Chemical transfer between mantle xenoliths and basic magmas: Evidence from oceanic magma chambers. The trinity ophiolite (northern California)

Lithos, 25 (1990) 243-259 Elsevier Science Publishers B.V., Amsterdam

243

Chemical transfer between mantle xenoliths and basic magmas: Evidence from oceanic magma chambers. The Trinity ophiolite (northern California) Christophe L6cuyer Laboratoire de GOochimie isotopique, CAESS-CNRS, 35042 Rennes (France) (Received November 27, 1989; accepted May 23, 1990 )

LITHOS

ABSTRACT L6cuyer, C., 1990. Chemical transfer between mantle xenoliths and basic magmas: Evidence from oceanic magma chambers. The Trinity ophiolite (northern California). Lithos, 25: 243-259. The Trinity ophiolite consists of small magma chambers inside a large mantle body. Xenoliths of mantle peridotite occur both in gabbroic cumulates along the walls and in the matrices of ultrabasic breccias on the floors of the magma chambers. Field relationships and petrographic data suggest that these fragments of original mantle peridotite were modified by contact with basic magmas by modal metasomatism. Quantitative elemental mass transfers determined from the composition, volume and density variations of reacting minerals demonstrate both closed and open system conditions for the major (Si, AI, Ti, Na, Ca, Fe and Mg) and trace elements (Cr, Ni). In the open system, material gains and losses provide information on the composition of the fluid taking part in the metasomatic reaction. During a first stage ofmetasomatism the mantle xenoliths were affected by high-temperature reactions at 600 to 925°C. They resulted from the interaction between solid mantle lherzolites and basic melts. The reactions are: ( 1 ) those forming orthopyroxene-magnetite simplectite (2) those forming plagioclase-magnetite corona ( 3 ) clinopyroxene + spinel I ~ pargasitic hornblende + spinel I I. Chemical interactions between the upper mantle and oceanic magma chambers occurred as soon as the basic magmas had ascended through the upper mantle. The chemically modified magmas, within oceanic magma chambers, were depleted in Ti, Fe and Na. This could partly explain regional variations of the chemical compositions of primary magmas produced beneath slow-spreading ridges. The breakdown of olivine to orthopyroxene and magnetite participates in the control of the partition of magnetic Fe-Ti oxides between oceanic crust and mantle. During the second stage, the serpentinization of olivine and the production of talc were superimposed on the products of the first stage. These reactions require large amounts of H20. The hydrothermal fluid was probably seawater. It circulated in the brecciated area along the walls and floors of the magma chambers located at shallow depths. Such structural discontinuities thus played the role of penetration channels favoring seawater circulation in the oceanic crust. All the chemical reactions examined suggest a significant open-system element transfer by infiltrating melts or circulating fluids. The results of this study suggest that caution is required in the interpretation of mineralogical and chemical information provided by mantle xenoliths carried to the surface by ascending magmas.

1. Introduction Mantle xenoliths have been studied extensively

0024-4937/90/$03.50

in different eruptive rocks of various volcanic provinces (review in Nixon, 1987). These xenoliths contribute to our understanding of the petrological

© 1990 - - Elsevier Science Publishers B.V.

244 and chemical interactions between crust and mantle. Metasomatism of mantle xenoliths is widely documented and is considered as a process responsible for generating particular geochemical characteristics in related magmas. In the context of metasomatism, many of the phenomena (such as modal metasomatism) described in mantle nodules appear to involve melt (or fluid derived from melt) infiltration. Quantitative information, however, on the metasomatic "fluid" in the mantle or lower crust is poorly assessed from mantle xenoliths since: - field relationships such as small-scale contact and intrusive features are not observed. - the mineralogy and chemistry of a xenolith prior to a metasomatic event and the contributions of original rock and metasomatic fluid to the chemical components of neoformed minerals are generally difficult to establish. The nature of mantle precursors to metasomatized mantle xenoliths may be inferred from the study of peculiar ophiolitic complexes in which the original mantle-oceanic crust boundary is well preserved. The Trinity ophiolite of northern California is a good example of a mantle diapir (The Trinity Peridotite; Quick, 1981 ) with associated small intrusive magma chambers (Cannat and L6cuyer, 1990 ). The geodynamic environment of the Trinity ophiolite has been discussed by Lapierre et al. (1987) and Brouxel and Lapierre (1988) who suggested that it formed in a marginal basin while Le Sueur et al. (1984), Boudier and Nicolas ( 1985/ 86) and Boudier et al. (1989) propose a slowspreading environment. Previous work (Brouxel and Lapierre, 1988; L6cuyer et al., 1990) attests to a strong rock alteration by oceanic hydrothermal activity. Xenoliths of mantle peridotite occur in the gabbroic cumulates along the walls and in the matrices of ultrabasic breccias on the floors of the magma chambers. The extreme variations within primary modal compositions of mantle rocks (Hamlyn and Bonatti, 1980; Michael and Bonatti, 1985) preclude a direct comparison of their geochemistry with associated metasomatized rocks. An understanding of metasomatism requires knowledge of the quantitative interaction between melts or fluids and their host rocks. The aim of this paper is to describe the successive mineralogical reactions recorded by mantle xenol-

C. LI~CUYER iths of the Trinity ophiolite and to discuss: ( 1 ) The quantitative elemental mass transfer between reacting minerals determined from density and volume relationships. (2) The importance of metasomatism (interaction between rocks and basic melts) as a process responsible for generating geochemical variations in MOR magmas. (3) The possible subsequent interaction between rocks and circulating bydrothermal water (modified seawater) in the deep levels of the oceanic crust.

