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A Statistical Description of Concurrent Mixing and Crystallization during MORB Differentiation: Implications for Trace Element Enrichment

Shorttle, Oliver and Rudge, John F. and Maclennan, John and Rubin, Ken H. (2016) A Statistical Description of Concurrent Mixing and Crystallization during MORB Differentiation: Implications for Trace Element Enrichment. Journal of Petrology, 57 (11-12). pp. 2127-2162. ISSN 0022-3530.

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The pattern of trace element enrichment and variability found in differentiated suites of basalts is a simple observable, which nonetheless records a wealth of information on processes occurring from the mantle to crustal magma chambers. The incompatible element contents of some mid-ocean ridge basalt (MORB) sample suites show progressive enrichment beyond the predictions of simple models of fractional crystallization of a single primary melt. Explanations for this over-enrichment have focused on the differentiation processes in crustal magma chambers. Here we consider an additional mechanism and focus instead on the deviation from simple fractionation trends that is possible by mixing of diverse mantle-derived melts supplied to magma chambers. A primary observation motivating this strategy is that there is significant chemical diversity in primitive high-MgO basalts, which single liquid parent models cannot match. Models were developed to simulate the compositional effects of concurrent mixing and crystallization (CMC): diverse parental melts were allowed to mix, with a likelihood that is proportional to the extent of fractional crystallization. Using a simple statistical model to explore the effects of concurrent mixing and crystallization on apparent liquid lines of descent, we show how significant departure from Rayleigh fractionation is possible as a function of the diversity of trace elements in the incoming melts, their primary MgO content, and the relative proportion of enriched to depleted melts. The model was used to make predictions of gradients of trace element enrichment in log[trace element]–MgO space. These predictions were compared with observations from a compilation of global MORB and provide a test of the applicability of CMC to natural systems. We find that by considering the trace element variability of primitive MORB, their MgO contents and degree of enrichment, CMC accurately predicts the pattern of trace element over-enrichment seen in global MORB. Importantly, this model shows that the relationship between over-enrichment and incompatibility can result from mantle processes: the fact that during mantle melting maximum variability is generated in those elements with the smallest bulk Kd. Magma chamber processes are therefore filtering the signal of mantle-derived chemical diversity to produce trace element over-enrichment during differentiation. Finally, we interrogate the global MORB dataset for evidence that trace element over-enrichment varies as a function of melt supply. There is no correlation between over-enrichment and melt supply in the global dataset. Trace element over-enrichment occurs at slow-spreading ridges where extensive steady-state axial magma chambers, the most likely environment for repeated episodes of replenishment, tapping and crystallization, are very rarely detected. This supports a model whereby trace element over-enrichment is an inevitable consequence of chemically heterogeneous melts delivered from the mantle, a process that may operate across all rates of melt supply.

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Additional Information:© 2016 The Author. Published by Oxford University Press. Received November 10, 2015; Accepted August 18, 2016. Ed Stolper and Paula Antoshechkina are thanked for their comments on an early version of this paper, and Paula additionally for her help calculating magmatic heat evolution in MELTS. Mike Perfit and Laurence Coogan are thanked for their detailed reviews, which helped to improve the paper; remaining errors are the authors’ own. The authors would like to thank the Isaac Newton Institute for Mathematical Sciences for its hospitality during the programme ‘Melt in the Mantle’, which was supported by EPSRC Grant Number EP/K032208/1. O.S. was supported by Trinity College Cambridge through a Title A Fellowship and at Caltech by a Geology Option Postdoctoral Fellowship. J.F.R. thanks the Leverhulme Trust for support.
Funding AgencyGrant Number
Engineering and Physical Sciences Research Council (EPSRC)EP/K032208/1
Trinity College, CambridgeUNSPECIFIED
Caltech Division of Geological and Planetary SciencesUNSPECIFIED
Leverhulme TrustUNSPECIFIED
Issue or Number:11-12
Record Number:CaltechAUTHORS:20170428-085929543
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Official Citation:Oliver Shorttle, John F. Rudge, John Maclennan, Ken H. Rubin; A Statistical Description of Concurrent Mixing and Crystallization during MORB Differentiation: Implications for Trace Element Enrichment. J Petrology 2016; 57 (11-12): 2127-2162. doi: 10.1093/petrology/egw056
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77048
Deposited By: Tony Diaz
Deposited On:28 Apr 2017 16:12
Last Modified:03 Oct 2019 17:52

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