Characterization of spinel peridotites by olivine-spinel ... - Science Direct

Chemical Geology, 113 ( 1994 ) 191-204 Elsevier Science B.V., Amsterdam

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Characterization of spinel peridotites by olivine-spinel compositional relationships: Review and interpretation Shoji Arai Department of Earth Sciences, Kanazawa University, Kanazawa, Ishikawa 920-11, Japan (Received April 27, 1992; revised and accepted August 3, 1993)

ABSTRACT A comprehensive review on igneous petrological characteristics of mantle-derived spinel peridotites was made on the basis of their olivine-spinel compositional relationships. The spinel peridotites (harzburgites and lherzolites), of both massif and xenolithic derivations, plot in a narrow band, the olivine-spinelmantle array, in terms of Fo of olivine and C # (Cr/(Cr + A1) atomic ratio) of chromian spinel. The Cr # of spinel grows rapidly within the olivine-spinelmantle array with a slight increase of Fo. Their Cpx/(Opx + Cpx ) volume ratio decreases towards the high-Fo, high-Cr # end; it is 0.1 for Cr # = 0.5-0.6. Peridotite suites from each tectonic setting lie in a particular part of the olivine-spinelmantle array. The olivine-spinel mantle array possibly consists of integrated Fo-Cr# residual trends formed at various conditions (Ptou~ and Pn2o ). Cr# of residual spinel coexisting with olivine of a particular Fo increases with a decrease of Ptot~Jand/ or with an increase of Pmo on partial melting. Origins of peridotite suites can be constrained to some extent in terms of the Fo-Cr# residual trends, for example, some of the Japan-arc mantle peridotites and fore-arc peridotites are low-pressure and/or hydrous restites, and subcontinental and oceanic hot-spot peridotites are high-pressure and/or anhydrous restites. Fertile alpine-type lherzolites with Cr # ~ 0.1 are of subcontinental origin. Other alpine lherzolites are most frequently of sub-arc origin, sometimes of sub-ocean floor origin, and rarely of sub-continental origin. Most of the alpinetype harzburgites are of fore-arc origin.

1. Introduction Spinel peridotites are interpreted to be mantle restites (e.g., Dick and BuUen, 1984; Arai, 1987) formed in relatively low-pressure regions of the upper mantle (e.g., Kushiro and Yoder, 1966; Green and Ringwood, 1967). Refractory low-Ca,A1 peridotites, however, can have modal chromian spinel both in the plagioclase lherzolite and in the garnet lherzolite stability fields (e.g., MacGregor, 1970). The spinel-bearing peridotites are the most cornmon of all mantle-derived rocks that we can obtain on the Earth's surface both as xenoliths in volcanic rocks and as alpine-type peridotite masses (e.g., Nixon, 1987; Arai, 1990). Many important magmas could be formed or released from the mantle restites within the spinel-peridotitefield (e.g.,Tatsumietal., 1983;

Fujii and Scarfe, 1985). Dick and Bullen (1984) are successful in characterizing abyssal and alpine-type spinel peridotites by using spinel chemistry, especially by its Cr# [ =Cr/ (Cr+A1) atomic ratio]. Later, Bonatti and Michael (1989) summarized petrological characteristics of spinel peridotites from continental rifts, ocean basins and fore-arc regions in terms of various parameters including Fo of olivine and Cr# of spinel. They concluded that there is a variation in the degree of depletion (or partial fusion) of mantle peridotites in response to the difference of tectonic setting. In this paper I intend to systematically compile igneous petrological characteristics of mantlederived spinel peridotites in terms of combination of Cr# of spinel and Fo content of coexisting olivine. To use simple Fo-Cr # relationships for the compilation is of great

0009-2541/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved.

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advantage; F o - C r # data for peridotites are easily available both from literature and from natural rocks in the laboratory. Reid and Woods (1978) also discussed the origin of mantle peridotite xenoliths by means of a similar plot, Fo of olivine vs. C r / ( C r + A1 + Fe 3+ ) atomic ratio, 2. Olivine-spinel mantle array 2.1. Definition The olivine-spinel mantle array was proposed by Arai ( 1987, 1990) as a mantle peridotite restite trend formed in the spinel lherzolite field in terms of Fo content of olivine and Cr # of spinel (Fig. 1 ). Xenolithic and alpine-type peridotites as a whole show essentially the same trend on the F o - C r # plane, although frequency of the rock type (or C r # of spinel) is different between the two (Fig. 1 ). Note that garnet-free spinel (chromite) harz~o Xe,o/,,h

+~ 0 5

ooo ° o ~'° i.° • .~'1 4 . :'.:.~ "'~.'~.~r"

~5 . . . .

