High Mantle Temperatures Research Articles

Anomalous lithosphere beneath the Proterozoic of western and central Australia: A record of continental collision and intraplate deformation?

A new surface wave seismic tomography model of Australia is presented which provides a means of investigating the lithospheric structure beneath the Proterozoic regions in the west, north and centre of the continent with improved resolution and reliability. The dominant feature of the model is a region of low seismic wavespeeds in the uppermost mantle, at approximately 75 km depth, beneath central Australia. The zone of slow wavespeeds is underlain by a region of fast wavespeeds, more typical of continental lithosphere. This layered velocity structure, and strong positive wavespeed gradient, makes the shallow anomaly hard to explain in terms of high mantle temperatures and typical steady-state continental geotherms. A possible thermal explanation requires the impact of the redistribution of high heat producing elements within the crust. Alternatively, a mineral or minerals with low seismic velocities, such as amphibole, in the shallowest part of the lithosphere, with a more conventional lithology in the deep continental root below, could explain the seismic wavespeeds. The anomaly is located directly beneath the zone where the Australian cratons amalgamated in the Proterozoic and where, subsequently, there have been periods of intraplate tectonic activity; suggesting a correlation between the prolonged history of deformation and a highly unusual lithospheric structure. In western Australia, the Capricorn Orogen and Pilbara Craton have a similar lithospheric thickness, whereas a thicker lithosphere is observed beneath the Yilgarn to the south. In northern Australia, large regions appear to be underlain by fast wavespeeds, similar to those observed beneath the Yilgarn Craton. Variations in the shear wavespeeds beneath the Arunta also indicate that there is not always an obvious correlation between the overlying surface geology and seismic structures observed in the upper mantle.

The internal activity and thermal evolution of Earth-like planets

We model the internal thermal evolution of planets with Earth-like composition and masses ranging from 0.1 to 10 Earth masses over a period of 10 billion years. We also characterize the internal activity of the planets by the velocity of putative tectonic plates, the rate at which mantle material is processed through melting zones, and the time taken to process one mantle mass. The more massive the planet the larger its processing rate ( ϕ), which scales approximately as ϕ ∝ M 0.8 – 1.0 . The processing times for all the planets increase with time as they cool and become less active. As would be expected, the surface heat flow scales with planet mass. All planets have similar declines in mantle temperature except for the largest, in which pressure effects cause a larger decline. The larger planets have higher mantle temperatures over all times. The less massive the planet, the larger the decrease in core temperature with time. The core heat flow is also found to decrease more rapidly for smaller planet masses. Finally, rough predictions are made for the time required to generate an atmosphere from estimates of the time to degas water and carbon dioxide in mantle melting zones. The degassing times depend strongly on the initial temperature of the planet, but for the temperatures used in our model all the planets degas within ∼32 Ma after their formation.

Variation in styles of rifting in the Gulf of California

Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width, with narrow rifts thought to form as necking instabilities (where extension rates outpace thermal diffusion) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings. Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism--from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting (extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism.

Controls on density stratification in the early mantle

Previous numerical models of convection in the hot early mantle, incorporating simulated subducted oceanic crust and some viscosity stratification, developed thermal and compositional stratification, with cooler, strongly depleted mantle above hotter, slightly enriched mantle. The robustness of these results is explored through variation of some important parameters. The stratification is minor at present mantle temperatures but stronger at higher mantle temperatures because mantle viscosities are reduced. The results are not much affected by variations in plate velocity, nor by whether internal heat sources are directly associated with the mafic component, nor by 20% of heat entering from below. However, stratification was reduced somewhat at higher temperatures by increasing the number of tracers, indicating that the effect was moderately overestimated in a previous paper. The present results still indicate that oceanic crust would be no more than 6–8 km thick at high mantle temperatures, because of the strong depletion of the upper mantle. When the effect of a phase transformation in the transition zone is included, the stratification is moderately enhanced and crustal thicknesses of 3–5 km result at 1650°C. Oceanic crustal thickness is highly variable in space and time, with some plates having essentially zero crust for significant periods. The new results support the previous conclusions that the stratification may explain the strong depletion of incompatible elements documented for the earliest mantle and that early plate tectonics may have been more viable that hitherto thought.

