Contrib Mineral Petrol (2005) 150: 486–504 DOI 10.1007/s00410-005-0036-5
O R I GI N A L P A P E R
Eric Hellebrand Æ Jonathan E. Snow Smail Mostefaoui Æ Peter Hoppe
Trace element distribution between orthopyroxene and clinopyroxene in peridotites from the Gakkel Ridge: a SIMS and NanoSIMS study
Received: 25 May 2005 / Accepted: 5 September 2005 / Published online: 28 October 2005 Springer-Verlag 2005
Abstract Clinopyroxenes (cpx) in abyssal and ophiolitic peridotites are commonly analyzed for lithophile trace element abundances in order to estimate degrees of melting and porosity conditions during melt extraction, assuming that these data reflect near-solidus conditions. During cooling, however, cpxs always exsolve into parallel lamellae of low-Ca enstatite and high-Ca diopside. This may potentially lead to redistribution of the initial trace element budget. Since orthopyroxene (opx) cannot significantly host most incompatible trace elements, exsolution will lead to an enrichment in the cpx lamellae. In order to address a possibly exsolution-controlled partitioning between cpx and opx, we have obtained major and trace element mineral compositions on 14 plagioclase-free ocean floor mantle rocks. They cover the entire abyssal peridotite compositional spectrum from very fertile to highly depleted compositions. The mean volume proportion of opx lamellae in cpx porphyroclasts lies around 15% of the original cpx. For the light to middle rare earth elements, the enrichment in the measured cpx exsolution is exclusively controlled by these phase proportions. Relative to these highly incompatible trace elements, solely Ti and Yb partition significantly into opx. Lamellar interpyroxene partition coefficients, estimated from NanoSIMS analyses, are
Electronic Supplementary Material Supplementary material is available for this article at http://dx.doi.org/10.1007/10.1007/ s00410-005-0036-5 and is accessible for authorized users. Editorial Responsibility: J. Hoefs E. Hellebrand (&) Æ J. E. Snow Æ S. Mostefaoui Æ P. Hoppe Max-Planck-Institut fu¨r Chemie, 55020 Mainz, Germany E-mail:
[email protected] J. E. Snow Department of Geosciences, University of Houston, 312 S&R, Houston, TX 77204, USA S. Mostefaoui Laboratoire d’Etude de la Matiere Extraterrestre, Museum National d’Histoire Naturelle, Paris, France
around three times as high as the ones for near-solidus bulk pyroxene. The equilibration temperatures for the exsolution lamella are slightly higher than 800C. The bulk cpx can be reconstructed using the lamellar proportions and their relative partitioning. The implication of such a reconstruction is that the cpx rare earth element patterns shift almost in parallel to lower values. These shifts, however, do not affect mantle melting models proposed thus far for mid-ocean ridges.
Introduction Clinopyroxene (cpx) trace element compositions of residual mantle peridotites from mid-ocean ridges (Johnson et al. 1990; Johnson and Dick 1992; Hellebrand et al. 2002) and ophiolites (e.g., Rampone et al. 1996; Batanova et al. 1998; Suhr et al. 1998) are an important source of information for interpreting the melting history of a region. They allow the quantitative estimation of the degree of partial melting, as well as conditions of melt formation and melt migration. After partial melting, these cpxs have undergone subsolidus exsolution (formation of orthopyroxene (opx) lamellae in porphyroclastic cpx and vice versa), producing a highCa diopside and low-Ca enstatite. This is because the solubility of the high- and low-Ca pyroxenes in each other along the diopside-enstatite join decreases with decreasing temperature, an expression of the well-known pyroxene miscibility gap (e.g. Wells 1977; Lindsley 1983). At slow cooling rates, the peridotite minerals are generally well equilibrated. For instance, mantle xenoliths display a good correlation between the wollastonite content of their pyroxenes (Fig. 1), which is commonly used to estimate equilibration temperatures (e.g. Brey and Kohler 1990). Pyroxene compositions obtained on residual (plagioclase- and vein-free) abyssal peridotites strongly deviate from this temperature-dependent relationship (Fig. 1). There is little overlap between both
