or collective redistirbution other or collective or means this reposting, of machine, is by photocopy article only anypermitted of with portion the approval The O of This article has been published in Oceanography published been This has article Mountains in the Sea

Jasper SPOTLIGHT 2 30°26.40'N, 122°44.40'W

By Jasper G. Konter, Hubert Staudigel, and Jeffrey Gee , Volume 1, a quarterly 23, Number The O journal of

Jasper Seamount is a D1 Dredge OBS -500 -1500 -2700 -3900 in the Fieberling-Guadalupe seamount -900 -2100 -3300 mbsl trail, located off the coast of Baja California, Mexico. It rises from the 4000-m-deep seafloor to a summit depth of 700 m (Figure 1). Detailed geophys- 30°30'N ical, geochemical, and geological studies D1 there provide an in-depth geological D9 D2 ceanography S understanding of a seamount that D14 D15 D11 00 approaches our knowledge of subaerial 0 -1 ociety. volcanoes. Active-source seismic experi- D12 D3 D18 © D4 00 ments at Jasper Seamount resulted in 30°25'N 0 2010 by The O D6 -2

00 the first seismic velocity models of an 0 D13 -3 intraplate seamount (Hammer et al., D19 1994). Marine gravity and magnetics D5 ceanography S surveys, combined with analyses of D8 the physical properties, , ceanography S ociety. A and geochronology of dredge samples, enabled scientists to develop a detailed 122°45'W 122°40'W Permission is reserved. ll rights granted to in teaching copy this and research. for use article Republication, systemmatic reproduction, model of its internal structure (Gee et al., ociety. S Figure 1. Gridded multibeam bathymetry map of Jasper Seamount (adapted

1988, 1991; Pringle et al., 1991). end all correspondence to: [email protected] Th e O or from Konter et al., 2009). The seamount shows a slight northeast–southwest The resulting model of Jasper elongated summit and several small cones near the summit. Dredge hauls Seamount displays many similarities covered most of the seamount, and ocean bottom seismometers (OBSs) were to models of the formation of typical located around and across the seamount during the seismic experiment. Gravity and magnetics tows were performed in multiple passes (not shown). Hawaiian volcanoes, in particular, the stage-wise eruption of increasingly alkalic rocks (Figure 2). More than 90% of the volcano is made up of tholeiitic define two distinct stages: the flank throughout the mantle melting history to alkalic transitional , defining alkalic series (FAS; 8.7–7.5 million years that brackets these three different the flank transitional series (FTS) that ago) followed by the summit alkalic eruptive periods. Konter et al. (2009) ceanography S erupted 11.5–10 million years ago. series (SAS; 4.8–4.1 million years ago). observed that the distinct evolutionary This volcanic foundation resembles At Jasper Seamount, the eruption of stages of Jasper also carry characteristic ociety, P the shield-building phase of a typical volcanic rocks containing increasing mantle source signatures for Sr, Nd, Pb,

Hawaiian oceanic intraplate volcano. fractions of alkalies at a given SiO2 and Hf isotopes that evolve in a way that MD 20849-1931, USA 1931, Rockville, O Box Subsequent lavas erupted at Jasper concentration is interpreted to result resembles typical Hawaiian volcanoes Seamount are increasingly alkalic and from a decreasing relative melt fraction (Figure 3). The seamount’s foundation

Jasper G. Konter ([email protected]) is Assistant Professor, Department of Geological Sciences, University of Texas at El Paso, El Paso, TX, USA. Hubert Staudigel is Research Geologist and Lecturer, Institute of and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA, USA. Jeffrey Gee is Professor and Department Head, Geosciences Research Division, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA, USA. .

40 Oceanography Vol.23, No.1 10 (or shield) has isotopic mantle charac- PT teristics that are typical for long-term enriched incompatible element concen- 8 Alkali s trations (e.g., low 143Nd/144Nd), and basalts later stages appear to be more depleted TholeiiteTA 143 144 8 O

(e.g., high Nd/ Nd). Altogether, 2 M SAS: 4.8-4.1Ma K 6 S: MMGG these similarities are likely to indicate A Lower degree melts FAS: 8.7-7.5Ma O + that processes generating magmas at 2 HHWW

Na S:S 11.5-10 Jasper Seamount and Hawai`i are quite T FTS:FTS 11.5-10Ma similar, even though Jasper is a much 4 smaller volcano (690 km3) compared 3 to a Hawaiian volcano (~ 30,000 km ). Increasing crystallization This size difference is significant because 2 the most widely accepted model for the B origin of oceanic intraplate volcanoes 42 4466 5500 54 58 62 (OIV) is almost exclusively based on SiO2 studies of Hawai`i, which is the largest Figure 2. Major element composition of Jasper Seamount lavas showing that individual stages of the flank transitional series (FTS), OIV globally. Expanding the Hawaiian flank alkalic series (FAS), and summit alkalic series (SAS) are distinct evolutionary model to a much smaller in composition and likely represent smaller degrees of melting with seamount offers a rationale for the use of time. Within each stage, fractional crystallization creates a trend in the this model for this much more common data. Key for fields with rock types: B = . HW = hawaiite. MG = mugearite. TA = trachy-andesite. PT = phonolitic tephrite. Adapted class of OIVs (see Wessel et al., 2010). It from Konter et al., 2009 suggests that the melting processes that produce the largest OIVs are actually .5134 similar to the processes that form much smaller . Pacific MORB

.5132 REFERENCES Gee, J., L. Tauxe, J.A. Hildebrand, H. Staudigel, and Hawaii P. Lonsdale. 1988. Nonuniform magnetization PE SAS of Jasper Seamount. Journal of Geophysical Research 93:12,159–12,175. .5130 Gee, J., H. Staudigel, and J.H. Natland. 1991. Nd PS 144 and petrology of Jasper Seamount. / FAS

Nd FTS Journal of Geophysical Research 96:4,083–4,105. Hammer P.T.C, L.M. Dorman, J.A. Hildebrand, and 143 .5128 B.D. Cornuelle. 1994. Jasper Seamount struc- Austral ture: Seafloor seismic refraction tomography. S Journal of Geophysical Research 99:6,731–6,752. Konter, J.G., H. Staudigel, J. Blichert-Toft, .5126 B.B. Hanan, M. Polvé, G.R. Davies, N. Shimizu, and P. Schiffman. 2009. Geochemical stages Samoa at Jasper Seamount and the origin of intra- plate volcanoes. Geochemistry, Geophysics, Pitcairn Geosystems 10, Q02001, doi:10.1029/ 2008GC002236. 17.0 18.0 19.0 20.0 21.0 22.0 Pringle, M.S., H. Staudigel, and J. Gee. 1991. Jasper 206Pb/ 204Pb Seamount: Seven million years of volcanism. Figure 3. Pb-Nd isotope compositions of the flank transitional series Geology 19:364–368. (FTS), flank alkalic series (FAS), and summit alkalic series (SAS) are Wessel, P., D.T. Sandwell, and S.-S. Kim. distinct and vary significantly compared to the range in oceanic 2010. The global seamount census. basalts. In addition, the evolution in isotopic composition changes Oceanography 23(1):24–33. toward mid-ocean-ridge-basalt (MORB)-like compositions similar to the shield (S), post-shield (PS), and post-erosional (PE) stages in Hawai`i. Adapted from Konter et al., 2009

Oceanography March 2010 41