Homogeneous superchondritic 142Nd/144Nd in the mid-ocean ridge basalt and ocean island basalt mantle



[1] 146Sm decays to 142Nd with a relatively short half-life (∼68 Ma). The142Nd/144Nd of modern terrestrial mantle-derived lavas is 18 ± 5 ppm higher than the chondrite reservoir. The difference in142Nd/144Nd between Earth and chondrites likely owes to Sm/Nd ratios 6% higher in the accessible Earth that arose within the first 30 million years following accretion. In order to constrain the early history of the mantle domains sampled by ocean island basalts (OIB) and mid-ocean ridge basalts (MORB), we present high-precision142Nd/144Nd measurements on 11 different lavas from five hot spots, and one lava each from the Indian and Atlantic ridges. The lavas examined in this study bracket much of the known Sr-Nd-Pb-He isotopic variability the in mantle. These data complement existing high-precision142Nd/144Nd data on MORB and OIB lavas. In agreement with previous studies, we find that MORB and OIB lavas examined for high-precision142Nd/144Nd exhibit ratios that are indistinguishable from the terrestrial standard and are 15–20 ppm higher than the average obtained for ordinary and enstatite chondrites. The uniform, superchondritic 142Nd/144Nd data in OIB and MORB are consistent with derivation from a common, early formed (<30 Ma after accretion) progenitor reservoir with Sm/Nd ∼6% higher than chondrites. If there exists any variability in 142Nd/144Nd in the OIBs and MORBs examined to date, it is too small to be resolved with the precision currently available.

1. Introduction

[2] The composition of the bulk silicate Earth (BSE)—the solid Earth minus the core—has long been assumed to be tied to chondrites: the refractory lithophile elements, like Sm and Nd, are thought to exist in chondritic relative abundances in the BSE [e.g., Jagoutz et al., 1979; Jacobsen and Wasserburg, 1980; Hart and Zindler, 1986; McDonough and Sun, 1995; Palme and O'Neill, 2003, Amelin and Rotenberg, 2004; Patchett et al., 2004]. Sm and Nd present two different radiogenic decay schemes that can be useful for evaluating the hypothesis that the Earth and chondrites have the same Sm/Nd ratios: 146Sm decays to 142Nd (T1/2 = 68 Ma [Kinoshita et al., 2012]) and 147Sm decays to 143Nd (T1/2 = 106 Ga). If chondrites and BSE have the same Sm/Nd ratio, then they should have identical 142Nd/144Nd and 143Nd/144Nd ratios.

[3] Boyet and Carlson [2005] measured small, but significant (18 ± 5 ppm) differences between chondrites and modern terrestrial lavas. If the 142Nd/144Nd anomaly in modern terrestrial lavas relative to chondrites is the result of radiogenic decay of 146Sm and not initial Solar System nucleosynthetic heterogeneity [Andreasen and Sharma, 2006, 2007; Carlson et al., 2007; Qin et al., 2011], then all modern terrestrial rocks originate from an ancient reservoir that had Sm/Nd ratios ∼6% higher than chondritic during the lifetime of 146Sm. A 4.568 Ga reservoir with 147Sm/144Nd = 0.208 ± 0.002 (6% higher than chondritic) will today have 143Nd/144Nd = 0.5130 ± 0.0001. This 143Nd/144Nd value is significantly higher than chondritic (0.51263 [Bouvier et al., 2008]), and only slightly lower than normal MORB (0.51313 [Su, 2003]). If the Earth is not chondritic, then the question arises as to whether any of the primitive (but not chondritic) BSE material has survived to the present-day. The range of superchondritic143Nd/144Nd ratios predicted for a non-chondritic BSE brackets the lavas with the highest terrestrial mantle-derived3He/4He signatures, providing evidence consistent with the highest 3He/4He mantle being parental to all mantle reservoirs that give rise to modern volcanism [Boyet and Carlson, 2006; O'Neill and Palme, 2008; Caro et al., 2008; Caro and Bourdon, 2010; Jackson et al., 2010].

[4] Alternatively, if the BSE is chondritic, then the high 3He/4He lavas sample surviving portions of an Early formed incompatible element Depleted Reservoir (EDR), that must be complemented by an Early formed Enriched Reservoir (EER) that has not yet been sampled at Earth's surface. If this is the case, then mantle plumes, which are thought to derive from the deep mantle [Morgan, 1971; Courtillot et al., 2003], may sample the EER. The EER is predicted to have 142Nd/144Nd 38–54 ppm lower than the terrestrial standard [Carlson and Boyet, 2008]. Therefore, if a component of the EER is entrained in mantle plumes and sampled by plume-fed hot spot lavas, then the extremely low142Nd/144Nd of the EER would be expected to shift the 142Nd/144Nd of OIB lavas to lower values, even if it is sampled by the plume in diluted form.

