Melt Eruption Research Articles

Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions.

The internal thermal and magnetic evolution of rocky exoplanets is critical to their habitability. We focus on the thermal-orbital evolution of Earth-mass planets around low-mass M stars whose radiative habitable zone overlaps with the “tidal zone,” where tidal dissipation is expected to be a significant heat source in the interior. We develop a thermal-orbital evolution model calibrated to Earth that couples tidal dissipation, with a temperature-dependent Maxwell rheology, to orbital circularization and migration. We illustrate thermal-orbital steady states where surface heat flow is balanced by tidal dissipation and cooling can be stalled for billions of years until circularization occurs. Orbital energy dissipated as tidal heat in the interior drives both inward migration and circularization, with a circularization time that is inversely proportional to the dissipation rate. We identify a peak in the internal dissipation rate as the mantle passes through a viscoelastic state at mantle temperatures near 1800 K. Planets orbiting a 0.1 solar-mass star within 0.07 AU circularize before 10 Gyr, independent of initial eccentricity. Once circular, these planets cool monotonically and maintain dynamos similar to that of Earth. Planets forced into eccentric orbits can experience a super-cooling of the core and rapid core solidification, inhibiting dynamo action for planets in the habitable zone. We find that tidal heating is insignificant in the habitable zone around 0.45 (or larger) solar-mass stars because tidal dissipation is a stronger function of orbital distance than stellar mass, and the habitable zone is farther from larger stars. Suppression of the planetary magnetic field exposes the atmosphere to stellar wind erosion and the surface to harmful radiation. In addition to weak magnetic fields, massive melt eruption rates and prolonged magma oceans may render eccentric planets in the habitable zone of low-mass stars inhospitable for life. Key Words: Tidal dissipation—Thermal history—Planetary interiors—Magnetic field. Astrobiology 15, 739–760.

On the thermal and magnetic histories of Earth and Venus: Influences of melting, radioactivity, and conductivity

The study of the thermal evolution of Earth’s interior is uncertain and controversial in many respects, from the interpretation of petrologic observations used to infer the temperature and dynamics of the interior, to the physics and material properties governing heat transport. The thermal history of Venus is even more uncertain, but the lack of a dynamo at present in an otherwise similar planet may provide additional constraints on terrestrial planet evolution. In this paper a one dimensional thermal history model is derived that includes heat loss due to mantle melt eruption at the surface to explore its influence on the thermal and magnetic history of Earth and Venus. We show that the thermal catastrophe of Earth’s mantle, which occurs for a present day Urey ratio of 1/3 and convective heat loss exponent of β=1/3, can be avoided by assuming a rather high core heat flow of ∼15TW. This core heat flow also avoids the new core paradox by allowing for the geodynamo to be thermally powered prior to inner core growth for core thermal conductivities as high as 130Wm-1K-1. Dynamo regime diagrams demonstrate that the mantle melt eruption rate has a minor effect on the history of mobile lid planets due to the efficiency of plate tectonic convective heat loss. However, if Earth were in a stagnant lid regime prior to 2.5Ga, as has been proposed, then at least ∼5% of mantle melt is required to erupt in order to thermally power the paleodynamo at that time. Dynamo regime diagrams for stagnant lid Venus models indicate that more than half of the melt generated in the mantle is required to erupt in order to overcome the insulation imposed by the stagnant lid and drive a dynamo. This implies that with an Earth-like mantle radioactivity the Venusian dynamo shut down ∼0.3Ga for an eruption efficiency of 50%, and ∼3Ga for an eruption efficiency of zero. Consequently, a stagnant lid alone does not prevent a core dynamo if melting of the upper mantle provides a substantial mantle heat sink.

Temporal changes of medium properties during explosive volcanic eruption

Volcanic evolutions typically accompany significant changes in medium properties. Monitoring the seismic properties may be useful in identifying the state of volcanic evolution. Temporal variation of shallow crustal VP∕VS ratios before and during the 2009 eruption of Redoubt Volcano, Alaska, is investigated using the P and S traveltimes of local earthquakes. High VP∕VS ratios of ∼1.9 are observed over a wide region in the precursory phase, suggesting the presence of partial melts in the medium at least several months before explosive eruptions. The high V P ∕V S ratios decrease until the early effusive phase with rates of −0.179 year −1 in the precursory phase (65 days) and −2.147 year −1 in the explosive phase (40 days), which is equivalent to a decrease in the bulk modulus of fluids by ∼1.1 and 7.9 GPa, respectively. The decreasing V P ∕V S ratios in the precursory phase may be due to increasing amounts of overpressurized gas and water vapor in the medium. The rapid decrease in VP∕VS ratios during the explosive phase may be caused by melt eruption and gas emission. The VP∕VS ratios were observed to be nearly stationary since the early effusive phase, suggesting rare presence of melts and low levels of pore pressures with degassing. These observations suggest that monitoring VP∕VS ratios may be useful to identify the eruption state.

