•'OURNALOF GEOPHYSICAL RESEARCH VOL.75, NO. 2, •'ANUARY10, 1970

Titanoclinohumite'A PossibleMineralogical Site for Water in the

T. R. MCGETCHIN,2 L. T. SILVER,AND A. A. CHODOS Divisiono/ GeologicalSciences, Cali/ornia Institute o/ Technology Pasadena, 91109

Titanium-rich clinohumite and layered structure are observedin kimberlite and as inclusionsin pyropicgarnets from the MosesRock dike, a kimberlite-bearingbreccia dike in San Juan County, . Associatedclinopyroxenes observed as inclusionswithin similar pyropesand also in kimberlite are estimatedto have equilibratedat depthsranging from about 50 to 150 km at modesttemperatures, generally less than 1000øC.The presenceof titanoclinohumite,a high-density hydrous phase, is of considerableinterest as a possiblesite for volatilesin the earth'supper mantle. The dehydrationof hydrousphases such as titano- clinohumitewithin the upper mantle (1) may providewater as a free phase,(2) could be importantin the genesisof kimberliteand alkali-basaltmagma, and (3) may be one means of producinga low-velocityzone in the upper mantle.

INTRODUCTION This implies significant local concentrationof Titanoclinohumite has been observed in thin volatiles in the upper mantle and raises the sectionsand in heavy concentrates.from question of where H20 might reside at depth. kimberlite collected at the Moses Rock dike If bound in hydrous minerals,amphibole [Ox- in San Juan County, Utah. This dike is one brough,1964], and phlogopite[Kushiro of a cluster of breccia-filledkimberlite-bearing et al., 1967] are prime candidatesbecause these diatremes located in northeast Arizona and phases are commonly observed in kimberlite southeast Utah, which are known to contain and peridotire. rock and mineral fragments believed to be It is our suggestion that group derived from a wide vertical column ranging minerals, specifically, titanoclinohumites, may from the surface extending into the upper be an important mineralogicalsite for bound mantle [Shoemaker,1956, 1962; Watson, 1967; water in the upper mantle and may be more McGetchin, 1968a; McGetchin and Silver, 1968, abundant than commonlyrecognized because of 1969]. Kimberlite dikes and pipes character- their similarity to in optical, physical, istically contain abundant hydrousphases, such and chemical properties. It is implied that de- as serpentine,, mica, chlorite, amphibole, composition of titanoclinohumite to olivine, in addition to olivine, pyroxene,, , rutile, or other -rich phases such as and other less abundant minerals. Kimberlite ilmeniteor geikieliteand free volatilesmay have generally is believedto be emplacedas volatile- played an important role in the genesis of rich multiphase (probably fluidized) systems kimberlite at the Moses Rock dike and may be [Davidson, 1964; Shoemaker, 1962; Watson, an important upper mantle process. 1967; McGetchin, 1968a, b], directly from the OCCURRENCE OF TITANOCLINOI-IUMITE AT upper mantle at depths of 100 to 200 km MOSES ROCK DIKE [O'Hara, 1967; McGetchin and Silver, 1968]. Titanoclinohumite is observed in several tex- x Contribution 1647, Division of Geological Sci- tural modesat Moses Rock dike, as grains in ences,California Institute of Technology. kimberlite commonly associatedwith olivine 2 Air Force Institute of Technology, Wright- and as mineralinclusions within pyropicgarnets. Patterson AFB, Ohio 45433. Presently Dept. of Kimberlite at Moses Rock dike is a highly Earth and Planetary Sciences, In- serpentinized, mica-bearing, peridotRe micro- stitute of Technology. breccia 6onsistingof millimeter-sizedangular Copyright ¸ 1970 by the American GeophysicalUnion. mineral fragments,mostly olivine and pyroxene, 255 256 McGETCHIN, SILVER, AND CHODOS more rarely, garnet, spinel, and mica, with The Mg/(Mg q- Fe) rangeis similarto