Hotspot Volcanoes Research Articles

Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust

Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials. However, the residence time of these subducted materials in the mantle is uncertain and model-dependent, and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust. Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago. Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur--probably derived from hydrothermally altered oceanic crust--was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative Δ(33)S values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions. This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots.

Scientific Drilling

Deep-Earth convection can be understood by studying hotspot volcanoes that form where mantle plumes rise up and intersect the lithosphere, the Earth's rigid outer layer. Hotspots characteristically leave age-progressive trails of volcanoes and seamounts on top of oceanic lithosphere, which in turn allow us to decipher the motion of these plates relative to deep-mantle plumes, and their (isotope) geochemistry provides insights into the long-term evolution of mantle source regions. However, it is strongly suggested that the Hawaiian mantle plume moved approximately 15 degrees south between 80 and 50 million years ago. This raises a fundamental question about other hotspot systems in the Pacific, whether or not their mantle plumes experienced a similar amount and direction of motion. Integrated Ocean Drilling Program (IODP) Expedition 330 to the Louisville Seamounts showed that the Louisville hotspot in the South Pacific behaved in a different manner, as its mantle plume remained more or less fixed around 48 degrees S latitude during that same time period. Our findings demonstrate that the Pacific hotspots move independently and that their trajectories may be controlled by differences in subduction zone geometry. Additionally, shipboard geochemistry data shows that, in contrast to Hawaiian volcanoes, the construction of the Louisville Seamounts doesn't involve a shield-building phase dominated by tholeiitic lavas, and trace elements confirm the rather homogenous nature of the Louisville mantle source. Both observations set Louisville apart from the Hawaiian-Emperor seamount trail, whereby the latter has been erupting abundant tholeiites (characteristically up to 95% in volume) and which exhibit a large variability in (isotope) geochemistry and their mantle source

Geochemical variations in Archean volcanic rocks, southwestern Greenland: Traces of diverse tectonic settings in the early Earth

The primary driving force behind present-day structural, magmatic, sedimentary and metamorphic processes is plate tectonics, resulting from the fl ow of matter and energy between the lithosphere and mantle along divergent, convergent, and transform plate boundaries. Operation of plate tectonics and eruption of hot spot lavas from mantle plumes, stemming from the core-mantle boundary, appears to be coupled in that subducting oceanic plates pile up at the core-mantle boundary and then rise as buoyant plumes to feed hot spot volcanoes (see Hofmann, 1997; Burke, 2011). How far back in Earth history were these geological processes driven by plate tectonics? Were Archean geological processes dominated by density-driven, vertical crustal overturns and diapirs, without modern analogs? How did Archean continents grow? How did Archean oceanic crust form? Did Archean oceanic crust recycle into the mantle at subduction zones? These questions remain controversial. Excellent exposures and a prolonged geological record spanning 3.85‐2.5 Ga in the Archean craton of southwestern Greenland provide a unique opportunity to test hypotheses proposed for the early evolution of Earth. This craton consists mainly of Eoarchean to Neoarchean (ca. 3.8‐

IODP Expedition 330: Drilling the Louisville Seamount Trail in the SW Pacific

Deep-Earth convection can be understood by studying hotspot volcanoes that form where mantle plumes rise up and intersect the lithosphere, the Earth's rigid outer layer. Hotspots characteristically leave age-progressive trails of volcanoes and seamounts on top of oceanic lithosphere, which in turn allow us to decipher the motion of these plates relative to "fixed" deep-mantle plumes, and their (isotope) geochemistry provides insights into the long-term evolution of mantle source regions. However, it is strongly suggested that the Hawaiian mantle plume moved ~15° south between 80 and 50 million years ago. This raises a fundamental question about other hotspot systems in the Pacific, whether or not their mantle plumes experienced a similar amount and direction of motion. Integrated Ocean Drilling Program (IODP) Expedition 330 to the Louisville Seamounts showed that the Louisville hotspot in the South Pacific behaved in a different manner, as its mantle plume remained more or less fixed around 48°S latitude during that same time period. Our findings demonstrate that the Pacific hotspots move independently and that their trajectories may be controlled by differences in subduction zone geometry. Additionally, shipboard geochemistry data shows that, in contrast to Hawaiian volcanoes, the construction of the Louisville Seamounts doesn’t involve a shield-building phase dominated by tholeiitic lavas, and trace elements confirm the rather homogenous nature of the Louisville mantle source. Both observations set Louisville apart from the Hawaiian-Emperor seamount trail, whereby the latter has been erupting abundant tholeiites (characteristically up to 95% in volume) and which exhibit a large variability in (isotope) geochemistry and their mantle source components. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.15.02.2013" target="_blank">10.2204/iodp.sd.15.02.2013</a>

