Geochimica et Cosmochimica Acta, Vol. 65, No. 23, pp. 4339 – 4357, 2001
Copyright © 2001 Elsevier Science Ltd
Printed in the USA. All rights reserved
0016-7037/01 $20.00 ⫹ .00
Pergamon
PII S0016-7037(01)00714-1
Near-solidus evolution of oceanic gabbros: Insights from amphibole geochemistry
LAURENCE A. COOGAN,1,* ROBERT N. WILSON,2 KATHRYN M. GILLIS,3 and CHRISTOPHER J. MACLEOD1
1
Department of Earth Sciences, Cardiff University, Cardiff, CF10 3YE, Wales, UK
Department of Geology, The University of Leicester, University Road, Leicester, LE1 7RH, UK
3
School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada
2
(Received November 3, 2000; accepted in revised form April 25, 2001)
Abstract—The near-solidus evolution of plutonic rocks formed at slow-spreading ridges is investigated using
the major and trace element compositions of amphiboles in a suite of gabbros from the Mid-Atlantic Ridge.
These new data allow unambiguous geochemical discrimination between amphiboles of magmatic and
hydrothermal origin. In turn, this allows the gabbro solidus to be constrained to 860 ⫾ 30°C, using
amphibole-plagioclase thermometry. This is consistent with temperatures from associated secondary clinopyroxene. Magmatic amphibole, which can be identified in almost all samples, formed during metasomatism of
a low-porosity crystal mush by an evolved hydrous silicate melt. These amphiboles are characterised by high
F, Nb, and F/Cl and low Cl contents. The amphibole-forming reaction involved melt, plagioclase, and
clinopyroxene. Amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of
some primitive clinopyroxene crystals. There is no evidence for a seawater component in the magmatic
amphibole, as would be expected if high-temperature seawater ingress leads to flux melting, as has recently
been suggested. However, the ingress of seawater-derived fluids did occur at temperatures within error of the
gabbro solidus forming amphibole in veins and replacing igneous phases. These amphiboles are characterised
by high Cl, B, and Cl/F and low Nb, F, and Nb/La. The fluids involved in the formation of these amphiboles
had compositions unlike seawater or hydrothermal vent fluids. Copyright © 2001 Elsevier Science Ltd
mometer of Holland and Blundy (1994) allows reasonable
estimates of the temperature of amphibole formation to be
made; and (c) the amphibole crystal structure can incorporate a
wide range of trace elements, which can be determined by ion
microprobe.
1. INTRODUCTION
Significant mass and heat transport from the earth’s mantle
into the crust and hydrosphere occurs at midocean ridges
(MORs). Within the crust a range of mass transport processes
operate that range from purely magmatic to purely hydrothermal. In this study the transition between magmatic and hydrothermal processes at near-solidus temperatures in the lower
ocean crust is investigated. The evolution and distribution of
near-solidus melts is important as these contain very high
concentrations of the geochemically important incompatible
trace elements. Furthermore, knowledge of the spatial distribution of melt within crystal mushes, and the solidus temperature
of the lower oceanic crust, are important for the interpretation
of remote sensing (e.g., seismic) data in terms of geological
structure. Finally, very high-temperature seawater-derived fluids have the capacity to transport significant solute contents and
dramatically alter rock compositions and mineralogy.
This study aims to provide a better understanding of the
complex physical and chemical processes operating during the
transition from the magmatic to hydrothermal systems at slowspreading ridges. The approach taken is a detailed study of
amphibole major and trace element compositions in oceanic
gabbros from the MARK area (Mid-Atlantic Ridge south of the
Kane Fracture Zone). Amphibole was chosen as the principal
tracer for the processes operating for three reasons: (a) It can
form by crystallisation of evolved hydrous silicate melts and by
reaction of seawater-derived fluids with gabbro at high temperatures and it is the principal high-temperature hydrous phase in
the lower oceanic crust; (b) the plagioclase-amphibole ther-
* Author to whom correspondence
([email protected]).
should
be
1.1. Near-Solidus Evolution in Oceanic Gabbros
Recent models predict that magma chambers beneath slowspreading ridges are transient features that, when present, are
dominated by crystal mushes (e.g., Sinton and Detrick, 1992;
Sinha et al., 1997). Interstitial melt may migrate within the
mush, modifying crystal compositions, or react with the crystals present to form new minerals. The progressive solidification of these regions leads to the interstitial melt becoming
highly evolved as the porosity decreases (Coogan et al., 2000a).
Volatile elements are concentrated in the interstitial silicate
melt and this may lead to their exsolution to form a separate
exsolved magmatic fluid (“degassing”). These may modify any
melts, mushes, or solid rocks through which they migrate
and/or may be added directly into the hydrothermal system.
Probably the best evidence that magmatic volatiles are exsolved comes from the study of fluid inclusions (e.g., Kelley,
1997). Between times of magmatic accretion, magma chambers
freeze and seawater-derived fluids penetrate into the gabbros.
Ingress of seawater-derived fluids into the lower oceanic crust
at slow-spreading ridges is thought to occur through a downward-migrating cracking front (Lister, 1974; Wilcock and
Delaney, 1996). Water-rock reaction in this dynamic region
may lead to important modification of both the fluid and rock
compositions. For example, it has recently been hypothesised
that penetration of seawater at near-solidus temperatures leads
to the mobilization of incompatible “immobile” trace elements,
addressed
4339
4340
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
Table 1. Representative Analyses of Amphibole
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
Total
Mg#
Li
Be
B
F
Cl
K
Sc
Ti
V
Cr
Y
Zr
Nb
Ti
Sr
Y
Zr
Ba
La
Ce
Pr
Nd
La
Ce
Pr
Nd
Sm
Eu
Gd
Dy
Ho
Yb
Hf
1r
2r
3r
4i
5i
6i
7i
8i
9i
10r
11v
12v
13v
14i
15l
16b
17b
18i
19i
20i
49.29
0.42
5.96
0.03
15.82
0.3
12.99
11.75
1.23
0.03
98.11
59
0.01
0.20
0.82
781
1081
240
13.2
2105
329
106
95.2
7.7
0.03
2195
28.9
78.2
7.0
0.75
1.33
7.78
2.20
17.44
1.37
8.16
2.20
17.62
8.37
1.83
12.57
15.93
