Melting of MORB at core–mantle boundary
Introduction
The core–mantle boundary (CMB) exhibits the largest change in conditions (chemistry, density, viscosity, and temperature) across any boundary in the Earth's interior. It is also a dynamic boundary between the rapidly convecting outer core and the slowly convecting Earth's mantle. Seismology also indicates that the lowermost 200–300 km of the mantle, referred as , exhibits radial and lateral heterogeneity in seismic velocities gradients, discontinuities in both shear and compressional velocities. is a thermal boundary layer where geochemical and geophysical observations favor the presence of chemical heterogeneities, and it has even been proposed as a possible origin of mantle thermal plumes (Jeanloz and Richter, 1979, Jellinek, 2004). Heterogeneities are thought to be either residues from core formation process (Albarede and van der Hilst, 2002), segregation of dense subducted slab components (Boehler, 2000, McNamara et al., 2010), effects of partial melting (Lay et al., 2004), iron rich post-perovskite (Mao et al., 2006), iron-rich (Mg, Fe)O (Wicks et al., 2010) and/or chemical interactions between the core and mantle (Boyet and Carlson, 2005, Brandon et al., 1998). Seismological observations (Garnero, 2000) have revealed the existence of areas of very low seismic velocity (ultra-low-velocity zones, ULVZs) at the base of layer. These visible discontinuities at the core surface have a thickness up to 50 km, are hundred kilometers wide or more (McNamara et al., 2010, Thorne et al., 2013) and are commonly interpreted as zones of partial melting (Revenaugh and Meyer, 1997, Rost et al., 2005, Williams and Garnero, 1996). Indeed, partial melting at the base of the mantle can account for the observed seismic anisotropy (Kendall and Silver, 1996) and anomalously slow P-wave velocities (Revenaugh and Meyer, 1997, Williams and Garnero, 1996). Partial melting can as well cause strong differentiation in chemistry and density by melt segregation. The presence of pockets of partial melts can thus have profound consequences for the dynamics of the deepest part of the Earth, the heat flow across the CMB, and the storage of heat producing and rare earth elements or noble gases. Accordingly, the melting phase relations and element partitioning at CMB conditions are critical to address the chemical evolution of the early Earth and the nature of ULVZs.
One of the hypotheses that have been advanced on the composition and nature of the ULVZs is that they may be remnants of an initial magma ocean (Labrosse et al., 2007). A recent study (Fiquet et al., 2010) aiming at the determination of the melting of fertile peridotite showed that it is possible to melt peridotites at CMB conditions. This argues in favor of possible existence of molten regions at the base of the present-day mantle, thus making the hypothesis of a remnant magma ocean physically possible. The same overall conclusions were supported by a similar study on chondritic samples (Andrault et al., 2011) or by the recent study of on hydrous pyrolitic composition (Nomura et al., 2014) which yields even lower melting temperatures.
Following these studies, it is pertinent to ask whether subducted oceanic lithosphere – the other likely component present in the layer – can also melt at CMB conditions. The oceanic lithosphere consists of two chemically distinct lithologies: a basaltic crust and an underlying harzburgitic component. The latter is more refractory than fertile peridotite and is unlikely to melt at mantle depths. In contrast, the basaltic crust can potentially undergo melting. MORB is a good representative of the basaltic oceanic crust bulk composition. The study by Kogiso et al. (1998) showed that the melting of a mixture of mid-ocean ridge basalt (MORB) and peridotites has chemical characteristics similar to those of ocean island basalts (OIBs) and suggests that a chemical component of MORB is necessary in the formation of these magmas. It is thus important to know about the possible melting of the MORB at CMB conditions and whether the partial melting products can significantly contribute to ULVZs. However, with high Si, Al, Fe and Na content in MORB, the mineral phases [i.e. Mg- and Ca-perovskite (MgPv and CaPv), CaFe2O4-type (CF) phase and SiO2 (St)] developed at lower mantle pressures are complex in nature compared with those obtained from transformed mantle peridotite [i.e. MgPv, CaPv and ferropericlase (Mg, Fe)O]. In peridotite-like compositions, aluminum is incorporated into the perovskite structures. In more Al2O3 and SiO2-rich systems such as MORB, presence of additional phases like the Al-rich CaFe2O4-type (CF) as well as SiO2 (St) phase (Funamori et al., 2000, Hirose et al., 1999, Irifune and Ringwood, 1993) makes the interpretation of experimental data more challenging.
