September 2016 LIP of the Month

Phanerozoic Large Igneous Provinces sample remnants of early differentiation events on Earth

Hanika Rizo

Assistant Professor, Dept. of Earth and Atmospheric Sciences, Université de Québec à Montréal, Geotop Research Center, 201 av. President Kennedy, Montreal, H2X 3Y7, Canada


Based on Rizo, H., Walker, R. J., Carlson, R. W., Horan, M. F., Mukhopadhyay, S., Manthos, V., Francis, D., Jackson, M. G., 2016. Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts. Science 352(6287), 809-812.

The restored geographical positions of large igneous provinces (LIP) have been associated with the large low shear wave velocity provinces (LLSVP) imaged by seismic tomography at the base of the mantle (e.g. Torsvik et al., 2008). This relationship suggest that LLSVPs could be the mantle source(s) of LIPs, and that these LLSVPs are long-lived domains (having stayed in the same approximate location for at least 360 million years), and have resisted subsequent homogenization by mantle convection. As seen by seismic tomography, the LLSVPs appear to be warmer and compositionally different from the surrounding mantle. Estimates of their volume range to as high as 7% of the mantle, or on the order of 6 × 1010 km3 suggesting that they are sufficiently large to source all LIP magmatism through Earth’s history. These observations suggest that LIPs could contain geochemical evidence from the most primitive silicate reservoirs in our planet.

Rizo et al. (2016a) reports geochemical data from two LIPs, the North Atlantic Igneous Province (NAIP) and the Ontong Java Plateau (OJP), that implies that these two LIPs have sampled remnants of the early differentiation events on Earth. The evidence comes from variations in 182W, the decay product of the short-lived radionuclide 182Hf. The 182Hf isotope was present when Earth formed, but decayed away relatively quickly, within 50 Ma after Earth’s formation. Therefore, variability in 182W found in the rocks from NAIP and OJP imply that their mantle sources were created during the first 50 Ma of the Earth’s history and have been isolated from convective stirring since that time.

Flood basalts are the largest volcanic eruptions identified in the geological record and represent the surface components of LIPs (Ernst, 2014). LIP events and their flood basalts occur regularly (on average every 20-30 Ma) and require the melting of large volumes of mantle during unusual thermal events in the history of mantle circulation. Flood basalts belonging to both North Atlantic Igneous Province (NAIP) and the Ontong Java Plateau in the western Pacific Ocean were studied in Rizo et al. (2016a) and the results are also summarized below.

Units from the NAIP were collected by Don Francis in Padloping Island (Fig. 1), a small island in the eastern coast of Baffin Island, located between Cape Searle and Cape Dyer.

Figure 1. Location of Padloping Island, Baffin Bay.

Lavas from this locality seem to be related to those occurring along the western coast of Greenland, between the Svartenhuk Peninsula and Disko Island. Both Baffin and West Greenland lavas were emplaced in the Paleocene about 62 Ma ago (e.g. Clarke and Upton 1971), and could have erupted contemporaneously with the opening of Davis Strait (Chalmers, 1991). The NAIP magmatism of these regions is geochemically and isotopically similar to the present-day Icelandic basalts, suggesting that the Baffin and West Greenland lavas were the first expression of the Iceland plume (e.g. Saunders et al., 1997). Estimated total magma volumes of the NAIP range between 6.6 to 10 Mkm3 (Eldholm and Grue, 1994; White and McKenzie, 1989). The lavas on the Baffin Bay locale are almost entirely picritic in composition and their cumulative thickness is ~ 750 km (Francis, 1985). Lower sections (300 m) of these lavas erupted subaqueously and exhibit pillow lava structures (Fig. 2), while upper sections (450 m) were emplaced in a subaerial mode.

Figure 2. Pillow lava structures of the lower sections of Baffin Bay picrites. Photo credits : Don Francis.

Figure 3. Location of the Ontong Java Plateau, north of the Salomon Islands.

Ontong Java is Earth’s largest known volcanic province covering a surface of ~ 2 Mkm2 and involving over 44 Mkm3 of magma (Courtillot and Renne, 2003). It can be reconstructed with the coeval Manihiki and Hikurangi oceanic plateaus to represent a single LIP of nearly 80 Mkm3 emplaced at ~ 123 Ma (Taylor, 2006; Chandler et al. 2012). The Ontong Java Plateau (OJP) rock sample analyzed in Rizo et al. (2016a) was collected from the eastern flank of the Ontong Java plateau by the Ocean Drilling Project (ODP) Leg 192, site 1187 (Fig. 3). The rock samples of this site are mainly basaltic pillow lavas, characterized by high MgO and low TiO2 contents, suggesting their formation through high percentages of partial melting (18-30%, Mahoney et al., 2001).

