The Geological Society of America Special Paper 430 2007 Potential effects of hydrothermal circulation and magmatism on heatflow at hotspot swells Carol A. Stein* Department of Earth and Environmental Sciences, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7059, USA Richard P. Von Herzen* Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Clark 244B, MS#22, Woods Hole, Massachusetts 02543, USA, and Department of Earth Sciences, University of California at Santa Cruz, Earth and Marine Science Building, Santa Cruz, California 95064-1077, USA ABSTRACT The lack of high heatflow values at hotspots has been interpreted as showing that the mechanism forming the associated swells is not reheating of the lower half of oceanic lithosphere. Alternatively, it has recently been proposed that the hotspot sur- face heatflow signature is obscured by fluid circulation. We re-examine closely spaced heatflow measurements near the Hawaii, Réunion, Crozet, Cape Verde, and Bermuda hotspots. We conclude that hydrothermal circulation may redistribute heat near the swell axes, but it does not mask a large and spatially broad heatflow anomaly. There may, however, be heatflow perturbations associated with the cooling of igneous intru- sions emplaced during hotspot formation. Although such effects may raise heatflow at a few sites, the small heatflow anomalies indicate that the mechanisms producing hotspots do not significantly perturb the thermal state of the lithosphere. Keywords: heatflow, swell, hotspot, mantle plume INTRODUCTION anomalies at hotspots (e.g., Crough, 1978). A consequence of this mechanism is unusually high surface heatflow, which After the recognition that hotspots, regions of excess vol- should be measurable by established methodologies. The first canism and higher elevation compared to surrounding areas, re- detailed study of the Hawaiian hotspot found high mean heat- sult from the rise of material through the mantle to the Earth’s flow (Von Herzen et al., 1982), but subsequent studies showed surface beneath both oceans and continents, marine geothermal that the anomaly was overestimated because the reference measurements were among the first geophysical techniques model predicted significantly lower heatflow than observed in used to assess their nature. Initially it was proposed that signif- unperturbed older lithosphere (Von Herzen et al., 1989; Stein icant reheating of the bottom half of the lithosphere over a dis- and Stein, 1993). The conclusion was that lithospheric reheat- tance of ~300 km in diameter was required to match the depth ing is not a major factor in hotspot formation. Thus, possible *E-mails: Stein, [email protected]; Von Herzen, [email protected]. Stein, C.A., and Von Herzen, R.P., 2007, Potential effects of hydrothermal circulation and magmatism on heatflow at hotspot swells, in Foulger, G.R., and Jurdy, D.M., eds., Plates, plumes, and planetary processes: Geological Society of America Special Paper 430, p. 261–274, doi: 10.1130/2007.2430(13). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rights reserved. 261 262 Stein and Von Herzen mechanisms producing the swells must be consistent with small on the answer to this question, different mechanisms have been or essentially zero heatflow anomalies, such as those expected proposed for hotspot formation. for dynamic models (e.g., Ribe and Christensen, 1994). Many mechanisms have been suggested to explain hotspot However, it has been recently proposed (Harris et al., formation. An important observation to model is the anom- 2000a,b) that hydrothermal circulation may obscure the signal alously shallow bathymetry and its subsequent subsidence (e.g., from the processes forming hotspots. Harris et al. (2000a, DeLaughter et al., 2005). For example, the reheating model re- p. 21,368) suggested that “fluid flow has the potential to obscure quires significant upwelling of mantle material that rapidly thins basal heatflow patterns associated with the Hawaiian hot spot,” the bottom half of the overlying plate (Crough, 1978, 1983). For which would “not bode well for capturing the form and magni- the dynamic plume model that incorporates a large viscosity tude of this signature.” Most hydrothermal circulation occurs in contrast between the plume and overlying lithosphere, most re- young oceanic crust (0 to ca. 65 Ma), resulting in lower mea- heating is at the base of the lithosphere (e.g., Liu and Chase, sured heatflow compared to conductive cooling models of the 1989; Ribe and Christensen, 1994). The compositional buoyancy lithosphere. Heatflow transferred by hydrothermal circulation model predicts shallower depths from relatively low-density in older oceanic lithosphere is thought to be much less (e.g., magmas emplaced either within the lithosphere or near its base Stein and Stein, 1994; Von Herzen, 2004). However, as sug- (e.g., Robinson, 1988; Sleep, 1994; Phipps Morgan et al., 1995). gested above, if the overall rough basement and islands and Different amounts of reheating and excess heatflow with time seamounts