487 0.06 1400 0.05 1300
Wo (Opx)
RAP
1200 0.03
TCa-in-Opx (˚C)
0.04
1100
Continental
0.02 1000 900
0.01
800 0.00 0.30
0.35
0.40
0.45
0.50
0.55
Wo (Cpx)
Fig. 1 Wollastonite content of cpx vs. associated opx in plagioclase-free spinel peridotites. White diamonds represent abyssal peridotites, grey diamonds represent continental mantle xenoliths (full reference list given in Hellebrand and Snow (2003)), black squares represent spinel peridotite xenoliths for which trace element mineral data are available as well (Witt-Eickschen and O’Neill
2005). Most mantle xenoliths display well-defined interpyroxene equilibration, whereas abyssal peridotites are strongly shifted to higher cpx wollastonite contents. Although this suggests lack of interpyroxene equilibration in abyssal peridotites, this offset results from the dominant analysis of Ca-rich exsolution lamellae, due to the preferred serpentinization of opx lamellae
groups: abyssal peridotites are shifted to much higher cpx wollastonite content and never reach the low opx wollastonite values observed in many xenoliths. There are two main explanations for this discrepancy: (1) the cooling rate of abyssal peridotites is continuous and much faster as they are undergo adiabatic decompression melt extraction to exposure onto the ocean floor in less than a few million years. (2) Seawater alteration near the ocean floor preferentially replaces opx lamellae in cpx by serpentine minerals, whereas cpx lamellae and recrystallized neoblasts are more resistent. These unaltered high-Ca cpx are then analyzed using microbeam techniques such as electron and ion microprobe, leading to a considerable overestimation of the near-solidus cpx calcium content. In addition to these well-known major element effects, there may in principle be substantial subsolidus effects on the trace element patterns of mantle cpxs that distort the mantle-related processes under study. In order to investigate such effects, we have measured major and trace element compositions of 14 abyssal peridotite opx–cpx pairs (Table 1).
times were 30 s for major and 60 s for minor elements. A subset of these samples was subsequently measured with a defocused beam of 25 lm diameter. This subset is mainly restricted to the cores of coarse opx porphyroclasts. As shown in the backscattered electron image of a coarse cpx (Fig. 2a), opx exsolution lamellae in cpx porphyroclast are generally strongly serpentinized. This is the primary reason why focused beam analyses on the fresh cpx lamellae are generally required to obtain a ‘primary mantle composition’, both for major and trace elements. The average pyroxene porphyroclast compositions are listed in Tables 2 and 3. The relative mineral proportions of exsolution lamellae were estimated from binarized secondary electron images, using the greyscale contrast between cpx and opx. For this purpose only undeformed and serpentine-free domains of coarse porphyroclastic cpxs from three exceptionally fresh spinel lherzolites from a single dredge haul from Gakkel Ridge (samples PS59: 235-9, -11, -17) were used. All images were taken at the same resolution (400·), resulting in a total area of 1.38 mm2. Three to six images per grain and up to five grains per sample were analyzed by this procedure. The sample average and standard deviations are reported in Table 4. These estimates are independent of the crystallographic orientation as long as the lamellae are not (sub-) parallel to the thin section surface. For others, even only slightly more altered peridotites, this approach turned out to be not successful, since differential expansion of partly serpentinized opx lamellae distort the primary phase proportions.
