O'Neill, Scott P.

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Senior Thesis MEASUREMENTS OF THE WATER CONTENT OF BASALT FROM PATMOS ISLAND, GREECE Presented in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in the Department of Geological Sciences by Scott P. O'Neill The Ohio State University 1993 Approved By: tY!&,i-6) Dr. Michael Barton Advisor Department of Geological Sciences TABLE OF CONTENTS LIST OF FIGURES................................. ii ACKNOWLEDGEMENTS .................... .'........... iii ABSTRACT........................................ iv INTRODUCTION.................................... 1-2 GEOLOGY......................................... 4 METHODS......................................... 6-7 RESULTS......................................... 9 DISCUSSION...................................... 13 CONCLUSIONS..................................... 14 BIBLIOGRAPHY.................................... 15 APPENDIX (MICROPROBE DATA)...................... 16-20 i LIST OF FIGURES MAP OF THE AEGEAN SEA...................................... 2 GEOLOGIC MAP OF PATMOS ISLAND.............................. 5 TABLE 1: [Ca/Na] Ratios for Plag. Phenocrysts.............. 8 TABLE 2: [Ca/Na] Ratios for magma.......................... 8 GRAPH OF [Ca/Na]piag vs [Ca/Na]uq• PAT 26................. 10 GRAPH OF [Ca/Na]p1ag vs [Ca/Na]uq• PAT 97................. 11 GRAPH OF [Ca/Na]piag vs [Ca/Na]liq• PAT 167................ 12 ii ACKNOWLEDGEMENTS AND DEDICATIONS There are many people I would like to thank for helping me reach the point of actually writing this thesis and preparing to graduate. My thanks to my advisor Dr. Michael Barton for suggesting and supplying me with this project. My thanks also go out to the late David Little for his microprobe instruction. I also would like to thank Mr. Michael Comerford for his help in answering some technical questions I had about this paper. I also must say TA! to my friend Mr. Robert Laumann for supplying me with information on the electron microprobe. My thanks and my love to my father, Angelo O'Neill and my mother Gail O'Neill for there patience, understanding and support. I cannot leave out my various friends including Brad and Ann Brannaman, Rob and Jennifer Hinchee, and Steve and Libby Tompkins for helping me last year when I needed places to stash my belongings before heading to Utah. My thanks to Michelle Jones and Chrissy Straub for putting up with me for the past year. My special thanks go out to my good friend Mr. James Turrin for proof reading this paper for me and for listening to my complaints about it. SP0-1993 iii ABSTRACT The focus of this study was the examination of basalt samples from Patmos Island, Greece. The samples were examined using microprobe analysis and the relative concentrations of the elements present determined. It was hoped that through this analysis it could be determined whether or not the magma from which the rock formed was generated by the interaction of a subducting slab and a mantle source. The concentrations of calcium and sodium in the plagioclase phenocrysts was plotted against whole rock concentrations. These values were then compared against known values of the distribution coefficient and an estimate of the water content was made. From the plots made, it was determined that the source of the parent magma was a mantle source interacting with a subducting slab. However, one of the samples had anomalously low amounts of water and calcium. It is suggested that further study of this sample be conducted to determine if the data used in this study was in error, or to determine the possible causes for these low abundances iv INTRODUCTION Patmos is a small island of the Hellenic arc, Greece. The island is located approximately fifty kilometers east of Turkey and 200 kilometers northwest of Crete in the Southern Aegean Sea. Like several islands of the Aegean Sea, Patmos is made of extrusive igneous rocks, mainly basalt. The examination of these basalt with the use of geothermometers and geobarometers give geologist an idea of the conditions of temperature and pressure that the magma was under while it crystallized. Another important factor determined in analyzing these rocks is the water content of the rock. Knowing the water content of a basalt is important for several reasons, including what effects the water may have had on the magma. Among the effects that water content has on magma are reduction of melt viscosity, increase of diffusion rates, and the decrease of the crystallization temperature. Water content can also indicate the source of the magma that formed the rock. A high water content is a strong indicator that the magma was derived from a mantle source interacting with a subducting slab. A low water content or no water indicates that the possible source of the magma was from the mantle. The focus of this paper is to estimate the water content of various basalt samples from Patmos Island. Several samples of Patmos basalt were analyzed with the electron microprobe, these samples have been labeled Pat 167, Pat 97, and Pat 26. The data collected was then analyzed and an approximate water content was found. In doing this, it was hoped that the source of the magma that 1 created Patmos would be detennined. 