2. Field

observations

Two areas were selected for this study: the Castle Lake and Toad Lake magma chambers (Fig. 1). These magma chambers occur as pockets of gabbro in the surrounding mantle peridotites. The gabbro/ peridotite contacts are not significantly deformed and the internal successions of the gabbro sections are not disrupted. Stratigraphic columns for the Castle Lake and Toad Lake magma chambers and their relative positions within a schematic magma chamber are shown in Fig. 2. The Castle Lake magma chamber The ultramafic tectonites enclosing the Castle Lake magma chamber are largely represented by foliated harzburgites containing discordant metresized dunite bodies. The contact with the basal breccia unit is roughly horizontal, gradational and characterized by an increase in the density of gabbro dykes over a few metres in the underlying mantle peridotite (Fig. 2 ). The basal breccia unit of the Castle Lake magma chamber is > 100 m thick and consists of a matrix of banded werhlites and pyroxene-rich gabbros (Fig. 2). containing ultrabasic xenoliths. The xenoliths vary in composition from dunites and harzburgites to werhlites and pyroxenites. The pyroxene-rich gabbros and werhlites are crosscut by dykelets, sills and pegmatitic patches of leucocratic gabbros. The overlying upper gabbroic unit consists of gabbros which range from 600 to 900 m in thickness and which are characterized by well developed layering. This layering is free of plastic deformation

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and is discordant with the layering of the gabbros of the basal breccia unit (Fig. 2). At the base of this unit, the layered gabbros are petrographically similar to those surrounding the ultrabasic fragments of the basal breccia unit. The lowermost 100 m of this upper unit is composed of

alternating leucocratic gabbros and pyroxene-rich gabbros and includes metamorphosed ultrabasic xenoliths similar to those present in the basal breccia unit (Fig. 2). These rocks are overlain by about 150 m of layered leucocratic gabbros devoid of ultrabasic xenoliths followed by 100 m of pyroxene-

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CHEMICAL TRANSFER MANTLE XENOLITHS BASIC MAGMAS

rich isotropic gabbros containing numerous pegmatitic patches. The top of the lithologic succession consists of layered gabbros containing dykelets and irregular masses of quartz diorites (Fig. 2).

The Toad Lake magma chamber The ultramafic tectonites enclosing the Toad Lake magma chamber range in composition from dunires to lherzolites. The dunites form horizontal pockets discordant with respect to the spinel foliation of the lherzolites (Fig. 2). These peridotites are crossed by locally folded clinopyroxene-rich veinlets cutting the spinel foliation and by coarse gabbroic dykes which fed the magma chamber. Locally, the peridotites are impregnated by feldspar in lenses evolving into veinlets, with anastomosing and cross-cutting patterns. The contact between the mantle unit and the cumulate gabbros is generally undeformed, straight and parallel to the cumulate layering. It is continuously underlain by a several metres thick layer of ultrabasic-basic breccias (Fig. 2 ). The vertical dips of the diabase dykes crosscutting the magma chamber and the absence of tilting of the Trinity massif especially in the Toad Lake area, suggest that this contact is a primary feature trending parallel to one of the walls of the magma chamber. This wall was characterized by active currents as illustrated by strong bedding layers in the cumulate gabbros which usually contain ultrabasic xenoliths. Towards the center of the chamber, dominantly isotropic gabbros intrude the layered gabbros and also contain ultrabasic xenoliths. The whole cumulate sequence is crosscut by subvertical microgabbros and diabase dykes with locally weakly deformed margins. This feature suggests that the dykes intruded the gabbros before these were consolidated completely (Fig. 2 ).

(A) The mantle peridotites The mantle origin of the PI-harzburgites-harzburgites and the PI-lherzolites-lherzolites of the Trinity peridotite has been demonstrated by the mineral chemistry studies of Quick ( 1981 ). An excellent record of a complex mantle history involving plastic deformation and partial melting is preserved in these ultramafic rocks. Quick (1981 ) has estimated equilibration temperatures for pyroxenes in two samples of plagioclase lherzolite and one sample of lherzolite. Equilibration temperatures were calculated using the geothermometer of Wells (1977). Porphyroclast cores preserve equilibration temperatures that range from 1159 to 1203°C. Porphyroclast rims and matrix grains appear to have equilibrated at lower temperatures ranging from 963 to 1097°C. Textures of the peridotites are porphyroclastic following the terminology of Mercier and Nicolas ( 1975 ). The sizes and shapes of olivine grains (Fo9o to Fo92; NiO from 0.2 to 0.45%) are highly variable (0.6-1 mm). Larger grains are commonly elongated and contain regular kink-bands. Recrystallized olivines are fine-grained (0.1-0.3 mm ), strainfree and constitute an equant mosaic texture. Orthopyroxene porphyroclasts (Wo~ 2 En89-9o Fs8_9; 1.5-2.5 mm) often contain clinopyroxene exsolution lamellae. The elongated grains are kinkbanded with slightly irregular grain boundaries. Discrete grains of clinopyroxenes (Wo47 Enso Fs3) are generally small ( < l mm) and undeformed with xenomorphic grain boundaries. Reddish-brown spinels (Magnetite2_6 Hercynite52_6o Chromite35_44; 0.4-0.6 mm) are anhedral, elongated and define a lineation interpreted as flow direction during peridotite deformation (Le Sueur et al., 1984).

