Experimental investigation of crust-mantle hybridization in the Earth’s shallow upper mantle
Doctor of Philosophy
Chemical heterogeneities in the Earth’s mantle, such as subducted sediments and oceanic crust, along with volatiles such as H2O and CO2 affect melting processes, hence, chemical differentiation of the Earth and their presence in the source of erupted magma has been unequivocally established through isotope and trace element geochemistry. Yet, the nature of major element contribution of recycled crustal lithologies to the erupted basalts on the Earth’s surface is poorly understood because direct partial melting of crustal lithologies at mantle depths produces siliceous melts that are unlike surface basalts or their estimated parental melts. In case of oceanic crust and sediments, partial melting initiates at lower temperatures and at deeper depths than the surrounding mantle, hence, an andesitic partial melt (±CO2) from recycled oceanic crust and a rhyolitic partial melt (±H2O) from subducted sediments, being out of equilibrium with the surrounding peridotitic mantle with a hotter solidus temperature, must undergo reactive crystallization. However, the impact of crustal melt impregnation into mantle peridotite on the potential formation of hybrid melts and lithologies remained largely uninvestigated. The phase equilibria of reaction of siliceous partial melts (derived from crustal heterogeneities) with the mantle has been investigated in this thesis with the aid of high pressure-temperature laboratory experiments that simulated conditions at depths of 80 – 100 km inside the Earth. Andesite evolves to a basanite upon partial reactive crystallization in a peridotite matrix (Chapter 2), and with increasing amount of CO2 in the system, the residual melt evolves even to a nephelinite (Chapter 3 and 4). This is the effect of reaction of the silica component in the melt with olivine in the peridotite to crystallize orthopyroxene, with the orthopyroxene stability field being enhanced under the influence of CO2, therefore, drawing down the SiO2 content of the reacted melt even further. Major element characteristics of alkalic ocean island basalts can be reproduced by the reacted melts from these studies by a two-stage hybridization process: Firstly, partial melt from recycled oceanic crust reacts with surrounding sub-solidus peridotite and undergoes partial reactive crystallization and secondly, the reacted, residual melt from the first step subsequently mixes with peridotite-derived partial melt. An empirical model has been proposed to estimate the source characteristics of alkalic ocean island basalts. The model predicts that 15 – 45 wt.% oceanic-crust derived melt and 0.2 – 2 wt.% CO2 are required, followed by mixing with 25 – 55 wt.% peridotite partial melt to reproduce major element characteristics of alkalic lavas from Canary Islands, Cape Verde and Cook Australs (Chapter 4). The results from the studies obviate the need for the presence of silica-undersaturated exotic lithologies in the source of alkalic ocean island basalts. Also, the studies demonstrate that high MgO (>15 wt.%) alkalic basalts from the mantle can be produced by a potential temperature of 1350 °C and do not require potential temperatures exceeding 1430 °C, as predicted by current thermometers. This is owing to the effect of CO2 dissolution in the melt in the form of MgCO3 complexes, which enhances the MgO content of melts at a given pressure and temperature. Flux of hydrous rhyolitic, sediment-derived melts, to the mantle wedge fertile peridotite leads rhyolites to evolve to ultrapotassic nepheline normative basalts similar in composition to ultrapotassic lavas from active and inactive arcs (Chapter 5). This evolution in melt composition from a highly siliceous rhyolite to a nepheline-normative ultrapotassic basalt is due to the formation of orthopyroxene at the expense of olivine as well as the dominance of phlogopite in the melting systematics, buffering the K2O content of the melt to produce ultrapotassic compositions. Thermal stability of phlogopite to the core of hot mantle wedge is established in conjunction with previous studies, which suggests that recycling of phlogopite to the deeper mantle may be important in deep flux of large ion lithophile elements and volatile elements such as fluorine and nitrogen. Potential long-term survival of phlogopite can potentially create Sr-isotopically enriched zones in the mantle, as evident in the source of several arc and intraplate lavas.
mantle melting; chemical heterogeneity, basalt petrogenesis; experimental petrology; phase equilibria