Beyond the thermal plume paradigm Farnetani, C. G.; Samuel, H.
Geophysical research letters,
April 2005, Letnik:
32, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Geodynamic models of thermo‐chemical plumes rising in a mantle wind suggest that we should abandon some paradigms based on the dynamics of purely thermal axisymmetric plumes. The head‐tail structure ...is possible but not unique and the lack of a plume head does not preclude a deep origin. Our results suggest that the surface expression of some thermo‐chemical plumes may be a headless, age‐progressive volcanic chain. Plume tails are laterally heterogeneous, rather than concentrically zoned, because deep heterogeneities are sheared into distinct and long‐lasting filaments that will be successively sampled by different volcanoes, as the oceanic plate moves over the plume tail. Finally, calculated S‐wave velocity anomalies are consistent with recent plume tomographic images, showing that compositional heterogeneities in the lowermost mantle favour the coexistence of a great variety of plume shapes and sizes.
Two views of Hawaiian plume structure Hofmann, Albrecht W.; Farnetani, Cinzia G.
Geochemistry, geophysics, geosystems,
December 2013, Letnik:
14, Številka:
12
Journal Article
Recenzirano
Odprti dostop
Fundamentally contradictory interpretations of the isotopic compositions of Hawaiian basalts persist, even among authors who agree that the Hawaiian hotspot is caused by a deep‐mantle plume. One view ...holds that the regional isotopic pattern of the volcanoes reflects large‐scale heterogeneities in the basal thermal boundary layer of the mantle. These are drawn into the rising plume conduit, where they are vertically stretched and ultimately sampled by volcanoes. The alternative view is that the plume resembles a “uniformly heterogeneous plum pudding,” with fertile plums of pyroxenite and/or enriched peridotite scattered in a matrix of more refractory peridotite. In a rising plume, the plums melt before the matrix, and the final melt composition is controlled significantly by the bulk melt fraction. Here we show that the uniformly heterogeneous plum pudding model is inconsistent with several geochemical observations: (1) the relative melt fractions inferred from La/Yb ratios in shield‐stage basalts of the two parallel (Kea‐ and Loa‐) volcanic chains, (2) the systematic Pb‐isotopic differences between the chains, and the absence of such differences between shield and postshield phases, (3) the systematic shift to uniformly depleted Nd‐isotopic compositions during rejuvenated volcanism. We extend our previous numerical simulation to the low melt production rates calculated far downstream (200–400 km) from shield volcanism. Part of these melts, feeding rejuvenated volcanism, are formed at pressures of ∼5 GPa in the previously unmelted underside of the plume, from material that originally constituted the uppermost part of the thermal boundary layer at the base of the mantle.
Key Points
Small-scale heterogeneities do not explain geochemical structure Hawaiian plume
Rejuvenated volcanism is derived from the fertile underside of the plume
Its source comes from the upper portion of the thermal boundary layer
Epidemiological studies reported a negative relationship between concentrations of heavy metals and phthalates in seminal fluid and semen quality, likely compromising male fertility potential. The ...aim of this study was to investigate the in vitro effects of cadmium chloride (CdCl2), a common heavy metal, and diisobutyl phthalate (DIBP), a common phthalate ester, on human sperm functions necessary for fertilization. After in vitro incubation of spermatozoa with 10 µM CdCl2 or 100 and 200 µM DIBP for 24 h, a significant decrease of sperm progressive and hyperactivated motility was observed. The exposure to each of the two toxic agents also induced spontaneous sperm acrosome reaction and blunted the physiological response to progesterone. Both agents induced an increase of caspase activity suggesting triggering of an apoptotic pathway. Our results suggest that acute exposure of spermatozoa to these pollutants may impair sperm ability to reach and fertilize the oocyte.
Experimental studies at pressure and temperature conditions of the Earth's lower mantle have shown that iron in ferropericlase (Fp) and in Mg-silicate perovskite (Pv) undergoes a spin state ...transition. This electronic transition changes elastic and transport properties of lower mantle minerals and can play an important role in mantle convection. Here we focus on the geodynamic effect of the spin-induced density modifications caused by the volume collapse of Fp and by the variation of Fe partitioning (KPv–Fp) between Fp and Pv. Since KPv–Fp behavior strongly depends on alumina content, we explore two end-member compositions, one Al-bearing (with 4.7 wt% Al2O3 in Pv) and the other Al-free. We use the theoretical model by Sturhahn et al. (2005) to calculate the spin configuration of Fp over a range of pressure–temperature conditions, and use experimental results to model Fe partitioning. We then apply the Mie–Grüneisen–Debye equation of state to obtain the density of the mineral assemblages. The calculated amplitude of the density change across the spin state transition is less than 1%, consistent with experiments by Mao et al. (2011); our density profiles differ from PREM by less than 1.5%. The spin-induced density variations are included in a three dimensional convection code (Stag3D) for a compressible mantle. We find small temperature differences between models with and without spin state transitions, since over billions of years the relative temperature difference is less than 50 K. However the relative RMS vertical velocity difference is up to 15% for an Al-free system, but only less than 6% for an Al-bearing system.