that subordinatesmall rock fragments.,all set in a observedin olivine grains from Moses Rock fine-grainedgroundmass of serpentine,ehlorite, kimberlite (89 to 94, in olivine); where olivine and tale. Serpentine,tale, and ehloriteconstitute and elinohumite were observed to coexist in the about 80% of the rock by volume.As a discrete same grain, both had approximatelythe same mappable rock unit, kimberlite is rare, occupy- Mg/(Mg + Fe) ratio. This is shownin the ing only about 1% of the area within the dike. analysesof coexistingolivine and titanoelino- Clinohumite grains in the kimberlite occur humitc (seeTable 1, analyses5, 6 and 10, 11). as individual angular fragmentsaveraging 1 mm It is also true of coexistingtitanoelinohumite in size, and less commonly as intergrowths and olivine inclusionsin pyrope (see Table 1, (epitaxial?) with olivine in a single grain. analyses8 and 9). Olivine elastsin kimberlite generally are partly Total water contentscan only be estimated serpentinized; titanoelinohumite is similarly by difference of the oxide summation from altered but to a somewhatlesser degree than 100%, but suggestedvalues are all between1 the olivine. This is clearly demonstratedwhen and 2 wt %, in agreementwith availablewet olivine and elinohumite occur together in the chemical data (Table 1, analysis 14). The same grain. content is assumed to be characteristi- Chrome-richpyropie ,up to about 5 cally low, in accordancewith other workers' mm in diameter occurring in kimberlite at observation on titanoelinohumite. Moses Rock, are concentratedat the surface of the ground in aprons of ejeeta surrounding OCCURRENCESAND STRUCTUREOF CLINOI-IUMITE ant hills. Mineral inclusionswithin thesegarnets Balk [1954] observedclinohumite at the Buell include olivine, elinopyroxene, orthopyroxene, Park diatreme near Fort Defiance, Arizona, rutile, , timehire, mica, and titano- about 95 miles southeast of the Moses Rock elinohumite.One particularly interestingpyrope dike. Chemicalanalyses of a Buell Park sample specimencontains olivine, mica (?), elinohumite, (Table 1, analysis14) showthat major chemical and geikielite within the same grain. Mineral constituentsfall within the ranges defined by chemistry of the garnets and a large suite of the Moses Rock specimensanalyzed so far. associatedinclusions is describedelsewhere [Mc- The principal anton in the Buell Park titano- Getchin, 1968a; McGetchin and Silver, 1968, clinohumiteis hydroxyl ion, not fluorine. 1969]. Titanium-rich clinohumitescontaining 2.92% TiO2 have been reported from Bhandara Dis- IDENTIFICATION AND COMPOSITION trict, , in phlogopiteand tremolite-bearing Titanoclinohumite was identified optically in marble [Muthuswami, 1958]. In the southern thin sectionsof kimberlite. It is distinguished Urals clinohumite containing 5.40% Ti02 was from olivine in ordinary light becauseof its described in association with low-titanium, brownish-yellowcolor. forsteritc-richolivine by Borneman-Starynke- Four grains from heavy mineral concentrates rich and Myasnikiv [1950]. were picked, mounted, and analyzed for major The structuresof the humitc group elements (St, Ti, Cr, Fe, Mg, Mn) with the minerals are currently under investigation. Caltech microprobe (Table 1, analyses1, 2, 3, Gibbs and Ribbe [1969] and Jones et al., 4). Data were reducedby the method of Bence [1969] have shownthat humite groupminerals and Albee [1968]. Grains from these separates consistof units of Mg(F, OH)O alternatingwith were powdered and positively identified as Mg•SiO3(F, OH). clinohumite by X-ray diffraction. Machatschki [1930] suggestedthat Ti re- The density of several titanoclinohumite placesSi in the humitc series,but Deer e• al. grains was determined by buoyancy tests in [1962, pp. 52, 53] argue that no apparent silica calibrated heavy liquids to be 3.27 ---+ 0.02, deficiencyaccompanies high titanium contents. identical within error limits to the density of Moses Rock titanoclinohumites are possibly the associated olivine. slightly silica-deficient,but only by about I to The most distinctive feature of these analyses 2%, a value essentiallyat the error limits. is their high TiO.• contents. Jones et al. [1969] suggestthat titanium sub- UPPER MANTLE WATER SITE 257

