f gm

8, Bokfu

Orthopyroxene

of gghthorne

ns is the most important workhe pasPT

rate ththanearly o

2. Valid geothermometers

It should be stated that different versions of two-pyroxene geother-

Lithos 142143 (2012) 288290

Contents lists available at SciVerse ScienceDirect

Lith

.e lduced, and thus selection of suitable versions of thermometers andbarometers is a priori quite critical. After an extensive comparativestudy of garnet-orthopyroxene geobarometry, Wu and Zhao (2011;referred to as WZ11 hereafter) concluded that the Taylor (1998)garnet-orthopyroxene geobarometer and the Taylor (1998) two-pyroxene geothermometer are the most suitable tools for determin-ing PT conditions of lithospheric mantle. However, Nimis andGrtter (in press); referred to as NG12 hereafter) disagree withWZ11's conclusion. They focus on: (a) the Taylor (1998) two-pyroxene thermometer and the Nimis and Taylor (2000) single-clinopyroxene thermometer and (b) the Nickel and Green (1985)

Nimis and Grtter (2010) that the Taylor (1998) two-pyroxene ther-mometer and the Nimis and Taylor (2000) single-clinopyroxenethermometer provide themost reliable temperature estimates in repro-ducing the experimental temperatures; equilibrium temperature deter-minations of mantle xenoliths, therefore, should preferably apply thesetwo thermometers.

As discussed by Nickel and Green (1985), garnet-orthopyroxenegeobarometry is very sensitive to temperature estimation, and anoverestimation of temperature will cause a pressure overestima-tion, and vice verse. In evaluating equilibrium pressures, Nickeland Green (1985) preferred the Wells (1977) two-pyroxene ther-garnet-orthopyroxene geobarometer as beinpartly agree with their opinion and here we pation of the two barometers.

Corresponding author. Tel.: +86 10 8825 6312; faxE-mail address: wucm@gucas.ac.cn (C.-M. Wu).

0024-4937/$ see front matter 2012 Elsevier B.V. Alldoi:10.1016/j.lithos.2012.01.018ne hundred versions ofreactions among ortho-spinel have been pro-

the best statistics (Fig. 4 inWZ11) in reproducing experimental temper-atures between 800 and 1750 C of complex chemical systems, withstandard errors of 42 C and 49 C, respectively. We agree withthermometers based on FeMgCa exchangepyroxene, clinopyroxene, olivine, garnet and1. Introduction

Accurately estimating PT conditioin understanding the thermal state of tospheric mantle. Naturally, reliableobtained by applying precise and accueters. Over the past decades moreorthopyroxene geobarometers and nt or the present-day lith-estimates can only beermometers and barom-ten versions of garnet-

mometry are not equivalent in precision, although evaluation of thevalidity of this geobarometry is beyond the scope of WZ11. In fact,among the more than thirty versions of two-pyroxene thermometers,the Taylor (1998) and Brey and Khler (1990) thermometers showDiscussion

Reply to comment on The applicability omantle xenoliths by Paolo Nimis and Her

Chun-Ming Wua,, Guochun Zhaob

a College of Earth Science, The Graduate School, Chinese Academy of Sciences, P.O. Box 458b Department of Earth Sciences, James Lee Science Building, The University of Hong Kong, P

a b s t r a c ta r t i c l e i n f o

Article history:Received 1 November 2011Accepted 23 January 2012Available online 14 February 2012

Keywords:GarnetGeobarometerGeothermometerMantle xenoliths

Equilibrium PT conditionsto be best determined throuthe Taylor (1998) garnet-orNickel and Green (1985) ga

j ourna l homepage: wwwg the most accurate. Weresent additional evalu-

: +86 10 8825 6012.

rights reserved.arnet orthopyroxene geobarometry inan Grtter

eijing 100049, Chinalam Road, Hong Kong, China

arnet-facies or garnet-spinel transition facies mantle xenoliths are suggesteda combination of the Taylor (1998) two-pyroxene thermometer either withpyroxene barometer for TiO2-rich orthopyroxenes (Al/Tib12.0), or with thet-orthopyroxene barometer for TiO2-poor orthopyroxenes (Al/Ti>12.0).

2012 Elsevier B.V. All rights reserved.

os

sev ie r .com/ locate / l i thosmometer, Taylor's (1998) garnet-orthopyroxene geobarometerwas combined with his two-pyroxene thermometer (Taylor,1998), whereas Brey et al. (2008a) suggested that their barometershould be combined with the two-pyroxene thermometer (Breyand Khler, 1990), respectively.