Volcaniclastic sedimentation on the submarine slopes of a basaltic hotspot volcano: Piton de la Fournaise volcano (La Réunion Island, Indian Ocean)

Volcaniclastic successions are well-described in volcanic arc setting but rare in hotspot environments. The present work proposes a facies model of volcaniclastic sedimentation related to basaltic hotspot volcanoes as exemplified by the Piton de la Fournaise volcano (La Reunion Island). The facies model is based on a multi-scale approach combining high-resolution multibeam and backscatter data, deep-water photographs, side scan imagery and Kullenberg piston cores. Data show that a wide range of gravity flows and erosional features develop in the active volcaniclastic sedimentary system. Coastal and submarine instabilities are the main processes shaping the volcano's submarine morphology. Meanwhile, the evolution and dynamics of the sedimentary system are strongly linked with the morpho-structural evolution of the subaerial part of the volcano. The proposed model is characterized by a cyclic succession of stages: (1) a growing stage during which sedimentary activity is mainly restricted to proximal and mid-slope areas; (2) a collapse stage that entirely reshapes the morphology of the submarine slopes; and (3) an erosional stage related to a slow down of volcanic activity, enabling the development of large deep-sea fans.

Dendritic Crystallization: A Single Process for all the Textures of Olivine in Basalts?

The olivine macrocrysts found in oceanites, picrites and magnesian basalts erupted at hotspot volcanoes are generally interpreted either as phenocrysts crystallized from the magma or as xenocrysts extracted from a deforming cumulate. To constrain the origin of these crystals we studied their texture and composition at Piton de la Fournaise volcano, La Reunion. We show that macrocrysts are organized and subdivided into parallel units; this suggests a crystallization by dendritic growth and ripening rather than by a complex combination of paired nucleation, crystal aggregation or synneusis. Dendritic growth is also evidenced by the occurrence of hollow faces, P-rich zones, melt and Cr-spinel inclusions formed from the accumulation of slow diffusing impurities (P, Cr, Al) in the liquid at the contact with rapid-growing olivine. We suggest that early dendritic crystallization may even cause branch misorientations and lattice mismatches, yielding subgrain boundaries, dislocation lamellae and to a certain extent undulose extinction, which have all been formerly interpreted in terms of plastic intracrystalline deformation. We interpret olivine macrocrysts as phenocrysts crystallized under a strong degree of undercooling (-ΔT > 60°C), and derived from a harrisitic mush formed on the cold walls of the magma reservoir. Given the growth shapes indicated by P zoning patterns and external faces, the olivine macrocrysts (which consist of groups of several subcrystals) have grown in suspension within the liquid and were neither aggregated into a dense cumulate nor corroded, shocked or deformed before or during their transport to the surface. The major consequence of our study is that most olivine macrocrysts are not xenocrysts, and very few of them, if any, have experienced intracrystalline deformation. The importance of deforming (creeping) cumulate bodies, thought to accommodate the spreading of basaltic volcanoes in La Reunion and Hawaii, may hence have been overestimated.

