Mantle Petrology: Field Observations and High Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd © The Geochemical Society, Special Publication No.6, 1999 Editors: Yingwei Fei, Constance M. Bertka, and Bjorn O. Mysen
TIMOTHYL. GROVE,!STEPHENW. PARMAN!and JESSEC. DANN,2 'Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 USA 2Department of Geological Sciences, University of Cape Town, Rondebosch 7700 South Africa
Abstract-This paper evaluates the significance of major element compositional variations and miner- alogical evidence in the olivine spinifex komatiites of the 3.49 Ga Komati Formation of the Barberton Mountainland, South Africa. Constraints derived from bulk rock and mineral chemical composition are used to infer the conditions of komatiite crystallization near the Earth's surface and melt production in the mantle source. The emphasis is on the 3.49 Ga komatiites of the Barberton Mountainland, South Africa, but we compare these rocks with other younger (2.6-2.7 Ga) komatiites from Munro and Boston Townships, Canada. Because all Archean komatiites are metamorphosed, the influence of metamorphic processes on the composition of the igneous protolith must be critically evaluated. We document the effects of open system metamorphic serpentinization. We also show how we identify the composition that is closest to the igneous protolith. We also apply the results of phase equilibrium experiments to interpret the compositional information preserved in igneous minerals in komatiites. New experimental data on a primitive Barberton komatiite composition containing 6 wt. % H20 is used to infer the temperature _ depth regime of melt generation. The liquidus phase relations were determined in AuPd alloy capsules from 2 to 2.5 GPa in a piston cylinder device. The komatiite composition is multiply saturated on its liquidus with olivine and orthopyroxene at 2.2 GPa and 1430°C. These average melting conditions provide a basis for evaluating mantle melting models for hydrous komatiite magmas. The favored regime
for melt generation is an Archean subduction zone. In this model H20 is supplied by dewatering of the slab. The H20 rises into overlying hotter mantle and melting begins at near H20-saturated conditions. The melt separates and ascends into the hotter, shallower portion of the wedge. The melt reacts with this
mantle, producing additional melt and diluting its H20 content. Melting progresses until the magma reaches the hottest part of the wedge and the final melt resembles a batch melt. The unique compositional characteristics of the Barberton komatiites result from higher H20 or higher temperature, or a combi- nation of both, in the Archean arc environment. Based on the depth range of melting, the favored source mantle is one that was depleted by a prior melt extraction process.
INTRODUCTION The komatiites from the Barberton Mountainland, South Africa are an appropriate focus for a discussion KOMATIITESare high MgO (18-33 wt. %) igneous of the role of H 0 in komatiite generation, because rocks that are found predominantly in Archean and 2 the Barberton komatiites have the highest estimated early Proterozoic greenstone belts ARNDTand NISBET MgO contents of all komatiites. They represent the (1982). Komatiites are one candidate for the melt that highest extents of mantle melting and indicate the was removed from Archean mantle to produce the highest Archean mantle temperatures. The Barberton depleted harzburgite residues that constitute stable komatiites are also the Earth's oldest, best-preserved buoyant tectosphere BOYD (1989). These igneous and best exposed komatiites (3.49 Ga, LOPEZ- rocks are also important, because the high anhydrous MARTINEZet al., 1992). BOYD (1989) proposed that liquidus temperature inferred for some komatiites komatiites and depleted Kaapvaal lithosphere might (1670°C, GREENet al., 1975) provide the only direct be genetically related. The ages of the melt depletion evidence for high Archean mantle temperatures. event recorded in the Kaapvaal harzburgites are com- These high-MgO melts of the mantle are most abun- parable to the age of the Barberton komatiites (PEAR- dant in Archean and early Proterozoic greenstone SONet al., 1995), strengthening the proposal of BoYD belts and rare in the Phanerozoic record. Thus, they (1989). represent a set of magma generation conditions in the early Earth that may no longer be operative, or that may operate only in rare circumstances. The assumed ESTIMATION OF PRE-ERUPTIVE MAGMATIC H 0 CONTENT high liquidus temperatures of komatiites were ques- 2 tioned by ALLEGRE(1982), who proposed that the The tools of petrology provide both direct and presence of dissolved magmatic H20 might lower the indirect methods for estimating crystallization condi- temperature required to produce komatiites. Petro- tions. In the case of komatiites, the early estimates of logic evidence provides support for Allegre's hypoth- high liquidus temperatures (GREENet al., 1975) de- esis (PARMANet al., 1997; STONEet al., 1987). pend on several assumptions: 1) the composition of 155 156 T. L. Grove et al. the komatiite sample represents the composition of a these metamorphic rocks must be critically evaluated liquid, 2) the liquid was anhydrous and 3) the liqui- in interpreting