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Melting of Plagioclase + Spinel Lherzolite at Low Pressures

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Melting of Plagioclase + Spinel Lherzolite at Low Pressures JOURNAL OF PETROLOGY VOLUME 51 NUMBER 11 PAGES 2349^2376 2010 doi:10.1093/petrology/egq060 An Experimental Study of Liquid Compositions in Equilibrium with Plagioclase þ Spinel Lherzolite at Low Pressures (0·75 GPa) FRANC°OISE CHALOT-PRAT1*, TREVOR J. FALLOON2, DAVID H. GREEN2,3 AND WILLIAM O. HIBBERSON3 1CRPG-CNRS/NANCY UNIVERSITE¤ , 15 RUE NOTRE DAME DES PAUVRES, BP20, 54501 VANDOEUVRE LES NANCY, FRANCE 2ARC CENTRE OF EXCELLENCE IN ORE DEPOSITS AND SCHOOL OF EARTH SCIENCES, UNIVERSITY OF TASMANIA, Downloaded from PRIVATE BAG 79, HOBART, TASMANIA 7001, AUSTRALIA 3RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, ACT 0200, AUSTRALIA petrology.oxfordjournals.org RECEIVED OCTOBER 19, 2009; ACCEPTED SEPTEMBER 8, 2010 Models of formation of basaltic crust at mid-ocean ridges by adia- 85 and 95) and with Cr/(Cr þAl).The compositional vectors are batic upwelling of fertile mantle lherzolite require knowledge of consistent with effects observed in the end-member simple systems phase relations and phase compositions during melting at appropri- (FCMAS) and (CrCMAS).In comparing liquids at the anorthite ate pressures. Spinel þ plagioclase lherzolites are found among peri- end of the five-phase þ liquid cotectic with those on the Cr-free dotite samples from the ocean floor and ophiolitic exposures. At low CMAS four-phase þ liquid cotectic at 0·75 GPa, it is evident that by guest on October 21, 2010 pressure (51·2 GPa) the five-phase assemblage (olivine þ orthopyr- the presence of Al-rich Cr^Al spinel shifts liquid compositions to oxene þ clinopyroxene þ plagioclase þ spinel) is present at the more silica-rich and silica-oversaturated composition.These experi- anhydrous lherzolite solidus. New experimental data on mineral mentally defined melt compositions in equilibrium with plagio- and melt compositions, at 0·75 GPa and 1140^12608Cinthe clase þ spinel lherzolite are unlike quenched glasses from mid-ocean (Cr þ Na þ Fe þ Ca þ Mg þ Al þ Si) system, demonstrate ridge settings. The data do not support models of mantle upwelling smooth covariant relationships between oxides for melt compositions at low potential temperature (12808C) that produces low melt frac- and in partition relationships both between mineral pairs and be- tions at low pressures, leaving residual plagioclase þ spinel lherzo- tween minerals and melts. Molecular normative projections demon- lite. The detailed mineral compositional data at the solidus provide strate that liquids on the five-phase þ liquid cotectic occupy a a template for comparison with natural plagioclase þ spinel narrow compositional range. Of the mineral solid solutions that con- lherzolites refertilized by porous reactive flow. trol the liquid composition, the [Ca/(Ca þ Na)] or anorthite/ albite content of the plagioclase is dominant and liquids vary from silica undersaturated and nepheline-normative at the sodic (oligo- KEY WORDS: experiments; lherzolite; low pressure; MORB; clase) end to orthopyroxene and quartz-normative at the calcic plagioclase; spinel (anorthite) end of the cotectic. Spinel (Cr^Al) solid solution has limited variation on the five-phase þ liquid cotectic. It is very Cr-rich at the sodic end and has limited compositional variation INTRODUCTION from 50 to 20 in Cr/(Cr þAl) at the anorthitic end. With fixed This study is a contribution towards understanding the plagioclase composition on the cotectic, melt compositions show melting behaviour of the Earth’s upper mantle, focused on small compositional shifts with Fe^Mg (at Mg# between liquid compositions formed at low pressure (0·75 GPa), at ß The Author 2010. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ *Corresponding author. E-mail: [email protected] oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 51 NUMBER 11 NOVEMBER 2010 near-solidus temperatures, from plagioclase þ spinel shallow mantle source. In addition, the inference of close lherzolites. In widely used models of melt extraction and coupling between mechanical extension characteristics lithosphere formation at mid-ocean ridges (MORs), as- (spreading rates, lithospheric mantle exhumation and thenospheric mantle of lherzolite composition [Primitive related distribution of magma conduits at the axial zone) Modern Mantle (PMM) or MOR Pyrolite] is conceived and eruptive process (magma emplacement and volcano as passively upwelling in response to lithospheric plate formation at the axial zone) also suggests shallow rather stretching and rupture (McKenzie & Bickle, 1988). If the than deep levels of mantle upwelling and magma broad-scale upwelling approaches adiabatic conditions, separation. then the upwelling lherzolite