Constraints on mantle melting and composition and nature of slab components in volcanic arcs from volatiles (H 2O, S, Cl, F) and trace elements in melt inclusions from the Kamchatka Arc

New and published data on the composition of melt inclusions in olivine (Fo 73–91) from volcanoes of the Kamchatka and northern Kurile Arc are used 1) to evaluate the combined systematics of volatiles (H 2O, S, Cl, F) and incompatible trace elements in their parental magmas and mantle sources, 2) to constrain thermal conditions of mantle melting, and 3) to estimate the composition of slab-derived components. We demonstrate that typical Kamchatkan arc-type magmas originate through 5–14% melting of sources similar or slightly more depleted in HFSE (with up to ∼ 1 wt.% previous melt extraction) compared to MORB-source mantle, but strongly enriched in H 2O, B, Be, Li, Cl, F, LILE, LREE, Th and U. Mean H 2O in parental melts (1.8–2.6 wt.%) decreases with increasing depth to the subducting slab and correlates negatively with both ‘fluid-immobile’ (e.g. Ti, Na, LREE) and most ‘fluid-mobile’ (e.g. LILE, S, Cl, F) incompatible elements, implying that solubility in hydrous fluids or amount of water does not directly control the abundance of ‘fluid-mobile’ incompatible elements. Strong correlation is observed between H 2O/Ce and B/Zr (or B/LREE) ratios. Both, calculated H 2O in mantle sources (0.1–0.4%) and degrees of melting (5–14%) decrease with increasing depth to the slab indicating that the ultimate source of water in the sub-arc mantle is the subducting oceanic plate and that water flux (together with mantle temperature) governs the extent of mantle melting beneath Kamchatka. A parameterized hydrous melting model [Katz et al. 2003, G 3, 4(9), 1073] is utilized to estimate that mantle melting beneath Kamchatka occurs at or below the dry peridotite solidus (1245–1330 °C at 1.5–2.0 GPa). Relatively high mantle temperatures (yet lower than beneath back-arc basins and ocean ridges) suggest substantial corner flow driven mantle upwelling beneath Kamchatka in agreement with numerical models implying non-isoviscous mantle wedge rheology. Data from Kamchatka, Mexico and Central America indicate that < 5% melting would take place beneath continental arcs without water flux from the subducting slab. A broad negative correlation appears to exist between crustal thickness and the temperature of magma generation beneath volcanic arcs with larger amounts of decompression melting occurring beneath thinner arc crust (lithosphere). In agreement with the high mantle temperatures, we observe a systematic change in the composition of slab components with increasing slab depth from solute-poor hydrous fluid beneath the volcanic front to solute-rich hydrous melt or supercritical liquid at deeper depths beneath the rear arc. The solute-rich slab component dominates the budget of LILE, LREE, Th and U in the magmas and originates through wet-melting of subducted sediments and/or altered oceanic crust at ≥ 120 km depth. Melting of the upper parts of subducting plates under water flux from deeper lithosphere (e.g. serpentinites), combined with high temperatures in the mantle wedge, may be a more common process beneath volcanic arcs than has been previously recognized.

Phase equilibrium investigations of the Adirondack class basalts from the Gusev plains, Gusev crater, Mars

Abstract— Phase equilibrium experiments have been performed on a synthetic analog of the Gusev plains basalt composition from the Spirit landing site on Mars. Near‐liquidus phase relations were determined over the pressure range of 0.1 to 1.5 GPa and at temperatures from 1125 to 1390 °C in a piston cylinder apparatus and 1 atm gas mixing furnace. The composition is multiply saturated with olivine, orthopyroxene, and spinel near its liquidus at 1320 °C and 1.0 GPa, or 85 km depth on Mars, placing an upper limit constraint on the thickness of the Martian lithosphere at the time of eruption. Our experimental work suggests that the Gusev basalts are anhydrous batch melts of a primitive Martian mantle similar to the composition estimated by Dreibus and Wänke (1984). The temperature of multiple saturation indicates the persistence of high mantle potential temperatures on Mars, similar to those on the modern Earth, until at least the very latest Noachian (3.7 Ga). These high mantle temperatures would be responsible for persistent basaltic volcanism throughout the southern highlands during the first billion years of Mars's history. The source for Gusev basalts differs strongly from the source for shergottite meteorites, reinforcing the idea of the absence of global mantle convection and mixing on Mars. The existence of a relatively primitive mantle reservoir requires that at least part of the mantle underwent little modification during early planetary differentiation.

