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Silicic differentiates of abyssal oceanic magmas: Evidence for late-magmatic vapor transport of potassium
Earth and Planetary Science Letters, 47 (1980) 423-430 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
423
I21
SIL1CIC DIFFERENTIATES OF ABYSSAL OCEANIC MAGMAS: EVIDENCE FOR LATE-MAGMATIC
VAPOR TRANSPORT OF POTASSIUM JOHN M. SINTON Hawaii Institute o f Geophysics, University o f Hawaii, Honolulu, HI 96822 (U.S.A.)
and GARY R. BYERLY Department of Geology, Louisiana State University, Baton Rouge, LA 70803 {U.S.A.)
Re&ived September 28, 1979 Revised version received January 8, 1980
Massive, nearly holocrystalline dolerites from DSDP Hole 417D contain from 0.5 to 1.5% of granophyric patches composed mainly of Na-plagioclase and quartz. These patches are compositionally similar to other crystalline silicic rocks from oceanic spreading centers and differ from rarer abyssal silicic glasses. Crystalline varieties with SiO2 ;> 60 wt.% generally have Na/K ~ 10, whereas silicic glasses have Na/K in the range 3-6. While crystal fractionation readily accounts for the Na20 and K20 contents of abyssal silicic glasses, both the 417D granophyres and other crystalline abyssal silicic rocks have much lower K20 than that predicted by any reasonable crystal-liquid fractionation model. We propose that high-temperature vapor phase transport is responsible for removal of potassium during late-stage crystallization of these rocks. This allows for the formation of cogenetic silicic glassy and crystalline rocks with greatly different Na/K ratios. These observations and interpretations lead to a more confident assignment of high Na/K silicic rocks of oceanic and ophiolitic environments to a cogenetic origin with basaltic oceanic crust.
1. Introduction It has long been emphasized that the predominant lithology erupted at modern spreading ridges is basaltic (e.g. [1 5]). Although rocks more silicic than basalt have been described from the world's oceans [ 4 , 6 - 1 2 ] , many o f these are either erratic (e.g., [6,10]) or related to oceanic island and aseismic ridge volcanism [ 7 - 9 ] . However, several intermediate to silicic samples from the Pacific, Atlantic and Indian oceans [ 4 , 1 0 - 1 3 ] may be related to basaltic or gabbroic rocks o f each area by fractionation models. These samples comprise two populations, glassy volcanic andesites and rhyodacites from the Pacific [4,13] and crystalline plutonic diorites and Hawaii Institute of Geophysics Contribution No. 1028.
aplites from the Atlantic and Indian oceans [ 1 0 - 1 2 ] . All the above mentioned samples were obtained by dredging, and therefore the relationships of the silicic rocks to spatially associated marie rocks are largely inferential. In contrast, doleritic samples obtained during the Deep Sea Drilling Project kegs 5 1 - 5 2 [14,15] actually contain in-situ silicic differentiates. These differentiates may be an important guide to interpreting other differentiated rocks in the oceanic environment and in ophiolites. 2. Granophyres in DSDP Hole 417D Hole 417D, 25°N, 68°W in the western Atlantic Ocean was drilled during Deep Sea Drilling Project
424 grained (<5/am) irregular intergrowths of quartz and sodic plagioclase in sub-equal proportions make up the patches. Tiny colorless needles of apatite are a minor accessory phase. Selected broad-beam microprobe analyses of the granophyric patches are given in Table 1. Although texturally granophyres [17], the patches can be compositionally characterized as trondhjemites [18] or plagiogranites [19,20]. TiOz and K20 are very low in all the analyses, irrespective of FeO*/MgO ratio or SiO2 content. Na/Ca is generally correlative with SiO2 content. Due to the fine grain size, reliable analyses of individual mineral grains within the patches are not generally possible. However, the composition of the constituent feldspars can be estimated within 3 - 4 mole % using the broad-beam patch analyses and assuming that the patches are pure quartz-feldspar mixtures. It is evident in Fig. 1 that the feldspar compositions in the intergrowths range from about An14 to An3a. This result is supported by analysis o f an oligoclase grain
Legs 51 and 52. The recovered basement section is dominated by plagioclase-phyric pillow basalts with interspersed massive (non-pillowed) basalts and dolerites [ 1 4 - 1 6 ] . At least three massive horizons in Hole 417D are more than 10 m thick. Grain size and textures in massive units vary from porphyritic, hyalopilitic near the margins to medium-grained, subophitic in the interiors. The coarser-grained samples contain sub-ophitic intergrowths of plagioclase (mn86-mn6s) and clinopyroxene with Mg/(Mg + Fe) from 0.806 to 0.698. Minor subcalcic augite (CaO < 14.0 wt.%)locally coexists with pigeonite [ 16]. Fe-Ti oxides, phyllosilicate pseudomorphs after olivine and interstitial clays also occur in some of these samples. Granophyric patches [ 17] up to 5 mm across have been observed in the uppermost (413-438 m, subbottom) and lowermost (629 to bottom of hole, 708 m) of the thicker massive units. These patches tend to be irregular in shape and have an interstitial relationship to the mineral grains of the host dolerite. Very fine-
TABLE 1 Selected analyses of abyssal oceanic differentiates 417D granophyres
Crystalline rocks
Volcanic glasses
1
2
3
4
5
6
7
8
9
10
11
SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P2Os H20
61.7 0.07 22.0 * 1.38" n.d. 1.02 5.74 6.21 0.24 n.d. n.d.
71.9 0.06 16.3 * 0.59" n.d. 0.21 2.92 6.94 0.28 n.d. n.d.
81.4 0.13 10.6 * 0.72 n.d. 0.30 1.75 5.06 0.19 0.03 n.d.
61.97 0.94 16.00 3.22 3.57 0.09 2.43 3.24 5.55 0.75 0.22 1.28
72.47 0.33 14.17 1.85 1.19 0.08 1.39 1.48 5.55 0.24 0.06 0.90
78.39 0.09 12.68 0.38 0.41 0.01 0.54 0.55 6.66 0.06 0.01 0.41
76.37 0.42 12.78 0.39 0.46 0.02 0.87 0.84 7.70 0.07 0.02 0.28
75.07 0.15 13.18 0.76 1.15 0.03 0.23 1.10 4.55 3.27 0.12 0.28
59.00 1.75 12.60 * 12.00" n.d. 1.70 5.60 4.25 0.65 n.d. 1.78
57.08 1.76 13.48 * 12.12" n.d. 2.74 6.87 3.31 0.58 0.18 n.d.
70.05 0.63 12.38 * 4.53" n.d. 0.24 2.63 4.30 1.61 0.06 n.d.