[5] In this study we prospect for surviving mantle reservoirs that host anomalous 142Nd/144Nd ratios. We examine OIB and MORB lavas sampling the four canonical mantle end-members (DMM, depleted MORB mantle; EM1, enriched mantle 1; EM2, enriched mantle 2; HIMU, high ‘μ’, or high 238U/204Pb [Zindler and Hart, 1986]) and FOZO-C (‘Focus Zone’ or ‘Common’ components, both with high 3He/4He [Hart et al., 1992; Hanan and Graham, 1996]).

2. Sample Descriptions and Locations

[6] The following OIB and MORB samples were selected to better characterize the 142Nd/144Nd of the convecting mantle (Table 1). MAG-B-47 is a subaerial sample collected from Mangaia and has one of the most radiogenic Pb-isotopic (HIMU) compositions measured in an OIB [Hauri and Hart, 1993]. Samoan lavas ALIA-115-18 and ALIA-115-21 (and clinopyroxene separates from these samples), dredged from the flanks of the island of Savai', were selected for their extreme87Sr/86Sr compositions (>0.721 [Jackson et al., 2007a]). Two other Samoan lavas (Ta'u island sample T54 and clinopyroxene separates from Malulalu seamount sample AVON3-78-1) with less extreme87Sr/86Sr compositions [Workman et al., 2004; Jackson et al., 2009] were also selected for 142Nd/144Nd measurement. The Hawaiian Koolau (KOO-8 and KOO-30) and Loihi (KK-18-18) samples were selected because they represent two extreme compositions in the Hawaiian mantle, where the Koolau location represents the most geochemically enriched extreme (highest87Sr/86Sr and lowest 143Nd/144Nd [Frey et al., 1994; Roden et al., 1994]) in Hawaii, and Loihi seamount has the highest 3He/4He ratios (>30 Ra [Kurz et al., 1982]). Lavas from Koolau and Loihi were previously examined for high-precision142Nd/144Nd by Murphy et al. [2010]. In order to further characterize the high 3He/4He reservoir, we also measured 142Nd/144Nd on the highest 3He/4He (>30 Ra) lavas from Ofu island in Samoa (Ofu-04-06 [Jackson et al., 2007b]) and Fernandina island in the Galapagos (NSK-97-214 [Kurz and Geist, 1999; Saal et al., 2007; Jackson et al., 2008]). A single sample from the Christmas Island (territory of Australia in the Indian Ocean) hot spot (70488–2) was selected for its EM1-type signature [e.g.,Hoernle et al., 2011]. Two MORB samples (2ΠD43 and D-113) were selected to better characterize the142Nd/144Nd of this reservoir. D-113 is from the Mid-Indian Ocean Ridge [Engel et al., 1965] and shows elevated 87Sr/86Sr (0.7032 [Subbarao and Hedge, 1973]) at high ϵ143Nd (+10.6 [Carlson et al., 1978]) typical of Indian Ocean MORB. D-113 was previously examined for high-precision142Nd/144Nd by Boyet and Carlson [2006]. 2ΠD43, a “popping” rock from the Atlantic mid-ocean ridge (∼14° N), exhibits a129Xe/130Xe anomaly [e.g., Moreira et al., 1998; Staudacher et al., 1989]. While high precision 142Nd/144Nd measurements were previously made on Loihi, Koolau and the Indian MORB samples [Murphy et al., 2010; Boyet and Carlson, 2006], we report the first high-precision142Nd/144Nd measurements on the Samoa, Mangaia, Christmas Island and Galapagos hot spots.

Table 1. Sample Locations and Justification for Analyzing 142Nd/144Nda
Sample IDLocationReason for 142Nd/144Nd AnalysisbLatitudeLongitude
  • a

    wr is whole rock analysis, cpx is clinopyroxene analysis.

  • b

    See text for references to original measurements.

  • c

    Locations are approximate as samples were taken before GPS availability and/or latitude and longitude were not reported in the original manuscript.

  • d

    The Christmas Island sample was actually dredged from Vening Meinesz Seamount, with a dredge depth of 1763 m.

