INORGANIC ORIGIN IN UPPER MANTLE SEEN LIKELY FOR SOLID HYDROCARBON IN SYRIA PLATEAU BASALT

Robert F. Mahfoud, James N. Beck
McNeese State University
Lake Charles, La.

Lower Pleistocene carbonatite with basanitic, ultrabasic, and alkaline basalt xenoliths, was locally disseminated with and cut by veinlets of a solid hydrocarbon in rifted and plateau basalt covered southern Syria.

Research in progress by the authors indicates that these rocks originated from the upper mantle (asthenosphere-lithosphere).

The purpose of this study was to determine the chemical compositions of the hydrocarbon and whether or not it was of abiogenic origin.

Petrographic, chemical, inductively-coupled plasma (ICP) and emission spectrometric, neutron activation, and gas chromatographymass spectrometric analysis showed 17 chemical compounds in the solid hydrocarbon, eight of which are n-alkanes and nine aromatic.

In addition, chlorine, bromine, carbon dioxide, carbon monoxide, silicon, sulfur, mercury, antimony, and numerous metallic elements previously found in basanite and ultrabasic xenoliths, were detected.

The sources of chlorine, bromine, carbon dioxide, carbon monoxide, silicon, sulfur, mercury, and antimony were the upper mantle, hot springs and/or hydrothermal systems surging from or crossing the thick rifted and fractured alkaline plateau basalt.

The lack of hydrous silicates in basanite, ultrabasic xenoliths and plateau basalt, and the oxidation and hydration of ferrous oxide only in xenolithic peripheral mafic minerals (olivine and pyroxenes), indicated a seepage of meteoric water along the rift and fractures. The depth reached by this seepage was probably hot enough to decompose water to elemental hydrogen and oxygen.

The oxidation of ferrous oxide to magnetite and hematite was done by the reaction of ferrous oxide and magnetite with carbon monoxide, carbon dioxide, or oxygen (from water). The iron oxides acted as catalysts.

The hematite changed to goethite, Fe2O3, by hydration. The formation of the hydrocarbon occurred by the hydrogenation of carbon or by other reactions between catalysts, carbon monoxide, carbon dioxide, and water.

Reactions probably occurred between 230-500 C. (7-16 km in depth).

Drilling to more than 1,100 m in the alkaline plateau basalt did not reveal the presence of sedimentary rocks or any mother rocks (petroleum bearing).

The absence of mother rocks along with the difficulty of explaining otherwise the sources of all mentioned compounds suggested an inorganic or abiogenic origin in the mantle and/or along rift and fractures in basalt for the concerned hydrocarbon.

This abiogenic origin explained with ease all reactions, sources of elements, and their relationship with the tectonic events in southern Syria.

INTRODUCTION

Southern Syria is covered by a more than 1, 1 00 m thickness of alkaline plateau basalt lavas of Upper Tertiary age found by drilling.

Solid hydrocarbons uncommonly imbibe and fill small fractures cutting Lower Quaternary carbonatite in southern Syria. 1 2

The gray-brown to reddish vesicular carbonatite, in the forms of lavas, dikes, and tuffs, was found in six lower Pleistocene scoriaceous basanitic cinder cones, extruding the alkaline plateau basalt, in Jebel el Arab near the border with Jordan (Fig. 1).

It is also exposed on the surface of Khaldieh basaltic cone, and at Beer Hamam, about 18 km north-northwest of Khaldieh's cone, where it is crossed by the same Damascus-Soueida highway (Fig. 1).

The carbonatite lavas and dikes were commonly loaded with ultrabasic and basanite xenoliths. The thick plateau basalt lavas, basanite cones, ultrabasic xenoliths, and carbonatite, of lithosphere and upper asthenosphere origins, suggested probable presence of an east-west trending rift related to that of Dead Sea-Jordan River .2

Abundant carbon dioxide of asthenosphere origin played a major role in the partial melting process in the dry mantle systems.

