Ken H. Rubin

Volcanic eruptions on mid‐ocean ridges: New evidence from the superfast spreading East Pacific Rise, 17°–19°S

uniform sediment cover were recovered from lava that buries older faulted terrain. The boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector, consistent with magmas from two separate reservoirs being erupted in the same event. Chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments. A comparison of our results to global data on submarine mid-ocean ridge eruptions suggests consistent dependencies of erupted volume, activated fissure lengths, and chemical heterogeneity with spreading rate, consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 8414 Volcanology: Eruption mechanisms; 8439 Volcanology: Physics and chemistry of magma bodies; 3655 Mineralogy and Petrology: Major element composition; KEYWORDS: lava flow, chemical heterogeneity, erupted volume, lava morphology, side-scan sonar

New insights into mid-ocean ridge volcanic processes from the 2005–2006 eruption of the East Pacific Rise, 9°46′N–9°56′N

Digital seafloor imagery collected on 37 camera tows and Alvin dives, in which we identify 186 contacts between new and old lava, are used to create the most detailed map of a mid-ocean ridge (MOR) eruption to date. Lava flows erupted in 2005–2006 at the East Pacific Rise (EPR) covered an area of 14.6 km2 along ∼18 km of the EPR crest between 9°46′ and 9°56′N. The 2005–2006 lava is characterized by inflated lobate and sheet morphologies in the flow interiors and pillow forms at terminal flow fronts. Numerous lava channels ∼10–50 m wide and 1–5 m deep trending approximately east-west served as distributory pathways. Eruptions were sourced from fissures within the EPR axial summit trough as well as fissures located on an off-axis fissure mound ∼600 m east of the EPR axis between 9°52′ and 9°56′N. Portions of the lava flow reached as far as ∼2 km east of the axis near 9°51.2′N. Using a conservative estimate of 1.5 m for the average flow thickness implies that the 2005–2006 eruptions produced ∼22 × 106 m3 of lava, 4–5 times larger than estimated volumes of 1991–1992 EPR lava flows. Estimated lava volume for the 2005–2006 eruptions represents <15% of the magma available in the axial magma chamber.

DEGASSING OF METALS AND METALLOIDS FROM ERUPTING SEAMOUNT AND MID-OCEAN RIDGE VOLCANOES : OBSERVATIONS AND PREDICTIONS

Recently, it has been reported that the element polonium degasses from mid-ocean ridge and seamount volcanoes during eruptions. Published and new observations on other volatile metal and metalloid elements can also be interpreted as indicating significant degassing of magmatic vapors during submarine eruptions. This process potentially plays an important role in the net transfer of chemical elements from erupting volcanoes to seawater in addition to that arising from sea floor hydrothermal systems. In this paper, a framework is constructed for predicting and assessing semiquantitatively the potential magnitude and chemical fingerprints in the water column of metal and metalloid degassing using (1) predictions from a summary of element volatilities during mafic subaerial volcanism worldwide and (2) limited data from submarine volcanic effusives. The latter include analyses of polonium and trace metals in near-volcano water masses sampled following a submarine eruption at Loihi seamount, Hawaii (1000 m bsl) in 1996. The element volatility predictions and observations show good agreement, considering the limited dataset. Some of the highest volatility main group and transition element enrichments in seawater over Loihi are predicted by the degassing mass transfer model I present. When expanded to cover all submarine volcanic activity, it is predicted that exit fluxes of these elements are up to 102–103 greater by degassing than by normal MOR hydrothermalism. In contrast, MOR exit fluxes of low volatility alkali and alkaline earth elements are likely 102–106 greater from hydrothermal inputs. Degassing inputs to the ocean are probably highly episodic, occurring almost entirely during eruptions; these are times of enhanced and abnormal hydrothermalism as well. Although major hydrothermal and degassing events may not be chemically recognizable in real water masses as wholly distinct entities, it is nevertheless possible to predict to what extent each process flavors the effluents of the other. Degassing at mid-ocean ridges may explain a variety of observations previously ascribed to complexities occurring during hydrothermal venting and/or fluid ascent in the buoyant hydrothermal plumes above ridges.

Long-term eruptive activity at a submarine arc volcano

Three-quarters of the Earths volcanic activity is submarine, located mostly along the mid-ocean ridges, with the remainder along intraoceanic arcs and hotspots at depths varying from greater than 4,000 m to near the sea surface. Most observations and sampling of submarine eruptions have been indirect, made from surface vessels or made after the fact. We describe here direct observations and sampling of an eruption at a submarine arc volcano named NW Rota-1, located 60 km northwest of the island of Rota (Commonwealth of the Northern Mariana Islands). We observed a pulsating plume permeated with droplets of molten sulphur disgorging volcanic ash and lapilli from a 15-m diameter pit in March 2004 and again in October 2005 near the summit of the volcano at a water depth of 555 m (depth in 2004). A turbid layer found on the flanks of the volcano (in 2004) at depths from 700 m to more than 1,400 m was probably formed by mass-wasting events related to the eruption. Long-term eruptive activity has produced an unusual chemical environment and a very unstable benthic habitat exploited by only a few mobile decapod species. Such conditions are perhaps distinctive of active arc and hotspot volcanoes.