2ΠD43 wrMORB (popping rock)MORB sample with 129Xe/130Xe anomaly relative to air.13.5°N45.0°Wc
D113 wrMORB, Indian OceanMORB sample previously analyzed for 142Nd/144Nd.23.343°S74.942°Ec
MAG-B-47 wrMangaia, Cook IslandsOIB lava with one of the most extreme HIMU (highest 206Pb/204Pb, 21.784) globally.21.93°S157.93°Wc
ALIA-115-21 wrSavai'i, SamoaOIB lava with the most extreme EM2 signature (highest 87Sr/86Sr, 0.7205) globally.14.090°S172.898°W
ALIA-115-21 cpxSavai'i, SamoaOIB magmatic phenocrysts with an extreme EM2 signature (87Sr/86Sr, 0.7216).14.090°S172.898°W
ALIA-115-18 wrSavai'i, SamoaOIB lava with the second most extreme EM2 signature (second highest 87Sr/86Sr, 0.7186) globally.14.090°S172.898°W
ALIA-115-18 cpxSavai'i, SamoaOIB magmatic phenocrysts with an extreme EM2 signature (87Sr/86Sr, 0.7202-0.7208).14.090°S172.898°W
78-1 cpxMalumalu, SamoaOIB lava with a high 87Sr/86Sr ratio (0.7089) hosting cpx with high 87Sr/86Sr ratios (0.7077-0.7083)14.623°S169.726°W
T54 wrTa'u, SamoaOIB lava with one of the lowest 87Sr/86Sr ratios (0.7047) from Samoa.14.265°S169.497°W
Ofu-04-06 wrOfu, SamoaOIB lava with one of the highest 3He/4He ratios (33.8 Ra) globally.14.168°S169.642°Wc
NSK-97-214 wrFernandina, GalapagosOIB lava with one of the highest 3He/4He ratios (30.3 Ra) globally.0.300°S91.628°W
70488-2 wrChristmas IslanddOIB lava from a known EM1 hotspot.11.625°S103.682°Ec
KK-18-8 wrLoihi, HawaiiOIB lava with one of the highest 3He/4He ratios (32.3 Ra) globally.18.96°N155.27°Wc
KOO-8 wrKoolau, HawaiiOIB lava with one of the most enriched geochemical signatures (high 87Sr/86Sr) from Hawaii21.31°N157.65°Wc
KOO-30 wrKoolau, HawaiiOIB lava with one of the most enriched geochemical signatures (high 87Sr/86Sr) from Hawaii21.31°N157.65°Wc

3. Methods

[7] New 142Nd/144Nd data on the OIB and MORB samples are reported in Table 2. The analytical procedures for 142Nd/144Nd measurement in OIB and MORB samples are similar to those reported by O'Neil et al. [2008]. 100–200 mg of sample powder (whole rock basalt or clinopyroxene [cpx]) was dissolved in concentrated HF-HNO3 and chemically separated following the procedures outlined in Boyet and Carlson [2005]. The procedure is designed to reduce interference from Sm and Ce isobars. For the samples studied here, the 147Sm/146Nd ratio is less than 3.2 × 10−6 for all sample runs (Table 2). The 140Ce/146Nd ratio is <7.8 × 10−5 for 17 of the 23 sample runs, and the remaining six of the runs have 140Ce/146Nd ratios between 1.0 × 10−4 and 8.1 × 10−4 (Table 2). Following correction for both Ce and Sm interferences all samples exhibit 142Nd/144Nd ratios that are indistinguishable from the terrestrial standard. The total lab blank for Nd is <20 pg, or <0.008% of the sample load, so no correction was made for blank contribution.

Table 2. 142Nd/144Nd of Standards and Samples (Including Replicate Runs) Reported in This Studya
  • a

    Abbreviations are as follows: whole rock (wr), clinopyroxene (cpx). The samples were not spiked, so the exact filament loads are not known (but should be between 250–500 ng of Nd). μ142Nd = (142Nd/144Ndsample/142Nd/144Ndstandard − 1)*106, where 142Nd/144Ndstandard is the average of the standard values run in the study (1.1418383 ± 0.0000063, ±2σ standard deviation, n = 12). Standard and sample data in this study were measured in four separate barrels: Barrel 218 (Aug 26–Sept 3, 2008), Barrel 223 (Sept 24–26, 2008), Barrel 232 (Oct 20–26, 2008), Barrell 233 (Oct 27–28, 2008). Data for 144Nd is expressed in volts (10−11 amps). The 146Nd/144Nd is not corrected for fractionation. Data were corrected for mass fractionation using the exponential law and normalized to a 146Nd/144Nd of 0.7219. The 143Nd/144Nd of the whole rock and cpx separates are offset for samples ALIA-115-18 and ALIA-115-21. This is consistent with earlier observations of cpx-whole rock isotopic disequilibrium in Samoan lavas [Jackson et al., 2009]. A high precision 142Nd/144Nd measurement was made on the Indian MORB sample (D113 [Boyet and Carlson, 2006]) and is shown together with our new measurement of this sample in Figure 1.