Previous melting studies showed that the anhydrous melting temperature of basaltic composition is lower than that of peridotitic mantle composition at all pressures except between 13 to 18 GPa (Herzberg and Zhang, 1996, Hirose et al., 1999, Yasuda et al., 1994, Zerr et al., 1998). Previous works on MORB compositions at high pressure have mostly been focused on the sub-solidus evolution of the mineralogy and density (Hirose et al., 2005, Perrillat et al., 2006, Ricolleau et al., 2008). Until recently (Andrault et al., 2014), melting studies were rather limited in pressures. Melting relations have been studied up to only 28 GPa (Litasov and Ohtani, 2005) while there is one report of a solidus curve of MORB up to 64 GPa (Hirose et al., 1999). The study by Hirose et al. (1999) suggests that MORB partial melting is possible near the CMB. However, uncertainties are high due to huge extrapolations in the P–T space. Very recently, melting has been examined at CMB pressure and temperature conditions (Andrault et al., 2014) using laser-heated diamond anvil cell (LHDAC) experiments indicating possible melting of MORB at CMB. Notably, the quantitative composition of the liquids obtained from partial melting have been reported only up to 27.5 GPa by multi-anvil experiments (Hirose and Fei, 2002) whereas Andrault et al. (2014) have provided some qualitative compositions. We have therefore conducted a series of experiments using laser-heating in diamond anvil cells (DAC) and determined the solidus curve for the MORB composition over a range of lower-mantle pressures, between 44 and 128 GPa. We also performed chemical analyses of recovered samples at high spatial resolution using analytical transmission electron microscopy (TEM). On the basis of melt texture and chemical composition, our study provides new constraints on the melting sequence of MORBs at deep lower mantle conditions and the partitioning behavior of iron between perovskite and silicate melt. We specifically discuss our results in the light of a very recent study by Andrault et al. (2014), where melting of MORB has also been examined at similar CMB conditions.
Section snippets
Starting material
A fresh natural MORB glass [chemical composition (in wt%) from electron microprobe analysis: SiO2 – 49.74, Al2O3 – 15.89, TiO2 – 1.33, MgO – 8.46, FeO – 9.73, CaO – 11.74, Na2O – 2.69, MnO – 0.18, K2O – 0.10, P2O5 – 0.11, Cr2O3 – 0.04, SO2 – 0.14] from the East Pacific Rise was used as starting material. Further details about the composition can be found elsewhere (Perrillat et al., 2006, Ricolleau et al., 2008). From infrared absorption analysis, the H2O and CO2 content was found to be
Melt detection
From in situ XRD measurements carried out at ESRF, we could clearly identify all the expected major phases (e.g. MgPv, CaPv, St, and CF-type phases) as we heated the MORB glass under pressure (Fig. 2). However, it is difficult to observe a clear disappearance of individual phase(s) up on further heating/melting. It is conceivable that during the melting process, there is a continuous inflow of the adjoining crystallites to the melt pool impeding the observation of loss of diffraction lines. It
Possibility of MORB melting at CMB
Local partial melting of the crustal material of the basaltic component in the region, the probable graveyard for subducted lithosphere, can be inferred by comparison between its melting curve and geothermal profile of . The thermal profile of the region has been tentatively constrained after the discovery of the post-perovskite phase of MgSiO3 (Murakami et al., 2004, Oganov and Ono, 2004). Observation of pairs of positive and negative S-wave velocity jumps in the region due to
Conclusions
We have determined the solidus curve of MORB up to CMB pressures. Our melting criterion is based upon temperature–power relationships crosschecked with textural analysis of recovered samples (both at solidus and subsolidus conditions). Our melt texture analysis shows the melting sequence to be MgPv → SiO2 → CaPv. The quenched last melt is enriched in iron. We report a solidus temperature for fresh MORB of at CMB pressure. Hence, MORB is found to be slightly more fusible at solidus
Acknowledgments
G.K.P., G.F. and A.L.A. acknowledge support from the French National Research Agency (ANR) grant ANR-10_BLAN_62201 CMBmelt. A.L.A and J.S. acknowledge the financial support from the PNP program from INSU-CNRS. J.S. acknowledges financial support from the ANR project VolTerre, grant no. ANR-14-CE33-0017-01. The authors deeply acknowledge the help of Imene Esteve (IMPMC) for her help with the FIB experiments. The FIB and SEM facility of the Institut de Minéralogie, de physique des Matériaux et de
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