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Titanian Clinohumite in the Carbonatites of the Jacupiranga

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Titanian Clinohumite in the Carbonatites of the Jacupiranga American Mineralogist, Volume 77, pages 168-178, 1992 Titanian clinohumite in the carbonatitesof the JacupirangaComplex, Brazil: Mineral chemistry and comparisonwith titanian clinohumite from other environments Josf C. Glspan Departamento de Mineralogia e Petrologia, Instituto de Geoci€ncias,Universidade de Brasilia, 70910 Brasilia DF, Brazil Ansrucr Titanian clinohumite (TiCl) occurs in the carbonatites of the Jacupiranga Complex, Brazil, either as the result of alteration of olivine (Mitchell, 1978) or in the reaction rock developed in the contact ofthe carbonatitesand the host magnetite pyroxenite, where it may not be related to olivine. Lamellae of titanian chondrodite (TiCh) are found, and the presenceof lamellae of polysomesrepresenting n > 4 is inferred. The TiCl samplescontain TiO, up to 3.78 wto/0,FeO up to 3.09 wto/0,and F up to 2.34 wto/o.Fe and Ti show a positive correlation. Complex zoning is always present. The only consistenttrend is that of increasingTiO, toward the margins, but even this trend is only observedin the crystals of the reaction rock. The partitioning of Fe and Mg between TiCl and olivine indicates enrichment of Mg in the TiCl and is different for samples from the three sovites. The characteristicsof the TiCl are thought to derive from a changing metasomatic environ- ment, and all TiCl so far reported from carbonatite complexesis possibly of metasomatic origin. The JacupirangaTiCl is similar in composition to TiCl from marbles, an obser- vation that cannot be extendedto TiCl from carbonatitesin general.Samples of TiCl from kimberlites have compositions similar to those from some Alpine peridotites. Fluorine TiCl is certainly stable in the upper mantle, but becauseof its rarity it is considered a mineral of no importance to mantle geology. INrnonucrrou and the host magnetite pyroxenite (Gaspar and Wyllie, presented Titanian clinohumite (TiCl) is not an abundant min- 198.3).Microprobe analysesare in this contri- petrological implications eral in the Earth's crust but is the most abundant species bution, and some structural and of the humite group. Specialattention was paid to it'*tren are discussed'Analytical results are compared with those geological in 1976 Aoki et al. suggestedthat it coulcl be a storage from other environments, and some consid- location of Hro in the mantle. However, tatei studils erations^regardingTicl occrurencein the upper mantle presented' generatedcontrary evidence(Smith, 1977;Mitchell, 1978; are briefly Engiand Lindsley, 1980;Trommsdorffand Evans,1980). Dymek et al. (1988) returned to the problem, sr ggesting that titanian chondrodite (Tich) and not Ticl "ought to Tnn JAcUpIRANGA cOprpr,ex be evaluated further as a possible upper mantle mineral, Melcher (1966) presentedthe geologyof the Jacupiran- as it is the expectedhigh-pressure phase." ga Complex including the carbonatites.Gaspar and Wyl- The crystal structureof the TiCl has been studied by lie(1983)summarizedtheavailableknowledgeaboutthe Robinson et al. (1973), Kocman and Rucklidge (1973), complex and presentednew data on the carbonatites.Five Fujino and Tak6uchi (1978), Ribbe (1979), and Abbott different carbonatite intrusions were recognized (Cl to et al. (1989), and TEM studies of this mineral were car- C5, from the oldest to the youngest).Cl, C3, and C4 are ried out by Muller and Wenk (1978) and White and Hyde sovites (calcite rock), C2 is a dolomitic sovite, and C5 is (1982). Experimental work on the stability of the hy- a rauhaugite (dolomite rock). Based on mineral chemis- droxyl-TiCl has been performed by Merrill et al. (1972), try, a sequenceof magmatic evolution from C2 to C5 was Yamamoto and Akimoto (1977), and Engi and Lindsley established,but Cl representsa chemically separatein- (1980). Reviews of the humite group minerals can be trusion (Gaspar and Wyllie, 1983, 1987). Roden et al. found in Ribbe (1980)and Deer et al. (1982). (1985) determinedthe Sr and Nd isotopic composition Clinohumite was reported in the Jacupirangacarbon- of the Jacupirangacarbonatites and found that Cl to C3 atites by Melcher (1966), and one analysis of it was pre- are isotopically different from C4 and C5, and magma sentedby Mitchell (1978). TiCl occurs in the Jacupiranga contamination is evoked. Gaspar (1989) presented a carbonatites as a product of alteration of olivine or in a comprehensive study of the geology and mineralogy of rock that resulted from the reaction of the carbonatites the complex. 