  • b

    Rerun original load of the same filament.

  • c

    Rerun of the same batch of chemistry as the first run, but using a different filament.

  • d

    Rerun using a new (the second) batch of chemistry on the same powder, never run on another filament.

  • e

    Rerun using a new (the third) batch of chemistry on the same powder, never run on another filament.

Sample IDLocationwr or cpx?Barrell ## Cycles144Nd Volts142Nd/144Nd Dynamic±2σμ142Nd±2σ142Nd/144Nd Static143Nd/144Nd Static145Nd/144Nd Static146Nd/144Nd Static148Nd/144Nd Static150Nd/144Nd Static140Ce/146Nd ppm147Sm/146Nd ppm
JNdi  2185203.01.14183290.0000048  1.14183860.51209980.34839410.72109200.24158260.2364578101.6
JNdi  2185021.91.14184050.0000058  1.14185360.51210430.34839580.72220010.24157940.2364538153.8
JNdi  2185023.21.14183930.0000039  1.14184290.51210320.34839630.72243370.24158180.2364563193.3
JNdi  22347514.71.14183820.0000024  1.14184230.51210260.34839560.72167620.24157900.236453152.4
JNdi  2234983.11.14183960.0000042  1.14184490.51210250.34839560.72256390.24158080.2364542252.9
JNdi  2324983.71.14183770.0000039  1.14184150.51210270.34839540.72121600.24158050.236454572.4
JNdi  2324953.31.14184070.0000040  1.14184250.51210470.34839590.72107140.24157840.2364515292.6
JNdi  2325003.31.14184060.0000040  1.14184360.51210320.34839600.72128820.24158150.2364570162.7
JNdi  2325002.51.14183130.0000047  1.14184880.51210440.34839630.72127190.24158060.2364568260.9
JNdi  2325023.11.14183970.0000043  1.14184700.51211010.34839600.72094140.24157880.2364523192.9
JNdi  2333433.11.14184120.0000057  1.14185160.51210770.34839650.72188540.24158140.2364561162.3
JNdi  2335002.51.14183790.0000048  1.14185110.51210600.34839640.72130630.24157970.2364560133.4
   Standard Mean  1.1418383   1.14184570.51210430.34839580.72157890.24158040.2364549  
   2σ (ppm, std. dev.)  0.0000063   0.00000940.00000540.00000130.00112430.00000260.0000040  
2ΠD43MORB (popping rock)wr2182011.91.14184030.00001191.710.41.14183280.51304300.34839750.72338890.24157340.236454439−0.9
D113MORB, Indian Oceanwr2324953.11.14184230.00000413.53.61.14184430.51308280.34839670.72253760.24157910.2364512580.2
MAG-B-47Mangaia, Cook Islandswr2324974.11.14183810.0000036−−1.7
MAG-B-47repbMangaia, Cook Islandswr2325003.51.14183670.0000041−
ALIA-115-21Savai'i, Samoawr2184973.01.14182920.0000041−
ALIA-115-21repbSavai'i, Samoawr2184942.61.14183200.0000044−−1.3
ALIA-115-21repcSavai'i, Samoawr2234413.41.14183600.0000071−
ALIA-115-21repdSavai'i, Samoawr2323394.21.14183920.00000500.84.41.14184540.51228690.34840590.72223100.24157720.23644631762.9
ALIA-115-21repeSavai'i, Samoawr2324974.21.14184210.00000353.33.11.14185210.51228540.34839870.72317050.24158210.236454231−0.6
ALIA-115-21Savai'i, Samoacpx2324983.11.14183680.0000043−
ALIA-115-21repbSavai'i, Samoacpx2324933.41.14184310.00000404.23.51.14184620.51227490.34839480.72351170.24157860.236453526−0.5
ALIA-115-18Savai'i, Samoawr2325032.61.14183820.0000048−−2.0
ALIA-115-18Savai'i, Samoacpx2325014.31.14183400.0000037−−1.4
ALIA-115-18repbSavai'i, Samoacpx2325013.91.14182940.0000039−−1.5
78-1Malumalu, Samoacpx2322983.41.14183920.00000580.85.11.14183780.51259940.34839360.72292310.24157860.23644952922.0
T54Ta'u, Samoawr2325013.71.14183140.0000041−−0.7
Ofu-04-06Ofu, Samoawr2184983.11.14183460.0000043−−0.4
Ofu-04-06repbOfu, Samoawr2182032.21.14184400.00000935.08.21.14185030.51281810.34839720.72462310.24157860.236455630.3
NSK-97-214Fernandina, Galapagoswr2334983.71.14183820.0000039−−0.8
70488-2Christmas Islandwr2335015.41.14183410.0000034−−0.3
KK-18-8Loihi, Hawaiiwr2184945.91.14183250.0000031−−0.9
KOO-8Koolau, Hawaiiwr2324064.21.14183520.0000045−
KOO-30Koolau, Hawaiiwr2332222.11.14183380.0000093−
   Sample mean  1.1418365   1.1418414 0.34839680.72299410.24157900.2364517  
   2σ (ppm, std. dev.)  0.0000084   0.0000109 0.00000470.00133300.00000420.0000055  

