Memories of Earth Formation in the Modern Mantle: W Isotopic Composition of Flood Basalt Lavas

Abstract:

Four and a half billion years of geologic activity has overprinted much of the direct evidence for processes involved in Earth’s formation and its initial chemical differentiation. Xenon isotopic ratios [1] and ³He/²²Ne ratios [2] suggest that heterogeneities formed during Earth's accretion have been preserved to the present time. New opportunities to learn about early Earth history have opened up with the development of analytical techniques that allow high precision analysis of short-lived isotopic systems. The Hf-W system (t_½ = 8.9 Ma) is particularly valuable for studying events that occurred during the first ~50 Ma of Solar System history. Here we report new data for ~ 60 Ma Baffin Bay and ~ 120 Ma Ontong Java Plateau lava samples. Both are large igneous provinces that may have sampled a primitive, less degassed deep mantle reservoir that has remained isolated since shortly after Earth formation [3,4]. Three samples analyzed have ¹⁸²W/¹⁸⁴W ratios that are 10 to 48 ppm higher than our terrestrial standard. These excesses in ¹⁸²W are the highest ever measured in terrestrial rocks, and may reflect ¹⁸²W ingrowth in an early-formed high Hf/W mantle domain that was produced by magma ocean differentiation [5]. Long and short-lived Sm-Nd systematics in these samples, however, are inconsistent with this hypothesis. The ¹⁸²W excessses could rather reflect the derivation of these lavas from a mantle reservoir that was isolated from late accretionary additions [6]. The chondritic initial Os isotopic compositions and highly siderophile element abundances of these samples, however, are inconsistent with this interpretation. Tungsten concentrations for the Baffin Bay and Ontong Java Plateau samples range from 23 ppb to 62 ppb, and are negatively correlated with their ¹⁸²W/¹⁸⁴W ratios. We propose that the source reservoirs for these flood basalts likely formed through Hf/W fractionation caused by core-forming events occuring over a protacted time interval during Earth’s initial growth, and seem to have survived in the mantle to the present.

[1] Mukhopadhyay, 2012, Nature. [2] Tucker and Mukhopadhyay, 2014, EPSL. [3] Starkey et al., 2009, EPSL [4] Jackson and Carlson, 2011, Nature. [5] Touboul et al., 2012, Science. [6] Willbold et al., 2011, Nature.