Solid–liquid iron partitioning in Earth’s deep mantle

Melting processes in the deep mantle have important implications for the origin of the deep-derived plumes believed to feed hotspot volcanoes such as those in Hawaii. They also provide insight into how the mantle has evolved, geochemically and dynamically, since the formation of Earth. Melt production in the shallow mantle is quite well understood, but deeper melting near the core-mantle boundary remains controversial. Modelling the dynamic behaviour of deep, partially molten mantle requires knowledge of the density contrast between solid and melt fractions. Although both positive and negative melt buoyancies can produce major chemical segregation between different geochemical reservoirs, each type of buoyancy yields drastically different geodynamical models. Ascent or descent of liquids in a partially molten deep mantle should contribute to surface volcanism or production of a deep magma ocean, respectively. We investigated phase relations in a partially molten chondritic-type material under deep-mantle conditions. Here we show that the iron partition coefficient between aluminium-bearing (Mg,Fe)SiO(3) perovskite and liquid is between 0.45 and 0.6, so iron is not as incompatible with deep-mantle minerals as has been reported previously. Calculated solid and melt density contrasts suggest that melt generated at the core-mantle boundary should be buoyant, and hence should segregate upwards. In the framework of the magma oceans induced by large meteoritic impacts on early Earth, our results imply that the magma crystallization should push the liquids towards the surface and form a deep solid residue depleted in incompatible elements.

Large volumes of rejuvenated volcanism in Samoa: Evidence supporting a tectonic influence on late‐stage volcanism

Hot spot volcanoes are commonly constructed in a characteristic sequence of stages. After the volumetrically dominant shield stage, a protracted period of quiescence ends with a final stage of activity: rejuvenated volcanism. The mechanism responsible for generating rejuvenated volcanism is not generally agreed upon. New data obtained for samples 200 m down‐section in a deeply incised canyon on Savai‘i (Samoa) are unusually enriched isotopically and indicate a relatively voluminous rejuvenated stage compared to other intraplate volcanoes. Using a modified flexural model originally proposed for Hawai‘i, we suggest that the location of Samoa near the Tonga Trench terminus causes plate flexure resulting in upward flow of the shallow mantle driving partial melting. In particular, subduction‐related plate bending in the Samoan region may cause a larger flexural amplitude than generated by volcanic loading in Hawai‘i. The larger amplitude may explain the larger volume of rejuvenated melt in Samoa, constrained by our new data. Moreover, we argue that Sr‐Nd‐Pb‐Os‐He‐Ne isotopes in Samoan rejuvenated lavas are all consistent with sampling of a lithospheric component that is characterized by a metasomatic imprint from the Pacific Plate's earlier passage over the Rarotonga hot spot. Furthermore, temperature estimates for the melts suggest a drop in temperature during the predicted shallower melting due to flexural uplift, compared to the conditions during shield volcanism. Thus, flexural bending and metasomatism of the Samoan lithosphere may have generated the voluminous and geochemically distinct Samoan rejuvenated lavas, implying the lithosphere may play an important role during this stage in non‐ Hawaiian hot spots.

Shallow lithospheric contribution to mantle plumes revealed by integrating seismic and geochemical data

Using an integrated geophysical and geological approach based on a comparative analysis of 27 different parameters, we investigate both the location and origin of the source material for hot spot volcanoes. We report a significant correlation between extreme geochemical compositions of lavas with both shallow seismic velocities and the depth extent to which a hot spot source can be traced with seismic tomography. These relationships provide the first integrated evidence for buoyantly rising plumes of mantle material (plume depth extent correlation), incorporating thermally eroded continental material in the shallow mantle (anti‐correlation with slow shallow seismic velocities). The extreme geochemical composition of the lavas is successfully modeled with this process, placing constraints on dynamical processes in the mantle and explaining the origin of a significantly debated mantle compositional end‐member (enriched mantle; EM1).