[8] Approximately 250–500 ng of Nd was loaded on a Re double filament and analyzed on the Thermo-Finnigan Triton thermal ionization mass spectrometer at DTM following the mass spectrometric procedures outlined inCarlson et al. [2007] and O'Neil et al. [2008]. A two-step dynamic routine was used to provide static measurements of all Nd isotope ratios and140Ce and 147Sm, with a dynamic measurement of 142Nd/144Nd. Each sample or standard run on the Triton takes approximately 5 h, including the time required for 540 cycles (where two 8-s integrations are made for each cycle to obtain a dynamic142Nd/144Nd measurement) and time for 30 s of baselines after each block of 30 cycles. We used amplifier rotation during measurement, and an amplifier gain calibration was made at the start of each day. Each separate analysis, including run lengths (expressed in number of cycles) and 144Nd intensities, are shown in Table 2. Five samples were run twice on the same filament (denoted by the superscript “b” in Table 2: ALIA-115-21, MAG-B-47, ALIA-115-21cpx, ALIA-115-18cpx, Ofu-04-06). For one sample (ALIA-115-21), the Nd separated from the same batch of chemistry was loaded on another filament and run in a new barrel. The total chemistry (including sample dissolution on the same batch of powder) was repeated two more times on this sample, and the Nd separated was run on two additional filaments in yet another barrel. In all cases, replicates follow the basic conclusion of the study: oceanic lavas have142Nd/144Nd that are indistinguishable from the terrestrial standard (Figure 1). The sample data show no correlation between dynamically measured 142Nd/144Nd and either statically measured 148Nd/144Nd or 150Nd/144Nd that would be suggestive of deviation from the exponential mass dependence that was used for mass fractionation correction in this study.

Figure 1.

New high precision 142Nd/144Nd measurements on global MORB (Indian MORB and popping rock) and OIB (Mangaia, Samoa, Hawaii, Galapagos and Christmas Island) lavas. The new data are plotted with published MORB data and data from the Hawaii, Pitcairn, Societies, Iceland and Gough hostpots. The published data are from Boyet and Carlson [2006], Caro et al. [2006], Andreasen et al. [2008] and Murphy et al. [2010]. Error bars on each sample represent either the 2σ standard error of the mean or 2σ of the error on the mean for replicate analyses. All measured OIB and MORB samples are indistinguishable from the terrestrial standard at 5 ppm external precision (gray bar), and offset to higher 142Nd/144Nd ratios than ordinary chondrites. 142Nd/144Nd analyses from two localities with demonstrated anomalies are shown for context: Isua, Greenland [Boyet and Carlson, 2006; Caro et al., 2006; Bennett et al., 2007; Rizo et al., 2011] and Nuvvuagittuq, Quebec [O'Neil et al., 2008]. The numbers to the left of the samples represent the following samples measured as part of this study (the values in parenthesis indicate how many times a sample was measured, if measured more than once; see Table 2): 1 = 2ΠD43, 2 = D113, 3 = MAG-B-47 (n = 2), 4 = ALIA-115-21 whole rock (n = 5), 5 = ALIA-115-21cpx (n = 2), 6 = ALIA-115-18 whole rock, 7 = ALIA-115-18cpx (n = 2), 8 = 78–1cpx, 9 = T54, 10 = Ofu-04-06 (n = 2), 11 = NSK-97-214, 12 = 70448–2, 13 = KK-18-18, 14 = KOO-8, 15 = KOO-30.