2 r-----r·----.,.----------.r---------' I I ,,../. -. ,.._ . ...._,,.. . ,_ . ..,,,... _,. l SALON I Kl "' "' " " BULGARIA Western Thrace ~ Sarnothorki 0 .~ ~ Chios<f;.J IJ'I Sa mos -y r':1!.7 ,--? ~( (../' ~Q Christiana rn ,·,· ~ ............ / 50 100km ......._ -- _ _ -- ..-0 Tr~/-1 - --Figure 1 Map of the Aegean Sea showing major outcrops of volcanic rocks The dashed lines mark the Hellenic Trench and the Hellenic Arc. (From Barton and Wyers, 1991) 3 GEOLOGY The geology of Patmos Island is described in Wyers and Barton (1986). The basalts of Patmos are alkaline in nature, that is they have high concentrations of alkalies (Na, K) relative to silica. Several phases of volcanic activity are recognized on Patmos, including the eruption of ne-trachybasalts, hy-trachybasalts and Q-trachytes which occured 5.5 - 5.7 million years ago; and the eruption of ne-trachybasalts which occured 4.5 - 4.6 million years ago (Barton and Wyers, 1991). These basalts account for a large amount of the rock that makes up the island (fig. 2) Other rock types found on Patmos include rhyolite, phonolite, pyroclastics and marble. With the possible exception of zircon, major and trace element data for the ne-trachybasalts, hy-trachybasalts, trachyandesites and the trachytes are consistent with an evolutionary model involving fractional crystallization in an intermediate depth (about 8 kilometers) magma chamber (Wyers and Barton, 1986). 4 C.GHENOUPAS ~ C.PRASSONISSI figure 2 ISLE OF PATMOS 0 I '--- -:-:.-- '----0 0 .. = .... Ne - Trachy basoll fly - Tr achy IJosalt Hy - T HJChy andesile 0 - Trnchyte Trochyl!! Rhyolile Phonolilo Py1 ocl11sllcs Volcanlc Breccins Silicilied Rocks Old Volconic series Mori.Jiu Quaternary I f Simplified geologic map of Patmos Island, Greece (from Barton and Wyers, 1986) 5 METHODS Three samples collected from Patmos Island were examined for this study, Pat 26, Pat 97, Pat 167. The samples were made into thin sections and then analyzed with the electron microprobe. The microprobe analyzes the sample by bombarding a specific part of the thin section with electrons. X-Rays are produced as a result of this bombardment. Using the wavelength and intensity of the lines in the X-ray spectrum the elements present in the thin section may be identified and their concentrations estimated. The analysis of the samples involved scanning the thin sections for plagioclase phenocrysts. When the phenocrysts were located, a probe analysi~ of the grain's core and rim was conducted. The evolution of the magma that the basalt formed from is be reflected by the change in composition, or zonation that takes place from the core of the grain to it's rim. This will give the researcher an idea of the conditions that exsisted as the magma cooled. Given that: K = (Ca/ Na) plag D (Ca/Na) liquid Values of [Ca/Na]ptag were calculated for all three samples by calculating the molar values of the CaO and Na20 concentrations that were determined by the microprobe analysis. The values for [Ca/Na]liquid were calculated from whole 6 rock analysis of the three samples conducted previous to this study (Barton and Wyers, 1991). The results of these calculations are displayed in Tables 1 and 2. It should be noted that the K0 ~alysis of the plagioclase phenocryst was limited to the more Anorthitic (more calcic) phenocryst. Very calcic plagioclase (>A.Jlw) form in basaltic melts with high water content, but cannot form from dry melts with normal arc NazO and CaO concentrations (Sisson and Groves, 1992). Since no plagioclase phenocryst with >~0 were found in these samples, the more calcic phenocryst present were examined. Since very calcic plagioclase will not form in dry melts, it was believed that examination of the most calcic phenocryst would be the best to study to confirm water was indeed present. The [Ca/Na] ratios tabulated were then plotted and compared against known values of K0 , and an estimate of water content was made. 7 SAMPLE PAT 167 PAT 167 PAT 167 PAT 167 PAT 167 /Ill/// PAT 26 PAT 26 PAT 26 PAT 26 PAT 26 PAT 26 PAT 26 !PAT 26 !PAT 26 !PAT 26 IPAT 26 !/////// IPAT 97 GRAIN # CORE/RIM [Ca/Na) pla~ 4 CORE 8.2583 I 7 CORE 7.7456 I 9 CORE 12.293 I 9 RIM 7.9796 24 CORE I 15.39 ///////// ////////// //////////// 1 CORE 10.186 2 CORE 18.29 2 RIM 12.55 4 RIM 10.18 6 CORE 10.03 8 CORE 9.51 8 RIM 16.21 9 CORE 12.92 9 RIM 11.36 10 CORE 11.68 11 I RIM 9.253 I ll////llll///llllll/ /lllll/lll/11 I CORE 7.41 I TABLE 1 [Ca/Na] Ratios for Plagioclase Phenocryst for Basalt Samples, Patmos, Greece SAMPLE I PAT 26 I PAT 97 I PAT 1671 [Ca/Nal liqui<il 3.66 2.31 2.535 TABLE 2 [Ca/Na] Ratios for magma Basalt Samples Patmos, Greece 8 RESULTS The data for the three samples was plotted against three known values of K0 • From the plots (see pp. 9-11) it was determined that the samples did indeed contain various amounts of water. Given that K0 values of 1.8, 3, and 5 roughly correspond to water percentages of 2, 4, and 6, the water weight percentage of the samples was estimated. The sample PAT 26 possessed phenocryst with water content varying from about 2% H20 to 6% H20. Sample PAT 97 possessed a water content of about 4% and PAT 167 had a water weight percentage that varied from 4% to values greater than 6%. 