A,p,,e-,ype o~ ~o ° °~,. ".ai~o .#~°

"":.'.'. "'~" •"" " "~..... 9~ . . . . 0'5 9's . . . . 9'0 . . . . A' Fo olivine Fo olivine

Fig. 1. Relationships between the Fo content of olivine and the C r / ( C r + A 1 ) atomic ratio ( = C r # ) of spinel in mantle-derived spinel peridotites (Arai, 1987). Xenoliths are mainly from alkali basalts in continental rifts and oceanic hot spots. Chromite peridotite xenoliths in kimberlite (e.g., Hervig et al., 1980) are omitted as mentioned below in the text. Open circles= harzburgite; closed circles = lherzolite. Note that the peridotites from the two derivations make a roughly common trend ( = olivinespinel mantle array). See text for further details. For data

sources see Arai (1990).

burgite (or lherzolite) xenoliths in African kimberlites (e.g., Boyd and Nixon, 1975; Hervig et al., 1980) are omitted from the data source for the olivine-spinel mantle array as mentioned below (Fig. 1 ). The C p x / (Opx + Cpx) volume ratio gradually decreases towards the refractory (high-Fo, high-Cr# ) end of the olivine-spinel mantle array; the ratio is 0.1 at C r # =0.5-0.6. In this article "harzburgite" and "lherzolite" are used in a different way from the standard IUGS nomenclature; the boundary between them is set on the C p x / ( O p x + C p x ) ratio=0.1 instead of the Cpx mode of 5%. The degree of refractoriness and other petrological characters ofperidotites depend rather on the ratio than on simple clinopyroxene volume (e.g., Arai, 1984; Dick and Fisher, 1984). Proportion and chemistry of minerals in peridotites are variable even within a specimen (cf., Dick et al., 1984). The F o - C r # relationships are, therefore, dependent on which grains are analyzed. For the purpose of this study the core compositions of large grains may be favorable. Fo content of olivine in peridotites is essentially unchanged during subsolidus stages except for local variations near clinopyroxene and chromian spinel. It could be significantly changed by metasomatism as mentioned below. Cr # of spinel may be kept almost constant during subsolidus stages because Cr-A1 partitioning between spinel and orthopyroxene, one of main phases which could contain appreciable amounts of A1 and Cr, is invariable at subsolidus temperatures (Fig. 2). Cr # of spinel rims, however, could be changed during subsolidus deformation stages (Ozawa, 1989). The volume of spinel may increase with temperature decrease as Tschermak's components are extracted from pyroxenes to form modal spinel (Green and

Ringwood,1967). The Fo-Cr# relationships in mantle-derived peridotites, which are m o r e or less recrystallized under subsolidus conditions, are essentially inherited from the igneous-stage ones (also see Ozawa, 1986 ). The

CHARACTERIZATION

OF

SPINEL

PERIDOTITES

BY

OLIVINE-SPINEL

COMPOSITIONAL

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scattering o f F o - C r # in peridotites (Fig. 1 ) i s partly real and partly due to some analytical problems, including analytical uncertainties and inadequate choice o f analyzed grains. We should notice how carefully olivine and spinel were analyzed when we use data from the literature,

2.2. Peridotites plotting off the olivine-spinel mantle array The spinel peridotites m e t a s o m a t i z e d by melts o f fluids enriched with incompatible elements are sometimes shifted off the olivinespinel mantle array in low-Fo directions (Irving, 1980; K. Goto and Arai, 1987) (Fig. 3 ). Arai and Takahashi (1989 ) demonstrate that formation o f secondary phlogopite and amphibole, for which alkali-rich aqueous fluids were responsible, did not alter original F o - C r # re-

,

I

,

i

,

85

Fo

Fig. 2. Cr-A1partitioning between spinel and orthopyroxene in peridotitic assemblageswith different equilibrium conditions. Noyamadake & Kurose=xenoliths from the SouthwestJapan arc (Table 1; Arai and Hirai, 1983; Hirai, 1986); CIM=low-temperature alpine peridotites from the Circum-Izu Massif region, central Japan (Arai, 1991); Pyrolite & Tinaquillo=residues of anhydrous melting experiments at 10 and 15 kbar (italic numbers= temperature ) by Jaques and Green (1980). Note that the partition coefficient does not depend on temperature,

i

,

I

i

i

,

80 olivine

Fig. 3. Fo-Cr # relationships of cumulates and modified peridotites. Data for Great Dyke, Zimbabwe, cumulates and Ryozen volcanics, northeast Japan (Yoshida et al., 1985 ) are after Wilson (1982) and S. Arai (unpublished data, 1990), respectively.Trends a and b are after N. Takahashi (1988) and K. Goto and Arai (1987), respectively. OSMA=olivine-spinel mantle array. lationships of the H o r o m a n peridotites, northern Japan. Subsolidus formation of other aluminous phases, especially plagioclase, makes the remaining spinel Cr-enriched by selective consumption of Al-spinel components (N. Takahashi, 1988 ). The subsolidus formation o f plagioclase or possibly garnet m a y shift the spinel peridotites o f f t h e olivine-spinel mantle array in high-Cr# direction at constant Fo (Fig. 3 ). If peridotites are of cumulate origin they plot within or in low-Fo area off the olivine-spinel mantle array (Fig. 3). Coexisting olivine-spinel pairs in ultramafic cumulates or in Mg-rich volcanics always plot in this way (Fig. 3 ). 3. Upper-mantle spinel peridotites from known tectonic settings Spinel peridotites from a particular tectonic setting lie, on average, in their particular area

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s. ARAI

within the Fo-Cr # plane (Fig. 4). In the diagrams apparent metasomafized peridotites, that is, peridotites which contain secondary A1rich mineral (s) such as pargasite and phlogopite, are excluded, 3.1. Ocean-floorperidotites

The ocean-floor peridotites from the Atlantic and Indian Oceans are extensively described by Hamlyn and Bonatti (1980), Dick and Bullen (1984), Dick et al. (1984), Michael and Bonatti (1985), and Dick (1989). Cr # of their spinels varies from 0.1 to 0.6 (Fig. 4A); relatively fertile lherzolites with C r #