4. Results

[9] This study confirms and extends the observation that oceanic lavas hosting a wide variety of mantle components exhibit 142Nd/144Nd ratios that are identical to the terrestrial standard. Within analytical uncertainty, none of the modern OIB and MORB lavas reported here or in other studies [Boyet and Carlson, 2006; Caro et al., 2006; Andreasen et al., 2008; Murphy et al., 2010] exhibit 142Nd/144Nd ratios different from the terrestrial standard (Figure 1 and Table 2). Data for lavas representing each of the mantle end-members—HIMU, EM2, EM1, MORB and FOZO (high3He/4He)—are reported in this study. This is the first data demonstrating that lavas with radiogenic Pb-isotopic (HIMU) compositions (206Pb/204Pb > 21) have 142Nd/144Nd like all other modern terrestrial lavas. The 142Nd/144Nd result on extreme Samoan EM2 lavas is consistent with earlier measurements on less extreme EM2 lavas from the Societies [Caro et al., 2006]. Additionally, we show that enriched Samoan lavas, including the high 3He/4He lava from Ofu (Ofu-04-06) and cpx separates from Malumalu seamount lava AVON3-78-1, have142Nd/144Nd ratios indistinguishable from the terrestrial standard. The new data on Koolau and Christmas Island lavas support the observation that lavas hosting an EM1 signature, like lavas from Pitcairn [Boyet and Carlson, 2006; Caro et al., 2006], have 142Nd/144Nd like all other modern terrestrial lavas. Together with measurements of Hawaiian and Icelandic lavas with high 3He/4He (Murphy et al., 2010; Andreasen et al., 2008), this study supports the contention that lavas with the highest 3He/4He (>30 Ra) from all the four hot spots with the highest 3He/4He (>30 Ra: Iceland, Hawaii, Galapagos, Samoa) exhibit 142Nd/144Nd that is identical to all other modern terrestrial lavas. Previous studies have shown that MORB lavas [Boyet and Carlson, 2006; Caro et al., 2006] and abyssal peridotites [Cipriani et al., 2011] have 142Nd/144Nd ratios that are indistinguishable from the terrestrial standard. We confirm these results with 142Nd/144Nd measurements on two different MORB lavas including sample 2ΠD43, a “popping” rock that exhibits a 129Xe/130Xe anomaly relative to the atmosphere [Staudacher et al., 1989].

5. Discussion

5.1. Superchondritic 142Nd/144Nd in the Mantle Sampled by OIBs and MORBs

[10] Together with previously published results [Boyet and Carlson, 2006; Caro et al., 2006; Andreasen et al., 2008; Murphy et al., 2010], our results show that all MORB and OIB lavas–from 5 different mid-ocean ridge locations and 9 different hot spots–host142Nd/144Nd that is identical to the terrestrial standard (Figure 1 and Table 2). For several different reservoirs to evolve simultaneously in this brief time interval (<30 Ma) to have the exact same Sm/Nd (6% higher than chondrite) and 142Nd/144Nd (18 ± 5 ppm higher than chondrite) would be fortuitous. Therefore, we suggest that all measured modern terrestrial mantle reservoirs that contribute to OIB and MORB volcanism descend from a single early formed reservoir with a superchondritic Sm/Nd ratio (Figure 2).

Figure 2.

143Nd/144Nd evolution of a chondritic and non-chondritic Earth. Both models are consistent with the observation that all accessible modern, mantle-derived terrestrial samples have142Nd/144Nd 18 ± 5 ppm higher than chondrites, and were derived from an early formed (<30 Ma after accretion) precursor reservoir with Sm/Nd ∼6% higher than chondrites. This precursor reservoir has a present-day143Nd/144Nd of ∼0.5130 (ε143Nd = 7.2 [Jackson and Carlson, 2011]) and is (bottom) an EDR (early depleted reservoir) if BSE is chondritic (which implies the existence of an EER), or (top) a non-chondritic BSE. HIMU, EM1, EM2, DMM and most of the continental crust (excluding only a few, rare Archaean/Hadean terranes) were extracted from either the EDR (if the BSE is chondritic) or the BSE (if the BSE is not chondritic). In the chondritic Earth case (bottom), terrestrial samples withε143Nd < 0 (143Nd/144Nd < 0.51263) are considered enriched relative to BSE, and are considered depleted if ε143Nd > 0 (143Nd/144Nd > 0.51263); in the non-chondritic Earth case (top), terrestrial samples withε143Nd < 7.2 (143Nd/144Nd < 0.5130) are considered enriched, and are considered depleted if ε143Nd > 7.2 (143Nd/144Nd > 0.5130). The choice of reference frame is important, as the bulk of OIB 143Nd/144Nd measurements plot within a range of 143Nd/144Nd ratios of 0.51263 and 0.5130. The ε143Nd of the highest 3He/4He Baffin Island lavas is also indicated [Stuart et al., 2003; Starkey et al., 2009], and coincides with the value of the EDR or the non-chondritic BSE. A ∼2 Ga formation age of the EM and HIMU reservoirs is chosen to be consistent with the secondary isochron age of the mantle array in207Pb/204Pb vs 206Pb/204Pb isotopic space [e.g., Zindler and Hart, 1986]. A 3 Ga formation age of DMM is chosen to be consistent with Workman and Hart [2005]. The figure is adapted from Jackson et al. [2007b]. ε143Nd = (143Nd/144Ndsample/143Nd/144NdCHUR − 1) × 104, where 143Nd/144NdCHUR = 0.51263 [Bouvier et al., 2008].

