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.
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
Shield to rejuvenated stage volcanism on Kauai and Niihau
We report and interpret new geochemical and Pb–Sr–Nd isotopic data from 325 samples of shield, late-shield, postshield, and rejuvenated stage lavas from Kauai and Niihau, the two most north- westerly islands in the Hawaiian island chain. Kauai is unique in the Hawaiian chain in that it exhibits a near-continuous geochemical transition from shield to postshield to rejuvenated stage volcanism between 4.4 and 3.6 Ma and has been continuously active over ~6 Myr. From c. 5.7 to 4.3 Ma, the shield stage of both islands produced tholeiitic basalts typical of other Hawaiian shield volcanoes. The Niihau basalts are more evolved and have high Gd/Yb compared with Kauai, indicating a higher residual garnet content in the source. Both Kauai and Niihau shield basalts have Kea-like trace element ratios, but isotopic ratios are transitional between Kea- and Loa-like compositions. The geochemical similarity of the two shields indicates that mantle sources in different regions of the plume source were similar, and that the <2 Ma Loa and Kea trends of the southeastern Hawaiian volcanoes are not observed. More Loa-like compositions are evident in shield lavas from eastern Kauai, where the enhanced Loa composition may reflect melting of low-melting temperature plume components as the island migrates off the hotter, more Kea-like, center of the Hawaiian plume. Postshield lavas and intrusive rocks on both islands are rare: Kauai includes alkalic basalts, hawaiites and mugearites that are isotopically homogeneous and include a significant depleted mantle component compared with the shield basalts, whereas the Niihau late-shield and postshield rocks consist of highly contrasting transitional tholeiites or basanites with variable but shield-like isotopic compositions. The Niihau postshield rocks represent variable, but lower degrees of melting of the shield mantle source. Large volumes of rejuvenated stage lavas cover both islands and also form submarine cone fields, but lava compositions are different. On Kauai, rejuvenated lavas range from melilitite to alkalic basalt with trace element, Nd isotope, and Pb isotope ratios that vary as a function of Th and SiO2 content. Low-degree (high-Th) melts are dominated by a mixed Kea–Loihi component and high-degree (low-Th) melts include more of a depleted rejuvenated component (DRC) typified by rejuvenated stage lavas and xenoliths from nearby Kaula Island. With the exception of a single basanite, the Niihau rejuvenated stage lavas are uniformly alkalic basalt, with Sr and Ba excesses combined with depleted Th and Nb abundances relative to the light rare earth elements. Rejuvenated stage alkalic basalts from both islands are dominated by contributions from the DRC, which have high Sr/Ce and 87Sr/86Sr but low 206Pb/204Pb. The Sr-rich, possibly carbonate-bearing, DRC component may be distributed patchily in the rejuvenated stage mantle source such that, where present, the degree of partial melting was enhanced compared with the degree of partial melting of the Sr-poor, mixed Kea–Loihi component. Given the lack of a hiatus between postshield and rejuvenated stages on Kauai, the rejuvenated mantle source is already able to melt at the tail end of shield stage activity and no secondary melting mechanism is required to explain the rejuvenated stage.
Reference: Cousens, B.L. and D.A. Clague (2015) Shield to rejuvenated stage volcanism on Kauai and Niihau, Hawaiian Islands. Journal of Petrology, doi: 10.1093/petrology/egv045. [Supplementary material]
Magma source heterogeneity in transect across Molokai to Penguin Bank
An important geochemical feature of younger Hawaiian volcanoes is that they define two sub-parallel spatial trends known as the Loa- and Kea-trends, which have persisted for 2 million years. On the Island of Hawaii, the <1.5 Ma shield lavas on the Loa and Kea spatial trends have distinctive geochemical characteristics that are designated as Loa-type and Kea-type. These geochemical differences are clearly expressed in Sr, Nd, Hf and Pb isotopic ratios, major element contents, and ratios of incompatible elements. They are interpreted to reflect varying proportions of sediment, basalt, gabbro and peridotite in subducted oceanic lithosphere. Pb isotopic ratios indicate that the Loa-type component reflects ancient subduction, >2.5 Ga, whereas the Kea-type component reflects younger subduction, <1.5 Ga. To evaluate the temporal persistence of these geochemical differences in the source of Hawaiian shield lavas, we analyzed lavas from the ∼1.5 to 2 Ma Molokai Island volcanoes, East and West Molokai, and the adjacent submarine Penguin Bank. The three volcanoes form a nearly east–west trend that crosscuts the Loa and Kea spatial trends at a high angle; consequently we can determine if these older lavas are Kea-type in the east and Loa-type in the west. All lavas collected from the subaerial flanks of East Molokai, a Kea-trend volcano, have Kea-type geochemical characteristics; however, dive samples collected from Wailau landslide blocks, probably samples of the East Molokai shield that are older than those exposed on the subaerial flanks, include basalt with Loa-type geochemical features. Shield lavas from West Molokai and Penguin Bank, both on the Loa-trend, are dominantly Loa-type, but samples with Kea-type compositions also erupted at these Loa-trend volcanoes. The Loa-trend volcanoes, Mahukona, West Molokai, Penguin Bank, and Koolau, have also erupted lavas with Kea-type geochemical characteristics, and the Kea-trend volcanoes, Mauna Kea, Kohala, Haleakala, and East Molokai, have erupted lavas with Loa-type geochemical characteristics. The presence of both Loa- and Kea-type lavas in a volcano provides constraints on the distribution of geochemical heterogeneities in the source of Hawaiian shield basalts. Two plausible models are: (1) source components with Loa- and Kea-type geochemical characteristics are present in the sources of all <2 Ma shields, but the Kea-to-Loa proportion is higher beneath Kea-trend than Loa-trend volcanoes, or (2) the magma source contains a uniform proportion of Loa- and Kea-type components, but these components have different solidi. Magmas derived from the low-temperature regions of the source preferentially sample the component with the lower solidus temperature and form Loa-type lavas. In contrast, magmas derived from the relatively high-temperature regions of the source sample both low and high solidus components in the source and form Kea-type lavas. This model is supported by the linear correlations between isotopic ratios and calculated temperatures of estimated primary magmas. Reference: Xu, G., Frey, F.A., Blichert-Tift, J., Abouchami, W., Clague, D.A., Huang, S., Cousens, B., Moore, J.G., and Beeson, M.H. (2014). The distribution of geochemical heterogeneities in the source of Hawaiian shield lavas as revealed by a transect across the strike of the Loa and Kea spatial trends: East Molokai to West Molokai to Penguin Bank, Geochemica Cosmochemica Acta, 132, 214-237, doi: 10.1016/j.gca.2014.02.002.
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.