Non-hotspot volcano chains originating from small-scale sub-lithospheric convection

M. D. Ballmer^1*, J. van Hunen², G. Ito³, P. J. Tackley¹ and T. A. Bianco³

¹Institute of Geophysics, ETH Zürich, Zürich, Switzerland
²Department of Earth Sciences, Durham University, Durham, UK
³SOEST, University of Hawaii at Manoa, Honolulu, HI, USA
^*corresponding author: ballmer@hawaii.edu

Whereas plume theory successfully predicts many observations at some volcano chains [Courtillot et al., 2003], it is insufficient to explain the whole spectrum of oceanic intraplate volcanism on Earth. A mantle plume is assumed to be the geodynamic explanation for a fixed hotspot. Many ridges, however, lack linear age-distance relationships and an association with an oceanic plateau as predicted by plume theory. On the Pacific plate, prominent chains with voluminous volcanism violate this prediction, such as the Cook-Australs, Marshalls, Gilberts, and Line Islands. Thus, these chains cannot have been formed by the Pacific plate overriding stationary hotspots [Koppers et al., 2003; Koppers et al., 2007; Davis et al., 2002; Bonneville et al., 2006].

A mechanism that has often been invoked to account for non-hotspot volcanic ridges is lithospheric cracking. Cracks may be induced by tensile stresses [Sandwell et al., 1995], by loading with volcanic edifices [Hieronymus & Bercovici, 2000], or by thermal contraction [Sandwell & Fialko, 2004; Gans et al., 2003]. Cracks are supposed to control where and when volcanism occurs [Natland, 1980]. However, they do not provide an explanation for magma generation itself. The cracking hypothesis rather presumes a broad reservoir of pre-existing partial melt that may be tapped. Such a layer of partial melt in the asthenosphere was originally proposed to account for anomalously low seismic-wave velocities [Anderson and Sammis, 1970]. However, recent studies show that partial melt is not required to explain seismic observations [Faul & Jackson, 2005; Stixrude & Lithgow-Bertelloni, 2005; Priestley & McKenzie, 2006]. Moreover, partial melting in the asthenosphere would tend to increase seismic wave velocities due to dehydration of the residue [Karato & Jung, 1998]. Since a reservoir of partial melt in the asthenosphere is at odds with geophysical observations, we neglect the lithospheric cracking hypothesis.

Another possible mechanism for intraplate oceanic volcanism without linear age-distance relationships is small-scale sublithospheric convection [SSC; Bonatti & Harrison, 1976; Haxby & Weissel, 1986; Buck & Parmentier, 1986; Marquart et al., 1999; Ballmer et al., 2007]. SSC spontaneously develops at the base of mature oceanic lithosphere, whenever the cold thermal boundary layer below the lithosphere exceeds a threshold thickness. Thereafter, convection is a more efficient heat transport mechanism than conduction. It self-organizes as rolls aligned with plate-motion [Richter & Parsons, 1975], with parallel upwellings spaced 200-300 km apart (Figure 1).

Figure 1: SSC develops in rolls aligned with plate motion, as soon as the thermal boundary layer exceeds a critical thickness. Its onset is earlier adjacent to lateral density heterogeneities and later for larger T_m or η_eff. (from Ballmer et al., 2007).

Whether and how much melting is generated from SSC depends on the onset age of the SSC. The onset age is most sensitive to:

1. the effective viscosity in the asthenosphere η_eff, and
2. the amount of pre-existing lateral density heterogeneity [Huang et al., 2005; Korenaga & Jordan, 2002; Dumoulin et al., 2005].

Low η_eff and lateral heterogeneity both tend to trigger SSC beneath younger seafloor. If SSC onsets beneath relatively old and thick lithosphere, the rather small thermal anomalies resulting (e.g., compared with a plume) are unable to generate significant melting. Higher mantle temperatures T_m also enable SSC melting beneath thicker lithosphere, for later onset ages (Figure 2).

Figure 2: Results of the 3D-numerical models of Ballmer et al. [2007]. Amount of volcanism vs. age of the underlying seafloor for different model runs (varying the parameter T_m and η_eff). Vertical axis is the total volumetric rate of melt extraction in vertical (y-z) cross-section of our model divided by the number of SSC cells. It represents the average volume of melt extracted from a single SSC cell per unit time and per km in the direction of plate motion. Curves are shaded according to η_eff as indicated in the scale above. Colors indicate different reference temperatures. Numbers indicate heights (in km) of volcanic edifices with a slope of 10°, that could be created if all of the melt extracted above an SSC upwelling accretes as chains of circular volcanoes spaced 100 km apart. Viscosity predominantly controls volcano height, whereas T_m controls both the age of seafloor during magmatism and volcano height.

SSC triggers melting by disrupting the thermal and compositional stratification of the uppermost mantle. The positive thermal anomalies in SSC upwellings that result from advection of asthenospheric mantle along the adiabat are usually insufficient to trigger melting in a depleted harzburgite layer which already experienced MOR melting. However, immediately after its onset, SSC removes this depleted layer in downgoing sheets and replaces it with fresh mantle from below, allowing subsequent melting (Figure 3). For larger T_m, removal of a thicker buoyant depleted layer from more extensive MOR melting is more difficult, and SSC therefore develops later with the possibility of deeper melting (Figure 2).