(B) Castle Lake: the ultrabasic breccias of the basal unit

3. Petrography and mineralogy The chemical compositions of minerals were determined by electron microprobe at the Service Commun d'Analyses of the University of Nancy I. Representative microprobe analyses of minerals from the Castle Lake and Toad Lake areas are given in Tables 1 and 2. The amphibole names follow the classification of Leake ( 1978 ).

Xenoliths contained in the ultrabasic breccias are metamorphosed mantle lherzolites consisting of olivine, chromian spinel, brown hornblende and orthopyroxene. The olivine shows various degree of serpentinization and the orthopyroxene is often replaced by bastite and talc. Olivine (0.15-0.8 mm) is characterized by high NiO (up to 0.42%) and Fo (Fo86_89) contents (Ta-

C. LECUYER

248 TABLEI Representative m i c r o p r o b e a n a l y s e s o f m i n e r a l s f f o m Castle Lakexenolithsand mantleperidotites. Sample Type Mineral Reference* SiO2

Ti02 -k1203 Fe2Ost MgO MnO CaO Na20 K20 NiO (~r203 Sum Sample Type Mineral Reference*

Si02 TiO2 AI203 Fe~,O3t MgO MnO ('aO Na20 K20 NiO ('r203 Sum

CL6 mantle peridotite Clinopyroxene {21

CL6 mantle xenolith Spinel {2}

CL6 mantle xenolith Spinel I1 {2}

CL6 mantle xenolith Pargasitic-Hb {2}

CL6 mantle xenolith Talc {3a-b}

CL6 mantle xenolith Olivine II {4}, {6}

CL6 mantle xenolith Serpentine I ~t6}

C16 CL6 manlle mantle xenolith xenolith Serpentine 11 Serpentine III {6} {6}

52.36 0.25 2.84 2.33 16.64 0.07 24.03 0.51 0,00 0.19 1.10 100.12

0 0.58 21.70 30.05 8.71 0.31 0 0 0.01 0.19 38.50 100.05

0.01 1.15 10.52 48.40 1.86 0.65 0.02 0 0 0.28 30.67 93.57

45.58 1.48 I 1.59 4.36 18.26 0 12.35 2.04 0.08 0.12 1.59 97.45

59.01 0 2.01 2.52 30.60 0.06 0 0.14 0 0.12 0.45 94.91

39.99 0 0 18.30 41.81 0.35 0 0 0 0.40 0 100.13

40.14 0.04 1.02 13.28 39.10 0.26 0.01 0.03 0 0.37 0 100.85

40.83 0 3.06 7.01 37.44 0.03 0.04 0.03 0 0.21 0 94.25

CL6 mantle xenolith Olivine {5}

CL6 mantle xenolith Serpenline 15}

CL6 Orthopyroxene

CL6 Bastite

{7}

{7}

56.42 0 2.42 6.30 33.27 0.19 0.54 0 0 0.05 0.43 99.66

43.23 0 2.43 5.16 37.24 0.04 0 0 0 0 0.55 88.65

40.64 0 0.04 10.44 48.36 0.21 0.04 0 0.04 0.36 0 87.50

43.23 0 0.06 4.77 38.85 0 0 0 0.01 0.17 0 87.10

44.23 0 0.65 4.81 37.65 0 0.02 0 0 0.14 0.01 88.65

*The number refers to mineralogical reactions discussed in the text. TABLE 2 representative microprobe analyses of minerals from Toad Lake mantle xenoliths and cumulate gabbronorites. Samples Type Mineral

TOADI2 TOADI 2 gabbronorite gabbronorite Clinopyroxene Orthopyroxene

TOADI 2 mantle xenolith Olivine {9a-b}

TOAD 12 mantle xenolith Orthopyroxene {9a-b}

TOADI2 mantle xenolith Orthopyroxene {9a-b}

TOAD 12 mantle xenolith Magnetite {9a-b}, {10}

TOADI 2 mantle xenolith Plagioclase {10}

TOADI 2 mantle xenolith Mg-hornblende {10}

TOADI 2 mantle xenolilh Mg-hornblende {10}

52.77 0.49 1.71 8.18 14.02 0.18 21.9 0.13 0,00 0.02 0 99.40

39.17 0 0 19.07 41.48 0.39 0 0 0.01 0.32 0 100.44

55.00 0.18 1.94 13.80 28.96 0.37 0.55 0.01 0 0.06 0 100.87

55.34 0.21 1.72 12.71 29.24 0.39 0.58 0.01 0.01 0.09 0.09 100.38

0 0.67 0.62 88.36 0.20 0.08 0 0.07 0 0.18 2.73 92.93

45.93 0 34.91 0.29 0 0 17.64 1.03 0.16 0 0 99.96

46.24 1.41 11.92 6.71 17.13 0.07 11.41 1.51 0 0 0 96.40

44.79 1.06 13.29 7.67 15.96 0.10 11.69 1.65 0.02 0.11 0.02 96.36

Reference* SIC)2 TiOz M203 Fe203t MgO MnO CaO Na20 KzO NiO Cr203 Sum

53.71 0.10 1.09 19.89 23.86 0.44 1.21 0 0 0.07 0.17 100.54

*the number refers to mineralogical reactions discussed in the text.