Total
98.4
99.2
100.2
99.26
9 9 . 7 1 100.19
100.22
98.89
99.33
98.12
96.43
Atomic Na/K
39
38
11
35
167
40
169
2
6
5
4
* All Fe reported as FeO. n.d. = element not determined. Analysis 1: granophyric patch in 417D, core 33, sec. 5 ; 2,3 = granophyric patches in 417D, core 69, sec. 1; 4,5 = diorites from the Mid-Atlantic Ridge [ 10] ; 6 = aplite, 24°N, Mid-Atlantic Ridge [ 11 ] ; 7 = aplite, Indian Ocean [ 12] ; 8 = quartz-monzonite dikelet in diabase, Indian Ocean [ 12] ; 9 = andesite, East Pacific Rise [4] ; 10,11 = andesite and rhyodacite from Galapagos spreading ridge [ 13]. Analyses 1-3, determined by microprobe; conditions, standards, etc. available from the authors;
425
Na
1.0
(0=8) 0.5
q a ~ 5 _
granophyric patches. Clay-rich pseudomorphs after olivine have high FeO and MgO and low A1203 and K20. In contrast, interstitial clay-rich patches have lower FeO and MgO and higher SiO2, A1203 and alkalis (Table 2). Low totals from the microprobe analyses of these patches indicate the presence of significant amounts of water.
°
~
4.0
3.0
2.0
Si (0=8) Fig. 1. Na vs. Si cations per 8 oxygen of granophyric patches in samples from core 69, section 1 (solid circles) and core 33, section 5 (open circles). Compositions of feldspars in the intergrowths can be estimated by projecting the patch compositions (Table 1) from quartz onto the Ab-An join. Plagioclase in core 69 granophyrcs ranges from An 14 to An31 ; plagioclase in core 33 granophyres ranges from An32 to An38.
(An26) that is 0.04 mm long (see Sinton and Byerly [16, plate IV, fig. 2 ] ) i n one of the granophyric patches. In addition to the granophyric patches, Hole 417D dolerites contain brown, clay-rich patches of two types. Those in core 33, section 5 are mainly pseudomorphs after olivine but some in core 33 and most of the clay-rich patches in core 69, section 1 are irregular in form and fill interstices. Brown clay-rich patches comprise about 2.5% of a sample from core 69, section 1 and are spatially associated with the
3. Compositional characteristics of abyssal silicic rocks and glasses Selected analyses of oceanic rocks, considered by the referenced authors to represent silica-rich differentiates from abyssal tholeiitic magmas, are given in Table 1. It is clear from these analyses that there is considerable range in SiO2 and Na/K of the recovered samples. The 417D granophyres have moderately high atomic Na/K ratios, broadly similar to diorites from the Mid-Atlantic Ridge [10] and to aplites from the Indian and Atlantic spreading ridges [ 12,11]. Andesite and rhyodacite glasses from the Galapagos spreading ridge [ 13], a glassy andesite from the East Pacific Rise [4], and a siliceous dikelet from the Indian Ocean [ 12] are more potassic than either the 417D granophyric patches or other silicic rocks from the ocean floor. It is evident from Table 1 that a significant number of silicic crystalline rocks derived from abyssal magmas have high atomic Na/K ratios
TABLE 2 Selected analyses * of clay-rich patches, Hole 417D, 69-1
1
2
3
4
5
6
P205
46.9 0.33 4.35 17.0 16.0 1.87 0.23 0.37 0.09
48.0 0.24 4.46 17.7 15.5 1.90 0.19 0.50 0.05
53.5 0.30 13.3 9.16 6.64 3.20 5.53 0.80 1.64
54.5 0.31 11.1 12.0 8.36 1.78 4.47 1.31 0.11
54.9 0.40 12.5 10.0 7.07 2.88 5.49 0.57 0.087
55.9 0.32 12.3 10.8 7.15 1.61 5.81 1.15 0.11
Anhydrous total
87.1
88.5
94.1
93.9
94.7
95.2
SiO2 TiO2 A1203 FeO ** MgO CaO Na20 K20
* All analyses by microprobe (Univ. Hawaii CAMECA) using 20-/~mbeam diameter 18-nA beam current. ** Total Fe as FeO. Analysis 1, 2 = clay pseudomorphs after olivine; 3-6 = interstitial clay-rich patches.
426 (Na/K > 10 for SiO2 > 60 wt.%), whereas abyssal silicic glasses have consistently low (<10) Na/K ratios. Except for the Indian Ocean monzonite dikelet, all the crystalline samples have higher Na/K than the glassy ones. These data suggest that Na/K ratios of silicic abyssal magmas may be systematically increased during crystallization.
4. Formation o f the 417D granophyres The textural relations of the studied samples suggest that the granophyric patches represent late-stage liquids, residual from at least 95-98% crystallization of the host dolerites. Furthermore, most of the analyses listed in Table 1, are of samples which the referenced authors consider to be derivatives of abyssal basaltic magmas. This data set allows an evaluation of processes that may be active in the advanced stages of differentiation of abyssal magmas.
4.1. Fractional crystallization Granophyric patches make up about 1-1.5% of a sample from core 69, section 1 and about 0.5% of a sample from core 33, section 5. The most primitive volcanic glasses present in Hole 417D have about 7.70 wt.% MgO, 2.25 wt.% Na20 and 0.08 wt.% K20 [21], with atomic Na/K of about 25. Treating K20 as a completely incompatible element during fractionation, and allowing 95% fractionation of a magma with 0.08 wt.% K20, indicates that the last 5% (by weight) liquid should contain 1.6 wt.% KzO. Allowing for 40% fractionation of plagioclase [21] containing 0.05 wt.% K=O [16] reduces the expected K20 in the last 5% residual liquid to 1.2 wt.%. The rhyodacitic glass (analysis 11, Table 1) has K20 (1.61 wt.%) in the range predicted by the above semi-quantitative treatment; K20 and other elements in the andesitic glasses (analysis 9, 10, Table 1) can be modeled by lower degrees of crystal fractionation (e.g. [13]). In contrast to the glasses, 417D granophyres and other abyssal crystalline silicic rocks have K=O values far below those pTedicted by simple crystal fractionation. Other than minor K20 in plagioclase, there is no evidence for fractionation of a potassic mineral phase in any of these samples and
the effects of separation of such a phase can be effectively excluded from consideration. Clearly some process in addition to fractional crystallization is responsible for the production of the compositions of the 417D patches. A comparison of the 417D granophyres with glasses of roughly comparable SiO2 (Table 1)indicates that although Na20 is slightly enriched in the former, the most striking compositional characteristic is the apparent depletion of K20 in the crystalline samples. Interstitial brown-clay patches contain between 0.5 and 1.3 wt.% K20 (Table 2) and it is possible that residual K20 is now concentrated in these patches. We have considered three possible addition',d mechanisms to account for the apparent mobility of K, and the fractionation of K from Na in these rocks, namely (1) liquid immiscibility, (2) sub-solidus metasomatism, and (3) late-stage vapor phase transport.