Analytical procedures Cpx and opx were analyzed for major element compositions on a five-spectrometer JEOL JXA 8900RL electron probe microanalyzer at the University of Mainz, using an acceleration potential of 15 kV, a beam current of 12 nA and a spot size of 2 lm. Standard counting
488 Table 1 Sample locations and petrological characteristics Sample
Lat. (N)
Long. (E)
Lithologya
Textureb
Modal cpx
Spinel Cr#
Spinel TiO2
HLY0102: 30-1 HLY0102: 39-3 HLY0102: 70-34 HLY0102: 70-35 HLY0102: 70-52 HLY0102: 70-58 HLY0102: 70-62 HLY0102: 70-64 HLY0102: 70-75 PS59: 235-20 PS59: 249-74 PS59: 249-79 PS59: 257-1 PS59: 313-76 ANTP126-1
84.7 85.4 86.8 86.8 86.8 86.8 86.8 86.8 86.8 84.7 85.4 85.4 86.0 85.9 –13.5
3.0 13.6 64.8 64.8 64.8 64.8 64.8 64.8 64.8 4.2 14.7 16.7 23.6 25.7 66.5
Sp-Lhz Sp-Hzb Ol-Web Sp-Lhz Sp-Hzb Sp-Lhz Sp-Hzb Sp-Hzb Sp-Lhz Sp-Lhz Sp-Hzb Sp-Hzb Sp-Lhz Sp-Hzb Fe-Gabbro
gr ig gr gr gr gr gr gr gr gr ig ig gr gr
10 0.15 45 20 2.0 18 2.3 2.5 7.1 7.6 0.05 0.05 9.5 3 30
0.16 0.57 0.08 0.13 0.19 0.13 0.28 0.21 0.16 0.16 0.58 0.59 0.22 0.30 –
0.04 0.04 0.03 0.07 0.04 0.07 0.11 0.04 0.04 0.05 0.05 0.04 0.05 0.04 –
a
Sp-Lhz spinel-lherzolite, Sp-Hzb spinel-harzburgite, Ol-Web olivine-websterite Refers to dominant cpx texture: gr granular cpx, ig exclusive intergranular cpx
b
Cpx exsolution lamellae were analyzed for trace elements (selected rare earth elements (REE), Ti, Sr, Y, and Zr; Table 3) by secondary ion mass spectrometry (SIMS) using a modified Cameca ims-3f at the MaxPlanck-Institut fu¨r Chemie in Mainz. Spots were inspected before and after analysis to exclude alteration, cracks, mixed opx–cpx analysis, or inclusions. A primary beam current of 8–12 nA was used, resulting in a spot size of 15–20 lm. An energy offset of –80 V was applied to filter for molecular interferences following the principles reported by Shimizu et al. (1978). In cpx with ‘normal’ LREE-depleted REE patterns, solely 174Yb requires significant (generally not exceeding 10–15%) correction from the interfering 158GdO (Hellebrand et al. 2002). However, in some strongly HREE-depleted harzburgites with L- to MREE-enriched REE patterns, this particular interference can amount to 40% of the signal. In addition to the extremely low count rate, this affects the Yb precision too much for reliable interpyroxene partition coefficient estimation. Porphyroclastic opx was measured in undeformed domains with representative (i.e. low) and very thin cpx exsolution density. In order to improve counting statistics and limit the ‘nugget effects’ of small cpx exsolutions, primary beam currents of 15–20 nA were used, leading to a spot size of 25 lm in diameter. Each reported analysis is the average of 3–6 individual measurements, including both core and rim measurements (Table 3). No significant zoning was detected in any of the pyroxenes. The detector background noise (90% of the signal), and cannot be interpreted quantitatively. However, the chemical distinction between opx and cpx is not affected, and is smaller than the beam size (100 nm). Considering the poor electrical conductivity of silicate, an electron gun was applied to eliminate surface charging effects, which would nearly completely suppress secondary ion extraction. Prior to element map acquisition, an area of 20·20 lm2 was sputtered for 15 min to remove the Au coating and obtain a stable signal. One multi-element map consists of stacked images (2–8), each of which requiring an acquisition time of 15 min. Data reduction was performed offline, using square box-sized regions of interest, in order to quantify the element to silicon ratio within opx and cpx lamellae. Values reported in Table 5, list the image size, the average measured isotopic ratios for each lamella, the side length used to define the region of interest box, as well as the results for two individual spot analyses obtained with an oxygen source.