Origin of Icelandic basalts: A review of their petrology and geochemistry

The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust. The study of glass inclusions in primitive phenocrysts is still in its infancy but already shows results unattainable by other methods. Such studies reveal the existence of mantle melts with highly variable compositions, such as calcium-rich melts and a low- 18 O mantle component, probably recycled oceanic crust. Future high-resolution seismic studies may help to identify and reveal the relative proportions of different lithologies in the mantle.

Imaging subduction from the trench to 300 km depth beneath the central North Island, New Zealand, withVpandVp/Vs

SUMMARY Recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central North Island of New Zealand. Here we use a subset of the best-recorded earthquakes in an inversion for the 3-D Vp and Vp/Vs structure. The data set includes 39 123 P observations and 18 331 S observations from 1239 earthquakes and nine explosions. The subducted plate is imaged as a high Vp, low Vp/Vs feature. Vp within the mantle of the subducted slab is almost always >8.5 km s−1, which requires the ca. 120 Myr slab to be unusually cold. The low Vp/Vs within the subducted plate closely parallels the lower plane of the dipping seismic zone. It most likely indicates fluid resulting from dehydration of serpentine in the slab mantle, and the earthquakes themselves are likely to be promoted by dehydration embrittlement. We identify a region with Vp 8.0 km s−1 directly above the dipping seismic zone can be interpreted as sinking, entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high-temperature mantle wedge. Material in the overlying low Vp region can be interpreted as rising within a return flow within the wedge. The volcanic front appears to be controlled by where this dipping low Vp region meets the base of the crust. The thickness of the backarc crust also shows significant variation along strike. In the central Taupo Volcanic Zone (TVZ) the crust is ca. 35 km thick, while southwest of Mt Ruapehu the crust thickens by ca. 10 km. There is no significant low Vp zone in the mantle wedge in this southwestern region, suggesting that this thicker crust has choked off mantle return flow. The seismic tomography results, when combined with constraints on mantle flow from previous shear-wave splitting results, provide a plausible model for both the distribution of volcanism in the central North Island, and the exceptional magmatic productivity of the central TVZ.

Giant swarms of Precambrian mafic dikes and potential diamond resources of ancient platforms

By the examples of the Siberian Platform and Canadian Shield, it is shown that spatial juxtaposition of Phanerozoic diamond-bearing kimberlite fields with giant swarms of Precambrian mafic dikes is caused by both systematic and incidental events. The first of these include (1) origination of mantle plumes and associated lenses of high-temperature mantle melting in the subequatorial “hot belt” of the early Earth, (2) formation of magma chambers that generated mafic dikes in these asthenospheric lenses, (3) shear stress, and (4) ultrahigh-pressure metamorphism of igneous and country rocks. As a result, the association of diamond-bearing high-density mafic and ultramafic rocks was formed under favorable thermal and fluidal conditions. These processes occurred first in the embryonic (multiplate) Neoarchean tectonic setting at a depth of 40–60 km (present-day elevation marks) and then at a deeper (100–150 km) level during the transition to the Proterozoic true plate tectonics. These processes left behind giant swarms of Precambrian mafic dikes, as well as structurally and genetically related deep-seated morphological and density anomalies. The relatively high position of two lithospheric units of diamond-bearing rocks, each underlain by a thick layer of the cold mantle, prevented these units from thermal and mechanical erosion during subsequent plate-tectonic stages characterized by deeper location of asthenospheric layers. The occurrence of clusters of Phanerozoic diatremes in ancient giant swarms of mafic dikes, as well as the enrichment of pipes in xenogenic diamond-bearing material derived from different levels of the tectonically delaminated lithosphere, may be attributed to incidental events that controlled the fertility of a relatively small number of kimberlite pipes.