4.2. Liquid immiscibility The spatial association of relatively Fe-rich interstitial brown clay patches with Si-rich granophyric patches suggests that liquid immiscibility may have been important in the crystallization of 417D dolerites. This process has received renewed attention in recent years [22-25] and Sato [26] has shown that immiscible liquids have developed in some abyssal oceanic magmas. However, existing evidence indicates that immiscible liquids will not fractionate K from Na (e.g. [22]). Furthermore, immiscibility experiments [25] predict that P should be strongly partitioned into the Fe-rich liquid. However, phosphorus distribution in core 69, section 1 is erratic (Tables 1,2) with tiny apatite needles present in both the granophyres and in some clay-rich patches. Although liquid immiscibility may have been important in the latest stages of crystallization of the Hole 417D magmas, this process does not account for the Na, K and P distributions in the rocks.
4.3. Sub-solidus metasomatic alteration K-rich clays and K-feldspar were identified by shipboard scientists as low-temperature alteration products in Hole 417A, and it is clear that potassium can be mobilized at very low temperatures in the
427 presence of seawater. However, such alteration and leaching effects are usually accompanied by hydration of the primary rock. The granophyres are essentially anhydrous and do not appear to have undergone sub-solidus hydrothermal alteration. In contrast, the brown clay-rich patches contain significant water as well as potassium. Although a reasonable case can be made for a hydrothermal source for the K in the clay-rich patches, it is unlikely that this process is responsible for the apparent depletion of K in the granophyric patches. We therefore favor a late magmatic, rather than a sub-solidus, mechanism as an explanation for the alkali concentrations in these rocks.
4.4. Vapor-phase transport in abyssal magmas The role of a vapor phase in modifying alkali concentrations in rocks was explored by Kennedy [27] and by Orville [28]. Orville demonstrated that, in the presence of a thermal gradient, K is lost from hotter portions of a rock and moves toward cooler portions. He also noted that high Ca-feldspar component (i.e., compositions more appropriate to 417D dolerites) promotes high K/Na in the vapor phase [271. Richter and Moore [29] found a zone enriched in alkalis, especially KzO, occurring in melt immediately below the upper crust of the cooling Kilauea Iki lava lake. Those authors suggested that the K-enrichment was produced by alkali transfer from deeper portions of the lake and proposed that this transfer takes place in water vapor. This work provides important documentation that vapor-phase transport of alkalis may operate in tholeiitic magnaas under appropriate conditions. Attempts to explain the low alkali contents of lunar basalts brought renewed interest in the volatilities of alkali elements [30-33]. Selective vapor phase transport has been proposed to explain potash variations in lunar plagioclase [30] and between lunar rocks and fines [31]. De Maria et al. [32] demonstrated that K is more volatile than Na below about 1050°C, that is at temperatures appropriate to the near-solidus region of abyssal basaltic magmas, and Gooding and Muenow [33] showed that terrestrial basalts have similar vaporization trends to lunar basalts.
Although detailed transport mechanisms are difficult to determine, the evidence of these studies indicates that at temperatures appropriate to the nearsolidus region of abyssal magmas, K may be fractionated into a vapor phase. Gooding and Muenow [33] point out that alkali vapor transport can occur with vesiculation under essentially anhydrous conditions appropriate to the moon, or with water present. Those authors also suggest that only in lava lakes would the requisite amount of gas and thermal energy be present to allow for continuous vesiculation over the necessary time. This is consistent with the proposal for vapor phase transport of alkalis in the Kilauea Iki lava lake [29]. The 417D granophyres occur in massive units greater than 10 m thick, giving rise to the interpretation of the formation of these units either as ponded flows (lava lakes) or as sills. Regardless of their specific occurrence, vesiculation was apparently active since these units contain welldeveloped amygdaloidal zones in the upper portions (personal observation of the authors while aboard ship). We feel that the above evidence supports the conclusion that the principal processes controlling K distribution in the granophyres were late magmatic and propose that vapor-phase transport be considered as playing an important role in producing the characteristically low KzO contents of these patches. This allows for volcanic silicic glasses and holocrystalline silicic rocks with widely different Na/K, and possibly other cation, ratios to have a cogenetic relationship to each other, as well as to abyssal oceanic basalts. However, for this interpretation to have validity it is important to consider why this effect is pronounced in abyssal oceanic magmas, but apparently not in many continental magmas. Coleman and Peterman [19] showed that high Na/K ratios appear to be a characteristic of oceanic silicic differentiates, and a reliable discriminant from continental differentiates (Fig. 2). We suggest that depletion of K by vapor phase transport may be nearly universal in the late stages of crystallization of plutonic oceanic rocks. We also propose that this phenomenon may be restricted to magmas with low initial K20 contents. Magmas with higher initial K20 contents commonly crystallize a potassic mineral phase before reaching 98-99% crystallization, thus fixing K20 in the resultant rocks. Although late-stage
428 An
vapors may be active in both oceanic and in continental magmas, their effects on alkali concentrations will only be pronounced where nearly all K20 is retained in the last I - 2 % residual liquid. Furthermore, this residual liquid must be present at low enough temperatures for effective vapor-induced fractionation of potassium from sodium (cf., volatility relations of Na and K versus temperature [30,31 ]). Apparently these conditions are realized in abyssal oceanic magmas, to the exclusion of most continental magmas.
5. Implications for the composition o f oceanic crust Silicic rocks, typically with high Na/K, commonly occur in the upper portion o f the plutonic parts or near the dike complex-erupted lava interface, in ophiolites. In many such complexes, silicic rocks account for about 1% of the total section [34], whereas in others the percentage of silicic rocks is much greater (e.g. [35]). The presence of silicic rocks in ophiolite complexes has been cited as evidence against ophiolite origin as oceanic crust [36]. Although we have not shown that the small patches of granophyre ever accumulate and segregate significantly from their basaltic hosts, our data suggest that small volumes of silicic rocks with high Na/K can be expected in oceanic crust. From an analysis o f Hole 417D glass data, Byerly and Sinton [21] suggested that even the least fractionated samples in that hole may have undergone as much as 50% crystallization and extraction before emplacement. Taking this figure as a reliable generalization for oceanic crustal processes, and a figure o f 0.5-1.5% for the proportion of granophyre in 417D dolerites, then high-Si, high-Na/K differentiates can be expected to comprise no more than about 1% of typical oceanic crust, or perhaps up to 2% of the plutonic part of typical oceanic crust. Thus, the evidence presented here indicates that large volumes of high-SiO2 igneous rocks are not to be expected in oceanic crustal sections, post-magmatic silica metasomatic enrichment notwithstanding. Coleman and Peterman [ 19] and Coleman and Donato [20] have shown that siliceous rocks from the Troodos ophiolite complex are characterized by high normative (Ab + An)/Or, a feature atypical of siliceous rocks from continental environments. Sili-
Ab
--
xOr
Fig. 2. Oceanic silicic rocks plotted in terms of their normative components An-Ab-Or. Hole 417D granophyres are shown as solid circles, Mid-Atlantic Ridge diorites [ 10] as squares, aplites [11,12] as inverted triangles, an Indian Ocean quartz-monzonite [12] as a triangle and Galapagos Ridge andesites and rhyodacites [ 13] as open circles. The fields for Troodos plaglogranites (TP) and continental granophyres (CG) are from Coleman and Peterman [19].
ceous rocks from other ophiolites tend to be similar [20,34,35] as are the oceanic rocks considered in the present paper (Fig. 2). These data support the conclusion [19,20] that Troodos and many other ophiolitic siliceous rocks have oceanic affinities.