Results Samples and petrography The 14 ultramafic samples investigated for this study were collected by dredging during the joint USA-German AMORE 2001 expedition along the ultra-slow spreading Gakkel Ridge (Michael et al. 2003). They comprise six lherzolites, seven harzburgites and one olivine websterite (Table 1). All samples are optically free of plagioclase and crosscutting magmatic veins. This is further supported by their mineral compositions (low Ti in spinel, no negative Sr anomaly in cpx, tight correlation between Cr# (= molar Cr/(Cr+Al)) in spinel and Cr# in pyroxene, no within-sample and within-grain compositional gradient). Both modally and compositionally, these samples are representative of the complete range of plagioclase-free abyssal peridotites, from the most fertile (cpx-rich, low spinel Cr#) to the most depleted (virtually cpx-free, high spinel Cr#) yet reported on the ocean floor (Table 1). Detailed discussion of the tectonomagmatic history of the mantle underlying Gakkel Ridge will be presented elsewhere. Texturally, the lherzolites and cpx-bearing harzburgites are subequigranular to moderately porphyroclastic, with mainly coarse subequant opx porphyroclasts. Their cpxs are generally fine- to medium-grained in the 0.1– 2 mm range, and always have fine opx exsolution lamellae. These low-Ca enstatite exsolution lamellae in cpx are thin (2–20 lm), parallel, and generally of relatively constant thickness for an individual lamella (Figs. 2a, b). Nevertheless, very thin lamellae may pinch out, thicker lamellae may sometimes display a sudden or
gradual increase (20%) in thickness (Fig. 2a). The spacing between the individual lamellae is highly variable. Often, the centres of these broad, ‘first generation’ cpx lamellae contain a set of finer, discontinuous, en-echelon arrayed, needle shaped opx exsolutions (Figs. 2a, b). The angle between these finer exsolutions and the wider continuous exsolution ranges between 10 and 30, which may partly result from cutting/orientation effects. In general, the broad opx exsolution lamellae are more susceptible to serpentinization, often initiated at cracks that crosscut the grains (Fig. 2a). Finally, cpx can contain more irregular, patchy opx inclusions/exsolutions in a variety of textures and sizes (Figs. 2b, c). Such patchy exsolutions are relatively rare. In transmitted light, many opx porphyroclasts appear to be optically devoid of cpx exsolution lamellae and seem entirely homogeneous. Backscattered electron images, however, clearly show that thin (1 lm), parallel cpx exsolution lamellae are always present in opx porphyroclasts. Their spacing is also much smaller than the exsolution lamellae in the associated cpx porphyroclasts (Figs. 2d, e). Second-generation en-echelon exsolutions, such as frequently observed in cpx, were never found in opx. Both at two-pyroxene grain boundaries and at pyroxene–olivine grain boundaries, many lamellae often pinch out 20–30 lm away from the grain boundaries (Figs. 2b, c). At numerous opx–olivine grain boundaries, the opxs of the three virtually cpx-free harzburgites (39-3, 249-74, 249-79) have deep irregular embayments that may have resulted from partial dissolution by reactively percolating melts. Rare interstitial cpxs mainly occur at opx–opx grain boundaries, or are associated with small spinels surrounded by olivine. These fine-grained, often vermicular or holly-leaf shaped cpxs are completely devoid of the continuous opx lamellae seen in the porphyroclastic cpx of the lherzolites and the cpx-rich harzburgites. Aside from these normal and representative textures, one sample (lherzolite 70–75) displays more unusual textural relationships between opx and cpx, and is therefore included in order to assess whether trace element partitioning between the two pyroxenes could be related to their textures. Lherzolite 70–75 has coarse (20–200 lm) flame-like opx-exsolutions and blebs that in places pinch out into regular thin lamellae (Fig. 2f). The reason to include this sample is to verify whether clear textural disequilibrium is chemically discernable by measuring the cpx porphyroclast and these opx ‘flames’. This textural two-pyroxene association has been found previously in plagioclase-bearing peridotites. This parTable 4 Digitally estimated proportions of orthopyroxene lamellae in clinopyroxene porphyroclasts Sample
n grains
(%) opx exsol
1sd
235-09 235-11 235-17
3 3 5
0.155 0.148 0.145
0.030 0.008 0.018
O
O
O
Cs+
6
Cs+
10a
Cs+
11
4
Cs+
Cs+
18
19
2·2
5·5
15·15
2·2
4·4
4·4
13·13
6·6
4·4
15·15
15·15