oo o• 258 McGETCHIN, SILVER, AND CHODOS stitutes for Mg with a general humitc formula necessaryvolatiles responsiblefor the eruption of the form and associatedemplacement of kimberlite at MosesRock dike. Rutile, chrome-bearingtimeh- n[ M•.S i04]. [ M•_•T i•(OI-I, ire, and geikielite are titanium-rich phasescom- where x ( 1, M includesoctahedral coordinated mon among the dense minerals concentrated eations (Mg, Fe, Mn, Ca, Zn, etc.), and n -- 4 from Moses Rock kimberlite and could resull for clinohumite.This formula suggeststhat the from decompositionreactions involving titano- ratios Mg'Ti' Si in clinohumitesshould vary clinohumite,in part. systematicallybetween 9'0'4 (for x -- 0) and Liberation of free volatiles in the upper 8' 1'4 (for x -- 1). mantle by the decompositionof hydrous min- Analysis of titanoclinohumitesfrom Moses erals including titanoclinohumite may be a Rock kimberlite and other previouslypublished geologicallyimportant pheonomenonand could analyses (Table 1) fall on the compositional be involved in the genesisof volatile-rich erup- join betweenMg(OI-I, F)2 ß 4Mg2SiO•and TiO• tive systems, such as kimberlite, , : 4MgSiO, in agreement,with the suggested and alkali-basalts, and may be a means of formula of Jones et al. [1969], and excludethe producing the low-velocity layer in regions of possibilityof Ti for Si substitution. low surface heat flow where calculated upper mantle temperatures appear to be too low to I)ISCUSSION AND CONCLUSIONS permit partial melting. The textural relationshipsalmost preclude the Titanoclinohumite may be added to the list genesisof titanoclinohumitein the MosesRock of possible volatile-bearing mineral phases kimberlite as a late stage alteration product within the mantle, along with phlogopite, of olivine and indicate that titaniclinohumite amphibole,and hydrouspyroxenes. coexistedwith chrome-pyropeand olvinie prior Acknowledgments. Microprobe data were re- to serpentinization.Titanoclinohumite coexists duced with the help of Mrs. Lily Ray, with a with olivine of both higher and lower Mg/(Mg computer program by A. L. Albee and A. E. -t- Fe) ratios. Olivine grains separated from Bence. The help of these individuals is gratefully Moses Rock kimberlite have Mg/(Mg •- Fe) acknowledged. This research was supported by a Penrose grant ratios between 88 and 94. Samples with low (1940-65) from the Geological Society of America Mg/(Mg •- Fe) ratios are believed to be and the U.S. Atomic Energy Commission, con- derivedfrom a spinel-bearingperidotire (lherzo- tract AT(04-3)-427, CALT-427-40. lite) assemblagewith aluminum-richpyroxenes; REFERENCES those with high Mg/(Mg •- Fe) ratios, from garnet-bearingperidotire (lherzolite) assemblage Balk, R., Petrology seclion, in Mineral resources with aluminum-poorpyroxenes. Pressure and of Fort Defiance and Tohatchi quadrangles,Ari- temperatureestimates on mineral phasesinti- zona and , .hr. Mex. Bur. Mines Mineral Res. Bull. 36, 192, 1954. mately associatedwith clinohumitein pyropes Bence, A. E., and A. L. Albee, Empirical correc- and in the kimberlite suggestthat theseminerals tion factors for the electron microanalysis of probablywere deriveddirectly from the mantle and oxides, J. Geol., 76, 383-403, 1968. [McGetchin and Silver, 1968]. Minerals in Borneman-Starynkevich,I.D., and V. S. Myasni- kimberlite from the