3.27
7.88
0.71
46.48
1.62
9.21
0.05
15.04
0.2
13.25
11.58
2.21
0.25
100.00
61
0.02
0.29
0.52
1542
119
1653
125.9
9140
551
117
96.2
103.6
0.22
8123
16.3
109.6
92.7
1.54
2.41
13.23
3.42
24.41
2.48
13.86
3.42
24.65
10.58
3.24
15.02
20.74
4.18
11.64
4.45
47.7
1.45
7.2
0.02
14.11
0.23
14.21
11.13
1.85
0.14
98.16
64
0.31
0.25
0.54
1089
249
1116
114.0
9069
371
147
48.7
107.9
0.61
10,014
24.1
55.5
118.4
3.01
2.19
8.92
1.94
13.44
2.25
9.35
1.94
13.57
5.18
2.01
6.66
9.59
2.22
7.45
3.58
43.90
3.01
10.68
0.02
13.10
0.21
13.59
11.26
2.51
0.27
98.66
65
0.19
0.49
0.44
1539
121
1810
155.9
18,068
481
178
64.5
169.9
2.22
18,639
43.6
70.4
174.9
3.98
2.32
9.82
2.32
13.75
2.39
10.29
2.32
13.89
5.35
2.96
6.99
10.69
2.58
8.67
4.44
44.66
2.29
10.23
0.00
13.36
0.22
13.60
11.14
2.32
0.28
98.17
64
0.31
0.40
0.77
1481
65
2029
168.7
15,702
405
119
60.5
185.0
1.66
16,632
38.3
50.2
172.8
4.26
2.05
8.94
1.95
11.68
2.10
9.37
1.95
11.79
4.07
2.85
5.39
7.72
1.81
6.84
4.98
46.43
1.69
8.68
0.02
13.84
0.22
13.89
11.38
1.93
0.20
98.35
64
0.17
0.19
0.44
1871
45
1158
66.9
8755
379
107
39.7
119.4
0.78
8429
16.6
34.6
111.0
1.32
1.11
5.43
1.17
7.92
1.14
5.69
1.17
8.00
3.07
1.81
4.40
5.61
1.22
3.92
3.09
43.71
3.83
11.65
0.05
9.69
0.13
14.99
11.89
2.72
0.16
99.04
73
0.58
0.16
0.51
3927
105
1101
144.3
21,970
808
595
87.6
349.2
7.93
22,767
46.1
92.5
385.0
2.41
1.16
8.03
2.13
16.94
1.19
8.42
2.13
17.11
7.95
1.53
12.34
16.22
3.20
8.28
8.81
43.48
4.02
11.57
0.03
9.79
0.15
14.91
11.69
2.58
0.16
98.69
73
0.63
0.18
0.40
4198
109
1034
125.7
23,666
699
631
85.3
352.3
12.27
23,100
42.3
86.1
355.3
2.18
0.99
7.20
2.17
16.95
1.02
7.54
2.17
17.12
7.55
1.37
11.08
14.75
3.29
8.15
7.63
43.85
3.48
11.60
0.09
9.74
0.17
14.99
11.84
2.46
0.17
98.64
73
0.70
0.15
0.58
3816
74
1115
124.8
20,213
681
695
84.2
330.4
7.73
20,213
39.2
86.2
351.3
2.30
1.13
7.69
2.16
16.31
1.16
8.05
2.16
16.47
8.01
1.44
11.08
15.17
3.16
8.35
8.00
52.58
0.27
4.46
0.02
10.40
0.19
17.49
12.33
0.87
0.08
98.85
75
0.27
0.05
3.70
246
651
463
23.8
1671
117
242
7.3
8.1
0.12
1849
3.8
5.9
8.0
0.65
0.84
1.66
0.25
1.45
0.87
1.74
0.25
1.46
0.61
0.38
0.88
0.92
0.22
0.93
0.27
47.32
0.26
8.21
0
11.89
0.18
15.03
12.48
1.55
0.26
97.31
69
0.47
0.07
4.76
447
1443
1589
8.6
1837
178
124
11.6
7.3
0.14
1577
8.2
11.8
6.2
1.51
3.42
6.48
0.51
2.74
3.52
6.79
0.51
2.77
0.94
0.72
2.21
1.92
0.45
1.22
0.24
47.42
0.25
8.42
0.02
12.1
0.18
14.88
12.28
1.57
0.27
97.63
69
0.53
0.08
4.03
444
1530
1612
14.8
2210
214
127
14.5
11.7
0.16
2173
7.4
11.6
9.6
1.87
4.64
9.95
0.88
2.85
4.77
10.43
0.88
2.88
1.13
0.79
1.46
2.01
0.43
1.54
0.31
46.79
0.38
8.27
0
11.98
0.18
14.17
11.86
1.63
0.19
95.63
68
0.57
0.07
3.45
440
1291
1318
24.7
2942
261
140
19.3
18.3
0.13
2756
9.0
17.3
18.6
1.40
1.35
2.76
0.49
3.23
1.39
2.90
0.49
3.27
1.57
0.68
2.42
3.36
0.58
1.93
0.49
43.34
3.94
11.19
0.18
10.57
0.14
14.76
11.04
2.85
0.15
98.31
71
0.42
0.22
0.48
1490
44
1025
156.9
23,367
936
1204
70.3
124.3
7.66
22,690
46.4
69.5
119.4
3.08
1.34
7.74
1.97
13.87
1.38
8.11
1.97
14.01
5.62
1.72
9.51
11.54
2.27
6.85
4.51
43.26
3.16
11.43
0.1
11.69
0.15
14.07
11.06
2.74
0.16
97.98
68
0.36
0.18
0.15
1534
25
1089
139.8
20,958
691
675
59.5
65.3
2.10
20,282
45.3
59.1
64.3
2.80
1.05
6.00
1.54
11.21
1.08
6.29
1.54
11.32
5.25
1.69
7.67
9.95
2.03
5.37
2.62
46.65
1.9
10.07
0.12
8.48
0.17
16.24
11.18
2.29
0.05
97.26
77
0.86
0.16
1.59
1109
32
380
139.1
13,487
656
1189
51.7
82.2
1.79
12,636
44.0
51.3
82.2
3.83
1.90
7.29
1.53
10.27
1.95
7.64
1.53
10.38
4.35
1.54
6.36
8.54
1.75
5.03
2.78
44.87
2.52
11.36
0.16
8.93
0.14
15.39
11.41
2.76
0.05
97.64
75
0.96
0.16
3.14
1006
34
391
139.9
15,193
671
1287
52.5
76.6
3.04
14,959
52.1
51.8
77.4
4.98
2.00
6.74
1.31
10.92
2.05
7.07
1.31
11.03
4.02
1.37
6.97
8.95
1.82
4.79
2.52
43.27
3.08
11.77
0.04
11.41
0.14
14.20
11.69
2.51
0.19
98.43
69
0.39
0.19
0.27
1180
52
1271
139.9
18,255
623
489
79.8
169.4
4.42
19,157
44.5
81.1
176.3
3.12
1.89
10.15
2.49
17.07
1.94
10.64
2.49
17.24
7.54
2.89
10.93
15.07
2.95
7.29
6.47
42.76
3.33
11.57
0.07
12.15
0.13
13.67
11.60
2.57
0.24
98.22
67
0.42
0.22
0.39
1344
128
1826
141.1
20,715
670
313
104.0
245.3
3.91
22,052
54.4
104.4
256.0
4.68
2.38
13.00
3.12
23.42
2.44
13.63
3.12
23.65
9.83
2.74
15.48
19.57
4.17
10.22
7.55
43.09
2.86
11.40
0.04
11.41
0.13
14.11
11.85
2.62
0.23
97.83
69
1.16
0.23
0.43
1528
60
1521
130.4
17,506
581
425
77.0
165.8
2.36
16,272
42.7
73.6
151.0
3.78
2.27
10.95
2.71
17.58
2.33
11.48
2.71
17.75
7.06
2.16
10.40
13.24
2.72
6.84
4.76
MARK ⫽ Mid-Atlantic Ridge south of the Kane Fracture Zone; r ⫽ 1 replacive; v ⫽ vein; b ⫽ bleb; i ⫽ interstitial. Representative major (wt.
7–9: 153 923A 8R1 126 to 131 cm, Troct; 10 –13: 153 923A 8R2 81 to 88 cm, OG; 14 –17: 153 923A 8R2 110 to 116 cm, OG; 18 –20: 153 923A
29 –30: 153 923A 14R1 100 to 112 cm, Troct; 31–32: 153 923A 16R1 17 to 19 cm, Troct; 33–34: Alvin 1012-12, G; 35– 40: 153 922A 2R5 69 to
possibly through flux melting, with little mineralogical change
(McCollum and Shock, 1998; Hart et al., 1999).
1.2. Formation of Amphibole in the Oceanic Crust
Amphibole is the principal high-temperature hydrous mineral to form in MOR gabbros whether during the crystallisation
of evolved hydrous silicate melt or as a product of hightemperature water-rock reactions. Because of this, many studies have documented the composition and texture of amphibole
in oceanic gabbros (e.g., Mével, 1988; Gillis et al., 1993;
Manning et al., 1996); however, the discrimination of magmatic and metamorphic amphiboles on major element grounds
has been somewhat unsatisfactory. Recent studies have used
trace elements to better constrain the origins of amphiboles
(Gillis, 1996; Tribuzio et al., 1995, 1999; Cortesogno et al.,
2000; Gillis and Meyer, 2001) but either concentrate on a
smaller suite of elements (e.g., just rare earth elements [REEs]),
or a more limited range of amphibole textures (e.g., only
interstitial amphibole), than this study.
1.3. The MARK Area and the Sample Suite Studied
The samples used in this study come from the MARK area of
the Mid-Atlantic Ridge (MAR). Most samples come from
Ocean Drilling Program (ODP) Hole 923A, which is located
⬃6 km south of the Kane Fracture Zone. Two samples come
from ⬃2 km south of Hole 923A; one from Alvin dive 1012
(Karson and Dick, 1983) and one from ODP Hole 922B (see
Fig. 4 in chapter 1 of Cannat et al., 1995 for a map of the
Near-solidus evolution of oceanic gabbros
4341
in Gabbros From the MARK Area.