the water contents of the glasses. dus mineral records crystallization conditions. The original estimates of high liquidus temperatures were INFLUENCE OF H20 ON PHASE APPEARANCE based assuming that 1) and 2) were true. Assumption AND LIQUIDUS BOUNDARIES 3) could not be fully evaluated, because experimental The experimental study of PARMANet al. (1997) data on solid -melt exchange for olivine in komatiite provides phase relations for a Barberton komatiite melt was not available at the time of the GREENet al. composition at 0.1 MPa, anhydrous and 100 and 200 (1975) work. MPa, H 0-saturated. The phase relations are summa- The solubility of H 0 in silicate melt is extremely 2 2 rized in Fig. 1. H 0 lowers the phase appearance pressure sensitive (KUSHIRO and YODER, 1969; 2 temperature for the crystallizing minerals to various HODGES,1974). At 2 GPa pure forsterite liquid is degrees. The effect of H20 on the liquidus tempera- vapor-saturated when 20 wt. % H20 is dissolved in ture (Fig. 1) is inferred from anhydrous and H20- % the liquid, but H20 solubility drops to 10 wt. at 1 saturated experiments on primitive, sub-alkaline ba- GPa, and 0.1 wt. % at 0.1 MPa. Liquids become saltic lavas (HAMILTONet al., 1964; SISSON and vapor-saturated as they ascend to crustal pressures GROVE,1993; GAETANIet al., 1994) to be ~80 DC at and lose their original high pressure magmatic H20 200 MPa. The phase appearance temperature of as the solubility of H20 drops to nil at surface pres- spinel was not directly determined. Experiments on sures. The pressure dependence of vapor saturation primitive, sub-alkaline basalts (SISSONand GROVE, can eradicate the pre-eruption magmatic H 0 content 2 1993) show little effect of H20 on spinel appearance and result in the preservation of H20 contents that are temperature, and this is inferred to be the case for the lower than the primary value. Olivine is the liquidus komatiite composition. The effect of H20 on the phase of komatiite. Dissolved H20 in silicate melt phase appearance temperature of low-Ca pyroxene is lowers the olivine liquidus temperature dramatically, dramatic. At 0.1 MPa, a low-Ca pyroxene, pigeonite, but H20 does not influence Fe-Mg exchange (SISSON and GROVE,1993; PARMANet al., 1997). Therefore, the composition of the liquidus olivine provides no 1500~------~------.-. direct information on pre-eruptive H20 content. Crystallization paths and phase appearance se- quences are modified by increases in magmatic H20, and subliquidus minerals crystallized after hypabys- 1400 ...... sal emplacement can provide direct evidence of mag- matic H 0. The pyroxenes preserved in Barberton 01 in - -- 2 komatiites (PARMANet al., 1997) and the amphibole - -- o chr in found in the Boston Township (STONEet al., 1987) o 1300 and Munro Township (ARNDTet al., 1977) provide Q).._ :::l direct evidence of dissolved magmatic H20 at sub- +oJ liquidus conditions. ~ • Q) in Silicate melt inclusions trapped within minerals 0- during crystallization can also provide direct esti- E • Q) mates of pre-eruptive volatile contents. In modern I- • subduction-related basalts the H20 contents of melt pig inclusions in olivine and pyroxene do record pre- 1100 • eruptive magmatic H20 contents (SISSONand LAYNE, r-~ ~o~p~xin • 1993) that are in agreement with petrologic estimates. .anhydrous Melt inclusions are abundant in olivine in Barberton II.H20 saturated I ~ , komatiites and more rarely preserved in spinel. How- 1000 100 200 ever, they are invariably serpentinized and consist of 0 the same metamorphic mineral assemblage present in Pressure MPa the metamorphosed glassy meso stasis outside of the olivine crystal. Therefore, these meta-melt inclusions FIG. 1. Phase appearance temperatures at 0.1 MPa (anhy- drous) and 100 and 200 MPa (H 0 saturated). Solid circles only record the bulk composition of the metamorphic 2 are 0.1 MPa experiments. Solid diamonds are H20- mineral assemblage. Even if melt inclusions were saturated experiments. Note the relatively large effect of preserved, the post crystallization mobility of H20 in H20 on 10w-Ca pyroxene stability. Generation of Archean komatiites 157
Cpx phase field expanded. Another way to think about changes in the behavior of Si02 in the melt is to calculate silica activity for the coexistence of pyrox- ene and olivine. For the reaction, Phase Boundaries 0.1 MPa 200 MPa Starting Compositions the effect of H20 on silica activity is shown in Fig. 3. • this study GAETANIand GROVE(1998) calculated the influence * Green et al. 1975 of H20 on silica activity in liquids saturated with olivine + orthopyroxene + hi-Ca pyroxene + spinel and found that the influence of H20 is significant and causes silica to behave more like an ideal component FIG. 2. Compositions of experimental melts with inferred in the melt. In Fig. 3 the calculated L\G from multiple saturation boundaries at 0.1 MPa (anhydrous) and rxn GROVEand JUSTER(1989, Table, 4, reaction Ib) was 200 MPa (H20 saturated). The starting compositions used in this study is shown as a solid black circle. The GREENet al. used to estimate the value of the equilibrium constant (1975) composition is shown as the star. The projection (K) for reaction , where scheme is described in GROVE(1993). Arrows show olivine fractionation paths.
