Vesicularity of Mid-Oceanic Pillow Lavas

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<ul><li><p>NOTES </p><p>Vesicularity of Mid-Oceanic Pillow Lavas </p><p>F. AUMENTO Department of'Geology, Dallzo~lsie University, Halifax, Nova Scotiu </p><p>Received March 11, 1971 Revision accepted for publication May 25, 1971 </p><p>In contrast to pillow lavas from the marine environment around Iceland and Hawaii, tholeiitic pillow lavas from the Mid-Atlantic Ridge at 45" N do not show a correlation between vesicle size and depth of recovery. Post-eruptive vertical block faulting may have displaced the lavas from their original extrusion depth by as much as 1500 m, explaining in part the lack of correlation. Calculation of the original depth of extrusion by compensation for these vertical movements indicates that many of the basalts now high on the Crest Mountains were extruded in deep water (2900-3500 m), possibly within the floor of the Median Valley. Nevertheless, they were able to produce numerous, com- paratively large vesicles, permitting the basalts to outgas radiogenic argon trapped in the magmas. </p><p>Bulk chemical differences between the 45' N basalts and those of Iceland and Hawaii cannot explain why those from 45" N are able to outgas under high hydrostatic pressure. Different histories prior to extrusion may contribute to these varying characteristics. </p><p>A l'inverse des pillow-lavas rencontres autour de 1'Islande ou de Hawaii, les tholeites de la dorsale atlantique 45" latitude N. ne montrent pas de correlation entre la dimension des vCsicules et la profondeur du prelkvement. Des mouvements tardi-Cruptifs ont pu dCplacer les laves sur des distances verticales de I'ordre de 1500 m, ce qui expliquerait ce manque de correlation. Si on calcule la pro- fondeur initiale d'extrusion en corrigeant l'effet des mouvements verticaux, on constate que beaucoup de basaltes qui sont en "altitude" sur la dorsale ont fait Cruption en eau profonde (2900 a 3500 m), probablement sur le plancher de la vallke midiane. NCanmoins, ces basaltes possederaient assez de vCsicules, suffisamment grandes, pour permettre la perte de I'argon radiogenique. </p><p>Les diffkrences de chimisme entre les basaltes de la latitude N. 45" et ceux de I'Islande ou d'Hawai ne peuvent expliquer pourquoi ceux de la dorsale subiraient en dkgazage a haute pression. Des differences prk-extrusives peuvent Ctre responsables de ces variations. </p><p>Introduction Vesicularity in sub-aqueously extruded basalts </p><p>has been considered to be indicative of the hydro- static pressure under which lavas solidified, and hence an indication of depth of extrusion. Both vesicle size and percentage vesicle volume from Hawaiian and Icelandic lavas have been plotted as a function of depth (Moore 1965; Jones 1969). Moore showed that lavas extruded at depths of less than 1 km and vesicle sizes greater than 0.5 mm diameter, the diameter decreasing rapidly to 0.1 mm at 3 km depth of extrusion, and prac- tically disappearing, i.e. tending to 0 at 4 km depth. The percent vesicle volume also tended to 0 from 3-5 km depth, whereas the specific gravity of the rocks approached a maximum of 3.0 at 4 km. </p><p>Decreasing vesicularity with water depth has been attributed to the inhibiting effect of hydro- static pressure to the exolution and outgassing of gases dissolved in extruding magmas. These gases </p><p>can contain He0 (90 by volume or greater), HCI, HzS, SOz, SOa, and COB (Macdonald 1944; White and Waring 1963; Murata 1966); radio- genic argon and helium, however, are important trace constituents. </p><p>The ability to correlate vesicularity, and hence outgassing, with depth of extrusion has had im- portant applications. Jones (1969), for example, was able to deduce that an Ordovician pillow lava sequence in Wales showed progressive shal- lowing of depth of emplacement in water of neritic-bathyal depth. Of great geochronological importance is the outgassing of radiogenic 40Ar. A fundamental assumption of the K/Ar method of dating rocks depends on the total outgassing of any radiogenic 40Ar present in a magma, per- mitting the radiogenic "clock" to re-set to 0 upon solidification of the magma. Should total out- gassing be impaired, the radiogenic age calcu- lated from measurement of K and 40Ar will yield an age older than the solidification event. The </p><p>Canadian Journal of Earth Sciences, 8. 