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Interaction of granitic and basic magmas: experimental observations on contamination processes at 10 kbar with H_2O

Johnston, A. Dana and Wyllie, Peter J. (1988) Interaction of granitic and basic magmas: experimental observations on contamination processes at 10 kbar with H_2O. Contributions to Mineralogy and Petrology, 98 (3). pp. 352-362. ISSN 0010-7999. doi:10.1007/BF00375185.

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Experiments designed to simulate the interaction of juxtaposed rhyolitic and basic magmas were conducted at 10 kbar with H_2O, using reaction-couples consisting of Westerly granite (WG) against basalt (DW-1) and WG against a synthetic mafic glass (SMG, enriched in MgO and Na_2O relative to DW-1). Each couple was run with ∼5 and ∼10 wt% H_2O corresponding respectively to H_2O-undersaturated and H_2O-oversaturated conditions. Experiments were run for 42–44 h at 920° C, above the liquidus of WG and within the melting intervals of DW-1 and SMG. WG was run above the basic material in all but one experiment. The composition of the granitic melt was altered only through material exchange with the adjacent basic melts, whereas that of the basic melts also changed (relative to the bulk basic composition) due to partial crystallization. Some crystallization also occurred within the zone of interaction. For control, the basic compositions were also run alone under the same conditions as the reaction-couple experiments. The crystalline phase assemblages in the basic ends of the coupled experiments differed from those produced from the basic materials alone, demonstrating interaction with the granite melt. Moreover, compositional gradients within the basic ends of coupled experiments are indicated by changes in phase assemblage and compositions with distance from the interface with WG. Microprobe analyses of glass collected along the length of the capsules confirm published observations that alkali diffusion is very fast: K_2O and Na_2O homogenized throughout the capsules in less than the two-day run times. This, coupled with the fact that introduction of K_2O into SMG stabilized biotite, produced the result that after interaction the bulk basic material (melt+crystals) contained more K_2O than the coexisting felsic melt. Only very gentle gradients for CaO, FeO, and MgO are preserved in our experiments, in contrast with published anhydrous results, suggesting that the difference in activity coefficients for these components between basic and felsic melts is reduced by the introduction of H_2O. Gradients for SiO_2 and Al_2O_3 are of comparable length to those of the divalent cations, confirming earlier results that the diffusivities of the network-formers limit the rate of diffusion of Ca, Fe, and Mg.

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Alternate Title:Interaction of granitic and basic magmas: experimental observations on contamination processes at 10 kbar with H2O
Additional Information:© 1988 Springer-Verlag. Received June 5, 1987; accepted November 5, 1987. This research was supported by the Earth Science Section of the National Science Foundation, NSF Grant EAR 850857. We thank M.R. Carroll, P. Keleman and EB Watson for critical reviews of the manuscript. Caltech Division of Geological and Planetary Sciences Contribution 4554.
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NSFEAR 850857
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Caltech Division of Geological and Planetary Sciences4554
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ID Code:64679
Deposited By: Tony Diaz
Deposited On:23 Feb 2016 21:27
Last Modified:10 Nov 2021 23:34

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