Compositional heterogeneity of the ancient Martian crust: Analysis of Ares Vallis bedrock with THEMIS and TES data

THEMIS multispectral and thermophysical information combined with TES hyperspectral and albedo data is used as a powerful high spectral/spatial resolution tool to investigate the mineralogic heterogeneity of ancient Martian crust exposed in Ares Vallis bedrock. Three major spectral units are present in the upper Ares Vallis region: (1) a regional unit that is composed primarily of a high‐silica (Si/O &gt; ∼0.35) component, with lesser amounts of plagioclase and pyroxene, and is associated with channel wall rock and surrounding plains; (2) a pyroxene‐ and olivine‐rich rock unit exposed as a ∼250 m thick contiguous layer in the wall rock of Ares Vallis, as well as in isolated exposures in the plains outside of the channel; and (3) a unit composed of plagioclase, pyroxene, and lesser high‐silica component(s) (Si/O &gt; ∼0.35) that primarily occurs as low‐albedo sand. The spatial and stratigraphic distribution of these units suggests that the pyroxene‐ and olivine‐enriched unit may be extrusive and/or intrusive in origin, indicating that this region experienced either a single stage or repeated episodes of olivine‐enriched magmatism during the first 1.5 Gyr of crust formation. This olivine enrichment may have been caused by a larger degree of mantle partial melting, facilitated by higher mantle temperature, melting from a more depleted mantle source, and/or less olivine fractionation relative to regional rock parent magmas. The olivine‐rich unit is similar in thermophysical character to previously published olivine‐bearing terrains on Mars, but the derived modal mineralogy consists of &gt;∼20% more pyroxene and less plagioclase than those terrains.

Anomalous uplift and subsidence of the Ontong Java Plateau inferred from CO2 contents of submarine basaltic glasses

The Ontong Java Plateau in the western Pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau. Paleoeruption depths (at 122 Ma) estimated from dissolved H2O and CO2 in submarine basaltic glass pillow rims vary from 1100 m below sea level (mbsl) on the central part of the plateau to 2200–3000 mbsl on the northeastern edge. Our results suggest maximum initial uplift for the plateau of 2500–3600 m above the surrounding seafloor and 1500 ± 400 m of postemplacement subsidence since 122 Ma. Our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere, and thus our results are inconsistent with formation of the plateau by a high-temperature mantle plume. Two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau (1) formed as a result of a giant bolide impact, or (2) formed from a mantle plume but has a lower crust of dense garnet granulite and/or eclogite; neither of these possibilities is fully consistent with all available geological, geophysical, and geochemical data. The origin of the largest magmatic event on Earth in the past 200 m.y. thus remains an enigma.

Serpentinitic Mélanges Associated with HP and UHP Rocks in Central Asia

Serpentinitic mélanges containing eclogites, blueschists, and jadeitic rocks have been studied within the following foldbelts: Borus (West Sayan), Chagan-Uzun (Gorny Altai), Chara (East Kazakhstan), Kokchetav (North Kazakhstan), and Maksyutov (Southern Urals). The West Sayan (Borus belt) and Gorny Altai (Chagan-Uzun belt) are Caledonian foldbelts formed after collisions of paleo-seamounts with primitive island arcs, which resulted in the exhumation of eclogites and jadeitic rocks. The Chara belt of Hercynian age was formed during the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents and represents a strike-slip zone comprising serpentinitic mélanges and HP units of different ages. In Caledonian time, the mélanges represent an accretionary wedge at the margin of the Kazakhstan continent. The Kokchetav and Maksyutov foldbelts of, respectively, Caledonian and Hercynian ages resulted from collision of microcontinents with island arcs and formation of sheeted complexes incorporating large lenses of UHP-HP rocks and less serpentinitic mélanges. Two types of serpentinitic mélange occur in all suture zones formed as a result of collision of island arcs with seamounts or other island arcs. Type I mélange contains HP rocks in antigorite or olivine-talc-antigorite schists associated with high-temperature mantle peridotites, which can be regarded as part of the hanging wall of a paleosubduction zone. Type II mélange is a part of an obducted oceanic crust and contains blocks and inclusions of LP and LT metamorphic rocks associated with island arc and/or seamount terranes. The suture zones formed by continent/island arc or microcontinent collision (the Kokchetav megamélange and the lower unit of the Maksyutov complex) comprise no large sheets of serpentinitic mélange, but only subordinate lenses of mélanges with HP rocks (the Maksyutov lower unit) or small and sporadic ultramafic lenses inside a tectonic mélange with a metasedimentary matrix.