Activities were calculated for the olivine and hi-Ca appears at 1285°C, while at 200 MPa, a low-Ca pyx (ANDERSENet al., 1993) that coexist with liquid pyroxene (now orthopyroxene) appears at ~ 1100°C. in the experiments of PARMANet al. (1997) and used to calculate values for a~:b2' The activities of both The effect of H20 content on the appearance temper- ature of hi-Ca clinopyroxene (hi-Ca pyx) is small solids drop by similar amounts. At the same time the (Fig. 1). Variations in magmatic H 0 affect the phase 2 0.8 boundaries and primary phase regions for the Bar- berton komatiite (Fig. 2). Though simplified into a 100 MPa mineral component representation (GRovE, 1993), 0.75 ~ 06 o 6.. this projection scheme illustrates most of the compo- ~ .. sitional variations for low-Al komatiites, like the > 0.7 . is Barberton rocks. This phase diagram provides an co explanation for the changes in low-Ca and hi-Ca • 0.65 l- 0 • pyroxene phase appearance (Fig. 1) that occur in o~ response to the increase of H20 in the melt with 200 MPa increasing pressure. The major effect of H20 in the 0.75 melt is to expand the olivine liquidus field and to r. displace the primary liquidus field of low-Ca pyrox- ~ 0.7 • ._ ene to higher silica contents. The increased H20 has 0 little effect on the hi-Ca pyx primary phase field or on ~0.65 the position of the olivine + hi-Ca pyx saturation :g boundary. co 0.6 KUSHIRO (1969, studied the system forsterite- diopside-silica with and without H20 and showed that H20 expands the liquidus field of forsterite at the 0.55 expense of the enstatite liquidus field. Therefore, 1000 1300 increased magmatic H20 has a similar influence on the olivine and pyroxene primary phase regions in both the simple system forsterite-diopside-silica and FIG. 3. Calculated activities of minerals and melts from experiments of PARMANet al. (1997). Circles are 0.1 MPa, the natural komatiite system. KUSHIRO(1973) sug- triangles show 100 MPa and diamonds are 200 MPa exper- gested that the olivine primary phase field expanded iments. Upper panel shows variation of activity of olivine because the addition of H20 broke up polymerized Si (open symbols) and hi-Ca pyroxene (solid symbols) calcu- - 0 chains in the melt, thereby favoring crystalliza- lated using ANDERSENet al. (1993). Lower panel shows tion of silicate minerals in which there was less variation in value of the equilibrium constant for reaction  (solid line) and calculated liquid silica activity (a~:b2) linkage of Si04 tetrahedra. Thus, the olivine primary for each experiment. 158 T. L. Grove et al. value of K for reaction  increases with decreasing CPX temperature. Since the calculated activities for the solids decrease in similar proportion and K increases, a~~)2 must increase. The expansion of the olivine stability field is caused by the temperature-lowering effect of H20. The olivine + pyroxene + melt as- semblage is stabilized to lower temperatures, and a~b2 is responding to the temperature dependence of the equilibrium constant. A similar effect was noted by GAETANIand GROVE(1998, see their Fig. 5) on the equilibrium constant governing forsterite-liquid equilibria. These changes in liquidus phase relations exercise important controls on the mineral assemblage that develops in a crystallizing komatiite and on the se- to chlorite quence of phase appearance and liquid line of descent followed by komatiite magmas with varying H20 -. 0.5 ,,;-'r--r-:--:-;-:-::--- content. If a komatiite magma crystallizes at elevated <5 0.4 • chill margins pressures, the mineral assemblage can provide a serpentine + means of estimating the pre-eruptive magmatic H20 + c::::l chlorite content. ~ 0.3 • () "'.• ~0.2 •• INFLUENCE OF BULK (STARTING) • COMPOSITION ON LIQUID LINE OF DESCENT o, 0.1 • Bulk composition of the initial liquid is another o o '--to olivine key variable in controlling the crystallization se- ~ 6 8 10 12 14 quence followed by a magma. In the case of the Wt% H20 Barberton komatiites there is a large variation in
major element composition of analyzed rocks (Fig. FIG. 4. Upper diagram shows compositions of Barberton 4). The rocks span a range from Iow-SrOj, 10w-CaO, komatiite chill margins and aphyric zones projected onto the high MgO (33 wt. %, recalculated on an anhydrous pseudotemary Oliv-Cpx-Qtz. Data are from SMITH and ER- basis) to high-Si02' high-CaO, 10w-MgO (24 wt. % LANK(1980) and SMITH et al. (1982). Also indicated are the compositions of the dominant metamorphic minerals in the recalculated on an anhydrous basis). The data set rocks, serpentine and chlorite. The line indicates the data (SMITH et al., 1980; SMITHand ERLANK,1982) in- trend expected if the primary compositional control was cludes only aphyric chill margins. These are the best olivine fractionation with the most olivine normative rocks candidates for liquid compositions. Early discussions being the primitive melts. Lower diagrams shows the com- on the influence of metamorphism on komatiite bulk positions of the chill margins projected onto the Ol-Cpx join and plotted against their H 0 content (loss on ignition). The composition (GREEN et al., 1975; BESWICK,1982; 2 H20 is hosted in the metamorphic mineral assemblage and SMITHand ERLANK,1982) concluded that there was therefore represents the extent of alteration. only a minor influence. In contrast, DE WIT et al. (1987), NISBET et al. (1993), and PARMANet al. the metamorphic rock (Fig. 4). The observed com- (1997) show that the H20 content of the metamor- phic rock, major element composition and metamor- positional variation records an open system reaction phic mineral assemblages are correlated. These au- whereby primary igneous hi-Ca pyroxene converts to thors argue that open-system metamorphic exchange metamorphic tremolite. Continued fluid influx and between the rock and a metamorphic fluid is respon- metamorphic reaction alters the tremolite to serpen- sible for the variation in major elements. Figure 4 tine and chlorite, resulting in an increase in MgO and illustrates the control of open-system metamorphism H20 and a decrease in CaO and Si02 of the meta- on major elements. In the olivine-clinopyroxene- morphic rock. Note that an igneous trend would be quartz pseudoternary, the compositions of the chill expected to be collinear with the liquidus mineral, margins vary along a linear trend (Fig. 4). This trend olivine, if the compositional variation was generated is collinear with the projected compositions of meta- by pre-emplacement fractionation. The observed morphic serpentine and chlorite minerals found in the trend is consistent with a metamorphic process that rocks. There is also a correlation with H20 content of depleted soluble elements (i.e. Na, Ca, K) from the Generation of Archean komatiites 159
