In this longer-format paper we examine in detail the record of basaltic rocks preserved in the continental crust, building on the weighted bootstrap resampling procedure of Keller and Schoene, 2012. A range of rapid trace element variations in the basaltic record can be explained as a result of mantle melting systematics (changing partition coefficients as a result of Grt and Cpx-out). Both trace element signatures of slab fluid input and major element signatures of calc-alkaline vs tholeiitic differentiation are remarkably stable and consistently arc-like, strongly suggesting the occurence of subduction and plate tectonics throughout the preserved rock record (back to at least 3.85 Ga). While some non-plate tectonic models may produce some flux melting, a virtually constant proportion of flux to decompression melting in the preserved continental record is a tall order for any non-plate model for Archean tectonics.
Basaltic magmas constitute the primary mass flux from Earth’s mantle to its crust, carrying information about the conditions of mantle melting through which they were generated. As such, changes in the average basaltic geochemistry through time reflect changes in underlying parameters such as mantle potential temperature and the geodynamic setting of mantle melting. However, sampling bias, preservation bias, and geological heterogeneity complicate the calculation of representative average compositions. Here we use weighted bootstrap resampling to minimize sampling bias over the heterogeneous rock record and obtain maximally representative average basaltic compositions through time. Over the approximately 4 Ga of the continental rock record, the average composition of preserved continental basalts has evolved along a generally continuous trajectory, with decreasing compatible element concentrations and increasing incompatible element concentrations, punctuated by a comparatively rapid transition in some variables such as La/Yb ratios and Zr, Nb, and Ti abundances approximately 2.5 Ga ago. Geochemical modeling of mantle melting systematics and trace element partitioning suggests that these observations can be explained by discontinuous changes in the mineralogy of mantle partial melting driven by a gradual decrease in mantle potential temperature, without appealing to any change in tectonic process. This interpretation is supported by the geochemical record of slab fluid input to continental basalts, which indicates no long-term change in the global proportion of arc versus non-arc basaltic magmatism at any time in the preserved rock record.
Keller, C.B. & Schoene, B. (2018). Plate tectonics and continental basaltic geochemistry throughout Earth history. Earth and Planetary Science Letters 481, 290-304.