A common precursor for global hotspot lavas

Abstract

Hotspot lavas exhibit chemical heterogeneity, much of which is ascribed to heterogeneous deep mantle sources that contain various components with distinct composition, origin and age. However, characterizing primary melt compositions and mantle heterogeneity directly is challenging. Here we investigate a global dataset of hotspot lavas to constrain the incompatible-element composition of their parental melts and sources. Trace-element ratios indicate that the compositional heterogeneity of global hotspot lavas is not primary, but reflects processes that hotspot melts undergo as they ascend to the surface. We find the parental melts of these lavas, as well as of kimberlites and basalts from large igneous provinces, to be uniform in their elemental, and radiogenic and noble-gas isotope, composition. We suggest that the parental melts to all of these lavas derive from a depleted and outgassed mantle reservoir that was replenished with incompatible element-enriched material during the Archaean. This interpretation explains the elemental, radiogenic and noble-gas isotope compositions of hotspot lavas without requiring a heterogeneous lower mantle or the long-term survival of undegassed relics from a primordial Earth.

Keywords: Geochemistry; Geology; Petrology.

Fig. 1. Ni–Nb–Cr systematics of OIB.

a,b, Nb/Fe (a) and Yb/Hf (b) versus Nb/Cr for isotopically enriched OIB distal (>1,000 km) and proximal (<1,000 km) to continental shelves, for OIB from plumes with high buoyancy flux, and for kimberlites and mantle xenoliths. Isl., Islands. The data for ‘other basalts’ in a represent non-hotspot basalts from continental rifts, within-plate settings and active margins. The inset in a shows the relative compositional shifts in melts with starting composition C_l caused by olivine (ol), clinopyroxene (cpx) and amphibole (amp) fractionation (fractional crystallization or crystallization during melt–rock interaction; vector shows 30% to scale of panel a). Partition coefficients and further information on melt modelling are presented in Supplementary Table 2 and Supplementary Note 1, respectively. The composition of L*, constrained through the procedure in Supplementary Note 2, is provided in Supplementary Table 1. Average PM as well as DM and MORB compositions are shown for reference. Diagrams for individual hotspots are provided in Supplementary Fig. 2.

Fig. 2. Isotope systematics of OIB.

a,b, ¹⁴³Nd/¹⁴⁴Nd (a) and ³He/⁴He (b) ratio (R) normalized to the atmospheric ratio R_a (1.38 × 10⁻⁶) versus Ni for OIB. Enriched Nd isotope values and relatively high and low ³He/⁴He values progressively occur in more Ni-poor basalts. Other radiogenic isotope ratios, as well as associated trace-element ratios, show similar systematics (Supplementary Fig. 5). Similar results are obtained using MgO given that Ni and MgO are strongly correlated (Supplementary Note 1). The data reflect increased elemental, and radiogenic and He isotope heterogeneity for more fractionated lavas. The symbols are as in Fig. 1.

Fig. 3. Trace-element diagram showing the composition of L* and DM*.

Incompatible element (X) compositions of MORB, the upper, middle and bulk continental crust (BCC) and DM are shown for reference. The composition of L* was estimated following the procedure in Supplementary Note 2 and is provided in Supplementary Table 1. The error band shows 2 s.d. around the mean. The composition of DM* is estimated from L* at different degrees of melting (numbers are per cent melting; 2–10%), using partition coefficients provided in Supplementary Table 3 and assuming a source with 60% olivine, 25% orthopyroxene, 20% clinopyroxene and 5% garnet (representative of Hawaiʻian mantle). All data are normalized to PM. Background information, including DM* compositions calculated for other source assemblages, are provided in Supplementary Note 3.

Fig. 4. Block diagram for the formation of L* and its DM* source.

The diagram illustrates the formation of L* from DM* and the chemical differentiation of L* in the uppermost mantle to form isotopically diverse OIB, kimberlites and basalts from large igneous provinces (LIP). The assimilation in basalts from high-buoyancy-flux plumes involves DM (Iceland) and MORB (other) materials.