rocks, and left behind an MgO-enriched metamor- Under equilibrium conditions the residual liquid will phic assemblage of serpentine and chlorite. remain at this boundary until all liquid is exhausted. Having identified the igneous protolith for the Bar- Crystallization paths under anhydrous conditions
berton komatiites, the effects of variable H20 on the and H20-saturated at 200 MPa follow two contrast- liquid line of descent can be evaluated. The least ing phase appearance sequences as a result of the altered sample lies at the high-CaO, high-SrO, end of expansion of the olivine and contraction of the the komatiite compositional array. This composition low-Ca pyroxene primary phase regions. Under an- was chosen for experimental study by PARMANet al. hydrous conditions, low-Ca pyroxene (pigeonite) ap- (1997) and used to generate the phase diagram (Fig. pears as the first pyroxene- before hi-Ca pyx. Under 2). The composition used by GREENet al. (1975) lies H20-saturated conditions hi-Ca pyx appears first, and
at the extreme high-MgO, low-Si02, high-Hjt) end crystallizes for a significant temperature interval of the compositional array of Barberton komatiite (~lOO°C). If one considers the crystallization path chill margins (Fig. 4) and corresponds to the most followed by a low-SrOj, high-MgO composition altered end of the metamorphic array. (e.g., the GREENet al.  composition) a distinc-
Variations in H20 content influence the liquid line tively different crystallization sequence would be in- of descent of komatiite magma (Fig. 2). This discus- ferred. Under anhydrous 0.1 MPa conditions, pigeon- sion neglects the influence of Cr-spinel, which ap- ite would again be the first low-Ca pyroxene to pears after olivine and has little influence on the path crystallize, but it would crystallize in reaction rela- followed by residual liquids in the Oliv-Cpx-Qtz tion along the olivine - pigeonite reaction boundary. pseudoternary projection scheme. Under 0.1 MPa Under conditions of fractional crystallization pigeon- anhydrous conditions, the high-CaO, high-SrO, com- ite crystallizes and the liquid enters the pigeonite position (BK in Fig. 2) has olivine as its liquidus primary phase region. Crystallization continues over a significant temperature interval (> lOODC) before phase. Olivine crystallization drives the residualliq- hi-Ca pyroxene joins the crystallizing assemblage. In uid toward the olivine - pigeonite reaction boundary the case of 200 MPa H 0-saturated conditions, oli- where olivine + liquid react to form pigeonite. The 2 vine would crystallize over a larger temperature in- liquid moves a short distance along this reaction terval, and augite + orthopyroxene would appear boundary to the reaction boundary: simultaneously at the 200 MPa reaction boundary olivine + liquid = pigeonite + hi-Ca pyroxene. (A) (B). Therefore, the pyroxene assemblages in a ko- matiite provide information to infer variations in The liquid composition will remain at this reaction crystallization conditions (anhydrous vs hydrous). If boundary (under equilibrium conditions) until allliq- there is a variation in parental magma compositions, uid is exhausted and the solid assemblage consists of the mineral assemblages in each contrasting compo- olivine + pigeonite + hi-Ca pyroxene. The 0.1 MPa sition should be consistent with the composition of experiments reported by PARMANet al. (1997) exhibit the rock relative to the phase boundaries. In the case this behavior. Under conditions of fractional crystal- of Barberton komatiites, high-MgO anhydrous ko- lization, olivine would be removed from reaction matiites should contain abundant pigeonite. with liquid, either by crystal settling in the lava flow Only a hi-Ca pyx is present in the olivine komati- or by reaction overgrowth of low-Ca pyroxene ites from the lower part of the Komati Formation. around olivine. The liquid would leave the reaction These units are inferred to form by crystallization of boundary and evolve along the pigeonite + augite near H20-saturated magmas with ~6 wt. % H20 and saturation boundary (dark line in Fig. 2 that extends are interpreted as sills emplaced at ~6 km depth (see toward the Qtz apex). At 200 MPa, H20-saturated, next section). PARMANet al. (1997) also find evidence olivine is again the liquidus phase. Olivine crystalli- for variable crystallization conditions of a parent zation follows the same path, but because the low-Ca magma similar to the high-CaO, high-Sifr, compo- pyx boundary has shifted to higher Si02 values, the sition (BK). Komatiites in the upper Komati Forma- residual liquid reaches the olivine + hi-Ca pyroxene tion contain pigeonite cores overgrown by augite saturation boundary (grey line in Fig. 2 that parallels rims (GROVEet al., 1997, Fig. 18.104.22.168 and PARMANet the pigeonite + augite boundary). Along this bound- al., 1997, Fig. 6). This assemblage indicates that ary olivine + hi-Ca pyx crystallize and the liquid these komatiite magmas contained less H20 than the becomes Si02 enriched. After ~60 % crystallization melt that produced the olivine spinifex komatiites in the residual liquid reaches the reaction boundary: the lower Komati Formation. If an initially hydrous komatiite magma was emplaced at shallow levels, the olivine + liquid = orthopyroxene + hi-Ca pyroxene. (B) sharp decrease in H20 solubility with depth would 160 T. L. Grove et al.
lead to vapor saturation, degassing of H20 and an- of clinopyroxene crystallized from hydrous melts hydrous crystallization that produced early pigeonite. (SISSONand GROVE,1993; GAETANIet al., 1994). The lowered crystallization temperature of the hi-Ca py-
Influence of H20 on mineral compositions roxene causes higher Wo contents, because the two- phase region (augite - orthopyroxene) expands as H 0 also influences the composition of the pyrox- 2 temperature drops. The magnitude of the thermody- ene that crystallizes. The compositional effects are namic non-ideality in the hi-Ca pyroxene is temper- primarily related to the lower temperature at which ature dependent. At lower temperatures the non-ideal pyroxene and melt coexist. Pyroxene Wo (Wo = term is relatively larger (LINDSLEYet al., 1981). CaSi03) contents in the 200 MPa, H20 saturated experiments are distinctly higher than the pyroxene Therefore, phase compositions are closer to end Wo contents in the 0.1 MPa experiments (Fig. 5). member compositions. Even when the augite is not High Wo contents are recognized as a characteristic saturated with a low-Ca pyroxene, the non-ideality is ... 0.5 C 0.4 ...Q) o C o 0.3 0.1 MPa o I H 0 saturated o 0.2 I~ e~periments S I ~ dry experiments . 01 MPa~ 0.1 . V' 'll • chill margins J interiors L 200MPa~ o o ~ 0.14 o Q) o 0> augites • ~o 0.10 o • • crystal .~ ~ fractionation en s.... 0.06 Q) pnetiCS a.
