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Onset and Progression of Serpentinization and Magnetite Formation in Olivine-Rich Troctolite from IODP Hole U1309D

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Onset and Progression of Serpentinization and Magnetite Formation in Olivine-Rich Troctolite from IODP Hole U1309D JOURNAL OF PETROLOGY VOLUME 50 NUMBER 3 PAGES 387^403 2009 doi:10.1093/petrology/egp004 Onset and Progression of Serpentinization and Magnetite Formation in Olivine-rich Troctolite from IODP Hole U1309D JAMES S. BEARD1*,B.RONALDFROST2,PATRICIAFRYER3, ANDREW McCAIG4, ROGER SEARLE5, BENOIT ILDEFONSE6, PAVEL ZININ3 ANDSHIVK.SHARMA3 1VIRGINIA MUSEUM OF NATURAL HISTORY, 21 STARLING AVENUE, MARTINSVILLE, VA 24112, USA 2DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF WYOMING, LARAMIE, WY 82071, USA 3HAWAI’I INSTITUTE OF GEOPHYSICS AND PLANETOLOGY, UNIVERSITY OF HAWAI’I, HONOLULU, HI 96822, USA 4SCHOOL OF EARTH AND ENVIRONMENT, UNIVERSITY OF LEEDS, LEEDS LS2 9JT, UK 5DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, DURHAM DH1 3LE, UK 6GE¤ OSCIENCES MONTPELLIER, CNRS, UNIVERSITE¤ MONTPELLIER 2, CC 60, 34095 MONTPELLIER CE¤ DEX 05, FRANCE RECEIVED AUGUST 16, 2008; ACCEPTED JANUARY 14, 2009 ADVANCE ACCESS PUBLICATION FEBRUARY 19, 2009 Serpentinization of olivine-rich troctolite from core 227, Integrated silica to the system converted the Mg-component of the brucite to Ocean Drilling Program (IODP) Hole U1309D ranges from serpentine. Magnetite forms one or more distinct bands in the 510% to 490%. Two episodes of serpentinization are recognized. interior of the vein and is never in direct contact with relict olivine. The first, dominant in weakly serpentinized samples, is an approxi- A brucite^serpentine mixture, similar to that found in type 1 veins, mately isochemical (except for water) replacement of olivine (Fo84^ but with lizardite instead of antigorite, is commonly present at the 85) by a mixture of serpentine (antigorite, Mg-number 92) and bru- margins of type 2 veins (i.e. where they are in reaction contact with cite (amakinite-rich; Mg-number 65). The compositions of the relict olivine). We interpret type 2 veins as a steady-state system minerals in type 1 veins are a reflection of Fe^Mg exchange between where brucite continually forms at the olivine^vein contact and then olivine and the brucite þ serpentine formed during early serpentini- reacts out in the interior of the vein. This continual formation and 5 Á zation. The early serpentinite veins (type 1) are thin ( 0 05 mm), destruction of brucite imposes an exceptionally low aSiO2 on the irregular, and exploit pre-existing cracks in olivine. The presence system. Magnetite and olivine are never in contact in type 2 veins of antigorite suggests that early serpentinization occurred at (or anywhere else) because the olivine-out reaction yields ferroan T43008C. Type 1 veins reflect rock-dominated serpentinization, brucite and not magnetite.The desilication of serpentine in the type became isolated early in their history, and persist as relicts in all 2 veins is a reflection of the inherent instability of Fe-rich serpentine but the most altered samples. The main episode of serpentinization with respect to magnetite at low silica activity.Thus, the composition is manifested by through-going lizardite (average Mg-number 96)^ of serpentine in equilibrium with magnetite in serpentinites is a func- magnetite veins (type 2). Type 2 veins define an anastomosing tion of serpentine^magnetite and not serpentine^olivine equilbria. foliation, may be several millimeters in width and appear to exploit pre-existing, favorably oriented type 1 veins.Type2 veins reflect open- system serpentinization. Magnetite in these veins formed by KEY WORDS: serpentinite; magnetite; brucite; antigorite; lizardite; oxidation of the Fe in brucite and serpentine, whereas addition of ocean drilling;1309D ß The Author 2009. 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 50 NUMBER 3 MARCH 2009 INTRODUCTION SETTING AND SAMPLES Some of the most extreme geochemical environments on IODP Hole U1309D was drilled during IODP Expeditions Earth are associated with the serpentinization of olivine. 304 and 305 (Blackman et al., 2006), and is located on the Serpentinization is associated with strongly reducing con- Atlantis Massif at 30810Á12’N, 42 87Á11’W on the western rift ditions that lead to the generation of H2, a fluid pH that flank of the Mid-Atlantic Ridge (Fig. 1a). The Hole pene- ranges from 3 (at high temperature) to 12.5 (at low T), trates 1415m of mostly (490%) gabbroic rocks including and among the lowest aSiO2 in terrestrial silicate systems olivine gabbro, gabbronorite, oxide gabbro, and troctolite, (Coleman, 1963; Barnes & O’Neil, 1969; Frost, 1985; along with lesser amounts of peridotite (a few tens of cen- Abrajano et al., 1988; Peretti et al., 1992; Muntener & timeters) and diabase. Recovery was 75%. The gabbroic Hermann, 1994; Charlou et al., 1998, 2002; Beard & rocks include some of the most pristine and primitive ever Hopkinson, 2000; Allen & Seyfried, 2003; Mottl et al., sampled in the ocean basins (Blackman et al., 2006; 2004; Palandri & Reed, 2004; Shervais et al., 2005; Frost & Ildefonse et al., 2006). Some troctolites are very rich in oli- Beard, 2007). As a consequence of this there are minerals vine, with modal olivine approaching 90%. In this study, (e.g. brucite, hydrogarnet) and mineral assemblages (e.g. we examine the serpentinization and associated alteration awaruite^andradite, which pairs a ferric iron garnet with of the olivine-rich troctolites from core U1309D 227R-3 a metallic iron alloy; e.g. Frost,1985) found in serpentinites (Fig. 1b). that occur almost nowhere else on Earth. Additionally, Cores U1309D 227R-2 and U1309D 227R-3 preserve a there is growing recognition that hydrothermal vents asso- zone of serpentinization in olivine-rich (80^90% olivine) ciated with serpentinizing rocks may have played a key troctolites where relatively unserpentinized (i.e.510% ser- role in the genesis of life in the early oceans (Sleep et al., pentine) troctolite progressively grades into almost com- 2004; Russell & Arndt, 2005). pletely serpentinized rock and grades out again to Most serpentinites contain magnetite. Frost & Beard relatively unserpentinized rock. We selected four samples (2007) argued that magnetite formation was a direct conse- from core U1309D 227R-3 that represent the sequence quence of the instability of ferroan serpentine at low aSiO2. from most to least altered (Fig. 1b). More recently, Evans (2008) has argued that magnetite forms because the Fe^Mg exchange between olivine and serpentine during serpentinization requires that the ser- PETROGRAPHY pentine be more magnesian than olivine. Magnetite is Overall important, not only for understanding serpentinite geo- chemistry (e.g. magnetite formation is largely responsible Four samples from a 70 cm length of core U1309D 227R-3 for the reducing conditions that characterize actively ser- (henceforth referred to as core 227R-3) were analyzed in 5 pentinizing systems), but also for understanding magneti- detail. Serpentinization ranges from 10% in the sample zation of serpentinite. In particular, Toft et al. (1990) have from 69^71cm to 30% in the sample from 41^43 cm to 4 suggested that the observed lag between the onset of ser- 70% in the sample from 14^16cm and to 90% in the pentinization (as indicated by a drop in density) and the sample from 2^4 cm (Figs 1b and 2a^d).The degree of ser- production of modal magnetite (as indicated by magnetic pentinization is inhomogeneous on the thin-section scale. susceptibility) is a consequence of a multi-stage serpentini- The estimates reflect an average value, but, particularly in the moderately serpentinized samples, there is grain-to- zation in which magnetite formation is delayed. Bach et al. grain variation in the degree of serpentinization. Overall, (2006) proposed a two-stage process involving early forma- we interpret the samples as representative of progressive tion of ferroan brucite and subsequent oxidation^silicifica- serpentinization of the olivine-rich troctolite. tion (during late fluid influx) of the brucite to magnetite plus serpentine that could account for the magnetic and density characteristics of serpentinized peridotites drilled Early intra-olivine veins (type 1) at Ocean Drilling Program (ODP) Site 1274. (early or incipient serpentinization) In this study we describe a sequence of serpentinized oli- These narrow (50Á05 mm wide) and irregular veins form vine-rich (70^90% olivine) troctolites from Integrated a network within single olivine grains (Fig. 2a and b).They Ocean Drilling Program (IODP) Hole U1309D. Detailed are the commonest vein type in weakly serpentinized sam- examination of several samples from this core allows us to ples. Type 1 veins are brownish in color, usually free of constrain several aspects of the timing and progression of opaque phases (but see Fig. 2c), and rarely pass from one serpentinization. In particular, we demonstrate that mag- olivine grain to another nor into any surrounding phases netite in serpentinites forms via a two-stage process as a (Fig. 2a). The color and textural characteristics of these consequence of both oxidation^silication of brucite and veins are distinctive, and relict examples of type 1 veins oxidation^desilication of ferroan serpentine (e.g. Frost & have been identified in all but the most strongly serpenti- Beard, 2007). nized samples (Fig. 2d and e). 388 BEARD et al. SERPENTINIZATION IN THE TROCTOLITE 42 15'W 42 10'W 42 05'W 42 00'W 41 55'W (b) -3000 -2000 cm alteration (a) sample locations 30 15'N 0 x -3000 10 >50% -3000 x -2000 -4000 -4000 20 hole -20001309D ssif 30 10'N 30 20-50% 40 Atlantis-1000 ma x -2000 0 Lost City -4000 50 -200-3000 -4000 30 05'N -3000 MAR Unit 562, Dunitic troctolite 60 -4000 x <20% Atlantis 70 -4000 FZ 305-U1309D-227R-003 (top = 1094.93 mbsf) 80 -4000 30 00'N Fig. 1. (a) Bathymetry of the Atlantis Massif region and the location of IODP Site 1309D. (b) Top 80 cm of core U1309D-227R-3W showing distribution of alteration (Ildefonse et al., 2006) and locations of samples (crosses) examined for this study.
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