•We model the effect of Fe2+ spin state transition in Ferropericlase on density.•We study two pyrolitic compositions, an Al-bearing and an Al-free.•We consider Fe partitioning between ferropericlase and perovskite.•We include the calculated density profile in the convection code Stag3D.•We find significant consequences on plumes and slabs flow velocity.
A thorough understanding of the internal structure of the Hawaiian plume conduit requires to link geochemical observations of surface lavas to fluid dynamic simulations able to quantify the flow ...trajectories of upwelling geochemical heterogeneities and their sampling by volcanoes. With the present work we fill a gap between the numerous geochemical studies of Hawaiian lavas and the paucity of dynamical models that relate the observed geochemical record to the internal plume structure. Our three-dimensional numerical simulation of a vigorous plume sheared by a fast moving oceanic plate shows that the dominant deformation in the conduit is vertical stretching, while horizontal spreading and vertical shortening prevail in the sublithospheric part of the plume (hereafter referred to as plume head). Flow trajectories indicate that a young volcano like Loihi samples the ‘upstream’ side of the plume, not its center, whereas volcanoes in the post-shield phase sample deep melts from the ‘downstream’ side of the plume. To constrain the internal conduit structure we focus on two geochemical observations: old (>350kyr) Mauna Kea lavas from the Hawaii Scientific Drilling Project are isotopically distinct from recent Mauna Kea lavas, but they are isotopically identical to present-day Kilauea lavas. By modelling a plume conduit with several long-lasting filaments of 10km radius, we find that the isotopic record of a volcano (e.g., Mauna Kea) is expected to change over time-scales of ∼400kyr. Furthermore, by requiring that two age progressive volcanoes (e.g., Mauna Kea and Kilauea) sample the same filament, we constrain the minimum filament length to be ∼600km. In this paper we adopt a ‘top-down’ approach: from geochemical observations of surface lavas, to dynamical models of the conduit structure, and further down to the ‘geochemical architecture’ of the thermal boundary layer feeding the plume. A conduit structure with filaments maps back into heterogeneous volumes with azimuthal and radial extents of several hundred kilometers in the source region of plumes.
Geochemical studies, including those made possible by the Hawaiian Scientific Drilling Project, have revealed the chemically and isotopically heterogeneous nature of hotspot lavas, yet their ...interpretation is highly controversial and there is little agreement as to how geochemical heterogeneities might be spatially arranged within the plume conduit. To address this issue we conduct high resolution numerical simulations of an axisymmetric purely thermal plume, focusing on the lower mantle part of the conduit and on the thermal boundary layer (TBL) feeding the plume. We explore the relation between length-scales of heterogeneities across the source region and the length- and time-scales of geochemical variations in the plume conduit. The vertical velocity inside the conduit decreases exponentially with the square of radial distance generating high strain rates (order 10
−
13
–10
−
14
s
−
1
) that modify the shape of upwelling heterogeneities into elongated and narrow filaments. Therefore, the preservation of ‘blob-like’ heterogeneities (i.e., with a 1:1 aspect ratio in a vertical section) is quite unlikely, even in the central part of the plume. For example, initial lenses of size 100
×
10 km in the TBL are stretched into filaments 500–1000 km long. These filaments constitute ‘long-lived’ structures in a rising plume, and their geochemical fingerprints may be registered at a given radial distance for several millions of years. We also consider an idealized heterogeneous architecture inside the TBL, consisting of ‘trains’ of small scale lenses. When such trains upwell in the conduit, they form high radial geochemical gradients. Their ‘geochemical record’, registered over time at a given depth and radial distance, will fluctuate over time, with shorter period and a larger amplitude at the conduit center than at its periphery. Finally, we demonstrate that material existing ‘side by side’ in the conduit originated from regions in the TBL that are separated by distances of several hundred kilometers. This implies that vigorous plumes are able to sample, and to bring side by side, very distant portions of their source region. Our results provide a fluid dynamically consistent framework to discuss the main aspects of the different (and to some extent mutually exclusive) models of conduit structure used to interpret the geochemical observations of the Hawaiian lavas.
Displaced helium and carbon in the Hawaiian plume Hofmann, Albrecht W.; Farnetani, Cinzia G.; Spiegelman, Marc ...