21b
22b
23b
24i
25i
26i
27i
28i
29i
30b
31i
32i
33r
34v
35r
36r
37r
38r
39r
40r
43.16 42.42 43.71 43.32 43.30 43.28 43.64 43.72 43.95 46.05 43.16 42.89 51.56 46.52 47.75 47.19 46.14 46.75 46.32 46.16
2.51
2.93
2.08
3.71
3.51
3.28
2.90
3.00
2.73
2.37
3.26
3.63
0.15
0.07 1.17 0.12
1.71
1.37
1.63
0.14
12.94 12.95 13.22 11.53 11.50 11.48 11.59 11.44 11.29
9.83
11.38 10.98 6.625 11.21 6.45 7.33
7.95
7.37
7.84 7.635
1.87
1.99
1.84
0.07
0.07
0.03
0.05
0.09
0.1
0.59
0
0.12
0.025
0
0.03 0.04
0.02
0.00
0
0.025
5.95
5.66
5.6
9.21
9.18
9.53
8.98
8.95
10.93
6.19
9.6
8.84
6.465 8.38 17.52 19.79 16.66 16.87 16.39 20.12
0.10
0.10
0.08
0.12
0.11
0.13
0.13
0.14
0.14
0.1
0.14
0.12
0.15
0.15 0.32 0.36
0.28
0.29
0.28 0.365
16.21 16.00 16.23 15.38 15.55 15.22 15.57 16.23 14.51 17.16
14.6
14.98 20.195 17.86 12.12 10.14 12.06 11.99 12.22 9.825
11.87 11.86 12.08 11.63 11.82 11.96 11.78 10.97 11.64 12.96 11.54 11.45 11.41
11 11.13 11.10 11.15 11.05 10.88 11.295
2.91
2.82
2.68
2.76
2.79
2.74
2.73
2.53
2.42
2.34
2.34
2.33
1.6
2.59 1.54 1.67
1.92
1.78
1.93
1.7
0.10
0.07
0.06
0.16
0.17
0.18
0.17
0.16
0.2
0.05
0.22
0.2
0.05
0.13 0.19 0.09
0.25
0.21
0.23 0.095
97.87 96.88 97.74 98.09 98.22 98.05 97.78 97.33 98.01 97.83 96.43 95.77
98.4 98.17 98.3 98.01 98.22 97.84 97.87 97.58
83
83
84
75
75
74
76
76
70
83
73
75
85
79
55
48
56
56
57
46
0.39
1.36
1.11
0.65
0.15
0.23
0.15
0.10
0.21
0.29
0.25
0.52
0.31
0.02 0.03 0.38
0.12
0.09
0.05
0.15
0.10
0.09
0.11
0.25
0.27
0.28
0.19
0.22
0.17
0.08
0.17
0.13
0.20
0.25 0.72 0.42
1.05
0.91
0.84
0.38
0.25
0.18
0.18
0.95
0.77
1.74
0.45
0.29
0.38
0.16
0.28
0.22
1.32
0.78 5.37 3.58
6.16
5.65
5.83
3
2429
2313
2378
2525
2396
2354
2467
2988
1322
4119
1988
2068
176
278
705
132
1419
1708 1680
193
26
18
31
50
85
118
20
99
108
18
62
56
602
1281 127 2535
214
118
237
1545
725
463
442
1102
955
1229
1088
906
1346
331
1407
1070
406
719 1368 649
1937
1824 1750 557.3
115.9 113.0 119.4 111.4 105.5
98.5
117.2 126.4 135.6 146.8 113.0 106.4
22.3
8.8 151.1 391.0 113.4 123.5 133.9
4
16,075 12,925 11,499 22,451 20,072 19,727 17,048 23,455 17,629 18,111 21,246 18,097 658
371 6890 762 11,309 10,691 9724
888
568
527
521
619
529
433
555
1194
695
654
626
550
63
64
195
72
244
234
213
76
16801 16028 17114
652
435
305
537
833
1000
7307
693
1230
188
114
118
83
103
107
119
82.1
31.2
37.4
32.5
55.2
46.6
52.6
57.4
95.1
80.3
41.7
76.6
50.7
7.6
1.2 280.2 107.6 190.5 161.3 242.8 88.7
64.7
71.0
47.7
98.0
91.4
93.6
75.5
383.1 132.1
52.8
126.7
64.0
11.2
1.0
47.1
0.6
112.4
96.6
79.2
0.64
17.19
4.63
7.63
5.08
4.42
4.11
2.99
8.86
3.40
5.17
3.93
1.43
0.10
0.33 0.62 0.18
2.25
1.87
1.46
0
16,075 14,693 10,280 23,241 19,960 18,938 17,730 22,886 16,942 15,473 20,235 20,108 675
338 7002 812 11,393 10,801 9989 941.4
86.7
75.4
66.5
46.3
44.7
41.6
39.9
46.8
39.8
61.1
39.2
32.4
3.8
6.1
9.9
19.3
21.7
20.5
15.9
10.4
31.5
35.8
31.5
58.7
45.6
61.6
60.6
95.6
81.1
35.3
76.1
56.7
7.7
1.4 265.9 118.1 195.5 153.7 276.8 89.9
65.8
64.7
45.4
101.6
85.5
102.5
83.3
382.1 131.1
42.6
128.7
72.7
10.8
1.1
48.5
0.7
119.7 104.0 83.4
0.85
3.83
2.33
1.87
2.80
2.26
4.34
2.46
1.95
2.83
1.94
2.96
1.76
0.50
0.42 2.83 4.48
5.73
5.37
4.21
2.04
0.68
0.76
1.45
1.54
1.25
1.43
1.44
1.17
1.61
0.73
1.36
0.96
4.85 13.70 13.75 1.54
9.53
8.76 14.46 1.46
2.97
3.75
4.60
7.49
6.28
7.69
7.41
7.18
8.88
3.73
8.94
5.89
13.33 38.40 70.12 10.20 39.79 36.66 70.93 9.10
0.68
0.83
0.87
1.82
1.32
2.00
1.84
2.02
2.24
0.90
2.41
1.63
1.70
4.18 16.52 3.22
8.72
7.61 16.13 2.65
4.78
6.95
6.11
12.69 10.15 14.92 14.06 15.86 17.43
5.92
17.51 11.99
6.69 12.37 103.7 25.74 54.49 46.99 106.5 20.60
0.70
0.78
1.49
1.59
1.28
1.47
1.48
1.20
1.65
0.75
1.40
0.99
4.99 14.08 14.13 1.58
9.80
9.00 14.87 1.50
3.11
3.93
4.82
7.85
6.58
8.06
7.77
7.52
9.30
3.91
9.37
6.17
13.97 40.24 73.48 10.69 41.70 38.42 74.34 9.54
0.68
0.83
0.87
1.82
1.32
2.00
1.84
2.02
2.24
0.90
2.41
1.63
1.70
4.18 16.52 3.22
8.72
7.61 16.13 2.65
4.82
7.02
6.17
12.82 10.25 15.07 14.20 16.02 17.61
5.98
17.69 12.11
6.76 12.50 104.8 26.00 55.04 47.46 107.6 20.81
2.47
2.85
2.35
4.84
3.57
6.04
6.17
6.82
7.54
2.51
8.14
5.20
1.44
0.88 36.96 12.68 19.96 16.07 36.91 9.65
0.82
1.06
0.90
1.64
1.30
1.86
1.81
1.46
1.98
1.16
1.57
1.21
0.92
1.54 4.93 3.78
5.02
4.69
6.27
2.89
3.71
4.50
3.85
7.48
5.76
8.35
8.13
11.34 11.81
4.42
10.61
7.41
1.30
0.83 44.46 16.33 25.73 20.97 44.71 12.82
5.34
6.42
5.32
9.96
7.21
11.13
9.50
15.62 14.81
5.93
13.17 10.25
1.36
0.31 51.52 22.24 35.50 26.71 49.35 17.44
1.14
1.31
1.16
1.97
1.52
2.14
2.26
3.32
2.92
1.31
2.61
2.03
0.29
0.07 9.29 4.54
7.04
5.48
9.96
3.66
3.34
3.38
3.20
5.26
5.30
5.61
5.24
9.58
7.78
3.07
6.80
5.60
1.14
0.28 20.98 11.60 20.59 16.39 25.94 9.85
2.08
1.94
1.70
2.82
1.97
3.11
2.84
9.43
3.96
1.47
4.26
2.38
0.29
0.03 4.25 1.97
7.69
7.02
5.88 1.0613
%) and trace (ppm) element analyses of amphibole. Sample number: 1–2: 153 923A 3R1 18 to 22 cm, OxG; 3– 6: 153 923A 3R2 39 to 45 cm, OxG;
10R2 1 to 4 cm, OG; 21–23: 153 923A 12R1 30 to 38 cm, Troct; 24 –27: 153 923A 12R2 66 to 73 cm, OG; 28: 153 923A 13R2 68 to 75 cm, Troct;
76 cm, G. Lithology/protolith: OxG ⫽ oxide gabbro; OG ⫽ olivine gabbro; Troct ⫽ troctolitic gabbro; G ⫽ gabbro.
sampling localities). These additional samples were included
because previous work has shown the Alvin sample to contain
light REE (LREE)– enriched amphibole (Gillis and Meyer,
2001) and has shown that Hole 922 has more LREE-enriched
bulk rock compositions (Barling et al., 1997), and more Cl-rich
apatite, than Hole 923A. These characteristics have been associated with exsolved magmatic fluids (Boudreau et al., 1986;
Flynn and Burnham, 1978).
Previous studies of near-solidus processes in gabbros from
this area suggest that the solidification of crystal mushes is
associated with extensive interstitial melt migration that causes
metasomatism of the crystal mush (“magmatic metasomatism”;
Ross and Elthon, 1997; Coogan et al., 2000a). The highest
temperature fluid-rock interactions are thought to involve the
exsolution of magmatic fluids from evolved hydrous silicate
melts at ⬎700°C (Kelley and Delaney, 1987; Kelley et al.,
1993; Kelley, 1997). At lower temperatures seawater ingress
has been thought to be predominantly tectonically controlled,
initially penetrating along shear zones and at lower temperatures through brittle fractures and cataclasites (Gillis et al.,
1993; Dilek et al., 1997).