[11] The origin of this progenitor reservoir is still unknown. The progenitor reservoir is either a non-chondritic BSE or the high Sm/Nd “depleted” reservoir (EDR) produced in an early differentiation event where an early enriched reservoir (EER) was extracted from the mantle [Boyet and Carlson, 2005; Caro et al., 2008; O'Neill and Palme, 2008; Caro and Bourdon, 2010; Jackson et al., 2010]. The non-chondritic BSE and the EDR are geochemically identical entities, characterized by the same superchondritic Sm/Nd (6% higher than chondrites),143Nd/144Nd (0.5130) and 142Nd/144Nd (18 ± 5 ppm higher than chondritic). An EER is only needed if the BSE has chondritic relative abundances of refractory lithophile elements (Figure 2). This study and previous studies have identified no geochemical evidence that any surface rock has been derived from the EER [e.g., Boyet and Carlson, 2006; Caro et al., 2006; Andreasen et al., 2008; Murphy et al., 2010]. Either the EER has been convectively isolated from participating in surface magmatism throughout Earth history, or there is no EER and the BSE does not have chondritic relative abundances of the refractory lithophile elements.

[12] Two explanations that have been proposed for a BSE deficient in the more incompatible of the refractory lithophile elements are impact erosion of planetesimal crusts prior to accretion to Earth [O'Neill and Palme, 2008; Caro et al., 2008] or volcanic ejection of partial melts from small planetesimals [Warren, 2008]. In modeling the trace element composition of the EDR/non-chondritic Earth [e.g.,Boyet and Carlson, 2005; Jackson et al., 2010; Jackson and Carlson, 2011] an important feature is the relative smoothness of the chondrite-normalized patterns when plotting the refractory incompatible elements according to their incompatibility during the relatively shallow mantle melting involved in the production of MORB or OIB [e.g.,Hofmann, 1988]. The shape of the incompatible element pattern of the EDR/nonchondritic BSE thus is most consistent with the loss of a partial melt generated at relatively low pressure [Boyet and Carlson, 2005] such as might occur on small planetesimals prior to their accretion to Earth. Alternatively, if this differentiation occurred on Earth, then a possible explanation of the EDR trace element pattern would be removal of an early formed terrestrial crust either through impact erosion or its subduction and permanent storage in the deep mantle. The timing of these early events—collision erosion or subduction of early crust—is not well known, but is likely to have happened well before 30 Ma, as suggested by recent modeling efforts [Korenaga, 2009] and the recent revision of the 146Sm half-life from 103 Ma to 68 Ma [Kinoshita et al., 2012].

[13] An important consequence of a non-chondritic BSE model is as follows: Because all modern terrestrial mantle reservoirs were derived from this non-chondritic BSE, a non-chondritic BSE becomes the standard for determining whether a reservoir is considered “enriched” (<0.5130) or “depleted” (>0.5130). For comparison, in a chondritic world, all reservoirs with143Nd/144Nd > 0.51263 [Bouvier et al., 2008] are considered depleted, and reservoirs with 143Nd/144Nd < 0.51263 are enriched. Therefore, it is the range of 143Nd/144Nd ratios from 0.51263 to 0.5130 that is affected by a change of reference frame: considered “depleted” in a non-chondritic world, lavas with143Nd/144Nd ratios from 0.51263 to 0.5130 would be considered “enriched” in the non-chondritic BSE model. The importance of this subtle difference is illustrated with HIMU lavas. In the non-chondritic reference-frame, extreme HIMU lavas from Mangaia with143Nd/144Nd of 0.51286 are considered enriched, not depleted. The non-chondritic BSE model suggests a history of incompatible element enrichment for the HIMU reservoir, instead of a history of incompatible element depletion. If HIMU forms from recycled oceanic crust [e.g.,Hofmann and White, 1982], then the history of isotopic enrichment in HIMU, as implied by a non-chondritic BSE, is consistent with the suggested origin of HIMU.