Surface ocean iron fertilization: The role of airborne volcanic ash from subduction zone and hot spot volcanoes and related iron fluxes into the Pacific Ocean

Surface ocean iron (Fe) fertilization can affect the marine primary productivity (MPP), thereby impacting on CO2 exchanges at the atmosphere-ocean interface and eventually on climate. Mineral (aeolian or desert) dust is known to be a major atmospheric source for the surface ocean biogeochemical iron cycle, but the significance of volcanic ash is poorly constrained. We present the results of geochemical experiments aimed at determining the rapid release of Fe upon contact of pristine volcanic ash with seawater, mimicking their dry deposition into the surface ocean. Our data show that volcanic ash from both subduction zone and hot spot volcanoes (n = 44 samples) rapidly mobilized significant amounts of soluble Fe into seawater (35–340 nmol/g ash), with a suggested global mean of 200 ± 50 nmol Fe/g ash. These values are comparable to the range for desert dust in experiments at seawater pH (10–125 nmol Fe/g dust) presented in the literature (Guieu et al., 1996; Spokes et al., 1996). Combining our new Fe release data with the calculated ash flux from a selected major eruption into the ocean as a case study demonstrates that single volcanic eruptions have the potential to significantly increase the surface ocean Fe concentration within an ash fallout area. We also constrain the long-term (millennial-scale) airborne volcanic ash and mineral dust Fe flux into the Pacific Ocean by merging the Fe release data with geological flux estimates. These show that the input of volcanic ash into the Pacific Ocean (128–221 × 1015 g/ka) is within the same order of magnitude as the mineral dust input (39–519 × 1015 g/ka) (Mahowald et al., 2005). From the similarity in both Fe release and particle flux follows that the flux of soluble Fe related to the dry deposition of volcanic ash (3–75 × 109 mol/ka) is comparable to that of mineral dust (1–65 × 109 mol/ka). Our study therefore suggests that airborne volcanic ash is an important but hitherto underestimated atmospheric source for the Pacific surface ocean biogeochemical iron cycle.

Cannibalism of olivine-rich cumulate xenoliths during the 1998 eruption of Piton de la Fournaise (La Réunion hotspot): Implications for the generation of magma diversity

Contrasting with its unusual isotopic homogeneity compared to other hotspot volcanoes, Piton de la Fournaise has produced a large diversity of basaltic magmas over its 0.5 Ma history: picrites and two types of transitional basalts with distinct petrological and chemical compositions. A minor group of evolved basalts (anomalous group of basalts or AGB) is enriched in both compatible (Mg, Fe, Ti, Cr, and Ni) and incompatible (K, Th, and La) elements and depleted in Ca and Si relative to the dominant group of evolved basalts. The 1998 eruption simultaneously produced the two basaltic types at two distinct vents (Hudson vent: AGB, Kapor vent: common basalt) but from the same feeding conduit. Glasses of both magmas are close in composition and belong to the single differentiation trend defined by all 1998–2007 glass compositions. Thermodynamic model (MELTS code) shows that AGB-type magmas cannot be produced by high pressure (> 1 GPa) clinopyroxene fractionation as previously proposed and that all melts of the 1998–2007 activity period are produced by low pressure (< 800 MPa) crystal fractionation from the most primitive basalt (MgO ~ 9%). Modal composition of 1998 lavas (mass balance calculation and SEM image analysis) and olivine crystal composition show that Hudson lavas have assimilated significant fractions of olivine xenocrysts contrary to Kapor lavas. In addition, the higher incompatible element contents of Hudson lavas suggest contamination by a differentiated (trachytic) melt. All AGB share the following characteristics: (i) evolved glass compositions, (ii) 5–10% olivine xenocrysts, and (iii) vents located in a narrow region at the summit of the edifice. They are interpreted as the result of the assimilation of olivine-rich xenoliths either by evolved melts or by basaltic melts contaminated by low fractions of differentiated melts produced from interstitial glass frequently coating cumulates minerals or resulting from partial melting of cumulates bearing pyroxene or plagioclase (wehrlitic to gabbroic cumulates). The scarcity of AGB magmas is attributed to their shallow transfer path in rarely intruded lateral zones of Piton de la Fournaise volcano: wehrlitic to gabbroic cumulates bodies are either heterogeneously distributed within the edifice or have been depleted in low melting point components in the ‘Rift Zone’ where most of the recent eruptive events are emplaced. These results emphasize the exceptional chemical homogeneity of the primary basaltic melt involved in volcanic activity of Piton de la Fournaise hotspot for 0.5 Ma and the increasingly recognized role of magma–wall rock interactions in erupted magma compositions.