In order to objectively evaluate the accuracy of the differentgarnet-orthopyroxene geobarometers (Nickel and Green, 1985,

referred to as NG85 hereafter; Taylor, 1998, referred to as TA98 here-after) without bias caused by different errors of input temperatures,we have combined the barometers of NG85 and TA98 with the two-pyroxene thermometers of Taylor (1998) as suggested by TA98.

3. The TA98 and NG85 barometers: which is the best?

NG12 argue that the correction of activity model of the Mg-tschermak component in orthopyroxene by TA98 served to im-prove agreement between the TA98 barometer and the TA98 ex-periments. We agree with NG12 that TA98's Ti-correction of theMg-Mschermak's activity (aMg-Ts=(Al+NaCr+Ti)/2) is purelyempirical and tentative. The correction relates to the observationthat for fertile peridotites and garnet websterites with high-TiO2

orthopyroxenes, the NG85 barometer clearly overestimates pres-sures for experimental data sets (see Fig. 5 in TA98).

In WZ11's Fig. 2, the precision of barometers was estimated by ap-plying them to experimental data (Table 2 inWZ11) without discrim-inating high-TiO2 and low-TiO2 orthopyroxenes. Here we use NG12'sdenition that orthopyroxenes with Al/Tib12 are high-TiO2 and thosewith Al/Ti>12 are low-TiO2 orthopyroxenes. When applying theNG85 and TA98 barometers to the experimental data with low-TiO2orthopyroxenes, the NG85 barometer is more precise than the TA98barometer (Fig. 1) with correlation coefcients of 0.947 (standarderror is 5.7 kbar) and 0.946 (standard error is 5.9 kbar), respec-tively. For the runs having no TiO2 components in the product ortho-pyroxenes in the collated experimental data (Table 2 in WZ11), theNG85 and TA98 barometers yielded identical pressure estimate.

For the experimental runs with high-TiO2 orthopyroxenes (MG78,B90 and TA98), the NG85 barometer is less precise than the TA98 ba-rometer (Fig. 2), with correlation coefcients of 0.812 (standard error

289C.-M. Wu, G. Zhao / Lithos 142143 (2012) 288290Fig. 1. Experimental pressures versus pressures computed by (a) the Nickel and Green(1985) and (b) the Taylor (1998) garnet-orthopyroxene geobarometers applied to theexperimental runs (Table 2 in WZ11) with low-TiO2 orthopyroxenes (Al/Ti>12.0).Dashed lines represent3 kbar deviations. Symbols for experimental data: A76=Akella(1976),MG78=Mori andGreen (1978), B90=Brey et al. (1990), GW95=Gudmundssonand Wood (1995), RW98=Robinson and Wood (1998), TA98=Taylor (1998), W98=

Walter (1998), B08a=Brey et al. (2008a), B08b=Brey et al. (2008b).Fig. 2. Experimental pressures versus pressures computed by (a) the Nickel and Green(1985) and (b) the Taylor (1998) garnet-orthopyroxene geobarometers applied to exper-imental runs (Table 2 in WZ11) with high-TiO2 orthopyroxenes (Al/Tib12.0). Dashed

lines represent3 kbar deviations. Experimental data are the same as in Fig. 1.

is 4.5 kbar) and 0.894 (standard error is 3.3 kbar), respectively.This suggests that the TA98 barometer is suitable for high-TiO2 ortho-pyroxenes whereas the NG85 barometer is suitable for low-TiO2orthopyroxenes. It should be stated that the TA98 experiments didnot only produce high-TiO2 orthopyroxenes (13 runs) but also low-TiO2 orthopyroxenes (11 runs).

NG12 point out that most of the natural orthopyroxenes are TiO2-poor in chemistry as collated in the go-forward dataset (Nimis andGrtter, 2010). This is partly conrmed by the collated natural mantlexenoliths (Table S2 in WZ11): among garnet-spinel transition faciesnatural xenoliths only 3% of the samples are TiO2-rich, 83% samplesare TiO2-poor and the remaining 14% samples are TiO2-free. As forthe collated natural garnet-facies mantle xenoliths (Tables S2 inWZ11), as many as 23% samples are TiO2-rich, 66% samples areTiO2-poor, and the remaining 11% samples are TiO2-free.

NG12 applied the NG85 and TA98 barometers to mantle xenolithscollected from South Africa, northern Lesotho and Canada to evaluatethe smoothness of xenolith geotherms to test the validity of these twobarometers (Fig. 2 in NG12). NG12 concluded that TA98's modica-tion invariably increases the scatter around xenolith geotherms andthis barometer should be abandoned. It should be noted that suchscatter falls well within error of both the NK85 and the TA98 barom-eters, and accordingly, cannot be used to verify that the TA98 barom-

which helped us in understanding the validity of garnet-orthopyroxenegeothermobarometers. Reviews by Thomas Stachel and an anony-mous reviewer improved the science and English of the originalmanuscript. This work was supported by the Chinese Academy ofSciences (XDA08110000) and the National Natural Science Founda-tion of China (40872125).