1315 (1971 ) </p><p>Can</p><p>. J. E</p><p>arth</p><p> Sci</p><p>. Dow</p><p>nloa</p><p>ded </p><p>from</p><p> ww</p><p>w.n</p><p>rcre</p><p>sear</p><p>chpr</p><p>ess.</p><p>com</p><p> by </p><p>HA</p><p>RV</p><p>AR</p><p>D U</p><p>NIV</p><p>ER</p><p>SIT</p><p>Y o</p><p>n 06</p><p>/09/</p><p>14Fo</p><p>r pe</p><p>rson</p><p>al u</p><p>se o</p><p>nly.</p></li><li><p>1316 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 8, 1971 </p><p>retention and detection of radiogenic argon, called "excess" argon, has been demonstrated to be a major problem in K/Ar dating of oceanic rocks (Dalrymple and Moore 1968; Noble and Naughton 1968; Funkhouser rt ul. 1968). The first two papers have shown that recent Hawaiian lavas give systematically older ages with sample depth, from 0 age up to 22 m.y.: these "ages" can be correlated to vesicularity and hence out- gassing. Funkhouser et al. (1968) presented similar evidence from rocks from the East Pacific Rise, and suggested that, in the absence of other criteria, basalts of a high glass content (&gt; 75 z), which would indicate rapid quenching, be re- garded with suspicion as regards to the retention of excess argon. </p><p>Pillow Lavas From 45" N Tholeiitic basalts from the Crest and High </p><p>Fractured Plateaus of the Mid-Atlantic Ridge at 45" N are characterized by classical "pillow lava" shapes with thick glassy rinds (up to 2 cm), very fine grained internal textures with pheno- crysts, and ubiquitous vesicles. These pillows were dredged from depths ranging from less than 1 km to more than 3 km. Vesicles range in size from huge lenticular vesicles some 3 cm thick and up to 15 cm diameter, to the more common spherical vesicles in the 0.1 mm to 2 cm diameter range. Some of the larger tabular vesicles are possibly the result of draining off some of the lava after solidification of an outer rind (Fuller 193 I), rather than gaseous expansion effects. </p><p>The total absence of an obvious correlation of vesicularity with depth of dredging, and the implications of the previous work discussed above prompted a systematic measurement of vesicle size and density of over 300 specimens. Density of the freshest specimens (those from the Median Valley) was found to be unrelated to depth of recovery; over the whole area sampled, it was more a function of distance from the axis, and hence of age, than of other parameters. The observed density decrease with age is due to ocean-floor weathering (Aumento 1971); this effect masks other criteria so effectively as to invalidate this line of investigation. Vesicle size, </p><p>I however, is unaffected by ocean-floor weather- ing, and hence can be used over the whole area. </p><p>Figure la shows the size distribution of all spherical vesicles from all the tholeiitic pillow lavas plotted against depth of recovery. The larger, lensoid vesicles of dubious origin are </p><p>omitted: these would have plotted in the 10-15 cm range, randomly distributed over the whole range of depths. It is apparent that ( i ) there is no obvious correlation of vesicle size with depth, and that ( i i ) vesicles are much larger than re- ported previously from similar depths from Hawaii and Iceland (Moore 1965; Jones 1969). Most of the vesicles range in size from 0.1 to 0.4 mm, but larger vesicles are quite common. </p><p>Discussion </p><p>Barrett and Aumento (1970) demonstrated the existence of vertical displacements on the Crest of the Mid-Atlantic Ridge, due to block faulting, in the order of 1500 m. These vertical movements would invalidate any correlation of vesicle size versus depth, as