SILICIC DIFFERENTIATES OF ABYSSAL OCEANIC MAGMAS: EVIDENCE EOR LATE-MAGMATIC VAPOR TRANSPORT OF POTASSIUM
J O H N M . SINTON
Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HL 96822 (U.S.A.)
and • ' " ' . .' . : . '
G A R Y R . B Y E R L Y ' ' '
Department of Geology, Louisiana State University, Baton Rouge, LA 70803 (U.S.A.) ,. .,.;
Rec'eived 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 rater abyssal silicic glasses. Crystalline varieties with SiOj > 6 0 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 K2O contents of abyssal silicic glasses, both the 417D granophyres and other '• crystalline abyssal silicic rocks have much lower K2O 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 oogenetic 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 oogenetic origin with basaltic oceanic crust.
1. Introduct ion
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—12], 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 ,10 -13 ] 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
aplites from the Atlantic and Indian oceans [10—12]. Al l the above mentioned samples were obtained by
dredging, and therefore the relationships o f the silicic rocks to spatially associated mafic rocks are largely inferential. In contrast, doleritic samples obtained during the Deep Sea Dril l ing Project Legs 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 4 I 7 D
Hole 417D, 25°N, 68°W in the western Atlantic Ocean was drilled during Deep Sea Dril l ing Project
Hawaii Institute of Geophysics Contribution No. 1028.
424 Legs 51 and 52. The recovered basement section is dominated by plagioclase-phyric pillow basalts with interspersed massive (non-pillowed) basalts and dolerites [14—16]. At least three massive horizons in Hole 417D are more than 10 m thick. Grain size and textures in massive units vary from porphyritic, hyalO' pilitic near the margins to medium-grained, sub-ophitic in the interiors. The coarser-grained samples contain sub-ophitic intergrowths of plagioclase (Angs—Angs) 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, subbot-tom) and lowermost (629 to bot tom 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
grained (<5 iim) 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 micro-probe analyses of the granophyric patches are given ir Table 1. Although texturally granophyres [17] , the patches can be compositionally characterized as trondhjemites [18] or plagiogranites [19,20]. TiOj and KjO are very low in all the analyses, irrespective of FeO*/MgO ratio or S i02 content. Na/Ca is generally correlative with SiOa 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 Ani4 to Augg. This result is supported by analysis of an oligoclase grain
TABLE 1 Selected analyses of abyssal oceanic differentiates
* All Fe reported as FeO. n.d. = element not determined. Analysis 1: granophyric patch in 417D, core 33, sec. 5;2,3 = granophyric patches in417D, 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.
granophyric patches. Clay-rich pseudomorphs after olivine have high FeO and MgO and low AI2O3 and K2O. In contrast, interstitial clay-rich patches have lower FeO and MgO and higher Si02, AI2O3 and alkalis (Table 2). Low totals from the microprobe analyses of these patches indicate the presence of significant amounts of water.
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 Si02 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]. Ande-site 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
T A B L E 2
Selected analyses * of clay-rich patches. Hole 417D, 69-1
1 2 3 4 5 6
S i 0 2 46.9 48.0 53.5 54.5 54.9 55.9 T i 0 2 0.33 0.24 0.30 0.31 0.40 0.32 A I 2 O 3 4.35 4.46 13.3 11.1 12.5 12.3 FeO ** 17.0 17.7 9.16 12.0 10.0 10.8 MgO 16.0 15.5 6.64 8.36 7.07 7.15 CaO 1.87 1.90 3.20 1.78 2.88 1.61 N a 2 0 0.23 0.19 5.53 4.47 5.49 5.81 K 2 O 0.37 0.50 0.80 1.31 0.57 1.15 i . P 2 O 5 . 0.09 0.05 1.64 0.11 0.087 0.11
Anhydrous 87.1 total 87.1 88.5 94.1 93.9 94.7 95.2
* All analyses by microprobe (Univ. Hawaii C A M E C A ) using 20-pm beam diameter 18-nA beam current. ** Total Fe as FeO. Analysis 1, 2 = clay pseudomorphs after olivine; 3 -6 = interstitial clay-rich patches.