10·10
13·13
Raster (lm2)
0.19
0.63
1.88
0.19
0.25
0.25
0.81
0.56
0.38
1.88
1.88
1.25
1.63
sd 5 sd 4 sd
5 sd 6 sd 5 sd 5 sd 6 sd 6 sd as 10a sd 5 sd 4
4 sd 4 sd 1 sd
Ca
4.7 0.11 4.4 0.14 4.2 0.10
44
Ti
5.3E-05 7.0E-04 3.2E-05 8.3E-04 5.3E-05
3.7E-02 6.7E-04 3.6E-02 8.8E-04 3.5E-02 8.0E-04 48 16 Ti O 6.0E-04 6.5E-05 7.1E-04 7.5E-05 4.6E-04 5.3E-05 4.1E-04 4.7E-05 3.8E-04 5.1E-05 6.8E-04 10.0E-05 6.8E-04 10.0E-05 6.4E-04 4.3E-05 6.6E-04
47
Y
2.4E-03 1.9E-04 2.1E-03 1.8E-04 1.8E-03 7.3E-05
89
0.19
0.47
0.47
0.25
0.38
0.25
0.81
0.75
0.50
1.41
1.88
1.25
1.63
ROI box size (lm)
Cpx mass/30Si
ROI boxb size (lm) n
Opx
Cpx
c
b
Image numbers refer to Figure 4 and electronic supplement Side length of square box region of interest (ROI) to measure mass to silicon ratios CaO in opx estimated assuming cpx CaO of 23 wt% d Temperature estimated from opx Ca content at 5 kbar after Brey and Kohler (1990)
a
4
Cs
4
3
17
+
Cs
10b
3
3
Cs+
9
+
3
Cs
2
2
2
2
1
1
Lam. nr
7
+
Cs
+
Cs+
16
16
16
Ion source
5
4
spot
2
1
Image nr a
sd 6 sd 5 sd
6 sd 6 sd 5 sd 6 sd 6 sd 4 sd 6 sd 4 sd 6
1
4 sd 4
n
Table 5 NanoSIMS opx and cpx lamellae compositions in cpx porphyroclast of lherzolite 235-20
Ca
9.2E-02 4.4E-03 5.7E-02 4.6E-03 8.7E-02 2.5E-03
44
Ti
4.7E-05 1.1E-04 2.4E-05 1.4E-04 2.1E-05
4.7E-03 6.6E-04 4.5E-03 8.6E-04 6.4E-03 1.3E-04 48 16 Ti O 1.2E-04 1.5E-05 1.4E-04 2.7E-05 6.9E-05 1.7E-05 1.1E-04 3.8E-05 5.8E-05 3.4E-05 7.3E-05 2.1E-05 7.0E-05 9.0E-06 9.6E-05 2.2E-05 6.1E-05
47
Opx mass/30Si Y
9.6E-05 2.2E-05 8.1E-05 2.2E-05 1.0E-04 2.6E-06
89
50 2 77 6 49 1
Ca
9.0 6.4 1.7 5.7 1.2
4.9 1.1 5.1 1.5 6.7 2.4 3.8 1.8 6.6 4.8 9.3 4.1 9.8 2.7 6.7 2.0 10.7
7.9 1.1 8.1 1.6 5.5 0.01
Ti
cpx lamella too thin for raster
thin cpx lamella
thin cpx lamella
25 6 26 7 17 0.2 Comment
Y
Kd (Cpx/Opx)
0.46 0.02 0.30 0.03 0.47 0.02
Opx c CaO (wt%)
830 10 760 10 840 10
Td Ca (Opx) (C)
494
495
ticular sample is well preserved and plagioclase is absent based on both optical and geochemical criteria. Also included in this study is a harzburgite-hosted ferrogabbro vein (ANTP126-1) from the Argo fracture zone along the intermediate spreading Central Indian Ridge (Hellebrand et al. 2002). This medium-grained gabbro crystallized from an injected evolved melt and has diffuse contacts to the host peridotite. Its fractionated nature is further supported by the occurrence of coarse accessory minerals such as apatite, ilmenite and brown hornblende. As already seen in the mantle rocks, the vein cpx has exsolved thin opx lamellae, whereas the associated opx is optically homogeneous. Backscatteredelectron images revealed fine cpx exsolution lamellae, similar to the magnesian opx of the residual peridotites. Bulk major element composition of ‘near-solidus’ pyroxenes Without extensive dynamic recrystallization, the formation of exsolution lamellae in pyroxenes of slowly cooled peridotites is the dominating process that affects their micro-scale major and trace element composition. As bulk grain, their compositions have not been drastically affected, particularly in the case of coarse porphyroclasts, since intramineral diffusion for major constituents and trivalent species as the REE is extremely slow (van Orman et al. 2001). Thus, bulk grain compositions would reflect near-solidus conditions. Opx: focused and defocused beam analyses are comparable If measured with a beam diameter several times larger than the width/thickness of the lamellae, opx bulk composition can be measured directly. Cpx exsolution lamellae in opx are generally very thin, on the order of 1 lm and are too small to be analyzed by EPMA. Using a 2 lm diameter spot size, occasionally, a mixed analysis with a relatively high proportion of cpx would yield a high CaO content. With a large number of randomly positioned individual analyses, the average opx composition would reflect its bulk, near-solidus composition. Most samples were routinely investigated with four to six analyses per grain core, with two to four crystals per sample. This approach is sufficient to provide a good estimate for all major elements, except for calcium, which displays a high standard deviation. For this reason, the opx porphyroclast cores from selected samples were reanalyzed with a defocused beam (25 lm diameter). For most samples, the average opx core obtained with this method does not strongly deviate from the composition obtained with a focused beam. This is particularly noteworthy for Mg# and Cr#, which are very strongly correlated (r2: 0.973 and 0.993, respectively). However, CaO content measured with a broad beam is almost 50% higher than with a