Minimum speed limit for ocean ridge magmatism from 210Pb-226Ra-230Th disequilibria.

Although 70 per cent of global crustal magmatism occurs at mid-ocean ridges—where the heat budget controls crustal structure, hydrothermal activity and a vibrant biosphere—the tempo of magmatic inputs in these regions remains poorly understood. Such timescales can be assessed, however, with natural radioactive-decay-chain nuclides, because chemical disruption to secular equilibrium systems initiates parent–daughter disequilibria, which re-equilibrate by the shorter half-life in a pair. Here we use 210Pb–226Ra–230Th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the Earths mantle can be transported, accumulated and erupted in a few decades. This implies that magmatic conditions can fluctuate rapidly at ridge volcanoes. 210Pb deficits of up to 15 per cent relative to 226Ra occur in normal mid-ocean ridge basalts, with the largest deficits in the most magnesium-rich lavas. The 22-year half-life of 210Pb requires very recent fractionation of these two uranium-series nuclides. Relationships between 210Pb-deficits, (226Ra/230Th) activity ratios and compatible trace-element ratios preclude crustal-magma differentiation or daughter-isotope degassing as the main causes for the signal. A mantle-melting model can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly. The likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied.

226Ra–230Th–238U disequilibria of historical Kilauea lavas (1790–1982) and the dynamics of mantle melting within the Hawaiian plume

Abstract The geochemical variations of Kilauea’s historical summit lavas (1790–1982) document a rapid fluctuation in the mantle source and melting history of this volcano. These lavas span nearly the entire known range of source composition for Kilauea in only 200 yr and record a factor of ∼2 change in the degree of partial melting. In this study, we use high-precision measurements of the U-series isotope abundances of Kilauea’s historical summit lavas and two ‘ingrowth’ models (dynamic and equilibrium percolation melting) to focus on the process of melt generation at this volcano. Our results show that the 226 Ra– 230 Th– 238 U disequilibria of these lavas have remained relatively small and constant with ∼12±4% excess 226 Ra and ∼2.5±1.6% excess 230 Th (both are ±2σ). Model calculations based mostly on subtle variations in the 230 Th– 238 U disequilibria suggest that lavas from the 19th to early 20th centuries formed at significantly higher rates of mantle melting and upwelling (up to a factor of ∼10) compared to lavas from 1790 and the late 20th century. The shift to higher values for these parameters correlates with a short-term decrease in the size of the melting region sampled by the volcano, which is consistent with fluid dynamical models that predict an exponential increase in the upwelling rate (and, thus, the melting rate) towards the core of the Hawaiian plume. The Pb, Sr, and Nd isotope ratios of lavas derived from the smallest source volumes correspond to the ‘Kilauea’ end member of Hawaiian volcanoes, whereas lavas derived from the largest source volumes overlap isotopically with recent Loihi tholeiitic basalts. This behavior probably arises from the more effective blending of small-scale source heterogeneities as the melting region sampled by Kilauea increases in size. The source that was preferentially tapped during the early 20th century (when the melt fractions were lowest) is more chemically and isotopically depleted than the source of the early 19th and late 20th century lavas (which formed by the highest melt fractions). This inverse relationship between the magnitude of source depletion and melt fraction suggests that source fertility (i.e. lithology) controls the degree of partial melting at Kilauea. Thus, rapid changes in the size of the melting region sampled by the volcano (in the presence of these small-scale heterogeneities) may regulate most of the source- and melting-related geochemical variations observed at Kilauea over time scales of decades to centuries.

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N: Implications for ridge crest plumbing and decadal changes in magma chamber compositions