SAMPLES, ANALYTICAL METHODS

Chemical tests were done on reddish-gray (nonhydrocarbon bearing) and dark (hydrocarbon bearing) carbonatite samples from Khaldieh's cone.

The samples are Kh1, dark carbonatite, deeply imbibed with and cut by anastomosing veinlets of solid hydrocarbon; Kh2, with about 50 vol % scattered pockets of dark solid hydrocarbon; and Kh3, without any obvious trace of hydrocarbon (Table 1).

Concentrated hydrochloric acid, with a specific gravity of 1.18, was used on equal, pulverized amounts from Kh1 (dark carbonatite), Kh2 (gray pockets without visible hydrocarbon), and Kh3 (gray carbonatite).

The purpose was to leach out and dissolve sulfur and other elements. To check for sulfur, small wires of fresh silver and rods of copper were dropped for 3 days into the filtered yellow liquid (specific gravity 1.20). ICP spectrometry, emission spectrometry, and neutron activation were also used to analyze the yellow liquid, along with Kh2, Kh3, and the black solid hydrocarbon (HC), for information on trace elements (Table 1).

Analysis by gas chromatography-mass spectrometry helped decipher the composition of the hydrocarbon compounds locked in the solid complex. For this purpose, three samples from Kh1 (with dark hydrocarbon) were pulverized and treated with hexane, chloroform, and diethyl ether (Table 2).

ANALYSES

Solid hydrocarbon from the dark carbonatite Kh1 and reddish carbonatite Kh2 samples were used for qualitative analysis by emission spectrometry.

Kh3 was used for quantitative analysis by neutron activation; whereas filtered yellow leachates from Kh1, Kh2, and Kh3 were analyzed by ICP.

The results are summarized (Table 1).

DISCUSSION

All elements listed in Table 1 except mercury, antimony, and perhaps tin, were derived from basanite and ultrabasic xenoliths (originally located in the upper asthenosphere-lower lithosphere) and carried to the surface by carbonatite magma of deeper origin .2

The concentration variations of those elements depended on the percentage of olivine, pyroxenes, spinel, and plagioclase found in xenoliths and included in carbonatite.

The mercury-concentration averages (Table 3), in six carbonatites, four ultrabasic xenoliths, and one basanite, from southern Syria, respectively were 0.007, < 0.005, and 0.47 parts per million weight (Ppmw). Therefore, mercury 4.14 ppmw (Table 1) probably originated from a shallower depth as the next section explains.

Mercury and antimony (mercury, as mercury (acetate-o) phenyl-, was also detected in the concerned hydrocarbon, by gas chromatography-mass spectrometry) presented relatively high concentrations (Table 1), and indicated shallow origin .3 4

Both metals also exhibit a strong affinity with the concerned hydrocarbon; and both originated from and were picked up by the vertically ascending hydrocarbon through Upper Tertiary fractured and/or faulted thick alkaline basaltic lavas in rifted southern Syria .2

Mercury and antimony are commonly associated with volcanic activity around the world, and in minerals such as cinnabar, livingstonite, corderoite, calomel, and others. 3 4 5

Tin, some zinc, and probably other elements, not listed in Table 1, may have also originated from those lavas.1 Sulfur was chemically detected, and its presence was confirmed by gas chromatography-mass spectrometry (Table 2). It was probably derived from the upper mantle6 and/or from the alkaline basalt. 3 4 5

Sulfur and mercury are commonly found together in nature as cinnabar and to a lesser degree as corderoite and livingstonite.4

The results (Table 4) indicated that sulfur was not present in the pure carbonatite Kh3 (i.e. without any trace of hydrocarbon); and sulfur showed a strong affinity with hydrocarbon, which is a common fact acknowledged by petroleum geologists all over 6 7 1 the world.

HYDROCARBON'S CHEMISTRY

In order to decipher the chemical compounds in the concerned hydrocarbon, three pulverized samples from Kh1 (Table 1 for descriptions) were treated with chloroform (trichloromethane), hexane, and diethyl ether, then analyzed by gas chromatography-mass spectrometry.