« 0.02 0.9 0.8 0.7 0.6 Mg#
FIG. 5. Upper figure shows wollastonite (Wo) content versus Mg# in hi-Ca pyx in chill margins (solid diamonds) and interiors of units (open grey circles). Lines-show compositions of pyroxenes produced in
0.1 MPa, anhydrous experiments (light grey) and 200 MPa, H20 saturated experiments (dark grey) from PARMANet al. (1997). Error bars show range of pyroxene compositions. The highest Mg# points are the cores of crystals. These are first pyroxene crystals to form and they are interpreted to record the emplacement conditions. Note the good correspondence between the cores of augite from the interior of
the units and the 200 MPa H20-saturated experimental data. Lower figure shows Al per six oxygens versus Mg# for natural and experimental augite. Both sets of experimental data are similar in Al content, with the 0.1 MPa data falling on the low side of the range. Augite cores from chill margins fall off the experimental trend because of their delayed nucleation and the effect of cooling rate on partitioning. Generation of Archean komatiites 161
still present, causing higher Wo contents. The effect result of crystal/melt disequilibrium. Increased cool- of increasing H20 in the melt stabilizes melt, pyrox- ing fate increases crystal growth rates and the lower ene and olivine to lower temperature. The activities temperature results in a decrease in melt diffusion of the (Fe,Mg)Si03 components in the hi-Ca pyx rates. Diffusion in the melt is not rapid enough to decrease with temperature (Fig. 3), which leads to an remove elements from or supply them to the advanc- increase in the CaSi03 component in the hi-Ca py- ing crystal face. Crystals are forced to grow from a roxene that coexists with olivine and melt. melt with higher concentrations of incompatible ele- In comparison, the pyroxene compositions pro- ments and lower concentrations of compatible ele- duced in the 0.1 MPa anhydrous experiments record ments. Therefore, as cooling rates increase, partition higher temperatures (~25 DC) for the coexistence of coefficients converge on a value of unity. So, the the first pigeonite and augite pair to crystallize. The composition of the pyroxene becomes increasingly Ca contents of the augite are lower, because the similar to the melt. Augite is closer in composition to pigeonite-augite miscibility gap is narrow at these the melt than pigeonite and, therefore, is relatively higher temperatures compared to the orthopyroxene- more stable in the disequilibrium system. Therefore augite two-phase region. Pressure has a negligible the increased disequilibrium at fast cooling rates fa- effect on the phase relations (LINDSLEY and vors the nucleation of augite over pigeonite. ANDERSEN,1983). The main effect of increased pres- The chill margins of Barberton olivine spinifex sure alone is to lower the Wo content of the augites komatiite units contain fresh hi-Ca pyroxene (Fig. 5). by raising the temperature at which they crystallize The pyroxene compositions reflect the rapid cooling under anhydrous conditions. rates at the chill margin. The composition of the first pyroxene to crystallize has lower Wo content and INFLUENCE OF COOLING RATE- Mg# and higher Al contents than the first pyroxene CRYSTALLIZATION KINETICS ON MINERAL that crystallized in the slowly cooled interiors of the COMPOSITIONS units or in the 200 MPa isothermal experiments. The major and minor element compositions of the py_ In the Komati Formation, the cooling units that roxenes from the slowly cooled interiors do not show have been studied are on the order of 1 to 55 meters the effects of rapid cooling rate. The compositions of thick. The results of isothermal crystallization exper- the cores of the pyroxenes from the slowly cooled iments discussed above are relevant to the slowly interiors are nearly identical to the pyroxenes pro- cooled interiors of the thicker units. It is important to duced in the isothermal experiments of PARMANet al. determine the range of cooling rates where kinetic (1997). Therefore, the effect of cooling rate is negli- effects may influence the appearance sequence and gible in the interiors of the units, and we can use the composition of crystallizing phases. PARMANet al. compositions of the pyroxenes in the isothermal ex- (1997) performed cooling rate experiments on the periments to infer emplacement conditions. BK composition at 3 and 10DClhour. These cooling rates would correspond to distances of 0.5 and 0.2 m, INTERPRETING THE MINERALOGICAL respectively, from a boundary of conductive heatloss EVIDENCE IN OTHER KOMATIITES in a cooling Barberton komatiite sill. In the cooling rate experiments, textures indicate that olivine is the ARNDTet al. (1998) equate the compositions of liquidus phase, followed by spinel and then followed pyroxenes produced in rapidly chilled margins in the by pigeonite and augite. The effect of cooling rate is komatiites of the Munro Township to those produced to suppress the appearance temperature of later in the isothermal experiments of PARMANet al. phases, specifically pigeonite and augite (PARMANet (1997). The hi-Ca augites with high Al203 contents al., 1997). in the chill margins of Munro Township komatiites Textural evidence in the experiments indicates that are not compositionally similar to the PARMANet al. faster cooling rates suppress pigeonite crystallization (1997) isothermally produced Iow-Al augites. A more than augite crystallization. In the lODClhr ex- quick comparison of minor element compositions periment the pigeonite cores are very small and con- shows that these Munro pyroxenes contain from 7.8 stitute <5 volume % of the crystal. At cooling rates to 8.5 wt. % A1203. In contrast, the pyroxenes in the faster than lOoClhr, pigeonite does not crystallize, a Barberton komatiites and the PARMANet al. (1997) result in accordance with the work of KINZLERand . experiments contain 1 to 1.5 wt. % A1203 KINZLER GROVE(1985). KINZLERand GROVE(1985) also found and GROVE(1985) produced high-Al pyroxenes sim- that increased cooling rate suppresses the nucleation ilar to those found in the Munro samples in rapid of pigeonite relative to augite. This relative suppres- cooling rate experiments and discuss the near surface sion of pigeonite in the cooling rate experiments is a origin for the Munro Township pyroxenes. 162 T. L. Grove et al.