CHEMICAL TRANSFER MANTLE XENOLITHS BASIC MAGMAS

249

Fig. 3. Transmitted light microphotographs of mantle xenoliths from Castle Lake. (a) talc ( 1 ) and secondary olivine (2) replacing orthopyroxene ( 3 ). (b) secondary spinel ( 1) associated with pargasitic hornblende (2). ( 3 ) is a primary cumulus olivine. ble 1 ). A less magnesian olivine (Fos0) may be intimately associated with orthopyroxene and talc (Fig. 3a; Table 1 ). The serpentine which replaces olivine is characterized by high NiO contents (0.140.37%: Table 1 ). Orthopyroxene (1.25 mm length by 0.6 mm width) is pseudomorphed by a Cr203 rich-bastite (0.52-0.59%; Table 1 ). Some magnesian ( M g O = 29.7-31.8%) and chromiferous (Cr203= 0.290.53%: Table 1 ) talc is also present as a replacement

of orthopyroxene and occurs as patches within the crystals. The Cr-spinels (0.3 m m ) show large chemical variations within a given sample, particularly in hercynite and spinel components (Table 1 ). Cr-spinels intimately associated with hornblende (Fig. 3b ) are characterized by having the lowest A1203 (10.5%) and MgO (1.9%) contents which contrast with their rather high FeO (48.4%) and TiO2 (1.15%) contents.

250 The poikilitic brown hornblende which occurs as a replacement o f c l i n o p y r o x e n e (0.6 m m length by 0.3 m m width) is a pargasitic hornblende (Table 1 ). It also usually contains small euhedral olivine inclusions. The matrix o f the ultrabasic breccias is heterogeneous in composition and is made up o f m e t a m o r -

C. LECUYER phosed clinopyroxenites with two generations o f amphibole which, respectively, are poikilitic magnesio-hornblende and actinolitic hornblende. These clinopyroxenites contain centimetre-sized ultrabasic xenoliths which are ancient tectonized dunires with high-magnesian olivine ( M G O = 4 6 . 9 49.8% ) and high N i O contents ( N i O = 0.35-0.48 ).

Fig. 4. Transmitted light microphotographs of mantle xenoliths from Toad Lake. (a) lherzolmc mantle xenoliths ( 1 ) within a layered gabbronorite cumulate (2). (b) ( 1 ) kink-banded orthopyroxene in the mantle xenolith core. (c) olivine ( 1 ) and orthopyroxene (2) enclosing subparallel sets of numerous fine platelets and vermicules of magnetite. (d) intergranular magnetite ( 1 ) and polygonal orthopyroxene grains (2) in partial or complete replacement of olivine (3). (e) magnetite grains ( 1 ) replaced by magnesio-hornblende (2) adjacent to plagioclase (3) altered to epidote.

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C. LECUYER

The matrix may also be gabbroic with poikilitic amphiboles and scarce cumulus plagioclase relicts (An91.4_95.1).

90 ¸

(C) Toad Lake." the layered cumulates and their xenoliths

80

The layered cumulates of the Toad Lake magma chamber are ilmenite-rich gabbronorites with mesocumulate to orthocumulate textures. The well developed magmatic layering in these rocks is defined by mineralogical and granulometric changes. Generally, the coarse-grained layers have plagioclase (altered to epidote or albite) dominant proportions. The pyroxenes are usually fresh, although orthopyroxene may be altered to chlorite and clinopyroxene to actinolite. The gabbronorites are characterized by early cumulate plagioclase followed by clinopyroxene, orthopyroxene and finally interstitial ilmenite. The pyroxenes have high Fs (ferrosilite) contents (Cpx=Fsl3.4_13.6; O p x = Fs29.8_3~.9; Table 2) which contrast with those from fresh mantle peridotites. The gabbronorites located near the walls of the magma chamber are rich in millimetre to centimetre-sized xenoliths of mantle peridotite (Fig. 4a). The cores of the largest xenoiiths often show textural and mineralogical features characteristic of mantle lherzolites: they contain Crspinel (0.3-0.4 mm), kink-banded grains (0.5-1 mm ) and unstrained neoblasts (0.2-0.5 mm) of olivine, kink-banded orthopyroxene porphyroclasts ( 2-3 mm in length; Fig. 4b) with exsolution lamellae of clinopyroxene (up to 15 ~tm) and aggregates of subhedral clinopyroxene and plagioclase lenses altered to epidote (50/zm). The small millimetresized xenoliths and the rims of the larger xenoliths contain orthopyroxene-magnetite symplectites and magnetite-plagioclase coronas. The symplectite consists of an association of magnetite and orthopyroxene that occurs as a replacement of olivine (FO76_78; Table 2). Two textural varieties of the symplectite are present with one consisting of numerous fine platelets and vermicules of magnetite (3 to 30 ltm) in subparallel sets enclosed in orthopyroxene (Fig. 4c). The other consists of polygonal orthopyroxene grains (0.1-0.3 mm ) and interstitial granular magnetite (30/tm to 0.25 mm) in partial or complete replacement of olivine (Fig. 4d). Orthopyroxene is characterized by high Fs

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(Fs19.9_23.5) and low Cr203 contents (0-0.1%) while the magnetite has TiO2 contents ranging from 0.3 to 2% and Cr203 from 1.79 to 2.76% (Table 2). The chemical compositions of olivine, orthopyroxene and spinel show large variations from the rims to the cores of the centimetre-sized mantle xenoliths. The TiO2, Fe203 and A1203 contents of the orthopyroxenes from the xenoliths lie between two poles corresponding to host cumulate and peridotite orthopyroxene compositions (Fig. 5). The local development of a plagioclase-magnetite reaction corresponds with the presence of a brown hornblende zone (coronas=25-40 pm in width; Table 2) including micrometric spinels in replacement of an An-rich plagioclase (An89.6_94.4; Table 2) when the latter is rimmed by magnetite (Fig. 4e). This magnesio-hornblende shows high TiO2 (1-1.4%) and Na20 (1.5-1.7%) contents. This mineralogical reaction occurs after the orthopyroxene-magnetite symplectite.