1.4. Terminology
In this study we are concerned with late-stage hydrous silicate magmas, fluids exsolved from these magmas, and seawater-derived fluids as well as mixtures between these end-members. We refer to (a) residual interstitial silicate melt derived
from the fractionation of basalt that has reached amphibole
saturation as hydrous silicate melt, (b) volatile-rich phases
4342
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
exsolved from a hydrous silicate melt as exsolved magmatic
fluids, and (c) phases derived from the heating of seawater and
the reaction of the resulting fluids with the oceanic crust as
seawater-derived fluids. Amphiboles formed from hydrous silicate melts are termed magmatic and those formed from seawater-derived fluids are termed hydrothermal. Formation of
amphibole from exsolved magmatic fluids is discussed explicitly. The term secondary clinopyroxene is used to describe
clinopyroxene that formed at a lower temperature than the main
igneous clinopyroxene. This is characterised by a clearer appearance; higher Ca; and lower Ti, Na, and Al than the main
igneous clinopyroxene. This term is not meant to suggest that
these clinopyroxene necessarily form under subsolidus conditions.
which contains approximately 3 times more Al than amphibole,
gave a Be abundance of only ⬃0.15 ppm, which, assuming that
all of this is interference, suggests that ⬍0.07 ppm of Be can be
explained by Al3⫹ interference. A Hoover Dam amphibole
standard was measured daily to monitor accuracy and precision. Results for this are compared with instrumental neutron
activation analysis data (Irving and Frey, 1984) in Table 2.
Comparison of electron probe K, Ti, and Cr abundances reveal
good correlations (Pearson product moment correlation coefficients squared [r2] of 0.92, 0.95, and 0.99, respectively), although the absolute abundances of K and Cr vary between the
techniques, with K being lower by ion probe (by ⬃15%) and Cr
being higher (by ⬃30%).
2. DATA PRESENTATION
1.5. Analytical Techniques
Major and minor element compositions were determined on
a JEOL 8600 electron microprobe at The University of Leicester using a 15 kV accelerating voltage and 30 nA beam
current with 20 s counting on the peak and background for Si,
Al, Fe, Mg, and Ca and 60 s for Mn, Ti, K, and Na. A 5 ␮m
beam was used for all amphibole analyses. Plagioclase was
analysed with a 10 ␮m beam and clinopyroxene with a focused
beam. The raw data were corrected using a ZAF correction
procedure.
Trace elements were determined using a Cameca IMS-4f ion
microprobe at the University of Edinburgh and representative
analyses are reported in Table 1. Before analysis samples were
washed individually in an ultrasonic bath in petroleum ether for
⬎5 min, dried on a hotplate and rewashed in deionised water,
redried, and then gold coated. Analyses were performed as two
spots per point with the light elements (Li, Be, B, F, Cl, K, Sc,
Ti, V, Cr, Y, Zr, and Nb) and heavy elements (Ti, Sr, Y, Zr, Ba,
the REEs, and Hf) analysed separately. Titanium, Y, and Zr
were analysed in both spots to ensure data compatibility between analyses. Data are reported in Table 1 only if the common elements in the light and heavy element groups (Ti, Y, Zr)
show ⬍20% difference between the two analyses. Ten cycles
consisting of 3 to 10 s counting on each element were measured
for each analysis. Silicon was used as the internal standard for
both groups of elements.
A primary 16O⫺ beam of 15 keV net energy was focused on
a 10 to 20 ␮m spot with an ⬃8 nA current. Sputtered positive
secondary ions were accelerated to a nominal energy of 4.5
keV; molecular ions were reduced using energy filtering so that
only ions with initial energies between 55 and 95 eV were
allowed into the mass spectrometer. Despite this energy filtering Ba and LREE oxide corrections on Eu and the heavy REEs
were necessary. Calibration was achieved via ion yields calculated based on those for National Institute of Standards and
Technology 610 assuming 500 ppm of all elements in this glass
except for F (295 ppm; Hoskin, 1999).
Potential interferences on 35Cl and 19F (e.g., LiSi, OH) are
not considered important as no correlation exists between these
elements and Li or H abundances, respectively. This is consistent with the findings of Tribuzio et al. (1995). There is no
evidence for a significant interference of Al3⫹ on Be; for
example, large variations in Be occur with minor, and uncorrelated, variations in Al. Furthermore, analysis of plagioclase,
Amphibole major and trace element compositions were determined on samples that range in degree of alteration from
almost unaltered through to samples in which amphibole has
completely replaced all primary pyroxene. All samples show
relatively minor degrees of alteration below the amphibolite
facies. Samples range between troctolitic gabbros, olivine gabbros, gabbros, and oxide gabbros and range in grain size from
microgabbro (⬃150 ␮m crystals) to very coarse-grained gabbro (⬎1 cm crystals).
2.1. Amphibole Textures
Amphiboles are divided into four textural types (Fig. 1): (a)
Interstitial amphibole: brown amphibole crystals interstitial to
the main igneous phases (plagioclase, clinopyroxene, and olivine). These are generally isolated from one another (i.e., not
clusters of crystals) and have well-defined and smoothly curved
or planar grain boundaries (Fig. 1a). This textural type includes
both subgranular crystals and elongate crystals appearing to
“wet” grain boundaries and triple junctions and is found in
almost all samples but is almost always ⬍1% of the mode. (b)
Amphibole blebs: brown or greenish-brown “blebs” enclosed
within clinopyroxene including in the core of clinopyroxene
crystals in coarse-grained gabbros. These blebs are generally
surrounded by secondary clinopyroxene, normally have
rounded margins, and commonly occur in clusters, but linear
arrays have not been observed. These are found in all lithologies and occur within clinopyroxene of all compositions, including the most primitive sampled (Mg#90; Fig. 1b). (c) Replacive amphibole: green or brownish-green amphibole
concentrated at clinopyroxene and plagioclase grain boundaries
commonly, but not always, composed of multiple crystals and
generally having more irregular grain boundaries than interstitial amphiboles. Secondary clinopyroxene is commonly found
adjacent to replacive amphiboles (Fig. 1c). (d) Vein amphibole:
amphibole that occurs in veins (Fig. 1d). Vein amphiboles
analysed for trace element contents in this study all occur
within plagioclase.
2.2. Major Element Compositions and Thermometry
2.2.1. Amphibole major and minor element compositions
Amphibole major element compositions span a similar range
to that previously reported for amphibole in oceanic gabbros
Near-solidus evolution of oceanic gabbros
Table 2. Comparison of the Average Concentrations Determined for
the Hoover Dam Amphibole Standard Analysed Daily With the
Values From Irving and Frey (1984).
Average
Li
Be
B
F
Cl
K
Sc
Ti3
V
Cr
Y3
Zr3
Nb
Ti2
Sr
Y2
Zr2
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Dy
Ho
Yb
Hf
0.80
0.59
0.12
2277
104
14,256
54.90
35,888
458
101
25.08
79.50
28.00
35,548
553
25.00
79.86
290
8.15
25.46
4.42
24.30
6.34
2.23
7.11
5.74
0.98
1.74
2.80
SD
0.03
0.02
0.01
181
5.60
289
0.77
591
4.78
8.00
0.49
2.38
0.86
621
12.50
0.64
2.01
11.83
0.22
1.09
0.16
1.36
0.33
0.20
0.38
0.22
0.06
0.14
0.21
Irving and Frey
(1984)
13,483
46
34,171
34,171
6.35
22
5.32
1.75
1.49
2.10
Also shown are the standard deviations of the six analyses as an
estimate of precision. Superscripts 1 and 2 for Ti, Y, and Zr refer to the
two spots per analysis (see section 1.5).
(Fig. 2) and are all calcic (⬎10 wt.% CaO). Interstitial brown
amphiboles and amphibole blebs have similar major element
compositions, although blebs have slightly more diverse compositions with generally slightly lower TiO2 and slightly higher
Na2O abundances (Fig. 2). Replacive and vein amphiboles
form a spectrum of compositions from the field of bleb and
interstitial brown amphiboles to higher SiO2 and lower TiO2
and Na2O. Al2O3 abundances are lower in replacive amphiboles than in interstitial and bleb amphiboles, but vein amphiboles show a range of Al2O3 concentrations from similar to
those of the replacive amphiboles to greater than in either
interstitial or bleb amphiboles. Replacive and vein amphiboles
commonly have higher Mn abundances, at a given Mg# (⫽Mg/
[Mg ⫹ Fetot]; in cations), than interstitial and bleb amphiboles
(not shown).
Interstitial brown amphiboles show little correlation between
major and minor element compositions. There is no correlation
between their Mg# and TiO2, Na2O, K2O or Cr2O3 abundances.
Average interstitial brown amphibole compositions are compared with the average compositions of clinopyroxene in the
same sample in Figure 3. The Mg# of interstitial brown amphibole correlates well with the average Mg# of clinopyroxene in
the same sample (Fig. 3a). This suggests buffering of the
Mg/Fe of the amphibole by the cumulate assemblage. Chromium, a highly compatible element, is expected to be depleted
4343
in any late-stage melts from which magmatic amphibole may
grow; however, Figure 3b shows a near one-to-one correlation
between the average Cr2O3 abundance in clinopyroxene and
that in interstitial brown amphibole. The one sample that shows
a much higher Cr2O3 abundance in the amphibole than clinopyroxene comes from a thin (a few cm wide) oxide gabbro within
a ⬎10-m thick section of primitive (high-Cr) gabbros. This
suggests that the Cr content of the amphibole may not simply
be related to the composition of the adjacent clinopyroxene
crystals but also to the surrounding rocks. The partitioning of
incompatible elements between amphibole and clinopyroxene
is more complex, with amphibole Ti and Na abundances showing no correlation with those of clinopyroxene in the same
sample but Mn correlating reasonably well.