0003-oo4x/92l0102-0168$02.00 168 GASPAR: TITANIAN CLINOHUMITE t69 The reaction rock A reaction that developed between carbonatites and CLINOHUMITE pyroxenite a-cl the host magnetite formed a rock that is com- o-c3 posed of an alternation of carbonate and silicate-rich tr-cq bands, millimeters to centimeters in width. The whole X - REACTIONROCK CHONDRODITE banded rock may reach more than 2 m in width. The o -cl E na silicate bands are composedmainly of brown phlogopite, u "- 6 acicular alkali amphibole, and magrretitewith platy exso- = l lution lamella of ilmenite. Calcite and apatite are also z z Xx common. TiCl and serpentinepseudomorphs after oliv- o present. pyroxenite F ine may be Near the magnetite con- ooz tact, the grain size is submicroscopic, becoming larger oo 1*"" near the carbonatite bands. Locally, meters-widepockets F of pegmatitic recrystallization occur where phlogopite crystals may reach up to 40 cm in diameter. In these portions, many other mineral speciesmay be found, but augite, perovskite, and severalzeolites are the most com- mon. The carbonate bands are composed of calcite with accessorybrown phlogopite, apatite, and amphibole. Clinohumite 02 03 The JacupirangaTiCl is orange to red-brown in color and displays a light yellow to colorless pleochroism in Fe CATIONNUMBER thin sections.It has two different types ofoccurrence.The Fig. l. TiCl and TiCh analyses(Table l) showinga general first is as reaction rims or patches in olivine crystals in positivecorrelation of Ti and Fe. Diferent total cation numbers the three sovites of the complex where some olivine crys- usedfor normalizations(see discussion in the text) make com- parisons valuesmeaningless. tals may be completely replaced (Mitchell, 1978). Rare basedon absolute coarse-grainedsamples ofthe Cl sovite contain no relicts of olivine. It was in these crystals that rims of chondro- dite have been found during the analysis, and no optical TiCl is the variable metal to Si ratios (Mri/Si) as dis- discontinuity has been observed. In C4, the olivines are cussedbelow. The highest FeO content is 3.09 wto/0,and strongly serpentinizedand only small patchesof TiCl are the highest TiO, is 3.78 wto/ofor the TiCl and 4.43 wto/o found. The second type of occurrenceis in the reaction for the TiCh. The TilSi ratio in TiCh is higher than that rock. In this case, the crystals are well developed and of coexisting TiCl but not by a factor of 2 as would be locally very abundant, and they have diameters up to 2 expectedin coexisting TiCh and TiCl (Aoki et al., 1976; mm. Habits vary from anhedral to euhedralshort prisms, Ehlersand Hoikens, 1987;Dymek et al., 1988).This fac- and inclusions of magnetite, ilmenite, and calcite are tor of 2 results because chondrodite contains twice as common. In a few casesthe TiCl is poikilitic to these many Mg(OHF)O "layers" as does clinohumite. The minerals. Mg(OHF)O "layer" is the part of the structure that con- lains the M3 site where Ti replacesMg (Ribbe, 1979). h ANll.yrrc,ll, PRoCEDURE is apparent from Figure l, despite some scatteringof the EDS analyseswere carried out using an ARL/EMX au- data, that Ti and Fe show a positive correlation in the tomated microprobe. WDS analyseswere perfbrmed in a TiCl ofthe Jacupirangacarbonatites, a behavior already Camebax Microbeam automated microprobe. Standard observed by Evans and Trommsdortr(1983) in the TiCl analysisconditions were usedfor both methods.Identical from an Alpine recrystallized garnet peridotite. In detail, results were obtained by the two methods for major ele- however, TiCl from the reaction rock does not display ments (Si, Ti, Fe, and Mg), with some differencesfor Mn this positive correlation: increasingTi is accompaniedby and Ca. As would be expected,the EDS analysesfailed almost constant Fe. to detect Mn and Ca or gave inaccurate results for them The F contents (2.34-1.13 wto/o)are in reasonable becauseof their low concentrations (Table l). Data col- agreement with observations by I-apin (1982) that cli- lection was done on crystal cores and rims. Four to six nohumite from carbonatite environments reach 1.5-2.0 analyseswere carried out on each crystal except in the wto/0.The highest F contents of the TiCh (3.0 and 4.43 chemical profiles. Averages were made according to the wt%) do not represent higher X. contents (=0.35) than homogeneity of M,,/Si in each crystal (seebelow). those of the TiCl (0.3-0.6). X. does not have a well- defrned and constant negative correlation with Ti as would ANlr.yrrcll. RESULTS be expected from a substitution of the type MgF, : TiO, The microprobe results of the Jacupiranga TiCl are (Evans and Trommsdorff, 1983; Dymek et al., 1988). presentedin Table l. The most remarkable feature of the Mg(OHF), : TiO, is certainly the substitution mecha- 170 GASPAR: TITANIAN CLINOHUMITE Tnsle 1. Microprobe analytical results of clinohumiteand chondrodite samples c1 12345C 5R 6C, 6C" 6R 78C8R (4) (4) (4) (4) (s) (2) (21 (3) (2) (3) (2t 0) sio, 37.35 37.55 37.85 37.80 38.74 35.30 38.85 38.47 35.00 38.47 38.60 38.10 Tio, 3.4s 3.26 2.05 2.08 3.36 4.43 3.55 3.78 2.93 2.54
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