4. M i n e r a l o g i c a l r e a c t i o n s - - e l e m e n t a l m a s s transfers

Field relationships and petrographic data suggest that fragments of original mantle peridotite were modified by contact with basic magmas by "modal metasomatism" (Dawson, 1980). In modal metasomatism, changes in bulk chemistry reflect the modifications in modal mineralogy. Many examples of this type of metasomatism observed in man-

2 53

CHEMICAL TRANSFER MANTLE XENOLITHS BASIC MAGMAS

tie xenoliths appear to result from the presence of either a melt or a fluid (Harte, 1987 ). In an attempt to characterize these fluids and to determine their contributions to the neoformed minerals, chemical components lost or gained during the mineralogical reactions were calculated as outlined below.

(A) Method

The gains and losses of chemical components by weight related to mineralogical reactions require knowledge of the chemical compositions and densities of reactants and the global volume change (Gresens, 1967). For a mineralogical reaction in which the amount a of mineral source A gives an amount b of produced mineral B with a total amount X of material subtracted from or added to A, Gresens ( 1967 ) has formulated an equation characteristic of composition-volume relationships involving mineral chemistry, specific gravities and volume factors ( 1 ). 100{fv- ( gB / gA ) . c n B - cnA } = xn

(l)

(B) Results and discussion Castle Lake (1) Origin of the pargasitic hornblende. The slow Si and high (AP v + Na + K )A contents of the pargasitic hornblende (Table 1 ) are consistent with its crystallization at magmatic temperatures (Spear, 1981 ). The high Cr203 contents of the hornblende and its close association with Cr-spinel also correspond to a high temperature reaction ( T = 9 2 5 ° C ; Spear, 1981 ; curve 8, Fig. 6) involving clinopyroxene and spinel in the presence of a fluid phase. The pseudomorphic replacement of clinopyroxene by pargasitic hornblende and the negligible difference in their densities allows the following reaction to be written: 100g Clinopyroxene + 100g Spinel I +22g F e O + 1.8g N a 2 0 + 2.25g TiO2 +2g H20-~ 100g Hornblende + 100g Spinel II +7g SiO2 + 1 lg C a O + 9.8g MgO

(2)

In this reaction, A1203, Cr203 and NiO ar redistributed between the minerals in a closed system P (Kb)

h, is defined as the volume factor, and when fv = l, replacement is isovolumetric. The specific gravities of minerals A and B are designated by gA and gB. cnA and cnB are the weight fractions of component n for each mineral and xn is the amount of material lost or gained. Generally, if N minerals participate in the reaction, N - 1 assumptions or additional data are necessary to solve the equation (Gresens, 1967). For eqn. ( 1 ), it is necessary to assume that either the reaction is isovolumetric (fv= 1 ) or that a component is immobile. The choice of one of these assumptions may be inferred from the textural relationships observed in thin section (for example, fi~= 1 for pseudomorphism) or by the knowledge of the chemical behavior of a component (for example, AI is considered to be relatively immobile during most alteration processes). In the following discussion, the amount of primary minerals is fixed at 100 g by the use of weight mineral analyses. The specific gravities of minerals are those given by Deer et al. (1966).

5 1

4 3

4

5

6

7

2

300

I

TALC

2

OLIVINE

3 : LOWER

SERPENTINE

7oo

T (°C)

~ 5102

, H20 = SERPENTINE LIMIT O F H O R N B L E N D E

4

OLIVINE + TALC

5

LOWER

+ H20

70RTHOPYROXENE HORNBLENDE

~ TREMOLITE

STABILITY

SERPENTINE

LIMIT O F S U P E R S O L V U S

60RTIIOPYROXENE

8

soo

, I120 = T A L C

HORNBLENDE

STABILITY

+ OLIVINE

+ 1120 - T A L C CLLNOPYROXENE

+ PLAGIOCLASE

~ OLIVINE

ILb'ENITE ~ t120

Fig. 6. Pressure-temperature diagram showing experimental curves for the reactions discussed in the text.

254

C.L~CUYER

while a low hydration accompanies gains of Na~O, TiO2 and FeO and losses of MgO, SiO2 and CaO.

volved in the destruction of orthopyroxene are characterized by large losses of SiO2.