An electron microprobe traverse across an amphibole bleb
within a clinopyroxene is shown in Figure 4. The amphibole
bleb is ⬃8 ␮m wide and is surrounded by an ⬃6 ␮m-wide zone
of secondary clinopyroxene. Although the lack of three-dimensional spatial information prevents a full mass balance of these
data, a simple mass balance can be calculated assuming no
mass input or removal. Explaining the abundances of SiO2,
TiO2, FeO, and Al2O3 requires that the volume of secondary
clinopyroxene is between 6 and 8 times as large as the volume
of amphibole. Calcium and Na2O require a larger volume of
secondary clinopyroxene and MgO requires a smaller volume,
suggesting that these elements may have been added and lost,
respectively.
2.2.2. Amphibole-plagioclase thermometry
Equilibration temperatures have been calculated for ⬃500
amphibole-plagioclase pairs using the Holland and Blundy
(1994) edenite-richterite thermometer. A detailed discussion of
the merits of this thermometer for calculating amphibole formation temperatures in oceanic gabbros can be found in Manning et al. (1996, 2001). These arguments are not repeated here,
but we briefly document a few details regarding amphiboleplagioclase equilibrium and the applicability of the thermometer. Wherever the plagioclase adjacent to amphibole showed a
different electron backscatter intensity to plagioclase further
away, it was ensured that the plagioclase closer to the amphibole was analysed. This was generally the case for replacive
and vein amphiboles but not for interstitial amphiboles. Furthermore, the average separation of analyses of plagioclase and
amphibole was only ⬃27 ␮m. The calibrant range of the
thermometer is for equilibration temperatures between 500 and
900°C and is thought to be accurate within ⫾40°C (Holland
and Blundy, 1994). Less than 6% of the data lie outside of this
range, between 900 and 949°C; these are included in the
calculation of the average amphibole formation temperatures to
avoid skewing this to lower temperatures. Titanium, Mn, F, and
Cl are not used in the thermometer, and thus, variations in these
elements are not accounted for in calculating equilibrium temperatures. However, only a very weak positive correlation
exists between the calculated temperature and the Ti content of
the amphibole and no correlation between temperature and the
other elements, suggesting that these elements do not significantly affect the thermometry and that crystallisation temperature does not control the abundances of these elements.
Very consistent equilibration temperatures for interstitial
4344
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
Fig. 1. Representative photomicrographs of amphibole textural types distinguished in this study: (a) interstitial amphibole
in sample 153 923A 8R1, 126 to 131 cm, a troctolitic gabbro; (b) amphibole blebs enclosed in clinopyroxene from sample
153 923A 14R1, 100 to 110 cm (note the clear secondary clinopyroxene around the amphibole); (c) replacive amphibole
in sample 153 923A 3R2, 39 to 45 cm, an oxide gabbro (note the clear secondary clinopyroxene along the contact of the
amphibole and clinopyroxene); and (d) vein amphibole in sample 153 923A 8R2, 81 to 88 cm, an olivine gabbro. Note:
plag ⫽ plagioclase; cpx ⫽ clinopyroxene; 2cpx ⫽ secondary clinopyroxene; amph ⫽ amphibole; ilm ⫽ ilmenite.
amphiboles are found. Within-sample average temperatures
range from 842 to 888°C with no correlation with lithology.
The overall mean, median, and modal equilibration temperatures for interstitial amphiboles are 864, 866, and 857°C, respectively, with one standard deviation of 27°C (Fig. 5). This
constrains the formation of interstitial amphibole to be within a
narrow temperature interval that corresponds closely (within
the uncertainty of the thermometer) with experimentally determined amphibole liquidus temperatures in hydrous basaltic
systems at low pressure (⬃900°C; Spear, 1981; Spulber and
Rutherford, 1983). Lower equilibration temperatures are observed for vein and replacive amphiboles, although more limited data are available for these textural types. No temperatures
can be calculated using this thermometer for blebs, as these
occur enclosed in clinopyroxene without plagioclase. However,
restricted clinopyroxene thermometry (Lindsley and Anderson,
1983) for secondary clinopyroxene around amphibole blebs
indicates minimum formation temperatures of ⬃850 to 900°C,
similar to that for interstitial amphibole.
2.3. Trace Element Compositions
2.3.1. REEs
REE patterns for interstitial and bleb amphiboles are relatively uniform and similar to those of clinopyroxene from the
same sample suite (Coogan et al., 2000a; Fig. 6). Both positive
and negative Eu anomalies exist in interstitial amphibole, and
these show a strong positive correlation with La/Sm that is not
observed in other textural types (see section 3.2). A broader
range of REE abundances and patterns are observed in replacive and vein amphiboles than in interstitial and bleb amphiboles, both in terms of REE slope and Eu anomalies. Replacive
amphiboles span a range of REE compositions between those
Near-solidus evolution of oceanic gabbros
4345
Fig. 2. Major element compositions of amphiboles studied here: (a) Al2O3 versus SiO2; (b) Al2O3 versus TiO2; (c) Al2O3
versus Na2O. Error bars based on counting statistics are smaller than the symbol sizes.
of clinopyroxene up to higher abundances and have slightly
flatter REE patterns and both positive and negative Eu anomalies. Vein amphiboles have high LREE abundances, the highest La/Sm, and all have positive Eu anomalies. Interestingly, a
positive correlation exists between Eu/Eu* and Yb/Ho in replacive amphiboles, suggesting the possibility that divalent Yb
may have existed, as is possible in aqueous solutions at high
temperatures (Wood, 1990).
2.3.2. High field strength elements (Ti, Zr, Nb, Hf)
The high–field strength elements (HFSE) generally occur
in higher concentrations in interstitial and bleb amphiboles
than in replacive and vein amphiboles (Figs. 2b and 7). This
is especially true for Nb, for which there is very little
overlap in abundance between these groups. This discrimination contrasts with the behaviour of the REEs, which are
4346
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
Fig. 3. Comparison of the average composition of clinopyroxene and interstitial amphibole in a given sample showing
a strong correlation between the abundances of compatible elements in clinopyroxene and amphibole: (a) amphibole Mg#
versus clinopyroxene Mg#; (b) amphibole Cr2O3 versus clinopyroxene Cr2O3. Incompatible elements do not correlate in this
manner.
not significantly depleted in concentration in vein and replacive amphiboles compared with interstitial and bleb amphiboles (Fig. 6). However, vein and replacive amphiboles
generally have higher HFSE abundances than plagioclase or
olivine and higher Nb abundances than clinopyroxene (Fig.
7).
The covariation of the geochemically similar elements Zr
and Hf is shown in Figure 8. Interstitial, bleb, and vein amphi-
boles have near chondritic Zr/Hf values. In contrast, replacive
amphiboles generally have a subchondritic ratio. This is true at
all HFSE abundances but is more obvious at lower abundances.
There is also a general separation of the textural types on the
basis of Zr and Hf abundances, with interstitial brown amphibole having the highest abundances, amphibole blebs having
lower abundances, and vein and replacive amphiboles still
lower abundances.
Near-solidus evolution of oceanic gabbros
4347
Fig. 4. Electron probe traverse across an amphibole bleb (shaded) and the surrounding clinopyroxene showing a “reaction
rim” of secondary clinopyroxene (hatched) around the amphibole bleb (e.g., Fig. 1b). Note the breaks in the scale. The
amphibole bleb contains 45 wt.% SiO2 and 12 wt.% CaO. Error bars based on counting statistics are smaller than the symbol
sizes.
2.3.3. Alkalis (Li, K), alkali earths (Be, Sr, Ba), and boron
Boron and Sr abundances vary significantly with texture
(Fig. 9). Interstitial and bleb amphiboles have higher Sr and
lower B abundances than vein and replacive amphiboles. Replacive and vein amphiboles have average B contents similar to
those of seawater and hydrothermal vent fluids, whereas interstitial and bleb amphiboles generally have lower B concentrations. The Sr abundances of vein and replacive amphiboles are
also similar to those of seawater and are considerably lower
than those of interstitial and bleb amphiboles. Bleb amphiboles
generally have Sr abundances higher than those of interstitial
amphibole and much higher than those of the host clinopyroxene (see section 3.2), as also observed by Cortesogno et al.
(2000).
The behaviour of Li, K, Be, and Ba is less systematic with
amphibole texture, although the lowest Li abundances occur in
replacive and vein amphiboles. All amphiboles have lower Li
contents (⬍1.5 ppm) than clinopyroxene (2 to 4 ppm) and
olivine (⬃4 ppm). Interstitial and bleb amphiboles can be
separated on the basis of K contents, with amphibole blebs
consistently having lower K abundances, a characteristic also
observed in electron probe data. Barium abundances are generally slightly higher in interstitial and bleb amphibole than in
replacive or vein amphiboles. The high mobility of these elements, and the relatively small differences in their abundances
in magmas, seawater, and vent fluids, make this group of
elements relatively poor discriminants between amphiboles of
different origin.