Other Archean komatiite localities have features dence indicates production of melts of widely vary- that point to variations in the H20 content of the ing H20 content closely related in space and time crystallizing komatiite magma. In the Munro Town- (BAKERet al., 1994). Such complexities could also ship locality (PYKEet al., 1973) olivine spinifex ko- exist in the komatiite melt generation regime. Careful matiites at Pyke's hill contain early crystallizing pi- field studies and detailed petrologic examination will geonite (ARNDTet al., 1977), permissive evidence for be required to infer the conditions recorded in these anhydrous crystallization at near surface conditions. important ancient igneous rocks. However, thick differentiated sills, also present in the Munro Township, contain igneous amphibole. Am- IMPLICATIONS OF ELEVATED PRE-ERUPTIVE phibole is an important indicator of the presence of H20 CONTENTS FOR ARCHEAN MANTLE CONDITIONS significant quantities of magmatic H20 (>2 wt. % H 0 at the minimum pressure of amphibole stabil- 2 Experiments ity). The thick, differentiated units (like Fred's Flow, ARNDTet al., 1977) and other late Archean komatiites Previous high pressure experimental studies on kornati- in the Abitibi belt (Boston Township, STONEet al., ites have investigated the conditions of anhydrous mantle melting that could lead to the production of komatiite mag- 1987) are likely to be intrusive. As a consequence of mas (GREEN et al., 1975; HERZBERG,1989). We used the their higher pressure of emplacement, they record a pre-eruptive H20 content estimate of PARMANet al. (1997) higher pre-eruptive H20 content. However, rather to investigate the influence of H20 on the high pressure than generalize the results from Barberton to all phase relations of a primitive 24 wt. % MgO Barberton komatiite. The liquidus phase relations are shown in Fig. 6, komatiites, a careful re-evaluation of the petrologic and experimental conditions are provided in Table 1. The evidence for the role of H20 in komatiitic magma- purpose of these experiments is to provide constraints on the tism is needed. conditions for melt generation in the mantle source of Barberton komatiites.
PRE-ERUPTIVE H20 CONTENTS IN MODERN Experimental methods. The experiments were performed BASALTIC MAGMAS in a piston cylinder apparatus (BOYD and ENGLAND,1960) over the pressure range of 2.0 to 2.5 GPa. The pressure Komatiites are found predominantly in Archean medium consisted of sintered BaC03, and pressure was and early Proterozoic greenstone belts. The Tertiary calibrated using the Ca-Tschermakite reaction (HAYS, 1966). Temperature was monitored and controlled using Gorgona Island komatiites (ECHEVERRIA,1980) are W97Re3-W75Re25 thermocouples, and no correction for the the closest Phanerozoic analogs to komatiites. The effect of pressure on thermocouple EMF was applied. The
pre-eruptive H20 contents for these rocks have not sample was positioned in the hot spot of the experimental been quantitatively investigated, but there is an inter- assembly. The temperature difference between the sample esting association of ultramafic tuff with the komati- ite units (ECHEVERRIA,1980). Modem primitive mag- mas with H20 contents similar to those inferred for 1480~----~---'~~----r------' •• ol-opx-pl Barberton komatiites (4-6 wt. % H20) are present in ...-. subduction zone environments. ANDERSON(1979) and U [;jjjI [;jjjI ol+gl 01460 I!I"'J opx-ql BAKERet al. (1994) report primitive basaltic andesite ,_Q) o gl (10.5 wt. % MgO) and andesite (9.1 wt. % MgO) :::l from the Southern USA Cascades that contain 4.5 to ~,_ 1440 6.5 wt. % H 0. Primitive basaltic rocks from the Q) 2 c. liq Oregon and Washington Cascades contain ~4 wt. % E 1420 oh-opx-Iiq Q) H20 (CONREYet al. 1997). MOOREand CARMICHAEL I- ol+Iiq (1998) use evidence from the compositions of phe- nocrysts preserved in Mexican basaltic andesites to 14001.8 2 2.2 2.4 2.6 (GPa) infer pre-eruptive H20 contents of ~6 wt. %. Thus, in modern environments, high-Hjt) primitive mag- FIG. 6. Melting experiments carried out on the least mas are found in subduction zone settings. By anal- altered Barberton komatiite composition of PARMANet al. ogy a possible Archean environment for the produc- (1997) containing 6 wt. % H20. Black squares show exper- tion of hydrous komatiites is in a subduction-related iments in unsaturated AusoPd2o capsules. Grey squares setting. In the final section we develop a model for show experiments in Fe-saturated capsules. The dashed line melt production in an Archean subduction setting. is the inferred melting point of the Fe-saturated AusoPd2o' Failed (capsule melted) experiments were used to define the Another factor must be considered in comparing capsule melting boundary, and these are omitted from the magmas from modem subduction zones with those diagram Other lines are phase stability fields inferred from found in Archean Greenstone Belts. Petrologic evi- experiments. Generation of Archean komatiites 163
Table 1. Conditions and results of elevated pressure experiments on Barberton komatiite composition BK.