focused beam, and also shows less variation. The correlation between focused and defocused beam is poor. For this reason, the average opx core analyzed with a defocused beam probably best reflect its bulk nearsolidus CaO content. Solely the two reanalyzed depleted harzburgites yielded almost identical CaO contents by both methods. Cpx: phase proportion estimates and lamella compositions Cpx bulk compositions are more difficult to obtain, as the opx exsolution lamellae are thicker and more widely and irregularly spaced. The most challenging analytical aspect, however, is the generally strong serpentinization of the opx lamellae. Cpx porphyroclasts in the ‘average abyssal peridotite’ are not as well preserved as the examples shown in Fig. 2. In order to obtain bulk cpx compositions that reflect pre-exsolution conditions, two sets of information are required: (1) the mineral proportions of cpx and opx lamellae in the cpx and opx porphyroclasts, and (2) the major element compositions of these lamellae. For eleven cpx porphyroclast grains in three very fresh (0.89 reported by Seitz et al. (1999), is in good agreement with our NanoSIMS results. This suggests that rather than an incorrect estimate in either of these studies, the temperate-controlled interpyroxene partitioning of titanium is not completely understood. Consequences of a subsolidus exsolution correction We applied the exsolution corrections to published data obtained on the Cameca IMS-3f in Mainz. The overall trace element pattern of the integrated cpx compositions is shifted downward subparallel to the uncorrected pattern. The precision and accuracy for the highly incompatible trace elements are not sufficient to address the exsolution correction issue. In any event, the effect
on modelled residual melt porosities, or amount of entrapped melt would be minor. For the moderately incompatible trace elements, however, there may be consequences that are significantly beyond the analytical precision and accuracy. For example, the 15% effect for a single analysis would be at or near the limit of the accuracy of Sm. Given that most reported data are the weighted means of three or more individual analyses, the accuracy is better than ten percent (2-sigma) for the HREE, Ti, Zr and Y. For a homogeneous sample set, such as the N-type peridotites from Lena Trough (Hellebrand and Snow 2003), there could therefore in principle be statistically significant consequences, which are evaluated below. Degree of melting The good correlation between the HREE in cpx and the Cr# in spinel has been used to calibrate the degree of melting, estimated from trace element melting models, on easily obtained spinel major element data (Hellebrand et al. 2001). This would result in a simple straightforward expression for the degree of melting (in percentage) based on the spinel Cr#: F=10.8 ln(Cr#sp) +24.5. If the true near-solidus cpx have lower HREE than the actually measured cpx, spinel peridotite nearfractional melting models that are used to produce this, then the estimated degree of melting would be only 1% higher. Therefore, a modified expression would change to: F=10.3 ln(Cr#sp)+25.0. Considering the relatively large scatter in the degree of melting at a given spinel Cr#, which can be caused by analytical uncertainty as well as variable degrees of melting in the stability field of garnet peridotite, this change is entirely insignificant. Garnet-field melting Hellebrand et al. (2002) reported evidence for initial melting in the stability field of garnet peridotite in some peridotites of the Central Indian Ridge. A strong fractionation between middle and heavy REE combined with relatively high HREE concentrations cannot be explained by melting of a spinel peridotite exclusively. Therefore, residual garnet is required to retain high HREE. The exsolution correction would lead to a shift towards lower HREE concentrations. As shown in Fig. 7, this leads to a slight decrease in absolute amount of garnet field melting. This is the direct result of the relative partitioning of Yb into the opx lamellae, generating a decrease to 85 and 88% of the measured concentration of Sm and Yb, respectively. However, the Sm/Yb ratio decreases by nearly 4%, as shown by the shift subparallel to the spinel peridotite fractional melting array (Fig. 7). In summary, exsolution correction does not strongly affect the garnet signature, it solely leads to a minor increase in the inferred degree of melting under spinel peridotite conditions.