6. Conclusions (1) Granophyric patches in DSDP Hole 417D dolerites represent direct evidence that abyssal tholeiitic magmas may evolve to high-SiO2, high-Na/K differentiates. (2) High atomic Na/K ratios appear to be typical of differentiated silicic abyssal crystalline rocks; known volcanic glasses from oceanic spreading centers are enriched in potassium relative to the 417D granophyres and to most other crystalline abyssal siliceous rocks. (3) Potassium contents of the granophyres are much lower than predicted from formation by crystal fractionation. We suggest that K may have been removed by vapor-phase transport during late-stage crystallization. Thus, a cogenetic suite of volcanic
429 silicic glasses a n d h o l o c r y s t a l l i n e silicic r o c k s m i g h t have very d i f f e r e n t N a / K , a n d possible o t h e r c a t i o n , ratios. ( 4 ) A l t h o u g h late-stage v a p o r t r a n s p o r t m a y be active in m a n y e n v i r o n m e n t s , this m e c h a n i s m h a s pron o u n c e d e f f e c t o n K d i s t r i b u t i o n s in abyssal o c e a n i c m a g m a s w h e r e n e a r l y all K 2 0 is residual in the last 1 - 2 % liquid at t e m p e r a t u r e s a p p r o p r i a t e for effective v a p o r f r a c t i o n a t i o n o f K f r o m Na. (5) High-SiO2, h i g h - N a / K i g n e o u s rocks c a n be e x p e c t e d to m a k e u p n o m o r e t h a n 1% o f typical o c e a n i c crust or a b o u t 2% o f t h e p l u t o n i c part o f o c e a n i c crust. (6) Siliceous r o c k s in m a n y o p h i o l i t e s are similar to o c e a n i c rocks d e s c r i b e d here a n d can n o w c o n f i d e n t l y be c o n s i d e r e d to h a v e a f f i n i t y w i t h abyssal o c e a n i c magmas.
Acknowledgements We b o t h t h a n k t h e S m i t h s o n i a n I n s t i t u t i o n for fellowship s u p p o r t , use o f facilities a n d invaluable help from numerous staff members, particularly Charles O b e r m e y e r III, T i m O ' H e a r n a n d R i c h a r d J o h n s o n . T h e Deep Sea Drilling P r o j e c t ( N S F ) provided s u p p o r t t o p a r t i c i p a t e o n D S D P Legs 51 (J.S.) a n d 5 2 (G.B.). The m a n u s c r i p t was p r e p a r e d w i t h the h e l p o f s t a f f at Hawaii I n s t i t u t e o f G e o p h y s i c s . T w o a n o n y m o u s reviewers gave h e l p f u l reviews o f an earlier version o f t h e m a n u s c r i p t , a n d David M. Christie m a d e m a n y valuable c o m m e n t s at various stages o f this s t u d y . This is H I G c o n t r i b u t i o n No. 1028.
References 1 I.D. Muir and C.E. Tilley, Basalts from the northern part of the rift zone of the Mid-Atlantic Ridge, J. Petrol. 5 (1964) 409. 2 I.D. Muir and C.E. Tilley, Basalts from the northern part of the Mid-Atlantic Ridge, II. The Atlantic collection near 30°N, J. Petrol. 7 (1966) 193. 3 A.E.J. Engel, C.G. Engel and R.G. Havens, Chemical characteristics of oceanic basalts and the upper mantle, Bull. Geol. Soc. Am. 76 (1965) 719. 4 R.W. Kay, N.J. Hubbard and P. Gast, Chemical characteristics and origin of oceanic ridge volcanic rocks, J. Geophys. Res 75 (1970) 1581.
5 A. Miyashiro, F. Shido and M. Ewing, Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 26 ° and 30 ° north latitude, Contrib. Mineral. Petrol. 23 (1969) 38. 6 E. Bonatti and G. Arrhenius, Acidic rocks on the Pacific Ocean floor, in: The Sea, Part 1 (Wiley-lnterscience, New York, N.Y., 1970). 7 I.V. Luchitskiy, Acidic magmatic rocks in the oceans, Geotectonics 5 (1973) 268. 8 R. Hekinian, Petrology of the Ninety East Ridge (Indian Ocean) compared to other aseismic ridges, Contrib. Mineral. Petrol. 43 (1973) 125. 9 B.M. Walker, T.A. Vogel and R. Ehrlich, Petrogenesis of Oceanic granites from the Ayes Ridge in the Caribbean Basin, Earth Planet. Sci. Lett, 15 (1972) 133. 10 F. Aumento, Diorites from the Mid-Atlantic Ridge at 45°N, Science 165 (1969) 1112. 11 A. Miyashiro, F. Shido and M. Ewing, Crystallization and differentiation in abyssal tholeiites and gabbros from mid-oceanic ridges, Earth Planet. Sci. Lett. 7 (1970) 361. 12 C.G. Engel and R.L. Fisher, Granitic to ultramafic rock complexes of the Indian Ocean Ridge system, western Indian Ocean, Geol. Soc. Am. Bull. 86 (1975) 1553. 13 G.R. Byerly, W.G. Melson and P.R. Vogt, Rhyodacites, andesites, ferro-basalts and ocean tholeiites from the Galapagos spreading center, Earth Planet. Sci. Lett. 30 (1976) 215. 14 Shipboard Scientists, DSDP Leg 51, Mid-ocean ridge in the Cretaceous, Geotimes 22, No. 6 (1977) 21. 15 Shipboard Scientists, DSDP Leg 52, Studying oceanic layers, Geotimes 22, No. 7 (1977) 22. 16 J.M. Sinton and G.R. Byerly, Mineral compositions and crystallization trends in DSDP Holes 417D and 418A, in: Initial Reports of the Deep Sea Drilling Project, 5 1 - 5 3 (in press). 17 The term "granophyric" is here used to denote irregular intergrowths of quartz and feldspar without reference to feldspar composition. This usage is consistent with definitions given in the AGI "Dictionary of Geological Terms" (Dolphin Books, 1962) and in P.W. Thrust, "A Dictionary of Mining, Mineral and Related Terms" (U.S. Department of the Interior, Bureau of Mines, 1968). However, see also D.S. Barker, Composition of granophyre, myrmekite and graphic granite, Geol. Soc. Am. Bull. 81 (1970) 3339. 18 A.L. Streickeisen et al., Classification and nomenclature of plutonic rocks recommended by the IUGS subcommission on the systematics of igneous rocks, Geotimes 18, No. 10 (1973) 26. 19 R.G. Coleman and Z.E. Peterman, Oceanic plagiogranite, J. Geophys. Res. 80 (1975) 1099. 20 R.G. Coleman and M.M. Donato, Oceanic plagiogranite revisited, in: Trondhjemites, Dacites and Related Rocks, F. Barker, ed. (Elsevier, Amsterdam, 1979) 149-168. 21 G.R. Byerly and J.M. Sinton, Compositional trends in natural basaltic glasses from DSDP Holes 417D and 418A, in: Initial Reports of the Deep Sea Drilling Project, 5 1 - 5 3 (in press).