Expt. Pressure T time Phases 1 K2 wt.%Fe %Fe D KD # (GPa) CCc) (hours) in capsule loss/gain ollliq opx/liq BK2.2 2.0 1400 7 q,ol 0.0 0.3 BK2.3 2.4 1450 9 q, 01, opx 0.0 na' BK2.4 2.4 1475 8 q,opx 0.0 -13 0.29 BK2.5 2.2 1440 6 g, q1 0.0 -8 BK2.6 2.25 1420 7.5 q, 01, opx 0.0 -5 0.27 0.25 BK2.7 2.3 1440 4.5 q, 01, opx 0.0 -8 0.29 0.26 BK2.9 2.2 1420 21 q, 01, opx 1.35 +10 0.32 0.29 BK2.12 2.2 1440 6 q,ol 0.94 +11 0.30 BK2.15 2.4 1450 8 q, 01, opx 0.67 +10 0.35 0.30
1 Abbreviations: q = quench crystals from liquid, gl = quenched glass, 01 = olivine, opx = orthopyroxene, 2 KD = [(XFe XlaJ*XMg Hq)/(XMg xtaJ*XFe -n 3 na = not analyzed.
and thermocouple was measured, and a 20°C correction are not desirable, the results of these experiments are judged was applied. to provide a reasonable approximation of the liquidus phase The starting composition is a synthetic analog of the least relations, for the following reasons. 1) We duplicated sev- altered Barberton komatiite chill margin used by PARMANet eral of the lower temperature experiments using Fe-doped al. (1997). The starting composition was fabricated using and undoped capsules to see whether there was a change in oxide starting materials following the technique described the phase relations (Table 1). The results were similar. 2) in GAETANIand GROVE(1998). In order to add H20 to the The Fe-Mg KD'S for olivine and orthopyroxene achieved starting material, a portion of the MgO was added as brucite the same value in doped and undoped capsules. Therefore, and the remainder was added as anhydrous MgO (BAKERet exchange equilibrium for these elements between melt and al., 1994). The resulting starting mix contained 6 wt. % solid was achieved in the experiments. For these reasons the H20. Approximately 10 to 15 mg of starting material was phase diagram presented in Fig. 6 is thought to be an loaded into AusoPd2o capsules that were welded shut. The accurate representation of a komatiite liquid containing 6 Au-Pd alloy capsule was placed into an MgO sleeve. This wt. % H20. assembly was placed into an outer Pt capsule, packed with
MgO at both ends and welded shut. The experimental con- Experimental results. With 6% H20 the Barberton com- ditions were very close to the melting point of the Au-Pd position is multiply saturated with olivine and orthopyrox- alloys. We initially used capsules that had been pre- ene on its liquidus at 2.2 GPa and 1430°C (Fig. 6). This saturated with Fe to minimize Fe exchange between the multiple saturation point is equivalent to a melt residue of sample and container (GAETANI and GROVE, 1998), but harzburgite. Thus, the melt segregated after high extents of some of these melted in our experiments. Figure 6 shows the melting. In contrast, HERZBERG(1992) concluded that Bar-
solidus of the AugoPd20 alloy as determined during the berton komatiites similar to the composition studied by course of the experiments. In order to determine the liquidus GREENet al. (1975) were derived from deep sources (260 to
of the Barberton komatiite, some unsaturated AugoPd20 cap- 330 km depth) from a residue consisting of olivine, garnet sules were used. and subcalcic augite at a temperature of 1800 to 2000 "C. Experimental products were analyzed using the JEOL 4- or 5- spectrometer electron microprobe at MIT. Liquids in these experiments did not quench to glass and consist of Models for komatiite melting micron-sized quench pyroxene crystals set in glass. This quench growth was analyzed using a broad electron micro- Early anhydrous models for komatiite generation probe beam (30 microns), and 20 to 30 replicate analyses of assumed that the experimentally determined multiple the glass phase were used to obtain an estimate of melt saturation point represented the depth and tempera- composition. Crystalline phases (olivine and orthopyrox- ture of melt segregation. Thus, melting was modeled ene) were analyzed using a 2 micron beam. The values of Fe-Mg exchange distribution coefficients (K~e-Mg) = [(x~~al as a batch process. A near fractional adiabatic de- * X~'k)/(x~~ * X~i)l for olivine-melt and orthopyroxene- compression melting model was described by HERZ- melt are provided in Table I and are comparable to the BERG (1995). His proposed adiabatic melting path values obtained in other elevated pressure experimental uses an estimate of the maximum depth of melting studies of basaltic compositions (e.g., KINZLER,1997). The from petrologic criteria (CaO/AI 0 of the melt) and calculated gain or loss of Fe from the silicate charge to the 2 3 the melting path starts at >300 depth. Estimates AugOPd20 capsule is also shown. All of the experiments km either lost or gained from 5 to 10 wt. % FeO. Although such of the amount of melt produced during decompres- changes in composition of the silicate portion of the charge sion melting [(dF/dP)s] range from about 1 to 3 wt. % 164 T. L. Grove et al. per 0.1 GPa. (AHERNand TURCOTTE,1979; MCKEN- reaches mantle that is at the temperature and pressure ZIE, 1984; KINZLERand GROVE,1992). If these melt- conditions of the vapor-saturated solidus, an H20- ing rates are applied to the HERZBERG(1995) dry rich melt is produced and melting begins. Figure 7 melting path, only a part of the adiabatic ascent path shows a mantle solidus calculated using the approach is necessary to generate a sufficient melt fraction to of KINZLERand GROVE(1992) and including the hy- produce a komatiite magma as a partial melt from a drous mantle melting experiments of GAETANIand primitive mantle (~50% melting). HERZBERG(1995) GROVE(1998). Also shown is a schematic cross sec- does not discuss what conditions or mechanisms tion of the mantle wedge with isotherms and two would lead to a cessation of melting. A drawback of possible ascent paths for the fluid. If a flux melting this melting model is that melting begins in a depth process is operative, it can occur when partial melting region where the ultramafic partial melt would be more dense than its surrounding solid mantle residue (AGEE and WALKER, 1993), and melt would sink rather than ascend. For hydrous decompression melting, a near-frac- tional melt generation process is not feasible. Near- fractional melting models assume that once a melt 6% batch is produced, it is removed and no longer inter- o 1300 ~\ H20 acts with the mantle. All the H20 would be removed ~s I \ ~ 1200 with the first melt fraction. Subsequent fractional \ ~~~~~~tiO \ Zone melts would have to be generated under anhydrous 1100 ~agmas conditions, which would require a large increase in
1000 1'-, temperature. Such a process does not seem relevant Vapor-sat...... to the generation of hydrous komatiite magmas. Mag- solidus ..... matic H20 could be retained in a batch melting 10 15 ~ ~ ~ ~ ® process. In the case of hydrous batch melting, the Pin kbar crystalline residue and melt ascend together and con- stantly re-equilibrate as melting proceeds. Once the melt is removed, it would retain the signature of the last temperature and pressure of equilibration. An- other process that could resemble batch melting is flux melting. In this process the H20-bearing melt separates from its residue and ascends through the mantle by porous flow. The melt continuously reacts and requilibrates with shallower mantle as it ascends and is subjected to changing pressure and tempera- ture conditions.