(2) Mineralogical reactions destroying orthopyroxene: production of talc and secondary olivine. (a) talc

(3) Serpentinization of olivine. (a) the example of

production. For this reaction, Cr (high and constant values: Table 1 ) may be considered as an immobile component: 100cm 30rthopyroxene-zfv= 1.1 --, 110cm 3 Talc

100cm 30livine-fv= 1.26-, 126cm 3 Serpentine

Then: 100g Orthopyroxene + 4g H20--* 88g Talc+ 6.34g MgO+4.5g SiO2 + 4.08g FeO+0.65g A1203 +0.54g CaO

Mg-olivine. The increasing serpentinization of olivine shows a progressive leaching of FeO, MgO and NiO (Fig. 7). The SiO2 contents, however, of the two minerals remain constant so that:

0,5-

NIO

(3a)

T=800°C (Hemley et al., 1977a; curve 7; Fig. 6). The formation of talc involves the hydration of orthopyroxene, a volume increase, and significant losses of MgO, FeO and SiO2. If reaction (3a) is considered as isovolumetric, then:

0.4

03

H20+

100g Orthopyroxene + 3.6g H20 80g Talc+ 8.8g MgO+ 9.2g SiO2 + 4.3g FeO + 0.54g CaO + 0.8g A1203

0.1 5

10

1

(3b) 20

The isovolumetric assumption results in a lesser amount of produced talc which is balanced by a greater loss of SiO2 and MgO. Taking into account the volume change (3a), the final proportions of secondary minerals and elemental mass transfers are obviously different. They must be considered with the aim to quantify more realistic elemental transfers for mineralogical reactions involving high hydration rates. (b) Production of secondary olivine Reaction (3a), involving orthopyroxene at T=725°C (Hemley et al., 1977a; curve 6; Fig. 6) leads to the formation of talc and a secondary magnesian olivine.

FeO

10

H20+ .

.

.

.

i

.

.

.

.

5

42

,

.

.

.

.

10

15

MgO

41

100cm 30rthopyroxene-fv= 1.07--, 67cm 3 Olivine II +40cm 3 Talc

40

Then: 39

100g Orthopyroxene + 6.8 lg MgO + 7.54g FeO+ lg H20-~ 34g Talc+ 71g Olivine II 7.97g SiO2+ 1.74g A1203 +0.54g CaO

38

(4)

• 37

.

.

.

.

, 5

This reaction utilizes the FeO and MgO released by reaction (3a) to form secondary Fe-enriched olivine at lower temperatures. All of the reactions in-

.

.

.

.

, 10

.

20+ .

.

. 15

Fig. 7. Variationof MgO, FeO and NiO versus HzO+ content of olivine with increasingserpentinization.Interval between microprobeanalysisis 10/~m.

CHEMICAL TRANSFER MANTLE X E N O L I T H S BASIC MAGMAS

100g Olivine + 14g H20-~ 97.3g Serpentine+ 6.14 FeO+ 11.2g MgO

(5)

At T=450°C (Hemley et al., 1977b; curve 2; Fig. 6). The serpentinization of olivine is characterized by a large volume increase and important losses of MgO and FeO. (b) The example ofa ferriferous olivine: serpentinization of secondary olivine. The equation becomes: 100 g Olivine+ 14g H20---~ (6) 97.36 g Serpentine+ 3.55g MgO+ 13.62g FeO The serpentinization of either Mg- or Fe-olivine is characterized by a similar rate of hydration and by a large increase in the FeO/MgO weight ratio of the serpentine which ranges from 0.55 to 3.84. This contrasts with the primary FeO/MgO weight ratio of olivine which only varies from 0.21 to 0.44. During the serpentinization of olivine, FeO is more mobile than MgO and forms magnetite.

(4) Serpentinization of orthopyroxene. Cr may be considered as an immobile component during the replacement of orthopyroxene by bastite. The equation is reduced to: 100 cm 30rthopyroxene-fv= 0.994~ 99.4 cm 3 Bastite 100 g Orthopyroxene + 8g H 2 0 ~ 78.2g Bastite + 22.6g SiO2 + 4.15g MgO + 2.27g FeO + 0.54g CaO + 0.52g A1203

(7)

The serpentinization of olivine and orthopyroxene is thus correlated with different degrees of hydration. In contrast to the volume increase of 26% during olivine alteration, the production of bastite corresponds to an isovolumetric reaction which agrees with the petrographic evidence for pseudomorphic replacement. Furthermore, the serpentinization of orthopyroxene induces an important SiO2 loss which is a typical feature ofpyroxene or pyroxenite alteration (Gresens, 1967 ). The mineralogical reactions discussed above for the formation of serpentine and talc are restricted to the ultrabasic breccia unit while the surrounding mantle peridotites are relatively fresh. Similar reactions have also been recognized in the abyssal peridotites of the Islas Orcadas Fracture Zone

255

(Dick, 1979; Kimball et al., 1985) and of the MidAtlantic Ridge near 45°N (Aumento and Loubat, 1971 ). The above authors suggest that these hightemperature alteration processes may reflect seawater circulation in the upper mantle. Sr isotopic data from a continuous section ofgabbroic cumulates within a magma chamber adjacent to Castle Lake have revealed that seawater reached the deep cumulate layers (L6cuyer et al., 1990). If the contributions of magmatic water and/or meteoric water are not to be excluded in the development of serpentine, the presence of dominantly modified seawater is more possible. Oxygen and hydrogen isotope studies on serpentinized ultramafic rocks are currently in progress to test the possible sources of water. Modified seawater probably circulated in the brecciated areas along the walls and the floor of magma chambers located at shallow levels (Cannat and L6cuyer, 1990). These structural discontinuities probably played the role of channels favouring seawater circulation in the oceanic crust. The results obtained in this study are consistent with those of Brikowski and Norton (1989) which highlight the role of magma chamber geometry on the control of seawater circulation through the oceanic crust. The high-temperature reaction which produces pargasitic hornblende is probably the result of the addition ofNa20, TiO2 and FeO to the mantle xenoliths from an infiltrating melt. This mineralogical reaction represents the interaction of mantle lherzolites in the solid state with the basic magmas that fed the magma chambers. Such interactions are better documented in the Toad Lake magma chamber.