503
10
1 (Sm/Yb) in cpx
4%
ld l -fie Gt tiona c fra
DMM cpx in sp-facies
gt
N
6%
s 2%
df fiel
-
tan
8% sp p 10% s
12% s p
sp
tF
14% s p
16%
18% sp
ns
p
co
p 4% s
0 .1
gt
tio
rac
Sp
gt
8%
l na
6% sp
Fig. 7 Chondrite-normalized Sm/Yb vs. Yb concentration in cpx of residual abyssal peridotites. White diamonds are uncorrected sample averages from Gakkel Ridge, SWIR, CIR. Grey and closed squares represent uncorrected and exsolution-corrected cpx from LREE-depleted Lena Trough residues, respectively. The consequence of an exsolution correction is a slight decrease in garnet-field (gt) melting signature and an increase of similar magnitude in the degree of melting in the stability field of spinel (sp) peridotite. This means that cooling-related exsolution, which all abyssal peridotites undergo before exposure on the ocean floor, does not significantly affect the partial melting signature. Initial melting in the presence of residual garnet is still required to account for the relatively strong MREE depletion in cpx of many abyssal peridotites
0 .0 1 1
10
20
Yb N in cpx
Conclusions Incompatible lithophile trace element compositions were obtained by SIMS on cpx–opx pairs in 14 plagioclasefree peridotites from the ultraslow spreading Gakkel Ridge (Arctic Ocean). (1) During exsolution, regularly spaced parallel lamellae of low-Ca opx are observed in Ca-rich cpx porphyroclasts, taking up 15% of the original crystal. (2) An equilibration temperature of 810C for the formation of opx lamellae in cpx lamellae can be estimated by the Ca content in the opx, measured by NanoSIMS with a 1 lm beam diameter. These Ca values are lower and more accurate than EPMA data, since the latter are distorted by secondary fluorescence effects, resulting from the thinness of the lamellae and the excitation volume of the electron beam. (3) Inter-grain opx-cpx Kd’s are relatively constant over the range of abyssal peridotite compositions found in the ocean basins. Kdcpx/opx appears to be independent of estimated equilibration temperature, modal composition, aluminium activity, or other indicators of depletion. (4) With respect to bulk pyroxene Kd’s, low-temperature intra-grain lamellar Kd’s for Ti and Y estimated from NanoSIMS analysis are higher by a factor 2.7 and 3.3, respectively. This suggests that coupled major and trace element diffusion combined with lamella formation at a micron-scale can continue to lower temperatures, and produce stronger trace element fractionation between individual cpx and
opx lamellae. Large-beam trace element analyses on opx, which integrate over larger volumes do not record such low-temperature partitioning. (5) The absolute concentration of incompatible lithophile trace elements in a cpx prior to exsolution is on average 15% lower than the measured values. There are thus no drastic consequences of an exsolution correction. This effect is just outside the analytical accuracy for a single analysis, but certainly significant if three or more analyses are made on the same homogeneous sample. (6) The quasi-exponential correlation of model HREE concentrations with spinel chrome number is not affected by the exsolution correction, independent of whether high-temperature correction factors are used. (7) For a set of peridotites from a locally homogeneous mantle exposed at Lena Trough, the exsolution correction would lead to a decrease of 1% in the degree of melting in the stability field of garnet peridotite and a 1% increase in the spinel field. This means that subsolidus exsolution, a process undergone by all peridotites collected from the ocean floor, does not significantly affect previous models that advocate initial melting of a garnet peridotite.In our opinion, at the current state of the art in microbeam analysis of lithophile trace elements in mantle pyroxenes, there is no justification for the routine correction of trace element concentrations determined by ion probe to pre-exsolution levels. Further, the results of previous inquiries into the nature of mantle melting are not significantly affected by the phenomenon of pyroxene exsolution.
504 Acknowledgements This work was inspired by discussions with Dave Green, Hugh O’Neill and Jon Blundy, who independently pointed out the discrepancy between observed ocean floor peridotite mineral compositions and those obtained in melting experiments. We are indepted to Elmar Groener for careful technical assistence on the ion probes. We thank Gu¨nter Suhr, Melanie Griselin, Anette von der Handt, Henry Dick, and Othmar Mu¨ntener for discussion, and Laurence Coogan and Kevin Johnson for their thoughtful and thorough reviews that significantly improved the manuscript. E.H. gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft grant SN15/2.
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