References

Akella, J., 1976. Garnet pyroxene equilibria in the system CaSiO3MgSiO3Al2O3 and ina natural mineral mixture. American Mineralogist 61, 589598.

Brey, G.P., Bulatov, V.K., Girnis, A.V., 2008a. Geobarometry for peridotites: experimentsin simple and natural systems from 6 to 10 GPa. Journal of Petrology 49, 324.

Brey, G.P., Bulatov, V.K., Girnis, A.V., Lahaye, Y., 2008b. Experimental melting of carbon-ated peridotite at 610 GPa. Journal of Petrology 49, 797821.

Brey, G.P., Khler, T., 1990. Geothermometry in fourphase lherzolites. II. New thermo-barometers, and practical assessment of existing thermobarometers. Journal ofPetrology 31, 13531378.

Brey, G.P., Khler, T., Nickel, K.G., 1990. Geothermometry in four-phase lherzolites. I.Experimental results from 10 to 60 kb. Journal of Petrology 31, 13131352.

Gudmundsson, G., Wood, B.J., 1995. Experimental tests of garnet peridotite oxygenbarometry. Contributions to Mineralogy and Petrology 119, 5667.

Mori, T., Green, D.H., 1978. Laboratory duplication of phase equilibria observed innatural garnet lherzolites. Journal of Geology 86, 8397.

Nickel, K.G., Green, D.H., 1985. Empirical geothermobarometry for garnet peridotitesand implications for the nature of the lithosphere, kimberlites and diamonds.Earth and Planetary Science Letters 73, 158170.

Nimis, P., Grtter, H., 2010. Internally consistent geothermometers for garnet perido-tites and pyroxenites. Contributions to Mineralogy and Petrology 159, 411427.

Nimis P., Grtter H., Discussion of The applicability of garnet-orthopyroxene geobaro-metry in mantle xenoliths, by Wu C.-M. and Zhao G. (Lithos, v. 125, p. 19). Lithos(in press). doi:10.1016/j.lithos.2011.09.006.

290 C.-M. Wu, G. Zhao / Lithos 142143 (2012) 288290eter is less valid. As shown in Fig. 2, the NG85 barometer is lessprecise than the TA98 barometer for TiO2-rich orthopyroxenes, iffull equilibration was indeed achieved during the experimental runsshown.

4. Conclusion

At present, equilibrium pressures and temperatures of garnet-facies or garnet-spinel transition facies mantle xenoliths may bebest determined by applying the Taylor (1998) two-pyroxene ther-mometer combined with the Taylor (1998) garnet-orthopyroxene ba-rometer for TiO2-rich orthopyroxenes, and combined with the Nickeland Green (1985) garnet-orthopyroxene barometer for TiO2-poororthopyroxenes.

Acknowledgements

We sincerely thank Paolo Nimis and Herman Grtter for theirdiscussion of garnet-orthopyroxene geobarometry in mantle xenoliths,Nimis, P., Taylor, W.R., 2000. Single-clinopyroxene thermobarometry for garnet perido-tites, Part I. Calibration and testing of a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer. Contributions to Mineralogy and Petrology 139, 541554.

Robinson, J.A.C., Wood, B.J., 1998. The depth of the spinel to garnet transition at theperidotite solidus. Earth and Planetary Science Letters 164, 277284.

Taylor, W.R., 1998. An experimental test of some geothermometer and geobarometerformulations for upper mantle peridotites with application to the thermobarometryof fertile lherzolites and garnet websterite. Neues Jahrbuch fr MineralogieAbhandlungen 172, 381408.

Walter, M.J., 1998. Melting of garnet peridotite and the origin of komatiite anddepleted lithosphere. Journal of Petrology 39, 2960.

Wells, P.R.A., 1977. Pyroxene thermometry in simple and complex systems. Contributionsto Mineralogy and Petrology 62, 129139.

Wu, C.M., Zhao, G.C., 2011. The applicability of garnet-orthopyroxene geobarometry inmantle xenoliths. Lithos 125, 19.

Reply to comment on The applicability of garnet orthopyroxene geobarometry inmantle xenoliths by Paolo Nimis and Herman Grtter1. Introduction2. Valid geothermometers3. The TA98 and NG85 barometers: which is the best?4. ConclusionAcknowledgementsReferences