Rocks from both the NAIP and OJP were perfect targets for a study of short-lived 182Hf-182W. Some Baffin Bay lavas were known to contain the highest 3He/4He ratios (Starkey et al., 2009, Figure 4) and lowest D/H ratios (Hallis et al., 2015) ever measured, suggesting their mantle source is relatively undegassed. Lead isotopic compositions for both Baffin and Ontong Java volcanic rocks indicate that their mantle sources were formed early in Earth’s history (Jackson et al., 2011, Figure 5). All of these observations were indicative of NAIP and OJP having mantle sources that are ancient and that got isolated since shortly after Earth’s accretion.

Figure 4. 3He/4He vs. 87Sr/66Sr ratios for different ocean island basalts, large igneous provinces (e.g. Baffin Island (BI)) and mid-ocean ridge basalts. Reference: Starkey et al. (2009).

Figure 5. Lead isotopic compositions for different rock samples from various large igneous provinces plot between the 4.43 Ga and 4.50 Ga geochrons. These observations are consistent with these lavas sampling an ancient mantle source. Reference: Jackson et al. (2010).

The results from the high precision W isotope analyses of NAIP and OJP rocks are shown in Figure 6, compared to two modern oceanic basalts and several W isotope standards. Both NAIP and OJP rocks show 182W excesses compared to the estimated composition of the modern mantle. The parent isotope of 182W is 182Hf, which was only actively decaying during the first 50 Ma of the Earth’s history. The 182W variations thus detected in the NAIP and OJP rocks could have only been produced prior 4.5 billion years ago. These are not the first 182W variations that have been discovered, other variations have recently been detected in rocks from the ~ 4.3 Ga Nuvvuagittuq greenstone belt from northern Quebec (Touboul et al., 2014), in the Acasta Gneisses from the Northwest Territories (Willbold et al., 2015), in the 3.8-3.7 Ga Isua Supracrustal belt from southwest Greenland (Willbold et al., 2011; Rizo et al., 2016b), in the 3.55 Ga Shapenburg komatiites from South Africa (Puchtel et al., 2016), and in the 2.7 Ga Kostomuksha komatiites from the Baltic Shield (Touboul et al., 2012). In these previous studies the 182W anomalies were found in rocks older than 2.7 Ga. This suggests that early-formed mantle domains (whose compositions were established by events occurring during the first 50 Ma of the Earth’s history) are preserved from mixing resulting from mantle convection until at least c. 2.7 Ga, i.e. for nearly half of Earth’s existence. The discovery of early-formed 182W isotope variations in rocks from the Phanerozoic LIPs such as NAIP and OJP imply that these early-formed mantle domains have been somehow preserved in the mantle throughout most of Earth’s history.

Figure 6. m182W values measured in for the Baffin and Ontong Java Plateau rock samples, the geological reference materials VE-32 (MORB) and BHVO-1 (OIB), and the Alfa Aesar W standard. The m182W values are expressed as deviations in ppm from the average value measured for the W standard. The gray shaded area represents 2s for the average W standard value. Reference: Rizo et al. (2016a).

The cause(s) of 182W variations in terrestrial rocks is still a matter of debate, but center principally on two hypotheses: 1) fractionation of Hf/W ratios during early (< 50 Ma) differentiation events leading to 182W variations in the differentiated reservoirs, and 2) incomplete homogenisation into the mantle of late-accreted materials having different W isotopic compositions (primitive meteorites have on average a m182W value of - 200 ppm). The different hypotheses won’t be developed here, but are well described in various recent publications (Puchtel et al., 2016; Rizo et al., 2016a; 2016b; Touboul et al., 2012; 2014; Willbold et al., 2012;2015).

Regardless of the origin of the 182W variability, the W isotope compositions, together with He and Pb isotopic compositions and D/H ratios of the two LIPs studied in Rizo et al. (2016a) seem to be consistent, and surprisingly imply that the LIP mantle sources were formed by very early processes and that still remain isolated in the mantle today, perhaps in the deep mantle as part of the LLSVPs.

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