4348
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
Fig. 5. Histogram of amphibole-plagioclase equilibration temperatures normalised to unity for each texture calculated
assuming O kbar equilibration pressure. Assuming a pressure of 2 kbars increases the temperatures ⬃1%. The calibration
only covers a temperature range up to 900°C; thus, the interstitial amphibole with calculated temperatures ⬎900°C (shown
as hatched symbol) should be treated with caution. The plagioclase and amphibole analyses were spaced between 13 and
53 ␮m apart (27 and 28 ␮m median and mean separations, respectively). Note: n ⫽ number of pairs analysed.
2.3.4. Transition elements (V, Sc, Cr)
Chromium abundances are generally similar in amphibole
and clinopyroxene in the same sample (Fig. 3b). Interstitial and
bleb amphiboles generally contain higher Cr abundances
(mainly between ⬃300 and ⬃1300 ppm) than replacive and
vein amphiboles (mainly between ⬃100 and ⬃200 ppm). Vanadium and Sc abundances are also lower in vein amphiboles,
and in approximately half of the replacive amphiboles, than in
interstitial and bleb amphiboles. The other half of the replacive
amphiboles, and interstitial and bleb amphiboles, have abundances similar to those of clinopyroxene (⬃300 to 700 ppm V
and ⬃90 to 170 ppm Sc).
2.3.5. Volatile elements (F, Cl)
Vein and replacive amphiboles have high Cl and low F
contents with respect to interstitial and bleb amphiboles (Fig.
10). The few replacive amphiboles with relatively high F and
low Cl (overlapping with the interstitial amphiboles) are all
texturally compatible with being interstitial amphiboles that
were pseudomorphed by replacive amphibole. Interestingly, all
amphiboles have F abundances greater than the main igneous
phases (olivine, plagioclase, and clinopyroxene) and seawater,
suggesting a more complex origin than simple seawater-rock
interaction. This is considered further in section 4.2.
There is little correlation between either Cl or F and other
trace elements in interstitial amphiboles. Amphibole blebs
show weak positive correlations of Cl with Be, Ba, and La and
weak negative correlations of F with REEs, Zr, and Hf; however, the limited amount of amphibole bleb data makes inter-
pretation of these observations equivocal. Vein amphiboles
show a positive correlation between Cl and K, although this
may be due to crystal chemical effects (Morrison, 1991; Oberti
et al., 1993). However, a single sample in which amphibole is
abundant both replacing clinopyroxene and in veins (Alvin
1012) shows strong positive correlations between Cl and K,
LREE, Eu/Eu*, and La(n)/Sm(n) (Fig. 11) and negative correlation of Cl with Sc, heavy REEs, Zr, and Hf. This is discussed
in section 4.2.
3. COMPOSITIONAL CONSTRAINTS ON AMPHIBOLE
PETROGENESIS
Amphibole trace element compositions are discussed and
modelled in this section with the aims of (a) distinguishing
criteria for the discrimination amphibole formed from hydrous
silicate melts and seawater-derived fluid-rock interactions and
(b) constraining the petrogenesis of the different textural type.
In the following discussion we do not attempt to account for
variations in amphibole partition coefficients because (a) the
compositions of the phases from which the amphiboles grew is
unknown; (b) amphibole crystal chemistry is sufficiently complex, including an unknown Fe2⫹/Fe3⫹, that its control on
partition coefficients is relatively poorly constrained, and the
sites into which some trace elements partition are poorly constrained; and (c) no experimental partitioning data are available
for some amphibole compositions (e.g., actinolites). This prevents us from using amphibole compositions to place quantitative constraints on the compositions of the phases they grew
from. However, we believe that the ⬃1 to ⬎3 orders of mag-
Near-solidus evolution of oceanic gabbros
4349
Fig. 6. Chondrite normalised (Anders and Grevesse, 1989) rare earth element (REE) patterns in different amphibole
textural types. Note the light REE enrichment in some replacive and vein amphiboles compared with interstitial and bleb
amphiboles, which have REE patterns much more similar to those of clinopyroxene from the same samples (grey field; from
Coogan et al., 2000a).
nitude variations in trace element abundances between amphiboles in the data are highly unlikely to be explained wholly by
variations in partition coefficients. Thus, the compositional
variations between amphiboles can be used to constrain the
origins of amphiboles.
3.1. Distinguishing Magmatic and Hydrothermal
Amphibole
Niobium abundances in the main silicate phases in oceanic
gabbros (plagioclase, olivine, and pyroxene) are very low (generally near or below the detection limit of the ion microprobe;
Fig. 7). Niobium is also generally relatively immobile, even
under amphibolite facies metamorphism (e.g., Weaver and
Tarney, 1981). Thus, interaction of seawater-derived fluids and
a gabbroic assemblage is likely to form amphibole with low Nb
abundances. Evolved hydrous silicate melts, and amphiboles
that crystallised from these melts, are likely to be enriched in
Nb due to its highly incompatible behaviour. In addition to Nb
abundances, ratios of Nb to elements that occur in the main
igneous phases (plagioclase and clinopyroxene) should be
higher in magmatic than hydrothermal amphibole. For example, Nb/La in magmatic amphibole is expected to greatly exceed Nb/La in hydrothermal amphibole. Thus, both Nb abun-
dances and Nb/La should be able to distinguish between
magmatic and hydrothermal amphiboles. Similarly, Cl is likely
to be high in seawater-derived fluids (⬃20,000 ppm in seawater; e.g., Von Damm, 1990) and low in silicate melts (⬃20 to
50 ppm in primitive magmas; Michael and Schilling, 1989). In
contrast, F is relatively high in evolved silicate melts (commonly ⬎300 ppm; Michael and Schilling, 1989) and low in
seawater-derived fluids (⬃2 ppm in seawater; Faure, 1991).
Chlorine incorporation into amphibole is dependent on its K,
Fe2⫹, (IV)Al, and Mg contents (Morrison, 1991; Oberti et al.,
1993). However, when all amphibole compositions are considered there is no correlation between the abundances of these
elements and that of Cl, suggesting that the activity of Cl in the
fluid phases (silicate or aqueous based) was the dominant
control on amphibole Cl contents. Thus, high Nb/La and high
F/Cl should characterise magmatic amphibole, and low Nb/La
and low F/Cl should characterise hydrothermal amphiboles.
These criteria suggest that interstitial and bleb amphiboles are
magmatic, and vein and replacive amphibole are hydrothermal
(Fig. 12). The amphiboles that fall between the end-members
(labelled complex origin in Fig. 12) may form either from the
interaction of exsolved magmatic fluids with igneous plagioclase and clinopyroxene or from seawater-derived fluids inter-
4350
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
Fig. 7. Nb versus Zr, showing a general geochemical discrimination between the textural types and the difference in Nb
content between amphibole and olivine, clinopyroxene and plagioclase (hatched field; Cortesogno et al., 2000; Coogan,
1998; Coogan, unpublished data). The compositional overlap between the replacive amphibole and the interstitial and bleb
amphiboles comes principally from a single sample (153 922A 2R5, 69 to 76 cm) in which it is difficult to tell whether the
amphibole is replacing clinopyroxene or interstitial amphibole. One sigma errors based on counting statistics are generally
smaller than the symbols for Zr (all ⬍10%) and for most Nb data. At the lowest Nb abundances one sigma counting statistic
errors are approximately twice the size of the symbol (always ⬍40% relative).
Fig. 8. Zr versus Hf, showing elevated Hf/Zr ratios in some replacive amphiboles. The dashed grey line shows the
chondritic ratio (Anders and Grevesse, 1989). One sigma errors based on counting statistics are generally smaller than or
comparable to the symbol sizes except at the lowest Hf abundances, for which they are approximately twice the size of the
symbol for Hf (always ⬍30% relative).
Near-solidus evolution of oceanic gabbros
4351
Fig. 9. Sr versus B, showing that replacive and vein amphiboles generally have higher B and lower Sr than interstitial
and bleb amphiboles. The grey box shows the composition of seawater and the hatched field is for vent fluids (Von Damm,
1990). Plagioclase and clinopyroxene in this suite of rocks contain 173 to 256 ppm and 7.5 to 11.7 ppm Sr respectively.
One sigma errors based on counting statistics are generally smaller than or comparable to the symbol sizes.
acting with magmatic amphibole. The former model is
favoured because in the latter, interaction of seawater-derived fluids with magmatic amphibole would have to modify
amphibole Nb/La more rapidly than Cl/F, which seems
unlikely. Other elements that show good discrimination between these textural groupings are B (high in vein and
replacive amphibole; Fig. 9) and Sc and Sr (low in vein and
replacive amphibole).