Deciphering Earth's Deep History: Drilling in Africa's Oldest Greenstone Belt

Earth's early evolution and the conditions under which life first emerged are highly controversial topics. Did Venus‐type tectonics dominate the early Earth, with vast landscapes of hot spot volcanoes belching lava and gases into a tenuous carbon dioxide/methane‐rich atmosphere, or was plate tectonics already in operation? Debates rage about when water first became dominant across Earth's surface, the temperatures and depths of early oceans, and when the first protocontinents began to form. These large uncertainties confound scientists' understanding of ecological niches available for the emergence of simple life, and a quantifiable image of the early Archean world is still missing.

Intrinsic and scattering attenuation of the Mt Fuji Region, Japan

Mount Fuji is the focus of intense study because of its potential hazard signaled by seismic, geologic and historical activity. Based on extensive seismic data recorded in the vicinity of Mt Fuji, coda quality factor (Q−1C) using a single scattering model hypothesis, and intrinsic and scattering quality factor (Q−1i and Q−1S) using the Multiple Lapse Time Window (MLTW) method were measured. To focus the study on the magmatic structure below Mt Fuji, the data were separated into two groups: a near-Fuji region of rays traversing an area with radius 5 km around the summit (R < 5 km), and a far-Fuji region of rays beyond a radius of 20 km around the summit (R > 20 km). This classification shows the largest discrepancy of Q−1C at a range of sampling volumes corresponding to overlapped sampling depth of about 80 km. Further, the spatial division shows significant difference of Q−1i and Q−1s at hypocentral distance of 80 km. The large difference of Q−1s in bandwidths 2–4, and 4–8 Hz indicates lithospheric heterogeneity beneath Mt Fuji with a characteristic heterogeneity scale length of about 1 km. The results have a small error range due to the large data sample, showing that all Q−1 values in the near-Fuji area are greater than those of the far-Fuji area, and Q−1i for both the near and far-Fuji areas is higher than Q−1s at high frequencies. The Q−1i and Q−1s values for far-Fuji are in the range of values for typical non-volcanic areas. The Q−1i values of the near-Fuji area are lower than those of other volcanic areas considered, where as values of Q−1s are not. The low Q−1i for the volcanic region of near-Fuji suggests that the magmatic activity, indicated by percent partial melt, in the vicinity of Mt Fuji is not as active as hot spot volcanoes, such as Kilauea, Hawaii.

Preserving noble gases in a convecting mantle

High (3)He/(4)He ratios sampled at many ocean islands are usually attributed to an essentially undegassed lower-mantle reservoir with high (3)He concentrations. A large and mostly undegassed mantle reservoir is also required to balance the Earth's (40)Ar budget, because only half of the (40)Ar produced from the radioactive decay of (40)K is accounted for by the atmosphere and upper mantle. However, geophysical and geochemical observations suggest slab subduction into the lower mantle, implying that most or all of Earth's mantle should have been processed by partial melting beneath mid-ocean ridges and hotspot volcanoes. This should have left noble gases in both the upper and the lower mantle extensively outgassed, contrary to expectations from (3)He/(4)He ratios and the Earth's (40)Ar budget. Here we suggest a simple solution: recycling and mixing of noble-gas-depleted slabs dilutes the concentrations of noble gases in the mantle, thereby decreasing the rate of mantle degassing and leaving significant amounts of noble gases in the processed mantle. As a result, even when the mass flux across the 660-km seismic discontinuity is equivalent to approximately one lower-mantle mass over the Earth's history, high (3)He contents, high (3)He/(4)He ratios and (40)Ar concentrations high enough to satisfy the (40)Ar mass balance of the Earth can be preserved in the lower mantle. The differences in (3)He/(4)He ratios between mid-ocean-ridge basalts and ocean island basalts, as well as high concentrations of (3)He and (40)Ar in the mantle source of ocean island basalts, can be explained within the framework of different processing rates for the upper and the lower mantle. Hence, to preserve primitive noble gas signatures, we find no need for hidden reservoirs or convective isolation of the lower mantle for any length of time.