Detailed mapping, sampling, and geochemical analyses of lava flows erupted from an ∼18 km long section of the northern East Pacific Rise (EPR) from 9°46′N to 9°56′N during 2005–2006 provide unique data pertaining to the short-term thermochemical changes in a mid-ocean ridge magmatic system. The 2005–2006 lavas are typical normal mid-oceanic ridge basalt with strongly depleted incompatible trace element patterns with marked negative Sr and Eu/Eu* anomalies and are slightly more evolved than lavas erupted in 1991–1992 at the same location on the EPR. Spatial geochemical differences show that lavas from the northern and southern limits of the 2005–2006 eruption are more evolved than those erupted in the central portion of the fissure system. Similar spatial patterns observed in 1991–1992 lavas suggest geochemical gradients are preserved over decadal time scales. Products of northern axial and off-axis fissure eruptions are consistent with the eruption of cooler, more fractionated lavas that also record a parental melt component not observed in the main suite of 2005–2006 lavas. Radiogenic isotopic ratios for 2005–2006 lavas fall within larger isotopic fields defined for young axial lavas from 9°N to 10°N EPR, including those from the 1991–1992 eruption. Geochemical data from the 2005–2006 eruption are consistent with an invariable mantle source over the spatial extent of the eruption and petrogenetic processes (e.g., fractional crystallization and magma mixing) operating within the crystal mush zone and axial magma chamber (AMC) before and during the 13 year repose period. Geochemical modeling suggests that the 2005–2006 lavas represent differentiated residual liquids from the 1991–1992 eruption that were modified by melts added from deeper within the crust and that the eruption was not initiated by the injection of hotter, more primitive basalt directly into the AMC. Rather, the eruption was driven by AMC pressurization from persistent or episodic addition of more evolved magma from the crystal mush zone into the overlying subridge AMC during the period between the two eruptions. Heat balance calculations of a hydrothermally cooled AMC support this model and show that continual addition of melt from the mush zone was required to maintain a sizable AMC over this time interval.

Geochemical heterogeneity within mid-ocean ridge lava flows: insights into eruption, emplacement and global variations in magma generation

Compositional heterogeneity in mid-ocean ridge (MOR) lava flows is a powerful yet presently under-utilized volcanological and petrological tracer. Here, it is demonstrated that variations in pre- and syn-eruptive magmatic conditions throughout the global ridge system can be constrained with intra-flow compositional heterogeneity among 10 discrete MOR flows. Geographical distribution of chemical heterogeneity within flows is also used along with mapped physical features to help decipher the range of conditions that apply to seafloor eruptions (i.e. inferred vent locations and whether there were single or multiple eruptive episodes). Although low-pressure equilibrium fractional crystallization can account for much of the observed intra-flow compositional heterogeneity, some cases require multiple parent magmas and/or more complex crystallization conditions. Globally, the extent of within-flow compositional heterogeneity is well correlated (positively) with estimated erupted volume for flows from the northern East Pacific Rise (EPR), and the Mid Atlantic, Juan de Fuca and Gorda Ridges; however, some lavas from the superfast spreading southern EPR fall below this trend. Compositional heterogeneity is also inversely correlated with spreading rate. The more homogeneous compositions of lavas from faster spreading ridges likely reflect the relative thermal stability and longevity of sub-ridge crustal magma bodies, and possibly higher eruption frequencies. By contrast, greater compositional heterogeneity in lavas at slower spreading rates probably results from low thermal stability of the crust (due to diminished magma supply and greater hydrothermal cooling). Finally, the within-flow compositional variations observed here imply that caution must be exercised when interpreting MOR basalt data on samples where individual flows have not been mapped because chemical variations between lava samples may not necessarily record the history of spatially and temporally distinct eruptions. fl 2001 Elsevier Science B.V. All rights reserved.

Rapid dike emplacement leads to eruptions and hydrothermal plume release during seafloor spreading events

The creation of ocean crust by rapid injection of magma at mid-ocean ridges can lead to eruptions of lava onto the seafloor and release of “event plumes,” which are huge volumes of anomalously warm water enriched in reduced chemicals that rise up to 1 km above the seafloor. Here, we use seismic data to show that seafloor eruptions and the release of hydrothermal event plumes correspond to diking episodes with high injection velocities and rapid onset of magma emplacement within the rift zone. These attributes result from high excess magma pressure at the dike source, likely due to a new influx of melt from the mantle. These dynamic magmatic conditions can be detected remotely and may predict the likelihood of event plume release during future seafloor spreading events.

Recent eruptive history and magma reservoir dynamics on the southern East Pacific Rise at 17°30′S

Submersible-based geologic observations and geochemical, magnetic paleointensity, and (210Pb/226Ra) radioactive disequilibria data indicate that at least five distinct lava sequences (three normal mid-ocean ridge basalt (N-MORB) and two transitional mid-ocean ridge basalt (T-MORB)) have been erupted within the last several hundred years along a 27-km-long portion of the fast spreading East Pacific Rise near 17°30′S. Isotopic and geochemical variations, both within and between eruptive units, indicate mixing of different primary magmas concurrently with differentiation in shallow-level subaxial magma reservoirs. Differentiation trends are linked to geographical variations in axial magma chamber (AMC) characteristics, with the lowest MgO samples erupted above the shallowest portion of the AMC, suggesting that pre-eruptive magma temperature is in part controlled by the depth-dependent efficacy of hydrothermal cooling. A third-order axial discontinuity at ∼17°29′S coincides with a narrowing of the subaxial melt lens and an increase in lava MgO to the south; we interpret the latter to reflect a sharp increase in the mixing proportion of recharge to low-MgO magma residing in the melt lens. Magmatic evolution of this area over the last few hundred years reflects continually evolving conditions in the subsurface and mantle melting processes that vary rapidly at rates that are at least as great as the eruption rate.