Fig. 10. F versus Cl, showing the higher F and lower Cl in interstitial and bleb amphiboles than in vein and replacive
amphiboles. Clinopyroxene, olivine, and plagioclase contain ⬍20 ppm Cl and ⬍80 ppm F. One sigma errors based on
counting statistics are generally smaller than or comparable to the symbol sizes.
4352
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
Fig. 11. Chondrite-normalised (Anders and Grevasse, 1989) spidergram showing amphibole in sample Alvin 1012–11
labelled by Cl content (ppm) and insets of Cl versus La and Cl versus La/Sm. Vein amphiboles are shown as solid lines
and replacive amphibole as dashed lines. Note also that the size of the Eu anomaly correlates with the light rare earth
element enrichment. Symbols as in Figure 10.
3.2. REE Constraints on Amphibole Petrogenesis
The REEs provide a useful group of elements to model when
trying to constrain the origin of different amphiboles because of
the generally coherent behaviour of this group of elements and
because of the large range of patterns observed. Also, due to the
relatively small variations in Ca in the amphiboles analysed,
and the substitution of REEs for Ca in amphibole, there is
probably only a minor crystal chemical effect on the distribution coefficients for REEs between the amphiboles. The main
variations in the REE patterns are shown in Figure 13. Interstitial brown amphibole shows a strong positive correlation
between La(n)/Sm(n) and Eu(n)/Eu(n)* that is almost at right
angles to that predicted by Rayleigh or equilibrium crystallisation of a gabbroic assemblage (Fig. 13). These data can be
modelled by assuming that amphibole forms in a reaction
between hydrous silicate melt and plagioclase. This is shown in
Figure 13 as an assimilation and fractional crystallization
(AFC) (DePaolo, 1981) trend with pure plagioclase being assimilated. Clinopyroxene may also be assimilated but is not
required in this modeling, and plagioclase must dominate the
reaction to produce the observed increase in Eu(n)/Eu(n)*. However, the correlated clinopyroxene and interstitial amphibole
Mg#s and Cr2O3 contents (Fig. 3) suggest that clinopyroxene
does play a role in the amphibole-forming reaction to buffer the
abundances of these elements in interstitial amphiboles. A high
mass assimilated to mass crystallised ratio is required, as otherwise the melt would dominate the trace element abundance of
the amphibole. A melt component is required, however, to
explain the high Nb and F abundances in interstitial and bleb
amphibole (see above). This reaction is plausible, as dissolution
of plagioclase occurs with increased water activity in the melt
(e.g., Brandriss and Bird, 1999) and is the favoured origin of
interstitial amphibole.
Replacive and vein amphiboles have higher La(n)/Sm(n) for a
given Eu(n)/Eu(n)* than clinopyroxene, plagioclase, and interstitial amphibole. Thus, their REE compositions are difficult to
explain in terms of grain-scale reactions involving clinopyroxene and plagioclase and require the addition or removal of
REEs from the local system (plagioclase-clinopyroxene mixing
lines on this figure are almost straight). Seawater has the
required REE pattern to produce the LREE enrichment in these
amphiboles. However, the very low REE concentrations in
seawater would require huge volumes of seawater to be
stripped of its REEs to enrich the amphibole (seawater/amphibole ⬃5 ⫻ 105; Fig. 13). An alternative and more plausible
explanation is that at depth in the ocean crust, seawater-derived
fluids may contain much higher REE concentrations than seawater or hydrothermal vent fluids (see section 4.2).
3.3. Petrogenesis of Amphibole Blebs
Amphibole blebs have major and trace element compositions
similar to those of interstitial amphibole and very different to
those of replacive and vein amphiboles (Figs. 2, 6 to 10, and
12). However, texturally they appear to replace, or at least have
reacted with, clinopyroxene, as they are surrounded by secondary clinopyroxene formed at ⬎850°C. With respect to primary
clinopyroxene, these amphiboles are strongly enriched in Nb,
Near-solidus evolution of oceanic gabbros
4353
Fig. 12. Cl/F versus Nb/La, showing the discrimination of textural types based on these tracers of magmatic and
hydrothermal origin. The field of amphibole labelled “complex origin” with low Nb/La and low Cl/F may be formed either
from exsolved magmatic volatiles or interaction of seawater-derived fluids with magmatic amphibole. See text for
discussion.
K, Ba, and F, suggesting addition of these elements. Mass
balance of Nb, Ba, and K, assuming that secondary clinopyroxene is completely free of these elements, requires ⬃50 times
the volume of secondary clinopyroxene to amphibole bleb, and
F requires ⬃400 times that volume. These are much greater
values than suggested by the major elements (⬃6 to 8 times;
see section 2.2.1), suggesting bulk addition of these trace
elements. The similarity of amphibole bleb and interstitial
amphibole trace element compositions, in particular their low
Cl contents and high F and Nb contents, strongly suggests a
role for a hydrous silicate melt in the formation of these
amphiboles (see section 3.1). The subtle compositional differences between amphibole blebs and interstitial amphiboles can
be at least partially explained simply by the lack of plagioclase
for blebs to chemically interact with. For example, the lack of
a correlation between La(n)/Sm(n) and Eu(n)/Eu(n)* in amphibole blebs as observed in interstitial amphiboles suggests a lack
of plagioclase to provide a high La(n)/Sm(n) and high Eu(n)/
Eu(n)* component.
The formation of amphibole blebs with “magmatic” compositions enclosed with clinopyroxene is problematic. Two models can be envisaged for this: (a) these may form by the reaction
of hydrous silicate melt and surrounding clinopyroxene after
fracturing of the clinopyroxene to allow ingress of evolved melt
to clinopyroxene crystal cores, and (b) pervasive alteration of
the clinopyroxene by a hydrous silicate melt or exsolved magmatic fluid followed by recrystallisation during subsolidus
cooling to produce the amphibole blebs (e.g., Buseck et al.,
1980).
Model (a) suggests that clinopyroxene crystals cracked at
temperatures above the solidus (⬎860°C), and melts traversed
through the pyroxene along fractures and reacted with the
pyroxene, probably during melt flow or possibly postentrapment. High strain rates or unexpectedly strong crystals are
necessary for brittle failure to occur at temperatures close to the
solidus. However, magmatic felsic veins and planar fluid inclusion arrays formed at temperatures ⬎700°C have been
found in gabbros from this part of the MAR (Kelley and
Delaney, 1987), indicating that brittle deformation can occur at
high temperatures. Furthermore, the formation of amphibole
veins, albeit with a seawater signature, at ⬎800°C (Fig. 5) is
consistent with a high-temperature onset of brittle deformation.
Plausible mechanisms of producing high enough strain rates
include overpressure related to the exsolution of magmatic
volatiles (e.g., Boudreau, 1992) or magmatic intrusion (e.g.,
Fournier, 1999). Alternatively, the brittle-plastic transition may
occur under normal ridge strain rates at around this temperature, at low pressures, if the lower crust is stronger than is
commonly assumed (Hirth et al., 1998). The lack of planar
arrays of amphibole blebs, as might be expected if they formed
along fracture traces, may plausibly be explained if the high
formation temperatures allowed extensive annealing.
In model (b), the amphibole components would be introduced pervasively into the pyroxene structure presumably
along microcracks, possibly forming a bipyribole structure,
followed by recrystallisation of the pyroxene to a lower temperature clinopyroxene and the amphibole blebs (e.g., Buseck
et al., 1980). Direct crystallisation of a bipyribole from a melt
4354
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
Fig. 13. La(n)/Sm(n) versus Eu(n)/Eu(n)* (Eu* ⫽ {10[log(Sm) ⫹ log(Gd)]}/2), showing amphibole compositions and those
predicted to form during crystallisation, melt-cumulate reaction, and fluid-rock interaction. The equilibrium crystallisation
trend (equil. crystallisation) assumes crystallisation of a gabbroic assemblage (DLa ⫽ 0.11; DSm ⫽ 0.13; DEu ⫽ 0.5; DEu* ⫽
0.13) and clearly does not fit any of the data. The assimilation and fractional crystallization (AFC) trend (DePaolo, 1981)
is calculated for assimilation of plagioclase and crystallisation of amphibole (plagioclase composition: La(n) ⫽ 3; Sm(n) ⫽
1.4; Eu(n) ⫽ 13; Eu*(n) ⫽ 0.7; amphibole distribution coefficients DLa ⫽ 0.08; DSm ⫽ 0.45; DEu ⫽ 0.40; DEu* ⫽ 0.45; and
an initial melt composition calculated to be in equilibrium with clinopyroxene: La(n) ⫽ 50; Sm(n) ⫽ 100; Eu(n) ⫽ 53;
Eu*(n) ⫽ 105). This AFC trend replicates the interstitial amphibole data very well (see inset) provided that plagioclase is
the main phase assimilated and the rate of assimilation to crystallisation is high (shown here as 0.97). A mixing line between
plagioclase and clinopyroxene is almost identical to this AFC trend. The two fluid-rock reaction trends show complete
stripping of rare earth elements (REEs) from seawater (Elderfield and Greaves, 1982) and their addition to average
plagioclase (plag ⫹ seawater) and clinopyroxene (cpx ⫹ seawater) compositions respectively. Both of these trends
terminate at seawater/rock ratios of 5 ⫻ 105. These high ratios are unreasonable, and thus, producing the hydrothermal
amphibole compositions requires a fluid composition with much higher REE abundances than seawater. Fields for
clinopyroxene and plagioclase from Ocean Drilling Program Hole 923A are from Coogan et al. (2000a, 2000b) and the field
for hydrothermal vent fluids is from Klinkhammer et al. (1994). Symbols as in Figure 12.
is unlikely, as this would require high H2O contents in even the
most primitive magmas. Gillis and Meyer (2001) have observed amphibole lamallae in clinopyroxene associated with
amphibole blebs, which may support bleb formation by pervasive alteration followed by recrystallisation.