Easter Island, SE Pacific: An end-member type of hotspot volcanism

Easter Island (Rapa Nui, Chile) is an intraoceanic volcanic island on the Easter hotspot, ~350 km E of the Eastern Pacific Rise. We match new field data with previously published age and petrochemical data to reconstruct the general evolution of the Island. This consists of three main volcanoes (Poike, Rano Kau, and the larger Terevaka), which experienced an overall similar and nearly coeval evolution, characterized by two periods: (1) buildup of a basaltic shield, culminating in the development of a summit caldera and the emission of more evolved highly porphyritic lavas (ca. 0.78–0.3 Ma); and (2) rifting along the shield flanks, by means of fissure eruptions (0.24–0.11 Ma). The trend of most eruptive fissures, NNE-SSW to NE–SW, appears to be controlled by the ~NE-SW elongated, emerged, and submerged morphology of the island. However, while the fissure-forming period at Rano Kau and Poike appears to be associated with reduced magma supply to the reservoir, at Terevaka it is characterized by the arrival of new basic magma, rejuvenating the system. The comparison to other intraoceanic volcanic islands suggests that, because of its tectonomagmatic features (low eruptive rate, scattered rift zones, and scarce lateral collapses), Easter Island represents an end-member type of hotspot volcano that is contrary to Hawaii, which represents the opposite end member.

Rapid change in drift of the Australian plate records collision with Ontong Java plateau

The subduction of oceanic plateaux, which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood-basalt provinces, is an important factor in many current models of plate motion and provides a potential mechanism for triggering plate reorganization. To evaluate such models, it is essential to decipher the history of the collision between the largest and thickest of the world's oceanic plateaux, the Ontong Java plateau, and the Australian plate, but this has been hindered by poor constraints for the arrival of the plateau at the Melanesian trench. Here we present (40)Ar-(39)Ar geochronological data on hotspot volcanoes in eastern Australian that reveal a strong link between collision of the Greenland-sized Ontong Java plateau with the Melanesian arc and motion of the Australian plate. The new ages define a short-lived period of reduced northward plate motion between 26 and 23 Myr ago, coincident with an eastward offset in the contemporaneous tracks of seamount chains in the Tasman Sea east of Australia. These features record a brief westward deflection of the Australian plate as the plateau entered and choked the Melanesian trench 26 Myr ago. From 23 Myr ago, Australia returned to a rapid northerly trajectory at roughly the same time that southwest-directed subduction began along the Trobriand trough. The timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the Pacific plate, including the archetypal Hawaiian chain, and thus provide strong evidence that immense oceanic plateaux, like the Ontong Java, can contribute to initiating rapid change in plate boundaries and motions on a global scale.

Geochronology of the Australian Cenozoic: a history of tectonic and igneous activity, weathering, erosion, and sedimentation*