The thermal effects of steady-state slab-driven mantle flow above a subducting plate: the Cascadia subduction zone and backarc

At subduction zones, geophysical and geochemical observations indicate that the arc and backarc regions are hot, in spite of the cooling effects of a subducting plate. At the well-studied Cascadia subduction zone, high mantle temperatures persist for over 500 km into the backarc, with little lateral variation. These high temperatures are even more surprising due to the juxtaposition of the hot Cascadia backarc against the thick, cold North America craton lithosphere. Given that local heat sources appear to be negligible, mantle flow is required to transport heat into the wedge and backarc. We have examined the thermal effects of mantle flow induced by traction along the top of the subducting plate. Through systematic tests of the backarc model boundary, we have shown that the model thermal structure of the wedge is primarily determined by the assumed temperatures along this boundary. To get high temperatures in the wedge, it is necessary for flow to mine heat from depth, either by using a temperature-dependent rheology, or by introducing a deep cold boundary through a thick adjacent lithosphere, consistent with the presence of a craton. Regardless of the thermal conditions along the backarc boundary, flow within an isoviscous wedge is too slow to transport a significant amount of heat into the wedge corner. With a more realistic stress- and temperature-dependent wedge rheology, flow is focused into the wedge corner, resulting in rapid flow upward toward the corner and enhanced temperatures below the arc, compatible with temperatures required for arc magma generation. However, this strong flow focusing produces a nearly stagnant region further landward in the shallow backarc mantle, where model temperatures and heat flow are much lower than observed. Observations of high backarc temperatures, particularly in areas that have not undergone recent extension, provide an important constraint on wedge dynamics. None of the models of simple traction-driven flow were able to simultaneously produce high temperatures beneath the volcanic arc and throughout the backarc. The most likely way to produce hot and isothermal conditions in the backarc is through vigorous small-scale-free convection in a low-viscosity asthenosphere.

Thermal evolution of the Earth as recorded by komatiites

Komatiites are rare ultramafic lavas that were produced most commonly during the Archean and Early Proterozoic and less frequently in the Phanerozoic. These magmas provide a record of the thermal and chemical characteristics of the upper mantle through time. The most widely cited interpretation is that komatiites were produced in a plume environment and record high mantle temperatures and deep melting pressures. The decline in their abundance from the Archean to the Phanerozoic has been interpreted as primary evidence for secular cooling (up to 500°C) of the mantle. In the last decade new evidence from petrology, geochemistry and field investigations has reopened the question of the conditions of mantle melting preserved by komatiites. An alternative proposal has been rekindled: that komatiites are produced by hydrous melting at shallow mantle depths in a subduction environment. This alternative interpretation predicts that the Archean mantle was only slightly (∼100°C) hotter than at present and implicates subduction as a process that operated in the Archean. Many thermal evolution and chemical differentiation models of the young Earth use the plume origin of komatiites as a central theme in their model. Therefore, this controversy over the mechanism of komatiite generation has the potential to modify widely accepted views of the Archean Earth and its subsequent evolution. This paper briefly reviews some of the pros and cons of the plume and subduction zone models and recounts other hypotheses that have been proposed for komatiites. We suggest critical tests that will improve our understanding of komatiites and allow us to better integrate the story recorded in komatiites into our view of early Earth evolution.

Nature and distribution of dykes and related melt migration structures in the mantle section of the Oman ophiolite

We conducted a comprehensive field, petrographic, and microprobe study of the dykes and porous flow channels cropping out in the Oman harzburgites. The 36 rock types we recognized among of about 1000 samples can be grouped in two main magma suites contrasted in terms of structural and textural characteristics, modal composition, order of crystallization, and phase chemistry. One suite (troctolites, olivine gabbros, opx‐poor gabbronorites, and rare oxyde gabbros) derives from MORB‐like melts. The other suite (pyroxenites, opx‐rich gabbronorites, diorites, and tonalite‐trondhjemites) derives from melts richer in silica and water than MORBs and ultradepleted in incompatible elements. Dykes and porous flow channels from the MORB suite are restricted to a few areas, covering only 25% of the mantle section. This is an unexpected result as the deep Oman crust is made essentially of cumulates from MORB‐like melts. Their composition, texture, and relations with the host harzburgites point to high mantle temperatures at the time of crystallization (likely above 1100°C, up to 1200°C for part of them), i.e., conditions close to the “asthenosphere/lithosphere” boundary. The largest outcrop of mantle harzburgites enclosing MORB like dykes is a 80 km long and 10 km wide corridor, parallel to the strike of the sheeted dyke complex and centered on an area where a former mantle upwelling has been unambiguously defined (the Maqsad “diapir”). A few other occurrences of mantle cumulates from the MORB suite are smaller than the Maqsad area and have a lesser abundance of troctolites (i.e., of high‐temperature cumulates). We interpret the troctolite zones of Oman as the witnesses of former diapirs frozen at various stages of their development. Dykes belonging to the depleted suite are the most common in Oman harzburgites. Their structural and textural characteristics show that they crystallized in a mantle colder than the melt (likely in the range 600°C to 1100°C). A possible origin for the parent melts of this suite is in situ partial melting of the shallow and partly hydrated lithosphere residual after MORB extraction. Our data support the view that feeding magma chambers with MORBs is a focused (and likely episodic) process involving the rise of hot mantle to the base of the crust through a lithospheric lid accreted during a previous diapiric event. They suggest also that the shallow mantle beneath spreading centers is a place of important petrologic processes, some of them predicted on the basis of MORB composition (e.g., fractionation inside melt conduits) and other ones unexpected (e.g., remelting of the depleted lithosphere).