430 22 A.R. Philpotts, Silicate liquid immiscibility in tholeiitic basalts, J. Petrol. 20 (1979), 99. 23 E. Roedder and P.W. Weiblen, Petrology of silicate melt inclusions, Apollo 11 and Apollo 12 and terrestrial equivalents, Proc. 2nd Lunar Sci. Conf. 1 (1971) 507. 24 T.N. Irvine, Metastable liquid immiscibility and MgOFeO-SiO2 fractionation patterns in the system Mg2SiO4Fe2SiO4-CaAI2Si2Os-KAISi3Os-SiO2, Carnegie Inst. Washington Yearb. 76 (1977) 597. 25 E.B. Watson, Two-liquid partition coefficients: experimental data and geochemical implications, Contrib. Mineral. Petrol. 56 (1976) 119. 26 H. Sato, Segregation vesicles and immiscible liquid droplets in ocean floor basalt of Hole 396B, IPOD/DSDP Leg 46, in: Initial Reports of the Deep Sea Drilling Project, 46 (1978) 283. 27 G.C. Kennedy, Some aspects of the role of water in rock melts, Geol. Soc. Am. Spec. Paper 62 (1955) 489. 28 P.M. Orville, Alkali ion exchange between vapor and feldspar phases, Am. J. Sci. 261 (1963) 201. 29 D.H. Richter and J.G. Moore, Petrology of the Kilauea Iki lava lake, Hawaii, U.S. Geol. Surv. Prof. Paper 537B (1966) B1. 30 B.J. Skinner and H. Winchell, Mineralogical evidence for subsolidus vapor phase transport o f alkalis in lunar basalts, Proc. 3rd Lunar Sci. Conf. 1 (1972) 243. 31 J.J. Naughton, J.V. Derby and V.A. Lewis, Vaporization from heated lunar samples and the investigation of lunar
32
33
34
35
36
erosion by volatilized alkalis, Proc. 2nd Lunar Sci. Conf. 1 (1971)449. G. De Maria, G. Balducci, M. Guido and V. Piacente, Mass spectrometric investigation o f the vaporization process of Apollo 12 lunar samples, Proc. 2nd Lunar Sci. Conf. 2 (1972) 1367. J.L. Gooding and D.W. Muenow, Activated release of alkalis during the vesiculation of molten basalts under high vacuum: implications for lunar volcanism, Geochim. Cosmochin~. Acta 40 (1976) 675. J.M. Sinton, Petrology and evolution of the Red Mountain ophiolite complex, New Zealand, Am. J. Sci., Jackson Vol. (in press). T.P. Thayer, The Canyon Mountain ophiolite complex, Oregon and some problems of ophiolites, Oreg. Dep. Geol. Mineral. Ind. Bull. 95 (1977) 93. A. Miyashiro, The Troodos ophiolite complex was probably formed in an island arc, Earth Planet. Sci. Lett. 19 (1973) 218.
NOTE ADDEDINPROOF Wright, Peck and Shaw [Am. Geophys. Union, Geophys. Monogr. 19 (1979) 385-387] found some of the original analyses listed in Richter and Moore [29] to be in error and concluded that there is no evidence for alkali transfer in Kilauea lava lakes. These findings in no way alter the conclusions of this paper, however.
423
I21
SIL1CIC DIFFERENTIATES OF ABYSSAL OCEANIC MAGMAS: EVIDENCE FOR LATE-MAGMATIC
VAPOR TRANSPORT OF POTASSIUM JOHN M. SINTON Hawaii Institute o f Geophysics, University o f Hawaii, Honolulu, HI 96822 (U.S.A.)
and GARY R. BYERLY Department of Geology, Louisiana State University, Baton Rouge, LA 70803 {U.S.A.)
Re&ived September 28, 1979 Revised version received January 8, 1980
Massive, nearly holocrystalline dolerites from DSDP Hole 417D contain from 0.5 to 1.5% of granophyric patches composed mainly of Na-plagioclase and quartz. These patches are compositionally similar to other crystalline silicic rocks from oceanic spreading centers and differ from rarer abyssal silicic glasses. Crystalline varieties with SiO2 ;> 60 wt.% generally have Na/K ~ 10, whereas silicic glasses have Na/K in the range 3-6. While crystal fractionation readily accounts for the Na20 and K20 contents of abyssal silicic glasses, both the 417D granophyres and other crystalline abyssal silicic rocks have much lower K20 than that predicted by any reasonable crystal-liquid fractionation model. We propose that high-temperature vapor phase transport is responsible for removal of potassium during late-stage crystallization of these rocks. This allows for the formation of cogenetic silicic glassy and crystalline rocks with greatly different Na/K ratios. These observations and interpretations lead to a more confident assignment of high Na/K silicic rocks of oceanic and ophiolitic environments to a cogenetic origin with basaltic oceanic crust.
1. Introduction It has long been emphasized that the predominant lithology erupted at modern spreading ridges is basaltic (e.g. [1 5]). Although rocks more silicic than basalt have been described from the world's oceans [ 4 , 6 - 1 2 ] , many o f these are either erratic (e.g., [6,10]) or related to oceanic island and aseismic ridge volcanism [ 7 - 9 ] . However, several intermediate to silicic samples from the Pacific, Atlantic and Indian oceans [ 4 , 1 0 - 1 3 ] may be related to basaltic or gabbroic rocks o f each area by fractionation models. These samples comprise two populations, glassy volcanic andesites and rhyodacites from the Pacific [4,13] and crystalline plutonic diorites and Hawaii Institute of Geophysics Contribution No. 1028.