The development and application of geochronological tools suitable for dating Cenozoic rocks and processes have been instrumental to our understanding of the modern history of Australia. Geochronology reveals a dynamic continent that traced a long and rapid trajectory from a position adjacent to Antarctica in the early Cenozoic to its present position near the tropics. The average travel velocity along this path is revealed by the age of hotspot volcanoes, derived by the K–Ar method, and is complemented by measured geomagnetic pole positions on dated igneous rocks and sedimentary deposits. K–Ar dating of volcanic rocks also provided constraints on rates of landscape evolution before and after volcanism and the timing and pattern of dispersion of life—including human inhabitation. K–Ar geochronological results reveal a history of faunal and floral evolution suggestive of a continent undergoing progressive cooling and dehydration with a few brief warm and humid excursions. In contrast, 40Ar/39Ar, SHRIMP U–Pb, fission-track thermochronology, luminescence techniques, and cosmogenic-isotope methods have played relatively minor roles in reconstructing the chronology of Cenozoic volcanism in Australia. Integrated application of these techniques will be critical to providing more precise constraints on the volcanic history of the continent and its climatic and biological evolution. While Cenozoic volcanism, uplift, and denudation were active along the eastern and southeastern margins, a significant part of Australia west of the Tasman Line remained relatively quiescent. The history of this part of the continent is marked by slow and subdued uplift and subsidence, with subtle displacements along major continental structures, and occasional invasion by shallow seas. Despite the general absence of Cenozoic igneous rocks west of the Tasman Line, Australia (east and west) is blanketed by Cenozoic sedimentary covers and weathering profiles. If we consider weathering as a fourth rock-forming process (in addition to igneous, metamorphic and sedimentary), Australia has one of the most complete Cenozoic rock covers of any continent. Retrieving information recorded in these weathering profiles is essential for unravelling its Cenozoic history. Paleomagnetic studies, calibrated δ18O curves, and weathering geochronology by K–Ar, 40Ar/39Ar, and (U–Th)/He provide insights into the imprint of climatic events and tectonic processes and illustrate the importance of erosion and weathering to the formation or enrichment of ore and mineral deposits. Except for diamondiferous lamproites of Western Australia and sapphire-rich volcanic rocks in eastern Australia, all other Cenozoic ore and mineral deposits in Australia are related to weathering and erosion. The widespread weathered blanket in Australia suggests low Cenozoic erosion rates. Numerical constraints on chronology and erosion rates are derived from the cooling and denudation histories retrieved from apatite and zircon fission-track and, more recently, (U–Th–Sm)/He thermochronology and cosmogenic isotope studies. Geochronological studies of veneers of sediments, lake and cave deposits, marine carbonates, organic matter and groundwaters provide information on sediment provenance, subtle tectonic movements, and the Australian Cenozoic climatic history. These studies reveal a continent sensitive to global climatic cycles and subject to active, but subtle, tectonism and erosion. This record shows that Australia suffered periods of extreme aridity during cyclical glaciation at high latitudes and precise dating of carbonate sediments and speleothems reveals the exact timing and duration of these glacial and interglacial periods. Cosmogenic isotopes also provide constraints on the age and migration paths of Australia's limited and finite groundwater resources. Lastly, age information extracted from surficial deposits reveals a protracted history of human occupation.

Extreme spatial and temporal variability of hydrothermal microbial mat communities along the Mariana Island Arc and southern Mariana back‐arc system

Twenty‐five microbial communities were sampled from 18 different hydrothermal systems located at seven different sites along the Mariana Island Arc and at a single site from the southern Mariana Spreading Center over a 3‐year period. Terminal restriction fragment length polymorphism (T‐RFLP) analysis of the small subunit rRNA gene revealed that the microbial community diversity is much greater along the Mariana Arc/back‐arc than at either hot spot volcanoes or mid‐ocean ridges along the same spatial scale. Cluster analysis of T‐RFLP fingerprints reveals the microbial communities formed three distinct clusters designated Mariana clusters I, II, and III. Microbial communities in Mariana Cluster I are all associated with iron‐rich microbial mats and are dominated by members of the ζ‐Proteobacteria and by unique phylotypes clustering deeply in the δ‐Proteobacteria and within the Nitrospira division. Mariana Cluster II communities are all from shallow hydrothermal systems and mostly from colder sediments or microbial mats that are dominated by putative heterotrophic phylotypes usually associated with seawater and sediments not generally associated with hydrothermal fluid inputs. Mariana Cluster III is generally from much hotter vent sites and is dominated by sulfur‐oxidizing ɛ‐Proteobacteria. Quantitative‐polymerase chain reaction (Q‐PCR) of Archaeal abundance reveal that all of the microbial communities are dominated by members of the Bacterial domain. Sampling of microbial mats from Iceberg Vent at NW Rota‐1 in 2004 and again in 2006 reveal the community has shown a transition from Caminibacter group ɛ‐Proteobacteria phylotypes to a mixed population of Caminibacter, Sulfurovum, and Sulfurimonas group ɛ‐Proteobacteria.

