Hot spot magmatic processes

Inferring magmatic processes from the erupted rocks

The history of volcanic rocks prior to eruption, in the mantle and within magma chambers, can be inferred from the composition of the melt and the mineralogy of included crystals and xenoliths (fragments of "foreign", older rock caught up in lava flows). The magmatic processes they record include melting in the mantle, transport to within the volcano, cooling and crystallization, assimilation of surrounding rocks, magma mixing, and degassing. The xenoliths can be compared with glassy rinds of submarine lava flows, which retain the chemical composition of the melt at the time of eruption because the lava surface cools so rapidly upon contact with cold seawater that it quenches and can not degas further or form crystals.

Lava (brown) containing olivine-rich xenoliths (greenish) collected offshore of Kauai
Photo © MBARI 2001

The chemical compositions among xenoliths and glass rinds will be different because as primitive magma cools during its travel from the mantle, to magma chambers, and to eruption, specific crystal suites will form in equilibrium with particular temperature and pressure conditions, differentiating and leaving behind a more evolved magma. Olivine, rich in magnesium-oxide (MgO) is the first to begin to crystallize as magma cools. Clinopyroxene, plagioclase and others are next.

Our research on magmatic processes at hot spot volcanoes

The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.

Evolution of magmatic systems

HAWAIIAN ISLANDS - It is widely understood that volcanoes can have short- and long-term effects on the atmosphere, hydrosphere, and biosphere. It is less widely recognized that the environment around a volcano affects the magmatic and eruption characteristics of the volcanic system. Extrinsic parameters that affect the evolution of magmatic systems within and beneath ocean island volcanoes include physical variables such as confining pressure, which controls magma degassing, and temperature of the underlying lithosphere and crust, which controls magma crystallization during ascent. Other extrinsic parameters are environmental variables coupled to the hydrosphere and atmosphere such as hydrothermal circulation systems and even rainfall. These extrinsic factors interact with intrinsic parameters, such as magma supply rates or composition, to modulate the evolution of magma chambers and the petrologic processes that take place within them.

Reference: D.A. Clague and J.E. Dixon (2000) Extrinsic controls on the evolution of Hawaiian ocean island volcanoes, Geochemistry, Geophysics, Geosystems, 1: 1999GC000023, 12 p. [Article]

Magma storage time

KILAUEA - Wide-ranging estimates of crustal storage time of magmas at Kilauea have lead to uncertainty in the time scales of processes of magmatic storage and differentiation. A new approach is used to determine magma residence times: dating plagioclase, pyroxene, and groundmass separates from lavas using ²²⁶Ra-²³⁰Th disequilibria, coupled with trace element measurements, to demonstrate significant fractionation of Ra from Ba during crystal growth. The lavas studied with this technique are from an early phase of the 1955 east rift eruption at Kilauea, and the data constrain the minimum magmatic residence time to be ~550 years, considerably longer than previous estimates of storage time at Kilauea. From this minimum residence time, a maximum constant cooling rate of 0.1 degrees C/yr is derived, which requires a complex cooling history where cooling rates are more rapid early in the storage history, rather than a constant cooling rate over the entire history of the magma chamber.

Reference: K.M. Cooper, M.R. Reid, M.T. Murrel, and D.A. Clague (2001) Crystal and magma residence at Kilauea Volcano, Hawaii: ²³⁰Th-²²⁶Ra dating of the 1955 east rift eruption, Earth and Planetary Science Letters, 184: 703-718. [Article]

Hualalai trachytes offer clues about the plumbing system

HUALALAI - Hualalai Volcano is unique among Hawaiian volcanoes in that evolved, trachytic lavas are relatively common and were erupted at the beginning rather than the end of the alkalic, postshield phase of volcanism. These evolved lavas yield insights into magma sources, magma supply rates, and the evolution of the magmatic plumbing system at this time.

New ⁴⁰Ar/³⁹Ar dates show that the Puu Waawaa and Puu Anahulu trachyte complex is 114 ka, a block from the Waha Pele maar on the south flank is 103 ka, and trachyte flows in a water well on the west flank range from 107 to 92 ka in age, indicating a range for trachyte volcanism of 20 ka. Nd and Pb isotopic compositions overlap with younger alkalic basalts from Hualalai but are distinct from Hualalai tholeiitic basalts and Pacific mid-ocean ridge basalts, linking the trachytes to alkalic parental magmas that underwent extensive crystallization to yield trachytic residual magmas. Both Sr and O isotopic ratios are higher in the trachytes than in Hualalai alkalic lavas, which is best explained by reaction with, or assimilation of, altered Hualalai shield basalts at shallow depth. Major, trace element, and isotopic variations between trachytes are consistent with their evolution by fractional crystallization from a Puu Anahulu parent. The short time gap between the end of tholeiitic volcanism (<133 ka) and the onset of trachytic, alkalic volcanism and the lack of deep-origin xenoliths place the magma reservoir within which the trachytes evolved rapidly at shallow (<7km) depth.

Whereas Mauna Kea and Kohala volcanoes produced small volumes of highly evolved lavas as magma supply rates dwindled through the postshield stage, postshield magma intrusion rates at Hualalai were lowest during trachyte formation and increased through a more recent period of alkalic basalt eruptions. Subtle rare earth element and radiogenic isotopic distinctions between trachytes from the three locations on Hualalai indicate that the roof of the shallow magma reservoir may have been irregular, trapping magma and allowing some trachytes to evolve independently from others.