Moses Rock dike probably kiv, On isomorphous replacements in clino- humite, Dokl. Acad. Sci. USSR, 71, 137, 1950. representmechanically disintegrated lherzolite Davidson, D. F., On diamantiferous diatremes, from the mantle, and to a much lesserextent Econ. Geol., 59, 1368-1380, 1964. from the vent walls, not associated Deer, W. A., R. A. Itowie, and J. Zussman,Rock with a silicatemelt [McGetchin, 1968a, 1969a]. Forming Minerals, vol. 1, Ortho- and Ring Silicates, John Wiley, , 1962. It is inferred that titanoclinohumite was pres- Gibbs, G. V., and P. It. Ribbe, The crystal struc- ent with both garnet-bearingand•spinel-bearing tures of the humite minerals, 1, Norbergite, assemblagesin the mantle under the Moses Amer. Mineral., 54, 376-390, 1969. Rock dike before transport to the surface dur- Jones, N. W., P. It. Ribbe, and G. V. Gibbs, Crystal chemistry of the humire minerals, ing the emplacementof kimberlite. Amer. Mineral., 54, 390-411, 1969. The decompositionof hydrousminerals, such Kushiro, I., Y. Syono, and S. Akimoto, Stability as titanoclinohumite,could supply part of the of phlogopite at high pressure and possible UPPER MANTLE WATER SITE 259

presence of phlogopite in the earth's upper Muthuswami, T. N., Clinohumite, Sausar series, mantle, Earth Planet. $ci. Lett., 3, 197-203, 1967. Bhandara District, India. Proc. Indian Acad. Machatschki, F., Ueber Titanolivin, Ein Beitrag Sci., 48A, 9, 1958. zur Frage der Rolle des Titans in Silikatan, O'Hara, M. J., Mineral paragenesisin ultrabasic C.entralb. Min., Abt. A, 191, 1930. rocks, chap. 12, part III, in Ultramafic and Re- McGetchin, T. R., The Moses Rock dike: Geol- lated Rocks, edited by P. J. Wyllie, John Wiley, ogy, petrology, and mode of emplacement of a New York, 1967. kimberlite-bearing breccia dike, San Juan Oxbrough, E. R., Petrological evidence for the County, Utah, Ph.D. thesis, California Institute presenceof amplibole in the upper mantle and of Technology, Pasadena, 1968a. its petrogenic and geophysical implications, McGetchin, T. R., Structure of the Moses Rock Geol. Mag., 101, 1, 1964. kimberlite dike, San Juan County, Utah (ab- Ribbe, P. H., G. V. Gibbs, and N. W. Jones, stract), Program Annual Meetings, Geological Cation and anion substitutions in the humitc Society of America, Mexico City, 194-195, Nov. minerals, Mineral. Mag., 37, 966-975, 1968. 1968b. .Shoemaker, E. M., Occurrence of uranium in McGetchin, T. R., Source and emplacement of diatremes on the Navajo and Hopi reservations, kimberlite at Moses Rock dike, Utah (abstract), Arizona, New Mexico, and Utah, in Contribu- Trans. Amer. Geophys. Union, 49, 360, 1969a. tions to the Geology o• Uranium and Thorium by the U.S. Geological Survey and Atomic McGetchin, T. R., A crustal-upper mantle model Energy Commission •or the United Nations for the plateau based on observations Internation Conference on Peaceful Uses o• of crystalline rock fragments in a kimberlite dike (abstract), Trans. Amer. Geophys. Union, Atomic Energy, Geneva, Switzerland, 1955, U.S. Geol. Surv. Pro[. Pap., 300, 179-185, 1956. 49, 360, 1969b. Shoemaker, E. M., Interpretations of lunar craters, McGetchin, T. R., and L. T. Silver, Compositional relations in kimberlite from the Moses Rock chap. 8, in Physics and Astronomy o• the Moon, edited by Z. Kopal, Pergamon Press, New dike, San Juan County, Utah (abstract), Trans. York, 1962. Amer. Geophys. Union, 49, 360, 1968. Watson, K. D., Kimberlite pipes of northeastern McGetchin, T. R., and L. T. Silver, Compositional Arizona, chap. 8, part IV, in Ultramafic and relations in minerals from kimberlite and re- R.elated Rocks, edited by P. J. Wyllie, John lated rocks from the Moses Rock dike, San Wiley, New York, 1967. Juan County, Utah, submitted to American Mineralogist, 1969. (Received July 11, 1969.)