Plume Generation Zones at the margins of Large Low Shear Velocity Provinces on the core–mantle boundary

Large Igneous Province (LIP) eruption sites of the past 300 My lie vertically above 1% slow shear wave velocity ( V s) contours bounding the African and Pacific Large Low Shear Velocity Provinces (LLSVPs) at the core–mantle boundary (CMB), or in the cases of the Siberian and Columbia River LIPs, bounding one or other of two smaller, Low Shear Velocity Provinces (LSVPs). Steep gradients in V s at the CMB coincide with those 1% slow contours. The sites of 24 active hotspot volcanoes project down to the same narrowly defined borders of the LLSVPs at the CMB. Plumes that have generated LIPs and major hotspot volcanoes have risen only from the immediate neighbourhoods of the 1% slow V s contours at the CMB which thus define Plume Generation Zones (PGZs). PGZs projected vertically upward approximately match the + 10 m elevation contour of the geoid showing that the LLSVPs are a dominant control on the positively elevated geoid. Minima in the frequency distribution of shear wave velocities in the lowermost mantle near V s = − 1% indicate that regions with more negative velocities, forming ∼ 2% of total mantle mass, are likely to be of material compositionally different from the rest of the mantle. Because all LIP eruption sites with ages younger than 300 Ma lie above the borders of LLSVPs or LSVPs at the CMB, PGZ footprints are inferred to have remained in the same places for the past 300 My. Because no plumes have risen from the interior of the LLSVPs and because no lithospheric slabs have penetrated those bodies the volumes of the LLSVPs are inferred to have also remained unchanged for the past 300 My. Because the LLSVPs are the dominant control on the positively elevated areas of the geoid those too must have remained as they now are since 300 Ma. The LLSVPs are not rising buoyant objects but stable features of the deep mantle. LIPs have been erupted throughout the past 2.5 Gy indicating that PGZs comparable to those of the past 0.3 Gy and LLSVPs (of which PGZs mark the margins at the CMB) have also existed for at least that long. LLSVPs could thus form the isolated reservoir invoked by some to explain the distinctive isotopic compositions of terrestrial rocks. PGZs lie at places where the boundaries of: (i) The outer core, (ii) one of the LLSVPs or LSVPs, and (iii) the seismically faster part of the deep mantle meet. Horizontal temperature gradients across the steeply inclined margins to the LLSVPs, the interiors of which are hotter than the surrounding mantle, at the CMB are key controls for the generation of plumes. Near the CMB the association of the high temperature of the outer core with an inclined thermal boundary layer at the margins of LLSVPs facilitates the generation of mantle plumes in the PGZs.

Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism

Radioactive decay of uranium and thorium produces 4He, whereas 3He in the Earth's mantle is not produced by radioactive decay and was only incorporated during accretion-that is, it is primordial. 3He/4He ratios in many ocean-island basalts (OIBs) that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts (MORBs). This is inferred to be the result of outgassing by melt production at mid-ocean ridges in conjunction with radiogenic ingrowth of 4He, which has led to a volatile-depleted upper mantle (MORB source) with low 3He concentrations and low 3He/4He ratios. Consequently, high 3He/4He ratios in OIBs are conventionally viewed as evidence for an undegassed, primitive mantle source, which is sampled by hot, buoyantly upwelling deep-mantle plumes. However, this conventional model provides no viable explanation of why helium concentrations and elemental ratios of He/Ne and He/Ar in OIBs are an order of magnitude lower than in MORBs. This has been described as the 'helium concentration paradox' and has contributed to a long-standing controversy about the structure and dynamics of the Earth's mantle. Here we show that the helium concentration paradox, as well as the full range of noble-gas concentrations observed in MORB and OIB glasses, can self-consistently be explained by disequilibrium open-system degassing of the erupting magma. We show that a higher CO2 content in OIBs than in MORBs leads to more extensive degassing of helium in OIB magmas and that noble gases in OIB lavas can be derived from a largely undegassed primitive mantle source.