The duration of volcanism due to SSC is limited by secondary cooling of the asthenosphere. Melting slowly ensues after the removal of the harzburgite layer. Intrinsic positive density anomalies of partially molten peridotite further drive decompression and melting [Tackley & Stevenson, 1993; Raddick et al., 2002; Hernlund et al., 2008]. Therefore, melt production and extraction (i.e., volcanism) successively increase to a maximum ~ 4 Ma after their onset (Figure 2). However, SSC itself diminishes temperatures in the asthenosphere, as it continues to mix the bottom of the cold thermal boundary (Figure 3) into the asthenosphere. Thus, the duration of volcanism is limited to ~ 8 Ma with a length-scale of the underlying melting anomaly of ~ 1500 km (for fast Pacific plate-motion). The ages along an associated volcano chain are not predicted to be time-progressive. As the melting anomaly is elongated (“hot line”) and not point-like, the age relationships are predicted to be more complex than a simple progression [Bonatti & Harrison, 1976; Ballmer et al., 2007; 2009a].

Figure 3: Isosurfaces of melt fraction and depletion, and cross-sections of the temperature and velocity field, for an example calculation (T_m = 1380°C and η_eff = 1.6 x 10¹⁹ Pa s). Lifted from the top is a plan view of the thickness of melt accumulated onto a plate moving over the box. The harzburgite layer is partly removed in downgoning sheets. Hence, partial melting emerges above SSC upwellings. Melting is limited by secondary cooling of the asthenosphere due to the SSC itself (greenish colors; figure is from Ballmer et al., 2007).

Such behaviour could explain key observations at some, previously enigmatic, volcanic chains in the Pacific. Figure 4 shows ages vs. distance along the Marshalls, Gilberts, Lines, Cook-Australs, and Pukapuka ridges. For the Marshalls and the Pukapuka ridges, most samples fall into bands with a finite width of 1500 km and 1000 km, respectively (cf. shaded fields in Fig. 4), as would be predicted for volcanism on a plate moving over a “hot line” as caused by SSC. For the Cook-Austral chain on the South Pacific Superswell, volcanism has been attributed to at least three [Bonneville et al., 2006], or even more [McNutt et al., 1997] “plumelets”. However, two different episodes of SSC-volcanism (the younger of which maybe reactivated beneath older seafloor by the South Pacific “Superplume” [e.g., McNutt, 1998]) provide an alternative explanation [Ballmer et al., 2009a]. The overall age-progression of at least one episode of SSC-volcanism at the Cook-Austral Islands (and of the Pukapuka ridges) does not correspond to absolute plate motion (cf., slopes of the fields in Figs. 4d, 4e), something that may be explained by the onset age of SSC systematically varying through geological time (e.g., for systematically decreasing T_m). For both the Cook-Austral and the Marshall-Islands, the lateral spacings of the individual (parallel) subchains are well consistent with the the typical wavelength of SSC, while older seafloor ages at the time of active volcanism (i.e., 50-100 Ma) than predicted by our simple models for T_m ≤ 1410°C are reconciled by accounting for a heterogeneous mantle source with small fractions of fusible lithologies such as enriched peridotite, and pyroxenite [Ballmer el al., 2009b].

Figure 4: Ages collected at the (a) Wake seamounts, the (b) Marshalls, the (c) Line Islands, the (d) Pukapuka ridges, and the (e) Cook-Australs plot within fields in age-distance space of width (a, b, e) 1500 km, (c) 2000 km, and (d) 1000 km, respectively. This behavior is consistent with volcanism formed on the Pacific Plate overriding a quasi-stationary elongated magma source (“hot line”). In order to avoid introducing additional scattering, we take ages only from a limited number of reliable sources [cf., Ballmer et al., 2009a]. In (b) and (e) distances are obtained by projecting the locations of the seamounts of each of the 5 and 2 lineaments (cf. legends), respectively, on top of each other (with a projection axis parallel to the lineaments). Locations of the volcano chains on the Pacific Plate are given in (f). (figure is taken from Ballmer et al., 2009a). Click here or on figure for enlargement.

The Line Islands are also reasonably well explained by SSC. These volcanoes display two events of quasi-synchronous volcanism that erupted laterally over ~ 2000 km and on seafloor aged 30-55 Ma [Davis et al., 2002]. This range of seafloor age is consistent with our models with T_m ~ 1380-1410°C. Lateral density heterogeneity in the asthenosphere beneath the Line Islands presumably triggered SSC locally (during two distinct events). Since neighbouring limbs of SSC away from the heterogeneity develop later, volcanism evolved along a single lineament as observed at the Line Islands. Local (and episodic) SSC implies slower asthenospheric cooling, and may therefore explain the larger length-scales of the melting anomaly (i.e., ~ 2000 km) than predicted by our models.

Volcanism due to SSC requires either a slightly lower η_eff or a slightly higher T_m in the asthenosphere than average. Smaller-than-average η_eff (or lateral density heterogeneity) are needed for early onset of SSC and volcanism on young seafloor without the requisite of anomalously large T_m. For chains evolving on middle-aged seafloor (e.g., the Marshalls and Cook-Australs), small excess temperatures of the order of 50°C are sufficient to produce large volumes of volcanism, whereas much larger thermal anomalies (>>100°C) are required for significant volcanism by the hotspot or lithospheric cracking mechanisms. These volumes of melt are generated by overturning the thermal and compositional stratification in the uppermost mantle inherited at the MOR. Several other ridges with poorly constrained geochronology potentially originate from SSC as well.

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