Assessment of relative Ti, Ta, and Nb (TITAN) enrichments in ocean island basalts

Abstract

The sensitivity of trace element concentrations to processes governing solid-melt interactions has made them valuable tools for tracing the effects of partial melting, fractional crystallization, metasomatism, and similar processes on the composition of a parental melt. Recent studies of ocean island basalts (OIB) have sought to correlate Ti, Ta, and Nb (TITAN) anomalies to isotopic tracers, such as 3 He/ 4 He and 187 Os/ 188 Os ratios, which may trace primordial deep mantle sources. A new compilation of global OIB trace element abundance data indicates that positive TITAN anomalies, though statistically pervasive features of OIB, may not be compositional features of their mantle sources. OIB show a range of Ti (Ti/Ti* 5 0.28-2.35), Ta (Ta/Ta* 5 0.11-93.4), and Nb (Nb/Nb* 5 0.13-17.8) anomalies that show negligible correlations with 3 He/ 4 He ratios, indicating that TITAN anomalies are not derived from the less-degassed mantle source traced by high-3 He/ 4 He. Positive TITAN anomalies can be modeled using variable degrees (0.1-10%) of nonmodal batch partial melting of garnet-spinel lherzolite at temperatures and pressures considered typical for OIB petrogenesis, and subjecting this partial melt to fractional crystallization and assimilation of mid-ocean ridge basalt-like crust (AFC). Correlations of TITAN anomalies with modal abundances of olivine and clinopyroxene in porphyritic Canary Islands lavas provide empirical support for this process and indicate that high abundances of these phases in OIB may create misleading trace element anomalies on primitive mantle-normalized spider diagrams. Because partial melting and AFC are common to all mantle-derived magmas, caution should be used when attributing TITAN anomalies to direct sampling of recycled or deep mantle sources by hotspots.

Osmium isotope evidence for a heterogeneous 3He/4He mantle plume beneath the Juan Fernandez Islands

“…These new data expand upon -and are in excellent agreement with -previously published Juan Fernandez datasets (Baker et al, 1987;Farley et al, 1993;Truong et al, 2018). Juan Fernandez lavas show enrichment relative to primitive mantle in high field strength elements (HFSE) Ti, Ta and Nb, as reported by Truong et al (2018), which are common features of most OIB (Jackson et al, 2008;Peters and Day, 2014). The majority of Robinson Crusoe and Alexander Selkirk main shield lavas show positive anomalies in Nb, Ta, Zr, and Sr, and slight negative anomalies in Y.…”

A proposed new approach and unified solution to old Pb paradoxes

“…For example, the distinctive Pb enrichment in arc lavas is counterbalanced by Pb depletion in oceanic basalts. More relevant to the present study is the hallmark depletion in some HFSE, particularly Nb, in arc lavas is counterbalanced by enrichment of these elements, or positive 'TITAN anomalies' (Jackson et al, 2008) that are pervasive in OIB (Peters and Day, 2014). What is also being advocated here is that although carbonatitic silicate melts have the bulk (Jenner and O'Neill, 2012) and ALL MORB (Gale et al, 2013).…”

The Jurassic magmatism of the Demerara Plateau (offshore French Guiana) as a remnant of the Sierra Leone hotspot during the Atlantic rifting

“…These patterns are common in Oceanic Island Basalts (OIB). Even if TITAN anomalies may be accounted for by partial melting, www.nature.com/scientificreports www.nature.com/scientificreports/ subsequent differentiations and crustal assimilations rather than deep source compositions 22 , they may still be indicative of a hotspot origin for the parental magma.…”