associated with hotspots cause fluid flow to occur since hotspot formation are expected from these different mech- both in the igneous basement and perhaps in the relatively per- anisms. Hence, heatflow data may help to discriminate among meable volcaniclastic sediments, then “capturing the maximum them. basal heat flux may be confounded by fluid flow” (Harris et al., For the Hawaiian chain, on the relatively fast-moving Pa- 2000a, p. 21,368). Evidence on the magnitude and extent of cific plate, matching the observed uplift and subsequent subsi- hydrothermal heat transfer may be provided by the detailed dence by the reheating mechanism requires that the bottom third heatflow at the five hotspots on crust older than ~65 Ma. to half of the oceanic lithosphere is rapidly reheated to as- In this article, we first examine heatflow anomalies near thenospheric temperatures over a zone 300 km wide centered on these hotspots. The anomalies are computed relative to the the axis (Von Herzen et al., 1982). At the time reheating begins, Global Depth and Heatflow reference model (Stein and Stein, the surface heatflow anomaly should be zero, but as the plate 1992) and with crustal ages from Mueller et al. (1997). Second, moves away from the upwelling, the heating of the lower litho- we examine which sites may be affected by hydrothermal cir- sphere propagates upward by conduction. The predicted anom- culation and thus mask higher heatflow. Last, we consider heat- aly gradually increases to a maximum of ~25 mW m–2 ~15–20 flow perturbations associated with the cooling of igneous struc- m.y. after reheating and then gradually decreases. The heatflow tures that result from the hotspot processes, including the igneous anomaly should be a maximum at the axis of the chain and de- material emplaced on the surface of pre-existing oceanic crust, crease to zero over ~600 km normal to the axis (Fig. 1A). and intruded sills at subcrustal depths. Although anomalously high heatflow was initially reported for the Hawaiian chain (Von Herzen et al., 1982), consistent with ESTIMATED HEATFLOW significant lithospheric reheating, subsequent data and analysis ANOMALIES FROM HOTSPOTS (Von Herzen et al., 1989; Stein and Stein, 1993) showed that the expected Gaussian distribution of anomalous heat flux orthogo- Although the depth in the Earth from which hotspots orig- nal to the axis did not exist, and many of the apparent anomalies inate is still debated (e.g., Sleep, 2003; Anderson, 2005; Foul- along axis result from comparing the data to reference thermal ger, this volume; Garnero et al., this volume), a geophysical models that underestimate heatflow elsewhere. Thus, the impli- challenge is to separate the effects of mechanisms forming cation is that heatflow data do not support swell formation by hotspots from shallower lithospheric processes, including the extensive lithospheric heating. “normal” lithospheric cooling as the plate ages. Lithospheric in- The small heatflow anomaly and absence of the pattern ex- teractions with the upward flow of magma, the absolute veloc- pected for lithospheric reheating at Hawaii and other marine ity of the plate, and near-surface physical processes (including hotspots (Réunion [Bonneville et al., 1997], Bermuda [Detrick excess volcanism and hydrothermal circulation) complicate the et al., 1986], Cape Verde [Courtney and White, 1986], and Crozet task of understanding the deeper mantle processes leading to the [Courtney and Recq, 1986]) imply that uplift may result from formation of hotspots. Also, because heat is largely transferred the dynamic effects of rising plumes. Their thermal effects are by conduction through the lithosphere, excess heat loss associ- concentrated at the base of the lithosphere and hence would raise ated with hotspot formation at a given location may occur long surface heatflow at most slightly, given that tens of millions after the hotspot upwelling has ceased. Although hotspots are of years are required for the conduction of heat to the surface. associated with anomalously shallow bathymetry, it is unclear For example, Ribe and Christensen (1994) predicted heatflow whether their surface heatflow is anomalously high. Depending anomalies for Hawaii of less than 1 mW m–2 (Fig. 1B). The heat- Potential effects of hydrothermal circulation and magmatism on heatflow 263 Figure 1. Predicted heatflow anomalies from reheating and dynamic plume mod- els. (A) The reheating model predicts heatflow anomalies up to ~25 mW m–2 (Von Herzen et al., 1982) in contrast to (B) a dynamic plume model of less than 1 mW m–2 (Ribe and Christensen, 1994). The location of heatflow measurements for Hawaii from Von Herzen et al. (1982, 1989) and Harris et al. (2000a) are shown in panel A. In panel B, the predicted sur- face heatflow from an upwelling plume (location indicated by the solid semicir- cle) beneath a moving plate is shown for up to 2400 km downstream of where the plume has passed (horizontal axis) and for up to 1600 km perpendicular (verti- cal axis) from the path of the plume.