9 Graph of [Ca/Na]piag vs [Ca/Na]uquid PAT 26 BASALT SAMPLES 17.00.------------;:::===:::::;----:r---------, 16.00 ---------------------------------------~---------------! I<:.cl::~ ~--- ---------"-----------------------------1!5.00 ------------------------------------------------------------------ --------------------------------------------14.00 13.00 12.00 11.00 .10.00 9.00 8.00 7.00 6.00 !5.00 4.00 3.00 2.00 1.00 ----------------------..--------------• I ~--1 I<:.d = 3 1----------------------------------- .------'r&·------------------O.OO·~~-.-~~-.-~--.-~~-r--~~~---.~~-r-~__,.~~~~--1 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 [Ca/Na ]11qu1d 10 Graph of [Ca/Na]plag vs [Ca/Na]uquid PAT 97 BASALT SAMPLE 15.oo-.-~~~~~~~~~~~~~~~~~~~~~~~~ 14.00 13.00 12.00 11.00 10.00 9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 1 Kd=5 r------------------------------------O.Oo-F-~--.-~~...--~--.-~___,~~-,--~-,...~~.,.--~--,..-~~,---~-i 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 [Ca/Na] uquid 11 Graph of [Ca/Na]plag vs [Ca/Na]uquid PAT 167 BASALT SAMPLES 16.oo-.--~~~~~~~~~~~~~~~~~~~~~~~~ 1 E>.00 --------------------------------------------------------~---------- --~ I<:cl == ~ 1--··-··-··-·---------------14.00 13.00 12.00 11.00 10.00 9.00 8.00 7 .00 -·-------· ·----------- ------ --· 6.00 E>.00 4.00 3.00 2.00 1.00 O.OO-+«-~-..,...~~~~---.-~~.,.--~-.-~~-r--~-...-~~..--~--.-~~ 0.00 O.E>O 1.00 1.E>O 2.00 2.E>O 3.00 3.E>O 4.00 4.E>O E>.00 [Ca/Na ]liquid 12 DISCUSSION The data collected and calculated seem to have shown that water was indeed present in the magmas that formed these rocks. Samples PAT 167 and PAT 26 have many high calcic plagioclase phenocryst present. From these two samples it appears that there is enough evidence to suggest that plate subduction did indeed play a part in the genesis of the magma. The problem arises when the data for PAT 97 is examined. Of all the plagioclase cores and rims examined in this sample, only one core was very calcic (An78). The other phenocryst had calcium concentrations ranging from An5 to A"<;3• A calculation of water content for the low calcic plagioclase, An5, revealed that even though small amounts of calcium were present, there was still water present in the sample ( = 1 % H20). The decreasing water content and low calcium content of PAT 97 may indicate that the magma from which this sample originated crystallized later than the parent magmas which produced PAT 26 and PAT 167. As the magma crystallized perhaps the water and calcium present in the magma was depleted early. As a result, later cooling of the magma produced rocks that were depleted in water and calcium relative to the earlier crystallized basalts. This would also explain why the rims of the plagioclase phenocryst in sample PAT 97 show a large change in calcium and water content from the cores to the rims. 13 CONCLUSIONS The water content of these samples of basalts from Patmos Islands strongly suggest that the original magma from which these samples originated was derived from a mantle source interacting with a subducting slab. Water content in the samples from Patmos Island ranges from 2% H20 to >6% H20. Sample PAT 97 has anomalously low percentages of calcium and water relative to the other samples. This may be a result of fractional crystallization. It might be worthwhile re-examine this sample for a future study. This would have to involve a very rigorous and detailed study of the sample with the electron microprobe, to determine if the sample possesses any amounts of highly anorthitic plagioclase. 14 BIBILOGRAPHY Barton, M. and Wyers, G.P. (1991), Estimates of P, T, Pn20 andf02 for Lavas from Patmos (Greece) and Implications for Magmatic Evolution: Journal of Volcaology and Geothermal Research, 47(1991), p. 265-297. Best, Myron G. (1982), Igneous and Metamorphic Petrology, New York W.H. Freeman and Co. Reed, S.J.B. (1975), Electron Microprobe Analysis, London Cambridge University Press Sisson, T.W. and Groves, T.L. (1993), Experimental Investigations of the Role of H20 in Cale-Alkaline differentiation and Subduction Zone Magmatisim: Contributions to Mineralogy and Petrology, 113, p.143-146 Wyers, G.P. and Barton, M. (1984), Transitional Alkaline - Sub Alkaline Lavas From Patmos, Greece: EOS Transactions, American Geophysical Union: 65(45), 1123 15 APPENDIX I 16 [. ! 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Measurements of the water content of basalt from Patmos Island, Greece

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Measurements of the water content of basalt from Patmos Island, Greece

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