The presence of trichloromethane and bromodichloromethane (when hexane was used as a solvent), and hexane (when chloroform was used) confirmed the existence of both compounds in the concerned hydrocarbon (Table 2).

The appearance of methane and hexane suggested that a series of straight-chain n-alkanes (paraffine or methane series CnH2+2) was included.8 9

On the other hand, the appearance of benzene compounds indicated the presence (Table 2) of aromatic hydrocarbons CnH2,-6. 8 9 The 17 discovered compounds included eight paraffinic and nine aromatic. The absence of naphthene series and the presence of aromatic compounds is worthy of note.

DISCUSSION

In addition to carbon and hydrogen, the hydrocarbon compounds contained the following substances nitrogen or nitrous oxide, carbon dioxide, oxygen, chloride, bromide, sulfur, mercury, and silicon (Table 2).

Nitrogen, probably derived from ammonia, has been found in volcanic hot spring emanations and hydrothermal systems along with CI2, CO2, H2S, Br2, and Si;5 8 or from the upper mantle .6 Carbon, CO2, CO, CI2, H2, Si, and hydrocarbons were also commonly found associated with ultrabasic, alkaline, and carbonatite rocks, which originated from the mantle (asthenosphere-lithosphere) beneath rift zones .6 10 11 12 13 14 15 16

Bubbles, exsolved in fractured and annealed olivine and pyroxene (in ultrabasic xenoliths) were observed petrographically by the authors. Such bubbles previously have been reported as CO2 by Wyllie.17

Hydrogen, N2 gases, and hydrocarbons were also detected above rift zones and were found associated with serpentinized peridotites and ultrabasic rocks.18 19

Ozocerite and asphalt (solid hydrocarbons), associated with silica, calcite, fluorite, and metallic sulfides were reported in hydrothermally altered traps.8

The carbon was probably derived from the reduction of carbon dioxide and carbon monoxide, from the carbonatite of mantle origin, by reacting with the catalyst ferrous iron (in mafic minerals) in the upper mantle 6 and during their vertical ascent through fractured ultrabasic and alkaline basaltic rocks.

The oxidation of ferrous oxide in mafic silicates to magnetite and hematite was petrographically commonly found in fractured olivine and pyroxenes (Fig. 2A). Goethite was also found along fractured mafic minerals (Fig. 2B).

The fracturing was caused by the expansion of ascending ultrabasic xenoliths from the mantle to the surface; and goethite formation by the downward seepage of meteoric water, through the rift and fractures (also reported by Gough 20), probably to a depth, hot enough to break the water to H2 and 02 (hematite and goethite are found together, Fig. 2B).

Part of the oxygen reacted with the catalyst ferrous oxide, or with magnetite, to form hematite; and H2 with C to form hydrocarbons.

It is worth mentioning here that part of the carbon could have been derived from CO2 dissolved in meteoric water. The following H2O-CO2-CO-reduction reactions, based on Fischer-Tropsch method, and applied by many, probably took place in rifted southern Syria to form the original hydrocarbons. Ferrous oxide, in the reactions, refers to ferrous magnesium silicates (olivine and pyroxenes) in the ultrabasic rocks and, to a lesser extent, in basalt.

2FeO + CO -Fe2O3 + C

3FeO + CO -Fe3O4 + C

3FEO + CO2 -Fe3O4 + CO

2Fe3O4 + CO- 3Fe2O3 + C

2FeO + H2O-Fe2O3 + H2

3FeO + H2O - Fe3O4 + H2

2Fe3O4 + H2O 3Fe2O3 + H2

Fe2O3 + H2O Fe2O3.H2O

Other reactions pertaining to the formation of hydrocarbons are numerous. 21 22 23 24 25 26

Rare earths, nickel, and perhaps cobalt and platinum, besides iron, in carbonatite and ultrabasic rocks, probably also acted as catalysts in speeding up the reactions and subsequent formation of hydrocarbons. 11 26

