Abstract Details
(2020) A Geochemical Comparison of the Galápagos and Hawai‘i Plumes
Harpp K & Weis D
https://doi.org/10.46427/gold2020.959
The author has not provided any additional details.
05h: Room 2, Saturday 27th June 05:36 - 05:39
Karen Harpp
View abstracts at 2 conferences in series
Dominique Weis View all 12 abstracts at Goldschmidt2020 View abstracts at 22 conferences in series
Dominique Weis View all 12 abstracts at Goldschmidt2020 View abstracts at 22 conferences in series
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
Submitted by Keith Putirka on Wednesday 24th June 17:33
Hi Karen - this looks like a compelling test of the Weis et al. (2011) model, with volcanoes migrating from one source to another and tracking that change. I am curious about what is perahps a minor detail: the Nb/Zr vs. delta-Nb plot. Do any plumes, anywhere, plot in the "plume-hosted depleted quadrant"? Given that Nb appears on both axes, I wonder if there is always a high correlation in such plots, and that most plumes would fall along a similar line. If that were the case, then perhaps the plot only distinguishes Nb-enriched from Nb-depleted? -Keith
Hi Karen - this looks like a compelling test of the Weis et al. (2011) model, with volcanoes migrating from one source to another and tracking that change. I am curious about what is perahps a minor detail: the Nb/Zr vs. delta-Nb plot. Do any plumes, anywhere, plot in the "plume-hosted depleted quadrant"? Given that Nb appears on both axes, I wonder if there is always a high correlation in such plots, and that most plumes would fall along a similar line. If that were the case, then perhaps the plot only distinguishes Nb-enriched from Nb-depleted? -Keith
Submitted by Karen Harpp on Thursday 25th June 15:49
Hi Keith, apologies for the delay in getting back to you. To be honest, I have not tried that plot with more than Galapagos and Iceland data, yet. Instead, we tested it using Galapagos Spreading Center data. Here's the description from the paper (just accepted to G-Cubed) (it's just easier to paste it in here, I hope that's ok!) To distinguish between depleted lower mantle (OIB) from upper mantle (DUM) in the Galápagos, we examined the data from Galápagos Spreading Center with Fitton et al. (1997, 2003) approach. Axial GSC lavas between 101o and 83oW exhibit a systematic increase in enrichment as the distance to the Galápagos hotspot decreases, reaching a peak ~92oW (Figure 3D; Schilling et al., 1982; Schilling et al., 2003; Verma & Schilling, 1982). Westward from ~92oW along the ridge, geochemical, morphological, and geophysical evidence of plume contributions decreases systematically, ceasing abruptly at the 95.5oW propagating rift. Only GSC axial lavas west of the 95.5oW propagating rift fall in the lower left quadrant of the Nb/Zr-?Nb diagram (depleted MORB; Figure 2B), consistent with independent morphological and geochemical evidence that there is little to no influence from the Galápagos plume on the axis west of 95.5oW (e.g., Schilling et al., 2003; Ingle et al., 2010). Samples from east of the 95.5oW propagating rift, where geochemical and geophysical studies have confirmed there is a strong influence from the plume (e.g., Behn et al., 2004; Canales et al., 2002; Detrick et al., 2002; Ingle et al., 2010; Schilling et al., 2003; Sinton et al., 2003) exhibit increasingly positive Nb/Zr and ?Nb values (Figure 2B, inset), defining what is presumably a mixing trend between depleted MORB mantle and more enriched Galápagos compositions as influence of the plume increases eastward along the GSC. Thus, we conclude that the Nb/Zr-?Nb discrimination diagram is an effective method for distinguishing depleted OIB-sourced from MORB-sourced material. So that was the primary intellectual grounding behind using this discrimination diagram. Your point is a good one and warrants further investigation, for sure, though. Happy to keep discussing.