will cross the anhydrous sol- Natural occurrences of plagioclase lherzolite have been idus of (C þ H)-free lherzolite or pass from the incipient reviewed by Piccardo et al. (2007, and references therein), melting regime [small liquid fraction controlled by Mu« ntener et al. (2010) and Borghini et al. (2010). These stu- 2^ ^ (C þ H) concentration, i.e. CO3 and OH solubility] dies have advocated the production of plagioclase lherzo- into the major melting regime of lherzolite þ (C, H, O) lite in the uppermost lithosphere by invasive percolation (Green & Falloon, 1998). The experimentally determined of basaltic melt into pre-existing high-temperature residual solidi of volatile-free lherzolite compositions exhibit a peridotite. Piccardo et al. (2007) and Mu« ntener et al. change in slope (increase of dT/dP) at pressures from (2010) described examples of melt^rock reaction or ‘referti- Downloaded from 0·7to1·3 GPa where plagioclase disappears from the sol- lization’ and considered that magma storage in the idus mineral assemblage (Green & Falloon, 1998).The role uppermost mantle in this way is a major process at of this ‘cusp’ on the solidus as a controlling feature in loca- magma-poor rifted margins and at magma-starved lizing mantle melting has been debated (Presnall & mid-ocean ridge segments. Borghini et al. (2010) empha- Hoover, 1987; Asimow et al., 2001; Presnall et al., 2002) and sized two origins for natural plagioclase lherzolites; that used in modelling of upwelling to argue for relatively low is, either by metamorphic recrystallization during decom- petrology.oxfordjournals.org mantle potential temperatures beneath mid-ocean ridges pression of spinel lherzolite at low pressure and high tem- (McKenzie & Bickle, 1988; Presnall et al., 2002). perature, or by basaltic melt impregnation into high- Estimates of upper mantle composition concur at lher- temperature residual peridotite at low pressure and conse- zolite with 3^4% CaO and 3^4% Al2O3 (Hart & quent reaction and crystallization. In these interpret- Zindler, 1986; Green & Falloon, 1998). Using natural or ations, the role of melting of plagioclase lherzolite has normative mineral compositions and mixing calculations, relevance to understanding the chemical evolution of it can be shown that such compositions have 10% modal a basaltic melt permeating and reacting with lherzolite plagioclase at low-pressure and high-temperature condi- that includes the five-phase mineralogy [olivine by guest on October 21, 2010 tions. Extraction of 510^12% melt fractions, whether by (Ol) þ enstatite (Opx) þ diopside (Cpx) þ plagioclase batch or fractional melting processes, will leave depleted (Plag) þ spinel (Sp)].Van Den Bleeken et al. (2010) have re- lherzolite compositions including residual plagioclase at cently investigated the reactive porous flow concept by ex- low pressure. Some geophysical models of mid-ocean perimentally passing basaltic melts through residual ridge upwelling relate ocean crust thickness to mantle po- spinel lherzolite layers at 0·8 GPa. In choosing a pressure tential temperature and suggest average melt depletion by of 0·8 GPa for their study, those researchers aimed to simu- 5^7% melting in the mantle column beneath normal late conditions within the transition zone or thermal oceanic crust of 510km thickness (McKenzie & Bickle, boundary layer between near-adiabatic upwelling mantle 1988). If fertile lherzolite compositions are preserved to during lithospheric thinning and the thermal conductive shallow depths in upwelling mantle, particularly at regime of the thinned lithosphere. The experiments mantle potential temperatures of 5130 0 8C, then the re- demonstrated melt^residue interaction and chemical evo- sidual lherzolite mineralogy from single-stage melting at lution of both lherzolite and melt compositions, com- depths 530 km will include plagioclase. monly with mineral zoning, except at the highest Detailed studies of recent and fossil spreading ridge set- temperatures and melt fractions. A few of their runs con- tings, including field, petrographical and geochemical ob- tained the five-phase plagioclase þ spinel lherzolite assem- servations, have led some workers (Stakes et al., 1984; blage and demonstrated a role for this mineral assemblage Chalot-Prat, 2005) to infer a shallow depth of melt segrega- in controlling the composition of melts at near-solidus tion from upwelling residual mantle. Sets of small volca- conditions. The present investigation complements the noes formed at the ridge axis show eruption of both work of Van Den Bleeken et al. (2010) by determining the parental magmas, interpreted as partial melts of a compositions of equilibrium liquids at the P,T conditions depleted asthenospheric mantle, and differentiates of of interest. such magmas. Rapid alternation between
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