[14] Indeed, with the exception of lavas from Koolau, lavas erupted from the highest-flux mantle plume, Hawaii, generally have geochemically depleted143Nd/144Nd ratios (>0.51263) relative to a chondritic Earth. This implies a history of depletion in the mantle source of Hawaiian lavas. However, if the Earth is not chondritic, the bulk of Hawaiian shield lavas are actually enriched (143Nd/144Nd < 0.5130 [see Jackson and Carlson, 2011]), which implies a history of geochemical enrichment for this plume relative to the bulk composition of the silicate Earth. Enrichment of the Hawaiian plume source is also consistent with its Pb isotopic composition that plots to the right of the Pb geochron, whereas a depleted source would be expected to plot to the left of the geochron. Therefore, the non-chondritic reference frame provides a view that is more consistent with the standard model for the origin of mantle plumes, which maintains that plumes are buoyantly upwelling regions ofenriched crustal material that was subducted into the mantle in the geologic past [Hofmann and White, 1982; White and Hofmann, 1982]. Indeed, most of the global OIB database has 143Nd/144Nd that lies in the range of 0.51263 to 0.5130 (the median lies near 0.5130 [Zindler and Hart, 1986; Jackson and Carlson, 2011]), suggesting that, in a non-chondritic reference frame, most plume-derived lavas sampled geochemically enriched material relative to the BSE. By contrast, in a chondritic world, approximately 90% of OIB lavas are considered depleted (143Nd/144Nd > 0.51263) relative to BSE, which is not consistent with the hypothesis that plumes originate from recycling of ancient subducted enriched crustal materials.

[15] Whether any of the EDR (or non-chondritic BSE) has survived differentiation or mixing with recycled crust over Earth history to survive in the modern mantle is not clear. The143Nd/144Nd predicted for the progenitor reservoir (0.5130) is similar to that measured in lavas with the highest terrestrial mantle 3He/4He ratios [Boyet and Carlson, 2006; Caro et al., 2008; Caro and Bourdon, 2010; Jackson et al., 2010]. The high 3He/4He mantle reservoir, including lavas with the highest 3He/4He (50 Ra) from Baffin Island [de Leeuw et al., 2010], have 142Nd/144Nd identical to all other modern terrestrial lavas and 18 ± 5 ppm higher than chondrites. The highest 3He/4He Baffin Island lavas also have Pb-isotopic compositions that lie near the geochron, which is consistent with an ancient origin for this mantle reservoir [Jackson et al., 2010].

5.2. Is the Hidden Early Enriched Reservoir Simply “Hidden” in the Measurement Precision?

[16] We cannot exclude the possibility that a hidden EER exists in the deep mantle, but if it exists, it is not efficiently entrained in mantle plumes and is thus not sampled at Earth's surface. Entrainment of basal mantle layers by plumes is the subject of significant discussion, and Bourdon and Caro [2007] review the limits on the amount of a basal layer than can be entrained. Here, we place limits on how much EER can be entrained without measurably modifying the Nd isotopic composition of the EDR. We model the minimum amount of EER mantle (that contributes to a melt) necessary to generate a clearly resolved 142Nd/144Nd anomaly relative to the terrestrial standard (i.e., −10 ppm relative to the terrestrial standard). We assume an EER composition from Carlson and Boyet [2008] (142Nd/144Nd ranges from −38 to −54 ppm relative to the terrestrial standard, with Nd concentrations ranging from 2.2 to 8.9 ppm, respectively), and we assume that the EER mantle was diluted with mantle material with 1.0 ppm Nd (similar to the EDR composition of Carlson and Boyet [2008]) and 142Nd/144Nd identical to the terrestrial standard. This scenario requires that approximately 2–14% of the Nd in the mantle plume is from the EER to generate a 142Nd/144Nd anomaly of −10 ppm. A similar calculation, with similar results, notes that with one exception [Upadhyay et al., 2009], there has been no variation in 142Nd/144Nd in mantle-derived rocks since 3.5 Ga [e.g.,Bennett et al., 2007]. Using the same EER and EDR concentrations mentioned above, the lack of a secular trend in 142Nd/144Nd in mantle-derived rocks since 3.5 Ga allows no more than the same 2 to 14 weight % entrainment of the EER into the EDR. The calculation shows that, given current measurement precision, 2 to 14 weight % of an EER may be entrained in plumes or into the EDR without generating a measurable difference142Nd/144Nd. This neither supports nor denies the presence on an EER, but does point out the relative insensitivity of the Nd system in resolving the presence of geochemical reservoirs left over from early Earth differentiation because of the limited fractionation of Sm and Nd and the low initial abundance of 146Sm.


[17] We thank Tim Mock and Mary Horan for analytical assistance. We also thank Stan Hart, Mark Kurz, Nobu Shimizu and Mark Javoy for supplying samples. Reviews by Bernard Bourdon and an unnamed reviewer are greatly appreciated.