Geochemistry, Nd isotopes and U–Pb SHRIMP zircon dating of Neoproterozoic volcanic rocks from the Central Eastern Desert of Egypt: New insights into the ∼750 Ma crust-forming event

Submarine tholeiitic and calc-alkaline basalt and andesite with subordinate dacite, metamorphosed to greenschist facies, are exposed at Wadi Kareim and Wadi El Dabbah, Central Eastern Desert of Egypt. These metavolcanic sequences are representative of early stages of Neoproterozoic crust formation in the northern part of the Arabian-Nubian Shield. The magmas were primitive to moderately fractionated, as indicated by average Mg# (54), Cr (191ppm) and Ni (69ppm). Their REE patterns are slightly fractionated [(La/Yb)N=1.3–3.9], and multi-element diagrams show Ba, Sr, K and Rb enrichments and Nb and Ta depletions, typical of modern convergent margin igneous rocks. Tectonic discrimination diagrams also show affinities with modern convergent margin magmas, suggesting that Wadi El Dabbah lavas formed in a volcanic arc that experienced cryptic crustal contamination, whereas Wadi Kareim lavas formed in a back-arc basin. Positive initial ɛNd (+5.1 to +8.9) and Nd model ages (0.64–0.79Ga) indicate that the crust is juvenile and was extracted from a depleted mantle source. Wadis Kareim and El Dabbah metavolcanic rocks are dated at ∼750Ma (U–Pb SHRIMP) but contain abundant pre-Neoproterozoic zircons which might be derived from underlying sediments or perhaps older continental crust. These results indicate that a major ∼750Ma crust-forming event involving ophiolite generation and eruption of juvenile melts was important for the formation of the crust.

Intermediate composition magma production in an intracontinental setting: Unusual andesites and dacites of the mid-Miocene Santa Rosa–Calico volcanic field, Northern Nevada

The mid-Miocene Santa Rosa–Calico volcanic field (SC) of northern Nevada provides an outstanding example of the role open-system magmatic processes play in producing calc-alkaline and tholeiitic andesite–dacite magmas in an intracontinental setting. SC volcanism commenced at ∼ 16.7 Ma and is associated with the initial manifestations of the Yellowstone hotspot, the Columbia River–Steens flood basalt event(s), and the formation of the Northern Nevada rift. Locally a diverse package of magmatic products ranging from tholeiitic basalt to high-Si rhyolite was produced during an ∼ 2 myr duration. Within this package are the products of at least four distinct intermediate composition magmatic systems that may represent as much as 40% of the SC volcanic pile. These help differentiate the SC from contemporaneous Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee, Northwest Nevada) that are dominated by bimodal basalt–rhyolite assemblages. All SC intermediate units are characterized by textural and mineralogic complexities including xenoliths and xenocrysts of local crust and crystal clots of plagioclase ± clinopyroxene ± orthopyroxene ± oxide. SC intermediate units are dominantly tholeiitic, but include lava flows with transitional to calc-alkaline affinities. Relative to locally erupted Steens Basalt, SC intermediate lava flows have similar elemental enrichments and depletions, but dissimilar Sr and Nd isotopic compositions. These isotopic differences, coupled with the abundant disequilibrium features and variable incompatible element ratios, indicate that open system magmatic processes played a major role in the genesis of the intermediate units. SC silicic magmas were produced primarily via upper crustal melting of chemically and isotopically heterogeneous Cretaceous granitoid. Interaction between fractionating mafic Steens flood basalt magmas and the more evolved crustal melts ± assimilation of local upper crust provides a general template for the generation of SC tholeiitic and calc-alkaline intermediate magmas. Between system heterogeneities primarily reflect magma- versus assimilation-dominated mixing. These complex processes and the resulting production and eruption of SC intermediate composition melts were stimulated by incipient, focused lithospheric extension and driven by the virtually continuous local availability of upwelling Steens Basalt, over at least a 2 myr duration. These results demonstrate that at least some of the mid-Miocene and younger calc-alkaline magmas that are found across the Pacific Northwest and are unrelated to subduction processes, were formed as a direct result of open-system processes affecting regional flood basalt magmas, rather than just melting of previously enriched (via subduction) lithospheric mantle as suggested by previous workers.