Oxygen isotopes of an Early Archaean layered ultramafic body, southern West Greenland: implications for magma source and post-intrusion history

A large number of metamorphosed ultramafic bodies are found in the region SW of the Isua Greenstone Belt in southern West Greenland, enclosed in Archaean banded tonalitic gneisses. These enclaves of dunite and harzburgite range in size from a few to several hundred meters. The Ujarassuit Nunat layered body occurs as a large enclave (800 m×100 m) entrained within tonalitic gneisses and preserves primary igneous layering and textures. This enclave has been subjected to amphibolite-grade metamorphism, but for the most part the magmatic mineralogy remains intact. Separated minerals have been analysed for δ18O by CO2 laser fluorination. δ18O values range from +4.49 to +4.89‰ for olivine and from +5.70 to +5.86‰ for orthopyroxene. All chromite values are <+2.5‰. Closure temperatures recorded for olivine–orthopyroxene pairs vary from 700 to 900 °C, much lower than expected for an ultramafic magma. Widespread late-magmatic re-equilibration has resulted in significant decreases in δ18O for chromites and small changes of <0.3‰ in olivine, but orthopyroxene values show little variation from typical magmatic signatures. The lowering of 18O in chromite is associated with significant Fe-enrichment. Thermometry suggests that the re-equilibration took place at 680–750 °C, with equilibration between plagioclase and amphibole in residual gabbro anorthosite melt around 700 °C. Hydration, possibly associated with this late melt stage, resulted in further changes to chromite chemistry, formation of secondary amphibole, phlogopite and chlorite and may have further enhanced 18O-depletion in disseminated chromites. All these events are related to late-magmatic processes. Only the later metamorphic mineral, ruby corundum, present in amphibole-harzburgite adjacent to an intrusive gneiss vein, clearly indicates an external oxygen input to this system. Olivines from an unlayered dunite body, 3.5 km away from Ujarassuit Nunat, have higher δ18O values around 5.25‰, typical of more recent high temperature mantle peridotite nodules, and suggesting that mantle oxygen composition at 3.8 Ga was already similar to the present composition.

High mantle temperature during Cretaceous avalanche

A new sequence of axisymmetrical spherical mantle convection simulations has been computed to monitor the influence of mantle avalanches on the mantle thermal state. During an avalanche that is characterized by a high rate of mixing between upper mantle and lower mantle, a complex perturbation of the thermal field occurs due to the advection of cold upper mantle into the lower mantle, and the hot return flows from the lower mantle. The computation shows that the avalanches trigger global upper mantle warming of several tens of degrees and make it possible to explain both core–mantle boundary and transition zone origins for mantle plumes. Recently, the chemical compositions of DSDP/ODP drill holes on crust older than 80 Myr have been interpreted to reflect a higher mean mantle temperature during the Mesozoic [Humler et al., Earth Planet. Sci. Lett. 173 (1999) 7–23]. Here, we have used and extended the data set for oceanic crust younger than 80 Myr. Combining the mean chemistry for old oceanic crust and the mean depth of the ocean basin versus the square root of age, we propose a continuous curve for the temperature evolution of the upper mantle for the last 130 Myr. It shows a mantle thermal high around 125 Ma with amplitude (50 K) and duration (30 Myr) comparable with those obtained from theoretical convection models. It is proposed that such a high mantle temperature may be due to the thermal effects of an avalanche which may have started 180 Myr ago.