Toad Lake Cr-spinels are typical of oceanic mantle xenoliths and are recognized as reliable indicators of fo2 (Maurel and Maurel, 1984; Murck and Campbell, 1986). Experimental studies have emphasized the relationships between the Fe203 contents of spinels, fo2, temperature and the FeO contents of silicate melts (Maurel and Maurel, 1984 ): (Fe203 )spinel 0.0045.104

= 8 1 3 / T . 117,.¢'~ ~ \ l ~ , , J ! total (liquid)

1 + 562" (fo2-°2185)" 10-55o2/v

(8)

For the mantle xenoliths of the Toad Lake magma chamber, the FezO3 contents of spinels have been

256

C.LECUYER

calculated from electron microprobe analyses assuming stoichiometric formulae. The spinels reveal a high fo2 (Logfo2 = - 5 ) with FeOt.~8% and Fe203 ~ 4% in the silicate melt (Fe 3+/Fetotal = 0.5 ). The development of the orthopyroxene-magnetite symplectite may result from the progressive breakdown of olivine at high fo: and Fe203 contents in the magma.

(5) Orthopyroxene-magnetite symplectite. (a) first assumption: isovolumetric reaction. The volume proportions of orthopyroxene and magnetite have been measured by pont counting (1400 points) of a xenolith in which the reaction is considered as complete since no relics of olivine remain. The selected counting area was also devoid of primary orthopyroxene porphyroclasts and amphibole corona. 100cm 3 Olivine-* 72cm s Orthopyroxene + 28cm 3 Magnetite Then: 100g Olviine + 26.6g FeO + 1.45g A1203 + 0.4g CaO + 0.43g TiO2 + 4g 0 2 4 70g Orthopyroxene + 41.6g Magnetite + 21 MgO + 0.4g SiO2

(9a)

The chemical balance confirms the presence of an iron-rich fluid which also contained A1203, TiO2 and CaO. It also reveals an important release of MgO. SiO2 remains relatively immobile which is in agreement with the previous work of Lamoen (1979). However, some authors (Goode, 1974; Lamoen, 1979) consider that the FeO released in the breakdown of olivine contributes to the ferrosilite component of orthopyroxene and results in the formation of magnetite. It is thus necessary to compare these results with those obtained from an equation constrained by the mass-balance conservation of SiO2 and FeO. (b) second assumption: conservation of Fe and Si. In this case, the volume factor for the reaction is different: 100cm 30livine-fv= 0.92--, 75cm 30rthopyroxene + 17cm 3 Magnetite Then: 100g Olivine + 0.22g TiO2 + 0.39g CaO + 1.28g A1203+ 1.5g 02-*

70.6g Orthopyroxene + l l.3g Magnetite+20.5 MgO

(9b)

This new reaction requires a lower fo2 than the previous one and results in the production of similar amounts of orthopyroxene and MgO. In contrast, a significantly lesser amount of magnetite is produced which is at variance with the modal proportions measured by point counting after corrections for mineral specific gravities. The volume change during the reaction is also at variance with the microscopic observation that primary polygonal structures of olivine are mimicked by orthopyroxene. Thus, the orthopyroxene-magnetite symplectite association appears to result from an isovolumetric reaction controlled by high fo~ and the presence of an iron-rich magmatic fluid (Johnson and Stout, 1984 ) according to the chemistry of the Cr-spinels. The formation of the symplectite is not simply the result of the oxidation of olivine in a closed system but rather is an open-system reaction (Ambler and Ashley, 1977) taking place in an iron-rich basic magma.

(6) The Plagioclase-magnetite corona. Plagioclase (An89.6_94.4) has reacted with magnetite to produce fine coronas of magnesio-hornblende containing inclusions of spinel. The limited mobility of Si and A1 deduced from diffusion models for corona formation (Grant, 1988) leads to the assumption that the total amounts of Si and AI are provided by the plagioclase and Fe by the magnetite to form both the magnesio-hornblende and the associated spinel. 100cm 3 Plagioclase + 8.7cm 3 Magnetitite-fv= 0.99-, 82.8cm 3 Magnesio-hornblende + 24.4cm 3 Spinel equivalent to: 100g Plagioclase + 16.6g Magnetite + 19g MgO + 0.9g TiO2 + 2.7g H20--* 97.5g Magnesio-hornblende + 35g Spinel 0.45g Cr203 + 6.24g CaO

(10)

The plagioclase-magnetite corona reaction is characterized by a negligible volume change, a gain of TiO2, and a loss of CaO. The reactants magnetite and the total amount of MgO necessary for the formation of magnesio-hornblende, for reaction (10)