Kimberlite petrogenesis: Insights from clinopyroxene-melt partitioning experiments at 6 GPa in the CaO-MgO-Al 2O 3-SiO 2-CO 2 system

In this experimental study, we examine the mineral-melt partitioning of major and trace elements between clinopyroxene and CO 2-rich kimberlitic melts at a pressure of 6 GPa and temperatures of 1410°C and 1430°C. The melts produced contain ∼ 28 wt% dissolved CO 2, and are saturated with olivine and clinopyroxene. To assess the effects of temperature, crystal and melt compositions on trace element partitioning, experiments were performed in the model CaO-MgO-Al 2O 3-SiO 2-CO 2 system. Our results reveal that all the elements studied, except Al, Mg, Si, and Ga, are incompatible in clinopyroxene. Partition coefficients show a considerable range in magnitude, from ∼ 10 −3 for D U and D Ba to ∼ 2.5 for D Si. The two experimental runs show similar overall partitioning patterns with the D values being lower at 1430°C. Rare earth elements display a wide range of partition coefficients, D La (0.012–0.026) being approximately one order of magnitude lower than D Lu (0.18–0.23). Partition coefficients for the 2+ and 3+ cations entering the M2-site exhibit a near-parabolic dependence on radius of the incorporated cations as predicted from the lattice strain model. This underlines the contribution made by the crystal structure toward controlling the distribution of trace elements. Using data obtained in this study combined with that in the published literature, we also discuss the effects that other important parameters, namely, melt composition, pressure, and temperature, could have on partitioning. Our partition coefficients have been used to model the generation of the Group I (GI) kimberlites from South Africa. The numerical modeling shows that kimberlitic melts can be produced by ∼0.5% melting of a MORB-type depleted source that has been enriched by small-degree melts originating from a similar depleted source. This result suggests that the source of GI kimberlites may be located at the lithosphere-asthenosphere transition. Percolation of small degree melts from the asthenosphere would essentially create a metasomatic horizon near the bottom of the non-convecting sublithospheric mantle. Accumulation of such small degree melts together with the presence of volatiles and conductive heating would trigger melting of the ambient mantle and subsequently lead to eruption of kimberlitic melts. Additionally, our model shows that the GI source can be generated by metasomatism of a 2 Ga old MORB source ca. 1 Ga ago. Assuming that MORB-type mantle is the most depleted source of magmas on earth, then this is the oldest age at which the GI source could have existed. However, this age most likely reflects the average age of a series of metasomatic events than that of a single event.

Petrogenesis of the Greenhills Complex, Southland, New Zealand: magmatic differentiation and cumulate formation at the roots of a Permian island-arc volcano

A Permian (265 Ma) intrusive complex which formed as a magmatic feeder reservoir to an immature island-arc volcano is fortuitously exposed in southern New Zealand. Known as the Greenhills Complex, this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes. Cumulates, which include dunite, olivine clinopyroxenite, olivine gabbro, and hornblende gabbro-norite, are related products of parent-magma fractionation. Both primary (magmatic) and secondary platinum-group minerals occur within dunite at one locality. Using the composition of cumulus minerals, mafic dykes and melt inclusions, we have determined that the parent magmas of the complex were hydrous, low-K island-arc tholeiites of ankaramitic affinities. Progressive magmatic differentiation of this parent magma generated fractionated melt of high-alumina basalt composition which is now preserved only as dykes which cut the Complex. Field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate-rock formation. Recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface. Similar processes are inferred to account for volcanic-rock compositions in other parts of this arc terrane and in modern island-arc systems.