The Central Basin Spreading Center in the Philippine Sea: Structure of an extinct spreading center and implications for marginal basin formation

Newly acquired marine geophysical data along the Central Basin Spreading Center (CBSC), the extinct spreading axis of the West Philippine Basin (WPB), display along‐axis variations of spreading style. We have described and analyzed the tectonic spreading fabric and segmentation patterns along a 1000‐km‐long section of the fossil ridge between 126°00′ and 133°30′E. “Slow‐spreading features” (deep rift valleys and nodal basins, rough abyssal hills on the ridge flanks, and mantle Bouguer anomaly lows beneath segment centers) are observed in the eastern CBSC. In contrast, “fast‐spreading” features (overlapping spreading centers, volcanic axial ridges, and smooth abyssal hill fabric) are identified along the western CBSC. We attribute the large morphological and geophysical variations along the CBSC to higher melt supply in the western region caused by high mantle temperature and/or mantle heterogeneity, which may be related to a relatively small‐scale mantle plume forming the oceanic plateaus located in the WPB. Another prominent feature of the CBSC is the development of a deep valley oblique to the spreading fabric. Reconstructions of the plate boundary geometry through time, using abyssal hill geometry as well as other measurements, provide evidence for a later stage of amagmatic extension (i.e., reactivation) along the CBSC after the formation of the main basin. This stage is dominated by tectonic deformation with minor amounts of volcanism (mainly located in eastern segments), resulting in the observed surface brittle deformation distributed within a broad zone of ductile deformation.

Global thermal and dynamical perturbations due to Cretaceous mantle avalanche

The numerical models of mantle convection agree to depict avalanches behaviour according to the level of endothermicity of the spinel → perovskite phase change. Their potential effects on the global thermal and dynamical states of the mantle have been computed thanks to a numerical code, which takes into account both the 400-km exothermic and the 660-km endothermic phase changes. The cycle followed by the avalanches is: local layering, destabilization of the 660-km thermal layer, travelling and spreading on the core, and reappearing of the local layering. Therefore, mantle convection is characterized by quiet periods of partial layering embedded in catastrophic events. During the avalanche, the amplitude of the surface velocity is multiplied by two, which would imply an enhanced plate tectonic and ridge activities. The global thermal effects of the avalanche are compatible with a high mantle temperature and an acceleration of Earth's rotation during the Cretaceous. They also offer a coherent explanation to locate the origin of mantle plumes both within the CMB and just below the transition zone.

On the relations between cratonic lithosphere thickness, plate motions, and basal drag

An overview of seismic, thermal, and petrological evidence on the structure of Precambrian lithosphere suggests that its local maximum thickness is highly variable (140–350 km), with a bimodal distribution for Archean cratons (200–220 km and 300–350 km). We discuss the origin of such large differences in lithospheric thickness, and propose that the lithospheric base can have large depth variations over short distances. The topography of Bryce Canyon (western USA) is proposed as an inverted analog of the base of the lithosphere. The horizontal and vertical dimensions of Archean cratons are strongly correlated: larger cratons have thicker lithosphere. Analysis of the bimodal distribution of lithospheric thickness in Archean cratons shows that the “critical” surface area for cratons to have thick (>300 km) keels is >6–8×10 6 km 2. Extrapolation of the linear trend between Archean lithospheric thickness and cratonic area to zero area yields a thickness of 180 km. This implies that the reworking of Archean crust should be accompanied by thinning and reworking of the entire lithospheric column to a thickness of 180 km in accord with thickness estimates for Proterozoic lithosphere. Likewise, extrapolation of the same trend to the size equal to the total area of all Archean cratons implies that the lithospheric thickness of a hypothesized early Archean supercontinent could have been 350–450 km decreasing to 280–400 km for Gondwanaland. We evaluate the basal drag model as a possible mechanism that may thin the cratonic lithosphere. Inverse correlations are found between lithospheric thickness and (a) fractional subduction length and (b) the effective ridge length. In agreement with theoretical predictions, lithospheric thickness of Archean keels is proportional to the square root of the ratio of the craton length (along the direction of plate motion) to the plate velocity. Large cratons with thick keels and low plate velocities are less eroded by basal drag than small fast-moving cratons. Basal drag may have varied in magnitude over the past 4 Ga. Higher mantle temperatures in the Archean would have resulted in lower mantle viscosity. This in turn would have reduced basal drag and basal erosion, and promoted the preservation of thick (>300 km) Archean keels, even if plate velocities were high during the Archean.