Earth and planetary science letters,
12/2011, Letnik:
312, Številka:
1
Journal Article
Recenzirano
High relative abundances of primordial
3He are commonly found in ocean island basalts (OIB) thought to be derived from mantle plumes, and high
3He/
4He ratios have been used to distinguish plume-type ...from non-plume OIBs. In simple plume models, one expects to find the highest
3He/
4He ratios in the axial part of the plume conduit, which is sampled during the shield building stage of the volcanoes. However, the actual locus of the highest
3He/
4He ratios is sometimes significantly displaced. This is best documented for the Hawaiian plume, where the highest-
3He/
4He basalts are found on Loihi, a volcano tens of kilometers ahead of the inferred plume center, and
3He/
4He ratios decrease systematically toward MORB-type values during the main and late phases of eruption. We propose that this effect is caused by small amounts of carbonatite melt formed in plumes as they rise through the transition zone. If the plume conduit is tilted by plate-driven upper mantle flow, the carbonatite melt infiltrates more vertically due to its low density and viscosity and is thus displaced from the plume center. Helium, if partitioned into the carbonatite melt, will also be displaced from the plume center. To test this model we use a numerical simulation of the Hawaiian plume interacting with the fast-moving Pacific lithosphere. We obtain vertical separation velocities of the carbonatite melt on the order of a meter/year. Consequently, helium and carbon, initially located in the plume center at >
450
km depth, are laterally displaced by 50 to 80
km in the shallow mantle, depending on grain size, porosity and melt production rate. This can explain why the highest
3He/
4HE ratios (R/Ra up to 39; R/Ra
≡
(
3He/
4He)
sample/(
3He/
4He)
atmosphere) occur on pre-shield Loihi, why they decline during the shield phases of Mauna Loa, Mauna Kea and Haleakala, and why post-shield and rejuvenated Hawaiian volcanism delivers only low
3He/
4He ratios (R/Ra
=
8–10). Our results quantify the potential role of carbonatite liquids in transporting helium in the Hawaiian conduit, and they appear to apply also to other plumes tilted by upper-mantle ‘wind’.
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We model ascending carbonatite liquids formed deep in the Hawaiian plume. ► They transport high
3He/
4He from conduit center to the leading edge of a tilted plume. ► This explains the
3He/
4He maximum in preshield Loihi lavas. ► It also explains lower values during later shield and post-shield eruptive stages.
Compositionally heterogeneous material may exist in the lowermost mantle van der Hilst and Kárason, Science 283 (1999) 1885–1888. Here we use a numerical model to investigate the dynamics of (i) the ...subducted oceanic crust and lithosphere, (ii) a deep layer chemically denser, relatively undegassed and enriched in radiogenic elements. Tracers carry U, Th, K, and He concentrations which vary due to radioactive decay and to partial melting and degassing processes. We investigate the stability of the denser layer and find that by considering a depth dependent thermal expansion coefficient and temperature dependent viscosity, a layer with a chemical density excess of 2.4% can remain stable and poorly mixed until present-day time. The calculated helium ratios are in good agreement with
3He/
4He observed at ridges and hotspots and show that the large spectrum of helium ratios of OIB can be explained by mixing between undegassed material, recycled oceanic crust and lithosphere. For MORB, the sharp spectrum of helium ratios may be due to a degassed, homogeneous and well mixed shallow mantle.
SUMMARY
During the accretion stage, large impacts provided sufficient energy to melt the entire mantle into a terrestrial magma ocean. Processes occurring in the magma ocean may have led to the ...formation of heterogeneities still found in modern ocean island basalts. So far, no definitive mechanism exists to explain the survival of early heterogeneities for approximately 4.5 Ga. Addressing this question requires understanding the efficiency of convective mixing during both the early molten and the solid-state stages experienced by the Earth’s mantle. While mixing in the solid mantle and in an essentially crystallized magma ocean has been relatively well documented, the efficiency of convective mixing in a liquid magma ocean has received less attention. In this paper we characterized the mixing efficiency of a convecting fluid in a rotating spherical shell, accounting for inertial effects, by computing finite-time Lyapunov exponents (i.e. the Lagrangian strain rate). We conducted a series of numerical experiments for a regime where the influence of the buoyancy force dominates that of rotation and we derived scaling laws to predict the mixing efficiency. We found that for a terrestrial magma ocean, in its fully liquid state, mixing time is of the order of a few minutes or less, even for initially large (∼1000 km) heterogeneities. Therefore, passive early mantle heterogeneities cannot survive a fully molten magma ocean stage. This suggests that short-lived heterogeneities (e.g. 182Hf−182W) were either created at the end of the accretional stage, or were stored in deeper regions of the Earth.
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