plbtted in Fig. la, since the depth plotted may bear little relation to the original depth of extrusion: the two depths may be displaced by as much as 1500 m. To eliminate the possibility of such a discrepancy, the data for only the very youngest pillow lavas from the Median Valley (less than 100 000 y; Aumento 1969), for which no vertical movements are to be expected, was replotted in Fig. 16. Although the reduced quantity of data makes interpretation less significant, there still appears to be no marked decrease in vesicle size with depth. </p><p>Another approach at eliminating the effect of vertical movements is to recalculate the depths for each site of recovery back to an original, pre- block faulting elevation. The data of Barrett and Aumento (1970) was used in applying the cor- rection, and the modified vesicle data is presented in Fig. Ic, using an expanded depth scale. The corrected depths suggest that all pillow lavas were extruded within the floor of the Median Valley: note that the calculated depths have a very narrow range between 2900 and 3500 m. We know that this deduction is not altogether valid, since extrusions from volcanic cones of different elevations have been demonstrated to have taken place well away from the valley floor (Muir and Tilley 1964; Fleischer et al. 1968). However, there is a definite case for suggesting that many of the basalts extruded within the narrow depth limits determined. This offers an explanation for the absence of a vesicle size correlation with depth of dredging, since the present scatter of depths is only a secondary effect resulting from block faulting. </p><p>There remains the problem of explaining the different vesicularities of Mid-Atlantic Ridge, </p><p>Can</p><p>. J. E</p><p>arth</p><p> Sci</p><p>. Dow</p><p>nloa</p><p>ded </p><p>from</p><p> ww</p><p>w.n</p><p>rcre</p><p>sear</p><p>chpr</p><p>ess.</p><p>com</p><p> by </p><p>HA</p><p>RV</p><p>AR</p><p>D U</p><p>NIV</p><p>ER</p><p>SIT</p><p>Y o</p><p>n 06</p><p>/09/</p><p>14Fo</p><p>r pe</p><p>rson</p><p>al u</p><p>se o</p><p>nly.</p></li><li><p>NOTES </p><p>ALL BASALTS A 8 tB </p><p>I J .. .. </p><p>PRESENT DEPTH I m ) </p><p>ONLY I </p><p>NO VERTICAL MOVEMENTS EXPECTED - </p><p>I i i </p><p>2 0 0 0 i A 3 0 0 0 </p><p>PRESENT DEPTH = DEPTH OF EXTRUSION (m) </p><p>3 0 0 0 3500 CALCULATED DEPTH OF EXTRUSION (3) </p><p>FIG. 1. Vesicle sizeldepth plots:-(a) all vesicle size data plotted against depth of recovery. (b) plots restricted to recent basalts only, for which the depth of recovery is expected to represent the depth of extrusion. (c) vesicle size data plotted against depths corrected for post-eruptive vertical movements. </p><p>Can</p><p>. J. E</p><p>arth</p><p> Sci</p><p>. Dow</p><p>nloa</p><p>ded </p><p>from</p><p> ww</p><p>w.n</p><p>rcre</p><p>sear</p><p>chpr</p><p>ess.</p><p>com</p><p> by </p><p>HA</p><p>RV</p><p>AR</p><p>D U</p><p>NIV</p><p>ER</p><p>SIT</p><p>Y o</p><p>n 06</p><p>/09/</p><p>14Fo</p><p>r pe</p><p>rson</p><p>al u</p><p>se o</p><p>nly.</p></li><li><p>1318 CANADIAN J O U R N A L OF EARTH SCIENCES. VOL. 8. 1971 </p><p>Hawaiian, and Icelandic lavas extruded in similar water depths. Whereas in Hawaii one can cor- relate incomplete Ar outgassing with depth of extrusion, on the Mid-Atlantic Ridge at 45O N there appears to be no indication for the presence of excess radiogenic argon. No measurable amounts of 4oAr, whatever the origin, were found in glassy pillows from the Median Valley, which later gave 10 000 y fission track dates (Aumento !969). Similarly, six other K/Ar dates from the area are perfectly concordant with fission track dates and ages inferred from ocean floor mag- netic anomalies (Aumento ri (11. 1968; York et (11. 