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A New Insight Into the Hawaiian Plume Earth and Planetary Science Letters 241 (2006) 438–453 www.elsevier.com/locate/epsl A new insight into the Hawaiian plume Jianshe Lei *, Dapeng Zhao Geodynamics Research Centre, Ehime University, Japan Received 19 June 2005; received in revised form 22 November 2005; accepted 27 November 2005 Available online 5 January 2006 Editor: S. King Abstract Although many geochemical, geophysical and seismological studies have suggested that the Hawaiian mantle plume originates from the core–mantle boundary (CMB), so far no tomographic model shows a continuous image of the Hawaiian plume in the entire mantle because of the few seismic stations on the narrow Hawaiian island chain. Here we present a new tomographic image beneath Hawaii determined by using simultaneously 10 kinds of seismic phases, P, pP, PP, PcP, Pdiff, PKPab, PKPbc, PKiKP, PKKPab and PKKPbc, extracted from the data set compiled by the International Seismological Center. Of these phases, PKiKP, PKKPab and PKKPbc are, for the first time, attempted to use in the global seismic tomography. Our results show a slow anomaly beneath Hawaii ascending continuously from the CMB to the surface, implying that the Hawaiian plume indeed originates from the CMB. This image is improved notably over the previous results in the whole mantle, particularly in and below the middle mantle, suggesting that later phases, PP, Pdiff, PKP and particularly PKiKP, are of great importance for better imaging the Hawaiian plume. This slow anomaly is considered to be a plume conduit being tilted, which is likely caused by the mantle flow. This indicates that the position of the Hawaiian hotspot on the surface is not stationary, as evidenced by the recent paleomagnetic and numerical modeling studies. D 2005 Elsevier B.V. All rights reserved. Keywords: mantle plume; Hawaiian hotspot; P-wave tomography; global tomography; mantle and core phases 1. Introduction the chain of the Hawaiian islands and seamounts (Fig. 1) is believed to mark the passage of the oceanic Hotspots are characterized by topographic swell, lithosphere over a mantle plume [4,5]. These Hawaiian higher temperature and recent volcanism with isotopic islands form a linear chain with progressively increas- signatures distinct from those that characterize mid- ing age away from the active region, perhaps reflecting ocean ridge or andesitic basalts [1–3]. Of all the hot- a nearly stationary hotspot beneath the moving litho- spots, Hawaii is a well-known example. Hawaii is also spheric plate. The formation of the Hawaiian hotspot one of the longest-lived hotspots and by far the stron- may be caused by an upwelling mantle plume formed gest in flux, as measured by the topographic swell it from instabilities at a hot thermal boundary layer in the created [1,2]. The volcanism responsible for creating Earth’s mantle [1–5]. The core–mantle boundary (CMB) source of the Hawaiian hotspot has been com- monly assumed [6–12], but the morphology of the * Corresponding author. Tel.: +81 89 927 8257; fax: +81 89 927 8167. mantle plume has long been debated because of the E-mail addresses: [email protected] (J. Lei), lack of seismic stations on the narrow Hawaiian volca- [email protected] (D. Zhao). nic chain. 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.11.038 J. Lei, D. Zhao / Earth and Planetary Science Letters 241 (2006) 438–453 439 Fig. 1. Map of the study area. White triangles and white dots denote the seismic stations and earthquakes, extracted from the ISC bulletins, in and around the Hawaiian island chain. The hotspot (red triangle) is currently centered just southeast of Hawaii. The seismic stations are mainly located on the islands of Hawaii, Maui and Oahu. The insert map shows the location of the study area. Many researchers have used geological, geochemi- anomaly beneath Hawaii in his global tomographic cal and geophysical approaches to study the Hawaiian models, perhaps because he used only mantle phases hotspot [6–22]. Geochemical and gravity observations (P, pP, PP, PcP and Pdiff). So far no tomographic results indicate the existence of a mantle plume beneath show a continuous low-V cylinder beneath Hawaii Hawaii [12,20–22]. A number of dynamic models extending from the CMB to the surface in the entire have suggested possible plume–lithosphere interaction mantle. around Hawaii [2,17,21]. Teleseismic tomography revealed low velocity (low-V) anomalies beneath the Hawaiian islands in the upper mantle [19]. Analyses of Rayleigh wave data showed low velocity and thinning of the lithosphere beneath Hawaii [13–15]. An analysis of P to S converted seismic phase indicates the exis- tence of a zone of very slow shear-wave velocity be- neath