aplites from the Atlantic and Indian oceans [ 1 0 - 1 2 ] . All the above mentioned samples were obtained by dredging, and therefore the relationships of the silicic rocks to spatially associated marie rocks are largely inferential. In contrast, doleritic samples obtained during the Deep Sea Drilling Project kegs 5 1 - 5 2 [14,15] actually contain in-situ silicic differentiates. These differentiates may be an important guide to interpreting other differentiated rocks in the oceanic environment and in ophiolites. 2. Granophyres in DSDP Hole 417D Hole 417D, 25°N, 68°W in the western Atlantic Ocean was drilled during Deep Sea Drilling Project
424 grained (<5/am) irregular intergrowths of quartz and sodic plagioclase in sub-equal proportions make up the patches. Tiny colorless needles of apatite are a minor accessory phase. Selected broad-beam microprobe analyses of the granophyric patches are given in Table 1. Although texturally granophyres [17], the patches can be compositionally characterized as trondhjemites [18] or plagiogranites [19,20]. TiOz and K20 are very low in all the analyses, irrespective of FeO*/MgO ratio or SiO2 content. Na/Ca is generally correlative with SiO2 content. Due to the fine grain size, reliable analyses of individual mineral grains within the patches are not generally possible. However, the composition of the constituent feldspars can be estimated within 3 - 4 mole % using the broad-beam patch analyses and assuming that the patches are pure quartz-feldspar mixtures. It is evident in Fig. 1 that the feldspar compositions in the intergrowths range from about An14 to An3a. This result is supported by analysis o f an oligoclase grain
Legs 51 and 52. The recovered basement section is dominated by plagioclase-phyric pillow basalts with interspersed massive (non-pillowed) basalts and dolerites [ 1 4 - 1 6 ] . At least three massive horizons in Hole 417D are more than 10 m thick. Grain size and textures in massive units vary from porphyritic, hyalopilitic near the margins to medium-grained, subophitic in the interiors. The coarser-grained samples contain sub-ophitic intergrowths of plagioclase (mn86-mn6s) and clinopyroxene with Mg/(Mg + Fe) from 0.806 to 0.698. Minor subcalcic augite (CaO < 14.0 wt.%)locally coexists with pigeonite [ 16]. Fe-Ti oxides, phyllosilicate pseudomorphs after olivine and interstitial clays also occur in some of these samples. Granophyric patches [ 17] up to 5 mm across have been observed in the uppermost (413-438 m, subbottom) and lowermost (629 to bottom of hole, 708 m) of the thicker massive units. These patches tend to be irregular in shape and have an interstitial relationship to the mineral grains of the host dolerite. Very fine-
TABLE 1 Selected analyses of abyssal oceanic differentiates 417D granophyres
Crystalline rocks
Volcanic glasses
1
2
3
4
5
6
7
8
9
10
11
SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P2Os H20
61.7 0.07 22.0 * 1.38" n.d. 1.02 5.74 6.21 0.24 n.d. n.d.
71.9 0.06 16.3 * 0.59" n.d. 0.21 2.92 6.94 0.28 n.d. n.d.
81.4 0.13 10.6 * 0.72 n.d. 0.30 1.75 5.06 0.19 0.03 n.d.
61.97 0.94 16.00 3.22 3.57 0.09 2.43 3.24 5.55 0.75 0.22 1.28
72.47 0.33 14.17 1.85 1.19 0.08 1.39 1.48 5.55 0.24 0.06 0.90
78.39 0.09 12.68 0.38 0.41 0.01 0.54 0.55 6.66 0.06 0.01 0.41
76.37 0.42 12.78 0.39 0.46 0.02 0.87 0.84 7.70 0.07 0.02 0.28
75.07 0.15 13.18 0.76 1.15 0.03 0.23 1.10 4.55 3.27 0.12 0.28
59.00 1.75 12.60 * 12.00" n.d. 1.70 5.60 4.25 0.65 n.d. 1.78
57.08 1.76 13.48 * 12.12" n.d. 2.74 6.87 3.31 0.58 0.18 n.d.
70.05 0.63 12.38 * 4.53" n.d. 0.24 2.63 4.30 1.61 0.06 n.d.
Total
98.4
99.2
100.2
99.26
9 9 . 7 1 100.19
100.22
98.89
99.33
98.12
96.43
Atomic Na/K
39
38
11
35
167
40
169
2
6
5
4
* All Fe reported as FeO. n.d. = element not determined. Analysis 1: granophyric patch in 417D, core 33, sec. 5 ; 2,3 = granophyric patches in 417D, core 69, sec. 1; 4,5 = diorites from the Mid-Atlantic Ridge [ 10] ; 6 = aplite, 24°N, Mid-Atlantic Ridge [ 11 ] ; 7 = aplite, Indian Ocean [ 12] ; 8 = quartz-monzonite dikelet in diabase, Indian Ocean [ 12] ; 9 = andesite, East Pacific Rise [4] ; 10,11 = andesite and rhyodacite from Galapagos spreading ridge [ 13]. Analyses 1-3, determined by microprobe; conditions, standards, etc. available from the authors;
425
Na
1.0
(0=8) 0.5
q a ~ 5 _
granophyric patches. Clay-rich pseudomorphs after olivine have high FeO and MgO and low A1203 and K20. In contrast, interstitial clay-rich patches have lower FeO and MgO and higher SiO2, A1203 and alkalis (Table 2). Low totals from the microprobe analyses of these patches indicate the presence of significant amounts of water.
°
~
4.0
3.0
2.0
Si (0=8) Fig. 1. Na vs. Si cations per 8 oxygen of granophyric patches in samples from core 69, section 1 (solid circles) and core 33, section 5 (open circles). Compositions of feldspars in the intergrowths can be estimated by projecting the patch compositions (Table 1) from quartz onto the Ab-An join. Plagioclase in core 69 granophyrcs ranges from An 14 to An31 ; plagioclase in core 33 granophyres ranges from An32 to An38.
(An26) that is 0.04 mm long (see Sinton and Byerly [16, plate IV, fig. 2 ] ) i n one of the granophyric patches. In addition to the granophyric patches, Hole 417D dolerites contain brown, clay-rich patches of two types. Those in core 33, section 5 are mainly pseudomorphs after olivine but some in core 33 and most of the clay-rich patches in core 69, section 1 are irregular in form and fill interstices. Brown clay-rich patches comprise about 2.5% of a sample from core 69, section 1 and are spatially associated with the
3. Compositional characteristics of abyssal silicic rocks and glasses Selected analyses of oceanic rocks, considered by the referenced authors to represent silica-rich differentiates from abyssal tholeiitic magmas, are given in Table 1. It is clear from these analyses that there is considerable range in SiO2 and Na/K of the recovered samples. The 417D granophyres have moderately high atomic Na/K ratios, broadly similar to diorites from the Mid-Atlantic Ridge [10] and to aplites from the Indian and Atlantic spreading ridges [ 12,11]. Andesite and rhyodacite glasses from the Galapagos spreading ridge [ 13], a glassy andesite from the East Pacific Rise [4], and a siliceous dikelet from the Indian Ocean [ 12] are more potassic than either the 417D granophyric patches or other silicic rocks from the ocean floor. It is evident from Table 1 that a significant number of silicic crystalline rocks derived from abyssal magmas have high atomic Na/K ratios
TABLE 2 Selected analyses * of clay-rich patches, Hole 417D, 69-1
1
2
3
4
5
6
P205
46.9 0.33 4.35 17.0 16.0 1.87 0.23 0.37 0.09
48.0 0.24 4.46 17.7 15.5 1.90 0.19 0.50 0.05
53.5 0.30 13.3 9.16 6.64 3.20 5.53 0.80 1.64
54.5 0.31 11.1 12.0 8.36 1.78 4.47 1.31 0.11
54.9 0.40 12.5 10.0 7.07 2.88 5.49 0.57 0.087
55.9 0.32 12.3 10.8 7.15 1.61 5.81 1.15 0.11
Anhydrous total
87.1
88.5
94.1
93.9
94.7
95.2
SiO2 TiO2 A1203 FeO ** MgO CaO Na20 K20
* All analyses by microprobe (Univ. Hawaii CAMECA) using 20-/~mbeam diameter 18-nA beam current. ** Total Fe as FeO. Analysis 1, 2 = clay pseudomorphs after olivine; 3-6 = interstitial clay-rich patches.