In either model, interaction of a hydrous silicate melt or
exsolved magmatic gas with the clinopyroxene is required by
the amphibole bleb compositions. Both processes may operate,
and confidently distinguishing between these will require a
more in-depth study.
4. NEAR-SOLIDUS EVOLUTION
Amphibole-plagioclase thermometry of interstitial amphiboles constrains the solidus of the MARK gabbros to be ⬃860 ⫾
30°C (Fig. 5). This suggests a solidification interval of ⬎300°C
for cumulates at slow-spreading ridges. The ingress of chemically modified seawater along fractures and grain boundaries
occurred from near-solidus temperatures downward, forming
vein and replacive amphiboles (Fig. 5). The physical and chemical processes active during the transition from magmatic to
seawater-controlled mass transport are discussed below.
4.1. Supra-Solidus Evolution
The occurrence of either amphibole blebs and/or interstitial
amphibole in all samples, but in small volumes, suggests that
evolved hydrous silicate melts are pervasively distributed
within low-porosity crystal mushes beneath slow-spreading
ridges. This is consistent with the close approach to an equilibrium grain boundary geometry observed for interstitial amphibole (Fig. 1a), which would probably lead to permeability
being maintained to a low porosity in a basaltic system where
the melt-matrix dihedral angle is likely to be ⬍60° (e.g.,
Hunter, 1987).
Interstitial amphibole probably formed in a reaction between
interstitial hydrous silicate melt and the crystal assemblage.
Near-solidus evolution of oceanic gabbros
The correlation between clinopyroxene and amphibole Mg#s
and Cr content suggests that clinopyroxene partially buffered
amphibole compositions, and the AFC modelling (Fig. 13)
suggests that plagioclase also played an important role in this
amphibole-forming reaction. As discussed above, the petrogenesis of amphibole blebs is uncertain, but it is likely that amphibole forms from a reaction between clinopyroxene and the
interstitial hydrous silicate melt either along pervasive microcracks in the pyroxene followed by pyroxene recrystallisation
to form amphibole blebs, or along larger, more distributed
fractures, followed by annealing.
It has recently been proposed that seawater ingress into
solidified cumulates may lead to flux melting (McCollum and
Shock, 1998; Hart et al., 1999). Potentially this could provide
a mechanism to form a hydrous silicate melt from which
magmatic amphibole could crystallise. In this scenario the melt
would be expected to acquire Cl from the seawater and thus
form amphibole with elevated Cl abundances. This is inconsistent with the low Cl abundances in interstitial and bleb amphiboles, suggesting that the melt from which they formed was not
formed by flux melting involving a seawater-derived fluid. It is
more likely that the amphibole-forming melt was simply the
residual interstitial silicate melt left as a crystal mush solidified.
4.2. Subsolidus Evolution
Ingress of seawater-derived fluids into the lower crust along
fractures at temperatures immediately below the solidus (Fig.
5) suggests that strain rates were high enough, or the crust
strong enough, to allow brittle deformation at these high temperatures. The temperature of the first ingress of these fluids is
higher than that found for high-level gabbros in the Oman
ophiolite and at Hess Deep (⬃700°C) but similar to those for
amphiboles from near the Moho in Oman (⬎800°C; Manning
et al., 1996, 2001). This may reflect the MARK gabbros having
formed at near-Moho levels. However, a more likely explanation is that there are inherent differences in the behaviour of the
hydrothermal system with spreading rate. Incipient cracking
probably occurs off-axis at fast-spreading ridges (i.e., ridges
with steady-state magma chambers), but on-axis at slowspreading ridges (i.e., ridges with transient magma chambers;
e.g., Wilcock and Delaney, 1996). Axial cracking at slowspreading ridges could occur in a downward-propagating
cracking front into newly solidified gabbro, as has been suggested to occur off-axis at fast-spreading ridges (Lister, 1974;
Manning et al., 1996, 2001). Alternatively, cracking at slowspreading ridges may occur around a magma body that is being
emplaced during renewed magmatism after an amagmatic period (see Hanson, 1995, and Fournier, 1999, for discussions of
the origins of fracturing in country rocks related to magma
emplacement). In either case cracking must be closely tied both
spatially and temporally to magmatism to occur at high temperatures.
The compositions of deep-seated high-temperature fluids in
MOR hydrothermal systems are unknown due to their inaccessibility. Some constraints can be placed on these fluid compositions from the compositions of vein and replacive amphiboles. Vein amphiboles probably provide the best constraints on
the fluid compositions, as these must have grown dominantly
from a fluid in an open fracture. Thus, their trace element
4355
compositions are probably dominated by the fluid composition.
This suggests that the highest temperature seawater-derived
fluids to interact with these samples were LREE enriched, with
a positive Eu anomaly, low HFSE contents, and high Cl and B
contents. The fluids from which these amphibole veins formed
must have contained much higher REE abundances than any
known fluids venting on the seafloor (see section 3.2).
Trace element concentrations may be high in deep-seated
hydrothermal fluids due to a number of factors such as the high
temperatures enhancing element solubilites, complexing of elements with Cl enhancing element solubilites, and the mixing
of exsolved magmatic fluids, with high solute contents, with
seawater-derived fluids. The fluids from which vein amphiboles
grew had temperatures up to ⬎800°C, whereas those that vent
on the seafloor are generally ⬍400°C. A suggestion of REE
complexing with Cl at high temperatures comes from the strong
correlation of LREEs and La/Sm with Cl in sample A1012–11
(see Fig. 11). Possible evidence of direct mass transfer from the
magmatic to hydrothermal systems comes from the F contents
of vein and replacive amphiboles (130 to 1700 ppm; Fig. 10).
These are higher than those in plagioclase, olivine, or clinopyroxene (Fig. 10) and much higher than in seawater or vent
fluids (⬍2 ppm; Von Damm, 1990; Faure, 1991; Oosting and
Von Damm, 1994). High F contents in hydrothermal amphiboles are most easily explained if they form from fluids that are
a mixture of exsolved magmatic fluids, with high F concentrations, and seawater-derived fluids. The alternative, that large
volumes of seawater are stripped of F during interaction with
the amphibole, would require surprisingly large seawater/amphibole ratios ⬎65 to 850.
Fractionation of Zr and Hf during fluid-rock interaction is
suggested by the low Zr for a given Hf in replacive amphibole
compared with all other amphiboles analysed (Fig. 8). However, the mobility of these elements in the fluids must have
been low, as they both occur in low concentrations in vein
amphiboles. These observations suggest that whole-rock analyses of gabbros that have interacted with fluids under amphibolite facies conditions may differ significantly from the magmatic compositions.
5. SUMMARY AND CONCLUSIONS
The near-solidus evolution of oceanic gabbros from the
slow-spreading MAR has been investigated using amphibole
compositions and textures. Amphibole begins to form as interstitial crystals within a crystal mush at ⬃900°C from a reaction
involving hydrous silicate melt, plagioclase, and clinopyroxene. The ingress of seawater-derived fluids occurs at temperatures immediately below the solidus both along grain boundaries and macroscopic fractures; however, there is no evidence
of flux melting due to this. Compositions of vein and replacive
amphiboles suggest that high-temperature fluids are capable of
significant mass transport and element fractionations. Melt
migration and metasomatism within the crystal mush at low
porosities, and very high-temperature fluid-rock interactions,
may lead to significant redistribution of elements generally
considered immobile (e.g., REEs).
Acknowledgments—Journal reviews by Bramley Murton and Ben
Harte, along with informal reviews by Bernard Leake and Mike
O’Hara, helped us considerably. Richard Hinton and John Craven are
4356
L. A. Coogan, R. N. Wilson, K. M. Gillis, and C. J. MacLeod
thanked for their invaluable help with ion microprobe analyses, without
which this project would not have been possible. Pamela Kempton is
thanked for introducing Laurence A. Coogan to the MARK area gabbros. Craig Manning is thanked for discussions concerning amphibole
formation in the lower oceanic crust. The ion microprobe time was
funded by Natural Environment Research Council (NERC) grant IMP
137/1098. Laurence A. Coogan was funded through NERC grant
GR3/10791. Kathryn M. Gillis acknowledges an operating grant from
the Natural Sciences and Engineering Research Council of Canada.
Associate editor: M. A. Menzies
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