1969). </p><p>Thus, the ability of these basalts to form vesicles under high hydrostatic pressures also permits them to outgas completely. This out- gassing capability remains to be explained. Jones (1969) had found lavas of similar composition in Iceland which behaved quite differently: his type 1 (Kalfstindar) pillow lavas show good vesicle size to depth correlations, whereas his type 2 (Raudefell) pillows show no depth trends. He explained the characteristics of type 2 pillows in terms of partial degassing of the lavas at an emergent vent prior to extrusion. However, he stressed that there are no significant composi- tional differences between the two types of pillow lava. </p><p>The pillow lavas from 45" N used in the plots of Fig. 1 are all tholeiitic, and have compositions similar to the Icelandic lavas of Jones (1969), except for a lower MgO content (8.2% com- pared to 10.5 z). MgO and TiO? are also con- siderably lower than for the Hawaiian lavas of Moore (1965); these parameters can hardly be expected to be responsible for varying degrees of vesicularity. There appear to be no other major element criteria with which to explain the differ- ent outgassing properties. It appears, therefore, that the different properties may depend either on other unknown chemical parameters (original water content of the magma, its viscosity), or on the physical conditions to which the magmas were subjected prior to extrusion (time, tempera- ture, pressure, and rate of crystallization). It is known, for example, that whereas on Hawaii magmas rise rapidly from depths of 60 km to the surface, those beneath the Mid-Atlantic Ridge at 45" N probably stagnate for considerable periods of time in large lopoliths a few km beneath the axis of the Ridge (Muir and Tilley 1964; Au- mento and Loubat 1971). Vesicularity may thus </p><p>be related to the "residence" time of the magma in a chamber prior to extrusion. The longer the residence, the greater the extent to which crystal- lization might proceed, thus increasing the rela- tive water-va~or content of the residual fluid. Upon extrusion, the excessive over-saturation might ensure outgassing, even under relatively high hydrostatic pressure. </p><p>Conclusions </p><p>Pillow lavas from the Mid-Atlantic Ridge at 45" N do not show any correlation between vesicle size and present depth of the lavas. How- ever, their present depth may not be indicative of their original depth of extrusion, since post- eruptive vertical block faulting may have modi- fied the topography by as much as 1.5 km. There is no vesicle size/depth correlation even for the more recent lavas for which no vertical move- ments are expected. </p><p>Attempts at deriving the original depths of extrusion by taking the vertical motions into consideration suggest that many of the lavas extruded within narrow depths limits, between 2900 and 3500 m; such extrusions would have occurred on the floor of the Median Valley. However. the abundance and size of vesicles formed at such depths contrasts the scarcity found in similar environments in the ocean off Iceland and Hawaii. </p><p>The vesicularity at 45' N is accompanied by outgassing during extrusion of any radiogenic 4oAr trapped in the magma. This feature allows K/Ar age determinations to be made with less interference from excess radiogenic argon. Geo- chemical differences between those basalts which are capable of outgassing at depth (45" N) and those which retain gases (Iceland and Hawaii) are small, and would not seem to be contributing factors. Different physical conditions for these basalts prior to extrusion may be the more likely criteria controlling outgassing. Outgassing under high hydrostatic pressure may have taken place upon extrusion as a result of the basalts being held for considerable periods of time in stagnant cupolas prior to extrusion, where crystallization proceeded to the extent of producing volatile- oversaturated residual magmas. </p><p>AUMENTO, F. 1969. The Mid-Atlantic Ridge near 45" N. V. Fission track and ferro-manganese chronol- ogy. Can. J. Earch Sci. 6, pp. 143 1-1440. </p><p>1971. Uranium content of mid-oceanic basalt. Earth and Planet. Sci. Latt. 11, pp. 90-94. </p><p>Can</p><p>. J. E</p><p>arth</p><p> Sci</p><p>. Dow</p><p>nloa</p><p>ded...</p></li></ul>