426 (Na/K > 10 for SiO2 > 60 wt.%), whereas abyssal silicic glasses have consistently low (<10) Na/K ratios. Except for the Indian Ocean monzonite dikelet, all the crystalline samples have higher Na/K than the glassy ones. These data suggest that Na/K ratios of silicic abyssal magmas may be systematically increased during crystallization.
4. Formation o f the 417D granophyres The textural relations of the studied samples suggest that the granophyric patches represent late-stage liquids, residual from at least 95-98% crystallization of the host dolerites. Furthermore, most of the analyses listed in Table 1, are of samples which the referenced authors consider to be derivatives of abyssal basaltic magmas. This data set allows an evaluation of processes that may be active in the advanced stages of differentiation of abyssal magmas.
4.1. Fractional crystallization Granophyric patches make up about 1-1.5% of a sample from core 69, section 1 and about 0.5% of a sample from core 33, section 5. The most primitive volcanic glasses present in Hole 417D have about 7.70 wt.% MgO, 2.25 wt.% Na20 and 0.08 wt.% K20 [21], with atomic Na/K of about 25. Treating K20 as a completely incompatible element during fractionation, and allowing 95% fractionation of a magma with 0.08 wt.% K20, indicates that the last 5% (by weight) liquid should contain 1.6 wt.% KzO. Allowing for 40% fractionation of plagioclase [21] containing 0.05 wt.% K=O [16] reduces the expected K20 in the last 5% residual liquid to 1.2 wt.%. The rhyodacitic glass (analysis 11, Table 1) has K20 (1.61 wt.%) in the range predicted by the above semi-quantitative treatment; K20 and other elements in the andesitic glasses (analysis 9, 10, Table 1) can be modeled by lower degrees of crystal fractionation (e.g. [13]). In contrast to the glasses, 417D granophyres and other abyssal crystalline silicic rocks have K=O values far below those pTedicted by simple crystal fractionation. Other than minor K20 in plagioclase, there is no evidence for fractionation of a potassic mineral phase in any of these samples and
the effects of separation of such a phase can be effectively excluded from consideration. Clearly some process in addition to fractional crystallization is responsible for the production of the compositions of the 417D patches. A comparison of the 417D granophyres with glasses of roughly comparable SiO2 (Table 1)indicates that although Na20 is slightly enriched in the former, the most striking compositional characteristic is the apparent depletion of K20 in the crystalline samples. Interstitial brown-clay patches contain between 0.5 and 1.3 wt.% K20 (Table 2) and it is possible that residual K20 is now concentrated in these patches. We have considered three possible addition',d mechanisms to account for the apparent mobility of K, and the fractionation of K from Na in these rocks, namely (1) liquid immiscibility, (2) sub-solidus metasomatism, and (3) late-stage vapor phase transport.
4.2. Liquid immiscibility The spatial association of relatively Fe-rich interstitial brown clay patches with Si-rich granophyric patches suggests that liquid immiscibility may have been important in the crystallization of 417D dolerites. This process has received renewed attention in recent years [22-25] and Sato [26] has shown that immiscible liquids have developed in some abyssal oceanic magmas. However, existing evidence indicates that immiscible liquids will not fractionate K from Na (e.g. [22]). Furthermore, immiscibility experiments [25] predict that P should be strongly partitioned into the Fe-rich liquid. However, phosphorus distribution in core 69, section 1 is erratic (Tables 1,2) with tiny apatite needles present in both the granophyres and in some clay-rich patches. Although liquid immiscibility may have been important in the latest stages of crystallization of the Hole 417D magmas, this process does not account for the Na, K and P distributions in the rocks.
4.3. Sub-solidus metasomatic alteration K-rich clays and K-feldspar were identified by shipboard scientists as low-temperature alteration products in Hole 417A, and it is clear that potassium can be mobilized at very low temperatures in the
427 presence of seawater. However, such alteration and leaching effects are usually accompanied by hydration of the primary rock. The granophyres are essentially anhydrous and do not appear to have undergone sub-solidus hydrothermal alteration. In contrast, the brown clay-rich patches contain significant water as well as potassium. Although a reasonable case can be made for a hydrothermal source for the K in the clay-rich patches, it is unlikely that this process is responsible for the apparent depletion of K in the granophyric patches. We therefore favor a late magmatic, rather than a sub-solidus, mechanism as an explanation for the alkali concentrations in these rocks.
4.4. Vapor-phase transport in abyssal magmas The role of a vapor phase in modifying alkali concentrations in rocks was explored by Kennedy [27] and by Orville [28]. Orville demonstrated that, in the presence of a thermal gradient, K is lost from hotter portions of a rock and moves toward cooler portions. He also noted that high Ca-feldspar component (i.e., compositions more appropriate to 417D dolerites) promotes high K/Na in the vapor phase [271. Richter and Moore [29] found a zone enriched in alkalis, especially KzO, occurring in melt immediately below the upper crust of the cooling Kilauea Iki lava lake. Those authors suggested that the K-enrichment was produced by alkali transfer from deeper portions of the lake and proposed that this transfer takes place in water vapor. This work provides important documentation that vapor-phase transport of alkalis may operate in tholeiitic magnaas under appropriate conditions. Attempts to explain the low alkali contents of lunar basalts brought renewed interest in the volatilities of alkali elements [30-33]. Selective vapor phase transport has been proposed to explain potash variations in lunar plagioclase [30] and between lunar rocks and fines [31]. De Maria et al. [32] demonstrated that K is more volatile than Na below about 1050°C, that is at temperatures appropriate to the near-solidus region of abyssal basaltic magmas, and Gooding and Muenow [33] showed that terrestrial basalts have similar vaporization trends to lunar basalts.