S i (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. Plagio-clase in core 69 granophyres ranges from An 14 to Ans 1; plagioclase in core 33 granophyres ranges from A n 3 2 to Ansg.
( A n 2 6 ) that is 0.04 mm long (see Sinton and Byerly [16, plate IV, fig. 2]) in 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
426 (Na/K > 10 for SiOj > 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 of 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.% NajO and 0.08 wt.% K^G [21] , with atomic Na/K of about 25 . Treating K2O as a completely incompatible element during fractionation, and allowing 95% fractionation of a magma with 0.08 wt.% K2O, indicates that the last 5% (by weight) liquid should contain 1.6 wt.% KjO. Allowing for 40% fractionation of plagioclase [21] containing 0.05 wt.% K2O [16] reduces the expected K2O in the last 5% residual liquid to 1.2 wt.%.
The rhyodacitic glass (analysis 11, Table 1) has K2O (1.61 wt.%) in the range predicted by the above semi-quantitative treatment; K2O 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 KjO values far below those predicted by simple crystal fractionation. Other than minor .K2O 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 Si02 (Table 1) indicates that although Na20 is slightly enriched in the former, the most striking compositional characteristic is the apparent depletion of K2O in the crystalline samples. Interstitial brown-clay patches contain between 0.5 and 1.3 wt.% K2O (Table 2) and it is possible that residual KjO is now concentrated in these patches. We have considered three possible additional 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 inter
stitial brown clay patches with Si-rich granophyric patches suggests that liquid immiscibility may have been important in the crystallization of 417D dole-rites. This process has received renewed attention in recent years [ 2 2 - 2 5 ] 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 mag-matic, 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 [27].
Richter and Moore [29] found a zone enriched in alkalis, especially K2O, 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 magmas 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 near-solidus 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 well-developed 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 K2O 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 oogenetic 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 K2O contents. Magmas with higher initial K2O contents commonly crystallize a potassic mineral phase before reaching 98-99% crystallization, thus fixing K2O in the resultant rocks. Although late-stage
428
vapors may be active in both oceanic and in continental magmas, their effects on alkali concentrations will only be pronounced where nearly all K2O is retained in the last 1—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 of oceanic crust
Silicic rocks, typically with high Na/K, commonly occur in the upper portion of 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 otliers 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 of 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 of 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--Si02 igneous rocks are not to be expected in oceanic crustal sections, post-magmatic silica metaso-matic 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 -1- An)/Or, a feature atypical of siliceous rocks from continental environments. Sili-
An
Ab
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 plagiogranites (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 DSD? Hole 417D dolerites represent direct evidence that abyssal tholei-itic magmas may evolve to high-Si02, 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 and holocrystalline silicic rocks might have very different Na/K, and possible other cation, ratios.
(4) Although late-stage vapor transport may be active in many environments, this mechanism has pronounced effect on K distributions in abyssal oceanic magmas where nearly all K2O is residual in the last 1—2% l iquid at temperatures appropriate for effective vapor fractionation o f K from Na.
(5) High-Si02, high-Na/K igneous rocks can be expected to make up no more than 1% o f typical oceanic crust or about 2% o f the plutonic part o f oceanic crust.
(6) Siliceous rocks in many ophiolites are similar to oceanic rocks described here and can now confidently be considered to have affinity w i t h abyssal oceanic magmas.
Acknowledgements
We both thank the Smithsonian Inst i tut ion for fellowship support, use o f facilities and invaluable help f rom numerous staff members, particularly Charles Obermeyer I I I , T i m O'Hearn and Richard Johnson. The Deep Sea Dri l l ing Project (NSF) provided support to participate on DSDP Legs 51 (J.S.) and 52 (G.B.). The manuscript was prepared w i t h the help o f staff at Hawaii Institute o f Geophysics. Two anonymous reviewers gave helpful reviews o f an earlier version o f the manuscript, and David M . Christie made many valuable comments at various stages o f this study. This is H I G contribution No. 1028.
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NOTE ADDED IN PROOF 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.