Reference: B.L. Cousens, D.A. Clague, and W.D. Sharp (2003) Chronology, chemistry, and origin of trachytes from Hualalai Volcano, Hawaii, Geochemistry, Geophysics, Geosystems, 4(9): 1078, doi:10.1029/2003GC000560. [Abstract] [Article]

Assimilation of seawater into magma

LOIHI - Many volcanic glass samples from Loihi have high Cl contents and Cl/K₂O ratios relative to other Hawaiian subaerial volcanoes, such as Mauna Loa, Mauna Kea and Kilauea. We suggest that this results from widespread contamination of Loihi magmas by a Cl-rich, seawater-derived component.

Assimilation of high-Cl phases such as saline brine or Cl-rich minerals can explain the range and magnitude of Cl contents in Loihi glasses, as well as the variations in the ratios of Cl to other incompatible elements. Brines and Cl-rich minerals are thought to form from seawater within the hydrothermal systems associated with submarine volcanoes, and Loihi magmas could plausibly have assimilated such materials from the hydrothermal envelope adjacent to the magma chamber. This model can also explain the observed contamination of Loihi glasses with atmospheric-derived noble gases. This is more likely for brines than for Cl-rich minerals, leading us to favor brines as the major assimilant. Cl/Br ratios for some Loihi samples are also seawater-like, and show no indication of the higher values expected if Cl-rich hydrothermal minerals had been assimilated.

Although Cl enrichment is a common feature of lavas from Loihi, submarine glasses from other Hawaiian volcanoes show little or no evidence of this process, suggesting that assimilation of seawater-derived components is more likely to occur in the early stages of growth of oceanic volcanoes. Summit collapse events such as the one observed at Loihi in October 1996 provide a ready mechanism for depositing brine-bearing rocks from the volcanic edifice into the top of a submarine summit magma chamber.

Reference: A.J.R. Kent, D.A. Clague, M. Honda, E.M. Stolper, I.D. Hutcheon, and M.D. Norman (1999) Widespread assimilation of a seawater-derived component at Loihi Seamount, Hawaii, Geochimica et Cosmochimica Acta, 63(18): 2749-2761. [Abstract] [Article]

Trace elements and the composition of primary magma

KILAUEA, MAUNA LOA, HALEAKALA - Submarine-collected high-MgO (6.7-14.8 wt%) tholeiite glasses from the Hawaiian volcanoes Kilauea, Mauna Loa and Haleakala were studied using ion-microprobe. Rare earth (RE), high field strength, and other selected trace element abundances of these glasses were examined, and the data used to establish their relationship to typical Hawaiian shield tholeiite and to infer characteristics of their source.

The glasses have trace element abundance characteristics generally similar to those of typical shield tholeiites, e.g. L(light)REE/H(heavy)REE_C1 < 1. The Kilauea and Mauna Loa glasses, however, display trace and major element characteristics that cross geochemical discriminants observed between Kilauea and Mauna Loa shield lavas. The glasses contain a blend of these discriminating chemical characteristics, and are not exactly like the typical shield lavas from either volcano. The production of these hybrid magmas likely requires a complexly zoned source, rather than two unique sources. When corrected for olivine fractionation, the glass data show correlations between CaO concentration and incompatible trace element abundances, indicating that CaO may behave incompatibly during melting of the tholeiite source. Furthermore, the tholeiite source must contain residual garnet and clinopyroxene to account for the variation in trace element abundances of the Kilauea glasses. Inversion modeling indicates that the Kilauea source is flat relative to C1 chondrites, and has a higher bulk distribution coefficient for the HREE than the LREE.

Reference: T. P. Wagner, D. A. Clague, E. H. Hauri, T. L. Grove (1998) Trace element abundances of high-MgO glasses from Kilauea, Mauna Loa and Haleakala volcanoes, Hawaii, Contributions to Mineralogy and Petrology 131: 13-21. [Abstract] [Article]

MgO, temperature, and viscosity of magma

KILAUEA - Xenocrysts and glass rinds from quenched lava flows on Puna Ridge (the submarine extension of the East Rift Zone of Kilauea) and turbidite sands cored in the Hawaiian Trough document magma chamber dynamics, such as variable primary magma compositions, olivine and multiphase crystallization and fractionation, degassing, wall-rock stoping and assimilation, mixing in the crustal reservoir and the rift zone, entrainment of olivine xenocrysts from a hot, ductile olivine cumulate body, and disruption of gabbro wallrocks in the rift zone. The glass grains in turbidite sands contain up to 15.0 wt% MgO, and the most forsteritic olivine xenocryst is in equilibrium with primary Kilauea melt containing an average of 16.5 wt% MgO. In contrast, the glass rinds of the pillow lava flows contain <7.0 wt% MgO. Comparison of chemical constituents trapped in xenocrysts indicate that diverse liquids in the magma chamber are mixed and homogenized before eruption, and provide evidence of a sub-crustal magma staging zone.

Low viscosities of the primary liquids facilitates settling of olivine crystals into a dense olivine cumulate body at the base of the magma reservoir. The less-dense, fractionated liquids near the top of the reservoir degas at low pressure, increasing their density so they sink to levels where they mix with resident undegassed, near-primary liquid. This represents a mechanism by which magma can turn over and mix in the chamber. When fresh dikes of magma intrude the hot, ductile dunite body, they disaggregate and entrain olivine crystals, and also dislodge fragments of gabbroic wallrocks, which erupt with Puna Ridge lava flows as xenoliths.

Reference: D.A. Clague, J.G. Moore, J.E. Dixon, and W.B. Friesen (1995) Petrology of submarine lavas from Kilauea's Puna Ridge, Hawaii. Journal of Petrology, 36: 299-349.

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Questions? Comments? Please contact Jenny Paduan
Last updated: Jan. 24, 2008