Hi Keith, apologies for the delay in getting back to you. To be honest, I have not tried that plot with more than Galapagos and Iceland data, yet. Instead, we tested it using Galapagos Spreading Center data. Here's the description from the paper (just accepted to G-Cubed) (it's just easier to paste it in here, I hope that's ok!) To distinguish between depleted lower mantle (OIB) from upper mantle (DUM) in the Galápagos, we examined the data from Galápagos Spreading Center with Fitton et al. (1997, 2003) approach. Axial GSC lavas between 101o and 83oW exhibit a systematic increase in enrichment as the distance to the Galápagos hotspot decreases, reaching a peak ~92oW (Figure 3D; Schilling et al., 1982; Schilling et al., 2003; Verma & Schilling, 1982). Westward from ~92oW along the ridge, geochemical, morphological, and geophysical evidence of plume contributions decreases systematically, ceasing abruptly at the 95.5oW propagating rift. Only GSC axial lavas west of the 95.5oW propagating rift fall in the lower left quadrant of the Nb/Zr-?Nb diagram (depleted MORB; Figure 2B), consistent with independent morphological and geochemical evidence that there is little to no influence from the Galápagos plume on the axis west of 95.5oW (e.g., Schilling et al., 2003; Ingle et al., 2010). Samples from east of the 95.5oW propagating rift, where geochemical and geophysical studies have confirmed there is a strong influence from the plume (e.g., Behn et al., 2004; Canales et al., 2002; Detrick et al., 2002; Ingle et al., 2010; Schilling et al., 2003; Sinton et al., 2003) exhibit increasingly positive Nb/Zr and ?Nb values (Figure 2B, inset), defining what is presumably a mixing trend between depleted MORB mantle and more enriched Galápagos compositions as influence of the plume increases eastward along the GSC. Thus, we conclude that the Nb/Zr-?Nb discrimination diagram is an effective method for distinguishing depleted OIB-sourced from MORB-sourced material. So that was the primary intellectual grounding behind using this discrimination diagram. Your point is a good one and warrants further investigation, for sure, though. Happy to keep discussing.
Submitted by Garrett Ito on Friday 26th June 17:27
Hi Karen. This is was a really great presentation! I like the test that you proposed suggesting that volcanoes that moved across the compositional divide in the plume, should be compositionally stratified. Are there any lavas sampled from the NE quadrant of the archipelago that are old-enough that they would have crossed the compositional divide in the mantle? If so, do their compositions reflect the bilateral asymmetry? Also, I think another prediction is that the bilaterally asymmetry should appear at the spreading axis. Does it, and if so, how?
Hi Karen. This is was a really great presentation! I like the test that you proposed suggesting that volcanoes that moved across the compositional divide in the plume, should be compositionally stratified. Are there any lavas sampled from the NE quadrant of the archipelago that are old-enough that they would have crossed the compositional divide in the mantle? If so, do their compositions reflect the bilateral asymmetry? Also, I think another prediction is that the bilaterally asymmetry should appear at the spreading axis. Does it, and if so, how?
Submitted by Karen Harpp on Friday 26th June 19:59
Hi Garrett! Thanks for the kind words, much appreciated. I really enjoyed your presentation as well, and your ideas to explain the anomalously warm lithosphere beneath Hawaii. I’m wondering whether you think the warmer lithosphere has an impact on the production of rejuvenated phase lavas in Hawaii, and whether they are somehow related to that property of the lithosphere? RE: your question about Galapagos: There are not many accessible samples that address your question in Galapagos, but there are two that do, at least at preliminary/sparsely sampled levels. Santa Cruz Island (central Galápagos): a) Older Platform Series, exposed in the northeast corner of the island, dates to >1.6 Ma; b) that’s overlain by the Main Shield series (as young as 74 ka). The Main Shield lavas have more depleted signatures than the Platform Series, and the Platform series was erupted near the current plume center. San Cristobal has a similar phenomenon, with samples getting more depleted as they get younger, but it’s not very thoroughly sampled. I think it’s harder to address this along the GSC. Schilling et al. (2003) noted a distinct difference in lead isotopic compositions E and W of the transform fault on the GSC, which does support the hypothesis to some extent. But we don’t really know how material gets to the ridge from the plume in enough detail to map it to how it’s expressed spatially along the ridge. So in theory, I agree with you, but if we have something like Sally Gibson’s recent proposal that the material is fed along a channel then distributed along the ridge plumbing system and sub-ridge flow, then that adds a convoluting process.
Hi Garrett! Thanks for the kind words, much appreciated. I really enjoyed your presentation as well, and your ideas to explain the anomalously warm lithosphere beneath Hawaii. I’m wondering whether you think the warmer lithosphere has an impact on the production of rejuvenated phase lavas in Hawaii, and whether they are somehow related to that property of the lithosphere? RE: your question about Galapagos: There are not many accessible samples that address your question in Galapagos, but there are two that do, at least at preliminary/sparsely sampled levels. Santa Cruz Island (central Galápagos): a) Older Platform Series, exposed in the northeast corner of the island, dates to >1.6 Ma; b) that’s overlain by the Main Shield series (as young as 74 ka). The Main Shield lavas have more depleted signatures than the Platform Series, and the Platform series was erupted near the current plume center. San Cristobal has a similar phenomenon, with samples getting more depleted as they get younger, but it’s not very thoroughly sampled. I think it’s harder to address this along the GSC. Schilling et al. (2003) noted a distinct difference in lead isotopic compositions E and W of the transform fault on the GSC, which does support the hypothesis to some extent. But we don’t really know how material gets to the ridge from the plume in enough detail to map it to how it’s expressed spatially along the ridge. So in theory, I agree with you, but if we have something like Sally Gibson’s recent proposal that the material is fed along a channel then distributed along the ridge plumbing system and sub-ridge flow, then that adds a convoluting process.
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