Browsing by Author "Mallik, Ananya"

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    Effect of variable CO2 on eclogite-derived andesite and lherzolite reaction at 3 GPa—Implications for mantle source characteristics of alkalic ocean island basalts
    (Wiley, 2014) Mallik, Ananya; Dasgupta, Rajdeep
    We have performed reaction experiments between 1, 4, and 5 wt % CO2-bearing MORB-eclogite (recycled oceanic crust)-derived low-degree andesitic partial melt and fertile peridotite at 1375°C, 3 GPa for infiltrating melt fractions of 25% and 33% by weight. We observe that the reacted melts are alkalic with degree of alkalinity or Si undersaturation increasing with increasing CO2 content in reacting melt. Consequently, an andesite evolves through basanite to nephelinite owing to greater drawdown of SiO2 from melt and enhanced precipitation of orthopyroxene in residue. We have developed an empirical model to predict reacted melt composition as a function of reacting andesite fraction and source CO2 concentration. Using our model, we have quantified the mutual proportions of equilibrated melt from andesite-peridotite (+ CO2) hybridization and subsequent peridotite (± CO2)-derived melt required to produce the major element composition of various ocean island basalts. Our model can thus be applied to characterize the source of ocean islands from primary alkalic lava composition. Accordingly, we determined that average HIMU source requires 24 wt % of MORB-eclogite-derived melt relative to peridotite containing 2 wt % CO2 and subsequent contribution of 45% of volatile-free peridotite partial melt. We demonstrate that mantle hybridization by eclogite melt-peridotite (± CO2) reaction in the system can produce high MgO (>15 wt %) basaltic melts at mantle potential temperature (TP) of 1350°C. Therefore, currently used thermometers to estimate TP using MgO content of primary alkalic melts need to be revised, with corrections for melt-rock reaction in a heterogeneous mantle as well as presence of CO2.
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    Experimental investigation of crust-mantle hybridization in the Earth’s shallow upper mantle
    (2014-11-13) Mallik, Ananya; Dasgupta, Rajdeep; Lee, Cin-Ty; Lenardic, Adrian; Brooks, Philip R
    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.
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    Reactive Infiltration of MORB-Eclogite-Derived Carbonated Silicate Melt into Fertile Peridotite at 3GPa and Genesis of Alkalic Magmas
    (Oxford University Press, 2013) Mallik, Ananya; Dasgupta, Rajdeep
    We performed experiments between two different carbonated eclogite-derived melts and lherzolite at 1375°C and 3 GPa by varying the reacting melt fraction from 8 to 50 wt %. The two starting melt compositions were (1) alkalic basalt with 11·7 wt % dissolved CO2 (ABC), (2) basaltic andesite with 2·6 wt % dissolved CO2 (BAC). The starting melts were mixed homogeneously with peridotite to simulate porous reactive infiltration of melt in the Earth’s mantle. All the experiments produced an assemblage of melt + orthopyroxene + clinopyroxene + garnet ± olivine; olivine was absent for a reacting melt fraction of 50 wt % for ABC and 40 wt % for BAC. Basanitic ABC evolved to melilitites (on a CO2-free basis, SiO2 ∼27–39 wt %, TiO2 ∼2·8–6·3 wt %, Al2O3 ∼4·1–9·1 wt %, FeO* ∼11–16 wt %, MgO ∼17–21 wt %, CaO ∼13–21 wt %, Na2O ∼4–7 wt %, CO2 ∼10–25 wt %) upon melt–rock reaction and the degree of alkalinity of the reacted melts is positively correlated with melt–rock ratio. On the other hand, reacted melts derived from BAC (on a CO2-free basis SiO2 ∼42–53 wt %, TiO2 ∼6·4–8·7 wt %, Al2O3 ∼10·5–12·3 wt %, FeO* ∼6·5–10·5 wt %, MgO ∼7·9–15·4 wt %, CaO ∼7·3–10·3 wt %, Na2O ∼3·4–4 wt %, CO2 ∼6·2–11·7 wt %) increase in alkalinity with decreasing melt–rock ratio. We demonstrate that owing to the presence of only 0·65 wt % of CO2 in the bulk melt–rock mixture (corresponding to 25 wt % BAC + lherzolite mixture), nephelinitic-basanite melts can be generated by partial reactive crystallization of basaltic andesite as opposed to basanites produced in volatile-free conditions. Post 20% olivine fractionation, the reacted melts derived from ABC at low to intermediate melt–rock ratios match with 20–40% of the population of natural nephelinites and melilitites in terms of SiO2 and CaO/Al2O3, 60–80% in terms of TiO2, Al2O3 and FeO, and <20% in terms of CaO and Na2O. The reacted melts from BAC, at intermediate melt–rock ratios, are excellent matches for some of the Mg-rich (MgO >15 wt %) natural nephelinites in terms of SiO2, Al2O3, FeO*, CaO, Na2O and CaO/Al2O3. Not only can these reacted melts erupt by themselves, they can also act as metasomatizing agents in the Earth’s mantle. Our study suggests that a combination of subducted, silica-saturated crust–peridotite interaction and the presence of CO2 in the mantle source region are sufficient to produce a large range of primitive alkalic basalts. Also, mantle potential temperatures of 1330–1350°C appear sufficient to produce high-MgO, primitive basanite–nephelinite if carbonated eclogite melt and peridotite interaction is taken into account.