|
|||||
|
|
Depths of Partial Crystallization of H2O-bearing MORB: Phase Equilibria Simulations of Basalts at the MAR near Ascension Island (7 11{degrees}S) |
|
| Authors: | Almeev Renat; Holtz Francois; Koepke Jurgen; Haase Karsten; Devey Colin |
| Institution: | 1Institute of Mineralogy, Leibniz University Of Hannover, Callinstrasse 3, 30167, Germany 2Department of Earth Sciences, University of Aarhus, C. F. Møllers Allé 110, DK-8000 Aarhus C, Denmark 3leibniz Institute For Marine Sciences (IFM-GEOMAR), WISCHHOFSTRASSE. 1–3, D-24148 Kiel, Germany |
| Abstract: | Phase equilibria simulations were performed on naturally quenchedbasaltic glasses to determine crystallization conditions priorto eruption of magmas at the Mid-Atlantic Ridge (MAR) east ofAscension Island (7–11°S). The results indicate thatmid-ocean ridge basalt (MORB) magmas beneath different segmentsof the MAR have crystallized over a wide range of pressures(100–900 MPa). However, each segment seems to have a specificcrystallization history. Nearly isobaric crystallization conditions(100–300 MPa) were obtained for the geochemically enrichedMORB magmas of the central segments, whereas normal (N)-MORBmagmas of the bounding segments are characterized by polybariccrystallization conditions (200–900 MPa). In addition,our results demonstrate close to anhydrous crystallization conditionsof N-MORBs, whereas geochemically enriched MORBs were successfullymodeled in the presence of 0·4–1 wt% H2O in theparental melts. These estimates are in agreement with direct(Fourier transform IR) measurements of H2O abundances in basalticglasses and melt inclusions for selected samples. Water contentsdetermined in the parental melts are in the range 0·04–0·09and 0·30–0·55 wt% H2O for depleted and enrichedMORBs, respectively. Our results are in general agreement (within±200 MPa) with previous approaches used to evaluate pressureestimates in MORB. However, the determination of pre-eruptiveconditions of MORBs, including temperature and water contentin addition to pressure, requires the improvement of magma crystallizationmodels to simulate liquid lines of descent in the presence ofsmall amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation |
| Keywords: | : MORB Mid-Atlantic Ridge depth of crystallization water abundances phase equilibria calculations cotectic crystallization pressure estimates polybaric fractionation |
| 本文献已被 Oxford 等数据库收录！ | |