If a komatiite magma containing 6% H20 is gen- erated by high extents of partial melting of a fertile peridotite source (~50% melting) the source must contain 3 wt. % H20. There are no hydrous mantle minerals that would be present in sufficient modal abundance to provide the required H20. The only other source of H20 is as free water released by the breakdown of hydrous minerals in the subducting lithosphere. SCHMIDTand POLl (1998) show that de- hydration of the downgoing hydrated mantle and slab FIG. 7. Subduction zone model for Barberton komatiite production. Top figure is a P-T diagram showing the solidus occurs over a broad range of depths, beginning at 70 temperature for dry (solid black line) and H20 saturated km with amphibole breakdown. Dehydration of ser- (dashed grey line) peridotite. Arrows connect experimen- pentine and chlorite brings mineralogically bound tally determined olivine + orthopyroxene saturation points (stars) for Barberton komatiites and modem high-Mg basal- H20 to depths of 150 to 200 km, and other dehydra- tic andesites with their initial melting conditions at the tion reactions can extend the depth of H20 release to H20-saturated peridotite solidus. Lower figure is a cross- ~300 km. The H20 is released, ascends into the section of a subduction zone comparing melting path for overlying mantle wedge and encounters an inverted Barberton komatiites with modem arc volcanism. Arrows temperature gradient. When the H20-rich fluid and stars correspond to P-T conditions in upper figure. Generation of Archean komatiites 165
creates an interconnected melt network and an equi- series of flux melting events experienced by the man- librium melt fraction. When the critical melt fraction tle wedge. Earlier hydrous flux melting of the wedge is exceeded, this melt segregates from its residue and produces basaltic magmas and depletes the source in rises into shallower, hotter mantle. The melt reacts high-Ca clinopyroxene and/or garnet. H20 continues with this mantle, producing additional melt and di- to ascend through the depleted mantle, initiates hy- luting its H20 content. As the hydrous melt ascends, drous melting and the komatiite is the last magma it is continuously superheated. If the melt resides in produced and extracted from the depleted source the interconnected network of grain boundaries, it region. constitutes only a small fraction of the total mass. An important geologic constraint is that the koma- Therefore, the heat of fusion and specific heat can be tiites are a small fraction of the mafic- ultramafic supplied from the surrounding solids without lower- section at Barberton and constitute ~ 10% by volume. ing the temperature of the mantle. As melt rises and As intrusions the komatiites follow emplacement of a interacts with surrounding mantle, it follows the 5 to 6 km accumulation of basalts and komatiitic H20-undersaturated liquidus boundary in the natural basalts. The magmatic stratigraphy is consistent with peridotite-water system. Melting ceases when the the komatiites as the last product of hydrous flux normal temperature-depth gradient is encountered. melting. Melt production is controlled by the slope of this If the subduction zone hypothesis is the correct boundary and by the temperature-depth conditions interpretation of the komatiite phase relations, then it of ascent. The melting process is essentially a batch implies different conditions in subduction zone melt process, because the melt reacts with its surroundings generation during the Archean and Early Proterozoic. at each step of the process. When the final melt is The melts produced in modern subduction zones extracted and delivered to the surface, it would record (BAKERet al., 1994) have comparable H20 contents, the final pressure and temperature of equilibration but are derived from shallower depths (30 to 45 km) conditions. In the case of the Barberton komatiites at lower temperatures (1200-1300DC). The differ- this would be the multiple saturation point shown in ence may be related to the vigor of Archean subduc- Fig. 6. The pressure of multiple saturation (2.2 GPa) tion or to the higher temperatures in the Archean corresponds to a depth of ~65 km, which is well mantle wedge, or both. Back-arc magmatism may within a possible depth range for wedge melting in a have more efficiently depleted the mantle that was subduction zone. The temperature of multiple satu- fed into Archean subduction zones. Hydrothermal ration (1430 DC) is at the upper limit of the range of alteration systems at mid-ocean or back-arc spread- predicted mantle temperatures in thermal models of ing centers may have been more active and basalt and subduction zones (FURUKAWA,1993). mantle may have experienced more extensive hydra- The flux melting model for hydrous komatiite op- tion prior to subduction. These processes would lead erates over a depth interval of about 60 km in the to an increase in the amount of H20 that was sub- lower part of the wedge (Fig. 7). The combined ducted and the depth to which it was subducted. 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