Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.v22h..05n&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #V22H-05
Physics
3035 Midocean Ridge Processes, 3040 Plate Tectonics (8150, 8155, 8157, 8158), 3640 Igneous Petrology, 3655 Major Element Composition, 8434 Magma Migration
Scientific paper
Abyssal peridotites (AP) are mantle melting residues for mid-ocean ridge basalts (MORB) [1-4]. Recent studies [5-8] have confirmed the complementary relationship between MORB and AP, but also revealed the hidden complexities in these peridotites such as olivine addition [5-7] and melt refertilization [7,9,10]. These same studies [5,6] have immediately excited serious debates on AP petrogenesis [11-15]. These debates are stimulating and healthy towards an improved understanding of ocean ridge magmatic processes. However, a straightforward interpretation [5,6] has been widely misunderstood because of the influence of [13]. A clarification is necessary. Niu, Langmuir and Kinzler [5] showed that a positive FeO-MgO trend exists in bulk-rock AP samples reconstructed using mineral modes and compositions [3,4]. Such a positive trend is inconsistent with AP being simple melting residues, but consistent with AP being melting residues plus excess olivine [5,6]. Using their site averages of reconstructed bulk-rock AP data, Baker and Beckett [13] countered that the positive FeO-MgO correlation by Niu et al. [5] is an artifact and there is no evidence for significant olivine accumulation in AP. The clarification here includes the following valid statements: (1) the site averages by [13] simply cannot be derived from their own unaveraged data; (2) the unaveraged data by [13], as expected, do show a positive FeO-MgO trend as shown by [5,6]; (3) the positive FeO-MgO trend is not an artifact as this trend is also clear in the unaveraged data of [13]; (4) excess olivine is already evident in the original modal data [3,4]; and (5) olivine addition is observed petrographically [7]. Therefore, the positive FeO-MgO trend defined by bulk-rock AP samples is characteristic of AP. Addition of olivine in AP is a consequence of cooling of ascending melts migrating through advanced residues in the "cold" thermal boundary layer beneath ocean ridges [5,6]. This is consistent with bulk-rock trace element data; excess olivine correlates with elevated abundances of incompatible elements (trapped melts) [7]. As olivine addition and incompatible element enrichments are observed on thin-section scales, AP record porous melt migration although dunite channels could be volumetrically more important in melt transport [16]. Massif and ophiolitic peridotites should differ from AP if they did not have the same histories as AP had beneath ocean ridges. References: [1] Dick et al., Earth Planet. Sci. Lett., 69, 88-106, 1984; [2] Michael and Bonatti, Earth Planet. Sci. Lett., 73, 91-l04, 1985; [3] Dick, Geol. Soc. Lon. Spec. Pub., 42, 71-105, 1989; [4] Johnson et al., J. Geophys. Res., 95, 2661-2678, 1990; [5] Niu et al., Earth Planet. Sci. Lett., 152, 251-265, 1997; [6] Niu, J. Petrol., 38, 1047-1074, 1997; [7] Niu and Hékinian, Earth Planet. Sci. Lett., 146, 243-258, 1997; [8] Niu and Hékinian, Nature, 385, 326-329, 1997; [9] Elthon, J. Geophys. Res., 97, 9015-9025, 1992; [10] Niu and Greig, Eos Trans. AGU, Suppl., 81 (48), F1282, 2000; [11] Walter, J. Petrol., 40, 1187-1193, 1999; [12] Niu, J. Petrol., 40, 1195-1203, 1999; [13] Baker and Beckett, Earth Planet. Sci. Lett., 171, 49-61, 1999; [14] Asimow, Earth Planet. Sci. Lett., 169, 303-319, 1999; [15] Lundstrom, Nature, 403, 527-530, 2000; [16] Kelemen et al., Nature, 375, 747-735, 1995.
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