied in detail can be divided into six geochemical groups. Samples within each group are similar in 87Sr/86Sr [1], and intra-group compositional variations may reflect low-pressure fractionation and different degrees of melting. In addition, crossing chondrite-normalized REE patterns within the alkalic basalt groups reflect equilibration of the magmas with garnet. In ratio-ratio plots involving abundance ratios of highly incompatible elements, e.g., La/P, Nb/P, K/P, Rb/P, Ba/P and Th/P, the geochemical groups define linear arrays suggestive of mixing. However, these data combined with the isotopic data are not consistent with two-component mixing.

Hawaiian xenolith populations, magma supply rates, and development of magma chambers

Hawaiian volcanoes pass through a sequence of four eruptive stages characterized by distinct lava types, magma supply rates, and xenolith populations. Magma supply rates are low in the earliest and two latest alkalic stages and high in the tholeiitic second stage. Magma storage reservoirs develop at shallow and intermediate depths as the magma supply rate increases during the earliest stage; magma in these reservoirs solidifies as the supply rate declines during the alkalic third stage. These magma storage reservoirs function as hydraulic filters and remove dense xenoliths that the ascending magma has entrained. During the earliest and latest stages, no magma storage zone exists, and mantle xenoliths of lherzolite are carried to the surface in primitive alkalic lava. During the tholeiitic second stage, magma storage reservoirs develop and persist both at the base of the ocean crust and 3–7 km below the caldera; only xenoliths of shallow origin are carried to the surface by differentiated lava. During the alkalic third stage, magma in the shallow subcaldera reservoir solidifies, and crustal xenoliths, including oceanic-crustal rocks, are carried to the surface in lava that fractionates in an intermediate-depth reservoir. Worldwide xenolith populations in tholeiitic and alkalic lava may reflect the presence or absence of subvolcanic magma storage reservoirs.

Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U– 230Th disequilibrium

A highly precise mass spectrometric method of analysis was used to determine238U—234U—230Th—232Th in axial and off-axis basalt glasses from Juan de Fuca (JDF) and Gorda ridges. Initial230Th activity excesses in the axial samples range from 3 to 38%, but generally lie within a narrow range of 12 to 15%. Secondary alteration effects were evaluated usingδ234U and appear to be negligible; hence the230Th excesses are magmatic in origin. n nDirect dating of MORB was accomplished by measuring the decrease in excess230Th in off-axis samples.238U—230Th ages progressively increase with distance from axis. Uncertainties in age range from 10 to 25 ka for U—Th ages of 50 to 200 ka. The full spreading rate based on U—Th ages for Endeavour segment of JDF is 5.9 ± 1/2 cm/yr, with asymmetry in spreading between the Pacific (4.0 ± 0.6 cm/yr) and JDF (1.9 ± 0.6 cm/yr) plates. For northern Gorda ridge, the half spreading rate for the JDF plate is found to be 3.0 ± 0.4 cm/yr. These rates are in agreement with paleomagnetic spreading rates and topographic constraints. This suggests that assumptions used to determine ages, including constancy of initial 230Th/232Th ratio over time, are generally valid for the areas studied. Samples located near the axis of spreading are typically younger than predicted by these spreading rates, which most likely reflects recent volcanism within a 1–3 km wide zone of crustal accretion. n nInitial230Th/232Th ratios and230Th activity were also used to examine the recent Th/U evolution and extent of melting of mantle sources beneath these ridges. A negative anomaly in 230Th/232Th for Axial seamount lavas provides the first geochemical evidence of a mantle plume source for Axial seamount and the Cobb-Eickelberg seamount chain and indicates recent depletion of other JDF segment sources. Large230Th activity excesses for lavas from northern Gorda ridge and Endeavour segment indicate formation from a lower degree of partial melting than other segments. An inverse correlation between230Th excess and 230Th/232Th for each ridge indicates that these lower degree melts formed from slightly less depleted sources than higher degree melts. Uniformity in230Th excess for other segments suggests similarity in processes of melt formation and mixing beneath most of the JDF-Gorda ridge area. The average initial230Th/232Th activity ratio of 1.31 for the JDF-Gorda ridge area is in agreement with the predicted value of 1.32 from the Th—Sr isotope mantle array.