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
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. [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]
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.
Next: Volcanic processesQuestions? Comments? Please contact Jenny Paduan