257

CHEMICAL TRANSFER MANTLE XENOLITHS BASIC MAGMAS

are provided by the previous breakdown of olivine (9a). The presence of scarce ilmenite-magnetite pairs in equilibrium yields the temperatures of last chemical equilibrium (Buddington and Lindsley, 1964 ). These range from 600°C to 400°C and correspond to the stability field of the magnesio-hornblende produced by reaction ( 10 ) involving magnetite and plagioclase. The early cumulate plagioclase crystallized at a temperature of around 1000 °C (Kudo and Weill, 1971 ). The breakdown of olivine to orthopyroxene and magnetite controls the partitioning of magnetic FeTi oxides between the crust and mantle. Mantle peridotites contain primary Cr-A1 spinels which are non-magnetic at mantle pressures and temperatures (Wasilewski et al., 1979; Wasilewski and Mayhew, 1982). However, field relationships show a transition zone of ultrabasic breccias at the oceanic crust-mantle boundary where mantle xenoliths reacted at high temperatures with basic magmas to form magnetite-orthopyroxene symplectites. The magnetic character across the oceanic crust-mantle boundary may be considered as a complex transition zone most likely resulting from the mixture of mantle rocks and cumulate layers (Wasilewski, 1987). A common feature of modally metasomatized mantle nodules, clearly derived by the replacement of pre-existing rocks, is the development of pargasitic hornblende and Fe-Ti-rich oxides (Wilshire and Trask, 1971; Lloyd and Bailey, 1975; Harte et al., 1975; Francis, 1976; Wass et al., 1980; Wilshire et al., 1980 ). Ti, Na, Fe and H20 are also added to the modally metasomatised rocks. Gurney and Harte (1980) have shown that the chemical compositions of many mantle nodules have been affected by metasomatism at the contact of intrusions. Consequently, the results of this study provide evidence for a metasomatic signature overprinted on depleted mantle compositions. The interpretation of the geochemistry of mantle xenoliths is thus difficult even though they have been studied extensively for clues to the evolution of the mantle. The orthopyroxene-magnetite symplectite has also been observed in the margins ofgabbroic dykes which crosscut the Trinity mantle peridotites. The dykes have been interpreted as the conduits to the overlying magma chamber (Cannat and L6cuyer, 1990). The mineralogical reactions which take place

in chemically open system conditions reveal that chemical interactions between mantle rocks and basic melts occur as soon as they ascend through the upper mantle. The chemically modified magmas within oceanic magma chambers could partly explain regional variations in the chemical composition of primary magmas.

5. Conclusions

Mineralogical assemblages of mantle xenoliths found in the oceanic Castle Lake and Toad Lake magma chambers of the Trinity ophiolite record two stages of metasomatism. The first stage consists of three high-temperature metasomatic reactions: ( 1 ) those forming orthopyroxene-magnetite symplectite (2) those forming plagioclase-magnetite corona (3) clinopyroxene+spinel I-> pargasitic hornblende + spinel II. These reactions represent the interaction of mantle lherzolites in the solid state with basic magmas. The chemistry of the metasomatic spinels indicates that the basic magmas had high Fe203 contents and fo2. The chemical interactions between the upper mantle and oceanic magma chambers occur as basaltic melts which ascend through the upper mantle and feed the magma chambers. The formation of pargasitic hornblende results in the depletion of the primary basaltic melts in Ti, Fe and Na. Such chemical modifications of magmas within the oceanic crust could partly explain regional variations in the chemical compositions of primary magmas related to particular geotectonic environments (slow-spreading ridge). The second stage of metasomatism, superimposed on the first stage, involved the serpentinization of olivine and the production of talc. Both serpentine and talc require large amounts of H20 for their formation. Seawater probably circulated in the brecciated areas along the walls and the floors of the magma chambers at shallow depths. These structural discontinuities have played the role of channels favouring seawater circulation in the oceanic crust. It thus appears that the geometry of magma chambers partly control seawater circulation in the oceanic crust. All the mineralogical reactions observed in this

258 paper suggest significant open-system elemental transfers between m a n t l e xenoliths a n d infiltrating melts or circulating fluids. C o n s e q u e n t l y , these results suggest that c a u t i o n is required in the interpretation of mineralogical a n d chemical i n f o r m a t i o n p r o v i d e d by m a n t l e xenoliths carried to the surface by ascending magmas.

Acknowledgments C o m m e n t s by Dr. S. O ' R e i l l y were helpful a n d greatly appreciated. H. P i n t s o n is t h a n k e d for editing the m a n u s c r i p t ,

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259 evidence from rare earth and other elements in apatiterich xenoliths in basaltic rocks from eastern Australia. Philos. Trans. R. Soc. Lond., A297: 333-346. Wasilewski, P.J., 1987. Magnetic properties of mantle xenoliths and the magnetic character of the crust-mantle boundary. In: P.H. Nixon (Editor), Mantle Xenoliths. Wiley, Chichester, 836 pp. Wasilewski, P.J., Thomas, H.H. and Mayhew, M.A., 1979. The Moho as a magnetic boundary. Geophys. Res. Lett., 6: 541-544. Wasilewski, P. and Mayhew, M.A., 1982. Crustal xenolith magnetic properties and long wavelength anomaly source requirements. Geophys. Res. Lett., 9: 329-332. Wells, P.R.A., 1977. Pyroxene thermometry in simple and complex systems. Contrib. Mineral. Petrol., 62:129-139. Wilshire, H.G. and Trask, N.J., 1971. Structural and textural relationships of amphibole and phlogopite in peridotite inclusions, Dish Hill, California. Am. Mineral., 56: 240255. Wilshire, H.G., Pike, J.E.N., Meyer, C.E. and Schwarzmann, E.C., 1980. Amphibole-rich veins in lherzolite xenoliths, Dish Hill and Deadman Lake, California, Am. J. Sci., 280A: 576-593.