Differential hydrogenations of carbon, probably between 230-500 C (7-16 km of depth), led to the synthesis of the mentioned paraffinic and aromatic compounds (Table 2).18 19 26 27 28 29

The other elements (CI2, Br2, Hg, S, Si, N2, etc.) were picked up during or shortly after the chemical reactions with catalysts started to prepare for the hydrocarbon formation probably in the upper mantle 6 30 3 1 and/or along the rift (deep fault as called by Porfr'ev6 32) , and fractures in basalt.6 32

Bromine, and probably the others, easily reacted with ethylene or methyl compounds, as confirmed by Lyday. 33

GENESIS OF HYDROCARBON

The intimate association of the concerned solid hydrocarbon with carbonatite suggested a close genetic relationship between the two.

The hydrocarbon occurred only in very small concentrations and mostly as irregular veinlets cutting carbonatite.

This kind of showing has been reported in many areas of the world,' and the carbonatite was probably formed by the combination Of CO2, originating from the asthenosphere, and calcium oxide that was the result of depletion of peridotite.

Some carbonatite thin sections showed fine dark grains of hydrocarbon, densely disseminated, and randomly mixed with carbonate, in addition to veinlets. Paragenetically, the hydrocarbon was probably syngenetically formed with, or slightly younger than, carbonatite.

As mentioned earlier, the necessary hydrogen must have been derived from meteoric water; first, because of the total absence of hydrous silicate minerals in the ultrabasic xenoliths and alkaline basalt; and second, because of the occurrence of secondary magnetite, hematite, and geothite only along peripheral fracture mafic minerals in the xenoliths (Fig. 2).

It is deduced, therefore, that the process of hydrocarbon formation has taken place in the crust after a deep infiltration of meteoric waters. No sedimentary mother rocks (petroleum-bearing sedimentary rocks) or any other sedimentary rocks, but only fractured basalts, were found by drilling to more than 1,100 m in southern Syria.

Therefore clear field evidence suggesting an organic origin for the concerned hydrocarbon is lacking.

The presence of carbonatite dikes, carrying ultrabasic xenoliths coated with basanite, indicated an origin from the asthenosphere; and along with the thick plateau basalt, suggested the presence of a rift more than 70 km deep probably connected to the Dead Sea-Jordan Valley rift.

This depth certainly discounted any organic origin for the concerned hydrocarbon, " and offered favorably the probability of an inorganic (abiogenic) genetic rift-upper mantle source.

This probability easily explained all relationships between the concerned hydrocarbon and the petrologic and tectonic history in southern Syria.

CONCLUSIONS

All chemical and analytical results favor an abiogenic origin for the concerned hydrocarbon in the upper mantle and/or along the rift and fractures in the plateau basalt in southern Syria.

Carbon and hydrogen, necessary for the formation of hydrocarbons, originated from the reactions Of CO2, CO, and H2O with catalysts, especially ferrous oxide and magnetite in mafic silicates (olivine and pyroxenes), at 230-500 C.

Carbon dioxide and carbon monoxide were probably mostly derived from the mantle, whereas H2O was derived from meteoric water.

Eight n-alkanes and nine aromatic compounds were found in the hydrocarbon. Included with the hydrocarbon were CI2, Br2, CO2, S, Hg, and Si; which probably originated from the upper mantle, hot springs, and/or hydrothermal systems that surged from or crossed the rifted and fractured alkaline plateau basalt.

All metallic elements listed in Table 1 probably originated from ultrabasic, basanitic, and, plateau basalt, rocks during the formation of the hydrocarbon.

ACKNOWLEDGMENTS

The first author sincerely thanks the General Establishment of Geology and Mineral Resources, Syria, for having provided samples used in the research. Both authors acknowledge the help of the chemistry department and that of McNeese State University by allowing them to use laboratory facilities. The authors are deeply appreciative of J.D. Tauber, G. Ramelow, and E. Murray, professors at McNeese, for their needed lab assistance, and of J. Batchelor, the geology coordinator, for his financial support.

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