Geochemical variability in a single flow from northern Iceland

Compositional variability is present for almost every element analyzed in 70 whole rock samples and 40 olivine‐hosted melt inclusions from the Borgarhraun lava flow in NE Iceland. The variation of incompatible element concentrations can be produced by incomplete mixing of fractional melts, while the compatible element variations in the whole rock samples can be explained by addition/removal of crystals found in the flow. The melt inclusion incompatible element compositions are more variable than the whole rock compositions, and the magnitude of variability of the whole rock samples can be matched if each hand specimen is made of magma that was formed by mixing of 20–30 batches of melt with the composition of the inclusions. Clinopyroxene barometry and the major element composition of the whole rock samples suggest that mixing took place at pressures of ∼0.9 GPa in sub‐Moho magma chambers. The spatial distribution of the concentrations of incompatible elements is not random in Borgarhraun and samples with a separation of <4 km have concentrations that are more similar than expected from a random distribution of geochemistry. This separation distance corresponds to a lava volume of 0.014–0.14 km3. This volume may be controlled by magma mixing and episodic eruption of melts from a sub‐Moho chamber. The concentration variations of compatible elements with position in the flow are little different to those expected for random variation. It is likely that the crystal removal and addition processes that control major element compositions generate variability on short length scales.

Temporal‐compositional trends in intraplate basalt eruptions: Implications for mantle heterogeneity and melting processes

Because primitive basaltic magmas travel from their mantle sources to the surface without significant cooling and crystallization en route, compositional variations within them provide direct information on the sources and processes of partial melting in the mantle. Most studies of basalt magmagenesis have focused on chemical variations in magmas erupted from multiple vents in regional complexes or on long timescale (105–106years) variations in magmas from single shield volcanoes; few studies have aimed to measure changes in melt chemistry with time in individual eruptions. Available observations of individual basaltic eruption sequences, however, show remarkably similar temporal‐compositional trends in the form of decreasing incompatible elements and MgO, with increasing SiO2, as the eruption (or sequence of related, closely‐spaced eruptions) continued. Both trace element and isotopic variations indicate that this does not reflect fractional crystallization nor can it be explained simply by varying extents of partial melting of a single source. Instead, these trends are consistent with systematically changing proportions of mixing between melts produced by different degrees of partial melting of two compositionally distinct but apparently common sources in the mantle. One potential explanation for this is that a relatively deep mantle source melts to large degrees, producing melts that ascend through and mix with small‐degree melts of the lithosphere. The small‐degree lithospheric melts are then progressively exhausted during melt migration and eruption, so that late‐erupting melts contain the smallest proportion of the shallow, small‐degree melt. This cannot explain Os isotopic correlations with melt composition and radiogenic Os isotope signatures of the late‐erupting melts in several eruption sequences, however. An alternative scenario is that systematic changes in erupted melt chemistry with time reflect sequential eruption of melt from increasing depths in mantle melting regions that are compositionally zoned because of different solidii and melt productivities of distinct peridotite and pyroxenite lithologic domains. In either case, compositional variation in many eruptions of primitive, intraplate basalts is dominated by mixing of melts from distinct lithologies, and temporal‐compositional changes in such eruptions reflect the distribution and composition of melt with depth in mantle melting and melt migration regions.

Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 from 3 to 7 GPa

We have experimentally determined the solidus position of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 (CMAS.CO2) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subsolidus reactions at the solidus involving carbonate generates two invariant points, I1A and I2A, which mark the transition from CO2-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO2, we find I1A at 2.6 GPa/1230 °C and I2A at 4.8 GPa/1320 °C. The variation of all phase compositions along the solidus has also been determined. In the pressure range investigated, solidus melts are carbonatitic with SiO2 contents of <6 wt%, CO2 contents of ˜45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO2 system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO2-rich and silica poor. The widespread evidence for the presence of CO2 in both the oceanic and continental upper mantle implies that such low degree SiO2-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface.

Phenomenological studies on melt-coolant interactions in the ALPHA program

Two series of experiments to investigate melt-coolant interactions have been performed as part of the ALPHA program at JAERI. In the melt drop steam explosion experiments, melt simulating a molten core was dropped into a pool of water. Volume fractions of the melt, water and steam in the mixing region prior to the occurrence of spontaneous steam explosions were quantified. Other characteristics of melt-coolant interactions were evaluated for settling velocity of the melt in water, propagation and expansion velocities, energy conversion ratio and debris size distribution. It was found that the probability of the occurrence of spontaneous steam explosions could be reduced by using a melt dispersion device. Measurement of void fraction in the mixing region clearly showed that the melt dispersion device enhanced steam generation. However, one experiment indicated that the use of the dispersion device could possibly result in a more energetic steam explosion. It was found that the mixing of non-condensable gas in the steam phase of the mixing region during melt dispersion played an important role for the suppression of the spontaneous steam explosion. Knowledge of the parametric effects of melt mass, ambient pressure and water temperature was extended. In the melt coolability experiments, water was poured onto the melt to investigate melt-coolant interactions in a stratified geometry where water overlies on a melt layer. Melt eruptions which could induce an explosive interaction were observed when the subcooled water was poured through a pipe nozzle. The eruption was not observed when the water was near the saturation temperature or supplied through a spray nozzle. The explosive interaction in the stratified geometry was found to be much smaller in magnitude than the steam explosion in the melt drop configuration.