Although detailed transport mechanisms are difficult to determine, the evidence of these studies indicates that at temperatures appropriate to the nearsolidus region of abyssal magmas, K may be fractionated into a vapor phase. Gooding and Muenow [33] point out that alkali vapor transport can occur with vesiculation under essentially anhydrous conditions appropriate to the moon, or with water present. Those authors also suggest that only in lava lakes would the requisite amount of gas and thermal energy be present to allow for continuous vesiculation over the necessary time. This is consistent with the proposal for vapor phase transport of alkalis in the Kilauea Iki lava lake [29]. The 417D granophyres occur in massive units greater than 10 m thick, giving rise to the interpretation of the formation of these units either as ponded flows (lava lakes) or as sills. Regardless of their specific occurrence, vesiculation was apparently active since these units contain welldeveloped amygdaloidal zones in the upper portions (personal observation of the authors while aboard ship). We feel that the above evidence supports the conclusion that the principal processes controlling K distribution in the granophyres were late magmatic and propose that vapor-phase transport be considered as playing an important role in producing the characteristically low KzO contents of these patches. This allows for volcanic silicic glasses and holocrystalline silicic rocks with widely different Na/K, and possibly other cation, ratios to have a cogenetic relationship to each other, as well as to abyssal oceanic basalts. However, for this interpretation to have validity it is important to consider why this effect is pronounced in abyssal oceanic magmas, but apparently not in many continental magmas. Coleman and Peterman [19] showed that high Na/K ratios appear to be a characteristic of oceanic silicic differentiates, and a reliable discriminant from continental differentiates (Fig. 2). We suggest that depletion of K by vapor phase transport may be nearly universal in the late stages of crystallization of plutonic oceanic rocks. We also propose that this phenomenon may be restricted to magmas with low initial K20 contents. Magmas with higher initial K20 contents commonly crystallize a potassic mineral phase before reaching 98-99% crystallization, thus fixing K20 in the resultant rocks. Although late-stage
428 An
vapors may be active in both oceanic and in continental magmas, their effects on alkali concentrations will only be pronounced where nearly all K20 is retained in the last I - 2 % residual liquid. Furthermore, this residual liquid must be present at low enough temperatures for effective vapor-induced fractionation of potassium from sodium (cf., volatility relations of Na and K versus temperature [30,31 ]). Apparently these conditions are realized in abyssal oceanic magmas, to the exclusion of most continental magmas.
5. Implications for the composition o f oceanic crust Silicic rocks, typically with high Na/K, commonly occur in the upper portion o f the plutonic parts or near the dike complex-erupted lava interface, in ophiolites. In many such complexes, silicic rocks account for about 1% of the total section [34], whereas in others the percentage of silicic rocks is much greater (e.g. [35]). The presence of silicic rocks in ophiolite complexes has been cited as evidence against ophiolite origin as oceanic crust [36]. Although we have not shown that the small patches of granophyre ever accumulate and segregate significantly from their basaltic hosts, our data suggest that small volumes of silicic rocks with high Na/K can be expected in oceanic crust. From an analysis o f Hole 417D glass data, Byerly and Sinton [21] suggested that even the least fractionated samples in that hole may have undergone as much as 50% crystallization and extraction before emplacement. Taking this figure as a reliable generalization for oceanic crustal processes, and a figure o f 0.5-1.5% for the proportion of granophyre in 417D dolerites, then high-Si, high-Na/K differentiates can be expected to comprise no more than about 1% of typical oceanic crust, or perhaps up to 2% of the plutonic part of typical oceanic crust. Thus, the evidence presented here indicates that large volumes of high-SiO2 igneous rocks are not to be expected in oceanic crustal sections, post-magmatic silica metasomatic enrichment notwithstanding. Coleman and Peterman [ 19] and Coleman and Donato [20] have shown that siliceous rocks from the Troodos ophiolite complex are characterized by high normative (Ab + An)/Or, a feature atypical of siliceous rocks from continental environments. Sili-
Ab
--
xOr
Fig. 2. Oceanic silicic rocks plotted in terms of their normative components An-Ab-Or. Hole 417D granophyres are shown as solid circles, Mid-Atlantic Ridge diorites [ 10] as squares, aplites [11,12] as inverted triangles, an Indian Ocean quartz-monzonite [12] as a triangle and Galapagos Ridge andesites and rhyodacites [ 13] as open circles. The fields for Troodos plaglogranites (TP) and continental granophyres (CG) are from Coleman and Peterman [19].
ceous rocks from other ophiolites tend to be similar [20,34,35] as are the oceanic rocks considered in the present paper (Fig. 2). These data support the conclusion [19,20] that Troodos and many other ophiolitic siliceous rocks have oceanic affinities.
6. Conclusions (1) Granophyric patches in DSDP Hole 417D dolerites represent direct evidence that abyssal tholeiitic magmas may evolve to high-SiO2, high-Na/K differentiates. (2) High atomic Na/K ratios appear to be typical of differentiated silicic abyssal crystalline rocks; known volcanic glasses from oceanic spreading centers are enriched in potassium relative to the 417D granophyres and to most other crystalline abyssal siliceous rocks. (3) Potassium contents of the granophyres are much lower than predicted from formation by crystal fractionation. We suggest that K may have been removed by vapor-phase transport during late-stage crystallization. Thus, a cogenetic suite of volcanic
429 silicic glasses a n d h o l o c r y s t a l l i n e silicic r o c k s m i g h t have very d i f f e r e n t N a / K , a n d possible o t h e r c a t i o n , ratios. ( 4 ) A l t h o u g h late-stage v a p o r t r a n s p o r t m a y be active in m a n y e n v i r o n m e n t s , this m e c h a n i s m h a s pron o u n c e d e f f e c t o n K d i s t r i b u t i o n s in abyssal o c e a n i c m a g m a s w h e r e n e a r l y all K 2 0 is residual in the last 1 - 2 % liquid at t e m p e r a t u r e s a p p r o p r i a t e for effective v a p o r f r a c t i o n a t i o n o f K f r o m Na. (5) High-SiO2, h i g h - N a / K i g n e o u s rocks c a n be e x p e c t e d to m a k e u p n o m o r e t h a n 1% o f typical o c e a n i c crust or a b o u t 2% o f t h e p l u t o n i c part o f o c e a n i c crust. (6) Siliceous r o c k s in m a n y o p h i o l i t e s are similar to o c e a n i c rocks d e s c r i b e d here a n d can n o w c o n f i d e n t l y be c o n s i d e r e d to h a v e a f f i n i t y w i t h abyssal o c e a n i c magmas.
Acknowledgements We b o t h t h a n k t h e S m i t h s o n i a n I n s t i t u t i o n for fellowship s u p p o r t , use o f facilities a n d invaluable help from numerous staff members, particularly Charles O b e r m e y e r III, T i m O ' H e a r n a n d R i c h a r d J o h n s o n . T h e Deep Sea Drilling P r o j e c t ( N S F ) provided s u p p o r t t o p a r t i c i p a t e o n D S D P Legs 51 (J.S.) a n d 5 2 (G.B.). The m a n u s c r i p t was p r e p a r e d w i t h the h e l p o f s t a f f at Hawaii I n s t i t u t e o f G e o p h y s i c s . T w o a n o n y m o u s reviewers gave h e l p f u l reviews o f an earlier version o f t h e m a n u s c r i p t , a n d David M. Christie m a d e m a n y valuable c o m m e n t s at various stages o f this s t u d y . This is H I G c o n t r i b u t i o n No. 1028.
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NOTE ADDEDINPROOF Wright, Peck and Shaw [Am. Geophys. Union, Geophys. Monogr. 19 (1979) 385-387] found some of the original analyses listed in Richter and Moore [29] to be in error and concluded that there is no evidence for alkali transfer in Kilauea lava lakes. These findings in no way alter the conclusions of this paper, however.