Hawaii extending deeper into the lower mantle [17]. Waveform modeling and receiver function studies suggested that the Hawaiian hotspot may originate from the lower mantle [10,16,17]. Some results from diffrac- tion wave and differential travel time tomography also displayed low-V anomalies existing in the lower mantle beneath Hawaii [9,23]. However, different global to- mographic models showed different images of the Ha- waiian plume. Fukao et al. [24] and Obayashi et al. [25] showed low-V anomalies beneath Hawaii limited to the middle mantle in depth. Montelli et al. [8] combined the Fig. 2. Sketch illustration of ray paths used in this study. The rays are International Seismological Center (ISC) data with the the direct P wave, depth phase (pP), surface reflected wave (PP), ray theory and travel times measured from waveforms CMB reflected wave (PcP), outer core diffracted wave (Pdiff), outer using the kernel theory and concluded that the Hawai- core transmitted waves (PKPab and PKPbc), inner core reflected wave (PKiKP) and CMB underside reflected waves (PKKPab and ian hotspot originates from the CMB, but the velocity PKKPbc). Star and inverted triangles denote earthquake and seismic structure of the lowermost mantle was less well re- stations. Two dashed lines denote the 410 and 660 km discontinuities, solved. Zhao [6,7] exhibited an intermittent low-V respectively. 440 J. Lei, D. Zhao / Earth and Planetary Science Letters 241 (2006) 438–453 Table 1 phases is considered to be an effective way to improve Mantle and core phases used in this study the tomographic image, particularly for the Southern Seismic phases Number of data Number of data Delta ranges Hemisphere and oceanic regions where few seismic (1964–1998) (1964–2003) stations and events exist. In this work, in additional to P 783,024 1,026,776 0–968 the direct P wave, nine kinds of later phases (pP, PP, pP 36,017 48,868 30–1008 PcP, Pdiff, PKPab, PKPbc, PKiKP, PKKPab and PP 8177 11,592 55–1808 PcP 4644 8698 25–408, 45–708 PKKPbc) are included to image the mantle structure Pdiff 13,401 19,839 100–1508 (Fig. 2). Comparing to the previous studies [6–8,24,27– PKiKP 5589 9488 100–1158 29], three kinds of core phases (PKiKP, PKKPab and PKPab 5312 7918 155–1808 PKKPbc) have, for the first time, been used in the PKPbc 26,768 35,624 146–1548 global seismic tomography. PKPdf and PKKPdf are PKKPab 157 587 105–1208 PKKPbc 515 2289 82–888, 92–1208 not taken into account in this study because they pass Total 883,604 1,175,212 through the inner core where strong anisotropy exists [30,31]. The data used were extracted from the ISC bulletins during 1964–1998 reprocessed by Engdahl et Here we present a new global tomographic model al. [32]. To identify all later-arriving phases of interest, obtained by applying the updated tomographic method a probabilistic method was used [32], which permits the of Zhao [6,7] to 10 kinds of seismic phases (Fig. 2). use of selected later phases in calculating hypocenters Our new model with all the phases shows a continuous by providing a basis for the relative weighting of low-V anomaly under Hawaii ascending from the CMB phases. Each earthquake in our data set has more than to the surface, which is a significant improvement over 50 recordings and the focal depths were well con- the previous results [6–8,24]. The images of other strained by pP depth phases. To avoid the effect of mantle plumes and subducted slabs are presented else- misidentified data and obtain a reliable tomographic where [26]. In this paper, we focus on the resolution image, we winnowed the data carefully, which is sim- analysis and discussion of the low-V anomaly observed ilar to the previous studies (e.g., [28,33–35]). Travel clearly beneath Hawaii. time residuals used in the inversion are confined to those less than 5.0 s. The total number of the data 2. Data and method (1964–1998) used in the inversion amounts to 883,604. For details, see Table 1. Later phases often sample the Earth’s mantle struc- Fig. 3 illustrates the distribution of bounce points of ture not ordinarily sampled by the direct P waves in PP on the surface, CMB legs of Pdiff, transmission most portions of the mantle. Therefore, adding later points of PKP, PKiKP and PKKP through the CMB, Fig. 3. Distributions of (a) bounce points (circles) of PP on the surface, (b) legs of Pdiff (lines) along the CMB, and transmission points (crosses) of (c) PKPab and PKPbc, (d) PKiKP and (e) PKKPab and PKKPbc through the CMB around Hawaii. J. Lei, D. Zhao / Earth and Planetary Science Letters 241 (2006) 438–453 441 in and around Hawaii. Bounce points of PP are mainly anomalies will appear more obviously under Hawaii distributed on the NW–SE orientation (Fig.
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