The Yellowstone hotspot in space and time: Nd and Hf isotopes in silicic magmas
Introduction
Hotspots provide a window into the earth's interior and yield information on the differentiation of the earth, the persistence of long-lived geochemical heterogeneities in the Earth's mantle, and geodynamic processes that recycle outer portions of the earth into its interior. Additionally, because hotspots originate at sublithospheric depths, they play an important role in plate tectonic theory and provide key reference points in assessment of relative plate motions. Hotspots are distinguished by their anomalous thermal character, a pronounced geoidal anomaly, a linear space- and time-transgressive volcanic history, and often, a geochemically distinct volcanic record.
The Yellowstone hotspot (YHS) is of interest because it is located on a continent and offers the opportunity to examine the interrelationship of coupled mantle and crustal igneous processes. Among continental hotspots, the Yellowstone hotspot is amenable to study because it is young, active, and the relative plate motion between the North American plate and the localized mantle heat source is sufficiently rapid so as to leave a well-defined hotspot trace. In addition, the Yellowstone plateau has been the locus of some of the most voluminous production of silicic magma on the planet in the Quaternary.
The YHS has been the focus of investigation for several decades and there continues to be disagreement as to its origin and evolution. Fundamental questions remain including whether or not the hotspot is the result of a mantle plume, and if so, where the plume first impinged on the North American lithosphere. The nature of the source of silicic magmas is speculative, and the mass and thermal contributions from the mantle are poorly constrained, although they are critical factors in crustal growth and maturation. A question of particular importance is whether large silicic provinces represent new material from the mantle or the recyling of older continental material. The ages of volcanic events and their distribution bear on the debate about the relative roles of plate motion and crustal extension in governing the apparent migration rate of the hotspot. In order to address these questions we have measured Nd and Hf isotopic ratios in glass from silicic tuffs erupted over the past 16 m.y. history of the hotspot.
Section snippets
The Yellowstone hotspot
Numerous lines of evidence support a hotspot origin for the Yellowstone system including extraordinarily high heat flow, a pronounced 1000 km diameter geoid anomaly, and a parabola of active seismicity and tectonism in the Yellowstone Plateau/Eastern Snake River Plain [1], [2]. The pronounced topographic decline to the west of the Yellowstone Plateau that defines the Snake River Plain can be modeled as the response of cooling lithosphere that has additional loading from injection of
Analytical methods
We analyzed glass from 54 representative YHS metaluminous rhyolite ash-fall tuffs ranging in age from 15.9 to 0.4 Ma for 143Nd/144Nd. The isotopic ratios of 5 proximal silicic volcanic units along the hotspot track were analyzed for comparative purposes. These units include: basal vitrophyre of the ∼ 16.4 Ma ash-flow tuff of Double H Mountains, a peralkaline tuff in the McDermitt volcanic field; basal vitrophyre from an early unit of the ∼ 16 Ma Jarbidge Rhyolite; the rhyolite of Birch Creek; and
Temporal variation in isotopic ratios
Results of isotopic analysis of YHS glass for present day ratios of 143Nd/144Nd and 176Hf/177Hf are provided in Table 1. The temporal pattern of isotopic variation in Nd and Hf in silicic magmas of the YHS is illustrated in Fig. 3. At ∼ 16 Ma, silicic ash fall tuffs exhibit εNd values of + 3 to + 4, within the range of contemporaneous Columbia River flood basalts [7]. Two distinct glasses from the younger Rattlesnake Tuff (7.05 Ma), erupted from the Harney Basin in central Oregon, yield similar εNd
Model for generation of hybrid melts
We propose a model for the generation of silicic magmas at the Yellowstone hotspot that involves the advection of mantle-generated basalt magma into the lower crust that undergoes partial melting producing a hybrid magma that may fractionate and is subsequently erupted at the locus of the hotspot. The evidence for such a process lies in Nd isotopic ratios in silicic magmas (εNd = − 10 to − 5) that are significantly elevated relative to the Precambrian continental crust and hence require a
Discussion
The age and isotopic relations of YHS silicic magmas are consistent with a mantle plume model for the hotspot. Evidence includes the broad distribution of silicic volcanism contemporaneous with mafic volcanism in the same region early in the history of the hotspot. Silicic magmas were being produced as early as 16.5 Ma, and silicic fallout tuffs, including those with sources west of the Sr .706 line such as sample buf94-614 (15.4 Ma), occur in sedimentary interbeds of the Columbia Plateau flood
Summary
Nd and Hf isotopes provide a continuous record of production of silicic magmas from the YHS from 16 Ma to the present. The silicic magmas result from hybridization of partially fused lower crust and mantle-derived basaltic magma, with a minimum mantle contribution ranging between 20% and 40%. From the isotopic record it is possible to trace the hotspot from early in its history in the accreted terranes west of the Precambrian craton onto the craton to its present location beneath the
Acknowledgements
BPN wishes to thank the inhabitants of the Radiogenic Isotope Geochemistry Laboratory at the University of Michigan for their hospitality and assistance during her sabbatical initiation into the nuances of Nd and Hf isotopes. Ongoing discussions with H. E. Cathey, R. B. Smith, M. Jordan and W.P. Leeman have been most beneficial. We very much appreciate careful reviews of versions of the paper by W. Leeman, D. Geist, A. Grunder, A. Glazner and P. Ihinger. Analytical support was provided by
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