Carbon isotope variations of CO 2 in Central Lau Basin basalts and ferrobasalts

CO 2 abundance and carbon isotope data are reported for basalt glasses from three spreading centres in the Central Lau Basin. Abundances of CO 2 dissolved in glasses vary from 9 ppm to 217 ppm with δ 13C ranging from −16.4‰ to −8.8‰. Evolved ferrobasalts from the propagating rift tip of the Central Lau Spreading Centre have both the lowest CO 2 concentrations and δ 13C values, while more magnesian basalts from the Intermediate and Eastern Lau Spreading Centres have higher CO 2 abundances with higher and less variable δ 13C values. Variations in abundance and carbon isotope ratios of dissolved CO 2 can be modelled by degassing during eruption of melts originally containing 410–440 ppm CO 2, with δ 13C of −7.7‰, using a vapour-melt isotope fractionation of 2.3‰. The type of degassing experienced by melts is related to their style of eruption, which is, in turn, governed by their eruption site. Central Lau Basin glasses, along with glasses from other marginal basins, display carbon isotope ratios which are lower than MORB. While the variations within the suite are adequately described by various degrees of degassing, carbon isotope heterogeneity between sources of oceanic basalts can not be discounted.

Tertiary picrites in West Greenland: melting at the periphery of a plume?

Abstract The West Greenland/Baffin Island Tertiary volcanic province differs from other CFB provinces in containing an unusually high proportion (30–50% by volume) of magnesian picritic lavas and hyaloclastites. Olivine-liquid equilibrium considerations suggest the presence during the earlier stages of eruption of picritic melts with MgO contents as high as 20%. Calculations based on McKenzie-Bickle melting models point to high degrees of melting (24–30%) at depths of 60–90 km in the underlying mantle, and require potential temperatures of 1540–1600°C. Such high potential temperatures are inconsistent with reconstructions that attribute the West Greenland volcanism to melting on the margins of the incipient Iceland plume-head. The distribution of Tertiary volcanic activity in Greenland, in particular its relation to Mesozoic-Tertiary extensional basins, indicates that lithospheric structure plays a part in determining where the plume-head can undergo melting. But to explain the restriction of high-temperature picrites to West Greenland, together with their distinctive trace element geochemistry, it is necessary to invoke (a) either an elongated Icelandic plume initially extending as far as West Greenland or a short-lived precursory plume head that developed directly beneath West Greenland, and (b) an active extensional regime that allowed rapid access of picritic melts to the surface.

Evolution of waning, subduction-related magmatism, northern Sierra Madre Occidental, Mexico

The stratigraphic record at the Tomochic volcanic center, Chihuahua, documents a change from andesitic to rhyolite-dominated volcanism, with late extrusion of mafic lavas, during the final ∼10 m.y. of subduction-related activity in the northern Sierra Madre Occidental. Andesitic volcanism, which started before 38 Ma and continued until ∼35 Ma, was followed by eruption of large-volume rhyolite ash flows from the Las Varas (34.1 Ma) and Tomochic (31.8-31.4 Ma) calderas. Basaltic andesite, the most mafic rock type in the area, was extruded near the end of calcalkalic volcanism at ∼30 Ma. The change in style of volcanism at the Tomochic volcanic center also occurred in other parts of the northern Sierra Madre Occidental and is interpreted to reflect the evolution of waning, subduction-related magmatism, as convergence between the North American and Farallon plates slowed before it eventually ceased. Slowed plate convergence resulted in a decreased flux of mafic melts from the mantle to the crust, leading, in turn, to the stagnation and coalescence of intermediate-composition magmas similar to those that fed early andesitic eruptions. Subsequent differentiation led to growth of zoned magma reservoirs from which the rhyolite ash flows were extruded. Slowed convergence was accompanied by the onset of crustal extension, which possibly aided the coalescence of andesitic magmas and which allowed the ascent and eruption of mafic melts near the end of silicic volcanic activity. Although minor differences exist, the history of mid-Tertiary magmatism and tectonism in the northern Sierra Madre Occidental was broadly similar to that in parts of the south-western United States.